Spectrum Sensing Time Optimization Algorithm for Spectrum Efficiency Maximization in the Low-power Cognitive Radio Ultra Wideband System

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1 Available online at ScienceDirect IERI Proceia 4 ( International Conerence on Electronic Engineering an Comuter Science Sectrum Sening ime Otimization Algorithm or Sectrum Eiciency Maximization in the Low-ower Cognitive Raio Ultra Wieban Sytem Liaoyuan Zeng a, *, Sean McGrath b a Univerity o Electronic Science an echnology o China, 2006 Xiyuan Ave., Wet High-tech Zone, Chengu, , China b Univerity o Limerick,Catletroy, Co. Limerick, Irelan Abtract For energy etection in a cognitive raio (CR ytem, the ectrum ening time require to ucceully etect a rimary uer (PU i inverely roortional to the PU ignal trength. When the PU oerate in low-snr regime, the eective tranmiion time o the CR ytem with a ixe tranmiion winow can be extremely hort or the CR ytem to ully utilize the ectrum. hi aer reent a ening time otimization algorithm aiming at maximizing the ectral eiciency o the Ultra Wieban (UWB bae CR ytem by ining the otimal traeo between the length o the ening winow an the etection robability in low-snr regime. Comare with the time ixe ening metho, the otimization algorithm can rouce a igniicantly longer tranmiion time or the UWB bae CR ytem to eectively utilize the etecte ectrum ban while guaranteeing rotection to the PU he Liaoyuan Author. Publihe Zeng, Sean by Elevier McGrath. B.V. Oen Publihe acce uner by CC Elevier BY-NC-ND B.V. licene. Selection an an eer eer review review uner reonibility uner reonibility o Inormation o Engineering Inormation Reearch Engineering Intitute Reearch Intitute Keywor: Cognitive Raio; OFDM; UWB; Sectrum Sening * Correoning author. el.: ; ax: are: lyzeng@uetc.eu.cn he Author. Publihe by Elevier B.V. Oen acce uner CC BY-NC-ND licene. Selection an eer review uner reonibility o Inormation Engineering Reearch Intitute oi: /j.ieri

2 Liaoyuan Zeng an Sean McGrath / IERI Proceia 4 ( Introuction Ultra Wieban (UWB oerate in a ectrum overlae with wirele ytem with higher acce riority. he UWB tranmit ower i require to be no greater than Bm er MHz to rotect the rimary uer (PU rom being interere [1], which may lea to low ectrum eiciency (SE becaue the overlae ectrum i unerutilize by the PU [2]. o enhance the SE, UWB ytem can integrate cognitive raio (CR technology to oortunitically acce the PU ectrum bae on the ening reult [3]. Since the characteritic o the PU ignal are generally unknown in ractice, energy etection i generally alie to the availability veriication o the overlae ectrum by non-coherent etection metho, which require the ue o the Fourier tranorm engine [4]. hu, the UWB ytem bae on the orthogonal requency iviion multilexing (OFDM cheme can utilize the energy etection technique to ene the ectrum. For a ucceul etection o the PU in the overlae ectrum, the require ectrum ening time o the UWB energy etector can be etermine by the PU ignal to noie ratio (SNR an the robability o etection/ale alarm threhol. For a tranmiion winow ixe UWB ytem [5], the uration o the ening time can etermine the eective tranmiion time, which can igniicantly aect the ytem SE. An exceive ening time can meet the PU rotection requirement but can reult in an inaequate eective tranmiion time which can limit the CR-UWB ue o the ectrum. In the aer, we rooe a ening time otimization algorithm aime at ining the traeo otimality o the ening time an the ening erormance which inclue the robability o ale alarm (PFA an the robability o etection (PD while PU oerate in low SNR regime. By eining the otimize time length or ening, the roortion between the CR-UWB eective tranmiion time to the whole tranmiion winow can be maximize, which enable the ully utilization o the overlae ectrum with a certain tye o tranmiion cheme while guaranteeing the PU oeration. hen, the SE o the overlae ectrum can be maximize. For ening time otimization algorithm, the baic rincile are et u by the work in [6][7]. Author in [6] ue Lagrange ual otimization metho to otimize the cheme o ower itribution cheme an the ectrum ening in term o the ening winow length o a to otimize the CR ytem' throughut. he iteration require or ining the value o the two Lagrange multilier i igniicantly high. In [8], the author ue aylor aroximation to in the relationhi between the PFA an PD, an minimize the CR ytem' overall outage robability by otimizing the ening time with linear rogramming algorithm [9]. he algorithm cannot be ue in low PU SNR regime, otherwie the reulte ening time woul be conierable. In thi work, we moel the SE otimization roblem into a convex otimization roblem. he contraint lie in CR-UWB PD an PFA, an the eciion variable i the ytem energy etector ening time. he otimization roblem i olve by uing linear rogramming algorithm, an the tranmiion cheme ue in the eective tranmiion winow i aume to be icrete water-illing bae algorithm [10]. 2. Sytem Moel In the work, the overlay ectrum haring aroach i ue. We aume that an ieal notch ilter i alie, thu the CR-UWB ieban intererence to the PU can be neglecte. hu, the roblem o maximizing the ytem SE can be ormulate a:

