Performance of Multicarrier CDMA Rake System over Rayleigh Fading Channel

Based on the theory of multicarrier (MC) technique and the Rake receiver, a multicarrier DS-CDMA Rake system is proposed, where a data sequence multiplied by a spreading sequence modulates multiple carriers. The receiver provides a Rake for each subcarrier, and the outputs of the Rakes are combined by a maximal-ratio combiner. The average probability of error of the system is derived from an uncorrelated subcarrier and frequency-selective fading channel model. The system performances are evaluated over Rayleigh fading channel with an exponential multipath intensity profile(MIP) and with a rectangular MIP, respectively, when multipath interference is present. It is found that this kind of model has larger superiority in an exponential MIP than in a rectangular MIP.     

Optimum Multiuser Detector for Multipath Slow Fading Asynchronous CDMA Channels

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A New Sequential Detection Based on Hopfield Neural Network in Frequency Selective Fading Channels

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DS—CDMA通信正在崛起

这篇论文介绍了ＣＤＭＡ的发展概况和相关的标准；论述了ＤＳ－ＣＤＭＡ的技术能力，直接序列扩系统的性能和码分多址的优点；举例说明了ＤＳ－ＣＤＭＡ在五种通信系统的应用可行性；最后分析比较了ＣＤＭＡ和ＧＳＭ系统的经济成本和运营收益。     

DS—CDMA中的空时处理技术

随着移动通信的发展，空时处理技术越来越引起人们的重视．本文首先介绍了DS-CDMA的特点，并在此基础上引入空时处理技术．通过对信号处理的比较，可获得系统性能改善的结论。     

Optimization of MQAM Modulation Schemes in Mobile Communications（Ⅱ）—Analysis of MQAM BER Performance in Rayleigh Channel

The transmission error code performance of square MQAM and star MQAM modulation schemes for the AGWN Rayleigh fading channel is analyzed.The corresponding BERformulas and computer aided numeric results are given.Therefore it provides a theoretical basis for choosing MQAM modulation schemes in mobile communications.     

Outage Probability Analysis of Relay Systems over K-μ Fading Channels

This paper provides an analytical framework for the outage probability evaluation of dual-hop decode-and-forward relay systems operating over K-μfading channels in the presence of co-channel interference. The interferers are independent non-identically distributed K-μfading. An accurate analytical-form expression for the cumulative distribution function of the received signal power to interference and noise ratio is derived. Various numerical results are compared with Monte Carlo simulation results in order to corroborate the accuracy of the proposed expressions. Our results show that increasing the value of kappa of main links will decrease the outage probability of relay systems. Furthermore, the system performance degrades with the number of interferences.     

DS／CDMA系统中多址干扰的消除

在直接序列扩频／码分多址系统中由于各用户之间的影响而使其容量受到很大限制。能常这种用户之间的影响是作为噪声处理的。本文讨论了多址干扰问题，从另一个不同的角度出发，即把用户之间的影响作为有用信息，对多个用户的数据信息进行联合判决，提出了提高判决准确性的方案。     

Spectrally Shaped DS–CDMA with Dual Sideband Combining for Increased Diversity on Dispersive Fading Channels

A new modulation scheme is proposed in this paper. This scheme uses sinusoidal chip waveforms to shape the spectrum of a direct sequence spread spectrum (DS-SS) signal such that the transmitted signal has two distinct spectral lobes, one from a lower sideband (LSB) and the other from an upper sideband (USB). By properly selecting the frequency of the sinusoidal chip waveforms, the two sideband signals can be made to undergo independent fading in a dispersive fading channel. These two independent sideband signals, when combined at the receiver, provide diversity gain. Our analysis and simulation results show that the bit error ratio (BER) performance of the proposed scheme is superior to that of the equivalent DS-SS system that uses conventional rectangular chip waveforms for severely faded channels.     

