Recently, we proposed a novel transmission scheme termed decomposed-quadrature optical single carrier frequency domain equalization (DQO-SCFDE) suitable for intensity-modulated direct-detection (IM/DD) diffuse optical wireless (DOW) communication. DQO-SCFDE has been shown to perform better than the conventional asymmetrically clipped optical OFDM (ACO-OFDM) when the non-linearity characteristics of the light emitting diodes (LED) are considered. In this paper we present a detail comparison of DQO-SCFDE and ACO-OFDM. We then propose a space-time coding scheme for DQO-SCFDE using two LED's at the transmitter to improve the bit-error rate performance.

Communication with low complexity decoding is an important attribute for many applications. From low cost, distributed sensors arrays with desirably long operative life to ultra smart handheld battery operated devices, low complexity decoding results in better power consumption and simpler hardware implementation. In this paper a linear decoding complexity is applied and analyzed to the semi-orthogonal space time codes which are rate 1 codes originated from a regular orthogonal space time codes. The proposed transmission and decoding schemes are very appealing in the sense that they improve the code rate of the orthogonal codes family, yet, maintain its simple, symbol by symbol decoding with no filter calculation complexity overhead. In addition, a performance boosting modifications are suggested and investigated for the basic transmission and decoding schemes under different system settings such as the availability of limited feedback and multiple receiving antennas.

The establishment of a common frequency reference in a distributed wireless network is a critical factor in enabling any degree of node cooperation in communication and sensing functions. In this paper we introduce Distributed Frequency-Locked Loops (D-FLL's) to control the carrier frequencies of autonomous nodes with wireless communication capabilities. A novel frequency difference detector (FDD) is introduced allowing each node to estimate the frequency difference with respect to the ensemble of its neighbors. D-FLL's are characterized in terms of stability and tracking accuracy when the synchronization network has a peer-to-peer (mutually coupled - MC) or hierarchical (master slave - MS) topology. In particular, synchronization error accumulates in MS architectures as the distance from the reference node increases. MC architectures, instead, trade slower convergence times for a smoother error distribution. Overall, D-FLL's prove to be a robust and accurate technique for carrier frequency synchronization purposes that can be readily employed with any network architecture.

A novel distributed scheme that combines cooperative ARQ with the spectrum leasing paradigm is proposed and analyzed. The strategy harnesses the opportunistic gains of cooperative communications, while inherently providing a spectrum-rewarding incentive for the otherwise non-cooperative relays to assist the source's transmission. As in cooperative ARQ, the source might decide to hand over the possible retransmission slots to nearby stations that were able to decode the original transmission. In the proposed scheme, however, in exchange for the cooperation, the relaying station is also awarded an opportunity to exploit the retransmission slot for its own traffic. Arbitration of relays' retransmissions is performed via an auction mechanism, with the source, the competing relays and the transmission slot acting as the auctioneer, the bidders and the bidding article, respectively. Auction theory (more generally, the theory of Bayesian games) is applied to analyze the scheme performance. It is noted that the setting here can be alternatively seen as a practical framework for implementation of property-rights cognitive radio networks. Numerical results and analysis show that the proposed scheme enables an efficient dynamic resource allocation that provides relevant gains (e.g., transmission reliability) for both the original source (primary) and the cooperating nodes (secondary users).

We propose and analyze a novel decentralized mechanism that motivates otherwise non-cooperative stations to participate as relays in cooperative ARQ protocol. Cooperation incentive is provided by the possibility for the source to lease a portion of retransmission slot for the traffic of relaying terminals. To further leverage the opportunistic nature of cooperative ARQ and obtain a fully decentralized solution, the (motivated) relaying nodes compete for access to the retransmission slot by trying to make the best retransmission offer. Effective arbitration of cooperative retransmissions is performed using auction theory (bidding), with the source in the role of the auctioneer, the relaying nodes acting as the bidders and the (use of the) retransmission slot as the bidding article. It is noted that the proposed solution can be seen as a practical framework for the implementation of cognitive radio networks running according to the property-rights model (spectrum leasing). Numerical results and analysis confirm the efficient dynamic resource allocation property of the proposed scheme, revealing the relevant gains in terms of expected number of (re)transmissions required for successful data delivery for both the source (primary) and the cooperating (secondary) nodes.

In this work, a multicell cooperative zero-forcing beamforming (ZFBF) scheme combined with a simple user selection procedure is considered for the Wyner cellular downlink channel. The approach is to transmit to the user with the ldquobestrdquo local channel in each cell. The performance of this suboptimal scheme is investigated in terms of the conventional sum-rate scaling law and the sum-rate offset for an increasing number of users per cell. We term this characterization of the sum-rate for large number of users ashigh-loadregimecharacterization, and point out the similarity of this approach to the standard affine approximation used in the high-signal-to-noise ratio (SNR) regime. It is shown that, under an overall power constraint, the suboptimal cooperative multicell ZFBF scheme achieves the same sum-rate growth rate and slightly degraded offset law, when compared to an optimal scheme deploying joint multicell dirty-paper coding (DPC), asymptotically with the number of users per cell. Moreover, the overall power constraint is shown to ensure in probability, equal per-cell power constraints when the number of users per-cell increases.

In this paper, we consider the energy efficiency of truncated hybrid-ARQ (HARQ) protocols in a single-user link (i.e., non-collaborative HARQ), or with the inclusion of a relay station (i.e., collaborative HARQ). The total energy consumption accounts for both the transmission energy and the energy consumed by the transmitting and receiving electronic circuitry of all involved terminals (source, destination and, possibly, the relay). Using the transmission time and transmission energy of each packet as optimization variables, the overall energy is minimized under an outage probability constraint for HARQ Type I, HARQ chase combining and HARQ incremental redundancy protocols (in the latter case, a tight lower bound is considered). Numerical optimization provides insight into the optimal design choices that enhance energy efficiency with HARQ protocols. It is shown, for instance, that, if the circuitry energy consumption is not negligible, selection of the transmission energy is not only dictated by the outage constraint, but is also significantly affected by the need to reduce the number of retransmissions. Our results also demonstrate the performance limitations of collaborative HARQ protocols in terms of energy efficiency, when circuitry consumption is properly accounted for.

Cyclostationarity is a significant feature of numerous physical and man-made processes. Specially in wireless communications, it has been explored and applied for many processing, like timing and frequency synchronization, channel estimation, and modulation classification. In this paper, we notice that the second order time varying auto-correlation of the received communication signal contains a cyclostationary real and a non-cyclostationary complex part. We propose a simple method to combine the positive and negative part of the cyclo-spectrum (the Fourier transform of the time varying autocorrelation) to enhance the performance of detecting the second order cyclostationarity of these signals. The idea redis related to the concept of cyclic Wiener filter put forth by Gardner in 1993 [4]. Monte Carlo simulations are given to verify the results.

According to the commons model of cognitive radio, the activity of secondary (unlicensed) nodes is required to guarantee quality-of-service (QoS) constraints on the transmission of primary (licensed) terminals. Towards this goal, vertical handover between different radio interfaces is currently being investigated as a promising solution to enhance flexibility in unlicensed channel access. In this paper, we propose an analysis of cognitive radio with vertical handover capability in a simple scenario with one secondary node and two primary nodes that employ different radio interfaces with packet-based transmission. The maximum stable throughput of the secondary node is evaluated under maximum-delay QoS constraints on the primary activity as a function of system geometry, QoS constraints and sensing errors. Numerical results show the relevant advantages of optimal vertical handover in terms of the throughput of secondary nodes.

The concept of cognitive radio (or secondary spectrum access) is currently under investigation as a promising paradigm to achieve efficient use of the frequency resource. In this paper, we consider a decentralized cognitive radio model based on spectrum leasing, whereby a primary (licensed) user leases its bandwidth for a fraction of time to a network of independent secondary (unlicensed) terminals in exchange for cooperation. On one hand, the primary user decides whether to exploit (space-time coded) cooperation from the network of secondary terminals in order to maximize its own transmission rate. On the other hand, secondary terminals accept to cooperate with the primary only if compensated with a large enough fraction of time for their own transmission, towards the goal of maximizing their rate discounted by the overall cost of transmitted power. The considered model is studied in the framework of Stackelberg games, with the primary and the set of secondary users modelled as the (Stackelberg) game leader and the follower, respectively. Numerical results show that spectrum leasing based on trading secondary spectrum access for cooperation is a promising framework for cognitive radio.

