Coded Caching for Wireless Backhaul Networks With Unequal Link Rates

Coded caching has emerged as a promising component of solutions to the exponential growth in network traffic. Previous approaches to network coding are all based on (simple) XOR coding, which is appropriate when links have the same rate. However, in typical wireless networks, different users experience different link rates. Thus, XOR coding is sub-optimal and cannot achieve full broadcast gain. In this paper, we consider the coded caching design for wireless networks with unequal link rates. More specifically, the backhaul networks of LTE-A or 5G system are considered, in which the link rates between the macroBS and microBSs are different. We leverage a new network coding scheme nested coded modulation (NCM) in the delivery phase and develop a novel file partition scheme for the placement phase based on unequal cache size allocation. This scheme adapts to unequal link rates for increasing broadcast gains. The achievable transmission time and the information-theoretic lower bound are derived; we show that transmission time of the NCM-based coded caching can achieve a constant gap to the lower bound. Moreover, the NCM-based coded caching can achieve significant performance improvement over the XOR-based coded caching; the example with normalized link rates from 1 to 6 achieves up to 250% throughput improvement. Numerical results also show that the NCM-based coded caching can well utilize the unequal link rates, which cannot be achieved by the XOR-based coded caching.