Grant-Free Random Access With Channel Differentiation in Massive MIMO Systems.

This paper investigates the uplink access for massive machine-type communication (mMTC) in massive multiple input multiple output (mMIMO) systems with coexisting line-of-sight (LOS) and non-LOS (NLOS) channels. A grant-free random access scheme with channel differentiation (CD-GFRA) is proposed to handle the massive access for this scenario, in which machine-type devices (MTDs) attempt to access the network with two different strategies according to their channel conditions. In CD-GFRA, MTDs with angle-dependent LOS channels access the network directly, while NLOS-channel MTDs transmit with the assistance of random relays. Moreover, an angle of arrival (AOA) based channel estimation is further proposed to improve the access capacity of LOS-channel MTDs. Numerical results show that the proposed CD-GFRA significantly improves the access success probability of MTDs in poor coverage areas and achieves almost triple the throughput of the conventional scheme for a high MTD density.