Dual-Purpose Spectrum Sensor UsingIEEE802.15.4a UWB PHY and Energy Detection

Yohannes D. Alemseged, Ha Nguyen Tran, Chen Sun, Hiroshi Harada

National Institute of Information and Communications Technology (NICT)New Generation Wireless Communications Research Center

3-4 Hikarino-Oka Yokosuka, 239-0847 Japan E-mail:{yohannes,sun,haguen,harada}@nict.go.jp

Abstract—Cognitive radio(CR) has become an enabling tech-nology to realize dynamic spectrum access through its spectrumsensing and reconfigurable capability. Robust and reliable spec-trum sensing is needed to discover spectrum opportunity. Singlecognitive radios often fail to provide such reliable informationdue to their inherent sensitivity limitation. Primary signals thatare subject to detection by cognitive radios may become weakdue to several factors such as fading and shadowing. Oneapproach to alleviate this problem is to perform the spectrumsensing by using multiple CRs or multiple spectrum sensors.This approach known as distributed sensing because sensing iscarried out through cooperation of spatially distributed sensors.In distributed sensing, sensors should perform spectrum sensingand forward the result to a destination where data fusionis carried out. In this paper we introduce a concept of adual-purpose spectrum sensor that can combine the sensingand communication functions. The dual-purpose sensor reusesits detection hardware for both functions resulting in a lesshardware complexity. We consider IEEE802.15.4a UWB PHYfor communication and energy detection for spectrum sensing.

Index Terms—Cognitive radio, spectrum sensing, energy de-tection, ultra-wideband, dual-purpose spectrum sensor

I. INTRODUCTION

In the last few decades, due to the steady fast growthof the wireless communication industry, the demand forradio frequency (RF) spectrum is increasing. So far, thetrend of spectrum allocation has relied on assigning differentportions of the spectrum for particular service. Howeverthis approach turned out inefficient to accommodate thecurrent spectrum demand. Recent studies show the assignedfrequencies are not occupied all the time implying thatthe traditional way of spectrum allocation has resulted inunder-utilization of the scarce spectrum [1]. The unusedspectrum, also known as white space, varies temporarily andspatially. FCC has reacted to this unveiled fact by allowing anopportunistic usage of the spectrum holes with a condition thatno harmful interference is induced to the licensed services [2].

Cognitive radio is a front runner technology to realizedynamic spectrum access (DSA). It performs spectrumsensing to discover unused or unoccupied frequency band andsubsequently performs decision and reconfiguration to exploitthe spectrum opportunity. The spectrum sensing capabilityof cognitive radios should provide sufficient protectionto the incumbent services while optimizing the discovery

of spectrum opportunity. Hence, reliable and fast sensingbecomes crucial. Recently the concept of utilizing distributedspectrum sensors for spectrum sensing has been suggested[3], [4], [5], [6], [7]. This paper introduces a dual-purposespectrum sensor that can be deployed in such scenario. Theproposed spectrum sensor combines both the transmit-receivefunction and spectrum sensing function through a commonsignal detection hardware. Using a common RF front-endfor both spectrum sensing and communication is not newon its own. For example, a software defined cognitive radiodiscussed in [8] reuses its RF board for both spectrumsensing and communication purposes. Unlike such devicesthat support different types of communication systems, thedual-purpose spectrum sensor we propose uses the IEEE802.15.4a UWB signaling and a low complexity non-coherentdetection [9]. Such devices are often known as ultra-widebandimpulse radio (UWB-IR). The detector part is designed forre-use during spectrum sensing yielding a low complexitydevice. Dual-purpose spectrum sensors would suite wheresmaller size and low power consumption is compulsory.

The rest of the paper is organized as follows. In SectionII, background information on UWB for communication andenergy detection based spectrum sensing is provided. In Sec-tion III, the system description of the dual-purpose spectrumsensor is introduced. In Section IV, different schemes of re-source sharing and factors affecting the spectrum sensing withsome simulation examples are explored. Section V presentsconclusive remarks.

