Coded caching provides an opportunity to reduce data traffic load in peak hours via coded multicast transmissions. This paper studies the coded caching problem for video contents in heterogeneous packet erasure channels. The video contents are encoded in a scalable manner, where each user's decoding quality depends on the number of layers it receives. Two main challenges lie in this problem. (1) In heterogeneous erasure broadcast channels, users may receive varying numbers of packets. Thus, the performance of the system will be limited by the worst channel condition of users, which is referred to as the “worst-user effect” in existing works. (2) In a scenario where users require different video qualities, different numbers of layers may be transmitted to the users, which indicates a reduction in the multicast opportunities. In this paper, we propose a novel coded caching scheme that enables video quality adaptation, addressing these two challenges at once. The insight is to align the number of layers each user requests with its channel condition, ensuring that the amount of data each user attempts to receive is the same, thus maximizing the multicast opportunities. Analyses show that the transmission rate is determined by the average erasure probability among users, thus alleviating the worst-user effect. Simulation results illustrate the superior performance of the proposed scheme compared to state-of-the-art methods.

Coded caching has emerged as a promising technique to alleviate traffic congestion by strategically creating coded multicasting opportunities, even for caches with different demands. For a two-layer cache-aided hierarchical network consisting of a central server, multiple helpers, and multiple users, prior works have characterized the fundamental performance limits of coded caching for this system with the constraint of a uniform user distribution (i.e., each helper serves an equal number of users). However, when the heterogeneity of user distribution is taken into account, there remain open questions. In this article, we consider a two-layer cache-aided hierarchical network with arbitrary user distributions, where a central server is connected via an error-free link to multiple helpers and each user can randomly access one helper. We introduce a new decentralized coded caching scheme and employ the cut-set technique to characterize lower bounds. Our results show that the gap between the upper and lower bound of the achievable rate from server to helpers is within a constant multiplicative (i.e., [1/32]) and additive (i.e., 2) factor, outperforming prior works under uniform user distribution. Moreover, we also show that the transmission rate from each helper to its attached users is at most a constant factor away from the corresponding lower bound. To our knowledge, this is the first work in hierarchical networks to eliminate the additive gap of the second layer. Finally, simulation results demonstrate the superiority of the proposed caching scheme compared with the state of the art.

Coded caching is an effective technique to reduce the downlink traffic on the network. While coded caching has been extended to many scenarios, coded caching in satellite networks has not been well investigated in the literature. In this paper, we first introduce a novel model of coded caching in satellite networks, which consists of P satellites periodically moving in a given orbit and K users on Earth. In this model, at each timeslot, every satellite (regarded as a server) serves Q consecutive users in a regime, while each user could access one or more satellites at the same time. Due to the cyclic mobility of satellites, the connections between satellites and users could be predictable but also dynamically change in a cyclic wrap-around fashion. Thus, the connections between different satellites and different users at different timeslots could be highly coupled. Taking advantage of the predictable connections given the satellite constellation, we propose a centralized achievable scheme such that different satellites can serve the users jointly. For the converse bound, we introduce a novel method to select user groups and construct request patterns, such that the connections between users and satellites involved could be decoupled. Moreover, the gap between the achievable rate and the converse bound is shown to be at most a constant. Numerical results show the superior performance of the proposed scheme and converse bound.

Coded caching was introduced in partially cooperative D2D networks where some selfish users keep silent during delivery process. In existing works, unselfish users were randomly selected as delivery proxies for selfish users, resulting in asymmetric utilization of unselfish users. We observe that averaging the transmission load uniformly over unselfish users may achieve better performance. With this motivation, we propose scheme A, which symmetrically employs all unselfish users in delivery, and its transmission rate outperforms the schemes in existing works. Moreover, existing schemes applied the same symmetric cache placement and file splitting strategy as in the fully cooperative D2D networks, which ignored the asymmetry brought by the silent selfish users and thus incurred extra transmission. Consequently, we propose scheme B with an asymmetric file division strategy where the subfiles are exclusively designated to be sent by unselfish users, thus eliminating the proxy transmissions. To evaluate the performance of proposed schemes, a new converse bound is derived by the index coding approach. The joint performance of schemes A and B in certain regime is shown to be exact-optimal under uncoded placement when$S=1$, where$S$represents the number of selfish users. Similar to existing works, schemes A and B require an assumption that$S\leq t-1$, where$t$represents the caching redundancy. When$S\geq t$, we further propose scheme$C$employing uncoded placement, which outperforms the existing MDS-code based scheme in certain regime due to reduction on coding overhead. Numerical simulations are conducted to verify the superior performance of the proposed schemes.

