Random Access in Millimeter-Wave Beamforming Cellular Networks: Issues and Approaches

The mmWave band has been utilized for indoor applications in IEEE 802.11ad and for point-to-point wireless backhaul solutions. Very recently, mmWave communications has come into the spotlight as an enabling technology for 5G cellular networks by virtue of development of mmWave beamforming technology and channel measurement campaigns in outdoor environments driven by academia and industry. A high path loss in the mmWave band can be alleviated by adaptive beamforming using antenna arrays; aligning transmit and receive beams in direction can result in high beamforming gains. When we consider the cellular network in the mmWave band, random access, which is primarily used for initial access and handover, is the very first issue in system design. Since random access cannot fully benefit from beamforming due to the lack of information on the best transmit-receive beam pair, the design of the random access channel, RACH, becomes more challenging, especially in non-line-of-sight channels. In this article, we analyze fundamental issues of RACH in mmWave cellular communications and present possible approaches to address these issues. Furthermore, research challenges and future directions are discussed.