Multiuser receivers for CDMA systems in Rayleigh fading channels

Multiuser demodulation in relatively fast fading channels is analyzed. The optimal maximum likelihood sequence detection (MLSD) receiver is derived and a general suboptimal receiver to approximate the MLSD is proposed. The performance of the parallel interference cancellation (PIC) and decorrelating receivers is compared. The PIC receiver is demonstrated to achieve better performance in known channels than the decorrelating receiver, but it is observed to be more sensitive to the channel coefficient estimation errors than the decorrelator. At high channel loads the PIC receiver suffers from bit error rate (BER) saturation, whereas the decorrelating receiver does not. The performance of data-aided (DA) and decision-directed (DD) multiuser channel estimation is also compared. DA channel estimation is shown to be more robust than DD channel estimation, which may suffer from BER saturation caused by hangups at high signal-to-noise ratios (SNRs). The impact of channel estimation filter impulse response on the BER is studied by comparing optimal and suboptimal channel estimation filters. The implementation complexity of the decorrelating and PIC receivers is compared in terms of required floating point operations and clock cycles in a practical communication scenario. It is observed that the PIC receiver is only moderately more complex to implement than the conventional matched filter bank receiver, whereas the decorrelating receiver is significantly more complex     

Differentially coherent decorrelating detector for CDMA single-pathtime-varying Rayleigh fading channels

This paper describes a differentially coherent decorrelating detector for a K-user reverse link code-division multiple-access environment that exhibits time-varying Rayleigh fading. The channel is modeled as providing only a single fading path for each user and with no additional means to achieve diversity. The design of the detector is based on using fractionally sampled matched filter outputs to simultaneously achieve two goals: 1) the novel realization of a one shot decorrelator with lower computational complexity and 2) the forming of the maximum-likelihood decision rule on the decorrelated outputs, which results in an effective increase of the correlation in the fading process. Analytical evaluation and simulation of the error probability of the detector demonstrates significant lowering of the error floor in comparison to the decorrelating detector that employs conventional differentially coherent detection     

Parallel group detection for synchronous CDMA communication overfrequency-selective Rayleigh fading channels

A group detector jointly detects a group of users, and a parallel group detection scheme is a bank of J independently operating group detectors, one for each group of a J group partition of the K transmitting users of a code-division multiple-access (CDMA) channel. In this paper, two group detectors are introduced for the frequency-selective Rayleigh fading (FSRF) CDMA channel. While the optimum multiuser detector has a time complexity per symbol (TCS) of O(M ^K/K) for M-ary signaling, each of the two group detectors has a TCS of O(M(|G|)/|G|) where |G| is the group size. Hence, there are parallel group detection schemes, based on each of the two group detectors, that satisfy a wide range of complexity constraints that result from the choice of partition. Each of the two group detectors is minimax optimal in the corresponding conditional group near-far resistance measure. Furthermore, a succinct indicator of the average BER over high SNR regions is defined via the asymptotic efficiency. A lower bound and an exact formula for the asymptotic efficiency are derived for the first and second group detectors, respectively. The group detection approach for the FSRF-CDMA channel generalizes previous approaches to the complexity-performance tradeoff problem. It yields the optimum detector when the group size is K. When the group size is equal to one, the first group detector results in a new optimum linear detector and the second reduces to a recently proposed suboptimum linear detector. All other nontrivial partitions yield new multiuser detectors whose performances are commensurate with their complexities     

An adaptive asynchronous CDMA multiuser detector forfrequency-selective Rayleigh fading channels

An adaptive asynchronous code-division multiple-access (CDMA) multiuser detector is proposed that uses a recently derived extended Kalman filter based algorithm (see Lim, T.J. and Rasmussen, L.K., IEEE Trans. Commun., vol.45, p.213-20, 1997) to perform joint data detection and parameter tracking in frequency-selective Rayleigh fading channels. A receiver structure based on this adaptive multiuser detector is presented and its performance in terms of parameter tracking and bit error rate (BER) is investigated. The receiver is a form of an adaptive RAKE that exploits multipaths to achieve performance gain     

Multiuser detectors for large CDMA random access systems over Rayleigh fading channels

