Adaptive WHTS-assisted SDMA-OFDM scheme for fair resource allocation in multi-user visible light communications

This paper proposes applying Walsh-Hadamard transform spreading (WHTS) in conjunction with space-division multiple access (SDMA) in an orthogonal frequency-division multiplexing (OFDM) system to provide enhanced multi-user downlink capability in visible light communication (VLC) scenarios. VLC environments are characterized by strong interference from the emissions of adjacent lamps. Thus, SDMA-OFDM schemes can be applied to enable multi-user capability while ensuring a highly efficient use of the limited available bandwidth. However, these SDMA-OFDM systems can only provide multiple access for a small number of users whenever there is enough channel diversity to separate the users' information. This is the case, for example, when each single user is associated with a specific emitting lamp and angle-diversity receiving techniques are applied. However, to accommodate a greater number of users requires a user's distribution in the frequency domain. In this sense, the proposed Walsh-Hadamard transform spreading scheme provides a fair assignment of the available subcarriers among the different users leading to an identical throughput and bit error rate (BER) for users allocated to the same emitting lamp. This BER is also controllable by means of an adaptive throughput mechanism. The numerical results show that the proposed adaptive WHTS-assisted SDMA-OFDM system in a full-user scenario presents a similar performance to that of an adaptive SDMA-OFDM system irrespective of the number of users per lamp, while providing enhanced multi-user capability. Finally, an adaptive algorithm for resource allocation is evaluated, demonstrating a high degree of satisfaction (regarding demanded data rates) when large numbers of users are simultaneously served.