IEEE TRANSACTIONS ON COMMUNICATIONS, VOL. XX, NO. XX, MONTH 20XX 1 Double-Loop Receiver-Initiated MAC for Cooperative Data Dissemination via Roadside WLANs Hao Liang, Student Member, IEEE, and Weihua Zhuang, Fellow, IEEE Abstract—In this paper, we investigate data dissemination in delay tolerant networks (DTNs) via roadside wireless local area networks (RS-WLANs). The data dissemination service is destined to a group of nomadic nodes roaming in a large network region with a low node density. The local nodes within the cover- age area of an RS-WLAN can provide packet caching and relay- ing capabilities. We consider a cooperative data dissemination approach where information packets are first pre-downloaded to the local nodes within the RS-WLAN before the visit of a pedestrian nomadic node, and then opportunistically scheduled to transmit to the nomadic node upon its arrival. In order to resolve the channel contention among multiple direct/relay links and exploit the predictable traffic characteristics as a result of packet pre-downloading, a double-loop receiver-initiated medium access control (DRMAC) scheme is proposed. The MAC scheme can achieve spatial and temporal diversity via the outer-loop and inner-loop MAC, respectively. A receiver initiated mechanism is used to reduce the signalling overhead, where the ACK message is used as an invitation of channel contention. An analytical model is established to evaluate the performance of the proposed MAC scheme. Numerical results demonstrate that the proposed MAC scheme can significantly improve the number of delivered packets from an RS-WLAN to a nomadic node as compared with the existing MAC schemes. Index Terms—Cooperation, data dissemination service, delay tolerant network (DTN), diversity, medium access control (MAC), wireless local area network (WLAN). I. I NTRODUCTION Data dissemination services in wireless networks have been widely studied because of the intensive service demands such as traffic information downloading, entertainment content distribution, and commercial advertising [1]–[3]. The existing services are typically generated by a content server in the Internet and destined to a group of nomadic nodes roaming in the network region. Since the data dissemination services provided by wireless wide area networks such as GPRS and 3G cellular networks suffer from high capital expenditure and low speed, extensive research has been carried out to exploit the roadside WiFi access points (APs) for data delivery. For instance, the Drive-thru Internet architecture addresses the Paper approved by B. Sikdar, the Editor for Networking and Cross-Layer Design of the IEEE Communications Society. Manuscript received November 30, 2011; revised May 1, 2012. The authors are with the Department of Electrical and Computer Engineer- ing, University of Waterloo, 200 University Avenue West, Waterloo, Ontario, Canada N2L 3G1 (e-mail: {h8liang, wzhuang}@uwaterloo.ca). This paper was presented in part at the IEEE Global Communications Conference (GLOBECOM), Miami, FL, USA, December 2010. Digital Object Identifier XXXX/TCOMM.XXXX short-term network access problem when a mobile user walks, drives, or passes (by other means) through the coverage area of an AP [4]. To improve the availability of the roadside APs, the notion of wireless metropolitan area sharing network (WMSN) is introduced, where publicly and/or privately owned wireless local area networks (WLANs) are shared [5]. Following the same concept as WMSN, FON has successfully established a business model to stimulate the sharing of the WLANs [6]. Based on two kinds of incentives, i.e., direct payment and co- operative sharing among FON subscribers, the nomadic nodes can obtain permission to access the privately owned roadside WLANs (RS-WLANs) which are typically deployed at the roadside restaurants, cafes, and residential houses. However, in a rural area and/or an urban area with a low market penetration rate, the densities of the shared RS-WLANs as well as the nomadic nodes are low such that an end-to-end path from an AP to a nomadic node can hardly be established. Moreover, the data rate of the wireline connection between an AP and the content server may be limited 1 , which further restricts the packet delivery rate from an RS-WLAN to a nomadic node. To achieve efficient data dissemination in such a sparse network or delay tolerant network (DTN) [9], a cooperative approach (also referred to as DTCoop [10]) can be used, in which not only the AP within each RS-WLAN is shared, but also the local nodes connected to the AP can provide packet caching and relaying capabilities. Information packets are first pre-downloaded to a group of storage local nodes within an RS-WLAN. Upon the arrival of a nomadic node in the RS-WLAN, both direct links from storage local nodes and relay links via non-storage local nodes can be used for packet delivery. Although packet pre-downloading (or pre- fetching) has been investigated in literature to address the bandwidth limitation of the wireline connection between an AP and the content server [8], [10]–[12], how to efficiently schedule packet transmissions among multiple direct/relay links for potential cooperative diversity gain is still an open issue. Specifically, an efficient medium access control (MAC) scheme should be designed to exploit both spatial and temporal diversity in the cooperative communication paradigm for a high packet delivery rate. At the link layer, spatial diversity can be achieved by scheduling the local node with the highest average transmission rate to transmit, based on the geographic 1 For instance, the rate can be limited to a few Mbps for some residential Internet service subscribers, which is much lower than the maximum rate of the wireless connection (e.g., 54 Mbps for IEEE 802.11a based RS- WLANs) [7] [8].