Full-Duplex Spectrum Sensing in Cognitive Radiosusing Optical Self-Interference Cancellation

Matthew P. Chang and Paul R. PrucnalElectrical Engineering Department

Princeton UniversityPrinceton, New Jersey, USA

Email: mpchang@princeton.edu

Yanhua DengBascom-Hunter

Baton Rouge, Louisiana, USAEmail: deng@bascomhunter.com

Abstract—We propose and experimentally demonstrate anoptical self-interference cancellation system to realize full-duplexspectrum sensing in cognitive radios. The optical system is ananalog radio-frequency front-end module, which cancels in-bandself-interference, enabling a radio to simultaneously transmitand receive signals. The system achieves 83 dB of narrowbandinterference cancellation, and 60 dB of cancellation of a 50 MHzfrequency-modulated signal. The center frequency of the opticalcanceler is freely tunable across the radio frequency spectrum,limited only by the bandwidth of the photodetector and theelectro-optic modulators to 10 GHz. The system is modulation-format independent and requires only one piece of hardware tooperate across a wide radio-frequency bandwidth. By reducingself-interference to acceptably low powers, a cognitive radio cancontinuously sense its radio-frequency environment to detect thepresence of a licensed user or scan for spectrum white spaceseven while transmitting simultaneously.

I. INTRODUCTION

Traditional radio-frequency (RF) spectrum management isperformed by allocating a static frequency band to an exclusivelicensed user. However, this method is inefficient because largeswaths of the licensed spectrum are significantly underutilizedduring certain times and across geographical regions [1], [2].

Cognitive radio, using dynamic spectrum access, hasevolved to enable secondary users to intelligently recycle theunderutilized spectrum by sensing the RF environment andexploiting available spectrum holes, or white spaces. Uponidentifying a spectrum white space, a cognitive radio may tem-porarily access the spectrum until a licensed user is detected oruntil network quality degrades below an application threshold,at which point the radio must relocate to a new spectrumwhite space, as shown in Fig. 1a [1]–[4]. Therefore, to providecontinuous service while avoiding interfering with other users,a cognitive radio must be able to constantly sense the RFspectrum, and process and quantify spectrum characteristicsat all times and across a wide bandwidth [1], [3].

Currently, radios cannot sense the RF environment whiletransmitting, because their own strong transmission over-whelms weak signals arriving from distant nodes, a problemknown as self-interference [5]. Thus, to perform spectrumsensing, the radio must stop transmitting, decreasing spectrumefficiency and adding costly overhead [1], [2]. Additonally,while transmitting, the radio may not be aware of the newpresence of a licensed user, resulting in interference.

In this paper, we present an optical system to cancel self-interference and enable a radio to simultaneously transmitand receive. By reducing the self-interference to the noisefloor, cognitive radios can continuously scan the spectrum,even while transmitting, to prevent interference and locate newspectrum white spaces, as shown in Fig. 1b.

The system leverages several well-known advantages of op-tics, such as wideband performance and high-precision [6], [7],that are highly advantageous for self-interference cancellation[8] and cognitive radio. The wideband performance enables acognitive radio to operate across a broad bandwidth withoutthe need to reconfigure internal hardware. Unlike typical RFfront ends, the optical system does not require a separate RFbandpass filter for each frequency band of operation, reducingsize, weight, and cost. The high-precision of optics allows thesystem to cancel the self-interference signal to very low levelsso that the radio can detect very weak user signals in theband. Finally, because the system is completely analog, it iscompatible with all modulation formats.

The paper is organized as follows: in section II, we presentthe setup of the optical system followed by experimentalresults in section III. In section IV, we discuss challengesthat the system faces and improvements that can be made toaddress them. We conclude our paper in section V.

II. EXPERIMENTAL SETUP

The optical self-interference cancellation system operates asan RF receiver front-end module, compatible with future andlegacy radios. The system accepts two RF inputs: the radio’sown transmitted signal, xTX , and the received signal, xRX

xTX(t) = n0(t) (1)

xRX(t) = s(t) + n1(t) (2)

where s(t) is the weak signal from other users that thecognitive radio is trying to sense, n0(t) is the radio’s owntransmitted signal, and n1(t) is the radio’s transmitted signalafter it propagates through the communication channel andenters the receiver i.e. the self-interference.

The goal of the optical self-interference cancellation systemis to remove the self-interference signal, n1(t), from thereceived signal by using the known transmitted signal, n0(t).

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!!!!

Ch1

Ch2

Ch3

Ch4

White Space

Time

Freq

uenc

y Occupied Spectrum

(a)

CR

Licensed User

Spectrum Hopping

Spectrum Sensing

!!

Licensed User

Licensed User

CR Range

(b) Fig. 1. (a) A cognitive radio (CR) may temporarily use spectrum white spaces to transmit and receive, but must switch to a new white space if a licenseduser comes online or network quality declines. (b) A cognitive radio simultaneously transmitting while sensing the RF environment.

