Table Of ContentThis full text paper was peer reviewed at the direction of IEEE Communications Society subject matter experts for publication in the ICC 2008 workshop proceedings.
A Cognitive Radio Receiver
Supporting Wide-Band Sensing
Volker Blaschke, Tobias Renk, Friedrich K. Jondral
Universita¨t Karlsruhe (TH), Institut fu¨r Nachrichtentechnik
{blaschke|renk|fj}@int.uni-karlsruhe.de
Abstract—IEEE 802.22 defines the world-wide first cogni- presented. Within the IEEE 802.22 frequency range several
tive radio (CR) standard. In the range between 41MHz and types of channel utilization can be identified. This a-priori
910MHz CR overlay-systems can be installed besides licensed knowledge combined with appropriate information processing
radio services such as radio and TV broadcasting. In order
lead to an optimized CR front-end structure supporting wide-
to fulfill the regulative guidelines for interference limitations,
adequatespectralsensinganduserdetectionhastobesupported band spectral sensing.
by the CR terminals. The wide frequency range specified in The paper is structured as follows: In the next section a
IEEE 802.22 and the high dynamic range of signals in this brief introduction to ADCs is presented. In section III a
band lead to high demands on the CR receiver’s front-end. detailed description of sensing algorithms in CR terminals
Especially the performance requirements on analog-to-digital
is given. Furthermore, expected signal characteristics within
converters (ADCs) increase significantly compared to current
wirelesssystems.Basedonmeasurementstakeninthefrequency the IEEE 802.22 frequency bands are identified. Based on
range between 41 MHz and 910 MHz requirements to CR’s these considerations, two general wide-band sensing methods
ADCs are figured out. Furthermore, the measurement results includingacomparisonoftheirdemandsonADCperformance
are analyzed regarding expectable allocation scenarios and their are described in section IV. In section V a CR receiver
impacts on spectral sensing. Derived from these results and a
structure is presented which includes adapted spectral sensing
comparisonofgeneralspectralsensingmechanisms,anapproach
for a CR receiver enabling wide-band sensing is presented. By and convenient information processing. Finally, a conclusion
combining a-priori information resulting from scenario analysis is given.
with adapted information processing in the CR terminal, the
ADC’s performance requirements can be reduced. II. ANALOGTODIGITALCONVERSIONINCRRECEIVERS
A suitable sampling and quantization of the analog input
I. INTRODUCTION signal is a precondition for digital signal processing. Most
Spectrum Sensing becomes more and more important, es- CR concepts described in literature only assume an appro-
pecially in the context of cognitive radio (CR). Due to the priate ADC as precondition without detailed specification.
increased demand on wireless transmission resources and the But supporting the performance constraints assumed in these
ongoing installation of new radio access technologies for approachescontainseveralchallengesinADCdesign.Thekey
broadbandaccess,enhancedresearchinthefieldofmobileCR parametersforsummarizingtheADC’sperformancearestated
receivers is necessary. Resulting form spectral measurements resolution, signal-to-noise ratio, spurious free dynamic range,
[1], [2] a low utilization over wide frequency ranges was and power dissipation [8]. Furthermore, aperture jitter as well
identified. In order to overcome this waste of resources the as the two-tone intermodulation distortion are important for
development of intelligent allocation mechanisms becomes characterizing ADCs. A detailed description and performance
necessary. Dynamic spectrum allocation (DSA) of unused analyzes can be found in [8] and [9]. In [8] the performance
radio resources is required for increasing the overall utiliza- of on-the-market ADCs is analyzed in order to describe the
tion. This approach is supported by the CR concept [3], [4]. evolution and trends in ADC technology. This evaluation was
For providing DSA, sufficient information about the spectral continued in [9] including also present-day trends.
environment has to be collected using different acquisition In this article ADC parameters which restrict an implemen-
methods.Ononeside,allinformationiscollectedbyacentral tation of current ADCs in mobile CR terminals are figured
control unit and distributed to the mobile terminals. In this out. These are sampling frequency fs, impacting the effective
casetrafficloadinformationcouldbeexchangedbetweenjoint resolutionbandwidth,effectivenumberofbitsNeff,describing
networks via backbone [5]. Other approaches are based on the dynamic range that is supported by the ADC, and power
distributed spectral sensing where all mobile terminals of a dissemination Pdiss influencing battery running time.
