Performance Investigation of Space‐Time Block Coded Multicarrier DS‐CDMA in Time‐Varying Channels

In this letter, we evaluate the system performance of a space‐time block coded (STBC) multicarrier (MC) DS‐CDMA system over a time selective fading channel, with imperfect channel knowledge. The average bit error rate impairment due to imperfect channel information is investigated by taking into account the effect of the STBC position. We consider two schemes: STBC after spreading and STBC before spreading in the MC DS‐CDMA system. In the scheme with STBC after spreading, STBC is performed at the chip level; in the scheme with STBC before spreading, STBC is performed at the symbol level. We found that these two schemes have various channel estimation errors, and that the system with STBC before spreading is more sensitive to channel estimation than the system with STBC after spreading. Furthermore, derived results prove that a high spreading factor (SF) in the MC DS‐CDMA system with STBC before spreading leads to high channel estimation error, whereas for a system with STBC after spreading this statement is not true.     

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.     

Multi‐carrier platform for wireless communications. Part 2: OFDM and MC‐CDMA systems with high‐performance,high‐throughput via innovations in spreading

Multi‐carrier technologies in general, and OFDM and MC‐CDMA in particular, are an integral part of the wireless landscape. In this second part of a two‐part survey, the authors present an innovative set of spreading codes known as CI codes, and demonstrate how these significantly increase performance and capacity in OFDM and MC‐CDMA systems, all the while eliminating PAPR concerns. Regarding OFDM: the spreading of each symbol over all N carriers using CI spreading codes (replacing the current one symbol per carrier strategy) are presented. CI codes are ideally suited for spreading OFDM since, when compared to traditional OFDM, CI‐based OFDM systems achieve the performance of coded OFDM (COFDM) while maintaining the throughput of uncoded OFDM, and, at the same time, eliminate PAPR concerns. When applied to MC‐CDMA, CI codes provide a simple means of supporting 2N users on N carriers while maintaining the performance of an N‐user Hadamard Walsh code MC‐CDMA system, i.e., CI codes double MC‐CDMA network capacity without loss in performance. The CI codes used in OFDM and MC‐CDMA systems are directly related to the CI pulse (chip) shapes used to enhance TDMA and DS‐CDMA (see part 1): hence, the CI approach provides a common hardware platform for today's multi‐carrier/multiple‐access technologies, enabling software radio applications. Copyright © 2002 John Wiley & Sons, Ltd.     

A near‐Nyquist rate transmission: A new minimum‐bandwidth direct‐sequence code division multiple access scheme

Bandlimited direct‐sequence code division multiple access (DS‐CDMA) attracts much attention for its compact spectrum and the ability to suppress inter‐symbol interference. Among the various bandlimited DS‐CDMA systems available, minimum‐bandwidth DS‐CDMA (MB‐DS‐CDMA) is the only realizable Nyquist rate transmission system. But, MB‐DS‐CDMA only applies to certain kinds of spreading codes. Accordingly, this study proposes a modified DS‐CDMA structure which extends the application of MB‐DS‐CDMA to all common spreading codes at the expense of a negligible reduction in the transmission rate. Additionally, the bit error rate of the proposed schemes adopting either single‐user or multi‐user detection receiver is analyzed and compared with that of the commonly‐used raised‐cosine‐pulsed DS‐CDMA over multipath fading channels. The numerical results show that given a sufficiently large number of users, the bit error rate performance of modified MB‐DS‐CDMA is comparable to that of the raised‐cosine‐pulsed DS‐CDMA scheme; meanwhile, the realizable modified MB‐DS‐CDMA approaches the ultimate transmission rate.     

