A Novel Method for Weighted Cooperative Spectrum Sensing in Cognitive Radio Networks

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1 Internatonal Conference on Industral Technology and Management Scence (ITMS 5) A ovel Method for Weghted Cooperatve Spectrum Sensng n Cogntve Rado etorks Xue JIAG & Kunbao CAI College of Communcaton Engneerng, Chongqng Unversty, Chongqng 444, P. R. of Chna ABSTRACT: Consderng that most exstng technques for eghted cooperatve spectrum sensng have not comprehensvely consdered the nfluence of the space factor n reless envronment on the detecton performance n cogntve rado systems, the authors of ths paper present a novel method for mprovng the detecton performance that combnes the sgnal-to-nose rato parameters and the path-loss parameters nto the eghts. The ne technque ncludes the desgn of the formulas for eght assgnment. Thus, the nformaton of the sgnal-to-nose ratos and the path losses can be reasonably fused so that the mathematcal model for enhancng detecton performance s ell establshed. Also, a spectrum sensng scenaro n the reless envronment s smulated. By applyng the randomly generated smulaton parameters and ther derved parameters, the smulaton experment s, respectvely, performed n the envronment of AWG, Raylegh and akagam reless channels. The expermental results sho that the ne technque substantally mproves the detecton performance. KEYWORD: Cogntve Rado etork; Cooperatve spectrum sensng; Weghted cooperatve; Energy detecton; Path loss 9, Ln et al. ). The fundamental requrement of the spectrum sensng s that the detecton tme duraton should be as short as possble, hle havng a hgher detecton probablty and a loer mssed or false alarm probablty, so as to mprove the spectrum utlzaton. The conventonal Equal Gan Combnng (abbrevated as EGC) s a typcal method for the eghted cooperatve spectrum sensng (Vsser et al. 8). Ths smple method assgns the same eght th unty to every Secondary User (abbrevated as SU). In other ords, the EGC method dd not consder the effect of the dfferent SRs receved by ndvdual SUs on the detecton performance of the cogntve rado, hch may leads a loer detecton lablty. Generally, the other eghted cooperatve methods are the modfcaton of the EGC method, hch mprove the detecton performance by usng dfferent eght factors (Pan, et al. 9, Vsser et al. 8, Zhou et al. 7, Ghasem et al. 5). It s also noted that the spatal varaton of the SUs has been consdered n the eghted cooperatve method (Shahd et al. 8), here the eght coeffcents determned by a rato of logarthms. On the bass of nvestgaton of the above eghted cooperatve algorthms, e present a novel cooperatve spectrum sensng method that ITRODUCTIO Wth the rapd development of reless communcaton technology, the frequency spectrum s becomng more and more croded. The spectrum resource has become the man bottleneck restrctng the development of future reless communcatons (Luo et al. 9). In terms of, hoever, the present reless communcatons, a consderable porton of the lcensed users dd not make full use of the fxed spectrum allocaton, so the research, development and utlzaton of ntellgent cogntve rado technology are expected to supply an effectve ay to solve such a problem n future reless communcatons. The cogntve rado th the help of the realtme sensng ablty of hte space can realze the dynamc management and allocaton of the frequency spectrum, th the result that the spectrum resource utlzaton s expected to be mproved greatly. Obvously, the spectrum sensng s one of the key technologes of cogntve rado systems, and s a premse and foundaton to realze cogntve rado systems. At present, most porton of the spectrum bands have been allocated, so the challenge s ho unlcensed users to share these bands thout nterferences th the lcensed users (Luo et al. 5. The authors - Publshed by Atlants Press 7