II. BACKGROUND INFORMATION

A. UWB for sensor network

This section gives brief background on the applicationof UWB for sensor networks. With improvements in powerconsumption, device size, communication and medium accesscontrol (MAC) algorithms, sensor networks are becomingmore popular for an ever increasing range of applications.Here, the emphasis is on the use of sensor network is in thecontext of distributed spectrum sensing. Spatially distributedsensors perform spectrum sensing over a particular or rangefrequency bands. The result is forwarded to a cognitiveradio terminal for further analysis and decision. Due to thebursty nature of the traffic (because low rate transmission

978-1-4244-4581-3/09/$25.00 ©2009 IEEE

is sufficient to exchange sensing information), the spectrumsensors may remain idle for long periods of time, thensending significant amounts of information when an eventoccurs. The event could be a request to provide spectrumsensing information by a cognitive radio for a given set ofparameters. Such low duty cycle operation of a spectrumsensor motivates design of efficient medium access protocols,low power consumption RF and baseband circuits, andinexpensive receiver architectures in terms of computingpower.

UWB-IR systems have a number of inherent propertiesthat are well suited to the scenario discussed above. Us-ing UWB for communication has a unique advantage thatthe extremely wide bandwidth characteristic provides veryrobust performance under harsh multipath and interferenceconditions. Furthermore, impulse based UWB systems can bemanufactured with low complexity and smaller size makingthem suitable for mass production.

B. The 802.15.4a UWB PHY symbol structure

The IEEE 802.15.4a standard specifies an alternativePHY based on UWB for low rate personal area networks(LR-WPAN) [9]. It is an amendment to the already existingIEEE 802.15.4 standard. The specification is designed toprovide robust performance for LR-WPAN applications byexploiting the appealing characteristics of UWB. The standardspecifies the use of UWB in sub-gigahertz (250-750 MHz),low band (3.1-5 GHz), and high band (6-10.6 GHz). TheUWB PHY provides a hybrid modulation that enables theuse of very simple, non-coherent receiver architectures likeIR-UWB to minimize power consumption and implementationcomplexity. The hybrid modulation scheme includes the wellknown binary phase shift keying (BPSK) modulation andbinary pulse position (BPM) modulation. In the BPM-BPSKmodulation scheme, a UWB PHY symbol is capable ofcarrying two bits information: one bit is used to determinethe position burst of pulses while an additional bit is usedto modulate the phase (polarity) of this same burst. Themotivation to use a combination of BPM and BPSK is tosupport both coherent and non-coherent receivers using acommon signaling scheme.

The structure and timing of a UWB PHY symbol isillustrated as follows. Each symbol consists of integer numberof possible chip positions, Nc, with duration Tc. The symbolduration denoted by Tdsym is given by Tdsym = NcTc.Each symbol is divided into two BPM intervals with durationTBPM = Tdsym/2, which enables binary position modulation.A burst is formed by grouping Ncpb consecutive chips andhas duration Tburst = NcpbTc. The location of the burst ineither the first half or second half of the symbol indicates onebit information. Additionally, the phase of the burst (either-1 or +1) is used to indicate a second bit of information.A condition Tburst << TBPM is needed to provide somemulti-user access interference rejection in the form of time

hopping. Total number of burst durations per symbol is givenby Nburst = Tdsym/Tburst. In order to limit the impact ofinter-symbol interference, only the first half of each TBPMperiod contains burst. Hence only the first Nhop = Nburst/4possible burst positions are candidate for hopping burstpositions with in each BPM interval. Each burst position canbe varied on a symbol-to-symbol basis according to a timehopping code. Fig. 2 illustrates the 802.15.4a UWB symbolstructure.

