  The millimeter wave (mmWave) bands have recently attracted considerable
interest for next-generation cellular systems due to the massive available
bandwidths at these frequencies. However, a key challenge in designing mmWave
cellular systems is initial access -- the procedure by which a mobile
establishes an initial link-layer connection to a base station cell. MmWave
communication relies on highly directional transmissions and the initial access
procedure must thus provide a mechanism by which initial transmission
directions can be searched in a potentially large angular space. Design options
are compared considering different scanning and signaling procedures to
evaluate access delay and system overhead. The channel structure and multiple
access issues are also considered. The analysis demonstrates significant
benefits of low-resolution fully digital architectures in comparison to single
stream analog beamforming.
