Joint design of access and fronthaul uplinks in cloud radio access networks

Cloud radio access networks (C-RANs) are a promising concept for the architecture of 5th generation mobile networks. In C-RANs, signal processing is not performed at the access points as in common mobile networks, but is instead centralized in large, cloud-based data centers. This approach promises many benefits, including smaller-footprint base stations, simplified network management, maintenance and upgrades, economies of scale and a more efficient implementation of cooperative processing techniques. However, such a centralized architecture comes at the heavy price of an extensive, so-called fronthaul network, which has to exchange the raw, unprocessed radio signals between the remote access points and the central processing unit. This requires the fronthaul network to fulfill challenging requirements in terms of data rate, latency, and synchronization. Currently, these fronthaul networks are designed, deployed, and operated separately from the radio access network, meaning that there is little cooperation and information exchange between the fronthaul and radio access network. To mitigate this, this thesis proposes a joint design of the radio access and fronthaul links, by considering the impact that they have on one another, and by exchanging more side-information to form a joint radio access/fronthaul link. A first step towards such a joint design is the re-design of the so-called functional split, which refers to the amount of processing performed at the remote access points versus that performed at the central unit. While the current approaches are two extreme cases of either full centralization or decentralization, an intermediate option can reduce the strict fronthaul requirements, while maintaining several of the benefits of centralization. Furthermore, the access and fronthaul links can cooperate on the level of the physical interface by exchanging information about signal statistics and their respective channel qualities. With this a joint minimum mean square error receiver is designed in this thesis, aiming to improve the performance of the joint radio access and fronthaul link. This receiver is especially beneficial when utilizing wireless millimeter wave fronthaul links, which suffer from a reduced channel quality due to their high attenuation by precipitation. Finally, it can be shown that the design of the quantizer, which is employed to digitize the radio signals before fronthaul transmission, has a considerable impact on overall performance. Accordingly, optimization schemes are proposed to limit this impact by optimized quantizer design. The three proposed approaches – intermediate functional split, joint receivers, and optimized quantizer design – are shown to improve the end-to-end performance of the joint radio access/fronthaul link considerably in a relevant scenario. An analysis of their implementation complexity further underlines their feasibility. In summary, the joint design of radio access and fronthaul is a promising novel approach to solve the challenges of today's fronthaul networks.