Coordinated Precoding for Multicell MIMO Networks

University dissertation from Stockholm : KTH Royal Institute of Technology

Abstract: Enabling multiple base stations to utilize the spatial dimension in a coordinated manner has been shown to be a fruitful technique for improving the spectral efficiency in wireless interference networks. This thesis considers multicell systems where base stations and mobile stations are equipped with multiple antennas. The base stations coordinate their spatial precoding, but individually serve their mobile stations with data. For such coordinated precoding systems, interference alignment (IA) is a useful theoretical tool, due to its ability to serve the maximum number of interference-free data streams. Three topics related to interference alignment and coordinated precoding are studied.First, the feasibility of IA over a joint space-frequency signal space is studied. A necessary condition for space-frequency IA feasibility is derived, and the possible gain over space-only IA is analyzed. An upper bound on the degree of freedom gain is shown to increase in the number of subcarriers, but decrease in the number of antennas. Numerical studies, using synthetically generated channels and real-world channels obtained from indoors and outdoors channel measurements, are used for sum rate performance evaluation. The results show that although a degree of freedom gain is noticeable due to the space-frequency precoding, the sum rate of the system is mainly improved due to a power gain.Second, distributed channel state information (CSI) acquisition techniques are proposed, which provide estimates of the information necessary to perform distributed coordinated precoding. The methods are based on pilot-assisted channel estimation in the uplink and downlink, and correspond to different tradeoffs between feedback and signaling, backhaul use, and computational complexity. Naively applying the existing WMMSE algorithm for distributed coordinated precoding together with the estimated CSI however results in poor performance. A robustification of the algorithm is therefore proposed, relying on the well known diagonal loading technique. An inherent property of the WMMSE solutions is derived and, when enforced onto solutions with imperfect CSI, results in diagonally loaded receive filters. Numerical simulations show the effectiveness of the proposed robustification. Further, the proposed robustified and distributed WMMSE algorithm performs well compared to existing state-of-the-art robust WMMSE algorithms. In contrast to our approach, the existing methods however rely on centralized CSI acquisition.Third, coordinated precoding systems with hardware impairments are studied. Assuming that impairment compensation techniques have been applied, a model is used to describe the aggregate effect of the residual hardware impairments. An iterative resource allocation method accounting for the residual hardware impairments is derived, based on an existing resource allocation framework. Numerical simulations show that the proposed method outperforms all benchmarks. In particular, the gain over impairments-aware time-division multiple access is substantial.