radio network are used. This requires sensing capabilities in In order to fulfill the Nyquist criterion the converter sampling
eachmobileentityandanefficientalgorithmforconsolidation frequencyfshastobemorethantwotimestheefficientanalog
oftheresults.Especiallyinsuchscenarios,swarm-intelligence bandwidth[10].Furthermore,theeffectivenumberofbitsNeff
algorithms could offer additional benefits [6]. Nevertheless, is lower than the stated number of resolution bits specified by
appropriate spectral sensing and information extraction forms the vendor. Due to hardware imperfections and quantization
a precondition for DSA. In order to avoid unacceptable inter- noise Neff is given by Neff = D6−.012.76, where D describes the
ferences reliable user detection has to be supported by CRs. effective dynamic range of the converter in dB [8]. Especially
This leads to additional demands on its front-end. indetectionandsensingapplications,ahighdynamicrangeis
In this paper the performance demands on analog-to-digital important. If the dynamic range of the expected input signals
converters(ADCs)inmobileCRreceiverssupportingspectral is higher than the ADC dynamic range, weak signals may not
sensingarediscussed.Basedonthefrequencyrangesspecified be detected due to resolution limitations.
in IEEE 802.22 [7] general aspects of wide-band sensing as Havingalooktotheresultsdepictedin[8]and[9]threemain
well as specific demands on the structure of mobile CRs are hardware architecture concepts become potential candidates
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for implementation in mobile CR terminals. These are: Flash
−40
Converters, Pipelined ADCs, and Σ∆ Converters.
−45
Flashconvertersofferveryhighsamplingratesduetoaparal-
−50
lel comparator structure. This enables sampling rates of about
−55
1higGhsppsowbuetr rceoqnusiurmesphtiiognhmhaarkdewsatrheemcomunpaltetxraitcyt.ivFeufrothremrmoobrilee, wer (dBm)−−6650
ParepisppoellilucinatietoidonnAos.fDDoCunselyNtoNetffehffecah=nig6hb..eh.8airndbcwriteasarseceacdnomubpeplsteouxpi1tpy5oarbtneidtes.ffUaetcstitinhvgee avgeraged po−−−877050
expenseofareducedsamplingfrequencyfs whichislessthan −85
500 Msps. The specific design of Pipelined ADCs requires
−90
theimplementationofanalogtrack-and-holdblocks.Thethird
−95
40 ch1 ch2 ch3 910
group of potential candidates offers an effective resolution of frequency (MHz)
up to 20 bits but the achievable sampling rate is less than 100
Msps.DuetotheirlowpowerconsumptionΣ∆Convertersare Fig.1. Averagepowervs.frequencyinIEEE802.22frequencyrange.
veryinterestingforanimplementationinmobileCRterminals.
FurtherinformationaboutthedifferentADCstructurescanbe 1 ch. 1
found in [10], [11]. 0.9 cchh.. 23
aunadtDiIouIcnIrhi.naanngSdnPetEuhlCseaeTlrRllaorUsectMqatuyiSioerEneaNmrsaSedInIsaNMtepsGvtPeewAAdraCeNlrTtDeoSapRptuhpEberLoliAacsTcuhhEreredDes.nHTtfoAhsrepRseDeDcWtScrAouAnmRcE[e1spi2tts]- averaged channel utilization0000000.......2345678
combine available transmission resources of several systems 0.1
optimizingthespectrumutilizationofallparticipatingsystems. 0
In order to provide traffic load information of each system to time
all other systems a global control entity and dedicated control
channels via the wired backbone network are necessary. This Fig.2. AveragechannelutilizationOtn ofchannel1,2and3.
means that each system has to be connected to this dedicated
control channel. Therefore, this architecture requires coopera- utilization, the full frequency range has to be supported by
tion between all radio access networks participating in DSA, the mobile terminals. This also includes the ability to fast and
which again results in an unflexible system configuration. efficient sensing of wide ranges in order to adapt the radio
Anotherapproachforincreasingspectralutilizationareoverlay transmission parameters to the licensed user’s allocation. The
systems [13]. Based on the observation that the frequency widesystembandwidthresultsinincreasedrequirementstothe
range of most established radio systems is under-utilized, this hardwarearchitecture.Especiallytheanalogsignalprocessing
approach describes the allocation of local wireless networks and the ADC performance limit the supported bandwidth. As
exploitingunoccupiedradiotransmissionchannels.Duetothe it is described in section II, there is a trade-off between the
basic precondition that established, licensed systems should bandwidth and the dynamic range of the converter.
