Spatio-Temporal Schedulers in IEEE 802.16 Sara Motahari, Ehsan Haghani, Shahrokh Valaee Department of Electrical and Computer Engineering, University of Toronto Email: {Sara, Ehsan, Valaee}@comm.utoronto.ca Abstract— With the growing interest in Broadband Wireless Access (BWA) and demand for mobile high-speed connection, there is a need to extend wireless connectivity to passengers travelling in highspeed vehicles. In this paper, we use the IEEE 802.16 standard as a backhaul communication technology for broadband wireless access to railway systems. The proposed architecture uses relay elements located in the vicinity of train track to repeat the signal between the base station and the mobile vehicle. The signal transmitted from the base station is received by repeaters and relayed to the train, and vice versa. We propose spatio-temporal scheduling as a means to increase downlink throughput. The proposed spatio-temporal scheduler distributes data traffic among the repeaters in the vicinity of the train and on the route of the train. Simulation results show that a substantial improvement can be obtained when data are scheduled in both temporal and spatial dimensions. I. I NTRODUCTION Ho and Valaee [1] [2] have recently proposed a link-layer solution for wireless communications to mass transportation systems such as trains, subways, and busses. In their architec- ture, a number of relay elements are placed along the track of the railway system acting as the repeaters between the base station and the train. They decompose data into smaller frag- ments transmitted towards train via multiple repeaters. They use erasure coding to secure data to fragment loss. They call this method information raining at which fragments of data are rained upon the moving vehicle from adjacent repeaters. Unfortunately, they only concentrate on the transmission link between the repeaters and the vehicle and do not investigate the backhaul link operating between the base station and the repeaters. In this paper, we use the IEEE 802.16 in the backhaul link. The wireless MAN standard, IEEE 802.16 [3], was origi- nally developed as a point to multipoint protocol to provide broadband connectivity for a large number of wireless users. Because of its adaptive physical and MAC layer characteristics and its superior performance, IEEE 802.16 promises robust and flexible high speed wireless communications [4]. How- ever, the details of switching among different modulation and coding schemes are left open for the vendors. In this paper, we will use the IEEE 802.16 standard to facilitate backhaul communication between the base station and relay elements located along the track. We will also use the adaptation algorithm of [5] to switch among the available modulations. The present paper proposes scheduling algorithms for down- link communication that can be used in base station to maximize total system throughput, defined as the number of bits received by the train. In particular, we will intro- duce the concept of spatio-temporal scheduling for downlink Zone Controller Internet Vehicle Station Internal Repeater Repeaters Vehicle Antenna Base Station R e pe a te r s IEEE 802.16 Fig. 1. Network Structure communication between the base station and the repeaters. A spatio-temporal scheduler transmits data in both temporal and spatial dimensions. If the direction and the speed of the mobile vehicle is known, the scheduler can transmit data to a repeater on the route of the vehicle. In Section III, we will introduce a novel scheduling algorithm to increase the system throughput and will show that in a highly loaded network, the proposed scheduler substantially decreases the total delay while increasing the throughput. II. SYSTEM ARCHITECTURE Mass transportation vehicles, such as trains, subways, and busses, usually have large physical size that allows them to carry complex equipment for networking without significant space or power restriction. The system architecture proposed in [2] for wireless communication to such vehicles is illustrated in Fig. 1. It is assumed that simple, cost-effective relay elements—called repeaters—are located along the trackside of trains, routes of busses, or inside subway tunnels. The relay elements repeat the signal arriving from the base station to the train, and vice versa. It is also assumed that multiple antennas are mounted on the exterior of the vehicle as illustrated in the figure. These antennas can be powerful antennas that provide line-of-sight between the mobile vehicle and adjacent repeaters. The antennas and repeaters located in the vicinity of the vehicle create a bipartite graph. A proper matching can be used to tune each antenna to a corresponding repeater. Several heuristics for such matchings have been proposed in [2]. The architecture proposed in [2] comprises two main com- ponents: zone controller (ZC) and vehicle station (VS). The ZC resides at the base station and is responsible for traffic