Fast Initial Access for mmWave 5G Systems with Hybrid Beamforming Using Online Statistics Learning

In cellular communication systems, user discovery or initial access (IA) procedures enable a terminal entering a cell to connect to the base station (BS). Up to 4G systems, users broadcast a cell-connect request omnidirectionally, and the BS replies to the request. In 5G systems operating at mmWave, directional transmissions (by means of beamforming) are needed to overcome strong attenuations. However, during IA the user's and BS's reciprocal positions are not known, and therefore a search procedure explores different directions, substantially increasing the IA overhead. Still, directions of arrival statistics largely depend on buildings and typical user entrance paths, which characterize the cell. By exploiting these statistics IA can be significantly accelerated, by searching more often where users typically are. In order to acquire the statistics, an online method is proposed and its performance is assessed for the first time using real data from a traffic data survey. We observe that the online method reduces the average discovery time up to 25 percent with respect to the conventional approach that does not exploit the user statistics.