Age-optimal Transmission Policy for Intelligent HARQ-CC aided NOMA Systems

Abstract

This paper investigates the timeliness performance of a downlink wireless communication system with a base station (BS) serving two users under the non-orthogonal multiple access (NOMA) system. The hybrid automatic repeat request with chase combining (HARQ-CC) in finite blocklength is considered. For minimizing the information freshness which is characterized by Age of Information (AoI), an intelligent system is adopted. The BS can adjust the power allocation to each user in NOMA, and decide to transmit old or new packets to each user, according to the users’ current AoI status and the positive/negative acknowledgement (ACK/NACK) feedback signal. First, the closed-form of the individual user’s outage probability with arbitrary power allocation combination of HARQ-CC aided NOMA system is derived. Based on the outage probability, the optimization of the system average AoI is achieved by minimizing the Lyapunov Drift function of each time slot. By introducing the HARQ-CC mechanism into the NOMA scheme, a trade-off between reliability and timeliness can be achieved, which is found that this policy can improve the AoI performance of NOMA scheme in low signal-noise ratio (SNR) and outperform existing works on NOMA and orthogonal multiple access (OMA).