Large Doppler shifts are a challenge in underwater wireless acoustic communication compared to terrestrial wireless radio communication. Resampling and Doppler shift compensation are used at the receiver to counteract the effect of the Doppler shift introduced during transmission. Imperfect resampling can degrade system performance. While the conventional choice of Doppler compensation factor minimizes the intercarrier interference (ICI) around the central subcarrier, we show that using a Doppler compensation factor that treats lower subcarriers preferentially in terms of ICI can significantly improve the system performance in the case of imperfect resampling. Two ad-hoc schemes are proposed in the paper to find suitable Doppler compensation factors for a given Doppler factor and percentage of error in the resampling factor. The knowledge of these parameters facilitates the performance analysis of the schemes, rather than focusing on design aspects. The proposed schemes are tested for various relative vehicular speeds. In addition, different Doppler factors for the paths are considered. In all cases, our proposed schemes achieve higher video peak-signal-to-noise ratio compared to conventional schemes.

This paper presents tile-based wireless streaming of 360-degree videos with rate adaptation using viewport estimation. We propose a probabilistic model for viewport location to design importance weights of tiles. Based on the tile weights, we improve the streaming performance by dynamically adjusting quantization parameters and forward error correction rates across tiles.

A joint channel estimation and channel coding scheme is presented for channels with memory using polar codes. Unlike the conventional approach of first estimating all channel parameters and then performing channel decoding separately, the proposed scheme incorporates a subset of reliable estimates of channel parameters into the decoding procedure and computes decoding metrics averaged over the statistical behavior of the channel. Specifically, decoding algorithms for finite-state Markov channels of any order, for the Gauss-Markov channel and for flat-fading channels are presented. Further, by adapting list decoding to identify reliably-decoded bits within a codeword, channel estimation and decoding steps are performed iteratively to boost the reliability of channel estimation as well as error correction. In order to improve the performance even further, a new pilot arrangement scheme is developed that utilizes the structure of polar codes and sends pilot symbols embedded within the polar codewords. This construction can be viewed as a new family of shortened polar codes that can be of independent interest. Simulation results demonstrate the benefit of the proposed approach compared to existing solutions.

We are interested in a communication system that operates under stringent power constraints, but is flexible with bandwidth constraints. Our approach is to consider some of the key elements in a transceiver and optimize them for low power consumption, as opposed to optimizing them to minimize, say, average probability of error. An obvious consequence of this is that high complexity components of the system, such as matched filters, forward error correction that employs iterative decoders, coherent demodulators, and bandwidth-efficient modulation formats, are not feasible for this research. Rather, our system is designed using M-ary FSK with non-coherent detection, optimized two-pole bandpass filters (BPF), and Reed-Solomon (RS) codes with hard-decision decoding. Among other things, we show that by properly optimizing the key parameters of the BPFs and RS codes, we can design the system to be significantly less complex than an optimal one, and only lose about 1.2 (M=16) to 1.5 (M=2) dB in terms of performance.

A generalized framework of spectrum sensing disruption for a power-limited adversary is proposed in this paper. In the literature, a conventional sensing attack typically assumes that the adversary has perfect knowledge of the spectral usage status. The framework in this paper considers a more general case where there are uncertainties in the estimates at the adversary. These uncertainties are modeled utilizing the probability of detection and the probability of false alarm. Then, the sum of the conditional probabilities of false detection at the secondary within the spectral range of interest, conditioned on the adversary's estimated spectrum usage status, is maximized. It is shown that the optimal sensing attack, given perfect estimation is a special case of the proposed framework. When the adversary has perfect spectrum usage information, this framework reduces to a previously demonstrated optimal sensing disruption. When the adversary has imperfect information on the spectral status, the proposed framework is significantly more robust than conventional sensing attacks. Further, when the adversary's power budget increases, it asymptotically approaches the sensing disruption performance upper bound.

The impact of joint partial-time, partial-band jamming on a multicarrier (MC) asynchronous direct-sequence code-division multiple access (DS-CDMA) system in a fast fading environment is studied in conjunction with two different subcarrier combining and decoding schemes. An easy-to-evaluate upper bound using the Chernoff bound is provided and compared to simulation results. Simulation results suggest that for soft-decision decoding systems, under Rayleigh fading, full-time, full-band jamming is most effective. In contrast to the Rayleigh case, when a sufficiently strong line-of-sight component exists in the channel, the jammer's optimal strategy of attacking in time or frequency depends on the strength and the type of error correction that the system is deploying for that dimension. For hard-decision decoding systems, in Rayleigh fading, partial-band jamming is recommended. For the coding strategies examined, in Rician fading, the jammer should switch from full-time, partial-band jamming to a strategy that jams a higher percentage of the more heavily protected dimension as the jamming power increases. Furthermore, for AWGN channels, the results from the system show that the jammer should always jam a higher percentage of the more heavily protected dimension.

A joint channel estimation and channel coding scheme is presented for finite-state Markov channels using polar codes. Unlike the conventional approach of first estimating all channel parameters and then performing channel decoding separately, the proposed scheme incorporates a subset of reliable estimates of channel parameters into the decoding algorithm and computes decoding metrics averaged over the statistical behavior of the Markov channel. By adapting list decoding (without any inner code), channel estimation and decoding steps can be performed iteratively to boost the reliability of channel estimation as well as error correction. In order to improve the performance even further, a new pilot transmission scheme is developed that utilizes the structure of polar codes and sends pilot symbols along with code symbols. This construction can be viewed as a new family of shortened polar codes that can be of independent interest. Simulation results demonstrate the benefit of the proposed approach compared to existing solutions.

Disposable wireless video sensors have many potential applications but are subject to stringent energy constraints. We studied the minimization of end-to-end distortion under an total energy constraint, by means of optimizing FEC code rate, number of source bits, and energy allocation between video encoding and wireless transmission. A two-step approach is employed. First, the FEC rate is optimized by exhaustive search. Then a binary-search-based algorithm is proposed to optimize the energy allocation and number of source bits. Experiments show that the algorithm achieves a PSNR gain up to 1dB over some reasonable baselines. A simpler suboptimal algorithm is also tested and exhibits similar performance.

In this paper, cross-layer resource allocation methods for wireless video transmission are proposed. We propose two practical metrics to measure the relative importance of each forward error correction (FEC) block to the user's quality of experience. They can be obtained from the header information, and require no overhead bits or amendment of the header format of the existing standard video codec. We discuss adaptive modulation and coding scheme allocation, and adaptive energy allocation for the FEC blocks of a group of pictures. We also provide low-complexity resource allocation solutions. In both approaches, we consider two types of slice packetization, which affect the priority of FEC blocks. The proposed cross-layer resource allocation methods have significant performance gain over equal error protection, and similar performance to more complicated algorithms.

In this paper, the sensing disruption for a power limited adversary with estimation uncertainty is formulated and analyzed. The estimation uncertainty of the adversary is modeled in terms of its probability of false alarm in vacant bands and the probability of detection in busy bands. The strategy for the adversary is obtained by maximizing the sum of the conditional probabilities of false detection within the spectral range of interest, conditioned on the adversary's estimated spectrum usage status. The proposed algorithm is shown to be significantly more robust than conventional algorithms. It is shown in simulation results that, as the adversary's power budget increases, the proposed algorithm asymptotically approaches the performance upper bound when the adversary has perfect information on the spectrum usage status.

In resource allocation for a device-to-device (D2D) video transmission system, the performance improvement by applying the exact symbol error rate (SER) is compared with the conventional signal-to-interference-plus-noise-ratio-based SER evaluation method that uses a Gaussian approximation (GA) for the aggregated interference. An analytical SER expression for a D2D system using multicarrier bandlimited QAM is derived and then used in the resource allocation algorithm. We consider centralized resource allocation for the D2D system, given knowledge of the channel state information and the rate distortion information of the video streams, and propose an iterative algorithm for subcarrier assignment and power allocation. Bit-level simulations for different numbers of D2D pairs demonstrate a considerable improvement on user capacity and video peak signal-to-noise ratio by incorporating the proposed SER expression compared with the GA. By invoking the conditions under which the central limit theorem holds, and comparing these conditions with the number of interferers and the power ratio of the dominant interferer in the simulated D2D system, we also study why the GA for the interference degrades performance.

Subcarrier assignment and power allocation for device-to-device (D2D) video transmission using a filter bank multicarrier waveform in a Rayleigh fading environment are investigated. We analyze the co-channel interference between D2D pairs, and propose a cross-layer algorithm with a subcarrier assignment outer loop and a power allocation inner loop, which aims to optimize the overall video quality. Unlike the non-convexity in physical layer power allocation for maximizing the total throughput, the cross-layer power allocation problem is convex under certain conditions, so a high quality solution for power allocation can be efficiently found. Simulation results demonstrate a higher overall video quality by the proposed cross-layer algorithm compared with baseline algorithms.

We consider centralized cross-layer resource allocation for a device-to-device (D2D) video transmission system, given knowledge of the channel state information and the rate distortion information of the video streams, and propose an iterative algorithm for subcarrier assignment and power allocation. In the resource allocation, the performance improvement by applying the exact symbol error rate (SER) is compared with the conventional signal-to-interference-plus- noise-ratio (SINR) based SER evaluation method that uses a Gaussian approximation (GA) for the aggregated interference. An exact SER expression for a D2D system using multicarrier bandlimited QAM is derived and then used in the resource allocation algorithm. Bit-level simulations for different numbers of D2D pairs demonstrate a considerable improvement on user capacity and video peak signal-to-noise ratio by incorporating the proposed SER expression compared to the GA.

A joint source-channel rate-distortion (RD) optimization is proposed for video communication systems. The source coding and channel coding options are optimized by seeking the best trade-off between the estimated end-to-end distortion of a video packet and the sum of the number of source bits and forward error correction bits used to encode that packet. The proposed RD algorithm controls the total bit rate by using a Lagrange multiplier. Compared to conventional RD optimization schemes, which only optimize over the source coding modes of macroblocks, our proposed RD algorithm achieves superior performance over an AWGN channel.

We consider video transmission over a mobile cognitive radio (CR) system operating in a hostile environment where an intelligent adversary tries to disrupt communications. We investigate the optimal strategy for spoofing, desynchronizing, and jamming a cluster-based CR network with a Gaussian noise signal over a slow Rayleigh fading channel. The adversary can limit access for secondary users (SUs) by either transmitting a spoofing signal in the sensing interval, or a desynchronizing signal in the code acquisition interval. By jamming the network during the transmission interval, the adversary can reduce the rate of successful transmission. We show how the adversary can optimally allocate its energy across subcarriers during sensing, code acquisition, and transmission intervals. We determine a worst-case optimal energy allocation for spoofing, desynchronizing, and jamming, which gives an upper bound to the received video distortion of SUs. We also propose cross-layer resource allocation algorithms and evaluate their performance under disruptive attacks.

We consider the joint source-channel coding (JSCC) problem of 3-D stereo video transmission in video plus depth format over noisy channels. Full resolution and downsampled depth maps are considered. The proposed JSCC scheme yields the optimum color and depth quantization parameters as well as the optimum forward error correction code rates used for unequal error protection (UEP) at the packet level. Different coding scenarios are compared and the UEP gain over equal error protection is quantified for flat Rayleigh fading channels.

We consider an uplink multicarrier system with multiple video users who want to send compressed video data to the base station. In the time domain, we model the time-varying channel using Jakes' model, and in the frequency domain, each subcarrier is assumed to be independently fading. The video is scalably coded in units of a group of pictures (GOP), and users have different video rate distortion (RD) functions. At the beginning of the GOP, the base station collects both the RD information and the instantaneous channel state information (CSI) for subcarrier allocation purposes. We design a cross-layer resource allocation algorithm to assign subcarriers to users based on both the demand of the video and the quality of the channel. Once the resource allocation decision is made, the users then periodically adapt the modulation format of the subcarriers allocated according to the evolution of the CSI for the duration of the GOP. We show that our cross-layer resource allocation robustly outperforms two baseline algorithms, each of which uses only one layer of information for resource allocation.

We examine the performance of a cognitive radio system in a hostile environment where an intelligent adversary tries to disrupt communications by minimizing the system throughput. We investigate the optimal strategy for spoofing and jamming a cognitive radio network with a Gaussian noise signal over a Rayleigh fading channel. We analyze a cluster-based network of secondary users (SUs). The adversary may attack during the sensing interval to limit access for SUs by transmitting a spoofing signal. By jamming the network during the transmission interval, the adversary may reduce the rate of successful transmission. We present how the adversary can optimally allocate power across subcarriers during sensing and transmission intervals with knowledge of the system, using a simple optimization approach specific to this problem. We determine a worst-case optimal energy allocation for spoofing and jamming, which gives a lower bound to the overall information throughput of SUs under attack.

We consider a wireless relay network with a single source, a single destination, and a multiple relay. The relays are half-duplex and use the decode-and-forward protocol. The transmit source is a layered video bitstream, which can be partitioned into two layers, a base layer (BL) and an enhancement layer (EL), where the BL is more important than the EL in terms of the source distortion. The source broadcasts both layers to the relays and the destination using hierarchical 16-QAM. Each relay detects and transmits successfully decoded layers to the destination using either hierarchical 16-QAM or QPSK. The destination can thus receive multiple signals, each of which can include either only the BL or both the BL and the EL. We derive the optimal linear combining method at the destination, where the uncoded bit error rate is minimized. We also present a suboptimal combining method with a closed-form solution, which performs very close to the optimal. We use the proposed double-layer transmission scheme with our combining methods for transmitting layered video bitstreams. Numerical results show that the double-layer scheme can gain 2-2.5 dB in channel signal-to-noise ratio or 5-7 dB in video peak signal-to-noise ratio, compared with the classical single-layer scheme using conventional modulation.

We consider the joint source-channel coding problem of a 3D video transmitted over an AWGN channel. The goal is to minimize the total number of bits, which is the sum of the number of source bits and the number of forward error correction bits, under two constraints: the quality of the primary view and the quality of the secondary view must be greater than or equal to a predetermined threshold at the receiver. The quality is measured in terms of the expected PSNR of an entire decoded group of pictures. A MVC (multiview coding) encoder is used as the source encoder, and rate compatible punctured turbo codes are utilized for protection of the encoded 3D video over the noisy channel. Equal error protection and unequal error protection are compared for various 3D video sequences and noise levels.

This paper considers a relay system using two-layer superposition coding to minimize the expected distortion of a Gaussian source at the destination node. For the system, we propose two types of layer- selective relaying based on the local decoding result at the relay and the decoding result at the destination node fed back to the relay. One type of the proposed scheme uses decode-and-forward (DF) in the design of the relay signals, while the other type uses both DF and amplify-and-forward (AF). For the proposed scheme, we analyze the outage probabilities and evaluate the expected distortion according to the relay location. The results reveal that the proposed scheme improves the finite SNR performance, in particular when the relay node is closer to the source node than it is to the destination node.

We consider a relay network that delivers a Gaussian source by employing successive refinement source coding and superposition coding of layers at the source node and successive decoding at the relay and destination nodes. For the network, making use of the decoding results at the relay and destination nodes of the first transmission, an efficient relaying strategy of layers is proposed to minimize the expected distortion (ED) when only the average channel state information is available at the source node. Three types of the proposed scheme, defined as Prop-DF, using decode-and-forward signals, Prop-AF, using amplify-and-forward signals, and Prop-MF, using mixed-forward signals, are addressed and analyzed in terms of the outage probability and distortion exponent. Unlike other studies, we have also taken the relay location into account in deriving the distortion exponent showing the high SNR behavior of the ED. The results show that the proposed scheme increases the distortion exponent up to twice that of the conventional relaying schemes when the relay is close to the source node, and that Prop-MF provides the best performance for most relay locations.

We consider a multiuser OFDM system in which users want to transmit videos via a base station. The base station knows the channel state information (CSI) as well as the rate distortion (RD) information of the video streams and tries to allocate power and spectrum resources to the users according to both physical layer CSI and application layer RD information. We derive and analyze a condition for the optimal resource allocation solution in a continuous frequency response setting. The optimality condition for this cross layer optimization scenario is similar to the equal slope condition for conventional video multiplexing resource allocation. Based on our analysis, we design an iterative subcarrier assignment and power allocation algorithm for an uplink system, and provide numerical performance analysis with different numbers of users. Comparing to systems with either only physical layer or only application layer information available at the base station, our results show that the user capacity and the video PSNR performance can be increased significantly by using cross layer design. Bit-level simulations which take into account the imperfection of the video coding rate control, the variation of RD curve fitting, as well as channel errors, are presented.

We study motion compensated fine granular scalable (MC-FGS) video transmission over multiinput multioutput (MIMO) wireless channels applicable to video streaming, where leaky and partial prediction schemes are applied in the enhancement layer of MC-FGS to exploit the tradeoff between error propagation and coding efficiency. For reliable transmission, we propose unequal error protection (UEP) by considering a tradeoff between reliability and data rates, which are controlled by forward error correction and MIMO mode selection to minimize the average distortion. In a high Doppler environment, where it is hard to get an accurate channel estimate, we investigate the performance of the proposed MC-FGS video transmission scheme with joint control of both the leaky and partial prediction parameters, and the UEP. In a slow fading channel, where the channel throughput can be estimated at the transmitter, adaptive control of prediction parameters is considered.

Rateless codes allow a user to incrementally send additional redundancy, so they can be useful for heterogeneous and time-varying networks for which the choice of redundancy level in advance is difficult. Rateless codes are an attractive application layer forward error correction solution due to their flexibility and capacity-approaching performance. The original rateless codes were developed for the delivery of equally important information. In many multimedia applications, some data symbols are more important than others. Unequal error protection (UEP) designs are attractive solutions for such transmissions. However previous UEP rateless code designs were aimed for coarsely layered sources and might exhibit poor performance for fine-grained progressive coding. The main objective of this paper is to introduce a more generalized coding scheme, parameters of which can be tailored for progressive multimedia transmission. We present the optimization of a generalized rateless code using two different progressive source transmission protocols. Proposed coding scheme is shown to exhibit better unequal protection and recovery time properties than other published results.

Optimization of multimedia transmissions over wireless channels should be aimed at maximizing the video quality perceived by the final user. For transmission of video sequences over an orthogonal frequency division multiplexing (OFDM) system in a slowly varying Rayleigh faded environment, we develop a cross-layer technique, based on a slice loss visibility (SLV) model used to evaluate the visual importance of each slice. In particular, taking into account the visibility scores available from the bitstream, depending on the scenario, we optimize the mapping of video slices within a 2-D time-frequency resource block and/or the channel code rates, in order to better protect more visually important slices. The proposed algorithm is investigated for several scenarios, with different levels of information about the channel available in the optimization process. Results demonstrate that, for different physical environments and different video sequences, the proposed algorithm outperforms baseline ones which do not take into account either the SLV or the CSI in the video transmission.

This paper addresses an adaptive multi-user resource allocation for a distributed multi-carrier direct sequence-code division multiple access (MC DS-CDMA) network. The packet throughput, which turns out to be nonconvex and nondifferentiable, is considered to measure the system performance. A sub-optimal non-cooperative power control game is proposed to adaptively allocate the transmit power, available subchannels and alphabet size by minimizing the transmit power with a transmit power constraint and a packet throughput requirement. Also, the paper investigates the effect of channel estimation error on the adaptive resource allocation in a distributed MC DS-CDMA network, and shows that using the Gaussian approximation for the signal-dependent noise can lead to a nontrivial discrepancy in system performance.

A state-of-the-art progressive source encoder is combined with a concatenated block coding mechanism to produce a robust source transmission system for embedded bit streams. The proposed scheme efficiently trades off the available total bit budget between information bits and parity bits through efficient information block size adjustment, concatenated block coding, and random block interleavers. The objective is to create embedded codewords such that, for a particular information block, the necessary protection is obtained via multiple channel encodings, contrary to the conventional methods that use a single code rate per information block. This way, a more flexible protection scheme is obtained. The information block size and concatenated coding rates are judiciously chosen to maximize system performance, subject to a total bit budget. The set of codes is usually created by puncturing a low-rate mother code so that a single encoder-decoder pair is used. The proposed scheme is shown to effectively enlarge this code set by providing more protection levels than is possible using the code rate set directly. At the expense of complexity, average system performance is shown to be significantly better than that of several known comparison systems, particularly at higher channel bit error rates.

The original design of standard digital fountain codes assumes that the coded information symbols are equally important. In many applications, some source symbols are more important than others, and they must be recovered prior to the rest. Unequal Error Protection (UEP) designs are attractive solutions for such source transmissions. In this study, we introduce a more generalized design for the first universal fountain code design, LT codes, that makes it particularly suited for progressive bit stream transmissions. We apply the generalized LT codes to a progressive source and show that it has better UEP properties than other published results in the literature. For example, using the proposed generalization, we obtained up to 1.7dB PSNR gain in a progressive image transmission scenario over the two major UEP fountain code designs.

Multiple description coding has been receiving attention as a robust transmission framework for multimedia services. This paper studies the iterative decoding of FEC-based multiple description codes. The proposed decoding algorithms take advantage of the error detection capability of Reed-Solomon (RS) erasure codes. The information of correctly decoded RS codewords is exploited to enhance the error correction capability of the Viterbi algorithm at the next iteration of decoding. In the proposed algorithm, an intradescription interleaver is synergistically combined with the iterative decoder. The interleaver does not affect the performance of noniterative decoding but greatly enhances the performance when the system is iteratively decoded. We also address the optimal allocation of RS parity symbols for unequal error protection. For the optimal allocation in iterative decoding, we derive mathematical equations from which the probability distributions of description erasures can be generated in a simple way. The performance of the algorithm is evaluated over an orthogonal frequency-division multiplexing system. The results show that the performance of the multiple description codes is significantly enhanced.

