Although first proposed a few decades ago, chirp-based modulation has recently seen a surge of popularity as a result of its application in the LoRa standard. Over the past years, this key Internet of Things (IoT) enabler has been well researched, and ever more advanced low-power wide-area networks (LPWANs) are being implemented across the globe, based on this technology. As a result of this international acclaim, multiple actors have invested efforts into implementing frequency-shift chirp modulation (FSCM), which is a more general term for the open-source physical layer modulation protocol also embedded in the LoRa standard, on software-defined radio (SDR) systems. However, while oversampling, advanced post-processing and other digital techniques have led to significant advances in the technology’s capabilities and reliability, real-world deployments of these SDR implementations have to overcome the excessive computational cost associated to these techniques. In response to this challenge, this article examines several new strategies for symbol detection methods operating on FSCM signals, such as those employed in LoRa modulation, enabling significant computational cost reductions. Examples of these are integrating frequency correction in the dechirping procedure and omitting the downsampling operation by using upsampled down-chirps when processing the received samples. In comparison to the standard detection method, computational efficiency gains between 19% and 36% are achieved. Hence, applying the methods presented in this work can yield significant reductions in power consumption for real-world SDR-based FSCM systems in state-of-the-art IoT deployments.

Physical-layer authentication (PLA) mechanisms exploit signals exchanged at the physical layer of communication systems to confirm the sender of a received message. In this paper, we propose a novel challenge-response PLA (CR-PLA) mechanism for a cellular system that leverages the reconfigurability property of a reconfigurable intelligent surface (RIS) (under the control of the verifier) in an authentication mechanism. In CR-PLA, the verifier base station (BS) sets a random RIS configuration, which remains secret to the intruder, and then checks that the resulting estimated channel is modified correspondingly. In fact, for a message sent by an attacker in a different location than the legitimate user equipment (UE), the BS will estimate a different channel and the message will be rejected as fake. Such a solution reduces the communication and computational overhead with respect to higher-layer cryptographic authentication. We derive the maximum a-posteriori attack when the attacker observes a correlated channel and the reconfigurable intelligent surface (RIS) has many elements, and the attacker transmits to Bob either directly or through the RIS. Using a generalized likelihood ratio test to test the authenticity at the base station (BS), we derive approximate expressions of the false alarm and misdetection probabilities when both the BS and the UE have a single antenna each, while the RIS has a large number of elements. We also evaluate the trade-off between security and communication performance, since choosing a random RIS configuration reduces the data rate. Moreover, we investigate the impact of various parameters (e.g., the RIS randomness, the number of RIS elements, and the operating signal-to-noise ratio) on security and communication performance.

In recent years, LoRa has been deployed in countless Internet of Things (IoT) applications across the globe. However, as LoRa is a proprietary technology, research into its physical-layer performance has been challenging. Implementing LoRa on software-defined radio (SDR) platforms yields valuable insight into the physical layer of the LoRa standard and paves the way for improvements in packet reception capabilities for LoRa receivers. This article presents an independently developed packet reception algorithm, which drastically improves the physical performance of LoRa communication links. The advanced signal presence detection, synchronization, and symbol detection strategies are shown to significantly increase packet reception ratios in extremely adverse noise conditions. Multiple algorithm variations are presented and compared in terms of bit error rate (BER) performance and computational cost. In comparison to a theoretical system with perfect channel state information, the simulated BER performance of the best performing algorithm only requires an increase of 1.6 dB in signal-to-noise ratio (SNR) to exhibit the same performance. Finally, SDR implementations of the algorithms exhibit average SNR performance gains up to 4.7 dB when compared to commercially available hardware.

Simultaneous wireless information and power transfer can be achieved using anintegratedreceiver, where the incoming signal is rectified, and both the energy harvesting (EH) and the information detection (ID) operate on this rectified signal. Because phase information is lost at the input of the ID, commonly-used modulation techniques cannot be straightforwardly applied and biased$M$-ASK is used instead. We consider downstream communication over a Rician block-fading channel, from a multi–antenna basestation to a number of single-antenna users. Multi-user interference is eliminated by applying zero-forcing precoding. Using a realistic rectifier model, we compare EH and ID performances of linear precoding (LP) and modified Tomlinson-Harashima precoding (THP). We add low-complexity algorithms to these precoders that optimize the user ordering and the constellation rotations. Moreover, we investigate the effect of the amplitude ratio$\rho _{\mathrm {ASK}}$of the constellation on the energy-rate trade-off. Compared to the same precoders without optimization, optimizing the user ordering, rotations and$\rho _{\mathrm {ASK}}$considerably reduces the transmit power required to achieve a harvested power of −30 dBm or an average mutual information of$(\log _{2}M)/2$. Both with and without optimizations, THP outperforms LP in terms of EH and ID, but THP exhibits much larger gains when optimizations are applied.

Simultaneous wireless information and power transfer can be realized by means of an integrated receiver, where both the energy harvesting and the information detection operate on the rectified received signal. In this contribution, we consider a MU-MISO communication system where all users terminals are equipped with a current-splitting integrated receiver. Biased binary ASK modulation with zero-forcing linear precoding is used. We demonstrate that, for a given transmit power, the received signal power can be considerably increased by applying appropriate rotations of the symbols and suitable row permutations of the channel matrix at the transmitter. We present low-complexity rotation and permutation optimization algorithms, resulting in significantly higher harvested power and information rate, compared to the case where no optimization is performed. We investigate the power-rate trade-off of the optimized system by varying the amount of the rectifier output current drawn by the harvesting circuit, and study the effect of selecting the ratio of the minimum-to-maximum symbol magnitudes in the biased binary ASK constellation. A numerical example shows that the proposed optimizations and selection of the symbol magnitude ratio result in SNR gains up to about 11 dB to 21 dB (depending on the information rate) and harvested power gains up to about 14 dB.

Non-Coherent Rate-Splitting (NCRS) was recently proposed as a practical multiuser coding and decoding scheme to increase the spectral efficiency of multibeam satellite communication systems. In this paper, we further study the practical realization of NCRS. We propose a modified coding scheme (NCRS*) that is robust to a nonzero time offset among beams. In NCRS*, as opposed to NCRS, the beams send independently channel encoded and modulated waveforms. We assess the performance of NCRS* in terms of the achievable rate region. It is shown that NCRS* performs worse than NCRS, but better than or comparable to other competing schemes, which, as opposed to NCRS*, require flexible bandwidth allocation or perfect synchronization at the transmitter. We also propose a new N-MAP algorithm for the practical implementation of NCRS* receivers. Similar to the existing UMAP algorithm, N-MAP takes into account the modulation used by, and the time offset between, the signals received from the different beams. In most cases, however, N-MAP has a significantly lower complexity than U-MAP.

Due to the continuous demand for higher bit rates, the management of the spatio-temporal intersymbol interference in frequency-selective multiple-input multiple-output (MIMO) channels becomes increasingly important. For single-input single-output channels, equalized precoded partial-response signaling is capable of handling a large amount of intersymbol interference, but, to date, no equalization scheme with general partial-response signaling has been presented for the frequency-selective MIMO channel. Not only does this contribution extend partial-response signaling to the MIMO channel by proposing a general spatio-temporal partial-response precoder, but it also develops a minimum mean-squared-error optimization framework in which the equalization coefficients and the spatio-temporal target response are jointly optimized. Three iterative optimization algorithms are discussed, which update (part of) a row of the target impulse response matrix in each iteration. In particular, the third algorithm reformulates this row optimization as a lattice decoding problem. Numerical simulations confirm that the general partial-response signaling clearly outperforms the traditional full-response signaling in terms of the mean squared error and the bit error rate. The third optimization algorithm has a better performance but a higher complexity, compared to the first and the second algorithm.

Partial-response signaling is known to facilitate the equalizer design because a controlled amount of residual interference is permitted. The design of the target impulse response of the partial-response precoder often assumes perfect channel state information, which is unfortunately not available at the transmitter in most practical applications. Consequently, this contribution focuses instead on the robust and joint design of a spatio-temporal target impulse response and the equalization coefficients for a frequency-selective fading multiple-input multiple-output communication channel based on current and/or previous noisy channel estimates. More precisely, the error in the channel estimates is statistically modeled, and robustness is achieved by minimizing the mean-squared estimation error averaged over the joint distribution of the actual channel and the available channel estimates. Numerical results of the bit error rate confirm that the proposed robust partial-response signaling not only provides a significant performance gain compared to traditional full-response signaling, but also outperforms the naive approach, which ignores channel estimation errors.

High-speed communication over mutually coupled channels can be severely affected by intersymbol interference (ISI) and crosstalk (XT). Multiple-input multiple-output (MIMO) equalization is known to mitigate both ISI and XT, but requires accurate channel information at the receiver (RX) and/or transmitter (TX), when the channel is stochastic. However, since adjusting the equalization filters to the specific channel realization involves a considerable implementational and computational complexity, it is desirable to keep the equalization filters fixed irrespective of the specific channel realization, especially when the channel variability is relatively small. Therefore, we propose a general MIMO transceiver scheme where the equalization filters depend on the channel statistics rather than the actual channel. More specifically, by considering the time-domain equalization with the fixed linear MIMO pre-equalization and adjustable MIMO decision feedback equalization, the complexity associated with passing channel information from the RX to the TX is avoided, and the computationally intensive and iterative calculations of the fixed pre-equalizer can be performed offline rather than on-chip. The proposed equalization scheme is shown to yield only a small performance degradation compared to fully adjustable equalization schemes, thereby enabling multi-Gbit/s chip-to-chip communication over low-cost electrical interconnects which are prone to manufacturing tolerances.

While the requirements for delivering high throughputs increase exponentially with every generation of access node hardware, the device cost is of primary concern. As a result, multiple- input multiple-output (MIMO) equalization, which has been shown to facilitate multi-Gbit/s communication over low-cost parallel electrical interconnects, is emerging as an attractive high- speed interconnect solution for next-generation access nodes. Because of the high operating frequencies, however, the transfer functions of the on- and off-chip interconnects become highly susceptible to manufacturing tolerances (MTs); hence, the equalization filters must be adjusted to the specific channel realization to achieve optimal performance, which involves a high implementation and computational complexity. Considering that the MTs are usually limited, we propose a robust low-complexity transceiver consisting of a fixed MIMO linear pre-equalizer (which avoids the need for feeding back the channel state information to the transmitter), with either a fixed or adjustable MIMO decision- feedback equalizer (DFE). For a specific chip-to- chip interconnect operating at 75 Gbit/s per line and a 26 dB signal-to-noise ratio, we show that the resulting bit error rate does not exceed 10^(-12) for MTs up to 10.5% (fixed DFE) and 17.7% (adjustable DFE) of the nominal line width.

This paper studies frame synchronization for use with the advanced communication link transmission unit format that was recently proposed for updating the telecommand synchronization and channel coding standard for space applications. With a view to improving the robustness against jamming, future satellite telecommand systems are planning to adopt direct-sequence spread spectrum modulation and advanced channel coding. Compared to the frame synchronization algorithm specified in the current Consultative Committee for Space Data Systems (CCSDS) recommendation, we consider a longer start sequence and relax the condition for declaring synchronization. We investigate the performance of this algorithm in the presence of jamming, and show that the frame synchronizer can be designed such that the overall system's robustness against pulsed jamming is limited by the robustness of the code rather than the synchronizer.

A cognitive radio scenario is considered where the signals transmitted by a secondary user (SU) are relayed by multi-antenna relays using an amplify-and-forward cooperation protocol. In this paper, the optimal power allocation and beamforming scheme is derived for the SU transmitters (SU-TXs), which minimizes the exact outage probability of the SU network with relay selection, under both a transmit power constraint and a constraint on the interference power generated at every primary user receiver. After deriving the optimal structure of the beamforming matrix, several distributed resource allocation algorithms are obtained for different levels of channel state information (CSI) at the SU-TXs: perfect CSI and imperfect CSI are considered, along with the case where only channel distribution information is available. The numerical results show that the multi-antenna relays can significantly improve the performance of the SU network, which would otherwise be severely limited by the harsh interference constraints. Furthermore, we also investigate how the number of relays, the number of antennas, and the level of CSI impact the performance of the SU network. Finally, we point out that the proposed algorithms outperform several algorithms presented in the literature.

