多载波CDMA技术简介

CDMA技术能提供高容量、软切换,可望用于多媒体业务,但其性能受到多址干扰和符号间干扰的影响;正交频分复用对频率选择性衰落不敏感,可以用于无线环境中高速数据的传送,综合两者的优点,所形成的多载波CDMA具有很强的竞争力,可望用于第三代移动通信系统。本文介绍了三种多载波CDMA技术,即MC-CDMA、多载波DS-CDMA和多音CDMA,并简单介绍了它们的发射和接收技术。     

低峰值平均功率比的多载波CDMA

多载波CDMA技术是CDMA和正交频分复用(OFDM)技术的综合,具有抗干扰能力强、频谱效率高、数据传输速率高等优点,在未来个人通信业务(PCS)和第三代移动通信中具有良好的发展前景。但是,多载波CDMA信号峰平比比较高,影响到功放的功率效率,这已成为目前多载波CDMA系统实用化的一大障碍。本文利用格雷互补序列的特性有效降低多载波CDMA信号的峰平比,对于任意载波数的多载波CDMA信号,其峰平比上限为6dB。     

几种CDMA技术及其性能比较

CDMA是一种码分多址技术，可提供远远超过传统接入技术(如TDMA和FDMA)的容量并能抗信道的频率选择性衰落。作为一种移动无线通信技术，CDMA技术在不断发展，出现了几种有竞争力的方案，诸如：正交频分复用(OFDM)的多载波CDMA，即所谓的MC—CDMA：直接序列扩频的多载波CDMA(MC—DS—     

基于正交互补码的CMDA系统仿真

给出一种用于未来移动通信的CDMA系统,即基于正交全补码的多载波CDMA系统,其中的核心技术是使用正交全补码编码,叙述了CDMA系统结构和多载波技术,介绍了正交全补码编码原理,分析了正交全补码的多载波系统编码领域的最新CDMA系统特点,计算机仿真误码率结果表明：提出的基于正交全补码的CDMA比传统的CDMA具有更好的性能。     

应用空时编码的正交多载波直接序列扩频CDMA系统

本文把空时编码(STC)技术应用于正交多载波直接序列扩频CDMA(MC—DS—CDMA)系统，给出了空时编码MC—DS—CDMA系统的发射机和接收机方案。计算机仿真给出了应用空时编码的MC—DS—CDMA系统性能，和普通MC—DS—CDMA以及应用空时编码的多载波CDMA(MC—CDMA)系统的性能比较也由计算机仿真给出。     

基于OFDM的码分多址技术及其性能仿真

给出了多载波CDMA系统构架。目前提出有3种方案,分别是:频域扩频(MC CDMA)、时域扩频(MC DSCDMA)和多音CDMA(MT CDMA)。以MC CDMA,MC DS CDMA为例,对两种模型在AWGN和Rayleigh信道的性能进行了仿真。仿真数据表明:多载波频率分集CDMA系统(MC CDMA)在无线宽带数据传输中性能大大优于时域扩频CDMA系统,具有良好的应用前景。     

从调制方式看4G

讨论了多载波技术(MCM)与CDMA技术相结合的多载波码分多址(MC-CDMA)技术的原理及其应用的优缺点,认为该技术将在下一代无线通信系统中发挥重要的作用.     

改进的多载波CDMA系统性能分析

多载波CDMA技术能够提供大的系统容量、对抗瑞利衰落和消除符号间干扰等优点,成为具有发展前景的调制技术之一。本文基于各用户服务指标(QoS)的不同,提出了一种自适应子载波分配的改进的多载波CDMA系统。通过在衰落信道下的仿真,结果表明:采用本文提出的自适应子载波分配算法的改进MC—CDMA系统较之传统的MC-CDMA系统的误码率性能有了较大程度的改善。     

瑞利衰落信道中MC-CDMA系统的载波频率偏移性能

多载波CDMA（MC－CDMA）技术结合了OFDM和CDMA技术的优点,在无线衰落信道中传输高速数据时具有突出的性能。但由于MC－CDMA采用了多载波技术,载波频率偏移对系统性能的影响非常大。本文对瑞利衰落信道下存在载波频率偏移的MC－CDMA系统性能进行了理论分析并对误码率进行了详细讨论。     

