Cache-Assisted Broadcast-Relay Wireless Networks: A Delivery-Time Cache-Memory Tradeoff

An emerging trend of next generation communication systems is to deploy caches at network edges to reduce file delivery latency. To investigate this aspect, we study the fundamental limits of a cache-aided broadcast-relay wireless network consisting of one central base station, $M$ cache-equipped transceivers and $K$ receivers from a latency-centric perspective. We use the normalized delivery time (NDT) to capture the per-bit latency at high signal-to-noise ratio (SNR). The objective is to jointly design the cache placement and file delivery in order to minimize the NDT. To this end, we establish two converse results and two achievability schemes. The first converse result is restricted to one-shot delivery schemes, while the second excludes this restriction. Similarly, the first achievable scheme is a general NDT-optimal one-shot scheme that synergistically exploits both multicasting and distributed zero-forcing opportunities. With respect to the second converse result, this scheme performs well for various parameter settings, particularly at higher cache sizes. The second scheme, effective at lower cache sizes, designs beamformers to facilitate both subspace interference alignment and zero-forcing. Exploiting both schemes, we are able to characterize the optimal tradeoff between cache storage and latency in networks satisfying $K+M\leq 4$ . The tradeoff illustrates that the NDT is the preferred choice to capture the latency of a system rather than the commonly used sum degrees of freedom (DoF). In fact, our optimal tradeoff refutes the popular belief that increasing cache sizes translates to increasing the achievable sum DoF. As such, we discuss cases where increasing cache sizes decreases both the delivery time and the achievable DoF.