Published in: Proceedings of the Workshop on Cognitive Radio, June 2013, Kista, Sweden

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1 Adapive nonlinear inerference suppressor for cogniive radio applicaions Habibi, H.; Pooh Ling, E.; Janssen, E.J.G.; Wu, Y.; Milosevic, D.; Bergmans, J.W.M.; Balus, P.G.M. Published in: Proceedings of he Workshop on Cogniive Radio, 8-9 June, Kisa, Sweden Published: // Documen Version Acceped manuscrip including changes made a he peer-review sage Please check he documen version of his publicaion: A submied manuscrip is he auhor's version of he aricle upon submission and before peer-review. There can be imporan differences beween he submied version and he official published version of record. People ineresed in he research are advised o conac he auhor for he final version of he publicaion, or visi he DOI o he publisher's websie. The final auhor version and he galley proof are versions of he publicaion afer peer review. The final published version feaures he final layou of he paper including he volume, issue and page numbers. Link o publicaion Ciaion for published version (APA): Habibi, H., Pooh Ling, E., Janssen, E. J. G., Wu, Y., Milosevic, D., Bergmans, J. W. M., & Balus, P. G. M. (). Adapive nonlinear inerference suppressor for cogniive radio applicaions. In Proceedings of he Workshop on Cogniive Radio, 8-9 June, Kisa, Sweden (pp. 6-) General righs Copyrigh and moral righs for he publicaions made accessible in he public poral are reained by he auhors and/or oher copyrigh owners and i is a condiion of accessing publicaions ha users recognise and abide by he legal requiremens associaed wih hese righs. Users may download and prin one copy of any publicaion from he public poral for he purpose of privae sudy or research. You may no furher disribue he maerial or use i for any profi-making aciviy or commercial gain You may freely disribue he URL idenifying he publicaion in he public poral? Take down policy If you believe ha his documen breaches copyrigh please conac us providing deails, and we will remove access o he work immediaely and invesigae your claim. Download dae:. Oc. 8

2 Adapive nonlinear inerference suppressor for cogniive radio applicaions H.Habibi, Pooh Ling E, E. J. G. Janssen, Y. Wu, D. Milosevic, J. W. M. Bergmans, P.G.M. Balus Deparmen of Elecrical Engineering, Eindhoven Universiy of Technology Den Dolech, 56 AZ Eindhoven, The Neherland Telephone: +()7 87 Caena Radio Design bv Science Park Eindhoven, Ekkersrij 6, 569 GA Son en Breugel, The Neherlands Telephone: +() Absrac To uilize he radio frequency specrum efficienly a Cogniive Radio (CR) can operae as a secondary user in a frequency band which is licensed o a primary user. To his end, he CR mus sense he specrum coninuously o find empy frequency channels for is ransmission. The ransmied signal by he local ransmier of he CR, however, induces a srong local inerference in he local receiver of he CR. Hence a half-duplex ransceiver is used where he ransmi and sense operaions are done in separae ime slos. The imesloed operaion hough, reduces he hroughpu of he CR. This paper proposes applicaion of an adapive Nonlinear Inerference Suppressor (NIS) o suppress his srong local inerference o enable simulaneous ransmi and sense. We presen experimenal resuls of a ransceiver esbed ha uses an implemenaion of he NIS, fabricaed in nm CMOS echnology. These experimens show ha he NIS can subsanially suppress he local inerference wih low complexiy and power consumpion. This suggess he applicaion of he NIS for specrum sensing in CR. Index Terms Cogniive radio, Mulimode ransceivers, Inerference suppression, Adapive circuis, Nonlinear sysems, adapive sysems. I. INTRODUCTION Cogniive Radio (CR) is recognized as a soluion o efficienly uilize he radio frequency specrum. A frequency band, which is comprised of several frequency channels, is officially allocaed o a Primary User (PU). The CR can emporarily occupy an empy channel in his band as a Secondary User (SU). To his end he CR mus coninuously sense he frequency specrum over a wide frequency range o find empy channels for is operaion []. If a duplex ransceiver is used hen he sense and ransmi operaions could be performed, simulaneously. The ransmied signal by he Local Transmier (LTX) of he CR, however, is received by he Local Receiver (LRX) of he CR, inducing a local inerference which can be several orders of magniude larger han wha would be received from he PU by