大规模MIMO系统下的全向STBC编码设计与性能分析

Design and Performance Analysis of Omnidirectional Space-Time Block Code in Massive MIMO Systems

摘要: 在蜂窝通信单元下的大规模多输入多输出(MIMO)系统中，公用信号应该被全向传输以确保全范围覆盖。空时分组编码(STBC)目前已被广泛地应用于MIMO系统中，本文研究下行同步传输情景下的全向STBC编码，该方案也为有效降低下行链路导频开销提供了新的思路。首先我们在采样后的离散角度域为2*2的Alamouti编码设计了一种全向编码，该方案可以在空间角度域获得恒定的传输功率，同时保证各天线上的瞬时功率相等。随后，为获得4分集阶数，我们针对天线数为4的星座旋转STBC编码，提出了一种全向编码设计，并给出了对应的STBC编码中信息符号的生成方法。同时，我们将该方法推广到了使用复信号OSTBC编码时的情形。然后我们针对连续角度域重新推导了全向传输的等价条件，运用相位互补序列分别构造了Alamouti码和星座旋转STBC码的全向编码。为评估上述编码的性能，我们从误码率，遍历信道容量，峰均比，中断概率四个方面推导了相关表达式。最后我们从上述四个方面对所提出的构造进行了性能仿真，结果证明其中一种编码能有效降低误码率，渐进地达到信道容量上界，并在OFDM系统中保持原信号峰均比。

Abstract: Common signals should be transmitted omnidirectionally to guarantee cell-wide coverage in massive MIMO systems under celluar communication units. Space-Time Block Code (STBC) has received widespread application in MIMO systems, in this paper, we focus on the omnidirectional STBC in massive MIMO downlink, which provides a new thinking for reducing the burden of downlink pilot overhead. We firstly design an omnidirectional precoding (OP) matrix for 2*2 Alamouti Code, which guarantees equal receiving power for user terminals in all spatial directions at any instant time as well as equal instantaneous power for each transmitting antenna. To obtain diversity order of four, OP matrices utilizing $4*4$ STBC and OSTBC Code are also proposed, respectively. What’s more, the condition of OP matrices is rewritten considering constant angle domain, and several examples of OP matrices are proposed. We analyse the system performance utilizing omnidirectional STBC in four aspects: bit error rate, peak-to-average ratio (PAPR), channel capacity and outage probability. Simulation results show that one proposed design can 1) reduce bit error bit; 2) maximize channel capacity; 3) preserve the PAPR of transmitted signals.