A New MAC Protocol with Control Channel Auto-Discovery for Self-Deployed Cognitive Radio Networks

Transcription

1 1 A New MAC Proocol wih Conrol Channel Auo-Discovery for Self-Deployed Cogniive Radio Neworks Ahanassios V. Adamis, Member, IEEE, Konsaninos N. Maliasos, Member, IEEE, and Prof. Philip Consaninou, Senior Member, IEEE Absrac The old-daed specrum regulaions have lead o a specrum shorage problem. This problem is acually no real since many frequency bands are no uilized efficienly. Specrum Pooling and unlicensed specrum access as enabled by Cogniive Radios (CR) can compensae for he specrum scarciy and opimize specral uilizaion. The common problem among many proposed CR archiecures ha operae in an unlicensed-overlay manner is he lack of a dedicaed conrol channel. In his paper a novel idea is proposed, o also use a differen unlicensed band as he conrol channel which, hough, mus have some special characerisics. This band is auo-discovered by CR erminals and is used o convey signaling and conrol messages. A new MAC has been also developed ha operaes on wo differen unlicensed bands, one for conrol and signaling informaion conveyance and he oher for acual daa raffic. The proocol has been implemened and evaluaed in an OPNET based Simulaor. I. INTRODUCTION COGNITIVE Radio echnology along wih a specrum agile sofware defined physical layer can provide he means for compensaing he arificial problem of specrum shorage [1], [2], ha resuls from he underuilizaion of many frequency bands. This can be done if Cogniive Radios (CR) are allowed o use specral areas, currenly licensed o various specrum owners (primary users) [3], and ha remain idle in frequency, ime and space domain. Idle specral ranges are referred o as specrum holes [4] ha CRs are able o sense and uilize while hey remain unused wihou disurbing he primary users communicaions. This idea of specrum pooling, and using he specrum as a secondary - overlay - unlicensed user, has been sudied for is poenials [5] [7] and has resuled o be a soluion o specrum scarciy. Many operaional nework archiecures have been proposed for overlay operaion in specrum pooling scenarios bu he majoriy is sharing one common problem; he lack of a dedicaed conrol channel which mus be licensed o he Cogniive Radio Nework (CRN). The CRs have o exchange essenial conrol informaion hrough such a conrol channel as in [8]. Bu he need for a dedicaed conrol channel for CRNs Manuscrip received November 2, 26; revised February 25, 27. This work was suppored by he Greek General Secrearia for Research and Developmen (GSRT) under Acion (Reinforcemen Programme of Human Research Manpower - PENED 23, Projec ID 3ED185. The auhors are wih he Mobile Radio-Communicaions Laboraory, School of Elecrical and Compuer Engineering, Naional Technical Universiy of Ahens, Zographou Campus, 9 Heroon Polyechneiou Sree, 15773, Ahens-Greece (phone: ; fax: ; {adamis,maliasos,fkons}@mobile.nua.gr). is conradicory o he very naure of he unlicensed specrum use idea as i demands for a licensed band for hem. In his paper we propose a novel idea ha release cogniive radio neworks from he need for a licensed specrum band o convey he conrol channel. We propose he uilizaion of wo differen specrum bands boh in an unlicensed - overlay manner, one o be used for he signalling and conrol messages raffic and he oher for he user-daa raffic. In his work an unconrolled, disribued, conenion based medium access conrol proocol has been also developed. The new proocol suppors he CR operaion on boh bands separaing daa raffic from signalling raffic and also compensaes for he rapid changes of he underlying physical medium ransmission rae ha resuls from he specrum pooling scheme, as described in Secion III. II. SYSTEM FRAMEWORK Unavoidably, every possible CRN ha would be allowed o operae as a secondary sysem would operae in an inermien manner in recurren cycles of sense-and-ransmi. This recurren operaion mus be frequen enough, depending on he licensed sysem ha owns he specral area, in such a way ha he overlay sysem mee is design arges [6] and respec he inerference crieria of he primary sysem. The more frequen his cycle happens, he less possible inerference he primary nework will encouner. The above descripion enails hree key design issues ha have o be considered in a cogniive radio implemenaion: 1) Synchronizaion beween CRs. The CRN mus simulaneously perform he described scan procedure. The broadcas of he resuls also requires synchronizaion. 