With progressive image or scalable video encoders, as more bits are received, the source can be reconstructed with progressively better quality. These progressive codes have gradual differences of importance in their bitstreams, which necessitates multiple levels of unequal error protection (UEP). One practical method of achieving UEP is based on a constellation of nonuniformly spaced signal points, or hierarchical constellations. However, hierarchical modulation can achieve only a limited number of UEP levels for a given constellation size. Though hierarchical modulation has been intensively studied for digital broadcasting or multimedia transmission, most work has considered only two layered source coding, and methods of achieving a large number of UEP levels for progressive transmission have rarely been studied. In this paper, we propose a multilevel UEP system using multiplexed hierarchical quadrature amplitude modulation (QAM). We show that multiple levels of UEP are achieved by the proposed multiplexing method. When the BER is dominated by the minimum Euclidian distance, we derive an optimal multiplexing approach which minimizes both the average and peak powers. We next propose an asymmetric hierarchical QAM which reduces the peak-to-average power ratio (PAPR) of the proposed UEP system without any performance loss. Numerical results show that the performance of progressive transmission over Rayleigh fading channels is significantly enhanced by the proposed methods.

In this paper, a 60-GHz channel model that incorporates the presence of human activity, and which is an extension of the standard channel model proposed by the IEEE 802.15.3c group, is presented. In the presence of human activity, various measurement results show that the attenuation due to human blockage can be 20 dB or more, and thus is non-negligible. Various measurement results in the literature are used to determine the distribution of the shadowing attenuation caused by human blockage. A multiuser single-carrier system with a single receiver is considered, and system performance in the presence of blockage because of human activity is evaluated. Numerical results show that the 60-GHz system can incur a large degradation in bit error rate due to human blockage.

We study an uplink video communication system with multiple users in a centralized wireless cell. The multiple access scheme is Orthogonal Frequency Division Multiple Access (OFDMA). Both physical layer channel state information (CSI) and application layer rate distortion (RD) information of video streams are collected by the base station. With the goal of minimizing the average video distortion across all the users in the system, we design an iterative resource allocation algorithm for subcarrier assignment and power allocation. Based on the physical layer resource allocation decision, the user will adapt the application layer video source coding rate. To show the advantage of this cross layer algorithm, numerical results are compared with two baseline resource allocation algorithms using only physical layer information or only application layer information. Bit-level simulation results are presented which take into account the imperfection of the video coding rate control, as well as channel errors.

We address channel code rate optimization for transmission of non-scalable coded video sequences over orthogonal frequency division multiplexing networks. A slice loss visibility (SLV) model is used to evaluate the visual importance of each H.264 slice. Based on both the SLV model and the frequency diversity order available from the channel, we propose a cross-layer technique to allocate video slices within a 2-D time-frequency resource block, and optimize the unequal channel code rate profile, in order to better protect more visually important slices. The proposed algorithm outperforms baseline ones which do not take into account the SLV.

The tradeoff between spoofing and jamming a cognitive radio network by an intelligent adversary is analyzed in this paper. Due to the vulnerabilities of spectrum sensing noted in recent studies, a cognitive radio can be attacked during the sensing interval by an adversary who puts spoofing signals in unused bands. Further, once secondary users access unused bands, the adversary can use traditional jamming to interfere with them during transmission. For an energy-constrained intelligent adversary, a two step procedure is formulated to distribute the energy between spoofing and jamming, such that the average sum throughput of the secondary users is minimized. That is, we optimally spoof in the sensing duration and then optimally jam in the transmission slot. In a cluster-based cognitive radio network, when the number of spectral vacancies required by secondary users increases, the optimal attack for the intelligent adversary will shift from jamming only, to a combination of spoofing and jamming, to spoofing only.

We study single-term exponential-type bounds (also known as Chernoff-type bounds) on the Gaussian error function. This type of bound is analytically the simplest such that the performance metrics in most fading channel models can be expressed in a concise closed form. We derive the conditions for a general single-term exponential function to be an upper or lower bound on the Gaussian error function. We prove that there exists no tighter single-term exponential upper bound beyond the Chernoff bound employing a factor of one-half. Regarding the lower bound, we prove that the single-term exponential lower bound of this letter outperforms previous work. Numerical results show that the tightness of our lower bound is comparable to that of previous work employing eight exponential terms.

We propose superposition multiple-input multiple-output (MIMO) coding for the transmission of unequally important sources in a point-to-multipoint system. First, a tradeoff between Alamouti code and spatial multiplexing (V-BLAST) is analyzed in terms of the average bit error rate (BER), where the maximum data rates of both MIMO schemes are set to be equal. The results show that for a given target bit error rate, Alamouti code is preferable for a low data rate, and spatial multiplexing is preferable for a high data rate. For layered sources such as scalable video, the more important component typically has lower data rate than does the less important component. Based on these, we construct a superposition MIMO scheme where two different MIMO techniques are hierarchically combined such that important data is Alamouti coded, less important data is spatially multiplexed, and then two unequally important data symbols are superposed.

This paper characterizes the SER for a non-ideal OFDM system with either M-PSK or M-QAM constellations operating over a frequency-selective, time-varying, Rayleigh fading channel. The imperfections that we consider are inter-carrier interference (ICI) and/or inter-symbol interference (ISI), imperfect time and frequency synchronization, and noisy channel estimates. We consider an MRC receiver with L diversity branches. We present SER characterizations with and without ISI, and show that the effect of ISI, especially for higher order modulation schemes, e.g., 8PSK and above, can be very severe, irrespective of the number of diversity branches.

Recently, multiple description source coding has emerged as an attractive framework for robust multimedia transmission over packet erasure channels. In this paper, we mathematically analyze the performance of n-channel symmetric FEC-based multiple description coding for a progressive mode of transmission over orthogonal frequency division multiplexing (OFDM) networks in a frequency-selective slowly-varying Rayleigh faded environment. We derive the expressions for the bounds of the throughput and distortion performance of the system in an explicit closed form, whereas the exact performance is given by an expression in the form of a single integration. Based on this analysis, the performance of the system can be numerically evaluated. Our results show that at high SNR, the multiple description encoder does not need to fine-tune the optimization parameters of the system due to the correlated nature of the subcarriers. It is also shown that, despite the bursty nature of the errors in a slow fading environment, FEC-based multiple description coding without temporal coding provides a greater advantage for smaller description sizes.

In this paper, we consider an adaptive multi-user resource allocation for the downlink transmission of a multi-cluster tactical multicarrier DS CDMA network. The goal is to maximize the sum packet throughput, subject to transmit power constraints. Since the objective function turns out to be noncovex and nondifferentiable, we propose a simple iterative bisection algorithm. At each iteration, a closed-form expression is derived for the transmit power, subchannel, and modulation assignment, which significantly reduces the computational complexity. We also provide an optimization algorithm for the downlink transmission under the condition of imperfect channel knowledge, and investigate the effects of both channel estimation error and partial-band jamming.

Average throughput maximization has commonly been used as an alternative to image or video average distortion minimization. However, as the throughput-distortion curve is non-linear, throughput maximization can be very different from distortion minimization if the variance of the throughput is large. Under high channel quality variability, the variance of the throughput will be high. Multiple description coding with unequal error protection is a promising technique for the transmission of progressive images when there exists a high variability of the channel conditions. In this paper, we consider both the variance and the average of the throughput when deciding the constellation size for adaptive modulation in an OFDM system used for transmitting progressively-coded images with multiple description coding. Simulation results show that in an adaptive modulated image transmission system, taking into account both moments of the throughput achieves better performance than a system considering average throughput alone.

We consider a convolutionally coded orthogonal frequency-division multiple access (OFDMA) based cognitive radio system where each user achieves perfect synchronization for its own signal, while different users are asynchronized due to random timing offsets. The presence of asynchronous secondary users introduces potential multiple access interference (MAI) to each primary user's receiver. The strength of the MAI is determined by the secondary users' transmission powers, and their distances and frequency separations to the primary user. Expressions for the primary user's pairwise error probability and average probability of error are derived, and an error floor prediction method is presented. Finally, the trade-off between the MAI to the primary user and the secondary users' performances are investigated through numerical examples and simulations.

We consider a simple ad-hoc cognitive scenario with two data up-links, one licensed to use the spectral resource (primary) and the other unlicensed (secondary or cognitive). It is assumed that the cognitive link accesses the channel only when the channel is sensed idle. An ON-OFF channel model is used for the primary link, where traffic statistical characteristics are taken into account. A closed-form expression for the signal-to noise-plus interference (SINR) statistics of the cognitive nodes is derived that can be used for estimating the network performance. Moreover, a M/G/1 queueing model is exploited for deriving a simple expression for the average packet delay. Finally, a MAC strategy based on a channel-and-queue aware scheduling is introduced.

The 20 papers in this special issue on wireless video transmissions are divided into four categories: wireless video streaming, optimization and scheduling; wireless video broadcast/multicast; retransmission techniques; and video quality assessment.

In this paper, we are concerned with the design and analysis of joint source-channel coding schemes for block fading channels with relay-assisted distributed spatial diversity. Assuming a progressive image coder with a constraint on the transmission bandwidth, we formulate a joint source-channel rate allocation scheme that maximizes the expected source throughput. Specifically, using Gaussian as well as BPSK inputs on flat Rayleigh fading channels, we lower bound the average packet error rate by the corresponding mutual information outage probability, and derive the average throughput expression as a function of channel code rates as well as channel SNR for both a frequency-division multiplexing-based baseline system without relaying, and a half-duplex relay system with a decode-and- forward protocol. At high signal-to-noise ratio (SNR), for the systems considered in this paper, we show that our rate optimization problem is a convex function of the channel code rates, and we show that a known recursive algorithm can be used to predict the performance of both systems.

This paper derives closed-form asymptotic symbol error rates (ASERs) for MPSK and MQAM signals with selection combining (SC) over Nakagami-m fading channels. It also presents a closed-form expression for the power loss of SC compared to maximal ratio combining (MRC) over a Nakagami- m fading channel. It is found that this power gap is a function of only the number (L) of independent Nakagami-m channels and the fading parameter m. For a fixed m, the gap monotonically increases with L. For a fixed L, the gap monotonically increases with m and approaches 10 log10L dB (the asymptotic gap between MRC and SC in an AWGN channel) as m ¿ ¿. In addition, numerical results show the accuracy of the ASERs obtained from closed-form expressions for both MRC and SC with MPSK and MQAM signals for large SNRs. They also show that the SNR gap is identical to that predicted by the analysis for arbitrary L and m.

Studies of channel estimation techniques for OFDM systems often assume idealized conditions such as a cyclic prefix (CP) long enough to absorb the entire channel delay spread, or perfect time and frequency synchronization at the receiver. This paper studies the impact of removing these assumptions. We characterize the mean square error (MSE) of various linear channel estimation methods, and we derive analytical expressions for the BER performance. The derivations are kept generic so that they apply to both time-division and frequency-division multiplexing of the pilot signal and the data subcarriers. The analysis is specialized to the BER performance characterization of OFDM-based UWB systems for different realistic channel estimation methods with residual time and frequency errors. The effect of inter-carrier interference (ICI) on channel estimation is shown to be small. However, inter-symbol interference (ISI) severely impacts the quality of the channel estimates and hence can yield a large (>2 dB) data demodulation performance degradation.

In this paper, we propose a cross-layer distributed power control and scheduling protocol for delay-constrained applications over mobile CDMA-based ad hoc wireless networks. Herein, we propose a novel scheme where power control is employed to combat delay occurring on multi-hop wireless ad hoc networks via cross-layer information exchange. Based on that, a distributed power control and scheduling protocol is proposed to control the incurred delay as well as the multiple access interference (MAI). Unlike other previous work on power control and scheduling, we also investigate the impact of Doppler spread upon the system performance, and provide a robust system which employs a combination of power control, and coding/interleaving to combat the effects of Doppler spread by exploiting the time diversity when the Doppler spread gets large. Hence, our proposed approach can function appropriately over a wide range of channel conditions.

In this paper, we investigate the issue of cooperative node selection in MIMO communications for wireless ad hoc/sensor networks, where a source node is surrounded by multiple neighbors and all of them are equipped with a single antenna. Given energy, delay and data rate constraints, a source node dynamically chooses its cooperating nodes from its neighbors to form a virtual MIMO system with the destination node (which is assumed to have multiple antennas), as well as adaptively allocates the power level and adjusts the constellation size for each of the selected cooperative nodes. In order to optimize system performance, we jointly consider the optimization of all these parameters, given the aforementioned system constraints. We assume that the source node either has CSI, or has no CSI. Heuristic algorithms, such as maximal channel gain (MCG) and least channel correlation (LCC) algorithms are proposed in order to exploit available system information and to solve the constrained optimization problem.

With Unequal Error Protection (UEP), more important symbols are given greater protection against channel errors than are less important symbols. The protection can be accomplished by various methods, including joint source-channel coding (JSCC) and hierarchical modulation. In this paper, a robust progressive source transmission system using forward error correction (FEC) and a bits-to-symbol assignment methodology that provides UEP is proposed. UEP is not only provided by hierarchical modulation but also by the packetization methodology combined with channel coding. It is demonstrated by simulation that our system improves performance compared to an Equal Error Protection (EEP) technique and to a baseline JSCC-only mechanism.

Progressive image and scalable video have gradual differences of importance in their bitstreams, which can benefit from multiple levels of unequal error protection (UEP). Though hierarchical modulation has been intensively studied as an UEP approach for digital broadcasting and multimedia transmission, methods of achieving a large number of UEP levels have rarely been studied. In this paper, we propose a multilevel UEP system using multiplexed hierarchical quadrature amplitude modulation (QAM) for progressive transmission over mobile radio channels. We suggest a specific way of multiplexing, and prove that multiple levels of UEP are achieved by the suggested method. When the BER is dominated by the minimum Euclidian distance, we derive an optimal multiplexing approach which minimizes both the average and peak powers. An asymmetric hierarchical QAM which reduces the peak-to-average power ratio (PAPR) without performance loss is also proposed. Numerical results show that the performance of progressive transmission over Rayleigh fading channels is significantly enhanced by the proposed UEP systems.

We study motion compensated fine granular scalable (MC-FGS) video transmission over multi-input multi-output (MIMO) wireless channels, where leaky and partial prediction schemes are applied in the enhancement layer of MC-FGS to exploit the tradeoff between error propagation and coding efficiency. For reliable transmission, we propose unequal error protection (UEP) by considering a tradeoff between reliability and data rates, which are controlled by forward error correction (FEC) and MIMO mode selection to minimize the average distortion. In a high Doppler environment where it is hard to get an accurate channel estimate, we investigate the performance of the proposed MC-FGS video transmission scheme with joint control of the leaky and partial prediction parameters and the UEP.

In this paper, we explore a novel approach to evaluate the inherent UEP (unequal error protection) properties of irregular LDPC (low-density parity-check) codes over BECs (binary erasure channels). Exploiting the finite-length scaling methodology, suggested by Amraoui et. al., we introduce a scaling approach to approximate the bit erasure rates of variable nodes with different degrees in the waterfall region of the peeling decoder. Comparing the bit erasure rates obtained from Monte Carlo simulation with the proposed scaling approximations, we demonstrate that the scaling approach provides a close approximation for a wide range of code lengths (between 1000 and 8000). In view of the complexity associated with the numerical evaluation of the scaling approximation, we also derive simpler upper and lower bounds.

Cognitive radios have been proposed as a means to implement efficient reuse of the licensed spectrum. Commonly, wireless networks are characterized by a fixed spectrum assignment policy. The limited available spectrum and the inefficiency in the spectrum usage necessitate a new communication paradigm to exploit the existing wireless spectrum opportunistically. We consider a simple single-cell scenario with two data up-links, one licensed to use the spectral resource (primary) and the other unlicensed (secondary or cognitive). It is assumed that the cognitive user accesses the channel only when the channel is sensed idle. An ON-OFF channel model is used for the primary link, where traffic statistical characteristics are taken into account. We study a practical resource allocation algorithm that assigns the uplink to the secondary users according to a channel-and-queues aware scheduler when primary link OFF periods are sensed. We fit the resource allocation algorithm to the widely investigated orthogonal frequency division multiple access (OFDMA) scheme and we exploit multiuser diversity by applying a smart power allocation within independent OFDMA subchannels. A video encoder rate control is introduced in order to limit the video frame loss due to overflow that trades the video frame loss probability with the overall encoding quality. Lastly, the performance of the cognitive network model is investigated under the proposed resource allocation algorithm.

The effects of channel estimation errors on the performance of a system employing Tomlinson-Harashima precoding and the QR decomposition, operating over multiple antenna frequency-flat fading channels with decentralized receivers, is evaluated. The QR decomposition of the channel matrix is employed to arrive at an equivalent channel where successive interference cancellation at the transmitter can be used to remove the effect of the multiuser interference. However, in the case of imperfect channel estimation, it is not possible to remove the effect of the multiuser interference due to a mismatch between the precoder and the channel. Consequently, it is necessary to include the estimation error in deriving the probability of symbol error. We also provide simulations to corroborate the analytical results.

In our previous work, we compared the theoretical bit error rates of multi-carrier direct sequence code division multiple access (MC-DS-CDMA) and multi-carrier code division multiple access (MC-CDMA). To ensure a fair comparison, we constrained both schemes to the same bandwidth, information rate, and energy-per-bit, and these constraints resulted in a possible trade-off between diversity gain and channel estimation errors between the two schemes. While only a single-input single-output (SISO) system was considered in our previous work, in this paper, we extend the comparison to a multiple-input multiple-output (MIMO) system which employs Alamouti space-time block coding at each sub-carrier frequency to achieve spatial diversity. We consider only those cases where MC-CDMA has higher frequency diversity than MC-DS-CDMA. Since increases in diversity yield diminishing gains, we conclude that the addition of spatial diversity to this multi-carrier comparison benefits MC-DS-CDMA more than MC-CDMA. To determine whether these gains for MC-DS-CDMA are enough to offset the difference in frequency diversity between the two schemes, we derive closed-form expressions for the bit error probabilities of both schemes, and we compare the MIMO results against those of the SISO system for different information rates, number of users, and number of pilot symbols per channel estimate.

Most OFDM analyses assume idealized conditions when it comes to the waveform at the receiver in terms of time and frequency synchronization. In addition, often a cyclic prefix (CP) longer than the channel delay spread is assumed, and thus the impact of the possible inter-symbol interference (ISI) from adjacent OFDM symbols is not a consideration. This work provides a consolidated framework for study and analysis of OFDM systems with imperfections. The imperfections include time and frequency synchronization errors, and channel delay spread beyond the CP duration. Transmit as well as receive filtering operations are explicitly modeled, and different receiver detection criteria are considered. The use of an OFDM waveform for UWB applications is known to be a promising choice to simplify the receiver design and to maximize the bandwidth scalability, as well as the adaptation to various spectrum allocations. We present bit error rate (BER) sensitivity analysis to key system parameters for OFDM-based UWB applications. From these sensitivity analyses, we see graceful performance degradation with decreasing CP length and increasing residual frequency error. The performance degradation can be much more critical for timing synchronization errors.

In this paper, we consider a time-slotted MIMO point-to-multipoint network, which, for example, could represent either a cellular downlink, or a cluster in a wireless mesh network. The transmitter sends an independent data stream from each transmit antenna to communicate simultaneously to a subset of the receivers in each slot. The transmitter sends training symbols at the beginning of each slot so that each receiver can estimate the channel and determine the data rate it could potentially receive from each transmit antenna after configuring its antenna weights appropriately. For this purpose, we develop a maximum-likelihood estimator of the signal to interference plus noise ratio (SINR) from the received training symbols. The transmitter decides which receivers to serve in each slot to maximize the minimum normalized average data rate realized by each receiver. This scheduling decision is made based on estimated SINR information fed back by receivers. Through some numerical examples, we discuss some general issues with regard to exploitation of multi-user diversity in the context of imperfect channel state information. For example, the scheduler introduces a bias for positive SINR estimation errors, which is important to take into account when the receivers declare a potential data rate in each slot.

Coding and diversity are very effective techniques for improving transmission reliability in a mobile wireless environment. The use of diversity is particularly important for multimedia communications over fading channels. In this work, we study the transmission of progressive image bitstreams using channel coding in a 2-D time-frequency resource block in an OFDM network, employing time and frequency diversities simultaneously. In particular, in the frequency domain, based on the order of diversity and the correlation of individual subcarriers, we construct symmetric n -channel FEC-based multiple descriptions using channel erasure codes combined with embedded image coding. In the time domain, a concatenation of RCPC codes and CRC codes is employed to protect individual descriptions. We consider the physical channel conditions arising from various coherence bandwidths and coherence times, leading to a range of orders of diversities available in the time and frequency domains. We investigate the effects of different error patterns on the delivered image quality due to various fade rates. We also study the tradeoffs and compare the relative effectiveness associated with the use of erasure codes in the frequency domain and convolutional codes in the time domain under different physical environments. Both the effects of intercarrier interference and channel estimation errors are included in our study. Specifically, the effects of channel estimation errors, frequency selectivity and the rate of the channel variations are taken into consideration for the construction of the 2-D time-frequency block. We provide results showing the gain that the proposed model achieves compared to a system without temporal coding. In one example, for a system experiencing flat fading, low Doppler, and imperfect CSI, we find that the increase in PSNR compared to a system without time diversity is as much as 9.4 dB.