We propose a novel resource allocation (RA) strategy for a cognitive radio packet-oriented bit-interleaved coded orthogonal frequency division multiplexing (BIC-OFDM) system with decode-and-forward (DF) relays. The aim of the RA is maximizing the goodput (GP) of the source-relay-destination link, which is the number of information bits correctly received at the destination node per unit of time. Therefore, we derive an accurate analytic approximation for this figure of merit, which allows us to find the optimum constellation size, code rate and energy allocation per subcarrier. Further, this expression also serves as a novel relay selection criterion. Finally, we validate the proposed RA method, and compare its performance to capacitymaximizing algorithms through numerical simulations.

In this contribution, we present a linear multiple-input multiple-output (MIMO) equalization scheme at the receiver side for high-speed electrical chip-to-chip communication. As opposed to traditional single-input single-output (SISO) equalization per lane, this MIMO approach enables cooperating receivers to treat crosstalk (XT) between neighboring channels as an information-bearing signal instead of a disturbing signal, allowing to mitigate both inter symbol interference and XT. Given a simulated 4 × 4 MIMO electrical chip-to-chip interconnect channel, we point out that, for a given total number of equalizer taps, MIMO equalization can outperform SISO equalization. Moreover, by further increasing the total number of equalizer taps, MIMO equalization allows to obtain performance gains that are substantially larger than for SISO equalization.

Precoding is a technique that counteracts the crosstalk in downlink digital subscriber line (DSL) transmission. While linear precoding is adequate in the VDSL2 technology because of the moderate crosstalk environment, this might no longer be the case in the G.fast technology, which uses higher frequencies and therefore is affected by higher crosstalk levels. In this contribution we compare linear and nonlinear precoding for G.fast transmission protected by trellis-coded modulation, and present two bitloading algorithms that are suited for both precoding schemes. We show that for a set of measured broad-band channels, each representing a binder of eight twisted-pair cables, these bitloading algorithms have substantially different complexities but yield a similar information bitrate, which is about 10% higher for nonlinear precoding as compared to linear precoding.

In this paper we consider a decode-and-forward cooperative network, which is active in the same bandwidth as a primary user network. The cooperative or secondary user network applies a dynamic resource allocation algorithm to maximize its own performance while limiting the interference caused at the primary user receivers. The main contribution of this paper is the proposition of a new accurate approximation for the outage probability, which takes into account that the channel state information available at the transmitter is imperfect and outdated. The accuracy of the approximation and the performance of the resource allocation scheme are validated through numerical simulations.

In this contribution, we present a simple but efficient importance sampling technique to speed up Monte Carlo simulations for bit error rate estimation of orthogonal space-time block codes on spatially correlated Nakagami-m fading channels. While maintaining the actual distributions for the channel noise and the data symbols, we derive a convenient biased distribution for the fading channel that is shown to result in impressive efficiency gains up to multiple orders of magnitude.

This paper considers the detection of coded phaseshift keying signals subjected to additive white Gaussian noise and oscillator phase noise. We propose a detector that partitions the received frame into smaller blocks and models the unknown phasor variations over each block as a truncated discrete cosine transform (DCT) expansion. Detection and decoding are iteratively performed between a soft-input soft-output (SISO) demodulator, a SISO demapper, and a SISO decoder based on the sum-product algorithm and the factor graph framework, whereas the expectation-maximization algorithm is used in the demodulator for the DCT coefficients estimation. The resulting demodulator is shown to have an excellent performance/complexity tradeoff and to be well-suited for parallel processing on multiple cores.

Several diversity techniques have been proposed to cope with fading in wireless networks. A recent and promising technique, which does not increase hardware demands, is cooperative communication. Here spatial diversity is created by using other terminals to relay the information from the source terminal to the destination terminal. Cooperative protocols proposed in literature are often studied under the simplifying assumption that all channel state information is available at the destination. This contrasts to a practical scenario where channel estimates need to be derived from the broadcasted signals. In this paper, we focus on the Amplify-and-Forward protocol and use the Space-Alternating Generalized Expectation-maximization (SAGE) algorithm to estimate the channel gain and noise variance in an iterative way, exploiting both the pilot part and the data part of the signal received at the destination. We consider a relay that performs a general affine transformation on the pilot symbols received by the source, and we optimize this transformation in terms of estimation accuracy. The resulting mean square error performance is shown to be close to the Cramer-Rao lower bound (CRB).

We consider Discrete Multitone Transmission over short Digital Subscriber Lines using bandwidths applicable to G.fast, in the presence of impulsive noise. Various combinations of coding and retransmission schemes are compared in terms of bit error ratio performance and error-free information bitrate (goodput). Our study shows that, under an error performance constraint, retransmission schemes allow to achieve larger good-put with smaller latency as compared to the traditional combination of Reed-Solomon coding and Trellis Coded Modulation with a large interleaver in between.

In this contribution, a cognitive radio scenario is considered, where a secondary user (SU) cooperative network is allowed to transmit concurrently with a primary user (PU), provided that the resulting average interference power at the PU receiver is kept below a certain threshold. The SU network uses a selection amplify-and-forward protocol. An optimum and a low-complexity suboptimum dynamic energy allocation strategy are proposed, that minimize the link outage probability between the source node and the destination node of the SU network under an average interference power constraint. Because the interference channels from the SU nodes to the PU are affected by fast fading, the realistic assumption is made that each SU node has access to the root mean-square value (rather than the instantaneous value) of the gain of its own interference channel to the PU. We show that both dynamic allocation strategies considerably outperform the optimum static allocation strategy, and that the low-complexity dynamic allocation performs nearly as well as the optimum dynamic allocation. Finally, we confirm that our dynamic algorithms show a significant performance improvement as compared to the optimal dynamic algorithm under a peak interference power constraint.

The quantize-and-forward (QF) cooperative protocol can effectively be used to combat fading in systems using half-duplex relay terminals. While the outage behavior of the QF protocol has been extensively investigated, few research has been performed on the impact of imperfect channel estimates. In this contribution, a lower bound (LB) on the estimation error is obtained for a one-hop relaying channel with data quantization at the relay. To this purpose the modified Cramer-Rao (MCRB) bound is calculated, which, compared to the true Cramer-Rao bound (CRB), is a looser and computationally less complex bound that converges to the CRB in the high signal-to-noise ratios. By using a general system model for the relay channel, the obtained results can be utilized to benchmark a wide variety of systems.

In this contribution, we present a novel closed-form bit error rate (BER) approximation for orthogonal space-time block codes employing rectangular quadrature amplitude modulation. The multiple-input multiple-output channel is assumed to be affected by arbitrarily spatially correlated Rayleigh fading. We consider a mismatched maximum-likelihood receiver that obtains the channel state information through pilot-based linear minimum mean-square error channel estimation. The presented BER approximation is shown to yield very accurate results over a wide range of signal-to-noise ratios.

The outage probability limit is a fundamental and achievable lower bound on the word error rate of coded communication systems affected by fading. This limit is mainly determined by two parameters: the diversity order and the coding gain. With linear precoding, full diversity on a block fading channel can be achieved without error-correcting code. However, the effect of precoding on the coding gain is not well known, mainly due to the complicated expression of the outage probability. Using a geometric approach, this paper establishes simple upper bounds on the outage probability, the minimization of which yields to precoding matrices that achieve very good performance. For discrete alphabets, it is shown that the combination of constellation expansion and precoding is sufficient to closely approach the minimum possible outage achieved by an i.i.d. Gaussian input distribution, thus essentially maximizing the coding gain.

Multiple antennas are used to increase reliability and bit rate for a given bandwidth. For a fixed transmission rate, discrete input alphabets and no channel state information at the transmitter, optimal space-time codes (STCs) achieving both gains (full rate and full diversity) are well known. However, the complexity of maximum likelihood decoding increases exponentially with the number of space and time dimensions of the STC. Despite praiseworthy efforts to reduce the decoding complexity, optimal STCs are not used in practice in the case of more than two transmit antennas because of complexity reasons. It is generally accepted that reducing the dimension of the STC leads to a reduced diversity order. We show that this is not true for coded communication, assuming the presence of an outer error-correcting code with any coding rate. We propose a new class of full-rate full-diversity STCs; more specifically, time-varying space-only codes. This new class is referred to as EMI codes. Full diversity is proven in terms of outage probability, for the case where the number of receive antennas is larger than or equal to the number of transmit antennas, and is numerically verified in terms of outage and word error probability using LDPC codes.

In a cooperative network, relaying nodes transmit to the destination information that is related to the message sent by the source. In literature the performance of such networks is usually studied under the (unrealistic) assumption of perfect channel knowledge at the destination. In this paper we consider the Amplify-and-Forward protocol and assume time-varying Rice fading components. Based on pilot symbols an initial estimate of the channel parameters that correspond to the actual channel realizations is obtained, and is subsequently refined in an iterative way by also exploiting the information contained in the data part of the received signals. The iterative estimator gives rise to substantial improvements in terms of error performance and bandwidth efficiency as compared to the initial pilot-based estimate.

Cooperative communication systems can effectively be used to combat fading. A cooperative protocol that can be used with half-duplex terminals is the quantize and forward (QF) protocol, in which the relay quantizes the information received from the source before forwarding it to the destination. Most studies on the QF protocol are carried out under the assumption of perfect channel state information (CSI) at the destination, which is not often the case in real-life systems. Therefore, in the present contribution, the effect of incomplete CSI is analyzed for flat Rayleigh fading channels with a frequency offset. To limit the complexity of the estimation, the destination terminal assumes that the relay operates in an amplify and forward (AF) mode. By using the expectation maximization (EM) algorithm to refine the initial pilot-based estimates, the resulting error performance can be made very close to that of a system with perfect CSI.

Cooperative communication is a promising technique that achieves spatial diversity without requiring multi-antenna receivers. In this paper we consider the cooperative Amplify-and-Forward protocol and assume the channel realizations to be unknown. We study several pilot-based channel estimation strategies at the relay and the destination. The obtained maximum-likelihood estimators are evaluated for transmission over Rice-fading channels, in terms of channel estimation accuracy and error performance.

We consider a cognitive radio (CR) scenario where the signals transmitted by a secondary user are relayed using an amplify-and-forward cooperation protocol. A distributed strategy for power allocation is derived that minimizes the outage probability of the cooperative CR communication system, under the constraint that the interference power experienced by the primary receiver does not exceed a given threshold. First, assuming that each CR node has ideal channel state information about its interference channel, we show that the average interference power constraint gives better outage probability performance than the peak power constraint. Then, when only channel statistics are available for the interference channel, the outage probability is verified to lie in between the results relevant to the average and peak power constraints, respectively, with ideal channel state information.

This paper presents a new iterative multiuser (MU) detection algorithm for asynchronous spectrally-efficient continuous-phase modulation in additive white Gaussian noise. The proposed detector is derived from the sum-product (SP) algorithm and the factor graph (FG) framework, and performs approximate maximum a posteriori bit detection. A convenient FG of the actual MU detection problem is considered, rather than only FGs of the individual single-user detection problems combined with ad-hoc inter-user interference cancellation. A suitable set of SP messages is approximated by a Gaussian distribution; this considerably reduces the computational complexity and memory size requirements as compared to a straightforward application of the SP rules. The resulting algorithm succeeds in achieving a good error performance.

We propose an iterative decision-directed joint frequency offset (FO) and channel estimation algorithm in a known symbol padding (KSP) orthogonal frequency division multiplexing (OFDM) system, where the guard interval is filled with pilot symbols. Besides those time domain pilot symbols, some additional pilot symbols are transmitted on the pilot carriers. The decision-directed algorithm is initialized by pilot-aided FO estimation without channel knowledge. We propose a possible initialization algorithm that operates in the frequency domain (FD). After the initialization phase, the iterative decision-directed estimation algorithm is applied. For the channel estimation step, an existing pilot-aided channel estimation algorithm is extended to a decision-directed algorithm which uses the Fast Fourier Transform outputs at both the pilot and data carrier positions. For the uncoded case, the proposed iterative decision-directed joint FO and channel estimation algorithm reaches the bit error rate performance of a receiver with perfect synchronization and perfect channel knowledge. For a coded system, there is small loss in performance of less than 1 dB when our proposed algorithm is applied compared to a receiver with perfect knowledge about the FO and the channel.