2) Operaing frequency band. CRs need a channel o find he CRN and negoiae on he frequency band o uilize. 3) Fair sharing of he resources. A channel is needed o make possible he exchange of signalling messages for he fair resource allocaion. Apar from he above issues, here are some oher imporan or/and desirable capabiliies of CRs ha necessiae he exisence of a conrol channel beween erminals, such as negoiaion of RF chain parameers as described in [9], from equalizer number of aps and coefficiens used, o operaing frequency range and RF filers descripions, for a rue CR sysem. This conrol channel can be a well-defined frequency band which would be licensed o CRs, bu, in his case CRN would

2 2 Slo: Ch1 Ch2 Ch3 Ch4 SCB GSM Frame Srucure: τguard Cogniive Radio Terminal SDR PHY TSLOT Used ime Slos Idle Time Slos f DTB Specrum Pool: Includes a plehora of licensed sysems Fig. 1. Aachmen of a CR Terminal a boh SCB for conrol messages ransmissions and DTB for acual daa conveyance. miss heir ideal arge of being able o offer services whenever wherever wihou coordinaion in a self-deployed way. We hus propose he specrum band for he conveyance of he conrol channel o be also an unlicensed band bu wih specific characerisics as described in he following secion. In his way he CR will uilize and ransmi on wo differen frequency bands, he Signalling and Conrol Band (SCB) and he Daa Band (DTB) as demonsraed in Fig.1. A. SCB and DTB The SCB mus no have all he drawbacks ha usual overlay operaion, as in DTB, has. Oherwise, using a second unlicensed band would have no meaning. So a vial characerisic of he SCB is he ransmissions here o be carried ou wih he highes possible safey wihou causing inerference o he primary users. By knowing he ime and he ype of poenial primary users ransmissions his can be achieved. So, he SCB mus be a commercial widely used and well known radio inerface (e.g. GSM, UMTS) band. CRs already can have he knowledge of he srucure of such a band s air inerface and hus he overlay ransmissions can be performed wih high confidence. An operaional scenario, depiced in 1 would be like his: A heir iniializaion phase, CR erminals search sequenially over all possible bands ha are candidaes o serve as SCB. The beacon frequency and he frame srucure of he primary sysems in hese bands are known a-priori o he CRs. Assuming ha a CR is searching in he GSM band, i deecs he beacon frequency of he GSM Base Saion [1], decode he FCCH (frequency correcion channel) ha is no encryped and hen obain synchronizaion by decoding SCH (synchronizaion channel). I hen can discover cell available frequencies by decoding CCD (cell channel descripion). By having obained he cell frequency and slo srucure, he CR can now find ime-frequency idle slos and use hem for CRN discovery using hem as he conrol channel. Idle slos can be found wih high confidence [1] by sensing all he channels f in he cell during a period of ime, a he beginning of each slo of T GSM = 577µs. If a CRN is idenified, hen he CR has become member and has obained synchronizaion wih he res of he erminals, by using he common GSM clock. On he oher hand, Daa Band (DTB) can be any frequency range ha has been chosen o saisfy crieria such as low primary user raffic and convenien ypes of primary sysems ha are using i. The DTB can be discovered by new members of CR, afer hey have aached o he SCB, hrough negoiaion - conrol messages. Convenien ypes of primary sysems are considered o be radio sysems ha use some combinaion of cenralized TDMA/FDMA or OFDMA access schemes. In such sysems specrum holes are easier o deec. Moreover if he primary sysem uses some Carrier Sense access scheme, he overlay CRN would obsruc normal primary