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OFDM for Cognitive Radio: Merits and ChallengesHisham A. Mahmoud, Tevfik Yucek, and Huseyin Arslan

Department of Electrical Engineering, University of SouthFlorida4202 E. Fowler Avenue, ENB-118, Tampa, FL, 33620

E-mail:{hmahmoud, yucek}@mail.usf.edu, arslan@eng.usf.edu

Abstract—Cognitive radio (CR) is a novel concept that allowswireless systems to sense the environment, adapt, and learnfromprevious experience to improve the communication quality.How-ever, CR needs a flexible and adaptive physical layer in ordertoperform the required tasks efficiently. In this paper, CR systemsand their requirements of the physical layer are discussed andorthogonal frequency division multiplexing (OFDM) techniqueis investigated as a candidate transmission technology forCR.The challenges that arise from employing OFDM in CR systemsare identified. The cognitive properties of some OFDM-basedwireless standards are also discussed in order to indicate thetrend toward a more CR.

I. I NTRODUCTION

With emerging technologies and with the increasing numberof wireless devices, the radio spectrum is becoming increas-ingly congested everyday. On the other hand, measurementsshow that wide ranges of the spectrum are rarely used mostof the time, while other bands are heavily used. Dependingon the location, time of the day, and frequency bands, thespectrum is actually found to be underutilized. However, thoseunused portions of the spectrum are licensed and thus cannotbe used by systems other than the license owners. Hence, thereis a need for a novel technology that can benefit from theseopportunities. cognitive radio (CR) arises to be a temptingsolution to spectral crowding problem by introducing theopportunistic usage of frequency bands that are not heavilyoccupied by licensed users (LU) [1]. CR can be defined asan intelligent wireless system that is aware of its surroundingenvironment through sensing and measurements; a system thatuses its gained experience to plan future actions and adapt toimprove the overall communication quality and meet user’sneeds.

A main aspect of CR is to autonomously exploit locallyunused spectrum to improve spectrum utilization. Other as-pects include interoperability across several networks, de-vices, and protocols; roaming across borders while beingable to stay in compliance with local regulations; adaptingthe system, transmission, and reception parameters withoutuser intervention; and having the ability to understand andfollow actions and choices taken by their users to learn andbecome more responsive over time. The focus of this paperis the first aspect,i.e. CR’s ability to sense and be awareof its operational environment, and dynamically adjust itsradio operating parameters accordingly. For CR to achieve thisobjective, the physical layer (PHY) needs to be highly flexibleand adaptable. A special case of multicarrier transmissionknown as orthogonal frequency division multiplexing (OFDM)

is one of the most widely used technologies in current wirelesscommunications systems. OFDM has the potential of fulfillingthe aforementioned requirements of CR inherently or withminor modifications. Because of its attractive features, OFDMhas been successfully used in numerous wireless standards andtechnologies. We believe that OFDM will play an importantrole in realizing CR concept as well by providing a proven,scalable, and adaptive technology for air interface.

In this paper, OFDM technique is investigated as a candidatefor CR systems. CR features and requirements are discussedin detail, and OFDM’s ability to satisfy these requirementsisexplained. In addition, we go through the challenges that arisefrom employing OFDM technology in CR.

The article is organized as follows. In Section II, OFDMtechnology is introduced and a basic system model for OFDM-based CR is presented. Section III discusses the merits ofOFDM technology and its advantages when employed by CRsystems. Challenges to a practical OFDM-based CR systemand possible solutions are addressed in Section IV. SectionVlooks into present and future technologies that use OFDM withCognitive features. Section VI concludes the article.

II. OFDM-BASED CR

OFDM is a multicarrier modulation technique that canovercome many problems that arise with high bit rate com-munications, the biggest of which is time dispersion. The databearing symbol stream is split into several lower rate streamsand these streams are transmitted on different carriers. Sincethis splitting increases the symbol duration by the numberof orthogonally overlapping carriers (subcarriers), multipathechoes affect only a small portion of the neighboring symbols.Remaining inter-symbol interference (ISI) is removed by ex-tending the OFDM symbol with a cyclic prefix (CP). Usingthis method, OFDM reduces the dispersion effect of multipathchannels encountered with high data rates and reduces theneed for complex equalizers. Other advantages of OFDMinclude high spectral efficiency, robustness against narrow-band interference (NBI), scalability, and easy implementationusing fast Fourier transform (FFT).

