Deng Lijun, Fan Yangyu
Appl Opt. 2020 May 20;59(15):4672-4684. doi: 10.1364/AO.386587.
In indoor multi-input-multi-output (MIMO) visible light communication (VLC) systems, spatial multiplexing (SMP) is employed to improve spectral efficiency. However, the performance of SMP in an indoor VLC system depends on a low channel correlation. In this paper, a receiver model with angular diversity detectors is considered. The objective is to reduce the channel correlation and hence the system performance in terms of bit error rate (BER) and channel capacity compared to vertically oriented detectors under no-line-of-sight (NLOS) channel conditions. For a vertical detector setup, the results show that the channel correlation cannot be further reduced by varying the transmitter separation, transmitter semi-angle, or field of view of the receiver in NLOS conditions due to the design of receiver separation being very small in small mobile devices. In comparison to vertical detector setups, by varying the detector axis of each photodetector (PD) detector axis in angular diversity detector setups, the channel matrix rank is improved under LOS conditions, and the channel correlation is effectively reduced under NLOS conditions without requiring any implementation complexity at the receiver. Therefore, it is found that the angular diversity detector setup can substantially improve the BER performance of SMP, since it makes each PD more spatially separated to improve channel conditions. Notably, we deduce the channel capacity expression to approximate the capacity of the indoor highly correlated MIMO channel and verify the theoretical analysis by numerical simulations. The results show that the angular diversity detector setup provides capacity improvement when compared with the vertical detector setup. Even though it diminishes the received power when the elevation angle exceeds the optimal elevation angle, it outweighs this degradation by providing reduced channel correlation.
在室内多输入多输出（MIMO）可见光通信（VLC）系统中，采用空间复用（SMP）来提高频谱效率。然而，室内VLC系统中SMP的性能取决于低信道相关性。本文考虑了一种具有角度分集探测器的接收机模型。目标是在非视距（NLOS）信道条件下，与垂直定向探测器相比，降低信道相关性，从而提高系统在误码率（BER）和信道容量方面的性能。对于垂直探测器设置，结果表明，由于小型移动设备中接收机间距设计非常小，在NLOS条件下，通过改变发射机间距、发射机半角或接收机视场，信道相关性无法进一步降低。与垂直探测器设置相比，在角度分集探测器设置中，通过改变每个光电探测器（PD）的探测器轴，在视距（LOS）条件下信道矩阵秩得到提高，在NLOS条件下信道相关性有效降低，且接收机无需任何实现复杂度。因此，发现角度分集探测器设置可以显著提高SMP的BER性能，因为它使每个PD在空间上更加分离，从而改善信道条件。值得注意的是，我们推导了信道容量表达式以近似室内高度相关MIMO信道的容量，并通过数值模拟验证了理论分析。结果表明，与垂直探测器设置相比，角度分集探测器设置可提高容量。即使仰角超过最佳仰角时接收功率会降低，但通过降低信道相关性，其优势超过了这种功率降低。
