Efficient processing of requests with network coding in on-demand data broadcast environments Jun Chen a,⇑ , Victor C.S. Lee b , Kai Liu b , G.G.M.N. Ali b , Edward Chan b a School of Information Management, Wuhan University, Wuhan, Hubei, China b Department of Computer Science, City University of Hong Kong, Kowloon, Hong Kong article info Article history: Received 16 September 2011 Received in revised form 27 December 2012 Accepted 31 December 2012 Available online 23 January 2013 Keywords: Network coding Mobile computing On-demand broadcast Data scheduling abstract On-demand broadcast is an effective wireless data dissemination technique to enhance system scalability and the ability to handle dynamic user access patterns. In traditional on-demand broadcast, only one data item can be retrieved by mobile clients during the course of each broadcast, which limits bandwidth utilization and throughput. In this paper, we consider data broadcast with network coding in on-demand broadcast environments. We analyze the coding problem in on-demand broadcast and transform it into the problem of finding the maximum clique in graph theory. Based on our analysis, we first propose a new coding strategy called AC, which exploits the cached information related to clients and data items requested by them, to implement a flexible coding mechanism. Then, based on AC, we propose two novel coding assisted algorithms called ADC-1 and ADC-2 which con- sider data scheduling, in addition to network coding. In ADC-1 data scheduling and coding are considered separately, while these two factors are fully integrated in ADC-2. The per- formance gain of our proposed algorithms over traditional and other coding assisted broad- cast algorithms is demonstrated through simulation results. Our algorithms not only reduce request response time but also utilize broadcast channel bandwidth more efficiently. Ó 2013 Elsevier Inc. All rights reserved. 1. Introduction Data broadcast has attracted many academic researchers’ attention as it has been increasingly used to disseminate infor- mation to large populations of mobile clients in many new mobile applications, such as location based services, where effi- cient data broadcast is critical to the system performance [14,31]. In general, there are two major data broadcast approaches [8,9]: (a) push-based and (b) pull-based. Push-based broadcast periodically broadcasts data according to a static schedule which is computed offline from clients’ historical data access statistics. Pull-based broadcast, commonly referred to as on- demand broadcast, compiles requests in the service queue and broadcasts data based on various attributes of pending data items at the server. Push-based broadcast is efficient with applications which require a small set of data items with stable access pattern, while on-demand broadcast is more widely used for dynamic, large-scale data dissemination [15,18]. In this paper, we focus our discussion on on-demand broadcast, where response time is one of the most important metrics to mea- sure the system performance [25]. In existing on-demand broadcast strategies, mobile users can retrieve only one data item from each broadcast unit. This constraint restricts full utilization of the limited broadcast bandwidth and leads to long response time to mobile clients. 0020-0255/$ - see front matter Ó 2013 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.ins.2012.12.044 ⇑ Corresponding author. E-mail addresses: christina.cj@hotmail.com (J. Chen), csvlee@cityu.edu.hk (V.C.S. Lee), kevin.liu@cityu.edu.hk (K. Liu), gnawazali2@student.cityu.edu.hk (G.G.M.N. Ali), csedchan@cityu.edu.hk (E. Chan). Information Sciences 232 (2013) 27–43 Contents lists available at SciVerse ScienceDirect Information Sciences journal homepage: www.elsevier.com/locate/ins