he LRX. This large local inerference desensiizes he LRX, and prevens simulaneous ransmi and sense. Hence Par of he aciviies is financed by he European Commission wihin he FP7 Marie Curie IAPP projec ParCR (Gran Agreemen Number 688). This research is mainly funded by Duch foundaion for echnology (STW) wihin he Digially Enhanced and Conrolled Fronends (DECAFE) projec. currenly a half-duplex ransceiver is used wih separae ime slos for ransmi and sense. This half-duplex operaion can cause inerference o he PU and also reduces he hroughpu of he CR []. Tradiionally duplexer passive filers are used o isolae he LTX and LRX in Frequency Division Duplex (FDD) ransceivers. These duplexers however, have fixed pass-bands and sop-bands and hence are no suiable for CR applicaion. The anenna cancelaion scheme as proposed in [], [], requires an addiional ransmi anenna. The analogue cancelaion mehod [], [], [5] canno achieve an adequae inerference suppression, wihou high analogue complexiy. An aracive approach is o suppress he local inerference by passing he received signal hrough an adapive memoryless nonlineariy [6]. This Nonlinear Inerference Suppressor (NIS) is placed before acive componens of he LRX o preven desensiizaion of he LRX. The NIS can significanly suppress a srong inerference and pass he weak desired signal wih a small amoun of nonlinear disorion, provided ha i is adaped as a funcion of he envelope of he received inerference a he NIS inpu. Assuming ha he coupling pah of he inerference from he LTX o he LRX is linear, he NIS adapaion signal can be digially esimaed using an adapive model of his pah and he baseband inerference as he inpu of his model. Boh he inerference coupling pah and he NIS are subjec o environmenal changes. Hence he pah model is adaped during he LRX operaion o rack hese changes such ha he average power of he inerference a he NIS oupu is minimized. In his paper, we presen experimenal resuls of a ransceiver esbed which uses he NIS chip, fabricaed in nm CMOS echnology [7]. Performance of he NIS in erms of inerference suppression and recepion qualiy of he weak desired signal is presened. Finally, he pros and cons of using he NIS are discussed. The NIS approach is originally developed for mulimode ransceivers, where a local ransmier of one communicaion sandard induces a large inerference on a local receiver of anoher one. Alhough he experimenal resuls are based on he mulimode operaion scenarios, he similariy of hese wo applicaions suggess he NIS as a poenial soluion for simulaneous ransmi and sense in CR. 6

3 A. Transceiver model II. SYSTEM MODEL The CR ransceiver, including he NIS, is shown in Fig.. The Local RX (LRX) is mean o receive a desired signal, ransmied by a Remoe TX (RTX), while he Local TX (LTX) is acive. The RTX here can be he PU ransmier (specrum sensing) or a ransmier of anoher sandard (mulimode operaion). The LTX FE up-convers he baseband inerference i() o a frequency f i and ransmis i as i (). A he LRX, he desired signal ransmied by he RTX is received in he presence of a par of i () coupled from he LTX. The combinaion of hese wo signals is passed hrough a Band Pass Filer (BPF), which selec he band of operaion. Afer he BPF, he NIS inpu x() includes boh he desired signal and inerference as: x()=a d ()cos(πf d +φ d ())+A i () cos(πf i +φ i ()), () where A d, φ d, f d, A i, φ i, and f i are envelope, phase and cener frequencies of he desired signal and inerference a he NIS inpu, respecively. The power of he desired signal and inerference a he NIS inpu are denoed by P d and P i. B. Nonlinear Inerference Suppressor, principle of operaion The NIS can be buil by combining a linear amplifier (wih gain of c) and a limier wih an adapable limiing ampliude as shown in Fig.. The limier gain for he weak signal is smaller han he gain for he srong signal, because of is compressive behavior. On he oher hand he amplifier has he same gain for boh weak and srong signals. By adaping proporional o he envelope of he received inerference, he gains of he limier and amplifier for he srong inerference can be made equal bu of opposie sign. Thus, here is an opimal adapaion signal ha leads o cancellaion while he weak desired signal is amplified. The described principle for inerference suppression is no resriced o he hard-limier NIS and can also be applied o a sof-limier, which is implemened in pracice. The high frequency componens a he NIS oupu around f i and higher harmonics can be simply filered