In this paper, we assume a CR system operating as asecondary user in a licensed band. The CR system identifiesavailable or unused parts of the spectrum and exploit them.The goal is to achieve maximum throughput while keepinginterference to primary/licensed users to a minimum. Anexample of such a CR system could be the IEEE 802.22standard-based system where the spectrum allocated for TV

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kl�Fig. 1. OFDM-based CR system block diagram. All of the layerscan interact with the Cognitive engine. OFDM parameters andradio are configured by theCognitive engine.

TABLE IOFDM-BASED WIRELESS STANDARDS.

Parameters

Standard FFT Size CP Size Bit per symbol Pilots Bandwidth Multiple Accessing

IEEE 802.11 (a/g) 64 1/4 of FFT size 1, 2, 4, 6 4 20 MHz CSMAa

IEEE 802.16 (d/e) 128, 256, 512,1024, 2048

1/4, 1/8, 1/16, 1/32of FFT size

1, 2, 4, 6 variable 1.75 to 20 MHz OFDMA∗/TDMA∗

IEEE 802.22 1024, 2048, 4096 variable 2, 4, 6 96, 192, 384 6, 7,and 8 MHz OFDMA/TDMA

DVB-T 2048, 8192 1/4, 1/8, 1/16, 1/32of FFT size 2, 4, 6 62, 245 8 MHz NA

acarrier sense multiple accessing (CSMA), orthogonal frequency division multiple access (OFDMA), time division multiple access (TDMA)

channels is reused. In this case, the TV channels are theprimary users and the standard-based systems are the sec-ondary users (see section V-B for more details). A blockdiagram of the CR-OFDM system considered in this paperis shown in Fig. 11. The cognitive engine is responsible formaking intelligent decisions and configuring the radio andPHY parameters. The transmission opportunities are identifiedby the decision unit based on the information from policyengine as well as local and network spectrum sensing data. Asfar as the PHY layer is concerned, CR can communicate withvarious radio access technologies in the environment, or itcanimprove the communication quality depending on the environ-

1Some OFDM functions are skipped or simplified in order to keepthefigure simple.

mental characteristics, by simply changing the configurationparameters of the OFDM system (see Table I for some exampleparameters) and the radio frequency (RF) interface. Note thatcoding type, coding rate, interleaver pattern, and other mediumaccess control (MAC) and higher layer functionalitiesetc.should also be changed accordingly.

III. W HY OFDM IS A GOOD FIT FOR CR

OFDM’s underlying sensing and spectrum shaping capa-bilities together with its flexibility and adaptivity make itprobably the best transmission technology for CR systems. Inthe following, we present some of the requirements for CR andexplain how OFDM can fulfill these requirements. A summaryof these requirements and strength of OFDM in meeting themare presented in Table II.

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TABLE IIOFDM CR

CR Requirements OFDM’s Strength

Spectrum sensing Inherent FFT operation of OFDM eases spec-trum sensing in frequency domain.

Efficient spectrumutilization

Waveform can easily be shaped by simplyturning off some subcarriers where primaryusers exist.

Adaptation/Scalability

OFDM systems can be adapted to differenttransmission environments and available re-sources. Some adaptable parameters are FFTsize, subcarrier spacing, CP size, modulation,coding, subcarrier powers.

Advanced antennatechniques

Techniques such as multiple-input multiple-Output (MIMO) are commonly used withOFDM mainly because of the reduced equal-izer complexity. OFDM also supports smartantennas.

Interoperability

With WLAN (IEEE 802.11), WMAN (IEEE802.16). WRAN (IEEE 802.22), WPAN (IEEE802.15.3a) all using OFDM as their physicallayer techniques, interoperability becomes eas-ier compared to other technologies.

Multiple accessingand spectral allocation

Support for multiuser access is already inher-ited in the system design by assigning groupsof subcarriers to different users (i.e. orthogonalfrequency division multiple access (OFDMA)).

