Downlink Resource Allocation in Cooperative Wireless Networks

Doktorsavhandling, 2015

Wireless cooperative networks, which is based on exploiting coordination among multiple access nodes, has been considered as a promising approach to improve the spectral efficiency, reduce the energy consumption, and extend the network coverage of future wireless communication systems. In practice, the actual benefit of multi-node cooperation is affected by a variety of factors, including the quality of channel state information (CSI), the constraints on the feedback and backhaul links, hardware impairments, resource allocation and data processing schemes. This thesis investigates the design of resource allocation algorithms and the performance of different coordinated transmission schemes for downlink wireless cooperative systems under practical constraints. First, we consider multi-node cooperation in homogeneous cooperative networks. In [Paper A], a power allocation scheme is proposed for a worst case scenario, where the carrier phases between base stations (BSs) are un-synchronized so that joint transmission must be performed without precoding. We show that in this scenario, joint transmission happens with higher probability when the maximum transmit power is high, or the users are in the overlapped cell-edge area. In practice, the network is divided into clusters of coordinated BSs, and the cooperation gain is limited by the inter-cluster interference. In [Paper B] different fractional frequency reuse schemes are proposed to coordinate inter-cluster interference. Simulation results show that the proposed schemes can efficiently reduce the inter-cluster interference and provide considerable performance improvement in terms of both the cell-edge and cell-average user data rate. [Paper C] compares different coordinated transmission schemes, considering the effects of the feedback and backhaul latency. Compared to zero-forcing coherent joint transmission, we show that non-coherent joint transmission and coordinated scheduling are more robust to channel uncertainly. The second part of the thesis focuses on heterogeneous cooperative networks. In [Paper D], we analyze the performance of amplify-and-forward two-way relaying with in-phase and quadrature-phase imbalance (IQI) at the relay node. Different design guidelines and power allocation schemes are proposed to improve the system reliability under a total transmit power constraint. [Paper E] investigates adaptive power allocation for hybrid automatic repeat request based relay networks. Our results demonstrate that depending on the relay positions and the total power budget, the system should switch between the single-node transmission mode and the joint transmission mode, in order to minimize the outage probability. Finally, [Paper F] studies the joint design of precoding and load balancing for energy-efficient small cell networks with imperfect CSI. An optimal BS association condition is parameterized, which reveals how it is impacted by different system parameters. Our results also show that putting BSs into sleep mode by proper load balancing is an important solution for energy savings in heterogeneous networks.