Adapt or Wait: Quality Adaptation for Cache-Aided Channels

Coded Caching is a technology that promises to reduce cacheable traffic by turning stored content at the users to multicast opportunities. In wireless channels, though, users experience different rates causing each message to be communicated with the group’s worst-user’s rate, which in turn impacts significantly the achieved performance. In this work we propose an adaptive quality transmission framework specifically designed for coded caching multicast communications, which uses superposition coding to overcome channel degradation. Our scheme combines coded caching, superposition coding, and scalable source coding, while keeping the caching oblivious to future channel rates and delivered file quality. The proposed framework covers all possible channel rate and quality configurations, while we further propose algorithms that can optimise the served quality. An interesting outcome of our work is that a modest quality reduction at the degraded users can counter the effects of significant channel degradation. For example, in a 100-user system with normalized cache size 1/10 at each user, if 10 users experience channel degradation of 60% compared to the rate of the non-degraded users, we show that our transmission strategy leads to a <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\thicksim 85\%$ </tex-math></inline-formula> quality at the degraded users and perfect quality at the non-degraded users.