Some Research Issues in Cognitive Radio NetworksGaurav Bansal, Md. Jahangir Hossain, Praveen Kaligineedi, Hugues Mercier, Chris Nicola, Umesh Phuyal, Md.

Mamunur Rashid, Kapila C. Wavegedara, Ziaul Hasan, Majid Khabbazian, and Vijay K. BhargavaUniversity of British Columbia, 2356 Main Mall

Vancouver, BC, V6T 1Z4, CanadaEmail: {gauravbs, vijayb}@ece.ubc.ca

Abstract—The cognitive radio (CR) technology will allow agroup of potential users to identify and access available spectrumresources provided that the interference to the users for whom theband has been licensed is kept below a prescribed level. However,this research area is at a very immature stage because variousresearch challenges have to be addressed and solved. In thispaper our objective is to present an overview of some researchissues for CR networks. Specifically, we present some researchand development in CR networks with focus on i) information-theoretic aspects, ii) spectrum sensing, iii) link adaptation, iv)advanced transceiver design, and v) admission control. Wealso discuss some important research problems related to thesespecific topics that needs to be addressed before deployment ofCR systems in practice.

Keywords: Cognitive radio, opportunistic spectrum ac-cess, information-theoretic analysis, spectrum sensing, OFDM,MIMO, MC-CDMA, admission controller.

I. INTRODUCTION

Given the fact that radio spectrum is one of the most scarceand valuable resources for wireless communications, newinsights have recently challenged the traditional approachesto spectrum management. Most of the allocated spectrum islargely underutilized as reported by field measurements [1],and similar views about the underutilization of the allocatedspectrum have been reported by the Spectrum Policy TaskForce appointed by Federal Communications Commission(FCC) [2].

The most promising way to significantly improve spectralefficiency is to give opportunistic access of the frequencybands to a group of users for whom the band has notbeen licensed (referred to as secondary or cognitive radiousers). Cognitive radio (CR) has been proposed as a way toimprove spectrum efficiency by exploiting unused spectrum indynamically changing environments. CR design is, therefore,an innovative radio design philosophy which involves smartlysensing the swaths of spectrum and then determining the trans-mission characteristics (e.g., symbol rate, power, bandwidth,latency) of a group of secondary users based on the primaryusers behavior to whom the spectrum has been licensed.Specifically, CR is likely to be built on software defined radio(SDR) [3], which allows it to dynamically adjust its transmittercharacteristics based on the interaction with the environmentin which it operates.

Due to the enormous potential of improving the spectralutilization by using CR, adaptive access system design forthe CR networks is one of the most important research areas

in wireless communications. The current main focus of CRnetwork design is to research and develop such enablingadaptive radio access technologies. The research scopes andchallenges for CR systems are being outlined in the literature(e.g., in [4], [5] and references therein). In general, it is alsoour view that the successful development of CR systems willrequire the accomplishment of following key objectives:

• Developing rate and capacity expressions for CR-basednetworks;

• Designing innovative spectrum sensing algorithms suchthat cognitive users can work without causing excessiveinterference to the primary users;

• Developing schemes that can answer the security threatsand requirements for CR networks;

• Devising efficient resource allocation strategies based onthe spectrum occupancy of the primary users;

• Developing advanced transceiver for the cognitive phys-ical layer;

• Designing adaptive medium access control (MAC) layerprotocols and admission controller which will enableefficient resource sharing by the cognitive users withrequired quality of service (QoS).

In this paper, our objective is to provide a survey on theabove mentioned challenging issues. Specifically, we presentsome research and development in these areas and mentionfurther interesting research problems involved with CR net-works. The organization of the rest of the paper is as follows.In Section II, we discuss information theoretic aspects of CRnetworks. Section III presents the various schemes for sensingthe spectrum and their security threats. In Section IV, wedescribe the resource allocation problem in CR systems. InSection V, we provide research issues in the transceiver designof CR systems and we discuss scheduling and admissioncontrol issues in the MAC layer of CR networks in SectionVI. Finally, Section VII concludes the paper.

