  In this paper, a proposal to improve the reliability of a downlink multiuser
(MU) MIMO transmission scheme is investigated with the use of a new approach in
orthogonal space-time block codes (OSTBC) and network coding with a
superposition modulated system and side information. With the new encoded OSTBC
approach, diversity is offered where each user receives all other users'
symbols, which allows the recovery of symbols in several ways. In addition,
multiple users can be accommodated with the same resource, which is quite
useful in a wireless system where resources are always restricted. By employing
superposition modulation, the side information needed for error recovery can be
transmitted over the same resource used for the normal information frame. In
addition, the proposed system exploits diversity through a novel technique of
sub-constellation alignment-based signal combining for efficient side
information dissemination. A detailed analysis of the new OSTBC approach is
carried out. It is shown that the performance of the MU-MIMO system can be
improved significantly in terms of block and frame error rates (BLER, FER)
considered as reliability measures. By accommodating a reasonable number of
multiple users, high reliability is achieved at the expense of the rate. To
compensate for the low rate, conventional OSTBC can be considered and
simulation results are shown, where, as a penalty to pay, multiple orthogonal
resources are required.