not be changed for overlay usage, the rental user must
A. Basics on Spectral Analysis
observe the communication channel in order to provide a
reliable detection of the licensed user’s channel allocation. Analyzing the receiving signals within the IEEE 802.22
Furthermore, the rental user’s signal has to be adapted to frequency range, several allocation characteristics can be ob-
the licensed system’s transmission parameters with respect served. In Fig. 1 the received signal power per frequency
to channel bandwidth, maximum channel allocation dura- averaged over a sensing period of 3hours is depicted. The
tion, transmission power, etc. Static system parameters, e.g., measurements presented here are taken in an urban area. As
channel bandwidth, can be defined in a database available it can easily be seen, wide frequency ranges are character-
for each rental user. For dynamic parameters, e.g., current ized by a very low average signal energy. But also highly
channel allocation, licensed user’s allocation statistics have to utilized bands, e.g., the European FM radio broadcast service
be observed and analyzed. In order to gain information about between 88MHz and 108MHz (cf. ch 1), can be figured
the current allocation of frequency bands, energy detection out. Furthermore, there are some TV broadcast signals as
can be used in overlay systems. If the received signal power wellastemporarilyallocatedchannels.Describingthechannel
inacommunicationchannelishigherthanthemeasurednoise allocation during sensing time a binary spectrogram can be
level, this channel is already occupied. This gives general defined:
(cid:2) (cid:1)
isTnihgfoenrraemlfsoatrthieoa,ntaaanrbaeolyucztliontshgeethcoiugnrhroeeirnsteourldteivelierzlasettianotenirs.gtiBycsudteoitfnecactiaossinegncoaaflnwmfeaaailyk. O{x(t)}(cid:1)(cid:1)f=fm = 10 ffoorrSS{{xx((tt))}}(cid:1)(cid:1)(cid:1)ff==ffmm >≤PPtthh , (1)
become necessary [14]. (cid:1)
IcnomIEmEuEnic8a0ti2o.n22chaanCneRl wteirtmhianablanisdwaildlothweBdchto=u6s.e..a8rMadHioz twhheesrepexct(rto)griasmthoefrxec(et)ivaetdfsreigqnuaeln,cSy{fxm(t,)a}n(cid:1)df=Pftmh ddeessccrriibbeess
inthefrequencyrangebetween41MHzand910MHz[7].So, thedetectionthreshold.Theresultingbinarydescriptionofthe
the overall system bandwidth is BS =869MHz. This is more spectralallocationcanbeaveragedoverthesensingtimeusing
than 100 times the signal bandwidth Bch. In order to provide a window length Nw. The average channel utilization can be
a flexible CR system which is able to optimize the spectral written as
500
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Fig.3. SuperheterodynereceiverwithtwoIFchainsandlowfrequencyADCfortheIEEE802.22specification.
frequency of 970MHz mixes the 1000MHz signal down to
Otn(cid:1)(cid:1)f=fm = N1w tn+(cid:3)Nw−1Otn(cid:1)(cid:1)f=fm, (2) fcaIFn2ea=sil3y0bMeHfizl.teIrmedagoeuftrebqyuetnhceyIiFs294fi0ltMer.HzBowthhicLhOasgaairne
t=tn controlledbyPLLsthatareconnectedtoareferenceoscillator
where tn is the time index. The resulting characteristics for to increase frequency accuracy. The automatic gain control
threedifferentfrequenciesaredepictedinFig.2.Thecurveof (AGC) block is followed by a tunable bandpass filter that
channel1describesatypicalutilizationofbroadcastchannels. determines the resolution bandwidth. The ideal case for the
Channel 2 offers a varying average occupation between 0.01 resolution filter is a rectangular filter with bandwidth BR. In
and 0.92. In channel 3 some time periods with an average ordertoachieveshortmeasurementtimes,however,optimized
utilization of 0 can be noticed. During this period the channel Gaussian filters are used that are temperature stable and
is not allocated by the licensed user and is thus an interesting possess a higher bandwidth accuracy as well. Nevertheless,
candidate for CR usage. In order to detect such periods that BR influences the sweep time Tsw that is necessary to scan
are also known as white spaces [4] a suitable observation of the whole frequency range BS. If measurement time falls
wide frequency ranges has to be provided. As can be noticed below Tsw, amplitude losses and signal distortions occur
in Fig. 1, the depicted signal possesses a high dynamic range. that eventually lead to frequency offsets. After filtering there
Duringmeasurementsamaximumsignalpowerof−41.8dBm are the envelope detector and ADC. The video filter is a
atchannel1wasobserved.Thegeneralnoiselevelmeasuredin lowpass with the bandwidth BV that suppresses noise and
unallocated sub-bands is −92dBm. So, the overall dynamic helps to smooth the signal spectrum. The micro-processing
range compared to the bandwidth of more than 850 MHz block (µP) includes among others averaging and threshold
marks the main challenge in finding suitable solutions for decision making.
