Simulation of MC-CDMA in the Presence of Timing Errors

1　ＩｎｔｒｏｄｕｃｔｉｏｎＴｈｅｔｈｉｒｄｇｅｎｅｒａｔｉｏｎｏｆｍｏｂｉｌｅｃｏｍｍｕｎｉｃａｔｉｏｎｓｙｓｔｅｍｓｗｉｌｌｂｅａｂｌｅｔｏｐｒｏｖｉｄｅａｗｈｏｌｅｖａｒｉｅｔｙｏｆｖｏｉｃｅ ,ｄａｔａａｎｄｖｉｄｅｏｓｅｒｖｉｃｅｓｆｏｒｔｈｅｕｓｅｒ.Ｍｕｃｈｒｅ ｓｅａｒｃｈｈａｓｂｅｅｎｄｏｎｅｏｎｔｈｅｕｓｅｏｆｗｉｄｅｂａｎｄ ＣＤＭＡｆｏｒｓｕｃｈｓｙｓｔｅｍｓ.ＴｈｅｍａｉｎａｄｖａｎｔａｇｅｓｏｆＣＤＭＡａｒｅｗｅｌｌｋｎｏｗｎ ,ｈｏｗｅｖｅｒ,ｉｔｓｃａｐａｃｉｔｙｉｓｌｉｍ…     

基于频分的MC-CDMA多用户系统

对多载波码分多址系统的多用户接收进行了研究,提出了基于频分的多载波码分多址(FD—MC—CDMA)系统,它是在MC—CDMA基础上结合FDMA的思想,发射端为每个用户分配一个特定的子载波集进行数据传输,减少相同子栽波上所叠加用户的数目;接收端使用最大似然检测器,利用所有接收信号进行多用户联合检测。该系统与传统的MC-CDMA系统相比有保证频率分集增益的同时降低多址干扰、有效利用多用户信息的优点。经过在Matlab平台上的仿真,结果表明,在频率选择性衰落信道和相同接入用户数的情况下,该系统与传统的MC—CDMA系统相比拥有更低的误码率,并且误码率随着用户数的增加呈阶梯状的缓慢递增。     

Narrow‐band interference suppression in wavelet packets based multicarrier multicode CDMA overlay system

The wavelet packets based multicarrier (MC) multicode (MCD) code‐division multiple‐access (CDMA) transceiver consists of the MCD part, which ensures the transmission for high speed and flexible data rate; the MC part contributing to robustness to frequency‐selective fading and flexibility for handling multiple data rates; and wavelet packets (WPs) modulation technique, which contributes to the mitigation of the interference problems. As WPs have lower sidelobes compared with sinusoidal carriers, this system is very effective in reducing the problem of inter‐carrier interference. Of course, like any CDMA system, the system can suppress a given amount of interference. This paper considers an interference suppression scheme which will enhance the performance of the system. The receiver employs suppression filters to mitigate the effect of narrow‐band jammer interference. The framework for the system and the performance evaluation are presented in terms of the bit error rate and the outage probability over a Nakagami fading channel. Also, we investigate how the performance is influenced by various parameters, such as the number of taps of the suppression filter and the ratio of narrow‐band interference bandwidth to the spread‐spectrum bandwidth. Finally, the performance of the system is compared with the performance of sinusoidal based MC/MCD‐CDMA system denoted Sin‐MC/MCD‐CDMA. Copyright © 2012 John Wiley & Sons, Ltd.     

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.     

载波误差对MC-CDMA系统性能的影响

该文对下行无线通信信道中MC-CDMA系统的载波误差性能进行了研究。对接收判决器的输入信噪比(SNR)和判决误码率(BER)进行了详细的理论分析和数值模拟。结果表明,MC-CDMA对载波频率偏移十分敏感,系统性能随载波数量的增加而迅速下降;在最大载荷下,载波相位抖动导致的性能下降与载波数无关。     

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.     

Performance of multicarrier CDMA with DPSK modulation anddifferential detection in fading multipath channels

Multicarrier (MC) direct sequence (DS) code division multiple access (CDMA) with differential phase-shift keying (DPSK) modulation and differential detection is proposed. Transmitted data bits are differentially encoded after serial-to-parallel conversion to a number of parallel streams. On each branch, encoded bits are direct sequence spread spectrum (SS) modulated and transmitted using different carriers. The system is analyzed with a differential detector in static Rayleigh fading multipath channel, in fast Rayleigh fading multipath channel and for variable overlapping between carrier spectra in static fading channel. Closed-form expressions are derived for the error probability and evaluated for many cases. The performance is compared to that of a system using phase-shift keying (PSK) with conventional matched filter (CMF) coherent receiver. For static fading channel, the error probability performance of the differential detector is close to that of CMF receiver. For fast fading, the performance degrades slightly with increasing fading rate. Finally; successive carriers of the system are allowed to overlap with various overlapping percentages. The condition of a single path can be achieved by increasing both the number of carriers and the separation between successive carriers. Also, for each number of carriers, there exists an optimum overlapping percentage at which the system performance is optimized. The performance of the proposed DPSK with differential detection system is close to that of PSK with CMF receiver, but the former is simpler to implement     

