Abstract—It is commonly believed that there is a crisis of spec-trum availability due to tremendous growth in the wireless basedservices and evolution of new radio communication technologies.In fact, there are many areas of the radio spectrum which arenot fully utilized. Cognitive radio (CR) has been proposed toenhance utilization of spectrum in opportunistic manner. Thefirst and most important requirement for deployment of CRtechnology is identification of primary user, which is not an easyjob. A spectrum user data bank is need to be established foridentification of primary users. This paper provides an overviewof spectrum monitoring requirement for maintaining of databasefor deployment of cognitive radio technology based services.
Index Terms—Radio spectrum, spectrum monitoring, cognitiveradio, NFAP 2008.
I. INTRODUCTION
Traditionally radio spectrum is managed worldwide bycommand and control method [1] in which radio spectrumis divided into different frequency bands and these bands areallocated to different radiocommunication services on exclu-sive basis. As per international radio regulation, there are 41different radiocommunications services such as mobile, fixed,broadcast, fixed satellite and mobile satellite services etc.This traditional method of spectrum management is efficientto avoid interference between users but leads to inefficientspectrum utilization and creates artificial shortage of spec-trum. Studies on spectrum utilization have been carried outworldwide and reported that spectrum is underutilized in mostof the frequency bands at given time and location meaningthat, part of the frequency band could be free and available ata particular time and location,although it has been allocatedto some primary services [2]. The Federal CommunicationCommission (FCC) estimates [3] that the variation of use oflicensed spectrum ranges form 15% to 85% whereas DefenceAdvance Research Projects Agency (DARPA) [2] claims thatonly the 2% of the spectrum is in use in US at any givenmoment, even if all bands are allocated.
Cognitive radio is considered a promising technology forexploiting underutilised spectrum, arises by current spectrumallocation policy.Cognitive Radio (CR) is a radio that canchange its transmitter parameters based on interaction withthe environment in which it operates and exploit the vacantspectrum spots/holes available in time and space [4]. Thepre-condition of deployment of cognitive radio technology isidentification of spectrum holes. Extensive studies are beingundertaken by the various government agencies and research
institutes for deployment of cognitive radio in real environ-ment. Several methodologies [5] have been proposed for detec-tion of primary signal under spectrum sensing concept. Thesemethods can detect the presence of signal but they cannotidentify signal which causes interference into primary userservices. For identification of primary user, a database aboutusage pattern of primary signal should to be maintained. Thisdatabase will provide statistical model of spectrum utilizationof the primary user as function of time, space and frequencyetc and the rules related to the use of the frequencies incertain locations [6]. By knowing the locations and havingaccess to the database, the cognitive radio devices can checkavailable frequencies from the database to be used for theirown transmissions. Database approach is useful to present thespectrum use of systems where the locations of the stationsremain stable and spectrum use does not change frequently andit will also help to deploy cognitive radio device at initial stage[7]. This paper gives an idea about what should be monitoredto develop database.
The paper is organized as follows: In section II, we intro-duce a brief about cognitive radio. In section III, we discussthe requirement of spectrum monitoring for deployment ofCR technology. Deployment of cognitive radio is discussed insection IV and the section V makes the final conclusion.
II. COGNITIVE RADIO
The term ’Cognitive Radio’ [8] [9], was first introduced byJoseph Mitola in an article published in 1999, he describedhow the flexibility in wireless services can be enhancedthrough cognitive radio. As per ITU definition [7], CognitiveRadio is a radio system employing technology that allows thesystem to obtain knowledge of its operational and geographicalenvironment, established policies and its internal state; to dy-namically and autonomously adjust its operational parametersand protocols according to its obtained knowledge in orderto achieve predefined objectives; and to learn from the resultsobtained. Some of the key features typically associated withCR include [10]:
• Maintains awareness of its operational and geographicalenvironment and its internal state.
• Autonomously adjusts its operating parameters to meetrequirements and goals.
• Learns from previous experiences to further improve itsperformance.
• Reasons on observations to adjust adaptation goals.• Take future decision based on anticipated events.• Collaborates with other devices to make decisions
through collective observations and knowledge.In cognitive radio, primary user(PU) is licensed user and
secondary user(SU) is unlicensed user, who can exploit spec-trum allocated to PU in such a way that it should not causeinterference into primary user services. Therefore, secondaryusers should have cognitive radio capabilities to check whetherspectrum is being used by primary users and to change radioparameters to exploit the unused part of the spectrum [11].
