Phase selection design for reconfigurable intelligent surfaces (RISs) is a significant research challenge, as a closed-form optimal solution for a multi-user (MU) system is believed to be intractable. While existing methods achieve strong near-optimal performance, they typically entail high computational complexity. In this work, we take a different approach and propose a practical method that achieves competitive performance while substantially reducing computational complexity. To do so, we consider a RIS divided into subsurfaces. Each subsurface is designed specifically for one user, who is served on their own frequency band. The other subsurfaces (those not designed for this user) provide additional uncontrolled scattering. We derive the exact closed-form expression for the mean signal-to-noise ratio (SNR) for the proposed subsurface design (SD) when all channels experience correlated Ricean fading. We simplify this to find the mean SNR for line-of-sight (LoS) channels and channels experiencing correlated Rayleigh fading. An iterative SD (ISD) process is proposed, where subsurfaces are designed sequentially, and the phases that are already set are used to enhance the design of the remaining subsurfaces. This is extended to a converged ISD (CISD), where the ISD process is repeated multiple times until the SNR increases by less than a specified tolerance. The ISD and CISD both provide a performance improvement over SD, which increases as the number of RIS elements increases. The SD is significantly simpler than the lowest complexity MU method we know of, and despite each user having less bandwidth, the SD outperforms the existing method in some key scenarios. The SD is more robust to strongly LoS channels and clustered users, as it does not rely on spatial multiplexing like other MU methods. Combined with the complexity reduction, this makes the SD an attractive phase selection method.

A key challenge facing reconfigurable intelligent surfaces (RISs) is channel state information acquisition. Passive RISs cannot generate pilot signals or process data, making rapid temporal changes in the channel problematic. Additionally, the impact of spatial changes in RIS channels has not been thoroughly investigated. Therefore, in this work, we use second order statistics to investigate the spatio-temporal behaviour of a single-user (SU) RIS system. Assuming a line-of-sight (LoS) RIS to base station (BS) link, we derive an exact expression for the level crossing rate (LCR) of the RIS link (user equipment (UE)-RIS-BS path) and propose a numerically stable approximation for the LCR of the global UE-BS channel. Each LCR expression attained is then utilised to find the corresponding average fade duration (AFD). The temporal signal-to-noise ratio (SNR) correlation is also derived assuming an LoS RIS-BS link. Assuming a Ricean RIS-BS link, expressions for the spatial correlation matrix of the global channel and the mean SNR loss due to channel ageing are derived. All of the analyses are verified by simulation, and the impact of key system parameters is investigated. We show that the use of an RIS does not significantly amplify changes in the channel.

We analyse the level crossing rate (LCR) of an uplink single-user (SU) reconfigurable intelligent surface (RIS) aided system. It is assumed that the RIS to base station (RIS-BS) channel is deployed as line-of-sight (LoS), and the user (UE)-RIS and UE-BS channels are correlated Rayleigh. For the optimal RIS reflection matrix, we derive a novel and exact analytical LCR expression for when the direct (UE-BS) channel is blocked, i.e. the RIS-only channel. Also, the existing exact expression for the direct-only channel (equivalent to classical maximal-ratio-combining (MRC)) suffers from extreme numerical precision problems when the BS has many elements. Therefore, we propose a new stable and accurate approximation to the LCR of the direct channel. The approximation is based on replacing any small similar eigenvalues of the channel correlation matrix by their average. We show that increasing the number of elements at the RIS or BS and decreasing channel correlation makes the LCR drop more rapidly for thresholds away from the mean SNR. Crucially, we find that RIS systems do not significantly amplify temporal variations in the channel. This is particularly beneficial for RIS systems considering the difficulty in acquiring channel state information (CSI).

In this paper, we analyse the performance of a reconfigurable intelligent surface (RIS) aided system where the RIS is divided into subsurfaces. Each subsurface is designed specifically for one user, who is served on their own frequency band. The other subsurfaces (those not designed for this user) provide additional uncontrolled scattering. A new subsurface RIS design is developed based on the optimal single-user design for a pure line-of-sight (LoS) base station (BS) to RIS channel. This is also extended to arbitrary BS-RIS channels. For our method, exact closed form solutions for the mean SNR and a mean rate upper bound are derived for the BS-RIS LoS scenario. For each user, the designed subsurface performs optimally in LoS conditions and is remarkably robust to non-LoS conditions. The system design drives down complexity to extremely low levels, reducing RIS design and receiver processing complexity and reducing the channel estimation requirements. We also quantify the complexity-performance trade-off for the new design relative to multi-user approaches.

