Low-complexity OTFS-based hybrid precoding in mmWave high mobility systems

The integration of aircraft into air-to-ground (A2G) communications with a variety of applications has attracted much attention, and it is critical to establish reliable communication links in A2G systems. However, suffering from the multipath effects and high Doppler frequency shifts, the quality of the transmission links is severely degraded in high mobility scenarios. To overcome the performance degradation caused by the double selective channel, a new modulation technique called orthogonal time-frequency-space (OTFS) is proposed recently. Moreover, millimeter wave (mmWave) massive multiple-input-multiple-output (MIMO) precoding is a simple and effective method to improve the system performance. Unfortunately, most of the existing MIMO-OTFS studies are implemented on the full-digital architecture, which will lead to high power consumption and high hardware cost. Besides, with the assumption of the practical waveforms in OTFS systems, it is complicated to work out the precoding algorithm. The objective of this paper is to develop a low-complexity OTFS-based hybrid precoding algorithm with rectangular waveforms for mmWave A2G communications. We first establish the delay-Doppler (DD) domain signal transmission process in hybrid precoding and derive the equivalent baseband DD domain channel matrix. Next, a two-stage hybrid precoding algorithm is proposed, in which the analog precoder based on steering vector is designed, and the recursive digital precoding algorithm is explored. Then a matrix approximation-based strategy is discussed to further reduce complexity. Furthermore, the computational complexity of the proposed algorithm is analyzed. Finally, the numerical results show the lower complexity and superior bit error rate performance of the proposed algorithm compared with the existing OTFS algorithms in high mobility scenarios.