Research Projects

Spectrum sensing techniques with cognitive radio applications

List of researchers

• Danyo Danev , associate professor, Communication Systems, ISY.
• Erik Axell , Ph.D. student, Communication Systems, ISY.

Short description

The goal of this project is to develop algorithms and methods that can detect the presence of very weak communications signals. The primary intended application is cognitive radio. Cognitive radio is an emerging technique with which licensed spectrum can be locally reused by unlicensed users. One of the fundamental problems in cognitive radio is to be able to detect the presence of weak signals from a primary (licensed) system, and this is the problem that we address.

The project proposal reflects the long-term research strategy and ambition of the Division of Communication Systems at ISY. In particular, the proposed work is closely connected to the FP7 sponsored project ``Sensor network for dynamic and cognitive radio access'' (SENDORA) in which the division will participate as a partner.

Background and industrial relevance

Fast and reliable wireless communication depends mainly on the available frequency bandwidth. The frequency spectrum is common to all radio systems. Conventional solution to the interference problem is to apply frequency planning. In this way the available spectrum is divided into frequency bands which are designated for particular wireless services. Some frequency bands are left free for everyone to use under certain power spectrum assumptions. Such bands are known as unlicensed frequency bands. In the other frequency bands only one actor (also known as licensed user) is allowed to make transmisions. The so-called spectrum holes arise when licensed users do not utilize their designated spectrum in a given area at a given time. Cognitive radio [1,3,4,6,7,8] is an emerging wireless communications concept in which a network or a wireless node is able to sense its environment, and especially spectrum holes, and change its transmission and reception chains to communicate efficiently without interfering with licensed users. Cognitive radio thus aims to improve the way the radio spectrum is utilized.

The motivation for cognitive radio stems from various measurements of spectrum utilization [5], which generally show that spectrum is under-utilized, in the sense that the typical duty cycle of spectrum usage at a fixed frequency and at a random geographical location is low. This means that there are many "holes" in the radio spectrum that could be exploited. Cognitive radios should be able to exploit these spectrum holes by detecting them and using them in an opportunistic manner. In particular, it has been argued that cognitive radios could be permitted to transmit if they cannot ``hear'' any primary transmission. The US the Federal Communications Commission (FCC), following requests from telecommunications companies like the Shared Spectrum Company (SSC), is planning to allow unlicensed usage of certain frequency bands [2]. This will challenge the conventional view, where no secondary users are allowed to use licensed spectrum. Currently this trend is US dominated, but the EU regulatory bodies are expected to follow. Strong indication for it is the support for the SENDORA project through the EU's seventh framework programme.

A central part in the design of cognitive radios is the so called spectrum sensing. This should provide efficient and reliable methods for the detection of signals from the primary licensed technology (PLT). The only way to tell, whether a PLT user is active, is by trying to detect its signal. However, this may in some cases be very difficult. It is not acceptable for the cognitive radio to exceed the allowed interference temperature levels to the primary user (PU), even when the PU is operating at its defined minimum sensitivity level.

Multipath fading and shadowing add a significant uncertainty to the power levels and signal-to-noise ratios (SNR) that the cognitive radio (CR) of the secondary user (SU) observes compared to what a nearby primary user might see. Consequently, the CR must be able to sense the PU's signal well below the detection threshold of the PU itself. Moreover, difficult propagation environments may require that secondary users perform the detection cooperatively in a decentralized manner. The detection task becomes even more difficult if the PLT signal is very weak, is not continuous in time or frequency or if it is a direct sequence spread spectrum signal. The main focus of this project will be the development of efficient ways of sensing the presence of weak PLT signals.

Project plan and goal

The main goals of the project are:

• To derive decision rules for detecting different types of primary user signals.
• To design sensing strategies and algorithms for sensing the primary users' signal.
• To evaluate sensing algorithms in terms of performance, complexity, and energy efficiency.

The project will be initiated by a study and characterization of potential primary user signals. In order to do this the statistical and structural properties of potential primary user signals such as different variants of OFDM, WCDMA and GSM will be investigated. The results will be used in the development of sensing algorithms that take advantage of those properties.