II. INFORMATION THEORETIC ASPECTS OF COGNITIVE

RADIO

There have been a number of recent papers which explorethe information theoretic analysis of CR channels, i.e., chan-nels where CR users operate in licensed bands in the presenceof legacy users. Commonly referred to as dynamic spectrumallocation, this is one of the more pressing issues in cognitiveradio since regulatory bodies have declared their intention toallow for secondary use of this underutilized spectrum.

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Figure 1. Simple two-switch channel model of [6].

A fundamental property of cognitive radio channels is thatCR users are capable of observing the local radio scene anddynamically adapting to it. In information theory this can bemodeled as a scenario where there is side information or chan-nel state information (CSI) which the radio obtains throughpassive observation. By defining a model for the cognitiveradio channel in information theoretic terms it is possible toderive equations for capacity, which can offer insight intohow to maximize performance. In the work considered herethe channel is defined in one of two ways. The first oneis the interference avoidance approach [6]. It assumes CRusers will always avoid transmitting in the presence of adetected primary user. The second one, called interferencemitigation approach [7], assumes that the secondary usertransmits simultaneously but will try to compensate for theinterference caused to the primary user, as well as that causedby the primary user.

In the interference avoidance approach of [6], the transmitterand receiver communicate on a channel only if they both detectthe channel free from primary user activity. In Fig. 1, theprimary user detection is represented by ST at the transmitterand SR at the receiver. In practice we expect ST and SR tobe correlated but also sub-optimal with false detections andmissed detections occurring with some probability. When aprimary user is detected the switch is open, otherwise it isclosed. One way for secondary users to communicate is toperform frequency-hopping, where the secondary users changechannels in a prearranged pseudo-random order communicat-ing on each channel only if it is detected to be unoccupied.The other is frequency-coding where the secondary users scanthe entire radio scene and communicate across all unoccu-pied channels simultaneously. The difference between theseapproaches is that side information for frequency-hoppingis causal since it is obtained throughout the transmission,whereas in the frequency-coding approach the side informationis considered non-causal since the entire radio spectrum mustbe observed before transmission. In both cases CSI is thedetected primary user activity.

The interference mitigation approach presented in [7] andshown in Fig. 2 is more complex. Instead of simply avoidinginterference with the primary user, the secondary transmitterutilizes CSI, represented by the side information channel σ,to try to compensate for the interference. The CSI is obtainedthrough σ, which may be observed in real-time (causal) orobtained a priori (non-causal) from the primary user. In theobserved case it might happen that σ is a noisy channel. Theother two paths, α and β, represent the signal received bythe primary receiver from the secondary transmitter and thesignal recceived by the secondary receiver from the primary

Figure 2. Simultaneous transmission model of [7].

transmitter, respectively. The secondary transmitter tries tochoose Xs using the side information from σ such that theinterference caused to the primary channel is minimized orcompensated via the signal α. It is noted in [7] that even inthe causal case it is possible for the secondary transmitter toachieve this by acting partly as a repeater for the primarytransmitter as long as the gain between the two transmittersis sufficiently greater than the gain between the primarytransmitter and the primary receiver. This assumes howeverthat the receiver is capable of benefiting from a secondarytime-delayed signal.

Channels with causal and non-causal CSI are not new toinformation theory. Causal channels were first proposed byShannon [8] and a detailed capacity analysis of a numberof causal CSI channels has been done in [9]. Non-causalside-information channels were first introduced by Gelfandand Pinsker [10]. A detailed exploration of non-causal CSIchannel capacity can be found in [11]. These types of channelshave also been considered for applications other than cognitiveradio. In [12] the authors examined the capacity of defectivememory with stuck-at defects. The authors of [6] noted thatthis is very similar to their frequency-coding case for cognitiveradio. The work of [7] relates to work done in informationhiding and digital watermarking techniques [13] where amessage or watermark is embedded in a host signal such asan image or video.

