基于离散粒子群算法的MC—CDMA多用户检测

多载波码分多址(MC-CDMA)系统中主要存在多径和多址两种干扰.多径干扰可通过正交频分复用(OFDM)技术来消除,多址干扰需采用多用户检测技术来消除.提出了一种基于离散粒子群算法的MC-CDMA多用户检测方法.因离散粒子群算法中需设置的参数较步.计算简单,故提出的MC-CDMA多用户检测法计算复杂度较低.仿真结果表明,该多用户检测方法的抗噪声能力、抗远近效应能力及系统容量均优于最小均方误差多用户检测,更加逼近最佳多用户检测的性能.     

空时分组码MC-CDMA系统多用户检测

本文研究了在频率选择性瑞利衰落信道中工作于时分双工(TDD)模式的多载波码分多址(MC-CDMA)系统上行链路,在用户终端处使用两个发射天线,采用基于空时分组码的发射分集。考虑对应于子载波的衰落系数是信道冲激响应的离散傅里叶变换,给出了基于空时分组码的MC-CDMA系统上行链路信号模型。采用适用于同步CDMA系统的低代价Steiner估计器来进行基于空时分组码的MC-CDMA系统的信道估计,每个用户终端的两个发射天线各分配一个midamble。研究了基于空时分组码的MC-CDMA系统的解相关多用户检测、最小均方误差(MMSE)多用户检测,进行联合的多用户检测和空时码解码。仿真结果验证了上述模型及算法的有效性。     

MC-CDMA系统中去相关多用户检测器的研究

多载波码分多址(MC-CDMA)结合了多载波传输技术和直接序列扩频技术,在带宽和用户数方面具有一定的优势,多用户检测作为一种提高CDMA和MC-CDMA系统容量的技术引起了人们的广泛关注。本文研究了MC-CDMA系统中的去相关多用户检测器,并对有无多用户检测算法下的MC-CDMA系统性能差异进行了仿真分析比较。     

MC-CDMA在VHF航空信道中的性能研究

文章分析多载波码分多址系统的信号传输模型和甚高频（VHF）航空信道模型，对多载波码分多址（MC-CDMA）系统在航空信道下的误码性能进行了研究和仿真。VHF航空通信信道拥挤，多径现象严重，时延和多普勒频移很大，MC-CDMA结合利用正交频分复用和码分多址技术的优点，能够有效的对抗多径干扰，并提供更大的通信容量，具有良好的应用前景。     

基于旋转不变技术实现多用户交织OFDM频偏估计

本文提出一种针对多用户交织OFDM载频偏移估计的算法。首先介绍上行链路中交织OFDM的信号结构,利用其周期性质,形成特定的数据矩阵,利用旋转不变技术(ESPRIT)实现载频偏移估计。这种估计方法不需要利用任何训练序列或导频信号,进一步提高了系统频带利用率,降低了计算复杂度。仿真表明,该估计算法在信噪比大于零的情况下也能获得较好的频偏估计。     

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。     

基于最小误码率准则的多载波CDMA系统多用户检测算法

本文介绍了一种将正交频分复用(OFDM)技术和码分多址(CDMA)技术相结合的多载波CDMA (MC-CDMA)系统,分析了MC-CDMA系统的发射端和接收端的结构,建立了MC-CDMA系统的信号模型,详细分析了MC-CDMA中的多址干扰形式,以及针对MC-CDMA系统的多用户检测原理,提出了一种基于最小误码率准则的多用户检测算法。理想的基于最小误码率准则的多用户检测算法在实际中难于实现,本文采用估计概率密度函数的方法,推导出了一种自适应实现的最小误码率多用户检测算法。仿真结果表明,该算法能够有效抑制多址干扰,降低误码率,同时,该算法还具有比基于最小均方误差准则的多用户检测器更低的误码率的优点。      

多用户交织OFDM频偏估计

同步技术是OFDM的关键技术之一。提出一种针对多用户交织OFDM载频偏移估计的算法。首先分析上行链路中交织OFDM的信号结构，利用其周期性质导出特定的数据矩阵，实现频偏估计。这种估计方法不需要利用任何训练序列或导频信号，提高了系统频带利用率。仿真表明，该估计算法在信噪比大于零的情况下都能获得较好的频偏估计。     

基于编码法降低MC-CDMA系统峰均比技术研究

分析了多载波码分多址（MC-CDMA）系统模型及扩频码序列性能,根据MC-CDMA系统特点,研究了采用GOLAY码、Gold码及Walsh码等不同扩频序列及不同序列长度对MC-CDMA系统峰均比的影响。仿真分析了序列长度为16、32及64位时MC-CDMA系统峰均比性能。     

第四代移动通信系统的调制技术

概要介绍了第四代移动通信系统特点及关键技术,系统介绍了OFDM载波技术,重点介绍了多载波码分多址(MC-CDMA)和正交频分复用时分多址(OFDM-TDMA)两种多载波调制(MCM)技术。     

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。     

On the performance and capacity of an asynchronous space-time block-coded MC-CDMA system in the presence of carrier frequency offset

In this paper, the bit-error rate (BER) performance and capacity of asynchronous space-time block-coded (STBC) multicarrier code-division multiple-access (MC-CDMA) systems in the presence of carrier frequency offset (CFO) between the transmitter and receiver oscillators are analyzed. The exact BER expression when using equal gain combining (EGC) and the approximate BER expression when using maximum ratio combining (MRC) are derived. These BER expressions are verified through simulations. Using these derived expressions, the achievable system capacity satisfying a minimum BER requirement can be studied for the two cases when EGC and MRC are used and, hence, it is possible to compare the achievable capacity of STBC MC-CDMA systems with that of MC-CDMA systems. It is concluded that small CFO has an insignificant effect on the BER and capacity of STBC MC-CDMA systems and that this range of CFO is important in transceiver design. Besides, STBC MC-CDMA systems with multiple receive antennas can achieve higher capacity than that of the MC-CDMA systems; this amount can be obtained analytically using the theoretical BER expressions derived.     