Cooperative communication systems can effectively be used to combat fading. A cooperative protocol that can be used with half-duplex terminals is the Quantize and Forward (QF) protocol, in which the relay quantizes the information received from the source before forwarding it to the destination. While the outage behavior and error performance of the QF protocol have been investigated extensively, only few research has been performed on the effects of channel parameter estimation. In this contribution, estimates are derived for all communication channels involved, while keeping the complexity of the relay terminal unaltered. First, channel estimates are calculated using known pilot symbols sent by the source and relay. Thereafter, these pilot-based channel estimates are refined using a code-aided Expectation Maximization (EM) approach, yielding an error performance that is very close to that of a system in which the channel parameters are assumed to be known.

In wireless communications, the block fading (BF) channel is an important channel model. A key quality indicator in coded transmission is the word error rate (WER), which is the fraction of packets that cannot be decoded correctly at the receiver. We study Low-Density Parity-Check (LDPC) coded modulation with precoding, with the aim to minimize the WER on BF channels without channel state information at the transmitter. In the literature, it was not yet known how to optimize the system parameters for this channel model, mainly due to the fading gain distribution. One of the existing approaches to combining coding and modulation is bit-interleaved coded modulation with iterative decoding (BICM-ID). This work uses precoding to optimize BICM-ID with LDPC codes for BF channels, and is a continuation of previous work that used precoding to minimize the outage probability limit. We present the selection of the precoding matrix, the mapping function and the error-correcting code yielding a WER that closely approaches this minimum outage probability. Using a geometric approach, the off-line system optimization effort for the BF channel is limited to at most B+1 times the effort for Gaussian channels, where B is the number of blocks in the BF channel.

Indoor off-body wireless MIMO links between a mobile user equipped with wearable textile patch antennas and a fixed base station exhibit specific channel behavior due to the near presence and movements of the human body. Therefore, they require a dedicated channel model that captures the effects of correlated small-scale Rayleigh fading and correlated lognormal shadowing. A methodology is presented to construct such a model, allowing to predict the bit error characteristics and channel capacity curves based on the shadowing and fading correlation matrices that are extracted from channel measurements. It is shown that by separating shadowing, including effects caused by movement and reorientation of the human body, from small-scale fading, the main mechanisms of the off-body communication link are accurately captured by the model. A clear dependence of the shadowing correlation values on the physical layout of the antenna system is found. In our measurements, shadowing is not significantly decorrelated by polarization diversity or front-to-back diversity whereas the small-scale fading is clearly decorrelated. From the model, MIMO channel realizations with identical bit error rate and channel capacity characteristics as the measured channel can be quickly generated for link emulation purposes.

We study precoding for the outage probability minimization of block fading (BF) channels and BF relay channels. Recently, an upper bound on the outage probability with precoding was established for BF channels, but only for high instantaneous SNR. This upper bound is much easier to minimize than the actual outage probability, so that optimal precoding matrices can be determined without much computational effort. Here, we provide a proof for the upper bound on the outage probability at low instantaneous SNR. Next, the structure of the precoding matrix is simplified so that it can be easily constructed for an arbitrary number of blocks in the BF channel. Finally, we apply this technique to cooperative communications.

The presence of a frequency offset (FO) and phase noise can cause severe performance degradation in digital communication systems. This work combines a simple FO estimation technique with a low-complexity phase noise estimation method, inspired by the space-alternating generalized expectation-maximization algorithm. Using a truncated discrete-cosine transform (DCT) expansion, the phase noise estimate is derived from the estimated DCT coefficients of the phase. A number of implementations of the proposed algorithm are discussed. Numerical results indicate that when estimating the FO from pilot symbols only, comparable performance can be reached as the computationally more complex case where the FO is updated iteratively, with small convergence time. The phase noise estimation step is well capable of compensating for the residual FO. For the considered scenario, performing FO compensation before iterative phase noise estimation yields a bit-error rate performance degradation close to the case where the FO is known.

We study Low-Density Parity-Check (LDPC) codes with iterative decoding on block-fading (BF) Relay Channels. We consider two users that employ coded cooperation, a variant of decode-and-forward with a smaller outage probability than the latter. An outage probability analysis for discrete constellations shows that full diversity can be achieved only when the coding rate does not exceed a maximum value that depends on the level of cooperation. We derive a new code structure by extending the previously published full-diversity root-LDPC code, designed for the BF point-to-point channel, to exhibit a rate-compatibility property which is necessary for coded cooperation. We estimate the asymptotic performance through a new density evolution analysis and the word error rate performance is determined for finite length codes. We show that our code construction exhibits near-outage limit performance for all block lengths and for a range of coding rates up to 0.5, which is the highest possible coding rate for two cooperating users.

We show that the common approach to channel and Time-of-Arrival (ToA) estimation for Impulse-Radio Ultra WideBand (IR-UWB), where the occurrence of pulse overlap is ignored, is bound to lead to considerable estimation errors in the presence of pulse overlap. We propose the use of a low-complexity extension of the sliding correlator as a Maximum Likelihood (ML) channel estimation method that can deal with sporadic pulse overlap due to multipath components with delay difference of less than the pulse duration. We also illustrate the superior performance of the improved estimator.

The outage probability limit is a fundamental lower bound on the word error rate of coded communications systems. It is mainly determined by two parameters: the diversity order and the coding gain. With linear precoding, the maximum achievable coding rate yielding full-diversity can exceed the upper limit given by the standard Singleton bound. However, the effect of precoding on the coding gain is not well understood. This paper analyzes linear precoding from an information theoretical point of view and tries to optimize the coding gain. For discrete constellations, it is shown that constellation expansion together with one optimized precoding parameter is sufficient to approach the best outage achieved by a Gaussian alphabet, thus maximizing the coding gain.

Streaming on-demand video services can be provided to an end user by transmitting video data as a sequence of Internet Protocol (IP) packets over the network. In order to maintain a sufficient video quality at the end user, video packets must be protected against erasures by means of a suitable form of error control. In this contribution we make a comparison of 2 classes of Forward Error Correction (FEC) codes: the Reed-Solomon (RS) codes and the Raptor codes. We present the decoding complexity analysis of these codes and compare their erasure decoding performance. For a given decoding complexity, the codewords of the Raptor code can be taken much longer than those of the RS code, because the decoding of the former code involves only XOR operations. For a performance target of less than one decoding error event in 4 hours, a video bitrate of 7.3 Mbit/s, a latency constraint of 10 seconds and a transmission overhead of 20%, equal erasure probabilities at the decoder input are allowed for the RS(254,212) code and for the Raptor code of size (8760,7300), whereas this RS code has a 12 times higher decoding complexity.

Cooperative communication offers a way to obtain spatial diversity in a wireless network without increasing hardware demands. The different cooperation protocols proposed in the literature are often studied under the assumption that all channel state information is available at the destination. In a practical scenario, channel estimates need to be derived from the broadcasted signals. In this paper, we study the Amplify-and-Forward protocol and use the expectation-maximization (EM) algorithm to obtain the channel estimates in an iterative way. Our results show that the performance of the system that knows the channels can be approached at the cost of an increased computational complexity. In case a small constellation is used, a low complexity approximation is proposed with a similar performance.

In wireless scenarios an effective protocol to increase the reliability for time-varying channels is the hybrid automatic repeat request (H-ARQ). The H-ARQ scheme with cross-packet channel coding (CPC) is a recently published extension of H-ARQ with several advantages. No full-diversity low-density parity-check (LDPC) code design for the whole range of coding rates yielding full-diversity has been published. In this paper the authors provide a new outage behavior analysis and a new structured LDPC code ensemble achieving full-diversity for H-ARQ with CPC by exploiting the rootcheck principle. Simulation results show that the new code design outperforms the previous approaches, providing full-diversity and good coding gain, also at high coding rates.

The quality of experience (QoE) of IP-packetized streaming video is affected by both packet loss and packet delay variations. When the network delivering the video content contains a wireless link, occasional deep fades give rise to bursts of packet losses. In order to maintain a sufficient video QoE at the end user, video packets must be protected against losses by means of a suitable form of error control. In this contribution, we consider an indoor radio MIMO-OFDM transceiver operating over a Rayleigh block-fading channel with arbitrary correlation in the time and frequency dimensions, which makes use of an application layer Automatic Repeat reQuest (ARQ) protocol to provide additional protection of the video content against packet loss. We analyze the resulting residual packet loss performance, under a latency constraint imposed by the requirement of a small TV channel switching delay. This analysis makes direct use of the fading characterization (correlation functions in time and frequency dimensions) of the indoor environment, rather than relying on a Markov model that only approximately describes the packet loss process. Numerical results are obtained by Monte Carlo integration combined with an efficient importance sampling technique devised for the problem at hand. Assuming a 2.4 GHz wireless link, we point out how to select the system parameters (number of antennas, number of retransmissions) in order to achieve a residual packet loss performance yielding a satisfactory QoE for HDTV transmission.

This contribution deals with estimation and compensation of phase noise in single-carrier digital communications. We present an iterative feedforward soft-decision-directed phase noise estimation algorithm, that is based on approximating the phase noise process by an expansion of DCT basis functions containing only a few terms. An initial estimate of the phase noise is obtained using pilot symbols inserted in the data sequence. The estimate is iteratively improved by exploiting also soft decisions from the data symbols, obtained in a previous iteration. We demonstrate that the resulting (linearized) mean-square estimation error consists of two contributions: a contribution from the additive noise, that equals the Cramer-Rao lower bound, and a noise-independent contribution that results from the phase noise modeling error. Performance can be optimized by a proper selection of the number of DCT coefficients in the expansion of the phase noise. Iterative soft-decision-directed phase noise estimation yields a considerable performance improvement as compared to estimation using only pilot symbols.

Cooperative communications is a well known technique to yield transmit diversity in a multi-user environment. Network coding can increase the spectral efficiency in networks. These two techniques can be combined to achieve a double diversity order for a maximum coding rate Rc= 2/3 on the Multiple Access Relay Channel (MARC), where two sources share a common relay in their transmission to the destination. However, codes have to be carefully designed to obtain the intrinsic channel diversity. Up till now, no full-diversity capacity-achieving code for the MARC at a coding rate Rc= 2/3 has been published. We present a strategy to produce excellent low-density parity-check (LDPC) codes with rate 2/3, i.e., exhibiting full-diversity and operating close to the outage probability limit. Simulation of the word error rate performance of the new proposed family of LDPC codes for the MARC confirms the full-diversity.

In this contribution, we provide an exact bit error rate (BER) analysis of orthogonal space-time block codes over arbitrarily distributed (possibly correlated) block fading channels with imperfect channel estimation. The transmitted symbols belong to a PAM or QAM signal constellation. With Ntand Nrdenoting the number of transmit and receive antennas, respectively, the resulting average BER can easily be written as an expectation over 4NtNrreal-valued random variables, but the computation time needed for its numerical evaluation increases exponentially with Ntand Nr. We provide a methodology that allows an efficient numerical evaluation of the exact BER with a computation time that is essentially independent of NtNrand the signal-to-noise ratio.

In this paper we present a time delay estimation algorithm for known symbol padding (KSP) orthogonal frequency division multiplexing (OFDM) systems. The estimator exploits both the pilot symbols in the guard interval and on the pilot carriers, and takes the frequency selectivity of the fading channel into account. Our simulations indicate that the time delay estimator gives rise to only a small degradation of the bit error rate (BER) performance, as compared to a receiver with perfect synchronization.

In this contribution, we examine the effect of imperfect channel estimation on the bit error rate (BER) performance of square orthogonal space-time block codes (OSTBCs). The propagation channels from the Nt transmit antennas to each of the Ntreceive antennas are assumed to be affected by (possibly correlated) flat block fading with an arbitrary distribution. The transmitted symbols belong to a PAM or QAM constellation. The resulting average BER can easily be written as an expectation over 4NtNrreal-valued random variables, but the computing time needed for its numerical evaluation increases exponentially with Ntand Nr. However, we point out that expressing the BER in terms of the distribution of the Frobenius norm of the channel matrix, allows to reduce the BER expression for any number of receive antennas to an expectation over only 2Ns+ 2 random variables (with Nsdenoting the number of information symbols in the OSTBC matrix). Due to the high diversity order of OSTBCs, the numerical evaluation of this expectation is much less time-consuming than a straightforward computer simulation. Moreover, the presented BER expression is useful not only when the fading distribution is given in closed form, but also when only experimental data (e.g. a histogram) on the fading are available.