III. SPECTRUM SENSING

One of the important challenges for a cognitive radiosystem is to identify the primary users over a wide rangeof spectrum. This process is very difficult as we need toidentify various primary users employing different modulationschemes, data rates and transmission powers in presence ofvariable propagation losses, interference generated by othersecondary users and thermal noise [14]. This is especially truein the case of broadcast TV channels, where the receivers arepassive, and as such it is not possible to detect the presenceof a nearby receiver. For example, if the channel betweenthe primary transmitter and the spectrum sensing device isunder a deep fade, it is possible that the sensing device maynot detect the primary signal. It might result into well-knownhidden terminal problem where the CR might transmit signalin the corresponding primary user band causing interferenceto the nearby primary receiver. To overcome this problem, thesensitivity of the cognitive radio sensing device has to be at

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least 20-30dB more than that of the primary receiver. Also,if the cognitive radio device wants to use a higher transmitpower, it should be even more sensitive to the primary usersignal. In particular, the sensing process must be very quick inorder to scan the entire wide band without significant delay.

Traditionally, there are two techniques which are used forspectrum sensing: energy detection and cyclostationary featuredetection [15]. The energy detector measures energy in eachnarrowband channel and determines the presence of a primaryuser if the energy detected in a narrowband channel is higherthan a certain threshold. However, to achieve high receiversensitivity, a low threshold has to be used. In some cases, thethreshold has to be lower than the noise floor, in which casethe detection fails. The problem is even more complicated dueto the fact that the noise is most likely non-Gaussian becauseof the presence of CR user’s interference. The other spectrumsensing technique is cyclostationary feature detection. Mostof the signals encountered in wireless communications arecyclostationary, whereas the noise is stationary. As a result,the cyclostationarity of the primary signals can be used todetect their presence. The cyclostationarity of a signal is notreflected in the power spectral density (PSD), however it isreflected in the spectral correlation density (SCD) functionwhich is obtained by taking the Fourier transform of thecyclic autocorrelation function. Therefore, spectral correlationanalysis of the received data can be used to identify the signal.Higher order spectral statistics have also been used to identifyweak users. Efficient search strategies to detect a signal in awideband using its cyclostationarity have been studied.

Many schemes have been proposed in the literature todetect weak signals (with signal power less than the noisepower) in presence of stationary Gaussian as well as non-Gaussian interference based on the above two techniques[16], [17]. These schemes depend upon the knowledge thedetector has regarding the signal frequency band and themodulation format and characteristics. These schemes couldbe applied for spectrum sensing in CR user systems whilealso taking into consideration their computational complexity,storage requirements, total search time, and knowledge ofprimary user signal characteristics.

Burden on the signal processing techniques can be alleviatedto a large extent by using cooperative diversity betweencognitive radio spectrum sensors. Few cognitive radio spec-trum sensors under independent fades can help in reducingindividual sensitivity requirements [18] and essentially helpin overcoming the hidden terminal problem by counteringthe shadowing and multi-path effects. The sensing devicescould be collocated with the cognitive radios, or a separatenetwork for spectrum sensors could be employed. Finding anefficient method to combine the decision statistics from varioussensing devices is an interesting problem which needs to beexplored and the combination scheme could be based on softor hard decision statistics. It was shown in [18] that presenceof malicious users can significantly affect the performance ofthe cooperative sensing system. Techniques to identify andtackle such malicious users need to be investigated.

IV. ADAPTATION IN OFDM-BASED COGNITIVE RADIO

SYSTEMS

Based on the information of available spectrum as deter-mined by the spectral sensing schemes, the next challengingtask is to allocate and utilize the available spectrum in anoptimal fashion. As such, the transmission capacity of theCR users should be maximized while keeping the interferenceintroduced to the primary user band within the tolerablerange. Orthogonal frequency division multiplexing (OFDM)has already been widely recognized in the literature as a po-tential transmission technology for CR systems as it providesgreat flexibility in dynamically allocating the unused spectrumamong the secondary users as well as an easy analysis of thespectral activity of the primary users [19]. Another inherent ad-vantage of the OFDM modulation technique is the possibilityof exploiting different channel qualities among the subcarriersin a given channel access and thus loading (power or bit orboth power and bit) different subcarriers according to theirrespective channel qualities [20].

