Performance optimisation and efficient user grouping of MU-MIMO for indoor optical wireless communication systems

Abstract

Multiple-input multiple-output (MIMO) in combination with orthogonal frequency division multiplexing (OFDM) is an attractive technique to provide the spectral efficiency and high data transmission rates in optical wireless communications (OWC) systems using light emitting diode (LED) light sources, where the modulation bandwidth of LED is limited. To investigate the performance of the system, indoor MIMO-OFDM OWC systems supporting a single user terminal is considered. However, the performance is limited because of strong correlation among propagation paths dominated by line-of-sight (LOS) components. To mitigate such effects, the receiver front-end design of MIMO-OFDM is investigated to reduce MIMO channel correlation in OWC. In addition, to utilize the OWC broadcast nature, multiuser MIMO-OWC systems have been investigated. When there are multiple user terminals in the same room, there is a challenge to eliminate the multi-user interference (MUI), which affects the transmission performance at each user’s receiver. In order to suppress the MUI, block diagonalization (BD) precoding is applied. However, BD prevents only interference among individual users. To further prevent interference among individual data streams of each user, singular value decomposition (SVD) is applied. Nevertheless, BD precoding is not applicable when the number of photo diodes (PDs) exceeds the number of LED transmitters. Therefore, to further improve the system performance measured in terms of the bit error rate (BER), this research considers LED transmitter distribution in addition to angular diversity based on tilting receiver PDs. In the first half of the thesis, the research studies the performance of indoor SVD-based MIMO-OFDM OWC systems. In addition, since SVD decomposes the MIMO channel into parallel sub-channels with different path losses, the bit loading technique is utilized. In the second half of the thesis, the research considers a lot of user devices in the same room, yielding more PDs than LEDs. To overcome the limitation of BD precoding, user grouping is performed, and time division multiplexing (TDM) is applied with BD precoding for each user group. In particular. this research focus on optimising the BER performance of SVD-based MIMO-OFDM OWC systems under the impact of the OFDM symbol design, the configuration of MIMO transmitters, the required data rates, the available bandwidth, and the angular diversity based on tilting receiver PDs. The results show that the 9×4-MIMO system is the optimal configuration in term of the large coverage of low BER regions and low computation complexity for the 5m×5m room. In addition, polar angle tilting of PDs can effectively improve the BER performance. In the case of light dimming control with a low total transmit optical power, the system can benefit from a high multiplexing gain with a large tilting polar angle, as indicated by improved condition numbers of MIMO channel matrices. For multiuser systems, user grouping methods are proposed based on pairwise interference considerations among users in the same group. The proposed method can be implemented through integer linear programming (ILP), which requires less computation than exhaustive search. Numerical results on the average minimum user throughputs over random scenarios indicate that the proposed hybrid method can significantly outperform random user grouping, and performs reasonably well compared to exhaustive search. Finally, it is demonstrated that, when BD precoding greatly attenuates desired user signals, user grouping can help improve the minimum user throughputs even through BD precoding can support all users as a single group.