MMSE RAKE Reception for Asynchronous DS/CDMA Overlay Systems and Frequency-Selective Fading Channels

In this work the authors introduce and analyze a detection scheme for simultaneous suppression ofmultiaccess and digital narrowband interference (NBI) for an asynchronousDS/CDMA system operatingover a frequency-selective fading channel. The proposed detector may possibly consider a processing interval greater than the bit-interval, andamounts to a RAKE structure, wherein each branch is designed under an MMSE optimization strategy, followedby a propercombining rule. It is shown that narrowband interference rejection entails ingeneral a time-varyingdetection structure. As to the performance assessment, we give closed-form formulas for the systemBit Error Rate (BER),Near-Far Resistance and Signal-to-Interference Ratio (SIR).Numerical results show that the proposed receiver proves effective in suppressing interference, and that itlargely outperforms conventional multiuser detectors which do not account forthe presence of theNBI, at the price of little complexity increase.Finally, we provide a blind adaptive implementation of the proposedreceiver, based on the exponentially weighted Recursive-Least-Squares (RLS)algorithm, andrequiring knowledge of the relevant parameters of the desired user only, andof the NBI signaling time.     

Adaptive Detection and Channel Estimation for Dual-Rate DS/CDMA Networks in Frequency-Selective Fading

In a Dual-Rate DS/CDMA system, a primary, ``high rate' network coexists witha secondary one with a slower rate. Demultiplexinghigh-rate users typically entails, at the receiverend, a time-varying processing ifVariable Spreading Length (VSL), Variable Chip Rate (VCR), or Variable ChipRate with Frequency Shift (VCRFS) modulation formats are used, and Rake-Typedemodulation schemes if the chip rate exceeds the channel coherence bandwidth.The realization of fully adaptive receivers thus requires the joint estimationof(a) the interference to be suppressed, whether from other users ofthe high-ratenetwork or from the secondary network, and (b) the channel tap weights.The solutionthat we consider here assumes that,for each user, a pilot signal is transmitted alongwith the information-bearing signal, whether in a time-multiplexed orin a parallel-channel format. We consider both the uplink and the downlink channel and, based on a Minimum-Mean-Square-Error(MMSE) optimality criterion, we have come up with a fully adaptive system,capable on the one handof tracking, through a cyclic Recursive-Least-Squares typealgorithm, the periodically time-varying variations of theinterference subspace andon the other of updating the channel tap weights estimates.A thorough performance assessment is presented, and the convergence dynamicsof the system are derived through computer simulations in order to assessthe impact of the prior uncertainty with regard to the other-users signaturesand thechannel tap weights realizations.     

Interference Modelling in DS/CDMA Cellular Systems Operating in a General Fading Environment

Multiple access interference and the mobile radio environment are the primary limitations on the performance of DS/CDMA cellular systems. This paper presents the analysis of a DS/CDMA cellular system operating in a general fading environment. In particular the issues of signal fading, multiple access interference, and power control are addressed. A computationally efficient statistical method is used in the estimation of system performance. It is assumed that the variability of each received signal can be represented by fast Nakagami_m fading plus slower log-normal shadowing. Average bit-error-rate (BER) and outage probability are estimated as system performance indicators. The analysis shows, that as the variability of the fast fading of the received signal reduces, the performance of the system improves. However, when the signal undergoes both fast fading and shadowing, it is largely the shadowing that determines system performance. Use of forward link power control (that compensates for the variability due to shadowing) results in minimal performance improvement. However, in the reverse link significant improvement in performance can be achieved using a similar power control scheme.     

衰落信道中的DS/CDMA系统盲差分自适应接收机

在本文中，我们针对标准MMSE自适应接收机在频率非选择性衰落信道中的相位滑动和失锁问题，提出了一种无需进行训练和信道参数估计的盲差分自适应接收机。为自适应地实现该接收机，我们同时提出了一种基于正交分解的盲平均随机梯度（Orthogonal Decomposition-based Blind Aver-aged stochastic Gradient，简称ODBAG）算法。仿真结果表明，这种以ODBAG算法实现的盲差分自适应接收机在瑞利（Rayleigh）衰落信道中，误码率性能比传统匹配滤波器（Matched Filter，简写为MF）接收机有显著的提高，并接近改进的MMSE自适应接收机的性能。     

Selection and MRC Diversity for a DS/CDMA Mobile Radio System through Nakagami Fading Channel

Spatial diversity is an attractive technology for coping with the fadingchannels encountered in mobile communications.In this paper, the bit error rate (BER) is analyzed theoretically fordiversity reception with a RAKE receiver in aNakagami fading environment using either selection or maximal ratio combining.A coherent binary phase-shiftkeying (CBPSK) direct sequence code division multiple access (DS/CDMA) systemis considered. An arbitrary branchcorrelation is also considered for any diversity order in the case ofidentical fading severity on the branches.     

