Periodic Reporting for period 1 - DISC (Distributed storage based on sparse-graph codes)
Berichtszeitraum: 2015-05-01 bis 2017-04-30
In this context, wireless caching has emerged as a powerful technique to overcome the backhaul bottleneck, by reducing the backhaul rate and the delay in retrieving content from the network. The key idea is to store popular content closer to the end users. Recently, a novel system architecture named femtocaching was proposed. It consists of deploying a number of small base stations (BSs) with large storage capacity, in which content is stored during periods of offpeak traffic. The mobile users can download content from the small BSs, resulting in a higher throughput per user. It was also proposed to store content directly in the mobile devices. Users can then retrieve content from neighbouring devices using device-to-device (D2D) communication or, alternatively, from the serving BS. The data loss in these systems happens if one or multiple storage nodes fail or leave the network, or if the fading channel characteristics make them unavailable.
In both scenarios, content may be stored using an erasure correcting code, which brings gains with respect to uncoded caching. The use of erasure correcting codes establishes an interesting link between distributed caching for content delivery and distributed storage (DS) for reliable data storage. The key difference is that in the wireless network scenario, data can be downloaded from the storage nodes (the small BSs or the mobile devices) but also from a serving macro BS, which has always the content available. Therefore, the reliability requirements in DS for reliable data storage can be relaxed.
The main objective of this project consists in evaluate the advantages of coded distributed caching with respect to the classical scenario where content is always downloaded from the BS.
Furthermore, we consider the design of large-scale cellular network using distribute caching. We study a network model that is closer to a large-scale network, where the cell is divided into clusters where D2D links can be activated in order to increase the spatial reuse, and hence to reduce the latency. In this context, we consider more practical aspects, such as the inter and intra-cluster interference, the fact that users may request files, of different popularity, from a library of files, and that the D2D link is not ideal. The main results of the project are collected in [5] and [6].
The novelty of project lies in the fact that for the first time the advantage of distributed caching for wireless content delivery has been analysed in terms of latency, that is a performance measure of very practical interest.
The main achieved scientific objective consists in showing that coded distributed caching can greatly improve the performance in terms of content download delay with respect to the case where content is downloaded from the BS, provided that the BS broadcasts the network information frequently enough. Interestingly, the results of the project also show that the performance improves when the length of the code increases. In particular, simple replication is very inefficient and much better performance are achieved using larger codes (of the same rate).
References:
[1] V. Bioglio, F. Gabry, and I. Land, “Optimizing MDS codes for caching at the edge,” in Proc. IEEE Global Commun. Conf. (GLOBECOM), San Diego, CA, 2015.
[2] J. Pääkkönen, C. Hollanti, and O. Tirkkonen, “Device-to-device data storage for mobile cellular systems,” in Proc. IEEE Global Commun. Conf. (GLOBECOM), 2013.
[3] J. Pedersen, A. Graell i Amat, I. Andriyanova, and F. Brännström, “Repair scheduling in wireless distributed storage with D2D communication,” in Proc. IEEE Inf. Theory Work. (ITW), Jeju Island, Korea, 2015.
[4] J. Pedersen, A. Graell i Amat, I. Andriyanova, and F. Brännström, “Distributed storage in mobile wireless networks with device-to-device communication,” IEEE Trans. Commun., vol. 64, no. 11, pp. 4862–4878, Nov. 2016.
[5] A. Piemontese and A. Graell i Amat, “MDS-coded distributed storage for low delay wireless content delivery,” in Proc. Int. Symp. Turbo Codes & Iterative Inform. Proc. (ISTC), Brest, France, Sep. 2016.
[6] A. Piemontese and A. Graell i Amat, “MDS-coded distributed caching for low delay wireless content delivery,” submitted to IEEE Trans. Commun.. Available on arXiv.