Cognitive radios in the TV whitespaces: challenges and opportunities
Technical Report Identifier: EECS-2011-151
December 16, 2011
Abstract: In the past decade, wireless devices have become increasingly popular which in turn makes spectrum more valuable and more scarce. In an effort to help alleviate the current spectrum shortage crisis in the United States, the Federal Communications Commission (FCC) has ruled that starting in 2008 unlicensed devices are allowed to transmit wirelessly in the television bands. This permits unlicensed devices such as a wireless router or cordless phone to transmit in the "unused" portions of the TV bands, also known as the TV whitespaces. The TV whitespaces are generally regarded as a great opportunity for unlicensed devices (also known as secondaries) to expand beyond the crowded Industrial, Scientific and Medical (ISM) bands. We explore the magnitude of this opportunity as well as the challenges it presents.
This thesis first quantifies the opportunity to secondaries in terms of available spectrum and achievable data rates via simulations involving real-world TV assignment and population data. Prior studies for the United States and the United Kingdom have bounded the amount of available spectrum but fail to account for significant effects such as an increased noise level and self-interference among secondaries. These effects mean that the traditional bandwidth metric is inadequate since spectrum users are not interested purely in spectrum but rather in what they can do with it, which can be characterized by the achievable data rate. Our work incorporates this metric in order to better quantify the impact of the TV towers and associated regulatory decisions on the utility of whitespaces to secondaries. We also develop models for both point-to-point and cellular systems in order to quantify the impact of self-interference among secondaries in the TV whitespaces.
Second, using these models, we compare simple spectrum reallocation to the use of whitespaces and conclude that the latter is better if we do not wish to experience drastic changes in TV availability. This improves the work done by Mishra, et al. in by more accurately modeling secondary utility.
Third, we find that the current FCC regulations may be inadequate to protect TV viewers from the harmful effects of aggregate interference because the rules are made implicitly with only a single transmitter in mind. The authors of reach the same conclusion regarding the safety of both FCC and ECC rules in Finland but they do not suggest a remedy. We suggest setting a maximum power density for secondaries rather than a per-device power limit to avoid this problem. Such an approach might seem difficult with sensing-only technologies, but we believe that the existence of TV whitespace databases enables this necessary fine-grain control.
Given that enforcing a power density is necessary for TV receiver protection, we then explore how this solution can be employed to improve utility as well as freedom for secondary users. The current FCC rules implicitly favor certain applications. We offer a principled way to help mitigate the tensions between two types of users, rural and urban, using an approximately-optimal algorithm to choose a power density.
Finally, we provide a toolkit which is publicly available online. This Matlab-based toolkit uses real-world data to produce all results herein and can easily be modified to extend these results. Details of the toolkit can be found in Appendix A and its limitations are examined in Appendix B.