双码本波束成形相关技术研究

发布时间：2018-08-18 16:21

【摘要】：多天线技术能够利用空间自由度实现空间分集和复用，有效提高频谱利用率。其中，波束成形是在多天线系统中实现空分多址的主要技术，主要分为两类，，一类是长时的波束成形技术，它应用于小间距的多根天线阵列，主要原理是利用空间信道的强相关性及波的干涉原理产生强方向性的辐射方向图，使辐射方向图的主瓣自适应地指向用户来波方向；另一类是短时的波束成形技术，它的主要原理是基于实时的信道冲击响应，利用天线阵元之间的弱相关性构建信号处理算法，当发送端和接收端都具有完整的信道信息，短时波束成形技术能够简单有效地实现MIMO信道的最大分集增益。当天线距离在半波长到10倍波长之间，角度扩散不大时，为长时波束成形和短时波束成形结合起来提供了条件，因而，能使二者结合的双码本技术被引入到LTE-Advanced（LTE-A）系统中，它同时用两个码本分别指示长时波束成形特性（码本1）和短时波束成形特性（码本2）。此外，由于长时特性和短时特性的变化周期不相同，决定了二者反馈周期也不同，这样就可以延长长时特性码本的反馈周期，从而减小平均反馈量，提高资源利用率。虽然在LTE-A中定义了双码本发送方案，但是双码本估计和反馈等都是双码本技术中亟待解决的关键问题。本论文就是围绕着双码本波束成形技术的研究展开的，通过对相关理论研究，提出了一系列解决方案。总的来说，本文主要研究内容和贡献如下：首先，研究了双码本波束成形技术的码本设计特性和码本估计算法。在宽带信号的正交频分多路复用（Orthogonal Frequency Division Multiplexing，OFDM）系统中，不同子载波之间存在频率偏移，所以在利用不同载波上的信道信息进行独立码本估计时存在偏差，本文提出相关估计算法，利用宽带信号中多个载波之间的相关性，将不同载波的信道信息聚焦到某个参考频点上获得等效的信道信息。此外，在将多采样点估计获得的信道矩阵进行平均来获得统计信道信息时，引入采样点时刻的信噪比作为加权因子进行加权平均，可以克服由于个别噪声干扰较大的采样信息带来的估计性能恶化现象。通过理论分析和数值仿真得出，在信号带宽较小时，优化算法对系统性能提升较小，主要得益于加权平均；当信号带宽较大时，系统性能提升较明显。接着，在双码本波束成形系统中，分析由于IQ（in-phase/quadrature-phase）不平衡对波束成形系统带来的影响，推导了在IQ不平衡情况下引起的码本估计偏差以及双码本估计特性。为了消除IQ不平衡对码本估计的影响，本文给出了一种完整的收发端IQ不平衡估计和补偿算法，通过该算法能够对收发端IQ不平衡参数进行分步估计补偿，校正该因素引起的码本估计误差，从而提高系统性能。然后，通常情况下为了减小上行反馈资源，对代表长时信道特征的码本1采用较长的反馈周期，然而信道变化快慢具有时变性，如果采用固定的反馈周期会影响码本1的准确性，因此本文利用修正的非等间距灰色预测算法对信道方向指示（CDI）信息进行预测，提出一种自适应反馈策略。此外，由于码本2估计实时性要求较高，过时的信道估计会严重降低码本2的估计性能，因此本文利用卡尔曼预测算法对信道估计矩阵进行预测，并且引入灰色预测算法对预测时用到的状态转移矩阵进行修正，从而提高码本2的估计性能。最后，重点研究在大规模天线阵列（Massive MIMO）系统模型下，随着天线数的增加，信道信息估计与反馈信息的复杂度成倍增加，同时，Massive MIMO系统中天线阵通常呈二维阵列分布，信道的方向性估计也需要扩展成二维方向估计。针对上述特性，本文提出基于在二维信道方向估计下的双码本波束成形技术方案，包括双码本设计和双码本估计方案。最后通过仿真验证该方案的有效性，并对多种码本估计算法进行对比仿真，从复杂度和性能两方面进行比较分析，其中似ESPRIT原理的码本估计算法复杂度较低，且性能也接近改进的二维双码本估计算法。
[Abstract]:Multi-antenna technology can realize spatial diversity and multiplexing by using spatial degree of freedom, and improve spectrum efficiency effectively. Beamforming is the main technology to realize space division multiple access in multi-antenna system. It can be divided into two types. One is long-time beamforming technology, which is applied to multi-antenna arrays with small spacing. The main principle is to utilize space. The strong correlation of the time channel and the interference principle of the wave produce a strong directional radiation pattern, which makes the main lobe of the radiation pattern self-adaptively point to the user's direction of arrival; the other is the short-term beam forming technology, whose main principle is based on the real-time channel impulse response, and uses the weak correlation between the antenna elements to construct the signal location. When the distance between the antenna is between half-wavelength and 10-fold wavelength, and the angle spread is small, it provides the conditions for combining long-time beam forming with short-time beam forming. The dual codebook technique, which combines the two codebooks, is introduced into LTE-Advanced (LTE-A) system. The two codebooks are used to indicate the long-time and short-time beamforming characteristics (codebook 1) respectively. In addition, the feedback periods of the two codebooks are different because of the different change periods of the long-time and short-time characteristics. Extending the feedback period of the long-time characteristic codebook to reduce the average feedback and improve the resource utilization. Although the dual codebook transmission scheme is defined in LTE-A, the estimation and feedback of the dual codebook are the key problems in the dual codebook technology.
This dissertation focuses on the research of double codebook beamforming technology, and puts forward a series of solutions through the related theoretical research. In general, the main research contents and contributions of this dissertation are as follows:
