Design Aspects of Coordinated Multipoint Transmission : A Study of Channel Predictions, Resource Allocation, User Grouping and Robust Linear Precoding for Coherent Joint Transmission

Sammanfattning: Shadowed areas and interference at cell borders pose great challenges for future wireless broadband systems. Coordinated Multipoint (CoMP) coherent joint transmission has shown the potential to overcome these challenges by turning harmful interference into useful signal power. However, there are obstacles to overcome before coherent joint transmission CoMP can be deployed. Some of these are the investigated in this thesis.First, coherent joint transmission requires very accurate Channel State Information (CSI), but unfortunately long system latencies cause outdating of the CSI. This can to some extend be counteracted by channel predictions. Two schemes are here investigated for predicting downlink Frequency Division Duplex (FDD) Orthogonal Frequency Division Multiplexing (OFDM) channels; Kalman filters and “predictor antennas”. The first is well suited for slow moving users, e.g. pedestrians or cyclists, as it does not require any special antenna setup. The second, which utilizes an extra antenna, located in front of the main receive antennas, is well suited for vehicular users, such as buses or trams, as these require long spatial prediction horizon.Second, a user grouping and resource allocation scheme is investigated. This scheme forms CoMP groups by local resource allocations and provides multi-user diversity gains very close to the optimal gains, found through an extensive combinatorial search. It has very low complexity, requires less feedback capacity than other schemes and places no demands on backhaul capacity.Finally, a linear precoder, which is robust to errors in the CSI, is investigated. This precoder takes the covariances of the channel errors into account while optimizing a Mean Squared Error (MSE) criterion. The MSE criterion includes design parameters that can be used as flexible tools for low dimensional searches with respect to an arbitrary optimization criterion, e.g. a weighted sum-rate criterion. The precoder design is also extended to handle backhaul constraints.Results show that with the combination of these three schemes: channel predictions, the proposed user grouping and resource allocation scheme and the robust linear precoder, then coherent joint transmission will indeed provide large capacity gains.