Internal
state
Learning
Obtaining
knowledge
Decision and
Adjustment
C R SOutside World
Fig. 1. cognitive radio system [7]
The component of cognitive radio cycle [7] is presented inFigure 1. Outside world includes information about establishedpolicies, other radio system, current status of spectrum usage,geo-location of PU and users’ needs etc. The internal stateincludes traffic load distribution, interference level, coveragearea, antenna orientation and transmission power levels ofthe cognitive radio system. The first key feature of CR isobtaining knowledge. Obtaining knowledge is the capabilityto obtain the knowledge of operational radio environmentand geographical environment, the established policies andits internal state; and to monitor usage patterns and users’needs and any subsequent changes. Outside knowledge canbe obtained through spectrum sensing and data base access.Database provides information about usage pattern of PU andregulatory policies etc. For example, the data base could tellthe cognitive device, what frequencies it can use in its currentlocation. Obtaining knowledge is generally known as cognitivecapability of CR.
The second key feature of the CR is re-configurabilityi.e. decision and adjustment of operational parameters andprotocols. Re-configurability of the CR is its capability todynamically and autonomously adjust its operational param-eters like operating frequency, modulation, transmit powerand communication technology and protocols according to
the obtained knowledge, which includes past experience, inorder to achieve some predefined objectives. The CR analysesthe obtained knowledge and dynamically and autonomouslymakes decisions on its reconfiguration for example, to getappropriate communication quality, to change the radio accesstechnology to be used in a certain connection or adjust theradio resources dedicated to a system, to adjust a transmissionpower to reduce interference.
The third key feature of CR is the capability to learn. Theobjective of the learning process is to improve its performanceby using stored information of its past actions and theirresults. CR evaluates each action and constantly optimizesthe parameters to further improve the performance. A keyfunctionality of the learning process is to create and maintainknowledge basis in the changing environment.
The cognitive capability is performed in receiver whereasreconfigurability and learning from past actions are performedin transmitter based on the feedback provided by the receiverabout available spectrum hole and channel conditions throughcognitive capability.
An ideal CR device can work in any frequency band. Since,cognitive radio technology is in nascent stage, deployment ofCR can be categorized into four different classes [12]. Firstcategory belongs to those assigned spectrum bands which arerarely utilized within a specific time and geographic location.The second category belongs to spectrum bands allocated topredictable nature of fixed signals, such as television trans-mission where the spectrum appears to be fully utilized buttime based opportunities are available for cognitive radio basedsystem. The third category belongs to those spectral regionsthat are infrequently utilized, where the cognitive radio mustbe capable in detecting primary user transmissions when theyoccur, and also strong in ceasing transmissions immediately,or moving the transmissions to other unoccupied channel.Military and civilian government bands are examples of thisclass. The last category is those regions where the spectrumis relatively well-utilized, but still has some capacity availableon time and location basis. In this case, the cognitive radiocapabilities must be highly capable to insure that the radiosdo not interfere with primary spectrum users.
Cognitive radio is not a radiocommunication service, butis a technology that can be implemented under any servicesdefined in the radio regulation of ITU-R [7]. No separateradio regulation need to be developed for cognitive radio.The key benefit for deploying cognitive radio is to improvethe overall spectrum efficiency and increase flexibility. Thisalso facilitates to enhance the economic value of spectrum.Cognitive radio system encourage the manufacturers to comeup with new ideas as spectrum scarcity would not hindrancefor new services. It also promotes green energy concept as hightransmit power would cause interference to existing primaryusers.
III. REQUIREMENT FOR SPECTRUM MONITORING FORCOGNITIVE RADIO
As stated, cognitive radio is the most promising technologyto enhance the spectrum utilisation. The first and foremostissue in deployment of cognitive radio is faithful detection ofprimary signal. Spectrum sensing in cognitive radio involvesdeciding whether the primary signal is present or not fromthe observed signals. The main objective of spectrum sensingis to provide more spectrum access opportunities to CR userswithout interference to the primary networks [13].
Presently there are three methods [6] of implementingspectrum sensing: match filtering, energy detection and cyclo-stationary feature detection. All these three techniques differin their sensing capabilities and computational complexities.Matched filtering [5] is an optimal way for signal detectionwhen the primary signal is known to the secondary user but theproblem is that secondary user needs a dedicated demodulatorfor each type of primary signal, which increases overheads andmake it more complicated and also it is not suitable for verylow SNR environment. Energy detection is used to determinethe presence of signals without prior knowledge.It simplymeasure the energy of received signal and compares it topredefined threshold level. This method is simplest one and haslow implementation complexities. The main drawback of theenergy detector is its inability to discriminate between sourcesof received energy (the primary signal and noise), makingit susceptible to uncertainties in background noise power,especially at low SNR [14]. Cyclostationary feature detectionis used to extract signal features in the background of noise[15].In cyclostationary detection, periodicity characteristics areused to sense the presence of primary user signal. Modulatedsignals are in general coupled with sine wave carriers, pulsetrains, repeating spreading, hoping sequences, or cyclic pre-fixes which result in built-in periodicity whereas noise don’thave such properties.This technique is a promising optionfor the spectrum sensing in cognitive radio, especially in thesituation where energy detection is not sufficient effective. Thedisadvantage of cyclstationary detection is that it is computa-tionally complex and requires significantly long observationtime [5] and failed when periodicity of primary signal is notknown. In nut shell, all the three detection methods are notvery much reliable and do not provide accurate information.
