  A new detection scheme for multiuser multiple-input multiple-output (MIMO)
systems is analytically presented. In particular, the transmitting users are
being categorized in two distinct priority service groups, while they
communicate directly with a multi-antenna receiver. The linear zero-forcing
scheme is applied in two consecutive detection stages upon the signal
reception. In the first stage, the signals of one service group are detected,
followed by the second stage including the corresponding detection of the
remaining signals. An appropriate switching scheme based on specific
transmission quality requirements is utilized prior to the detection so as to
allocate the signals of a given service group to the suitable detection stage.
The objective is the enhancement of the reception quality for both service
groups. The proposed approach can be implemented directly in cognitive radio
communication assigning the secondary users to the appropriate service group.
The exact outage probability of the considered system is derived in closed
form. The special case of massive MIMO is further studied yielding some useful
engineering outcomes; the effective channel coherence time and a certain
optimality condition defining both the transmission quality and effective
number of independent transmissions.
