Delay Analysis and Optimization of Beam Scanning-Based User Discovery in Millimeter Wave Systems

In order to satisfy higher requirements for data transmission rate and system capacity, millimeter wave (mmWave) with abundant spectrum resources has be exploited and become a research hotspot of the fifth generation (5G) mobile communication systems. However, introducing mmWave technologies into cellular networks also faces many severe challenges, especially the initial user equipment (UE) discovery. In the UE discovery, the narrow-beam directional scanning is often used to search the UE, but it increases the control overhead of the system and brings about a non-negligible search delay. In this paper, we will analyze the average search delay of mmWave systems under two different UE access methods, i.e., the random scanning access method in independent networks and the search method based on UE location in heterogeneous networks. Taking the average search times discovering UEs as a measure and the number of search sectors in beam-space as an optimization parameter, the optimal number of sectors with the maximum capacity is obtained so that the search delay and the capacity can be balanced. The simulation results verify the correctness of the analysis and show that the performance of the user discovery method in heterogeneous networks is better than that of the random scan access method in mmWave independent networks.