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基于BICM调制的MIMO-OFDM系统的高效编译码技术研究

Resaearch on the High Efficient Coding Technology of MIMO-OFDM Based on Bit-Interleaved Coded Modulation

【作者】 张驰

【导师】 郭黎利;

【作者基本信息】 哈尔滨工程大学 , 通信与信息系统, 2010, 博士

【摘要】 多输入多输出(Multiple Input Multiple Output, MIMO)系统因其自身物理特点,在传播中可以有效地利用多径分量,因此在一定程度上可以抗多径衰落,但当MIMO技术由窄带向宽带系统移植时,信道中出现的频率选择性衰落对其性能影响较大。正交频分复用(Orthogonal Frequency Division Multiplexing, OFDM)技术虽然自身具有极强的抗频率选择性衰落能力,但提高频谱利用率的能力却有限。因此二者的结合既可以提供更高的数据传输速率,提高频谱利用率,也可以有效地对抗多径及频率选择性衰落。为了进一步提高整个系统性能需要采取增加传输带宽、增加发射功率、提高调制阶数或增加调制星座点数等相关措施。这无疑将导致整个系统的实现复杂度和功率损耗大幅增加。因此,通过物理层链路的有效变化来降低整个系统的复杂度及功耗显得尤为关键,而结合高效的编码调制技术就可以无需增加额外的系统带宽获得较高的编码增益,降低系统误码率,非常适合带宽受限信道。因此将比特交织编码调制(Bit-Interleaved Coded Modulation, BICM)与OFDM和MIMO技术相结合,构成BICM-MIMO-OFDM系统,能利用时间、空间、频率多重分集,很好地实现了信道编码和空间复用的有机结合,有效提高信息传递速率,充分发挥MIMO信道的潜力,从而满足在频率选择性衰落信道下的可靠、高效的无线通信。本论文以联合BICM-OFDM编码调制的MIMO系统发射分集方案的选择和接收端译码算法的简化及优化等方面为切入点,对复杂度和性能之间具有合理折中的编码调制方案和信号检测方法等技术做深入的研究。论文重点介绍了BICM技术及OFDM技术,分析了BICM及OFDM的理论基础、主要特点及将两者相结合在衰落信道中的的优势。探讨了频率选择性衰落信道下BICM技术联合OFDM技术在MIMO通信系统中的应用及检测算法的研究,论文主要包括以下内容:1.系统的编码调制方案的选择。BICM联合OFDM是一种可以有效抗频率选择性衰落信道影响的技术。论文将BICM技术与OFDM技术联合应用到MIMO系统中,实现了基于BICM编码调制的MIMO-OFDM系统,并对其性能进行分析和仿真,分析其性能界,为该系统编码调制技术方案的选择提供理论依据。2.系统性能分析及预编码方案设计。讨论BICM技术中映射方式、卷积码状态数、帧长等参数的改变对整个系统性能的影响。针对本系统特点,提出了一种基于分组形式的全速率预编码方案,并采用旋转星座及网格生成矩阵两种矩阵构造方式,通过仿真可以看出,其可以最大化编码增益,从而改善系统的传输性能,在不改变传输速率的情况下进一步降低系统误码率。3.系统的检测算法的简化。针对在发射天线数较多的高阶星座调制下最优检测算法的选择以及目前的次最优的检测算法复杂度较高的问题,结合球形算法的优点,提出一种改进的球形检测算法应用到本系统中。改进算法的球形半径可以随信道参数和调制阶数的大小自适应改变,大大简化系统实现的复杂度。相对于其他算法和方案,其在不同的信道条件、天线配置以及调制方式情况下有很好的适应性和稳健性。4.系统的迭代译码方式的选择及检测算法的研究。当前一些迭代方式及译码算法,尤其进一步结合MIMO技术提高数据率时,由于其译码复杂度太高,因此在一定程度上影响了它的实际应用。论文提出了一种在BICM-ID迭代译码结构中联合球形解码算法的系统模型。将迭代译码过程中的MAP解调部分用改进的球形算法来代替,以逼近其原有MAP译码性能,随着信噪比以及迭代次数的增加,该算法能够有效降低系统误码率。相对于最大似然检测算法,在损失很小性能同时,大大降低系统复杂度。

