An Agile and Efficient MAC for Wireless Access over TV Whitespaces

Abstract

The FCC mandate of allowing TV Whitespaces for unlicensed access has the potential for dramatic improvements in wireless access data rates. We argue that an ideal MAC should account for diverse user-location and spectrum dependent channel rates to provide fair data rates and efficient utilization. Furthermore, due to limited tunable bandwidth of a radio and fragmented spectrum, the AP should support multiple radios. We make the following contributions by designing a MAC for wireless LAN access over TV Whitespace. (i) We propose an architecture and beaconing mechanism to enable such a MAC. Our MAC is an evolution of 802.11 MAC. (ii) We propose an algorithm that chooses the Whitespaces for the different radios of the AP and assigns clients to the radios. Our algorithm has provable guarantee and is near-optimal in many scenarios. (iii) Extensive simulation over OMNET platform demonstrates the benefit of our design over a frequency and client-location agnostic Wi-Fi-like MAC. The typical throughput gain is 30-76 percent, whereas, the reduction in collisions is up to 80 percent. (iv) We implemented a proof-of-concept prototype (by modifying madWiFi drivers) that demonstrates feasibility of our design, robustness to temporal variation of available spectrum, and system throughput.Index Terms-TV whitespace, CSMA, distributed MAC Ç 1 INTRODUCTION I N November 2008, the FCC in a landmark ruling, stated that unused Digital Television spectrum, or Whitespaces, could be used for unlicensed access. OFCOM in United Kingdom is also actively considering a similar ruling. The FCC ruling is significant because the TV spectrum is located in the VHF (50-174 MHz) and UHF (512-698 MHz) bands which are significantly better propagation bands than current cellular and WiFi spectrum.Moreover several studies estimate that the available spectrum could range from a few tens of megahertz in crowded urban areas to over 100 MHz in sub-urban and rural areas [19]. The ruling allowed two types of access-(i) infrastructure based, cellular type access which could range across the entire TV band and (ii) short range WLAN like access in the UHF band, apart from channel 37. Design of a standard for the first type of access is being actively worked upon by the IEEE 802.22 group. The WLAN type of access has garnered intense interest from several key players from the industry (Google, Microsoft, Dell, etc.) who have banded together to form the Whitespace Coalition. Media and technology pundits believe that WLAN access over this spectrum will be like "WiFi on steroids". In fact a recent market study by Perspective Associates [4] estimates that the market potential for devices and services over this spectrum could be over USD 100Bn over the next 15 years! Beyond the hype, the unique characteristics of this spectrum opens up a rich set of technological problems that have been barely addressed.A major research issue stems from designing networking solutions for this fragmented spectrum. In this paper we focus on designing a MAC for WLAN access where each AP is...

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