An intelligent cooperative sensing strategy with low overhead for cognitive radios

Transcription

1 WIRELE COMMUNICATION AN MOBILE COMPUTING Wrel. Commun. Mob. Comput. 015; 15: Publshed onlne October 01 n Wley Onlne Lbrary (wleyonlnelbrary.com)..444 REEARCH ARTICLE An ntellgent cooperatve sensng strategy wth low overhead for cogntve rados Zeyang a 1, Jan Lu * and Kepng Long 1, 1 chool of Communcaton and Informaton Engneerng (CIE), Unversty of Electronc cence and Technology of Chna (UETC), Chengdu 61171, Chna Insttute of Advanced Network Technology and New ervces (ANT), Unversty of cence and Technology Bejng (UTB), Bejng 10008, Chna ABTRACT As s well known, cooperatve sensng can remarkably mprove the sensng accuracy by explotng the spatal dversty of dfferent secondary users. However, a large number of cooperatve secondary users reportng ther local decsons would nduce great detecton delay and traffc burden, whch degrades the performance of secondary spectrum access. Ths paper proposes an ntellgent cooperatve sensng (IC) strategy wth selectve reportng and sequental detecton to enhance the sensng relablty as well as reduce the sensng overhead for cogntve rados. The tradeoff n the sensng tme allocaton s studed for IC and then two novel fuson rules are developed to effcently obtan the optmum sensng tme allocaton wth dfferent objectves. The performance of IC s analyzed n terms of mss detecton probablty and average sensng tme, where ther closed-form expressons are derved over Raylegh fadng channels. mulaton results reveal that IC acheves hgher sensng relablty wth less sensng overhead than the tradtonal strategy. It s also shown that the mss detecton probablty and average sensng tme of IC can be mnmzed by optmzng the sensng tme allocaton. Copyrght 01 John Wley & ons, Ltd. KEWOR cogntve rado; spectrum sensng; cooperatve detecton; selecton procedures; detecton probablty *Correspondence Jan Lu, Insttute of Advanced Network Technology and New ervces (ANT), Unversty of cence and Technology Bejng (UTB), Bejng 10008, Chna. E-mal: lujan@ustb.edu.cn 1. INTROUCTION Cogntve rado s regarded as a promsng technque to solve the spectrum scarcty problem by allowng the secondary users (Us) to access the lcensed spectrum band wthout causng harmful nterference to the prmary users (PUs) [1,]. In cogntve rado networks (CRNs), the U requres effcent and effectve spectrum sensng to opportunstcally access the lcensed spectrum whle guaranteeng the qualty-of-servce (Qo) for both the PU and tself []. For example, the U should sense the actvtes of the PU before t uses the lcensed spectrum for secondary transmssons. If the PU s not detected, the U wll drectly send ts messages over the spectrum. Otherwse, the U wll access the spectrum va power control so as to ensure the Qo of prmary transmssons [4], where such secondary transmsson manner s commonly called underlay. However, f the mss detecton occurs, that s, the PU s undetected when t s actually present, the secondary transmssons would cause harmful nterference to the prmary transmssons. On the other hand, f the false alarm occurs, that s, the PU s detected when t s ndeed absent, the performance of secondary transmssons would be degraded because of the power control. Hence, one goal should be to make spectrum sensng as accurate as possble under a sustanable false alarm probablty [5]. In general, the spectrum sensng technques nvestgated n lterature can be classfed nto two types: local sensng and cooperatve sensng []. In local sensng, a U detects the states of PU by tself, where three dfferent methods are wdely used n applcaton: energy detecton, matched flter, and cyclostatonary feature detecton [6 9]. Among these methods, energy detecton s the smplest way that detects the PU s states by measurng the power of the receved sgnal. However, t s a challengng job to mprove the accuracy of local sensng n practcal 1518 Copyrght 01 John Wley & ons, Ltd.

2 Z. a, J. Lu and K. Long Intellgent cooperatve sensng envronments wth channel uncertantes such as shadowng and fadng [5]. To combat such channel uncertantes, cooperatve sensng has been ntroduced, where an anchor U detects the PU s states wth the help of other elgble Us [10]. Typcally, n cooperatve sensng, each sensng phase s dvded nto several subslots that are used for the local sensng and decson reportng [11]. More specfcally, each cooperatve U ndependently makes a local sensng n the frst subslot. Then, all the cooperatve Us report ther local decsons to the anchor U (or fuson center) durng the remanng subslots. Fnally, the anchor U combnes the receved detecton results to produce a global decson by usng a gven fuson rule, such as AN, Majorty, OR, and so on []. In [1] and [1], the authors appled cooperatve dversty to spectrum sensng to mprove the detecton performance n CRNs. In [14], the authors ncorporated the double threshold energy detecton technque and empster hafer theory nto cooperatve sensng to promote the detecton relablty under a sustanable false alarm probablty. In [15], the authors studed the energy detecton based cooperatve sensng strateges wth data fuson and decson fuson over fadng channels, respectvely. Although the cooperatve sensng can sgnfcantly promote the sensng accuracy compared wth the local sensng, a large number of cooperatve Us reportng ther local decsons may nduce sgnfcant