Quad-LED OTFS Modulation in Indoor Visible Light Communication Systems

In this paper, we investigate orthogonal time frequency space (OTFS) modulation in multi-LED indoor visible light wireless communications. Specifically, we propose two quadLED OTFS schemes, namely, 1) quad-LED complex modulation OTFS (QCM-OTFS), and 2) spatial modulation dual-LED complex modulation OTFS (SM-DCM-OTFS). The proposed QCM-OTFS scheme sends the magnitudes of real and imaginary parts of complex signals through intensity modulation (IM) 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. The proposed schemes do not require Hermitian symmetry and DC bias operations to obtain real positive valued signals suited for IM of LEDs. We obtain upper bounds on the bit error rate (BER) performance of the proposed schemes, which are tight at high signal-to-noise ratios (SNR). Compared to the QCM-OFDM and SM-DCM-OFDM schemes known in the literature, the proposed schemes achieve significantly better BER performance. We also analyze the spatial distribution of the SNR gain in the proposed QCM-OTFS/SM-DCM-OTFS schemes compared to QCM-OFDM/SM-DCM-OFDM schemes using the ratio of minimum distance of normalized received signal sets as a metric.