C. Energy detection based spectrum sensing

In energy detection based spectrum sensing, the energyfeature of a captured signal is used to infer if it containsprimary signal (PS) or not. For example, let’s denote anobserved signal as x(t) and its sampled form as x[n] witha sampling period of Ts s. We write x[n] as,

x[n] = w[n] + hs[n] in PS presence (1)

x[n] = w[n] in PS absence (2)

where w[n] is an additive white Gaussian noise (AWGN) withstatistics N (0, σ2

w). s[n] is a narrowband a PS with statisticsN (0, σ2

s) propagating through a fading channel with channelgain h having Rayleigh distribution. Energy detector collectsn = 1, . . . , N samples of the received signal to performsquaring and summation operation and obtain a test statistic zgiven by

z =N−1∑

n

x[n]2. (3)

The duration over which N samples are acquired is knownas integration interval. It is chosen to meet a given targetdetection performance. Using notations of the widely knownNeyman Pearson (NP) hypothesis testing, namely the null-hypothesis (H0) and signal-hypothesis (H1), (3) can be writtenas

z =

{ ∑Nn=1(w[n])2, H0∑Nn=1 (hs[n] + w[n])2, H1

(4)

For large value of N , the distribution of z under H1 and underH0 can be approximated by Gaussian distribution with thestatistics,

z ∼{ N (Nσ2

w, 2Nσ4w), H0

N (N(σ2w + |h|2σ2

s), 2N(|h|2σ2s + σ2

w)2), H1(5)

Then the NP test for PS presence (H1) is written as

Λ(z) =p(z;H1)p(z;H0)

> γ′

(6)

where γ′

is a threshold value. The term L(z) is knownas likelihood ratio and the test procedure is also known aslikelihood ratio test (LRT). The NP test assumes all statisticalparameters known a priori. However, this may not be a caseoften. Alternatively, we can carry a binary hypothesis testingby comparing z with another threshold value γ as follows

bz ={

1 if z > γ;0 otherwise.

(7)

where bz is a binary decision variable. In other words this is amapping process of x(t) during NTs s observation period toa one bit information. The thresholds γ′ and γ are related byγ = {z : Λ(z) > γ

′}. The performance of the above detectoris often measured by the pair of metrics Pfa (probability offalse alarm) and PD (probability of detection) given by

Pfa = Q

[γ − Nσ2

w√2Nσ4

w

](8)

PD = Q

[γ − N(σ2

w + |h|2σ2s)√

2N(|h|2σ2s + σ2

w)2

]

For a given Pfa value the threshold γ in (7) is given by

γ = Q−1(Pfa)√

2Nσ4w + Nσ2

w. (9)

If both PD and PD are provided, the number of samples Nrequired would be given by

N =√

2[Q−1(Pfa) + Q−1(PD)(|h|2ξ + 1)

|h|2ξ]

(10)

where ξ = σ2s

σ2w

is SNR of the PS.

III. DUAL-PURPOSE SPECTRUM SENSOR

Following the discussions in Section II, we propose an IRbased dual-purpose spectrum sensor illustrated in Fig. 1 thatdetects the 802.15.4a UWB signal. The choice of IR withenergy detection as transceiver module of spectrum sensor en-hances the re-usability of components for both communicationand spectrum sensing purposes. The main functional blocksof the device consists of a controller, detector and filter bankwhich are described as follows.

chain

a

fa

fb1

fb2

Spectrum sensor

B1

B2

Bo

spectrum sensing band

communication band (UWB frequency band)

BN

integration interval selector

delay selector

+

+

zUWB

sensingz

2

x(−1)

Controller

function selector/band selector

Receiver

f

Fig. 1. Schematic diagram of dual-purpose spectrum sensor

A. Controller

The main function of the controller is to perform theswitching between the transmit-receive action and spectrumsensing action. In principle, the device is a basic UWBreceiver that can detects the signal described in SectionII-B. It first communicates with other spectrum sensors orcognitive radio to initiate the spectrum sensing. In addition, itobtains an instruction on which particular frequency band thesensing should be carried out. Then the controller switchesthe device to spectrum sensing function by disconnecting thereceiver front-end filter and connecting the required filters.The controller has also additional tasks of managing thespectrum sensing filters, the integration interval and the delayinterval. The controller accomplishes the switching betweentransmit-receive and spectrum sensing through the followingactions.

