多用户检测技术的应用与发展

第三代移动通信系统采用码分多址(CDMA)方式。由于CDMA系统是一个自干扰系统,存在多址干扰及远近效应,因而限制了系统的容量和性能。多用户检测技术能从根本上消除多址干扰,提高系统的性能。本文主要介绍了多用户检测的思想、分类和性能指标,还提出了多用户检测技术的新发展和在几个具体系统中的应用。     

MIMO系统中基于干扰子空间投影的盲空时多用户检测算法

该文针对多天线码分多址系统,提出了一种盲空时多用户检测算法。该算法结合空域、时域二维信息,利用Alamouti空时分组码与MIMO信道的结构特征,通过跟踪干扰子空间对多天线信道进行估计。在此基础上,通过将接收信号投影到干扰子空间的正交补空间上,实现了抑制多址干扰的空时多用户检测。该算法不仅能够自适应地跟踪信道的变化,且具有较低的算法复杂度。仿真结果表明该算法能够有效地抑制多天线码分多址系统中的多址干扰,改善系统的性能。     

Minimum Cross Correlation Spreading Codes

Multiple access interference (MAI) appears in Code Division Multiple Access (CDMA) systems when the communication channel is a multi-path channel and the spreading codes are not orthogonal. Orthogonality between spreading codes cannot be maintained at the CDMA receiver because the codes may be asynchronous due to channel delay and multi-path spread. The receiver cannot perfectly separate the different signals of the multiple access users, and the resultant MAI limits the capacity of CDMA systems. MAI is a function of the cross correlation property between used spreading codes. In this paper we focus on the cross correlation of the spreading codes, we propose a method to find spreading codes with minimum magnitude of cross correlation. Employing these codes will reduce the resultant MAI in the CDMA system; hence it will increase the system capacity. A great enhancement is shown by comparing found minimum cross correlation spreading codes (MCCSC) with Hadamard and Gold codes.

TDRSS接收机码分多址干扰抑制方案设计

为了在跟踪与数据中继卫星系统( TDRSS)中实现码分多址( CDMA)应用,需要完成多用户检测( MUD)及码分多址干扰( MAI)抑制。采用减性多级迭代对消算法,在数字信号处理芯片中完成信号再生、延时校正、时域相减对消,可以完成至少12通道的多用户检测,并能将多址干扰抑制至最小1 dB以内。该算法可作为通用MUD算法的有效补充,并能应用于各类非最优的CDMA系统中,具有迭代级数可选、通道扩展性强、工程实现性好的优点。     

ITERATIVE "ONE-SHOT" DECORRELATION IN DS-CDMA SYSTEMS

In this paper, an iterative "one-shot" decorrelator for asynchronous code Division Multiple Access(CDMA) systems is proposed. This decorrelator feedbacks the decorrelated symbols of interfering users before decision to mitigate their interferences. As a result, this decorrelator is a linear detector and does not need any amplitude information of users as in decision-feedback detectors. Compared with existing "one-shot" decorrelator, it has reduced complexity and provided better performance while eliminating all Multiple Access Interference(MAI). The performance of this decorrelator is analyzed thoeritically, and analytical result is given. Numerical results show near-far resistant capability of this decorrelator in both AWGN and fading channel.     

ITERATIVE “ONE－SHOT” DECORRELATION IN DS－CDMA SYSTEMS

In this paper, an iterative “one-shot“ decorrelator for asynchronous code Division Multiple Access(CDMA) systems is proposed. This decorrelator feedbacks the decorrelated symbols of interfering users before decision to mitigate their interferences. As a result, this decorrelator is a linear detector and does not need any amplitude information of users as in decisionfeedback detectors. Compared with existing “one-shot“ decorrelator, it has reduced complexityand provided better performance while eliminating all Multiple Access Interference(MAI). The performance of this decorrelator is analyzed thoeritically, and analytical result is given. Numerical results show near-far resistant capability of this decorrelator in both AWGN and fading channel.     

