下一代无线通信系统中的时频同步技术研究
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摘要
近十年来,随着人们对高速率、高质量的无线通信需求的飞速增长,下一代无线通信系统的设计和实现都面临着越来越严峻的挑战。在物理层,为了有效地克服高速率无线通信带来的符号间干扰,以及解决大量用户的大容量通信需求和有限的频谱资源供给之间的矛盾,以正交频分复用(Orthogonal Frequency DivisionMultiplexing, OFDM)技术为代表的多载波技术和能充分利用空间资源的多天线多输入多输出(Multiple-Input Multiple-Output, MIMO)技术得到了学术界和工业界的关注和研究,并被广泛接受为下一代无线通信系统的首选框架。
同步技术是数字通信系统的关键技术之一。同步可以分为两个方面:时间同步和载波同步。由于各种基于OFDM的调制和多址技术,如单入单出正交频分复用(Single-Input Single-Output OFDM, SISO-OFDM)、多入多出正交频分复用(MIMO-OFDM)、以及正交频分多址(Orthogonal Frequency Division Multiple Ac-cess, OFDMA)技术等对载波频率偏移(Carrier Frequency Offset, CFO)都非常敏感;另一方面,协作通信系统(Cooperative Communication Sysmtes)作为一种分布式的多天线MIMO系统,也存在着其内禀的时、频异步特性,因此研究这两类系统中的时频同步问题具有重要意义。
本论文以下一代无线通信系统中的时间、频率同步问题做为主要研究对象,较为系统地考察了各种基于OFDM的调制和多址系统,包括SISO-OFDM、MIMO-OFDM和OFDMA系统,以及分布式协作通信系统中的时频同步问题。研究内容涉及参数估计、信号检测和空时编码设计等多个方面。本论文的主要创新点包括:
1.研究了SISO-OFDM、MIMO-OFDM以及OFDMA系统中的频偏估计问题。认识到上述各种OFDM系统中的频偏估计问题本质上均为噪声中的多个不同幅度和初相的谐波频率估计问题,将上述各种OFDM系统的频偏估计问题纳入一个统一的框架进行讨论,充分利用各个系统的频域、时域和空域资源,提出一系列的频偏估计方案,为频偏估计问题提供了一个新的视角。具体而言:
1)充分利用频域资源。利用频偏信息在OFDM频谱上存在冗余的特点,提出了一种可以应用于SISO-OFDM和MIMO-OFDM系统的基于欠采样的频偏估计方法。充分利用频偏信息在OFDM频谱结构上的冗余性,新方法通过设计均匀间隔的虚拟子载波(空导频),并在接收端通过对接收信号的欠采样重新构造低维度的接收信号矩阵。这种欠采样构造导致的OFDM信号频域的混叠不会造成频偏信息的损失,反而压缩了待估参数的个数,提高了估计的性能。而在频率选择性信道中,这种频谱折叠还能提供频率分集,从而克服了传统的基于虚拟子载波的频偏估计方法在频率选择性信道下的性能损失,使得即使在信道频谱存在深衰落甚至零点的情况下仍能保持其超分辨的性能。同时,欠采样还降低了自相关矩阵的维度从而降低了整个算法的计算复杂度。
2)充分利用时域资源。利用实际的OFDM时域信号均采用足够长的循环前缀以克服符号块间干扰,因此系统中存在着没有被符号块间干扰污染的“干净”循环前缀的特点,提出了一种应用于SISO-OFDM系统的自适应联合符号块定时和载波频偏估计方法。与传统的利用循环前缀的定时和频偏估计方法相比,新方法能够在无需已知“干净”循环前缀的长度和起始位置的情况下,以自适应的方式尽可能地充分利用“干净”的循环前缀样点,避免了块间干扰的影响。新方法在时间弥散信道中性能稳健,且计算复杂度低。而且当自适应的过程收敛后,可以得到频偏的最大似然估计。
3)充分利用空域资源。利用OFDMA系统配置均匀线性阵列天线的特点,提出了一种应用于OFDMA上行系统的多频偏估计方法。基于窄带信号假设,新方法充分利用了阵列天线带来的空间分辨能力,从空域将来自不同用户的信号进行分离,从而可以对各个用户的频偏进行逐个估计。由于充分利用了空间资源,与已有的方法相比,新方法: a)不依赖于任何特定的子载波分配方案,适用于子载波任意分配的系统,因此能够支持系统的动态信道分配,更加灵活实用;b)不依赖于空子载波,所有载波都可用于数据发送,因此频谱利用率较高;c)可以同时得到多个用户的入射角信息,用于上行多用户分离和下行选择性发送,提高系统性能。