We present a simple closed-form expression for the BER of orthogonal space-time block codes (OSTBCs) with MMSE channel estimation, assuming PAM and QAM constellations and flat Rayleigh block fading. The BER expression is exact for a certain class of OSTBCs, and provides a very accurate approximation for other OSTBCs.

In a recent paper, Garg et al. present an expression for the exact decoding error probability (DEP) of square orthogonal space-time block codes (OSTBCs) with imperfect channel estimation. We show that their DEP expression is only asymptotically correct and point out how to obtain the exact result for arbitrary signal-to-noise ratio.

In this paper, we propose an iterative joint DA/DD channel estimation algorithm for known symbol padding (KSP) OFDM. The pilot symbols used to estimate the channel are not only located in the guard interval, but also on some of the OFDM carriers. Initially, the channel is estimated using the pilot symbols only. Next, a decision is made with respect to the transmitted data symbols. We consider both hard and soft decision of the data symbols. The decisions on the data symbols are then used to update the channel estimate in a joint DA/DD estimation algorithm. The algorithm iterates between data detection and channel estimation until convergence is reached. At high SNR, the MSE performance of the iterative estimator converges to the MSE performance of the case where all data symbols are prior known at the receiver, i.e. the all pilot DA estimator. It turns out that the MSE performance of hard decision of the data symbols reaches the MSE of the all pilot DA estimator at lower SNR than that of soft decision of the data symbols; this is caused by some approximations needed to simplify the estimation algorithm.

In this contribution, we investigate the effect of imperfect channel estimation on the bit error rate (BER) performance of uncoded quadrature amplitude modulation (QAM) with maximal-ratio combining (MRC) multichannel reception. The propagation channels from the transmitter to each of the Nr receive antennas are assumed to be affected by (possibly correlated) flat block fading with an arbitrary fading distribution. The MRC receiver makes use of estimated channel coefficients, obtained from known pilot symbols sent among the data. The resulting average BER for QAM can easily be written as an expectation over 4Nr random variables, but the computing time needed for its numerical evaluation increases exponentially with Nr. We point out that the BER can be expressed in terms of the distribution of the norm of the channel vector, rather than the joint distribution of all channel coefficients. This allows to reduce the BER expression to an expectation over only 4 random variables, irrespective of the number of receive antennas. Moreover, we show that for real-valued constellations and/or real-valued channels, the BER expression reduces to an expectation over less than 4 variables. For practical BER levels, the numerical evaluation of the BER is much less time-consuming than a straightforward computer simulation. The presented BER expression is useful not only when the fading distribution is given in closed form, but also when only experimental data (e.g. a histogram) on the fading are available.

This paper investigates linear precoding for bit-interleaved coded modulation (BICM) on additive white Gaussian noise (AWGN) channels. Concatenating a linear precoder as an inner encoder with an outer (convolutional) encoder produces a powerful code with a limited decoding complexity. Linear precoders are examined and optimized for two scenarios: using (i) a noniterative decoding strategy and (ii) an iterative decoding strategy under a perfect feedback assumption. The precoder design is based on an information-theoretical approach, on the one hand, and a pair-wise error probability (PEP) analysis, on the other hand. Both approaches render convenient precoder design rules. For a binary phase-shift keying (BPSK) signal set, the optimal precoders that result from these rules are also derived. Numerical results confirm the analytical findings and simulations illustrate the effectiveness of the approach.

In this contribution, we propose a new code-aided synchronization and channel estimation algorithm for uplink MC-CDMA. The space alternating generalized expectation- maximization (SAGE) algorithm is used to estimate the channel impulse responses, propagation delays and carrier frequency offsets of the different users. The estimator, multi-user detector, equalizer, demapper and channel decoder exchange soft information in an iterative way. The performance of the proposed algorithm is evaluated through Monte Carlo simulations. Impressive performance gains are visible as compared to a conventional data-aided estimation scheme.

This contribution deals with code-aided hypothesis testing for wireless digital receivers. We provide a theoretical justification for a hypothesis testing algorithm that was previously introduced in (Wymeersch et al., 2006) based on ad-hoc arguments. Contrary to conventional hypothesis testing methods, the algorithm from Wymeersch et al. exploits the code structure within the received signal and does not require any pilot symbols. By doing so, it allows to improve the bandwidth-efficiency of the transmission. The present contribution shows that, under mild conditions, the performance of the algorithm from Wymeersch et al. coincides with the performance of the optimal Maximum A Posteriori (MAP) hypothesis test. Computer simulations support this result.

In this paper, we derive the Cramer-Rao bound (CRB) for data-aided channel estimation for OFDM with known symbol padding (KSP-OFDM). The pilot symbols used to estimate the channel are distributed over the guard interval and OFDM carriers, in order to keep the guard interval length as small as possible. An analytical expression for the CRB is obtained by performing a proper linear transformation on the observed samples. At low SNR, the CRB corresponds to the low SNR limit of the CRB obtained in [1], where it is assumed that the influence of the data symbols on the channel estimation can be neglected. At high SNR, the CRB is determined by the observations that are independent of the data symbols; the observations that are affected by data symbols are neglected. The CRB depends on the number of pilots and slightly increases with increasing guard interval length, but is essentially independent of the FFT size and the used pilot sequence. Further, a low complexity ML channel estimation technique is derived based on the linear transformation. Although in this estimation technique only a part of the observation is used, the mean squared error (MSE) performance of this estimate reaches the CRB for a large range of SNR, but a high SNR, the MSE reaches an error floor caused by the approximations made in the derivation.

This paper compares parametric and nonparametric channel estimation methods in a multipath environment. For both methods, the MSE on the received symbol pulse is shown to consist of a modeling error term and an estimation error term. We lower bound the latter term by computing the associated Cramer-Rao bounds. We point out that the MSE can be minimized by a proper selection of the number of parameters to be estimated. In addition, we present simulation results from practical estimators that confirm our theoretical analysis. Our numerical results indicate that the parametric method outperforms the nonparametric approach.

In this contribution, we present a novel multiple-input multiple-output (MIMO) detection scheme for quadrature amplitude modulation (QAM). The proposed method is based on the slowest descent (SD) method and is suitable for both iterative and non-iterative detectors. Optimal MIMO detection entails maximizing or marginalizing a likelihood function over a very large set of vectors. Similar to other low-complexity detection schemes such as sphere decoding, the SD approach restricts the maximization/marginalization to a small set of candidate vectors. As the size of this set scales linearly with the number of transmit antennas, the SD method bears an exceptionally low complexity. Furthermore, simulation results indicate that the method achieves a close-to-optimal performance, provided that the diversity order is sufficiently high.

The introduction of turbo and low-density parity-check (LDPC) codes with iterative decoding that almost attain Shannon capacity challenges the synchronization subsystems of a data modem. Fast and accurate signal synchronization has to be performed at a much lower value of signal-to-noise ratio (SNR) than in previous less efficiently coded systems. The solution to this issue is developing specific synchronization techniques that take advantage of the presence of the channel code and of the iterative nature of decoding: the so-calledturbo-synchronizationalgorithms. The aim of this paper within this special issue devoted to the turbo principle is twofold: on the one hand, it shows how the many turbo-synchronization algorithms that have already appeared in the literature can be cast into a simple and rigorous theoretical framework. On the other hand, it shows the application of such techniques in a few simple cases, and evaluates improvement that can be obtained from them, especially in the low-SNR regime.

In this paper, we address the issue of symbol timing recovery for a coded burst transmission system. As direct maximum-likelihood (ML) estimation is intractable, we resort to the expectation-maximization (EM) algorithm in order to derive a receiver that iterates between data detection and synchronization. Conventional data-aided (DA) and decision-directed (DD) synchronizers can be interpreted as special cases of the proposed algorithm. The EM-based technique takes into account code properties and is especially well suited to scenarios where conventional schemes fail to provide the detector with a reliable timing estimate. The performance of the proposed algorithm is compared with conventional techniques through computer simulations, both in terms of mean-square estimation error (MSEE) and bit error rate (BER).

This paper investigates the effectiveness of a (non-)code-aided ML-based FB phase synchronizer at the low operating signal-to-noise ratio of capacity-approaching codes. We show that the performance of the code-aided synchronizer is very close to that of a data-aided synchronizer that knows all data symbols in advance. This illustrates the optimality of the code-aided synchronizer. For the non-code-aided and the data-aided synchronizer, the linearized mean square phase error (MSPE) is evaluated analytically in the case of a first order loop. We demonstrate that, the MSPE of the non-code-aided synchronizer equals that of the data-aided synchronizer when the carrier phase is essentially constant and the loop filter gain is the same for both synchronizers, but that the non-code-aided synchronizer (as compared to the data-aided synchronizer) yields a larger MSPE due to phase fluctuations. This proves that code-aided FB phase estimation outperforms non-code-aided FB phase estimation when that the phase to be estimated is time-varying.

In this paper, based on a sequential Monte Carlo method, a computationally efficient algorithm is presented for blind data detection in the presence of residual phase noise generated at the output the phase tracking loop employed in a digital receiver. The basic idea is to treat the transmitted symbols as "missing data" and draw samples sequentially of them based on the observed signal samples up to time t. This way, the Bayesian estimates of the phase noise and the incoming data are obtained through these samples, sequentially drawn, together with their importance weights. The proposed receiver structure is seen to be ideally suited for high-speed parallel implementation using VLSI technology.

The use of linear precoding in bit-interleaved coded modulation with iterative decoding leads to a significant performance improvement on AWGN channels. In this contribution, we approach linear precoding from an information theoretical perspective. We compute the bitwise capacity as seen by each of the precoder inputs in order to investigate the impact of precoding for two scenarios: using (i) a non-iterative decoder and (ii) an iterative decoder under a perfect feedback assumption. When restricting to BPSK signaling, the bitwise capacity reveals simple precoding design rules for both scenarios. Simulation results confirm our analytical findings

This paper deals with joint data detection, synchronization and channel parameter estimation for direct-sequence code-division multiple-access systems over frequency-selective channels. In low-signal-to-noise ratio environments, conventional data-aided (DA) estimation algorithms may require an unacceptably large number of pilot symbols in order to obtain sufficiently accurate estimates of the channel and synchronization parameters. Especially, frequency offset estimation results in a significant loss in spectral efficiency. In this contribution, we consider several code-aided estimation schemes which can be incorporated in an iterative turbo detection scheme. We consider the expectation maximization algorithm, as well as the space alternating generalized expectation maximization algorithm as tools to develop code-aided estimation algorithms for a variety of scenarios. We pay special attention to the issue of computational complexity, and propose some complexity-reducing approximations. We show through computer simulations that the proposed code-aided estimation techniques considerably outperform their conventional DA counterparts.

In this paper, we present a novel code-aided joint synchronization and channel estimation algorithm for downlink multicarrier code-division multiple access. The expectation-maximization algorithm is used to locate the maximum-likelihood estimate of the channel impulse response, propagation delay, and carrier frequency offset. The estimator accepts soft information from the decoder in the form of a posteriori probabilities of the coded symbols, and can be interpreted as performing joint estimation and data detection. The performance of the proposed algorithm is verified through computer simulations. Impressive performance gains are visible as compared with a conventional data-aided estimation scheme.

