COOPERATIVE COMMUNICATIONS FOR WIRELESS INFORMATION ASSURANCE

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1 AFRL-IF-RS-TR Fnal Techncal Report July 005 COOPERATIVE COMMUNICATIONS FOR WIRELESS INFORMATION ASSURANCE State Unversty of New York at Bnghamton APPROVED FOR PUBLIC RELEASE; DISTRIBUTION UNLIMITED. AIR FORCE RESEARC LABORATORY INFORMATION DIRECTORATE ROME RESEARC SITE ROME, NEW YORK

2 STINFO FINAL REPORT Ths report has been revewed by the Ar Force Research Laboratory, Informaton Drectorate, Publc Affars Offce (IFOIPA) and s releasable to the Natonal Techncal Informaton Servce (NTIS). At NTIS t wll be releasable to the general publc, ncludng foregn natons. AFRL-IF-RS-TR has been revewed and s approved for publcaton APPROVED: /s/ E. PAUL RATAZZI Project Engneer FOR TE DIRECTOR: /s/ WARREN. DEBANY, JR., Techncal Advsor Informaton Grd Dvson Informaton Drectorate

3 REPORT DOCUMENTATION PAGE Form Approved OMB No Publc reportng burden for ths collecton of nformaton s estmated to average 1 hour per response, ncludng the tme for revewng nstructons, searchng exstng data sources, gatherng and mantanng the data needed, and completng and revewng ths collecton of nformaton. Send comments regardng ths burden estmate or any other aspect of ths collecton of nformaton, ncludng suggestons for reducng ths burden to Washngton eadquarters Servces, Drectorate for Informaton Operatons and Reports, 115 Jefferson Davs ghway, Sute 104, Arlngton, VA 0-430, and to the Offce of Management and Budget, Paperwork Reducton Project ( ), Washngton, DC AGENCY USE ONLY (Leave blank). REPORT DATE 3. REPORT TYPE AND DATES COVERED JULY TITLE AND SUBTITLE COOPERATIVE COMMUNICATIONS FOR WIRELESS INFORMATION ASSURANCE 6. AUTOR(S) Xaohua (Edward) L Fnal May 04 Aug FUNDING NUMBERS C - FA PE F PR - 558B TA - II WU - RS 7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) State Unversty of New York at Bnghamton Bnghamton New York PERFORMING ORGANIZATION REPORT NUMBER N/A 9. SPONSORING / MONITORING AGENCY NAME(S) AND ADDRESS(ES) Ar Force Research Laboratory/IFGB 55 Brooks Road Rome New York SPONSORING / MONITORING AGENCY REPORT NUMBER AFRL-IF-RS-TR SUPPLEMENTARY NOTES AFRL Project Engneer: E. Paul Ratazz/IFGB/(315) / Paul.Ratazz@rl.af.ml 1a. DISTRIBUTION / AVAILABILITY STATEMENT APPROVED FOR PUBLIC RELEASE; DISTRIBUTION UNLIMITED. 1b. DISTRIBUTION CODE 13. ABSTRACT (Maxmum 00 Words) Ths report descrbes research n the 004 Summer Vstng Faculty Research Program from May 04 Aug 04. Three of our research topcs wthn the feld of wreless communcaton networks are ncluded: theory of physcal-layer securty, realzng physcal-layer securty for 80.11b wreless LAN, and cooperatve communcatons n dstrbuted sensor networks. 14. SUBJECT TERMS WLAN, Physcal-Layer Securty, Cooperatve Communcatons, Wreless Informaton 15. NUMBER OF PAGES PRICE CODE 17. SECURITY CLASSIFICATION OF REPORT 18. SECURITY CLASSIFICATION OF TIS PAGE 19. SECURITY CLASSIFICATION OF ABSTRACT 0. LIMITATION OF ABSTRACT UNCLASSIFIED UNCLASSIFIED UNCLASSIFIED UL NSN Standard Form 98 (Rev. -89) Prescrbed by ANSI Std. Z