In infrastructure-less wireless systems network-wise time and frequency synchronization can be achieved by exchanging mutual synchronization errors among neighboring nodes. Cooperative synchronization is based on the use of distributed digital locked loops (D-DLLs), as the extension to distributed systems of the classical concept of (analog or digital) locked loops. The convergence to a synchronized state depends ultimately on the degree of connectivity of the network. D-DLLs can be specialized for time or frequency synchronization by adopting an appropriate error detector, but preserving the same control loop. The focus of this paper is on distributed frequency synchronization for packet-based communication. A novel detector is proposed, which approximates the local mean frequency error from the uncoordinated transmission of packets by neighboring nodes. The performance of distributed frequency locked loops (D-FLLs) is evaluated for a wireless network employing packet-based cooperative relaying. Numerical validations are used to compare different frequency synchronization protocols in terms of speed of convergence and degradation of end-to-end performances.

The concept of cognitive radio (or secondary spectrum access) is currently under investigation as a promising paradigm to achieve efficient use of the frequency resource by allowing the coexistence of licensed (primary) and unlicensed (secondary) users in the same bandwidth. According to the property-rights model of cognitive radio, the primary terminals own a given bandwidth and may decide to lease it for a fraction of time to secondary nodes in exchange for appropriate remuneration. In this paper, we propose and analyze an implementation of this framework, whereby a primary link has the possibility to lease the owned spectrum to an ad hoc network of secondary nodes in exchange for cooperation in the form of distributed space-time coding. On one hand, the primary link attempts to maximize its quality of service in terms of either rate or probability of outage, accounting for the possible contribution from cooperation. On the other hand, nodes in the secondary ad hoc network compete among themselves for transmission within the leased time-slot following a distributed power control mechanism. The investigated model is conveniently cast in the framework of Stackelberg games. We consider both a baseline scenario with full channel state information and information-theoretic transmission strategies, and a more practical model with long-term channel state information and randomized distributed space-time coding. Analysis and numerical results show that spectrum leasing based on trading secondary spectrum access for cooperation is a promising framework for cognitive radio.

Achieving the sum-capacity of a multiple-input-multiple- output (MIMO) Gaussian broadcast channel is known to require full channel state information (CSI) at the base station, which implies the need of a large amount of feedback information from the users. Different asymptotics of the sum-capacity, such as its scaling law with respect to the number of users n or the multiplexing gain, are conventionally used to assess the performance of suboptimal schemes with reduced feedback, or equivalently with partial channel state information at the transmitter. In this correspondence, the optimal scaling law of the sum-rate with respect to n, for fixed signal-to-noise ratio (SNR), fixed number of transmit antennas M and any number of receiving antennasN(i.e.,Mlog log nN), is proved to be achievable with a deterministic feedback of only one bit per user. Moreover, the amount of feedback is shown to be further reduced with no asymptotic optimality loss by applying the selective feedback principle, leading to an average feedback rate that scales as log n. Finally, the asymptotic performance with respect to SNR is studied, by assessing how fast the number of users needs to increase with the SNR in order to guarantee a noninterference limited behavior.

In this correspondence, joint (cooperative) decoding at the base stations combined with collaborative transmission (relaying) is investigated as a means to improve the uplink throughput of current cellular systems over fading channels. Intracell orthogonal medium access control (e.g., TDMA, FDMA, or orthogonal CDMA) and Decode-and-Forward relaying by either a mobile terminal or a fixed relay are assumed. Moreover, the cellular system is modeled according to a simplified framework introduced by Wyner. The per-cell achievable ergodic throughput is calculated for different system configurations and then characterized in the low-power (wideband) regime in terms of minimum energy per bit required for reliable communication and slope of the spectral efficiency. The analysis allows to clearly assess the relative merits of both cooperation among base stations and at the terminal level within the considered model.

In this letter, packet-by-packet vertical handover is investigated as a means to improve the performance of unlicensed spectrum access. The analysis focuses on the optimization of a channel access strategy for a cognitive multi-standard radio node that has the capability to switch between two orthogonal uplink radio interfaces, based on long-term channel state information. The radio interfaces differ for coverage and quality of service (QoS) requirements of primary users. The proposed strategy prescribes a cross-layer selection of physical-layer (transmitting powers) and medium access control layer (handover probability) parameters and is designed to maximize secondary throughput while guaranteeing the primary QoS constraints. Analysis and numerical results bring insight into the impact of network topology, measurement errors and primary QoS constraints on the optimal system design and corresponding performance.

In this paper, we investigate one of the MIMO classification problem: distinguishing Space Time Coding (STC) from BLAST schemes. With STC the transmitted signals are correlated in time, while with BLAST scheme the transmitted signals do not have such correlation, that is, the signal emitted from different antenna are uncorrelated. We exploit the cyclostationarity test to distinguish between these two schemes. Different linear combining process of the received signal have been proposed to check the cyclostationarity. Finally, we verify our method using Monte Carlo simulations.

We proposed a hybrid automatic repeat request (HARQ) packet transmission scheme for a multiple input multiple output (MIMO) system in a frequency selective fading channel. The scheme combines Alamouti space-time block coding with MIMO-HARQ and single-carrier frequency domain equalization (SC-FDE). It is shown that the technique works in a frequency flat fading channel as well.

The performance of wireless multicasting is negatively affected by multipath fading. To improve reliability, cooperation among the nodes of the network can be used to create spatial diversity. This paper analyzes a two phase, space-time coded, cooperative multicast protocol and investigates two transmission and combination strategies: Maximal Ratio Combining and Incremental Redundancy. Moreover, it addresses two different channel state information (CSI) scenarios: i) no transmit CSI within the network and ii) broadcast transmit CSI at the source.

Cooperation is increasingly regarded as a key technology for tackling the challenges of a practical implementation of cognitive radio. In this paper, we first give a brief overview of the envisioned applications of cooperative technology to cognitive radio, distinguishing among cooperative sensing for detection of the primary activity, cooperative transmission between secondary nodes and cooperative transmission of primary traffic by secondary users (cognitive relaying). Then, we focus on the latter scenario and investigate a simple wireless network, where one secondary transmitter has the option to relay traffic of the primary. Assuming that the primary is oblivious to the presence of the secondary (thus excluding the possibility of spectrum leasing), the secondary transmitter optimizes transmission/ relaying parameters towards the goal of maximizing the rate towards the secondary receiver. Numerical results are provided in order to discuss the advantage and limits of cognitive relaying.

As the distance between terminals in modern wireless networks tends to decrease, the energy consumption issue, conventionally assumed to be exclusively dominated by the transmission power, needs to be revaluated. In particular, retransmission (ARQ) protocols that typically reduce the transmission energy required to obtain a given error probability on the channel (at the expense of a larger delay), also increase the energy consumed by the circuitry other than the power amplifier. In this paper, the energy efficiency of hybrid-ARQ type I, chase combining and incremental redundancy protocols in Rayleigh fading channels, is analyzed by accounting for the energy consumed by the transmitting and receiving electronic circuitry. It is shown that the advantages of hybrid-ARQ protocols in terms of energy consumption strictly depend on the transmission range.

In this paper, we consider downlink throughput performances of multiuser orthogonal frequency division multiplexing (multiuser OFDM) with reduced channel information feedback schemes. Specifically, two types of reduced feedback schemes, namely, 1-bit per sub-carrier and selective feedback scheme are considered and compared with each other in terms of average network throughput. For the latter, since the exact analysis is complicated, we resort to an approximate analysis. Simulations results will also be provided to verify the approximate analysis. Since the strict throughput comparison for given number of feedback bits per user is quite difficult, rather we compare their general behaviors in various system configurations with different system parameters, which can give us an insight into practical system design with those reduced feedback schemes.

A novel "permuted sequences combination" method is proposed in this paper, which uses the conventional frame structure wherein two sync words, each comprising M complex symbols, are appended at the frame start. The goal of this new method is to reduce the large variance of the timing estimation error inherent in the conventional correlation method. Essentially, the first sync word, stilde1, is a predetermined constant amplitude zero autocorrelation (CAZAC) sequence. The second sync word, stilde2, which is a certain permutation of the first so that the combination of the two received sync signal vectors perform sliding window processing with peak occurs at the correct frame start. We show that the symmetric correlation method previously presented in (Park et al., 2003) for orthogonal frequency division multiplexing (OFDM) systems can be regarded a special case of the proposed permuted sequences combination method. As a special case of permuted sequences combination method, symmetric correlation method is robust to the frequency offset. The proposed methods have good simulation results in both AWGN channel and multipath environments. Simulation results support the analysis.

