Aizaz U. Chaudhry

Improving throughput and fairness by improved channel assignment using topology control based on power control for multi-radio multi- channel wireless mesh networks

Multi-radio multi-channel (MRMC) wireless mesh networks (WMNs) achieve higher throughput using multiple simultaneous transmissions and receptions. However, due to limited number of non-overlapping channels, such networks suffer from co-channel interference, which degrades their performance. To mitigate co-channel interference, effective channel assignment algorithms (CAAs) are desired. In this article, we propose a novel CAA, Topology-controlled Interference-aware Channel-assignment Algorithm (TICA), for MRMC WMNs. This algorithm uses topology control based on power control to assign channels to multi-radio mesh routers such that co-channel interference is minimized, network throughput is maximized, and network connectivity is guaranteed. We further propose to use two-way interference-range edge coloring, and call the improved algorithm Enhanced TICA (e-TICA), which improves the fairness among flows in the network. However, the presence of relatively long links in some topologies leads to conflicting channel assignments due to their high interference range. To address this issue, we propose to utilize minimum spanning tree rooted at the gateway to reduce conflicting channels, and in turn, improve medium access fairness among the mesh nodes. We call the improved algorithm e-TICA version 2 (e-TICA2). We evaluate the performance of the proposed CAAs using simulations in NS2. We show that TICA significantly outperforms the Common Channel Assignment scheme in terms of network throughput, and e-TICA and e-TICA2 achieve better fairness among traffic flows as compared to TICA. It is also shown that e-TICA2 leads to improved network throughput, as compared to TICA and e-TICA.

Throughput Improvement in Multi-Radio Multi-Channel 802.11a-Based Wireless Mesh Networks

Single-radio mesh routers operating on a single channel suffer from low throughput due to collisions. Equipping mesh routers with multiple radios operating on non-overlapping channels can significantly improve the throughput. However, the assignment of channels to radios in a multi-radio mesh network is a challenging task. In this paper, we propose a channel assignment algorithm, TICA (Topology-controlled Interference-aware Channel-assignment Algorithm), which significantly improves network throughput by minimizing interference within the mesh network using a novel approach of controlling the network topology based on power control before intelligently assigning the channels to the multi-radio mesh routers, as well as guaranteeing network connectivity.

On the impact of interference models on channel assignment in multi-radio multi-channel wireless mesh networks

We study the impact of three different interference models on channel assignment in multi-radio multi-channel wireless mesh networks, namely the protocol model, the signal-to-interference ratio (SIR) model and the SIR model with shadowing. The main purpose is to determine the minimum number of non-overlapping frequency channels required to achieve interference-free communication among the mesh nodes based on a realistic interference model. We propose novel, effective, and computationally simple methods for building the conflict graph based on the SIR model with shadowing, and for finding channel assignments from the resulting conflict graph. We find that channel assignment using a realistic interference model (SIR model with shadowing) requires more frequency channels for network throughputs at different node-degree constraints as compared to using simpler interference models.

Channel Requirements for Interference-Free Wireless Mesh Networks to Achieve Maximum Throughput

In a multi-radio multi-channel wireless mesh network, a channel assignment that is based on a fixed number of available frequency channels may cause co-channel interference, which degrades the network throughput. We address this problem by ensuring interference-free communication among the mesh nodes. The main purpose of this work is to determine the minimum number of non-overlapping frequency channels required for interference-free channel assignment in order to achieve the maximum network throughput while maintaining fairness among the multiple network flows, given the location of the mesh nodes and the number of their half-duplex radio interfaces. To minimize the number of channels required, we apply our Select x for less than x Topology Control Algorithm to build the connectivity graph instead of using the classical approach based on maximum power (MP). We show that our approach outperforms the MP-based approach in terms of the number of channels required as well as the links to channels ratio for all node-degrees.

Fault-tolerant and scalable channel assignment for multi-radio multi-channel IEEE 802.11a-based wireless mesh networks

Wireless Mesh Networks aim to provide high-bandwidth broadband connections to a large community and thus, should be able to accommodate a large number of users accessing the Internet. Due to high estimated traffic volume in Wireless Mesh Networks, scalability and fault tolerance become important requirements in algorithm design. We propose a Failure Recovery Mechanism (FRM) for the channel assignment algorithm, TICA (Topology-controlled Interference-aware Channel-assignment Algorithm), whose goal is to provide automatic and fast failure recovery. We also investigate the performance of TICA with respect to scalability and show that TICA performs well in large-scale networks.

