Abstract — Despite improvements in wireless access technologies such as 3G or 802.11x, ubiquitous data access has remained a challenge, mainly due to the lack of inexpensive, pervasive backhaul connections from access points to the Internet. With the recent WiMAX standard for high-speed, non-line-of-sight fixed wireless links, multihop wireless backhauls might now overcome this bottleneck. However an important remaining challenge is to provide rate and delay guarantees for customer connections similar to wired backhauls. We provide several schemes for performing admission control for connections with QoS requirements over a multihop wireless backhaul. This is the first work to address both rate and delay requirements for connections. Our admission control algorithms first construct appropriate tree-based topologies connecting wireless backhaul nodes to a wired gateway and then admit the best subset of connections while respecting their rate and delay requirements. Alternately, we admit all the connections with appropriate degradation of their QoS requirements. I.

Abstract — We introduce a low-complexity distributed slotted MAC protocol that can support all feasible arrival rates in a wireless backhaul network (WBN). For arbitrary wireless networks, such a maximum throughput protocol has been notoriously hard to realize because (i) even if global topology information is available, the problem of computing the optimal link transmission set at each slot is NP-complete (ii) no bounds exist on the number of steps required for such a computation (per-slot overhead). For the logical tree structures induced by the WBN traffic matrices, we first introduce a centralized algorithm that solves the optimal scheduling problem in a number of steps at most linear in the number of nodes in the network. This is achieved by discovering and exploiting a novel set of graph-theoretical properties of the WBN contention graph. Guided by the centralized algorithm, we design a distributed protocol where, at the beginning of each slot, nodes coordinate and incrementally compute the optimal link transmission set. We then introduce an algorithm to compute the minimum number of steps to complete this computation, thus minimizing the per-slot overhead. Using both analysis and simulations, we show that in practice our protocol yields low overhead when implemented over existing wireless technologies and significantly outperforms existing suboptimal distributed slotted scheduling mechanisms. I.

Abstract—We design and implement Soft-TDMAC, a software Time Division Multiple Access (TDMA) based MAC protocol, running over commodity 802.11 hardware. Soft-TDMAC has a synchronization mechanism, which synchronizes all pairs of network clocks to within microseconds of each other. Building on pairwise synchronization, Soft-TDMAC achieves network wide synchronization. With, out-of-band, network wide synchronization Soft-TDMAC can schedule arbitrary TDMA transmission patterns. We summarize hundreds of hours of testing Soft-TDMAC on a multi-hop testbed. Our experimental results show that Soft-TDMAC synchronizes multi-hop networks to within a few microsecond sized TDMA slots. Soft-TDMAC can schedule transmissions to take end-to-end demands into account and in a way that decreases end-to-end delay [1], [2]. With no collisions, under good channel conditions, TCP achieves almost the full wireless channel bandwidth.

Time division multiple access (TDMA) based medium access control (MAC) protocols can provide QoS with guaranteed access to the wireless channel. However, in multihop wireless networks, these protocols may introduce scheduling delay if, on the same path, an outbound link on a router is scheduled to transmit before an inbound link on that router. The total scheduling delay can be quite large since it accumulates at every hop on a path. This paper presents a method that finds conflict-free TDMA schedules with minimum scheduling delay. We show that the scheduling delay can be interpreted as a cost, in terms of transmission order of the links, collected over a cycle in the conflict graph. We use this observation to formulate an optimization, which finds a transmission order with the minmax delay across a set of multiple paths. The min-max delay optimization is NP-complete since the transmission order of links is a vector of binary integer variables. We devise an algorithm that finds the transmission order with the minimum delay on overlay tree topologies and use it with a modified Bellman-Ford algorithm, to find minimum delay schedules in polynomial time. The simulation results in 802.16 mesh networks confirm that the proposed algorithm can find effective min-max delay schedules.

IEEE 802.16, popularly known as WiMAX, is at the forefront of the technology drive because of the growing demand for high-speed wireless broadband networks. Multihop WiMAX networks are particularly useful as they increase the coverage area without the need to deploy expensive base stations. There are two kinds of multihop WiMAX networks- WiMAX mesh and Mobile Multihop Relay Networks. Scheduling is very important in both of these multihop WiMAX networks. The links have to be scheduled in such a way so that they do not interfere with each other while maximizing the throughput of the networks. As WiMAX networks are geared towards broadband applications, any scheduling scheme should accomodate the rate, latency and jitter requirements of the applications. This article provides an insight to the scheduling framework presented in the IEEE 802.16 standard. It also presents a few representative research proposals for centralized scheduling in WiMAX networks. We discuss some of the research issues and challenges that need to be addressed for multihop WiMAX networks to realize their full potential. I.