Application of Rate Compatible Punctured Turbo Coded Hybrid ARQ to MC‐CDMA Mobile Radio

MC‐CDMA, a multicarrier (MC) modulation scheme based on code division multiple access (CDMA), is the most likely candidate for the next generation of mobile radio communications. The rate compatible punctured turbo (RCPT) coded hybrid automatic repeat request (HARQ) has been found to give improved throughput performance in a direct sequence (DS) CDMA system. However, the extent to which the RCPT HARQ improves the throughput performance of an MC‐CDMA system has not been fully understood. In this paper, we apply the RCPT HARQ to MC‐CDMA and evaluate by computer simulations its performance in a frequency selective Rayleigh fading channel. We found that the performance of RCPT HARQ MC‐CDMA is almost insensitive to channel characteristics. The performance can be drastically improved with receive diversity combined with space‐time transmit diversity. In addition, the comparison of RCPT HARQ MC‐CDMA, orthogonal frequency division multiplexing, and DS‐CDMA shows that under similar conditions the throughput of MC‐CDMA is the best in a frequency selective fading channel.     

采用广义正交序列的多载波DS-CDMA系统在瑞利衰落信道中的性能分析

在CDMA系统中，采用正交序列可以有效地减小多址干扰的影响。然后，当由于多径衰落的影响使码片间的相对移位增加时，系统中用户扩频序列间的相互正交性减弱，导致性能大大恶化。本文基于新近提出的广义正交序列，分析了采用该序列的多载波直接序列码多分址系统在瑞利衰落信道中的误比特性能。数值结果表明，当扩频序列间相互移位在零相关区之内时，扩频序列仍可保持正交，因而大大提高了系统误比特性能。     

A truncated adaptive transmission scheme for hybrid multicarrier CDMA/FDM systems in forward link

A truncated adaptive transmission scheme for the hybrid multicarrier CDMA/FDM system is considered in forward link. In the proposed scheme, a data substream is transmitted over the subchannels of which the channel gains are greater than a given threshold, based on the feedback information from the mobile station. We analyze the performance of the proposed system when orthogonal and random signature sequences are used in single- and multiple-cell environment. In the single-cell environment, when orthogonal signature sequences are used, the proposed scheme outperforms the adaptive FH/DS system as well as the conventional MC DS/CDMA system, and accommodates more users than the adaptive FH/DS system while maintaining the orthogonality between users. In the multiple-cell environment also, the proposed scheme has better performance characteristics than the adaptive FH/DS system when orthogonal or random codes are used as spreading sequences.     

Multi‐carrier platform for wireless communications. Part 1: High‐performance,high‐throughput TDMA and DS‐CDMA via multi‐carrier implementations

Multi‐carrier technologies in general, and OFDM and MC‐CDMA in particular, are quickly becoming an integral part of the wireless landscape. In this first of a two‐part survey, the authors present the innovative transmit/receive multi‐carrier implementation of TDMA and DS‐CDMA systems. Specifically, at the transmit side, the pulse shape (in TDMA) and the chip shape (in DS‐CDMA) corresponds to a linear combining of in‐phase harmonics (called a CI signal). At the receiver side, traditional time‐domain processing (equalization in TDMA and RAKE reception in DS‐CDMA) is replaced by innovative frequency based processing. Here, receivers decompose pulse (or chip) shapes into carrier subcomponents and recombine these in a manner achieving both high frequency diversity gain and low MAI. The resulting system outperforms traditional TDMA and DS‐CDMA systems by 10–14 dB at typical BERs, and, by application of pseudo‐orthogonal pulse shapes (chip shapes), is able to double system throughput while maintaining performance gains of up to 8 dB. Copyright © 2002 John Wiley & Sons, Ltd.     

MC DS/SFH-CDMA systems for overlay systems

We introduce the slow frequency hopping (SFH) technique to the multicarrier (MC) code-division multiple-access (CDMA) systems for overlay situations. Using lower chip rate, which results in a narrower spectrum for each carrier and hopping the signal in frequency, the MC direct sequence (DS)/SFH system achieves better performance than the MC DS CDMA system in most cases, especially when the bandwidth of the narrowband interference (NBI) is narrower than one subchannel. It also exhibits a stable performance against the variations of the NBI location and bandwidth. When there is no NBI, the two systems perform approximately the same. The evaluation is performed over a frequency selective Rayleigh fading channel, with both Gaussian approximation and Monte Carlo simulation     

