等衰落轨道角动量索引调制系统的信道容量性能分析

射频轨道角动量(orbital angular momentum,OAM)提供了一种可以在同一载波频率物理信道上复用一组正交模式以提高信道容量的方法。现有研究大多专注于信道容量提升，对系统复杂度的研究相对较少。在基于均匀圆形阵列(uniform circular array,UCA)的OAM通信系统中，发射机需要完成大量调制运算以生成OAM信号。于是，提出了一种等衰落轨道角动量索引调制(equal fading orbital angular momentum index modulation,EOAM-IM)方案，其中，IM有效降低了OAM调制运算复杂度，且等衰落发射机也保证了不同模式OAM子信道传输增益。此外，还详细推导了所提EOAM-IM方案的系统信道容量。仿真结果表明，所提EOAM-IM方案能以更低的调制复杂度获得比传统OAM方案更高的信道容量。

Channel capacity analysis of the equal fading orbital angular momentum index modulation system

Radio frequency orbital angular momentum (OAM) provides a novel method of reusing a set of orthogonal modes on the same carrier frequency channel to improve the channel capacity. However, the existing technical research on OAM communications is mainly focused on the improvement of channel capacity rather than the reduction of system complexity. For a uniform circular array (UCA) based OAM communication system, the transmitter needs to perform a lot of modulation operations to generate the multiple orthogonal OAM signals. In this paper, the equal fading OAM index modulation (EOAM-IM) scheme is proposed, where the index modulation (IM) technique reduces the OAM modulation complexity and the equal fading transmitter ensures the sub-channel transmission gain of each OAM mode. Furthermore, the channel capacity of the proposed EOAM-IM system is derived in detail. Simulation results show that the proposed EOAM-IM scheme can achieve higher channel capacity than the traditional OAM scheme with lower modulation complexity.