The degrees of freedom (DoF) of the two-user multiple-input single-output (MISO) broadcast channel (BC) are studied under the assumption that the form, I i , i = 1, 2, of the channel state information at the transmitter (CSIT) for each user's channel can be either perfect (P ), delayed (D) or not available (N ), i.e., I 1 , I 2 ∈ {P, N, D}, and therefore the overall CSIT can alternate between the 9 resulting states I 1 I 2 . The fraction of time associated with CSIT state I 1 I 2 is denoted by the parameter λ I1I2 and it is assumed throughout that λ I1I2 = λ I2I1 , i.e.,Under this assumption of symmetry, the main contribution of this paper is a complete characterization of the DoF region of the two user MISO BC with alternating CSIT. Surprisingly, the DoF region is found to depend only on the marginal probabilities (λ P , λ D , λ N ) = I2 λ P I2 , I2 λ DI2 , I2 λ N I2 , I 2 ∈ {P, D, N }, which represent the fraction of time that any given user (e.g., user 1) is associated with perfect, delayed, or no CSIT, respectively. As a consequence, the DoF region with all 9 CSIT states, D(λ I1I2 : I 1 , I 2 ∈ {P, D, N }), is the same as the DoF region with only 3 CSIT states D(λ P P , λ DD , λ N N ), under the same marginal distribution of CSIT states, i.e., (λ P P , λ DD , λ N N ) = (λ P , λ D , λ N ). The sum-DoF value can be expressed as DoF = min 4+2λ P 3 , 1 + λ P + λ D , from which one can uniquely identify the minimum required marginal CSIT fractions to achieve any target DoF value as (λ P , λ D ) min = 3 2 DoF − 2, 1 − 1 2 DoF when DoF ∈ 4 3 , 2 and (λ P , λ D ) min = (0, (DoF − 1) + ) when DoF ∈ 0, 4 3 . The results highlight the synergistic benefits of alternating CSIT and the tradeoffs between various forms of CSIT for any given DoF value. *

A fog-aided wireless network architecture is studied in which edge-nodes (ENs), such as base stations, are connected to a cloud processor via dedicated fronthaul links, while also being endowed with caches. Cloud processing enables the centralized implementation of cooperative transmission strategies at the ENs, albeit at the cost of an increased latency due to fronthaul transfer. In contrast, the proactive caching of popular content at the ENs allows for the low-latency delivery of the cached files, but with generally limited opportunities for cooperative transmission among the ENs. The interplay between cloud processing and edge caching is addressed from an information-theoretic viewpoint by investigating the fundamental limits of a high Signal-to-Noise-Ratio (SNR) metric, termed normalized delivery time (NDT), which captures the worst-case coding latency for delivering any requested content to the users. The NDT is defined under the assumptions of either serial or pipelined fronthauledge transmission, and is studied as a function of fronthaul and cache capacity constraints. Placement and delivery strategies across both fronthaul and wireless, or edge, segments are proposed with the aim of minimizing the NDT. Information-theoretic lower bounds on the NDT are also derived. Achievability arguments and lower bounds are leveraged to characterize the minimal NDT in a number of important special cases, including systems with no caching capabilities, as well as to prove that the proposed schemes achieve optimality within a constant multiplicative factor of 2 for all values of the problem parameters. Index TermsCaching, Cloud Radio Access Network (C-RAN), Fog Radio Access Network, edge processing, 5G, degreesof-freedom, latency, wireless networks, interference channel. the fronthaul capacity is small. This is because, with pipelined transmission, the ENs need not wait for the fronthaul transmission to be completed before communicating to the users on the edge links. For the same reason, pipelined fronthaul-edge transmission generally improves the NDT compared to serial transmission. In particular, even with partial caching, that is, with µ < 1, the ideal NDT δ * = 1 is achievable with pipelined fronthaul-edge transmission, while this is not the case with serial transmission. More details can be found in Sections V-C and VI-D.Related Work: The line of work pertaining to the information-theoretic analysis of cache-aided communication systems can be broadly classified into studies that consider caching at the end-users' devices or at the ENs. This research direction was initiated by [9], [10] for a set-up that consists of a multicast link with cache-aided receivers. This work demonstrates that coded multicasting enables global caching gains to be reaped, as opposed to the conventional local caching gains of uncoded transmission. Follow-up papers on related models with receiver-end caching include [11]-[24]. The present paper is instead inscribed in the parallel line of work that concerns caching at the ENs of a wireless n...

Caching is emerging as a vital tool for alleviating the severe capacity crunch in modern content-centric wireless networks. The main idea behind caching is to store parts of popular content in end-users' memory and leverage the locally stored content to reduce peak data rates. By jointly designing content placement and delivery mechanisms, recent works have shown order-wise reduction in transmission rates in contrast to traditional methods. In this work, we consider the secure caching problem with the additional goal of minimizing information leakage to an external wiretapper. The fundamental cache memory vs. transmission rate trade-off for the secure caching problem is characterized. Rather surprisingly, these results show that security can be introduced at a negligible cost, particularly for large number of files and users. It is also shown that the rate achieved by the proposed caching scheme with secure delivery is within a constant multiplicative factor from the information-theoretic optimal rate for almost all parameter values of practical interest.

Abstract-We investigate the fundamental information theoretic limits of cache-aided wireless networks, in which edge nodes (or transmitters) are endowed with caches that can store popular content, such as multimedia files. This architecture aims to localize popular multimedia content by proactively pushing it closer to the edge of the wireless network, thereby alleviating backhaul load. An information theoretic model of such networks is presented, that includes the introduction of a new metric, namely normalized delivery time (NDT), which captures the worst case time to deliver any requested content to the users. We present new results on the trade-off between latency, measured via the NDT, and the cache storage capacity of the edge nodes. In particular, a novel information theoretic lower bound on NDT is presented for cache aided networks. The optimality of this bound is shown for several system parameters.Index Terms-Caching, 5G, degrees of freedom, latency.

The problem of cache enabled private information retrieval (PIR) is considered in which a user wishes to privately retrieve one out of K messages, each of size L bits from N distributed databases. The user has a local cache of storage SL bits which can be used to store any function of the K messages. The main contribution of this work is the exact characterization of the capacity of cache enabled PIR as a function of the storage parameter S. In particular, for a given cache storage parameter S, the information-theoretically optimal download cost D * (S)/L (or the inverse of capacity) is shown to be equal to (1 − S K ) 1 + 1 N + . . . + 1 N K−1 . Special cases of this result correspond to the settings when S = 0, for which the optimal download cost was shown by Sun and Jafar to be 1 + 1 N + . . . + 1 N K−1 , and the case when S = K, i.e., cache size is large enough to store all messages locally, for which the optimal download cost is 0. The intermediate points S ∈ (0, K) can be readily achieved through a simple memory-sharing based PIR scheme. The key technical contribution of this work is the converse, i.e., a lower bound on the download cost as a function of storage S which shows that memory sharing is information-theoretically optimal.