Orthogonal Time Sequency Multiplexing Modulation: Analysis and Low-Complexity Receiver Design

This paper proposes orthogonal time sequency multiplexing (OTSM), a novel single carrier modulation scheme that places information symbols in the delay-sequency domain followed by a cascade of time-division multiplexing (TDM) and Walsh-Hadamard sequence multiplexing. Thanks to the Walsh Hadamard transform (WHT), the modulation and demodulation do not require complex domain multiplications. For the proposed OTSM, we first derive the input-output relation in the delay-sequency domain and present a low complexity detection method taking advantage of zero-padding. We demonstrate via simulations that OTSM offers high performance gains over orthogonal frequency division multiplexing (OFDM) and similar performance to orthogonal time frequency space (OTFS), but at lower complexity owing to WHT. Then we propose a low complexity time domain channel estimation method. Finally, we show how to include an outer error control code and a turbo decoder to improve error performance of the coded system.

I. Introduction

Orthogonal frequency division multiplexing (OFDM) is the physical-layer modulation scheme deployed in 4G and 5G mobile systems, where the wireless channel typically exhibits time-varying multipath fading due to mobility. OFDM is known to achieve a near-capacity performance over such channels when the Doppler effect is limited [1]–[3], but suffers from severe performance degradation in high-mobility environments [4]. Hence, new modulation techniques that are robust in both slow and fast time-varying channels are needed.