4 Table of Contents Part 1 Randomzed Array Transmssons for Physcal-layer Secured Wreless Communcatons Introducton System descrpton A randomzed transmsson scheme and computatonal secrecy Transmsson and recevng procedure Transmttng weghts desgn Transmsson power Transmsson secrecy of Algorthm Random matrx method for ntentonal ambguty Transmsson wth ntentonal ambguty Transmsson secrecy of Algorthm Perfect secrecy Secure transmsson n dspersve channels Smulatons Realzng Physcal-layer Secured WLAN Channel-based approach Tmng-based approach A smple testbed for demonstratng the concepts Part Applcaton of STBC-encoded Cooperatve Transmssons n Wreless Sensor Networks Introducton LEAC wth cooperatve transmsson Synchronzaton among cooperatng sensors Synchronzaton and channel models Long-term effect of frequency and tmng offsets Energy effcency Improvement on transmsson power effcency Overall sensor energy effcency Part 3 Conclusons References... 19

5 Lst of Fgures Fgure 1. System model for secured array transmsson wth ether J cooperatve transmtters or one transmtter wth a physcal antenna array.... Fgure. The block dagram of array transmsson.... P, / P, Fgure 3. Total transmsson power P t t t j and power rato of the th transmtter to the j th transmtter ( j ) when h s selected n (10). J = 4 for (a). α = 1 for (b). Sold lnes: total power. Dashed lnes: power rato... 6 Fgure 4. Recevng performance comparson. (a) For Algorthm 1. (b) For Algorthm. J = 4. o :Algorthms 1 or wth flat-fadng channels. :Algorthms 1 or wth dspersve channels. + :transmt beamformng. :theoretcal BER curve wth Raylegh fadng channel. :blnd detector of unauthorzed user Fgure 5. Transmsson power and standard devaton. Standard devaton s shown by above the power value. (a) Total transmsson power. (b) Power of a sngle transmtter. :Algorthm 1. o :Algorthm. :transmt beamformng... 1 Fgure 6. (a) Illustraton of LEAC wth cooperatve transmsson for wreless sensor networks. : prmary heads. : secondary heads. (b) Compare energy effcency wth/wthout cooperatve transmsson n LEAC... 18

6 Ths report conssts of two parts. The frst part develops physcal-layer securty theory and proposes ts realzaton n WLAN. The second part addresses cooperatve communcatons n sensor networks. To save space, some detals have been skpped, but can be referred n [31]-[34]. Part 1 Randomzed Array Transmssons for Physcal-layer Secured Wreless Communcatons 1.1. Introducton For the rapdly growng wreless communcatons, securty has become one of the major concerns [1]. Compared wth wrelne networks, wreless networks lack a physcal boundary due to the broadcastng nature of wreless transmssons. Ths unque physcal-layer weakness calls for nnovatve physcal-layer securty desgns n addton to, and ntegrated wth, the tradtonal data encrypton approaches. Exstng physcal-layer securty technques may be classfed nto three categores: ) power approach lke beamformng and drectonal transmssons, ) code approach lke spread-spectrum [], and ) channel approach lke [3,4,5]. They usually depend on some strong assumptons for secrecy, e.g., the unauthorzed user has null-recevng energy, or has no nformaton about the spreadng codes or the propagaton channel. If these assumptons hold, then secrecy s trvally acheved, otherwse secrecy s lost. As a result, t s dffcult to conduct a meanngful secrecy analyss that measures the performance of a technque under varyng condtons and assumptons. Unfortunately, such strong assumptons can be easly volated. Beamformng technques can only reduce, not completely nullfy, the sgnal energy toward the unauthorzed users, especally for those nsde the transmsson beam. Spreadng codes may be easly estmated by the unauthorzed user from the receved sgnals [6]. The unauthorzed user may use blnd equalzaton algorthms [7, 8] to estmate channels, whch causes many channel-based approaches such as [3] to lose secrecy. Even for the tmngbased approach [4] whch explots the channel recprocty, certan brute-force methods may effcently break the secrecy by examnng all possble tmng. It s well known that data encrypton technques realze computatonal secrecy nstead of perfect secrecy [9] because perfect secrecy requres transmttng a key as long as the data. The key dstrbuton usually remans as a weakness for encrypton technques. Interestngly, perfect secrecy s suggested n [5,3] as achevable wth physcal-layer technques, although some unrealstc assumptons have to be made, such as channels are unknown to the unauthorzed user or the channel of the unauthorzed user s noser than that of the authorzed user. We propose new physcal-layer transmsson technques to realze secrecy under more reasonable assumptons. We assume that the unauthorzed user may have better receved sgnal qualty and knows all the transmsson protocols. There are no secret keys shared by the transmtters and the authorzed user before transmsson, and both of them have no knowledge of the unauthorzed user. We depend on two specal propertes of wreless transmssons for secure desgns. Frst, sgnals receved by the authorzed user and the unauthorzed user are dfferent because ther channels are dfferent. Second, channels between the transmtters and the authorzed user can be recprocal [10] and can be adjusted ntentonally [11]. The frst property s due to multpath propagaton and ndependent fadng [1], whereas the other one has been wdely accepted n lterature [1] wth some supportve demonstraton from tme-reversal mrror experments [13]. These propertes make physcal-layer securty technques qute dfferent from data encrypton approaches. Our prmary objectve s to develop randomzed array transmsson schemes for computatonal secrecy, though perfect secrecy s shown to be realzable under some crcumstances. The transmsson schemes are presented wthn the framework of a cooperatve array formed by a group of cooperatng transmtters, each of whch may have only a sngle transmttng antenna. Physcal antenna array s ncluded n ths framework as a specal case. Cooperatve transmtters are not only more cost-effectve for mplementng large arrays, but also more flexble for creatng desrable channel condtons. On the other hand, cooperatve array s more challengng n terms of synchronzaton among the transmtters. 1