overhead []. Recently, a great research nterest has grown n alternate models for reducng the overhead of cooperatve sensng [11,16 18]. Recently, user selecton based cooperatve sensng schemes have been proposed. In [11], only the cooperatve Us that do not detect the presence of PU are allowed to report ther local decsons such that the reportng overhead s reduced. In [16], the authors proposed three methods for selectng the cooperatve Us wth the best detecton performance to reduce the sensng overhead and total energy consumpton. fferent from [11,16], [17,18] tred to reduce the sensng overhead by usng the sequental detecton technque. In [17], the local test statstcs are reported to the fuson center n descendng order of magntude, whch sgnfcantly reduces the reportng tme. In [18], the authors proposed a dstrbuted sensng strategy wth sequental ordered transmssons to make a quck, yet relable, decson regardng prmary actvty, where the fuson center mposes a maxmum on the number of reportng Us. In ths paper, an ntellgent cooperatve sensng (IC) strategy s proposed to mprove the sensng accuracy whle reducng the overhead. The man contrbutons of ths paper can be summarzed as follows: (1) Intellgent cooperatve sensng naturally ntegrates the selectve reportng and sequental detecton technques to reduce the sensng overhead. Unlke exstng works [17,18] where each cooperatve U reports a quantzed verson of ts local detecton statstc, the reported Us n IC only send ther 1-bt hard local decsons to the anchor U for mplementaton smplcty when performng sequental detecton. In ths context, once the number of the local decsons successfully receved at the anchor U equals to a predefned threshold, called fuson threshold, the anchor U wll stop spectrum sensng mmedately and then declare the PU s presence. Thus, IC can reduce the sensng tme compared wth [11,16] due to the use of proposed sequental detecton. Besdes, snce IC only allows the cooperatve Us whch detect the PU s presence to report ther local decsons, t has lower reportng overhead than [17,18] where the cooperatve Us whch do not detect the PU s presence may also need to report. () In IC, whle a cooperatve U reports ts local decson, other cooperatve Us wll proceed wth makng the local sensng. As a result, the local sensng tme for cooperatve Us n IC s longer than that n tradtonal cases [11,16 18], whch mples that IC has hgher local sensng relablty. Thus, IC can acheve lower mss detecton probablty than tradtonal cases. () Ths paper also studes the optmzaton problems n IC wth two dfferent objectves. One objectve s to mnmze the mss detecton probablty so as to protect prmary transmssons as much as possble. The other s to mnmze the sensng tme to promote the secondary throughput and also to reduce the overhead. Accordngly, we develop two optmum fuson rules for IC. It s shown by smulatons that, usng the optmum fuson rules, IC can reduce not only the mss detecton probablty but also the sensng tme compared wth the tradtonal cooperatve sensng strategy. (4) The performance of IC s analyzed n terms of mss detecton probablty and average sensng tme (AT) whose closed-form expressons are derved over Raylegh fadng channels wth consderaton of the errors n the reportng channel. Ths paper manly focuses on reducng the sensng tme when PU s present, whch s very mportant for the underlay spectrum sharng models n CRNs [19]. That s because Us wll have more tme for underlay transmssons f the PU s detected more quckly. Consequently, the secondary throughput could be mproved owng to the fast detecton. In addton, snce IC can acheve lower mss detecton probablty than tradtonal cases, t nduces less nterference to prmary transmssons. Fnally, the smulaton results demonstrate the enhanced performance of proposed IC strategy. The rest of the paper s organzed as follows: ecton presents the system model and descrbes the proposed IC strategy. ecton analyzes the performances of IC and tradtonal strateges, respectvely, then derves the closedform expressons of mss detecton probablty and AT. In ecton 4, the optmzaton problems for IC are dscussed and two optmum fuson rules are developed accordngly. Wrel. Commun. Mob. Comput. 015; 15: John Wley & ons, Ltd. 1519

3 Intellgent cooperatve sensng Z. a, J. Lu and K. Long The smulaton results are presented to llustrate the advantages of IC n ecton 5. Fnally, the paper s concluded n ecton 6.. PROPOE COOPERATIVE ENING.1. ystem model Consder a tme-slotted CRN [11] organzed by a source PU P, an anchor U and a set of cooperatve n Us fu 1 ; ;U N g. P transmts the sgnal x P E jx P j o 1 to ts destnaton wth power E P.The gan of the lnk I! J (I fp;u g, J f;u g;i J ), denoted as h IJ, s Raylegh fadng wth varance IJ. n J s the addtve whte Gaussan nose at J wth zero mean and varance n. Then, the sgnal from P receved at J s y J p E P h PJ x P C n J (1) where denotes the state of P,thats, 0 ndcates P s absence whle 1 represents P s presence. Bnary hypothess testng s consdered for spectrum sensng wth the followng two hypotheses: (1) H 0,thats, 0; () H 1,thats, 1. Ths paper lets O J denote a local decson of J,where O J 0 means that J does not detect P s presence whle O J 1 means the opposte. Wthout loss of generalty, energy detector [8] s used to evaluate the performance of proposed strategy. Followng [8], the false alarm and detecton probabltes