【作者】 韩春雷；

【导师】 葛建华；

【作者基本信息】 西安电子科技大学 ， 通信与信息系统， 2011， 博士

【摘要】 在未来的无线通信系统中,无线信道的衰落特性仍是影响系统可靠性和有效性的重要因素。多输入多输出(MIMO)技术是对抗衰落的重要手段,它可以在不增加系统带宽和发送功率的情况下,有效提高通信系统的频谱利用率和可靠性。为了使无法装配多天线的移动终端也获得MIMO技术带来的各种好处,学者们提出了协作分集技术。该技术通过单天线移动终端之间的协作共享彼此天线,形成了虚拟的天线阵,从而获得了空间分集。协作分集技术是低硬件成本条件下MIMO技术的有效补充,因此成为了目前无线通信领域的研究热点。本论文针对协作分集技术展开了研究,主要对信道编码和网络编码协作技术在无线通信系统中的应用进行了深入的分析和探讨。论文的主要工作如下:1.编码协作方案中断率与功率优化分配首先分析了基于中继系统的编码协作方案的中断率,通过一种近似计算方法化简了中断率积分,随后推导了系统中断率近似界的简单表达式,最后以最小化系统中断率为目标,提出了一种功率优化分配方案。该方案以系统中断率近似界为目标函数,通过求导得到了功率优化分配系数的数学闭式解。仿真结果表明理论推导的近似界是一条紧界。同时发现,随着中继节点与目的节点之间信道状况的改善,采用功率优化分配方案可以有效改善系统的中断率性能。其次,在此基础之上又分析了Nakagami信道下编码协作方案的中断率性能,通过近似计算推导了中断率近似界的数学闭式解,并通过对该闭式解的分析给出了高信噪比下编码协作系统完全协作时的分集阶数。最后利用计算机仿真分析了功率分配和协作度对系统中断率的影响。2.编码协作系统的中继选择方案针对编码协作中采用循环冗余校验(CRC)方案会降低系统传输速率的问题,提出了一种改进的门限选择中继方案。该方案首先推导了使用速率匹配穿孔卷积码情况下系统的误码率上界,并以误码率最小为原则得到了最佳判决门限值,然后利用中继节点接收信噪比与该门限值比较判断是否协作。计算机仿真表明,在使用最优门限和功率分配系数的条件下,与CRC方案相比,该方案使系统误码率性能获得一定提升。3.联合网络-信道编码的传输方案将网络编码与信道编码相结合,提出了一种可获得高阶分集增益的中继协作方案。该方案在中继节点处将多个时隙接收的信息分别进行信道编码,再将所有码字进行网络编码后转发。目的节点采用最大后验概率算法迭代译码,利用多个时隙收到的码字恢复源节点发送信息。对Rayleigh衰落信道下的成对错误概率分析表明,该方案可使源节点发送信息在不同时隙得到多次转发从而增加时间分集增益。仿真结果表明,该方案在快衰落和慢衰落信道下均能提高系统分集增益,并且随着中继节点接收信噪比的提升,系统误码率性能明显改善。4.多址接入协作中继方案首先,针对现有多址接入协作中继方案存在不能充分利用编码冗余的问题,提出了一种在理想用户-中继信道条件下,联合网络与信道编码的多址接入协作中继方案。该方案在中继节点利用网络编码将不同用户的编码冗余同时转发,目的节点通过对某个用户信息的正确译码恢复出其他用户的编码冗余。通过对译码错误事件概率的分析,推导了该方案的中断率与误码率近似值表达式。仿真结果表明,随着信噪比的提升,所提方案可获得比编码协作更好的中断率和误码率性能。其次,针对非理想用户-中继信道,提出了一种改进方案。随后基于对目的节点译码过程的描述,详细分析了该方案的译码错误事件,并推导了其在Nakagami信道下的误码率理论近似值表达式,最后通过仿真验证了理论推导的正确性,并分析了用户-中继信道状况对系统误码率性能的影响。更多还原

