Computer Communications 83 (2016)45-55 Contents lists available at ScienceDirect computer cmmuncatons Computer Communications ELSEVIER journal homepage:www.elsevier.com/locate/comcom Joint storage assignment for D2D offloading systems CrossMark Wei Wang",Xiaobing Wu,Lei Xie,Sanglu Lu State Key Laboratory for Novel Software Technology.Nanjing University.Nanjing 210093.China ARTICLE INFO ABSTRACT Article history: D2D offloading reduces the load of cellular network by asking mobile nodes to download content directly Received 3 September 2014 from storage of neighboring helpers via short range links.In this paper.we introduce a novel storage Revised 18 December 2015 Accepted 22 February 2016 assignment scheme that can enhance storage utilization for D2D networks that have different types of Available online 3 March 2016 storage nodes.Unlike traditional D2D systems that only use storage as static content cache,our scheme uses on-demand relaying to enhance storage utilization.Our on-demand relaying scheme replicates rare Keywords: content when it is requested.Therefore,the proposed scheme can greatly increase the amount of con- Device-to-Device communication tent supported by the offloading system.We develop a convex optimization based algorithm to find the DTN optimal storage assignment tradeoff between static caching and on-demand relaying.Numerical results Opportunistic networks and real-world trace-driven simulations show that our algorithm can achieve 30%reduction in offloading failure rate compared to static schemes. 2016 Elsevier B.V.All rights reserved. 1.Introduction starts downloading the movie directly through the cellular network 5.To make offloading successful,the movie must be cached in at Mobile traffic has increased at a compound annual growth rate least one of the helpers in a small virtual region that Alice can of more than 70%in recent years [1].Meeting such a large surge contact before she becomes impatient.The size of the "region"is in traffic demand is a great challenge for both cellular network de- determined by the user mobility pattern and the time that Alice signers and operators.One way to relieve the pressure on current can wait.Because the total storage contributed by helpers in this cellular networks is to offload part of the mobile traffic through virtual region is limited,the offloading system can only provide a Device-to-Device (D2D)communication [2.Because most mobile limited amount of content to Alice.Even if the offloading system traffic comes from content downloading [1].one of the impor- covers a large area and has a huge number of helpers,it can only tant offloading targets is mobile content,which includes applica- offload a constant amount of content due to the locality of the con- tion updates and video clips.In D2D offloading systems for con- tent in the static caching scheme. tent downloading,mobile helpers serve as content caches for other In this paper,we improve the scalability of D2D systems by in- nodes.These helpers directly deliver content to neighboring de- troducing different roles to mobile helpers.We assign some of the vices through short-range and low-cost wireless links (e.g.,Blue- mobile helpers as on-demand relays instead of static caches,which tooth,WiFi Direct or LTE D2D links [3]).In this way,the download- are referred to as seeds in this paper.Due to the inherent non- ing traffic does not pass through base stations and core networks uniformity in the contact pattern,relays may have a higher con- so that the limited resource of cellular networks can be saved [4. tact probability to the subscriber than seeds.For example,Alice Existing D2D offloading systems "statically"assign content to may request her friends to act as mobile relays to download the mobile helpers based on content popularity [5.6].Subscribers need movie.Although it might be difficult for Alice to directly contact to directly contact mobile helpers that have the content of interest a seed,there will be a higher chance that one of her friends can in their cache.Such static caching schemes have limited scalabil- successfully download the movie.Afterwards,Alice can download ity with respect to network size.Consider the case where a sub- the movie from her friends when she intentionally or accidentally scriber called Alice wishes to download a movie through the D2D meets them.In this way.the number of helpers that Alice can con- offloading system.Due to limited mobility.Alice can only contact a tact is effectively enlarged with the help of her friends. small number of mobile helpers before she becomes impatient and In addition to using friends as relays,mobile nodes mounted on buses or static nodes deployed at public places (e.g..entrance of subways)can also act as relay nodes.These types of nodes have Corresponding author.Tel:+86 13851658076. regular contact patterns to a given subset of subscribers.For exam- E-mail addresses:ww@nju.edu.cn,wangwei.ww@gmail.com (W.Wang). ple,a subscriber may pass by a given subway station every day.If wuxb@nju.edu.cn (X.Wu).Ixie@nju.edu.cn (L Xie).sanglu@nju.edu.cn (S.Lu). http://dx.doiorg/10.1016/j.comcom.2016.02.012 0140-3664/6 2016 Elsevier B.V.All rights reserved.Computer Communications 