An Asynchronous Grant-Free Multiple Access Scheme with Rateless Codes for URLLC

Grant-free multiple access is an efficient way to achieve low latency access and is becoming a promising technique for massive machine type communications in future cellular system. However, the collision nature of grant-free multiple access is a major challenge for reliable and low-latency communications. Although rateless codes can enhance the reliability due to the rate adaptation property, existing grant-free access schemes with rateless codes are synchronous in that the users with arrived packets are not allowed to start access until the ongoing transmissions are finished, which incurs additional waiting delay. To reduce the latency without damaging the reliability, an asynchronous grant-free access scheme with rateless codes is proposed in this paper. It is asynchronous in that a user can start to access the shared channel instantly even when other users remain transmitting, without waiting for the finish of current transmission period. The stability and delay performance of this scheme are analyzed. Numerical results show significant reduction of the latency from the arrival of a packet to its successful decoding at the base station.