基于优化多带复小波的多载波CDMA及反向链路性能

在分析多载波码分多址的原理基础上,利用优化生成的多带复小波,提出了一种基于优化多带复小波的多载波CDMA系统反向链路模型,并采用基于多天线的空间分集合并(SDC)技术进一步完善所提系统.该系统可利用优化多带复小波的优良特性来避免通常多载波CDMA由于插入循环前缀所带来的频谱效率的下降,在此基础上我们研究了所提系统采用空间分集合并技术时在瑞利衰落信道下的反向链路性能;给出相应的误比特率分析.理论分析和仿真结果表明采用基于最大比合并的SDC技术可显著提高所提系统抗空间衰落和各种干扰的能力,而且基于SDC技术的所提系统要好于同样基于SDC技术的通常多载波CDMA系统和基于实小波包的多载波CDMA系统.     

多载波CDMA容量的研究

该文对多载波CDMA容量规划进行了详细的理论和数学分析,推导出多载波CDMA上下行链路的容量的表达式,利用拉格朗日乘数求极值的方法,分别得到多载波CDMA话音和数据业务上下行链路最大容量;同时得出一个非常重要的结论:只有当即基站最大发射功率平均分配给每个子载波时,多载波CDMA系统才能获得最大的容量。该文还对影响容量的因素进行了仿真,各业务的容量随平均路径损耗的增加急剧下降。由此揭示了容量和覆盖的内在关系:容量和覆盖是相互制约的一对矛盾,容量的增加就意味着覆盖的减小,反之亦然。无论上下行链路,当功率达到一定程度时,再通过增大功率来提高容量并不是行之有效的方法。最后,通过对GSM,WCDMA,多载波CDMA 3个系统容量的比较,得出多载波CDMA下行链路频谱效率为WCDMA频谱效率的1.7倍,GSM 的2.7倍;上行链路频谱效率为WCDMA频谱效率的2.1倍,GSM 的2.4倍的结论。     

双正交多码CDMA系统在衰落信道的性能分析

在直接序列扩频通信（ＤＳ／ＣＤＭＡ）系统中,为了有效地实现用户高数据率的传输,以适应宽带ＣＤＭＡ技术发展的要求,本文提出了一种双正交多码ＣＤＭＡ系统（称为ＢＭＣ－ＣＤＭＡ）。相似于多载波调制,传输的高数据泫首先进行串－并转换,然后对变换后的低数据流用双正交码进行扩频调制。不同于多载波调制,各支路扩频信号用同一载波进行调制。并对此系统在衰落信道性能进行了计算,还对数值结果进行了分析。     

A unified analysis for coded DS-CDMA with equal-gain chip combining in the downlink of OFDM systems

This paper presents a novel unified analysis for the bit-error rate in the downlink of convolutionally coded and single-user detected multicarrier code-division multiple access (CDMA), multicarrier direct-sequence CDMA, time and frequency-domain spreading CDMA, and orthogonal frequency-division multiple access. Performance is analyzed under the assumption of Hadamard spreading codes, equal-gain chip combining, and a highly correlated frequency-selective Rayleigh-fading channel. Closed-form expressions are presented for the cumulative distribution function, probability distribution function, and moment-generating function for the signal-to-noise plus interference ratio after despreading. The presented results assume error-free channel estimates, a perfectly synchronized receiver, and are found to reasonably well agree with simulation results.     

Comparison of MC-CDMA with DS-CDMA using frequency domain and time domain RAKE receivers

