Downlink Transmission and Caching Strategies for Backhaul-Limited Cloud Radio Access Networks

This thesis considers a downlink cloud radio access network (C-RAN) where all the base stations (BSs) are connected to a central processor (CP) via capacity-limited backhaul links. Two fundamental and different downlink transmission strategies are studied, namely the data-sharing strategy and the compression strategy. In the data-sharing strategy, the backhaul links connecting the CP and the BSs are used to carry user messages; each user's messages are sent to a cluster of BSs, called BS clustering, which locally form the beamforming vectors to cooperatively transmit the messages to the user. In the compression strategy, the user's messages are precoded centrally at the CP, which forwards a compressed version of the analog beamformed signals to the BSs for cooperative transmission. The first part of this thesis focuses on optimizing the data-sharing strategy and the BS clustering while explicitly accounting for the per-BS backhaul capacity limits in each time slot. We show that with the proposed static BS clustering algorithms, the data-sharing strategy can achieve most of the gain brought by the dynamic BS clustering. The second part of this thesis compares the performance of data-sharing and compression strategies from two different perspectives, maximizing the energy efficiency and maximizing the spectral efficiency of the network. From numerical simulations, we observe that although both data-sharing and compression strategies outperform traditional cellular downlink transmission strategies, their comparative performance highly depends on the backhaul capacity. In the final part of this thesis, we study a promising approach to improve the performance of the data-sharing strategy for C-RAN by augmenting the backhaul with BS caching, where the BSs pre-store contents of popular files. We consider a downlink C-RAN consisting of a single cluster of BSs and wireless backhaul channel, and study the optimal cache size allocation strategy among the BSs and the optimal beamforming transmission strategy at the CP such that the users' requested messages are delivered from the CP to the BSs in the most efficient way. We show that more cache sizes should be allocated to BSs with weaker channels and that it is in general not optimal to entirely cache the most popular files first. Numerical results show considerable performance improvement of the optimized cache allocation scheme over the uniform allocation and other heuristic schemes.

Department dc:contributor.department

Electrical and Computer Engineering

Year dc:date.issued

2018

Author dc:creator

• Dai, Binbin

Advisor dc:contributor.advisor

• Yu, Wei

• backhaul
• caching
• cloud radio access network
• clustering
• compression
• data-sharing

Handle dc:identifier.uri

http://hdl.handle.net/1807/89898

OAI identifier oai:identifier

oai:utoronto.scholaris.ca:1807/89898