Impact of frequency offsets and IQ imbalance on MC-CDMA reception based on channel tracking

New air interfaces are currently being developed to meet the high spectral efficiency requirements of the emerging wireless communication systems. Multicarrier code-division multiple access (MC-CDMA) is seen as a promising candidate for the fourth-generation (4G) cellular communication systems because it can interestingly deal with the multipath propagation at a low processing complexity. Besides spectral efficiency and power consumption, the production cost of the transceiver should also be optimized. Direct conversion radio frequency (RF) receivers are appealing because they avoid costly intermediate frequency (IF) filters. However, they imply RF IQ separation, introducing a phase and amplitude mismatch between the I and Q branches. A communication system based on MC-CDMA is sensitive to synchronization errors and front-end non-idealities because it uses a long symbol duration. The goal of this paper is to evaluate the impact of the carrier frequency offset, the sampling clock offset, and the IQ imbalance on the MC-CDMA downlink system performance, considering a receiver based on channel tracking designed to cope with high mobility conditions. It is demonstrated that part of the effects is compensated by the channel estimation and an expression of the variance of the remaining symbol estimation error is provided. For the cellular system and the target performance considered in this paper, specifications are defined on the non-idealities. The results are validated with bit-error rate simulations.     

Effect of carrier frequency offset on performance of MC-CDMAsystems

The bit error probability of the downlink of an MC-CDMA system is analysed taking into account the effect of a carrier frequency offset for Rayleigh and Rician fading channels. Derived results show that the performance of an MC-CDMA system is very sensitive even to a relatively small frequency offset     

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。     

Performance of Up-link MC-CDMA System with Frequency Offset and Imperfect Channel Estimation Simultaneously over Nakagami Fading Channels

In this paper, effects of carrier frequency offset on performance of uplink MC-CDMA (Multi-Carrier Code Division Multiple Access) system in Nakagami fading channel are investigated through the theoretical analysis and Monte Carlo computer simulations. Both perfect maximal-ratio combining (MRC) and equal gain combining (EGC) receivers are analyzed; the impact of imperfect channel fading estimation on the performance of MRC is also explored. The performance of MC-CDMA system is also compared with that of the conventional single-carrier DS-CDMA system. Our results indicate that the performance of MC-CDMA system is sensitive to even small values of carrier frequency offset and that the performance of MC-CDMA system improves as number of subcarriers increases. In perfect channel fading estimation, the overall performance of MRC is superior to EGC. However, when imperfect or inaccurate channel fading estimation exists, which leads to serious performance degradation, EGC becomes superior to MRC. This revised version was published online in July 2006 with corrections to the Cover Date.     

Performance comparison of MRC and EGC on a MC-CDMA system with synchronization errors over fading channels

This work derives the average bit error rate (BER) of the uplink and downlink multicarrier code division multiple access (MC-CDMA) systems using maximum ratio combining (MRC) and equal gain combining (EGC) with synchronization errors over fading channels. The derived equation can simultaneously incorporate the parameters of the fading channel and all of the synchronization errors, including frequency offset, carrier phase jitter, and timing jitter. Numerical results indicate that those two combining schemes on the uplink and downlink MC-CDMA systems are degraded by all of the normalized synchronization errors over 10⁻². The comparison outcomes between MRC and EGC reveal that the MRC generally outperforms EGC in the uplink MC-CDMA system. However, EGC achieves better performance when the number of users is small, the normalized synchronization errors are low and the signal to noise ratio (SNR) is high. In the downlink system, EGC mainly outperforms MRC when the SNR and the number of users are gradually increased and the normalized synchronization errors are low. Therefore, the selection of MRC or EGC depends on the SNR, the synchronization errors and the number of users in uplink and downlink MC-CDMA systems.     

The effect of carrier frequency offsets on downlink and uplinkMC-DS-CDMA

In this paper, we study the sensitivity of uplink and downlink MC-DS-CDMA to carrier frequency offsets, assuming orthogonal spreading sequences. For both uplink and downlink MC-DS-CDMA, we show that the performance rapidly degrades for an increasing ratio of maximum frequency offset to carrier spacing. We point out that the degradation in the uplink is larger than in the downlink because only the former is affected by multiuser interference. For a given (small) ratio of maximum frequency offset to carrier spacing, enlarging the spreading factor in a fully loaded system does not affect the downlink degradation but strongly increases the uplink degradation. Finally, we show that the downlink degradations of MC-DS-CDMA and fully loaded MC-CDMA are the same, provided that for both systems the ratio of frequency offset to carrier spacing is the same     

一种基于快跳频技术的多载波CDMA系统

为进一步提高多载波CDMA系统的频带效率与抗干扰能力,提出了一种跳频多址技术结合多载波调制的传输方案,利用跳频技术将部分子载波传输与随机交织引入到多载波传输系统中,给出了系统实现的模型,并对该传输方案的系统带宽和误码性能进行了分析。理论分析与仿真结果表明：与以往的多载波CDMA系统相比,所建议系统的结构简单,性能优异,消除了已有系统中峰均功率比高和远近效应等问题,具有较强的抗多址干扰与窄带干扰的能力,并能有效地抗信道中突发错误的能力。     

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。