【Abstract】 Multipath fading is an important factor in wireless channel which still seriously degrades the efficiency and reliability of wireless communication systems in the future. Multiple input multiple output (MIMO) technology is an effective way to combat fading over wireless channels, which can improve the spectral efficiency and reliability of wireless communication systems without additional power and bandwidth consumption. In order to take full advantage of MIMO technology on mobile terminals which may not have multiple antennas, cooperation diversity has been proposed as a promising solution in recent years. This technology creates a virtual antenna array through sharing terminal’s antennas, which can obtain space diversity on single antenna terminals. It is particularly attractive in wireless communication filed as it can be viewed as an effective complement of MIMO technology at low hardware cost. This dissertation intends to focus on some key technologies in cooperative diversity systems. Specifically, we mainly investigate the applications of channel coding and network coding technologies in wireless communication systems. The main contribution of our works can be summarized as follows:1. Outage probability analysis and power allocation scheme of coded cooperationAn optimum power allocation (OPA) approach is proposed base on the outage probability analysis of coded cooperation over Rayleigh fading channels. An approximation method is proposed for simplifying the outage integral, and a simple expression of the outage probability approximation bound is derived. Then, the closed-form expression of OPA coefficient is obtained based on the approximation bound. Simulation results demonstrate the validity of the theoretical analysis and the tightness of the approximation bound. In addition, the OPA method can improve the outage performance substantially with a better channel condition between relay and destination. Based on the approximation method mentioned above, the outage probability of coded cooperation over Nakagami fading channel is investigated, and then, the closed-form expression of outage probability approximation bound is derived. With the analysis of the closed-form expression, the diversity order is obtained in terms of full cooperation case at high average signal to noise ratio region. Moreover, the relationships of outage probability with cooperation level and power allocation are discussed through simulations.2. Relay selection scheme of coded cooperation For the rate reduction induced by using cyclic redundancy check (CRC) in coded cooperation, an improved threshold selection relaying scheme is proposed for coded cooperation. The upper bound of the bit error rate is derived when the rate-compatible punctured convolutional codes are used, and an optimum threshold (OT) is obtained by minimizing the bit error rate. The relay node decides whether to relay or not by comparing the estimated signal to noise ratio with the threshold. Simulations show that with the OT and power allocation method, the scheme has a better improvement over the CRC scheme.3. Joint network-channel coding cooperation schemeA high-order diversity scheme is proposed for relay cooperation with the network-channel coding approach. The information received by the relay node in different time slots is encoded with a channel coding method, and all the codes are then forwarded after network coding. With iterative maximum a posteriori decoding method, the information can be obtained at the destination node by decoding the codes received in different time slots. The pairwise error probability analysis under Rayleigh fading channel show that the proposed scheme can increase time diversity gain by forwarding the information several times in different time slots. Simulation results show that the scheme can obtain higher diversity gain in both fast and slow fading channels. Furthermore, the bit error rate performance can be improved when increasing the received signal to noise ratio of the relay node.4. Multiple-access relay cooperation schemeAs for the conventional multiple-access relay cooperation scheme, the coding redundancy is not fully utilized. In order to solve this problem, a novel cooperative scheme based on network and channel coding is proposed over ideal user-relay channel. In this scheme, the coding redundancy of different users is simultaneously forwarded through network coding at the relay node, and once the codes of one user are correctly decoded at the destination, the coding redundancy of other users can be retrieved. The approximation of outage probability and bit error rate of the proposed scheme are then deduced by analyzing the error event probability. Simulation results show that the proposed scheme is of better performance than the coded cooperation in terms of outage probability and bit error rate when the signal to noise ratio is getting higher.In addition,an improved cooperative scheme is proposed for non-ideal user-relay channel. The error event is investigated through analyzing the decoding process at the destination node. Then, the approximation of bit error rate over Nakagami channel is deduced. Simulation verified the validity of the theoretical analysis, and also analyzed the relationship between user-relay channel and the bit error rate of this scheme.更多还原