We evaluate the performance of a low-power narrowband system utilising higher-order sectorisation (HOS) at the base stations (BS) and directional antennas (DA) at fixed user equipment (UE). We use a specific rotational frequency reuse scheme (RFR) for directional UEs and extend it for HOS (3, 6 and 12 sectors) by considering antenna orientations for interference avoidance. We propose two novel schemes for directional UEs with a cluster size less than one, named double frequency reuse (DFR) and triple frequency reuse (TFR). These schemes are tested and compared to conventional frequency reuse (CFR) through simulations in a realistic geographic terrain. The simulation results indicates that the proposed schemes are helpful to achieve higher capacity while using different BS and UE antenna beamwidths. It is shown that scheme RFR provides a performance enhancement compared to CFR only with wider UE antenna beamwidths. The implementation of DFR achieves reduced outage probability and offers a large increase in average spectral efficiency per cell (ASE) of up to 84.8-81.6% in 6 and 12 sectored system. The scheme TFR provides a maximum ASE of up to 149-153% in 6 and 12 sectored system. However, in a six-sectored system, TFR only meets the coverage criteria with a narrow beamwidth antenna of 20°.

Millimeter-wave channel measurements for a meeting room, lecture room and open plan office floor were used to analyse and model the spatial consistency of channel clusters. Particularly, how the angles of arrival and delays of extracted multi-path components in each cluster varied with small changes in receiver location. We extended the KPowerMeans algorithm, for classifying captured multi-path components into clusters from the delay/angular domains, to include the location domain to allow the spatial consistency of the clusters between locations to be analysed. The observed spatial consistency in measurements was then used to validate whether the 3GPP spatial consistency procedure (3GPP SC-I) reflects real world spatial consistency. The 3GPP spatial consistency procedure for ensuring spatial consistency of cluster parameters during mobile user simulations was then applied for each environment, allowing for comparison between the ‘predicted’ cluster parameters and the measured cluster parameters. The 3GPP model/procedure showed a good fit for all environments but highlighted the impact of the environment on the amount of accuracy of the procedure.

Continuous sensing and transmission by secondary users (SUs) in cognitive radio (CR) allows efficient use of available spectrum. In this approach, the SU receiver continuously senses the spectrum while it transmits in the same frequency band. Leveraging this advantage, we study the performance of error vector magnitude (EVM) based detection to detect the reappearance of the primary user (PU) signal during ongoing SU data transmission. We exploit the pilot tones that are inherent to many OFDM based standards to measure the EVM. We consider two models of PU namely an unknown deterministic signal and a Gaussian random signal with additive white Gaussian noise (AWGN). We derive an analytical form of the probability density function (PDF) of the EVM based detector based on the exact representation of Chi-square distributions. We analyze the performance of our detector by deriving the type I (false positive) and type II (false negative) error probabilities. Simulation results show that the EVM based approach achieves better performance than energy detection.

Fifth generation cellular networks must support dense user environments where close proximity users and highly correlated channels are prevalent. This study presents a novel decorrelating zero‐forcing (DZF) scheme. It improves downlink zero‐forcing (ZF) performance over multi‐user multiple‐input multiple‐output channels when highly correlated users are present. DZF groups correlated users into pairs where the stronger user is labelled ‘near’ and the weaker user, ‘far’. For the far and unpaired users, the proposed design is similar to conventional ZF (CZF) in that the base station performs traditional ZF and the users employ the leading left singular vectors of the channel for processing. In contrast, the near users employ the second strongest left singular vector which decorrelates their effective channel with the paired far user. This reduces the noise inflation inherent in ZF for correlated channels. From simulations and analysis the authors demonstrate that DZF has both rate and fairness advantages over CZF when highly correlated users are present. Furthermore, they design a hybrid ZF (HZF) scheme, which harnesses the advantages of both CZF and DZF, hence providing robustness against the joint scheduling of semi‐orthogonal and highly correlated users with very little additional complexity. Simulations clearly demonstrate that HZF has higher rates than CZF and DZF.

In cognitive radio (CR) systems, secondary users (SUs) equipped with a single transceiver are unable to sense and transmit simultaneously. Due to this limitation, most of the models intended to describe spectrum sensing focus on pre‐transmission sensing and consider that primary users (PUs) only change their activity state in the beginning of each SU's operation cycle. This study characterises the more practical scenario which considers the reappearing PU during ongoing SU communication. The authors study a quickest detection scheme that uses an error vector magnitude (EVM) of the received signal. They consider two models of PUs namely an unknown deterministic signal and a Gaussian random signal with additive white Gaussian noise. The probability density function of the EVM test statistic is derived for both cases. The authors develop an exact quickest detection scheme by using a traditional cumulative sum (CUSUM) test. They show that CUSUM EVM outperforms CUSUM ED using theoretical analysis and numerical simulations.