7 Ths part s organzed as follows. In Secton I., a framework of cooperatve array transmsson s formulated wth synchronous flat-fadng channels. In Secton I.3, a transmsson scheme s developed for securty based on the nherent ambguty of blnd equalzaton. Then, assumng the unauthorzed user knows ts own channels, a random-matrx scheme s developed n Secton I.4. In Secton I.5 these schemes are extended to dspersve channels wth mperfect synchronzaton. Smulatons are gven n Secton I.6. In Secton I.7, we descrbe ther realzatons n WLAN. 1.. System descrpton We consder a wreless network where moble users communcate wth a base-staton whch has J transmttng antennas. The base-staton has ether one transmtter wth a physcal antenna array, or J cooperatve transmtters. We consder the latter snce t ncludes the former as a specal case. The J transmtters communcate wth each other usng a secure lnk, such as the wrelne Ethernet or some cables that drectly connect them together. Packets are transmtted by the J transmtters cooperatvely, durng whch any unauthorzed user should be deprved of sgnal ntercepton capablty, as llustrated n Fg. 1. Authorzed User Secure Lnk Tx1 Tx Tx J Tx Antenna Array (Physcal or Cooperatve) Unauthorzed User Fg. 1. System model for secured array transmsson wth ether J cooperatve transmtters or one transmtter wth a physcal antenna array. A beamformng-lke array transmsson procedure shown n Fg. s used by the J transmtters. A symbol sequence { b ( }, obtaned va any tradtonal modulaton scheme, s fed to all J transmtters. Before transmsson, the sequence s processed by the transmtters. Though more complex flters can be used, we consder sngle-tap weghts w ( for smplcty. In addton, each of the transmtters may approprately delay (or advance) the sgnal by δ. The transmtted sgnal from the transmtter s thus s (, whereas the authorzed user receves sgnal x (. symbol sequence b ( w 1 ( Tx 1 delay δ 1 s 1( channel h 1 (k) x 1( w J ( delay δ J channel s ( J h x J (k) J ( Tx J Fg.. The block dagram of array transmsson. x( User Recever b ( If a physcal antenna array s used and the propagaton channel s Raylegh flat fadng, the receved sgnal at the authorzed user s J * x( = h s ( + v( = h s( + v(, (1) = 1 where v ( denotes AWGN wth zero-mean and varance σ v, channel coeffcents h * are ndependent complex crcular symmetrc Gaussan dstrbuted wth zero-mean and unt varance, and

