Multi-user MIMO channel estimation in the presence of carrier frequency offsets

During the uplink of a multi-user MIMO system, a compensation of different users' carrier frequency offsets (CFOs) at the receiver's side is possible only after the spatial separation of the single user signals. As spatial separation requires knowledge of the communication channel, this issue longs for a channel estimation scheme capable to estimate the channel from received sequences that are distorted by CFOs from different users. We propose orthogonal sequences to be used as pilots for the simultaneous estimation of all user channels in the time-domain, which satisfy this requirement.