Rayleigh衰落信道中的CDMA多用户分集接收性能

本文针对频率选择性Ｒａｙｌｅｉｇｈ衰落信道，设计了一种ＣＤＭＡ多用户检测器分集结构，通过该结构，高斯信道多用户检测算法可以有效地应用到Ｒａｙｌｅｉｇｈ衰落信道中，仿真结果表明，分集合并实现的多用户中以显著改善衰落信道的误码性能，其处理多址干扰能力并不因分集而受到影响，另外，本文还对不同的合并以及不同的检测地作了性能上的比较。     

Performance Analysis of Optimum and Suboptimum Selection Diversity Schemes on Rayleigh Fading Channels With Imperfect Channel Estimates

In this paper, we present a comparative analysis on the effects of channel estimation errors on the performance of optimum and suboptimum selection diversity (SD) receivers on Rayleigh-fading channels. By modeling the estimation errors as independent complex Gaussian random variables, we derive simple closed-form expressions for the average probability of error for both optimum and suboptimum SD schemes with noisy channel estimates. With dual diversity and imperfect estimates, we establish a connection between optimum SD and maximal-ratio combining (MRC), and between suboptimum SD and equal-gain combining diversity schemes. Interestingly, we show that the optimum SD receiver structure and the resulting performance for differential binary coherent phase-shift keying (DBPSK) signaling can be obtained, in a straightforward way, as a special case of the performance of the optimum SD scheme with binary PSK signaling and channel estimation errors. For a fixed average power and bit duration, in conjunction with pilot-assisted minimum mean-square error channel estimation, we show that the optimum coherent SD scheme coincides with that of the optimum noncoherent SD scheme with binary frequency-shift keying (BFSK) signaling, whereas the coherent MRC scheme coincides with the optimum noncoherent receiver (i.e., the square-law combiner) for BFSK. The optimum number of diversity channels, under an energy-sharing mode of operation, is also studied. Finally, we formulate the problem of optimal pilot placement, consider channel estimation with a practical pilot-symbol-assisted modulation technique, and present some numerical results illustrating the comparative performances of various SD receivers     

Performance of Differentially Detected DPSK Over Nonselective Rayleigh Fading Channels With Maximal Ratio Combining and Multiple Cochannel Interferers

In this paper, we investigate the performance of differentially detected differential phase-shift keying (DPSK) modulation with postdetection maximal ratio combining, in nonselective Rayleigh fading channels with multiple asynchronous cochannel interferers. The approach is based on an analytical technique we have presented earlier in the literature. Exact bit-error probability (BEP) results for binary DPSK and quaternary DPSK are derived. More specifically, we look into the effects of symbol-timing offsets between the interfering signals and the desired signal on the error performance. Our results show that when all the interfering signals are synchronous with the desired signal, the impairment caused by the cochannel interference to the desired user is maximum. On the other hand, when all the interfering signals are half-symbol-duration-delayed with respect to the desired user, they introduce the minimum impairment. Based on these findings, upper and lower bounds on the BEP are derived in simple closed form. Our explicit BEP results also show that the error probabilities of different transmitted symbols of the desired user are affected differently by the interfering signal     

Combined Space Diversity Reception and Trellis Coding for Rayleigh Fading Channels

Space diversity reception, in which several signals received at different antennas are combined, is a well known method that can be used to combat the effects of fading in wireless systems. Also, trellis coded modulation (TCM), when combined with interleaving of sufficient depth, is known to provide some form of time diversity that allows the achievement of good error performance in fading environments. In this paper we consider the analysis of the error performance of reference-based Maximal Ratio Combining (MRC) systems when used in conjunction with trellis-coded MPSK modulation techniques over a Rayleigh fading channel. We also consider the analysis of MRC trellis-coded MPSK systems with conventional differential detection. The results are obtained by using a combination of theoretical analysis and simulation. Exact and near-exact expressions for the pairwise error-event probability in Rayleigh fading are derived. Monte-Carlo simulation results, which are more indicative of the exact system performance, are also given.     

Reception of PSK Signals Over Fading Channels Via Quadrature Amplitude Estimation

We consider here a new approach to PSK signal detection over a slow nonselective Rayleigh fading channel which does not require a carrier recovery loop. The receiver achieves coherent demodulation by making use of estimates of the quadrature amplitudes of the received PSK signals in its likelihood ratio test. The receiver is assumed to have a memory containing information on the past received signals which enables it to generate the estimates. The error rate of the receiver can be evaluated analytically and computer simulation results are presented to verify the predicted performance.     

