Performance analysis of cooperative multi-relay spatial modulation based OTFS system in V2V communication

Owing to the characteristics of spatial modulation (SM) and orthogonal time frequency space (OTFS) modulation, SM based OTFS (SM-OTFS) system has an excellent capability to improve the effectiveness and reliability of vehicle-to-vehicle (V2V) communication. To further combat the severe shadow effect, improve the system performance, and extend the transmission range of V2V communication in the urban scenario with dense buildings, we propose a cooperative multi-relay SM-OTFS system with the optimal relay selection in this paper, where both amplify-and-forward (AF) and decode-and-forward (DF) protocols have been employed and only the optimal relay participates in the signal forwarding. Moreover, the fast time-selective fading is mainly considered, the closed-form outage probability and the system capacity of the cooperative SM-OTFS (CSM-OTFS) system are derived from the viewpoint of signal transmissions in the time domain. Numerical results verify the superiority of the proposed CSM-OTFS system over the non-cooperative SM-OTFS system and illustrate the influence of the main system parameters on the system outage probability and capacity, such as the relaying protocol, the number of relays, the estimation rate of channel state information (CSI), and the time-varying channel characteristic.