sysems operaion as i also senses before ransmission. If a Carrier Sense ype of primary sysem is he case, he only way secondary-overlay ransmission could possibly be feasible is by reducing secondary ransmied power or using some spreading echnique, o keep ransmied power below primary sysem s sensiiviy, hus reducing exremely CRN operaional range and limiing is applicaions. B. PHY Layer The efficien physical layer design of a CR erminal is a hard and complicaed ask. The Sofware Radio perspecive is he only soluion for such a fully adjusable and reconfigurable ransceiver [11]. The firs significan problem in he SDR-PHY design is he way how he wideband operaion range will be digiized given he oday s ADC/DAC echnology. UWB sysem design has deal wih similar problems ha can convenienly be used in his case [12]. The nex challenge is he inegraion of he receiver algorihms in he processing unis. An OFDM- FDMA daa ransmission scheme has been chosen for he proposed CR srucure, because i provides decen hroughpu, adequae receiver complexiy and i can be used appropriaely o fill specrum gaps currenly no used by primary users. The mos compuaionally demanding block of an SDR receiver, which is he channelizer, can efficienly be implemened wih he use of proper mulirae conversion sages and polyphase filer banks [13]. Oher funcional blocks such as equalizaion and synchronizaion blocks can be suiably embodied in he filering chain o minimize he workload in respec o he insananeous channel requiremens. III. DUAL UNLICENSED BAND MAC (DUB-MAC) I can be concluded from he preceding secions ha he Medium Access Conrol (MAC) proocol of a cogniive radio operaing in an overlay-unlicensed manner on wo differen frequency bands will have o face he following criical poins: The coninuously alering ransmission raes provided by he PHY layer on each band The frequen sensing procedures The rapidiy in which ransmission rae is alered may reach he order of magniude of one housand changes every

3 3 second or more. Addiionally MAC proocol has o monior he sensing procedures and paricularly o be aware for he saring imes and heir duraion. I is desirable for he MAC as well o pause is sae during scans, which are no performed insananeously, as no o sar a frame ransmission if here is no enough ime because of an upcoming sensing procedure. Wihin his already demanding conex he CR MAC proocol mus use efficienly he limied idle specrum and use i in a fair manner among all CR erminals. In his work we developed a MAC proocol ha akes ino accoun he above criical poins by modifying he CSMA/CA proocol found in he Disribued Coordinaion Funcion (DCF) [14]. The developed proocol mees he special needs of a CR erminal ha operaes over wo unlicensed bands MAC was modified o aach o he GSM radio inerface (SCB) for conrol packes ransmission and o any oher band (DTB) for useful daa ransmission. The reason why he DCF funcion was preferred is ha for a self-deployed nework ha ses up auonomously, disribued conrol is necessary a all levels as no pre-insalled infrasrucure is here and no coordinaion processes are needed. Conrol packes are considered o be he Reques o Send (), Clear o Send () and Acknowledgmen (ACK) packes as, serving for he virual carrier sensing and collision avoidance mechanisms, hey conain conrol informaion for he upcoming or foregone daa ransmissions. These hree ypes of packes are conveyed via he SCB channel on he GSM specrum holes discovered. Daa raffic which is conveyed in he DTB is considered o be he normal daa frames (MSDUs) of he proocol. In he operaion descripion ha follows, focus is given on he modificaions developed for he proocol o mee he design challenges of an inermien specrum pooling environmen. A. Access Scheme The proocol is in operaional sae beween successive specrum scans procedures, exacly for ime T τ scan, where T is he scan Period and τ scan he duraion of one specral scan and measuremen broadcas. During hese periods ransmission raes are considered consan. During he scanning procedure, ha is for ime τ scan he proocol ceases ransmission aemps and pause is sae. The basic