Performance of DS/CDMA systems with differential M-ary orthogonal modulation and RS-coding for LEO satellite communications

This paper presents a coded modulation scheme based on M-ary orthogonal modulation by means of Walsh–Hadamard (WH) sequences, suitable for low-earth-orbit (LEO) direct sequence/code division multiple access (DS/CDMA) satellite communication systems. Based on the IS-95 scheme, we consider Reed–Solomon (RS)-coded M-ary orthogonal modulation with error or erasures decoding, which presents good performance enhancement with low complexity. LEO satellite links are characterized by large Doppler frequency shifts caused by the difference in velocity between the satellite and the earth mobile terminal, which make conventional non-coherent detection ineffective. In order to overcome the phase shift variations during the symbol period, which result in orthogonality loss of the WH sequences, we applied a differential encoding process to the spreading sequences or the WH chips prior to transmission. A special diversity process suitable for the environment under consideration is also applied. Simulation results show that the proposed diversity/coding/modulation scheme attains very good performance at low transmitter/receiver complexity. © 1998 John Wiley & Sons, Ltd.     

Interference cancellation technique using channel parameterestimation in DS/CDMA system with M-ary orthogonal modulation

A channel estimation technique in the uplink of a noncoherent DS/CDMA system with M-ary orthogonal signalling over multipath fading channels is considered. This method is used for the multiuser interference cancellation with coherent detection. The performance of the proposed multiuser detector is compared with that of a conventional detector and a significant improvement of performance is shown     

Performance of multicode DS/CDMA with noncoherent M-ary orthogonal Modulation in multipath fading channels

This paper presents the performance analysis and simulation of a multicode direct-sequence code-division multiple-access system with noncoherent M-ary modulation, in a multipath fading environment. This type of transceiver is specified for the reverse link of the IS-95B and cdma2000 (radio configurations 1 and 2) systems and is intended to serve high-rate applications such as data transfer and video communications. While previous studies considered the analytical error performance of coherent multicode systems, little attention has been devoted in the literature to the noncoherent case. We provide concise and useful expressions for the interference terms as a function of the commonly used aperiodic cross correlation functions. After a statistical characterization of these terms, we make use of the Gaussian approximation (GA) in order to obtain the bit-error rate (BER). However, unlike some other analyses (for coherent detection) relying on the GA, in our derivation, we take into account the fact that all the codes transmitted by a mobile user fade in unison. As demonstrated via computer simulations, this fact is crucial to obtain a reliable estimate of the BER, especially when equal-gain combining (EGC) is used at the receiver. The analysis is also extended to include a simple closed-loop power control algorithm and hard handoff between multiple cells. In particular, we verify-for the multicode case-previous observations that the use of EGC allows improvement only for a certain range of values of the total interference seen at the receiver: When either the number of interfering users is too large, or too many codes are assigned to the high-rate user, the noncoherent combining loss becomes such that the use of many diversity branches can decrease the performance as compared to a system with little or no diversity.     

Adaptive receiver for DS/CDMA communications over impulsive noisechannels

A new adaptive algorithm is proposed for training soft-limiter based correlation receiver in which the direct sequence code division multiple access signals corrupted by impulsive symmetric a-stable noise are demodulated. The new adaptation algorithm allows simpler implementation and faster convergence speed in comparison with the traditional adaptive stochastic gradient-based algorithms     

A low complexity reduced-rank MMSE receiver for DS/CDMA communications

The minimum mean squared error (MMSE) receiver is a linear filter which can achieve optimal near-far resistance in direct-sequence code-division multiple-access communications. However, one of the main problems of this receiver is the required number of filter taps, which is typically large. This is especially true in systems with a large processing gain in which case the receiver's computation burden becomes very high. As a result, methods for reducing the complexity of the MMSE receiver have been of great interest in recent years. We propose an efficient partitioned MMSE receiver based on a classification algorithm. It is shown that the computational complexity (in terms of the filter taps) of the proposed receiver can be reduced significantly while good performance is maintained. Based on the special structure of our proposed receiver, we also propose a release-merge adaptive partition algorithm which can update the partition and the receiver's coefficients simultaneously. In particular, it is demonstrated that the proposed receiver can perform much better than previously proposed reduced-rank MMSE receivers, such as the partial despreading MMSE receiver and the cyclically shifted filter bank receiver, with even a smaller number of taps.     