We study the problem of scheduling in OFDMA-based relay networks with emphasis on IEEE 802.16j based WiMAX relay networks. In such networks, in addition to a base station, multiple relay stations are used for enhancing the throughput, and/or improving the range of the base station. We solve the problem of MAC scheduling in such networks so as to serve the mobiles in a fair manner while exploiting the multiuser diversity, as well as the frequency selectivity of the wireless channel. The schedulingresources consist of tiles in a two-dimensional scheduling frame with time slots along one axis, and frequency bands or sub-channels along the other axis. The resource allocation problem has to be solved once every scheduling frame which is about 5−10 ms long. While the original scheduling problem is computationally complex, we provide an easy-to-compute upper bound on the optimum. We also propose three fast heuristic algorithms that perform close to the optimum (within 99.5%), and outperform other algorithms such as OFDM 2 A proposed in the past. Through extensive simulation results, we demonstrate the benefits of relaying in throughput enhancement (an improvement in the median throughput of about 25%), and feasibility of range extension (for e.g., 7 relays can be used to extend the cell-radius by 60 % but mean throughput reduces by 36%). Our algorithms are easy to implement, and have an average running time of less than 0.05 ms making them appropriate for WiMAX relay networks.

Wireless mesh networks are a recent architecture for multihop wireless networks. Also, standards for realizing mesh networks are being actively developed, especially in the IEEE working groups. In contrast with mobile ad hoc networks, mesh networks consist of static nodes communicating with each other over wireless links. The static nodes are essentially wireless routers. Such networks can be used, for example to provide a cost effective alternative to a wireline Internet access network. As opposed to the nodes in mobile ad hoc networks, the nodes in mesh networks are not energy constrained and node mobility is not a concern in protocol scalability. Instead, the main technical problems relate to achieving high user data rates over multihop wireless paths by using advanced MAC/routing layer solutions. This report presents a state-of-the-art analysis of wireless mesh networks, both from the point of view of standardization and academic research activities. In the standardization, we focus on the recent developments on defining new physical layer and MAC layer standards for mesh network in the IEEE 802.11 and 802.16 working groups. At the IP layer, in addition to routing, mobility management is a key issue, and these are reviewed from the point of view of recent IETF activities in the field. In academic research, the emphasis has been on identifying feasible mechanisms that can be used to mitigate the impact of interference

Abstract—In this paper we propose an adaptive scheduling algorithm for IEEE 802.16j based wireless broadband networks. Computation of an optimal schedule for prioritized traffic in OFDMA based IEEE 802.16 wireless network is an NP-Hard problem. Hence, we propose a scheduling heuristic for an OFDMA based WiMAX relay network. The ORS (OFDMA Relay Scheduler) heuristic computes the zone boundaries (relay and access) in an uplink scheduling frame based on the number of RSs and MSs, the bandwidth demands and the link conditions. The ORS heuristic determines a schedule which assigns subchannelstimeslot to prioritized traffic based on the demand for various nodes while implementing frequency selectivity. The ORS adapts zone boundaries and the schedule to link and demand conditions at every scheduling period. We perform extensive simulations to demonstrate the effectiveness of adaptive zone scheduling and changes in rate conditions for various topologies.

Abstract—Long-distance multi-hop wireless networks have been used in recent years to provide connectivity to rural areas. The salient features of such networks include TDMA channel access, nodes with multiple radios, and point-to-point longdistance wireless links established using high-gain directional antennas mounted on high towers. It has been demonstrated previously that in such network architectures, nodes can transmit concurrently on multiple radios, as well as receive concurrently on multiple radios. However, concurrent transmission on one radio, and reception on another radio causes interference. Under this scheduling constraint, given a set of source-destination demand rates, we consider the problem of satisfying the maximum fraction of each demand (also called the maximum concurrent flow problem). We give a novel joint routing and scheduling scheme for this problem, based on linear programming and graph coloring. We analyze our algorithm theoretically and prove that at least 50 % of a satisfiable set of demands is satisfied by our algorithm for most practical networks (with maximum node degree at most 5). I.

Abstract—In wireless mesh networks, the end-to-end throughput of traffic flows depends on the path length, i.e., the higher the number of hops, the lower becomes the throughput. In this paper, a fair end-to-end bandwidth allocation (FEBA) algorithm is introduced to solve this problem. FEBA is implemented at the medium access control (MAC) layer of single-radio, multiple channels IEEE 802.16 mesh nodes, operated in a distributed coordinated scheduling mode. FEBA negotiates bandwidth among neighbors to assign a fair share proportional to a specified weight to each end-to-end traffic flow. This way traffic flows are served in a differentiated manner, with higher priority traffic flows being allocated more bandwidth on the average than the lower priority traffic flows. In fact, a node requests/grants bandwidth from/to its neighbors in a round-robin fashion where the amount of service depends on both the load on its different links and the priority of currently active traffic flows. If multiple channels are available, they are all shared evenly in order to increase the network capacity due to frequency reuse. The performance of FEBA is evaluated by extensive simulations. It is shown that wireless resources are shared fairly among best-effort traffic flows, while multimedia streams are provided with a differentiated service that enables quality of service. Index Terms—Access protocols, packet reservation multiaccess, scheduling, wireless LAN. I.