2 comprehensvely utlzes the SRs receved by SUs and the path losses caused by the dstance beteen the ndvdual SUs and a Prmary User (abbrevated as PU). The key pont of the ne method les n the desgn of a reasonable parameter fuson scheme hch ll be ntroduced n ths paper. By a smulated spectrum sensng scenaro, the ne method th a varety of smulaton envronment parameters s expermented n the AWG channel, the Raylegh and akagam fadng channels. The expermental results sho that the detecton performance s substantally mproved, hle the mplementaton complexty s only slghtly ncreased. SIGLE-ODE EERGY DETECTIO Under the condton of sngle node, the detecton problem of an unknon determnstc sgnal n AWG can be summarzed nto a bnary hypothess test as follos: n( y ( () s( n( here y ( s the receved sgnal by a SU, s ( s the transmtted sgnal by a PU, n ( s the AWG th zero mean. Under the hypothess, the receved sgnal y ( s the sum of the transmtted sgnal and the nose,.e., the PU s usng the present frequency band thn the observaton duraton from to T seconds. Under the hypothess, the sgnal y ( conssts only of the nose, that s, the frequency band s not beng used. For the stuaton that only the orkng frequency band s knon, but there s no any other pror knoledge of the determnstc sgnal transmtted by the PU, t s recognzed that the energy detecton method s perhaps the best choce for the above bnary hypothess test problem (Urkotz, 967, Dgham, 3). The basc structure of an energy detector s shon n Fgure, here the deal bandpass flter th banddth W n z s used to flter out-ofband nose. The output y o ( of the flter s then sent to the square la devce hose output s ntegrated Fgure. Energy detector by the follong ntegrator. The ntegrator output T V y o ( dt T s just the average poer of the fltered sgnal. The test statstc s selected as a T normalzed energy, Y y o ( dt, n hch the quantty s the to-sded poer spectral densty of the nose. Then the test statstc for the energy detector can be represented as (Dgham et al. 3, Bn et al. 8) TW Y TW ( ) () here the tme-banddth producttw s assumed to be an nteger value. Ths says that under the hypothess, the test statstc Y s subject to a central dstrbuton th TW degrees of freedom; and under the hypothess, Y follos a noncentral dstrbuton th TW degrees of freedom and a noncentral parameter. ere, E s / s defned as the receved SR by the SU. Thus, n the AWG channel, the detecton and false-alarm probabltes are, respectvely, gven by Pd P( Y ) Qu (, ) (3) and P( Y ) Γ( u, / ) / Γ( u) (4) P f here TW Q u a, b s the generalzed Marcum Q -functon, Γ( ) and Γ(, ) are the complete and ncomplete gamma functons, respectvely. The average detecton probablty n a fadng envronment can be expressed as (Ghasem et al. 5, Zhou et al. 7) P u, s the decson threshold, d d ) x P f ( x dx (5) here P d s determned by (3), f (x) represents the probablty densty functon of the SR n dfferent fadng envronments. Because P f s unrelated to the sgnal transmtted by the PU, t s fxed, as shon n (4). 3 WEIGTED COOPERATIVE METOD 3. Basc Prncple In the conventonal method of the eghted cooperatve spectrum sensng, each SU frst detects the sgnal energy n the current band, and then sends a normalzed energy value to a central Access Pont (Access Pont, AP). After recevng all of the normalzed energy values, the AP calculates the eghted sum of these values n terms of a set of prescrbed eghts, and then compares the sum to a detecton threshold to make a decson. ere the eghted sum can be represented as (Shahd et al. 8) 7

3 Y W Y (6) here s the total number of SUs ho partcpate n the cooperatve frequency sensng; Y s the normalzed energy value detected by the th SU; W s the theght. The test statstc Y follos the ch-square dstrbuton, and can be expressed as Y u u( ) ere, W (7) s the eghted sum of the ndvdual SRs for,,,. If all of the eghts take a value of unty, the eghted cooperatve method s degenerated nto the EGC method. 