Performance of RS-Coded DS/CDMA Microcellular Systems with M-ary Orthogonal Signaling

This paper investigates the application of Reed-Solomon coding on DS/CDMA systems with M-ary orthogonal signaling for the reverse link of a microcellular environment. The performance of voice and data communications are analytically evaluated for a Rician or Rayleigh faded channel with lognormal shadowing and a two-slope path loss model. The effects of sectorization and antenna diversity are also considered. Numerical results, in terms of bit error probability and throughput, show that with proper selection of the coding rate and spreading, the proposed system presents enhanced performance with moderate complexity.     

Performance of selection diversity for DS/CDMA communications overRayleigh fading channels

An analytical model to evaluate the performance of selection diversity is presented. Results indicate that the conventional selection diversity scheme. in which the branch with the largest signal-to-noise ratio (SNR) is chosen, does not accurately reflect the performance of the more commonly implemented selection systems, in which the largest signal-plus-interference and noise (S+I) is chosen. Owing to the statistical nature of the noise, S+1 selection diversity performs better than conventional selection diversity model     

Adaptive rate and power DS/CDMA communications in fading channels

We investigate the average information rates attained by adapting the transmit power and the information rate relative to channel variations in code division multiple access communication systems. Our results show that the rate adaptation provides a higher average information rate than the power adaptation for a given average transmit power, and the rate increase when using rate adaptation is more significant for channels with a faster decaying multipath intensity profile and weaker line-of-sight component     

Efficient use of DS/CDMA signals for broadband communications over power lines

This paper investigates the use of direct‐sequence/code‐division multiple access (DS/CDMA) signals for broadband communications over power lines. Each user is assumed to utilize all available spreading codes for sending the information to the destination. The transmitter and the receiver are assumed to have perfect channel knowledge with the receiver employing a zero‐forcing multiuser detector. Based on channel knowledge we attempt to maximize the data throughput by suitable choice of the number of codes used and the power and the constellation size (bit‐load) assigned to the data modulating each spreading code. We employ Gold codes, in addition to special codes derived based on the channel knowledge for ISI minimization, termed ‘eigen codes’. In contrast to some earlier results concerning CDMA and OFDM, we show that DS/CDMA signals can be optimized to achieve an overall data throughput of approximately 80% of that achieved by OFDM systems. This result shows that DS/CDMA signaling can be a good candidate for broadband power line communications. Copyright © 2011 John Wiley & Sons, Ltd.     

DS/CDMAM-ary orthogonal signalling in a shadowed Rayleigh channel for mobile communications

In this paper a modified RAKE receiver is studied for a frequency selective mobile radio channel. The reverse link (Mobile to base station) is analysed, assuming lognormal shadowing and Rayleigh fading andK asynchronous users, withM orthogonal sequences per user. The analysis is based on the consideration of the quadrature components of the signal and noise, taking advantage of the multipath effects. The performance evaluation is carried out in terms of both the bit error rate and outage probability in order to qualify completely the proposed receiver. The positive results assure the possibility of applying this system in a microcellular mobile radio environment.     

Performance analysis of DS/CDMA with noncoherent M-ary orthogonalmodulation in multipath fading channels

The performance of a DS/CDMA system using M-ary orthogonal modulation with noncoherent demodulation is evaluated. The system operates in a multipath fading channel. A RAKE receiver structure with equal gain combining is used for demodulation. An approximation to the bit error probability is given which depends only on the first- and second-order moments of the multipath energies. The analysis results are compared with the results from computer simulations. It is seen that the approximation is accurate for multipath energies with realistic coefficient of variation. The system performance is also evaluated in terms of the capacity, which is defined as the number of users that can be supported at a given bit error probability. The approximation is used to evaluate the capacity reductions due to power variations caused by multipath fading     

Code-aided interference suppression for DS/CDMA communications. I.Interference suppression capability

A code-aided technique for the simultaneous suppression of narrow-band interference (NBI) and multiple-access interference (MAI) in direct-sequence code-division multiple-access (DS/CDMA) networks is proposed. This technique is based on the linear minimum mean-square error (MMSE) algorithm for multiuser detection. The performance of this technique against MAI has been considered previously. In this paper, its performance against NBI as well as combined NBI and MAI is addressed. Specifically, the performance of this technique against three types of narrow-band interferers, namely, multitone interferers, autoregressive (AR) interferers, and digital interferers, is analyzed. The NBI suppression performance of this method is then compared with performance bounds of the linear and nonlinear estimator-subtracter NBI suppression techniques. It is seen that this method outperforms all of the previous linear or nonlinear methods of NBI suppression, while simultaneously suppressing the MAI     

Power-adaptation strategies for DS/CDMA communications with successive interference cancellation in Nakagami-fading channels

In this paper, power adaptation for direct-sequence code-division multiple-access communications that employs a successive interference cancellation (SIC) receiver is considered. The transmission power is adapted so that, with the channel variations, the received power levels of each user have appropriate disparities. Under the constraint of average transmission power, we consider two strategies in adjusting the disparity between received signal powers. With the first strategy, the average bit-error rate (BER) for a given user averaged over channel fading statistics is minimized, while with the other, the instantaneous BER is equal for all users. We find that the performance difference between the two strategies becomes negligible as the average transmission power or line-of-sight component increases. We also discuss the impact of appropriate disparity in received power levels on the BER performance of SIC receivers.     

