This study attempts to exploit the potential of link directionality to increase the achievable capacities of ad hoc networks. When an IEEE 802.11 ad hoc network achieves capacity C by using a single channel, the targeted capacity by using two channels should be 2C. However, most of the dual-channel 802.11 protocols proposed in the literature appear only to be able to achieve less than 60% of the 2C targeted capacity. The authors thus propose a link-directionality-based dual-channel medium access control protocol in an attempt to double the capacities of networks using the single-channel IEEE 802.11 protocol. The main idea is to assign channels according to link directionality to allow a link to transmit simultaneously within the carrier-sensing region of another link provided that these transmissions do not interfere with each other. Simulations show that our proposed scheme can achieve more than 85% of our targeted capacities, 0.85×2C=1.7C, in large-scale random topologies. In lattice and irregular topologies, the throughput is boosted up to 2.83C and 2.13C, respectively. An approach for capacity analysis is also introduced to determine the throughput improvements that can be achieved by our proposed protocol. We believe using link directionality for channel allocations is a key step that yields significant potential for multiplying the capacity of ad hoc networks.

In this paper, we clarify inter-link interference in wireless ad-hoc networks by using link-directional interference graphs (I-graph). Most of the interference graphs in the literature simply model the DATA and ACK traffic of a link by a single vertex. They fail to capture the link-directionalities. In fact, some instances of directional traffic can actually transmit simultaneously but are prohibited by the interference graphs. Thus, in our link-directional interference graph, a link is represented by two vertices, one for DATA traffic and the other for ACK traffic. We then apply a colouring algorithm in the I-graphs. The colouring results provide insights in order to boost network capacities in TDMA or FDMA ad-hoc networks. We show that the network capacities can be improved by 100% in a triangular topology and 33% in a lattice topology. Simulations also show that a distributed dual channel protocol assigning channels according to link-directionalities can boost the capacities by 70% in large-scale random networks. We believe this is a first paper in the literature to take into account link-directionalities in interference graphs.

Multi-carrier code division multiple access (MC-CDMA) was proposed to improve capacities of wireless networks. Instead of increasing the data transmission rates of wireless links, the authors propose to assign independent data streams obtained by MC-CDMA for transmitting uplink and downlink traffic separately in order to eliminate the hidden-node and exposed-node problems between cochannel BSSs in WLANs. Simulations show that our scheme can improve network throughputs by 106% in an exposed-node scenario and by 113% in a hidden-node scenario. The authors also consider the hardware and MAC requirements of our scheme under different settings of physical carrier sensing range. The authors believe this is a first paper in the literature to assign MC-CDMA generated data streams in WLANs so as to mitigate the hidden-node and exposed-node problems.

In this paper, we propose a link-directionality-based dual channel MAC protocol in an attempt to double the capacities of networks using the single-channel IEEE 802.11 protocol. When an IEEE 802.11 ad-hoc network achieves capacity C by using a single channel, the targeted capacity by using two channels should be 2 - C. However, most of the multichannel 802.11 protocols proposed in the literature only appear to be able to achieve less than 60% of the 2 ldr C targeted capacity. Simulations show that our proposed scheme can achieve more than 106% of our targeted capacities, 1.06*2 ldr C = 2.12 ldr C. We believe this is a first paper in the literature to propose a MAC protocol to transmit RTS/DATA and CTS/ACK of a link on different channels, a key step that yields significant potential formultiplyingthe network capacities of ad-hoc networks.

In this paper, we propose a link-directionality-based dual channel MAC protocol in an attempt to double the capacities of networks using the single-channel 802.11 protocol. Simulations show that the proposed scheme can achieve more than 78% of our targeted capacities. We believe this is a first paper in the literature to propose a MAC protocol to transmit RTS/DATA and CTS/ACK of a link in different channels. This yields a large potential for multiplying the network capacities of ad-hoc networks by using only two channels.