operaion of he MAC proocol is by successive frame exchange sequences (FES) --DATA-ACK as in DCF. Each ransmier before aemping o ransmi, i mus find he carrier o be idle for a period of DIFS seconds. If a collision occurs he saions ha were involved back-off and heir rery couners are being se accordingly. Iniially, i doesn seem necessary o use an addiional band (SCB) for conrol packe ransmissions insead of only he DTB. And ha s why in a CSMA ype disribued proocol only one ransmission akes place a a ime. Bu we propose his kind of operaion for he following reasons some of which have already been menioned in Secion II: 1) CRs, by aaching o SCB, obain a necessary common reference clock for he DTB scanning procedures and measuremen broadcas o be operaed concurrenly TABLE I PACKET FIELDS IN DUB-MAC Case: Muliple Case: Single MPDU MPDU Packe More DuraionID More DuraionID Type Fragmen Fragmen - DATA SIZE - DATA SIZE - DATA SIZE - DATA SIZE DATA 1 NEXT DATA SIZE ACK - NEXT DATA - SIZE 2) The frequen ransmission of conrol packes in DTB increases he probabiliy of causing inerference o primary users. For a single packe ransmission he proocol uses four disinc packe ransmissions. The probabiliy P i of causing inerference o primary users is he probabiliy of he primary user o sar ransmiing while some CR erminal is ransmiing and is sraighly proporional o he ransmission duraion. In each FES he hree ou of four packes are conrol packes of he same duraion T CP while one is he daa packe wih duraion T D. If all packes are being ransmied in DTB he oal probabiliy of causing inerference o primary users is: P i = λt D + 3λT CP (1),where λ is he proporional facor for inerference. By removing he conrol packes from DTB and ransmi hem in SCB where he primary sysem s radio inerface srucure is known and he possibiliy of inerference is sligh we decrease he P i in DTB by he amoun 3λT CP. 3) In case ha DTB scanning periods are oo small, or he scanning delay is oo long and here is no enough acual ime o be used for ransmission i s preferable o use his ime only for daa ransmission. 4) By using an addiional band as he SCB, he implemenaion of a collision deecion scheme is enabled by using ou of band feedback ransmission. B. Virual Carrier Sensing and Timeous The virual carrier sensing mechanism is implemened as a decreasing ime couner called NAV (Nework Allocaion Vecor) ha shows in ime slos unis a prognosis for how much longer he medium is expeced o be busy for he curren FES. Bu in a specrum pooling environmen a FES is very likely o finish afer one or more scanning procedures (in eiher SCB or DTB). The ransmission raes can be differen afer he scans and hus a differen virual carrier sensing mechanism mus be implemened. In DUB-MAC he DuraionID and More Fragmen field of he ransmied packes is being se as shown in Table I. Each erminal ha does no paricipae in he FES mainains he NAV couner for he virual carrier sensing mechanism and ses i each ime i receives a packe ha is no desined o i. In virual carrier mechanism he DuraionID of a

4 4 (a) (b) (c) Nex DTB Scan Procedure Sar Nex GSM Slo Boundary Nex GSM Slo Boundary Fig. 2. The hree cases when an or a DATA packe is received. (a) The upcoming conrol packe can be ransmied immediaely, (b) Terminal has o wai for DTB scan before ransmiing and (c) Terminal mus wai for SCB scan o finish. packe carries direc informaion on he ime ha is needed for he FES o complee. In DUB-MAC he informaion carried in DuraionID field is he packe size of he upcoming Daa Packe or zero, as noed in Table I. So his size mus be convered accordingly ino ime unis. In DUB-MAC NAV is se no for he whole remaining FES bu only for he nex expeced packe eiher i is a conrol packe (, ACK) or i is a daa packe. DuraionID carries, when necessary, only he upcoming daa packe sizes because conrol packe sizes are consan and known. The reason for his is ha i is no known how many scan procedures will ake place and how many imes he ransmission rae will change. Thus, NAV is se according o he ype of he received packe: 1) or DATA received: Here, wha is expeced o be ransmied