Random-phase beamforming for initial access in millimeter-wave cellular networks

The utilization of the millimeter-wave frequency band (mm-wave) in the fifth generation ({5G}) of mobile communication is a highly-debated current topic. Mm-wave MIMO systems will use arrays with large number of antennas at the transmitter and the receiver, implemented on a relatively small area. With the inherent high directivity of these arrays, algorithms to help the user equipment find the base station and establish a communication link should be carefully designed. Towards that, we examine two beamforming schemes, namely, random-phase beamforming (RPBF) and directional beamforming (DBF), and test their impact on the Cram\'er-Rao lower bounds (CRB) of jointly estimating the direction-of-arrival, direction-of-departure, time-of-arrival, and the complex channel gain, under the line-of-sight channel model. The results show that the application of RPBF is more appropriate in the considered scenario as it attains a lower CRB with fewer beams compared to DBF.