In this paper, large code division multiple access (CDMA) random access systems employing the decorrelator and minimum mean square error (MMSE) detectors are investigated over Rayleigh fading channels under the assumption that both the number of users and the spreading gain tend to infinity, but their ratio converges to a constant. The signal to interference ratio (SIR) is shown to converge almost surely to a constant and the bit‐error rate (BER) is expressed as a function of the traffic load, transmission probability, channel coefficient, and distribution of transmission power. Furthermore, the throughput, the spectrum efficiency, and the stability region are analyzed and simulated. For dominating systems, it is shown that the MMSE detector achieves much higher throughput and spectral efficiency than decorrelator detector. Besides, it is also disclosed that, when the signal to noise ratio (SNR) is larger than an optimum value, the spectrum efficiency increases as the ratio of bit energy to noise power spectrum density (E_b/N₀) increases; however, when SNR is smaller than the optimum value, the spectrum efficiency decreases as E_b/N₀ increases. For ordinary stable systems, it is demonstrated that their stability region gets narrower as the traffic load increases. Copyright © 2010 John Wiley & Sons, Ltd.     

Robust VB 2D-CELP image transmission over CDMA Rayleigh fading channels

This paper describes a new error resilient analysis-by-synthesis coding scheme and its application for robust still image transmission over CDMA Rayleigh fading channels. For the purpose of providing reliable transmission, a type-I hybrid ARQ protocol is used in a concatenated Reed–Solomon/convolutional coding scheme. However, for delay-constrained applications, the protocol is used with a limited number of retransmissions in addition to limited RS interleaving. Results of coded images are provided for the uplink of a communication system, where a slowly Rayleigh fading channel is considered.     

Multi-branch DFE for Rayleigh fading multipath channels

Multipath Rayleigh fading channels which have very fast time variation can cause loss of tracking in equalisers, from which recovery without retraining is unlikely. A new multi-branch decision feedback equaliser is suggested to improve bit error rate performance while maintaining very low complexity. Simulation results demonstrate its suitability     

Finite-state Markov model for Rayleigh fading channels

We form a finite-state Markov channel model to represent Rayleigh fading channels. We develop and analyze a methodology to partition the received signal-to-noise ratio (SNR) into a finite number of states according to the time duration of each state. Each state corresponds to a different channel quality indicated by the bit-error rate (BER). The number of states and SNR partitions are determined by the fading speed of the channel. Computer simulations are performed to verify the accuracy of the model     

GQR models for multipath Rayleigh fading channels

In this paper, reduced complexity statistical models for the representation of wide sense stationary-uncorrelated scattering doubly selective fading channels are developed. Their derivation is based on the evaluation of Fourier integrals by means of Gaussian quadrature rules. The accuracy and the complexity of the proposed models is assessed, and is then compared to that provided by other modeling techniques available in the literature     

Convolutionally coded binary PSAM for Rayleigh fading channels

Recently proposed, pilot symbol assisted modulation (PSAM) uses a known pilot sequence to derive amplitude and phase references at the receiver. The authors present convolutional coding for such systems and derive the exact pairwise error probability and the Chernoff upper bound of it. A comparison among PSAM, coherent and differential detected coded systems indicates that, even at 5% Doppler fading rate, coded PSAM requires 3.5 dB more than the ideal coherent case but less than the differential case.<>     

多径非相关瑞利信道生成的改进

提出一种多径非相关瑞利信道生成改进方法。和目前其他瑞利信道生成方法相比，该方法不仅能满足瑞利信道所具备的统计特性，而且保证信号多径传播中各径信号不相关特性。通过仿真可以看出该方法的单径信道二阶特性和现有模型统计特性相仿，而多径非相关性明显较好。     

Robust space-time codes for correlated Rayleigh fading channels

Space-time (ST) coding has emerged as an effective strategy to enhance performance of wireless communications in fading environments. Many different ST coding schemes have been proposed to achieve reliable communications in independent fading channels. However, a design of robust ST codes for correlated fading channels has not been addressed. We propose a simple robust ST coding scheme that achieves robust performance over a wide range of fading conditions. The key to achieve robust performance is to formulate code design criteria that are not dependent on the channel correlation statistics. A provably robust scheme can be formulated by concatenating a full-rank ST block code with an outer encoder. We derive several robust code examples via the concatenated orthogonal ST block code and TCM construction. The simulation results show that some traditional ST codes perform poorly, whereas the proposed codes achieve robust performance over a broad range of fading conditions.     