The automatic recognition of the modulation format of a detected signal, the intermediate step between signal detection and demodulation, is a major task of an intelligent receiver, with various civilian and military applications. Obviously, with no knowledge of the transmitted data and many unknown parameters at the receiver, such as the signal power, carrier frequency and phase offsets, timing information and so on, blind identification of the modulation is a difficult task. This becomes even more challenging in real-world scenarios with multipath fading, frequency-selective and time-varying channels. With this in mind, the authors provide a comprehensive survey of different modulation recognition techniques in a systematic way. A unified notation is used to bring in together, under the same umbrella, the vast amount of results and classifiers, developed for different modulations. The two general classes of automatic modulation identification algorithms are discussed in detail, which rely on the likelihood function and features of the received signal, respectively. The contributions of numerous articles are summarised in compact forms. This helps the reader to see the main characteristics of each technique. However, in many cases, the results reported in the literature have been obtained under different conditions. So, we have also simulated some major techniques under the same conditions, which allows a fair comparison among different methodologies. Furthermore, new problems that have appeared as a result of emerging wireless technologies are outlined. Finally, open problems and possible directions for future research are briefly discussed.

A scenario with two single-user links, one licensed to use the spectral resource (primary) and one unlicensed (secondary or cognitive), is considered. According to the cognitive radio principle, the activity of the secondary link is required not to interfere with the performance of the primary. Therefore, in this paper, it is assumed that the cognitive link accesses the channel only when sensed idle. Moreover, the analysis includes: (1) random packet arrivals; (2) sensing errors due to fading at the secondary link; (3) power allocation at the secondary transmitter based on long-term measurements. In this framework, the maximum stable throughput of the cognitive link (in packets/slot) is derived for a fixed throughput selected by the primary link. The model is modified so as to allow the secondary transmitter to act as a ldquotransparentrdquo relay for the primary link. In particular, packets that are not received correctly by the intended destination might be decoded successfully by the secondary transmitter. The latter can, then, queue and forward these packets to the intended receiver. A stable throughput of the secondary link with relaying is derived under the same conditions as before. Results show that benefits of relaying strongly depend on the topology (i.e., average channel powers) of the network.

Cooperation between base stations and collaborative transmission between mobile terminals are two technologies currently under study as promising paradigms for next generation communications systems. In this paper, we provide a first look to the interplay between these two approaches by studying the per-cell achievable sum-rate (throughput) of different cooperative protocols under a simplified model for the uplink of a TDMA cellular system. The analysis is limited to non-regenerative (amplify-and-forward) cooperation schemes between terminals for their'simplicity and appeal to a practical implementation. A closed form expression for the (asymptotic) achievable rate of multicell processing combined with amplify-and-forward collaboration at the terminals is derived for an AWGN (i.e., no fading) scenario. Moreover, the impact of fading is investigated numerically, allowing to draw some conclusions on the impact of multicell diversity (or macrodiversity) on the performance of collaborative schemes among the terminals. In particular, we show that while AF cooperation is generally advantageous for single cell processing (i.e., with no collaboration between base stations), its benefits when combined with multicell processing are limited to the regime of low to moderate transmission rates.

Likelihood ratio test (LRT)-based linear modulation classifier is sensitive to unknown parameters, such as carrier frequency offset (CFO), phase shift, etc. An antenna array-based quasi-hybrid likelihood ratio test (qHLRT) approach is proposed to cope with the problem. A non-maximum likelihood (ML) estimator is employed to reduce the computational burden of multivariate maximization. A two-stage CFO estimation scheme is also proposed to increase the accuracy of CFO estimation. To combat channel fading, maximal ratio combining (MRC) technique is applied for CFO estimation as well as the computation of the likelihood functions. The Cramer-Rao lower bound (CRLB) of the proposed CFO estimation method is derived. It is shown that with nonlinear least-squares (NLS) algorithm and method- of-moment (MoM) algorithm to estimate phase and amplitude respectively, our scheme offers an effective and practical solution to recognize linear modulation formats in fading channels.

Both OFDM and single-carrier block transmission (SCBT) are promising candidates for the future high date rate communication, which well handle the frequency selective channels with a low implementation complexity. However, the sensitivity to carrier frequency offset, caused by Doppler effect or the carrier frequency difference between transmitter and receiver, is always a major concern of these block transmission systems. Actually, a controversy in comparing the carrier frequency offset effect on OFDM and SCBT systems has already been witnessed in the literature for a long time. In this paper, we first analyze the effects of carrier frequency offset on these two systems, then by carefully investigating existing works, we classify the problem into two categories: carrier frequency offset analysis with compensation and without compensation. Finally, we draw conclusions based on theoretical and numerical results that the BER performance of OFDM systems is more sensitive to carrier frequency offset than SCBT systems for the BPSK modulation in AWGN channels, although the difference of the BER performance between two systems is much smaller than theoretically predicted in previous works, if a fair comparison environment is used. Moreover, this small performance gap vanishes in higher level constellation like QPSK.

For a multiple-input single-output (MISO) downlink channel with M transmit antennas, it has been recently proved that zero-forcing beamforming (ZFBF) to a subset of (at most) M "semi-orthogonal" users is optimal in terms of the sum-rate, asymptotically with the number of users. However, determining the subset of users for transmission is a complex optimization problem. Adopting the ZFBF scheme in a cooperative multi-cell scenario renders the selection process even more difficult since more users are involved. In this paper, we consider a multi-cell cooperative ZFBF scheme combined with a simple sub-optimal users selection procedure for the Wyner downlink channel setup. According to this sub-optimal procedure, the user with the "best" local channel is selected for transmission in each cell. It is shown that under an overall power constraint, a distributed multi-cell ZFBF to this sub-optimal subset of users achieves the same sum-rate growth rate as an optimal scheme deploying joint multi-cell dirty-paper coding (DPC) techniques, asymptotically with the number of users per cell. Moreover, the overall power constraint is shown to ensure in probability, equal per-cell power constraints when the number of users per-cell increases.

OFDM system suffers from significant performance degradation due to phase noise. Many phase noise analysis methods have been proposed in the literature showing its effect as a loss in the ratio of the signal power to the statistical average of the intercarrier interference (ICI). It would be more desirable, however, to predict phase noise effect on bit error rate (BER) directly. In this paper, by investigating the phase noise properties and the impacts caused by phase noise on an OFDM system, we derive approximate closed-form expressions for BER performance in AWGN channels with BPSK and QPSK modulations. In general, it is impossible to get such expression for multipath Rayleigh fading channels. Hence we establish upper and lower bounds for the degradation caused by phase noise, which give insightful view of the system performance. Simulation results show that the proposed simple analytical forms are quite accurate for different phase noise levels, which lead to the conclusion that a phase noise mitigation mechanism is needed to obtain acceptable performance in OFDM systems.

The sum-capacity of a multi-antenna broadcast Gaussian channel is known to be achieved by dirty paper coding techniques, or, asymptotically in the number of users n, by beamforming methods, that require full channel state information at the base station. Based on the opportunistic beamforming principle, it has been recently shown that the optimal scaling law of the sum-rate with respect to n, for fixed signal to noise ratio and number of transmitting antennas M, (i.e., M log log n) can be achieved by employing a feedback of only one real and one integer number per user. Moreover, it was proved that a linear scaling with respect of M can be guaranteed only if M scales no faster than log n. In this paper, the optimal scaling law of the sum-rate with respect to n is proved to be achievable with only one bit of feedback per user. The proof builds on opportunistic beamforming and binary quantization of the signal to noise plus interference ratio. Moreover, the linear scaling of the sum-rate with M is demonstrated to hold for M growing no faster than log n even with such a reduced feedback. Finally, the results above are extended to the MIMO case, where each user is equipped with multiple antennas

The sum-capacity of the multi-antenna Gaussian broadcast channel is known to be achieved by Dirty Paper Coding techniques, that require full channel state information at the base station. It has been recently shown that a sum-rate having the same scaling law of the sum-capacity with respect to the number of users n for a fixed signal to noise ratio (i.e., M log log n where M is the number of transmitting antennas) can be achieved by using reduced feedback (or equivalently reduced channel state information at the transmitter). In particular, it has been proved that n real and n integer numbers are enough to guarantee the optimal scaling law. In this paper, the optimal scaling law of the sum-rate is shown to be achievable with an even smaller amount of feedback and, more precisely, with 1) n log2(M + 1) bits, if a deterministic feedback scheme is employed; 2) an average number of feedback bits that scales as M log2M log n with the number of users n, if a selective (random) feedback scheme is employed.

