  Frequency domain (FD) multi-user detection (MUD) has been shown to be an
effective means of approaching the theoretical per-user capacity for single
carrier modulation (SCM) schemes in massive MIMO scenarios with highly
dispersive channels. When a cyclic prefix is added to the SCM waveform, the
circulant structures of the resulting convolutional channel matrix allows for
relatively simple expressions for the FD detection. In this paper, we develop a
computationally efficient minimum mean squared error (MMSE) FD-MUD technique in
a time-division duplexing (TDD) massive MIMO setup and show how processing
steps can be shared between uplink and downlink.
