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CDMA移动通信系统干扰分析与抵消算法研究
Research on Interference Analysis and Cancellation Algorithms for CDMA Mobile Communication Systems
【作者】 陈玉;
【导师】 尤肖虎;
【作者基本信息】 东南大学 , 通信与信息系统, 2006, 博士
【摘要】 码分多址(CDMA)以其频率规划简单、频谱利用率高和独特的抗多径衰落、软容量、软切换、宏分集以及灵活的变速率传输等技术特点,目前已经成为第三代移动通信系统的主流技术。论文以宽带CDMA系统关键技术中的干扰抵消与接收机技术作为主要研究方向,以cdma2000系统的物理层传输技术为切入点,研究和探讨适用于宽带CDMA移动通信系统的干扰抵消与接收机技术。首先,论文研究了量化处理和有限字长效应对cdma2000系统下行链路RAKE接收机和Viterbi解码器性能的影响,通过仿真研究了A/D转换器字长、信道估计区间长度、移动台速度以及导频功率增益因子等参数的选取对定点仿真系统性能的影响;基于对量化噪声统计特性的分析,提出了通过合理设计滤波器以提高量化信噪比,增加量化器有效字长的方法,可以方便地应用于实际系统的硬件电路以及ASIC设计之中。针对具有连续导频辅助的DS-CDMA系统,论文分析了接收信号中导频干扰、用户多址干扰、多径干扰和噪声等各分量的统计特性;推导了单小区与多小区环境中、均匀多径分布和(非)理想信道估计条件下,导频干扰抵消前后系统平均符号错误概率的理论公式;分析了均匀多径分布条件下,滑动平均信道估计误差的统计特性,给出了导频功率增益因子与信道估计噪声方差之间的数学关系,并通过数值仿真对于理论计算的结果进行了分析。在理论分析的基础上,论文给出了码片级导频干扰抵消算法和符号级导频干扰抵消算法的具体实现步骤,说明了两种算法的等价性;系统仿真表明,符号级导频干扰抵消算法不仅可以有效提高接收机的性能,而且在符号速率上处理时将大大降低硬件实现的复杂度。该算法可以进一步推广到多个蜂窝小区的情形,有效地消除相邻小区基站发送的导频信号对于本小区中用户产生的多址干扰,更好地满足话音和数据业务的传输要求。针对具有连续导频辅助的DS-CDMA系统,论文给出了扩频序列部分互相关函数的精确描述,得到了低扩频比时关于多址和多径干扰更为精确的数学表达式。基于对导频干扰、用户多址干扰、多径干扰和噪声等各分量统计特性的分析,推导了单小区环境与多小区环境中、均匀多径分布和(非)理想信道估计条件下,低扩频比DS-CDMA系统平均符号错误概率的理论公式,并通过数值仿真对于理论计算的结果进行了分析。在理论分析的基础上,论文提出了RAKE合并与符号级Viterbi均衡及多级干扰抵消相结合的联合接收算法,并给出了相应的接收机结构。既可以利用RAKE接收技术合并多径信号,实现接收分集;又可以利用符号级Viterbi均衡及多级干扰抵消算法消除各径信号中的符号间干扰,从而有效地克服在低扩频比情况下、传统RAKE接收机中存在严重的符号
【Abstract】 CDMA (Code Division Multiple Access) becomes main stream in the 3rd generation mobile communication systems due to its attractive features, such as simple frequency deployment, high spectrum efficiency, multipath fading mitigation, soft hand-off and macro-diversity. This dissertation focuses on the wideband CDMA based technologies, including interference cancellation and receiver techniques.The effect of quantization and finite wordlength on the performance of RAKE receiver and Viterbi decoder is addressed for fixed-point simulations in cdma2000-1x downlink. Selections of parameters as ADC wordlength, channel estimation interval length, mobile velocity and pilot power gain factor on the system performance are investigated via simulations. Detailed discussions and comparisons of results between the floating-point and fixed-point simulations lead to some important conclusions, which provide references for practical systems. Furthermore, based on statistical analyses of quantization errors, mathematic relationship is obtained between the quantization SNR and the wordlength of the baseband sampled signal. Design methods of special filters improving the quantization SNR hence the equivalent quantizer wordlength are proposed, which not only demonstrate good performance but also enable easy ASIC implementation.As for continuous pilot aided DS-CDMA systems, statistical properties of pilot interference, multiple access interference (MAI), multipath interference (MPI) and background noise are investigated in detail for both single-cell and multiple-cell environments under the conditions of uniform multipath intensity profile and ideal/non-ideal channel estimation, which leads to average symbol error probabilities for DS-CDMA systems with/without pilot cancellation evaluated via numerical results and system simulations. Besides, based on the statistical analysis of moving averaging channel estimation error, mathematic relationship is obtained between the pilot power gain factor and the variance of channel estimation error.Based on theoretical analyses, two practical algorithms—symbol-level and chip-level pilot cancellation schemes are proposed and their equivalence is also verified. Simulation results show that the symbol-level pilot cancellation scheme can not only improve the RAKE receiver performance but also enable relatively low implementation complexity. Besides, this algorithm can be extended to multiple-cell environments to effectively eliminate the pilot interference from