of J by usng energy detecton are respectvely gven by n o Pf J Pr OJ 1jH 0 u J ; J ().u J / f.u J ; J / n o Pd J Pr OJ 1jH 1 e J u J X k0 J 4e.1CN J / e J 1 k J 1 uj 1 CNJ C kš N J u J X k0 1 kš J N J.1CN J / k 5 d.u J ; N J ; J / () where u J s the tme bandwdth product of energy detector, J s the power threshold, N J u J P PJ,and P E P =n s sgnal-to-nose rato (NR) of E P.For notaton smplcty, ths paper defnes f.u J ; J / as a functon of u J and J n () and d.u J ; N J ; J / as a functon of u J, N J and J n (). When Pf J s equal to (0 < <1), J 1 f.u J ; /s obtaned from (), where 1 f s the nverse functon of f... Tradtonal cooperatve sensng As shown n Fgure 1, each sensng phase wth the tme duraton T n tradtonal cooperatve sensng strategy s separated nto a set of subslots ft 0 ;t 1 ; ;t N g used for the local sensng and decson reportng. t 0 occupes ˇ (0 < ˇ < 1) fracton of T whle the remanng N equal subslots ft 1 ; ;t N g occupy 1 ˇ fracton. Typcally, n t 0, all Us ndependently make local sensng. Then, the cooperatve Us fu 1 ; ;U N g wll send ther local decsons to the anchor U durng the remanng subslots ft 1 ; ;t N g n a selectve reportng manner [11]. For nstance, f U detects the presence of P by tself n t 0, t wll report ts local decson n t ; otherwse, f U does not detect the presence of P by tself n t 0, t wll keep quet. A common control channel s assumed for the nformaton exchange between U and, whch s not on the spectrum band beng sensed [17]. Lettng x U represent the reported sgnal of U, the local decson of U receved at s expressed as y./ O U qe U h U x U C n./ (4) where E U s the transmt power of U.Next, attempts to retreve U s local decson usng y./ n (4). Unlke prevous studes where perfect decson reportng s assumed, ths paper takes nto consderaton of the errors n reportng channel. Let denote whether successfully decodes the reported decson from U or not, that s, 0 represents the faled decodng whle 1 represents the opposte. In an nformaton-theoretc sense and followng [11], the probablty of the case 1 s derved as P U Pr f 1g Pr nlog 1 C O ˇ U U ˇˇhU ˇ o R U ( e U = U U ; O U 1 0; O U 0 where U E U =n, U R U 1, R U 1.1 ˇ/TB r =N and B r s the bandwdth of reportng channel. Fgure 1. The tme slot structure of tradtonal cooperatve sensng strategy. (5) 150 Wrel. Commun. Mob. Comput. 015; 15: John Wley & ons, Ltd.

4 Z. a, J. Lu and K. Long Intellgent cooperatve sensng After all cooperatve Us report n a selectve fashon, wll combne the successfully receved local decsons wth ts own local decson to make a global decson by usng a gven fuson rule at the end of t N.Inthspaper, the general fuson rule K-out-of-N s adopted at. Let O ; ndcate whether successfully receves a local decson from U or not, that s, O ; 0 represents the faled recevng whle O ; 1 represents the opposte. Hence, the decson strategy of tradtonal cooperatve sensng s expressed as If n0 O ; K; declareh1 otherwse; declareh 0 (6) where O ;0 O, O ; U O ( 1; ;N)andK (1 K N C 1) s the fuson threshold. In general, the sensng phase s arranged at the begnnng of each secondary medum access control frame [1]. If P s detected at the end of a sensng phase, wll response to other Us mmedately. Then, the Us may swtch to underlay model so as to acheve secondary spectrum access wthout affectng prmary operatons. Otherwse, f P s not detected, Us wll send ther messages over the spectrum drectly. Ths paper manly nvestgates the cooperatve sensng problem. The data transmsson after spectrum sensng s out of the scope of ths paper, whch wll be studed n our future works... Intellgent cooperatve sensng strategy The slotted structure of IC s llustrated n Fgure. mlar to the tradtonal case as gven n Fgure 1, each sensng phase n IC s also dvded nto N C 1 subslots. But, IC has a completely dfferent sensng process. In IC, should mantan a counter whose ntal value s set as 0. The value of ths counter, denoted by C, wll be added by 1 when ether of the followng two cases occurs: (1) detects the presence of P by tself n t 0 ;() successfully receves a local decson ndcatng P s presence from the cooperatve Us. Thus, the counter value n t, denoted as C, s gven as C C 1 C O ; (7) In (7), the ntal counter value of C s C 1 0. The detecton process of IC conssts of two steps, whch are respectvely descrbed as follows: tep 1 (Local sensng): Int 0, all Us ndependently perform local sensng. Next, uses (7) to calculate C 0 and compares t wth K. IfC 0 K, nforms other Us that P s detected and stops spectrum sensng mmedately; otherwse, t wll try to make a fnal decson wth the help of cooperatve Us durng ft 1 ; ;t N g, that s, step wll be executed. tep (Cooperatve sensng): Int 1, U 1 wll report ts local decson f t detects P s presence, whle fu ; ;U N g contnue local sensng. Meanwhle, attempts to decode the reported decson from U 1. Next, calculates C 1 by usng (7) and compares t wth K.IfC 1 K, declares P s presence and stops spectrum sensng mmedately; otherwse, cooperatve sensng wll proceed. Then, n t, U attempts to report whle fu ; ;U N g contnue sensng. In ths case, C s calculated and compared wth K to make a global decson agan. Ths contnues untl a global decson n favor of H 0 or H 1 s made, or the current sensng phase expres. From the prevous dscussons, the decson strategy of IC s wrtten as If C K; declareh1 <K for all ; declareh 0 (8) As shown n Fgures 