An Agile and Efficient MAC for Wireless Access over TV Whitespaces

The FCC mandate of opening up TV whitespaces for unlicensed use has the potential for dramatic improvements in wireless access data rates. In this paper, we design and develop MAC for wireless LAN kind of access over TV whitespace. We look into detail the unique nature of this spectrum, namely spatio-temporal variations of available spectrum, frequency dependent propagation, and highly fragmented nature of the spectrum. We argue that an ideal MAC should account for diverse user-location and spectrum dependent channel rates so as to provide fair data rates and efficient utilization. Furthermore, due to hardware limitation, the AP should support multiple radios. In addition to challenges, we impose the constraint that the medium access mechanism should be evolution of 802.11-like CSMA. Our main contributions are as follows: (i) we propose an architecture and control channel mechanism to enable such a MAC design, with minimal changes to 802.11-like CSMA. (ii) we propose algorithms, that sit on the top of the MAC of the AP, and intelligently chooses the whitespaces for the different radios of the AP and also, assigns clients to the different radios. Our algorithms have provable guarantees and close to optimal in many scenarios. (iii) We perform extensive simulation over OMNET platform to demonstrate that the benefit our design over a frequency and client-location agnostic Wi-Fi-like MAC. The typical gain in total throughput is between 33%-50%, whereas, the number of collisions drops by as much as by a factor of 5.