Non-orthogonal multiple access based zero-forcing beamforming (NOMA-ZFBF) systems require both semi- orthogonal users (SU) in beams and highly correlated (HC) users within a beam to maximize the spectral efficiency of the system. Hence, channel models where different users are likely to experience both similar and near-orthogonal channel vectors are likely to give good NOMA performance. In order to explore this, we analyze the variance of the interference power between two users, where a large variance indicates this channel behavior is more likely. Moreover, we have compared the Rician and Rayleigh channels in correlated and independent and identically distributed (I.I.D.) environments in order to provide some new insights into the role of line- of-sight (LoS) and correlated components with respect to NOMA beam formation and clustering of HC users. Analysis and numerical results show that correlated channels with similar channel statistics result in a higher variance that in turn increases the cluster percentage and enhances the cluster rate.

In the communication systems that use a linear modulation scheme for transmission, the fractionally-spaced (FS) samples of the received signal constitute a wide-sense cyclostationary time series. Hence, the standard Fourier transform techniques cannot be used to study the spectral characteristics of the received FS samples or to derive the transfer function (TF) of the corresponding digital minimum mean-square error (MMSE) receiver. In this paper, an analytical expression for the TF of the FS MMSE equalizer is derived, which includes the effects of the continuous-time to discrete-time (C/D) converter used at the receiver front end. Using this TF, the sources of instability of the FS least-mean-square (LMS) algorithm and the effects of the equalizer length and sampling phase on convergence of the LMS algorithm are explained. For stabilization of the FS LMS algorithm, conditions on the front-end C/D converter are provided, such that, when satisfied the LMS algorithm becomes more stable and the learning characteristics of the modified receiver are better than the leaky-FS LMS algorithm. Theoretical results are corroborated by simulations.

The neighbourhood area network (NAN) in smart grid (SG) is responsible for data exchange from smart meters (SMs) to the control center and vice versa. Efficient and reliable communication from a control center to individual SMs is an essential part of enabling a green power supply system. Different packet loss at SMs causes information delay. Network coding (NC) is a promising technique to improve throughput and reduce this delay by recovering lost packets in a broadcast scenario. In this paper, to recover lost packets, a codebook is designed for SG NAN downlink (DL) communication without feedback information. The codebook is designed for a small number of retransmissions under the assumption of an optimal decoder/receiver. The key performance metrics include decoded packets and packet loss ratio. Simulation results show the proposed codebook outperforms existing codebooks in terms of throughput for similar performance metrics.

Reconfigurable antenna arrays provide a means for efficient use of the spatial domain in wireless communication systems. Despite its potential, the topic is only briefly explored in the literature. In this paper, we present a comprehensive theoretical analysis of the performance of reconfigurable systems. We consider a receiver equipped with multiple reconfigurable antennas that pick the best state based on the channel between the transmitter and the receiver. For such a system, we derive a new expression for the moment generating function (MGF) of the received signal-to-noise ratio by employing maximal ratio combining. Based on the MGF, we analyze three important performance measures, specifically, achievable rate, error probability, and outage probability. Furthermore, we conduct an asymptotic analysis incorporating the correlation between reconfigurable states and show that a reconfigurable system can achieve a diversity order of the number of antennas times the number of reconfigurable states. Finally, we discuss the applicability of reconfigurable antennas in novel wireless networks with large antenna arrays and distributed antenna systems, highlighting the performance gains and requirement for fewer RF chains.

The periodic sensing requirements in cognitive radios cause interruptions for secondary user (SU) communication. As a result, the throughput of the secondary system during sensing can be very low. In this paper, we propose a monitoring scheme for Orthogonal Frequency Division Multiplexing (OFDM) based cognitive radio which can monitor the spectrum during ongoing communication and detect the emergence of primary users (PUs). Our scheme permits secondary cognitive radios to perform spectrum monitoring while receiving packets. The proposed method exploits the pilot tones that are inherent to many OFDM based standards to measure the error vector magnitude (EVM) of the group of pilots (GOPs). A step change in the EVM curve signifies the emergence of the PU.

We investigate multiple input multiple output (MIMO) orthogonal frequency-division multiplexing (OFDM) systems that operate with insufficient cyclic prefix (CP). Using a CP shorter than the channel delay spread can enable a significant improvement in bandwidth utilization or range extension for OFDM networks at the cost of increased intersymbol interference (ISI) and inter-carrier interference (ICI). We first analyze the effect of ICI and ISI on the received signal. A bi-directional M-algorithm (BDMA) is then proposed for high performance trellis-based equalization to construct an iterative interference mitigation and detection process. Simulations show that, after only 2 iterations, the bit error rate (BER) of the proposed equalization scheme can converge to that of a sufficient-CP system even when the channel delay spread is 6 times longer than the insufficient CP.