Space-Time Receiver for Multi-Rate CDMA Systems: Multi-Code and Variable Spreading Length Access Methods Comparison

This work proposed the linear minimum mean square error (LMMSE) space-timereceivers for synchronous multi-rate direct sequence code devision multipleaccess (DS/CDMA) systems. The performance of the proposed receivers isanalyzed. The high-rate (HR) LMMSE space-time receiver with combiningtechnique can provide essentially the same performance for low-rate (LR) usersas LR LMMSE space-time receiver while eliminate the time delay for the HRusers. The performance of the proposed multi-rate LMMSE space-time receiversare studied in the context of two multi-rate access methods: multi-code (MC)access where high data rate users multiplex their information streams ontomultiple codes, and variable spreading length (VSL) access where signaturesequences of different lengths are assigned to users with different datarates.Through the simulation results, we show that: (a) the proposed multi-rateLMMSE space-time receivers are optimum near-far resistant; (b) the proposedmulti-rate LMMSE space-time receivers have a clear performance improvementcompared to the time only LMMSE dual rate receiver; (c) there is a performancegain of the proposed multi-rate LMMSE space-time receivers compared to themulti-rate decorrelating space-time receiver when near-far ratio (NFR) is low;(d) The performance difference between MMSE combining and MRC in multi-ratesystem is much more pronounced than in single-rate system. Simulations alsoshow that the VSL access method is more robust than MC access methodconsidering changing rate-ratio in multi-rate systems.     

A Toeplitz displacement method for blind multipath estimation forlong code DS/CDMA signals

The problem of blind channel identification for direct-sequence/code-division multiple-access (DS/CDMA) multiuser systems is explored. For wideband DS/CDMA signals, multipath distortion is well modeled by a finite-impulse response filter. In this work, a blind channel identification technique based on second-order statistics is investigated. The method exploits knowledge of the spreading code of the user of interest via matched filtering, as well as properties of spreading codes. The current scheme focuses on a method appropriate for randomized long sequence DS/CDMA. This access scheme poses special challenges as the spreading codes are time varying. An analytical approximation of the mean-squared error is derived using perturbation techniques. The performance of the algorithm is studied via simulation and through the mean-squared error approximation, which is observed to be tight     

Performance Analyses of OFDM–CDMA Receivers in Multipath Fading Channels

Orthogonal frequency-division multiplexing (OFDM) is a promising scheme for multicarrier (MC) transmission. The combination of OFDM and code-division multiple access (CDMA) which is referred to as OFDM–CDMA, has recently attracted much research interest in wireless communications. In this paper, we evaluate the downlink performance for two forms of adaptive OFDM–CDMA receivers in multipath fading channels: 1) MC–CDMA and 2) MC–direct sequence (DS)–CDMA. We propose theoretical minimum mean square errors (MMSEs) for MC–CDMA and MC–DS–CDMA to compare the performances in different situations. Moreover, one drawback of OFDM schemes is the rate reduction due to the cyclic prefix (CP) overhead, which significantly affects the channel utilization in MC–DS–CDMA systems. We propose adaptive receiver structures with both subcarrier- and code-interval equalizations to improve the performance of MC–DS–CDMA systems with shorter or no CP. Simulation results demonstrate the effectiveness of the proposed receivers and the correctness of the theoretical derivations.      

基于多用户去相关的波达方向估计新方法

以直接序列码分多址(DS/CDMA)系统中的天线阵为背景,讨论了一种在多径慢衰落信道环境下的波达方向(DOA)估计新方法.该方法在基站对各用户各路径同步信息的基础上,根据扩频序列表现出的周期性和非完全正交的性质,利用用户多径与多址干扰表现出的信息,相关处理后再去相关,实现对各用户各路径的DOA及幅值估计.它不是简单地忽略掉解扩后残留的多径与多址干扰,或仅将其当作噪声处理,而是通过去相关的方法消除它们的影响.理论推导与仿真实验表明,该方法提高了DOA的估计性能,很好地实现了多径信道的空间分离,为后面进一步的空时处理提供了可能.     