ou. By neglecing hese componens y() can be wrien as [6]: y() A d,y () cos(πf d + φ d ()) + A i,y () cos(πf i + φ i ()) () + A IM () cos(π(f i f d ) + φ i () φ d ()), where A i,y, A d,y, and A IM are envelopes of he inerference, desired signal and he main inermodulaion a he NIS oupu. The NIS power gains for he desired signal and inerference are defined as: g d = A d,y A, g i = A i,y d A i There is an opimal adapaion signal, which minimizes he inerference power P i,y =E(A i,y ) a he NIS oupu. For he hard-limier NIS, assuming ha A i A d, by adaping he NIS according o l HL () = π ca i(), one can obain: g i =, () RTX LRX LTX BPF i() x () NIS TX FE y () Fig. : CR ransceiver wih he NIS. y l x ya = cx x To subsequen sages of FE y i () Fig. : NIS inpu-oupu characerisic wih a hard-limier. hn [ ] BPF Swiching mixer NIS l ˆ() DAC l ˆ[ n] L( Aˆ i ) Aˆ[ i n ]. gn [ ] TX FE+DAC in [ ] ADC x To subsequen sage of he receiver η() η[ n] Adapaion Fig. : Closed-loop adapaion of he NIS. g d = c, and A IM = A d [6]. For he general case of a soflimier NIS, is a funcion of A i (), which will be denoed as = L(A i ()). C. NIS adapaion Fig. shows closed-loop adapaion of he NIS [8]. Complex-valued signals are shown wih bold lines. We assume ha he coupling pah of he inerference is linear wih a baseband impulse response h[n]. Hence l[n] = L( h i[n] ), where denoes convoluion. To generae an esimae ˆl[n] of l[n], an adapive finie impulse response filer g[n] is used wih i[n] as is inpu. The envelope of g i[n] is calculaed and he funcion L() is applied on he esimaed envelope  i [n] = g i[n]. Finally ˆl[n] = L(Âi[n]) is convered o he analogue signal ˆ. Since he coupling pah and he NIS are subjec o environmenal changes, g[n] mus be adaped o rack hese changes. To his end an error signal η() ha is approximaely proporional o ˆ is exraced by downconvering y() using x() as he local oscillaor of a 7

4 swiching mixer, which is implemened in he same package as he NIS circui. By processing η[n] and i[n] ogeher, g[n] is adaped such ha he power of he residual inerference a he NIS oupu is minimized. For a narrow-band consan-envelope inerference A i () changes only because of variaions in he inerference coupling pah. Since hese variaions are slow, ˆ can be esimaed direcly by inegraing η[n] [6]. III. TRANSCEIVER TESTBED A esbed, as shown in Fig., is developed o characerize he NIS circui and invesigae he NIS performance in he receiver. The NIS and he swiching mixer are fabricaed as one chip in nm CMOS echnology. The chip is packaged and mouned on a PCB and he PCB is enclosed in a Faraday cage o fix he circui s elecromagneic condiion. Block diagram of he esbed is shown in Fig. 5. The baseband desired signal and inerference are generaed in a PC. They are combined digially, uploaded o Naional Insrumens (NI) Flex RIO module #, convered o he analog IQ signals by NI578 module #, and up-convered by a Vecor Signal Generaor (VSG). The oupu of he VSG includes he inerference (f i =.85 GHz) and desired signal (f d =.87 GHz) and is conneced o he NIS inpu direcly by a coaxial cable. Flex RIO# and NI578 module# are used o generae and digiize η(). The adapaion is done via he PC. The NIS oupu signal is down-convered by a commercial IQ mixer, digiized using he adaper module #, and sen o he PC by FPGA module # for furher processing. defined as IS = g d gi and equals he amoun of improvemen in Signal o Inerference Raio (SIR) from he NIS inpu o oupu. For dbm < P i < dbm, alhough g i is less han - db, i is no zero as would be expeced for a memoryless NIS. This nonzero g i originaes from a phase misalignmen beween he limier and he amplifier. The misalignmen depends on P i. A P i - dbm and dbm, a perfec alignmen is obained and g i becomes exremely small. For dbm < P i < dbm, g d is approximaely consan and drops when P i goes below dbm. When P i becomes small, he sof limier behaves linearly. Hence he inerference is suppressed by subracing he oupus of wo linear blocks which have he same gain for he inerference and he desired signal. Hence boh signals are suppressed. For a varying envelope inerference, variaion of A i () leads o variaion of g(a i ()). Hence he ampliude modulaion of he inerference is ransferred o he modulaion of he desired signal. This leads o Cross Modulaion (CM) disorion of he weak desired signal. To exend he range ha g d is consan o smaller P i, he slope of he sof-limier can be increased. Alernaively he CM disorion can be compensaed digially. An