An Uplink Short-Term Fading Prediction-Based Power Control Method for DS/CDMA Cellular Radio Systems

In previous research, a new multi-step SIR-based powercontrol, referred to as short-term fadingprediction-based (SFP-based) method, has been proposedfor DS/CDMA downlink power control. Simulation resultsshowed that the SFP method outperforms the originalmulti-step SIR-based power control. In this paper, wefurther investigate the behavior of the SFP methodapplied to uplink. Simulation results show that theSFP method still performs better than the originalmulti-step SIR-based method. Although the uplinkperformance is still poorer than the downlink usingSFP power control method instead of the originalmulti-step SIR-based method, the improvement amount ofSFP-based method compared to the multi-step SIR-basedmethod in uplink is larger than that in downlink.     

Vector Channel Estimation and Multiuser Detection for Multicarrier DS CDMA in Time and Frequency Selective Fading Channels

In a multicarrier direct-sequence code-division multiple access (MC DS CDMA) system, different fading channels for different users and/or different carriers are correlated in general; thus a vector channel model is more appropriate than disjoint scalar channel models. For multiuser MC DS CDMA systems, we propose (1) a generalized vector autoregressive model which accounts for correlation between different user/carrier fading channels, (2) the use of a two-phased algorithm to obtain the proposed model’s parameters, and (3) a receiver structure that consists of a generalized decorrelator followed by maximal-ratio combining (MRC) of uncorrelated carrier channel outputs of each user. The estimated fading coefficients provide the necessary quantities to MRC. The computer simulation results show that the proposed scheme has performance close to the case in which the channel is perfectly known, and outperforms separate scalar channel estimation case.     

Blind Channel Estimation for Multi-Rate Multicarrier DS/CDMA Communications

Multi-rate multicarrier DS/CDMA is a potentially attractive multiple access method for future wireless communications networks that must support multimedia, and thus multi-rate, traffic. Several receiver structures exist for single-rate multicarrier systems, but little has been reported on multi-rate multicarrier systems. Considering that high-performance detection such as coherent demodulation needs the explicit knowledge of the channel, based on the finite-length chip waveform truncation, this paper proposes a subspace-based scheme for timing and channel estimation in multi-rate multicarrier DS/CDMA systems, which is applicable to both multicode and variable spreading factor systems. The performance of the proposed scheme for these two multi-rate systems is validated via numerical simulations. The effects of the finite-length chip waveform truncation on the performance of the proposed scheme is also analyzed theoretically.     

O<Superscript>3</Superscript>BPSK-based Linear Decorrelating Detector for Asynchronous DS/CDMA Systems over Frequency-Selective Rayleigh Fading Channels

In this paper, a convenient signaling scheme, called orthogonal on–off BPSK (O³BPSK), along with a simple one-shot linear decorrelating detector (LDD) and a whitening Rake bank, is proposed for near–far resistant detection in asynchronous DS/CDMA systems. Based on the maximum multi-path spreading delay, a minimum duration of “off” is suggested, during which the temporally adjacent bits (TABs) that contain multi-user interference (MUI) and inter-symbol interference (ISI) from different users at the receiver are decoupled. The O³BPSK signaling scheme is combined with the whitening Rake receiver to preserve multi-path diversity gain in multi-path fading CDMA channels. The scheme offers low complexity, no detection delay, near–far resistance, and compensation for fading channels.

Performance of a Multicarrier CDMA System with Rayleigh Fading Multipath Channels

This paper examines the performance of a multicarrier CDMA system in a Rayleigh fading channel without a RAKE receiver. Since the multicarrier technique is a diversity reception technique used in the frequency domain, the signal is transmitted at different frequency bands and undergoes different fading. The superimposed signal of the multipath components after going through the mobile channel will be processed by an Fourier Transform to enter the frequency domain, where the time delays in various paths are converted to the phase shifts in the compound signal. In this paper, we use sounding bits to extract the characteristics of the mobile channel and to partly remove the multipath effect. From the simulation, it is found that the multicarrier technique is simpler than the technique used in the single carrier system with a RAKE receiver while producing a better performance than that of the RAKE receiver.