User Selection Algorithms for Block Diagonalization Aided Multiuser Downlink mm-Wave Communication

Rakshith Rajashekar, Lajos Hanzo
2017 IEEE Access  
In order to meet the ever increasing data rate demands, the next generation of wireless communication systems is being designed for exploiting the large amounts of unused spectrum in the millimeter-wave (mm-wave) band. Since operating at mm-wave frequencies imposes several challenges, such as high-path loss, as well as both spatial and temporal channel sparsity, there is a significant research interest focused on designing feasible solutions for establishing reliable and high-throughput links
more » ... -throughput links at mmwave frequencies. In this paper, we consider a cellular system relying on hybrid beamforming aided base station (BS) as well as user equipment and study the user selection problem, which has not been hitherto studied in the literature. More specifically, we study the problem of selecting K -users by the BS for communication out of K -users whilst ensuring that the sum rate is maximized. Specifically, we propose a user selection algorithm, which relies on the knowledge of both the channel gains and the angle of departure (AoD) of the channel paths spanning to the various users, which is termed the AoD aided user selection (AoD-US). Furthermore, we devise a pair of subspace metrics based on: 1) the angle between the subspaces spanned by the BS array response vectors; 2) the ratio of interference and signal space dimensions of various users, in order to reduce the user search space in AoD-US. This modified user selection algorithm is termed the AoD aided user selection with user set pruning (AoD-US-P). Furthermore, we study the attainable sum-rate performance of the block-diagonalization aided downlink and show that the proposed selection algorithms guarantee both multiuser diversity and multiplexing gains. Additionally, the proposed algorithms are studied in the round-robin scheduling scenario, where all the K -users are scheduled for achieving fairness. Our simulation results revealed that the AoD-US-P achieves nearly the same performance as that achieved by the AoD-US despite having a small user set, while both are observed to outperform the channel power-based selection scheme. INDEX TERMS User selection, scheduling, block diagonalization, mm-wave communication, precoding and combining. I. INTRODUCTION The large unused spectral resources available in the millimeter wave (mm-wave) frequencies are expected to be utilized for meeting the throughput demands of the next generation wireless communication systems [1]- [5] . While attaining higher throughputs is the main goal, as a benefit of having higher user bandwidths, operating at mm-wave frequencies comes with its own challenges [3] . A major challenge imposed by the mm-wave channel is the high signal attenuation and the channel sparsity in the spatial and temporal domains [6], [7] . Beamforming (BF) is a popular solution conceived for mitigating the pathloss imposed by the channel, where both the transmitter and the receiver are aligned to the dominant singular vectors of the channel matrix in order to improve the received signal quality. Furthermore, the performance of the hybrid BF (HBF) architecture has been extensively studied in the literature [8]-[20], where several analog phase shifters are employed in conjunction with the digital processing implemented with the aid of a reduced number of RF chains compared to the number of antennas available 5760 2169-3536
doi:10.1109/access.2017.2687078 fatcat:25lby5sqbjarxooa6zfw2ianke