The cross-layer design is a promising strategy to maximize the spectral efficiency in communication systems. In this paper we focus on packet-oriented systems and we propose to combine adaptive modulation and coding (AMC) at physical layer and hybrid automatic repeat request (ARQ) at the data link layer. In the proposed multilevel hybrid ARQ (M-HARQ) strategy, the transmitted symbols are decomposed into a set of parallel sub-channels with an independent ARQ protocol over each sub-stream. It follows a layered structure where each sub-channel can be decoded only once the lower sub-channels have been resolved, thus subchannels are sequentially detected. The number of parallel streams is adaptively arranged according to the channel state information (CSI). Considerable throughput gain is achieved using the same encoder/decoder structure for all the sub-channels. Moreover, M-HARQ permits an adaptive and flexible implementation even when CSI is not available. The drawback is an increase in the decoding delay, which might be tolerable in some applications

A likelihood ratio test (LRT)-based modulation classifier is sensitive to unknown parameters, such as carrier frequency offset (CFO), symbol rate, etc. To deal with the limited knowledge of CFO, in this paper, a quasi-hybrid likelihood ratio test (qHLRT)-based approach is proposed for linear modulation classification. In the qHLRT algorithm, a non-maximum likelihood (ML) estimator is used to reduce the computational burden of multivariate maximization. Several of blind, non-ML CFO estimators are studied and their performance are compared with both single and multiple receiving antennas systems. It is shown that the nonlinear least-squares (NLS) CFO estimator is the best choice for the qHLRT algorithm, particularly with antenna arrays, which are introduced to combat the effect of channel fading on modulation classification

OFDM suffers from severe performance degradation in the presence of phase noise. In particular, phase noise leads to common phase error (CPE) as well as intercarrier interference (ICI) in the frequency domain. Some approaches in the literature mitigate phase noise by directly evaluating and then compensating for CPE or ICI, while others choose to correct phase noise in the time domain. A new parametric model of OFDM signals is proposed in this paper which shows that, in the presence of phase noise, each received frequency-domain subcarrier signal can be expressed as a sum of all subcarrier signals weighted by a vector parameter. Then, two reduced-complexity techniques are presented to estimate this weighting vector. The first is a maximum likelihood (ML) method whereas the second one is a linear minimum mean square error (LMMSE) technique. Using the obtained estimates, we also propose two approaches, i.e., a decorrelator and an interference canceler, to mitigate phase noise. It is shown that most conventional methods can be readily obtained from our approaches with some approximation or orthogonal transform. Theoretical analysis and numerical results are provided to elaborate the proposed schemes. We show that the performance of both approaches is superior to that of conventional methods. Furthermore, LMMSE gives the best performance, while ML provides a much simpler yet effective way to mitigate phase noise

The performance of MC-CDMA is limited by multiple access interference (MAI). To mitigate this problem, equal BER power control (PC) was proposed for MMSE-SIC receiver, which provides a powerful solution for MAI suppression (M. Tan and Y. Bar-Ness, IEEE Commun. Lett., vol. 8, pp. 348-350, 2004), wherein the power distribution on different users was derived under the assumption of perfect channel state information (CSI) at the receiver. In practice, CSI is obtained from channel estimation, in which errors are inevitable. Therefore, the analysis of the robustness of the MMSE-SIC with the equal BER PC to channel estimation errors (CEE) is of great importance. In this paper, a method of second-order approximation is applied to estimate the mean excess MSE (MEMSE) of different users in a given decision order. The accuracy of the approximation is confirmed by simulation results. Furthermore, it is interesting to find out that the MMSE-SIC receiver with the equal BER PC presents significant robustness to CEE.

The optimal diversity-combining technique is investigated for a multipath Rayleigh fading channel with imperfect channel state information at the receiver. Applying minimum mean-square error channel estimation, the channel state can be decomposed into the channel estimator spanned by channel observation, and the estimation error orthogonal to channel observation. The optimal combining weight is obtained from the first principle of maximum a posteriori detection, taking into consideration the imperfect channel estimation. The bit-error performance using the optimal diversity combining is derived and compared with that of the suboptimal application of maximal ratio combining. Numerical results are presented for specific channel models and estimation methods to illustrate the combined effect of channel estimation and detection on bit-error rate performance.

Direct sequence (DS) and multicarrier (MC) are code-division multiple-access (CDMA) schemes based on single-carrier and MC modulations, respectively. While in DS CDMA, spreading is performed by increasing the transmission rate, or in the time domain, in MC CDMA, spreading is performed using several subcarriers, or in the frequency domain. In this paper, it is shown that MC and DS CDMA are time-frequency dual systems. Their analysis in time- and frequency-dispersive multiple-input multiple-output channels revealed that they can be described equally in terms of dual-channel functions, therefore exhibiting the same performance on dual channels. Furthermore, the duality property is used to derive matched filters for highly time-variant and wide-band channels. Finally, other issues involved in the comparison of DS and MC, namely spreading-sequence design and multiuser detection, are addressed.

The outage capacity of a Rayleigh flat fading channel completely unknown at the transmitter and partially known at the receiver is considered in this paper. Analytical expressions for the upper and lower bounds of the outage capacity of such channels are obtained as a function of the channel estimation error at the receiver. Those results reveal how the noise level and the channel uncertainty combine in reducing the outage capacity of the fading channel. In addition, by deriving an expression for the estimation error variance of pilot symbol assisted modulation (PSAM), we are able to establish direct relations among estimation parameters, channel characteristics (in terms of SNR and normalized Doppler spread) and its impact on the outage capacity.

Multicode multi-carrier CDMA (MC-CDMA) is a promising candidate for supporting multimedia services in future wireless communications. However, the performance of this system is limited by self-interference (SI) of user's multicodes and multi-user interference (MUI). In this paper, to mitigate this problem, a minimum mean square error successive interference cancellation (MMSE-SIC) receiver is applied in the multicode MC-CDMA. Furthermore, to increase the efficiency of SIC, a simple power distribution control (PDC) algorithm, based on equal BER criterion, is investigated under a short-term constraint of average transmit power. In correlated Rayleigh fading channels, the MMSE-SIC receiver integrated with the equal BER PDC can suppress interference effectively, resulting in a performance close to the single user bound (SUB).

An adaptive solution to the joint frequency and time offset for OFDM (orthogonal frequency division multiplexing) systems is presented in this paper. Two schemes are proposed herein, whose important feature is that the correction is performed blindly (without any information about the value of its parameters) and without the need for any pilot symbols, therefore without any bandwidth efficiency loss. Within the acquisition range, the output of both proposed schemes is made completely independent of the frequency and time offset values.

In this paper we develop an algorithm based on higher-order cyclic cumulants for the automatic recognition of QAM signals. The method is robust to the presence of carrier phase and frequency offsets. Theoretical arguments are verified with simulations performed for 4-QAM and 16-QAM signals.

We consider transmission power adaptations in multicarrier code-division multiple-access (MC-CDMA) communications, where the transmission power is adapted in frequency and/or time domain in response to channel variations. With frequency domain power adaptation, we propose to allocate the transmission power over N'(1/spl les/N'/spl les/N) strongest subcarriers (having highest channel gains) rather than over all possible N subcarriers. We examine the effect of choosing N' on the performance of the frequency domain power adaptation scheme, and find that there exists an optimal N' that minimizes the average BER. In the time domain, we consider adapting the transmission power so that desired signal strength at the receiver output maintains at a fixed level. We analyze the BER performances of frequency domain, time domain, and simultaneous frequency-time domain power adaptations with an average transmission power constraint. Our results show that the frequency domain and the time domain power adaptation schemes outperform the nonadaptive (i.e. constant uniform power over all subcarriers) scheme especially for noise-limited and interference-limited regions, respectively. The combined frequency-time domain power adaptation is shown to have a significant performance gain over the power adaptation in only frequency or time domain as well as over the nonadaptive scheme.