瑞利衰落电离层信道的自适应盲均衡

研究通过具有加性高斯白噪声的瑞利慢衰落电离层信道的恒模盲均衡技术。著名的恒模算法(CMA)是一种重要的盲信道均衡方法。但是，普通恒模算法存在收敛速度慢和相位旋转等不足。采用一种修正恒模算法(MCMA)，该算法使修正的误差函数最小并且自适应学习率由接收序列即时调整。两种算法用8—PSK信号进行了盲均衡比较，模拟结果显示修正恒模算法比普通恒模算法的收敛速度快，符号间干扰(ISI)小，另外该算法均衡器输出具有既无相位旋转也无延迟等优点。     

Spread-time CDMA resistance in fading channels

We study and examine the effect of fading channels on the performance of spread-time/code division multiple access (ST/CDMA). We consider two different models for fading channel and obtain the average signal to interference plus noise ratio (SIR) for ST/CDMA in both cases. We also obtain the SIR for spread-spectrum/CDMA (SS/CDMA) and compare the results with that of ST/CDMA. Depending upon the transmitted chip pulse shape, and in the absence of thermal noise, spread-time (ST) performance is equal to or greater than spread spectrum (SS). For example ST/CDMA has 0.62 dB higher SIR value compared to SS/CDMA with root raised cosine pulse shape with rolloff factor 0.22 used in W-CDMA. The spread-time technique was first proposed by Crespo et al. (1991, 1995), and US Patents no. 5177768, 5175744, 5175743, 5173923, and is considered as the time-frequency dual of spread spectrum technique.     

FHMA receiver over Rayleigh fading channels

A novel FH/MFSK receiver is proposed which utilises the side information of interfering signals for asynchronous frequency hopping multiple-access (FHMA) systems in the presence of Rayleigh fading. It is shown that the novel receiver performs much better than the conventional receiver for a wide range of signal to noise ratios.     

Bandwidth efficient block codes for Rayleigh fading channels

It is shown that the effective code length (ECL) of a short ECL block modulation code (BCM) is the dominant factor in its performance over a Rayleigh fading channel. To demonstrate this, three new BCM codes are presented as examples. Their performances are evaluated and compared on both Gaussian and fading channels.<>     

Multiuser detection with neural network and PIC in CDMA systems for AWGN and Rayleigh fading asynchronous channels

In this paper, multiuser detection in code division multiple access (CDMA) was performed by using neural network (NN) and parallel interference cancellation (PIC). Neural network is used as a front-end stage of one stage PIC circuit. PIC is a classical technique in multi user detection process and its bit error rate (BER) performance is not good in one stage for most of the applications. For improving its BER performance, generally multi stage PIC which has high computational complexity is used. In this study, we have got better BER performance than the three stage PIC receiver in AWGN channel, almost same BER performance with the three stages PIC receiver in the Rayleigh fading channel.     

Multilevel coded 16-QAM schemes for Rayleigh fading channels

Multilevel codes show better performance compared with trellis codes on Rayleigh fading channels at comparable decoder complexity and bandwidth. However, they suffer from performance degradation due to error propagation in the multistage decoder. The authors, with a view to minimising the error propagation, compare three multilevel coded 16-QAM schemes which are four-level codes, I/Q separated two-level codes and I/Q separated two-level codes with a new set partitioning     

Error analysis for nonuniform signaling over Rayleigh fading channels

We investigate the error performance of a communication system where a nonuniform memoryless binary source is transmitted via Gray-mapped M-ary phase-shift keying or quadrature amplitude modulation over memoryless Rayleigh fading channels, and demodulated via optimal maximum a posteriori detection. Using recently derived upper and lower bounds on the probability of a general union of events, which are tight and can be efficiently computed, the system symbol-error (P/sub s/) and bit-error (P/sub b/) rates are evaluated for a wide range of channel conditions. Since for nonuniform signaling, Gray mapping is not necessarily optimal for minimizing P/sub s/ or P/sub b/ (as was recently shown by Takahara et al.), we also evaluate the system performance under the map obtained by Takahara et al. and compare it with a Gray-mapped system.     

Matched filter bound for OFDM on Rayleigh fading channels

A matched filter bound (MFB) analysis is presented, in which the normalised Doppler rate is unrestricted, and thus applicable to OFDM signalling. In contrast to the static channel case, the optimal matched filter receiver is shown to be time varying and the probability of error is shown to depend on the transmitted pulse shape