2.研究了时、频异步的协作通信系统中的可靠通信问题。从发射端和接收端两个方面着手,分别研究了发射端的满分集分布式空时编码的设计和接收端低计算复杂度的信号检测算法的设计。具体而言:
1)研究了频率异步协作通信系统中的协作发射方案的设计问题。由于协作通信系统中多个不同频偏的存在造成的系统等效信道的时变性,破坏了传统分布式空时编码的结构,从而恶化了接收性能。针对这一问题,首先通过时频变换将时域中由频偏引起的时变平坦衰落信道变换为频域中的块时不变符号间串扰(Inter-Symbol Interference, ISI)信道。基于这样的信道,充分利用时-频对偶特性,分别提出了两种分布式空频编码的设计方案:频率反转空频编码和频域线性卷积空频编码。新的空频码不但能在多个频偏存在的情况下仍然获得满协作分集增益,而且接收机并不需要采用计算量巨大的最大似然解码,而仅仅需要采用低复杂度的线性均衡方法,如迫零均衡或最小均方误差均衡即可获得满分集增益。在此基础上,将该设计思路推广到任意个协作节点存在的情况,提出了一族具有较高频谱效率,在线性接收机下达到满频率异步协作分集的空频编码。新编码达到了性能、复杂度和频谱效率的良好折中。
2)研究了时、频异步协作通信系统中的可靠通信问题。针对由协作通信系统的分布式特点带来的时频异步问题,以及导致的等效信道的时变性和频率选择性,从发射和接收两个方面入手以实现系统的可靠接收。在协作发射端,采用分布式线性卷积空时码,以保证目的节点采用线性接收机即可获得满的时间异步协作分集增益;而在接收端,设计了一种低计算复杂度的快速均衡算法。由于等效信道的时变性仅有多个频偏的存在而引起。而如果接收端得到各个协作节点频偏的正确估计,那么这种时变性的规律就完全确定了。这就意味着上一个时刻的信道矩阵和当前时刻的信道矩阵之间是存在某种特定的关系的。基于这样的认识,设计了基于递推思想的最小均方误差和最小均方误差判决反馈均衡器。在每一个时刻,新的均衡器可以从上一个时刻已经得到的均衡器递推而来,而并不需要完全重新设计,这样大大减小了目的节点的计算复杂度,实现了快速均衡。
In the last decade, the fast growing demands for high-data-rate and high-quality communi-cations over wireless channels make the design and implementation of the next-generationwireless communication systems more and more challenging. In physical layer, to eliminatethe inter-symbol interference (ISI) caused by the high-rate wireless transmission effectively,and to solve the basic contradiction between the high-rate wireless communication require-ments of large number of subscribes and the scarcity of electromagnetic spectrum, multi-carrier technique, such as orthogonal frequency division multiplexing (OFDM) technique,and multiple-input multiple-output (MIMO) technique, which is able to exploit the potentialspatial resource effectively, attracts much attention and extensive research in both academicand industrial society. They have already been widely recognized as the most promisingcandidates for the next-generation wireless communications systems.
