An Efficient NOMA V2X Communication Scheme in the Internet of Vehicles

This paper proposes a new graph-based practical encoding and joint belief propagation (BP) decoding techniques for two-vehicle communicating simultaneously with another vehicle or a roadside unit. The proposed scheme achieves any rate pair close to the capacity region without using time sharing. While at the encoders, the corresponding parity check matrices are randomly built from a half-rate matrix, the joint BP decoder employs the associated Tanner graphs of the parity check matrices to iteratively recover the erasures in the received combined messages. Specifically, the joint decoder performs two steps in each decoding iteration: 1) simultaneously and independently runs the BP decoding process at each constituent sub-graph to recover some of the common erasures, 2) update the other sub-graph with newly recovered erasures and vice versa. When the number of erasures in the received message is less than or equal to the number of parity check constraints, the decoder may successfully decode both messages, otherwise the decoder declares decoding failure. Furthermore, the probability of decoding failure and the outage capacity are computed. Additionally, it is shown how the erasure probability evolves with the number of decoding iterations and the maximum tolerable loss. Simulations show that any rate pair close to the capacity boundary is achievable without using time sharing.