Combining Pilot-Symbol-Aided Techniques for Fading Estimation and Diversity Reception in Multipath Fading Channels

A novel pilot-symbol-aided (PSA) fading estimation technique that combines the estimates from a conventional PSA technique and a bandwidth-efficient PSA technique to achieve better performances is proposed for digital signals in multipath fading channels. The conventional technique has better performances at low signal-to-noise ratios (SNRs), while the bandwidth-efficient technique is superior at high SNRs. Monte Carlo computer simulation has been used to assess the effects of the proposed combining technique on the bit-error-rate (BER) performances of 16-ary quadrature-amplitude-modulation (16QAM), with and without two-branch diversity reception, in a flat Rayleigh fading channel. Results have shown that the combining technique has the advantages of both of the conventional technique and the bandwidth-efficient technique and is more preferred for use with diversity reception. Man-Hung Ng received a BSc degree in Computer Studies from City University of Hong Kong in 1991. He worked as a computer programmer in Hong Kong from 1991 to 1995. In 1996, he obtained a MSc degree in Communication and Radio Engineering from King's College London. He joined the University of Hong Kong as a research assistant in 1997, and completed a PhD degree in mobile communications in 2001. He joined Lucent Technologies N.S. UK in 2001 and is now a principal standards engineer. Sing-Wai Cheung received the BSc degree in Electrical and Electronic Engineering from Middlesex University, U.K. in 1982 and the PhD degree from Loughborough University of Technology, U.K. in 1986. From 1986–1988, he was a post-doctorate research assistant in the Communications Research Group of King's College, London University. During 1988–1990, he was with the Radio and Satellite Communications Division in British Telecom Research Laboratories (now British Telecom Laboratories). He joined the Department of Electrical and Electronic Engineering at the University of Hong Kong in 1990 and is now an Associate Professor. He contributes regularly courses on mobile and satellite communications systems. His current research interests include modulation, coding, fading compensation and diversity and MIMO for mobile and satellite communications systems.     

Performance Analysis of Joint Adaptive Modulation and Diversity Combining Over Fading Channels

Both adaptive modulation and diversity combining represent important enabling techniques for future generations of wireless communication systems. In this paper, capitalizing on recent developments in adaptive combining, we propose three joint adaptive modulation and diversity combining (AMDC) schemes. With these schemes, the modulation mode and diversity combiner structure are adaptively determined based on the fading channel condition and error-rate requirement. We accurately analyze these three AMDC schemes in terms of processing power consumption, spectral efficiency, and error-rate performance. Selected numerical examples show that the proposed AMDC systems meet the target error-rate requirement while achieving high spectral efficiency with low processing power consumption     

采用两条支路分集接收的相关瑞利衰落信道容量

本文研究采用两条支路最大比合并(MRC)或选择合并(SC)分集接收的相关瑞利衰落信道理论容量推导恒定发射功率自适应M进制正交幅度调制(M-QAM)的频谱效率,并将它们与独立同分布瑞利信道理论容量进行比较,其结果对收发信机之间无视距分量路径、接收机上分集天线之间的距离小于半个波长的无线通信系统设计具有指导作用.     

Switched Dual Diversity Reception of M-ary DPSK Signals on Nakagami Fading Channels

The paper deals with dual diversity reception of M-ary differential phase-shift keying modulated signals in the presence of additive white Gaussian noise and Nakagami-distributed slow and nonselective fading. The performance of a switched diversity system is analysed and compared to that of the predetection selection diversity combining scheme. The general case of correlated diversity branches is considered, without restrictions on the fading severity parameter. Average symbol error rate formulas are analytically derived in terms of integral expressions that can be easily computed via numerical integration routines. Moreover, the numerical evaluation of the optimum switching threshold is carried out and the influence of the fading severity parameter, the branch correlation, and the cardinality of the symbol alphabet is analysed. Finally, three fixed switching threshold strategies that allow to obtain a satisfactory diversity gain are considered.     