3 70 Liaoyuan Zeng an Sean McGrath / IERI Proceia 4 ( ( o P1 arg max Be B( 1 P 1 ( P( H 1 (1 o.t. P P 1 0 P P 22 ( Q ( P Q ( P where B enote the number o bit tranmitte uring CR-UWB acce to the overlae ectrum, e B rereent the theoretical tranmit bit number when the CR-UWB ha ull acce to the overlae ectrum, o i the re-eine tranmiion winow, rereent the ening time, P i the PFA, an P( H 1 rereent the robability that one PU activate uring o in the PU ectrum. he robability that one PU i activate uring o i aume to ollow a Poion roce [11] an i comute t x e ( t by P( H 1 ( x; t, where x enote the execte number o occurrence uring the erio o t. x! In the contraint, P enote the value o PD, rereent the receive PU ignal SNR in the CR- UWB ie, enote the UWB ytem amling rate, an P an P rereent the target PFA an PD, 2 1 y reectively. Furthermore, Q( x ex y i the one-imenional Gauian Q-unction [12]. x 2 2 Note P, P an are relate by [3] Q 1 ( P 2(2 N P Q 2N an B i not aecte by P2 arg max F ratio ( ( o D, the objective unction P1 can be equivalent to o Q 1 ( P 2(2 N 1 Q 2N D Auming the PU are WiMAX bae incumbent ytem, Fig. 1 illutrate that the CR-UWB SE grow exonentially with the increae in an reache an otimum at a certain time ot. A the value o grow beyon the otimal time ot, the SE ecreae monotonically becaue the eective tranmiion time, o i hortene. Fig. 1 inicate that an otimal exit uner eciic P an value.,.

4 Liaoyuan Zeng an Sean McGrath / IERI Proceia 4 ( Fig. 1: the maximum SE veru the ening time in UWB channel moel 1 (CM1 3. Sening ime Otimization Algorithm Contraine by P, the otimize value o can be given by uing linear rogramming metho to in the root or Fratio ( ratio ( 0, where 1 1 (2 ratio( Q ( ( Q( ( Q( ( 0 o, where ( rereent a unction o an can be exree a an the ( ( irt orer ierentiation i ( 1 Q ( P 2 (2 Q 1 ( P 2( ( Q ( P (4 2, 4( 3/2 Neverthele, ientiying the otimal in cloe orm i igniicantly comlex. Hence, the numerical metho i ue to aroximate the otimal. 4. Numerical Reult he numerical imulation i carrie out in CM1 which rereent a line-o-ight UWB channel. When the ectrum hole i etecte by the CR-UWB, the CR-UWB will aly the icrete water-illing bae algorithm within the eective tranmiion time to achieve the maximum SE value. Furthermore, or the behavior o PU in the overlae ectrum, i et to 1000/ (i.e., one thouan time er econ. he reult comare the SE achieve uing the ixe ening time with the SE reache by uing the ening time otimization metho. Fig. 2 how that the rooe ectrum.

5 72 Liaoyuan Zeng an Sean McGrath / IERI Proceia 4 ( Fig. 2: the otimize SE veru in CM1 ening algorithm increae the SE igniicantly, eecially in low SNR regime o the PU. Note that when i -19 B, the ytem SE i twice o the SE achieve with the ixe ening time algorithm. With the increae o, the two line ga ecreae in an exonential manner. When PU oerate with a high SNR (i.e., -10 B, the ga i minor becaue the large reult in a mall or a re-eine P. 5. Concluion In thi aer, a new ening time otimization algorithm or SE maximization i rooe or the OFDM bae CR-UWB oerating in a low PU SNR regime. he rooe algorithm i eveloe bae on the energy etection metho an can in the otimal length o the ening time through numerical metho, which can maximize the eective tranmiion time or the tranmiion winow ixe CR-UWB contraine by the target PD an PFA. By combining the rooe algorithm with the exite ectrum haring an management algorithm, the CR-UWB SE i conierably imrove comare with the ening time ixe ening algorithm. Reerence [1]. D. Porcino an H. Walter, Ultra-wieban raio technology: otential an challenge ahea. IEEE Commun. Mag., 2003; 41: [2]. I. F. Akyiliz, W. Y. Lee, M. C. Vuran, an S. Mohanty, NeXt generation/ynamic ectrum acce/cognitive raio wirele network: a urvey. Comuter Network, 2006; 50: [3]. J. Mitola III, Cognitive raio or lexible mobile multimeia communication. In Proc. IEEE MoMuC, San Diego, Caliornia, 1999: [4]. F. F. Digham, M. S. Alouini, an M. K. Simon, On the energy etection o unknown ignal over aing channel. IEEE ran. Commun., 2007; 55: [5]. L. Yang an G. B. Giannaki, Ultra-wieban communication: an iea whoe time ha come. IEEE Signal Proce. Mag. 2004; 21: [6]. S. Stota an A. Nallanathan, Otimal ening time an ower allocation in multiban cognitive raio network. IEEE ran. Commun., 2011; 59:

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