无线通信系统协作传输和信道互易性问题研究
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摘要
为了满足人们对数据速率和数据传输可靠性的不断提升的要求,在未来无线通信系统中,MIMO技术成为必不可少的基础技术之一。MIMO技术提供的复用增益和分集增益分别为提升数据传输速率和数据传输可靠性提供了技术保证。基于上述原因,MIMO及其相关技术得到了广泛的研究和应用,其中就包括协作传输技术和多天线发送预处理技术。
     协作传输技术是MIMO技术在单天线环境下的自然推广。很多时候在廉价的小型终端上无法安装多个天线,而多个单天线终端互相借用对方的天线进行协作传输,可以构成虚拟的多天线系统以应用MIMO技术的众多研究成果。但不同于传统MIMO系统,协作传输系统中的多个分布式天线无法达到理想同步,因此导致许多可应用于传统MIMO系统的技术无法直接应用于协作传输系统中。因此,需要对异步环境下的协作传输进行研究以解决上述问题。
     另一个MIMO相关技术是多天线发送预处理技术。在发送端获得信道状态信息(CSI)的情况下,使用发送预处理可明显提升系统性能。FDD系统的上下行链路工作于不同频点,因此上下行链路的信道不具有互易性,移动台(MS)需要使用专门的反馈信道把测量到的下行链路CSI发送到基站(BS),从而浪费了系统资源,增加了复杂度;而TDD系统的上下行链路工作于同一频点,因此上下行信道互易。这样BS可以根据上行检测到的CSI进行发送预处理,而不需要使用反馈信道,这成为TDD系统的一个天然优势。但有多种因素都会对信道互易性产生影响,因此,需要对这些影响因素进行研究并补偿。
     本文针对上述异步协作传输问题和信道互易性问题进行了研究,所取得的主要研究成果为:
     1.对异步空时协作传输技术进行了研究。针对异步环境下无法使用正交空时分组码的问题,提出了一种基于单载波块传输的异步协作传输机制。在该机制中,中继节点将来自源节点的数据进行分段后按既定协作规则使用单载波块传输方式传输;目的节点首先对接收到的对多个传输块进行FFT,然后通过线性组合分离相互叠加的数据块,再经频域均衡和IDFT及符号判决以恢复原始数据块。通过块传输技术,所提算法克服了异步所致的码字正交性丧失及信道弥散等问题,实现了满分集增益。接下来基于上述思想,给出了一般化的适合异步环境的空时协作传输码字。码字的设计基于OSTBC码字的设计思想,通过块传输克服传统OSTBC码字无法在异步环境下直接使用的缺点,实现了满分集增益。
     2.对I/Q不平衡引起的信道互易性损失问题进行了研究。存在I/Q不平衡的TDD系统中,I/Q不平衡的影响最终反映到系统的等效信道上,这样BS上行检测到的CSI已不能表征下行链路的信道状态,此时BS基于该CSI进行的发送预处理会出现偏差,进而导致系统性能的严重下降。针对此问题,提出了一种互易性补偿方法。该方法基于正式通信之前的双向信道测量来补偿I/Q不平衡的影响。BS和MS在正式通信之前,首先进行各自接收方向上的信道测量,得到上下行链路的CSI,接着MS将检测到的下行链路CSI反馈到BS,由BS计算出校准矩阵,然后BS将用于MS的校准矩阵发送给MS,这样BS和MS在发送时,首先使用校准矩阵进行预处理,使实际检测到的上下行信道的互易性得到保持。
     3.对信道时变特性引起的信道互易性损失进行了研究。在TDD系统中,BS的上行信道检测和下行发送具有一定的时间间隔。此间隔内的信道时变会导致上下行信道的互易性丧失。针对此问题,提出了一种基于变换域显著分量检测和预测的互易性补偿算法。首先将初步的LS信道估计所得到的导频信道CFR序列进行DFT转换到变换域,接着在变换域中使用MDL准则进行显著分量检测,再对每个显著分量使用基于修正协方差的AR模型进行预测以补偿由信道时变特性所致的信道互易性损失。所提算法有效地降低了信道估计和预测误差,弥补了由时变引起的信道非互易性所带来的系统容量损失,同时与频域直接预测方法相比,大大降低了系统复杂度。
     4.对信道估计误差引起的信道互易性丧失进行了研究。在存在虚载波和非严格等距导频的TDD-MIMO-OFDM系统中,把初步LS估计得到的导频信道CFR序列通过IFFT转换到时域时,出现了严重的时域CIR能量泄漏,从而使得FFT插值算法等基于时域处理的算法不能使用,无法实现估计误差的抑制。针对此问题,提出了一种低复杂度的信道估计算法以对抗由信道估计误差导致的互易性损失。该算法首先通过频域线性插值对虚载波位置的CFR进行添加,然后重新选取严格等距子载波作为导频,并对此新的导频CFR序列进行IFFT变换到时域,然后对所得的时域CIR序列进行基于噪声门限的去噪,从而有效地抑制了信道估计误差,使信道互易性得到保持,弥补了由信道非互易所致的系统容量损失。
     对以上方法都在理论分析的基础上通过仿真实验验证了其正确性和可行性。
In order to meet the increasing demands on higher rate and reliability of datatransmission,Multiple Input Multiple Output (MIMO) technique has been chosen asone of the indispensably basic techniques used in future wireless communicationsystems. The multiplexing gains and diversity gains obtained by MIMO techniqueprovide technical guarantee for higher data rate and reliability respectively. For thereason above, MIMO technique as well as some related techniques have been studiedand applied widely, including cooperative transmission and multi-antenna transmitpreprocessing.
