Feasibility of Index-Coded Retransmissions for Enhancing Sidelink Channel Efficiency of V2X Communications

To support the vehicular communication services that require low end-to-end transfer latency, the long-term evolution (LTE) specification recently defined a new channel sidelink, which enables direct data transfer among vehicle user equipments (UEs) without the aid of network infrastructure. Since a sidelink transmitter is not able to have the receiver information due to the lack of feedback channels, the vehicle UE unconditionally triggers sidelink retransmissions regardless of the receiver's status. Although this retransmission scheme is essential in a vehicular environment, the scheme has an inherent limitation in a performance aspect due to its redundant retransmissions, which eventually reduces the amount of the radio resource available for new sidelink transmissions. To resolve this issue, we propose to exploit the concept of index coding, which is effective for transferring multiple data blocks in an efficient way, to the sidelink retransmissions. We add an index-coding process to the sidelink retransmission procedure to make it consume the radio resource more efficiently. We also investigate how the sidelink protocol can be modfied so that the proposed scheme can be realized in practical LTE systems. The intensive simulations and numerical analysis validate the feasibility of the proposed scheme for the enhancement of the sidelink performance in the aspect of channel efficiency.