1 and, the tradtonal cooperatve sensng strategy makes a global decson at the end of each sensng phase whle the proposed IC strategy s able to gve a global decson before a sensng phase expres. Thus, IC has the potental to reduce the sensng tme. Note that the saved sensng tme could be used for possble underlay transmssons, whch mproves the secondary throughput. Furthermore, when K 1, a global decson may be gven at step 1, and thus step s not requred, that s, the cooperatve sensng can be removed. In ths case, IC sgnfcantly reduces the sensng overhead compared to the tradtonal case where cooperaton s always needed. On the other hand, because the cooperatve Us n IC also can perform local sensng durng ft 1 ; ;t N g rather than only n t 0, IC acheves hgher detecton relablty than the tradtonal case because of the longer local sensng tme. Besdes, ecton 4 wll study the optmzaton problems wth dfferent objectves for IC. It s shown that the sensng tme allocaton can be optmzed to maxmze the sensng accuracy and mnmze the sensng tme for IC, respectvely..4. Implementaton Fgure. The tme slot structure of ntellgent cooperatve sensng strategy. In tme dvson multple access based CRNs, the proposed IC strategy can be executed n a dstrbuted manner, Wrel. Commun. Mob. Comput. 015; 15: John Wley & ons, Ltd. 151

5 Intellgent cooperatve sensng whch does not requre any coordnaton by a centralzed controller. To be specfc, an anchor U executes the proposed IC strategy descrbed n ecton. perodcally durng the sensng phase n each secondary medum access control frame. Once the anchor U detects PU s presence n a subslot of the current sensng phase, t wll broadcast a 1-bt global decson ndcatng PU s presence to other Us and stop spectrum sensng mmedately. However, f the anchor U does not detect PU s presence n a sensng subslot, t wll broadcast nothng. In ths context, because cooperatve Us cannot receve the global decson, they know that the PU has not been detected yet and then the spectrum sensng wll proceed. Ths process wll not stop untl PU s detected or the current sensng phase expres. Lke many exstng works [11 18], t s assumed that other Us can correctly receve the global decson ndcatng PU s presence sent from the anchor U n ths paper. If the anchor U does not detect PU s presence n any subslot of the current sensng phase, a fnal decson ndcatng PU s absence s made. In IC, the anchor U does not need to broadcast the fnal decson ndcatng PU s absence, because other Us wll know that PU s sensed to be absent f they do not receve a fnal decson ndcatng PU s presence n any subslot of the current sensng phase. On the other hand, the anchor U can utlze the optmal fuson rules gven n ecton 4 to perform spectrum sensng wth dfferent objectves n practce. As shown n ecton, the closed-form expressons of mss detecton probablty and AT are derved, where these dervatons only requre average channel gans that are relatvely stable and can be estmated at Us n pror. Thus, the sensng parameters, such as local power threshold and optmal sensng tme allocaton factor, can be easly obtaned, whch facltates the mplementaton of IC n real CRNs.. PERFORMANCE ANALI Ths secton wll analyze the performance of IC n terms of mss detecton probablty and AT, and derve ther closed-form expressons over Raylegh fadng channels accordngly. Here, the AT s defned as the average tme requred for to make a fnal decson under H 1 n a sensng phase. In ths paper, t s supposed that all local false alarm probabltes are equal to each other, and the overall false alarm probablty s fxed at 0 [11]. For the purpose of comparson, the performance analyss for tradtonal cooperatve sensng strategy s also gven..1. Mss detecton probablty.1.1. Tradtonal cooperatve sensng. The tradtonal cooperatve sensng strategy has been ntroduced n ecton.. From () and (), the local false alarm and detecton probabltes of U n tradtonal strategy are respectvely obtaned as Pf Tra U f u Tra U ; Tra U Tra (9) Pd Tra U d u Tra U ; N Tra U ; Tra U Z. a, J. Lu and K. Long (10) where u Tra U ˇT B e, B e s the bandwdth of energy detector, N U Tra u Tra U P PU and the superscrpt Tra denotes the tradtonal strategy. In (9), the local false alarm probablty s denoted as Tra for notaton smplcty. Then, the power threshold Tra U s derved from (9) as Tra U 1 f u Tra ; Tra U (11) mlarly, the local false alarm and detecton probabltes of n tradtonal strategy are respectvely gven by Pf Tra f where u Tra utra U equals to u Tra Pd Tra d u Tra and N Tra U,wehave Tra u Tra ;Tra Tra ; N Tra ;Tra (1) (1) utra P P. Because utra Tra U. From ecton., t s known that the probablty of the case O ; 1 under H 0,thats, successfully receves a false alarm from U, can be gven as n o Pf Tra ;U Pr O; 1jH 0 n o Pr OU 1jH 0 Pr f 1g Pf Tra U P U (14) For ease of representaton, ths paper lets denote asetofusf;u j 1; ;Ng and m;n denote ts nth nonempty subcollecton that contans m dfferent Us, N C 1 where m f1; ;N C 1g and n 1; ;. m Then, the overall false alarm probablty of tradtonal K-out-of-N strategy s 8 9 < = Pf Tra Pr O : ; KjH 0 ; n0 N C1 C X m XN C1 4 Pf Tra ;I mk n1 I m;n where C m N C1 N C 1 m J m;n 1 Pf Tra 5 ;J and Pf Tra ; Pf Tra. (15) efne the functon ' 1. Tra / Pf Tra for notaton smplcty. Because Pf Tra s assumed to be equal to 0, Tra s obtaned as 15 Wrel. Commun. Mob. Comput. 015; 15: John Wley & ons, Ltd.