The assignment of the unused band to the secondary usersin an OFDM fashion causes mutual interference between theprimary and secondary users due to the non-orthogonality ofthe transmitted signals. Specifically, it has been shown that theamount of interference introduced by allowing secondary userstransmission in a given subcarrier depends on the location ofthe subcarrier with respect to the primary users spectrum aswell as the amount of power transmitted in that subcarrier [19].Thus, the classical subcarrier allocation and bit and powerloading algorithms, e.g., greedy and water-filling algorithms,which increase the overall capacity of the cognitive system,may result in higher mutual interference to the primary users.For example, allocation of a given subcarrier to a CR user whoexperiences better channel quality and allocating more powerto this carrier as suggested by the water filling algorithm mayincrease the capacity of CR systems. However, the locationof that specific carrier with respect to the primary users bandmay restrict transmitting more power in order to reduce theinterference. Consequently, the subcarrier allocation as wellas the power and rate adaptation in CR networks is notstraightforward.

In [21], optimal power and bit loading algorithms for anOFDM-based CR system have been studied. Specifically, anoptimal scheme which maximizes the downlink transmissioncapacity of the CR users while keeping the interferenceinduced to the primary users below a specific threshold hasbeen proposed using Lagrange formulation. Since the com-plexity of the optimal scheme can be quite high for practicalsystems, two suboptimal schemes, namely scheme A andscheme B, have also been proposed. Fig. 3 shows the plot ofachievable transmission rate of CR user versus interferenceintroduced to the primary user band for different schemesunder consideration as done in [21]. The results show that fora given interference threshold the proposed schemes allows theCR users to transmit more power in order to achieve highertransmission rate than the classical water-filling and uniformpower loading algorithms.

In [19], the effect of nulling the subcarriers have been

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Figure 3. Maximum transmitted data rate of CR user vs Interferenceintroduced to the primary users band as in [21].

studied. It is shown that by nulling the subcarriers which areadjacent to the primary user band, the interference introducedto the primary user band can be reduced. The reason behind itis that the subcarriers which are adjacent to the primary userband produce the maximum amount of interference. It alsoimplies that for a given interference threshold, more powercan be allocated to the far away subcarriers than the amountof power allocated to the neighboring subcarriers. One mightexpect that higher transmission rate can be achieved by nullingsome of the subcarriers as more power can be loaded into theremaining ones. However, by nulling subcarriers we loose thedegrees of freedom as even if a nulled subcarrier has a verygood channel gain, zero power is assigned to it. Therefore,nulling creates a trade-off. In [21], the effects of subcarriernulling have been studied on various schemes.

The study in [21] is based on the assumption that thetransmission rate can be varied continuously. Researchershave developed discrete loading algorithms for OFDM-basedsystems as most of the coding and modulation schemes that areused in practice provide a discrete or integer transmission rate.In this context, a number of algorithms have been proposedin the literature (e.g., Hughes-Hartogs algorithm [22], Chowet al. algorithm [23]). Unfortunately, these algorithms are notdirectly applicable to OFDM-based CR systems as there ismutual interference between CR and primary users when bothgroups co-exist in side by side bands. These conventionalalgorithms can introduce significantly high interference to theadjacent primary user band. In [24], a suboptimal algorithmhas been presented for an integer bit loading case such that itis applicable for OFDM-based CR system. The algorithm issuboptimal in the sense that it approximates the optimal con-tinuous rate value to the nearest integer value. Also, two otherschemes tailored for a CR scenario have been proposed basedon modifications to the existing Hughes-Hartogs algorithm andChow et al. scheme.

The work in [21] and [24] has been extended for themultiuser scenario in [25]. The schemes proposed in [25]

tries to maximize the transmitted data rate for a group ofCR users under the interference introduced to the primaryuser band and total power constraint. However, to the best ofour knowledge no research has been done for MIMO-OFDMbased CR systems. This research is important because multipleantenna systems significantly improve the spectral efficiency.

