Suboptimal Receiver Research Articles

Bandwidth-Efficient Selective Retransmission for MIMO-OFDM Systems

In this work, we propose an efficient selective retransmission method for multiple-input and multiple-output (MIMO) wireless systems under orthogonal frequency-division multiplexing (OFDM) signaling. A typical received OFDM frame may have some symbols in error, which results in a retransmission of the entire frame. Such a retransmission is often unnecessary, and to avoid this, we propose a method to selectively retransmit symbols that correspond to poor-quality subcarriers. We use the condition numbers of the subcarrier channel matrices of the MIMO-OFDM system as a quality measure. The proposed scheme is embedded in the modulation layer and is independent of conventional hybrid automatic repeat request (HARQ) methods. The receiver integrates the original OFDM and the punctured retransmitted OFDM signals for more reliable detection. The targeted retransmission results in fewer negative acknowledgements from conventional HARQ algorithms, which results in increasing bandwidth and power efficiency. We investigate the efficacy of the proposed method for optimal and suboptimal receivers. The simulation results demonstrate the efficacy of the proposed method on throughput for MIMO-OFDM systems.

Resource-efficient fusion over fading and non-fading reporting channels for cooperative spectrum sensing.

Recently, a novel resource-efficient technique for the reporting channel transmissions in cooperative spectrum sensing was proposed. In this technique, secondary users are allowed to simultaneously send their local decisions to the fusion center, saving time and frequency resources. Expressions for the probabilities of detection and false alarm for the unitary-gain AWGN reporting channels were derived, while simulation results were given for both the AWGN and Rayleigh fading channels. Here, we provide an expression that is applicable to AWGN channels with different real-valued gains and to time-varying real-valued gains. A simple suboptimum receiver is proposed for the general complex-valued fading and non-fading channels, with an improved performance in the low signal-to-noise ratio condition. Numerical results are shown for both the AWGN and Rayleigh fading reporting channels, demonstrating the accuracy of the derived expressions and the attractive performance of the proposed receiver.

Nested Maximum Likelihood Group Detection in Generalized Spatial Modulation MIMO Systems

This letter presents a new detection scheme for the generalized spatial modulation MIMO system. The strategy is based on the successive use of maximum likelihood detectors, applied in a nested fashion, to variable-length partitions of the post-equalization received signal. The choice of the antenna combination mapping, named spatial constellation, as well as the detection ordering strategy are key design factors to the implementation of the high-performance and low-complexity detector. Simulation results show the substantial performance advantage of the proposed detector compared with current suboptimal receivers.

PLC Performance Analysis Assuming BPSK Modulation Over Nakagami- <inline-formula> <tex-math notation="TeX">$m$</tex-math></inline-formula> Additive Noise

In this letter, we derive a maximum likelihood receiver of binary phase shift keying signals over Nakagami- $m$ distributed additive noise in power line communication system. The decision variable is characterized by using copula approach. The analytical average bit error rate of the considered scheme is numerically evaluated by using the cumulative distribution function of the decision variable. It is shown by simulations that the proposed receiver performs significantly better than an existing suboptimal receiver.

Optimum and Sub-Optimum Receivers for OFDM Signals with Strong Nonlinear Distortion Effects

In this paper we consider the optimum detection of OFDM (Orthogonal Frequency Division Multiplexing) signals with strong nonlinear distortion effects. It is shown that the optimum performance with strong nonlinear distortion effects is not as bad as one might expect and can even be better than the performance with conventional, linear transmitters. To achieve these excellent performances we should employ receivers able to take advantage of the information associated to transmitted data symbols that is inherent to the nonlinear distortion component, in opposition to traditional OFDM implementations where nonlinear distortion effects are regarded as an undesirable noise-like component. We study the achievable gains of the optimum receiver both analytically and by simulation. Since the complexity of optimum receivers is extremely high when we have nonlinear distortion effects, even for OFDM signals with a small number of subcarriers, we propose several sub-optimum receivers and evaluate their performance. Our sub-optimal receivers allow remarkable performance improvements, being able to reduce significantly the gap between the optimum performance and the performance of typical OFDM receivers.