Synchronization, which consists of timing synchronization and carrier synchronization,is a key technique of digital communication systems. Various modulation and multiple-access techniques based on OFDM, such as SISO-OFDM, MIMO-OFDM and orthogonalfrequency-division multiple-access (OFDMA), are very sensitive to carrier frequency offset(CFO). On the other hand, cooperative communication system, as a special form of dis-tributed MIMO system, is also inherently time-frequency asynchronous. Consequently, it isvery important to do the research on the time-frequency synchronization for these systems.
This dissertation focuses on the synchronization issue for next generation wireless communi-cation systems. Time-frequency synchronization in various modulation and multiple-accesstechniques based on OFDM, such as SISO-OFDM, MIMO-OFDM and OFDMA, and co-operative communication systems are investigated. Several aspects of communications andsignal processing are involved in the dissertation, including parameter estimation, signaldetection and space-time coding design. The main contributions of the dissertation lie in:
1. The CFO estimation issue in SISO-OFDM, MIMO-OFDM and OFDMA systems areinvestigated. It has been indicated that the CFO estimation is essentially the frequenciesestimation issue of multiple harmonics with different amplitudes and initial phases in noise.Consequently, the CFO estimation of various OFDM systems can be discussed in an unifiedframework. Time-domain, frequency-domain and spatial-domain resources of these OFDM systems can be fully exploited and a series of CFO estimation approaches are proposed,which provide a new perspective of the CFO estimation issue. Especially:
1) Taking full advantage of the frequency-domain resource. The redundancy of CFOinformation in the spectrum structure of OFDM signals has been fully taken advan-tage of, and a novel CFO estimation method based on virtual subcarriers (zero pilots)has been proposed applying to both SISO- and MIMO-OFDM systems. It has beenshown that the CFO information is redundant in the spectrum structure of the receivedOFDM blocks. By allocating virtual subcarriers uniformly and down-sampling the re-ceived blocks, the method remodels each received OFDM block and provides a signalmatrix with lower dimension. The spectrum aliasing induces by the down-samplingdoes not cause the CFO information loss but compresses the dimension of signal spaceand enhances the estimation performance. Moreover, in frequency-selective channels,this spectrum aliasing provides frequency diversity and hence avoids the performancedegradation due to the deep fading (even spectrum-zeros) of the channel. The pro-posed method retains the high-resolution feature and the computational complexity isalso very low.
2) Taking full advantage of the time-domain resource. It has been shown in practicalOFDM systems that, sufficiently long cyclic prefix (CP) is always adopted to elimi-nate the inter-block interference (IBI) completely, hence there always exists“clean”CP samples, i.e., CP samples do not corrupted by the previous OFDM block. A noveladaptive joint block timing and CFO estimation algorithm for SISO-OFDM systemshas been proposed. Compared with similar methods exploiting CP, the adaptive ap-proach can utilize clean CP samples to avoid IBI effectively in a way without knowingthe exact length and location of them. Consequently, the algorithm is robust in time-dispersive channels. Furthermore, when the adaptive process converges, we show thealgorithm provides the maximum likelihood (ML) estimation of the CFO.
3) Taking full advantage of the spatial-domain resource. The adoption of uniform lineararray (ULA) at the base station of OFDMA systems has been fully taken advantageof, and a novel multiuser CFOs estimation method has been proposed for OFDMA up-link systems. Based on the narrow-band signal assumption, the new approach exploitsthe spatial division capability of ULA to separate signals from different users in thespatial-domain, and hence is able to estimation the CFOs of multiusers independently.Due to the utilization of spatial resource, the novel method has a series of merits: a) Itdoes not rely on any specific carrier assignment scheme (CAS) and applies to systemswith generalized CAS, so it is more ?exible and dynamic channel assignment is avail-able; b) It applies to fully loaded systems, i.e., all subcarriers can be assigned to users,which results in higher bandwidth efficiency; c) In addition, the directions of arrival(DOAs) of all active users are also obtained, which can be used for downlink selective transmission and mobile user orientation in frequency division duplex (FDD) systems.