NBI ImmunityNBI affects only some subcarriers in OFDMsystems. These subcarriers can be simplyturned off.

A. Spectrum Sensing and Awareness

One of the most important elements of CR concept is theability to measure, sense, learn, and be aware of importantoperating conditions. This includes parameters related totheradio channel characteristics, availability of spectrum,inter-ference temperature, and radio’s operational environments. Inaddition, the system should be aware of user requirementsand applications, available networks infrastructures andnodes,local policies and other operating restrictions. CR shouldbeable to identify and exploit the unused parts of the spectrumin a fast and efficient way. In OFDM systems, conversionfrom time domain to frequency domain is achieved by usingFFT. Hence, all the points in the time-frequency grid of theOFDM system’s operating band can be scanned without anyextra hardware or computation thanks to the hardware reuseof FFT cores. Using the time-frequency grid, the selection ofbins that are available for exploitation (spectrum holes) can becarried out using simple hypothesis testing. In [2, 3], FFT isapplied to the received signal. By using the output of FFT, thereceiver tries to detect the existence of a primary user in theband. In [3], more than one FFT output (averaging in time)is used. However, averaging in time increases the delay ortemporal overhead. In [4], the averaging size (number of FFTs)is adapted in order to increase the efficiency in a cooperativesensing environment. Primary user’s signal is usually spreadover a group of FFT output samples as the bandwidth ofprimary user is expected to be larger than the consideredbandwidth divided by the FFT size. Using this fact, the FFToutput is filtered for noise averaging in order to obtain a better

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performance [5]. In these sensing algorithms, the availabilityof FFT circuitry in OFDM systems eases the requirements onthe hardware. Moreover, the computational requirements ofthespectrum sensing algorithm is reduced as the receiver alreadyapplies FFT to the received signal in order to transform thereceived signal into frequency domain for data detection.

B. Spectrum Shaping

After a CR system scans the spectrum and identifies activeLUs, other rental users, and available opportunities, comesthe next step: spectrum shaping. Ideally, it is desired to allowcognitive users to freely use available bands in the spectrum.It is desired to have a flexible spectrum mask and controlover waveform parameters such as signal bandwidth, powerlevel, and center frequency. OFDM systems can provide suchflexibility thanks to the unique nature of OFDM signaling. Bydisabling a set of subcarriers, the spectrum of OFDM signalscan be adaptively shaped to fit into the required spectrummask2. Assuming the spectrum mask is already known to theCR system, choosing the disabled subcarriers is a relativelysimple process.

An example of spectrum sensing and shaping procedures inOFDM-based CR systems is illustrated in Fig. 2. The two LUsare detected using the output of the FFT block, and subcarriersthat can cause interference to these LUs are turned off. Thetransmitter then uses the unoccupied part of the spectrum forsignal transmission.

C. Adapting to the Environment

Adaptivity is one of the key requirements of CR. Bycombining gathered information (awareness) with knowledgeof current system capabilities and limitations, CR can performvarious tasks. CR can adapt its waveform to interoperate withother friendly communication devices, choose the most appro-priate communication channel or network for transmission,and allocate best frequency to transmit in a free band ofthe spectrum. The system waveform can also be adapted tocompensate for channel fading, and null any interfering signal.OFDM offers a great flexibility in this regard as the numberof parameters for adaptation is quite large [6].

2See Section IV-E for more details and for more advanced algorithms.

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An OFDM-based system can adaptively change the mod-ulation order, coding, and transmit power of each individualsubcarrier based on user needs or the channel quality [7]. Thisadaptive allocation can be optimized to achieve various goalssuch as increasing the system throughput, reducing bit errorrate (BER), limiting interference to LUs, increasing coverage,or to prolong unit battery life. In multiuser OFDM systems,subcarriers allocation to users can be done adaptively as wellto achieve the same goals [8].

One of the attractive features of OFDM for broadbandcommunications is its ability to operate using simple one tapequalizers, in the frequency domain. To maintain this feature,the subcarrier spacing in set to be less than the channelcoherence bandwidth. In addition, to avoid ISI, the systemappends a CP to each symbol with a duration longer than thechannel maximum delay spread. Based on estimated channelparameters, an OFDM-based CR system can adaptively changethe length of the CP to maintain an ISI-free signal whilemaximizing the system throughput [9].