When an IEEE 802.11 ad-hoc network achieves capacity C by using a single channel, the targeted capacity by using two channels should be2middotC. However, most or the multi-channel 802.11 protocols proposed in the literature only appear to be able to achieve less than 60% or the 2middotC targeted capacity. In our paper (2006), we proposed a link-directionality-based dual channel MAC protocol (DCP) to boost the network capacities up to 78% of our targeted capacities, 78%*2middotC = 1.56middotC. However, DCP still failed to reach the 2middotC capacity target due to the overheads incurred by the protocol. In this paper, we implement a power exchange algorithm on top of the DCP in an attempt to double the capacities of networks using the single-channel 802.11 protocol. This algorithm incurs relatively small overheads and can further release the protocol constraints imposed by DCP. Simulations show that the proposed scheme (DCPwPEA) can achieve more than 132% of our targeted capacities, 132%* 2middotC = 2.64middotC. We believe this protocol can be extended to allow multi-link simultaneous transmissions which can multiply the network capacities of ad-hoc networks by using only two channels

In large-scale wireless networks, interference among nodes limits channel spatial re-use and is a main hurdle for scalable performance. There are two types of interference: (1) physical interference due to the receiver's inability to decode a signal when the power received from other signals is large; (2) protocol interference imposed by the specific multi-access protocol being used. The paper models both interference types in terms of a set of inequality constraints for the IEEE 802.11 CSMA/CA protocol. Based on the inequalities, we investigate the impact of some parameters (basic-rate, data-rate, and physical-preamble-rate) on channel spatial re-use. Regardless of the parameter settings, the total capacity in an 802.11 network reaches a ceiling as the number of nodes, n, increases. We identify the fundamental causes for the non-scalable performance, and show that 802.11 can be made to be scalable with a simple modification, achieving O(n) throughput without adaptive power control. We believe that this is the first paper to demonstrate this.

TCP throughput instability is a well-known phenomenon in IEEE 802.11 multi-hop ad-hoc networks. However, we find that this problem is not restricted to TCP traffic only, but also occurs in UDP traffic. The associated throughput oscillations are not acceptable for real-time applications such as video conferencing and voice-over-IP. The paper re-defines this throughput fluctuation as a "re-routing instability problem" since it is caused by the triggering of the re-routing function. In particular, we show that the throughput instability is mainly induced by re-routing, not the binary exponential back-off of the IEEE 802.11 MAC protocol. Turning off the re-routing function, for example, eliminates the problem. We believe that this is the first paper to study this phenomenon in the context of re-routing instability. We propose to modify the ad-hoc routing protocols with a "don't-break-before-you-can-make" strategy. The scheme does not require modifications of the IEEE 802.11 standard, making it readily deployable using existing commercial wireless LAN (WLAN) products. Simulations show that the proposed scheme can significantly reduce the throughput variation in a traffic flow by 50-70% and improve the average throughput by up to 11%.

In a multi-hop ad-hoc network, stations may pump more traffic into it than can be supported, resulting in high packet-loss rate, rerouting instability and unfairness problems. The paper shows that controlling the offered load at the sources can eliminate these problems. In addition, we provide an analysis to estimate the optimal offered load that maximizes the throughput of a multi-hop traffic flow. We use this result to devise schemes that can achieve fairness when there are multiple flows from different sources to different destinations. We believe this is the first paper in the literature to provide a quantitative analysis (as opposed to simulation) for the impact of hidden nodes, exposed nodes, and signal capture on sustainable throughput. The analysis is based on the observation that a large-scale 802.11 network with hidden nodes is a network in which the carrier-sensing capability breaks down partially. Its performance is therefore somewhere between a carrier-sensing network and an ALOHA network. Indeed, our analytical closed-form solution has the appearance of the throughput equation of the ALOHA network. Our approach allows one to identify whether the performance of an 802.11 network is hidden-node limited or spatial-reuse limited.