Another concern with CR is the hidden node problem [5].This situation arises when a CR is unable to detect all ofthe radios with which it might interfere, not because its ownspectrum sensing is ineffective, but because some radios arehidden from it. Receive-only devices (e.g. television, radio,astronomy), devices with very weak transmit signals and anydevices whose transmissions are prevented from reaching theCR by the environment or terrain are example of hidden node.
Presently, sensing technology is under development, the CRdevice may need to use updated database about usage patternof primary users. This database can be maintained throughextensive spectrum monitoring of frequency bands with anymonitoring equpment such as spectrum analyzer. Presently
spectrum monitoring is being performed for band occupancy,channel occupancy, bandwidth of a signal, transmitter poweretc. Now the question arises that what should be the aimof developing database in respect of deployment of cognitiveradio system. The aims are given below [6]:
• to collect data to build a statistical model for usage ofthe spectrum;
• to collect parameters for signal identification;• to validate information on legitimate users;• to investigate the usage variation of signals in different
environments;• to investigate the cases of congestion of signals;• to collect the information for effective regulations of new
bands; and• to identifying areas for further use, sharing or realloca-
tion.Successful inclusion of any CR based system in spectrum
band is depends upon identification of primary signal. Spec-trum sensing provides idea about presence of signal but someother information is also required to identify the type of signallike signal bandwidth, power level, center frequency, type ofmodulation, carrier information etc. These new parametersenhance the spectrum sensing capabilities of cognitive radiossuch as allocation of spectrum to secondary users can be donedepending on the identified signal to reduce interference withthe primary users. If the signal type of the primary user isknown, alternative methodologies can be applied to allocatethe secondary user.
Now the question is that what should be monitored toachieve the above mentioned spectrum monitoring aims. Inour view the following should be monitored in respect of PUto create database for cognitive radio system.
1) Bandwidth/modulation2) Transmitted power level3) Carrier frequency4) Radio access technology5) Duration or duty cycle of operation6) Geographical location7) Outdoor or indoor operation8) Adjacent channel interference informationBased on the above information, a centralized database
should be maintained location wise. For effective deploymentof CR based system, this database should be updated regularlyand the database either be made available to secondary users ormay be kept in central server. When CR equipped user desireto transmit, first it would scan the entire spectrum band andidentify the white spot/spectrum hole in the band thereafterit would exchange the information with updated databaseto identify the primary signal. Based on the information,CR based SU would reconfigure its parameters and starttransmission. Spectrum sensing should be carried out by SUat regular interval as it may need to shift another spot. Thisdatabase server can also be worked as access point in multiuser cooperative sensing environment.
There are many challenges related to database management.
The authenticity of collection of information to the databaseand keeping this information consistent and up-to-date iscritical for the successful use of databases. Another problemis overhead cost involve creating, maintaining and updatingof the database. The management of databases includes alsosecurity and privacy aspects that need to be addressed. Aprotocol need to be developed to access this database by theCR system. This protocol should not be much complicatedotherwise additional power and time require accessing thisdatabase. This would further enhance overhead cost in CRsystem.
IV. COGNITIVE RADIO IN INDIAN PROSPECTS
Spectrum regulatory body in various countries are studyingthe pro and cons of CR devices. Some countries have alreadymade provisions for Cognitive radio. FCC has already madeprovision for use of cognitive radio device in TV bands [16].They have allowed unlicensed operation in the TV bandsat locations where frequencies are not in use by licensedservices. For the purpose of minimizing interference, theyhave classified unlicensed devices in two general functionalcategories. The first category consist of lower power ”per-sonal/portable” unlicensed devices, such as Wi-Fi-like cardsin laptop computers or wireless in-home local area networks(LANs). The second category consist of higher power ”fixed”unlicensed devices that would operate from a fixed locationand could be used to provide commercial services such aswireless broadband Internet access. However, studies show thatCR devices are not working satisfactorily. Most of the time,they failed to ”sense” the presence or absence of an occupiedTV channel. Ofcom, UK has also studied CR in their spectrumframework review and made provision for use of unlicensedcognitive devices in TV band by using TV spectrum hole[17]. CEPT has also allowed cognitive radio based device towork in TV band exploiting TV white space [18]. Internationalagencies like International Telecommunication Union (ITU)are also studying the feasibility of CR technology to enhancespectrum efficiency through its various study groups.
