Fundamental tradeoffs in robust spectrum sensing for opportunistic frequency reuse

Under the current system of spectrum allocation, spectral bands are allocated for time-scales ranging from years to decades and spatial scales ranging from counties to continents. Robust operation of devices is enabled by the use of guard bands in space and frequency. This paradigm has resulted in vastly underutilized spectrum bands, even in urban locales. However, advances in wireless technology now allow devices to operate on much smaller time and spatial scales. Like sand and pebbles poured into the gaps between larger rocks, such devices have the potential to greatly improve our overall spectrum utilization. The fundamental constraint is robustly guaranteeing non-interference to privileged users of the band. We focus on cognitive radios that perform sensing and adapt their output to avoid interfering. We show that uncertainty in fading poses a serious challenge by forcing high sensitivity. These challenges are exacerbated by the presence of multiple users, but gains are available through cooperation. However, cooperative gains are limited by trust/reliability and the network’s cooperation footprint. Furthermore, uncertainty regarding noise and interference imposes fundamental limits on how sensitive robust sensing can be. Local cooperation is necessary to provide fairness by reducing the uncertainty impact of other users’ transmissions.