nex, is a conrol packe, which is of course or ACK. So, NAV mus be se for an upcoming conrol packe ransmission. Fig. 2 shows he hree sub-cases upon receiving. In case a, he conrol packe ransmission can ake place immediaely before any scan and hus NAV is se as: NAV = L C R SCB (2),where L C is he conrol packe lengh in bis and R SCB is he SCB ransmission rae in bps. In cases b or c, erminals will have o wai for SCB or DTB specrum scans respecively o finish, in order o obain he updaed valid ransmission rae for he SCB and se NAV using 2. If a daa packe is received he only difference is ha a daa packe is being conveyed in he DTB channel. 2) or ACK wih DuraionID no zero received: Here he upcoming packe is a DATA packe and we are ineresed in seing he NAV for his. The sub-cases here appear in fig. 3. In DTB: (a) DTB: (b) Nex GSM Slo Boundary DATA Nex DTB Scan Procedure Sar Nex GSM Slo Boundary DATA Nex DTB Scan Procedure Sar Fig. 3. The wo possibiliies for a DATA ransmission. (a) TX mus wai for he DTB scan o ake place and (b) TX can ransmi immediaely. No case for GSM scan, since i s crucial o uilize he DTB wihou spending ime. case a, he daa packe ransmission can ake place immediaely before any scan and hus NAV is se as: NAV = L D R DT B (3),where L D is he daa packe lengh in bis and R DT B is he DTB ransmission rae in bps. In case b he NAV will be se afer he end of he DTB scan procedure. Noice here ha here are only wo sub-cases because we are only ineresed in he DTB scan procedure. Daa ransmission is crucial and should no be inerruped by SCB scan. 3) ACK wih DuraionID= received: Here, NAV is immediaely rese as i marks end of a FES. The Timeous for he ransmier erminal are being se following similar assumpions and calculaions. A maximum value of propagaion delay is also aken ino accoun. If he CR erminal is waiing for a scan o finish before seing he NAV, afer he scan i has o re-calculae wheher he expeced packe can be sen a ha ime and before he nex scan, following similar procedure. C. Incremening Back-off The inermien way ha DUB-MAC operaes necessiaes he implemenaion of wha we call he incremening Back-

5 5 CR1 CR2 CR1 CR2 (a) (b) Nex GSM Slo Boundary Las Opporuniy Time Poin exp τ exp2 2 1 Collision Back-Off sops here Fig. 4. (a) A usual case ha wo differen saions back-off values goes o zero a ime ha hey are no able o ransmi he message. This resuls in a collision. (b) The same case, bu wih a B inc increase in he back-off couner. Collision is avoided and prioriies are preserved off, ha is a back-off mechanism ha back-off couner doesn only decrease bu also increases by a cerain calculaed value a specific cases. The firs case can be explained hrough he following example ha is depiced in fig. 4. Here, wo CR erminals conend for he medium by decreasing heir back-off couners. As soon as a back-off couner reaches zero he erminal win he conenion and ransmis is conrol packe. In fig. 4a he back-off procedure is presened wihou modificaions and resuls o a collision because boh back-off couners zeroed afer he las opporuniy ime poin and boh had o wai for he signal scan o finish. The las opporuniy ime poin is defined as he ime: lasopporuniy = nexscbscan L C R SCB (4) ha is, he ime a which he packe o be ransmied can jus fi in ime before he upcoming GSM slo, when CR erminals have o perform scan for a period of. This is no a rare case during MAC operaion and hus, in DUB-MAC he back-off procedure is convered o include an increase in back-off couner when his happens. The increase in back-off is given by: B inc = exp lasopporuniy T slo (5),where B inc is he incremen quaniy of he back-off downcouner, exp is he ime a which back-off down-couner zeroes and T slo is he slo duraion of he MAC. This way, as shown in fig. 4, he collision is avoided and he prioriy is preserved correcly. The second case ha back-off couner is increased exiss o preserve fairness beween conended nodes. The back-off couner mus also be incremened by K inc if he coun-down passes over an SCB scan procedure. This case is illusraed in fig. 5. I would be unfair for CR2 if CR1 coninued