Multicode multicarrier CDMA (MC-CDMA) is a promising candidate for supporting multimedia services in future wireless communications. However, the performance of this system is limited by self-interference (SI) and multiuser interference (MUI). To mitigate this problem, a nonlinear zero-forcing successive interference cancellation (ZF-S1C) receiver is applied in this paper. Furthermore, to minimize each user's bit error rate (BER), optimal power distribution control (PDC) is considered. Results show that in correlated Rayleigh fading channels, the multicode MC-CDMA system based on the ZF-SIC receiver with the optimal PDC remarkably out performs other receivers proposed in the literature. Moreover, the ZF-SIC integrated with the optimal PDC present significant performance advantage over that with the equal BER PDC, particularly under a short-term total transmit power constraint.

We provide an exact analysis of orthogonal frequency-division multiplexing (OFDM) performance in the presence of phase noise. Unlike most methods which assume small phase noise, we examine the general case for any phase noise levels. After deriving a closed-form expression for the signal-to-noise-plus-interference ratio (SINR), we exhibit the effects of phase noise by precisely expressing the OFDM system performance as a function of its critical parameters. This helps in understanding the meaning of small phase noise and how it reflects on the proper parameters selection of a specific OFDM system. In order to combat phase noise, we also provide in this paper a general phase-noise suppression scheme, which, by analytical and numerical results, proves to be quite effective in practice.

In orthogonal frequency division multiplexing (OFDM) systems over fast-varying fading channels, channel estimation and tracking is generally carried out by transmitting known pilot symbols in given positions of the frequency-time grid. The traditional approach consists of two steps. First, the least-squares (LS) estimate is obtained over the pilot subcarriers. Then, this preliminary estimate is interpolated/smoothed over the entire frequency-time grid. In this paper, we propose to add an intermediate step, whose purpose is to increase the accuracy of the estimate over the pilot subcarriers. The presented techniques are based on the observation that the wireless radio channel can be parametrized as a combination of paths, each characterized by a delay and a complex amplitude. The amplitudes show fast temporal variations due to the mobility of terminals while the delays (and their associated delay-subspace) are almost constant over a large number of OFDM symbols. We propose to track the delay-subspace by a subspace tracking algorithm and the amplitudes by the least mean square algorithm (or modifications of the latter). The approach can be extended to multiple input multiple output OFDM or multicarrier code-division multiple-access systems. Analytical results and simulations prove the relevant benefits of the novel structure.

A transceiver structure for frequency-flat multiple-input multiple-output (MIMO) systems that comprises linear/nonlinear preequalization/equalization is optimized according to the minimum mean square error (MMSE) criterion under the assumption that only long-term channel state information (i.e., correlation matrices of fading channel and noise) is available at the transmitter. The structure generalizes different techniques known from the literature, such as BLAST, linear preequalization and equalization, and Tomlinson-Harashima precoding (THP). Simulations show that relevant benefits can be obtained by exploiting the long term channel state information at the transmitter in both dense multipath channels with relatively large correlation at the transmitter side and in sparse multipath channels.

In this two part paper, we consider a packet data communication over code-division multiple-access (CDMA) system. As a preliminary, this part of the paper highlights the practically achievable spectral efficiency of a CDMA system with turbo codes and multiuser detection, assuming the use of hybrid type-I automatic-repeat-request. We also investigate the effect of code rates on the performance of systems with matched filter and linear minimum mean square error (MMSE) receivers, respectively. For a given ensemble distance spectra of punctured turbo codes, we use an improved union bound to evaluate the error probability of maximum likelihood turbo decoder with matched filter receiver and with linear MMSE receiver front-end, and then compute the corresponding throughput and spectral efficiency as a function of system load. The results show that with 1/3 rate turbo code and matched filter receiver a spectral efficiency of around 1.1 b/s/Hz can be obtained, while, by employing a linear MMSE receiver, the spectral efficiency can be improved further up to 1.6 b/s/Hz. The results also show that the spectral efficiency with a linear MMSE receiver saturates at a code rate of about 4/5, i.e., does not improve further with lower rate coding, while, for matched filter receivers, better performance can be obtained with lower code rate. Theoretically, as the system load goes to infinity, the spectral efficiency of linear MMSE receiver converges to that of matched filter receiver. In a practical situation, however, there exists a noticeable performance gap between the two receivers unless a very low rate code is used.

In this part of the paper, we investigate the throughput bound of CDMA unslotted ALOHA system with a hybrid type II automatic-repeat-request (ARQ) implemented using rate compatible punctured turbo codes. The system under consideration can be regarded as an enhanced version of the classical CDMA packet radio network. When applying a hybrid type-II ARQ, the probability of packet success and packet length is generally different from attempt to attempt. Since the conventional analytical model, customarily employed for the ALOHA system with pure or hybrid type-I ARQ, cannot be applied for this case, we introduce an expanded analytical model, which can be regarded as a network of queues, and apply Jackson and Burke's theorems to simplify the analysis. When using code combining, soft-decision decoding is usually considered, in which case, it is very difficult to find the actual probability of packet success, especially when the number of terminals changes during a packet time duration. Hence, instead of finding the actual probability of packet success, we evaluate a lower and an upper bound. Together with the expanded model and the packet success probability bound, we obtain the corresponding throughput bounds by iteratively searching the steady-state arrival rate, /spl lambda/, and the average departure rate, /spl mu/, for given composite offered traffic, G=/spl lambda///spl mu/. The results show that significant improvement in throughput can be achieved by exploiting high coding gain of turbo codes and channel utilization efficiency of incremental redundancy retransmission protocol.

A signal scrambling method based on combined symbol rotation and inversion (CSRI) is proposed in this paper for the peak-to-average power ratio (PAPR) reduction of OFDM signals. By dividing OFDM sequences into subblocks and performing symbol rotation and inversion in each subblock, high degrees of freedom are available to offset the possibility of encounting poor sequences with large PAPR. In order to reduce the complexity of this scheme, two suboptimal CSRI schemes; one based on a successive approach combined with symbol grouping, termed successive suboptimal CSRI (SS-CSRI) and another based on a random approach combined with threshold control, termed random sub-optimal CSRI (RS-CSRI) are also investigated. With these two schemes, the complexity can be reduced significantly with only slight performance degradation. Compared with the partial transmit sequence (PTS) scheme, simulation results show that performing symbol rotation is more effective for PAPR reduction than increasing the number of phase weighting factors with PTS.

It has been proposed in the literature that phase noise causes severe performance degradation of OFDM systems and therefore need to be corrected . A combined OFDM/SDMA approach has been proposed in the literature allowing simultaneous transmitted user terminals which give rise to multiple phase noise. Conventional correction methods proposed in the literature are targeted for single phase noise and do not apply for multiple phase noise. We therefore propose a new multiple phase noise correction (MPNC) scheme which takes advantage of spatial diversity and mitigates the effects of multiple phase noise on the OFDM/SDMA receiver performance. The MPNC scheme presents quite good performance with few pilots involved per user. Numerical results are given to illustrate the effectiveness of the proposed scheme for an uplink OFDM/SDMA system.

We derive a throughput bound for CDMA unslotted ALOHA with memory ARQ in general. Since the conventional 'single queue with feedback' model, customarily employed for an ALOHA system with pure or hybrid type-I ARQ, cannot be applied to this case, we introduce an expanded model, which consists of a network of queues, and apply the Jackson theorem to simplify the analysis. When using code combining, it is very difficult to find the actual probability of packet success. Hence, instead of finding actual values, we evaluate a lower bound on it. Together with the expanded model and the packet success probability bound, we obtain the throughput bound for given composite offered traffic by using an iterative search procedure.

In this paper, we consider OFDM channel estimation in the presence of frequency offset and phase noise. In the literatures, most channel estimation methods assume perfect frequency synchronization and the knowledge of channel statistics. Phase noise and residual frequency offset cause inter-carrier interference (ICI), which consequently impairs the accuracy of channel estimation. The lack of knowledge of channel statistics can make channel estimation much harder. To resolve these problems, we propose with the aid of cyclic prefix (CP) based frequency offset estimation statistics-independent channel estimation. We iteratively search for the most likely channel impulse response (CIR) length, and use it not only for the optimum compensation of frequency offset, but also for finding the optimum window to filter the least square (LS) channel estimate which further suppress the effects of ICI and noise. The proposed scheme is compared with conventional methods for both non-interpolation and interpolation cases. Numerical results are presented to illustrate the effectiveness of the proposed scheme.

