Space-Time Coded OTFS Modulation in High-Doppler Channels

Orthogonal time frequency space (OTFS) modulation is a 2-dimensional modulation technique designed in delay-Doppler domain, specially suited for doubly- dispersive fading channels. OTFS modulation uses additional transform operations at the transmitter and receiver on top of conventional multicarrier modulation operations, and is shown to achieve superior performance compared to OFDM in high- Doppler channels. It has been shown in the recent literature that the asymptotic diversity order of OTFS modulation is one. Also, OTFS with phase rotation using transcendental numbers has been shown to achieve full diversity in the delay Doppler domain. In this paper, for the first time in the literature, we propose and investigate the use of space-time coding (STC) in OTFS modulation in a MIMO setting. We use the structure of Alamouti code, generalized to matrices, to achieve full transmit diversity in OTFS. We also show that the use of STC-OTFS along with phase rotation can achieve full diversity in both spatial and delay Doppler domains. Also, STC-OTFS black with phase rotation is found to achieve good diversity performance even with small frame sizes making it suited for low-latency applications.