2. The reliable communication issue in cooperative communication systems with both timeand frequency asynchronization is investigated. From the transmitting end to the receivingend, distributed space-time coding schemes and signal detection algorithms are discussedand proposed. Especially:
1) The cooperative transmission scheme design for frequency-asynchronous cooperativecommunication systems with multiple CFOs is investigated. The existence of multi-ple CFOs in cooperative systems makes the equivalent channels time-varying, whichwill destroy the elaborately designed structure of the distributed space-time codes, anddegrade the performance of the system. To deal with this, by time-frequency trans-form, a ?at fading channel with CFO in the time domain has been converted to a blocktime-invariant ISI channel in the frequency domain. Based on such channel, two dis-tributed space-frequency codes (SFC), frequency-reversal SFC and frequency-domainlinear convolutive SFC, are proposed to achieve the cooperative spatial diversity. Ithas been shown that even under the frequency-asynchronous case, full cooperative di-versity can be achieved with more computationally efficient linear receivers, such aszero-forcing (ZF) and minimum mean square error (MMSE) receivers, instead of thecomputationally exhausted maximum likelihood (ML) decoding. The design schemeis then generalized to the case with any number of relay nodes. A family of bandwidthefficient SFCs which can achieve full diversity with linear receivers under frequency-asynchronous case is proposed. The new family of codes achieves a better tradeoffamong performance, decoding complexity and spectrum efficiency.
2) The reliable communication issue in time-frequency asynchronous cooperative com-munication systems is investigated. To deals with the time-selective and frequency-selective equivalent channel caused by the multiple time and frequency offsets in co-operative communication systems, the design of transmitting and receiving schemesare addressed. At the transmitter end, distributed linear convolutive space-time code isadopted to collect full time-asynchronous cooperative diversity with only linear re-ceivers. Meanwhile at the receiver end, computationally efficient fast equalizationmethods are proposed. The time-selectivity of the equivalent channel is only intro-duced by the multiple CFOs, which can be determined once these CFOs are correctlyestimated. This implies that there exists special relationship between the channel ma-trices in the consecutive two symbol durations. Based on this observation, iterativeminimum mean square error (MMSE) equalizer and MMSE decision feedback equal-izer (MMSE-DFE) are proposed, where the equalizer at current symbol duration canbe designed using the one obtained at the last symbol duration instead of re-designcompletely, which therefore significantly reduces the computational complexity for the equalization.
