# Indoor MIMO-VLC Using Angle Diversity Transmitters
###### Abstract  
In this paper, we, for the first time, apply angle diversity transmitters (ADTs) to enhance the performance of multiple-input multiple-output visible light communication (MIMO-VLC) systems. The ADT is designed to consist of one center light emitting diode (LED) and multiple inclined side LEDs. We calculate the line-of-sight (LOS) channel gain of the MIMO-VLC system using ADTs and further derive the average achievable rate of the system. We show that the average achievable rate is related to both the inclination angle of the side LEDs and the spacing between two adjacent ADTs in the MIMO-VLC system. Simulations are conducted to verify that the average achievable rate of the ADT-enhanced MIMO-VLC system can be maximized by setting the optimal inclination angle of the side LEDs and the optimal spacing between adjacent ADTs. The obtained results further show that the average achievable rate of the ADT-enhanced MIMO-VLC system can be greatly improved when there are more LEDs in each ADT. Specifically, a substantial 42.9% average achievable rate improvement can be achieved by using the optimized ADT in comparison to using a conventional non-angle diversity transmitter.  
visible light communication (VLC); multiple-input multiple-output (MIMO); angle diversity transmitter (ADT) 2022 22 22 22 22 22 2  
In order to enhance the performance of MIMO-VLC systems, many emerging techniques have been considered for MIMO-VLC systems, such as hybrid space-frequency domain pre-equalization [12], two-dimensional power allocation [13], user-centric MIMO techniques [14], generalized MIMO techniques [15], advanced multiple access techniques [16; 17] and angle diversity receiver (ADR)-based advanced MIMO reception techniques [18; 19; 20; 21].  
The use of ADR with a proper diversity combining scheme in MIMO-VLC systems can reduce the channel correlation and, hence, enhance the system performance. Similarly, angle diversity can also be introduced at the transmitter side by an angle diversity transmitter (ADT) for VLC systems. Thus far, ADT has been applied in various VLC systems in the literature. In [22], ADT was applied in visible light positioning (VLP) systems to enhance the robustness against accelerometer measurement errors.  
In [23], ADT was used to realize space division multiple access (SDMA) in optical attocell network, which was shown to achieve improved area spectrum efficiency compared with traditional time division multiple access (TDMA). In [24], ADT was adopted to improve the bit error rate (BER) performance of indoor single-cell systems. However, ADT has not yet been considered in MIMO-VLC systems, and whether the use of ADT can enhance the performance of MIMO-VLC systems has not been verified.  
In this paper, for the first time, to the best of our knowledge, we introduce ADT into MIMO-VLC systems, and the designed ADT includes one center LED and multiple inclined side LEDs. The line-of-sight (LOS) channel gain of the MIMO-VLC system using ADTs is calculated, and the average achievable rate of the ADT-enhanced MIMO-VLC system is further derived. We found that the average achievable rate of the ADT-enhanced MIMO-VLC system can be maximized by setting the optimal inclination angle of the side LEDs and the optimal spacing between adjacent ADTs. Simulations are performed to verify the superiority of applying ADTs in MIMO-VLC systems. The impact of the transmitted SNR and the PD spacing is also analyzed.