1 Assisted-Multicast Scheduling in Wireless Cognitive Mesh Networks Hisham M. Almasaeid and Ahmed E. Kamal Dept. of Electrical and Computer Eng., Iowa State University, Ames, IA 50011, USA E-mail:{hisham,kamal}@iastate.edu Abstract—In this work, we consider the multicast problem in a single cell in a cognitive mesh network. Due to the potential heterogeneity in channel availability among the members of a multicast group(s), the total multicast time could be longer due to transmitting the multicast data over multiple channels. We propose, in this work, an assisted multicast strategy with the objective of minimizing the total multicast time. This assistance is composed of two main activities, first, allowing the receivers in a multicast group to forward the data they have received to other members of the multicast group(s), and second, allowing the transmission of coded (bitwise XORed) packets so that receivers belonging to different multicast groups can decode and extract their data concurrently. We show, in this paper, that the proposed assistance paradigm achieves a considerable reduction in the total multicast time, which in turn increases the system throughput. I. I NTRODUCTION Empowered by the technology of software-defined radios [1], cognitive radio networks have emerged as a solution for spectrum underutilization [2]. The new technology allows cognitive radio users, usually referred to as secondary users (SUs), to opportunistically utilize unused licensed spectrum provided that they vacate a channel once a licensed user, usually referred to as a primary user (PU), starts using it. As the channel availability is both time and location dependent, SUs may observe heterogeneous sets of idle channels. This heterogeneity raises a number of challenges to the operation of cognitive radio networks. Besides the channel sensing problem (or equivalently PU detection), transmission coordination is one of the essential and most challenging functionalities in cognitive radio networks. The channel sensing problem has been the focus of mainstream research in the area of cognitive radio networks. Cooperative sensing [3] and sensor network- aided-sensing [4], [5], where an infrastructure sensor network takes over the task of channel sensing, are two promising solutions for the PU detection problem. On the other hand, the most adopted solution for the transmission coordination problem is the use of a common control channel between all SUs on which the coordination takes place. In this paper, we are interested in the multicast problem in wireless cognitive mesh networks [6]. The property of heterogenous channel availability in cognitive radio networks may cause the multicast process to take place over multiple channels, causing longer multicast periods and consequently lower throughput. We study, in this work, the problem of minimizing the total multicast time in a single cell of a cognitive mesh network by scheduling the multicast activity over both time and frequency and by also using the tech- nique of network coding [7]. For this purpose, we propose a multicast mechanism that relies on three operations. The first operation is called assistance, in which some of the receiving members of a mutlicast group assist in the multicast process by forwarding the data to other members of their group (or other groups). The second operation is called overhearing, in which some receiving members of a multicast group overhear the data destined to another group. This operation has two advantages; it first enables the inter-group assistance (forward- ing) between different multicast groups, and also enables the delivery of multiple packets to different groups at the same time by using the third operation; the codeword exchange operation. In the codeword exchange operation, coded packets (bitwise XORed packets) ,which we refer to as codewords, are used in the assistance operation so that members of different multicast groups can decode and extract their own data using packets they overheard from previous transmissions. Formal definitions of these operations will be given in Section III-A. A very limited number of studies have addressed the multicast problem in cognitive radio networks. In [8], a multicast-tree construction protocol was proposed for mobile ad hoc networks. Video multicast in infrastructure cognitive radio networks was studied in [9]. The cognitive base station chooses the multicast channels opportunistically such that the collision probability with the primary system is minimized, and the unused spectrum is exploited as much as possible. In both works, there was no notion of assistance whether by multicast receivers or by the use of network coding. The rest of this paper is organized as follows. The system model is presented in Section II. In Section III, we formally define the assisted multicast scheduling problem and present some motivational examples. Then, in Section IV, we elaborate on the problem complexity and propose ILP formulations for unassisted multicast scheduling and assisted multicast scheduling problems. A heuristic approach to solve the assisted multicast scheduling problem is proposed in Section V. We evaluate the performance of the proposed scheme in Section VI, and conclude in Section VII. II. SYSTEM MODEL We consider a wireless cognitive mesh network that consists of a number of mesh routers (MRs) connected over multiple hops to a gateway node(s) that provides access to the backbone network. Each MR manages a number of mesh client (MCs)