couning down as SCB scan akes place. Here K inc mus be: CR1 CR Las Opporu niy Time Nex GSM Slo Boundary exp2 Incremen by Kinc No change: Unfair K inc = nexscbscan + lasopporuniy T slo (6) D. Dynamic Packe Size Finally, one las addiion is made o classic CSMA/CA proocol o efficienly operae in he inermien environmen of a specrum Pool. The case in which a daa packe is no being sen because here is no enough ime before he upcoming DTB scan saring ime, is quie frequen. If he size of he packe ha would fi in he remaining ime is larger han a minimum Packe Size hreshold hen he Daa packe is fragmened o wo in order no o wase he ime unil he upcoming DTB scan. Fig. 5. The second case ha back-off incremen is needed. In his case if no change is implemened he algorihm becomes unfair for erminal CR2, while wih a K inc incremen fairness is preserved and erminals will ransmi in he correc order. IV. SIMULATION RESULTS The DUB-MAC proocol was implemened in an OPNET modeler 12. [15] based simulaor and is performance was evaluaed for various sysem parameers. The sysem parameers are lised in Table II. The GSM sysem o which he CRN aaches o serve as he SCB is modeled as a ypical

6 6 TABLE II SIMULATION PARAMETERS Symbol Descripion Value N U Number of CR Terminals Varying ϱ GSM GSM hroughpu Varying 1/µ GSM GSM mean call duraion Varying C GSM GSM Cell frequency channels 7 GSM Scan Delay 5µs T DTB Scan Period Varying τ scan DTB Scan delay Varying ϱ DT B DTB Primary Sysem hroughpu Varying λ DT B DTB Primary Sysem arrival rae Varying N SUB Toal subcarrier number in DTB 3 Specrum Pool B SUB Subcarrier bandwih 1 khz mod Subcarrier modulaion QPSK GSM cell ha owns 7 frequency channels which is a ypical value. Every frequency channel is decomposed o 8 slos. New subscribers calls arrive wih a rae λ GSM and say for mean call duraion of 1/µ GSM occupying 2 sysem slos, one for uplink and one for downlink, a differen frequencies. A every GSM-slo beginning, CRs idenify idle slos and ransmi on hem by urning on and off he appropriae OFDM subcarriers and so he maximum rae on SCB is 7 imes he rae of a single subcarrier of 2kHz BW while he minimum is zero. We model he DTB as an M/M/m/m Markov chain wih arrival rae λ DT B and hroughpu ϱ DT B. The parameer m = N SUB from Table II is for he m-server loss sysem. We assume ha DTB scans are performed wih no errors and ogeher wih measuremens broadcas and agreemen ake up τ scan. T values are normalized o he GSM slo duraion and τ scan o he T. The DUB-MAC was found o be a fair MAC proocol wih normalized rms fairness offse F off,rms =.68% where (ϱi ϱ T OT N U ) F off,rms = (7) N U ϱ fair and ϱ i is i h user s hroughpu and ϱ T OT is he oal nework hroughpu. Nework Throughpu was measured in erms of Daa Packes only, when all erminals coninuously conend o ransmi. In fig. 6-9 hroughpu is presened for various sysem parameers. Fig.6 shows how DTB licensed sysem parameers affec he DUB-MAC hroughpu. Fig.7 presens he effec of he SCB (GSM) sysem parameers. Fig.8 demonsraes he effec of differen scanning periods and scanning delays o he performance of he sysem. As i was expeced scanning delay is of grea imporance for he sysem hroughpu. Fig.9 shows he oal hroughpu behavior as more cogniive radio erminals are added o he CR nework. Packe Delay is presened in fig.1 as a funcion of nework hroughpu for various DTB and GSM hroughpu raes and scan delays. Finally, we define specral uilizaion for an overlay MAC proocol as he fracion of ime ha he discovered specrum Nework Throughpu Primary user Arrival Rae.1 Primary user Arrival Rae 1 DTB Licensed Sysem Throughpu Fig. 6. Throughpu for varying DTB sysem and primary user access raes. N U = 1, T = 1, τ scan =.1, ϱ GSM = 3, 1/µ GSM = 45s, ϱ DT B = 2.andλ DT B =.5accesses/s Nework Throughpu GSM 1/µ = 2s GSM 1/µ = 2s SCB-GSM sysem Throughpu Fig. 7. Throughpu in bps for varying SCB licensed sysem hroughpu and mean call duraions. N U = 1, T = 1, τ scan =.1, ϱ DT B = 2.andλ DT B =.5accesses/s holes are being used for useful ransmissions. For a specific implemenaion, his of course is upper bounded each ime due