Achievable Synchronisation Gain In Uncalibrated Large Scale Antenna Systems

Large scale antenna systems are used to exploit spatial multiplexing gains in massive MIMO systems. To realise those gains, channel state information has to be acquired at a base station. However, an initial control channel has to be provided to synchronise time and frequency at the user. This control channel should be undirected to cover the base stations operational area and can therefore not exploit the coherent array gain without additional strategies. Beam sweeping has been proposed to provide increased spatial coverage. Its performance for large scale antenna systems in Rayleigh and Rician fading environments is analysed. Even an orthogonal basis of antenna weights for full spatial coverage can not provide the full array gain. The results quantity the gap between achievable synchronisation and full array gain for uncorrelated antennas. Closed form solutions for the distribution of the gain gap under Rayleigh fading conditions are derived.