This contribution deals with carrier frequency offset estimation for flat-fading multiple-input multiple-output (MIMO) channels. Both data-aided (using training symbols) and iterative code-aided (using the unknown coded symbols) estimation is considered. In both scenarios, maximum-likelihood (ML) frequency offset estimation involves solving a maximization problem with no closed-form solution. Since numerical calculation of the ML estimates is computationally hard, we derive a simple closed-form approximation. Simulation results indicate that the ML algorithm and the proposed reduced-complexity algorithm operate closely to the Cramer-Rao bound (CRB)

We compare the maximum achievable common signal to interference and noise ratio (SINR) target level Kmaxin the downlink of various code division multiple access (CDMA) systems with binary sequence sets. We derive an upper bound to Kmaxfor binary systems with symbol-aligned users, and we compare this upper bound to the value of Kmaxfor quasi-orthogonal sequences (QOS), Grassmannian signatures and binary Pados-Karystinos signatures. Secondly, we turn our attention to systems where the (binary) signatures of the users are displaced with respect to each other. In this context, we show that the value of Kmaxof improved random O(rthogonal)CDMA/OCDMA (O/O) can further be increased as compared to the original improved. O/O system (with square-root cosine rolloff pulses) by an appropriate choice of the chip pulse. In addition to this, improved O/O systems with fixed signature set can achieve values of Kmaxthat are as high as those in the improved random O/O system, while not having the drawback of a spread on Kmax. Applying a steepest descent search to such a fixed improved O/O system, can result in a system with a binary signature set that has even higher values for Kmaxwell beyond the upper bound on Kmaxfor systems with aligned chip pulses

We describe a class of low-complexity channel parameter and frequency offset estimation algorithms, applied to a space-time bit-interleaved coded modulation (ST-BICM) scheme for burst-mode asynchronous DS-CDMA uplink transmission over frequency selective channels. Conventional estimation algorithms rely mainly on the presence of training symbols within the data stream, reducing both the energy- and bandwidth efficiency. Our main goal is to remove the need for long training sequences by exploiting the presence of an underlying error-correcting code. We derive an iterative estimation technique, based on the space alternating generalized expectation maximization (SAGE) algorithm, which iterates between detection and estimation stages. Performance results are illustrated through computer simulations

This paper deals with space-time detection and channel parameter estimation in a bit-interleaved coded modulation (BICM) scheme for asynchronous direct-sequence code division multiple access (DS-CDMA) transmission over frequency selective channels. Using a pilot sequence to obtain sufficiently accurate channel estimates for this system requires an unacceptably large number of pilot symbols. In this contribution, we consider several code-aided estimation schemes which can be incorporated in an iterative space-time (ST) turbo detection scheme. We point out that the EM algorithm, which has recently been addressed as a convenient tool for code-aided estimation, has some serious drawbacks because of the large number of parameters to be estimated. These can be resolved using the space alternating generalized expectation maximization (SAGE) algorithm. We show through computer simulations that the proposed low complexity ST receiver with SAGE estimation considerably outperforms conventional estimation schemes.

Code-aided parameter estimation is considered for bit-interleaved coded modulation with iterative decoding (BICM-ID). The computational overhead caused by the estimation is minimized by embedding the parameter estimation into the iterative decoding. It is apparent that the performance of the estimation depends on the availability of accurate symbol a posteriori probabilities (SAPPs). Since the decoder computes bit a posteriori probabilities (BAPPs), the SAPPs are traditionally computed through a straightforward bit-to-symbol probability conversion. For advanced mappings which are optimized for BICM-ID, however, the latter method does not result in reliable SAPPs. Therefore, a new method is proposed and simulation results verify that the resulting iterative estimator-detector yields a close to perfectly synchronized performance.

The performance of multi-user direct-sequence code division multiple access (DS-CDMA) transmission over frequency selective, slowly fading channels strongly depends on the reliability of the channel parameter estimates: state-of-the-art detection algorithms that exploit multiple-access-interference and inter-symbol-interference require very powerful estimation algorithms. In this contribution we consider a system with bit-interleaved coded modulation (BICM) with an iterative detector consisting of minimum mean squared error (MMSE) interference cancellation, followed by a soft demapper and a MAP decoder. We derive an estimator that makes use of the a posteriori probabilities computed by the detector. This multi-user estimator is based on the iterative space alternating generalized expectation maximization (SAGE) algorithm. We show through computer simulation that the proposed receiver considerably outperforms conventional channel estimation schemes. Finally, the impact of different mappings in the BICM scheme is evaluated.

This contribution deals with symbol rate detection algorithms for multi-rate receivers. From the maximum likelihood criterion, we derive two blind rate detection algorithms: the first follows from a low SNR approximation of the likelihood function, while the second makes use of the expectation-maximization (EM) algorithm. We compare the proposed detectors with a cyclic correlation-based scheme from literature in terms of their rate detection error probability. The proposed ML-based algorithms turn out to substantially outperform the correlation-based algorithm.

We evaluate the symmetric capacity of the OCDMA/OCDMA (O/O) signature sets that maximize the sum capacity in quasi-scalable CDMA systems. It is found that the symmetric capacity is equal to the sum capacity for randomly scrambled O/O

We compare the sum capacity of strictly scalable and quasi-scalable CDMA systems. It is demonstrated that the requirement of strict scalability inevitably results in a capacity loss as compared to the optimal quasi-scalable OCDMA/OCDMA signature set.

We consider the sum capacity of code-division multiple-access (CDMA) signature-sequence sets in overloaded channels that contain an orthogonal subset of dimension N. We introduce a family of signature-sequence sets that consists of a hierarchy of orthogonal subsets (HOS), and show that various signature-sequence sets designed for channel overloading belong to this family. Assuming an equal average energy (EAE) constraint on the user signals, an expression for the sum capacity of the HOS family is derived. We point out that the maximum sum capacity over the HOS family is achieved by the multiple-orthogonal (m-O) CDMA sequence sets. The sum capacity of m-O is only slightly less than the upper bound imposed by the Gaussian multiple-access channel. Moreover, m-O is superior to a system with N orthogonal sequences and (K-N) pseudonoise (PN) sequences (PN/O), which, in turn, outperforms a system with K PN sequences. Both for PN/O and sequence sets over HOS, there is no additional loss in sum capacity if we require the sequences to consist of chips that are binary or quaternary valued.

Signature sets that maximize the sum capacity of an oversaturated code-division multiple-access (CDMA) system are derived, under the constraint that the signature sets have to be quasi-scalable. It turns out that the optimal signature set is composed of orthogonal signature sets, with at most one incomplete orthogonal set. As compared to the Gaussian multiple-access channel (GMAC), the loss in spectral efficiency for this multiple-orthogonal CDMA (OCDMA) system remains very low.

The paper deals with carrier frequency offset estimation for a flat-fading multiple-input multiple-output (MIMO) channel using a training sequence. The resulting maximum likelihood (ML) estimation entails solving a maximization problem with no closed-form solution. Since numerical calculation of the estimate is computationally hard, we propose a sub-optimal closed-form solution. In contrast with single-input-single-output (SISO) systems, however, self-noise arises in MIMO closed-form frequency offset estimation. Through proper training sequence design, we show how to avoid this self-noise and achieve a performance close to ML-performance and the Cramer-Rao bound (CRB).

This contribution deals with the problem of combining (non-binary) error-correcting codes with higher-order modulation schemes on AWGN and flat fading channels through interleaved coded modulation. We extend the idea of bit-interleaved coded modulation (BICM) to a more general form. With the aid of factor graph representations, we show how higher-order modulation can be combined with non-binary codes so that noniterative detection becomes optimal. This is in contrast with conventional BICM, for which non-iterative detection can be far from optimal. Through computer simulations, we compare an optimal non-iterative scheme with conventional BICM. It turns out that the gain due to optimal detection is outweighed by the loss in time-diversity as compared to BICM-ID.

The current paper addresses the issue of estimating the user propagation delays, received carrier phase offsets and received amplitudes in an asynchronous DS-CDMA environment with frequency non-selective propagation channels. The proposed synchronizer is based on the expectation-maximization (EM) algorithm and takes benefit from the soft information delivered by the receiver which is of the turbo type. The performance of the proposed synchronizer is illustrated by simulation results. In particular, the mean and the mean squared error of the estimator, as well as the bit error rate reached by the synchronized system, are reported.

This contribution deals with the comparison of the sum-product algorithm (SPA) and its log-domain version (log-SPA) for decoding LDPC (low density parity check) codes over general binary extension fields. For both algorithms, we determine their computational complexity based on the number of real-valued operations and investigate their sensitivity to quantization effects. Whereas the log-SPA yields the shorter decoding time in the case of binary LDPC codes, we point out that increasing the field size tends to favor the SPA, especially when a multiplication takes only slightly more time than an addition. Further, we show that log-SPA requires fewer quantization levels and suffers less from a quantization induced error-floor.

This contribution considers the joint estimation of the carrier phase and the frequency offset from a noisy linearly modulated burst signal containing random data symbols (DS) as well as known pilot symbols (PS). The corresponding Cramer-Rao lower bound (CRB) is derived. This bound indicates that it is potentially more accurate to estimate carrier phase and frequency from such a 'hybrid' burst than from a burst without PS or from the limited number of PS only. The new bound is then compared with the performance of new and existing carrier synchronizers. We present the iterative Soft-Decision-Directed estimator with combined Data-Aided/Nondata-Aided initialization, which performs closely to the CRB, and provides a large improvement over the classical Nondata-Aided estimator at low and moderate Signal-to-Noise Ratio.

This paper introduces a log-domain decoding scheme for LDPC codes over GF(q). While this scheme is mathematically equivalent to the conventional sum-product decoder, log-domain decoding has advantages in terms of implementation, computational complexity and numerical stability. Further, a suboptimal variant of the log-domain decoding algorithm is proposed, yielding a lower computational complexity. The proposed algorithms and the sum-product algorithm are compared both in terms of simulated BER performance and computational complexity.

We evaluate the sum capacity of the binary signatures of [G. N. Karystinos etal., (2001)(2003)] that achieve the lower bound on total squared correlation for binary signatures. Analytical expressions are derived for this sum capacity, and it is found that the restriction to binary-valued signatures can have a small but nonnegligible impact on the sum capacity.

This contribution proposes a new spatial mapping technique for bit-interleaved coded modulation (BICM) over multiple-input multiple-output (MIMO) channels. It is an extension of conventional BICM whereby the serial-to-parallel converter is replaced with a more general mapping strategy. Genie performance analysis of spatial mapping schemes results in very simple design criteria for the mapping function. Using an iterative detector, a significant performance gain is observed compared to a conventional BICM scheme, at no significant increase in computational complexity. The predicted performance gains are verified through computer simulations.

We discuss some extensions to the conventional OCDMA/OCDMA (O/O) system. First of all, synchronous O/O, defined for spreading gain N = 2/sup n/ is extended to any spreading gain that is a multiple of 4. In a second step, we consider the performance of O/O systems, where the signatures of the two orthogonal sets are displaced with respect to each other. It is found that this displacement can result in a significant performance improvement.

This paper considers the Cramer-Rao bound (CRB) related to estimating the carrier frequency of a noisy phase-shift keying signal. The following scenarios are discussed: 1) carrier frequency estimation irrespective of the carrier phase, based on either known or random data and 2) joint carrier phase and frequency estimation, based on either known or random data. Ideal symbol timing is assumed. We compare the results obtained from a (commonly used) simplified observation model against those resulting from the correct model. Because of the presence of nuisance parameters (random data and/or random carrier phase), the analytical computation of the corresponding CRBs is often not feasible. Here we present results that are based upon a combined analytical/numerical approach. Our results show that the choice of the observation model has essentially no effect on the CRBs at moderate and high signal-to-noise ratios. We also show that of the two scenarios considered, joint frequency and phase estimation yields the smaller CRB; the penalty resulting from frequency estimation, irrespective of the carrier phase, decreases with increasing observation interval.

This paper derives the Cramer-Rao bound (CRB) related to the estimation of the time delay of a linearly modulated bandpass signal with unknown carrier phase and frequency. We consider the following two scenarios: joint estimation of the time delay, the carrier phase, and the carrier frequency; and joint estimation of the time delay and the carrier frequency irrespective of the carrier phase. The transmit pulse is a bandlimited square-root Nyquist pulse. For each scenario, the transmitted symbols constitute either an a priori known training sequence or an unknown random data sequence. In spite of the presence of random data symbols and/or a random carrier phase, we obtain a relatively simple expression of the CRB, from which the effect of the constellation and the transmit pulse are easily derived. We show that the penalty resulting from estimating the time delay irrespective of the carrier phase decreases with increasing observation interval. However, the penalty, caused by not knowing the data symbols a priori, cannot be reduced by increasing the observation interval. Comparison of the true CRB to existing symbol synchronizer performance reveals that decision-directed timing recovery is close to optimum for moderate-to-large signal-to-noise ratios.

