Beam Selection for Two-Step Random Access in MTC With a Small Number of Antennas

In 5th generation (5G) systems, two-step random access has been introduced for machine-type communication (MTC) to lower signaling overhead. It is also shown that when a base station (BS) is equipped with a large number of antennas, the notion of massive multiple-input multiple-output (MIMO) can be exploited to improve the performance in terms of throughput and spectral efficiency. In this paper, we consider the case that a BS is equipped with a small number of antennas, in which a sufficiently high spatial selectivity cannot be obtained, and propose an approach to two-step random access based on beam selection that can perform well with a small number of antennas. In the proposed approach, spreading sequences are also used in conjunction with beam selection to mitigate interference due to limited spatial selectivity. To analyze the performance of the proposed approach, the distribution of the signal-to-interference-plus-noise ratio (SINR) is derived as a closed-form expression and the throughput is found. We compare the throughput of the proposed approach with those of conventional two-step random access approaches through analysis and simulations. While the theoretical results agree with simulation results, we can see that the proposed approach outperforms conventional ones when the number of antennas is small for a wide range of traffic intensity.