A Fair Channel Access Protocol for IEEE 802.11af

The explosive growth of Internet of Things (IoT) applications requires many wireless spectrum resources to support connections of billions of sensors and actuators. Unfortunately, the licensed bands such as 2/3/4G networks are very expensive while the unlicensed 2.4GHz/5.8GHz bands are very crowded. The good news is that although many of the licensed bands are already assigned, the actual usage is low, especially in the very-high frequency (VHF) and the ultra-high frequency (UHF) television (TV) bands. In views of this fact, IEEE 802.11af PHY/MAC standard, an emerging technology in the IEEE 802.11 standard family, is developed so that wireless devices can operate in these bands, provided that no harmful interference is caused to the TV broadcasters and the wireless microphones. Thanks to the low-frequency operation, the IEEE 802.11af protocol has excellent propagation/penetration characteristics and thus, a larger coverage. These characteristics make the IEEE 802.11af standard a valuable solution for various applications, but also cause much serious unfairness problems due to the capture effect, the hidden terminal problem, and designs of IEEE 802.11 MAC protocol. In this thesis, we first evaluate the three unfairness issues in an IEEE 802.11af network, and then propose a distributed-based scheme. In our scheme, every station in a basic service set (BSS) selects a fixed contention window (CW) size rather than doubling it after a failed transmission. Stations choose their CW sizes based on the number of captured and hidden stations. In order to detect the number of captured and hidden stations, some mechanisms are implemented. In addition, rather than freezing the backoff slots when the channel is busy, stations continuously count down the backoff slots. With our design, all stations are guaranteed to successfully transmit the data/control frames with an equal probability. The performance of our solution is evaluated by Opnet-based simulations. Both the analytical and simulation results show that the proposed scheme leads to 95% fairness in terms of the probability of the successful transmissions.