一种TD-SCDMA载波频偏与信道联合估计算法

通过分析时分同步码分多址(TD-SCDMA)系统的载波频偏和信道特性,提出一种频偏与信道联合估计算法。该算法通过对训练序列部分相关,然后做差分将频偏和信道特性解耦,并通过逐次迭代最终精确估计出频偏和信道参数。仿真结果表明,算法频偏估计性能接近Cramr-Rao界,准确度较高;全链路仿真误码率性能优越,信道估计准确。算法复杂度不高,实用性强,同样适用于其它的TDD模式的CDMA系统。     

PMCAP/CDMA系统中一种新的多址干扰控制技术

多址干扰(MAI)是限制CDMA系统容量的瓶颈,而在无线通信系统中,为了实现数据的高速无差错传输,混合ARQ已经得到越来越多的应用。该文利用第二类混合ARQ(HARQ type-II)中每次重传的数据分组有不同的纠错能力和抗多址能力的特性,提出了一种新的联合区分多址干扰控制方法。仿真结果显示该方法在CDMA系统中有比传统方法更好的性能。     

A class of band-limited chip waveforms for DS-CDMA systems

In Direct Sequence Code Division Multiple Access （DS-CDMA） systems, the chip waveform affects the implementation, system bandwidth, envelope uniformity, eye pattern and Multiple user Access Interference （MAI）. In this paper, based on an elementary density function of a second order polynomial, a class of second order continuity pulses is proposed. From this class of pulses, we can find some members having faster decaying rate, bigger eye opening, more uniform envelope and stronger anti-MAI capability than the Nyquist waveform. The normalized-bandwidth-pulseshape-factor product, the decaying rate of the tail of the time waveform, the opening of the eye diagram, and the envelope uniformity of the second order continuity pulses are addressed in the paper that provide the basic information for the selection of the chip pulse for CDMA systems.     

Combined ICA and FCA Schemes for a Hierarchical Network

A combined idle channel assignment (ICA) and fixed channel assignment (FCA) scheme is proposed to improve the traffic performance in a hierarchical network. This dual-mode network integrates the Frequency Division Duplex (FDD) and Time Division Duplex (TDD) modes of the Universal Mobile Telecommunication System (UMTS) and Terrestrial Radio Access (UTRA) in a given cell. This approach includes a high traffic load area and a blocked area as an example to evaluate the traffic performance. The ICA threshold and network timeout period effects on the traffic performance of this integrated dual-mode network are also investigated. The analytical results show that the handoff failure probabilities of the integrated dual-mode network can be reduced significantly with a minimal increase in the new call blocking probability when the combined ICA and FCA scheme replaces the FCA scheme. The integrated dual-mode network using the combined ICA and FCA scheme also increases the carried traffic. The traffic performance improvements for non-uniformly generated new calls are more significant than those for uniformly generated new calls when the combined ICA and FCA scheme is used. An increase in the high ICA threshold will result in an increase in the total carried traffic and an increase in the new call blocking and handoff failure probabilities for higher-tiered and low-tiered systems located in the high traffic load area. The traffic performance was evaluated using the discrete time simulation method to validate the analysis results.     

多无人机扩频测控链路的扩频码分析

在码分多址（CDMA）的多无人机扩频测控链路中,多址干扰（MAI）是多目标系统需考虑的重要因素。在此类系统中,针对抗多址干扰的要求,设计具有相应互相关性能的扩频码是首要任务。从多无人机对抗多址能力的需求入手,分析了码速率、用户个数以及码的互相关性能对抗多址干扰能力的影响,得出了Kasami码型互相关性较好、序列部分相关会降低互相关性能的结论。仿真结果验证了理论分析的正确性。     

频率选择性衰落信道中MC-CDMA信号检测技术

正交多载波码分多址(MC-CDMA)是近年无线宽带通信研究的热点.本文介绍了MC-CDMA信号检测技术,在分析频率选择性衰落信道中单小区MC-CDMA系统上/下行多址干扰(MAI)的基础上,指出上行接收引入多用户检测技术(MuD)的必要性和可能性,并通过与直接序列(DS)CDMA的对照和对偶性分析,讨论了MC-CDMA MuD的发展方向.     

自适应波束形成器在CDMA系统中抗干扰技术的研究

本文对码分多址系统中使用自适应波束形成器抗多址干扰性能作了研究,主要探讨检测在既定方向的有用信号的同时,对其它方向的多址干扰进行抑制,并在理论和实际上对系统的抗多址干扰的性能进行了分析模拟。     

基于子空间盲多用户检测算法的设计和仿真

随着数字移动通信技术的飞速发展,以码分多址(CDMA)为核心的第三代移动通信(3G)技术已经走入人们的日常生活。CDMA系统具有容量大、软容量、抗多径衰落强等优点,但多址干扰(MAI)的存在却严重的影响了系统的性能和容量,因此如何消除和抑制这种干扰就成了CDMA技术研究的热门话题之一,第三代移动通信系统已经将多用户检测技术(MUD)作为克服多址干扰的有效方法之一。文中主要研究了直接序列扩频码分多址(DS-CDMA)系统中的盲多用户检测算法,深入的研究了基于卡尔曼滤波的盲多用户检测算法,通过在卡尔曼滤波算法上引入子空间提高了原有卡尔曼滤波算法的效率。     