3. Improvement Prncple For a fxed threshold, the hgher SR of a receved sgnal the hgher detecton probablty of a SU ll have. Consderng that the dstance beteen the SU and PU s tghtly related to a path loss. Partcularly, the relaton beteen the average poer loss value P and the sgnal propagaton dstance (km) can be descrbed by (Cho et al. 8) P n log( d) (db) (8) here n s the poer loss exponent hch usually takes a constant n the range of ~4. It s noted that for a suffcent poer supply, the coverage of the IEEE8. WRA base staton can arrve at km, and the path loss n ths range can be up to dozens of db (Wang et al. 8). Thus, the effect of the dstance on the detecton performance s very bg. Fgure. Spectrum sensng scenaro Fgure depcts a spectrum sensng scenaro for cogntve rado netorks, here 5 SUs are detectng the sgnal transmtted by the PU. In the eghted cooperatve spectrum sensng, a reasonable selecton of eght coeffcents s an effectve ay to further mprove the detecton probablty. As shon n Fgure, the SUs can be dvded nto to classes as follos. ) The secondary users SU3, SU4 and SU5 are far from the PU, hch leads to bgger path loss. Thus, they should be assgned by smaller dstance eghtng coeffcents. On the other hand, the SRs of the sgnals receved by these users ll be loer. Therefore, these users should be allocated smaller SR eghtng coeffcents. ) The secondary users SU and SU are closer to the PU, hch leads to smaller path loss. Thus, they should be assgned by bgger dstance eghtng coeffcents. oever, these to users ll have a shado fadng, and thus they ll have smaller SRs. Therefore, they should be assgned by smaller SR eghtng coeffcents. From the above dscusson, t s knon that on the bass of comprehensvely consderng the effect of the SR and dstance path loss on the detecton performance, a reasonable selecton of eghtng coeffcents can be expected to mprove the detecton probablty further. Ths paper presents an mproved method for the eghted cooperatve spectrum sensng. In ths method, a dstance eght s ntroduced, except for the SR eght. The SR eght s defned as follos: (9) W here s the SR for the th SU. It mples that f the SR s smaller ts nfluence on the fnal decson ll be less. On the other hand, the total sum of the SR eghts s. The dstance eght s defned as W d ( ) log ( d ) log d ( ) () here d s the dstance beteen the thsu and the PU. It can be seen that a bgger d has less nfluence on the fnal decson. The total sum of the dstance eghts also s. Consderng that the total sum of the eghts for the EGC method s, the fnal eght n ths paper s desgned as W r d ( / ) W W () hose total sum s also the value of. 3.3 Probablty Descrpton Accordng to the method proposed n ths paper, the 7

4 basc process of the eghted cooperatve spectrum sensng follos that: each secondary user sends the current dstance parameter d, normalzed energy value Y and sgnal-to-nose rato to the central AP; values of the fnal eght W are determned by usng (9), () and (); the eghted energy sum Y s obtaned by usng (6); fnally the energy sum Y s compared th a decson threshold to decdes hether the PU s occupyng the lcensed frequency band. Under varous reless envronments, the detecton probablty Ψ d and the false-alarm probablty Ψ f for the eghted spectrum sensng can be expressed as follos (Shahd et al. 8): ) AWG For the AWG channel, the detecton and falsealarm probabltes are respectvely, gven by Ψ Y Qu, d AWG p () and Γ u, Ψ f p Y (3) Γ u In the case of ndependent dentcally dstrbuted fadng, the detecton probablty s based on nstantaneous SG. Thus, the detecton probablty s gven by Ψ d fadng ) u Q ( x, ) f ( x dx (4) here f () s the probablty densty functon of the eghted sum of SRs. Because the false-alarm probablty s ndependent of SRs, the false-alarm probablty for fadng case s the same as that of the AWG case. ) Raylegh Fadng For the Raylegh fadng channel, the probablty densty functon s f here x ( x) Γ( ) x exp s determned by W (5) (6) 3) akagam Fadng For the akagam fadng channel, the probablty densty functon can be expressed as m m m m exp xm x f x Γ here m s the akagam parameter. (7) 4 SIMULATIO EXPERIMET Wth Matlab as the platform for smulatng the algorthm presented by ths paper, e perform the smulaton experment on the sngle-node energy detecton, the EGC detecton and the detecton presented by ths paper under the AWG, Raylegh and akagam channels. As shon n Fgure, the number of users partcpatng n the cooperatve spectrum sensng s 5, here SU and SU suffer from shado fadng. Assume that the value of SR for each SU can be randomly selected n a range of -5 ~ db; the dstance beteen each SU and the PU can be randomly set thn km range. Furthermore, a set of randomly selected smulaton parameters th