Full Coded Caching Gains for Cache-less Users
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The work identifies the performance limits of the multiple-input single-output broadcast channel where cache-aided users coincide with users that do not have caches. The main contribution is a new algorithm that employs perfect matchings on a bipartite graph to offer full multiplexing as well as full coded-caching gains to both cache-aided as well as cache-less users. This performance is shown to be within a factor of at most 3 from the optimal, under the assumption of linear one-shot schemes. An interesting outcome is the following: starting from a single-stream centralized coded caching setting with normalized cache size γ, then every addition of an extra transmit antenna allows for the addition of approximately 1/γ extra cache-less users, at no added delay costs. For example, starting from a single-stream coded caching setting with γ = 1/100, every addition of a transmit antenna, allows for serving approximately an additional 100 more cache-less users, without any increase in the overall delay. Finally the work reveals the role of multiple antennas in removing the penalties typically associated to cache-size unevenness, as it shows that the performance in the presence of both cache-less and cache-aided users, matches the optimal (under uncoded cache placement) performance of the corresponding symmetric case where the same cumulative cache volume is split evenly across all users.
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