The main concern of the project is weak signal detection. It will focus on decision making in the very low SNR regime. Detection of known weak signals in Gaussian and non-Gaussian i.i.d. noise has been studied thoroughly. A convenient performance criterion used for such scenarios is to maximize local detection power, i.e., find locally optimum (most powerful) detectors. Such detectors maximize the slope of the detector power function at the origin among the detectors that have the specified false alarm probability. One major difficulty is that the signals to be detected (which originate from the primary system) are unknown or at least have unknown parameters. A possible approach to deal with this problem is to exploit cyclostationarity of the signals. The statistics of man-made signals typically display periodicities related to symbol rate, chip rate, used channel code or cyclic prefix. Cyclostationary statistics may be used to reveal these periodicities and construct statistical tests. In order to control the probability of false alarms in detectors, algorithms for estimating noise and interference statistics are developed.

The proposed spectrum sensing algorithms will be evaluated. The performance of the detection algorithms will be studied using statistical tools including ROC curves, asymptotic relative efficiency (ARE) and robustness. Controlling the false alarm probability is crucial in order to make sure that the cognitive radio system is not overwhelmed with alarms and to ensure that some data will get transmitted. Algorithms will also be compared from the implementation point of view by estimating the hardware cost and energy efficiency and how they relate to the computational complexity of the algorithm.

Application of the designed algorithms in other areas than cognitive radio will be considered. Spectrum sensing and detection of weak signals is central point in situations like indoor GPS positioning, positioning in GSM networks, astronomy, geophysics etc. Possibility for implementation of the suggested techniques in such areas will be investigated.

Project visions

The long term visions of this project is to to put the Communication Systems group among the leaders in Europe in the cognitive radio technology.

In a three years therm this project should result in one PhD thesis as well as several published research papers.

Relevance to other CENIIT financed projects

There are an indirect connections to several other projects supported by CENIIT. Potential cooperation can be established with some of the following projects:

• Flexible frequency band reallocation, Per Löwenborg, ISY - the utilization of the detected ``spectrum holes'' can be achieved with help of filter bank (FB) based frequency-band reallocation methods suggested by this project.
• Design and Implementation of DSP Systems, Håkan Johansson, ISY - filters with adjustable frequency responses are needed for implementation of signal detection algorithms. The reduced complexity of the suggested fractional-delay FIR filters will lead to faster and more power-efficient operation of the detection equipment.
• Algorithm-hardware co-design for FPGAs, Oscar Gustafsson, ISY - adaptivity of cognitive radios is enabled by reprogrammable and reconfigurable processors. To achieve programmability a field programmable gate arrays and digital signal processors are used.

Bibliography

1. R. W. Broderson, A. Wolisz, D. Cabric, S. M. Mishra, and D. Willkomm, Corvus: A cognitive radio approach for usage of virtual unlicensed spectrum, Tech. rep. (white paper), 2004, Available at http://bwrc.eecs.berkeley.edu.
2. FCC, FCC 04-113, May 2004,
   http://hraunfoss.fcc.gov/edocs_public/attachmatch/FCC-04-113A1.pdf.
3. S. Haykin, Cognitive radio technology: brain-empowered wireless communications, IEEE Journal on Selected Areas in Communications 23 (2005), 201-220.
4. E. G. Larsson and M. Skoglund, Cognitive radio in a frequency planned environment: Can it work?, Proc. IEEE Globecom, Nov 2007.
5. M. A. McHenry, NSF spectrum occupancy measurements project summary, Tech. report, SSC, August 2005, Available at http://www.sharedspectrum.com/.
6. I. J. Mitola, Software radios: Survey, critical evaluation and future directions, IEEE Aereosp. Electro. Syst. Mag. 8 (1993), 25-36.
7. R. Rubenstein, Radios get smart, IEEE Spectrum 44 (2007), 46-50.
8. A. Sahai, N. Hoven, and R. Tandra, Some fundamental limits on cognitive radio, Proc. 42nd Allerton Conference on Communication Control and Computing, Oct 2004.

Note

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