     Cooperative transmission is the natural extend of MIMO technique in single-antennaenvironment. Usually multi-antennas are not available for a cheap and small-sized userterminal, whereas several single-antenna user terminals can share their antennas to forma virtual MIMO system to reap the various benefits of MIMO technique. However, it isdifferent from the conventional MIMO systems that the distributed antennas in acooperative transmission system can’t achieve perfect synchronization, so that manytechniques which are suitable for conventional MIMO systems can’t be directlyemployed. Therefore, it’s necessary to study the cooperative transmission inasynchronous environment so as to solve this problem.
     Another technique related to MIMO is multi-antennas transmit preprocessing. Withchannel state information (CSI) at transmitter, transmit preprocessing can significantlyimprove system performance. In FDD systems, the uplink and downlink use differentfrequency bands, which means no reciprocity between the two channels can be tookadvantage of, so that a mobile station (MS) can’t help sending CSI to base station (BS)via a feedback channel. It is not only a waste of system resource but also an increasingin system complexity. However, the uplink and downlink channels in a TDD systemshare the same frequency band, which means that there exists reciprocity between theproperties of the two channels. Thus, BS can perform transmit preprocessing accordingto the CSI obtained by means of uplink channel estimation rather than via a feedbackchannel, which is believed to be an inherent superiority of TDD systems. Unfortunately,various factors can damage the channel reciprocity; so that it’s a vital issue how tocompensate the influence resulted from such factors.
     This dissertation mainly researches on the problems of asynchronous cooperativetransmission and channel reciprocity mentioned above. The author’s main contributionsare as follows.
     1. Considering non-feasibility of OSTBC in asynchronous environment, the dissertationproposes an asynchronous cooperative transmission scheme based on single-carrierblock transmission. In the scheme, the relay nodes segment the signal received from thesource node into several blocks and retransmit them based on single-carrier blocktransmission mode according to the predefined cooperation rules. For the multipleblocks of received signal, the destination node performs FFT respectively, separates theoverlapped ones among them by means of linear combination, and then performsfrequency domain equalization, IFFT and symbol decision to recover the original data.The proposed scheme can solves the problems of orthogonality damage and channeldispersion resulted from the asynchronization, and can achieve full diversity gains.Based on this idea, a generalized space-time cooperative transmission code withmodified OSTBC which is suitable for asynchronous environment is presented. It cansolve the problem of non-feasibility of OSTBC in asynchronous environment andachieve full diversity gains.
     2. In TDD systems with I/Q imbalance, the negative effect of I/Q imbalance will resultin the non-reciprocity between uplink and downlink channels, so as to degrade thesystem performance. Aiming at this problem, a compensation scheme is proposed,which is based on bidirectional channel estimations. Before starting formal datatransmission, BS and MS perform channel estimations to get the CSI of uplink anddownlink respectively, then MS send the downlink CSI to BS which is responsible forcalculating the calibration matrices used for BS and MS, and at last BS send MS’scalibration matrix to MS. In this way, BS and MS can use calibration matrices fortransmit preprocessing to maintain the channel reciprocity.
     3. Time-variance property of wireless channel will lead to differences between uplinkand downlink CSI of TDD systems to damage the channel reciprocity. Aiming at thisproblem, a compensation method based on transform-domain detection and prediction isproposed. Firstly the channel frequency response (CFR) sequence of pilot channelsobtained by least square estimation is transformed into transform-domain by DFT, andthen significant components in transform-domain is picked up with minimumdescription length (MDL) criterion, and at last prediction is made for each component tocompensate channel time-variances. It is through transform domain processing that theproposed method effectively reduces the estimation and prediction errors, and henceremedies the system capacity loses. Meanwhile the proposed method greatly lowers thesystem complexity compared with the frequency domain prediction method.
     4. Channel estimation error is another causation of channel non-reciprocity. InOFDM systems with virtual subcarriers and non-strictly equidistant pilots, seriousenergy leakage of channel impulse response (CIR) emerges when the pilots’ CFRsequence obtained by LS criterion is transformed into time domain using IFFT, so thatthe algorithms based on time domain processing including FFT interpolation can’t beused to suppress estimation error. Aiming at this problem, a low complexity channelestimation method is proposed to compensate the channel non-reciprocity caused byestimation error. Firstly the CFRs of virtual subcarriers are added by linear interpolation,then strictly equidistant subcarriers are chosen as new pilot channels for CIR recoveryby IFFT, and at last a threshold-based denoising is performed to the recovered CIRsequence to suppress the estimation error and compensate the channel non-reciprocity.
     All the proposed methods mentioned above are validated for their correctness andfeasibility by means of theoretical analysis and simulation experiments.
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