【作者】 蔚娜；

【导师】 杨莘元；

【作者基本信息】 哈尔滨工程大学 ， 通信与信息系统， 2008， 博士

【摘要】 如何高效地利用有限的通信资源来提供高速、可靠的宽带数据业务是无线通信技术发展的焦点所在。多天线技术能充分利用空间资源,在不增加系统带宽和天线总发送功率的情况下,有效对抗无线信道衰落的影响,大大提高通信系统的频谱利用率和信道容量。而获得多输入多输出(MIMO)系统这些好处的一个行之有效的方法就是空时编码,该方法联合考虑信道编码、调制、发送和接收分集,将它们有机结合,有效地提高了MIMO系统的传输性能。然而,与固定有线通信不同的是,无线通信中的信号要通过自由空间来传输,这一信道的随机时变性和严重的传输缺陷决定了无线通信技术的特殊性和复杂性。因此,本文的主要工作就是致力于选择性衰落信道下多天线系统编译码的研究。本文的主要研究内容和取得的研究成果包括以下几方面的内容:首先,研究了时变性对MIMO系统性能的影响,以及如何使用多普勒分集来提高MIMO系统的传输性能。基于多天线系统的基扩展时变信道模型,系统总结了时间选择性衰落信道下传统的编译码方法,在此基础上,提出了高速率、可获得更高分集增益的空时多普勒编译码方法,并提出了码字设计准则。该码字构造时基于分层的代数方法,即每一层编码后的符号都被分到不同的发送天线、时间片以及不同的符号块中发送。仿真结果表明该码字能在符号速率达到发送天线数的情况下同时获得最大分集增益为发送天线数、接收天线数、编码符号块的数目以及信道复指数基的个数的乘积。其次,研究了信道状态信息未知时时间频率双选择性衰落信道下的多天线差分编译码方法。在某些情况下,信道估计困难,比较有效的方法就是采用差分编码。然而,大多差分编码方案都是仅基于时间选择性衰落MIMO信道或者仅基于频率选择性衰落MIMO信道设计的,对于双选择性衰落MIMO信道下的差分方案研究较少,针对这一问题,基于多天线时间频率双选择性衰落信道的基扩展模型,提出了一种应用于时间频率双选择性衰落信道的块差分编译码方案,该方案可以配置任意数目的发送天线和接收天线,并且能够获得包含空间分集、多径分集以及多普勒分集的最大分集增益。该方法利用基扩展模型中各条路径各复指数基的系数在一个块内为恒定的特性,将一个块分为多个子块并在各子块间进行差分编码,通过发射端和接收端相应的处理,信号检测不需要信道状态信息,从而避免了对双选择性衰落MIMO信道的估计。仿真结果表明了所设计差分编译码方案的高效性。再次,对多用户STBC系统下行链路预处理算法进行了研究。利用矩阵的奇异值分解定理和QR分解定理以及矩阵的正交基给出多用户STBC系统的下行链路预处理算法,并在此基础上,提出了两种可获得更高分集增益的特征模式选择算法和天线选择算法。当基站有额外的射频模块和天线时,可以采用特征模式选择算法;当基站只有额外的天线时,可以采用天线选择算法。两种优化算法都基于最大化等效信道矩阵的Frobenius范数。对不采用选择算法和采用选择算法的系统性能作了仿真分析和比较,结果表明,优化的预处理算法可以获得更高的分集增益。还对信道存在估计误差以及存在相关性时对系统性能的影响作了仿真分析,在同样情况下,采用优化的预处理算法的系统性能优于未采用选择算法的系统性能。最后,对空时分组编码-单载波-码分多址(STBC-SC-CDMA)系统上行链路进行了研究。对于频率选择性衰落信道,传统DS-CDMA系统会受到符号间干扰(ISI)和多用户干扰(MUI)的影响,性能下降。为了能够有效对付多径,发挥DS-CDMA系统优越性能,并且同时能够获得空间-多径分集增益,提出了将STBC与SC-CDMA相结合的编译码方案,推导出了整个系统的收发关系的表达式,从而将系统等效为一个多用户空间复用系统。为了降低移动台的设计难度,主要考虑在基站实行联合检测算法,介绍了三种基本的联合检测算法:MF-BLE算法、ZF-BLE算法和MMSE-BLE算法,并对三种算法进行了分析比较。根据推导出的收发端的表达通式,接收端译码时采用基于MMSE准则的联合检测算法,即联合考虑不同天线的接收信号的最大比合并、CDMA解扩以及空时译码。仿真结果表明,所提出的STBC-SC-CDMA系统即使满载用户时也能获得优良的性能。在本论文的研究基础上,还需不断深入,使得MIMO系统的传输速率、系统容量以及性能得到更大程度地提高。更多还原

