Opportunistic Spectrum Sensing and Transmissions

1.1 Introduction

The demand for wireless communications tremendously increased in time and to cope up with this demand, cognitive radio (CR) is a solution of huge prospect. CR can be described as an intelligent and dynamically reconfigurable radio that can adaptively regulate its internal parameters (or similar) in response to changes in the surrounding environment. CR has been made feasible by recent advances such as software-defined radio (SDR), machine learning techniques and smart antennas, etc (e.g., see Bixio et al, 2011). The use of CR technology allows in principle flexible and agile access to the spectrum as well as improving spectrum efficiency substantially. In this sense, CR represents a possible solution to the problem of spectrum scarcity, due to the variety of bandwidth demanding newly developed wireless communication techniques. In particular, spectrum scarcity occurs due to the use of traditional static frequency allocation planning for different communication protocols. Federal Communications Commission, FCC (2002) disclosed that the utilization of the continuously assigned spectrum still only ranges between 15% and 85%. This means that the primary users (also known as licensed or legacy users) do not often occupy the allocated radio resources (code, temporal and spatial domain) incessantly and this leads to their underutilization. Allocated but not used spectrum bands are usually known as spectrum holes or white spaces (WSs) in literature (Zhao & Sadler, 2007). WSs can opportunistically be brought into play in CR network by introducing efficient techniques expressed as dynamic spectrum access (DSA). DSA make it possible to a CR user to sense the vacant spectrum before using it temporarily, thus making it more likely to occur an improved efficient spectrum employment (Zhao & Sadler, 2007).

When a CR user aims at using WSs, interferences with the PU licensees may occur. Therefore, one of the main challenges in CR network is related to the management of the available radio resources among the PUs and cognitive users for satisfying the respective quality-of-service (QoS) requirements while limiting the interference to the PU licensees (Hsien-Po & Schaar, 2009). The growing interest of DSA in CR is specially related to the fact that it is considered as a possible solution of the static spectrum assignment. DSA techniques for the cognitive radio can be classified as dynamic exclusive-use, shared-use or hierarchical access and open sharing or spectrum common models as suggested by several researchers as E. Hossain et al (2009) and Couturier & Scheers (2009). In those papers DSA models were described; in the dynamic exclusive-use model, a licensed user can grant to a cognitive user the right to have exclusive access to the spectrum. In the hierarchical access model, an unlicensed user accesses the spectrum opportunistically without interrupting a licensed user when a legacy user is not interested to utilize that portion of the frequency spectrum. In an open sharing model, an unlicensed user can access the spectrum freely.