8 h1 s1( w1 ( h =, s( M = M = M = w(. () hj s J ( wj ( * T In ths part, ( ), ( ), and ( ) denote conjugaton, transposton and ermtan, respectvely. Snce channel estmaton s requred, we assume that h s block fadng [3],.e., t s constant or slowly tmevaryng when transmttng a block of symbols but may change randomly between blocks. The symbols b ( are ndependent unformly dstrbuted wth zero-mean and unt varance. The unauthorzed user may use multple recevng antennas for better ntercepton, and the ntercepton becomes much easer wth a flat-fadng channel model. Therefore, we consder the worst case (to the transmtters and the authorzed user) where the unauthorzed user receves sgnals from M recevng antennas xu,1 ( hu,1,1 L hu,1, J w1 ( n d u,1 ) n d u,1 ) vu,1 ( M = M M M + M. (3), ( ),,1,, (, ) (, ), ( ) xu M n hu M L hu M J wj n d u J b n d u J vu M n The notatons are smlar to (1) except that ( ) u s used to denote the unauthorzed user. The delays d u, may not be zero because the transmtters adjust δ n favor of the authorzed user. Whle ntroducng such delays s an mportant way for enhancng securty, we assume zero delays for smplcty,.e., d u, = 0 for all. The equaton (3) can then be wrtten as x u ( = u w( + v u (. (4) Each element of the channel matrx u has the same dstrbuton as h, but s ndependent from h. We focus only on the securty of the downlnk transmsson (from the base-staton to the authorzed user). Once the downlnk s secured, the uplnk can be easly secured by usng smlar technques and/or by exchangng encrypton keys frequently A randomzed transmsson scheme and computatonal secrecy In ths subsecton, we assume that the unauthorzed user does not know the channels h and u. But t may try to estmate them by tranng/blnd methods, or by a brute-force search of all possble channels. The transmtters and the authorzed user do not know all channels ether, and have no ways to estmate u. Ways have to be desgned for them to estmate h and symbols, durng whch no nformaton should be obtaned by the unauthorzed user for successful ntercepton Transmsson and recevng procedure We frst gve the downlnk transmsson and recevng procedure wth the consderaton of the sgnal model (1)-(). Accordng to the receved sgnal x( = h w( + v(, (5) the transmtters need to use specal transmttng weghts w ( to fulfll the securty objectve. Our basc dea s to make h w( determnstc but u w( changng randomly n each symbol nterval. For ths purpose, w ( should be random snce the transmtters do not know u. We desgn the transmttng weghts vector w ( such that h w( = h, (6) 3