IS of a leas 5 db is observed for dbm < P i < dbm. db 6 g i g d IS SMIQB Rohde & Schwarz VSG DAC DAC NI578 # NI Flex RIO# NIS DAC ADC NI578 # NI Flex RIO# η() MAX IQ down converer ADC ADC NI578 # NI Flex RIO# P i, dbm Generaion of I,Q signal Baseband desired signal Baseband Inerference Closed loop adapaion Naional Insrumen (NI) PXIe PC received daa is analyzed (LRX signal processing) Fig. 5: Block diagram of he esbed, including LTX, RTX, and LRX wih he NIS. IV. MEASUREMENT RESULTS A. Measuremen resuls for Single Tone (ST) signals As he firs sep we find he opimal adapaion signal l(a i ) for a ST inerference wih envelope of A i such ha A i,y is minimized. The obained funcion l(a i ) = L(A i ) is sored in a look-up able and is used in he following experimens. To measure he NIS impac on he desired signal and he inerference, we use a ST as he inerference and anoher ST as he desired signal 6 db smaller han he inerference. The adapaion signal is se o l = l(a i ) and g d and g i are measured and are shown in Fig. 6. The Inerference Suppression (IS) is Fig. 6: g i and g d for a single one inerference and desired signal. B. Closed-loop adapaion for consan envelope inerference In his secion, for he inerference a GMSK modulaion and for he desired signal a 6QAM modulaion is used, wih MHz and 6 MHz bandwidhs, respecively. The SIR a he NIS inpu is - db and P i = dbm. The closed-loop adapaion mehod as described in [6] is used o adap he NIS. Fig. 7 shows he NIS oupu specrum. An inerference suppression of abou db is observed. Also he inermodulaion componen of he desired signal and he inerference is seen a he image frequency of he desired signal wih respec o he inerference. C. Closed-loop adapaion for varying-envelope inerference In his secion, for boh he inerference and he desire signal a 6 QAM modulaion is used, wih.5 MHz and 6 MHz bandwidhs, respecively. Roo raised cosine pulse shaping 8

5 Fig. : Transceiver esbed. 5 Inermodulaion Desired signal 5 dbm dbm Inerference 5 Inermodulaion Desired signal Inerference Frequency, GHz Frequency, GHz Fig. 7: NIS oupu specrum, for GMSK inerference. Fig. 8: NIS oupu specrum, for 6QAM inerference. wih roll-off-facor of.5 is used for boh signals. The closedloop adapaion mehod, shown in Fig., is used o adap he NIS. The SIR a he NIS inpu is - db and Pi = dbm. Fig. 8 shows he NIS oupu specrum afer convergence of he adapaion loop o he seady sae condiion. An inerference suppression of abou db is observed. Also he inermodulaion componen is seen a he image frequency of he desired signal wih respec o he inerference. Consellaion diagram of he desired signal is shown in Fig. 9a. The consellaion poins of he ransmied signal and he received signal are shown in red and blue, respecively. The cross-modulaion disorion of he received signal is observed in Fig. 9a. By muliplying he received signal by he cross x x (a) before compensaion. x x (b) afer compensaion. Fig. 9: Consellaion diagram of he received desired signal. g(a i [n]) bandwidh, he required bandwidh o accuraely generae he adapaion signal is abou 5 MHz. The power consumpions of a bi DAC and 6 bi ADC wih MSPS are 6 mw and 5 mw, respecively [9], []. A deailed analysis of power consumpion of he digial par of he NIS adapaion can be found in [] which resuls in mw for MHz sample frequency. The oal power consumpion of he NIS mehod hen would be abou 66 mw. I mus be noed ha: firsly, hese numbers are loose upper bounds, secondly, power consumpion of he ADC, DAC and digial processing decreases as bandwidh of he inerference decreases, hirdly, power consumpion of he digial par can be reduced by modulaion is compensaed. Fig. 9b shows he consellaion diagram afer he compensaion. We see ha he consellaion is significanly improved. As he resul of he compensaion, modulaion error raio is improved from 8 db o 5 db. V. P OWER CONSUMPTION The NIS approach includes analog and digial circuis. The analog circuis include he NIS circui, DAC and ADC. The power consumpion of he curren realizaion of he NIS circui is proporional o Pi and reaches o 5 mw for Pi = dbm. For a varying envelope inerference wih abou 5 MHz 9