Spectrum utilization in India is similar to other countries.Plot for spectrum usage at different location and time forfrequency band 600-800 MHz (fig. 2 & 3) and 2-3.1 GHz(fig.4 & 5) has been taken. These plots show that spectrumin these bands in India is underutilized.These bands can beopened for CR application.
Presently no specific regulation has been made for intro-duction of CR technology in India. In our views CR devicesmay be permitted to bands in which spectrum activity islow, location of base stations are known and receivers arerobust against interference bands whereas bands allocated foraeronautical services, marine services and safety services maynot be opened. Broadcasting,Radar, amateur bands are mostprominent candidate for CR applications as activities in thesebands is not high and these services are almost outdated.
The 698-806 MHz band has been primary allocated forbroadcasting services worldwide. This is most suitable bandfor cognitive radio application due to its good propagation
Fig. 2. Frequency utilization in 600-800 MHz band at one location
Fig. 3. Frequency utilization in 600-800 MHz band at other location
properties for long-range communications and also spectrumsensing conditions can be relaxed in this band being TVbroadcast is primary user. Most of the countries e.g. USA,Canada, European Union have already made provisions for useof cognitive radio application in this band. As per NationalFrequency Allocation Plan (NFAP) 2008 of India [19], thisband has been earmarked predominantly for broadcastingservices which include mobile TV. IEEE 802.22 [20] standardon cognitive wireless regional area network (WRAN) which isa cognitive air interface for fixed, point-to-multipoint WRAN’sthat operate on unused channels in the VHF/UHF TV bands.Use of IEEE 802.22 standard based WRAN has already beenallowed in most of the countries. Equipment in the market isalso available. Usage pattern in 600-800 MHz bands in Indiaalso support that 700MHz band is most suitable band for CR
Fig. 4. Frequency utilization in 2-3.1 GHz band at one location
Fig. 5. Frequency utilization in 2-3.1 GHz band at other location
application.The next suitable band is 2.3-2.4 GHz. In Asia region,
China and South Korea has already been opened for IMTapplication. In India [19], this band has been earmarked forfixed and mobile services for primary use and requirementof IMT applications including BWA can be considered. Thisband can be utilized for CR application as unlicensed lowpower devices are working perfectly in adjacent band (2.4GHz). However, primary services in adjacent bands shall beprotected from any interference due to cognitive radio.
V. CONCLUSION
Cognitive Radio (CR) has been attracted much attention inboth academic and industrial circles during the last years for itscapability to solve the problem of spectrum underutilization.Cognitive radio will gain acceptance only if the model of PU
detection is accurate enough to ensure no interference. Beforedeployment of CR Sustem, extensive spectrum monitoringneeds to be carried out to get he first hand knowledge ofactual spectrum utilization at different location & time. Basedon these monitoring data, bands in which spectrum usage islow may be opened first for use of CR technology.
REFERENCES
[1] Cristian Ianculescu, Andy Mudra. Cogni-tive radio and dynamic spectrum sharing.http://www.sdrforum.org/pages/sdr05/4.4%20Spectrum%20Mgmt%20and%20Cognitive%20Radio%202/4.4-03%20ianculescu%20et%20al.pdf
[2] Matteo Gandetto, Andrea F. Cattoni, Carlo S. Regaz-zoni. A Distributed Approach to Mode Identifica-tion and Spectrum Monitoring for Cognitive Radios.http://www.sdrforum.org/pages/sdr05/4.3%20Spectrum%20Mgmt%20and%20Cognitive%20Radio%201/4.3-02%20Gandetto%20et%20al.pdf
[3] FCC, ET Docket No 03-222 Notice of proposed rule making and order,December 2003.
[4] Ian F. Akyildiz, W Y Lee, M C Varun, S Mohanty, A survey on Spec-trum Management in Cognitive Radio Networks, IEEE CommunicationMagzine, April 2008.
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[6] Ali Gorcin, Bhaskar Thiagarajan. A Signal Identification Application forCognitive Radio. SDR Forum Technical Conference 2007.
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[9] Simon Haykin, ”Cognitive radio: brain empowered wireless communica-tions,” IEEE J. Select. Areas Commun, vol. 23, no.2 Feb. 2005.
[10] ”IEEE 802 Tutorial: Cognitive Radio”, Scott Seidel, Raytheon, Presentedat the IEEE 802 Plenary, 18 July 2005
[11] A. Sahai and D. Cabric. Spectrum sensing: fundamentals limits andpractical challenges. IEEE International Symposium on New Frontiers inDynamic Spectrum Access Network(DySPAN), November 2005.
[12] Dennis A. Roberson, Cynthia S. Hood, Joseph L. LoCicero,John T. MacDonald. Spectral Occupancy and InterferenceStudies in support of Cognitive Radio Technology Deployment.http://www.wemi.ece.iit.edu/publications/Cognitive
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