Scalable Content Delivery With Coded Caching in Multi-Antenna Fading Channels

We consider the content delivery problem in a fading multi-input single-output channel with cache-aided users. We are interested in the scalability of the equivalent content delivery rate when the number of users, <inline-formula> <tex-math notation="LaTeX">$K$ </tex-math></inline-formula>, is large. Analytical results show that, using coded caching and wireless multicasting, without channel state information at the transmitter, linear scaling of the content delivery rate with respect to <inline-formula> <tex-math notation="LaTeX">$K$ </tex-math></inline-formula> can be achieved in some different ways. First, if the multicast transmission spans over <inline-formula> <tex-math notation="LaTeX">$L$ </tex-math></inline-formula> independent sub-channels, e.g., in quasi-static fading if <inline-formula> <tex-math notation="LaTeX">$L = 1$ </tex-math></inline-formula>, and in block fading or multi-carrier systems if <inline-formula> <tex-math notation="LaTeX">$L>1$ </tex-math></inline-formula>, linear scaling can be obtained, when the product of the number of transmit antennas and the number of sub-channels scales logarithmically with <inline-formula> <tex-math notation="LaTeX">$K$ </tex-math></inline-formula>. Second, even with a fixed number of antennas, we can achieve the linear scaling with a threshold-based user selection requiring only one-bit feedbacks from the users. When CSIT is available, we propose a mixed strategy that combines spatial multiplexing and multicasting. Numerical results show that, by optimizing the power split between spatial multiplexing and multicasting, we can achieve a significant gain of the content delivery rate with moderate cache size.