In-Band Omnidirectional Initial Access via Alamouti Scheme in Millimeter-Wave Cellular Networks

The ever-increasing volume of data traffic and demand for higher data rates in current wireless networks have posed the need for new technologies to improve spectral efficiency in next-generation cellular networks. Among such technologies, millimeter wave (mmWave) communications have surfaced as a key candidate by taking advantage of abundant and unexploited frequency bands, coupled with dense small cell deployments and a large number of antenna elements at both ends of the link. However, severe path loss and signal blockage are important challenges to be overcome at mmWave bands. To compensate for that, beamforming techniques have been proposed to achieve high directional gains. Unfortunately, the use of beamforming in the initial access (when the user equipment needs to connect to the base station) may incur significant delay overhead, since it will require beam searching algorithms or out-of-band channel estimation for proper beam set up and alignment. To avoid this problem, this paper investigates the use of the Alamouti scheme to implement in-band omnidirectional initial access by leveraging its diversity gains. Simulation results for an outdoor cell at 28 GHz are presented for estimation of the user connection probability and uplink average signal-to-noise (SNR) ratio as a function of the distance between base station and user equipment. The simulation results indicate that, with appropriate chosen parameters, the Alamouti scheme can deliver user connection probabilities similar to beamforming, and without incurring the delay overhead caused by beam searching algorithms or out-of-band solutions.