【作者】 李静；

【导师】 黄爱苹；

【作者基本信息】 浙江大学 ， 通信与信息系统， 2008， 博士

【摘要】 无线通信业务种类繁多、对数据率要求日益提高的发展趋势,与频带资源极其有限的客观现状,要求未来宽带无线通信系统必须具备强抗干扰能力、高频谱利用率、高自由度和灵活性。CDMA在3G系统中得到了广泛的应用,有着成熟的商用基础。CDMA可以提供码域的自由度,广义的CDMA涵盖了TDMA、FDMA、随机CDMA和OFDMA等接入技术。因此针对未来系统的应用特点对CDMA技术做进一步的研究,具有重要且普适的意义。传统CDMA系统面临的干扰受限问题在宽带通信中将更加突出,有效、可行的干扰消除技术是CDMA系统的研究难点之一。分析表明,扩频序列的自相关/互相关特性和干扰强度密切相关,直接影响CDMA系统的性能。因而从扩频序列方面进行突破,是解决CDMA系统干扰问题的最直接的途径。本文首先介绍了反映扩频序列特性的基本参数,并提出了新的序列特性度量指标,为后面的序列设计和性能分析提供依据;用广义正交码的定义将现有各种正交、ZCZ序列纳入统一的框架;分析正交扩频序列的抗干扰能力和集合大小之间的关系,提出了新的序列设计应用准则。其次,研究了同步时域扩频系统中的序列设计和应用。对于基于完美互补(PC,perfect complementary)序列扩频的CDMA系统,采用偏移叠加的调制方式灵活实现多速率业务,理论上分析了信号多径分离的条件,从而提出了非递归的接收机结构,实现无干扰性能的同时解决了递归接收机错误累积的问题。为了提高系统稳健性,针对具有零相关区(ZCZ)的宽同步(LS,loosely synchronoized)码,作者分析了码集合内部的ZCZ分布特点,进而提出了新的LS码的分配策略。新策略提高了已分配码集合的平均ZCZ长度,使得LS-CDMA系统对抗多径信道的稳健性得到提高。针对现有广义正交序列的ZCZ长度均为2的幂次的局限,利用m序列的自相关特性,基于循环构造的方法,设计了一种称为m-ZCZ的新序列,产生简单,其ZCZ长度可以在整数范围取值。提出了基于m-ZCZ码的CDMA系统,设计了发送扩频码与接收端本地扩频码的帧结构,通过选择与信道最大多径时延最匹配的ZCZ长度参数,新系统的频谱效率和基于传统ZCZ码的系统相比有明显改善,尤其在信道时延非2的整数倍时。为了支持更多用户,我们提出了一种最小ZCZ长度为1的ZCZ-one序列,研究了其在系统中的应用和分配策略,研究了新系统的带宽在编码和扩频之间的分配,使得新系统既有支持大量用户接入的能力,同时又具有较强的干扰抑制能力。本文针对基于各类GO码的下行同步系统,研究了基于导频序列的信道估计方案,理论分析了信道估计准确性对系统性能的影响,对导频功率因子、导频符号数等系统参数进行优化,为系统的实用提供指导和依据。接着,针对异步时域扩频系统,本文提出了一种基于互补序列和正交矩阵扩展的新型广义正交序列——组间互补(IGC)码。该序列具有分组的二值相关特性。将IGC码应用于异步CDMA系统,根据IGC码的分组相关特性,提出新的码分配策略,使得用户间相对时延和序列间ZCZ长度最大程度匹配,达到提高码资源利用率及系统性能的目的。最后,本文研究了两种混合接入系统。针对频域扩展的MC-CDMA系统,分析了干扰强度和序列的频域相关特性之间的关系,设计了频域相关函数具有ZCZ的一种新序列——GW序列。提出一种基于GW序列的MC-CDMA系统,利用可变扩展因子的GW码支持多数据率业务,同时有效避免了基于Walsh序列的传统MC-CDMA系统存在的严重干扰。针对多天线CDMA系统,提出了互补码扩频与延迟复用技术结合的结构,证明了系统无用户间干扰及天线间干扰,并且可以灵活支持分集和复用。更多还原

【Abstract】 Due to the demand of high data rate for future wireless services and limited spectrum resources,future wireless communication systems are required to provide strong interference-resistant ability,high spectrum efficiency,high freedom degree and flexibility.CDMA is a mature technology which has been broadly applied in 3G systems.The generalized conception of CDMA includes TDMA,FDMA and OFDMA as special cases. However,the interference-limited problem of traditional CDMA systems is serious,especially in broadband communications.Interference cancelation is one of the key technologies of CDMA systems.Since the auto-/cross-correlation properties of spreading sequences are directly related to the interference intensity,break through from the spreading sequences would be a direct and effective way to solve the interference-limited problem in CDMA system.First,the fundamental parameters and metrics of spreading sequences are introduced. Two new metrics are proposed to provide guidance for sequence design and performance analysis in later chapters.The definition of GO code is given to include all the existing orthogonal codes in a generalized frame.The development of various orthogonal spreading sequences is summarized,based on which the new sequence design criterion is proposed.Then,we study the sequence design and application in synchronized spreading spectrum systems.Perfect complementary(PC)code spreading and offset-stack modulation have been proposed in the CDMA system which can support multi-rate services flexibly. We analyze the multipath separation condition theoretically and propose a non-recursive receiver,which achieves interference-free performance and avoids the error propagation caused by the recursive receiver.By investigating the distribution of ZCZ of the Loosely synchronized(LS)code,We propose a new code allocation algorithm to improve the robustness of LS-CDMA system against channel deterioration.Different from the existing GO codes whose ZCZ length must be the power of 2,m-ZCZ code proposed by the author is a kind of GO code whose ZCZ length can be selected in integer field.We give the cyclic construction method for m-ZCZ code,and design the application style for m-ZCZ code in CDMA system.To support more users,we also propose a code with minimum ZCZ length of one,called ZCZ-one code.We study the code allocation scheme and the bandwidth allocation between spreading and coding,enable the new system with both large capacity and strong interference-resistant ability.Channel estimation is important for the above systems to realize interference-free performance ,hence we study the pilot-based channel estimation for general GO code based synchronized system.The impact of accuracy of channel estimation on system performance is theoretically analyzed,based on which the parameters such as pilot power and pilot number are optimized.Next,a novel GO code called Intergroup complementary(IGC)code is proposed for asynchronized spreading spectrum systems.IGC code has group-wise correlation properties ,based on which we propose a new code allocation scheme to match the ZCZ length with the relative time delay between users.Simulation results show that IGC-CDMA system has better BER performance and spectral efficiency than both traditional gold code based CDMA system and PC code based interference-free CDMA system.At last,two hybrid multiple access systems are studied.For MC-CDMA system,we analyze the relation between interference intensity and spectral correlation properties of the sequences.Then we design a novel sequence called generalized Walsh(GW)code with spectral ZCZs.The GW code based MC-CDMA system is then proposed,which can effectively eliminate interference and support multi-rate services flexibly with variable spreading factor GW(VSF-GW)code.For MIMO-CDMA system,we employ PC code spreading and delay multiplexing technique.The resultant system can eliminate interferences from both other users and other antennas.更多还原