“Our goal was to get the highest data rate possible, without compromising the fidelity of the signal,” says Dr. Alexandra Duel-Hallen, a professor of electrical and computer engineering at NC State and co-author of a paper describing the work.
Transmitting data within MANETs is challenging because every node that transmits and receives data is in motion – and the faster they are moving, the harder it is for the network to identify effective relay “paths” for transmitting data. This is because the power of the data-transmission channels fluctuates much more rapidly at high speed.
In other words, a transmitter may try to send a message through Relay A, because Relay A has a strong signal. However, because the transmitter and Relay A are both moving quickly, Relay A’s signal might be weak by the time the message actually gets there. And a weak signal could result in the message being garbled.
To address this issue, researchers developed a method to improve the ability of each node in the network to select the best path for relaying data, as well as the best for transmitting the data that ensures reliable reception.
When a node needs to transmit a message, it first measures the strength of transmissions it is receiving from potential relays. Those data are then plugged in to an algorithm that predicts which relay will be strongest when the message is transmitted. By predicting the strength of the relay, the algorithm also tells the node the rate at which it should transmit the data. If it tries to send too much data too quickly, the data quality will suffer – the data could be compromised. If the rate of data transmission is too slow, the network won’t be operating at peak efficiency.
The paper, “Enabling Adaptive Rate and Relay Selection for 802.11 Mobile Ad Hoc Networks,” will be presented at IEEE’s International Conference on Communications in Ottawa, June 10-15. The paper is co-authored by Neil Mehta, an NC State Ph.D. student; Duel-Hallen; and Dr. Wenye Wang, associate professor of electrical and computer engineering at NC State. The research was supported by the National Science Foundation and the U.S. Army Research Office.
Note to Editors: The study abstract follows.
“Enabling Adaptive Rate and Relay Selection for 802.11 Mobile Ad Hoc Networks”
Authors: Neil Mehta, Alexandra Duel-Hallen and Wenye Wang, North Carolina State University
Presented: June 12, 2012, at IEEE’s International Conference on Communications in Ottawa
Abstract: Mobile ad hoc networks (MANETs) are self-configuring wireless networks that lack permanent infrastructure and are formed among mobile nodes on demand. Rapid node mobility results in dramatic channel variation, or fading, that degrades MANET performance. Employing channel state information (CSI) at the transmitter can improve the throughput of routing and medium access control (MAC) protocols for mobile ad hoc networks. Several routing algorithms in the literature explicitly incorporate the fading signal strength into the routing metric, thus selecting the routes with strong channel conditions. While these studies show that adaptation to the time-variant channel gain is beneficial in MANETs, they do not address the effect of the outdated fading CSI at the transmitter. For realistic mobile node speeds, the channel gain is rapidly varying, and becomes quickly outdated due the feedback delay. We analyze the link throughput of joint rate adaptation and adaptive relay selection in the presence of imperfect CSI. Moreover, for an 802.11 network that employs geographic opportunistic routing with adaptive rate and relay selection, we propose a novel method to reduce the effect of the feedback delay at the MAC layer in the presence of Rayleigh fading. This method exploits channel reciprocity and fading prediction and does not require significant modification to the existing 802.11 frame structure. Extensive network simulations demonstrate that the proposed approach significantly improves the throughput, delay, and packet delivery ratio for high mobile velocities relative to previously proposed approaches that employ outdated CSI at the transmitter.
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