o he Scan Delay during which specrum holes are no used, alhough hey exis. Even furher, he specrum uilizaion obained by a MAC proocol is upper bounded by he fracion of he ransmission ime used for daa ransmission o he oal ransmission ime of a complee FES. Specral uilizaion obained for specrum holes is presened in fig.11. V. CONCLUSION The exended simulaions performed showed ha he design of a Self-Deployed Cogniive Radio Nework is feasible. Cogniive neworks can obain auonomy by auo-searching he conrol channel ha is so imporan o heir operaion, wihou exernal coordinaion and regulaions processes. The performance of he Dual Unlicensed Band Medium Access Conrol proocol can be improved by implemening back-off algorihms specially designed for such inermien operaion. Furhermore he collision deecion possibiliy mus be invesigaed for feasibiliy in a Sofware Defined Radio PHY layer,

7 7 1 Nework Throughpu Scan Delay.1 Scan Delay.4 Scan Delay.7 Idle Specrum Uilizaion (%) average max 5 Scan Period Fig. 8. Nework hroughpu in bps for varying DTB scanning parameers. N U = 1, ϱ GSM = 3, 1/µ GSM = 45s, ϱ DT B = 2.λ DT B =.5accesses/s DTB Licensed Sysem Throughpu Fig. 11. Specral Uilizaion as a funcion of DTB primary sysem hroughpu. N U = 1, T = 1, ϱ GSM = 3, 1/µ GSM = 45s, τ scan =.1andλ DT B =.5accesses/s wih feedback ransmission o be carried ou in he SCB. This, if viable, would also increase hroughpu performance. Nework Throughpu Number of CRs Fig. 9. Nework hroughpu in bps as Cogniive radio erminals are added o he sysem. T = 1, τ scan =.1, ϱ GSM = 3, 1/µ GSM = 45s, ϱ DT B = 2.andλ DT B =.5 Mean Packe Delay (s) 4,5 4 3,5 3 2,5 2 1,5 1 τdelay=,1 - GSM ρ = 3 τdelay=,1 - GSM ρ = 7 τdelay=,5 - GSM ρ = 3 τdelay=,5 - GSM ρ = 7,5 Nework Throughpu (bps) Fig. 1. Packe End-To-End delay for differen Nework Throughpu and for various Scan Delays and SCB sysem hroughpu. N U = 1, ϱ DT B = 2 REFERENCES [1] J. M. Peha, Approaches o Specrum Sharing, IEEE Commun. Mag., vol. 43, no. 2, pp. 1 12, Feb. 25. [2] P. Leaves, K. Moessner, and R. Tafazolli, Dynamic Specrum Allocaion in Composie Reconfigurable Wireless Neworks, IEEE Commun. Mag., vol. 42, no. 5, pp , May 24. [3] T. A. Weiss and F. K. Jondral, Specrum Pooling: An Innovaive Sraegy for he Enhancmen of Specrum Efficiency, IEEE Commun. Mag., vol. 42, no. 3, pp. S8 14, Mar. 24. [4] S. Haykin, Cogniive Radio: Brain-Empowered Wireless Communicaions, IEEE J. Selec. Areas Commun., vol. 23, no. 2, pp , Feb. 25. [5] D. Cabric, S. M. Mishra, D. Willkomn, and R. Brodersen, A Cogniive Radio Approach for Usage of Virual Unlicensed Specrum, in Proceedings of he 14h IST Mobile and Wireless Communicaions Summi, 25. [6] U. Berhold and F. Jondral, Guidelines for Designing OFDM Overlay Sysems, in Proceedings of New Froniers in Dynamic Specrum Access Neworks, 25. [7] D. Willkomm, J. Gross, and A. Wolisz, Reliable Link Mainenance in Cogniive Radio Sysems, in Proceedings of New Froniers in Dynamic Specrum Access Neworks, 25. [8] D. Raychaudhuri and J. Xiangpeng, A Specrum Eiquee Proocol for Efficien Ccoordinaion of Radio Devices in Unlicensed Bands, in Proceedings of 14h Personal, Indoor and Mobile Radio Communicaions, 25. [9] J. MiolaIII, Cogniive Radio An Inegral; Agen Archiecure for Sofware Defined Radio, Ph.D. disseraion, Royal Insiue of Technology (KTH), 2. [1] P. Papadimiraos, S. Sankaranarayanan, and A. Mishra, A Bandwidh Sharing Approach o Improve Licensed Specrum Uilizaion, IEEE Commun. Mag., vol. 43, no. 12, pp. supl.1 supl.14, Dec. 25. [11] J. M. III, Sofware Radio Archiecure: A Mahemaical Perspecive, IEEE J. Selec. Areas Commun., vol. 17, no. 4, pp , Apr [12] W. Namgoong, A Channelized Digial Ulrawideband Receiver, IEEE Trans. Wireless Commun., vol. 2, no. 3, pp , May 23. [13] J. Qiang and P. Zhang, Filer Bank Based Muliuser Receiver for Wireless OFDMA Sysems, in Proceedings of IEEE Inernaional Symposium on Microwave, Anenna, Propagaion and EMC Technologies for Wireless Communicaions, 25. [14] I. S , Par 11: Wireless LAN Medium Access Conrol (MAC) and Physical Layer (PHY) Specificaions, IEEE, Tech. Rep., [15]