1) Band selection: The band selection is achieved throughselecting the required filters B1 to BN for spectrum sensingand B0 for communication as shown in Fig. 2. Discussion onthe actual realization of the switching is beyond the scope ofthis paper.

2) Integration interval selection: Depending on the selectedfunction (comm. or sensing) and by using the parameters inSection II-B, the integration interval can be given by

fa ={

Tburst + τrms, for comm.TGI , for sensing

(11)

where τrms is the rms delay spread of the UWB signal andTGI stands for the guard interval period. In Section IV-A, weshow that at low data rates, the value of TGI is significantenough to perform spectrum sensing. However, the actualintegration interval needed depends on the target PD and Pfa

values. Therefore an integration over several UWB symbolperiods might be needed. In case of short TGI other schemesto be discussed in Section IV-A can be followed.

3) Delay selection: Depending on the selected function(comm. or sensing) and by using the parameters in SectionII-B, the delay values to start the integration are given by

fb1 ={

kTsym + Dk, for comm.kTsym + Tburst, for sensing

(12)

fb2 ={

kTsym + TBPM + Dk, for comm.kTsym + TBPM + Tburst, for sensing

(13)

where fb1 and fb2 represent the delay selection functions forthe first half of symbol interval and the second half of symbolinterval respectively. k stands for symbol index while Dk

is a time hoping code that may change from symbol to symbol.

B. Detector

The detector portion of the sensor is a common hardwarefor both sensing and transmit-receive purpose. Detection isachieved through energy detection. It is known that suchdetectors provide a sub-optimal low complexity reception of

radio signals. The detector includes one squaring unit, twointegration and damp units, one inverter, one adder and finallya decision device Fig. 1. During spectrum sensing, the energyfrom the two integrators is added before decision. Hencea separate adder and decision device is needed. The twodecision devices employ different thresholds. For spectrumsensing decision takes place only when sufficient samples arecollected and not necessarily per UWB symbol basis.

C. Filter bank

The filter bank consists of B0 to BN filters as indicated inFig. 1. The filter B0 is committed for the communication andmay have the following frequency ranges, sub-gigahertz (250-750 MHz), low band (3.1-5 GHz), high band (6-10.6 GHz) ortheir combinations [9]. The filters from B1 to BN are com-mitted for spectrum sensing. Multiband sensing is achievedthrough these switchable bank of filters where band of interestcan flexibly be adjusted by connecting or disconnecting someof the filters. Depending on the need of the cognitive radio acognitive engine can distribute the task of sensing differentbands to different sensors. This approach would improvethe sensing time while reducing the computational burden ateach sensor. The coordination is targeted at attaining the bestsensing (in terms of sensing time and sensing quality). In caseof sensing a particular PS, all sensors can collaborate to yielda better aggregate result.

IV. SENSING AND TRANSMIT-RECEIVE

A. Resource sharing

1) Time-bonding: The concept of time-bonding is similarto channel-bonding where non-adjacent channels can be usedas one logical band. In time-bonding, spectrum sensing iscarried out in between signal bursts as in the case of LDCUWB signaling. In this approach sensing can be performedparallel to receiving signals.

For time-bonding, if Tint is a required total integrationtime and Ts is one time slot for integration, then Tint = KTs,where K is the total number of time slots used for integration.One time slot for integration or Ts can be the guard periodindicated in Fig. 2. For instance, the 802.15.4a supports anoptional low data rate of 110kb/s to enable long links orprovide high processing gain to cost-effective PHYs. Suchscheme allows sufficiently longer guard interval that makes itsuitable for sensing using time-bonding.

2) Continuous sensing: This kind of sensing can beimplemented when sufficient time interval is available tocollect the required samples and obtain a unit sensing result(binary detection outcome). In terms of the time slot forsensing, Tint = Ts. The sensing can be performed in betweencommunication packet bursts and idle times for instanceduring synchronization and time acquisition

b)B

0B

0

Tburst

Tchip T dsym

Possible burst position Guard Interval Guard IntervalPossible burst position

T TBPM BPM

Communication band Communication bandSensing band Sensing band

B1 B BB 1N N

a)

Fig. 2. a) IEEE802.15.4a UWB PHY symbol structure and b) Example offrequency band switching for communication and spectrum sensing

3) Scheduled sensing: In this approach, sensing is per-formed in scheduled manner in coordination with the transmit-receive of the radio and in coordination with other cooperatingspectrum sensors.

