Polar Coded HARQ NOMA in Block Fading Channel with Short Code Length

Polar codes are known for their good performance relative to other channel codes in the short-code regime. Thus they are suitable for internet of things (IoT) applications that have short transmission bursts and require low latency. To improve the communication reliability and throughput without significantly increasing the transmission power, hybrid automatic repeat request (HARQ) is often used. In this paper, we propose two new algorithms to decide on the number of re-transmission bits in an incremental redundancy (IR) HARQ system with short code length. The first algorithm is based on achieving a certain sequence of probabilities of successful decoding in retransmissions. The second maximizes the expected throughput of the HARQ system. We consider additive white Gaussian noise (AWGN) and Markov model for the block fading channel. The equivalent-length-based method is applied in our system to achieve higher throughput. For both designs, we derive an upper bound on the throughput. Simulations show that the proposed methods outperform the existing methods. We also apply our designs to a non-orthogonal multiple access (NOMA) system. In the NOMA system, we approximate parameters used in our algorithms to prevent storing too many parameters of the combination of the signal-to-noise ratio (SNR). It is found that the set of information bits should be chosen carefully to avoid puncturing bits with high reliability.