【Abstract】 The major driver for broadband wireless communications has been reliable high-data-rate services.This,together with the scarcity of bandwidth resources, motivate research toward developing efficient coding and modulation schemes that improve the quality and bandwidth efficiency of wireless systems. Multiple-input multiple-output systems,which deploy multiple antennas at both ends of the wireless link,explore the extra spatial dimension,to significantly increase the spectral efficiency,and to improve the link reliability relative to single antenna systems.It has been widely acknowledged that the space-time coding techniques can effectively exploit the spatial diversity created by multiple transmit antennas.However,these developments must cope with several performance limiting challenges that include mobile radio channel impairments, multiuser interference and size/power limitations at the mobile units.So,this paper is concentrated on the MIMO system codec over selective fading channels. The main contents and the contributions of this paper are as follows:Firstly,the impact of time variability over MIMO system as well as how to use Doppler diversity to improve the transmission performance are investigated. Relying on a basis expansion model for time-selective channels,the conventional codecs over time-selective channels are systematically summarized.On the basis of the conventional codecs,a systematic design of high-rate full-diversity space-time-Doppler(STD) codes is proposed for MIMO time-selective channels and the design criteria of full-diversity STD codes are shown.The proposed STD codes are constructed from a layered algebraic design,where each layer of algebraic coded symbols are parsed into different transmit antennas,time slots and symbol blocks without rate loss.It is shown that the proposed STD codes can achieve rate N_t and full-diversity N_tN_rN_b(Q+1),i.e.,the product of the number of transmit antennas N_t,receive antennas N_r,coding blocks N_b and channel complex exponential bases(Q+1).Simulation results have shown the proposed STD codes are efficient.Secondly,block-differential coding with multiple antennas for unknown doubly-selective channels is investigated in this paper.Coherent decoding requires reliable estimation of the underlying multi-channels at the receiver.This is a challenging and costly task,especially when the channel experiences high mobility induced fast-channel fading.Differential coding,which circumvents the need for channel estimation,is an attractive alternative in such environments. However,most differential coding schemes are designed only for time-selective channels or only for frequency-selective channels.With the scarcity of the differential coding schemes for doubly-selective channels,a new block-differential codec for systems that are equipped with an arbitrary number of transmit and receive antennas and operate in time and frequency selective(doubly selective) channels is proposed.It can achieve full diversity gains,while affording low-complexity maximum-likelihood decoding.Relying on the fact that the coefficients of complex exponential basis of BEM of each path are fixed for a block,the proposed scheme subdivide a block into multiple subblocks and performs differential encoding across the subblocks.By corresponding processing at the transmitter and receiver,signal detection does not need channel state information,and hence avoid estimation of the doubly-selective MIMO channels. Numerical results are presented to illustrate the performance of the proposed scheme over four styles of channel.Thirdly,the preprocessor for downlink of multi-user STBC system is investigated.Using the SVD decomposition theorem and the QR decomposition theorem,the precoder for downlink transmission is shown.Then,this paper proposes two precoding methods that use extra transmit antennas,beyond the minimum required,to provide additional degrees of diversity.Two proposed designs are:eigenmode selection and transmit selection.The principle of eigenmode selection is that every user signals on the best orthogonal basis, according to maximizing the Frobenius norm of the equivalent channel,which is the key parameter to determine the diversity performance of a single-user STBC system,and yet maintaing the zero interuser interference constraint.Multiuser antenna selection operates similarly to eigenmode selection with the additional constraint that only a subset of the available transmit antennas are employed. Simulations show that the proposed downlink precoder can effectively cancel the interference between mobiles,while still providing good diversity performance.In addition,the impacts of channel estimation error and channel correlation are researched and analyzed.Finally,considering the uplink of the communication system,a new air interface that combines STBC with SC-CDMA is proposed.DS-CDMA system suffers from inter-symbol interference(ISI) and multiuser interference(MUI) caused by multipath propogation,leading to a significant loss of performance.In order to enable the design of low complexity transceiver that can cope with multipath channels while still benefiting from the good properties of DS-CDMA and the space-multipath diversity,a new air interface that combines STBC with SC-CDMA is proposed.We get the expressions of input/output relationship.Three basic joint detection algorithms are shown:MF-BLE,ZF-BLE and MMSE-BLE. At the receiver,a joint multi-user detector and ST block decoder,optimized according to the MMSE criterion is designed.At last,simulations are done to corroborate our theoretical analysis.More work is needed to improve the transmission rate,channel capacity and link performance of MIMO system.更多还原