mobile CR receivers supporting this wide frequency range. For the definition of the required sweep time, two cases must
be taken into consideration. One where the video bandwidth
is higher than the resolution bandwidth and vice versa [15]:
IV. SPECTRUMSENSINGMETHODS
Due to the fact that currently available ADCs possess a k· BB2S forBV >BR
limitedbandwidth,itisnotpossibletodigitizelargefrequency Tsw = R , (3)
s(FpaFnTs) aatnaolynzceer.sTahreereofnolrye,ussaob-cleallfeodr lfoawst fFroequurieenrcytrasnigsfnoarlms, k· BRB·SBV forBV <BR
because the input signal is lowpass filtered and converted where the parameter k denotes a proportional factor that
into the digital domain directly [15]. In order to investigate is usually in the range of 1 to 3. The first case of
high frequency signals, superheterodyne receivers must be (3) is illustrated in Fig. 4. System parameters were cho-
applied. Here, the overall input frequency span is mixed to sen according to the IEEE 802.22 specification. The verti-
a common intermediate frequency (IF) by a tunable local cal dashed lines define the channel bandwidth of Bch =
oscillator [15]. Spectral resolution is directly determined by 6...8MHz [7]. For one single scan through the whole
the IF filter. The smaller the resolution bandwidth BR, the frequency range of 869MHz approximately 3 · 10−4s
higher the spectral resolution. A well-known problem occurs are necessary if we consider a resolution bandwidth of
iftheinputfrequencyrangeismorethantwotimeshigherthan BR = 2.5MHz which means we take two to four frequency
theIF,becausethensuppressionoftheimagefrequencyisnot bins per channel. As can be seen in Fig. 3, the ADC is
possible without affecting the input signal. Hence, a tunable
bandpass is necessary which would be very complicated for a
frequency range of several hundred MHz. This problem can 102
be overcome if several IF stages are used, where the first one 101
transforms the input signal to a higher frequency. It is then
possible to suppress the image frequency without affecting 100
tFfhoierg.itnh3peusIhtEosEwigEsnaa8l0.s2u.p2e2rhsepteecriofidcyantieonre.cTeihveerRwFitihnptuwtosiIgFnaclhafiirnsst weep time (s)1100−−21 Bch
s
passes RF attenuation that helps to prevent overload and
10−3
distortion. Afterwards, a preselector lowpass filters out higher
frequency signals. In order to mix the RF signal up to 10−4 k = 1
i1n00a0MfreHqzue(fnIcFy1)r,atnhgeefirfsrotmloca1l0o4s1cMillaHtozrt(oLO1911)0mMusHtzo.peTrahties 10 4 kk == 23 Bs = 86190 5MHz 106 107
resolution bandwidth (Hz)
leads to an image frequency that ranges from 2041MHz to
2910MHz which can easily be suppressed by the following Fig.4. Tsw inrelationtoBR withparameterk=1,2,3
IF1 filter. After this, a second local oscillator (LO2) with a located after the resolution filter and the envelope detector.
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This means that only a bandwidth of BR has to be digitized for a successful operation of flexible overlay systems. On the
whichleadstoahigheramplituderesolution.Moreover,aquite other side, each concept has significant drawbacks that can
simpleADCissufficientinthatcase.Drawbacksare,however, not be solved within the next couple of years and preclude
that pretty much analog hardware is necessary and that the an implementation in mobile terminals. So, a combination of
measurement time is increased (cf. section V). Additionally, both gives the opportunity to combine the advantages. This
no feature detection is possible since only energy detection is approach is discussed in section V.
performed.
