Toward Optimizing Delivery Probability in NR Sidelink by MCS Selection

To further enhance the reliability and throughput of the Vehicle-to-Everything (V2X) communications via New Radio (NR) sidelink, the flexible Modulation and Coding Scheme (MCS) was approved by 3GPP in Release 16. However, most available works neglect the variations in message data size and adopt fixed MCS, which unavoidably limits the NR sidelink spectral efficiency. Using the theory of stochastic geometry, we are able to present a mathematical framework and analytically investigate the impact of MCS selection on the achievable message delivery probability in NR sidelink, following the general message data size in 3GPP. Then, the optimal MCS is selected by enlarging the surplus between the Signal to Interference-plus-Noise Ratio (SINR) measured at the receiver and the SINR Threshold Required for Successful Delivery (TRSD). The numerical results prove that the optimal MCS is extremely beneficial for the V2X message transmission.

I. Introduction

In order to support higher reliability and throughput requirements, 3GPP Release 16 introduced the flexible Modulation and Coding Scheme (MCS) selection into the New Radio (NR) sidelink [1]. Based on the combination of modulation schemes and code rates, 3GPP provides 29 optional MCSs, which are strictly specified by the MCS indexes of the MCS table in [2]. For the NR sidelink, the first-stage sidelink control information between the transmitter and receiver can indicate the MCS index of the transmitting subchannels, and the MCS index determines the effective bits number carried by each resource element. The MCS selection plays an important role in NR sidelink transmission [3]. A lower MCS index may result in excessive subchannel occupancy, severe interference, and thus degraded delivery performance [4]. On the contrary, a higher MCS index is inappropriate and causes a higher BLock Error Rate (BLER) due to compromised robustness [5]. This obviously leads to a trade-off between the high and low MCS indexes in terms of the delivery probability in NR sidelink.