V. ADVANCED TRANSCEIVER AND RADIO INTERFACE

DESIGNS FOR THE COGNITIVE PHYSICAL LAYER

As CR systems are expected to operate in dynamicallychanging environments, maintaining QoS requirements of ser-vices offered by a CR system is challenging. Hence, properdesign of the cognitive physical layer to facilitate high datarate access with high spectral efficiency is very important. Toachieve this goal, it is crucial to incorporate recent technicaladvances such as multicarrier code division multiple access(CDMA), space-time (ST) coding and turbo detection anddecoding into CR systems.

An efficient multiple access scheme facilitating dynamicspectrum allocation is essential for CR systems. As mentionedin section IV, OFDM has received tremendous attention inthe literature as a potential transmission technology for CRsystems. Hence, orthogonal frequency division multiple access(OFDMA) is a natural choice for multi-user CR systems.On the other hand, novel multiple access schemes, whichare based on combination of CDMA and OFDM techniques,have recently drawn significant attention. These schemes yieldinherent advantages of both OFDM and CDMA techniques.There are two types of schemes widely discussed in the lit-erature: multicarrier (MC)-CDMA and direct spectrum (DS)-CDMA [26]. In MC-CDMA, as in DS-CDMA systems, eachuser is assigned a distinct spreading sequence, but spreading isperformed in the frequency-domain (FD). The main advantageof MC-CDMA over OFDM(A) is that the data loss caused bysevere FD fading in OFDM(A) can be avoided due to FDspreading in MC-CDMA.

In MC-CDMA systems, the number of subcarriers Nc isgiven by the relationship Nc = PG, where P is the number ofsubstreams and G is the processing gain [26]. Hence, whenMC-CDMA is employed in CR systems, satisfaction of thisrelationship between Nc and G may be an issue to be addressedas the number of subcarriers available for the CR users is de-termined by the pattern of primary system spectrum utilization.On the contrary, in MC-DS-CDMA, the user symbol stream isfirst serial-to-parallel converted into NC substreams and then,each subcarrier stream is separately spread in the time-domainusing the same user-specific spreading sequence. As such, wedo not foresee any technical barrier which prevents employingMC-DS-CDMA in CR systems. Nonetheless, the effectivenessof employing MC-CDMA and MC-DS-CDMA schemes infuture CR systems as an alternative to OFDMA should bethoroughly investigated.

ST coding has evolved as an effective transmit diversitytechnique. Furthermore, when the knowledge of channel infor-mation is available at the transmitter, beamforming techniquescan be used in combination with ST coding to further improvethe system performance under hostile conditions. Combined

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transmit beamforming and ST block coding (STBC) schemeshave been developed for single-carrier narrowband systems(e.g., [27]). Recently, in [28], joint transmit-beamforming andSTBC in combination with adaptive modulation has beenconsidered for OFDM systems. However, ST block codes,which were originally designed for flat fading channels, cannotyield inherent frequency diversity of wireless channels. There-fore, space-time-frequency (STF) codes, which are capable ofachieving maximum diversity and coding gains, have been de-veloped for OFDM systems (e.g., [29]). One of the importantresearch issues is to develop hybrid transmit-beamforming andSTF coding schemes for MIMO-OFDM based CR systems.Unlike for MIMO-OFDM (non-CR based) systems, whenhybrid transmit-beamforming and STF coding schemes aredesigned for CR systems, two additional issues have to betaken into account. The first one is that all subcarriers ofthe given frequency band are not available for the secondarycognitive system and subcarriers are dynamically allocated tothe CR system. More importantly, the second issue is that it isrequired to keep the interference to the primary system fromthe secondary cognitive system as low as possible.

Turbo detection and decoding techniques can be used toobtain additional performance improvements compared toconventional non-iterative hard-decision based approaches inchannel-coded MIMO systems (e.g., see [30] and referencestherein). Therefore, another important research problem isto develop efficient turbo receivers for MIMO-OFDMA andMIMO multicarrier-CDMA based CR systems to further im-prove the system performance.

