We consider the use of distributed storage (DS) to reduce the communication cost of content delivery in wireless networks. Content is stored (cached) in a number of mobile devices using an erasure correcting code. Users retrieve content from other devices using device-to-device communication or from the base station (BS), at the expense of higher communication cost. We address the repair problem when a device storing data leaves the cell. We introduce a repair scheduling where repair is performed periodically and derive analytical expressions for the overall communication cost of content download and data repair as a function of the repair interval. The derived expressions are then used to evaluate the communication cost entailed by DS using several erasure correcting codes. Our results show that DS can reduce the communication cost with respect to the case where content is downloaded only from the BS, provided that repairs are performed frequently enough. If devices storing content arrive to the cell, the communication cost using DS is further reduced and, for large enough arrival rate, it is always beneficial. Interestingly, we show that MDS codes, which do not perform well for classical DS, can yield a low overall communication cost in wireless DS.Index Terms-Caching, content delivery, device-to-device communication, distributed storage, erasure correcting codes. 2 general and can be used to analyze the overall communication cost incurred by any erasure correcting code for DS. As an example of the application of the proposed framework, we analyze the overall communication cost incurred by MDS codes, regenerating codes [10], and locally repairable codes (LRCs) [11]. We show that wireless DS can reduce the overall communication cost as compared to the basic scenario where content is only downloaded from the BS. However, this is provided that repairs can be performed frequently enough. Moreover, in the case when nodes storing content arrive to the cell, the communication cost using DS is further reduced and, for large enough arrival rate, it is always beneficial as compared to BS download. The repair interval that minimizes the overall communication cost depends on the network parameters and the underlying erasure correcting code. We show that, in general, instantaneous repair is not optimal. The derived expressions can also be used to find, for a given repair interval, the erasure correcting code yielding the lowest overall communication cost.Non-instantaneous repairs, the so-called "lazy" repairs, have already been proposed for DS in data centers [12], [13] to reduce the amount of data that has to be transmitted within the storage network during the repair process, known as the repair bandwidth. However, contrary to [12], [13], in the wireless scenario considered here the non-instantaneous repairs impact both data repair and download. We show that, somewhat interestingly, erasure correcting codes achieving a low repair bandwidth do not always perform well in a wireless DS setting. On the other hand, MDS codes, ...
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