On the performance of layered transmission with optimum decoding ordering and multiuser diversity for a K-user downlink scenario

In this work, the performance of layered transmission with optimum decoding order and multiuser diversity is analyzed in a K-user downlink network. The transmitter is equipped with two antennas whereas every user has r (\(r\ge 2\)) antennas. In each training period, the transmitter opportunistically schedules a single user based on a set of scalar feedback (FB) values sent back from the users. Investigating on what should be fed back, three distinct FB strategies are studied and an exact joint distribution expression is derived in each case for the evaluation of the average outage probability. Analytical results are obtained without disregarding the impact of error propagation and broadly differ from the former approximated analyses. It is shown that one of the inspected strategies clearly outperforms the others and can achieve full multiuser diversity gain of K. We show that by just a scalar FB from the users to the transmitter, which can easily be quantized in accordance with the desired accuracy, a significant power gain can be accomplished at the transmitter as compared to a single-user scenario. For the mathematical analysis, we rely on the fact that all orderings are equiprobable under independent and identical fading. To that end, we obtain the joint distribution of the ordered gains from the joint distribution of the unordered gains.