An algorithm is given for placing relays at spatial positions to improve the reliability of communicated data in a sensor network. The network consists of many power-limited sensors, a small set of relays, and a receiver. For each sensor, the receiver receives a direct signal as well as an indirect signal from one of the available relays. The relays rebroadcast the transmissions in order to achieve diversity at the receiver. Both amplify-and-forward and decode-and-forward relay networks are considered. Channels are modeled with Rayleigh fading, path loss, and additive white Gaussian noise. Performance analysis and numerical results are given.

This paper derives closed-form asymptotic symbol error rates (ASERs) for MPSK and MQAM signals with selection combining (SC) over Nakagami-m fading channels. It also presents a closed-form expression for the power loss of SC compared to maximal ratio combining (MRC) over a Nakagami-m fading channel. It is found that this power gap is a function of only the number (X) of independent Nakagami-m channels and the fading parameter m. For a fixed m, the gap monotonically increases with L. For a fixed L, the gap monotonically increases with m and approaches10log10L dn (the asymptotic gap between MRC and SC in an AWGN channel) as m rarr infin . In addition, numerical results show the accuracy of the ASERs obtained from closed-form expressions for both MRC and SC with MPSK and MQAM signals for large SNRs. They also show that the SNR gap is identical to that predicted by the analysis for arbitrary L and m.

This paper addresses the use of adaptive modulation in progressive image transmission with multiple description coding in conjunction with an Orthogonal Frequency Division Multiplexing (OFDM) system. Specifically, two adaptive systems are considered: variable rate with fixed power, and variable rate with variable power. An algorithm is proposed to allocate power and constellation size at each subchannel by maximizing the throughput. Simulation results confirm that cross-layer optimization with adaptive modulation enhances system performance.

In this paper, we investigate the issue of cooperative node selection in MIMO communications for wireless ad hoc/sensor networks, where a source node is surrounded by multiple neighbors and all of them are equipped with a single antenna. Given energy, delay and data rate constraints, a source node dynamically chooses its cooperating nodes from its neighbors to form a virtual MIMO system with the destination node (which is assumed to have multiple antennas), as well as adaptively allocates the power level and adjusts the constellation size for each of the selected cooperative nodes. In order to optimize system performance, we jointly consider the optimization of all these parameters, given the aforementioned system constraints. We assume that the source node either has CSI, or has no CSI. Heuristic algorithms, such as maximal channel gain (MCG) and least channel correlation (LCC) algorithms are proposed in order to exploit available system information and to solve the constrained optimization problem.

This paper proposes a new approach for decoding LDPC codes over MISO channels. Since in an nTtimes 1 MISO system with a modulation of alphabet size 2M, nTtransmitted symbols are combined and produce one received symbol at the receiver, we propose considering the LDPC-coded MISO system as an LDPC code over 2MnT-ary alphabet. Consequently, we propose a modified Tanner graph to introduce belief propagation for decoding MISO-LDPC systems. As a result, the MISO symbol detection and binary LDPC decoding steps are merged into a single message passing decoding. We also propose an efficient method that significantly reduces the complexity of belief propagation decoding in MISO-LDPC systems. Furthermore, we show that our proposed decoder outperforms the conventional decoder for short length LDPC codes in unknown channel scenarios.

The thirteen papers in this special issue focus on multiuser detection for advanced communication systems and networks. The papers can be divided into three thematic groups: Multiuser detection (MUD) in i) CDMA; ii) MIMO and Multicarrier CDMA/OFDM; and iii) Cooperative Communications.

The 17 papers in this special issue focus on the theory and applications of cognitive radio.

In this paper, we propose a cognitive radio based multi-user resource allocation framework for mobile ad hoc networks using multi-carrier DS CDMA modulation over a frequency-selective fading channel. In particular, given preexisting communications in the spectrum where the system is operating, in addition to potential narrow-band interference, a channel sensing and estimation mechanism is provided to obtain information such as subcarrier availability, noise power and channel gain. Given this information, both frequency spectrum and power are allocated to emerging new users (i.e., cognitive radio users), based on a distributed multi-user resource allocation framework, in order to satisfy a target data rate and a power constraint of each cognitive radio user, while attempting to avoid interference to the existing communications as well as to minimize total power consumption of the cognitive radio users.

This paper investigates the trade-off between diversity and channel estimation errors for asynchronous multi-carrier CDMA (MC-CDMA) and multicarrier direct-sequence CDMA (MC-DS-CDMA). We construct a side-by-side comparison between the two systems by assuming equal operating and channel conditions. Both systems perform pilot-symbol- aided channel estimation and maximal ratio combining at the receiver, where the weighting coefficients are determined by the channel estimates at each sub-carrier. Closed-form expressions for the probability of error of both systems are derived under two separate channel scenarios. These two scenarios assume a frequency-selective Rayleigh fading channel, where the coherence bandwidth is equal to either the bandwidth of a MC-DS-CDMA sub-band or the bandwidth of a MC-CDMA sub-band. Finally, we compare the bit error rates of the two systems for different information rates, number of users, and number of pilot symbols used in each channel estimate.

In this paper, we design jointly optimum transmitter and receiver beamforming structures in correlated MIMO Rayleigh fading channels with noisy channel estimates, and evaluate their performance through a closed-form expression for the probability of error. At the receiver, an estimate of the channel is obtained based on the transmitted pilot symbols. The knowledge of the estimate is then made available to the transmitter through a feedback channel. It is assumed that spatial correlation exists, either at the transmitter or at the receiver. A new exact closed-form expression for the probability of error is derived and the joint effects of diversity order and channel estimation accuracy on system performance are analyzed.

Motion-compensated fine-granularity scalability (MC-FGS) with leaky prediction has been shown to provide an efficient tradeoff between compression gain and error resilience, facilitating the transmission of video over dynamic channel conditions. In this paper, we propose an n-channel symmetric motion-compensated multiple description (MD) coding and transmission scheme for the delivery of scalable video over orthogonal frequency division multiplexed systems, utilizing the concepts of partial and leaky predictions. We investigate the proposed MD coding and transmission scheme using a cross-layer design perspective. In particular, we construct the symmetric motion-compensated MD codes based on the diversity order of the channel, defined as the ratio of the overall bandwidth of the system to the coherence bandwidth of the channel. We show that knowing the diversity order of a physical channel can assist an MC-FGS video coder in selecting the motion-compensation prediction point, as well as on the use of leaky prediction. More importantly, we illustrate how the side information can reduce the drift management problem associated with the construction of symmetric motion-compensated MD codes. We provide results based on both an information-theoretic approach and simulations.

In this paper, taking the view that a linear parallel interference canceller (LPIC) can be seen as a linear matrix filter, we propose new linear matrix filters that can result in improved bit error performance compared to other LPICs in the literature. The motivation for the proposed filters arises from the possibility of avoiding the generation of certain interference and noise terms in a given stage that would have been present in a conventional LPIC (CLPIC). In the proposed filters, we achieve such avoidance of the generation of interference and noise terms in a given stage by simply making the diagonal elements of a certain matrix in that stage equal to zero. Hence, the proposed filters do not require additional complexity compared to the CLPIC, and they can allow achieving a certain error performance using fewer LPIC stages. We also extend the proposed matrix filter solutions to a multicarrier DS-CDMA system, where we consider two types of receivers. In one receiver (referred to as Type-I receiver), LPIC is performed on each subcarrier first, followed by multicarrier combining (MCC). In the other receiver (called Type-II receiver), MCC is performed first, followed by LPIC. We show that in both Type-I and Type-II receivers, the proposed matrix filters outperform other matrix filters. Also, Type-II receiver performs better than Type-I receiver because of enhanced accuracy of the interference estimates achieved due to frequency diversity offered by MCC.

In this paper, we analyze the impact of imperfect channel quality feedback on the scheduling algorithms that exploit multi-user diversity. We consider a time-slotted MIMO broadcast network where the transmitter uses its multiple antennas for spatial multiplexing to communicate simultaneously to a subset of the receivers in each slot. The transmitter sends training symbols at the beginning of a slot so that each receiver can estimate the data rate it could potentially receive from each transmit antenna. For this purpose, we develop a maximum-likelihood estimator (MLE) of SINR. The receivers report the rate information, which is noisy due to estimation errors, to the transmitter to assist its scheduling decisions. Through numerical examples, we discuss some general issues regarding exploitation of multi-user diversity with imperfect channel state information. For example, the scheduler introduces a bias for positive SINR estimation errors, which is important to take into account when the receivers declare a potential data rate in each slot.

In this paper, taking the view that a linear parallel interference canceller (LPIC) can be seen as a linear matrix filter, we propose new linear matrix filters that can result in improved bit error performance compared to other LPICs in the literature. The motivation for the proposed filters arises from the possibility of avoiding the generation of certain interference and noise terms in a given stage that would have been present in a conventional LPIC (CLPIC). In the proposed filters, we achieve such avoidance of the generation of interference and noise terms in a given stage by simply making the diagonal elements of a certain matrix in that stage equal to zero. Hence, the proposed filters do not require additional complexity compared to the CLPIC, and they can allow achieving a certain error performance using fewer LPIC stages.

This paper presents a simple way to obtain the asymptotic performance of various digital communications systems operating over fading channels. The resulting expression for performance contains only the inverse average SNR raised to the system's diversity order, by averaging the conditional error probability (conditioned on the fading) with the dominant term in the fading density function. The proposed approach often renders simple closed-form asymptotic error probabilities (ASEPs), even when the corresponding closed-form actual error probabilities (ACEPs) are too difficult to find, such as those in generalized selection combining, denoted by GSC (m,L), where the strongest m branches from L (1lesmlesL) total branches are selected and combined Using the proposed approach, the paper derives simple closed-form ASEPs for GSC (m,L) which show its diversity order and SNR gaps among different m, over both iid and non-iid Rayleigh fading channels for both MPSK and QAM. It is found that the SNR gaps of GSC (m,L) are not a function of modulation type or orders. In addition, the paper also presents simple closed-form ASEPs for a Nakagami-m fading channel with both MPSK and QAM in conjunction with maximal ratio combining (MRC).

We are concerned with transmit power optimization in a wireless relay network with various cooperation protocols. With statistical channel knowledge (in the form of knowledge of the fading distribution and the path loss information across all the nodes) at the transmitters and perfect channel state information at the receivers, we derive the optimal power allocation that minimizes high signal-to-noise ratio (SNR) approximations of the outage probability of the mutual information (MI) with amplify-and-forward (AF), decode-and-forward (DF) and distributed space-time coded (DSTC) relaying protocols operating over Rayleigh fading channels. We demonstrate that the high SNR approximation-based outage probability expressions are convex functions of the transmit power vector, and the nature of the optimal power allocation depends on whether or not a direct link between the source and the destination exists. Interestingly, for AF and DF protocols, this allocation depends only on the ratio of mean channel power gains (i.e., the ratio of the source-relay gain to the relay-destination gain), whereas with a DSTC protocol this allocation also depends on the transmission rate when a direct link exists. In addition to the immediate benefits of improved outage behavior, our results show that optimal power allocation brings impressive coding gains over equal power allocation. Furthermore, our analysis reveals that the coding gain gap between the AF and DF protocols can also be reduced by the optimal power allocation.

The performance of a pre-Bell Laboratories Layered Space-Time Architecture (BLAST)-decision feedback equalization (DFE) technique with Tomlinson-Harashima precoding and decentralized receivers, operating over multiple-input single-output (MISO) frequency-selective fading channels, is evaluated. First, we derive the probability of symbol error for Tomlinson-Harashima precoding operating over single-input single-output channels with intersymbol interference. We then generalize this result to MISO frequency-selective fading channels with decentralized receivers by using the QR decomposition technique. The effect of optimal ordering of the decentralized receivers, which minimizes the total transmit energy, is investigated. We obtain a closed-form expression for the probability density function of the squared diagonal elements of the upper right triangular matrix belonging to the optimal QR decomposition when two transmit antennas and two receivers are used. We also provide simulations to corroborate the analytical results.

It is known that a major practical implementation challenge of ultra-wideband (UWB) receivers is the design of the coarse acquisition stage. Due to the fine time resolution of UWB signals, the acquisition stage has to acquire a large number of low-energy multipath components, with no or little knowledge of the state of the channel. In addition, the complexity further increases with the presence of narrowband interference due to the proposed spectral overlay. Our goal in this paper is to evaluate the affects of the lack ofa prioriknowledge of the channel state and the presence of narrowband interference during acquisition. Maximum-likelihood and maximuma posterioriprocedures for estimation in the presence of narrowband interference are formulated, and two different interference mitigation techniques are evaluated. In particular, this paper considers UWB communication systems that use spectral encoding as both the multiple access scheme and the interference suppression technique. The qualitative results are, however, believed to be valid for any UWB system implementation. It is shown that the acquisition performance strongly depends on the amount ofa prioriknowledge of the channel state at the receiver, and on whether or not interference suppression is employed.

It is known that a major practical implementation challenge of ultra-wideband (UWB) receivers is the design of the coarse acquisition stage. Due to the fine time resolution of UWB signals, the acquisition stage has to acquire a large number of low-energy multipath components, with no or little knowledge of the state of the channel. In addition, the complexity further increases with the presence of narrow-band interference (NBI) due to the proposed spectral overlay. Our goal in this paper is to evaluate the affects of the lack ofa prioriknowledge of the channel state and the presence of NBI during acquisition. Maximum likelihood and maximuma posterioriprocedures for estimation in the presence of NBI are formulated, and two different interference mitigation techniques are evaluated. In particular, this paper considers UWB communication systems that use spectral encoding as both the multiple access scheme and the interference suppression technique. The qualitative results are, however, believed to be valid for any UWB system implementation. It is shown that the acquisition performance strongly depends on the amount ofa prioriknowledge of the channel state at the receiver, and on whether or not interference suppression is employed.

This paper presents a practical implementation of the vertical Bell Laboratories layered space-time (V-BLAST) type system, in which the multiple-input multiple-output (MIMO) open-loop capacity can be approached with conventional scalar coding, using adaptive modulation with appropriate channel codes, e.g., low-density parity-check (LDPC) codes and optimum successive detection (OSD). The density evolution (DE) technique is employed to determine the maximal achievable rate of an LDPC code for each transmit antenna for a given channel realization at a given SNR. Numerical results show that the average sum rate of the adaptively modulated LDPC-encoded system is quite close to the V-BLAST capacity with both rate and power adaptations. Considering the performing degradation caused by error propagation due to the imperfect feedback and relatively long decoding delay in the OSD, we use parallel interference cancellation (PIC) followed by minimum mean square error (MMSE) filtering in the bit error rate (BER) performance simulation. Simulation results show that a target BER of 10-5 can be achieved by the optimally designed LDPC codes. To simplify the code design, we replace the LDPC codes optimally designed for each channel realization with rate-compatible punctured LDPC codes, at the cost of a slight sum rate loss. If the fading process is nonergodic, the outage capacity corresponding to a given outage probability is used to measure the channel performance. As an example, we design the LDPC codes for an adaptively modulated 2×2 V-BLAST system to approach its outage capacity for a given outage probability.

The performance of multicode direct-sequence spread-spectrum multiple-input multiple-output (MIMO) systems in the presence of frequency-selective fading is evaluated. We derive the asymptotic distribution of the multiple-antenna interference when the processing gain is sufficiently large. The probability of error is derived for the conventional RAKE receiver, and its performance is compared for various system configurations. We consider system tradeoffs for both fixed rate and fixed diversity. For a fixed total data rate, we demonstrate the advantage of decreasing the number of transmit antennas while increasing the number of codes, and for a fixed total diversity order, we demonstrate the advantage of decreasing the number of RAKE taps while increasing the number of receive antennas.

Generalized diversity combining (GDC), also known as hybrid selection/maximal ratio combining or generalized selection combining, is a low-complexity diversity combining technique by which a fixed subset of a large number of available diversity channels is chosen and then combined using the rules of maximal ratio combining. In this paper, we analyze the performance of GDC on time-correlated Rayleigh fading channels with noisy channel estimates. We derive expressions for the probability of error for various linear modulation schemes with coherent detection, and discuss the conditions under which the analysis can be extended to noncoherent and differentially coherent receiver structures. Throughout the paper, using a fundamental approach to obtain the decision statistic at the combiner output, a number of new expressions for the error probabilities are obtained in a rigorous way, along with a presentation of their performance with channel estimation errors. The final expressions have roughly the same complexity of evaluation as that for the channel with only additive Gaussian noise. Our results correct various inaccuracies in the literature, and show that coherent receivers based on imperfectly estimated channel knowledge incur a significant performance loss.

Research in recent years has been focusing on the role of multiuser diversity in enhancing the throughput of wireless networks. Channel fading is now seen as a source of randomization that can be exploited by a smart scheduler with partial channel information, for example, in terms of data rates requested by the users. In this paper, we analyze the performance of a multiuser diversity system, taking into account the feedback errors due to channel variability. Channel estimation errors are modeled as Gaussian random variables with variance depending on the Doppler spread. Based on a block fading channel model, we present an expression for the throughput, both as a function of the key system parameters, such as packet length and data rate thresholds, and channel characteristics, such as Doppler spread. From the analysis of the performance, we present a tradeoff between multiuser diversity and mobility, and also between rate adaptivity and channel estimation errors. It is shown that multiuser diversity gain is reduced for increasing receiver mobility, and that the throughput loss depends also on the distribution of the data rate thresholds. Finally, we use the expressions derived in the paper in order to adaptively reduce the loss due to feedback errors. Given a set of possible data rates, the users can choose the most suitable one based not only on the set of thresholds and signal strength, but also on mobility conditions and system parameters, such as packet length.

In a mobile ad-hoc network scenario, where communication nodes are mounted on moving platforms (like jeeps, trucks, tanks, etc.), use of V-BLAST requires that the number of receive antennas in a given node must be greater than or equal to the sum of the number of transmit antennas of all its neighbor nodes. This limits the achievable spatial multiplexing gain (data rate) for a given node. In such a scenario, we propose to achieve high data rates per node through multicode direct sequence spread spectrum techniques in conjunction with V-BLAST. In the considered multicode V-BLAST system, the receiver experiences code domain interference (CDI) in frequency selective fading, in addition to space domain interference (SDI) experienced in conventional V-BLAST systems. We propose two interference cancelling receivers that employ a linear parallel interference cancellation approach to handle the CDI, followed by conventional V-BLAST detector to handle the SDI, and then evaluate their bit error rates.

In this paper, we develop first-order Markov models that characterize the packet error processes on Rayleigh fading channels considering binary DPSK/NCFSK modulation. Such models available in the literature so far consider only the fading process ignoring the underlying modulation used. Our contribution in this paper is that we consider first-order Markov models for binary DPSK/NCFSK modulation. To derive the Markov model parameters, we first derive expressions for the second-order statistics of the channel error process (specifically, the auto-correlation function of the bit error process as well as the packet error process), and obtain the Markov model parameters, in closed-form, as a function of normalized Doppler bandwidth, average received SNR and packet length. We also verify the accuracy of the proposed Markov model by deriving closed-form expressions for the mutual information of the channel error process

The proposed spectrum-overlay scenario for ultra-wideband systems brings new technical challenges that must be considered during the system design. In this scenario, the receiver has to perform functions, such as multipath combining and channel estimation, on a large number of low-energy multipath components in the possible presence of strong in-band interference. This paper proposes an analytical formulation that allows for the analysis of the channel estimation in the presence of narrowband interference. It is shown that the affect of narrowband interference is exacerbated by channel estimation errors, and that the number of training bits used for channel estimation is strongly dependent on whether or not interference suppression is employed. It is also shown that sources of degradation, such as imperfect channel estimation and the presence of narrowband interference, impose a practical limit on the number of multipath components that should be combined.

Diversity can be used to combat multipath fading and improve the performance of wireless multimedia communication systems. In this work, by considering transmission of an embedded bitstream over an orthogonal frequency division multiplexing (OFDM) system in a slowly varying Rayleigh faded environment, we develop a cross-layer diversity technique which takes advantage of both multiple description coding and frequency diversity techniques. More specifically, assuming a frequency-selective channel, we study the packet loss behavior of an OFDM system and construct multiple independent descriptions using an FEC-based strategy. We provide some analysis of this cross-layer approach and demonstrate its superior performance using the set partitioning in hierarchical trees image coder.

A robust scheme is presented for the efficient transmission of packet video over a tandem wireless Internet channel. This channel is assumed to have bit errors (due to noise and fading on the wireless portion of the channel) and packet erasures (due to congestion on the wired portion). First, we propose an algorithm to optimally switch between intracoding and intercoding for a video coder that operates on a packet-switched network with fixed-length packets. Different re-synchronization schemes are considered and compared. This optimal mode selection algorithm is integrated with an efficient channel encoder, a cyclic redundancy check outer coder concatenated with an inner rate-compatible punctured convolutional coder. The system performance is both analyzed and simulated. Last, the framework is extended to operate on a time-varying wireless Internet channel with feedback information from the receiver. Both instantaneous feedback and delayed feedback are evaluated, and an improved method of refined distortion estimation for encoding is presented and simulated for the case of delayed feedback.

We present an adaptive video transmission scheme for use in a code-division multiple-access network, which incorporates efficient bandwidth allocation among source coding, channel coding, and spreading under a fixed total bandwidth constraint. We derive the statistics of the received signal, as well as a theoretical bound on the packet drop rate at the receiver. Based on these results, a bandwidth allocation algorithm is proposed at the packet level, which incorporates the effects of both the changing channel conditions and the dynamics of the source content. Detailed simulations are done to evaluate the performance of the system, and the sensitivity of the system to estimation error is presented.