Packet Radio Performance Over Slow Rayleigh Fading Channels

Expressions for the throughput and average packet delay for a Pure-ALOHA single-hop packet radio system operating in slow Rayleigh fading are derived. For noncoherent frequency-shift-keying (NCFSK), an exact closed form expression is presented. For coherent phase-shift-keying (CPSK) an excellent approximation for large packet sizes is derived. This approximation technique is valid in general for other modulation schemes and for other fading channel statistical characterizations. The packet length which maximizes the useful data throughput in slow Rayleigh fading is found. The results of this investigation indicate that a packet radio system can be designed with a modest link margin for fading and achieve identical throughput performance over a nonfading channel and a fading channel with only a small increase in average packet delay for the fading channel.     

Optimized Selection Diversity for Rayleigh Fading Channels

This paper considers the choice of the number of diversity branches for minimizing the bit error rate of a selection diversity system using noncoherent binary frequency-shift keying modulation for transmission over a Rayleigh fading Gaussian channel. An exact expression for the optimum number of branches,L_{sel}^{ast}, is given. The resulting probability of bit error decreases exponentially with the square root of the energy per bit.     

Error Performance of DQPSK with EGC Diversity Reception over Fading Channels

This paper derives the average bit error probability (BEP) of differential quaternary phase shift keying (DQPSK) with postdetection equal gain combining (EGC) diversity reception over independent and arbitrarily correlated fading channels. First, using the associated Legendre functions, the average BEP of DQPSK is analyzed over independent Rayleigh, Nakagami-m, and Rician fading channels. Finite-series closed-form expressions for the average BEP of DQPSK over L-branch independent Rayleigh and Nakagami-m fading channels (for integer Lm) are presented. Besides, a finite-series closed-form expression is given for the average BEP of differential binary phase shift keying (DBPSK) with EGC over independent Rician fading channels. Second, an alternative approach is propounded to study the performance of DQPSK over arbitrarily correlated Nakagami-m and Rician fading channels. Relatively simple BEP expressions in terms of a finite sum of a finite-range integral are proposed. Moreover, the penalty in signal to noise ratio (SNR) due to arbitrarily correlated channel fading is also investigated. Finally, the accuracy of the results is verified by computer simulation.     

瑞利衰落信道MDPSK相位比较解调性能

本文提出一种适合于瑞利衰落信道多进制差分相移键控系统相位比较解调的性能分析方法，推导出符号错误率解析表示式，考虑了瑞利衰落、时间选择性衰落和高斯白噪声对体制性能的影响，并针对衰落功率谱为高斯型和矩形分别对二进制、四进制和八进制情形进行了数值计算。     

SER Performance of Amplify-and-Forward Cooperative Diversity Over Asymmetric Fading Channels

In this paper, the performance of amplify-and-forward (AF) cooperative diversity is analyzed over asymmetric fading channels. The source–relay and the relay–destination links experience Rayleigh fading while the source–destination link is subject to generalized Gamma fading. First, the probability density function (PDF) and the moment generating function (MGF) of the source–relay–destination link and the MGF of the source–destination link are derived. Then, the symbol error rate (SER) is determined based on the MGF of the total end-to-end signal-to-noise ratio (SNR). Moreover, the SER performance of N-relay assisted AF cooperative diversity is illustrated for M-ary phase shift keying (M-PSK) and M-ary quadrature amplitude modulation (M-QAM). Based on the derived MGF expressions, the numerical results are obtained by varying the modulation types and channel parameters for different scenarios.     

Performance Analysis of Diversity Combining Algorithms in Shadowed Fading Channels

A compound fading model incorporating short term fading and shadowing proposed recently is used to analyze the performance of wireless systems employing microscopic diversity to mitigate the effects of flat fading. This model can account for the presence of different levels of fading and shadowing and provide an analytical solution for the probability density function of the signal-to-noise ratio. Using that model, the performances of MRC and SC diversity combining algorithms were studied. The amount fading (AF) following diversity implementation was calculated and it is seen that the decline in the amount of fading is bound by the level of shadowing present, with the MRC providing a larger decrease in the amount of fading than the SC algorithm. The effect on the error rates was studied using the example of the coherent BPSK modem. Results show that the performances of wireless systems can be analyzed using the compound model for the shadowed fading channels. P.M. Shankar received his M. Sc (1972) in Physics from Kerala University, India, M. Tech (1975) in Applied Optics and Ph. D. in Electrical Engineering (1980) from Indian Institute of Technology, Delhi, India. He was a visiting scholar at the School of Electrical Engineering, University of Sydney, Australia, from 1981 to 1982. He joined Drexel University in 1982 and is currently the Allen Rothwarf Professor of Electrical and Computer Engineering. He is the author of the textbook ‘Introduction to Wireless Systems’, published by John Wiley & Sons, 2002. His research interests are in Fading Channels, Wireless communications, and Statistical signal processing for medical applications.     