V. WIDE-BANDCRRECEIVERFORIEEE802.22
A. Wide-Band Sensing
A. Sub-band Spectrum Sensing
Another possibility for energy detection is the down-
As we presented above, a digitization of the complete
conversion of the wide-band radio signal to an intermediate
frequency used for ADC. This reduces the number of inter- system bandwidth BS is not useful with respect to technical
andeconomicalconstraints.ButenhancementsinCRspectrum
mediate frequency stages required for sweeping the detection
sensing are necessary to increase spectral utilization. In order
windowasitisusedinwide-bandspectrumanalyzers.Butthe
to simplify spectrum sensing and to reduce the hardware
performance requirements to the ADC increase significantly.
requirements mentioned above we analyze expectable sig-
Ideally, the incoming analog signal is filtered by a bandpass
withsystembandwidthBS.Afterwards,thesignalisamplified nal characteristics resulting from measurements in the IEEE
and down converted from the radio frequency fRF to an 802.22 frequency range. Having a closer look at the results
intermediate frequency fIF. After a second filter stage and depictedinFig.1andFig.2,thechannelutilizationinthesub-
bandallocatedbybroadcastservices(cf.ch1)doesnotchange
AGC there is the analog to digital conversion. The following
duringthemeasurementtime.Furthermore,thereceivedsignal
dataprocessingincludesanFFTinordertoextractthecurrent
power of broadcast transmitters averaged over the sensing
powerallocationoverthefrequency.Afterdigitizationthesig-
time is significantly higher than the noise level. In general,
nal contains information of the complete observed frequency
the average power level of the broadcast signals is higher
range. Of course, the information depth is characterized by
than−70dBmandevenhigherthan−50dBmconsideringthe
the resolution performance of the ADC, which is a main
strongest signal. Therefore, a continuous sensing of these fre-
drawback of this approach. As it was shown in the section
quencyrangesdoesnotofferanyadditionalinformationforan
before,theobservedsignalischaracterizedbyahighvariation
operatingCRsystem.Thisleadstotheopportunitytoexclude
of the spectral power density. The overall dynamic range
is more than 50dB. Furthermore, under-utilized small-band such quasi-static frequency ranges considering its information
entropy during the spectrum sensing phase. Additionally, the
signalswithweaksignalamplitudescanbenoticed.Inorderto
resulting decreased dynamic range of the input signal reduces
provide a reliable detection of the licensed user, these signals
the ADC hardware requirements. Without loss of generality
still have to be noticeable after the ADC, otherwise signal
statistical independence of observed communication channels
detection fails. Therefore, the dynamic range of the ADC is
animportantparameterforCRterminals.BasedonNeff given canbeassumed.Thus,thespectrumcanbedividedintoseveral
sub-bands in order to detect the average channel allocation.
above the presented measurements would require a minimum
resolution of Neff =9bits. Besides a high bit resolution, a These sub-bands do not need to be observed at the same
time but can be sensed sequentially as long as the sensing
high sampling rate is required for wide band digitization.
is repeated periodically and the sensing interval as well as
Following the example of IEEE 802.22, the sampling rate is
fsamp = 1.82Gsps meeting the Nyquist criterion. Due to the sensing period is suitable to the licensed user’s signal.
the overall frequency range from 41MHz up to 910MHz, Based on this knowledge and assuming a suitable sensing
a bandpass sub-sampling cannot be used for reduction of sequence the system bandwidth BS can be split into M sub-
fsamp. As stated in [9], the maximum sampling rate for bandsresultinginBsub =BS/M.Eachsub-bandisseparately
an effective resolution of Neff = 9bits is about fsamp = digitized reducing the sampling rate of the ADC. Due to the
500Msps...1Gsps.Theanalysesin[8]and[9]alsodiscover resultingdecouplingofcommunicationchannelswithdifferent
utilizations, the dynamic range of the ADC’s input signal
that sampling rates significantly higher than 100 Msps can
can be optimized for sensing weak signals. Additionally, the
onlybehandledbyFlashorPipelinedconverters.Asdescribed
suppression of strong signals using analog notch filter is not
in section II, these two architectures are characterized by a
necessary, because the reduced sampling rate enables higher
high power consumption and, therefore, not preferable for
an implementation in mobile terminals. The group of Σ∆- bit resolution.