【Abstract】 Multiple Input Multiple Output(MIMO) system can use multipath components to combat multi-path fading for its physical features, but when MIMO tranplants from narrowband system to wideband system, it is inevitable that channel will frequency-selectively fade. On the other hand, Orthogonal Frequency Division Multiplexing(OFDM) has the ability of anti-multipath interference and anti-frequency selective fading. But the raise of the spectrum efficiency limits. Therefore, the combination can not only increase the data transmission rate and spectrum efficiency, but also combat multi-path and frequency selective fading. In order to improve the performance of the whole system, it is necessary to take some measures such as increasing the transmission bandwidth or transmit power, enhance the order of the modulation or the point of the constellation, which will undoubtedly lead to the complexity of system realization and the increase of power loss. Accordingly, It is a key to reduce the complexity of system realization and power loss by the change of physical link. And the combination with high-effective coded modulation can obtain higher coding gain and lower bit error rate without increasing extra system bandwidth, which is suitable for limited bandwidth channel. If Bit-Interleaved Coded Modulation(BICM) is combined with OFDM and MIMO, namely BICM-MIMO-OFDM System,it can make effective use of multi-diversity of space, time and frequency, and realize the organic combination of channel code and spatial multiplexing, effectively raise the data rate and develop the potentiality of MIMO channel, so that make sure of reliable, high efficient wireless communication over frequency selective fading channels. In this thesis the technology of modulation and coding scheme and signal detection methods which can compromise the complexity and performance was studied based on the choice of the transmit diversity and the optimization of decoding algorithm over the receiver in the MIMO system combined with BICM-OFDM coded modulation. This thesis emphatically introduced BICM and OFDM technology, analysis the theoretical basis, main characteristics of the BICM and OFDM and the advantage of the combination of the two over the fading channels. The application of BICM together with OFDM on the MIMO system over frequency selective fading channels and the study of detection algorithm were discussed. The primary contributions included in this dissertation can be summarized as follow:1.The choice of modulation and coding scheme. BICM together with OFDM was a technology which can combat the effects of the frequency selective fading. In this thesis, the application of BICM and OFDM on the MIMO system constituted MIMO-OFDM system based on BICM coded modulation. The performance was analyzed and simulated, and the performance bound provided the basis for the choice of modulation and coding scheme.2.Performance analysis and precoding design. The influence of mapping mode, convolution code, frame length and other parameters over the system performance was discussed. A kind of sectional full-rate precoding method was introduced, and rotated constellation and mesh generation matrix methods are both used for the characteristics of this system. Simulation showed that it could maximize the coding gain to improve the transmission performance, greatly increased the system capacity, and lower system bit error rate.3.The optimization of detection algorithm. To solve the problem that transmit antenna in higher order modulation constellation optimal detection algorithm and the current suboptimal detection algorithm with higher complexity, according to the advantages of spherical algorithm, a simplified improved spherical detection algorithm was applied to this system. The spherical radius of the improved algorithm can adaptively change with the channel parameters and the size of the modulation order, which greatly simplified the system complexity. Compared with other algorithms and programs, it had good adaptability and robustness in different channel conditions, antenna configuration and modulation.4.The simplification of iterative decoding and detection. The high requirements of the spectral efficiency made it necessary to consider the higher order modulation constellation, especially when further integrates MIMO to increase data rate, some of the current iteration decoding algorithms affected its practical application due to their complexity. The thesis introduced a system model of BICM-ID iterative decoding structure united with spherical decoding algorithm.Replaced the MAP demodulation in the iterative decoding process with the improved sphere algorithm to approximate the original MAP decoding performance. And as the signal-to-noise ratio and iteration number increase, the algorithm could effectively reduce the error rate. Relative to the maximum likelihood algorithm, system complexity was significantly reduced while losing a small performance.

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