This paper studies the fundamental tradeoff between storage and latency in a general wireless interference network with caches equipped at all transmitters and receivers. The tradeoff is characterized by an information-theoretic metric, fractional delivery time (FDT), which is defined as the delivery time of the actual load normalized by the total requested bits at a transmission rate specified by degrees of freedom (DoF) of the given channel. We obtain achievable upper bounds of the minimum FDT for the considered network with N T (≥ 2) transmitters and N R = 2 or N R = 3 receivers. We also obtain a theoretical lower bound of the minimum FDT for any N T transmitters and N R receivers. Both upper and lower bounds are convex and piece-wise linear decreasing functions of transmitter and receiver cache sizes. The achievable bound is partially optimal and is at most NT +1 NT and NT +2 NT times of the lower bound in the considered N T × 2 and N T × 3 network, respectively. To show the achievability, we propose a novel cooperative transmitter/receiver coded caching strategy. It offers the freedom to adjust file splitting ratios for FDT minimization. Based on this caching strategy, we then design the delivery phase carefully to turn the considered network opportunistically into more favorable channels, including X channel, broadcast channel, multicast channel, or a hybrid form of these channels. Receiver local caching gain, coded multicasting gain, and transmitter cooperation gain (interference alignment and interference neutralization) are thus leveraged in different cache size regions and different channels. Wireless cache network, coded caching, content delivery, multicast, and interference management. This paper will be presented in part at IEEE ISIT 2016. freedom (DoF) and clustered cooperative beamforming in cellular networks with edge caching via a hypergraph coloring problem. The authors in [19] studied the transmitter cache strategy in the cache-aided interference channel under an information-theoretical framework. It is shown that splitting contents into different parts and caching each part in different transmitters can turn the interference channel into X channel, broadcast channel, or hybrid channel and hence increase the system throughput via interference management. The authors in [20] presented a lower bound of delivery latency in a general interference network with transmitter cache and showed that the scheme in [19] is optimal in certain region of cache size. The above literature reveals that caching at the receiver side can bring local caching gain and coded multicasting gain, and that caching at the transmitter side can induce transmitter cooperation for interference management and load balancing. It is thus of theoretical importance and practical interest to investigate the impact of caching at both transmitter and receiver sides. In this paper, we aim to study the fundamental limits of caching in a general wireless interference network with caches equipped at all transmitters and receivers as shown in Fig. 1 . The performance metric to characterize the gains of caching varies in the existing works. For the broadcast channel with receiver cache, the authors in [8] characterized the gain by memory-rate tradeoff, where the rate is defined as the load of the shared link normalized by the file size in the delivery phase. For the interference channel with transmitter cache, the authors in [19] characterized the gain by the standard DoF from the information-theoretic studies in the delivery phase. In [20] , the authors proposed the storage-latency tradeoff, where the latency is defined as the relative delivery time with respect to an ideal baseline system with unlimited cache and no interference in the high signal-to-noise ratio (SNR) region. In our considered wireless interference network with both transmitter and receiver caches, the standard DoF is unable to capture the potential reduction in the traffic load due to receiver caching, and the rate is unable to capture the potential DoF enhancement due to cache-induced transmitter cooperation. Interestingly, the latency-oriented performance metric introduced in [20] can reflect not only the load reduction due to receiver cache but also the DoF enhancement due to transmitter cache, since it evaluates the delivery time of the actual load at a transmission rate specified by the given DoF. As such, we adopt the storage-latency tradeoff to characterize the fundamental limits of caching in this work. In specific, we measure the performance by the fractional delivery time (FDT), denoted as τ (µ R , µ T ), which is a function of the normalized receiver cache size µ R and the normalized
The study in REF reveals that, with a novel cooperative transmitter and receiver caching strategy, the interference network can be turned opportunistically into more favorable channels, including X channel, broadcast channel, multicast channel, and a hybrid form of these channels.
Fundamental Tradeoff between Storage and Latency in Cache-Aided Wireless Interference Networks
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