Filter-Enhanced Multi-User Scheme for Spatial Modulation Based Optical Wireless Communication Systems

The spatial modulation (SM) technology has been widely studied to increase the spectral efficiency and data rate in optical wireless communication systems by exploiting the spatial domain. To cope with the multi-user access requirement, the use of SM in conjunction with the orthogonal multiple access (OMA), transmitting antenna grouping (TAG) and non-orthogonal multi-access (NOMA) approaches has been proposed and investigated. However, these approaches have fundamental limitations, namely, OMA reduces the effective spectral efficiency and data rate per user, TAG introduces significant multi-user interference (MUI), and NOMA requires relatively complicated successive interference cancellation (SIC) to limit the MUI. To overcome these limitations, we propose and experimentally demonstrate a filter-enhanced TAG multi-user access scheme in SM-based indoor optical wireless communication systems. By using spatial waveform shaping filters in the transmitters, multiple users can be served simultaneously, thus maintaining high data rate per user, while eliminating MUI, which typically degrades the bit-error-rate (BER) performance. In the conducted experiments, over gigabit-per-second (Gbps) optical wireless communication with SM is provided to two users simultaneously, and results show that the impact of MUI on the BER performance is negligible. Results also show that the proposed multi-user access approach is capable of supporting users with different modulation formats simultaneously. The adaptive user data rate allocation via the spatial domain according to requirements is also demonstrated. The performance of proposed filter-enhanced TAG scheme is compared with the conventional TAG method. Results show that the BER performance can be improved by more than two orders of magnitude. The proposed filter-enhanced scheme is a promising solution that provides high-speed SM optical wireless communications to multiple users.