广义多载波CDMA技术研究

本文描述了几种不同的多载波CDMA技术 ,并将这几种技术统一成一种格式 ,这被称为广义多载波CD MA ,并给出了仿真结果     

Design of next-generation cdma using orthogonal complementary codes and offset stacked spreading

This article presents an innovative code-division multiple access system architecture that is based on orthogonal complementary spreading codes and time-frequency domain spreading. The architecture has several advantages compared to conventional CDMA systems. Specifically, it offers multiple-access-interference-free operation in AWGN channels, reduces co-channel interference significantly, and has the potential for higher capacity and spectral efficiency than conventional CDMA systems. This is accomplished by using an "offset stacked" spreading modulation technique followed by quadrature amplitude modulation, which optimizes performance in a fading environment. This new spreading modulation scheme also simplifies the rate matching algorithms relevant for multimedia services and IP-based applications.     

DS/CDMA系统中的一种自适应单用户接收机

本文提出了一种在异步多径DS/CDMA系统中基于独立分量分析算法的单用户接收机,这种接收机仅需要已知期望用户的定时和特征波形,计算量小、结构简单.它不但能用于在基站进行多用户信号的上行接收,而且适合于用在移动台进行下行接收.其性能较之匹配滤波器和盲MMSE多用户检测器有很大提高.     

Performance analysis and improvement of decorrelating detection for multirate DS/CDMA

We study access strategies for decorrelating detection applied in multirate direct-sequence code-division multiple-access (DS/CDMA) systems, including multimodulation (MM), multicode (MC), and variable-spreading-length (VSL) schemes by jointly considering signal constellations and multiple-access interference. The mathematical analysis shows that when the number of active users is large, the MM scheme outperforms MC and VSL schemes especially for high-rate transmission. We also conclude that the design of modulation is important in MC and VSL schemes. Numerical analysis demonstrates that applying 4-PSK instead of 2-PSK in MC and VSL schemes can improve about 9 dB performance gain. In addition, by considering cross-correlation of noise components, we propose a detector that minimizes the symbol error probability under the constraint that the complexity grows linearly with the number of active users as decorrelating detectors. Simulations show that about 4 dB performance gain over conventional decorrelating detectors can be achieved for multirate DS/CDMA communications.     

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.     

Throughput Analysis of a Decentralized RLAN Based on Asynchronous DS/CDMA Using Random Periodic Spreading Sequences

Packet throughput figures are obtained for a decentralized radio local area network (RLAN) which is based on asynchronous direct-sequence code-division multiple-access (DS/CDMA). Packets arrive at the receiver nodes with different power levels. Techniques are developed to derive the probability of packet success for a system employing random periodic spreading sequences. It can be shown, that this system performs far better than a network using random spreading sequences. Based on the packet error probability, throughput figures of slotted DS/CDMA-ALOHA are presented for various scenarios. The effect of applying forward error correction (FEC) is investigated. For finite user environments, additional sources of errors have to be considered. These have a major impact and reduce the overall system performance. Finally, the throughput figures of a system applying the binary exponential backoff algorithm to avoid unstable behavior is investigated. The performance figures of the various systems described in this paper show that DS/CDMA forms a valuable choice for future RLANs.     

Effect of antenna array on the uplink orthogonal multicarrier DS‐CDMA system with imperfect power control

In this paper we analyze the effects of antenna array (AA) and imperfect power control on the performance of the uplink synchronous and/or asynchronous orthogonal multicarrier (MC) direct sequence‐code division multiple access (DS‐CDMA) system in multipath Rayleigh fading channels. Bit error rate (BER) performance is evaluated in terms of the number of antennas, the number of total subcarriers, power control error (PCE), and the number of users. Our numerical results show that the available user capacity of synchronous uplink is more than 1.5 times higher than that of asynchronous uplink, even though PCE increases. Copyright © 2009 John Wiley & Sons, Ltd.