This is performed in the analog domain, because such a strongsignal is likely to saturate the subsequent analog-to-digitalconverter. As such, the optical system is inserted between thereceiver antenna and low-noise amplifier, as shown in the toppart of Fig. 2. It receives a tap of the known transmitted signal.

The optical system is a fiber-optic setup which beginswith a bank of n distributed-feedback lasers to produce n

optical carriers, each with a distinct wavelength, �1...�n, asshown in the bottom part of Fig. 2. The first optical carrier,with wavelength �1 = 1550.12 nm, is modulated by the totalreceived signal, xRX , through an electro-absorption modulator.The modulated optical signal propagates through a length offiber to the positive port of a balanced photodetector. Theremaining lasers are each modulated by a copy of the knowntransmitter signal, xTX . Each copy of xTX is weighted anddelayed by a variable optical attenuator (VOA) and tunabledelay line (TDL), respectively, before being combined intoa single fiber and detected by the negative port of the bal-anced photodetector. In other words, xTX is processed byan optical matched filter. Different wavelengths are used toprevent coherent beating noise at the detector. The balancedphotodetector effectively subtracts the combined weighted anddelayed copies of xTX from the received signal, xRX . Theresulting RF output from the balanced photodetector is

y(t) = s(t) + [n1(t)�nX

i=2

↵in0(t� ⌧i)] (3)

where ↵i and ⌧i are the weight and delay introduced by thei� th VOA and TDL, respectively. The term in the bracketsof Eqn. 3 is the residual self-interference after the system.

The ability of the system to cancel self-interference isdetermined by how well the combined sets of optical weightsand delays can model the communication channel i.e. thetransformation between n0(t) and n1(t). Although correlated,n0(t) and n1(t) can be quite different because of the distor-tion introduced by the transmitter-to-recevier communication

xTX

xRX

xRX

xTX

Optical System

Fig. 2. The optical self-interference cancellation system. (Top) The systemacts as an RF receiver front end module inserted between the receiving antennaand the low-noise amplifier (LNA). (Bottom) The optical subsystem, whichconsists of an optical matched filter that weights (↵) and delays (⌧ ) multipletaps of the transmitter signal (xTX ) before subtracting them from the receiversignal (xRX ) to cancel the self-interference. PA = Pre-Amplifier, VOA =Variable Optical Attenuator, TDL = Tunable Delay Line.

channel. To first order, n1(t) will be an attenuated and delayedversion of n0(t) after propagating through a lossy commu-nication channel; therefore, the system can approximate thisby a single pair of weights and delays. However, a practi-cal communication channel exhibits more complex channeleffects. For example, the transmitted signal can be distorted bynonlinear effects in the RF circuitry, random amplifier noise,and multipath reflections [9]. In addition, the signal may leakinto the receiver circuitry by means of crosstalk. Thus, it isimportant to tap n0(t) as close to the transmitting antenna as

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Self-Interference

Licensed User Signal

Licensed User Signal 50 dB

60 dB

Licensed User

Licensed User

Self- Interference

Before Cancellation After Cancellation

60 dB

Licensed User

After Cancellation

Fig. 3. Experimental results showing the output receiver spectrum before (left)and after (right) activating the optical self-interference cancellation system.The licensed user signal is a CW signal centered at 500 MHz. The self-interfering signal is a 50 MHz frequency-modulated signal also centered at500 MHz.

Fig. 4. Experimental results showing the output receiver spectrum before (left)and after (right) activating the optical self-interference cancellation system.The self-interfering signal is a CW signal centered at 500 MHz. There is nolicensed user signal.

possible to capture these effects. The additional sets of weightsand delays in the system, as shown in the bottom part of Fig.2, are added to give the system more flexibility to compensatefor complex channel effects. One use of the multiple weightsand delays is to dedicate each weight and delay to represent adifferent multipath component of n0(t) as it propagates to thereceiver [10]. Given enough weights and delay taps, anotherapproach is to model the impulse response of the channel [9].

III. EXPERIMENTAL RESULTS

The optical system was implemented using a single pair ofweights and delays to cancel the strongest component of theinterference (i.e. the line-of-sight). A weak, continuous-wave(CW) RF signal was used as the licensed user signal that thecognitive radio is attempting to sense. Meanwhile, a strongtransmitter signal was introduced, and the system attempted torecover the licensed user signal by canceling the transmittersignal. Both narrowband and broadband interference cancella-tion was characterized to examine the wideband performanceof the system. To tune between different operating frequencybands, slight adjustments were manually made to the VOAand TDL in the optical system.

Figure 3 shows the results of the experimental test. Alicensed user signal centered at 500 MHz was introducedinto the system (Fig. 3a). A 50 MHz bandwidth frequency-modulated (FM) signal was used as the self-interference

transmitted signal. The receiver output spectrum was measuredboth before (left) and after (right) the optical self-interferencecancellation system was activated. The comparison of the twoplots shows that the system is able to cancel the vast majorityof the self-interference across the 50 MHz bandwidth. A smallamount of residual interference is visible just above the noisefloor, but it is reduced by about 60 dB from the strongestpart of the original interference. Most importantly, the licenseduser signal is observable, even while the radio is transmitting.The cognitive radio can therefore characterize the signal anddetermine if it needs to relocate to a different band. Note thatthe licensed user signal is recoverable even when it coexistswith the self-interference signal in the same band, makingthis type of processing fundamentally different from spectralfiltering. It is this critical feature that enables a cognitive radioto detect if a licensed user is operating in the same band.

