  Limited backhaul bandwidth and blockage effects are two main factors limiting
the practical deployment of millimeter wave (mmWave) networks. To tackle these
issues, we study the feasibility of relaying as well as caching in mmWave
networks. A user association and relaying (UAR) criterion dependent on both
caching status and maximum biased received power is proposed by considering the
spatial correlation caused by the coexistence of base stations (BSs) and relay
nodes (RNs). A joint UAR and caching placement problem is then formulated to
maximize the backhaul offloading traffic. Using stochastic geometry tools, we
decouple the joint UAR and caching placement problem by analyzing the
relationship between UAR probabilities and caching placement probabilities. We
then optimize the transformed caching placement problem based on polyblock
outer approximation by exploiting the monotonic property in the general case
and utilizing convex optimization in the noise-limited case. Accordingly, we
propose a BS and RN selection algorithm where caching status at BSs and maximum
biased received power are jointly considered. Experimental results demonstrate
a significant enhancement of backhaul offloading using the proposed algorithms,
and show that deploying more RNs and increasing cache size in mmWave networks
is a more cost-effective alternative than increasing BS density to achieve
similar backhaul offloading performance.