In this study, a reconfigurable antenna codebook feedback (FB) scheme is investigated to reduce precoding codebook loss without increasing the number of FB bits. Analysis and simulations show that increasing the number of receive antenna states ( S ) has the same effect as codebook size expansion when the codeword and state selection is based on minimising the distance between channel and codeword. In addition, the authors propose an optimum selection approach. The signal to noise ratio (SNR) and rates achieved with perfect FB in a traditional non‐reconfigurable antenna system, can be exceeded with only S = 2, using the proposed optimum selection. In fact doubling S gives an approximate SNR gain of 40% compared with S = 1. The authors also consider the effects of channel estimation errors and correlated antenna states. The authors show that the performance losses due to correlation can be quantified with a simple closed form result.

We derive closed-form expressions to approximate the distribution of detection delay for a time-invariant cumulative sum detector. In addition, we derive approximations for the distribution of detection delay for the general case, which can be applied to any independent and identically distributed channel. We also employ random walk and Brownian motion theory to derive approximate expressions. The validity of these expressions is verified using simulations. We also approximate the probability of missed detection and investigate the probability of long detection delays via analysis and simulations. Numerical results show that different signal-to-noise ratio and detection delay conditions require different approaches to achieve good approximations of the detection delay distribution. Results also illustrate that the remarkably simple Brownian motion approach provides the best approximation for longer delays and the type of fading channel has very little impact on long detection delays.

In this study, the authors design a multi‐user ultra‐wideband system for indoor wireless communication. The proposed system is useful in multipath radio reception as the full multipath diversity gain is achieved. The system has multiple transmitters and receivers. They use polyphase spreading waveforms which feature low levels of mutual interference and enable each of the channel impulse responses to be measured free from distortion continuously with each data symbol transmission. A key feature of the proposed system is that transmitted waveforms are used which produce outputs from the receiver cross correlators, which are zero on both sides of the main correlation peak, for each radio path. This zero response lasts for a duration in excess of the expected delay spread of the radio channel enabling full Rake processing of the multipath signal. The effects of the low levels of mutual interference are reduced further by using time and frequency hopping, forward error correction and soft decision decoding.

We present a novel class of space time trellis codes based on super quasi-orthogonal space time codes for a reconfigurable antenna system. We call them space time state trellis codes (STSTCs). We design for 4 trellis states and both 2 and 4 branch trellis structures for BPSK and QPSK constellations. The proposed codes take advantage of the additional degrees of diversity offered by reconfigurable antennas and changing propagation states. They achieve full rate, full diversity and high coding gain. Simulation results demonstrate their good performance. STSTCs achieve the diversity order expected of four transmit antennas while using only two reconfigurable transmit antennas. Our code out performs the existing super orthogonal space time trellis code, which fails to achieve full diversity in a reconfigurable antenna system. The proposed 4 trellis state STSTC is designed based on the following performance criteria: minimum coding gain distance of the minimum return path (CGD MRP), performance factor (PF), rank of the partial valid path (RPVP) on top of traditional minimum coding gain distance of parallel path (CGD PP). We show through analysis and simulations that minimum CGD PP is not a sufficient criteria and that additional performance gains are provided by considering the CGD MRP.

Pilot contamination occurs when cells simultaneously transmit the same pilot sequences, creating interference. Unsynchronizing the pilots can reduce pilot contamination, but it can produce data to pilot interference. In this paper, we investigate the impact of pilot contamination and other interference, namely data to pilot interference, on the performance of finite massive MIMO systems with synchronized and unsynchronized pilots. Two unsynchronized pilot schemes are considered. The first is based on an existing time-shifted pilot scheme, where pilots overlap with downlink data from nearby cells. The second time-shifted method overlaps pilots with uplink data from nearby cells. Results show that if there are small numbers of users, the first time-shifted method provides the best sum rate performance. However, for higher numbers of users, the second time-shifted method has advantage compared to other methods. We also show that time-synchronized pilot is not necessarily the worst case scenario in term of sum rate performance when shadowing effect are considered.

Pilot contamination is an issue affecting massive MIMO systems. It is known that a time-shifted pilot method can be used to reduce pilot contamination from neighboring cells when the number of antennas at the base station is infinitely large [1]. In our research, we analyze the effect of a finite number of antennas on the sum rate of the time-shifted method. Numerical results show that the time-shifted pilot method can improve uplink and downlink transmission rate compared to time-synchronized method for small number of spatial multiplexing users. However, for higher number of spatial multiplexing users, the time-shifted method will need more antennas at the base station in order to have noticeable improvement over the time-synchronized method.

We investigate orthogonal frequency-division multiplexing (OFDM) systems that operate with limited or insufficient cyclic prefix (CP). Using a shorter CP can enable a significant reduction in bandwidth usage or enable range extension for OFDM networks. The cost is increased inter-symbol interference (ISI) and inter-carrier interference (ICI). Here, the effect of ICI and ISI on the received signal is analyzed. We then propose a two-stage detector for single input multiple output (SIMO) systems with two receive antennas. Taking advantage of the spatial diversity, the first detection stage uses a low complexity zero-forcing (ZF) or minimum mean square error (MMSE) method to estimate the transmitted data for aiding the second stage trellis based detector. The proposed schemes provide satisfactory performance for delay spreads up to 3.5 times the CP.