Similarly, OFDM system can adaptively change the sub-carrier spacing to reduce inter-carrier interference (ICI) orpeak-to-average-power ratio (PAPR) [9], the data subcarrierinterleaving to reduce BER [10], or even the used pilotpatterns [11].

The adaptivity in OFDM systems can be performed eitherat algorithm level or at parameter level. In classical wirelesssystems, usually algorithm parameters,e.g. coding rate, havebeen adapted in order to optimize the transmission. However,in cognitive OFDM systems, algorithm type,e.g. channelcoding type, can also be adapted in order to achieve interop-erability with other systems and/or to further optimize systemperformance. To achieve such adaptivity, a fully configurablehardware platform would be needed.

D. Multiple Accessing and Spectral Allocation

The resources available to a cognitive system have to beshared among users. Several techniques can be used to achievesuch a task. OFDM supports well-known multiple accessingtechniques such as time division multiple access (TDMA),frequency division multiple accessing (FDMA) and carriersense multiple accessing (CSMA). Moreover, code divisionmultiple access (CDMA) can be used together with OFDM,in which case the transmission is known as multi-carrier codedivision multiple access (MC-CDMA) or multicarrier directspread code division multiple access (DS-CDMA).

orthogonal frequency division multiple access (OFDMA), aspecial case of FDMA, has gained significant attention recentlywith its usage in fixed and mobile Worldwide Interoperabilityfor Microwave Access (WiMAX). In OFDMA, subcarriers aregrouped into sets each of which is assigned to a different user.Interleaved, randomized, or clustered subcarrier assignmentschemes can be used. Therefore, OFDMA offers very flexiblemultiple accessing and spectral allocation capability forCRwithout any extra hardware complexity. The allocation ofsubcarriers can be tailored according to the spectrum avail-ability. The flexibility and support of OFDM systems forvarious multiple accessing techniques enable interoperabilityand accelerate the adoption of CR in future wireless systems.

E. Interoperability

Interoperability is defined asthe ability of two or moresystems or components to exchange information and to use theinformation that has been exchanged[12]. Since CR systemsmay have to deal with LUs as well as other cognitive users,the ability to detect and encode existing users’ signals canexpedite the adoption and improve the performance of CRsystems. Furthermore, some recent unfortunate disasters man-ifested the importance of interoperability in terms of wirelesscommunications for the first responders. CR has the potentialto improve the disaster relief operations by developing thecoordination among first responders [13].

To achieve interoperability, OFDM is one of the bestsignaling candidates. OFDM signaling has been successfullyused in various technologies including IEEE 802.11a and IEEE802.11g Wireless LAN standards, digital audio broadcast-ing (DAB), digital video broadcasting (DVB), and WiMAX.OFDM has been used in both short range and long rangecommunication systems. Hence, a CR system employingOFDM can communicate with systems using other OFDM-based technologies with much ease. Only the knowledge ofsignal parameters of intended users is needed (see Table I).However, for such task to be successful, the system needsto know all standard-related information required to decodethe signal, such as the data and pilot mapping to the fre-quency subcarriers, frame structure, and the coding type andrate. More importantly, the RF circuitry of the CR systemneeds to be flexible enough to accommodate different signalbandwidths and center frequencies. As a result, CR should bebuilt around a software-defined radio architecture to providerequired flexibility to the system.

IV. CHALLENGES TO COGNITIVE OFDM SYSTEMS

As an intelligent system with features such as awareness,adaptivity and learning, CR represents the future of wirelesssystems with the promise of offering solutions to variouscommunication problems. However, with this new technology,new challenges appear, raising interesting research topics.These challenges can be grouped into three categories asillustrated in Fig. 3. The first category includes the challengesthat are unique to classical OFDM systems such as PAPR,and sensitivity to frequency offset and phase noise. The secondcategory includes problems faced by all CRs such as spectrumsensing, cross layer adaptation, and interference avoidance.Our main focus in this article is on the third category:challenges that arise when OFDM technique is employed byCR systems. In the following, we discuss major challengesto a practical system implementation as well as some of theproposed approaches for solving these challenges.