In this paper, we consider the effect of timing jitter on the performance of a multicarrier direct-sequence code-division multiple-access system for both uplink and downlink transmission, assuming orthogonal spreading sequences. Theoretical expressions are derived for the performance degradation caused by the timing jitter, in the presence of a multipath channel. Assuming an additive white Gaussian noise channel, perfect power control, and full load, it is shown that the performance degradation for the downlink transmission is independent of the number of subcarriers, of the spreading factor, and of the spectral contents of the jitter at the receiving mobile station, but only depends on the jitter variance. Under the same assumptions, we point out that, if the jitter spectra of all transmitting mobile stations are the same, the degradation on the uplink is the same as the degradation on the downlink.

The paper addresses the issue of estimating the sampling instant in turbo receivers. The proposed synchronizer is based on the expectation-maximization (EM) algorithm and takes benefit from the soft information delivered by the turbo system. Performance of the proposed synchronizer is illustrated by simulation results. In particular, the mean and the variance of the estimator as well as the bit error rate reached by the synchronized system are reported.

The paper deals with the design of an all digital multirate receiver with nonsynchronized IF-sampling and digital timing correction, that can be used in software defined radios. By performing timing correction prior to matched filtering, the complexity of the matched filter is reduced, but at the expense of additional aliasing. Design parameters, yielding low degradations with respect to synchronized baseband sampling, are provided.

This paper is devoted to turbo synchronization, that is to say the use of soft information to estimate parameters like carrier phase, frequency offset or timing within a turbo receiver. It is shown how maximum-likelihood estimation of those synchronization parameters can be implemented by means of the iterative expectation-maximization (EM) algorithm [A.P. Dempster, et al., 1977]. Then we show that the EM algorithm iterations can be combined with those of a turbo receiver. This leads to a general theoretical framework for turbo synchronization. The soft decision-directed ad-hoc algorithm proposed in V. Lottici and M. Luise, [2002] for carrier phase recovery turns out to be a particular instance of this implementation. The proposed mathematical framework is illustrated by simulations reported for the particular case of carrier phase estimation combined with iterative demodulation and decoding [S. ten Brink, et al., 1998].

This contribution considers the Cramer-Rao bound (CRB) related to the joint estimation of the carrier phase and frequency of a noisy linearly modulated signal with random data symbols, using the correct continuous-time model of the received signal. We compare our results with the existing results obtained from a (common used) simplified discrete-time model of the matched filter output samples, that ignores useful signal reduction and ISI caused by nonzero frequency offset. We show that both models yield the same CRB for phase estimation. As far as frequency estimation is concerned, we point out that: (i) the correct observation models yield the smaller CRB, and (ii) the difference between the CRBs resulting from the two models is apparent only at very small SNR. This indicates that at practical SNR values, the frequency offset affects the likelihood function mainly through the signal rotation (rather than signal magnitude) at the matched filter output.

In this paper, we consider the effect of a carrier phase offset on the performance of a low-density parity-check (LDPC) coded QAM modulated system. We investigate an ML-based carrier phase synchronization algorithm that makes use of the posterior probabilities of the data symbols, provided by the iterative decoder. The resulting carrier phase synchronizer is an extension, to LDPC coded systems, of the iterative phase estimator for turbo coded systems, presented in V.Lottici and M.Louse [2002], and has negligible BER performance degradation as compared to the ideally synchronized system.

We compare the achievable load of three types of overloaded signature sets with multistage successive interference cancellation: random spreading (PN), Welch-bound-equality (WBE) sequences and overall permuted OCDMA/OCDMA (o-O/O) or pseudonoise/OCDMA (PN/O). It is found that o-O/O outperforms PN and WBE for spreading factors N, varying from 16 to 256.

Two parallel interference cancellation (PIC) schemes, based on the probabilistic data association (PDA) approach are evaluated with respect to the allowable load. The N-dimensional PDA detector achieves an acceptable load that is more than three times higher than that of the one-dimensional detector. An improvement of these PIC schemes is proposed by replacing the a priori probabilities in every stage of the cancellation process by the a posteriori probabilities of the previous stage. This results in a critical load that is twice as high as that of the original PDA detector for the one-dimensional version of the PDA-PIC detector.

A parallel interference cancellation scheme, based on the iterated soft decision interference cancellation with optimized soft cancellation (ISDIC-OSC) procedure is improved dramatically by replacing the a priori probabilities in every stage of the cancellation process by the a posteriori probabilities of the previous stage. The introduction of a forgetting factor results in an additional gain in acceptable load. With this multiuser detector, loads close to unity can be achieved for PN-CDMA without significant loss as compared to the single user performance for a spreading factor of 256. Over a wide range of spreading factors, loads of about twice the acceptable load of the original ISDIC-OSC detector can be achieved with this refined detector.

This contribution considers the Cramer-Rao bound (CRB) related to phase-independent carrier frequency estimation from a noisy PSK signal. Instead of estimating the frequency jointly with the carrier phase, we treat the phase as a nuisance parameter. Ideal symbol timing is assumed. Both cases of known data (training sequence) and random data are considered. We show that frequency estimation irrespective of the carrier phase yields a larger CRB than does joint frequency and phase estimation; the penalty resulting from the former strategy vanishes with increasing observation interval.

In this contribution we consider the Cramer-Rao bound (CRB) for the estimation of the time delay of a noisy linearly modulated signal with random data symbols. In spite of the presence of the nuisance parameters (i.e., the random data symbols), we obtain a closed-form expression of this CRB for arbitrary PSK, QAM or PAM constellations and a bandlimited square-root Nyquist transmit pulse.

In this paper, we present a code-division multiple access (CDMA) system with a spreading factor of N that can accommodate up to mN users, where m/spl ges/2. The m sets of orthogonal spreading sequences are constructed by overlaying the same N orthogonal Walsh-Hadamard sequences with a set-specific pseudo-noise (PN) sequence. Interference between user signals from different sets is cancelled by means of soft-decision iterative interference cancellation. We show that this technique, which is referred to as m-OCDMA, can accommodate more users and is computationally less complex than conventional PN-CDMA with the same type of interference cancellation. Assuming BPSK transmission over an additive white Gaussian noise (AWGN) channel and perfect power control, a degradation of 0.4 dB at the BER of 10/sup -5/ in m-OCDMA is obtained with 2.56N users in the uplink or 3.03N users in the downlink. In PN-CDMA, this figure is 2.16N for both the uplink and the downlink.

We consider a multiple access system with oversaturation of the channel by means of a combined OCDMA/OCDMA scheme with scrambled orthogonal bases. We investigate the performance and the associated maximum channel capacity (channel load) for uncoded BPSK modulation with binary valued chip sequences and optimum multiuser detection for spreading factor N = 4, 8, 16 and 32. It is found that, for scrambling sequences that change randomly from symbol-interval to symbol-interval, the allowable channel overload is zero for N = 4, 8, 16, and is only about 3% for N = 32. Major improvements on the allowed channel capacity are achieved by selecting a 'good' scrambling sequence and keeping it the same for all symbol-intervals. In this way the allowed channel overload can be increased to 25% for N = 8, to 31.25% for N = 16 and to at least 31.25% for N = 32.

The sum capacity of the OCDMA/OCDMA (O/O) signature sequence set is investigated when the users of the two orthogonal sets are allowed to have a different average-input-energy constraint and with perfect synchronization of the users. It is found that the sum capacity is dependent on the load and the respective SNR only, but independent of the choice of the orthogonal sets. For a fixed total power and when the power constraints of the two user sets are allowed to be different, it is shown that the sum capacity of the O/O system can be made arbitrarily close to the sum capacity of the Gaussian multiple access channel.

We present a new encoded signaling concept which consists of transmitting an additional data rate of 50 % to the original data rate that is determined by the channel bandwidth, the modulation scheme, and other physical-layer functions of the transmission system. This is accomplished by overlaying an encoded version (using a rate 1/2 code with Hamming distance 4) of N/2 excess symbols to each block of N primary symbols. The resulting interference between primary and excess symbols is handled using a joint symbol detection technique with low complexity for N=8 and N=10. Our analysis shows that the transmission of the excess data rate involves an asymptotic gain in SNR of 0.8 dB as compared to conventional BPSK or QPSK signaling. Our simulation results indicate that this gain is noticeable at bit error rate (BER) values below about 10/sup -7/.

We study the effect of carrier phase jitter on the performance of both uplink and downlink multicarrier direct-sequence CDMA (MC-DS-CDMA). We determine the resulting performance degradation in terms of the various systems parameters. Assuming an AWGN channel, perfect power control and full load, it is shown that this degradation is independent of the number of carriers, of the spreading factor, and of the jitter spectrum shape, but only depends on the jitter variance.

In this paper, we study the sensitivity of uplink and downlink MC-DS-CDMA to carrier frequency offsets, assuming orthogonal spreading sequences. For both uplink and downlink MC-DS-CDMA, we show that the performance rapidly degrades for an increasing ratio of maximum frequency offset to carrier spacing. We point out that the degradation in the uplink is larger than in the downlink because only the former is affected by multiuser interference. For a given (small) ratio of maximum frequency offset to carrier spacing, enlarging the spreading factor in a fully loaded system does not affect the downlink degradation but strongly increases the uplink degradation. Finally, we show that the downlink degradations of MC-DS-CDMA and fully loaded MC-CDMA are the same, provided that for both systems the ratio of frequency offset to carrier spacing is the same.

This paper investigates the sensitivity of several multiple-access techniques to narrow-band interference. The analysis covers time-division multiple access (TDMA), code-division multiple access (CDMA), and orthogonal frequency-division multiple access (OFDMA). The study is carried out under the assumption that all the considered multiple-access systems occupy the same total bandwidth, and the bit rates of all active users are identical. A major finding of this study is that CDMA with pseudonoise spreading sequences is more sensitive to narrow-band interference than TDMA. We point out that the signal-to-jammer power ratio at the decision device input is in fact identical for both multiple-access techniques, but the amplitude distribution of the jammer term at the threshold detector input is more favorable to TDMA, which turns out to be more robust in terms of bit-error rate. Another finding is that in terms of sensitivity to narrow-band interference, orthogonal CDMA (OCDMA) is closer to TDMA than to CDMA with pseudonoise sequences, because the degradation is not the same for all users. Finally, we discuss the relationship of OCDMA and TDMA and highlight the superiority, in terms of capacity over the narrow-band interference channel, of TDMA to the other multiple-access techniques considered in this paper.

We present a new signaling concept which consists of transmitting an additional data rate to the original data rate that is determined by the channel bandwidth, the modulation scheme, and other physical-layer functions of the transmission system. This is accomplished by overlaying M excess symbols to each block of N primary symbols, with the excess symbol period being equal to N times the primary symbol period. The resulting interference between primary and excess symbols is handled using a joint symbol detection technique. Our analysis shows that for BPSK or QPSK modulation and M=1 or M=3, transmission of the excess data rate does not degrade the minimum Euclidean distance when the excess symbol ratio M/N does not exceed 25%. Furthermore, our simulation results indicate that at bit error rate (BER) values in the order of 10/sup -3/ to 10/sup -5/, transmission of the additional data in these cases is achieved at the expense of a moderate to very small penalty in signal-to-noise ratio (SNR).

We present a multiple access technique which accommodates more than N users on a channel whose bandwidth is N times the bandwidth of the individual user signals. It makes use of two sets of orthogonal signal waveforms, one for the first N users, and one for the additional users. An iterative multistage detection technique is used to cancel interference between the two sets of users. At each stage of the detection process, the best estimate of the multiuser interference (MUI) is synthesized using the decisions available from the previous stages, and this interference is subtracted from the user signals of interest before entering these signals to a threshold detector. The new concept is described using a combination of time-division multiple access (TDMA) and orthogonal code-division multiple access (OCDMA), but it is equally applicable to any other sets of orthogonal signal waveforms. Using a binary phase-shift keying modulation (BPSK), it is shown that the proposed technique supports an excess user rate of 40% at the expense of a very small degradation of the signal-to-noise ratio (SNR).