Iterative Decoding of Coded THP with Quantized Output

This paper proposes an improved receiver for the Tomlinson-Harashima precoder (THP). In this paper we demonstrate that most of the capacity loss associated with conventional THP is due to the suboptimal receiver, which is a simple modulo operation. We then demonstrate how this loss can can be avoided with the optimization of the receiver. In the suboptimal receiver the modulo operation ignores the dependency between the implicitly added sequence of symbols removed by it. We then implement a THP with quantized outputs. The quantized outputs enable to implement a Maximum A-Posteriori Probability (MAP) receiver that exploits the dependency between the received symbols. The MAP receiver is named Enhanced MODulo operation (EMOD), and operates as an inner decoder within an iterative scheme, either with outer Low Density Parity Check (LDPC) code or outer Turbo Code. Simulation results show considerable gains relative to coded conventional THP, and also relative to the Turbo-Equalizer (TE) (which does not use a precoder in the transmitter).

Improving the Spectral Efficiency of Nonlinear Satellite Systems through Time-Frequency Packing and Advanced Receiver Processing

We consider realistic satellite communications systems for broadband and broadcasting applications, based on frequency-division-multiplexed linear modulations, where spectral efficiency is one of the main figures of merit. For these systems, we investigate their ultimate performance limits by using a framework to compute the spectral efficiency when suboptimal receivers are adopted and evaluating the performance improvements that can be obtained through the adoption of the time-frequency packing technique. Our analysis reveals that introducing controlled interference can significantly increase the efficiency of these systems. Moreover, if a receiver which is able to account for the interference and the nonlinear impairments is adopted, rather than a classical predistorter at the transmitter coupled with a simpler receiver, the benefits in terms of spectral efficiency can be even larger. Finally, we consider practical coded schemes and show the potential advantages of the optimized signaling formats when combined with iterative detection/decoding.

Monobit Digital Receivers for QPSK: Design, Performance and Impact of IQ Imbalances

Future communication system requires large bandwidths to achieve high data rates, thus rendering analog-to-digital conversion (ADC) a bottleneck due to its high power consumption. In this paper, we consider monobit receivers for QPSK. The optimal monobit receiver under Nyquist sampling is obtained and its performance is analyzed. Then, a suboptimal but low-complexity receiver is proposed. The effect of imbalances between In-phase (I) and Quadrature (Q) branches is carefully examined. To combat the performance loss due to IQ imbalances, monobit receivers based on double training sequences and eight-sector phase quantization are proposed. Numerical simulations show that the low-complexity suboptimal receiver suffers 3dB signal-to-noise-ratio (SNR) loss in additive white Gaussian noise (AWGN) channels and only 1dB SNR loss in multipath channels compared with matched-filter monobit receiver with perfect channel state information (CSI). It is further demonstrated that the amplitude imbalance has essentially no effect on monobit receivers. In AWGN channels, receivers based on double training sequences can efficiently compensate for the SNR loss without complexity increase, while receivers with eight-sector phase quantization can almost completely eliminate the SNR loss caused by IQ imbalances. In dense multipath channels, the effect of imbalances on monobit receivers is slight.

Normalisation‐based receiver using BCGM approximation for α ‐stable noise channels

Aiming at signal detection in α-stable (SαS) noise, a receiver with a new zero-memory nonlinearity limiter (ZMNL) is designed. The proposed ZMNL serves to transform the SαS noise into a standard Gaussian one by using the bi-parameter Cauchy-Gaussian mixture (BCGM) model to approximate the SαS density. Simulation results indicate that the proposed receiver outperforms the conventional sub-optimal receivers for α > 1.5.

Sub-Optimum Receiver Architecture for AWGN Channel with Symmetric Alpha-Stable Interference

In many scenarios, the undesirable disturbance at a communication receiver has to be modeled as the sum of Gaussian noise and impulsive interference for which closed form probability density function (pdf) generally does not exist. This makes optimum receiver implementation difficult. In this paper, motivated by the heavy tail nature of the resultant pdf, an alternate parametric solution is proposed. After estimation of the appropriate parameters, the alternate solution may then be written in closed form, leading to a relatively simple architecture. Simulation results show that the proposed receiver performance, in terms of bit error rate (BER), is almost indistinguishable from the optimum receiver while outperforming other sub-optimum receivers. Noise parameter estimation using empirical characteristic functions is also discussed.