Successive beamforming,multiuser detection and diversity reception for multicarrier DS-CDMA system with antenna array

Code-Division Multiple-Access (CDMA) systems are interference limited,and therefore efficient interference management is necessary to enhance the performance of a CDMA system.In this paper,a successive beamforming (spatial filtering),linear decorrelating MultiUser Detection (MUD, temporal filtering) and diversity reception structure for uplink multicarrier Direct Sequence CDMA (DS-CDMA) system with antenna array are proposed.By beamforming,the antenna array suppresses interference according to the distinct array signature.Subsequently,linear decorrelating MUD is ap- plied to separate the signals of different users and eliminate Multiple Access Interference (MAI).Finally, the decorrelated signals at different subcarriers that belong to the same user are combined to achieve frequency diversity.Simulation results show that the proposed structure offers significant Bit Error Rate (BER) performance improvement by successively exploiting the space-time-frequency processing.     

UTRA/FDD RF Transceiver Requirements

Unified RF requirements are derived for an UMTS Terrestrial RadioAccess/Frequency Division Duplex (UTRA/FDD) compliant mobile transceiver. Aset of transceiver requirements are proposed with consideration to systemissues including duplex aspects. From these design-compatible requirements areproposed for each functional block in the transceiver.     

Equalization and Carrier Frequency Offsets Compensation for the SC-FDMA System

Recently, the Single-Carrier Frequency Division Multiple Access (SC-FDMA) system has attracted the attention as an efficient alternative to the Orthogonal Division Multiple Access (OFDMA) system in the uplink communications. In this system, the Carrier Frequency Offsets (CFOs) disrupt the orthogonality between subcarriers, and give rise to Inter-Carrier Interference (ICI), and Multiple Access Interference (MAI) among users. In this paper, the impact of the CFOs on the performance of the Discrete Fourier Transform SC-FDMA (DFT-SC-FDMA) and the Discrete Cosine Transform SC-FDMA (DCT-SC-FDMA) systems is investigated. Based on the Minimum Mean Square Error (MMSE) criterion, a new low-complexity joint equalization and CFOs compensation scheme is proposed. It is refered to as the MMSE scheme. The MMSE weights of the proposed scheme are derived taking into account the MAI and the noise. Furthermore, a hybrid scheme comprising the proposed MMSE scheme and a Parallel Interference Cancellation (PIC) stage is also suggested and investigated to further enhance the performance of interleaved subcarriers mapping systems. From the obtained simulation results, it is clear that CFOs disrupt the orthogonality between the subcarriers in SC-FDMA systems and degrade the Bit Error Rate (BER) performance. The proposed compensation schemes are able to enhance the system performance, even in the presence of the estimation errors.     

Interference Cancellation in CDMA Systems Using Spectral Redundancy

Most modulated signals are cyclostationary, which results in redundancy in the signal frequency spectrum. In this paper, the spectral redundancy which exists in Code Division Multiple Access (CDMA) signals is used in the receiver to improve performance. In particular, we employ a Frequency Shift-Decision Feedback Equalizer (FRESH-DFE) to exploit the cyclostationary. We show that FRESH-DFE is superior to conventional DFE in multiple access CDMA systems, including long code systems.     

Effect of fading,diversity and interference on the performance of multitone CDMA systems

The performance of a multitone Code Division Multiple Access (CDMA) system is investigated in this paper by considering the effect of multipath fading, diversity and narrowband interference. The performance is evaluated in terms of the average bit error rate when both multiple access and partial band interference are present. Results are obtained by deriving the statistics of the receiver output parameters and then tested by numerical evaluation. The bit error rate performance is analyzed as a function of bit rate, bandwidth, delay range and different diversity techniques, while considering issues relevant to system implementation. Comparisons were made between conventional single‐tone CDMA systems and multitone CDMA systems. The effectiveness of multitone CDMA in suppressing interference compared to conventional CDMA system is analyzed. Results reveal considerable performance improvement of multitone CDMA system over conventional CDMA system. Copyright © 2002 John Wiley & Sons, Ltd.     

一种新的CDMA系统信干比估计方法

本文根据CDMA系统中解扩后基带信号的特点和基带信号模型,分析了基于QPSK调制的基带信号信噪比估计方法,提出了一种新的适用于CDMA解扩后基带信号的信干比估计方法.计算机仿真结果表明,在低信干比情况下该方法较其他方法,有较高的可信度.