Summary form only given. A robust scheme for the efficient transmission of packet video over a tandem wireless Internet channel is extended to a time varying scenario with a feedback channel. This channel is assumed to have bit errors (due to noise and fading on the wireless portion of the channel) and packet erasures (due to congestion on the wired portion). Simulation results showed that refined estimation can dramatically improve the performance for varying channel conditions, and that combined feedback of both channel conditions and ACK/NACK information can further improve system performance compared with the feedback of just one type of information.

A sample-decision photon counting receiver architecture is proposed for use in optical communication systems operating in low-flux conditions such as proposed deep space optical links. The performance of this system using PPM signaling is analyzed and compared to the performance of an ideal photon counter and a slot-energy detection receiver architecture at representative operating points

Diversity can be used to combat multipath fading and improve the performance of mobile wireless multimedia communication systems. In this work, by considering transmission of an embedded bitstream over a slow varying Rayleigh faded environment, we develop a cross-layer diversity technique which takes advantage of both multiple description source coding and frequency diversity techniques. More specifically, assuming a frequency-selective channel, we study the packet loss behavior of an OFDM system and construct multiple independent descriptions using an FEC-based strategy. We demonstrate the superior performance of this approach using the set partitioning in hierarchical trees (SPIHT) coder.

The ability of impulse radio to resolve a large number of multipaths in a dense frequency selective channel suggests only a marginal improvement in performance by the use of multiple antennas from a diversity perspective. Furthermore, the small size of UWB receivers for in-home or sensor networking would result in a very high correlation among antenna elements, reducing even more the exploitable diversity. Therefore, we concentrate on the use of multiple antennas for interference suppression. Specifically, we investigate the performance of a quadrature receiver for impulse radio systems with multiple receive antennas and minimum-mean-square-error beamforming to combat interference

In general, a power amplifier utilizes battery energy more efficiently with a higher transmission power. For a given message, a given bandwidth constraint and a given performance constraint, different allocations of the bandwidth among source coding, channel coding and modulation result in different amounts of battery usage. We propose a method to optimize the bandwidth allocation and minimize the battery consumption due to transmission. Our results show that with the optimal allocation, significant reduction in battery consumption can be achieved without sacrificing the system performance.

For a fixed total bandwidth expansion factor, we consider the problem of optimal bandwidth allocation among the source coder, the channel coder, and the spread-spectrum unit for a direct-sequence code-division multiple-access system operating over a frequency-selective fading channel with narrowband interference. Assuming a Gaussian source with the optimum scalar quantizer, and a binary convolutional code with soft-decision decoding, and further assuming that the self-interference is negligible, we obtain both a lower and an upper bound on the end-to-end average source distortion. The joint three-way constrained optimization of the source code rate, the channel code rate, and the spreading factor can be simplified into an unconstrained optimization problem over two variables. Upon fixing the channel code rate, we show that both upper and lower bound-based distortion functions are convex functions of the source code rate. Because an explicit solution for the optimum source code rate, i.e., one that minimizes the average distortion, is difficult to obtain, computer-based search techniques are employed. Numerical results are presented for the optimum source code rate and spreading factor, parameterized by the channel code rate and code constraint length. The optimal bandwidth allocation, in general, depends on the system and the channel conditions, such as the total number of active users, the average jammer-to-signal power ratio, and the number of resolved multipath components together with their power delay profile.

We design serial concatenated multi-input multi-output systems based on low-density parity-check (LDPC) codes. We employ a receiver structure combining the demapper/detector and the decoder in an iterative fashion. We consider the a posteriori probability (APP) demapper, as well as a suboptimal demapper incorporating interference cancellation with linear filtering. Extrinsic information transfer (EXIT) chart analysis is applied to study the convergence behavior of the proposed schemes. We show that EXIT charts match very well with the simulated decoding trajectories, and they help explain the impact of different mappings and different demappers. It is observed that if the APP demapper transfer characteristics are almost flat, the LDPC codes optimized for binary-input channels are good enough to achieve performance close to the channel capacity. We also present a simple code-optimization method based on EXIT chart analysis, and we design a rate-1/2 LDPC code that achieves very low bit-error rates within 0.15 dB of the capacity of a two-input two-output Rayleigh fading channel with 4-pulse amplitude modulation. We next propose to use a space-time block code as an inner code of our serial concatenated coding scheme. By means of a simple example scheme, using an Alamouti inner code, we demonstrate that the design/optimization of the outer code (e.g., LDPC code) is greatly simplified.

This paper considers a particular implementation of an ultra-wideband communication system that uses spectral encoding as both the multiple-access scheme and the interference-suppression technique. The main advantage of this technique is that the transmitted signal spectrum can be conveniently shaped to suppress narrowband interference and to not cause noticeable interference to overlaid systems. An extensive analysis of a possible implementation of this system by using surface acoustic-wave models is presented, and general expressions for the system performance are obtained. Numerical results show that a significant improvement in the system performance is obtained when the proposed interference-suppression method is used.

In this paper, the uplink of an asynchronous multi-carrier direct-sequence code-division multiple-access (MC-DS-CDMA) system with multiple antennas at both the transmitter and the receiver is considered. We analyze the system performance over a spatially correlated Rayleigh fading channel with multiple-access interference (MAI), and evaluate the antenna array performance with joint fading reduction and MAI suppression. Assuming perfect channel knowledge available at the transmitter, maximal ratio transmission is employed to weight the transmitted signal optimally in terms of combating signal fading. At the receiver, adaptive beamforming reception is adopted to both suppress MAI and combat the fading. Note that while correlations among the fades of the antennas in the receive array reduce the diversity gain against fading, the array still has the capability for interference suppression. We examine the effect of varying the number of transmit and receive antennas on both the diversity gain and the interference suppression.

In this paper, we analyze the error probability performance of maximal-ratio combining (MRC), equal-gain combining (EGC), selection combining (SC), and generalized-selection combining (GSC) diversity schemes with coherent binary phase-shift keying (BPSK) signaling on Rayleigh fading channels with Gaussian channel-estimation errors. We first show that, with weighting errors, averaging the conditional probability of error with the density function of the instantaneous signal-to-noise ratio (SNR) at the output of the combiner yields a lower bound on the exact probability of error. Later, we derive the exact probability of error for MRC, EGC, SC, and GSC diversity schemes and show that, for the case of nonnegative values of the in-phase correlation coefficient between the actual and the estimated channel gains, the exact probability of error with weighting errors is the same as that of the case with perfect channel estimation, but with the average SNR per diversity branch /spl gamma/~, for the case of perfect channel estimation, replaced by the effective SNR /spl gamma/~/sub /spl rho//, due to weighting errors. The effective SNR is a function of both /spl gamma/~ and /spl rho/, the magnitude of the normalized cross correlation between the actual and the estimated channel gains. Finally, we show that, as /spl rho//spl rarr/0, the average probability of error approaches 0.5, irrespective of the order of diversity and the diversity-combining rule.

This work considers the performance of a DS/CDMA system in the presence of narrow-band interference and imperfect channel estimation. A receiver based on MMSE channel estimation and maximum-likelihood data detection is assumed. An adaptive MMSE receiver, based on the LMS algorithm, is also explored. The performance of the DS/CDMA system is compared with that of a recently proposed UWB spectral-encoded system. The formulation and numerical results presented allow for a better understanding of the spectral overlay effects for UWB communication systems.

In this paper, we investigate the effect of fade correlation and the level of imbalance in the mean signal strength on the capacity of Rayleigh fading channels with dual antenna receive diversity. We consider maximal ratio combining (MRC), equal gain combining (EGC) and selection combining (SC) diversity schemes and derive expressions for the average and the outage capacities of the channel. For the case of MRC and SC schemes we obtain closed-form expressions for the average and the outage capacities, whereas efficient Gauss-Hermite quadrature rule-based expressions are provided for ECG diversity. Numerical results are provided to illustrate the capacity degradation due to channel correlation and unbalanced diversity branches.

The optimum selection combining (SC) scheme, proposed recently by Kim, requires estimation of complex channel fades on every diversity channel for minimizing the average probability of error. In this paper, we quantify the effect of imperfect channel estimation on the performance of the optimum SC receiver. By modeling the estimation errors as independent Gaussian random variables, we obtain a simple closed-form expression for the average probability of error. We also analyze a simple sub-optimum SC scheme, based on the output of the coherent matched filter alone, without requiring the knowledge of the fade amplitudes, in the presence of channel estimation errors. Finally, we provide some numerical results to illustrate the comparative performances of various SC receivers with pilot-aided minimum mean-square error (MMSE) channel estimation.

In this paper, we provide an analytical performance comparison of various selection combining (SC) schemes for binary noncoherent frequency-shift keying (NCFSK) signals operating on independent and nonidentically distributed (i.n.d.) Rayleigh fading channels. With this motivation, we first derive the receiver structure for the optimum SC scheme, which combines one out of the available L diversity branches so as to minimize the probability of bit error. We show that the optimum SC scheme chooses the diversity branch having the largest magnitude of the logarithm of the a posteriori probability ratio (LAPPR) of the transmitted information bit. We also show that: 1) the optimum noncoherent diversity receiver, for binary NCFSK signals, is equivalent to combining the LAPPRs of all the diversity branches; 2) the SC scheme proposed by Neasmith and Beaulieu is a special case of the optimum SC scheme for independent and identically distributed (i.i.d.) Rayleigh fading; and 3) for i.n.d. fading with dual diversity (i.e., L=2), the performance of the optimum SC scheme is the same as that of the optimum noncoherent diversity receiver, whereas the Neasmith and Beaulieu SC scheme gives the performance of the suboptimum equal gain combining receiver. Bit-error rate (BER) results show that, at 10/sup -4/ BER, for i.i.d. Rayleigh fading, the proposed optimum SC scheme performs better than the existing SC schemes by 0.5-0.9 dB for L=3 and by 0.8-1.5 dB for L=5, and performs within 0.3 dB of the optimum noncoherent diversity receiver for L=3,5, and for i.n.d. Rayleigh fading with L=5, the optimum SC scheme gives an additional gain of about 2.0 dB over the Pierce SC scheme. Further, for i.i.d. Rayleigh fading, we derive the bit-error probability expression for a (3,L) selection scheme which combines three branches whose LAPPR magnitudes are the largest among the available L branches. Numerical results for this (3,L) selection scheme show that for L=5 at a BER of 10/sup -4/, combining the three branches with the largest LAPPR magnitudes yields almost the full performance of the optimum noncoherent diversity receiver, whereas, for L=7, it is just about 0.2 dB away from the latter.

In this paper, we develop analytical tools for the performance analysis of coded, coherent communication systems on independent and identically distributed Nakagami-m fading channels with selection combining (SC) diversity. First, we derive an exact expression for the moment generation function (MGF) of the signal-to-noise ratio (SNR) of a code symbol at the output of the selection combiner. Next, based on Gauss-Chebyshev quadrature and Gauss-Laguerre quadrature rules, we propose a simple to compute, yet accurate, numerical solution for the pairwise error probability (PEP) of coded M-phase-shift keying (PSK) signals. Using the PEP expressions, we present the union bound-based bit-error performance of trellis-coded modulation schemes and turbo codes. Finally, we derive an exact expression for the computational cutoff rate of a coded system with M-PSK signaling and SC diversity, and show that the cutoff rate expression is a simple function of the MGF of the SNR at the output of the diversity combiner.

The uplink of a space-time spreading code-division multiple-access system with dual transmit and dual receive antennas is analyzed with the effect of imperfect channel estimation. With the help of pilot signals, the Rayleigh multipath fading channel experienced by the transmitted signal is estimated by simple correlators, and subsequently used to coherently combine the multipath signals in a RAKE-like space-time combiner. As a system becomes more wideband, more multipaths are resolved, and the energy in each path is reduced. This reduction in signal strength causes increased estimation error and impacts the system performance. Through the derivation of the probability of error in a space-time spreading system with channel-estimation errors, this paper studies the tradeoff between diversity and estimation errors. It is shown that an optimal bandwidth exists and the optimal power allocation for the pilot signals is a function of the quality of the channel estimates.

This letter presents a mathematical model for the statistics of wideband code-division multiple-access signals which experience multipath fading. The statistics are obtained from numerically generated signals, and are then modeled with an offset noncentral chi distribution with three degrees of freedom. The mathematical model is tested against the numerically generated data by the use of a chi-square goodness-of-fit test.

In this letter, the performance of multicarrier code-division multiple-access (CDMA) systems is studied in the presence of narrowband interference for future ultra-wideband (UWB) communications. A Nakagami fading channel is assumed, and notch filters along with diversity techniques are used in the multicarrier CDMA receiver. A complete performance analysis of error probability is given. It is shown that when the number of subcarriers jammed by narrowband interference is small, the multicarrier receiver without notch filters can work well, due to the gain of frequency diversity from nonjammed subcarriers. On the other hand, when the number of subcarriers jammed by the narrowband interference is large, using notch filters can improve the multicarrier system performance significantly.

This paper investigates the tradeoffs between source coding, channel coding, and spreading in code-division multiple-access systems, operating under a fixed total bandwidth constraint. We consider two systems, each consisting of a uniform source with a uniform quantizer, a channel coder, an interleaver, and a direct-sequence spreading module. System A is quadrature phase-shift keyed modulated and has a linear block channel coder. A minimum mean-squared error receiver is also employed in this system. System B is binary phase-shift keyed modulated. Rate-compatible punctured convolutional codes and soft-decision Viterbi decoding are used for channel coding in system B. The two systems are analyzed for both an additive white Gaussian noise channel and a flat Rayleigh fading channel. The performances of the systems are evaluated using the end-to-end mean squared error. A tight upper bound for frame-error rate is derived for nonterminated convolutional codes for ease of analysis of system B. We show that, for a given bandwidth, an optimal allocation of that bandwidth can be found using the proposed method.

The effect of mutual coupling and correlated fading on a receive diversity system employing a compact linear array of dipoles and noisy channel estimates is studied analytically in this paper. For a compact antenna array, the system performance depends on the interplay between diversity and mutual coupling, and it may not always improve with the addition of an antenna element in the presence of mutual coupling.

In this paper, we propose and analyze the bit error performance of a generalized selection combining (GSC) receiver for M-ary non-coherent frequency shift keying (NCFSK) signals on i.i.d. Rayleigh fading channels with L antennas at the receiver. For each of the M hypotheses, the receiver combines the K largest outputs among the L available square-law detector outputs before proceeding to the bit detection process. We derive a closed-form expression for the bit error probability of the proposed (K, L) GSC receiver, and present numerical results to illustrate the bit error performance of this receiver for different values of M, K, and L. We also show that our generalized (K, L) GSC scheme and analysis encompass the previously reported schemes/analyses by Chyi et al. and Hahn a special cases for K = 1 and K = L, respectively.

For pt.I see ibid., vol.50, p.2018-31 (2002). In part I, we proposed and investigated a hybrid pulse position modulation/ultrashort light pulse code-division multiple-access (PPM/ULP-CDMA) system for ultrafast optical communication networks. In this scheme, the large bandwidth of a ULP is efficiently utilized by virtue of the very high time resolution of a time-space processor. More detailed analysis and discussion on the receiver scheme using the time-space processor is now presented; nonideal performance of the time-space processor, including the reference pulse realization problem, as well as amplifier and detector noise, are taken into account. Discussions on physically achievable ranges of the system parameters that determine the performance of the proposed PPM/ULP-CDMA system are also made based upon current, state of the art technology. As remedies to overcome the physical limitations on the system parameters, two modified modulation/demodulation schemes are proposed and investigated to enhance the performance of the hybrid PPM/ULP-CDMA system.

To accommodate high-speed data transmissions, it may be necessary to substantially reduce the processing gain of a direct-sequence spread-spectrum (DSSS) system. As a result, intersymbol interference effects may become more severe. In this paper, we present a new structure for maximum-likelihood sequence estimation equalization of DSSS signals on a multipath fading channel that performs the function of despreading and equalization simultaneously. Analytical upper bounds are derived for the bit-error probability when random spreading sequences are used, and comparisons to simulation results show that the bounds are quite accurate. The results also show that significant performance improvement over the conventional RAKE receiver is obtained.

We design multilevel coding (MLC) and bit-interleaved coded modulation (BICM) schemes based on low-density parity-check (LDPC) codes. The analysis and optimization of the LDPC component codes for the MLC and BICM schemes are complicated because, in general, the equivalent binary-input component channels are not necessarily symmetric. To overcome this obstacle, we deploy two different approaches: one based on independent and identically distributed (i.i.d.) channel adapters and the other based on coset codes. By incorporating i.i.d. channel adapters, we can force the symmetry of each binary-input component channel. By considering coset codes, we extend the concentration theorem based on previous work by Richardson et al. ( see ibid., vol.47, p.599-618, Feb. 2001) and Kavc/spl caron/ic/spl acute/ et al.(see ibid., vol.49, p.1636-52, July 2003) We also discuss the relation between the systems based on the two approaches and show that they indeed have the same expected decoder behavior. Next, we jointly optimize the code rates and degree distribution pairs of the LDPC component codes for the MLC scheme. The optimized irregular LDPC codes at each level of MLC with multistage decoding (MSD) are able to perform well at signal-to-noise ratios (SNR) very close to the capacity of the additive white Gaussian noise (AWGN) channel. We also show that the optimized BICM scheme can approach the parallel independent decoding (PID) capacity as closely as does the MLC/PID scheme. Simulations with very large codeword length verify the accuracy of the analytical results. Finally, we compare the simulated performance of these coded modulation schemes at finite codeword lengths, and consider the results from the perspective of a random coding exponent analysis.

We analyze the influence of mobility on downlink performance of a wireless communication system. We first summarize some previously obtained information theoretic results, followed by a review of multiuser diversity systems. Then we introduce several first-order Markov chain models for use with multiuser diversity scheduling algorithms, in order to obtain quantitative performance results. Finally we propose an improved scheduling algorithm and compare its performance against that of a known scheduling algorithm.

In this paper, we derive a new selection combining (SC) scheme for noncoherent binary FSK signals on independent (but not necessarily identically distributed) Rayleigh fading channels with L-antenna diversity reception. With this combining scheme, we choose the diversity branch having the largest magnitude of the logarithm of the ratio of the a posteriori probabilities (log-APP ratio - LAPPR) of the transmitted information bit. We show that this scheme minimizes the probability of bit error, thus proving the optimality. We also show that a) the traditional square-law combining of all L diversity branches is equivalent to combining the LAPPR of all the L diversity branches, and b) the SC scheme proposed by Neasmith and Beaulieu (1998) is a special case of the proposed optimum SC scheme for independent and identically distributed (i.i.d) Rayleigh fading. Bit error probability results show that, at 10/sup -4/ BER, a) for i.i.d Rayleigh fading, the proposed optimum SC combining scheme performs better than the existing SC schemes by 0.5 dB for L = 3 and 1.5 dB for L = 5, and performs within 0.5 dB of the scheme which square-law combines all the L diversity branches, and b) for independent Rayleigh fading, the proposed optimum SC scheme gives an additional gain of 2.0 dB over the SC schemes of Pierce (1958) and Chyi et al. (1989).

In a previous paper, we derived the optimum selection combining (OSC) scheme for binary noncoherent FSK (NCFSK) signals on independent (but not necessarily identically distributed) Rayleigh fading channels with L-antenna diversity reception. In the OSC scheme, the diversity branch having the largest magnitude of the logarithm of the ratio of the a posteriori probabilities (log-APP ratio - LAPPR) of the transmitted information bit is chosen. In this paper, we derive the bit error performance of a (3, L) selection combining scheme for binary NCFSK signals which combines the three branches whose LAPPR magnitudes are the largest among the available L branches in i.i.d Rayleigh fading. Numerical results for this (3, L) selection scheme show that, for L=5, combining the three branches with the largest LAPPR magnitudes yields almost the full performance of the square-law combining of all the L=5 branches. For L=7, the performance of combining the three branches with the largest LAPPR magnitudes is just about 0.2 dB worse from the performance of square-law combining of all the L=7 branches. We also compare the performance of the proposed (3, L) selection scheme with the (3, L) selection combining scheme of Chyi et al. (1989).

In this paper, we analyze the performance of trellis coded modulation (TCM) schemes with generalized selection combining (GSC) on fading channels. We first derive the computational cutoff rate, R/sub 0/, for coherent TCM schemes on i.i.d. Rayleigh fading channels with (K, L) GSC diversity which combines the K paths with the largest instantaneous SNR among the L available diversity paths. The cutoff rate is shown to be a simple function of the moment generating function (MGF) of the SNR at the output of the (K, L) GSC receiver. The cutoff rate results show that, at a cutoff rate of 1 bit/sec/Hz, 8-PSK modulation with (1, 3) GSC requires about the same E/sub b//N/sub 0/ as QPSK modulation with (2,3) GSC. Also, at 1.5 bits/sec/Hz, 8-PSK with (1, 3) GSC and QPSK with (3,3) GSC require about the same E/sub b//N/sub 0/. This illustrates that in TCM schemes with GSC diversity, the modulation complexity (i.e., alphabet size, M) and the GSC receiver complexity (i.e., the number of combined diversity paths, K) can be traded off to achieve a desired performance. Next, we derive the union bound on the bit error probability of TCM schemes with (K, L) GSC reception in the form of a simple, finite integral. The effectiveness of this bound is verified through simulations.

Transmit diversity has been studied recently as an effective technique to provide high-performance wireless transmission. Space-time spreading is a transmit diversity scheme that achieves full spatial diversity while maintaining high spectral efficiency. This paper analyzes how channel estimation errors in diversity combining affects the uplink of a space-time spreading CDMA system with two transmit and two receive antennas. Pilot and data from K asynchronous users are transmitted through a Rayleigh multipath fading channel. The channel estimates are obtained at the outputs of correlators and are used in the RAKE combiner. As a system becomes more wideband, more multipaths are resolved, and the energy in each path is reduced. This reduction in signal strength causes increased estimation error and impacts the system performance. This paper studies the tradeoff between diversity and estimation errors.