A. Multiband OFDM System Design

So far we have considered the more conventional singleband systems. In single band CR-OFDM systems, the availableportion of the spectrum is occupied by a single OFDM signal.If LU exist within the used band, the CR system shapesthe OFDM signal as to avoid interference to those usersas shown in Fig. 2. For systems utilizing wide bands of

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the spectrum, multi-band signaling approach –where the totalbandwidth is divided into smaller bands– can prove to bemore advantageous over using single band signaling. Thisappears to be more significant if the detected free parts ofthe spectrum are scattered over a relatively wide band. Whileusing a single band simplifies the system design, processinga wide band signal requires building highly complex RFcircuitry for signal transmission/reception. High speed analogto digital converters (ADC) are required to sample and digitizethe wideband signal. In addition, higher complexity channelequalizers are also needed to capture sufficient multipath signalenergy for further processing. On the other hand, multi-bandsignaling relaxes the requirements on system hardware assmaller portions of the spectrum are processed separately.Dividing the spectrum into smaller bands allows for betterspectrum allocation as well.

For OFDM-based CR, the question becomes when to usemulti-band and when to use single band. Given a certainscanned spectrum shape, choosing the number of bands de-pends on various parameters. Required throughput, hardwarelimitations, computational complexity, number of spectrumholes and their bandwidth, and interference level are examplesof what could affect a cognitive system choice.

It is worth mentioning that multi-band OFDM (MB-OFDM)is employed in ultra wide band (UWB) systems. Instead ofusing a single band UWB signal, the spectrum is divided intosub-bands (with approximately 500 MHz bandwidth each) andOFDM signals are used to transmit data over each band [14].However, while UWB is one of the applications of MB-OFDM, it is only limited to a specific scenario where allsub-bands have almost equal size, and OFDM signals usedin sub-bands are identical in other parameters such as CP sizeand subcarrier spacing.

From a practical point of view, designing a cost effectivemulti-band transceiver with high performance has been studiedin the literature [15–19]. On most proposed transceivers, directconversion architecture is used to eliminate the need for imagerejection filters, and relax the bandwidth requirements forthebaseband filters and converters [15, 16]. The challenges thatface the implementation of a broadband multi-band OFDMsystem includes the need for wide range frequency synthe-sizers, broadband circuits and matching, gain switch in thelow noise amplifier (LNA) without degrading the input match,

broadband transmit/receive switch at the antenna, desensitiza-tion due to adjacent LU interferers, and fast band hopping toavoid interference to occupied bands [17]. Frequency synthe-sizers that can operate in seven bands [18, 19], nine bands [16],and even 12 bands [17] have been presented recently.

If single-band transmission is employed, there might bemany subcarriers that are deactivated. In such a case, theefficiency of FFT algorithms can be increased and the ex-ecution time can be decreased by removing operations oninput values which are zero; a process known as pruning.Designing effective pruning algorithms specific to CR-OFDMis an important subject for achieving higher performance [20].

B. Location Awareness

Geolocation information can be used to enable location-based services, optimize the network traffic, and adapt thetransceiver to the environment. Applications utilizing locationinformation can be grouped into four categories; location-based services, network optimization, transceiver algorithmdevelopment and optimization, and environment character-ization. Although some of the existing wireless networkshave a miniature utilization of location information, CR isexpected to have a more comprehensive location informationutilization [21, 22].

OFDM signaling can be used to obtain the geolocationinformation in CRs [23] without the need for any externalpositioning systems. Pilot sequences (preambles), which arecommonly used in OFDM systems for synchronization, canbe used for acquisition and tracking of units’ locations. Inthe literature, both time and frequency domain techniquesare proposed to estimate the time of arrival (TOA) usingreceived OFDM signal. Existing wireless local area network(WLAN) systems are being studied for indoor positioningapplications while MB-OFDM based UWB is proposed forhigh precision applications. Such positioning capabilities helpOFDM to fulfill another requirement of CR.

