  The rate loss is an important issue for multi-user beamspace multiple-input
multiple-output (MIMO) millimeter-wave systems. In such systems, multi-cluster
channel propagation leads as a function of the quantization error and, as a
result, each active user suffers a significant rate loss. Particularly, a
conventional single beam selection (SBS) scheme is used in an equivalent
beamspace MIMO channel to reduce the required number of radio frequency (RF)
chains, but this scheme does not properly mitigate the quantization error due
to the limited number of the channel propagation clusters per-user. In
addition, optimally selected beams or RF chains are operated at the same number
of the precoded data streams. Consequently, each active user inherently loses a
huge number of data streams utilizing the conventional SBS scheme. Therefore,
we propose a hybrid beam selection (HBS) scheme in an equivalent beamspace MIMO
channel to implement a multiple beam or RF chain group selection opportunity
and choose a reliable channel propagation cluster per-active-user, which
directly affects to mitigate the quantization error. The numerical result is
verified by computer simulations in terms of a millimeter-wave MIMO downlink
channel environment.