9 J where h = h = 1. Although (6) looks smlar to transmsson beamformng [10], the major dfference s that w ( changes randomly after each symbol b ( s transmtted. Ths can be realzed by selectng randomly the elements of w ( whle satsfyng the constrant (6). Obvously, f the channel h s constant or slowly tme-varyng, we need J transmtters. Ths explans why array transmsson s requred. The authorzed user can detect symbols after estmatng the receved sgnal power h, where h can be estmated as 1 N n = N 1 x( ˆ 1 = h x(, (7). If b ( s desgned wth constant magntude b (, e.g., usng PSK modulaton, then we can smply use x ( n place of h,.e., use the phase of x ( for symbol detecton. To mplement ths transmsson scheme, the channel h has to be known to the transmtters nstead of the recever. There are at least two ways for the transmtters to estmate the channel h. Frst, f the downlnk and uplnk channels are recprocal, the transmtters can estmate h drectly from the uplnk receved sgnals. Ths s the case n fast tme-dvson-duplexng (TDD) transmssons [10], [1]. The second way s to ask the authorzed user to feedback some receved sgnal nformaton to the transmtters. Snce explct tranng should be avoded, the transmtters can send a tranng sequence randomzed by w ( whch are known to themselves only. The authorzed user only estmates and feedbacks y( = h w(, wth whch the transmtters can estmate channel h based on ther knowledge of w (, w1 (1) L w1 ( J ) ˆ h = [ y(1) L y( J )] M M. (8) wj (1) L wj ( J ) Note that only J samples are requred for feedback f the weghts w ( are chosen properly. An alternatve method s that the authorzed user sends some x ( drectly back to the transmtters Transmttng weghts desgn Before presentng our desgns, we frst show that tradtonal transmt beamformng methods do not guarantee secrecy although they are optmal n terms of performance and power effcency. A typcal transmt beamformng method uses w ( n ) = h / h, whch has unt total transmsson power snce * E[ s ( ] = E[tr( w( b ( w ( )] = E[ w( ] = 1. Obvously, w ( s not random f the channel h s constant or slowly tme-varyng. The receved sgnal of the unauthorzed user becomes x ( = ( h / h ) + v (, from whch many blnd equalzers ncludng the constant modulus u u u algorthm (CMA) [15] can be appled for symbol detecton. The same concluson holds for other desgns of w ( that are not random. Ths explans why we should make w ( random for randomzed array transmssons. More generally, w ( can be obtaned from the sngular value decomposton (SVD) of h,.e., h = UDV [16]. In ths specal case, = 1 U, dag{ h, 0, L, 0 } D =, and V s a J J untary matrx whose frst column equals h / h. For transmt beamformng, w ( can be calculated as T T ( ) V[1, z(,, zj ( ] = V[1, z1 T w n = L ( ], where z j (, j =, L, J, can be arbtrary. Such a classc 4

10 approach does not have any secrecy even f w ( s randomzed by choosng randomly z 1(. For example, CMA may be used to estmate symbols from 1 x u( = uv + vu (. z1( (9) In summary, n order to guarantee secrecy, we may not acheve the optmal unt transmsson power. Ths can be further demonstrated by the followng observatons. For J =, f we guarantee unt transmsson power, then there s no degree of freedom n w ( left for randomzaton. In addton, f we solve (6) by frst choosng randomly w (, 3 J, and then lookng for w 1( and w ( for both (6) and unt power, t turns out that there may not have solutons. Based on such observatons, we desgn transmttng weghts whch trade transmsson power for secrecy. We frst select randomly an h from h. We can select a threshold α and choose those h that satsfy h > α. Then we choose randomly w j (, where 1 j J and j. Wthout loss of generalty, we can draw them from an..d. complex Gaussan random process. Denote T T z ( n ) = [ w1(, L, w 1(, w + 1(, L, wj ( ] and h ( n ) = [ h1, L, h 1, h + 1, L, hj ]. The weghts vector s calculated as h h z ( w ( = P * h. (10) z ( The matrx P s a J J commutaton matrx whose functon s to nsert the frst row of the followng vector nto the th row. Snce h s chosen randomly, P s also random. Ths approach s lsted below as Algorthm 1. Algorthm 1. Desgn weghts vector w ( for each symbol 1. Select randomly h, 1 J, such that h > α.. Generate..d. random varables w j (, 1 j J, j. 3. Calculate w ( by (10). One of the major advantages of Algorthm 1 s ts lnear computatonal complexty. Effcent computaton s mportant because w ( are recalculated n each symbol nterval Transmsson power Although we do not explctly apply any power constrants on w (, the transmsson power can be statstcally controlled by adjustng the mean and varance of the random varables w j (, j. Let us consder the case that the mean and varance are zero and σ, respectvely. Then the total transmsson power s P h h [ w ( w( h, P ] = ( J 1 σ + + σ = E (11) h h t, h ) for a gven channel realzaton h and a gven choce of h. Equaton (11) shows that small h ncreases the total transmsson power, so the threshold α should be carefully selected. Snce h s a complex Gaussan random varable wth zero mean and unt varance, h s exponentally dstrbuted wth unt mean. The probablty for the selected channel coeffcent h to have energy h greater than α s 5