C. Signaling the Transmission Parameters

In a CR system, communication units sense the surroundingenvironment and gather up information that can be used to im-prove the communication link. Based on gathered information,the system selects transmission parameters such as LU bands,spectral mask, operating frequency, coding, and modulation.While some of these parameters can be detected blindly byintended receivers, other parameters need to be known priorto establishing a communication link. Distributing informationamong communication units rather than using local sensingreduces the complexity and improves the performance of thesystem. Thus, it is crucial for the success of CR to successfullydistribute such information to other cognitive units.

One approach is to dedicate a communication channel toexchange measured information and transmission parametersamong cognitive units. However, this requires that a channelbe predefined (or licensed) for that purpose. As a result, theability of cognitive units to adaptively operate within anygiven unlicensed band becomes dependent on the availabilityof such channel. Moreover, as the number of units in the

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same cell increases, the amount of information that needs tobe distributed increases as well. This can result in a hugeoverhead that the dedicated channel can not handle.

Other approaches solve the distribution problem by eitherreducing the information overhead or by improving the per-formance of blind detectors. For example, in OFDM systems,based on the scanned channel, waveform is adjusted by turningoff some subcarriers in order to exploit the available spectrumholes (see Fig. 2). The receivers, however, should be informedabout detected spectrum holes (or which subcarriers are deacti-vated). The overhead is reduced by sending a vector containingdisabled subcarriers rather than sending the spectrum sensingresults. One method to further reduce the overhead is proposedin [24]. The activation/deactivation of subcarriers is performedover a block of subcarriers instead of individual subcarriers.Hence, the signaling overhead can be reduced by a factor of theblock size. On the other hand, instead of sharing the spectrumsensing information, tone-boosting can be used [25]. Once acognitive unit detects a LU signal within the band, it sends atone with maximum power but with a very short time durationover the detected signal band. The purpose is to inform otherusers that a LU exists within this band. Thus, the probabilityof interference to LUs is reduced, which is one of the mainpurposes of spectrum sensing. Meanwhile, the short durationof these tones causes no interference to LUs.

D. Synchronization

Synchronization is an important issue that needs to beaddressed in OFDM system design. With the introductionof CR, conventional synchronization methods become insuffi-cient. The NBI, which can interfere with the preamble, is oneof the problems [26]. Furthermore, the incomplete subcarrierset might be an issue for preambles. Pilots as well may fallinto unused subcarriers. Moreover, if multiple accessing is em-ployed, subcarriers are assigned to different users. To keep theorthogonality between subcarriers and to avoid interference,all users should be synchronized to the receiver. In [26], itis shown that longer preambles are required in CR-OFDMsystems as compared to conventional systems. In addition, newpreamble structures are introduced and their performancesfortime and frequency synchronization are investigated.

E. Mutual Interference

The sidelobes of modulated OFDM subcarriers are knownto be large. As a result, there is power leakage from OFDMsignals to adjacent channels. In addition, used subcarriers’power leaks to nulled subcarriers which causes interference,known as mutual interference, to LUs. Various techniques areproposed in the literature to reduce this leakage and to enableco-existence of cognitive-OFDM systems with primary users.One technique is to window the time domain OFDM symbols.However, spectrum shape improvement comes at the cost oflonger OFDM symbol duration and thus reduces the spectrumefficiency of the system. Another approach is to increase thenumber of nulled subcarriers to achieve lower interferencelevels to LU bands as most of the interference is caused by

neighboring subcarriers. However, the obvious disadvantageof this method is again the reduction of spectral efficiency.

A method that reduces interference to spectrum holeswhile keeping high spectrum efficiency is proposed in [27]and [28] and is referred to as active interference cancellationand cancellation carriers, respectively. Instead of disablingsubcarriers adjacent to spectrum holes, a much smaller numberof those adjacent subcarriers is used to reduce the interferenceleaked to spectrum holes. The cancellation subcarriers arepre-calculated to reduce subcarrier sidelobes inside spectrumholes. This technique achieves significant reduction of adjacentchannel interference. The disadvantage of this technique is theincrease in overall system complexity due to the calculationof cancellation carrier values for each symbol. In addition, forlarger spectrum holes, more cancellation carriers are needed tomaintain the desired interference level. Analog or digitalfilterscan also be used to filter-out the unwanted spectral componentsof the OFDM signal prior to transmission. However, sincethe spectrum mask on a CR signal needs to be adaptive,the use of analog filters is not practical. On the other hand,digital filters introduce an increase in the system computationalcomplexity and processing delay. Other methods to reduceOFDM interference to adjacent channels are presented in [29,30].