In this paper we propose a novel class of space time trellis codes based on super quasi-orthogonal space time codes for a reconfigurable antenna system. We call them Space Time State Trellis Codes (STSTCs). The proposed codes are capable of taking advantage of the additional degrees of diversity offered by reconfigurable antennas and changing propagation states. They achieve full rate, full diversity and high coding gain. Simulation results demonstrate their good performance. STSTCs achieve the diversity order expected for four transmit antennas while using only two reconfigurable transmit antennas. Our code out performs the existing super orthogonal space time trellis code, which fails to achieve full diversity in a reconfigurable antenna system.

We consider single-h continuous phase modulation (CPM) systems that use linear prediction receivers with coefficient look up table (LUT) for mitigating the effect of multipath fading. This type of receiver shows superior performance over conventional ones; however, high channel delay spread and fast fading can still lead to a high error floor in its performance. A sub-optimal soft/iterative detection based on the soft output Viterbi algorithm (SOVA) is presented for reducing such error floors while maintaining low complexity. The design is extended with a simple spatial diversity combining scheme and the utilization of multiple coefficient LUTs. The proposed approach allows a significant performance gain to be achieved.

The performance of maximal ratio combining (MRC) in Rayleigh channels with co-channel interference (CCI) is well-known for receive arrays which are co-located. Recent work in network MIMO, edge-excited cells and base station collaboration is increasing interest in macrodiversity systems. Hence, in this paper we consider the effect of macrodiversity on MRC performance in Rayleigh fading channels with CCI. We consider the uncoded symbol error rate (SER) as our performance measure of interest and investigate how different macrodiversity power profiles affect SER performance. This is the first analytical work in this area. We derive approximate and exact symbol error rate results for M-QAM/BPSK modulations and use the analysis to provide a simple power metric. Numerical results, verified by simulations, are used in conjunction with the analysis to gain insight into the effects of the link powers on performance.

The prospect of base station cooperation leading to joint combining at widely separated antennas has led to increased interest in macrodiversity systems, where both sources and receive antennas are geographically distributed. In this scenario, analytical investigation of channel capacity is extremely challenging for finite-size systems since the channel matrices have a very general form where each path may have a different power. Hence, in this paper, we consider the ergodic sum capacity of a macrodiversity multiple-input multiple-output system with arbitrary numbers of sources and receive antennas operating over Rayleigh fading channels. For this system, we compute the exact ergodic capacity for a system with at most two transmit antennas and a compact approximation for the general system, which is shown to be very accurate over a wide range of cases. Finally, we compare our results with previous asymptotic results and bounds. Results are verified by Monte Carlo simulations and the impact on capacity of various channel power profiles is investigated.

Consider a multiuser system where an arbitrary number of users communicate with a distributed receive array over independent Rayleigh fading paths. The receive array performs minimum mean squared error (MMSE) or zero forcing (ZF) combining and perfect channel state information is assumed at the receiver. This scenario is well-known and exact analysis is possible when the receive antennas are located in a single array. However, when the antennas are distributed, the individual links all have different average signal to noise ratio (SNRs) and this is a much more challenging problem. In this paper, we provide approximate distributions for the output SNR of a ZF receiver and the output signal to interference plus noise ratio (SINR) of an MMSE receiver. In addition, simple high SNR approximations are provided for the symbol error rate (SER) of both receivers assuming M-PSK or M-QAM modulations. These high SNR results provide array gain and diversity gain information as well as a remarkably simple functional link between performance and the link powers.

The performance of linear receivers in the presence of co-channel interference (CCI) in Rayleigh channels is a fundamental problem in wireless communications. Performance evaluation for these systems is well-known for receive arrays which are co-located. In contrast, there are almost no analytical results available for macrodiversity systems (such as network MIMO) where both the sources and receive antennas are widely separated. In general, this appears to be an extremely difficult problem. However, progress is possible for the two-user scenario. In this paper, we derive closed form results for the probability density function (pdf) and cumulative distribution function (cdf) of the output signal to interference plus noise ratio (SINR) and signal to noise ratio (SNR) of minimum mean squared error (MMSE) and zero forcing (ZF) receivers in independent Rayleigh channels with arbitrary numbers of receive antennas. The results are verified by Monte Carlo simulations. The results enable further system analysis such as the evaluation of outage probabilities, bit error rate (BER) and capacity.

The prospect of base station cooperation has led to increased interest in macrodiversity systems, where both sources and receive antennas are geographically distributed. In this scenario, little is known about channel capacity since the channel matrices have a very general form where each path may have a different power. Hence, in this paper we consider the ergodic sum capacity of a macrodiversity MIMO system with arbitrary numbers of sources and antennas operating over Rayleigh fading channels. For this system, we compute a compact approximation to ergodic sum capacity, which is shown to be very accurate over a wide range of cases. Finally, we develop a highly simplified upper-bound which leads to insights into the relationship between capacity and the channel powers.