Multicarrier DS/SFH-CDMA systems

Multicarrier direct-sequence/slow-frequency-hopping (MC DS/SFH) code-division multiple-access (CDMA) systems are proposed, in which multiple carriers are modulated by the same DS waveform and hopped in frequency according to a random hopping pattern. The receiver dehops the received signal with the same pattern, provides RAKE receivers for each carrier, and combines the outputs with a maximal ratio combiner (MRC). The performance of the proposed system is investigated over a frequency-selective Rayleigh-fading channel and compared to that of the MC DS-CDMA systems. It is shown that for the same diversity order, the MC DS/SFH-CDMA systems are superior in reducing multiple-access interference (MAI) while preserving the good capability of narrowband interference suppression, when the system parameters are selected properly.     

Rayleigh衰落信道下相位误差对相干跳频多址接入MRC分集接收系统的影响

该文研究一种新型混合通信系统,即基于多载波BPSK调制的相干跳频多址接入(FH/CDMA)系统,接收端采用最大比合并(MRC)分集接收。由于采用相位调制,接收端载波相位恢复的精确度直接影响系统性能。假设载波同步通过一阶锁相环电路(PLL)完成,由于载波恢复环路中存在高斯噪声,当系统达到同步稳态时不可避免地会产生稳态相位误差。该文采用Beaulieu级数方法和特征函数方法,得到理想相干和部分相干(存在稳态相位误差)时FH/CDMA系统误码率性能表达式。仿真结果表明,相对于单用户系统,多用户系统误码率受到相位误差影响较小,误码率性能降低的主要因素是多用户干扰。MRC分集接收能提高系统抵抗相位误差和信道衰落的能力。     

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.     

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.     

Effect of jamming and diversity on the performance of a wavelet packet based multicarrier multicode CDMA communication system

In this paper, the performance of multicarrier (MC), multicode (MCD), code division multiple access (CDMA) system employing wavelet packets (WPs) as subcarriers for modulation in the presence of narrowband interference is investigated. The performance analysis include the effects of diversity techniques such as selection, equal gain and maximal ratio combining. The system performance is evaluated by means of bit error rate (BER) and outage probability. The effects of system parameters such as channel intensity profile, diversity order and fading parameter on the system performances were investigated by means of analytical methods and numerical results. Also, the effect of jamming power is investigated. The performance of the system is compared to that of MC/MCD-CDMA based on sinusoidal carrier. Results reveal considerable performance improvement of the system that employ diversity techniques over the one without diversity and of our proposed system over the other system.     

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     

Performance of a Modified Multicarrier Direct Sequence CDMA System

In this paper, we present an improved multicarrier direct sequence (DS) code division multiple access (CDMA) scheme by modifying the system originally proposed by Kondo and Milstein [13]. In this modified system, different spreading sequences multiplied by a data sequence modulate different carriers. This is to prevent the multiple access capability from reducing when the fading characteristics of different carrier frequencies are highly correlated. We have derived a formula which determines the mean values of the relative received signal strength in a single carrier DS CDMA rake system and in a multicarrier DS-CDMA system. We present results on the comparison of the bit error rate (BER) performance of the two systems including the effect of correlation between fading characteristics of different frequencies under various multipath fading conditions. The results indicate that with 50 users the modified multicarrier DS CDMA system can achieve an uncoded irreducible BER of 1.7 × 10^??3 with an average received signal-to-noise ratio per bit of 10 dB, which is better than 3.0 × 10^??3 achieved by the single carrier DS CDMA rake system, and also show that if multicarrier CDMA system is used with respect to single carrier CDMA system, the SNR gain is up to 4.5 dB for the uncode BER of 10^??3 being achieved.     

Low‐Power‐Adaptive MC‐CDMA Receiver Architecture

This paper proposes a novel concept of adjusting the hardware size in a multi‐carrier code division multiple access (MC‐CDMA) receiver in real time as per the channel parameters such as delay spread, signal‐to‐noise ratio, transmission rate, and Doppler frequency. The fast Fourier transform (FFT) or inverse FFT (IFFT) size in orthogonal frequency division multiplexing (OFDM)/MC‐CDMA transceivers varies from 1024 points to 16 points. Two low‐power reconfigurable radix‐4 256‐point FFT processor architectures are proposed that can also be dynamically configured as 64‐point and 16‐point as per the channel parameters to prove the concept. By tailoring the clock of the higher FFT stages for longer FFTs and switching to shorter FFTs from longer FFTs, significant power saving is achieved. In addition, two 256 sub‐carrier MC‐CDMA receiver architectures are proposed which can also be configured for 64 sub‐carriers in real time to prove the feasibility of the concept over the whole receiver.     

