Two-Step Random Access in 5G New Radio: Channel Structure Design and Performance

Abstract

A common design of the random access procedure on the physical random access channel (PRACH) is required for the diverse usage scenarios in the fifth generation new radio (5G NR) mobile networks. Based on the latest 3GPP specifications and evaluation assumptions agreed for Release 16, the 2 step-RACH (2SR) enhancement, composed of the denoted MsgA and MsgB, not only reduces the latency but also the control-signalling overhead due to the reduced number of messages transmitted. The channel structure of MsgA comprises RACH preamble and data in the physical uplink shared channel (PUSCH) while MsgB combines the random access response and the contention resolution. This procedure should operate in local area (LA), medium range (MR) and wide area (WA) cells despite the lack of time alignment (TA) in the PUSCH part of MsgA. The demodulation performance degradation observed without time offset compensation at the base station (gNB), specially for MR or WA cells, highlight that practical gNB implementations relying in MAC control element-based TA command for PUSCH time alignment are not conceivable for 2SR. Furthermore, in the case that all preambles from multiple users (UEs) trying to perform the initial access are mapped to the same PUSCH physical resources, the associated data parts overlap and may result in unsuccessful decoding. There is therefore a trade-off between the collision probability of the PUSCH part of MsgA and the resource overhead for 2SR. This paper addresses the channel structure design of this procedure for the preamble and data parts of MsgA together with the receiver processing framework. The performance results suggest that using lower payload sizes provide higher resource utilization and allow more UEs to be multiplexed within the same PUSCH occasion. In addition, using different DMRS ports for UEs sharing same physical resources decrease the probability of failure in the decoding of the data part of MsgA while reduces the resource overhead for 2SR.