6 Z. a, J. Lu and K. Long Intellgent cooperatve sensng Tra ' / (16) where ' 1 1 s the nverse functon of ' 1. On the other hand, the probablty of the case O ; 1 under H 1,thats, successfully receves a detecton from U, s gven as Pd Tra ;U Pr Pr n O; 1jH 1 o n OU 1jH 1 o Pr f 1g Pd Tra U P U (17) Therefore, the overall detecton probablty of tradtonal K-out-of-N strategy s obtaned as 8 9 < = Pd Tra Pr O : ; KjH 1 ; n0 N C1 C X m XN C1 4 Pd Tra (18) ;I mk n1 I m;n J m;n 1 Pd Tra 5 ;J where Pd Tra ; PdTra. From (18), the overall mss detecton probablty of tradtonal K-out-of-N strategy s calculated as Pm Tra 1 Pd Tra (19).1.. Intellgent cooperatve sensng strategy. The proposed IC strategy has been llustrated n ecton.. Compared wth the tradtonal strategy, IC has longer local sensng tme for cooperatve Us. Thus, the local false alarm and detecton probabltes of U n IC are respectvely gven as Pf IC U f u IC U ; IC U IC (0) where u IC 1 f U Pd IC U d u IC U ; N IC U ; IC U Œˇ C. 1/.1 ˇ/ =N T B e, IC U (1) u IC; IC U and N IC U u IC U P PU. Furthermore, the local false alarm and detecton probabltes of n IC are where u IC u IC P P. Pf IC Pd IC f utra, IC u IC d u IC ; N IC 1 f ;IC IC () ;IC () u IC ; IC and N IC mlar to (14) and (17), the probabltes of the case O ; 1 under H 0 and H 1 n IC are respectvely calculated as Pf IC ;U Pf IC U P U (4) Pd IC ;U Pd IC U P U (5) By usng the results of Appendces A and B, the overall false alarm and detecton probabltes of IC are respectvely derved as Pf IC Pd IC Pr fc KjH 0 g K 1 ( A1 ;K 1 A ;1<K N C 1 Pr fc KjH 1 g K 1 ( B1 ;K 1 B ;1<K N C 1 (6) (7) mlarly, we defne the functon '. IC / Pf IC 0. Then, IC s gven by IC ' 1. 0/ (8) where ' 1 s the nverse functon of '. Fnally, from (7), the overall mss detecton probablty of IC can be easly obtaned as Pm IC 1 Pd IC (9) Remark 1. In fact, there exsts a tradeoff between the local sensng tme and the decson reportng tme [11]. pecfcally, ncreasng the local sensng tme mproves the local sensng relablty, but t comes at the expense of a reducton n reportng relablty, because less tme s now avalable for the decson reportng. Thus, a mnmzed mss detecton probablty can be acheved va adjustng ˇ to an optmal value n both tradtonal and proposed IC strateges. Besdes, because cooperatve Us n IC have longer local sensng tme as compared wth the tradtonal strategy, IC can acheve hgher sensng relablty. These wll be valdated by the smulaton results n ecton 5... ensng tme..1. Tradtonal cooperatve sensng. For performance comparson, the AT s examned for tradtonal strategy frst, where the AT has been defned Wrel. Commun. Mob. Comput. 015; 15: John Wley & ons, Ltd. 15

7 Intellgent cooperatve sensng Z. a, J. Lu and K. Long at the begnnng of ecton. As shown n ecton., tradtonal strategy makes a global decson at the end of each sensng phase. Thus, the AT of tradtonal strategy s AT Tra T (0)... Intellgent cooperatve sensng strategy. Then, the AT s examned for IC. Let t R denote the requred sensng tme for to gve a fnal decson n a sensng phase under H 1. Clearly, f IC makes a fnal decson n t (K 1 < N), the value of t R s equal to t R Œˇ C.1 ˇ/=N T (1) From Appendx B, the probablty of the case t R s Pr ft R gpr fc KjH 1 g () Note that, f a fnal decson s gven n t N, there exsts two possble sensng cases for IC: (1) clams P s presence n t N ; and () the mss detecton occurs. Thus, the probablty of the case t R N s gven as Pr ft R N gpr fc N KjH 1 gcpm IC () Followng the total probablty law, the AT of IC s calculated as AT IC K 1 Pr fc KjH 1 g C Pm IC T (4) Remark. It can be easly verfed that AT IC < AT Tra holds. Besdes, AT IC! ˇT as Pd IC! 1 under K 1, whch mples that, n ths case, when the local detecton relablty of s hgh n IC, the cooperatve sensng durng ft 1 ; ;t N g s seldom used. Thus, IC can sgnfcantly reduce the sensng overhead compared wth the tradtonal strategy. Besdes, ˇ can be optmzed to mnmze the AT for IC because of the fundamental tradeoff n the sensng tme allocaton. 4. PERFORMANCE OPTIMIZATION FOR INTELLIGENT COOPERATIVE ENING In CRNs, t s well known that the mss detecton probablty mpacts on the Qo of prmary transmssons; and the sensng tme under H 1 determnes the throughput of secondary underlay transmssons [5]. Accordngly, two effcent fuson rules are developed to obtan the related optmal sensng tme allocaton factor ˇ amng to mnmze the overall mss detecton probablty and AT for IC, respectvely. Mn-mss-detecton-probablty (MM) rule: IfthePUs are hghly senstve to the nterference from Us, t s of great mportance to mnmze the overall mss detecton probablty under a sustanable overall false alarm probablty. Therefore, for a gven overall false alarm probablty 0, the optmzaton problems of overall mss detecton probablty for the tradtonal and IC strateges are respectvely formulated as mnmze Pm Tra ˇ (5) subject to Pf Tra a 0 mnmze Pm IC ˇ (6) subject to Pf IC a 0 Clearly, the optmal values of ˇ n (5) and (6) can be easly found by lne search methods. Mn-AT (MA) rule: From the secondary performance consderatons, f the Us want to mprove the throughput of secondary underlay transmssons and/or reduce the sensng overhead, t s necessary to take best effort on mnmzng the AT for a gven overall false alarm probablty 0. Hence, the optmzaton problem s gven by mnmze AT IC ˇ (7) subject to Pf IC a 0 mlarly, the optmal value of ˇ n (7) can be easly obtaned usng the lne search methods. It wll be shown by smulatons n ecton 5 that, usng the proposed MM and MA rules, IC cannot only mprove the sensng relablty but also reduce the sensng overhead. 5. IMULATION REULT In ths secton, some numercal and smulaton results are presented to evaluate the performance of IC and compare t wth the tradtonal strategy as dscussed n ecton.. In these examples, the number of cooperatve Us s set as N 4, the tme duraton of each sensng phase as T 0 ms, the bandwdths of the energy detector and reportng channel as B e B r 10 Hz, the reportng NR as U 0 db and the channel varances as P 0: and PU U 1. It s noted that the lnes represent the theoretcal results derved n ths paper, and the dscrete marks denote the Monte Carlo smulaton results, where 100,000 Monte Carlo smulatons are performed usng MATLAB Mss detecton probablty Frst, Fgure plots the overall mss detecton probablty versus the overall false alarm probablty 0 for OR, Majorty and AN based IC and tradtonal strateges, respectvely, where the smulated results match well 154 Wrel. Commun. Mob. Comput. 015; 15: John Wley & ons, Ltd.

