In this paper, we consider the coded-caching broadcast network with user cooperation, where a server connects with multiple users and the users can cooperate with each other through a cooperation network. We propose a centralized coded caching scheme based on a new deterministic placement strategy and a parallel delivery strategy. It is shown that the new scheme optimally allocate the communication loads on the server and users, obtaining cooperation gain and parallel gain that greatly reduces the transmission delay. Furthermore, we show that the number of users who parallelly send information should decrease when the users' caching size increases. In other words, letting more users parallelly send information could be harmful. Finally, we derive a constant multiplicative gap between the lower bound and upper bound on the transmission delay, which proves that our scheme is order optimal. Index TermsCache, cooperation, delay I. INTRODUCTIONDue to the continuous growth of traffic in the network and the growing needs for higher Internet speed from users, it's imperative to improve the performance of the network. One of the promising directions to improve the quality of service is to utilize the cache memories in the network. In [1] Maddah-Ali and Niesen proposed a novel scheme, namely coded caching scheme, to improve the transmission efficiency of each transmission. It obtains a global caching gain by creating multicasting opportunities for different users. This new class of caching system has attracted significant interests [2]- [7].To further improve the quality of service, one can combine caching with user cooperation. It is particularly common and useful in fog network [8], where the edge users can carry out some amount of communication. This can include, for example, device-to-device (D2D) networks and ad-hoc networks. In [9], coded caching schemes were proposed for a D2D noiseless network with the absence of server. In [10], the caching problem on a two-user D2D wireless network with the presence of a server was studied. In [12], a maximum distance separable (MDS) coded caching scheme was proposed to reduce the communication load in highly-dense wireless networks considering device mobility.In this paper, we study a K-user (K ≥ 2) coded-caching broadcast network with user cooperation. In this network, a server connects with all users through a noiseless shared link, and the users can communicate with each other through a noiseless cooperation network. The cooperation network is parameterized by a positive integer α max ∈ {1, . . . , K/2 }, denoting the maximum number of users allowed to parallelly send data in the cooperation network. For example, when α max equals to 1, the cooperation network operates as a simple shared link connecting with all users, which is easy and low-cost to implement in fog network. The main contributions are summarized below.• We propose a novel coded caching scheme that fully exploits user cooperation and optimally allocates communication loads between the server and users. The sche...
Caching technique is a promising approach to reduce the heavy traffic load and improve user latency experience for the Internet of Things (IoT). In this paper, by exploiting edge cache resources and communication opportunities in device-to-device (D2D) networks and broadcast networks, two novel coded caching schemes are proposed that greatly reduce transmission latency for the centralized and decentralized caching settings, respectively. In addition to the multicast gain, both schemes obtain an additional cooperation gain offered by user cooperation and an additional parallel gain offered by the parallel transmission among the server and users. With a newly established lower bound on the transmission delay, we prove that the centralized coded caching scheme is order-optimal, i.e., achieving a constant multiplicative gap within the minimum transmission delay. The decentralized coded caching scheme is also order-optimal if each user’s cache size is larger than a threshold which approaches zero as the total number of users tends to infinity. Moreover, theoretical analysis shows that to reduce the transmission delay, the number of users sending signals simultaneously should be appropriately chosen according to the user’s cache size, and always letting more users send information in parallel could cause high transmission delay.
We consider cache-aided broadcast networks with user cooperation, where a server connects with multiple users and the users can cooperate with each other through a cooperation network. A new definition of transmission delay is introduced to characterize the latency cost during the delivery phase for arbitrary cache-aided networks. We investigate the deterministic caching and decentralized random caching setups respectively. For the deterministic caching setup, we propose new coded caching scheme that fully exploits time resource by allowing parallel transmission between the server and users. A constant multiplicative gap is derived between the information theoretic lower and upper bounds on the transmission delay. For the decentralized random caching setup, we show that if the cache size of each user is larger than a small threshold that tends to zero as the number of users goes to infinity, then the proposed decentralized coded caching scheme approaches an upper bound with a constant multiplicative factor. For both centralized and decentralized scenarios, we characterize cooperation gain (offered by the cooperation among the users) and parallel gain (offered by the parallel transmission between the server and multiple users) that greatly reduce the transmission delay. Furthermore, we show that due to a tradeoff between the parallel gain, cooperation gain and multicast gain, the number of users who parallelly send information should be chosen dynamically according to the system parameters, and letting more users parallelly send information could cause high transmission delay.
Ocean tide loading (OTL) displacements are sensitive to the shallow structure of the solid Earth; hence, the high-resolution spatial pattern of OTL displacement can provide knowledge to constrain the shallow Earth structure, especially in coastal areas. In this study, we investigate the sensitivity of the modeled M2 OTL displacement over Taiwan Island to perturbations of three physical quantities, namely, the density, bulk modulus, and shear modulus in the upper mantle and crust. Then, we compare the sensitivity of the modeled M2 OTL displacement to Earth models with the sensitivity to ocean tide models using root mean square (RMS) differences. We compute the displacement Green’s function and OTL displacement relative to the center of mass of the solid Earth (CE) reference frame, analyze the sensitivity to the three physical quantities in the CRUST1.0 model and the Preliminary Reference Earth Model (PREM), and present their spatial patterns. We find that displacement Green’s functions and OTL displacements are more sensitive to the two elastic moduli than the density in the upper mantle and crust. Moreover, their distinctive sensitivity patterns suggest that the three physical quantities might be constrained independently. The specific relationships between the perturbed structural depths and the distance ranges of peak sensitivities from the observation points to the coastline revealed by the shear modulus can mitigate the nonuniqueness problem in inversion. In particular, the horizontal tidal components observed by the Global Positioning System (GPS) can yield better results in inversions than the vertical component owing to the smaller OTL model errors and the higher structural sensitivity (except for the shear modulus in the asthenosphere).
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