Fig. 4 shows an example of a CR system which is usingan OFDM signal with FFT size of 256 subcarriers and cyclicprefix of 8 samples. A LU signal spanning three subcarriers 25,26, and 27 is detected. It is desired to minimize the interferenceto the LU. In case I, the CR system disables subcarriers 23though 29. A spectrum hole with 15 dB depth is achieved.In case II, the OFDM symbols are windowed using a raisedcosine window with a roll-off factor of 0.25 while keepingsubcarriers 23 through 29 disabled. The cyclic prefix is alsoextended to 64 samples in order to preserve the orthogonalityof the signal. In this case, the interference power is reducedto 30 dB of the original signal power. Finally, in case III, theCP is kept to 8 samples, subcarriers 25 to 27 are disabled,and subcarriers 23, 24, 28, and 29 are used as cancellationsubcarriers. A significant spectrum hole deeper than 70 dB isachieved in this case.

V. A STEP TOWARDS COGNITIVE-OFDM: STANDARDS

AND TECHNOLOGIES

As CR concept is attracting more interest everyday, recentlydeveloped standards are considering more cognitive features.Dynamic frequency selection (DFS), transmit power control(TPC), and spectrum sensing are just a few examples offeatures that are included in some of the current standards.These standards can be considered as a step towards thefuture implementation of a CR. In this section, some exampleOFDM-based standards which utilize cognitive features areintroduced.

A. IEEE 802.16

One of the technologies that is getting a fair amount ofinterest lately –in both academia and industry– is IEEE 802.16

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18 20 22 24 26 28 30 32 34

−80

−70

−60

−50

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−30

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−10

0

10

20

Normalized frequency (subcarrier index)

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dB)

Case ICase IICase III

Licenseduser band

Fig. 4. PSD of cognitive system OFDM signal with a spectrum hole overLU band.

(WiMAX). OFDMA PHY mode is probably the most interest-ing mode supported by WiMAX. In this mode, users can beassigned different bandwidths, time durations, transmit powerlevels, and modulation orders based on various parameterssuch as user carrier-to-interference-plus-noise ratio (CINR),received signal strength indicator (RSSI) or the availablebandwidth. Moreover, OFDMA PHY offers multiple FFTsizes, CP sizes, and pilot allocation schemes. The FFT sizecan be selected as 128, 256, 512, 1024 or 2048 dependingon the transmission bandwidth3. Similarly, the CP lengthcan be set to 1/4, 1/8, 1/16 and 1/32 times the OFDMsymbol length. The CP size can be changed depending on theenvironment characteristics. With all these adaptive features,WiMAX has the ability to adapt to various channel conditionsand communication scenarios.

WiMAX standard is also rich in terms of advanced an-tenna techniques as well. Available methods include adaptiveantenna systems (AAS), space time coding (STC), selectedmapping (SM), collaborative SM, antenna selection, antennagrouping, MIMO precoding, STC sub-packet combining, fre-quency hopping diversity combining (FHDC), and adaptiveMIMO switch. The standard use of these techniques is notdirectly related to CR, but rather to increase the spectralefficiency and increase the overall throughput of the system.However, advanced antenna techniques could be used toachieve some of the CR goals as well. For example, theCR transmitter can exploit location awareness to focus itsradiation only in the direction of intended receivers usingadaptive beamforming [31, 32]. On the other hand, the CRreceiver can use the same technique to adaptively cancel theinterference caused by unintended transmitters [33]. The goalis to achieve coexistence of WiMAX devices in unlicensedbands. Furthermore, methods for coexistence with primaryusers are also developed.