8 Z. a, J. Lu and K. Long Intellgent cooperatve sensng Fgure. The overall mss detecton probablty versus 0 for the ntellgent cooperatve sensng and tradtonal strateges wth the sensng tme allocaton factor ˇ 0: and the PU sgnal-to-nose rato P 0dB. Fgure 5. The average sensng tme versus 0 for the ntellgent cooperatve sensng and tradtonal strateges wth ˇ 0: and P 0dB. Fgure 4. The overall mss detecton probablty versus ˇ for the ntellgent cooperatve sensng and tradtonal strateges wth 0 10 and P 0dB. Fgure 6. The average sensng tme versus P for the ntellgent cooperatve sensng and tradtonal strateges wth 0 10 and ˇ 0:. wth the theoretcal results. All cases n Fgure demonstrate that IC acheves lower mss detecton probablty than the tradtonal strategy, whch s due to the fact that the cooperatve Us n IC have longer local sensng tme. Ths valdates the conclusons made n Remark 1. Besdes, for both the IC and tradtonal strateges, OR has the best performance among these three fuson rules whle AN has the worst performance. econd, Fgure 4 depcts the overall mss detecton probablty versus ˇ for the IC and tradtonal strateges. As shownnfgure4,ˇ can be optmzed to acheve the mnmum mss detecton probabltes for both the IC and tradtonal strateges snce there exsts a fundamental tradeoff n the sensng tme allocaton. Therefore, ths confrms the results obtaned n Remark 1. Clearly, t also can be observe from Fgure 4 that IC outperforms the tradtonal strategy due to the longer local sensng tme. 5.. ensng tme Fgure 5 llustrates the AT versus 0 for the IC and tradtonal strateges. From Fgure 5, one can see that IC sgnfcantly reduces the sensng tme under H 1 as com- pared wth the tradtonal strategy whose AT s always equal to the tme duraton of one sensng phase. In addton, the AT of IC decreases as 0 ncreases. Ths s because ncreasng 0 can mprove the local sensng relablty of Us. As a results, a fnal decson can be made earler under H 1 owng to such an mprovement. Because the OR rule gves a fnal decson faster than the Majorty rule, ts AT s lower. Note that, when AN rule s used n IC, the correspondng AT always equals to T 0 ms. Fgure 6 shows the AT versus P for the IC and tradtonal strateges. On can easly observe from Fgure 6 that the AT of IC decreases wth ncreasng P.Ths s due to the fact that the local sensng relablty of Us s mproved as P ncreases, thus a fnal decson can be made faster under H 1 n IC. Fgure 6 also shows that the AT of IC usng OR rule approaches to ˇT 4 ms n hgh P regons. Ths s because the local sensng relablty of s hgh when P s large. In ths context, by usng IC OR rule, s more lkely to make a global decson under H 1 n the frst sensng subslot. Hence, ths valdates the results obtaned n Remark. Fnally, Fgure 7 depcts the AT versus ˇ for the IC and tradtonal strateges. It s shown n Fgure 7 that the AT of IC can be mnmzed by adjustng ˇ Wrel. Commun. Mob. Comput. 015; 15: John Wley & ons, Ltd. 155

9 Intellgent cooperatve sensng Z. a, J. Lu and K. Long Fgure 7. The average sensng tme versus ˇ for the ntellgent cooperatve sensng and tradtonal strateges wth 0 10 and P 0dB. to an optmal value due to the fundamental tradeoff n the sensng tme allocaton. Ths confrms the conclusons n Remark. 5.. Performance optmzaton Frst, the AT of the IC strategy usng MM rule s compared wth that of the tradtonal strategy usng MM rule n Fgure 8. Clearly, n ths case, IC strategy stll can remarkably reduce the sensng tme under H 1 compared wth the tradtonal case. Note that the tradtonal strategy usng MM rule only can mnmze the overall mss detecton probablty but can not reduce the sensng tme. Then, Fgure 9 llustrates the performance comparson of the overall mss detecton probablty between the IC strategy wth MA rule and the tradtonal strategy wth MM rule. It can be easly observed from Fgure 9 that, whle mnmzng the AT, IC s stll able to acheve lower mss detecton probablty than the tradtonal strategy wth mnmzed mss detecton probablty. Ths demonstrates the advantages of proposed IC strategy. Besdes, both the AT of MM rule and the mss detecton probablty of MA rule n IC are reduced wth ncreasng P because of an mprovement n the local sensng relablty of Us. Fgure 9. The overall mss detecton probablty versus P for the ntellgent cooperatve sensng strategy usng MA rule and the tradtonal strategy by usng mn-mss-detecton-probablty rule wth CONCLUION In ths paper, an effcent cooperatve sensng strategy, called IC, s proposed to mprove the sensng accuracy and at the same tme reduce the sensng overhead n CRNs. To acheve ths, the sequental detecton and selectve reportng technques are naturally