同步技术是数字通信系统的关键技术之一。同步可以分为两个方面:时间同步和载波同步。由于各种基于OFDM的调制和多址技术,如单入单出正交频分复用(Single-Input Single-Output OFDM, SISO-OFDM)、多入多出正交频分复用(MIMO-OFDM)、以及正交频分多址(Orthogonal Frequency Division Multiple Ac-cess, OFDMA)技术等对载波频率偏移(Carrier Frequency Offset, CFO)都非常敏感;另一方面,协作通信系统(Cooperative Communication Sysmtes)作为一种分布式的多天线MIMO系统,也存在着其内禀的时、频异步特性,因此研究这两类系统中的时频同步问题具有重要意义。
本论文以下一代无线通信系统中的时间、频率同步问题做为主要研究对象,较为系统地考察了各种基于OFDM的调制和多址系统,包括SISO-OFDM、MIMO-OFDM和OFDMA系统,以及分布式协作通信系统中的时频同步问题。研究内容涉及参数估计、信号检测和空时编码设计等多个方面。本论文的主要创新点包括:
1.研究了SISO-OFDM、MIMO-OFDM以及OFDMA系统中的频偏估计问题。认识到上述各种OFDM系统中的频偏估计问题本质上均为噪声中的多个不同幅度和初相的谐波频率估计问题,将上述各种OFDM系统的频偏估计问题纳入一个统一的框架进行讨论,充分利用各个系统的频域、时域和空域资源,提出一系列的频偏估计方案,为频偏估计问题提供了一个新的视角。具体而言:
1)充分利用频域资源。利用频偏信息在OFDM频谱上存在冗余的特点,提出了一种可以应用于SISO-OFDM和MIMO-OFDM系统的基于欠采样的频偏估计方法。充分利用频偏信息在OFDM频谱结构上的冗余性,新方法通过设计均匀间隔的虚拟子载波(空导频),并在接收端通过对接收信号的欠采样重新构造低维度的接收信号矩阵。这种欠采样构造导致的OFDM信号频域的混叠不会造成频偏信息的损失,反而压缩了待估参数的个数,提高了估计的性能。而在频率选择性信道中,这种频谱折叠还能提供频率分集,从而克服了传统的基于虚拟子载波的频偏估计方法在频率选择性信道下的性能损失,使得即使在信道频谱存在深衰落甚至零点的情况下仍能保持其超分辨的性能。同时,欠采样还降低了自相关矩阵的维度从而降低了整个算法的计算复杂度。
2)充分利用时域资源。利用实际的OFDM时域信号均采用足够长的循环前缀以克服符号块间干扰,因此系统中存在着没有被符号块间干扰污染的“干净”循环前缀的特点,提出了一种应用于SISO-OFDM系统的自适应联合符号块定时和载波频偏估计方法。与传统的利用循环前缀的定时和频偏估计方法相比,新方法能够在无需已知“干净”循环前缀的长度和起始位置的情况下,以自适应的方式尽可能地充分利用“干净”的循环前缀样点,避免了块间干扰的影响。新方法在时间弥散信道中性能稳健,且计算复杂度低。而且当自适应的过程收敛后,可以得到频偏的最大似然估计。
3)充分利用空域资源。利用OFDMA系统配置均匀线性阵列天线的特点,提出了一种应用于OFDMA上行系统的多频偏估计方法。基于窄带信号假设,新方法充分利用了阵列天线带来的空间分辨能力,从空域将来自不同用户的信号进行分离,从而可以对各个用户的频偏进行逐个估计。由于充分利用了空间资源,与已有的方法相比,新方法: a)不依赖于任何特定的子载波分配方案,适用于子载波任意分配的系统,因此能够支持系统的动态信道分配,更加灵活实用;b)不依赖于空子载波,所有载波都可用于数据发送,因此频谱利用率较高;c)可以同时得到多个用户的入射角信息,用于上行多用户分离和下行选择性发送,提高系统性能。
2.研究了时、频异步的协作通信系统中的可靠通信问题。从发射端和接收端两个方面着手,分别研究了发射端的满分集分布式空时编码的设计和接收端低计算复杂度的信号检测算法的设计。具体而言:
1)研究了频率异步协作通信系统中的协作发射方案的设计问题。由于协作通信系统中多个不同频偏的存在造成的系统等效信道的时变性,破坏了传统分布式空时编码的结构,从而恶化了接收性能。针对这一问题,首先通过时频变换将时域中由频偏引起的时变平坦衰落信道变换为频域中的块时不变符号间串扰(Inter-Symbol Interference, ISI)信道。基于这样的信道,充分利用时-频对偶特性,分别提出了两种分布式空频编码的设计方案:频率反转空频编码和频域线性卷积空频编码。新的空频码不但能在多个频偏存在的情况下仍然获得满协作分集增益,而且接收机并不需要采用计算量巨大的最大似然解码,而仅仅需要采用低复杂度的线性均衡方法,如迫零均衡或最小均方误差均衡即可获得满分集增益。在此基础上,将该设计思路推广到任意个协作节点存在的情况,提出了一族具有较高频谱效率,在线性接收机下达到满频率异步协作分集的空频编码。新编码达到了性能、复杂度和频谱效率的良好折中。
2)研究了时、频异步协作通信系统中的可靠通信问题。针对由协作通信系统的分布式特点带来的时频异步问题,以及导致的等效信道的时变性和频率选择性,从发射和接收两个方面入手以实现系统的可靠接收。在协作发射端,采用分布式线性卷积空时码,以保证目的节点采用线性接收机即可获得满的时间异步协作分集增益;而在接收端,设计了一种低计算复杂度的快速均衡算法。由于等效信道的时变性仅有多个频偏的存在而引起。而如果接收端得到各个协作节点频偏的正确估计,那么这种时变性的规律就完全确定了。这就意味着上一个时刻的信道矩阵和当前时刻的信道矩阵之间是存在某种特定的关系的。基于这样的认识,设计了基于递推思想的最小均方误差和最小均方误差判决反馈均衡器。在每一个时刻,新的均衡器可以从上一个时刻已经得到的均衡器递推而来,而并不需要完全重新设计,这样大大减小了目的节点的计算复杂度,实现了快速均衡。
In the last decade, the fast growing demands for high-data-rate and high-quality communi-cations over wireless channels make the design and implementation of the next-generationwireless communication systems more and more challenging. In physical layer, to eliminatethe inter-symbol interference (ISI) caused by the high-rate wireless transmission effectively,and to solve the basic contradiction between the high-rate wireless communication require-ments of large number of subscribes and the scarcity of electromagnetic spectrum, multi-carrier technique, such as orthogonal frequency division multiplexing (OFDM) technique,and multiple-input multiple-output (MIMO) technique, which is able to exploit the potentialspatial resource effectively, attracts much attention and extensive research in both academicand industrial society. They have already been widely recognized as the most promisingcandidates for the next-generation wireless communications systems.
