【作者】 霍永青；

【导师】 彭启琮；

【作者基本信息】 电子科技大学 ， 信号与信息处理， 2008， 博士

【摘要】 下一代移动通信系统需要支持高速率、高可靠性的数据传输业务,如何在恶劣的信道环境下实现这些业务,提供端到端的服务质量(Quality of Service,QoS)保证,是未来移动通信系统关注的一个重要问题。正交频分复用(OrthogonalFrequency Division,OFDM)技术能有效克服多径衰落,提高传输速率;多入多出(Multiple Input Multiple Output,MIMO)技术能有效改善系统性能或显著提高系统容量;随着信道变化调整系统传输参数和结构以适应无线环境的自适应技术能进一步提高频谱效率,改善系统性能。因此,OFDM技术、MIMO技术以及自适应技术已经被认为是未来移动通信系统中的主要候选技术。本论文研究多载波系统中的自适应比特功率分配算法。针对不同约束条件,提出了几种自适应比特功率分配算法。在对MIMO-OFDM系统模型分析的基础上,将所提的几种算法推广到了MIMO-OFDM系统中。分析研究了联合低密度校验码(Low-Density Parity-Check,LDPC)编码及自适应分配算法在多载波系统中的性能。此外,本论文还研究了不完整信道信息对自适应分配算法性能的影响,提出了一种能减小该影响的自适应分配算法。同时,论文还对LDPC编码,OFDM技术以及自适应分配算法在图像传输中的应用做了研究。论文的第一章为绪论,介绍本论文的研究背景、国内外研究现状,以及本文的结构和创新点。作为研究基础,第二章介绍了OFDM、自适应技术以及MIMO-OFDM技术的原理,分析了这些技术的优缺点,给出了不同性能要求下,自适应源分配算法的多种数学模型。第三章研究多载波系统中的自适应比特功率分配算法。提出了一种最小化系统误码率的自适应比特功率分配算法。该算法可以在给定目标传输速率和总功率约束条件下,实现最小化系统误码率(Bit Error Rate,BER)。算法首先构建包含比特和功率值的拉格朗日函数,对其求导,获得每个子载波的最佳比特功率值。分析结果表明,该算法在低计算复杂度的情况下,可以很好地改善系统的误码率性能。在对MIMO-OFDM系统模型分析的基础上,将该算法推广到了空间复用(Spatial Multiplexing,SM)OFDM系统中,对其在SM-OFDM系统中的性能做了仿真分析。结果表明,算法仍高效可行。第四章提出了两种最大化系统容量的自适应比特功率分配算法。两种算法都可以在给定目标误码率和总功率约束条件下,实现最大化系统容量。一种在满足每个子载波的误码率都小于目标误码率的情况下,进行功率再分配;另一种在满足所有子载波的平均误码率小于目标误码率的情况下,进行功率再分配。分析结果表明,两种算法都可以实现很好的吞吐量性能。基于第三章对MIMO-OFDM系统模型的分析,研究了两种算法在SM-OFDM系统中的性能。仿真结果表明,两种算法仍然可靠有效。在本章中,还对不完整信道状态信息(反馈时延和估计误差)对自适应算法性能的影响做了研究,提出了一种可以减小不完整信道信息对自适应算法性能影响的比特功率分配算法,仿真表明,该算法在对抗反馈时延和估计误差带来的信道误匹配上具有有效性。第五章研究了结合LDPC编码的多载波系统中的自适应比特功率分配算法的性能。首先介绍了LDPC码的基本原理以及编译码算法。进而分别研究了最小化误码率自适应源分配算法、最小化发射功率自适应源分配算法,和不同码长及码率的LDPC码相结合,应用于多载波系统中的性能。对该性能做了仿真验证,结果表明,采用联合LDPC编码和自适应源分配算法,比采用单独的未编码自适应或非自适应的LDPC码的性能都要好。这说明了结合LDPC码和自适应源分配算法在多载波系统中的有效性。结果还表明,LDPC编码带来的增益,要比自适应源分配算法带来的增益大。第六章研究了自适应比特功率分配算法、LDPC编码以及OFDM技术对图像传输性能的影响。提出了结合OFDM、LDPC编码以及本文所提的自适应比特功率分配算法的图像传输方案。分别研究了最小化误码率自适应比特功率分配算法和最小化发射功率自适应比特功率分配算法结合LDPC编码及OFDM的图像传输方案的性能。分析结果表明,同时采用OFDM、LDPC编码以及本文所提的自适应比特功率分配三种技术的图像传输质量,要好于采用OFDM+LDPC或OFDM+自适应或单纯的OFDM。说明OFDM、LDPC编码以及自适应分配算法三者的结合应用在图像传输中是高效可行的。第七章总结论文的主要创新成果与研究结论,提出了一些需要进一步深入研究的问题。更多还原