Firstly, the codebook design characteristics and the codebook estimation algorithm of the dual codebook beamforming technique are studied. In the Orthogonal Frequency Division Multiplexing (OFDM) system of broadband signals, frequency offset exists between different subcarriers, so the channel information on different carriers is used for independent codebook estimation. In this paper, a correlation estimation algorithm is proposed to obtain the equivalent channel information by focusing the channel information of different carriers on a reference frequency point according to the correlation between multiple carriers in a wideband signal. Carved signal-to-noise ratio (SNR) as a weighted average factor can overcome the deterioration of estimation performance caused by sampling information with large noise interference. Theoretical analysis and numerical simulation show that when the signal bandwidth is small, the optimization algorithm improves the system performance less, mainly because of the weighted average; when the signal bandwidth is large. The performance of the system is improved obviously.
Then, the influence of IQ (in-phase/quadrature-phase) imbalance on beamforming system is analyzed in a dual codebook beamforming system. The error of codebook estimation caused by IQ imbalance and the characteristics of double codebook estimation are deduced. In order to eliminate the influence of IQ imbalance on codebook estimation, a complete transceiver and receiver is given in this paper. The IQ unbalance estimation and compensation algorithm can compensate the IQ unbalance parameters of the transceiver step by step, correct the code book estimation error caused by this factor, and improve the system performance.
Then, in order to reduce the upstream feedback resources, a long feedback period is usually used for codebook 1, which represents the characteristics of long-term channel. However, the speed of channel change is time-varying. If a fixed feedback period is adopted, the accuracy of codebook 1 will be affected. Therefore, the modified unequal-interval grey prediction algorithm is used to indicate the direction of the channel. In addition, because of the high real-time requirement of codebook 2 estimation, outdated channel estimation will seriously degrade the performance of codebook 2 estimation. Therefore, this paper uses Kalman prediction algorithm to predict the channel estimation matrix, and introduces grey prediction algorithm to predict the state transition used in the prediction. The matrix is modified to improve the estimation performance of codebook 2.
Finally, with the increase of the number of antennas in Massive MIMO system, the complexity of channel information estimation and feedback information increases exponentially. At the same time, the antenna array in Massive MIMO system is usually two-dimensional array distribution, and the channel direction estimation also needs to be extended to two-dimensional direction estimation. In this paper, a two-codebook beamforming scheme based on two-dimensional channel direction estimation is proposed, including two-codebook design and two-codebook estimation scheme. Finally, the validity of the scheme is verified by simulation, and a variety of codebook estimation algorithms are compared and simulated. Comparisons and analyses are made from the aspects of complexity and performance. The codebook estimation algorithm has low complexity and the performance is close to the improved two dimensional codebook estimation algorithm.
【学位授予单位】：重庆大学
【学位级别】：博士
【学位授予年份】：2014
【分类号】：TN919.3

【参考文献】