ther derved parameters are shon n Table. In ths table, randomly selected fve values of the SR parameter are sequentally assgned to each SU; the derved out parameter e s the eghted SR sum for EGC method, hose average value s gven by e e. In the smulaton, the tme-banddth product s set asu 3. For reasonably comparng the detecton performance of dfferent sensng methods, the SR for the sngle-node energy detecton s ntentonally selected as a maxmum value, that s, max 9.394dB. Table. Parameters and derved parameters for smulaton. SU (db) d (km) W W d W SU e (db) (db) e (db) (db) In ths paper, the detecton performance s evaluated by the curves of complementary Recever Operatng Characterstc (abbrevated as ROC), hose horzontal axs represents the false-alarm probablty Q f and vertcal axs descrbes the mssed-detecton probabltyq m. For the AWG channel, a set of complementary ROC curves for three detecton methods are shon n Fgure 3. Ths fgure shos that the method presented n ths paper has the best detecton performance. For example, hen the false-alarm probablty takes a value of., the msseddetecton probablty for the presented method s.5, but the EGC and sngle-node methods have hgher values of.48 and.39, respectvely. Comparng th the EGC method, the detecton 73

5 probablty for the presented method s ncreased by 68.3%. Fgure 4 shos the detecton performance for the Raylegh fadng channel, hch verfes that the presented method has the best detecton performance. Takng a fxed value of. for the false-alarm probablty, t can be fgured out that the detecton probablty of the presented method s ncreased by 53.%, comparng th the EGC method. Fgure 5 ndcates the detecton performance for the akagam fadng channel th a akagam parameter m 3, hch also shos that the presented method has the best detecton performance. For the convenence of comparson, a value of the false-alarm probablty s stll taken as.. It can be verfed that the detecton probablty of the presented method s ncreased by 53.%, comparng th the EGC method. Fgure 3. Complementary ROC for AWG channel Fgure 4. Complementary ROC for Raylegh channel 5 COCLUSIO The method presented by ths paper for the eghted cooperatve frequency sensng has comprehensvely consdered the effects of the sgnal-to-nose ratos and the path losses on the detecton performance. The desgned eghtng formulas can reasonably fuse the sgnal-to-nose ratos and dstance parameters, hch provde a good mathematcal model for mprovng the detecton performance. Usng randomly produced smulaton parameters, the smulaton results for the AWG, Raylegh and akagam fadng channels sho that the presented method has the best detecton performance, comparng th the sngle-node and EGC methods. REFERECES [] Cho, yun-o et al. 8. Adaptve sensng threshold control based on transmsson poer n cogntve rado systems. Thrd Internatonal Conference on Cogntve Rado Orented Wreless etorks and Communcatons ():-6. [] Dgham, E. F. et al. 3. On the energy detecton of unknon sgnals over fadng channels. IEEE Internatonal Conference on Communcatons (5): [3] Ghasem, A & Sousa, E. S. 5. Collaboratve spectrum sensng for opportunstc access n fadng envronments. IEEE DYSPA: [4] Ln, Yng-pe et al.. A spectrum sensng method based on hgh order cyclc statstc. Journal of Shangha Jaotong Unversty 44():9-3. [5] Luo, Tao et al. 9. Technology of Multuser collaboratve communcatons and spectrum sensng. ZTE Communcatons 5():6-64. [6] Pan, Jan-guo & Za, Xu-png. 9. Spectrum sensng n cogntve rado based on energy detecton. Journal of Shangha Unversty 5(): [7] Shahd, M. I. B. & Kamruzzaman, J. 8. Weghted soft decson for cooperatve sensng n cogntve rado netorks. the 6th IEEE Internatonal Conference ():-6. [8] Urkotz, Energy detecton of unknon determnstc sgnals. Proceedngs of the IEEE 55(4): [9] Vsser, F. E. et al. 8. Multnode spectrum sensng based on energy detecton for dynamc spectrum access. Vehcular Technology Conference 5: [] Wang, Jan-png et al. 8. Cogntve Rado. Bejng: atonal Defence Industry Press. [] Zhou, Xao-fe & Zhang, ong-gang. 7. Cogntve Rado Prncples. Bejng: Bejng Unversty of Posts and Telecommuncatons Press. Fgure 5. Complementary ROC for akagam channel 74