ntegrated nto cooperatve sensng. The optmzaton problems are also studed wth dfferent objectves n IC and then two optmal fuson rules are developed accordngly. The performance of IC s analyzed n terms of the mss detecton probablty and AT whose closed-form expressons are derved over Raylegh fadng channels. Fnally, smulaton results are provded to confrm the effectveness of proposed IC strategy. The performance of IC s also compared wth that of the tradtonal case. It s shown that IC can remarkably reduce both the mss detecton probablty and the sensng tme. Besdes, the results obtaned n ths paper can be easly extended to other local detector cases. APPENIX A: CALCULATION OF (7) Let ˆl denote the set organzed by the frst l Us n. ˆl;m;n denotes the nth nonempty subcollecton of ˆl, whch contans m dfferent Us. The calculaton of (7) nvolves two cases K 1 and 1<K N C 1, whch are respectvely gven as follows: Case 1.K 1/ W In ths case, a global decson may be made n any subslot of a sensng phase. If a global decson s gven n t 0 under H 0, t mples that makes a false alarm by local sensng. Thus, the probablty of that a global decson s made n t 0 under H 0 s gven as Fgure 8. The average sensng tme versus P for the ntellgent cooperatve sensng and tradtonal strateges by usng mn-mss-detecton-probablty rule wth Pr fc 0 1jH 0 gpf IC IC (A.1) On the other hand, f a global decson s gven n t (0 < N ) under H 0, t mples that success- 156 Wrel. Commun. Mob. Comput. 015; 15: John Wley & ons, Ltd.

10 Z. a, J. Lu and K. Long Intellgent cooperatve sensng fully receves a false alarm n t, but t does not make a false alarm durng ft 0 ; ;t 1 g. Then, the probablty of that a global decson s gven n t under H 0 s calculated as Pr fc 1jH 0 g Pf IC ;U U j ˆ IC P U U j ˆ 1 Pf IC ;U j 1 IC P Uj (A.) Followng the total probablty law, the overall false alarm probablty of IC under K 1 can be derved as P IC f Pr fc 1jH 0 g A 1 K 1 (A.) Case (1<K N C 1)W In ths case, a global decson could be gven durng ft K 1 ; ;t N g. If a global decson s made n t (K 1 N ) under H 0, t s known that successfully receves a false alarm from U n t, and t wrongly detects the presence of PU K 1 tmes durng ft 0 ; ;t 1 g. Hence, the probablty of that a global decson s gven n t (K 1 N ) under 1<K N C 1 and H 0 s obtaned as Pr fc KjH 0 g CX K 1 Pf IC 4 ;U n1 4 U j ˆ;K 1;n U k ˆ ˆ;K 1;n C K 1 X P U 4 4 n1 U k ˆ ˆ;K 1;n Pf IC 5 ;U j 1 Pf IC 5 ;U k U j ˆ;K 1;n P Uj 5 1 PUk 5 (A.4) Fnally, the overall false alarm probablty of IC under 1<K N C 1 s calculated as P IC f Pr fc KjH 0 ga K 1 APPENIX B: CALCULATION OF (8) (A.5) mlar to the calculaton of (7) as gven n Appendx A, the calculaton of (8) also nvolves two cases: K 1 and 1<K N C 1. Case 1 (K=1)W In ths case, the probablty of that a global decson s made n t 0 under H 1 s gven by Pr fc 0 1jH 1 gpd IC (B.1) On the other hand, the probablty of that a global decson s gven n t (0 < N ) under H 1 s calculated as Pr fc 1jH 1 gpd IC ;U 1 Pd IC ;U j U j ˆ (B.) Then, the overall detecton probablty of IC under K 1 s P IC d Pr fc 1jH 1 gb 1 K 1 (B.) Case (1<K N C 1)W The probablty of that a global decson s gven n t (K 1 N ) under 1<K N C 1 and H 1 s obtaned as Pr fc KjH 1 g CX K 1 Pd IC 4 ;U n1 4 U k ˆ ˆ;K 1;n U j ˆ;K 1;n Pd IC 5 ;U (B.4) j 1 Pd IC 5 ;U k Fnally, the overall detecton probablty of IC under 1<K N C 1 s calculated by P IC f Pr fc KjH 1 gb ( B. 5) K 1 ACKNOWLEGEMENT Ths work s supported by the Natonal Major Projects (nos. 01ZX and 01ZX ), the Natonal Natural cence Foundaton of Chna (nos and ), the 97 Project (no. 01CB15900), and the Fundamental Research Funds for the Central Unverstes (no. ZGX009J005, FRF-TP-1-080A). REFERENCE 1. Mtola J, Magure G. Cogntve rados: makng software rados more personal. IEEE Personal Communcatons 1999; 6(4): Haykn. Cogntve rado: bran-empowered wreless communcatons. IEEE Journal on elected Areas n Communcatons 005; (): Wrel. Commun. Mob. Comput. 015; 15: John Wley & ons, Ltd. 157