83 (2016) 45–55 Contents lists available at ScienceDirect Computer Communications journal homepage: www.elsevier.com/locate/comcom Joint storage assignment for D2D offloading systems Wei Wang∗ , Xiaobing Wu, Lei Xie, Sanglu Lu State Key Laboratory for Novel Software Technology, Nanjing University, Nanjing 210093, China a r t i c l e i n f o Article history: Received 3 September 2014 Revised 18 December 2015 Accepted 22 February 2016 Available online 3 March 2016 Keywords: Device-to-Device communication DTN Opportunistic networks a b s t r a c t D2D offloading reduces the load of cellular network by asking mobile nodes to download content directly from storage of neighboring helpers via short range links. In this paper, we introduce a novel storage assignment scheme that can enhance storage utilization for D2D networks that have different types of storage nodes. Unlike traditional D2D systems that only use storage as static content cache, our scheme uses on-demand relaying to enhance storage utilization. Our on-demand relaying scheme replicates rare content when it is requested. Therefore, the proposed scheme can greatly increase the amount of con￾tent supported by the offloading system. We develop a convex optimization based algorithm to find the optimal storage assignment tradeoff between static caching and on-demand relaying. Numerical results and real-world trace-driven simulations show that our algorithm can achieve 30% reduction in offloading failure rate compared to static schemes. © 2016 Elsevier B.V. All rights reserved. 1. Introduction Mobile traffic has increased at a compound annual growth rate of more than 70% in recent years [1]. Meeting such a large surge in traffic demand is a great challenge for both cellular network de￾signers and operators. One way to relieve the pressure on current cellular networks is to offload part of the mobile traffic through Device-to-Device (D2D) communication [2]. Because most mobile traffic comes from content downloading [1], one of the impor￾tant offloading targets is mobile content, which includes applica￾tion updates and video clips. In D2D offloading systems for con￾tent downloading, mobile helpers serve as content caches for other nodes. These helpers directly deliver content to neighboring de￾vices through short-range and low-cost wireless links (e.g., Blue￾tooth, WiFi Direct or LTE D2D links [3]). In this way, the download￾ing traffic does not pass through base stations and core networks so that the limited resource of cellular networks can be saved [4]. Existing D2D offloading systems “statically” assign content to mobile helpers based on content popularity [5,6]. Subscribers need to directly contact mobile helpers that have the content of interest in their cache. Such static caching schemes have limited scalabil￾ity with respect to network size. Consider the case where a sub￾scriber called Alice wishes to download a movie through the D2D offloading system. Due to limited mobility, Alice can only contact a small number of mobile helpers before she becomes impatient and ∗ Corresponding author. Tel.: +86 13851658076. E-mail addresses: ww@nju.edu.cn, wangwei.ww@gmail.com (W. Wang), wuxb@nju.edu.cn (X. Wu), lxie@nju.edu.cn (L. Xie), sanglu@nju.edu.cn (S. Lu). starts downloading the movie directly through the cellular network [5]. To make offloading successful, the movie must be cached in at least one of the helpers in a small virtual region that Alice can contact before she becomes impatient. The size of the “region” is determined by the user mobility pattern and the time that Alice can wait. Because the total storage contributed by helpers in this virtual region is limited, the offloading system can only provide a limited amount of content to Alice. Even if the offloading system covers a large area and has a huge number of helpers, it can only offload a constant amount of content due to the locality of the con￾tent in the static caching scheme. In this paper, we improve the scalability of D2D systems by in￾troducing different roles to mobile helpers. We assign some of the mobile helpers as on-demand relays instead of static caches, which are referred to as seeds in this paper. Due to the inherent non￾uniformity in the contact pattern, relays may have a higher con￾tact probability to the subscriber than seeds. For example, Alice may request her friends to act as mobile relays to download the movie. Although it might be difficult for Alice to directly contact a seed, there will be a higher chance that one of her friends can successfully download the movie. Afterwards, Alice can download the movie from her friends when she intentionally or accidentally meets them. In this way, the number of helpers that Alice can con￾tact is effectively enlarged with the help of her friends. In addition to using friends as relays, mobile nodes mounted on buses or static nodes deployed at public places (e.g., entrance of subways) can also act as relay nodes. These types of nodes have regular contact patterns to a given subset of subscribers. For exam￾ple, a subscriber may pass by a given subway station every day. If http://dx.doi.org/10.1016/j.comcom.2016.02.012 0140-3664/© 2016 Elsevier B.V. All rights reserved