This paper gives an overview of a previously introduced channel overloading concept that is applicable to both multiuser communications and single-user communications. We first describe it for multiuser communications using a hybrid multiple access scheme based on a combination of time-division multiple access (TDMA) and code-division multiple access (CDMA). Next, we extend it to full CDMA by defining two sets of orthogonal signal waveforms. Using a binary phase-shift keying (BPSK) modulation, it is shown that this multiple access concept can increase the number of users by 40% with respect to the hard limit of orthogonal waveform multiple access (OWMA) which includes TDMA, orthogonal CDMA (OCDMA), and orthogonal frequency-division multiple access (OFDMA). Then, we apply the same concept to single-user communications to transmit an excess bit rate in addition to the primary data rate. It is shown that an excess data rate of up to 25% can be transmitted at the price of a very small penalty in signal-to-noise ratio (SNR).

We investigate the sensitivity of MC-CDMA systems to synchronisation errors. We show that the signal-to-noise ratio (SNR) of the MC-CDMA system at the input of the decision device in the presence of a carrier frequency offset or a clock frequency offset is a rapidly decreasing function of the number of carriers. For a maximal load, carrier phase jitter and timing jitter give rise to a degradation that is independent of the spectral content of the jitter; moreover, the degradation caused by carrier phase jitter is independent of the number of carriers. A constant carrier phase offset and a constant timing offset cause no degradation of the MC-CDMA system performance.

We investigate the influence of the carrier phase jitter of a phase-locked local oscillator on the performance of a multicarrier code-division multiple-access (MC-CDMA) system in a nondispersive channel. This carrier phase jitter degrades the signal-to-noise ratio at the input of the decision device. When all users have the same power level and phase jitter spectrum, it is shown that for the highest load, the degradation only depends on the jitter variance but not on the specific shape of the jitter spectrum nor on the number of carriers.

In mobile radio communication, the fading channels generally exhibit both time-selectivity and frequency-selectivity. Orthogonal frequency-division multiplexing has been proposed to combat the frequency-selectivity, but its performance is also affected by the time-selectivity. We investigate how various parameters, such as the number of carriers, the guard time length, and the sampling offset between receiver and transmitter, affect the system performance. Further, we determine the optimum values of the above parameters, which minimize the degradation of the signal to-noise ratio at the input of the decision device.

This paper deals with the derivation and comparison of some promising TDMA and OFDM/CDMA receiver structures for communication over the return path channel of the cable TV (CATV) network. Regarding TDMA, it is shown that the performance substantially improves by using equalization. Given the slowly varying, frequency selective nature of the ingress noise, OFDM seems an interesting possible access technique to employ. Here it is considered in combination with CDMA. With respect to the performance in terms of the SNR at the input of the decision device, the TDMA structure gives better results than the OFDM/CDMA schemes. Also with respect to the design complexity TDMA seems more interesting.

This letter shows that the synchronization error resulting from a timing recovery loop, operating on nonsynchronized samples of a noisy sinusoid, contains periodic components. This periodic timing jitter is caused solely by nonideal interpolation between the nonsynchronized signal samples and is absent when using synchronized sampling. The results of our linearized tracking analysis are confirmed by computer simulations.

We analyze the cycle-slipping behavior of the Mth-power nondecision-aided (NDA) feedforward carrier synchronizer for M-ary phase-shift keying (MPSK). The averaging filter is a block window accumulator, and postprocessing is used to eliminate equivocation. An asymptotic (high E/sub s//N/sub o/) expression for the cycle-slip probability is derived and verified by computer simulation. A "hybrid" approach, combining analytical results with importance sampling simulation techniques, allows for the fast and accurate determination of the cycle-slip probability for all E/sub s//N/sub o/ values of practical interest.

We consider the Cramer-Rao bound (CRB) for the estimation of a scalar parameter in the presence of nuisance parameters. Whereas this true CRB is hard to evaluate in general, we present a simple analytical expression for its high SNR asymptote, i.e., the asymptotic CRB (ACRB). We show that this ACRB is related to the CRB for the joint estimation of the scalar parameter and the nuisance parameters. Previously, a modified CRB (MCRB) for the estimation of a scalar parameter in the presence of nuisance parameters has been derived. This MCRB is also simple to evaluate and is related to the CRB for the estimation of the scalar parameter assuming that the value of the nuisance parameters is a priori known. We show that the MCRB can be quite loose at high SNR, when the scalar parameter is coupled with the nuisance parameters. In the case of synchronization parameter estimation, we find that the ACRB equals the MCRB, whether or not (some of) the nuisance parameters are a priori known.

We investigate the sensitivity to carrier phase jitter of an orthogonal frequency-division multiple-access (OFDMA) system. When all OFDMA carriers have the same power level and jitter spectrum, the degradation caused by the jitter is shown to be equal to the degradation of an OFDM system. Also, traditional FDMA is found to be slightly more robust than OFDMA.

The claims in the comment of Popken (see ibid., vol.45, no.1, p.19-20, 1997) are wrong. We point out that the discrepancy of the cycle slip rates obtained in our paper and that of Meyr, Popken and Mueller (see ibid., vol.COM-34, no.5, p.436-45, 1986) is not due to a misconception in our paper, but rather to a different modeling of the loop noise. In order to obtain cycle slip rates by means of Fokker-Planck techniques, Meyr et al. were forced to use an oversimplified model, yielding essentially flat loop noise. In our paper, cycle slip rates were obtained by means of level crossing techniques, while preserving the smooth narrowband nature of the loop noise.

Presents a simple approximate expression for the BER degradation, caused by random carrier phase and timing errors, of the maximum-likelihood (ML) receivers operating on narrowband M-PSK and bandlimited direct-sequence/spread-spectrum (DS/SS) M-PSK signals; this expression is very accurate for (small) degradations of practical interest, and reveals that the BER degradation (in dB) is essentially proportional with the tracking error variance. In the absence of ISI at the optimum decision instants, carrier phase errors yield exactly the same degradation for narrowband M-PSK as for bandlimited DS/SS M-PSK. When the product of the r.m.s. timing error and the minimum signal bandwidth is held constant, the degradation for bandlimited DS/SS M-PSK is considerably smaller than for narrowband M-PSK, especially for large values of M and small values of the rolloff.>

Two maximum-likelihood (ML)-oriented data-aided (DA) symbol synchronization algorithms are investigated in this paper. They operate on a noisy M-PSK signal (M>2) transmitted over a slowly time-varying nonselective Rician fading channel. The first algorithm is channel-aided (CA): it makes use of an estimate or the instantaneous channel gain. The second algorithm is nonchannel-aided (NCA): timing is acquired independently of the channel gain estimation. Both algorithms operate either with a known training sequence or in a decision-directed mode, and are suitable for fully digital implementation. We find that the CA algorithm yields a smaller tracking error variance than the NCA algorithm, but the difference in performance decreases with increasing carrier-to-multipath (C/M) ratio.>

In this paper we investigate the tracking performance or two maximum-likelihood (ML)-oriented nondecision-aided (NDA) symbol synchronization algorithms, operating on a noisy M-PSK signal (M>2) transmitted over a time-varying nonselective Rician fading channel. The first algorithm is nonchannel-aided (CA): timing is acquired independently of the channel gain estimation. The second algorithm is channel-aided (CA): it makes use of (an estimate of) the instantaneous channel gain. As the channel gain estimator requires timing information, use of the CA algorithm implies that the timing and the channel gain must be acquired jointly. Both algorithms are suitable for fully digital implementation, and have a similar complexity. For E/sub 3//N/sub 0/ values of practical interest, we show that the NCA algorithm is to he preferred over the CA algorithm.>

In this contribution the transmission of M-PSK and M-QAM modulated orthogonal frequency division multiplexed (OFDM) signals over an additive white Gaussian noise (AWGN) channel is considered. The degradation of the bit error rate (BER), caused by the presence of carrier frequency offset and carrier phase noise is analytically evaluated. It is shown that for a given BER degradation, the values of the frequency offset and the linewidth of the carrier generator that are allowed for OFDM are orders of magnitude smaller than for single carrier systems carrying the same bit rate.>

Investigates the bit error rate (BER) performance of a passband single-carrier QAM transceiver for asymmetric digital subscriber lines (ADSL), with additive white Gaussian noise (AWGN) and self far-end crosstalk (FEXT) as the main disturbances. The major difference with previous contributions on ADSL performance is that the authors present a simple approximate analytical expression fur the signal-to-noise ratio (SNR) margin, which clearly shows the effect of various cable and system parameters (such as loop length, transmit power, QAM constellation size, bit rate, ...) on ADSL performance. From this expression, they determine the optimum constellation size and the maximum achievable bit rate as a function of the loop length.>

Discusses the carrier synchronization performance of a fully digitally implemented receiver, operating on a offset-QPSK (OQPSK) signal. OQPSK signals can be demodulated either by a parallel or a serial receiver structure. Both types of receiver structures are examined. The considered digital receivers operate on samples of the complex envelope of the received signal, taken by a free running clock which is not synchronized to the transmitter clock. The synchronized samples necessary for the carrier synchronization algorithm are computed by interpolating between the available non-synchronized samples. Because of finite memory, interpolation is non-ideal; hence, some amount of distortion is introduced, which affects not only the performance of the decision device but also the carrier synchronizer. By means of theoretical analysis, the authors investigate the tracking performance of a specific decision-directed feedback carrier synchronizer, assuming interpolation of orders zero, one and two.

Investigates a maximum-likelihood (ML) decision/data-aided (DA) channel-aided (CA) symbol synchronization algorithm operating on a noisy M-PSK signal transmitted over a slowly time-varying nonselective Rician fading channel. Each of the channel gain estimates used by the DA CA algorithm is held constant over a frame of F symbols. Compared to the hypothetical case where perfect channel gain estimates are available, the tracking performance of this algorithm is degraded by these imperfect channel gain estimates. The authors investigate how the linearized timing error variance depends on the channel characteristics (carrier-to-multipath ratio C/M, fading bandwidth) and on the frame length F. They show that the value of F that minimizes the tracking error variance, decreases with increasing fading bandwidth and decreasing C/M.

This paper deals with the bit error rate (BER) performance of a fully digitally implemented receiver performing coherent detection on a narrowband BPSK or (O)QPSK signal, transmitted over a slow non-frequency selective Rician fading channel. The considered digital receiver operates on samples of the received signal, taken by a fixed clock which is not synchronized to the transmitter clock. Signal samples needed for detection of the information sequence are obtained from interpolation between the available samples. In the case of nonideal interpolation a BER degradation occurs. By means of theoretical analysis, we demonstrate that even simple interpolators, operating at only a few samples per symbol, yield BER degradations that are limited to a small fraction of a dB. This paper is an extension of the work of Erup, Gardner and Harris (see ibid., vol.41, no.6, p.998-1008, 1993) where only the AWGN channel was considered and results were obtained by computer simulations.>

We point out that the nondecision-aided (NDA) carrier synchronizer, maximizing the low E/sub s//N/sub o/ limit of the likelihood function averaged over a general 2/spl pi//N-rotationally symmetric signal constellation, reduces to the familiar timing-aided Nth power synchronizer; this extends a result of D'Andrea, Mengali and Reggiannini (1988) where only M-PSK constellations have been considered. Whereas in the case of M-PSK the tracking error variance of this NDA ML synchronizer is known to converge to the Cramer-Rao bound (CRB) with increasing E/sub s//N/sub o/. We show that for other rotationally symmetric constellations (such as QAM) the tracking error variance is substantially larger than the CRB.>

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.>

A multiple-hopes-per-symbol fast-frequency-hopped spread-spectrum multiple-access (FFH-SSMA) system for indoor radio communication is treated. The data are Reed-Solomon (RS) encoded and then modulate a carrier by means of M-ary frequency-shift-keying (MFSK) The indoor radio channel is described as a multipath (Rayleigh or Rician) slowly fading channel. The receiver uses predetection diversity by linearly combining the squared enveloped of the different hops of the same MFSK symbol. The bit error rate (BER) performane is evaluated, and how the performance is influenced by various parameters, such as the number of hops per symbol, the size of the symbol alphabet, the ratio of specular power to fading power, and the ratio of nonreference user power to reference user power, is investigated.>

The authors derive close upper and lower bounds on the average bit error probability for hybrid direct-sequence/slow-frequency-hopped spread-spectrum multiple-access (DS/SFH-SSMA) systems with noncoherent DPSK demodulation, using predetection diversity (selection combining and equal gain combining) in conjunction with interleaved channel coding (Hamming