We propose a first-order Markov model for generalized Nakagami-m (1960) flat fading channels. Using a moment generating function (MGF) approach, we derive the parameters of the proposed model for any value of the fading severity index m /spl ges/ 0.5. The proposed model is a generalized version of the one proposed earlier by Zorzi et al. (1995) for flat Rayleigh fading channels. For m = 1, we show that our generalized model gives the same parameter values obtained using Zorzi's approach. Our generalized model thus encompasses Zorzi's model as a special case. We illustrate the usefulness of our proposed Markov model by applying it to the analysis of the throughput and energy efficiency performance of a link layer (LL) protocol with backoff on wireless fading links with different values of the m-parameter (= 0.5,1,4).

Space-time block codes (STBC) have been proposed as a less complex alternative to space-time trellis codes for achieving transmit diversity. Application of such STBCs to code division multiple access (CDMA) systems is called space-time spreading (STS). STS schemes have been considered under the assumption of independent transmit fades and ideal channel state information (CSI). However, the performance of these schemes deteriorates when the above assumptions are relaxed in a realistic system. We study analytically the performance degradation due to both the fade correlation and the channel estimation errors.

Multicarrier (MC) code division multiple access (CDMA) is an attractive way of implementing wideband CDMA systems while being backward-compatible with the existing narrowband CDMA systems. Initial acquisition in MC-CDMA has been studied for the case where the subcarriers experience independent fades. For many of scenarios, the assumption about the independence is unreasonable. We relax this assumption, and employ a realistic model for frequency correlation. We obtain an optimum receiver for initial acquisition that accounts for the frequency correlation, and present a technique for the analysis of this receiver. Finally, we study, by simulation, the tradeoff between the diversity gain available and noncoherent loss incurred as the number of subcarriers is increased.

We propose an online signal-to-noise ratio (SNR) estimation scheme for Nakagami-m (1960) fading channels with L branch equal gain combining (EGC) diversity. We derive the SNR estimate based on the statistical ratio of certain observables over a block of data, and use the SNR estimates in the iterative decoding of turbo codes on Nakagami-m fading channels with L branch EGC diversity. We evaluate the turbo decoder performance using the SNR estimate under various fading and diversity scenarios (m = 0.5, 1, 5 and L = 1, 2, 3) and compare it with the performance using perfect knowledge of the SNR and the fade amplitudes.

We propose a hybrid pulse position modulation/ultrashort light pulse code-division multiple-access (PPM/ULP-CDMA) system for ultrafast optical communications. The proposed system employs spectral CDMA encoding/decoding and PPM with very short pulse separation. The statistical properties of the encoded ULP are investigated with a general pulse model, and the performance of the proposed PPM/ULP-CDMA system is investigated. It is shown that we can improve upon the performance of other ULP-CDMA systems, such as those using on-off keying, by employing PPM. It is also shown that we can improve the performance of the proposed system by increasing the effective number of chips, by increasing the number of PPM symbols, and by reducing the ULP duration. The performance analysis shows that the aggregate throughput of the proposed PPM/ULP-CDMA system could be over 1 Tb/s.

We propose an adaptive scheme to reject multiuser interference using as little as 20-30 taps in direct-sequence code-division multiple-access systems employing arbitrary processing gains (31-400 considered here). We demonstrate that our receiver using this scheme does not require training signals, either initially when the desired user powers on in the presence of several strong interferers, or during transmission, as other users (possibly near-far interferers) gain access to the system. Moreover, knowledge of interfering users spreading codes, timing, or phase information is not required with our adaptive receiver.

As larger diversity and coding gains are introduced into spread-spectrum systems to mitigate the effects of multipath fading and thermal noise, system performance is increasingly being limited by channel estimation error. To properly design these systems, performance measures incorporating the effects of estimation errors are needed. A lower bound on the probability of bit error for a RAKE receiver operating with imperfect phase estimates is presented along with a method to obtain a closed-form expression for its evaluation. Further, this lower bound is compared to an upper bound so that an accurate assessment of system performance can be obtained.

We design multilevel coding (MLC) schemes with low-density parity-check (LDPC) codes as component codes at each level. We develop a method to analyze the performance of an LDPC code at any level as the codeword length goes to infinity, even if the equivalent binary-input component channels are not symmetric. By joint optimization of code rates and degree distributions, the optimized irregular LDPC codes at each level are capable of achieving reliable transmission at signal-to-noise ratios (SNR) very close to the capacity of the additive white Gaussian noise (AWGN) channel as the codeword length tends to infinity. Simulation results show that the optimized LDPC codes also perform very well at moderate codeword lengths.

We are concerned with the performance of turbo codes on Rayleigh fading channels when noncoherent detection is employed. Performance of turbo codes with noncoherent detection on AWGN channels has been analyzed by E.K. Hall and S.G. Wilson (see Proc. IEEE Commun. Theory Mini-Conf, p.66-70, 1997). They compared the performance of turbo codes with the achievable information theoretic channel capacity for noncoherent detection on AWGN channels. We derive the information theoretic channel capacity for noncoherent detection on Rayleigh fading channels, and show that the noncoherent turbo decoder achieves performance close to this theoretical capacity (within 1.5 dB). We further show that the performance of noncoherent turbo decoder without the knowledge of channel fades is quite close (within 0.5 dB at 10/sup -5/ BER) to that with perfect knowledge of the channel fades. Optimum decoding of turbo codes requires the knowledge of channel SNR. With this requirement in mind, we propose online SNR estimation schemes, based on the ratio of certain observables at the output of the noncoherent detector, which when used in the turbo decoder gives performance very close to that with perfect knowledge of channel SNR.

We derive bit error performance bounds for turbo codes on Nakagami fading channels with diversity combining. We first derive the average pairwise error probability expressions for turbo codes with maximal ratio combining (MRC), equal gain combining (EGC), and selection combining (SC) on Nakagami fading channels. Using the pairwise error probability expressions and the union bounding technique, we then obtain bounds on the bit error probability of turbo codes for MRC, EGC, and SC diversity schemes. We present the modified log-MAP turbo decoder for MRC, EGC, and SC, and compare the analytical bounds with simulation results for the special case of 2-antenna diversity with Nakagami parameter m=1 (i.e., Rayleigh fading). Results indicate that the EGC scheme with turbo coding performs close to MRC scheme for i.i.d Rayleigh fading.

We propose an online SNR estimation scheme for generalized fading channels. We derive the SNR estimate based on a statistical ratio of observables over a block of data, when the channel undergoes Nakagami fading. An online SNR estimator for AWGN channels has been derived by Summers and Wilson (see IEEE Trans. Commun., vol.46, no.4, p.421-23, 1998). Our SNR estimation scheme in this paper is for a general Nakagami fading channel, where the SNR estimates in Rayleigh fading and AWGN can be obtained as special cases corresponding to the Nakagami parameter m=1 and m=/spl infin/, respectively. As an example, we use our SNR estimate in the iterative decoding of turbo codes on both i.i.d and correlated Rayleigh fading channels. We show that the turbo decoder performance on Rayleigh fading channels using our SNR estimate is close to the performance using perfect knowledge of the fade amplitudes and the SNR.

We design low-density parity-check (LDPC) codes for bandwidth efficient modulation using a multilevel coding (MLC) technique. We develop a method to analyze the asymptotic performance of the LDPC codes using message-passing decoding at each level of the MLC scheme as the codeword length goes to infinity. Simulation of very large block size LDPC codes verifies the accuracy of the analytical results. We jointly optimize the code rates and code parameters of the LDPC codes at each level of the MLC scheme, and the asymptotic performance of the optimized irregular LDPC codes is very close to the channel capacity of the additive white Gaussian noise (AWGN) channel.

A numerical method has been presented to determine the noise thresholds of low density parity-check (LDPC) codes that employ the message passing decoding algorithm on the additive white Gaussian noise (AWGN) channel. In this paper, we apply the technique to the uncorrelated flat Rayleigh fading channel. Using a nonlinear code optimization technique, we optimize irregular LDPC codes for such a channel. The thresholds of the optimized irregular LDPC codes are very close to the Shannon limit for this channel. For example, at rate one-half, the optimized irregular LDPC code has a threshold only 0.07 dB away from the capacity of the channel. Furthermore, we compare simulated performance of the optimized irregular LDPC codes and turbo codes on a land mobile channel, and the results indicate that at a block size of 3072, irregular LDPC codes can outperform turbo codes over a wide range of mobile speeds.

This paper presents a turbo coded multicarrier direct sequence code division multiple access (DS/CDMA) system, where the outputs of a turbo encoder are repetition coded at multiple rates and transmitted in parallel over a number of subchannels. A performance bound useful in the so-called error floor region is obtained for Rayleigh fading channels when different diversity orders are given to each turbo code symbol. Simulation results are also provided for the low signal-to-noise ratio (SNR) region where the bound is not applicable. It is observed that the error floor can be lowered, with some performance loss in the low SNR region, by applying nonuniform repetition coding to the turbo code symbols.

In wideband code-division multiple-access (CDMA) systems, where large diversity gains are used to mitigate the effects of multipath fading, system performance is increasingly limited by channel estimation error. In a packet-based system, the estimation error can be reduced by increasing the header length; however this, for a fixed bandwidth, reduces the amount of transmit energy available to the data symbols and reduces the processing gain of the system. To determine the allocation of the transmit energy between the data and estimation symbols in order to minimize the probability of bit error, we use an approximate upper bound on the probability of bit error for a RAKE receiver operating with imperfect phase estimates.

Antenna diversity is an effective way of improving the performance of a communication system, especially for initial acquisition in a narrowband code division multiple access (CDMA) system that operates in a flat fading environment. Earlier attempts to analyze such systems assume independent fading at various antenna elements. However, the diversity gain predicted by these analyses is often unrealizable, since the assumption about the independence of the fades is unrealistic in a spatially constrained system. We relax the assumption about the spatial independence of the fades by employing a realistic model for spatial correlation. Also, we obtain an optimum receiver for initial acquisition that takes into account the particulars of the spatial correlation and present a technique for the analysis of this receiver. We also study the diversity gain available for various parameters of the spatial correlation model and conclude that there exists an optimum number of the antenna elements for an initial acquisition system, based on the spatial correlation of the fades. Further, we consider a wideband multicarrier CDMA system with correlated fades among its subcarriers, which also employs antenna diversity, and apply the techniques used in the narrowband case to obtain an optimum receiver and analyze its performance. Finally, we study the tradeoff between the diversity gain available and the noncoherent loss incurred as the numbers of subcarriers and antenna elements are changed.

The performance of a turbo-coded code division multiaccess system with a minimum mean-square error (MMSE) receiver for interference suppression is analyzed on a Rayleigh fading channel. In order to accurately estimate the performance of the turbo coding, two improvements are proposed on the conventional union bounds: the information of the minimum distance of a particular turbo interleaver is used to modify the average weight spectra, and the tangential bound is extended to the Rayleigh fading channel. Theoretical results are derived based on the optimum tap weights of the MMSE receiver and maximum-likelihood decoding. Simulation results incorporating iterative decoding, RLS adaptation, and the effects of finite interleaving are also presented. The results show that in the majority of the scenarios that we are concerned with, the MMSE receiver with a rate-1/2 turbo code will outperform a rate-1/4 turbo code. They also show that, for a bit error rate lower than 10/sup -3/, the capacity of the system is increased by using turbo codes over convolutional codes, even with small block sizes.

The performance of turbo-coded asynchronous direct sequence code division multiple access (DS-CDMA) using long and short spreading sequences is compared by both analysis and simulation. For coded systems with a conventional matched filter (MF) receiver, three analytical methods with different complexity are compared: the standard Gaussian approximation, the improved Gaussian approximation (IGA), and the density function approach. It is shown that while the standard Gaussian approximation is fairly accurate for the long sequences, it is too optimistic for the short sequences. For the short-sequence systems, the IGA gives an accurate estimate for the performance with much less complexity than the density function approach. The analysis shows that for either the additive white Gaussian noise (AWGN) channel or the flat Rayleigh fading channel and a MF receiver, there is a degradation in the average performance of the turbo-coded short-sequence systems compared to the long-sequence systems due to the fact that the cross-correlations are not time-varying. However, the short-sequence systems are amenable to the use of an interference suppression technique designed to minimize the mean square error. Such a minimum mean square error (MMSE) receiver in the turbo-coded system is shown to outperform the long-sequence system with the MF receiver, especially when there is a near-far problem, as previously observed in a convolutionally-coded system. Finally, similar results are obtained by computer simulations for the turbo-coded CDMA systems on a frequency-selective Rayleigh fading channel.

This article presents a successive interference cancellation (SIC) scheme for a multicarrier (MC) asynchronous DS/CDMA system, wherein the output of a convolutional encoder modulates bandlimited spreading waveforms at different subcarrier frequencies. In every subband, the SIC receiver successively detects the interferers' signals and subtracts them from that of the user-of-interest. The SIC receiver employs maximal-ratio combining (SIC-MRC) for detection of the desired user, and feeds a soft decision Viterbi decoder. A comparison is made among SIC-MRC, matched filter detection with MRC (MF-MRC), and N-tap minimum mean-squared error (MMSE) receivers with optimal tap coefficients, assuming a slowly varying, frequency selective, Rayleigh fading channel, where N is the processing gain. Analysis and simulation results show that the SIC-MRC can obtain performance close to that of N-tap MMSE receivers, and both of them have better ability to suppress multiple-access interference (MAI) than does MF-MRC. Finally, with timing or phase tracking errors, the results show that SIC-MRC can still retain a performance advantage over MF-MRC.

Adaptive linear interference suppression for direct-sequence (DS) code-division multiple access (CDMA) is studied in the presence of time- and frequency-selective fading. Interference suppression is achieved with an adaptive digital filter which spans a single symbol interval. Both decision-directed and blind adaptive algorithms, which do not require a training sequence, are considered. Modifications to least squares adaptive algorithms are presented which are compatible with differential coding and detection. For frequency-selective fading, adaptive algorithms are presented based upon different assumptions concerning knowledge of the desired user's channel. Specifically, the cases considered are as follows: (1) perfect knowledge of the desired channel; (2) knowledge of only the relative path delays; and (3) knowledge of only one delay corresponding to the strongest path. Computer simulation results are presented which compare the performance of these algorithms with the analogous RAKE receivers. These results show that for case (3), even slow fading can cause a significant degradation in performance. Effective use of channel parameters in the adaptive algorithm reduces the sensitivity to fade rate, although moderate to fast fading can significantly compromise the associated performance gain relative to the RAKE receiver.

This paper presents a coded multicarrier direct-sequence code-division multiple-access (DS-CDMA) system that, by the use of a minimum mean-squared-error receiver, achieves frequency diversity (instead of path diversity as in a conventional single-carrier RAKE DS-CDMA) and has the ability to suppress the intermodulation distortion and partially compensate for the signal distortion introduced by a nonlinear amplifier at the transmitter. A frequency-selective Rayleigh fading channel is decomposed into M frequency-nonselective channels, based on the channel coherence bandwidth. A rate 1/M convolutional code, after being interleaved, is used to modulate M different DS-CDMA waveforms. The new system is shown to effectively combat intermodulation distortion in the presence of multiple-access interference.

In this paper, we integrate the multicarrier signaling technique and the RAKE receiver to design a DS-CDMA system with the capability to increase the data rate, mitigate the effect of correlation among the various subcarriers, and suppress partial-band interference. In the proposed system, a data sequence is serial-to-parallel converted, and multicarrier DS-CDMA is used on each of the parallel data streams. The receiver provides a RAKE for each subcarrier, and the outputs of the RAKEs are combined by a maximal-ratio combiner. We employ a correlated subcarrier and frequency-selective fading channel model to derive the average probability of error of the system and compare the results with those of both a single carrier RAKE system and a multicarrier DS-CDMA system in a frequency selective Rayleigh fading channel with an exponential multipath intensity profile, when multiple access interference and partial-band interference are present.

We consider interference suppression for direct sequence code division multiple access systems when long codes are used for spreading. In particular, we evaluate the performance of a least mean square (LMS) type adaptive receiver on AWGN and correlated Rayleigh fading channels. The receiver uses a chip matched filter followed by an adaptive equalizer structure which performs despreading and interference suppression. The receiver requires only the knowledge of the long code and timing of the desired user. The probability of the error performance of the long code LMS (LC-LMS) receiver is estimated for varying degrees of near-far ratio (NFR), mobile user speed (equivalently, the Doppler bandwidth), and long code period (expressed in number of information bits). Performance results indicate that the given adaptive receiver outperforms the conventional linear correlation receiver by a considerable margin in high NFR and low mobility (i.e., small Doppler bandwidth) scenarios.

Bit error rate is evaluated for a multicarrier CDMA system with channel estimation errors. We consider a multicarrier CDMA system with pilot and data channels. The signal experiences independent flat Rayleigh fading on each subcarrier. Simple integrators produce the channel estimates which are used to maximal-ratio-combine the data from all the subcarriers, Keeping the total transmitted energy fixed, as the number of subcarriers increases, the weaker signal in each subcarrier makes channel estimation more difficult and results in larger errors in the combiner weights. We quantify how errors in channel estimation affect a multicarrier CDMA system as the system becomes more wideband. We show that there is a tradeoff between diversity order and SNR available for channel estimation.

This paper investigates the behavior of multi-code DS/CDMA systems in the presence of nonlinear distortions caused by high power amplifiers. A technique that uses a small number of parallel transmitted sequences is proposed for combating the nonlinearity. This technique can be modified to have a constant envelope for achieving further performance improvement. It is seen from performance comparisons that the proposed scheme is very effective for combating the nonlinearity.

It has previously been shown that employing a higher order constellation in conjunction with CDMA can improve the performance of the multiuser system in the presence of severe ISI and narrowband Gaussian interference. We study a similar scenario here, except that the narrowband Gaussian interferer is replaced with a tone interferer. It is shown that the use of the higher order alphabet size is also useful in the tone interference case. Also, we derive lower and upper bounds on the performance of an MMSE receiver in a multiuser, flat fading scenario. Later we apply the same technique to a multiuser, frequency selective fading scenario with negligible or mild ISI.

To determine the end-to-end delay in block coded transmission over the Gilbert-Elliott (1960, 1963) channel with interleaving and a retransmission strategy, the probability distribution of the number of transmissions that a packet has to go through before it succeeds has to be found. We propose an approximate method to evaluate the distribution when the error statistics are dependent. The end-to-end delay is then examined. Our study provides a way to choose an error-control scheme (i.e., an interleaver of an appropriate size with a retransmission protocol), which meets both the delay constraint and a reliability requirement.

This paper investigates the behavior of multicode direct sequence code division multiple access (DS/CDMA) systems in the presence of nonlinear distortions caused by high power amplifiers. The performance of the multicode system can be degraded due to a large variation in the envelope of the multicode signal. A technique that uses a small number of parallel transmitted sequences is proposed for combating the nonlinearity. This technique ran be modified to have a constant envelope for achieving further performance improvement. Comprehensive comparisons are made for different systems, based on both theoretical analysis and computer simulation. Our study shows that the technique employing a few parallel transmitted sequences, in conjunction with quasi-ML (maximum likelihood) detection, and the use of the modified scheme, are very effective for combating the nonlinearity.

To satisfy ever-increasing demands for higher data rates, as well as to allow more users to simultaneously access the network, interest has peaked in what has come to be known as wideband code division multiple access (WCDMA). We discuss those basic characteristics of WCDMA waveforms that make them attractive for high data rate transmission over wireless and mobile channels. We emphasize how the choice of spread bandwidth affects the bit error rate of the system, as well as how it affects the reliability of various subsystems, such as those that perform coarse acquisition and adaptive power control. We then discuss how some of the waveforms being considered for cellular WCDMA systems differ from those in use in the narrowband CDMA cellular system, and emphasize, as an example, multicarrier CDMA. Finally, we discuss other potential enhancements to WCDMA systems, such as the use of interference suppression at the receiver, or multiple antennas at the transmitter.

We present a successive interference cancellation (SIC) scheme for a multicarrier (MC) direct-sequence code-division multiple-access (CDMA) system, using band-limited spreading waveforms to prevent self-interference. In every subband, the SIC receiver successively detects the interferers' signals and substracts them from the user-of-interest. A comparison is made among SIC, a minimum mean-squared error (MMSE) receiver, and matched filter (MF) detection with maximal-ratio combining (MRC). We also consider suboptimal combining using pilot symbol-assisted modulation (PSAM) to make the system more realistic. Analytic bit-error probabilities for SIC, MMSE, and MF in correlated Rayleigh fading channels are derived. The theoretical results for SIC, MMSE, and MF are shown to agree well with simulations. In particular, SIC and MMSE are shown to achieve better performance than MF with MRC; when the number of users is small, SIC provides better performance than does MMSE. Further, the correlation among different subcarriers is studied, and only large subcarrier correlation coefficients result in an obvious worsening of performance. Finally, we derive results for the performance degradation that an SIC scheme experiences in MC CDMA due to phase and timing tracking errors. It is shown that SIC can still retain a performance advantage over MF-MRC, although the advantage of SIC decreases with increasing tracking errors, especially when subcarrier correlation coefficients are small.

