D2D Assisted Beamforming for Coded Caching

Abstract

Device-to-device aided multicast beamforming design for multi-antenna coded caching is explored to improve the per-user rate and mitigate the beamformer complexity. Novel beamforming and resource allocation schemes are proposed where the local cache content exchange among nearby users is exploited. The transmission is split into two phases: local D2D content exchange and downlink transmission. In the D2D phase, subsets of users are selected to share content with the adjacent users directly. The downlink phase utilizes multicast beamforming by the base station to simultaneously serve all users to fulfill the remaining content requests. A low complexity D2D-multicast mode selection algorithm is proposed with comparable performance to the optimal exhaustive search. Furthermore, D2D transmission scenarios and conditions useful for minimizing the overall delivery time are identified. Finally, we show that introducing the new device-to-device phase to the existing works significantly reduces the beamformers' complexity in the downlink phase. The results further highlight that by exploiting the direct D2D exchange of file fragments, the common multicast rate for delivering the remaining file fragments in the downlink phase is increased. As a result, overall content delivery performance is greatly enhanced, especially in the finite signal-to-noise (SNR) regime.

Complexity Reduction in Multicast Beamforming for D2D Assisted Coded Caching

“…At the same time, the complexity of the delivery scheme can be reduced both at the BS and at the end users. The results in this paper show that introducing the D2D phase to [14] significantly reduces the optimization complexity in addition to improving the delivery performance [16]. Moreover, we provide upper/lower bounds on the number of conditions which bound the computation complexity of the problem.…”

“…In this section, we investigate the effects of D2D transmission in computational complexity for the general case. A more thorough investigation from the performance (delivery time, symmetric rate) improvement perspective is provided in [16]. It was shown in [15], that the number of MAC conditions and quadratic terms in the SINR constraints dominate the complexity of the DL beamformer design in finite SNR CC.…”

“…Please note that, in this paper we assume a b = 0, for b > a. Proof of the theorem is omitted due to the lack of space, proofs are available in extended version of this paper [16]. The number of MAC conditions vary between these two boundaries based on which subsets have been chosen for D2D subphase.…”

“…In this manner, the multicast beamforming [15] of file fragments is complemented by allowing direct device-to-device (D2D) exchange of local cache contents. By allowing direct D2D exchange of file fragments, the interference management between different downlink multicast streams becomes easier and more efficient as compared to the multicast only case [15], which results in a higher per user rate as demonstrated in [16]. At the same time, the complexity of the delivery scheme can be reduced both at the BS and at the end users.…”

“…Moreover, a is defined in (15), b is defined in (16), X and U are defined in (18), U 1 is defined in (19) and Y is defined in (20). See [16] for proof. Fig.…”