Spectrum sensing for cognitive radio based on flexible RF filtering

In this paper we investigate a spectrum sensing technique suitable for cognitive radio (CR) considered a means to mitigate congestion in the future multiple access communication systems. The ultimate objective is opportunistic use of unoccupied frequency bands called spectrum holes or white spaces, which belong to another system. For this purpose, first, we identify the spectrum sensor (SS) nonlinearities and scan the available spectrum in wideband mode where the primary task is to identify strong interference. Next, by a complementary analysis we pick up channels which are likely to be spectrum holes. In the second stage the SS is tuned to a selected sub-band by making use of a built-in flexible RF filter which largely attenuates interference and the related intermodulation distortions (IMD). The scan process carried out in this stage is aimed at detection of a vacant channel, i.e. containing only noise that must be distinguished from a possible weak signal that usually requires a significant computation overhead, but as the scan is narrowband and the S/N ratio can be high, the related overhead is largely reduced compared to the wideband sensing approach. The strength of this technique is in the flexible RF filter eliminating IMD which typically tend to obscure the spectrum holes.

I. Introduction

Over the last two decades, the demands on wireless services have been increasing rapidly. The data rates in wireless communications experience an exponential growth leading to a demand for more transmission spectrum. Fortunately, the occupation of different radio frequency bands largely varies. While some bands are fully occupied, others are largely underutilized. The reported utilization of spectrum between 30 MHz and 3 GHz is about 5.2%, and it is less than 0.5% between 3 GHz and 5 GHz [1], [2]. In this situation one of the most promising solutions is the deployment of cognitive radio (CR), which enables spectrum sharing by dynamic and opportunistic spectrum access [3]–[5]. It is assumed that a radio in a CR system is capable of observing whether a particular frequency band is being occupied and, if not, it can use the band without interfering with other authorized systems. If the primary system of the band restarts transmission, the CR jumps to another frequency band or stays in the same band, altering its transmission power level or modulation scheme to avoid interference [3]. The available frequency bands can be, in particular, the ISM bands, the TV White Spaces or the licensed frequency bands of other operators.