【Abstract】 The next-generation mobile communication systems are required to provide high data-rate, high reliable transmission. How to provide these services and guarantee end-to-end QoS under abominable channel environment has been one challenge of the future wireless communication systems. OFDM can resist multipath fading and increase the transmission rate; MIMO can improve the system performance and capacity; adaptive technology which automatically adjusts the transmission parameters and structure to meet the wireless environment can improve the spectral efficiency and the system performance. So, OFDM, MIMO and adaptive technology are regarded to be key technologies of the future wireless mobile communication system.This dissertation researches on adaptive bit and power allocation algorithms in multicarrier systems. Several adaptive bit and power allocation algorithms are proposed according to different constraints. Based on the analysis of MIMO-OFDM system model, the proposed algorithms are extended to MIMO-OFDM systems. The performance of combined LDPC coding and adaptive allocation algorithms in multicarrier systems is studied. Furthermore, the effect of the imperfect channel state information on the adaptive algorithms is studied, and an adaptive algorithm is presented which can lower the effect. The dissertation also researches on the performance of LDPC coding, OFDM and adaptive algorithms in image transmission.Section I is the preface of this dissertation, which introduces the background knowledge and the current researching status. The organization and main contributions of this paper are presented also.As a theoretical foundation, In Section II, the principle of OFDM, adaptive technology and MIMO-OFDM is introduced and their advantages and disadvantages are analyzed. In this Section, several mathematical models of adaptive algorithms are given according to different service requests.In Section III, adaptive bit and power allocation algorithms in multicarrier systems are researched. An adaptive bit and power allocation algorithm is presented which minimize the BER under the constraints of total transmission rate and total power. Firstly, the algorithm constructs Lagrange function containing bit and power values, then obtains the optimal bit and power values of each subcarrier by the differential of Lagrange function. Simulation results show that the proposed algorithm can improve the system performance greatly with low computational complexity. Based on the analysis of MIMO-OFDM system, the proposed algorithm is extended to SM-OFDM system. The simulation results show that the algorithm is still effectiveIn Section IV, two bit and power allocation algorithms are presented, which can maximize the system capacity under the constraints of target BER and total power. One reallocates power based on the fact that BER value of each subcarrier is lower than the target BER; another one reallocates power based on the fact that the average BER of all subcarriers is lower than the target BER. The simulation results show that two algorithms can implement high throughput. Based on the analysis of MIMO-OFDM system model in Section III, the performance of two algorithms in SM-OFDM system is studied. The results show that two algorithms are effective and reliable in SM-OFDM system. In this Section, the effect of imperfect channel state information on adaptive algorithms is researched, and an adaptive algorithm which can lower this effect is presented. The results show that this algorithm is effective to resist the channel mismatch result form feedback delay and estimation error.In Section V, the performance of combined LDPC coding and adaptive bit power allocation algorithms in multicarrier system is researched. Firstly, the basic principle of LDPC codes is introduced. Then, the performance of combined LDPC coding with two adaptive algorithms (one is to minimize the BER and another is to minimize the transmission power) in multicarrier system is researched respectively. The simulation results show that the performance with combined LDPC coding and adaptive algorithm is better than LDPC coding or adaptive algorithm alone. This indicates that the combination of LDPC coding and adaptive algorithm is effective in multicarrier system. The results also show that the gain result from LDPC coding is higher than that result from adaptive algorithm. In Section VI, the performance of adaptive bit power allocation algorithm, LDPC coding and OFDM in image transmission is researched. Image transmission scheme which use the combination technology of adaptive bit and power allocation algorithm、LDPC coding and OFDM is proposed. The performance of two image transmission schemes is studied respectively. One scheme ues LDPC coding、OFDM and adaptive algorithm which is to minimize the BER, another scheme uses LDPC coding、OFDM and adaptive algorithm which is to minimize the transmission power. The simulation results show that the performance of image transmission with OFDM, LDPC and adaptive algorithm is better than that with OFDM+LDPC or OFDM+adaptive algorithm or OFDM. This indicates that the combination of OFDM, LDPC and adaptive algorithms is effective in image transmission.In Section VII, the dissertation is summarized and some problems need to be further researched are presented.更多还原