B. Factors affecting spectrum sensing

Referring to the energy detection scheme discussed inSection II, the performance of spectrum sensing is mainlyaffected by the number of samples captured to evaluate asignal energy. The number of samples also depends on theoverall integration interval and the filtered signal bandwidth.To show the impact of these parameters on the spectrumsensing we consider the following scenario.

For communication, we assume the UWB PHY usessymbol duration Tdsym) = 8205.13 ns and burst sizeNburst = 32 (cf.[9]). Accordingly, the guard interval TGI

will be 2051.3 ns. For non-line of sight (NLOS) officeenvironment, UWB exhibits an rms delay spread in the order20-30 ns. Hence all the TGI value can be considered forspectrum sensing with minimal impact on the performanceof the UWB communication. Assuming the time-bondingconcept in Section IV-A1 can be applied here, several guardintervals from UWB symbols burst are spanned to obtain theover all sensing period. During the UWB symbols burst, thePS’s channel is assumed to stay unchanged.

Fig. 3 shows the performance of the energy detector interms of the PD vs. SNR value of the PS for Pfa = 0.05.Equation (9) is used to compute the detection threshold.The first solid line assumes detection of a PS with 500KHz.The integration interval is obtained considering a 512 UWBsymbols burst. At 0 dB, 0.9 probability of detection can beachieved. The next line (dashed) shows a performance curveobtained by increasing the size of the UWB symbols burstto 1024. We observe a gain of ≈ 1.5 dB from the previousplot. The next two performance curves are obtained whenthe BW of the PS is increased to 2 MHz. The curves areshifted to the right with 1.5 dB compared to the previoustwo curves. Significant performance improvement is observedwhen both the bandwidth of the PS and as well as the sizeof the UWB symbols burst are increased. Note also that

−20 −15 −10 −5 0 5 10 150

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

SNR of primary signal [dB]

Pro

babi

lity

of d

etec

tion

Probability of detection for Pfa=0.05

500KHz PS BW and 512 UWB symbols burst2MHz PS BW and 512 UWB symbols burst500KHz PS BW and 1024 UWB symbols burst2MHz PS BW and 1024 UWB symbols burst

Fig. 3. Plot showing probability of detection versus SNR of PS for constantprobability of false alarm (Pf=0.05)

an optimum integration period can be obtained by usingequation (10) along with the bandwidth information of the PS.

V. CONCLUSIONS

A dual-purpose spectrum sensor is proposed for cognitiveradio.Different functional blocks of the dual-purpose spec-trum sensor such as a controller, detector, and filter-bankare described. The proposed spectrum sensor assumes theIEEE802.15.4a UWB PHY for communication and and anenergy detection method to performed a combined spectrumsensing and communication. The proposed spectrum sensorperforms both transmit-receive and spectrum sensing alter-nately by reusing the detection hardware yielding low com-plexity. Such dual-purpose spectrum sensor can be applicablein distributed spectrum sensing where low power consumption,

low complexity and smaller size are crucial. The communi-cation function is used for the inter sensor communicationor sensor to cognitive engine communication. Three schemesof how the hardware resource is shared between communi-cation and spectrum sensing is provided. Concepts such astime-bonding, continuous sensing, and scheduled sensing arediscussed. Finally, a simulation example by using some ofIEEE802.15.4a UWB PHY system parameters, is providedto show the impact of PS bandwidth and the duration ofintegration interval on spectrum sensing.

VI. ACKNOWLEDGMENT

This research was conducted under a contract of R&Dfor radio resource enhancement organized by the Ministry ofInternal Affairs and Communications, Japan.

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