Realistic interference-free channel assignment for dynamic wireless mesh networks using beamforming

To make the most efficient use of scarce bandwidth, channel assignment methods for wireless mesh networks (WMNs) should try to minimize the number of frequency channels used while achieving maximum network throughput. Beamforming is a well-known technique that improves spatial reuse in wireless networks. However, there are no channel assignment methods for WMNs that use beamforming to reduce the number of frequency channels. We develop the first channel assignment method for dynamic WMNs that incorporates beamforming in the conflict graph and matrix. This reduces co-channel interference significantly, thereby reducing the number of frequency channels required (NCR) to ensure interference-free communication among the mesh nodes while achieving maximum network throughput. Our novel Linear Array Beamforming-based Channel Assignment (LAB-CA) method significantly increases the spectrum utilization efficiency of WMNs at the expense of increased hardware complexity. It outperforms classical omni-directional antenna pattern-based channel assignment (OAP-CA) in terms of NCR. In a heterogeneous WMN where mesh nodes have differing numbers of radio interfaces, LAB-CA also outperforms OAP-CA in terms of NCR in both sparse and dense scenarios. A further significant reduction in NCR is achieved when the number of antennas in the linear antenna arrays of mesh nodes is increased.

Significantly reducing the number of frequency channels required for wireless mesh networks using beamforming

In a classical multi-radio multi-channel Wireless Mesh Network (WMN) architecture, mesh nodes use omni-directional antennas. Due to the circular radiation pattern of such antennas, when a mesh node communicates with its neighbor on a certain frequency channel, other mesh nodes within its range must remain silent. Directional antennas have been proposed as a way to improve spatial reuse. Since these antennas are non-steerable, they are not suitable for a dynamic WMN. In this paper, we address the problem of co-channel interference in a dynamic WMN environment by using beamforming based on utilizing the multiple omni-directional antennas of a multi-radio mesh node in the form of a linear antenna array. Our novel Linear Array Beamforming-based Channel Assignment method reduces the number of frequency channels required (NCR) for interference-free communication among the mesh nodes. It significantly outperforms the classical omni-directional antenna pattern-based channel assignment approach in terms of NCR for all node-degrees.

On the number of channels required for interference-free wireless mesh networks

We study the problem of achieving maximum network throughput with fairness among the flows at the nodes in a wireless mesh network, given their location and the number of their half-duplex radio interfaces. Our goal is to find the minimum number of non-overlapping frequency channels required to achieve interference-free communication. We use our existing Select x for less than x topology control algorithm (TCA) to build the connectivity graph (CG), which enhances spatial channel reuse to help minimize the number of channels required. We show that the TCA-based CG approach requires fewer channels than the classical approach of building the CG based on the maximum power. We use multi-path routing to achieve the maximum network throughput and show that it provides better network throughput than the classical minimum power-based shortest path routing. We also develop an effective heuristic method to determine the minimum number of channels required for interference-free channel assignment.

Enhanced Topology-controlled interference-aware channel assignment for Multi-Radio Multi-Channel Wireless Mesh Networks

One of the challenges in Multi-Radio Multi-Channel Wireless Mesh Networks is the assignment of channels to links such that connectivity is ensured and interference is minimized. Two links that are within interference range of each other could be assigned the same channel, which causes co-channel interference and decreases network throughput. In this paper, we identify the above problem in the Topology-controlled Interference-aware Channel-assignment Algorithm (TICA) [1] and propose an enhanced version called e-TICA. The new algorithm solves the un-foreseen interference problem through two-way interference-range edge coloring. We show through simulations that e-TICA outperforms TICA in terms of fairness among flows without sacrificing the average network throughput.

User Association Algorithm for Throughput Improvement in High-Density Wireless Networks

The default mechanism for a wireless station to select which access point (AP) to associate with is based on the strength of the received signal by the station. This leads to stations associating to the closest AP. In situations where user stations are concentrated around one AP, the default algorithm (DA) results in an unbalanced distribution of station-AP associations. This causes congestion at the AP with many associated stations, which degrades the network throughput. We propose a novel association algorithm called Time-Load Algorithm (TLA) that associates stations to an AP based on the time-loading of that AP as well as its signal strength. We evaluate the performance of our TLA in comparison with the DA in different scenarios based on network throughput and packet delivery ratio. The simulation results clearly indicate that the TLA outperforms the DA in all scenarios alleviating highly loaded APs and improving throughput experience.