The authors deal with two types of feedforward carrier-independent non-decision-aided (NDA) symbol synchronizers for PAM, which are based upon the maximum-likelihood criterion. Their timing error variances are compared, for two important classes of signal constellations.>

The authors investigate the bit-error-rate (BER) performance of hybrid direct-sequence/slow-frequency-hopped spread-spectrum multiple-access (DS/SFH-SSMA) systems operating over a multipath Rician-fading channel (which models indoor radio propagation in factories). They consider both phase-shift-keying (PSK) modulation with coherent demodulation and differential phase-shift-keying (DPSK) modulation with noncoherent demodulation. Predetection multipath diversity (maximal ratio combining for coherent reception and equal gain combining for noncoherent reception) and simple interleaved channel coding are employed for improving the BER performance. The BER of both coherent and noncoherent hybrid systems is obtained using a Gaussian interference approximation.>

The authors study the problem of maximum-likelihood sequence estimation and synchronization from samples of the output of a matched filter taken at integer multiples of the symbol rate, which is assumed perfectly known by the receiver. A general analysis is presented of sampled receivers that handle arbitrary baseband pulse shapes and arbitrary sampling rates. It is observed that the optimal processing of the matched-filter samples consists of digital interpolation, followed by symbol-by-symbol decoding when sampling is at (or above) the Nyquist rate or Viterbi decoding when sampling is below the Nyquist rate. Performance is studied through the Cramer-Rao bound on mean-square estimation error and a lower-bound on error-probability.>

The carrier synchronization and symbol timing recovery for two-dimensional trellis-code modulated signals are considered. For many popular carrier and clock synchronizers, and tracking and cycle-slipping performance depends on the trellis code only through the second- and fourth-order moments of the coded channel symbols. For three successively more complicated classes of coded symbol alphabets, simple conditions on the trellis code are derived which are sufficient to yield the same second- and fourth-order moments as for uncoded symbols. For trellis codes satisfying these conditions, the synchronizer performance does not depend on the specific code, and is the same as for uncoded symbols. These sufficient conditions are fulfilled by many codes which have been presented in the literature, some of which are discussed.>

A private communication system for two-wire full-duplex transmission at an information rate of 144 kb/s that uses time compression multiplex (TCM) over short cables is considered. A low-cost receiver without pulse-shaping prefilter or equalizer that derives its timing from the estimated start-bit arrival times is presented and analyzed. The worst-case analysis involves the derivation of the timing error distribution and the bit error rate, along with an accurate approximation for the timing error variance. It is shown that the bit error rate degradation due to timing errors can be limited to less than 1.5 dB in signal-to-noise ratio by using a recursive start-bit detection algorithm instead of a conventional memoryless algorithm.>

The cycle slipping in synchronizers subject to smooth narrowband loop noise is discussed. Such loop noise is shown to occur in repeater chains and in ring local area networks. The average rate of cycle slips and of bursts of cycle slips is evaluated, and closely approximated by a very simple expression that clearly indicates the influence of the loop noise bandwidth and the phase-detector characteristic. A comparison to the case of white loop noise reveals that, for most phase-detector characteristics, the slip rate caused by smooth narrowband loop noise is larger by several orders of magnitude than for white loop noise.>

This paper deals with the problem of symbol timing recovery at the subscriber end of a full duplex two wire digital subscriber loop employing echo cancellation. Both nondecision-aided and decisionaided synchronization are considered. It is demonstrated that the presence of a residual echo signal can substantially degrade the synchronizer performance. This degradation is shown to be larger for nondecisionaided synchronizers than for decision-aided synchronizers. The smaller the synchronizer bandwidth and the signal-to-echo ratio, the more the decision-aided synchronizer outperforms the nondecision-aided synchronizer.

In this paper, we evaluate the error probability of a PAM receiver, which derives symbol timing from the received waveform by means of a synchronizer. We point out that the conventional way (using the distribution of the synchronizer's modulo-2π- reduced phase error) of evaluating this error probability does not take into account the effect of the synchronizer's cycle slips. We present a correct expression for the error probability, which uses the distribution of the so-called renewal phase. Also, we derive simple but accurate approximations for both the conventional and the correct error probability, which clearly show the influence of the characteristics of the PAM receiver. These approximate expressions indicate that, for a decreasing additive noise level at the receiver input, the conventional error probability becomes vanishingly small, whereas the correct error probability approaches a nonzero value, which is inversely proportional to the mean time between slips. Hence, cycle slipping imposes a (sometimes rather large) lower limit on the attainable error probability.

The paper considers a class of continuous ARQ strategies, whereby multiple copies of each data block are sent contiguously (instead of one single copy), and whereby the data blocks are delivered at the receiver side in their order of arrival at the transmitter. In order to achieve the best possible throughput efficiency-within this class-an optimum value for the number of copies to be sent is determined, in terms of the block error probability and the propagation delay. It turns out that, depending on these parameters, the optimum scheme is one of the two following alternatives: 1) sending each data block repeatedly until a positive acknowledgment is received for it; 2) sending each block a constant (optimum) number of times. A throughput comparison of the optimum scheme with other related ARQ strategies is given.

This paper deals with a generalized stop-and-wait ARQ scheme, whereby data blocks are sent to the receiver in sets containing an arbitrary number of identical copies. The main idea behind this scheme is a reduction of the idle time of the transmitter.

The spectral density of the loop noise in the equivalent loop model of a symbol synchronizer depends on the phase error. Although this dependence has only a minor influence on the linearized mean-square synchronization error, we demonstrate that it has a major effect on the mean cycle slip time of the symbol synchronizer. A simple approximate expression for the mean cycle slip time is presented, by means of which this effect can be estimated quite accurately.

In this paper we derive a fundamental lower bound on the linearized mean-square phase and time delay errors resulting from a practical synchronizer operating on a carrier-modulated PAM waveform which is corrupted by stationary Gaussian noise. Our bound applies to a large class of synchronizers and, unlike the linearized performance itself, it is easily evaluated from the modulation format, the baseband PAM pulse shape, the additive noise spectrum, and the synchronizer's closedloop transfer function matrix.

This paper deals with a general class of PCM formats, which includes the well-known NRZ, Manchester, and Miller coding. We show that the Cramér-Rao lower bound on the mean-square error, resulting from a synchronizer which estimates the delay of a noisy PCM waveform, is inversely proportional to a quantity which depends on the PCM format only. This quantity is termed the "synchronizability" of the PCM format. For the general class of PCM formats we consider in this paper, we obtain as a simple result that the synchronizability equals four times the mean number of zero-crossings of the PCM waveform during one symbol period. Numerical results are presented for NRZ, Manchester, and Miller coding.

In this paper we consider a data-aided and a non-data-aided synchronizer, which are both based upon the maximum-likelihood estimation principle. For both synchronizers, the linearized mean-square synchronization error, the mean acquisition time, and the acquisition range are evaluated in the case of NRZ-PAM. We demonstrate that the linearized mean-square synchronization error is very close to the theoretical lower bound, and about 10 dB smaller than for a conventional synchronizer with the same closed-loop bandwidth. This superior tracking performance is confirmed by computer simulations. Comparing the acquisition times on the basis of equal acquisition ranges, we find that the non-data-aided ML synchronizer is about a factor of 2 faster than the conventional synchronizer. For the data-aided ML synchronizer, the result of the comparison depends highly on the amount of frequency detuning.

In this paper we derive a tight upper bound on the symbol error probability of a PAM receiver, consisting of a matched filter, a sampler which is subject to symbol timing errors resulting from a synchronizer, and a Viterbi detector. Our results pertain to four types of symbol synchronizers which have recently been treated in literature, and whose influence on the error probability has not been examined before. For both cases of NRZ-PAM and a cosine rolloff spectrum, we present graphs of the upper bound versus the receiver signal-to-noise ratio (SNR), with the synchronizer bandwidths as a parameter. From these graphs the design engineer directly reads the synchronizer bandwidth that is needed to achieve the desired error probability at the specific receiver SNR. Finally, we point out when an infinite SNR yields either a zero or a nonzero error probability.

In the above paper several strategies for joint phase and timing recovery have been investigated. Here we point out that some results from the paper about the mean-square error method for one-dimensional modulation are faulty.

For most symbol synchronizers operating on a PAM waveform corrupted by noise, the timing error consists of a noisedependent term and a data-dependent term. When the signal-to-noise ratio (SNR) is high, the data-dependent term, which is often called self-noise, dominates, and the timing error variance becomes independent of SNR. In this paper we present two types of symbol synchronizers for which self-noise is absent: the modified maximum-likelihood (MML) synchronizer and the minimum mean-square error (MMSE) synchronizer. As a consequence of the absence of self-noise, their timing error variances at high SNR are inversely proportional to the SNR. The performances of these synchronizers are evaluated and compared with the Cramér-Rao bound.

This paper presents an expression for the optimum closed-loop transfer function of a phase-locked loop (PLL) which estimates the phase of an unmodulated sinuosoid corrupted by an additive random disturbance. This matter has been extensively treated (e.g., [1, ch. 4]) in the conventional PLL theory, where the disturbance is a stationary noise process. However, for an important class of synchronizers operating on a PAM waveform corrupted by stationary noise, the disturbance at the input of the PLL cannot be treated as a stationary process. In this case we show that the PLL transfer function which is optimized according to the conventional PLL theory is far from optimum. Therefore, we derive the closed-loop transfer function which is optimum for a more general disturbance. This extension of the conventional PLL theory applies to most synchronizers.

We show that the synchronizer described in the above paper is not a maximum a posteriori (MAP) synchronizer, as it searches the minimum instead of the maximum of the log likelihood ratio. Only a slight modification is required to transform it into a MAP synchronizer. By a theoretical analysis, we show that this modification substantially reduces the rms timing error.

In this paper we present a lower bound on the linearized mean-square timing error resulting from a practical synchronizer estimating the delay of a baseband PAM waveform which is corrupted by stationary Gaussian noise. This lower bound is easily evaluated, and applies to a broad class of synchronizers.

Most practical synchronizers operating on a PAM waveform corrupted by additive noise can be analyzed by means of the theory of the phase-locked loop (PLL). In this paper, we present a class of synchronizers for which the equivalent phase detector characteristic is such that the timing instants generated by the voltage controlled clock (VCC) are unbiased with respect to the ideal sampling instants to be used in the data reconstruction path. As a consequence of this, the VCC output signal can directly activate the sampler in the data reconstruction path, without being properly delayed for bias compensation.

The method for evaluating the tracking performance in the above paper obliterates the contribution to the mean square error which is due to the cyclostationarity of the disturbances. Beside this, the pattern-dependent jitter has not been considered in the paper. Our results include the effect of cyclostationarity and the pattern-dependent jitter.

In the above paper, the linearized mean-square phase error resulting from a digital PLL has been related to the ratio of the power of the spectral line in the input at the VCO frequency and the value of the noise spectrum at the same frequency. We show that this is fundamentally incorrect.

This paper presents a general method for evaluating the linearized tracking performance of phase-locked loops (PLL's) estimating the phase of an unmodulated sinusoid corrupted by an additive disturbance which is described in terms of cyclostationary processes. Such disturbances are frequently encountered in carrier modulated systems where the sidebands are responsible for the error in estimating the carrier phase, and in baseband pulse amplitude modulation (PAM) systems where the timing error is caused by the so-called pattern-dependent jitter. The tracking performance has already been determined in the case where the disturbance is band limited [12]. In this paper, a general solution which applies to both cases of carrier extraction and clock synchronization is derived. One might be tempted to replace the disturbance by a stationary process with the same power spectral density, and then evaluate the linearized tracking performance by means of conventional linear PLL theory. It is shown that the mean-square phase error can be severely overestimated by applying this procedure.

In a PAM system with unknown carrier phase and timing phase the receiver has to estimate these parameters. The "dataaided" approach involves the joint estimation of the data symbols and the unknown parameters. This is in contrast with the "nondataaided" approach, where the carrier phase and timing phase are estimated independently of the data symbols. We first establish the equivalence of these two strategies for high signal-to-noise ratios. Then we demonstrate that several PAM systems (baseband, VSB, QAM) perform better when the data symbols are independent equiprobable bits, instead of independent identically distributed Gaussian random variables (a high signal-to-noise ratio is assumed). Finally, some generalizations of the previous conclusions are treated.