Diversity Performance of a Wavelet-Packet-Based Multicarrier Multicode CDMA Communication System

In this paper, we propose and analyze a novel multicarrier (MC) multicode (MCD) code-division multiple-access (CDMA) system employing wavelet packets (WPs) for modulation. This system can achieve robust performance against multipath fading due to the localization of WPs in the time and frequency domains. The analytical framework is presented, and the system performance with diversity is evaluated by means of bit error rates and the outage probability . Since WPs have lower sidelobes compared to sinusoidal carriers, our system is very effective in reducing the problem of intercarrier interference. The effects of system parameters (e.g., order of diversity, fading parameters, and WP type) were investigated. The major contribution is to compare the performance of the system to that of the MC/MCD-CDMA system that is based on sinusoidal carriers. The results reveal a considerable performance improvement of our proposed system over the MC/MCD-CDMA system.     

Tone diversity for OFDMA in broadband wireless communications

This paper presents two group‐based tone diversity schemes, namely, group‐orthogonal tone‐combining‐diversity (GO‐TCD) and group‐optimal tone‐selection‐diversity (GO‐TSD), for orthogonal frequency division multiple access (OFDMA) system in broadband wireless communications. In both schemes, the entire frequency band of N sub‐carriers is divided into N_G = N/L_f sub‐carrier groups with each having L_f sub‐carriers. When the channel knowledge is not available at the transmitter, GO‐TCD, which has a split‐and‐group structure to reduce the peak‐to‐average ratio (PAR) and employs the multi‐user detection for maximum‐likelihood (ML) estimation to increase the number of active users, is proposed and shown to provide a lower PAR and similar performance as compared with the group‐orthogonal multi‐carrier CDMA, and outperforms the random‐hopping (RH)‐OFDMA and matched‐filter based MC‐CDMA. When the channel knowledge is available at the transmitter, GO‐TSD can be used to select the best sub‐carrier in a given sub‐carrier group for any active user and shown to offer a throughput comparable to the optimal tone selection but with much less complexity. Copyright © 2006 John Wiley & Sons, Ltd.     

多载波直接序列扩频码分多址系统的性能分析

本文将多载波发送信号技术应用于直接序列扩频码分多址系统中。发射机中直接扩频序列与数据序列相乘,然后调制多个载波;接收机对每个载波进行相关,相关器的输出用最大比值合并。这种发送信号具有很好的窄带干扰压缩性能。文中估计了存在部份带宽干扰下系统的性能,并与单载波直扩码分多址系统进行了比较。     

Performance analysis of Doppler shift effects on OFDM‐based and MC‐CDMA‐based cognitive radios

The present development of high data rate wireless applications has led to extra bandwidth demands. However, finding a new spectrum bandwidth to accommodate these applications and services is a challenging task because of the scarcity of spectrum resources. In fact, the spectrum is utilized inefficiently for conventional spectrum allocation, so Federal Communications Commission has proposed dynamic spectrum access mechanism in cognitive radio, where unlicensed users can opportunistically borrow unused licensed spectrum, which is a challenge to obtain contiguous frequency spectrum block. This also has a significant impact on multicarrier transmission systems such as orthogonal frequency division multiplexing (OFDM) and multicarrier code division multiple access (MC‐CDMA). As a solution, this paper develops non‐contiguous OFDM (NC‐OFDM) and non‐contiguous MC‐CDMA (NC‐MC‐CDMA) cognitive system. The implementation of NC‐OFDM and NC‐MC‐CDMA systems provides high data rate via a large number of non‐contiguous subcarriers without interfering with the existing transmissions. The system performance evaluates NC‐OFDM and NC‐MC‐CDMA for mobile scenario where each propagation path will experience Doppler frequency shift because of the relative motion between the transmitter and receiver. The simulation results of this paper proved that NC‐OFDM system is a superior candidate than NC‐MC‐CDMA system considering the mobility for cognitive users. Copyright © 2013 John Wiley & Sons, Ltd.     

多载波CDMA系统中的频域线性相位内插算法

多载波CDMA系统结合了正交频分复用（OFDM）技术和CDMA技术的优点,自1993年提出以来就受到广泛的重视。本文主要讨论了载波频偏对连续OFDM符号的影响,提出了一种新的频域线性相位内插算法来估计信道响应,以对抗载波频偏对连续OFDM符号引起的相位旋转。理论分析和仿真试验都证明我们提出的算法能有效减少载波频偏对系统性能的影响。这种新算法适用于使用导频信号块状分布的多载波CDMA系统或OFDM系统。