This paper analyzes the performance of a convolutionally coded code-division multiple-access system, which employs a linear, minimum mean-square error (MMSE) receiver for interference suppression. A flat, Rayleigh fading channel is considered, where convolutional encoding and interleaving are employed in order to combat the effects of the fading. Theoretical results are derived for the average bit-error probability of the MMSE receiver, where the optimum tap weights for the adaptive filter are determined by the solution of the Wiener-Hopf equations. Simulation results showing the average bit-error rate of the MMSE receiver are also presented, which incorporate the effects of recursive least squares adaptation, channel estimation using pilot symbol-assisted modulation, and finite interleaving. Results show that the MMSE receiver with coding can provide a substantial gain over the matched-filter receiver in a rapidly varying, Rayleigh fading channel. The results also reiterate the fact that lower rate codes are not necessarily the best choice when used with the MMSE receiver.

Previous results have shown that high rate codes tend to yield a lower average bit-error rate than low rate codes when employing a minimum mean-square error (MMSE) receiver for a direct-sequence code-division multiple-access (CDMA) system in either an additive white Gaussian noise channel or a flat Rayleigh fading channel. we consider the use of larger signal constellations with both trellis-coded modulation and bit-interleaved coded modulation (BICM) to determine if further gains can be achieved in either the Rayleigh or Ricean fading channel. The average bit-error probability is derived for both coding schemes using the general Ricean fading channel model, based upon the common assumptions of infinite interleaving, perfect channel state information, and optimal MMSE receiver coefficients. New bounds are presented for BICM with 8-PSK and 16-QAM symbols, which take advantage of the symmetries inherent in the signal constellations with Gray code mapping. In addition, simulation results are presented which show the important effect a finite interleaving delay constraint has on the comparison of various codes. The results show that there are cases when coded modulation does yield a significant improvement in performance for a CDMA system using an MMSE receiver, compared to standard convolutional coding. However, the best coding strategy depends upon several factors, including the nature of the fading process (Rayleigh or Ricean), the operating signal-to-noise ratio, the interleaving delay constraint, the time-variability of the channel, the number of users in the system, and the severity of the near-far problem.

A closed-form expression for the average signal-to-noise-ratio (SNR) of generalized diversity selection combining, using the m largest (in instantaneous SNR) diversity signals, for arbitrary m, assuming that the Rayleigh fading statistics on each diversity branch are identically, independently distributed (i.i.d.), already exists in the literature. In this paper, a similar closed-form expression, but for nonidentically distributed statistics, is derived, This expression specializes to known results, such as the average SNRs of maximal-ratio combining with either i.i.d. or non-i.i.d. diversity statistics and that of conventional selection combining (CSC) with i.i.d. diversity statistics. In addition, it provides, for the first time, a simple closed-form solution to the combined SNR of CSC with non-i.i.d. diversity statistics. Further, the closed-form solution for the SNR of selecting the m, largest diversity signals has negligible computational complexity.

The performance of the minimum mean-squared error (MMSE) receiver for the detection of direct sequence code division multiple access is considered in various fading channel models. Several modifications to the basic MMSE receiver structure which have been previously proposed for use on nonselective fading channels are reviewed and shown to represent different approximations to a single common form. The performance of this general structure is analyzed as well as various extensions suitable for frequency-selective fading channels. Particular attention is given to the performance advantage gained through knowledge of the fading parameters of the various transmission paths of each user's signal. It is shown that having this knowledge is not particularly useful on a flat fading channel unless the loading is very heavy and even then the difference in performance is only minimal. On the other hand, having this knowledge is crucial in a multipath fading channel and the inability to learn the fading channel parameters will lead to substantial degradation in capacity. A heuristic explanation to support this result based on a dimensionality argument is also presented.

This paper introduces a hybrid forward-error correction/automatic repeat-request (ARQ) system that employs rate compatible punctured turbo (RCPT) codes to achieve enhanced throughput performance over a nonstationary Gaussian channel. The proposed RCPT-ARQ system combines the performance of turbo codes with the frugal use of incremental redundancy inherent in the rate compatible punctured convolutional codes of Hagenauer (1988). Moreover, this paper introduces the notion of puncturing the systematic code symbols of a turbo code to maximize throughput at signal-to-noise ratios (SNRs) of interest. The resulting system provides both an efficient family of achievable code rates at middle to high SNR and powerful low-rate error correction capability at low SNR.

The least-mean-square (LMS) estimator is a nonlinear estimator with information dependencies spanning the entire set of data fed into it. The traditional analysis techniques used to model this estimator obscure these dependencies; to simplify the analysis they restrict the estimator to the finite set of data sufficient to span the length of its filter. Thus the finite Wiener filter is often considered a bound on the performance of the LMS estimator. Several papers have reported the performance of the LMS filter exceeding that of the finite Wiener filter. We derive a bound on the LMS estimator which does not exclude the contributions from data outside its filter length. We give examples of this bound in cases where the LMS estimator outperforms the finite Wiener filter.

In Kong and Milstein (1998) a simple closed form expression for the average signal-to-noise-ratio (SNR) of combining the m largest (in instantaneous SNR) diversity signals, for arbitrary m, assuming that the Rayleigh fading statistics on each diversity branch were identically independently distributed (iid), was found. In this paper, a similar closed form expression, but for non-identically distributed statistics, is derived. This expression not only specializes to the known results, such as the average SNRs of maximal ratio combining (MRC) with either iid or non-iid diversity statistics and that of conventional selection combining (CSC) with iid diversity statistics (Jakes 1974), but also provides, for the first time, a simple closed form solution to the combined SNR of the conventional selection combining with non-iid diversity statistics. Further, the closed form solution for the SNR of selecting the m largest diversity signals has negligible computational complexity.

This paper presents a coded multicarrier (MC) DS-CDMA system which, by the use of a linear convolutional code (CC) and minimum mean-squared error (MMSE) receiver, achieves frequency diversity instead of path diversity as in a conventional single carrier RAKE DS-CDMA, and has the ability to partially compensate for the distortion introduced by a nonlinear amplifier at the transmitter. A frequency selective Rayleigh fading channel is decomposed into M frequency non-selective channels, based on the channel coherence bandwidth. A rate 1/M convolutional code, after being interleaved, is used to modulate M different DS-CDMA waveforms. The new system is shown to effectively suppress multiple access interference in the presence of intermodulation distortion.

A general model for M-ary direct sequence-spread spectrum (DS/SS) communications is presented to provide multiple services in direct sequence-spread spectrum multiple access (DS/SSMA). The main idea is to allow each user to use parallel orthogonal spreading sequences, depending upon its needs. More specifically, each service level is associated with certain combinations of the sequences. We use a code, which we refer to as a "mapping code", to form a multidimensional signal constellation. Thus, the data rate can be changed by changing the mapping code. Further, techniques of sequence sharing (reuse) and sequence arrangement (allocation), suitable for packet switching, are proposed; this allows support of multimedia services, while resulting in a reduction of sequence consumption. We evaluate error rates in the presence of multiple access interference (MAI) and additive white Gaussian noise (AWGN). Numerical results are given to show the effects of the system configuration, the data rates, and other system performance measures.

It is well known that power control is critical in CDMA mobile systems. In this paper, the error probability of a multicell CDMA system, operating with imperfect power control over a frequency selective multipath fading channel, is derived. Asymptotic expressions (for high signal-to-interference-plus-noise ratios) of the error probability show that the use of diversity combining exacerbates the effect of the power control error (PCE), since the power control is identical on each diversity channel; also, the error probability increases exponentially with PCE, while (asymptotically) increasing linearly with the number of users and decreasing inverse linearly with the processing gain. In addition to the asymptotic results mentioned above, both Chernoff bounds on, and approximations to, the error probability are found. Numerical results reveal the tightness of the Chernoff bounds, and show that the approximations are actually much tighter than the bounds, at least for PCE standard deviations of 1.5 dB or less.

The performance of a multicarrier direct sequence code-division multiple-access (CDMA) system employed in the forward link of a cellular system operating over a Rayleigh fading channel is analyzed and compared to the performance of both single-carrier CDMA and hybrid multicarrier CDMA/frequency division multiplexing systems. A RAKE receiver is provided for each subcarrier. We compare the performance of all three systems for various multipath intensity profiles. It is found that for a service requiring high quality and a small number of users, the multicarrier system is the best, but for a service requiring low quality and a large number of users, the hybrid system can support more users than the others. Also, for the case when nonorthogonal codes are used, the multiple-access interference in different resolvable paths are correlated. In that case, to maximize the signal-to-noise ratio in a correlated interference environment; maximal-ratio combining (MRC) is not optimal. However, we found that there is not much difference between the optimum combining and the conventional MRC.

We present two code-acquisition schemes for a multicarrier direct-sequence code-division multiple-access system, one that uses equal gain combining and the other that uses selection combining. The code-acquisition performance of the two multicarrier systems, as well as that of a single-carrier system, are analyzed in both nonfading and Rayleigh fading channels under the assumption that the receiver is chip-synchronized; the effect of partial-band interference (PBI) on the performance is also included. It is demonstrated that in an additive white Gaussian noise channel, the single-carrier system has a better code-acquisition performance than both multicarrier systems. However, in a Rayleigh fading channel, the code-acquisition performance of a multicarrier system with equal gain combining is better than that of the single-carrier system, while a multicarrier system with selection combining has the same performance as the single-carrier system. Further, the presence of PBI more severely affects the code-acquisition performance of the single-carrier system than those of both multicarrier systems. Finally, the code-acquisition performance of a multicarrier system with equal gain combining is always better than that of the selection combining system.

Several receivers for data demodulation in an asynchronous direct-sequence code-division multiple access (DS-CDMA) system operating without prior knowledge of the propagation delays are proposed and compared. Special attention is paid to the near-far problem, and the proposed schemes are numerically shown to be near-far resistant. The near-far resistance is obtained by estimating the a priori unknown propagation delay using a subspace-based technique. Quantities obtained in the estimation procedure are used to design a filter used for suppression of interference, according to the minimum mean square error criterion. Either a decision feedback technique or a simple two-state Viterbi algorithm is subsequently used for the data demodulation in the uncoded case. By extending the trellis used in the Viterbi algorithm, error correcting coding is easily implemented.

This paper presents a multicarrier asynchronous direct-sequence code-division multiple-access (DS-CDMA) system wherein the output of a convolutional encoder modulates multiple band-limited DS-CDMA waveforms, which are transmitted in parallel at different carrier frequencies. The receiver detects and combines signals for the desired user and feeds a soft-decision Viterbi decoder. The performance of this system is compared to that of a conventional single-carrier DS-CDMA system with a RAKE receiver, assuming a slowly varying frequency-selective Rayleigh fading channel and assuming the presence of additive white Gaussian noise and multiple-access interference. Results will demonstrate similar performance at roughly equal receiver complexity.

For pt.I see ibid., vol.47, p.1570-92 (1999). In Rowitch and Milstein (1999), a convolutionally encoded multicarrier asynchronous direct-sequence code-division multiple-access (DS-CDMA) system was proposed and compared to a classical single-carrier DS-CDMA system employing a RAKE receiver and the same convolutional code, in the presence of additive white Gaussian noise and multiple-access interference. This paper considers the additional impact of various forms of narrow-band interference on the performance of these two systems and the ability of the multicarrier system to effectively suppress the interference using the innate structure of its receiver. At roughly equivalent receiver complexity, results demonstrate superior performance of the multicarrier system in the presence of such interference, without requiring the addition of a front-end interference suppression filter.

A direct-sequence spread spectrum (DS/SS) receiver for suppressing multiple-access interference in direct-sequence code-division multiple-access (DS/CDMA) communication systems is introduced. The proposed receiver does not require knowledge of other users' spreading codes, timing, or phase information. Moreover, the receiver allows the number of taps to be chosen independently of the processing gain and, hence, is easily applicable to CDMA systems employing either a small or a large processing gain. Performance analysis, including average probability of error and signal-to-noise ratio, is provided, and results are presented for systems varying from lightly loaded (for example, eight user/255 chip) to heavily loaded (for example, 50 user/200 chip). Performance results indicate that the proposed receiver outperforms the linear correlation receiver and, in many cases, it does so by a considerable margin.

By using the Gilbert-Elliott (1960, 1963) model to study the performance of block-coded transmission over the land mobile channel, a new analytical expression illustrating the effect of various parameters, e,g., mobile speed, delay constraint, and parameters for the error correcting code, is found. Comparisons between the results obtained by this analytical expression and results obtained by computer simulations show that the analytical results are accurate for a broad range of channel parameters. The Gilbert-Elliott model is then used to compare the performance of different binary BCH codes when the delay constraint does not allow the assumption of infinite interleaving. In contrast to the memoryless case, where the performance typically is improved with increased block length, short codes are found to be as good, or even superior, due to the fact that the interleaver works better for shorter codes.

The least-mean-square (LMS) estimator is a nonlinear estimator with information dependencies spanning the entire set of data fed into it. The traditional analysis techniques which are used to model this estimator obscure this, restricting the estimator to the finite set of data sufficient to span the length of its filter. The finite Wiener filter is thus often considered a bound on the performance of the LMS estimator. Several papers have reported the performance of the LMS filter exceeding that of the finite Wiener filter. We demonstrate a bound on the LMS estimator, which does not exclude the contributions from data outside its filter length, and which demonstrates the ability of the LMS estimator to outperform the finite Wiener filter in certain cases.

The average signal-to-noise-ratio (SNR) of a generalized selection combining (GSC) scheme, in which the m diversity branches (m/spl les/L, where L is the total number of diversity branches available) with the largest instantaneous SNRs are selected and coherently combined, is derived. A Rayleigh fading channel is assumed, and a simple closed-form expression for the SNR is found which is upper bounded by the average SNR of maximal ratio combining, and lower bounded by average SNR of conventional selection combining.

When user terminals powered by a finite battery source are used for wireless communications, energy constraints are likely to influence the design/choice of media access protocols. We analyze the energy efficiency of a multichannel wireless access protocol using a finite energy source in a mobile radio environment. The average number of correctly transmitted packets for a given amount of allocated energy is used as the appropriate energy efficiency metric. The mobile radio channel itself is characterized by a correlated Rayleigh fading process, whose memory parameters depend on the speed of the user terminal, the data rate, and the physics of the channel. We show that the protocol which recovers erroneous data packets through retransmission is more energy efficient at low channel correlations.

Third generation wireless systems are expected to be characterized by high data rates and multimedia traffic. This paper is a preliminary investigation to determine if two dimensional higher order constellations, in conjunction with code division multiple access (CDMA), can be used to support the data communication needs of such systems. We compare the performance of QPSK and 16-QAM in a high multipath environment when either conventional matched filters (MF) or minimum mean square error (MMSE) receivers are used. We substantiate the results of semi-analytical calculations of the error rate with simulations, and conclude that the use of a higher order constellation can improve the system performance when the multipath is sufficiently severe.

The throughput performance of a wireless media access protocol taking into account the effect of correlated channel fading, capture, and propagation delay is analyzed. For efficient access on the uplink (mobile-to-base-station link), the protocol makes use of the uplink channel status information which is conveyed to the mobiles through a busy/idle flag broadcast on the downlink (base-station-to-mobile link). A first-order Markov model is used to describe the correlation in the packet success/failure process on a Rayleigh-fading channel. The analytical results obtained through the first-order Markov approximation of the channel are compared to those obtained from an independent and identically distributed (i.i.d.) channel model. The Markovian-fading channel model is shown to provide better performance results than the i.i.d. channel model. Simulations show that a first-order Markov approximation of the Rayleigh-fading process is quite accurate. An enhanced version of the access protocol to take advantage of the memory in the fading channel behavior is proposed and analyzed. The effect of retransmission of erroneous data packets and propagation delay on the throughput is also analyzed. It is shown that the access protocol with an error detect (ED) feature is efficient in slow fading (e.g., pedestrian user speeds), whereas a retransmission protocol is more efficient in fast fading (e.g., vehicular user speeds).

The effect of phase noise on the capacities of coherently demodulated hybrid frequency-division multiple access/code division multiple-access (FDMA/CDMA) systems operating over a multipath Rayleigh fading channel is investigated. Using an approximate upper bound on the bit-error rate performance, which has been derived and presented in a previous paper, the capacities of the FDMA/CDMA systems are estimated for several combinations of channel and system parameters. Simulation results are also included to show the effect of the bounding error.

Based on the observation of Kim, Kwon, Hong and Whang (see ibid., vol.46, no.9, p.1109-10, 1998), we discovered that in our derivation (see ibid., vol.44, p.1117-29, 1996), a factor of one-half in the exponent of one of three exponentials was inadvertently dropped and this, in turn, affected two other equations.

We present Walsh-quadrature phase-shift keying (Walsh-QPSK) pseudonoise (PN) modulation schemes for both coherent and noncoherent direct-sequence code-division multiple-access (DS-CDMA) systems, wherein the PN spreading sequences for in-phase and quadrature data in a conventional QPSK PN modulation scheme are coded by Walsh sequences indexed by a special rule to reduce the envelope variation of the transmitted signal. The signal characteristics of the two schemes are analyzed when a rectangular-shaped PN chip pulse is used, and it is shown that the proposed coherent DS-CDMA system has a constant envelope even in the presence of a transmitted phase reference. We simulate the signals to obtain the envelope variations when a spectrally efficient shaped PN chip pulse is used, and compare the results with those of conventional QPSK and orthogonal QPSK (OQPSK) PN modulation schemes. The results show that both the noncoherent and coherent Walsh-QPSK schemes have smaller envelope variations than the conventional noncoherent QPSK and OQPSK PN modulation schemes, even though in the coherent Walsh-QPSK scheme the pilot channel is added to the signal channel.

The focus of this paper is on the initial acquisition of a direct sequence (DS) spread-spectrum signal utilizing a purely parallel search strategy. A parallel search strategy is utilized because it reduces the acquisition time compared to either serial or partially parallel strategies. In particular, the purpose of this paper is to derive the optimal decision rule, based on the maximum-likelihood criterion, for frequency-selective fading channels. The performance of the conventional decision rule, optimized for additive white Gaussian noise and flat-fading channels, the optimal decision rule derived, and a suboptimal decision rule, also presented in this paper, are compared. It is shown that the optimal and suboptimal decision rules for Rayleigh-fading channels outperform the conventional decision rule. For Rician-fading channels, it is shown both that the optimal decision rule outperforms the conventional decision rule, and that the optimum decision rule for Rayleigh-fading channels, when implemented on a Rician-fading channel, yields approximately optimum performance.

We investigate the behavior of block errors which arise in data transmission on fading channels. Our approach takes into account the details of the specific coding/modulation scheme and tracks the fading process symbol by symbol. It is shown that a Markov approximation for the block error process (possibly degenerating into an identically distributed (i.i.d.) process for sufficiently fast fading) is a good model for a broad range of parameters. Also, it is observed that the relationship between the marginal error rate and the transition probability is largely insensitive to parameters such as block length, degree of forward error correction and modulation format, and depends essentially on an appropriately normalized version of the Doppler frequency. This relationship can therefore be computed in the simple case of a threshold model and then used more generally as an accurate approximation. This observation leads to a unified approach for the channel modeling, and to a simplified performance analysis of upper layer protocols.

The effect of both temporal and spatial dispersion on the performance of a combined configuration of a RAKE and a phased array system is analyzed theoretically. The mean power improvement due to an antenna array system, and fading reduction by means of the RAKE configuration, are described in the presence of the spatial and temporal dispersion of multipath waves.

The minimum mean square error (MMSE) receiver for a DS-CDMA system is capable of suppressing multiple access interference (MAI), unlike the conventional digital matched filter (DMF) receiver. Moreover, this type of receiver offers implementation advantages over optimum receivers for DS-CDMA systems. However, the performance of the MMSE receiver is affected by the quantization of the adaptive filter coefficients and the analog-to-digital converter (ADC) output. In this paper, the performance degradation of the MMSE receiver due to finite precision arithmetic of a digital signal processor is simulated for both non-fading and fading channels. The performance comparison between the MMSE receiver and the DMF receiver, for a given number of bits per sample, is also evaluated.

The bit-error rate (BER) performance of a direct sequence spread spectrum (DS-SS) signal, operating over a multipath Rayleigh fading channel, is investigated when corrupted by phase noise as well as additive white Gaussian noise (AWGN). The phase noise arises from phase locked loop (PLL) dynamics and results in imperfect receiver phase estimates whereby the phase errors assume Tikhonov densities. The phase estimates are used by a multipath-combining RAKE receiver for demodulation. Approximate upper-bounds on the bit error probability are obtained and evaluated for different combinations of channel parameters and for various values of the average loop signal-to-noise ratio (SNR). Results indicate that for a PLL with loop SNR 10 dB above the system E/sub b///spl eta//sub 0/, the degradation is less than 3 dB, and for a loop SNR of 20 dB above E/sub b///spl eta//sub 0/, the degradation is less than 1 dB.

This paper extends the analysis presented by Eng and Milstein (see IEEE J. Select. Areas Commun., vol.12, no.5, p.938-951, 1994) of a hybrid frequency division multiple access/code-division multiple-access [FDMA/CDMA (F/CDMA)] system in a Rayleigh fading environment to include the presence of a direct-path component (i.e., Ricean fading). The results show that a "wideband" CDMA system out performs a hybrid system, although the combination of a specular component and an exponentially decaying multipath intensity profile (MIP) with a large decay factor may result in only a minimal degradation of system performance for a hybrid system.

The use of antenna diversity is investigated for improving the parallel acquisition performance of a direct-sequence spread-spectrum signal in both flat and frequency-selective fading channels. Noncoherent combining techniques are analyzed because coherent diversity techniques, such as maximal ratio combining, cannot be employed prior to initial acquisition. A major application of this work is to cellular communications.