We propose an iterative soft multiuser detection technique for detecting multiple BPSK modulated cochannel signals in Rayleigh flat fading environments. Ant colony optimization is used to produce soft estimates of the transmitted signals. Our attention is focused on the challenging case of overloaded transmission, where there are fewer receive antennas than transmitted cochannel signals. Simulation results show our approach can achieve near maximum likelihood (ML) performance at significantly reduced complexity for critically loaded systems, and it continues to provide good performance under overloaded scenarios.

In this paper, the non-selective Decode-and-Forward (DF) scheme proposed in is analyzed using approximate Gaussian density evolution (DE). Based on the DE analysis, performance is shown to be improved by adaptively changing the transmit code at the relay node in response to the channel conditions. Simulation results show that the proposed adaptive scheme improves performance, particularly when the relay is closer to the destination than to the source. In addition, the adaptive scheme does not increase network overhead.

The performance of linear receivers in the presence of co-channel interference in Rayleigh channels is a fundamental problem in wireless communications. Performance evaluation for these systems is well-known for receive arrays where the antennas are close enough to experience equal average SNRs from a source. In contrast, almost no analytical results are available for macrodiversity systems where both the sources and receive antennas are widely separated. Here, receive antennas experience unequal average SNRs from a source and a single receive antenna receives a different average SNR from each source. Although this is an extremely difficult problem, progress is possible for the two-user scenario. In this paper, we derive closed form results for the probability density function (pdf) and cumulative distribution function (cdf) of the output signal to interference plus noise ratio (SINR) and signal to noise ratio (SNR) of minimum mean squared error (MMSE) and zero forcing (ZF) receivers in independent Rayleigh channels with arbitrary numbers of receive antennas. The results are verified by Monte Carlo simulations and approximations and high SNR analysis are also derived. The results enable further system analysis such as the evaluation of outage probability, bit error rate (BER) and capacity.

We investigate the use of the minimum mean-squared- error decision-feedback-equalizer (MMSE-DFE) as a preprocessor to reduce the computational complexity of symbol detection in frequency-selective multiple- input multiple-output (MIMO) channels. The preprocessor delays and focuses the received signal energy which reduces the effects of inter-symbol- interference and multiple-access-interference. This property of the MMSE-DFE preprocessor is exploited to order and to reduce the number of symbol candidates considered by an iterative maximum- likelihood (ML) detector. Simulations show that the proposed detection scheme can achieve lower bit- error-rates than existing reduced-complexity algorithms in overdetermined and underdetermined receive scenarios. Near-optimum performance is achieved when the number of symbol candidates is increased.

In this paper, we propose two solutions to the problem of joint transmit-receive antenna selection in a multiple-input multiple-output (MIMO) cognitive radio (CR) system. Our objective is to maximize CR data rates and satisfy interference constraints at the primary user (PU) receiver(s). In the first we approximate the original non-convex optimization problem using an iterative approach solving a series of smaller convex problems. Second we present a novel, norm-based transmit receive antenna selection technique that simultaneously improves throughput while maintaining the PU interference constraints. We show that this approach yields near optimal results with massive complexity reductions. We make a performance comparison between the proposed approaches and the optimal exhaustive search approach. We provide an analysis of the exhaustive search and relate selection gains to system parameters such as the shadow fading standard deviation, the path loss exponent and the number of PUs per square kilometer. Our results establish that antenna selection is a promising option for future MIMO CR devices in sparse PU environments.

Co-channel interference (CCI) occurs in a wireless receiver when multiple signals are present. The receiver experiences excessive CCI under overload which occurs when there are more signals than receive antennas. This makes separation and estimation of the transmitted data signals difficult. We develop a novel reduced-complexity receiver structure for the separation and symbol detection of multiple co-channel signals in frequency-flat Rayleigh fading channels. Moreover, we show that list-based group-wise processing can achieve better performance at low and moderate signal-to-noise ratios than existing group-wise soft information processing schemes. The receiver is equipped with multiple antennas and developed to work under overload. Its structure consists of a linear preprocessor followed by a nonlinear reduced-complexity symbol detector. The proposed list group-search detection (LGSD) algorithm relies on list feedback and reduces complexity by iteratively searching over groups of transmitted symbols. It outputs a list of likely data symbols which is well suited to further processing using soft input error control decoders. Simulation shows that LGSD can achieve near optimum joint maximum likelihood performance under overload at significantly lower complexity.