VI. SCHEDULING AND ADMISSION CONTROL IN MACLAYER FOR COGNITIVE RADIO NETWORKS

Although opportunistic spectrum access could allow CRusers to identify and access available spectrum resources,one of the main concerns is to utilize the available spectrumresources in an efficient manner. As such CR users’ QoS hasto be met without violating the priority right of the primaryusers. In a cognitive network, the availability of radio spectrumfor CR users depends mainly on the activity of the primaryusers. On the other hand, wireless channel quality of the CRusers is time varying in nature. Therefore, communicationresources available for CR users is highly dynamic. As such,the MAC layer in a CR network should be able to adapt tothe availability of the communication resources. An adaptiveMAC layer which efficiently uses communication resourcesgiven the QoS requirement is an important component forthe adaptive wireless access system of CR networks. Meetingthe QoS requirement of the transmission requests by the CRusers, however, still remains impossible if no proper admissioncontrol scheme is integrated into the MAC. Therefore, anadmission controller design that takes the peculiarity of CRnetworks into consideration remains an important researchobjective.

Motivated by the aforementioned challenging task, we de-velop, as a part of our research project, an analytic frame-work for studying MAC performances of CR users. Basically,we study buffer statistics and queuing delay of the packets

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belonging to CR users by modeling primary user’s activityas a two state Markov chain and the channels as finite stateMarkov chains (FSMC) [31]. Since opportunistic schedulingoffers throughout gain with increased number of users inthe system [32], we consider opportunistic channel allocation(OCA) scheme at the MAC layer scheduler. This frameworkfacilitates the design of an admission controller for CR users’network in order to provide proper QoS for data or multimediacalls served by the CR users. Numerical results derived fromthe developed analytical model show that as the activity levelof the primary users increases, the delay statistics of CRusers degrades and the packet injection rate of the CR usersdecreases for a given statistical delay constraint. If the numberof communication channels increase, the delay distributionimproves and the packet injection rate can be increased.

Based on the queueing analytic model developed, a model-based admission controller can be designed. It works asfollows: the CR user base station keeps track of the primaryuser channel occupancies and the channel transitions. Foreach CR user, the CR user base station runs the queueingmodel to derive the relationship between different traffic arrivalrates and the corresponding delay distributions of the packetsserved from the CR user queue. The admission controllerwill then use this information to determine the maximumallowable packet arrival rate for a given delay requirement.More specifically, if the delay requirement of the CR users isgiven in the form Pr(delay> Dth) < Pt , it is possible to find themaximum allowable average traffic rate for a given numberof connections. The admission controller then regulates theinjection rate of the packets into the CR user queue accordingto this maximum allowable rate. For an uplink queue, eachCR user has an admission controller with the queueing modeloutput and other relevant parameters fed by the CR user basestation.

As an example, Fig. 4 shows the maximum allowablepacket arrival rates versus average channel fading gain per

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channel for different average activity levels of the primaryusers. In this example, it is assumed that an incoming packetshould be delivered within Dth=20 time slots with probabilityPt=0.99. Consider the scenario of an average channel fadinggain of 16dB. For the considered low activity level of primaryusers, the admission controller will allow a maximum rateof 0.4 packet/slot, whereas for the medium activity level theadmission controller will cut down the maximum allowablerate to 0.33 packet/slot. In brief, the admission controller willbe able to effectively use the model output and other relevantparameters to perform an efficient packet level admissioncontrol.

VII. CONCLUSION

In this paper we have presented the state of the art ofCR research with specific focus on i) information-theoreticaspects, ii) spectrum sensing, iii) link adaptation, iv) advancedtransceiver design, and v) admission control. We also discusssome of the open research issues in these areas. These researchchallenges are very important in order to exploit the potentialbenefits of CR technology. Information theoretic analysisprovides insight into methods to optimize performance andminimize interference for CR users operating in the primaryuser band. We have also discussed various spectrum sensingschemes to determine the presence of primary users. Moreover,we have shown that judicious link adaptation for OFDM-basedCR networks can significantly increase their capacity, and wehave discussed some key research issues in the CR physicallayer that needs to be addressed. Finally, we have presented aproper admission controller to provide QoS of CR users in adynamic environment.

ACKNOWLEDGEMENT

This research was supported by Natural Sciences and Engi-neering Research Council of Canada under a Strategic ProjectGrant.

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