Orthogonal Time Frequency Space Modulation in Wideband Doppler Channel

Recently, the coherent optic wireless communication (OWC) has received extensive attention due to its superiority over traditional intensity modulated direct detection (IMDD) systems. Yet, the Doppler effect could be a showstopper for coherent OWC which is sensitive to the frequency offset and spread. Thus we introduce a new modulation scheme, which called orthogonal time frequency space (OTFS) modulation, into the coherent OWC to solve the Doppler problem. OTFS transforms traditional time-varying channel into delay-Doppler (DD) domain, which ensures all transmit symbols experience an almost identical and slowly varying sparse channel. Thus full channel diversity in time and frequency can be obtained when a suitable receiver is used. In addition, most of the research on Doppler effect only consider the random Doppler spread and average frequency shift to the signal, while ignoring the spectral spread caused by the frequency-dependent Doppler shift of a wideband signal such as radar signals and terahertz signals. To accurately model the Doppler effect, it is necessary to calculate the Doppler frequency shift according to the frequency bin of each sub-carrier, thereby the overall frequency offset and spectral spread are included. This manuscript proposes a Doppler channel model based on frequency-domain subband partition (FDSP), and for the first time, this wideband Doppler channel model is applied into wideband OTFS and orthogonal frequency division multiplexing (OFDM) systems. We carried out simulation for OTFS and OFDM under different velocities. Simulation results show that the OTFS can resist high Doppler frequency shift in high mobility scenarios, and in the case of large subcarrier spacing, OTFS is less affected by the Doppler frequency shift of each subcarrier than OFDM.

I. Introduction

Increasingly for optical wireless communications (OWC), coherent modulation of a laser beam is studied as it can substantially enhance the throughput [1], [2]. Yet, the transition from intensity modulation to optical carrier-based modulation radically changes the vulnerability to Doppler shifts, and in a non-line sight scenario, to Doppler spreads [3], [4].