This paper presents the performance of a direct sequence spread spectrum acquisition scheme in a mobile terrestrial communications system. The effects of fading, multipath, power control, shadowing, multiple access interference, out-of-cell interference, vehicle speed, voice activity, and sectorization are examined. The acquisition scheme uses noncoherent detection and a parallel search strategy. The analysis is done for the reverse link of a mobile code-division multiple-access (CDMA) system. The purpose of this paper is to derive the acquisition performance of a mobile communications system under practical assumptions, and give realistic capacity estimates based on acquisition performance criteria.

We investigate the behavior of block errors which arise in data transmission on fading channels. Our approach is more detailed than previous studies, in that it takes into account the specific coding/modulation scheme and it tracks the fading process symbol by symbol. It is shown that a Markov approximation for the block error process (possibly degenerating into an i.i.d. process for sufficiently fast fading) is a very good model for a broad range of parameters. Also, it is observed that the relationship between the marginal error rate and the transition probability is largely insensitive to parameters such as block length, degree of forward error correction and modulation format, and only depends on an appropriately normalized version of the Doppler frequency. This observation leads to a unified approach for the channel modelling which simplifies the performance analysis of upper-layer protocols.

Several practical methods are proposed for the implementation of linear filters that reject multiple-access noise in direct sequence code-division multiple-access (DS-CDMA) receivers. These methods rely on taking advantage of variations in the received chip spectrum, and do not require locking and despreading multiple signals. The resulting structures are simple, and some can operate at a high chip rate. Adaptive structures are also proposed. It is shown that a close relationship exists between noise whitening and chip-equalization for a DS-CDMA receiver. System issues affecting the choice of the chip spectrum, and thus the applicability of these methods, are described.

The coarse acquisition performance of a code-division multiple-access (CDMA) overlay system operating in a mobile communications environment is considered. Specifically, a CDMA system supporting communication between several mobile units and one base station shares the frequency band with an existing narrowband user. At the CDMA base station receivers, narrowband interference rejection filters are used to suppress the narrowband user's energy. It is demonstrated that in a nonfading environment the presence of the narrowband user does not severely affect the acquisition performance when the ratio of its bandwidth to the CDMA bandwidth is small. As the ratio becomes larger, the acquisition performance degrades, but the use of the interference rejection filter still significantly decreases the time to acquire. When flat Rician fading is introduced, the acquisition performance of the overlay system degrades, especially when the power in the direct component is small.

This paper extends and complements previous research we have performed on the performance of nonadaptive narrowband suppression filters when used in cellular code-division multiple-access (CDMA) overlay situations. An adaptive least mean square (LMS) filter is applied to a cellular CDMA overlay in order to reject narrowband interference. An accurate expression for the steady-state tap-weight covariance matrix is derived for the real LMS algorithm for arbitrary statistics of the overlaid interference. Numerical results illustrate that when the ratio of the narrowband interference bandwidth to the spread spectrum bandwidth is small, the LMS filter is very effective in rejecting the narrowband interference. Furthermore, it is seen that the performance of the LMS filter in a CDMA overlay environment is not significantly worse than the performance of an ideal Wiener filter, assuming the LMS filter has had sufficient time to converge.

Several methods of diversity combining for a Rayleigh-faded channel are evaluated and compared. The methods considered are, for coherent reception, maximal ratio combining (MRC), selection combining (SC), and a generalization of SC, whereby the two (three) signals with the two (three) largest amplitudes are coherently combined. We will call this method second (third) order SC, and denote it SC2 (SC3). Similar techniques are also investigated for noncoherent reception, with equal gain combining (EGC) replacing MRC, and noncoherent versions of SC2 and SC3. Numerical results indicate that SC2 and SC3 significantly enhances the bit-error rate (BER) performance relative to that achievable with SC, and under certain conditions approaches the performance achieved by MRC or EGG. The performance enhancement of SC2 and SC3 is especially noticable for noncoherent reception, where EGC is seen to provide the best performance only for low BER values. In fact, when the BER is 10/sup -3/ or greater, SC2 and SC3 performed comparably to EGG, and in some cases performed better than EGC.

In this paper, we apply a multicarrier signaling technique to a direct-sequence CDMA system, where a data sequence multiplied by a spreading sequence modulates multiple carriers, rather than a single carrier. The receiver provides a correlator for each carrier, and the outputs of the correlators are combined with a maximal-ratio combiner. This type of signaling has the desirable properties of exhibiting a narrowband interference suppression effect, along with robustness to fading, without requiring the use of either an explicit RAKE structure or an interference suppression filter. We use bandlimited spreading waveforms to prevent self-interference, and we evaluate system performance over a frequency selective Rayleigh channel in the presence of partial band interference; we also compare system performance with that of a single-carrier RAKE system.

In order to combat large scale shadowing and distance losses in a land mobile satellite system, an adaptive power control (APC) scheme is essential. Such a scheme, implemented on the uplink ensures that all users' signals arrive at the base station with equal average power as they move within the satellite spot beam-an important requirement in a CDMA system. Because of the lengthy round-trip delay on a satellite link, closed-loop power control systems are only of marginal benefit. Therefore, an open-loop APC scheme is proposed to counteract the effects of shadowing and distance loss. A fairly general channel model, consisting of log-normal shadowing and Rician fading, is assumed. This can be applied to a specific two-state land mobile satellite channel model, involving shadowed intervals with Rayleigh fading and unshadowed intervals with Rician fading. It is found that the power control error can be approximated by a log-normally distributed random variable. To quantify the performance of the APC, the standard deviation of the power control error in decibels is analyzed as a function of the specular power-to-scatter power ratio, the measurement time and the vehicle velocity. To illustrate the usefulness of the results, we analyze the effect of the power control error on the system capacity of a CDMA mobile satellite link.>

We are interested in determining the feasibility of having multiple service providers share the same spectrum in transponder-type. Satellite-based, code-division multiple-access mobile communications. To accomplish this, we compare the total capacity when n systems are operational to that when only a single system uses the frequency band. This comparison is made for both the uplink and the downlink under one of two possible constraints, either a maximum power flux density (PFD) limit per system, or an aggregate PFD limitation. While there are circumstances when the total capacity of multiple systems can achieve a greater value than that of a single service provider, conditions depend on the degree of shadowing, and whether or not the PFD limit is on a per system basis or on an aggregate basis from all systems.>

The coherent reception of direct sequence-code division multiple access (DS-CDMA) signals in a multipath fading channel is considered. The channel model assumes independent paths with Nakagami fading statistics; this model includes the Rayleigh channel os a special case and is a reasonable model for a frequency-selective fading channel. The bit error rate (BER) performance of a RAKE receiver under various multipath fading conditions is derived and evaluated. The results indicate that over 50 users may access the channel (assuming 127 chips per bit) simultaneously under typical fading conditions, and can achieve an uncoded BER of 0.01 with an average received signal-to-noise ratio per bit of 10 dB.>

We consider the problem of intercepting a direct sequence spread spectrum waveform which is embedded in both narrowband interference and additive white Gaussian noise. The receiver we analyze is a broadband radiometer which has been enhanced in the following manner. Prior to entering the square-law device in the radiometer, the received waveform is passed through a transform domain processor which serves as a notch filter to null out the narrowband interference before it can contribute unwanted energy to the output of the radiometer. A central-limit theorem is established for the receiver's output which forms the basis for analyzing the performance of the receiver. For various combinations of system parameters, numerical results are presented in the form of receiver operating characteristics.>

A model based on some results from queueing theory is developed which takes into account the possibility of overlapping bursts in a meteor-burst (MB) communication system. Application of this model to a simple broadcast MB system shows that for meteor arrival rates that correspond to times of slowest meteor activity the assumption of non-overlapping bursts is justified, while for arrival rates corresponding to times of highest meteor activity the problem of overlapping trails may cause significant performance degradation.>

Direct sequence code division multiple access communications is a promising approach to cellular mobile communications, which operates in an environment characterized by multipath Rician fading. In this paper, the CDMA network is assumed to share common spectrum with a narrowband microwave user. Because of the presence of the narrowband waveform, an interference suppression filter at each CDMA receiver is employed to reject the narrowband interference. The problem of interference from adjacent cells is also considered. Average power control is assumed to combat the near/far problem, and multipath diversity, in conjunction with simple interleaved channel coding, is considered for improving the performance of the CDMA system.>

A DS CDMA satellite uplink with power control error is considered. CDMA system performance is normally interference limited, and individual users typically experience the average performance over all users. This is particularly true in a Gaussian, additive white noise channel, with many users, sufficient processing gain, and effective power control. In this case, the other users appear as Gaussian noise and each user experiences the same environment. Under fading channel conditions however, the average behavior notion is flawed, since the fade conditions for each user are different, even if the fading statistics are identical. An alternative measure of performance under these conditions is the probability of outage. For coded signals, the fading must be averaged over all the symbols in the codeword. For properly interleaved signals and i.i.d. Rayleigh fade statistics, the average must be performed over a chi-squared variate. The probability of outage, defined as the probability that the decoded bit error rate exceeds a specified threshold, is monotonic with the probability that the chi-squared variate is less than a specified threshold. The authors derive upper and lower bounds on the outage probability for soft decision decoding.

In a conventional quasi-synchronous system, synchronized users, so-called in-sync users, experience less interference than do out of sync users. The authors propose a new technique to cancel the cochannel interference for an out-of-sync user in the forward link of a cellular CDMA system. They use multicarrier direct sequence waveforms, which can yield both robustness against multipath fading and narrowband interference suppression, in addition to cochannel interference suppression.>

A hybrid FDMA/CDMA scheme operating over a frequency selective Rayleigh fading channel is described and analyzed. The performance of the hybrid system is then compared with that of a wideband CDMA system occupying the same total bandwidth. Both coherent and noncoherent modulation formats are investigated; it is found that, for coherent modulation with a RAKE receiver, wideband CDMA has greater capacity than does the hybrid. However, for the noncoherent modulation formats (either DPSK or square-law detected orthogonal signalling), a hybrid system can have a greater capacity if a high channel error rate can be tolerated. Otherwise, a wideband noncoherent system remains optimal.>

Minimum probability of bit error is difficult to achieve in a DS-CDMA receiver. Since multiple-access noise is the sum of many independent random processes, it is reasonable to approximate it by a Gaussian process of the same power spectral density. This leads to the criterion of maximizing signal-to-noise ratio (SNR). In this paper, receivers that maximize SNR in a particular DS-CDMA system model under various constraints are proposed and analyzed. The method proposed here does not require locking and despreading multiple arriving CDMA signals. The maximization of SNR is compared with the minimization of probability of error, when the receiver is constrained to operate bit-by-bit, in the absence of knowledge of the other users' spreading codes, timing, and phase.>

The analysis of both the performance and capacity of direct sequence CDMA in terrestrial cellular systems has been addressed in the technical literature. It has been suggested that CDMA be used as a multiple access method for satellite systems as well, in particular for multispot beam low Earth orbit satellites (LEOS). One is tempted to argue that since CDMA works well on terrestrial links, it will nominally work as well on satellite links. However, because there are fundamental differences in the characteristics of the two channels, such as larger time delays from the mobile to the base station and smaller multipath delay spreads on the satellite channels, the performance of CDMA on satellite links cannot always be accurately predicted from its performance on terrestrial channels. In the paper, the authors analytically derive the performance of a CDMA system which operates over a low Earth orbiting satellite channel. They incorporate such effects as imperfect power control and dual-order diversity to obtain the average probability of error of a single user.>

A method of equalization for a coherent frequency hopped spread-spectrum receiver, operating in a heavily multipath-contaminated channel, is presented. The performance in the presence of thermal noise and either noise or multi-tone jamming is obtained and compared with the performance of a noncoherent frequency hopped spread-spectrum system operating in the same environment. It is shown that significant improvement in performance, in the presence of partial-band jamming, is achieved, primarily due to the equalization.>

This paper investigates the average bit error rate performance of a direct sequence code division multiple access system operating over a multipath Rician fading channel (which models indoor radio propagation in factories). We consider both phase-shift-keying modulation with coherent demodulation and differential phase-shift-keying modulation with noncoherent demodulation. Predetection multipath diversity (equal gain combining) is considered for improving the BER performance.>

Design and performance analyses of a fast frequency-hopped (FFH) spread spectrum communication system employing differential binary phase shift keying (DBPSK) modulation and differentially coherent demodulation are presented. The receiver utilizes a hop timing tracking loop to lock the hop clock. Through this tracking loop, the differentially coherent demodulation of an FFH signal is made possible. The authors present the system model of an FFH/DBPSK system. A digital, full-time early-late gate timing error estimator is investigated. The tracking loop is shown to be a Markov chain, and various performance characteristics of the tracking loop are studied.>

The purpose of the paper is to present the design and performance analysis of a fast frequency-hopped (FFH) spread spectrum communication system employing differential binary phase-shifted-keying (DBPSK) modulation and differentially coherent demodulation. The receiver utilizes a hop time tracking loop to "lock" the hop clock. The authors present an analysis of the average bit error probability of the overall system when it is operating in additive white Gaussian noise plus partial-band noise jamming. The effect of channel phase distortion is also studied.>

A systematic way to design nonuniformly spaced tapped-delay-line (TDL) equalizers is described, and the performance of such equalizers is compared to that of uniformly spaced TDL equalizers with the same number of tap coefficients. It is shown that the signal-to-mean-squared-error ratio at the output of a TDL equalizer can be improved by optimally choosing the positions of the tap weights. An algorithm to find both the tap positions and the corresponding tap weights for a given delay span and a given minimum tap spacing of the equalizer is presented. Typical results are illustrated by using, as an example, the magnetic recording channel. For two target waveforms at different densities of recording, it is shown that there is a potential for saving up to seven equalizer taps.>

The performance of finite intersymbol interference (ISI) channels in the presence of additive white Gaussian noise is evaluated by interpreting the ISI channel as a trellis code. Closed form results are obtained for ISI channels with one and two symbols of interference. An efficient algorithm is also introduced to evaluate the minimum free Euclidean distance of any ISI channel.>

A DS-CDMA receiver is typically implemented as a matched filter. Such a receiver, while optimum in additive white Gaussian noise, may yield poor performance when multiple-access noise is the dominant interference. Previously proposed multiple-user receivers greatly improve performance in this situation, but quickly become prohibitively complex for systems of realistic size. The authors propose a receiver which takes advantage of the colored power spectrum of the multiple-access noise to reject it. The receiver is simple to implement, can run at high rates, and can be tuned adaptively. It is designed to maximize signal-to-noise ratio, but is also shown to yield a substantial improvement over the conventional receiver in average probability of error.>

The authors consider the performance of a cellular radio, direct-sequence code-division multiple access, (CDMA) system. The base-to-mobile link is modeled as a flat Rayleigh fading channel, with all signals transmitted from a given base station fading in unison. For the mobile-to-base link, the authors use a similar model, except that the waveforms from all users are assumed to experience independent fading. The effects of imperfect power control are shown.>

Because of the continually increasing demand for mobile communications, it has been suggested that personal communication networks (PCNs) be established in the 1850-1990 MHz range. However, that band of frequencies is currently occupied by various microwave signals transmitted by users ranging from utility companies to state and local agencies. In order to allow both sets of users to occupy these frequencies as well as improve the spectral efficiency of this band, a spread-spectrum overlay is proposed, whereby a code-division multiple-access (CDMA) PCN would share the spectral band with the existing narrowband microwave traffic. The results of several field tests which have been designed to demonstrate the feasibility of an overlay of this type are discussed.>

The cutoff rate of a discrete memoryless channel whose output sequences are from a (d, k) encoder is investigated. A rational rate (d, k) encoder is considered as a finite state machine and maximum-likelihood decoding is used to compute the cutoff rate. Some commonly used (d, k) codes, such as the rate 1/2 (1, 3) code with a two-state encoder, the IBM rate 2/3 (1, 7) code having a five-state encoder, and the IBM rate 1/2 (2, 7) code with a seven-state encoder, are used to illustrate the cutoff rate computation. Results are presented for both the binary symmetric channel (BSC) and the Gaussian noise channel. The performance of a decoder designed for noiseless transmission of (1, 3) code is compared to that of a maximum-likelihood decoder for the (1, 3) code. It is also shown that for the case of the Gaussian noise channel, a gain of about 1.7 dB in signal-to-noise ratio is possible by using 3-bit soft decisions over hard decisions.>

Code Division Multiple Access (CDMA) is a digital multiple-access technique, whereby each signal has its own unique binary sequence, and all signals share the same spectrum. In the United States, personal communication networks using CDMA, will provide to each user, a separate code, which will change from cell-to-cell. We call this code reuse, just like the frequency reuse employed in the FDMA or TDMA systems. However, there are many more codes, than frequency bands.

A novel approach to slow frequency hopping (FH) spread-spectrum signal tracking is presented and analyzed. This approach is based on utilization of additional synchronization data carrying information for FH signal tracking purposes which is added to the digital data stream. Using a renewal process model, the steady-state probability mass function for the phase error process is derived, along with expressions for the tracking error variance, and the mean time to lose lock. Results are presented for several different tracking algorithms and a rough comparison with a standard early-late gate tracking loop is made.>

For pt.I see ibid., vol.39, no.2, p.304 (1991). A novel algorithm for discrete tracking of slow frequency hopping spread-spectrum signals is described and analyzed. In this system, each Mth hopping interval is completely used for transmission of synchronization data. The motivation for introducing this algorithm is that it allows a large degree of flexibility with respect to trading off system performance, complexity, and the redundancy introduced for tracking purposes. The analysis of the system performance is based on the utilization of results from discrete renewal process theory. However, because of the difficulty in obtaining the exact values for the transition probabilities, approximate results are presented.>

The performance of a compressive receiver is compared to that of a wideband radiometer in order to determine if there is any significant loss in detection performance that accompanies the added feature of frequency estimation in the compressive receiver. The waveforms to be detected are either sine waves or phase-shift keyed signals, where the latter waveforms can represent either narrow-band binary phase-shift keyed (PSK) signals or direct sequence spread spectrum waveforms. It is shown that the compressive receiver outperforms the radiometer in detecting the presence of a sine wave. The compressive receiver is also shown to exhibit superior performance when the input contains a binary PSK signal and the system is operating under the conditions of low input signal-to-noise power ratio and low probability of false alarm.>

A fast frequency-hopped (FFH) receiver which uses a prewhitening filter to reject narrowband interference is described. By using an appropriate fractional tap spacing, it is shown that interference can be estimated independently of the desired signal. Bit error rate results are presented for the receiver for linear square-law combining. The results compare favorably to those obtained by a near-optimal automatic gain-control (AGC) combining technique. The performance of the prewhitening filter interference rejection method is shown to be superior to that of a nonparametric self-normalizing receiver. Finally, simulation results for an FFH receiver using the complex least mean-square (LMS) algorithm to update the prewhitening filter coefficients are presented.>

The time-varying signal-to-noise ratio (SNR) model for the meteor burst (MB) channel is reviewed. Bounds on the capacity of the channel are derived for both a constant SNR model and a time-varying SNR model. These bounds show that there is a significant throughput improvement to be gained by using forward error correction. Two methods are given for determining the performance of an MB system when packets of information are encoded with an (n,k) linear block code. Numerical results are generated using high-rate BCH codes, and it is found that about 25% improvement over uncoded systems can be obtained by choosing the code rate properly. In addition, some suggestions for techniques that provide further improvement are given.>

A digital phase/timing tracking loop for a coherent frequency-hopped spread-spectrum receiver is analyzed. A training period is introduced to aid the system in entering the tracking mode; once in this latter mode, decision-directed measurements are used. The S-curve, the conditional variance of the phase estimator, and the linear model of the loop are derived. Then, under minor assumptions, the phase error is modeled as a homogeneous finite Markov chain. Based on this model, the length of the training period and the approximate probability of entering the tracking range are obtained; in the tracking mode, the steady-state average probability of error and the mean-time to loss-of-lock are obtained. By limiting the maximum output magnitude of the phase estimator, it is shown that the tracking performance is close to that of coherent binary phase-shift keying (BPSK) in the absence of jamming, or that assuming perfect synchronization, in the presence of partial-band jamming. The mean-time to loss-of-lock is shown to be several orders of magnitude larger than the expected length of training period. The effects of a cubic channel phase response and a constant RF phase error are presented.>

The optimization between the processing gain and the coding gain of a constant-bandwidth direct-sequence spread-spectrum communication system operating in the presence of either a tone jammer or a Gaussian jammer such that the bit-error rate (BER) is minimized is presented. Both bipolar phase-shift keyed pseudo-noise (PN) modulation and quaternary phase-shift keyed PN modulation are considered. Binary BCH codes are used to illustrate the techniques for optimization. The exact BER and its Gaussian approximation are derived and compared. During the optimization, two different approaches are used to obtain the indicator functions of the optimum code. These indicators are tested and shown to yield good results.>

A design and analysis of a coherent frequency-hopped (C-FH) spread-spectrum system operating in the presence of both jamming and a nonideal channel response is presented. A simple channel equalizer and a joint maximum-likelihood RF and C-FH phase track loop operate simultaneously. By assuming that the channel response is smooth over every slot, and that the effect of decision error is negligible, the convergence properties of the equalizer are analyzed in the presence of jamming. The numerical results are shown for the average error probability, the steady-state mean-square-error, and the convergence time when the system is operating in the presence of both frequency error and arbitrary channel phase response. The overall performance is very close to that of an ideally synchronized system, except when a coherent multitone jammer is present having the same phase of the received signal. In this latter case, the system experiences slightly more degradation.>

