OTFS Modulation in MIMO Visible Light Communication Systems

Abstract

Visible light communication (VLC) technology is emerging as an attractive technology for wireless communication in indoor and vehicular environments. In VLS systems, light emitting diodes (LED) and photo diodes (PD) are used for wireless signal transmission and reception, respectively, using the visible light spectrum. VLC systems are gaining popularity because of their ability to provide lighting and short-range communication simultaneously. Recently, a new modulation scheme called orthogonal time frequency space (OTFS) modulation has been introduced in the radio frequency (RF) communications domain, where it has been shown that OTFS achieves significantly better performance compared to the widely used orthogonal frequency division multiplexing (OFDM). OTFS is a two-dimensional (2D) modulation scheme, where MN information symbols are multiplexed in the delay-Doppler (DD) domain using M Doppler bins and N delay bins. This is in contrast to OFDM where information symbols are multiplexed in the time-frequency (TF) domain. Because of the demonstrated superiority of OTFS in the RF communication domain, research in OTFS for VLC systems has gained importance recently. In this context, it is of interest to 1) adapt OTFS waveform to VLC settings, and 2) investigate the performance OTFS in indoor VLC environments. In this thesis, we propose efficient multi-LED OTFS modulation schemes suited for visible light communication and analyze their bit error performance in indoor communication environments.

First, we propose two dual-LED transmission schemes, namely, non-DC-biased OTFS (NDC-OFTS) scheme and dual-LED complex modulation OTFS (DCM-OTFS) scheme. The NDC-OTFS scheme uses 2D Hermitian symmetry operation to convert complex signals to positive, real-valued signals suitable for transmission in the optical domain, and the DCM-OTFS scheme achieves this by exploiting the polar representation of complex signals. We obtain analytical upper bounds on the bit error performance of the proposed schemes, which are found to be tight at high signal-to-noise ratios (SNRs). Our analytical and simulation results show that the proposed OTFS schemes achieve better performance compared to their OFDM counterparts reported in the VLC literature. Further, using the ratio of the minimum distance of different normalized received signal sets as a metric, we quantify the spatial distribution of the SNR gain of the OTFS schemes compared to their OFDM counterparts.

Next, we propose two quad-LED architectures, namely, quad-LED complex modulation OTFS (QCM-OTFS) scheme and spatial modulation DCM-OTFS (SM-DCM-OTFS) scheme. The QCM-OTFS scheme sends the magnitudes of real and imaginary parts of complex signals through intensity modulation and their sign information through spatial indexing of LEDs. The proposed SM-DCM-OTFS scheme sends the magnitude and phase of complex signals (polar representation) through a pair of LEDs and frame indexing across two pairs of LEDs. We also propose two multi-LED OTFS schemes, namely, quadrature spatial modulation OTFS (QSM-OTFS) scheme and dual mode index modulation OTFS (DMIM-OTFS) scheme. The proposed schemes use the dual-LED complex modulation block as the basic building block and offer enhanced rates compared to conventional index modulation schemes. The proposed schemes have the advantage of not requiring the Hermitian symmetry and DC bias operations to obtain real, positive-valued signals suited for intensity modulation of LEDs. Our results show that the proposed OTFS based schemes perform better than their OFDM counterparts. Finally, to mitigate the effect of channel correlation and achieve improved system performance, we investigate the use of the pairwise coding technique and singular value decomposition precoding in multiple-input multiple-output (MIMO) VLC systems. Our simulation results show that the pairwise coding is effective in alleviating the effect of spatial correlation.