Coded caching is a novel technique to reduce network traffic by exploiting multicast opportunities over users. Practical networks may have two layers of caches, where the central server is first connected to an internal node called a “mirror” (e.g., a base station or wifi router) and this internal node links to end users through a broadcast channel. Previous work [1] shows that there exists a tension between rates in these two layers, and the optimal tradeoff is obtained in a simplified model with one mirror and two users. In this paper, we further consider a more general setting with one mirror and multiple users. The performance limits of coded caching on two-layer networks are obtained under uncoded placement. For converse bounds, it is shown that a linear combination of the worst-case rates in each layer is not less than a threshold. An optimal achievable scheme is designed accordingly to match the novel converse bounds.

Coded caching is an effective method to reduce the traffic load on network bottleneck. While the heterogeneities on the number of requests and cache sizes in coded caching have been studied independently, their joint impact is still unclear. This paper investigates coded caching in scenarios with heterogeneous number of requests and cache sizes. We propose two achievable schemes. The first scheme, based on file grouping and multi-round decentralized coded caching, is demonstrated to be order optimal under the worst setting, i.e., when user with the i-th smallest cache has the i-th largest number of requests. Moreover, we obtain an important insight that the lower bound of the achievable rate is predominantly influenced by users with high$\frac {X_{i}}{M_{i}}$ratios, where$X_{i}$and$M_{i}$represent the number of requests and cache size of user i, respectively. Since the achievable rate of our first scheme is difficult to analyze in the general setting, we further propose the second scheme to derive a tractable upper bound. Based on the insight, the second scheme rearranges the users according to their$\frac {X_{i}}{M_{i}}$ratios and employs a threshold to divide them into the head users who may have a large impact on the lower bound, and the tail users who may have a small impact on the lower bound. The server transmits the demands of the head users directly while applying the first scheme in groups among the tail users. Under the general setting, the gap between the rate of our second scheme and the lower bound is proved to be within a logarithmic factor. Simulations are conducted to verify the superior performance of our proposed schemes.

Coded caching is a promising technique for reducing traffic load in wireless networks. In this paper, coded caching is studied for heterogeneous lossy channels, where each user independently suffers packet loss with a distinct and fixed probability. In the original MAN transmission [1], each packet is destined for a specific combinational subset of users. When a packet is received by users beyond the destined subset, it is undecodable and will be dropped. We term these packets as the Overheard Packets (OPs). Interestingly, we find that the OPs can be exploited as side information to increase the multicast gain of the retransmitted packets. With this motivation, we propose a reliable coded caching scheme to reduce the retransmission rate over the shared link. This method enables the users to recover their own lost packets (could be different) from one encoded retransmitted packet. Aided by the OPs, the multicast gain of retransmissions can be increased up to K in certain cases, where K represents the number of users. In other cases, retransmission achieves at least the same multicast gain as the original transmission. Two well-known techniques for reliable transmission are employed in coded caching and serve as baseline schemes. Through both simulations and theoretical analysis we show that the proposed scheme significantly outperforms the baseline schemes in terms of the retransmission rate.

Coded caching is an effective technique to reduce the downlink traffic on the network. While coded caching has been extended to many scenarios, coded caching in satellite networks has not been well investigated in the literature. In this paper, we introduce a novel model of coded caching in satellite networks, which consists of P satellites periodically moving in a given orbit and K users on the earth. In this model, at each timeslot, every satellite (regarded as a server) serves Q consecutive users in a regime, while each user can access one satellite. Due to the cyclic mobility of satellites, the connections between satellites and users could be predictable but also dynamically change in a cyclic shift pattern. Thus, the connections between different satellites and different users at different timeslots could be highly coupled. Taking advantage of the predictable connections, we propose a centralized achievable scheme such that different satellites can serve the users jointly. For the converse bound, we introduce a novel method to construct request patterns such that the connections between users and satellites involved could be decoupled. The gap between the achievable rate and the converse bound is shown to be at most a constant. Numerical results for the performance of our scheme are also demonstrated.

Coded caching is an effective method to reduce the traffic load on the network bottleneck link by exploiting the joint optimization of caching and transmission. In most of previous works, one user may request only one file. However, users could have distinct number of requests in practice. The number of files requested by the users may also follow different distributions. In this paper, we investigate coded caching with distinct number of user requests. We propose a decentralized coded caching transmission scheme for user requests whose number may follow an arbitrary distribution and analyze the upper bound of the transmission rate. We also derive the corresponding lower bound for any achievable scheme. We show that the gap between these two rates is within a constant factor of 12. And this result holds for any distribution on the number of user requests. For the proposed transmission scheme and a given distribution, we show that when the number of users exceeds a threshold, the transmission rate will be bounded without regard to the number of users, due to the benefit of multicast opportunities brought by coded caching. Simulation results demonstrate the superior performance of the proposed scheme.