6 furher opimizaion of he adapaion algorihms. VI. ADVANTAGES AND DISADVANTAGES OF THE NIS APPROACH Advanages of he NIS approach can be summarized as: -Significan inerference suppression, -Low complexiy and power consumpion, -Possibiliy of a fully inegraed soluion. Despie is advanages, using he NIS leads o he following undesired effecs: -Specral mirroring: All he componens around f i f d a he NIS inpu are mapped o f d a he NIS oupu. This phenomenon which is referred o as specral mirroring, leads o a db floor on he noise figure of he NIS. Also if an exernal inerference is presen a around f i f d, hen i would be mapped o he frequency channel of he desired signal. This necessiaes using a BPF before he NIS o suppress his componen. For he CR applicaions, where a wide frequency agiliy is required, a unable BPF may be used. -Harmonic generaion: Owing o he srong nonlinear effecs in he NIS, higher order harmonics of he inerference are generaed. These harmonics are far away from f i and can be filered ou easily. They mus be however, filered immediaely o preven sauraion of he NIS. To his end, he curren implemenaion of he NIS circui has a bandpass filer wih a db bandwidh from.75ghz o.95ghz a he NIS oupu. This filer limis he frequency range of he desired signal. The curren implemenaion has a db bandwidh from.75ghz o.95ghz for he desired signal. [] E. Janssen, D. Milosevic, M. Herben, and P. Balus, Increasing isolaion beween colocaed anennas using a spaial noch, IEEE Anennas and Wireless Propagaion Leers, vol., pp ,. [] A. Raghavan, E. Gebara, E. Tenzeris, and J. Laskar, Analysis and design of an inerference canceller for collocaed radios, IEEE Transacions on Microwave Theory and Techniques, vol. 5, no., pp , nov. 5. [5] T. O Sullivan, R. York, B. Noren, and P. Asbeck, Adapive duplexer implemened using single-pah and mulipah feedforward echniques wih bs phase shifers, Microwave Theory and Techniques, IEEE Transacions on, vol. 5, no., pp. 6, 5. [6] H. Habibi, Y. Wu, J. Bergmans, E. Janssen, and P. Balus, Suppression of consan modulus inerference in mulimode ransceivers by closedloop uning of a nonlinear circui, in IEEE 75h Vehicular Technology Conference (VTC Spring),, may, pp. 6. [7] E. Janssen, D. Milosevic, and P. Balus, A.8ghz amplifier wih 9db frequency-independen smar self-inerference blocker suppression, in Radio Frequency Inegraed Circuis Symposium (RFIC), IEEE, june, pp. 97. [8] H. Habibi, Y. Wu, J. Bergmans, E. Janssen, and P. Balus, Closed-loop adapaion of a nonlinear inerference suppressor for local inerference in mulimode ransceivers, in Proceedings of h Symposium on Informaion Theory in he Benelux, May. [9] Y. Cong and R. Geiger, A.5-v -bi -ms/s self-calibraed dac, IEEE Journal of Solid-Sae Circuis, vol. 8, no., pp. 5 6, dec.. [] J. Moon, S. Jung, S. Hwang, and M. Song, A 6b ms/s.8mm 5mw.8um cmos f/i adc wih a novel folder reducion echnique, in h IEEE Inernaional Conference on Elecronics, Circuis and Sysems, 6. ICECS 6., dec. 6, pp.. [] E. Janssen, D. Milosevic, P. Balus, A. van Roermund, and H. Habibi, Digial hardware resources for seering a nonlinear inerference suppressor, in Mixed Design of Inegraed Circuis and Sysems (MIXDES), Proceedings of he 9h Inernaional Conference, may, pp. 8. VII. CONCLUSION A cogniive radio (CR) mus sense he specrum coninuously o find empy frequency channels for is ransmissions. The ransmied signal by local ransmier of he CR, however, induces a srong local inerference in local receiver of he CR, forcing he ransmi and sense o be done in separae ime slos. To enable simulaneous ransmi and sense an adapive memoryless nonlineariy can be used o significanly suppress he local inerference. In his paper, we presened experimenal resuls of a ransceiver esbed wih his Nonlinear Inerference Suppressor (NIS) implemened in nm CMOS echnology. Measuremen resuls show ha he srong inerference can be subsanially suppressed wih a small penaly o he qualiy of he desired signal. A closed-loop adapaions mehod is implemened which shows up o db improvemen in he desired Signal o Inerference power Raio (SIR) for a GMSK inerference. For a 6 QAM inerference up o db of improvemen in SIR is measured. The power consumpion of he NIS approach is esimaed o be below 66 mw. The curren implemenaion has a db bandwidh from.75ghz o.95ghz for he desired signal. REFERENCES [] H. Su and X. Zhang, Opporunisic mac proocols for cogniive radio based wireless neworks, in s Annual Conference on Informaion Sciences and Sysems, 7. CISS 7., 7, pp [] M. Jain, J. I. Choi, T. Kim, D. Bharadia, S. Seh, K. Srinivasan, P. Levis, S. Kai, and P. Sinha, Pracical, real-ime, full duplex wireless, in Proceedings of he 7h annual inernaional conference on Mobile compuing and neworking, ser. MobiCom. New York, NY, USA: ACM,, pp.. [Online]. Available: hp://doi.acm.org/.5/6.67