In this paper, the circular ring model is proposed to model the signal statistics of a wideband multiple-input multiple-output (MIMO) channel. Based on the proposed model, a new space-time frequency cross-correlation function is derived for the MIMO channel, in a compact mathematical form. The cross-correlation function includes various parameters of interest such as the distance between the base station and the user, spacing among antenna elements, user's speed and direction, as well as the angle spread and delay spread. In addition, the time of arrival probability density function and power delay profile of the circular ring model are derived and compared with measured data. The proposed model and the associated wideband space-time cross-correlation provide a convenient unified framework for the simulation of frequency selective mobile MIMO fading channels and the design of proper space-time coding and signal processing techniques over such channels.

We consider power adaptation strategies for binary phase-shift keying signals in Rayleigh fading channels under the assumption that channel state information is provided at both the transmitter and the receiver. We first derive a closed-form expression for the optimal power adaptation that minimizes average bit-error rate (BER) subject to average and peak transmission power constraints. Then, we analyze the average BER for channel inversion power adaptation with the same constraints. Our results show that the performance difference between the optimal power adaptation and the channel inversion becomes negligibly small as available average transmission power increases and/or peak-to-average power ratio decreases. We also find that an optimal peak-to-average power ratio exists that minimizes the average BER in the channel inversion scheme.

An adaptive method is proposed for multiple-input-multiple-output (MIMO) decision feedback equalization (DFE) in the absence of channel knowledge at the receiver and without requiring a training sequence. The zero-forcing (ZF) DFE is shown to consist of an orthogonal matrix (feedforward section) and an upper triangular matrix (feedback section). The feedforward matrix is expressed as the multiplication of Givens rotations terms, where each term is characterized by a rotation angle. An adaptive algorithm is presented for the blind determination of the unknown angles and of the elements of the feedback matrix. The adaptation criteria are decorrelation conditions. Simulation results are included demonstrating the performance of the method.

We propose an algorithm to allocate power among subcarriers in an MC-CDMA system. The optimal power allocation coefficients turn out to be the maximum eigenvalue eigenvector of the modified projection matrix orthogonal to the interference signal subspace. We also present a simple on-off control algorithm to decrease feedback information.

We analyze the system capacity of turbo coded CDMA packet system with hybrid FEC/ARQ scheme. In particular, assuming the use of punctured turbo codes, we investigate the system capacity in terms of the number of users that can be supported by the given rate of punctured turbo codes in both the AWGN and multipath fading channel. For a multipath fading channel, we derive a simple closed form expression for the PDF of signal to noise plus interference ratio (SNIR) at the output of matched filter receiver. Then, using an improved union bound, we evaluate the codeword error probabilities and the corresponding throughput performance for various rates of punctured turbo codes. With these results, we obtain the CDMA system capacity and compare its performance with theoretical capacity bounds obtained with idealized packet communication systems in which packet overheads, transmission and decoding delay can be ignored.

The performance of a linear decorrelating detector (LDD) and a minimum mean square error (MMSE) detector is analyzed for random spreading waveforms. The performance of the LDD and MMSE detectors is expressed in terms of the so-called near-far resistance, defined by a reciprocal of a diagonal component of inverse matrix. For random code division multiple access, which employs random spreading waveforms, the near-far resistance can be regarded as a random variable. Many papers have dealt with the analysis of multiuser detectors for random spreading sequences. In most cases, however, these analyses derived only the expectations or bounds for the near-far resistance. In this paper, we directly derive the approximate probability density function (PDF) of the near-far resistance and corresponding bit error rate expression for random spreading sequences. It is based on Gaussian approximation of the cross correlation between any two randomly generated spreading codes. The resulting PDF turned out to be a reversed-and-scaled version of chi-square distribution. The approximate expressions, both the PDF and the corresponding bit error rate expression, were verified via Monte Carlo simulations. The results showed that the approximation is quite close to the simulation results when the number of users is less than half the processing gain.

The bit error rate of a coded Multiuser OFDM (M-OFDM) system and coded multicarrier CDMA (MC-CDMA) system with multiuser detection is analyzed. The paper focuses on downlink wireless applications, where the channel suffers from frequency-selective fading. We investigate both systems when long time interleaving is possible, though introducing large delays, and when time interleaving is limited. Bit error rate lower bounds and simulation results show that, when the system load is high, M-OFDM performs similar to MC-CDMA while its implementation is less complex as it does not require multiuser detection.

In multi-carrier CDMA (MC-CDMA) system, timing of the starting point of the desired signal must be estimated to demodulate the desired signal. Especially for asynchronous MC-CDMA systems, rather a complicated timing estimation is needed, due to the random delays of all users. Zong, Wang and Bar-Ness (see VTC2001)proposed a partial sampling MMSE (PS-MMSE) receiver which does not require timing estimation. However, it requires several times more filter's taps than the conventional MC-CDMA receiver. The number of taps is a multiple of, and depends on, the number of subcarriers (i.e., the processing gain). A reduced complexity partial sampling MMSE (RPS-MMSE) receiver is proposed, which can be adaptively implemented by using the least-mean-square (LMS) algorithm. The proposed structure allows one to choose the number of taps independent of the number of subcarriers, hence having more flexibility in choosing the number of taps than with PS-MMSE receiver. With this structure, we can use fewer taps than with the PS-MMSE receiver and still can obtain comparable performance.

Channel estimation, particularly delay estimation in an asynchronous CDMA channel, is essential for most receivers. The correlated time domain data sequence becomes asymptotically uncorrelated in the frequency domain. By taking the discrete Fourier transform (DFT) of the received signals, we can take advantage of this asymptotical property to simplify the estimation algorithm. The proposed scheme can jointly estimate the path delay and the complex channel response with lower computational complexity. Furthermore, having such estimates, it is possible to use an adaptive subspace MOE (minimum output energy) detector, previously implemented in synchronous channels, for asynchronous multiuser CDMA. By simulation, it was shown that this scheme can provide comparable performance to that obtained with a Wiener filter.

Channel estimation, particularly delay estimation in asynchronous CDMA channel, is essential for most receivers. The correlated time domain data sequence will become asymptotically uncorrelated in the frequency domain. By taking the discrete Fourier transform (DFT) of the received signals, we can take advantage of this asymptotical property to simplify the estimation algorithm. The proposed scheme can jointly estimate the path delay and the complex channel response with lower computational complexity. Even with such a simplified channel estimation, performance results are shown to be better.

A new polynomial model for nonlinear power amplifiers (PA) was proposed by Lai and Bar-Ness (see IEEE Vehicular Technol. Conf., Amsterdam, the Netherlands, Sept. 1999) in their earlier work which takes into account both the input modulation format and the operating point of a PA. Such a model has been applied to analog predistorter (PD) design using a zero forcing scheme. In this paper, this model is applied to PD designs using the approach of approximating the ideal predistorter of the PA of interest. Based on the metric of the total degradation of a digital radio link with 64-QAM, the proposed PD has comparable performance of another PD obtained by the ideal approximation approach of D'Andrea, Lottici and Reggiannini (1996), but has less complexity in its implementation than the other one. In addition, based on the same metric, significant improvement is shown of the new PD over several PD designs using the zero forcing scheme, including the one proposed by Lai et al. However, the MDP-PM of the ideal PD has the same level of complexity as the PDs compared in the same order.

A novel coding scheme termed redundant bit position rotation sub-block-coding (RBPR SBC) is proposed to reduce the peak-to-average power ratio (PAPR) of OFDM signals. Simulation results show that by using this new scheme, a better performance compared with the standard sub-block-coding (SSBC) can be achieved. According to the probability distribution characteristics of the peak envelope power (PEP) of OFDM signals, a threshold control scheme is proposed to reduce the complexity of this novel coding scheme.

A novel transmit diversity scheme is proposed. Instead of using orthogonal codes in orthogonal transmit diversity technique, Gold codes are assigned to different transmit antennas. Jointly with a minimum variance decorrelating receiver, the proposed scheme can be applied to the asynchronous frequency selective fading channel which is one of the major obstacles for applications of other transmit diversity schemes.

