Resolving the Feedback Bottleneck of Multi-Antenna Coded Caching

Abstract: Multi-antenna cache-aided wireless networks have been known to suffer from a severe feedback bottleneck, where achieving the maximal Degrees-of-Freedom (DoF) performance required feedback from all served users. These costs matched the caching gains and thus scaled with the number of users. In the context of the $L$-antenna MISO broadcast channel with $K$ receivers having normalized cache size $\gamma$, we pair a fundamentally novel algorithm together with a new information-theoretic converse, and identify the optimal tradeoff between feedback costs and DoF performance, by showing that having CSIT from only $C<L$ served users implies an optimal one-shot linear DoF of $C+K\gamma$. As a side consequence of this, we also now understand that the well known DoF performance $L+K\gamma$ is in fact exactly optimal. In practice, the above means that we are now able to disentangle caching gains from feedback costs, thus achieving unbounded caching gains at the mere feedback cost of the multiplexing gain. This further solidifies the role of caching in boosting multi-antenna systems; caching now can provide unbounded DoF gains over multi-antenna downlink systems, at no additional feedback costs. The above results are extended to also include the corresponding multiple transmitter scenario with caches at both ends.