Diversity Analysis of Non-Coherent Wireless Network Coding

Network coding has been widely used in the wireless uplink. To do coherent detection at the receiver, channel estimation is performed to collect the channel state information (CSI). For multi-user channels, the channel estimation overhead is sometimes formidable and may even outweigh the network coding gain. To address this issue, we study the non-coherent network coding in this work. Both the analog network coding (ANC) and digital network coding (DNC) are studied. Depending on the available CSI, we first develop the coherent and non-coherent maximum likelihood receivers. For ANC, as the non-coherent receivers have no closed form, we develop two suboptimum receivers according to the average link qualities. Next we study the error rates, and show that full diversity can always be achieved at extremely high signal-to-noise ratios (SNR) regardless of the CSI assumptions. However, the lack of perfect CSI would incur some diversity loss at modest SNRs. Besides, the performance loss of ANC is more serious due to the incapability to efficiently suppress multi-user interferences at the receivers. Extensive simulations are performed to verify our analytical results.

Complexity reduction of RBF multiuser detector for DS-CDMA using a genetic algorithm

The optimal receiver for detecting direct sequence code division multiple access (DS-CDMA) signals suffers from computational complexity that increases exponentially with the number of users. Several suboptimal multiuser detectors (MUDs) have been proposed to overcome this problem. Due to the nonlinear nature of the decision boundary of the optimal receiver, it is known that nonlinear receivers outperform linear receivers. Radial basis function (RBF) MUD is a nonlinear suboptimal receiver that can perfectly approximate this decision boundary and it needs no training since it is fully determined when the spreading codes of all users and the channel impulse response (CIR) are known. However, the RBF MUD suffers from structural complexity since the number of hidden nodes (center functions) in its structure increases exponentially with the number of users. In this study, we propose a new method to minimize the number of center functions of the RBF MUD using a genetic algorithm (GA) and the least mean squares (LMS) algorithm. With simulations performed in AWGN and multipath channels it is shown that the proposed method immensely reduces the complexity of the RBF MUD with a negligible performance degradation.

A NEW PRACTICAL RECEIVER FOR A DECODE-AND-FORWARD COOPERATIVE CDMA SYSTEMS BASED ON A BLIND λ-COMBINER

This paper develops a practical receiver suggested for cooperative systems using decode-and-forward transmission and compares it to the theoretical sub-optimum ‚-MRC receiver model. The proposed receiver model adopts a channel blind ‚-combiner and employs a practical estimation of the combiner's weight ‚ that changes adaptively for each received bit. The ‚ estimation process relies on a dynamic-blind calculation performed on the incoming bit stream using an approximate formula. The accuracy of the estimated values of ‚ against the numerical (optimum) values is verifled by comparing their efiects on the performance curves. Next, the performance of the proposed receiver is evaluated against the sub-optimum receiver using the closed-form performance equations then verifled using an actual implementation of the decode-and-forward cooperative algorithm. The use of the proposed receiver is shown to have reliable performance under difierent channel assignments and provides adaptivity to channel variations without complexity or exaggerated signal processing.

Large-System Analysis of Joint Channel and Data Estimation for MIMO DS-CDMA Systems

This paper presents a large-system analysis of the performance of joint channel estimation, multiuser detection, and per-user decoding (CE-MUDD) for randomly-spread multiple-input multiple-output (MIMO) direct-sequence code-division multiple-access (DS-CDMA) systems. A suboptimal receiver based on successive decoding in conjunction with linear minimum mean-squared error (LMMSE) channel estimation is investigated. The replica method, developed in statistical mechanics, is used to evaluate the performance in the large-system limit, where the number of users and the spreading factor tend to infinity while their ratio and the number of transmit and receive antennas are kept constant. The performance of the joint CE-MUDD based on LMMSE channel estimation is compared to the spectral efficiencies of several receivers based on one-shot LMMSE channel estimation, in which the decoded data symbols are not utilized to refine the initial channel estimates. The results imply that the use of joint CE-MUDD significantly reduces rate loss due to transmission of pilot signals, especially for multiple-antenna systems. As a result, joint CE-MUDD can provide significant performance gains, compared to the receivers based on one-shot channel estimation.