DS/CDMA with two sets of orthogonal spreading sequences anditerative detection

In this letter we introduce a direct-sequence code-division multiple access (DS/CDMA) concept which accommodates a higher number of users than the spreading factor N. Each of the available orthogonal spreading sequences of length N is assigned to one of the first N users which employ a common pseudonoise (PN) scrambling sequence. When the number of users K exceeds N, say K=N+M with MN. The proposed technique thus accommodates N users without any mutual interference and a number of additional users at the expense of a small signal-to-noise ratio penalty     

Interference suppression for DS/CDMA

A direct-sequence spread spectrum (DS/SS) receiver for suppressing multiple-access interference in direct-sequence code-division multiple-access (DS/CDMA) communication systems is introduced. The proposed receiver does not require knowledge of other users' spreading codes, timing, or phase information. Moreover, the receiver allows the number of taps to be chosen independently of the processing gain and, hence, is easily applicable to CDMA systems employing either a small or a large processing gain. Performance analysis, including average probability of error and signal-to-noise ratio, is provided, and results are presented for systems varying from lightly loaded (for example, eight user/255 chip) to heavily loaded (for example, 50 user/200 chip). Performance results indicate that the proposed receiver outperforms the linear correlation receiver and, in many cases, it does so by a considerable margin     

Multistage decision-directed channel estimation scheme for DS-CDMAsystem with M-ary orthogonal signaling

A multistage decision-directed channel estimation scheme is proposed for a coherent direct-sequence code-division multiple-access (DS-CDMA) system with M-ary orthogonal signaling. The receiver with a multistage decision-directed channel estimator makes alternate use of symbol detection and channel estimation in an iterative manner, resulting in a multistage structure where each stage consists of an estimator and a detector. The estimator obtains the estimates of the channel by using the decisions from the detector in the previous stage, and the detector uses these estimates for coherent detection of the transmitted symbols. Modifications to the process of generating the channel estimates and the decision variables are made to mitigate the error propagation in successive stages. It is shown that the performance of the proposed estimation scheme is better than the conventional scheme, especially for channels with high diversity order. The gain is mostly achieved by two stages for channels with low diversity order and by five stages for channels with high diversity order. The proposed scheme can be used to increase the capacity of DS-CDMA systems currently operating in such environments     

Code-aided interference suppression for DS/CDMA communications. II.Parallel blind adaptive implementations

For pt.I see ibid., vol.45, no.9, p.1101-11 (1997). An adaptive code-aided technique for the simultaneous suppression of narrow-band interference (NBI) and multiple-access interference (MAI) in direct-sequence code-division multiple-access (DS/CDMA) networks is proposed. This technique is based on the recursive least-squares (RLS) version of the minimum mean-square error (MMSE) algorithm for multiuser detection. The convergence dynamics of the RLS blind adaptive algorithm for suppressing the combined NBI and MAI are analyzed. The steady-state performance of this algorithm in terms of the signal-to-interference ratio (SIR) is also derived. Systolic array structures for parallel implementations of the RLS adaptive interference suppression algorithms are then proposed. Versions of the rotation-based QR-RLS algorithms for both the blind adaptation mode and the decision-directed adaptation mode are derived. These algorithms exhibit high degrees of parallelism, and can be mapped to VLSI systolic arrays to exploit massively parallel signal processing computation     

Combined rate and power adaptation in DS/CDMA communications overNakagami fading channels

We consider combined rate and power adaptations in direct-sequence code-division multiple-access communications, where the transmission power and the data rate are adapted relative to channel variations. We discuss the power gain that the combined adaptations provide over power adaptation. Then, we consider an integrated voice and data transmission system that offers a constant bit rate voice service, using power adaptation and a variable bit rate data service with rate adaptation. We present an expression for the required average transmission power of each traffic type having different quality-of-service specifications and discuss the capacity gain over power adaptation for voice and data     

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。     

DS／CDMA系统中的干扰抑制

WCDMA技术是无线宽带通信发展方向之一，其抗干扰技术成为WCDMA系统中的关键技术。本文就DS/CDMA系统的抗干扰性及其干扰抑制进行分析和探讨。