11 Intellgent cooperatve sensng Z. a, J. Lu and K. Long. Letaef K, Zhang W. Cooperatve communcatons for cogntve rado networks. Proceedngs of the IEEE 009; 97(5): a Z, Lu J, Wang C, Long K. An adaptve cooperaton communcaton strategy for enhanced opportunstc spectrum access n cogntve rados. IEEE Communcatons Letters 01; 16(1): Axell E, Leus G, Larsson E, Poor H. pectrum sensng for cogntve rado: state-of-the-art and recent advances. IEEE gnal Processng Magazne 01; 9(): Farhang-Boroujeny B. Flter bank spectrum sensng for cogntve rados. IEEE Transactons on gnal Processng 008; 56(5): Gardner W. Explotaton of spectral redundancy n cyclostatonary sgnals. IEEE gnal Processng Magazne 1991; 8(): gham F, Aloun M, mon M. On the energy detecton of unknown sgnals over fadng channels, In Proceedngs of the IEEE Internatonal Conference on Communcatons (ICC), Anchorage, Alaska, UA, 00; a Z, Lu J, Long K. Improved energy detecton wth nterference cancellaton n heterogeneous cogntve wreless networks, In IEEE Global Telecommuncatons Conference 01 (GLOBECOM), Anahem, Calforna, UA, 01; Mshra, aha A, Brodersen R. Cooperatve sensng among cogntve rados, In Proceedngs of the IEEE Internatonal Conference on Communcatons (ICC), 006; Zou, ao -, Zheng B. A selectve-relay based cooperatve spectrum sensng scheme wthout dedcated reportng channels n cogntve rado networks. IEEE Transactons on Wreless Communcatons 011; 10(4): Ganesan G, L. Cooperatve spectrum sensng n cogntve rado part I: two user networks. IEEE Transactons on Wreless Communcatons 007; 6(6): Ganesan G, L. Cooperatve spectrum sensng n cogntve rado part II: multuser networks. IEEE Transactons on Wreless Communcatons 007; 6(6): Lu J, L J, Long K. Enhanced asynchronous cooperatve spectrum sensng based on empster hafer theory, In IEEE Global Telecommuncatons Conference 011 (GLOBECOM), Houston, Texas, UA, 011; Atapattu, Tellambura C, Jang H. Energy detecton based cooperatve spectrum sensng n cogntve rado networks. IEEE Transactons on Wreless Communcatons 011; 10(4): Khan Z, Lehtomak J, Umebayash K, Vartanen J. On the selecton of the best detecton performance sensors for cogntve rado networks. IEEE gnal Processng Letters 010; 17(4): Noh G, Wang H, Jo J, Km B-H, Hong. Reportng order control for fast prmary detecton n cooperatve spectrum sensng. IEEE Transactons on Vehcular Technology 011; 60(8): Hesham L, ultan A, Nafe M, gham F. strbuted spectrum sensng wth sequental ordered transmssons to a cogntve fuson center. IEEE Transactons on gnal Processng 01; 60(5): Zou, Zhu J, Zheng B, ao -. An adaptve cooperaton dversty scheme wth best-relay selecton n cogntve rado networks. IEEE Transactons on gnal Processng 010; 58(10): AUTHOR BIOGRAPHIE Zeyang a receved hs BEng degree n nformaton engneerng from Anhu Normal Unversty (ANHU), Wuhu, Chna, n 007 and hs MEng degree n electrcal engneerng from Chengdu Unversty of Informaton Technology (CUIT), Chengdu, Chna, n 010. He s currently workng toward hs Ph degree at the chool of Communcaton & Informaton Engneerng (CIE), Unversty of Electronc cence and Technology of Chna (UETC), Chengdu, Chna. Hs research nterests nclude cogntve rados, cooperatve communcatons, and green rados. Jan Lu receved hs B degree n Automatc Control Theory and Applcatons from handong Unversty, Chna, n 000, and hs Ph degree at the chool of Informaton cence and Engneerng from handong Unversty n 008. He s currently an assocate professor of Unversty of cence and Technology Bejng (UTB), Bejng, Chna. Hs research nterests nclude cogntve rado networks, moble mesh networks, and LTE-A. He s an IEEE member snce 009. Kepng Long receved hs M and Ph degrees at the UETC n 1995 and 1998, respectvely. From eptember 1998 to August 000, he worked as a postdoctoral research fellow at Natonal Laboratory of wtchng Technology and Telecommuncaton Networks n Bejng Unversty of Posts and Telecommuncatons (BUPT). From eptember 158 Wrel. Commun. Mob. Comput. 015; 15: John Wley & ons, Ltd.

12 Z. a, J. Lu and K. Long Intellgent cooperatve sensng 000 to June 001, he worked as an assocate professor at the Bejng Unversty of Posts and Telecommuncatons (BUPT). From July 001 to November 00, he was a research fellow n ARC pecal Research Centre for Ultra Broadband Informaton Networks (CUBIN) at the Unversty of Melbourne, Australa. He s now a professor and dean at the chool of Computer & Communcaton Engneerng (CCE), Unversty of cence and Technology Bejng (UTB). He s an IEEE senor member and a member of the Edtoral Commttee of cences n Chna eres F and Chna Communcatons. He s also a TPC and an IC member for COIN00/04/05/06/07/08/09/10, IEEE IWCN010, ICON04/06, APOC004/06/08, cochar of organzaton member for IWCMC006, TPC char of COIN005/008, TPC Co-char of COIN008/010, He was awarded for the Natonal cence Fund for stngushed oung cholars of Chna n 007, selected as the Chang Jang cholars Program Professor of Chna n 008. Hs research nterests are optcal nternet technology, new generaton network technology, wreless nformaton network, value-added servce, and secure technology of network. He has publshed over 00 papers, 0 keynotes speaks, and nvted talks n the nternatonal conferences and local conferences. Wrel. Commun. Mob. Comput. 015; 15: John Wley & ons, Ltd. 159