Resource Allocation in Downlink Coordinated Multi-Point Systems

Licentiatavhandling, 2013

The raised user expectations of quality of service and the rapid growth of the data traffics impose a great challenge for mobile communication systems. The performance, e.g., the spectrum efficiency, peak data rate, cell-edge data rate, of current cellular systems is mainly limited due to the presence of inter-cell interference. One way to combat inter-cell interference is by exploiting cooperation between base stations (BSs), which is known as coordinated multi-point (CoMP) transmission/reception. Depending on the levels of BS cooperation, CoMP techniques can either coordinate or exploit the interference to improve the system throughput and the user fairness. In the design of realistic CoMP systems, the actual benefit of BS cooperation is affected by a variety of factors, including the quality of channel state information (CSI), the constraints on the over-the-air feedback links and the backhaul links between BSs, user mobility, resource allocation and data processing schemes. This thesis investigates the resource allocation algorithm design and the performance for downlink CoMP systems under practical constraints. The main contributions are summarized as follows. First, we consider a CoMP cluster where all BSs are inter-connected via backhaul links with perfect CSI and data sharing. Joint optimization of user selection and power allocation across multiple subchannels and multiple BSs is studied in [Paper A] with zero-forcing joint transmission. Based on general duality theory, two centralized resource allocation algorithms are proposed. We show that the two proposed algorithms achieve a performance very close to the optimal, with much lower computational complexity. Multi-BS joint transmission requires tight phase synchronization between different BSs, which can be extremely difficult in practice due to the effect of carrier frequency offset, or/and phase noise from local oscillators in each BS. In order to deal with this situation, a power allocation scheme is proposed in [Paper B] considering a worst case scenario where the carrier phases between the BSs are un-synchronized. The second part of the thesis focus on the investigation of the consequences of imperfect CSI and backhaul constraints on CoMP. In [Paper C], three CoMP transmission schemes are studied under different network architectures that introduce different backhaul latencies and feedback errors, resulting in imperfect CSI at the transmitter side. It is shown that different schemes are performing better in different scenarios, thus, motivating a transmission mode switching functionality in order to improve the system performance. In any network architecture, the use of CoMP transmission is restricted to a cluster with limited number of cells due to practical constraints. The BS cooperation gain is then mainly limited by the inter-cluster interference, especially for the users located at the cluster edge area. In [Paper D], different fractional frequency reuse schemes are proposed to coordinate inter-cluster interference, therefore, reducing the cluster edge effect.