In this paper a multicarrier CDMA (MC-CDMA) system with a soft decision differential phase shift keying (DPSK) frequency domain RAKE receiver is described. We compare a MC-CDMA system with a direct sequence CDMA system using RAKE receivers. In contrast with previous MC-CDMA systems, guard intervals are not used and the carriers are spaced at the reciprocal of the bit rate, optimising the usage of the bandwidth. In this way a comparison can be made between the multicarrier CDMA system described and a direct sequence (DS-CDMA) system with the same bandwidth. The results presented are received bit error rates from Monte Carlo simulations. The simulations are conducted in a multipath channel with Rayleigh fading and 300 Hz Doppler spectrum with additive white Gaussian noise. It is shown that the multicarrier CDMA matched filter receiver performs favourably compared to the direct sequence CDMA matched filter receiver for 1 -path fading. For a single user at a receive bit error rate of 1×10^–3 in the 4-path fading channel the multicarrier RAKE receiver requires no knowledge of the channel delay spread and performs 3 dB worse than the DS-CDMA RAKE receiver simulated. The performance of the MC-CDMA RAKE receiver for a single user increases with increasing channel dispersion. The performance of the DS-CDMA RAKE receiver for multiple user is superior to that of the MC-CDMA RAKE receiver.     

A Novel Joint Detection of Frequency Offset and MAI for MC-CDMA Systems

While multicarrier CDMA techniques are used to reduce interference and improve the performance of the system in fading channel, carrier frequency offset and multiple access interference remain major obstacles for a multicarrier CDMA system's performance. In this article we propose a novel joint detection method called modified maximum likelihood multistage parallel interference canceller (M-ML-MPIC) to eliminate the frequency offset and MAI simultaneously. The main idea of our approach is to combine PIC multiuser detection and frequency offset estimation based on a maximum likelihood function with guard interval. We discuss our scheme and report on its performance using a set of simulation experiments.     

Overview of multicarrier CDMA

The authors present an overview of new multiple access schemes based on a combination of code division and multicarrier techniques, such as multicarrier code-division multiple access (MC-CDMA), multicarrier direct sequence CDMA (multicarrier DS-CDMA), and multitone CDMA (MT-CDMA)     

Multicarrier CDMA for cellular overlay systems

A code-division multiple access (CDMA) cellular overlay system is investigated, employing the idea of multicarrier CDMA, which has previously received significant attention as an alternative to traditional single-carrier CDMA. Overlay is pursued here as a means of long-term transition from narrowband cellular to CDMA cellular. A major result of this paper is the demonstration that the use of multicarrier CDMA in a fading channel is particularly beneficial to the narrowband system, as the CDMA users can reduce their transmitted powers as a result of diversity. Another significant conclusion is that the use of transmitter notching in the CDMA system in order to avoid active narrowband users outperforms a strategy in which a narrowband user is avoided by simply dropping the particular carrier which overlays it. Finally, results on the use of the minimum mean-squared error (MMSE) receiver in a fading channel are extended for use in the overlay scenario     

Asynchronous multicarrier DS-CDMA using mutually orthogonalcomplementary sets of sequences

The use of sets of multiple spreading sequences per user in multicarrier code-division multiple-access (CDMA) is investigated. Each user is assumed to have a distinct set of spreading sequences, with a different spreading sequence for each carrier in each user's set. We show that when these sets of sequences are chosen to be the mutually orthogonal (MO) complementary sets of sequences, multiple-access interference is minimal on a nonfading channel. As a result of the autocorrelation sidelobe cancellation properties of the MO complementary sequences, it is possible to pack symbols more closely together on the nonfading channel, resulting in a higher data rate than in multicarrier CDMA using the same spreading sequence for each carrier. The resulting communication system scheme results in an easily parallelized receiver architecture that may be useful in nonfading coherent channels, such as the optical fiber channel or the Rician channel with a strong line-of-sight component. On the Rayleigh fading channel, the performance of the system is identical to that of multicarrier CDMA employing a single spreading sequence per user, with only a minimal increase in receiver complexity     

Multicarrier CDMA overlay for ultra-wideband communications

In this letter, the performance of multicarrier code-division multiple-access (CDMA) systems is studied in the presence of narrowband interference for future ultra-wideband (UWB) communications. A Nakagami fading channel is assumed, and notch filters along with diversity techniques are used in the multicarrier CDMA receiver. A complete performance analysis of error probability is given. It is shown that when the number of subcarriers jammed by narrowband interference is small, the multicarrier receiver without notch filters can work well, due to the gain of frequency diversity from nonjammed subcarriers. On the other hand, when the number of subcarriers jammed by the narrowband interference is large, using notch filters can improve the multicarrier system performance significantly.