Padmashree Dr. D. Y. Patil Institute of Engineering and Technology, Pimpri, Pune – 411 018.
Abstract- Opportunistic unlicensed access to the (temporarily)unused frequency bands across the licensed radio spectrum is currently being investigated as a means to increase the efficiency of spectrum usage. Such opportunistic access calls for implementation of safeguards so that ongoing licensed operations are not compromised. Among different candidates, sensing-based access, where the unlicensed users transmit if they sense the licensed band to be free, is particularly appealing due to its low deployment cost and its compatibility with the legacy licensed systems. The ability to reliably and autonomously identify unused frequency bands is envisaged as one of the main functionalities of cognitive radios. In this article we provide an overview of the regulatory requirements and major challenges associated with the practical implementation of spectrum sensing functionality in cognitive radio systems. Furthermore, we outline different design trade-offs that have to be made in order to enhance various aspects of the system’s performance.
Driven by consumers’ increasing interest in wireless services, demand for radio spectrum has increased dramatically. Moreover, with the emergence of new wireless devices and applications, and the compelling need for broadband wireless access, this trend is expected to continue in the coming years. The conventional approach to spectrum management is very inflexible in the sense that each operator is granted an exclusive license to operate in a certain frequency band. However, with most of the useful radio spectrum already allocated, it is becoming exceedingly hard to find vacant bands to either deploy new services or enhance existing ones. On the other hand, as evidenced in recent measurements, the licensed spectrum is rarely utilized continuously across time and space .The spectrum utilization in the frequency bands between 30 MHz and 3 GHz averaged over six different locations . The relatively low utilization of the licensed spectrum suggests that spectrum scarcity, as perceived today, is largely due to inefficient fixed frequency allocations rather than any physical shortage of spectrum.
This observation has prompted the regulatory bodies to investigate a radically different access paradigm where secondary (unlicensed) systems are allowed to opportunistically utilize the unused primary (licensed) bands ,commonly referred to as white spaces. In particular, the Federal Communications Commission (FCC) has already expressed its interest in permitting unlicensed access to white spaces in the TV bands . This interest stems in part from the great propagation characteristics of the TV bands and their relatively predictable spatiotemporal usage characteristics. Building on this interest, the IEEE has formed a working group (IEEE 802.22) to develop an air interface for opportunistic secondary access to the TV spectrum.In order to protect the primary systemsfrom the adverse effects of secondary users’ interference, white spaces across frequency, time and space should be reliably identified. Table 1 lists a variety of approaches that may be employed for this purpose. The first two approaches charge the primary systems with the task of providing secondary users with current spectrum usage information by either registering the relevant data (e.g., the primary system’s location and power as well as expected duration of usage) at a centralized database or broadcasting this information on regional beacons . While leading to simplified secondary transceivers, these methods require some modifications to the current licensed systems and, as such, are incompatible with legacy primary users. Moreover, their deployment is costly and requires positioning information at the secondary users in addition to either a ubiquitous connection to the database or a dedicated standardized channel to broadcast the beacons. Spectrum sensing, on the other hand, solely relies on the secondary system to identify white spaces through direct sensing of the licensed bands. In this case the secondary system monitors a licensed frequency band and opportunistically transmits when it does not detect any primary signal.
Volume -01, Issue -05 , December 2013.