11 P α [ > σ ] = t h e dt = e σ. (1) Proposton 1. Wth Raylegh fadng channels, f the coeffcents are selected wth energy threshold α (1), then the expected total transmsson power s P t = ( J 1) σ + 1+ ( J 1)(1 + σ ) Γ(0, α). (13) Proof. See [31]. From (13), the total transmsson power P t s a functon of the number of transmttng antennas J, the varance σ of the random varables w j (, and the threshold α for selectng h. Fg. 3 llustrates ther relatons. From Fg. 3(a), wth J = 4, we see that P t ncreases when σ ncreases or α decreases. Sold: total power. Dashed: power rato α=.5 α=1 α= σ (a) Sold: total power. Dashed: power rato σ =. σ =.5 σ = Number of Transmtters: J (b) P P Fg. 3. Total transmsson power t and power rato t Pt, j of the th transmtter to the j th transmtter ( j ) when h s selected n (10). J = 4 for (a). α = 1 for (b). Sold lnes: total power. Dashed lnes: power rato. If the channel h s slowly tme-varyng or even constant for a long tme, we need to avod the case that the power of one of the transmtters s exceptonally larger than the others. Otherwse the array transmsson behaves as that wth a sngle transmtter, and securty can be compromsed. Therefore, we have to constran the rato of the transmsson power of the th transmtter Pt, = ( h + h σ ) / h to that of the j th transmtter P =. The power rato can be obtaned from (13) as t, j σ P t, 1+ ( J 1)(1 + σ ) Γ(0, α) =. (14) Pt, j σ Obvously, t s usually mpossble to obtan unt rato unless we change the probablty of choosng h accordng to the value of h. From Fg. 3, the power rato s a decreasng functon of both, / σ and α Transmsson secrecy of Algorthm 1 We have removed explct tranng so that the unauthorzed user has no tranng avalable for channel estmaton. If the channels are recprocal, then the transmtters can estmate channel h from any uplnk sgnal transmtted by the authorzed user n TDD, wthout leakng channel nformaton to the unauthorzed user. Otherwse, the transmtters depend on feedback from the authorzed user for channel estmaton. In ths latter case, the secrecy reles on the securty of the feedback data. If the feedback data are not secure and can be obtaned by the unauthorzed user, whether they are y( = h w( or raw receved samples, the secrecy of the downlnk transmsson can be lost. For example, f the unauthorzed 6

