【作者】 吴莉莉；

【导师】 保铮；

【作者基本信息】 西安电子科技大学 ， 信号与信息处理， 2004， 博士

【摘要】 近年来，全球移动通信事业经历着飞速的发展，随着用户数的急剧增加和需求呈现多样化发展，现有的以提供话音业务为主的移动通信系统已难以满足需求，因此，具有更大的系统容量且能提供多媒体业务的未来移动通信系统已成为研究热点。为了克服它所面临的一些技术难题，人们发展并创新了许多新技术。本论文主要针对未来移动通信系统中的接收问题，研究多用户检测、智能天线以及正交频分复用(OFDM)技术在其中的应用。主要工作概括如下： 1．研究了具有未知频率选择性衰落的OFDM信号的最优接收问题，提出了一种基于马尔可夫链蒙特卡罗(MCMC)方法的全盲贝叶斯接收机，它在不作信道估计的条件下，用MCMC方法近似计算用户发送的比特的后验概率，与直接实现贝叶斯检测的复杂度相比，其运算量大为减少，而且，由于我们利用了OFDM的正交性，避免了矩阵求逆，又进一步减少了计算复杂度。同时，它还结合了OFDM系统的特点，使用如下两个方法：一．对后验分布函数中的信道参数进行积分，推导出与信道参数无关的分布作为目标分布；二．用差分编码消除双模现象。从而使算法的收敛速度得到较大提高，其性能接近信道未知时的最优检测的性能。 2．研究了有小区外干扰用户时的异步多径DS-CDMA信号的多用户检测问题，提出了一种新颖的非线性多用户检测方法，它把Gibbs采样与线性群盲解相关技术相结合，使在异步多径及有小区外干扰用户的情况下，仍然能够利用Gibbs采样进行贝叶斯检测，并且也适于在有信道编码的系统中做Turbo检测。由于该方法在非线性处理阶段利用了不同用户的噪声具有相关性这一特点，与线性解相关方法相比，其检测性能有显著提高。同时，它也具有良好的抗远近效应能力。 3．针对共信道干扰与频率偏移共存的情况，提出了一种OFDM接收机，其思想是进行两级处理，即先做干扰抑制，然后再进行频偏估计。为了解决在有未知频偏情况下的干扰抑制这一问题，提出在发射端为每个用户分配一个扩频序列，并在时域对用户信号进行扩频，在接收端对接收数据进行一定的排列，通过波束形成进行干扰抑制，然后再根据干扰抑制的输出，利用导频序列得到期望用户的频偏和信道的最小二乘估计。考虑到有限快拍会在波束形成一级引起信号相消现象，所未来移动通信系统的几种新型接收机研究 以使用迭代处理的方法，即用第二级估计的参数重构期望信号，并反馈到第一级， 从接收信号中减掉重构的期望信号，可以提高干扰抑制的性能。仿真实验表明， 该方法只需要进行2一3次迭代就己经很好地接近理想接收的性能，而且它还具有 抗远近效应的能力，即使在用户数比较多的情况下，也能够很好地工作，当归一 化频偏落在卜告，+们这一范围内，该方法亦能很好地进行干扰抑制及频偏估计。4.研究了通道频率响应特性失配的校正问题，提出了一种基于自适应干扰抑制的智 能天线校正方法，它利用自适应均衡的原理补偿通道间的失配，并使用自适应干 扰抑制的结构削弱注入信号，从而保证校正与用户检测互不影响。通过仿真实验 定性研究了均衡器性能、干扰抵消器的性能与一些重要参数之间的关系，给出了 时延的最佳取值的经验值。关键词:多用户检测，智能天线，正交频分复用(OFDM)，贝叶斯检测，马尔可夫链蒙特卡罗(MCMC)方法，频率偏移，波束形成，干扰抑制，校正。西安电子科技大学博士学位论文更多还原

【Abstract】 With the development of universal mobile telecommunication systems, the number of subscribers is increasing rapidly and the service requirements are becoming diversified, which can not be satisfied by current mobile telecommunication systems. Therefore, future mobile communication systems with large system capacity and multimedia service support ability have absorbed much attention. To combat the technical obstacles, several new techniques have been innovated. In this dissertation, the focus is on the application of multiuser detection, smart antenna and Orthogonal Frequency Division Multiplexing (OFDM) techniques to the design of receivers for future mobile communication systems. The main contributions and original ideas included in the dissertation are summarized as follows.1. We investigate the problem of the design of an optimal OFDM receiver against unknown frequency selective fading. A fast convergent Monte Carlo receiver is proposed. The Markov chain Monte Carlo (MCMC) methods are employed for the approximate computation of bit posterior probability without channel estimation, which results in a substantial complexity reduction compared with the direct implementation of Bayesian symbol estimate. Moreover, no matrix inversion is needed with the use of the orthogonality of OFDM modulation and hence the computational load is again significantly reduced. Meanwhile, with the exploitation of the characteristic of OFDM systems, two methods are employed to improve the convergence rate and enhance the efficiency of MCMC algorithms. One is the integration of the posterior distribution function with respect to the associated channel parameters, which is involved in the derivation of the objective distribution function; the other is the differential coding for the elimination of the bimodality problem resulting from the presence of unknown fading channels. Thanks to the convergence acceleration tricks, the convergence rate is obviously improved and the performance approaches that of the optimal detection with unknown channel parameters.2. The multiuser detection for asynchronous DS-CDMA system with inter-cell interference and multipath fading is studied. The Gibbs sampler combined with the lineargroup-blind decorrelator is proposed as a novel nonlinear multiuser detector, which makes the Gibbs sampler applicable to the case of asynchronous CDMA systems in the presence of unknown interference and multipath fading. Furthermore, in a coded system the proposed detector is well suited for the Turbo detection. The proposed method has good near-far resistance and outperforms the linear group-blind decorrelator to a great extent.3. An OFDM receiver is designed to combat both frequency offset and cochannel interference, which involves the following two-staged procedure. First, the strong cochannel interference is effectively suppressed before frequency offset compensation. To this end, we propose to assign each user a spreading sequence in time domain and perform beamforming to suppress cochannel interference by aligning the data samples in proper order. Second, pilot symbols assisted frequency offset and channel least square estimation is achieved based on the output of the first stage. In particular, there is usually certain degradation in beamforming process due to the finite data samples, an iterative structure is used where signal waveform is reconstructed by exploiting the estimated parameters and subtracted from the beamformer input data sent to the next iteration. Simulation results show that within 2 or 3 iterations, the receiver has almost the same performance as the ideal receiver even in a heavily loaded system. The proposed receiver has a good near-far resistance and works well when the normalized frequency offset falls into the range of [-1/2,+1/2].4. The calibration of frequency response mismatch among antenna array channels is studied. A calibration method based on adaptive interference suppression is proposed, in which adaptive equalizers are employed for the compensation of channel mismatch. To mak更多还原