converters offer lower power consumption but cannot provide
B. Receiver Structure
thehighsamplingrateavailablewithFlashconverters.Inorder
to suppress the undesired signals analog notch filters can be The system architecture described in section III-A defines
applied, too. This requires a first scan for identification of the the basis for our wide-band CR receiver supporting sub-band
strongest signals. After tuning the notch filter to these fre- sensing. In contrast to the structure depicted in Fig. 3 the
quencies a second scan is used for detection of weak signals. ADC is placed directly after the IF 2 filter. This position
Besidesdoublingthescantimealsotheanaloghardwareeffort is marked with the letter ‘A’. The adapted signal processing
increases significantly. structure is depicted in Fig. 5. Up to marker ‘A’ the analog
Comparedtotheenergydetectiondescribedabove,wideband signalprocessingisthesameasdescribedinsubsectionIII-A.
digitizationoffersabettertimeresolutioninawidefrequency In order to support the IEEE 802.22 specifications the sub-
band. Thus, a detailed extraction of temporal features is pos- band bandwidth is defined to Bsub = 50MHz including six
sible. A high time resolution becomes important for detection sub-channels with a bandwidth of 8MHz up to eight sub-
of very short channel allocations or for detailed analysis of channels with a bandwidth of 6MHz, respectively. Similar to
the licensed user’s allocation statistics. the number of sweep points defined in analyzer detection the
Both concepts offer different advantages which are required LO1 can be tuned to 20 predefined frequency steps resulting
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the sub-band offering the lowest utilization is selected for the
upcoming sub-band switch of all overlay users. In Fig. 6 the
described sensing process is depicted. The data transmission
including sensing in the currently used sub-band is named
S1, the sensing of the sub-band(s) offering the next lowest
utilization is named S2, and the full range sensing is named
S3. With this sensing process the amount of irrelevant data
that have to be processed in the CR terminal can be reduced.
Fig.5. Structureofwide-bandCRreceiversupportingsub-bandsensing.
VI. CONCLUSION
In order to increase spectral utilization future CR receivers
have to provide enhanced spectrum sensing capabilities for
Fig.6. Sensingprocessforwide-bandCRsupportingsub-bandprocessing identification of the spectral environment. In IEEE 802.22
a frequency range from 41 to 910MHz is specified for CR
in a small sub-band overlap. For a more flexible sub-band overlay-system usage. In order to provide reliable licensed
configuration, a continuous oscillator tuning could also be usersdetection,theCRterminalfront-endhastofulfillseveral
implemented. Generally, digitization of a 50MHz sub-band preconditions. In this paper demands on ADCs in wide-
requiresasamplingrateofabout100Msps.Afterwards,digital band scenarios are discussed. As presented in section II, the
sensing processing like energy detection or feature detection generalperformanceofADCsischaracterizedbythetrade-off
algorithms could be applied. In Fig. 5 the block structure for between supported bandwidth and dynamic range defined by
energy detection is depicted. Due to the digital processing the the effective number of bits. Today’s ADCs support sampling
singlescantimecanbereducedbyafactorof1/20compared rates up to several Gsps at the expense of low dynamic
to analog processing using BR =2.5MHz. range and high power consumption. But, in order to meet
Besides decreasing scan time also the proposed sub-band the opposite wish for high dynamic ranges at low power
digitization offers the possibility of sensing adjacent channels consumption,theinputbandwidthhastobereduced.Insection
ofthecurrentlyallocatedcommunicationchannel.Asdepicted III the different radio services allocated in the considered
in Fig. 5 the received signal is used for communications frequency range are analyzed. Due to significantly varying
and sensing processing in parallel. This means, all spectral utilization, different demands for sensing in the sub-bands
information within the observed sub-band can be collected are observable. Especially sub-bands allocated by broadcast
along with current data transmission. Furthermore, the extra servicesdonotofferadditionalradioresourcesbutincreasethe
time released due to parallel sensing and communication demands on ADC’s performance significantly. Based on the
processing can be used for additional sensing of other sub- sensingalgorithmsdescribedinsectionIV,areceiverforwide-
bands. The gained information increases the CR’s knowledge band mobile CRs based on the superheterodyne principle is
about its spectral environment. But besides efficient sensing, presentedinsectionV.Duetoadistributioninto20sub-bands
also suitable information processing and knowledge storage highly utilized sub-bands can be masked while the resolution
has to be applied to CR. In the next subsection an algorithm in under-utilized sub-bands can be increased.
is presented that uses the proposed sub-band sensing.
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