Linear minimum mean square error (LMMSE) receivers for asynchronous multicarrier code division multiple access (MC-CDMA) system under frequency-selective Rayleigh fading channel is studied. The performance of this LMMSE receiver is evaluated and shown to be superior to that of the other two schemes, equal-gain combining (EQC) and maximum-ratio combining (MRC). However, a perfect timing estimation of the desired user is needed for these receivers, as a misaligned sampling interval of these receivers results in severely self intersymbol interference (ISI) and intercarrier interference (ICI) for the desired signal at the output. In order to remove the timing acquisition requirement of a receiver for an asynchronous MC-CDMA system, we proposed a novel partial sampling MMSE (PS-MMSE) receiver. Numerical result shows that the PS-MMSE receiver without timing knowledge provides significantly stronger interference suppression capability than the LMMSE receiver with known timing. Moreover, a so-called reduced complexity partial sampling MMSE (RPS-MMSE) receiver is proposed to make the number of the receiver's taps independent of the number of subcarriers. Results show that with a proper grouping parameter, a much less complicated RPS-MMSE receiver achieves almost the same performance as the PS-MMSE receiver. Thus, one is able to implement an MMSE receiver without a prior timing requirement to perform multiuser detection for the MC-CDMA system in an asynchronous scenario at the expense of a slight complexity increase.

We propose a novel dual-rate synchronous multi-carrier code division multiple access (MC-CDMA) system. In this system, all users are designed to have the same processing gain and share the same set of subcarriers. Two decorrelator-based receivers are implemented and analysed in this paper for the dual-rate MC-CDMA system; a high-rate decorrelator (HRD) matches high data rate and a low-rate decorrelator (LRD) matches low data rate. It has been proved that for low-rate users, HRD and LRD perform similarly, while for high-rate users, LRD outperforms HRD when the signature sequences of high-rate and low-rate users are not orthogonal. Furthermore, we propose a blind multiuser detection method, which can be implemented adaptively by applying the bootstrap algorithm. It is shown that the blind adaptive detector is near-far resistant and has performance similar to that of minimum mean square error (MMSE) detector.

Many of today's mobile channel models use the far-field approximation quite liberally without further validation. Strictly speaking, this approximation does introduce electric phase errors to the antenna signals. We quantify the probability of their occurrence within the framework of two elliptic channel models using a two element antenna array. For typical picocell design parameters, we find that errors are acceptably small for a great majority of realisations. Providing vector channel descriptions we extend the original models used for a single antenna element to cover an antenna array. However, we notice that these extensions do not account for fading correlation between array elements. Therefore, we explicitly discourage researchers in mobile communications to use this type of extension for picocell simulation. Finally, we conclude that the far-field assumption remains a valuable argument in support of a simplified evaluation of element correlation.

A new MMSE receiver, which is termed prediction MMSE (PMMSE) receiver, is proposed for joint multiple-access and narrow-band interference suppression in asynchronous DS/CDMA systems. Numerical results indicate that the PMMSE receiver provides considerable advantages over conventional MMSE receiver.

We examine a mobile data terminal equipped with a dual antenna receiver. A potential 4th generation communication system, namely Infostation, is taken as a framework to study smart antenna signal processing at the mobile receiver. We introduce a new criterion to mobile communications and compare the performance of the new smart antenna control method and a classic smart antenna receiver. Based on computer simulations, we draw conclusions with respect to the new receiver as well as to the Infostation system concept.

A code division multiple access (CDMA) detector that employs a combination of a decorrelator and an adaptive interference canceller was proposed by Siveski, Bar-Ness and Chen (1994) for the synchronous channel. For the asynchronous channel, the one-shot approach was suggested. It requires a (2K-1) dimensional decorrelator and a canceller (K is the number of users) and must be repeated once for every user if adequate performance for all users is sought. Instead, we propose in this paper a decorrelator, data combiner, canceller, data combiner arrangement. Using maximum signal-to-noise ratio (SNR) combining at both data combining stages, we show that the resulting decorrelator canceller performs with an asynchronous channel as does that for the synchronous case.

Adaptive interference cancellation schemes can offer a low-complexity means for alleviating the near-far problem pertinent in cellular CDMA systems. Bootstrap decorrelators have been proposed for simultaneous separation of all mutually interfering signals. Here, without any loss in performance, the bootstrap is simplified into a new adaptive scheme suitable for the downlink situation, i.e. for the detection of one desired user's signal.

The conventional CDMA signal detector suffers from MAI (multiaccess interference). Even with perfect power control, MAI always exists and accumulates when the number of users increases, such that the system capacity is limited. Under a wireless mobile channel, the case becomes worse and the detection of the desired CDMA signal, particularly in a fading environment, becomes more difficult. There have been numerous proposals for near-far resistant detectors, adaptive or non-adaptive, most of which assumed an additive white Gaussian noise (AWGN) channel. This paper evaluates the performance of the conventional detector (which uses solely a filter to match to the desired user) and the bootstrap adaptive detector under a slow fading environment. It compares the performance of a conventional detector and a bootstrap adaptive detector with that of a single user detector (no MAI) under a flat Rayleigh fading channel. Simulation and theoretical results show that the bootstrap adaptive detector gives a performance close to the single user detector and it remains near far resistant as it is under an AWGN channel.

Orthogonal frequency division multiplexing (OFDM) is believed to be an important technique for future wireline and wireless communications. One of the major disadvantages of such a multi-carrier modulation is the sensitivity of the performance to a frequency offset. The frequency offset can result from a Doppler shift due to mobile movement as well as from a carrier frequency synchronization error. Such a frequency offset causes a loss of the carriers' orthogonality, and hence inter-carrier interference (ICI). A previously proposed method to estimate and correct the frequency offset uses the retransmission of a data symbol, thus reducing the bandwidth efficiency. Another method, which merely reduces the effects of the ICI instead of completely correcting for effects of the frequency offset is based on repetition coding in the frequency domain, hence also reducing the throughput. In this paper, we propose a blind method using the correlation of the samples at the output of the discrete Fourier transform (DFT) to estimate the frequency offset. As no training sequence or symbol retransmission is required there is no loss in bandwidth efficiency. Additionally, we develop an adaptive algorithm for the frequency offset correction. The derived algorithm is basically a stochastic gradient algorithm which minimizes the mean squared frequency offset error.

The capacity of wireless CDMA systems in the forward link is limited by both intra-cell and inter-cell cochannel interferences. In particular, when the mobile station is close to a cell boundary, the desired signal is disturbed by relatively strong interference from neighboring base stations. In this paper, a receiver architecture with smart antenna followed by a RAKE demodulator is suggested at a IS-95 CDMA mobile station for interference suppression and diversity combining. Such a structure can work well only when the channel vector of desired signal is correctly estimated. We use the identifying spreading codes (as in IS-95 for example) to provide and adaptively correct a channel vector estimate, suppress cochannel interference, hence improve the system capacity.

Almost all near-far resistant detectors, adaptive or nonadaptive, assume knowledge of the relative delays of the different users' signals. Estimation and tracking errors of these delays cause errors in the correlation matrix as they all perceived by the receiver. Some linear detectors, like decorrelating detector, minimum mean-square error (MMSE) detector, or nonlinear detectors, such as zero-forcing decision-feedback (ZF-DF) detector, all directly perform some manipulation on the correlation matrix and hence they are very sensitive to delay shift or estimation error. The bootstrap detector, on the other hand, performs adaptive transformation on the output of the matched filter directly, taking into account any such errors if they exist. Therefore one may expect this detector to be more robust to delay errors. We analyse the bootstrap detector when delay errors exist. We examine analytically as well as by simulation, the BER obtained with this detector and compare it to the conventional decorrelating detector. We show that while the latter becomes non-near-far resistant when error exists, the bootstrap stays near-far resistant in the limit although with some degradation in performance when the condition number of the correlation matrix becomes large.

This paper presents the generalised maximum signal-to-interference-plus-noise ratio (MSINR) array processor, which is the extension of MSINR from the single to the multiple desired-signal case. The problem of optimal multiple desired signal processing is of particular importance in a multipath fading environment. Therefore, the performance of the MSINR processor is evaluated for personal communication systems (PCS) in a Rayleigh fading channel with interference. Strong interference is expected in the PCS channel, due to the overlay in the spectrum allocated to PCS. It is shown that the generalised MSINR processor outperforms other methods the minimum variance distortionless response (MVDR) and the minimum mean square error (MMSE) interference cancellers, yielding the best bit error rate (BER).

