Latency-Limited Broadcast Channel With Cache-Equipped Helpers

Decreasing communication latency is one of the main challenges for many applications in future communication systems. As a solution, we study in this work the benefits of using some cache-equipped helpers in a wireless communication scenario. More precisely, a wireless network with a transmitter, two cache-equipped helpers, and two receivers is studied from the latency point of view. While the transmitter has access to the whole data, the helpers store some data in their cache in order to support the transmitter in fulfilling the demands of the receivers. The main challenge is to develop a joint caching placement and the transmission strategy, which minimizes the latency of the communication. To this end, a general encoded caching is considered, in which each message is split into four types of sub-messages. The sub-messages of the first and second types are stored only in the cache of the first and second helper, respectively. The sub-messages of the third type are cached at both helpers, while the sub-messages of the fourth type are not cached at any helper. The size of the sub-messages is optimized in a way such that the delivery time of one bit for the worst-case demand is minimized. Moreover, a general lower bound on the delivery time is provided. Using this lower bound as a benchmark, the performance of the proposed caching policy is evaluated. It is shown that facilitating the cache-equipped helpers decreases the latency of broadcasting. Furthermore, it is shown that for the proposed caching policy, storing identical sub-messages in the cache of different helpers cannot be optimal if the cache size of the helpers is insufficiently large.