The authors present and analyze a model for both hybrid and frequency-hopped spread-spectrum systems in which a fully digital coherent receiver is used to demodulate the data. A receiver for a hybrid DS/FH (direct-sequence/frequency-hopped) system using a digital delay-lock loop is considered. In the absence of frequency uncertainty, it is shown that the tracking error can be modeled as an ergodic Markov chain with a finite-state set, and the probability density function of the steady-state tracking error is evaluated. When there is a frequency uncertainty, the dynamics of the resulting nonstationary phase error can be obtained, and an expression to evaluate the probability distribution of the first time at which the phase error hits predetermined boundary values is derived. Bit error rate performance is determined in the presence of both additive white Gaussian noise and various types of interference, and the performance is compared to that of noncoherent FSK systems.>

For pt.I see ibid., vol.37, no.6, p.601-11 (1989). A pure phase-coherent frequency-hop (FH) system is described, and a digital receiver based on maximum-likelihood estimation of the phase of the frequency-hopped carrier is suggested. The theory developed in pt.I is used in the analysis of the receiver whose performance in the presence of both additive white Gaussian noise and partial-band interference is evaluated. By comparison to the bit error rate performance of noncoherent FSK systems, it is found that for channels without excessive phase distortion, an improvement is achievable with the coherent system, but at a price of a more complex transmitter and receiver.>

An intercept receiver which uses a transform-domain-processing filter is described. This receiver detects direct-sequence binary-phase-shift-keyed (DS-BPSK) spread-spectrum signals in the presence of narrowband interference by employing adaptive narrowband interference rejection techniques. The improvement in the system performance over that of conventional detection techniques is shown by presenting the results of experimental measurements of probability of detection versus false alarm for an enhanced total power detector. Also presented are certain results corresponding to detection of the spectral lines generated at twice the carrier frequency, wherein the goal is often not just signal detection, but also carrier frequency estimation. The receiver uses one of two transform-domain-processing techniques for adaptive narrowband interference rejection. In the first technique, the narrowband interference is detected and excised in the transform domain by using an adaptive notch filter. In the second technique, the interference is suppressed using soft-limiting in the transform domain.>

The performance of a meteor-burst (MB) communication system using three different protocols is examined. The protocols were chosen to study the relative advantages of using a simple stop-and-wait automatic repeat request (ARQ) scheme as well as the ability to sense the presence of a channel. It was found that for long messages, the use of the ARQ scheme gave significant performance improvement; however, when the message length was on the order of the average burst length, the use of the ARQ scheme was not helpful. For packet sizes on the order of the average burst length, it was found that the ability to sense the presence of the channel gave about a 20% reduction in the time necessary to complete a message, but for much shorter packet sizes, the channel probing was not needed. The model used to analyze these systems takes into account the time-varying signal-to-noise ratio (SNR) present in a MB channel. The performance predicted by this model is compared to a less sophisticated constant SNR (signal-to-noise) model and its is found that the author's model predicted significantly more optimistic performance.>

It is argued that the ability of a spread-spectrum system to withstand interference, both intentional and unintentional, is probably its greatest asset. Any spread spectrum receiver can only suppress a given amount of interference; if the level of interference becomes too great, the system will not function properly. Even under these latter circumstances, however, other techniques, which enhance the performance of the system over and above the performance improvement that comes automatically to systems from using spread spectrum, are available for use. These techniques typically involve some type of additional signal processing and are examined here. Two general types of narrowband interference suppression schemes are discussed and an overview is presented for several other techniques. The two classes of rejection schemes emphasized are (1) those based on least-mean-square estimation techniques, and (2) those based on transform-domain processing structures.>

The acquisition and tracking systems of a spread-spectrum receiver are probably the most critical components of the receiver, since if they fail to function properly, it is doubtful that the desired signal can be successfully detected. This means that the affect of interference (such as jamming) on the receiver while it is attempting to learn the correct phase position of the incoming code might be especially harmful, since the interference might not allow the receiver to acquire the signal. To address this problem, a narrow-band interference suppression filter is used to enhance the performance of a serial search acquisition scheme for a direct-sequence spread-spectrum receiver. Analytical expressions for the probabilities of error in both the search and lock modes are derived, and numerical results are used to illustrate the sensitivity of the receiver to various system parameters. It is shown that the presence of the rejection filter can be significantly improves the performance of the acquisition system.>

The performance of direct-sequence spread-spectrum systems using a suppression filter in the presence of multiple narrow band interference with rational spectral densities is analyzed. Both BPSK and QPSK systems are considered, and analytical expressions for both the means-square error of the filter output and the performance improvement are established. When the bandwidths of the narrow band components of the interference are all small, approximate expressions are obtained and used to provide further insight into the behavior of the system. Results are presented for the limiting case when the bandwidths approach aero (i.e. when multiple narrow band interference becomes multiple sinusoidal interference).>

In this paper, we briefly describe the physical characteristics of a meteor-burst (MB) communication channel, and then present the results of an analysis showing the expected throughput and the probability of completing a given message under two different scenarios. In the first scenario, the transmitter is presumed to have knowledge (obtained by appropriate probe signals) of the entire duration of a given MB. In the second, only the starting time of each MB is known. Numerical results based on the analysis are provided, and it is shown what types of delays must be tolerated in order to achieve a given degree of reliability of having the transmitted message correctly received at its destination.

In this paper, the worst-case power spectral density of a narrow-band Gaussian jammer operating in a direct sequence (DS) spread-spectrum system employing an interference-rejection filter is derived under a joint power and bandwidth constraint on the jammer. The effectiveness of the suppression filter under these conditions is examined and average probability of error results for the system's performance are presented.

In this paper, the results of comparing the performance of various forward error correction techniques and several modulation formats when used over a nonselective Rayleigh fading channel in the presence of a pulse-burst jammer are presented. Both binary and nonbinary codes are considered, as well as concatenated codes consisting of either a block or a convolutional inner code and a Reed-Solomon outer code. Finally, the use of side information to allow the decoding of both erasures and errors is also analyzed.

This paper describes the structure of a digital implementation of the Widrow-Hoff LMS algorithm which uses a burst processing technique to obtain some hardware simplification. This adaptive system is used to suppress narrow-band interference in a direct-sequence spread-spectrum communication system. Several different narrow-band interferers are considered, and probability of error results are presented for all cases. While, in general, the results show significant improvement in performance when the LMS algorithm is used, certain disadvantages are also present and are discussed in this paper.

An erasure-control decoding algorithm is discussed for frequency-hopped spread-spectrum systems operating in the presence of pulsed or partial-time interference, and some properties of the resulting error probability which are useful in designing effective decoding algorithms are derived. Based on numerical results, specific decoding algorithms which correct both erasures and errors are then presented for use with block codes employing hard decisions.

The coded error probability for direct sequence (DS) BPSK or QPSK spread-spectrum systems, used with or without interleaving, and operating in the presence of pulsed multiple-tone interference, is investigated. We consider the worst-case channel error probability under conditions of pulsed multiple-tone jamming for the various systems, and present coded error probability as a function of interleaving delay for(n, k)block codes with hard-decision decoding. It is shown that when the maximum level of continuous multiple-tone interference does not exceed the level of the desired signal, the duty cycle of the corresponding pulsed interference which yields the worst-case error probability is usually small, and that the smaller the duty factor of the jamming, the more considerable is the performance improvement due to the use of interleaving in conjunction with the coding.

The Widrow LMS algorithm is considered for the implementation of an adaptive prewhitening filter in a direct-sequence (DS) spread-spectrum receiver. Exact expressions for the steady-state tapweight covariance matrix and resulting average excess mean square error are developed for the real LMS algorithm when the input contains a random binary sequence (used to model a pseudonoise spreading sequence). It is shown here that the output samples of the adaptive filter possess approximately Gaussian statistics under the conditions of slow convergence and a large number of filter taps. Using this approximation, expressions for the resulting bit error rate (BER) when the adaptive algorithm is used to suppress a fading gone jammer are developed, and numerical results obtained from these expressions are compared to simulation results for the DS receiver.

In this paper, a technique is described which uses multiple surface acoustic wave (SAW) devices in parallel to reduce the acquisition time of a direct sequence spread-spectrum communication system. Analysis of system performance in both the search and lock modes is presented, and key quantities such as probability of false alarm, probability of correct detection, mean dwell time, and mean time to lose lock are derived.

This paper describes a rapid acquisition technique which can be used in direct sequence spread-spectrum systems. It is sequential in nature and is based on a double threshold which defines when a decision can be made. The values of the thresholds change at every examination instant. The procedure makes use of a bound to the partial crosscorrelation function of PN sequences which ensures that only the sequence length needs to be given as far as the PN sequence particulars are concerned. A decision is made only if the probability of error associated with a decision at that point is equal to or better than a prespecified value. Finally, it is shown that most of the computational difficulties associated with the bound as given by a recursive algorithm can be eliminated when using a low-order polynomial approximation to the bound. However, either by using the bound or its polynomial approximation, a significant improvement in the acquisition time is obtained.

Upper and lower bounds on the average probability of error of direct sequence (DS) spread-spectrum communication systems operating in the presence of either multiple tone or multiple access interference are presented. The bounds are quite tight and can be used without the restriction that the peak level of the interference be less than the desired signal level. A simple method for evaluating approximations to the bounds for multiple access systems, in which the peak interference does indeed exceed the desired signal level, is presented as well. The tightness of both the bounds and the approximations are illustrated with numerical results.

Linear least squares estimation (LLSE) techniques can provide an effective means of suppressing narrow-band interference in direct sequence (DS) spread-spectrum systems. In the results presented here, analytical expressions for bit error rate are derived for two DS spread-spectrum systems under the conditions of either tone or narrowband Gaussian interference. It is shown that the most common LLSE filter design can lead to performance inferior to that of various other filter designs. However, results are also presented demonstrating that an LLSE filter design motivated by the structure of the maximum-likelihood receiver leads to consistently superior performance. The performance of a system using this new design criterion is compared with that of an approximation to the maximum-likelihood (ML) receiver for the tone interference model and with that of the exact ML receiver for the Gaussian interference. Finally, it is shown that the bit error rate estimate obtained from application of a Gaussian approximation for the test statistic is overly pessimistic for the systems studied here.

Analytical results have shown that adaptive filtering can be a powerful tool for the rejection of narrow-band interference in a spreadspectrum receiver. However, the complexity of adaptive filtering hardware has hindered the experimental verification of these results. This paper describes a new adaptive filter architecture for implementing the Widrow-Hoff LMS algorithm while using only two multipliers regardless of filter order. This hardware simplification is achieved through the use of a burst processing technique. A 16-tap version of this adaptive filter constructed using charge-transfer devices (CTD's) is used to suppress a single tone jammer in a direct sequence spread-spectrum receiver. Probability of error measurements demonstrating the effectiveness of the adaptive filter for suppressing the single tone jammer along with simulation results for the optimal Weiner-Hopf filter are presented and discussed.

In this paper, the performance of direct sequence QPSK spread-spectrum systems using complex adaptive filters in the presence of pulsed CW interference is analyzed. Both adaptive prediction error filters and adaptive transversal filters with two-sided taps are considered. It is shown that the time constant of the tap weight adaptation in the interference off-interval is usually much greater than the time constant in the on-interval, and that this is beneficial for the system since it results in retaining the rejection property of the filter. Under steady-state conditions, the tap weights are calculated. Analytical expressions for the signal-to-noise ratio improvement under the least favorable interference condition are given. It is shown that the performance of the two-sided transversal filter is better than that of the prediction error filter.

In this paper, the performance of QPSK systems using complex transversal filters with additional decision-feedback taps, in the presence of Gaussian noise and a single CW interferer, is analyzed. Both one-sided (with lagging taps) and two-sided transversal filters with additional decision-feedback taps are considered. Analytic expressions for the tap weights and the minimum mean square errors are obtained. The effect of error propagation on the error probability is discussed and an approximate solution for the error probability is obtained. The transient behavior of the filters using the LMS adaptation algorithm is analyzed. It is shown that if the filter is used for rejecting CW interference only, the one-sided decision-feedback filter is preferred.

Employing the technique of transform domain filtering described in Part I of this paper, a system is presented to detect a binary spread-spectrum signal embedded in a wide-band, stationary, colored Gaussian noise process. Using a specific example of a colored noise process, the performance of the system is analyzed. In addition, a specific technique using surface acoustic wave devices is presented which allows the estimation of the spectral density of the interference when such information is initially unknown.

Certain moments of the output of a bandpass nonlinear system whose input is a stationary Gaussian random process will be used to evaluate the performance (i.e., the average probability of error) of the system. The main application of this procedure is the analysis of a frequency translating saturating satellite link. The basic analytical technique to be employed will be the use of moment space bounds, it will be shown that by using at most a two-dimensional Gaussian quadrature formula to perform a numerical integration, the bounding procedure presented in this paper can be used in a relatively straightforward manner with virtually any bounded bandpass nonlinearity. Numerical results Corresponding to a hard limiter with only an AM/AM characteristic will be presented, as well as results corresponding to a soft limiter with both AM/AM and AM/PM characteristics. In the latter model, the effects of intersymbol interference will also be included. However, computationally, the procedure is only feasible for a small number of interfering terms.

In the presence of narrow-band interference, the performance of PN spread-spectrum communication systems can he improved by using various interference rejection filters. In this paper, both prediction error filters and transversal filters with two-sided taps are used and the performance of each is analyzed. Analytic expressions for the signal-to-noise ratio improvement factors are obtained. If the frequency of the jamming tone is near the carrier frequency of the spread-spectrum signal, the performance of the transversal filter with two-sided taps is better than that of the prediction error filter with the same number of taps.

The Chernoff bound is used to upper bound the average probability of error of a direct sequence spread spectrum communication system under the conditions of multiple-tone interference. A special case is analyzed using a Gaussian approximation to the interference, and this latter result is compared to that predicted by the Chernoff bound. The limitations of the Gaussian approximation are then carefully explained.

The performance of four digital modulation techniques are compared to one another when each is received in the presence of adjacent channel interference. The interfering waveforms that each modulation format sees are identical to the modulation format under consideration. The four types of modulation are QPSK, 8-PSK, 16QASK, and QPR. Upper bounds to the probability of error are derived and used to evaluate the relative merits of the different schemes.

The Chernoff bound is used to estimate the performance of a direct sequence spread spectrum communication system operating over a frequency-selective Rician fading channel. In addition to the degradation caused by the selectivity of the channel, the system is further degraded by the presence of a tone interfering signal. A closed form expression for the bound is presented, and it is seen that at low probabilities of error, the bound appears to be within 1 dB or so of the true value. Numerical results are also presented for typical combinations of system parameters.

The performance of a binary noncoherent frequency-hopped/frequency-shift-keyed system operating in the presence of partial-band tone interference and transmitted over a frequency-selective Rician fading channel is derived. The probability of error for the system is derived both with and without the use of error-correction coding, and the two cases are compared on the basis of equal information rates and equal spread bandwidths.

Spread-spectrum communications, with its inherent interference attenuation capability, has over the years become an increasingly popular technique for use in many different systems. Applications range from antijam systems, to code division multiple access systems, to systems designed to combat multipath. It is the intention of this paper to provide a tutorial treatment of the theory of spread-spectrum communications, including a discussion on the applications referred to above, on the properties of common spreading sequences, and on techniques that can be used for acquisition and tracking.

The probability of error of both a 16-ary QASK system and an MSK system, each operating over a Rician fading channel, is considered. The channel model has two key features: short term variations are modeled by making the channel frequency selective, and long term variations are accounted for by allowing the specular component of the received signal to fade according to a Rayleigh density. The improvement gained by using maximal-ratio diversity combining is derived, as well as the degrading effects of an adjacent channel. Finally, a comparison between MSK and 16-ary QASK operating over the same channel is presented, and it is shown that MSK results in a smaller probability of error.

The maximum-likelihood estimates of the angular position and angular extent of a target relative to the boresight of a monopulse radar system is derived, and the statistics of the estimates are determined. It is shown that the moments of the estimates do not exist when the thermal noise power in the receiver is nonzero. However, conditional moments are determined which do exist, as well as conditional density and/or distribution functions.

This paper provides a detailed analysis of the performance of systems recently proposed by various authors in which the received signal is Fourier transformed in real time (usually with a surface acoustic wave device) and then filtered by a multiplication of the transformed signal by an appropriate transfer function. We shall refer to this operation as transform domain filtering. General expressions for the output waveforms of the system will be derived, and a specific example illustrating the performance of the system when operating in the presence of a narrow-band interferer plus additive white Gaussian noise will be presented. It will be shown that certain filtering techniques not feasible in conventional systems, such as filtering with ideal bandpass filters, become straightforward using transform domain filtering. Finally, a receiver will be described which will eliminate that intersymbol interference between adjacent data symbols due to filtering at the receiver.

It is desired to determine the performance of a noncoherent FSK frequency-hopped spread spectrum communication system using MARK and SPACE tones which are not necessarily orthogonal, and in addition using an error-correction code to reduce the susceptibility of the system to jammer-induced errors. The overall bandwidth of the system (i.e., the spread bandwidth) will be held constant so that the processing gain of the system will always be adjusted to maintain this constant bandwidth as other factors (e.g., the bandwidth expansion due to the error correction coding) change the underlying baseband bandwidth.

In most realistic communication systems, be they spread spectrum or conventional systems, some overall bandwidth constraint is usually placed upon the system. In the case of spread spectrum communications, this constraint limits the amount of processing gain that the system can employ. In addition to processing gain, factors such as alphabet size affect the total bandwidth. It is the purpose of this paper to combine these two effects to determine how one should trade off the amount of processing gain one uses relative to the underlying modulation bandwidth when one is operating under the constraint of fixed total system bandwidth.

Summary form only given, as follows. On page 1398 of the above paper (ibid., vol. COM-28,Part II,pp. 1389-1398, Aig. 1980), the Acknowledgment was published incorrectly. The correct Acknowledgment is as follows: "ACKNOWLEDGMENT The authors would like to thank Prof. C. Ziegler for generating the numerical results, and J. Bartow and S. Hall for their helpful suggestions."

In this paper, various potential implementations of spread spectrum systems using surface acoustic wave (SAW) devices are discussed and experimental results for a phase-shift keyed (PSK) signal embedded in unwanted interference is presented. An overview of acoustic surface wave technology as it applies to signal processing in spread spectrum communication systems is presented. The ability to implement most of the basic elements of such systems using SAW devices is discussed, with emphasis on the space charge coupled convolver and Fourier transformer. SAW implementation of a fairly general block diagram applicable to a variety of signal and interference situations is described, and the relative advantages of SAW technology with respect to other technologies such as charge coupled devices is discussed.

Certain communication systems which employ spread spectrum techniques as a means of supporting multiple users need very long codes to guard against the threat of intentional jamming, but cannot tolerate the lengthy acquisition time which long codes usually require. As a possible solution to this problem, the use of combination sequences has been suggested, and this paper presents some new results, both analytical and numerical, on this technique.

Combination sequences have long been used successfully for ranging applications, but very little attempt has been made to determine their performance in a multiple access environment. Well known bounds are used to evaluate the probability that either the total cross correlation or the partial cross correlation exceeds a given value; and it is demonstrated that when a relatively small number of users are in need of long but rapidly acquirable codes, combination sequences are an attractive choice.

Extreme-value theory has been suggested by various investigators as an efficient tool for use in monitoring the performance of digital communication systems. One drawback to such a procedure is that it requires learning periods to estimate certain parameters of the extremal distributions. Clearly, whenever an update of the estimates is needed, transmission of information must temporarily cease since a new learning period must begin. To avoid this problem, a technique is proposed for obtaining estimates of the parameters when the learning period is eliminated (i.e., estimates are obtained using noisy samples), and these parameter estimates are in turn used to estimate the probability of error of the communication system. The accuracy of this procedure is tested by computer simulation, and it is indicated how one can verify (in a statistical sense) whether or not this technique is appropriate for any given system.

The performance of a noncoherent frequency-shift keying (FSK) system employing postdetection filtering is determined. The FSK pulses are allowed to be amplitude modulated (AM-FSK) and the postdetection filter is allowed to be arbitrary. The analysis for the general case will be shown to simplify the previously known results when the AM is removed and the postdetection filter is an integrate-and-dump. The numerical results will take into account the effects of intersymbol interference. Both computer simulation results and experimental results will be presented and will be shown to agree quite closely with the analysis.

Various forms and properties of the maximum-likelihood (ML) bit detector structure for binary signals with intersymbol interference in the presence of additive stationary Gaussian noise are presented and two receiver implementation forms are considered. A simple analytical approximation which also has a heuristic interpretation as a lower bound on the bit probability of error for this detector is presented. The performance of this detector for typical small and moderate degrees of intersymbol interference is presented and compared to other detectors. Both cases indicate that the analytical approximation is quite tight and thus useful in predicting the performance of this highly nonlinear device.

This paper examines the performance of a digital communication system operating in the presence of additive Gaussian noise when a bandpass hard limiter is inserted into the system. In contrast with earlier published work for large time-bandwidth product systems, small signal-time-duration-noise-bandwidth products at large SNR's are considered. It is found that the degradation introduced by the limiter is more severe under these conditions.

This paper examines the performance of a two-link digital communication system operating in the presence of Gaussian noise on both links when bandpass limiters are inserted into the system. The product of the signal time duration and the noise bandwidth for the systems under consideration is small. It is concluded that under certain conditions, the presence of a limiter can improve the performance of the system. Also, a simplification of previous work is presented when an infinite clipper is introduced at baseband in the detector.

The use of extreme-value theory (EVT) in the detection of a binary signal in additive, but statistically unknown, noise is considered. It is shown that the optimum threshold and the probability of error of the system can be accurately estimated by using EVT to obtain properties of the initial probability density functions on their "tails." Both constant signals and slowly fading signals are considered. In the case of a fading signal, the detector becomes adaptive. Detection of the constant signal, both with and without an initial learning period, is studied by computer simulation.