3This is known as scalable OFDMA. Various FFT sizes are used tokeepthe subcarrier spacing constant for different transmission bandwidths

B. IEEE 802.22

IEEE 802.22 standard is known asCR standardbecauseof the amount of cognitive features that are employed. Thesecognitive features include channel sensing, LUs detection,DFS, and TPC. Even though IEEE 802.22 standard is notfinalized yet, the current draft proposal is based on OFDMtransmission and it is anticipated that the final version will bethe same. The IEEE 802.22 standard is designed for a fixedpoint-to-multipoint communication topology where the basestation (BS) acts as the master mandating all the operationparameters of users within the cell. And while the users(slaves) can share sensing information with the BS throughdistributed sensing, it is up to the BS to change a user transmitpower, modulation, coding or operating frequency.

One of the most distinctive feature of IEEE 802.22 standardis its sensing requirements which is based on two stages: fastand fine sensing. In the fast sensing stage, a coarse algorithmis employed,e.g. energy detector. The fine sensing stage isinitiated based on the previous stage results. However, a moredetailed and powerful sensing methods are used in this stage.A BS can distribute sensing load among subscriber stations(SS). The results are returned to BS which uses these resultsfor managing transmissions.

Another challenge in designing the IEEE 802.22 standard isthe initialization of new users who desire to communicate withthe BS. Unlike current wireless technologies, frequency andtime duration of the initialization channel is not predefined.In other words, initial users have to scan parts (if not all)of the TV bands to find the BS operating frequency andtime. In addition, users should be able to differentiate betweenincumbent signals and the BS signal. This could prove tobe very challenging especially if the BS is operating over acombination of multiple frequency bands.

C. IEEE 802.11

The WLAN standard, IEEE 802.11a/g, is probably the mostcommonly known OFDM-based standard. The main standardis upgraded to have cognitive features with IEEE 802.11h andIEEE 802.11k standards. IEEE 802.11h is designed to allowestimation of channel characteristics and DFS. In addition,TPC is incorporated as well, providing the system with morecontrol over signal range and interference level. The purposeof the IEEE 802.11h standard is to allow WLAN systems toshare the 5-GHz spectrum with primary users (e.g. militaryradar systems).

Note that the DFS proposed for the aforementioned stan-dards can be considered as channel switching or frequencyhopping technique. Thus, it can be applied to any transmissiontechnology and is not exclusive to OFDM. However, it isalso noteworthy to mention that many recent published articleshave proposed new methods to facilitate DFS use for OFDMsystems. OFDM subcarriers are split into subchannels that canbe disabled, enabled, or assigned to multiple users, accordingto sensing measurements. These recent studies can be dividedinto two categories: addressing the reduction of interferencebetween different subchannels [28–30], and addressing theoptimization of the subcahnnel allocation process [34, 35].

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In following amendments to wireless standards, we expectthat the above advanced OFDM-based techniques will beconsidered for DFS.

On the other hand, IEEE 802.11k standard is proposedfor radio resource management. It defines several types ofmeasurements such as channel load report, noise histogramreport, and station statistics report. The noise histogramreportprovides methods to measure interference levels that displayall non-IEEE 802.11 energy on a channel as received bythe subscriber unit. The access point (AP) collects channelinformation from each mobile unit and makes its own mea-surements. This data is then used by the AP to regulate accessto a given channel.

Other features such as tracking of hidden nodes and sharingclients statistics are included in the standard. By applying bothIEEE 802.11h and IEEE 802.11k standards to current IEEE802.11-based WLAN systems, the performance and efficiencyof wireless networking can be improved significantly. Addingcognitive features such as channel sensing and estimation,statistics distribution, DFS, TPC to WLAN devices will soonbe possible.

VI. CONCLUSION

CR is an exciting and promising technology that offersa solution to the spectrum crowding problem. On the otherhand, OFDM technique is used in many wireless systemsand proven as a reliable and effective transmission method.OFDM can be used for realizing CR concept because ofits inherent capabilities that are discussed in detail in thispaper. By employing OFDM transmission in CR systems;adaptive, aware and flexible systems that can interoperate withcurrent technologies can be realized. However, the challengesidentified in this paper need to be researched further in orderto address the open issues. Practical CR systems can bedeveloped using two approaches: current wireless technologiescan evolve to support more cognitive features over time,or new systems that support full cognitive features can bedeveloped. In either case, we foresee that OFDM will be thedominant PHY technology for CR.

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