We propose a cooperative transmission scheme for two users with one common relay using superposition modulation. It uses distributed Turbo codes (DTCs) for each user. The relay always decodes, then interleaves and re-encodes the decoded data. A new packet is formed by combining the re-encoded packets from both users using superposition modulation. This packet is forwarded to the destination with the average receive SNRs of each user. The destination uses cooperative Turbo decoders to improve performance. Simulation results show the proposed scheme outperforms XOR based schemes and is simple to implement.

We consider a wireless communication system where multiple co-channel users transmit data via a synchronous, frequency-flat Rayleigh fading channel. The receiver employs an antenna array. The paper develops an iterative list multiuser detector for overloaded applications where the number of transmitted signals exceeds the number of receive antennas. The receiver uses a linear preprocessor to reduce co-channel interference followed by a multiuser detector with an iterative groupwise symbol detection algorithm that extracts a list of the most likely user symbols. Simulation results show that the proposed detector provides good complexity-performance trade offs.

This paper investigates the design and performance of a two-user satellite communication system. Each user is independently encoded using a structured Turbo code with identical symbol interleavers. This permits Turbo decoding to be performed using the combined component code trellises, which provides significant gains over independent decoding. The design of the code-matched symbol interleavers is described. Both ideal and imperfect knowledge of the relative phase shift between users is considered. Performance is compared to the single user case and the performance degradation due to a third unknown user is considered. These investigations show that the proposed system performs well in a variety of conditions.

In this comment we position the work of Z. Chi, et al, 2004,2001 in terms of the prior literature and note that many of its results are subsumed by previous papers.

In this paper, a frequency domain (FD) receiver architecture implemented using estimated channel parameters is derived for broadband single carrier modulations. Co-channel and inter-symbol interferences are compensated by a minimum mean squared error based integrated space-frequency-equalizer (SFE) using the estimated parameters. The integrated SFE in the FD consists of coupled FIR structures, that are jointly optimized by maximizing the desired signal to interference plus noise ratio. We develop analytical expressions and present simulation results for the integrated SFE. Simulation results demonstrate that excellent receiver performance is achieved even for channels with large ISI spans. The proposed SC-SFE outperforms previous layered space frequency (LSF) receivers with imperfect channel knowledge. Further, the parallel receiver architecture yields equal diversity gains to all data streams without the error propagation that is common to most LSF schemes.

To date most frequency-domain (FD) turbo equalization schemes assume ideal channel state information (CSI) is available. In this paper, a system combining FD turbo linear equalization with time-domain channel estimation is developed and evaluated for single-carrier modulation formats. The effect of estimated CSI on the equalizer form is shown. Performance results employing convolutionally encoded QPSK and 16-QAM transmissions show the efficacy of the proposed system and its capability to operate in different wireless scenarios.

Abstract — We present a space-time coded system which achieves high throughput and good performance using low-complexity detection and decoding. We focus on a Rate 2 quasi-orthogonal space-time block code structure which enables us to achieve an overall throughput of 5.6 bits/symbol period with good performance and relatively simple decoding using iterative parallel interference cancellation. We show that this can be achieved through the use of a bit-mapped coded modulation (BMCM) structure using parallel short low density parity check codes. The proposed system is shown to perform well on flat Rayleigh fading channels with a wide range of normalized fade rates, and to be robust to channel estimation errors. A comparison with bit-interleaved coded modulation (BICM) is also provided. Index Terms — Error correcting coding, iterative methods, low density parity check codes, MIMO systems, space-time coding. I.

Excessive co-channel interference (CCI) occurs in overloaded wireless environments where the number of transmitted signals exceeds the number of available receive antennas. We present a list-based multiuser detection scheme that effectively mitigates CCI in overloaded receivers with a circular array. The algorithm is named parallel symbol detection with reduced complexity interference estimation (PSD-RCIE). It employs an iterative parallel detector with list feedback of the best symbol estimates. Simulations show that our algorithm achieves near joint-maximum likelihood detection at a fraction of the complexity. Its good complexity-performance trade-off together with its parallel structure make PSD-RCIE well suited for practical implementation.

The use of multiple-input multiple-output (MIMO) configurations in wireless systems is becoming increasingly popular, due to the potential capacity enhancing properties. The use of such configurations, requires deployment of multiple RF chains and results in an increase in cost and complexity. This paper focuses on transmitter design, where cost is often dominated by the need to utilise linearized power amplifiers. Thus, the effect of reducing the number of active transmit antennae through selection can yield a significant cost advantage. Three criteria for transmit antenna selection are evaluated and characterised through the use of real channel data in the UHF band. The criteria are based only on the channel gains and have relatively low complexity; thus are suitable for practical purposes. This study examines the performance of the tested selection algorithms under a variety of operating conditions. The paper also considers the issue of delayed switch time due to reverse-link communication latency