Synchronization, which consists of timing synchronization and carrier synchronization,is a key technique of digital communication systems. Various modulation and multiple-access techniques based on OFDM, such as SISO-OFDM, MIMO-OFDM and orthogonalfrequency-division multiple-access (OFDMA), are very sensitive to carrier frequency offset(CFO). On the other hand, cooperative communication system, as a special form of dis-tributed MIMO system, is also inherently time-frequency asynchronous. Consequently, it isvery important to do the research on the time-frequency synchronization for these systems.
This dissertation focuses on the synchronization issue for next generation wireless communi-cation systems. Time-frequency synchronization in various modulation and multiple-accesstechniques based on OFDM, such as SISO-OFDM, MIMO-OFDM and OFDMA, and co-operative communication systems are investigated. Several aspects of communications andsignal processing are involved in the dissertation, including parameter estimation, signaldetection and space-time coding design. The main contributions of the dissertation lie in:
1. The CFO estimation issue in SISO-OFDM, MIMO-OFDM and OFDMA systems areinvestigated. It has been indicated that the CFO estimation is essentially the frequenciesestimation issue of multiple harmonics with different amplitudes and initial phases in noise.Consequently, the CFO estimation of various OFDM systems can be discussed in an unifiedframework. Time-domain, frequency-domain and spatial-domain resources of these OFDM systems can be fully exploited and a series of CFO estimation approaches are proposed,which provide a new perspective of the CFO estimation issue. Especially:
1) Taking full advantage of the frequency-domain resource. The redundancy of CFOinformation in the spectrum structure of OFDM signals has been fully taken advan-tage of, and a novel CFO estimation method based on virtual subcarriers (zero pilots)has been proposed applying to both SISO- and MIMO-OFDM systems. It has beenshown that the CFO information is redundant in the spectrum structure of the receivedOFDM blocks. By allocating virtual subcarriers uniformly and down-sampling the re-ceived blocks, the method remodels each received OFDM block and provides a signalmatrix with lower dimension. The spectrum aliasing induces by the down-samplingdoes not cause the CFO information loss but compresses the dimension of signal spaceand enhances the estimation performance. Moreover, in frequency-selective channels,this spectrum aliasing provides frequency diversity and hence avoids the performancedegradation due to the deep fading (even spectrum-zeros) of the channel. The pro-posed method retains the high-resolution feature and the computational complexity isalso very low.