Simple Detection Based on Soft-Limiting for Binary Transmission in a Mixture of Generalized Normal-Laplace Distributed Noise and Gaussian Noise

In this letter, a simplified suboptimum receiver based on soft-limiting for the detection of binary antipodal signals in non-Gaussian noise modeled as a generalized normal-Laplace (GNL) distribution combined with Gaussian noise is presented. The suboptimum receiver has low computational complexity. Furthermore, when the number of diversity branches is small, its performance is very close to that of the Neyman-Pearson optimum receiver based on the probability density function obtained by the Fourier inversion of the characteristic function of the GNL-plus-Gaussian distribution.

SAGE Based Suboptimal Receiver for Downlink MC-CDMA Systems

An iterative joint data detection and channel estimation algorithm for downlink multi-carrier code division multiple access (MC-CDMA) systems is proposed. The resulting algorithm is a space alternating generalized expectation-maximization (SAGE) algorithm which updates the data sequences serially and the channel parameters in parallel, leading to a receiver structure that also incorparates interchannel interference cancellation. Its performance is compared with various Minimum Mean Square Error (MMSE) estimators for a multipath frequency selective fading channel using simulations. It is illustrated that the proposed SAGE algorithm performs better than the MMSE estimators considered in this paper.

Optimal and suboptimal receivers for code‐multiplexed transmitted‐reference ultra‐wideband systems

ABSTRACTIn this study, optimal and suboptimal receivers are investigated for code‐multiplexed transmitted‐reference (CM‐TR) ultra‐wideband systems. First, a single‐user scenario is considered, and a CM‐TR system is modeled as a generalized noncoherent pulse‐position modulated system. Based on that model, the optimal receiver that minimizes the bit error probability is derived. Then, it is shown that the conventional CM‐TR receiver converges to the optimal receiver under certain conditions and achieves close‐to‐optimal performance in practical cases. Next, multi‐user systems are considered, and the conventional receiver, blinking receiver, and chip discriminator are investigated. Also, the linear minimum mean‐squared error (MMSE) receiver is derived for the downlink of a multi‐user CM‐TR system. In addition, the maximum likelihood receiver is obtained as a performance benchmark. The practicality and the computational complexity of the receivers are discussed, and their performance is evaluated via simulations. The linear MMSE receiver is observed to provide the best trade‐off between performance and complexity/practicality. Copyright © 2011 John Wiley & Sons, Ltd.

Large-SNR Outage Analysis for the DF Relay Channel with Randomized Space-Time Block Coding

A simple half-duplex decode-and-forward relay channel is presented and analyzed. Relays access the common channel by means of a randomized linear-dispersion space-time block code which is flexible with respect to the number of relays and the coding rate α. When the dimensions of the linear dispersion matrices grow large, but with constant ratio α, the spectral efficiency of the system converges fast to a deterministic quantity. Simulation results show that this asymptotic value is an extremely good approximation of the finite reality, even for not-so-large codes. Then, this asymptotic spectral efficiency is used to characterize the outage probability in the high-SNR regime. With maximum-likelihood reception, the proposed randomized coding scheme is shown to achieve full diversity order L+1, with L the total number of relays. On the contrary, with the sub-optimal LMMSE receiver, relays add diversity to the system only if the coding rate is small enough.

Optimum Receiver Performance With Binary Phase-Shift Keying for Decode-and-Forward Relaying

Cooperative diversity through decode-and-forward (DF) relaying is an effective method for improving the performance of mobile wireless systems. We consider here the performance of a cooperative diversity system in which the relay employs the DF protocol to transmit data from a source to a destination using binary phase-shift keying (BPSK). A flat Rayleigh fading environment with statistically independent links is assumed. The optimum receiver for this system is presented, and from the decision rule, the end-to-end symbol error probability (SEP) is derived. Numerical results on the SEP are obtained and compared with those for a suboptimum receiver, which does not need the channel state information of the source-to-relay link.

Iterative Suboptimal Maximum Likelihood Receiver for Nonlinearly Distorted SC-FDMA Symbols

Iterative Suboptimal Maximum Likelihood Receiver for Nonlinearly Distorted SC-FDMA Symbols