Sensitivity gains from co-operation in cognitive sensing

Abstract
...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.

ABSTRACT (excerpts):
...In this paper we study spectrum-sharing between a primary licensee and a group of secondary users. In order to enable access to unused licensed spectrum, a secondary user has to monitor licensed bands and opportunistically transmit whenever no primary signal is detected. However, detection is compromised when a user experiences shadowing or fading effects. In such cases, user cannot distinguish between an unused band and a deep fade. Collaborative spectrum sensing is proposed and studied in this paper as a means to combat such effects. Our analysis and simulation results suggest that collaboration may improve sensing performance significantly.

Abstract
The MAC protocol for a cognitive radio network should allow access to unused spectrum holes without (or with minimal) interference to incumbent system devices. To achieve this main goal, in this paper a distributed cognitive radio MAC (DCR-MAC) protocol is proposed for wireless ad hoc networks that provides for the detection and protection of incumbent systems around the communication pair. DCR-MAC operates over a separate common control channel and multiple data channels; hence, it is able to deal with dynamics of resource availability effectively in cognitive networks. A new type of hidden node problem is introduced that focuses on possible signal collisions between incumbent devices and cognitive radio ad hoc devices. To this end, a simple and efficient sensing information exchange mechanism between neighbor nodes with little overhead is proposed. In DCR-MAC, each ad hoc node maintains a channel status table with explicit and implicit channel sensing methods. Before a data transmission, to select an optimal data channel, a reactive neighbor information exchange is carried out. Simulation results show that the proposed distributed cognitive radio MAC protocol can greatly reduce interference to the neighbor incumbent devices. A higher number of neighbor nodes leads to better protection of incumbent devices.

Abstract
Measurements performed at several locations clearly show that frequency spectrum is under-utilized. Cognitive radio (CR) is a strong candidate to ensure better spectrum utilization by providing access in an opportunistic manner, i.e., when the spectrum is temporary or spatially unutilized (a.k.a. white-space). The key enabling technology for realizing better frequency spectrum utilization in CR system is spectrum sensing. In particular, spectrum sensing in television (TV) bands is of great interest due to the potential availability of spectrum as a result of the planned migration of analog TV broadcasting to digital TV broadcasting. In this paper, we present the TV white-space prototype that we developed to detect vacant spectrum in TV bands. The prototype is implemented on a real-time platform and tested in practical environments. Results show that our prototype is able to detect vacant channels up to sensitivities from -116 to -120 dBm under realistic conditions. This confirmation will give the much needed confidence in the capability of CR systems to detect the operation of primary users and protect their use of spectrum.

Abstract
Spectrum measurement studies have shown that substantial portions of the allocated wireless spectrum are highly underutilized. Frequency-agile radios (FARs) have the potential to make opportunistic use of such spectrum holes without causing harmful interference to users of the allocated spectrum. Toward this goal, we develop a framework for modeling the interference caused by FARs employing spectrum access mechanisms based on the simple Listen-Before-Talk (LBT) scheme. Two variations of LBT are considered: individual LBT, whereby the FARs act independently of each other; and collaborative LBT, whereby the FARs communicate with each other in order to more accurately identify the spectrum holes. Our analysis of the LBT scheme reveals the fundamental interdependencies among key system design metrics and provides a basis for analyzing more complex spectrum access methods. In particular, the analysis of LBT provides a lower bound on the capacity gain achievable by FARs employing spectrum-sharing schemes. Our numerical results show that the individual LBT scheme can provide substantial capacity gains, while even more gain can be achieved using the collaborative LBT schemes. Our analysis suggests that much greater gains should be achievable via spectrum access schemes that incorporate location information and/or more sophisticated group behaviors.

Abstract
"A crucial task for a network of cognitive radios is to detect occupied frequency bands, to protect transmissions of primary users, and to identify spectrum holes (unoccupied bands) to maximize the utilization of wasted resources. This paper is motivated by the need to account for challenging constraints that naturally arise in such applications such as channel model uncertainties and demanding sensitivity constraints of the sensing devices. We propose False Discovery Rate (FDR) based cooperative strategies to sense the occupancy of the spectrum. The strategies we propose could either be used to maximize bandwidth utilization or to provide guarantees on incurred interference levels. The proposed strategies are robust to significant uncertainties such as lack of CSI, fading and shadowing effects. The key idea of the paper is that the twin objectives of bandwidth utilization and interference control can significantly benefit from group testing across all channels in contrast to conventionally employed channel-by-channel detection strategy. Furthermore, it is shown that the cooperative sensing strategy significantly reduces sensitivity requirements. We quantify the effect of channel occupancy rate on the required cooperation degree for achieving a guaranteed level of primary user protection.

ABSTRACT:
Spectrum sensing has been identified as a key enabling functionality to ensure that cognitive radios would not interfere with primary users, by reliably detecting primary user signals. Recent research studied spectrum sensing using energy detection and network cooperation via modeling and simulations. However, there is a lack of experimental study that shows the feasibility and practical performance limits of this approach under real noise and interference sources in wireless channels. In this work, we implemented energy detector on a wireless testbed and measured the required sensing time needed to achieve the desired probability of detection and false alarm for modulated and sinewave-pilot signals in low SNR regime. We measured the minimum detectable signal levels set by the receiver noise uncertainties. Our experimental study also measured the sensing improvements achieved via network cooperation, identified the robust threshold rule for hard decision combining and quantified the effects of spatial separation between radios in indoor environments.