Performance Analysis of Multiuser DS-CDMA in MIMO Systems Over Rayleigh Fading Channels

The performance of space-time transmit diversity is examined in a multiuser direct-sequence code-division multiple-access (DS-CDMA) system over fast- and slow-fading channels. The underlying space-time system employs transmit antennas and receive antennas at the mobile user and receiver base station, respectively. We consider the performance of the space-time multiuser system when using the linear decorrelator detector to combat the effect of multiuser interference. In our analysis, we derive a closed-form expression for the probability of bit error for both fast- and slow-fading channels. These theoretical results are shown to be very accurate when compared to system simulations. Both simulations and theoretical results prove that, regardless of the system load, the full diversity order of for fast-fading channels and for slow-fading channels is always maintained, and only a signal-to-noise ratio (SNR) loss is incurred. This SNR loss is proved to be a function of only the number of users (i.e., level of interference) and independent of the number of transmit and/or receive antennas. Using our theoretical results, we show that the loss in SNR from the single-user bound can be well approximated by , where represents the level of multiuser interference.     

Performance Analysis of MIMO Based CDMA Code Acquisition Over Rayleigh Fading Channels

This paper proposes a new MIMO based CDMA code acquisition scheme. The pilot codes consist of a number of short Gold code sequences which are transmitted in parallel using a group of transmit antennas. Reception diversity is performed by multiple receive antennas at the receiver. Three different acquisition detection techniques are proposed and compared. Corresponding threshold optimizations are investigated as well. Detection and false alarm probabilities are derived in closed form based on the outputs of non-coherent matched filters. The acquisition performance is evaluated in terms of mean acquisition time (MAT) in Rayleigh fading environment. It is shown that the proposed MIMO acquisition scheme exhibits a much better MAT performance than the conventional single-antenna acquisition scheme. The results reveal that multiple receive antennas can be utilized to significantly reduce the MAT at the expense of receiver complexity increase. On the other hand, increasing the number of transmit antennas makes the MAT performance more robust in the presence of strong interference.     

Chaos-Coded Modulations Over Rician and Rayleigh Flat Fading Channels

In this brief, we analyze a kind of chaos-coded modulations over both Rician and Rayleigh frequency non-selective uncorrelated fading in the presence of additive white Gaussian noise. We provide bounds both for the case when perfect channel-state information (CSI) is available at the decoder and when there is no CSI. We show that the bounds proposed can be tight enough to give reason of the behavior of these systems in a flat fading channel. We compare the results with a related trellis-coded modulation and show that the degradation in performance can be at least as low as with conventional coded modulation systems.     

相关瑞利分集系统容量的研究

一般来说,充分利用无线信道的特性能够极大地提高通信系统的容量,但是对于相关瑞利信道的平均容量讨论的不多。文章借助于一种线性变换的方法,得出了一种相关瑞利分集情况下容量的近似表达式,简化了在通常情况下的复杂的表达式。数值计算的结果也形象地说明了相关系数对分集容量的影响,以及分集能够有效的提高系统的容量。     

Cross Layer Design for Multiaccess Communication Over Rayleigh Fading Channels

An information theoretic queueing model is proposed in a wireless multiple access communication setup. The proposed symmetric N user model captures physical layer parameters such as the encoding rate, transmit power and Medium Access Control (MAC) layer metrics such as queue stability. Two alternative medium access strategies are considered: centralized scheduling and ALOHA. Next, a cross-layer approach is taken wherein the maximum stable throughput of the system is achieved by a joint optimization over the MAC parameters (viz., scheduling set size with scheduling and transmission probability with ALOHA) and the encoding rate. Performance comparisons with traditional layered designs are given. It is shown that in the low and high SNR regimes, layered designs are close to optimal whereas in the moderate SNR range, cross-layer designs outperform layered schemes. Exact characterizations of the ";low"; and ";high"; SNR regimes are given quantitatively. It is also shown that ALOHA with transmission probability one is optimal in the low SNR regime.