An Adaptive Protocol for 802.11 Wireless LANs

Expert Analysis

Bandwidth is the width of a signal on the radio spectrum. As the transmitted data rate increases on an RF channel, a signal's bandwidth must be spread wider to occupy more frequency space so it can carry more information. As data spreads a signal wider, the "per bit" signal peak strength falls so it becomes more susceptible to errors generated by noise and the transmission distance falls. However, by processing a signal through signal coding that spreads the bandwidth wider than necessary to carry the information, spread spectrum signaling techniques provide processing gain, which aids in error detection and correction to protect the signal from RF noise, interference from other signals, and jamming interference. Adding processing gain helps protect the signal from interference if the spread RF bandwidth is wider than the data signaling bandwidth, but as the data signaling rate approaches the spread spectrum coding rate, processing gain from the coding technique is lost and the range decreases. The fundamental problem we want to overcome is the decrease in effective range as the data rate increases and the processing gain decreases over a wireless link.

Power control allows point-to-point communication networks to maintain link quality under fading and interference conditions. Emerging mobile ad-hoc networking (MANET) technology adds dynamic multi-hop routing to extend the range of communications networks beyond point-to-point links. The new MANET technology adds a new constraint to power adaptation. That is when the effective transmit radius of a MANET radio node changes it can cause proactive routing protocols to generate additional overhead in the form of route change messages. These observations motivated us to develop a scheme for dynamically setting the waveforms and transmission power levels for IEEE 802.11 wireless LAN MANETs that does not generate changes to the routing table.

This work is about a method for dynamically setting 802.11 wireless LAN waveforms and transmission power levels based on the wireless channel's signal to noise ratio. This consultant's method, known as Signal-to-Noise Ratio-Waveform Power Adaptation (SNR-WPA), changes the power in discrete steps matched to each of the 802.11 data rate-waveform steps. By matching the power to the spreading symbol rate, the technique maximizes the network throughput while minimizing MAC layer contention. Unlike other power adaptation methods, this method does not increase the Wireless LAN (WLAN) station's overall effective operational range and does not change the minimum-spanning tree used to calculate routing. This consultant found through experimentation that the power adaptation in SNR-WPA yields up to a 30% increase in throughput in a mobile wireless LAN network. Unlike other power adaptation methods, the scheme does not increase the Wireless LAN (WLAN) station's overall effective operational range and does not change the minimum-spanning tree used to calculate routing. Sample results for scenario 2 of this consultant's work-ten workstations clustered around a single router are shown below.

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