In MIMO optical wireless, locating all photodiodes (PDs) in one side of the smartphone or user equipment may lead to high correlation between the elements of channel gain matrix. This will cause ill-condition channel matrix and high BER. Hence, the aim is to break this symmetry. Multi-directional Receiver (MDR) is one practical solution that has been investigated in the following paper and compared with the traditional method of locating all PDs on one side of the user's device.
https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=8790655
Please let me know your opinion on this paper.
I would suggest to utilize the angle diversity receiver (ADR) with narrow field of view angles. This will have many benefits either in enhancing the channel correlation r reducing both ISI and IUI.