Single-User and Multi-User MIMO Channel Estimation for LTE-Advanced Uplink

In LTE-Advanced (LTE-A) demodulation reference symbols are employed for pilot aided channel estimation to perform coherent detection. Since these reference symbols are allocated on the same time-frequency positions for all users in Multi-User MIMO (MU-MIMO) operation or for all spatial layers in Single-User MIMO (SU-MIMO), they are designed to be code-domain orthogonal in order to be separable at the receiver. This orthogonality is obtained by cyclically shifting a reference signal base sequence, where the specific cyclic shift values are signaled to each user. In this work we show formal equivalence of SU-MIMO and MU-MIMO for LTE-A uplink in the context of channel estimation. We propose a standard compliant mapping that assigns cyclic shifts to users such that SU-MIMO estimation methods are applicable also for MU-MIMO transmissions. Further we show the trade-off between the number of active users and the channel's frequency selectivity in MU-MIMO operation due to the residual channel estimation error.