Optimization of Spectrum Sensing for Opportunistic Spectrum Access in Cognitive Radio Networks

Motivated by the low utilization of the licensed spectrum across many frequency bands, sensing-based oppor- tunistic spectrum access has recently emerged as an alternative to the outdated exclusive spectrum access policy. Under this new paradigm, a secondary (unlicensed) user monitors a primary (licensed) frequency band for a given time and opportunistically transmits if it does not detect any ongoing licensed operations. Evidently, selection of the sensing parameters involves balanc- ing a tradeoff between the speed and the quality with which the secondary user senses the licensed band. With the average throughput as the performance criterion, we obtain the sensing parameters so as to optimize the performance of the secondary user while providing the primary user with its desired level of interference protection. I. INTRODUCTION As evidenced by recent measurements, many frequency bands across the licensed spectrum are significantly under- utilized (1), (2). This finding suggests that the spectrum scarcity, as perceived today, is largely due to the inefficient fixed frequency allocations rather than the physical shortage of the spectrum and has led the regulatory bodies to consider the opportunistic access to the temporally/spatially unused licensed bands (a.k.a. the white spaces) as a means to improve the efficiency of spectrum usage. In the absence of cooperation or signalling between the primary licensee and the secondary users, spectrum availability for the opportunistic access may be determined by direct spectrum sensing where the secondary user monitors a licensed band for a given "sensing time" and opportunistically transmits if it does not detect any ongoing licensed operations. This approach is particularly appealing due to its low deployment cost and its compatibility with legacy primary users and is being considered for inclusion in the upcoming IEEE 802.22 standard for opportunistic access to the TV spectrum (3). Due to their ability to autonomously detect and to react to the changes in the spectrum usage, secondary users equipped with the spectrum sensing capability may be considered as a primitive form of the cognitive radio (4). Design of any sensing scheme involves balancing a tradeoff between the quality and the speed of sensing through an appropriate selection of the sensing time. As we shall illustrate, in the context of spectrum sensing, sensing time may be fine- tuned to enhance the secondary users' perceived quality-of- service (QoS) as long as the regulatory constraint for the protection of the primary users against harmful interference is satisfied. In particular, we will obtain the optimum sensing times at different stages of the spectrum sensing to maximize the average throughput of the secondary user. In this paper, simple energy detection (a.k.a. radiometry) (5) is chosen as the underlying detection scheme. In general, when some information about the structure of the primary signal is available, ad hoc feature-detectors offer a better performance (6). We note, however, that the methodology employed in this paper may be applied to optimize different spectrum sensors once the sensing quality is characterized in terms of the sensing time. The remainder of this paper is organized as follows. The regulatory constraints on spectrum sensing are described in the following section. Section 3 provides an overview of the energy-based spectrum sensing. The optimum sensing times for different stages of the spectrum sensing are derived in Section 4. Finally, this paper is concluded in Section 5.