In this paper, we focus on an advanced space-time system using a rate 2 quasi-orthogonal space-time block code which enables us to achieve very good performance for an overall throughput of up to 4.3 bits/s/Hz. This is done through the use of a bit-mapped coded modulation structure using short low density parity check component codes. At the receiver, parallel interference cancellation (PIC) and belief propagation (BP) decoding are employed. Iterative decoding is performed between the PIC and BP decoding stages. Given a fixed total number of decoding iterations, we investigate the frequency of PIC updates required to achieve good performance

We propose a simple class of encoding/decoding techniques that can be used to improve the information outage rate of certain multiple-input-multiple-output (MIMO) systems. Gains in outage rate can be achieved at low to moderate signal-to-noise ratios in MIMO systems that have fewer receive antennas than transmit antennas. This performance improvement is due to extra "virtual" receive antennas that are created with low-complexity signal processing. A simple space-time block code system using virtual receive antennas is also investigated

To date there has been little work done on multilevel codes for the space-time environment. In this paper we develop multi-dimensional space-time multilevel codes (ST-MLCs). Several construction methods are proposed, including a coset code approach. The proposed space-time multilevel encoders partition a 2Nt-dimensional signalling space, which spans all Nttransmit antennas. The multi-dimensional partitioning can be designed to reduce the complexity of detection/decoding. We develop a space-time multistage decoder for the proposed ST-MLCs. It allows the complexity of soft decision decoding to be significantly reduced compared to a single level approach. In addition, significant performance gains over a single level approach are obtained

This paper describes the high-throughput error correcting space-time block codes. To design STBCs, systematic binary and nonbinary Reed-Solomon (RS) and Bose-Chaudhuri-Hocquenghem (BCH) ECCs, embedded error correction codes (ECCs) are considered. The resulting STBCs provide significantly more coding gain and throughputs with the same number of transmit and receive antennas. The cost is the increased decoding complexity. The increased throughput is used to concatenate an external ECC with the STBC for providing a good performance.

This paper describes a new approach to list-based soft-input soft-output (SISO) decoding based on order-i reprocessing. Approximations to both the log-maximum a posteriori (MAP) and max-log-MAP algorithms are developed. Additional decoding steps are proposed to correct common types of errors remaining after iterative decoding. These steps can significantly improve performance at low bit-error rates in later iterations. The proposed algorithms offer a wide range of complexity versus performance tradeoffs, which are explored through Monte Carlo simulations of product code decodings. The algorithms improve performance over previous approaches.

The box and match decoding algorithm (BMA) significantly reduces the computational complexity of the ordered statistic decoding algorithm at the expense of increased memory requirements. A soft-input/soft-output version of the BMA is developed. Additional complexity-reduction techniques are also described.

Single parity-check (SPC) codes are applied in both parallel and serial concatenated structures to produce high-performance coding schemes. The number of concatenations or stages, M, is increased to improve system performance at moderate-to-low bit-error rates without changing the overall code parameters (namely, code rate and code block length). Analytical bounds are presented to estimate the performance at high signal-to-noise ratios. The SPC concatenated codes are considered with binary phase-shift keying and with 16-quadrature amplitude modulation bit-interleaved coded modulation on the additive white Gaussian noise channel and the independent Rayleigh fading channel. Simulations show that the four-stage serial or parallel concatenated SPC codes can, respectively, outperform or perform as well as 16-state turbo codes. Furthermore, decoding complexity is approximately 9-10 times less complex than that of 16-state turbo codes. The convergence behavior of both serial and parallel concatenated SPC codes is also discussed.

This paper describes an approach to combined equalization and decoding using iterative techniques and decision feedback equalization. We show that performance can be improved by using a bank of feedback filters, whose inputs are taken from the output of a list decoder. The terrestrial line-of-sight microwave radio channel is considered in this paper.

This article develops an alternative adaptive iterative Chase (1972) based decoding algorithm for block turbo/product codes. The decoder considers only a small subset of codewords, so that estimates of the extrinsic information are required in some cases. This article develops such an estimate based on code distance properties.

This paper develops an approach to iterative multistage decoding of multilevel codes. This involves passing reliability information to previous and subsequent decoders instead of only hard decisions to subsequent decoders. The paper also develops an adaptive version of the suboptimal soft output decoding algorithm of Picart and Pyndiah (1996). This adaptive algorithm provides a gain of approximately 0.24 dB at a bit error rate (BER) of 10/sup -5/ after four iterations and approximately 0.43 dB after ten iterations over the algorithm of Picart et al. If the adaptive algorithm is used in conjunction with iterative multistage decoding then a gain of approximately 0.62 dB is obtained at a BER of 10/sup -5/ after four iterations and approximately 0.9 dB after ten iterations over the algorithm of Picart et al.

This paper develops an adaptive approach to iterative decoding of block turbo codes. The method developed in this paper allows the receiver to adapt to the conditions of the channel, the codes and the modulation scheme. The approach yields improved performance especially in early iterations, for a very small increase in complexity.