A reduced-state sequence estimator (RSSE) for linear dispersive channels is described. The technique is based on partitioning the set of all channel states in a way that defines a trellis with fewer states. The new technique generalizes trellises that were previously obtained by Duel-Hallen and Heegard (1989) and Eyouboglu and Qureshi (1988). While the approach followed by Duel-Hallen results in powers of two-state trellises, the new technique generates trellises with an arbitrary number of states. Using the state partitioning approach one could also generalize the trellises obtained by Eyouboglu. Therefore, the approach described in this paper provides a better performance/complexity tradeoff.

Vast research was recently performed on signal detection of multiuser code division multiple access (CDMA). Particularly for uplink (users to base station) the signals are asynchronous and the near-far problem is an important issue to deal with. All near-far resistant detectors, adaptive or non-adaptive, assume knowledge of the relative delays of the different users signals. Among these are the one-shot detectors suggested by Verdu. In this paper we suggest an adaptive algorithm to decorrelate the outputs of the one shot matched filters that assumes no knowledge of the relative delay. The performance of this approach is shown to be better than the non-adaptive "zero-forcing" method previously used and which implements linear transformation via the inverse of matched filters output cross-correlation. For simplicity a two-user case is presented. Extension to a higher number of users is relatively simple.

High-speed communications suffer from intersymbol interference (ISI) introduced by the channel. In order to combat ISI one commonly uses equalizers that need a training sequence to adjust the equalizer tap weights. When sending a training sequence is not appropriate, blind equalization has to be used. A new blind linear equalizer is proposed. The equalizer has a recursive structure. To avoid stability problems, a soft limiter is used in the feedback loop. The weights are controlled by means of the decorrelation algorithm. For channels without precursors, the equalizer converges to the desired weights in around 500 iterations, depending on the amount of distortion. The novel blind equalization technique is globally convergent for minimum and non-minimum phase channels.

Using a one-shot matched filter followed by a decorrelating detector, results in outputs corresponding to the main user and the left (earlier bit) and right (current bit) portion of the code of the other users. Due to reduced power, these outputs will result in a degradated probability of error. The purpose of this paper is to propose a method of combining for the data of the split code users. We suggest to use combining gain in proportion to the relative user delays. The resulting decorrelating detector always performs better than the split-output detector. Such diversity data combining eliminates the need for the multiple one-shot filters, one for each user.

For personal, indoor, and mobile wireless radio communications (PIMRC) uplink multiuser CDMA channels, received signals are time asynchronous, and suffer from amplitude fading. The relative signals' bit delays, energy variations, and hence the cross-correlation matrix of the received signal, are unknown. Using a novel adaptive algorithm, we propose in this paper a decorrelating detector suitable for particular CDMA uplink channels. The proposed asynchronous CDMA adaptive decorrelator consists of a bank of matched filters (matched to the users' signature sequences), followed by a sampler at the bit-rate of 1/T and controllable weights. It is near-far resistant, and can be used by itself if signal-to-noise ratios are high, or as a first stage in the two-stage adaptive canceller previously proposed for the synchronous CDMA case.

Multi-carrier (MC) multi-user CDMA is a new transmission method in which every user bit is sent simultaneously over multiple subcarriers (or multiple tones), each of which has a time-constant 0 or /spl pi/ phase offset. The offsets corresponding to each user form a unique code signature sequence, orthogonal to those of any other user, or pseudo orthogonal as with Gold codes or Kessami code sequences. We address a moderately dispersive channel in which each user-subcarrier channel has a different random amplitude and phase. We estimate the performance of a receiver matched to such a multicarrier, multi-user scheme. Then we implement a decorrelating interference canceler at the output and examine its effect on performance.

A multiuser detector in an additive white Gaussian noise, code division multiple access channel is proposed and analyzed. The detector utilizes the decorrelated tentative decisions obtained in the first stage to estimate and subtract multiple access interference in the second stage. The novel feature of the detector is the adaptive manner in which the multiple access interference estimates are formed, which renders prior estimation of the received signal amplitudes unnecessary. The detector is near-far resistant, and is shown to provide substantial performance improvement over the conventional and decorrelating detector, particularly in the presence of strong interfering signals.

A fast multidimensional adaptive algorithm, Bootstrap, is proposed for multiple signal separation. It separates multiple uncorrelated signals imposed on each other. The bootstrap adaptive algorithm, which does not require training sequences, uses an optimization criteria that is based on minimization of output signal correlations. The learning process of this algorithm is compared with that of the least mean square (LMS) algorithm for different eigenvalue spreads. It has been found from computer simulations that the Bootstrap algorithm converges much faster than the LMS algorithm. The learning process of the Bootstrap algorithm is almost independent of eigenvalue spread.>

A special thinned array structure, the minimum redundancy array (MRA), is proposed for interference cancellation. This array structure has been proven in the past to have advantages in direction finding applications. It is also shown that MRA performs as a canceler better than a comparable uniform array structure. Simulation results supplement this conclusion.>

This paper discusses cross-pol cancellation for microwave radio system applications. A model for cross-pol interference coupling during multipath fades is postulated and some preliminary performance goals are presented. A design for an IF canceller consisting of cross-coupled adaptive tapped delay lines and a digital controller is presented and discussed. Predicted canceller performance as a function of tap spacing and number is discussed and compared to measured data. The design of a two-channel, dual-polarized lab fade simulator is presented, and methods to characterize canceller performance and fade parameters are also briefly discussed.

In this paper, we address the problem of element placement in a linear aperiodic array for use in spatial spectrum estimation. By making use of a theorem by Caratheódory, it is shown that, for a given number of elements, there exists a distribution of element positions which, for uncorreleated sources, results in superior spatial spectrum estimators than are otherwise achievable. The improvement is obtained by constructing an augmented covariance matrix, made possible by the choice of element positions, with dimension greater than the number of array elements. The augmented matrix is then used in any of the known spectrum estimation methods in conjunction with a correspondingly augmented search pointing vector. Examples are given to show the superior detection capability, the larger dynamic range for spectral peak to background level, the lower sidelobes and the relatively low bias values, when one of the known spectrum estimation techniques based on eigenstructure is used.

In this letter, we address the problem of element placement in a linear aperiodic array for use in spatial spectrum estimation. By making use of a theorem by Caratheodory, it is shown that, for a given number of elements, there exists a distribution of element positions which, for uncorrelated sources, results in superior spatial spectrum estimators than are otherwise achievable. The improvement is obtained by constructing an augmented covariance matrix, made possible by the choice of element positions, with dimension greater than the number of array elements. The augmented matrix is then used in any of the known spectrum estimation methods in conjunction with a correspondingly augmented search pointing vector. Examples are given to show the superior detection capability, the larger dynamic range for spectral peak to background level, the lower sidelobes, and the relatively low bias values, when one of the known spectrum estimation techniques based on eigenstructure is used.

In this work a new version of the cross-coupled phaselocked loop (CCPLL) interference canceller is presented. This detector consists of two phase-locked loop (PLL) demodulators interconnected in such a manner as to permit one PLL to lock onto and track the instantaneous frequency of the stronger received signal, while the other loop tracks and demodulates the weaker of two received signals. In addition, the receiver includes control loops to estimate the instantaneous amplitude of both received signals. This demodulator, therefore, has the capability of detecting both the desired and interfering signals, even though the interferer may be stronger than the desired signal and may share the same frequency band. The defining nonlinear differential equations for the CCPLL structure are derived and transient acquisition responses are obtained using computer-aided analysis. The CCPLL has numerous technological applications in suppressing a wide variety of cochannel and adjacent channel interferers.

In this correspondence we prove that the optimization process used in the paper leads to bounded optimal phase detectors. The bounds can be applied in normalizing the functions of these detectors, rendering Meidan's argument invalid.

In order to improve the operation of phase-locked loops (PLL) in both the acquisition and tracking modes, loops with two phase detectors are considered. The optimization problem of choosing the phase detectors in order to minimize the, variance of loop phase error is formulated. The optimization problem is solved under several approximations. Despite the approximation, the resulting nonlinearities can be implemented and results in a PLL with improved performance in both acquisition and tracking modes.