Generalized Spatial Modulation for Multi-user in Visible Light Communication

Recently, research in visible light communication (VLC) and optical wireless communication (OWC) has grown. The benefits of these technologies like ubiquitous availability, high bandwidth, license free spectrum are well known and many additional applications are still being explored. Though, the VLC can’t substitute the most popular radio frequency (RF) based system, however, it is expected that VLC can be widely deployed and used in indoor environment for multiple applications including Giga bit data rate. At the same time, there exists a technological difference in operation. However, many of the techniques of RF are being tested and evaluated in VLC. For example, multiple light emitting diodes (LEDs) which form VLC emitter and multiple photo detectors (PD) of VLC receiver are considered as multiple input multiple output (MIMO) based system. It has been shown that MIMO techniques can provide gains in VLC systems under line of-sight (LOS) conditions.

Several such optical MIMO techniques including repetition coding (RC), spatial modulation (SM), spatial multiplexing (SMP) have been investigated and their performance are analyzed. They have also been discussed for the multi user VLC. It is shown that SM offers improved spectral efficiency, especially at low SNR, and robustness to channel correlation compared to RC and SMP. However, RC requires larger constellation size to achieve high data rate and SMP improves this performance by making full use of multiplexing gains.

Furthermore, generalized spatial modulation is another scheme to improve spectral efficiency and reduce the number of transmitters in RF system. In optical domain also, authors in investigated this generalized spatial modulation. Authors compared the BER performance of generalized spatial modulation with other MIMO schemes including SMP and SM. It is shown that generalized spatial modulation gives better performance. Further it is concluded that performance of generalized spatial modulation based VLC improves if half power semi angle of the LEDs is decreased.

In this work, we extended the study for the case of multiuser detection point of view. Two user terminals, each with two optical receivers are considered at the VLC receiver. The generalized spatial modulation scheme in comparison with conventional MIMO and SM MIMO schemes for multi user VLC is investigated. Furthermore, single LED power and signal to noise ratio (SNR) performance by utilizing different field of view (FOV) at the receiver for multi user VLC system is also discussed and presented. In this work, the room area of 5m×5m×3m for multi user MIMO indoor VLC systems is considered. First, we analyse the conventional MIMO and SM MIMO and their BER performance. Next, we analyse generalized spatial modulation MIMO and its BER performance. Precoding algorithm is used to eliminate multi user interference (MUI) in transmitter by pre-processing the data of each user terminal. Results show that the generalized spatial modulation offers better performance.

The remainder of this paper is organized as follows. The multi user MIMO VLC system is presented in section II, while different transmission techniques are discussed in section III. The simulation results and discussion is outlined in section IV. Section V presents the conclusion.