2) Taking full advantage of the time-domain resource. It has been shown in practicalOFDM systems that, sufficiently long cyclic prefix (CP) is always adopted to elimi-nate the inter-block interference (IBI) completely, hence there always exists“clean”CP samples, i.e., CP samples do not corrupted by the previous OFDM block. A noveladaptive joint block timing and CFO estimation algorithm for SISO-OFDM systemshas been proposed. Compared with similar methods exploiting CP, the adaptive ap-proach can utilize clean CP samples to avoid IBI effectively in a way without knowingthe exact length and location of them. Consequently, the algorithm is robust in time-dispersive channels. Furthermore, when the adaptive process converges, we show thealgorithm provides the maximum likelihood (ML) estimation of the CFO.
3) Taking full advantage of the spatial-domain resource. The adoption of uniform lineararray (ULA) at the base station of OFDMA systems has been fully taken advantageof, and a novel multiuser CFOs estimation method has been proposed for OFDMA up-link systems. Based on the narrow-band signal assumption, the new approach exploitsthe spatial division capability of ULA to separate signals from different users in thespatial-domain, and hence is able to estimation the CFOs of multiusers independently.Due to the utilization of spatial resource, the novel method has a series of merits: a) Itdoes not rely on any specific carrier assignment scheme (CAS) and applies to systemswith generalized CAS, so it is more ?exible and dynamic channel assignment is avail-able; b) It applies to fully loaded systems, i.e., all subcarriers can be assigned to users,which results in higher bandwidth efficiency; c) In addition, the directions of arrival(DOAs) of all active users are also obtained, which can be used for downlink selective transmission and mobile user orientation in frequency division duplex (FDD) systems.
2. The reliable communication issue in cooperative communication systems with both timeand frequency asynchronization is investigated. From the transmitting end to the receivingend, distributed space-time coding schemes and signal detection algorithms are discussedand proposed. Especially:
1) The cooperative transmission scheme design for frequency-asynchronous cooperativecommunication systems with multiple CFOs is investigated. The existence of multi-ple CFOs in cooperative systems makes the equivalent channels time-varying, whichwill destroy the elaborately designed structure of the distributed space-time codes, anddegrade the performance of the system. To deal with this, by time-frequency trans-form, a ?at fading channel with CFO in the time domain has been converted to a blocktime-invariant ISI channel in the frequency domain. Based on such channel, two dis-tributed space-frequency codes (SFC), frequency-reversal SFC and frequency-domainlinear convolutive SFC, are proposed to achieve the cooperative spatial diversity. Ithas been shown that even under the frequency-asynchronous case, full cooperative di-versity can be achieved with more computationally efficient linear receivers, such aszero-forcing (ZF) and minimum mean square error (MMSE) receivers, instead of thecomputationally exhausted maximum likelihood (ML) decoding. The design schemeis then generalized to the case with any number of relay nodes. A family of bandwidthefficient SFCs which can achieve full diversity with linear receivers under frequency-asynchronous case is proposed. The new family of codes achieves a better tradeoffamong performance, decoding complexity and spectrum efficiency.
2) The reliable communication issue in time-frequency asynchronous cooperative com-munication systems is investigated. To deals with the time-selective and frequency-selective equivalent channel caused by the multiple time and frequency offsets in co-operative communication systems, the design of transmitting and receiving schemesare addressed. At the transmitter end, distributed linear convolutive space-time code isadopted to collect full time-asynchronous cooperative diversity with only linear re-ceivers. Meanwhile at the receiver end, computationally efficient fast equalizationmethods are proposed. The time-selectivity of the equivalent channel is only intro-duced by the multiple CFOs, which can be determined once these CFOs are correctlyestimated. This implies that there exists special relationship between the channel ma-trices in the consecutive two symbol durations. Based on this observation, iterativeminimum mean square error (MMSE) equalizer and MMSE decision feedback equal-izer (MMSE-DFE) are proposed, where the equalizer at current symbol duration canbe designed using the one obtained at the last symbol duration instead of re-designcompletely, which therefore significantly reduces the computational complexity for the equalization.
引文
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