To determine the limits of the optical self-interferencecancellation system, a strong 500 MHz CW signal was usedas the interfering signal, and the system’s VOA and TDLwere optimized to perform cancellation only at that frequency.No licensed user signal was used in order to observe onlythe effects of the self-interference cancellation. The self-interference cancellation was characterized by measuring theinterference power before and after activating the optical self-interference cancellation system, as shown in the left and rightparts of Fig. 4, respectively. The system is able to achieve 83dB of narrowband interference cancellation.

IV. DISCUSSION

In this section, we discuss some of the challenges that theoptical self-interference cancellation system faces in practicaloperation as well as some methods to meet these challenges.

If a cognitive radio is sensing the local RF environmentwhile simultaneously transmitting, it needs to ensure that theresidual self-interference is low enough to keep the detectionerror rate below an acceptable threshold. A significant amountof residual self-interference can cause more harm than benefitby preventing the radio from accurately sensing the RF envi-ronment. The radio’s self-interference level must remain lowat all times and across all bandwidths of potential operation,including those that the radio is not currently occupying, sothat the radio can accurately sense spectrum white spaces.Therefore, the optical self-interference cancellation systemmust maintain wideband operation and be able to adapt quicklyto a fluctuating RF environment.

Maintaining wideband operation is challenging becauseof the need to model the communication channel betweenreceiver and transmitter at all frequencies of operation. Thecommunication channel not only includes the free spacebetween the transmitting and receiving antennas, but alsothe antennas themselves and the receiver and transmitter RFcircuitry. We find that the lack of frequency flatness of theRF components, particularly the antennas, contribute the mostto complex channel frequency responses that are difficult toapproximate with a set of weights and delays.

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(a) (b)

(c) (d)

(e)

Fig. 5. Simulated wideband self-interference cancellation of the opticalsystem as a function of RF center frequency and bandwidth.

To characterize the wideband performance of the system,we measured the communication channel frequency response(S21), and compared it to the frequency response of theoptical system’s set of weights and delays. By measuring themismatch between the two frequency responses, the amountof self-cancellation can be determined by treating the opticalsystem as a finite-impulse-response (FIR) filter. The simulatedaverage cancellation, across center frequencies from 400 MHzto 2.6 GHz and with bandwidths ranging from 10 MHzto 500 MHz is shown in Fig. 5. The simulation indicatesthat the average cancellation degrades with increasing band-width because the system has more difficulty modeling acommunication channel’s frequency response across largerbandwidths. Nonetheless, the system still predicts at least 40dB of interference cancellation at bandwidths as large as 500MHz. It should be mentioned that this simulation does not in-clude the antenna response, which will add additional channelresponse mismatches, therefore degrading cancellation depthand bandwidth. A combination of precise RF engineering, tomatch the frequency responses of different antennas and RFcomponents thereby reducing frequency response mismatch,and additional optical taps, each with its own weight anddelay to introduce more degrees of freedom, can help improvebandwidth performance.

Cognitive radios employing the optical self-interferencecancellation system require the ability to maintain high-qualityinteference cancellation at all times. Another challenge is todevelop algorithms and technologies that allow the opticalsystem adapt quickly to changing RF environments by usingthe tunable weights and delays of the optical system. If thesystem does not adapt quickly, then the advantages of beingable to simultaneously transmit signals while sensing the RFenvironment may be lost by the overhead required to opti-mize self-interference cancellation. Optimization algorithmsare commonly used to minimize the power of an interference

signal. Two algorithms that we are currently investigatingare the Least Means Squared algorithm and the Nelder-MeadSimplex algorithm. The required convergence time depends onthe RF environment and how frequently the cognitive radioneeds to sense the RF environment.

V. CONCLUSION

In this work, we presented an optical system to cancel theself-interference of a cognitive radio so that it can simultane-ously transmit and sense the RF environment for other usersand spectrum white spaces. The fiber-optic system uses anoptical matched filter to perform self-interference cancellationacross a multi-GHz bandwidth, saving space, weight, andcost. The system has demonstrated 83 dB of narrowbandcancellation, 60 dB cancellation of a 50 MHz FM signal,and the potential for >40 dB of cancellation over 500 MHzinstantaneous bandwidth. By properly adapting to the changingRF environment, a cognitive radio employing the opticalself-interference cancellation system can significantly reducespectrum sensing overhead, improve spectrum management,and simplify the hardware required to operate over multiplechannels.

ACKNOWLEDGMENT

The authors would like to thank their colleagues in theLightwave Lab at Princeton University and Bascom-Hunterfor their invaluable assistance on this project.

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