Efficient User Subset Selection for Multiuser Space-Time Line Code Systems

This paper considers the problem of user subset (US) selection for minimizing the bit error rate (BER) of multi-user space-time line code (MU-STLC) multiple-input multiple-output systems with fairness-aware per-user power allocation. The optimal selection criterion suitable for MU-STLC transmissions based on zero forcing (ZF) precoding is given and two efficient algorithms are then proposed. First, an incremental search approach is presented for US selection in the MU-STLC systems. The proposed suboptimal solution to BER minimization starts an empty US and adds users one by one, where the low-complexity recursive computation of the block matrix inverse is further performed. Second, by avoiding recurring matrix computations in each incremental procedure of the first algorithm, a more efficient algorithm is developed. It is observed through simulation results that the proposed incremental-based algorithms achieve most US selection gains with very low complexity. In addition, it is demonstrated that when there are
$N_{T} $
transmit antennas,
$U$
users (each user has 2 receive antennas), and
$K$
selected users, the achievable upper diversity order of the ZF precoding-based MU-STLC systems with optimal US selection is given as
$2(N_{T} -K+1)(U-K+1)$
. The analytical diversity order is well-matched with simulation results.