12 user has ntercepted the feedback data y (, then together wth ts own estmatons y ( = w(, t 1 u can derve a vector h. By ths vector, t can ntercept symbols b ( from x u (. Therefore, before usng feedback, a secure ntalzaton method has to be adopted to secure the frst transmsson for the subsequent feedback-based data transmsson to become secure. We may explot the recprocal channel property to realze ths objectve. For example, the authorzed user can frst send a tranng sequence to the transmtters usng the downlnk frequency. After the transmtters estmate the channel, secure downlnk transmsson s setup by Algorthm 1. Feedback methods can then be used for channel estmaton for normal data transmsson, durng whch the feedback data can be secured va, e.g., Algorthm 1 employed at the authorzed user or nstantly exchanged keys. The advantage s that no secret keys are requred before transmsson, whch s mportant consderng that key dstrbuton s usually a major weakness for tradtonal securty technques. Wthout tranng, the unauthorzed user may turn to blnd equalzers. It s necessary for the transmtters to remove any constant modulus nformaton from s j ( = w j ( to prevent the applcaton of a major category of blnd equalzers: the constant modulus method [15], [17]. Ths s realzed n Algorthm 1 by choosng w j ( approprately. If w j ( s Gaussan, then s j ( s satsfactory because b ( s ndependent from w j ( and s unformly dstrbuted wth a fnte number of values. In partcular, f b ( s constant, then s j ( s Gaussan because the Gaussan probablty densty functon (pdf) of w j ( s phase symmetrc. Although s ( s not jontly Gaussan due to (16), t s determned completely by the frst and second order moments whereas hgher-order moments are zero. In ths scenaro, the secrecy of Algorthm 1 comes from the fact that the receved sgnal (4) of the unauthorzed user s wth a multple-nput multple-output (MIMO) channel model. It s well known that blnd MIMO channel estmaton has an nherent matrx ambguty f no source property can be exploted [17], [18]. In our case, snce sgnals s j ( are not drawn from a fnte alphabet, there may not be any modulus nformaton for the unauthorzed user to remove such an ambguty. For example, the frst-order moment of x u ( does not provde the unauthorzed user wth any useful nformaton because t s dentcally zero even f w j ( may not have zero mean. For the secondorder moments, the unauthorzed user may obtan R u = ue[ w( b ( w ( ] u = ur su. There exst some J J untary matrces Q such that R u = uq R squ as long as Q R sq = R s. Snce the unauthorzed user does not know R s, t has no nformaton of Q. Moreover, the unknown R s makes the ambguty matrx arbtrary, not only untary. The concluson about the ambguty matrx can be easly checked by the subspace method [19] wth N > J. Therefore, f the unauthorzed user can not dscrmnate u from uq, t can not dscrmnate w ( from Qw (. Ths makes the ntercepton mpossble as Q s unknown. If the blnd equalzaton s not applcable, the last way left for the unauthorzed user s to try a brute-force search of all possble channels u (or, strctly speakng, Q ) and h. Let us assume that the unauthorzed user uses K -level quantzaton for each sngle value (a complex number has two such ( J ) values). Then the brute-force search needs to consder at least K possble combnatons of u and J K J (J + 1) possble combnatons of h. Ths gves an overall complexty K. Wth J = 4 and QPSK transmsson, n order to acheve bt-error-rate (BER) under 0. 1, by smulatons we fnd K 4 even n the noseless case. When K = 4, the complexty becomes 4 ( 4+ 1) =, whch gves securty well above the encrypton wth a 18 -bt key [1]. If consderng a more realstc BER of at sgnal-to-nose-rato (SNR) 5 db per recevng antenna, then K should be 644 at least 18, whch gves a complexty over. Snce the complexty of the brute-force search ncreases rapdly wth J, computatonal secrecy of Algorthm 1 can be guaranteed. * u u 7

13 1.4. Random matrx method for ntentonal ambguty The transmsson secrecy of Algorthm 1 depends on the nherent ambguty of blnd channel equalzaton. owever, n practce, t may not be a trval task to prevent every possble blnd/non-blnd equalzaton method, especally snce networkng protocol nformaton or even the source correlatons may be exploted by the unauthorzed user for equalzaton [0], [1]. Source scramblng, networkng protocols, as well as w ( have to be carefully desgned. Instead of focusng on the ssues relatve to the overall network desgn, we develop another transmsson algorthm wth the objectve of achevng secrecy even f the unauthorzed user knows ts own channel u. Ths would effectvely smplfy the desgn of physcal-layer secured wreless networks. We assume n ths subsecton that the unauthorzed user knows u but not h, and has extremely hgh SNR or even noseless sgnal. Such assumptons make our approach dstnct from most exstng physcal-layer securty studes such as [3] Transmsson wth ntentonal ambguty Wth the known u, the sgnals of the unauthorzed user (4) can be smplfed to x u ( = w(, (16) where the nose s skpped under the assumpton of hgh SNR. Snce the unauthorzed user may know the sgnal model of the authorzed user (5)-(6) (but does not know h, w ( and b ( ), a brute-force search wth much reduced complexty can be appled, durng whch t smply checks every possble h wth (16) to see whether the rule of fnte symbol alphabet s satsfed. Ths procedure may break the secrecy wth a J complexty K only. To resolve ths weakness, one way s to make h tme-varyng, whch can ncrease the complexty of the brute-force method n low SNR but s not effectve n hgh SNR or noseless cases. To guarantee secrecy under (16), we propose to ntroduce ntentonal ambguty nto w ( n addton to creatng tmevaryng channels. Instead of usng (10) to fnd w (, we generate a J ( J 1) random matrx F = [ f1, L, fj 1], where each f s a J 1 vector. Let f1 c1( a( = M, (17) 1 1( ) f J cj n where { c ( }, 1 J 1, are secret sequences known only to the transmtters. Wthout loss of generalty, we assume that c ( = ± 1,, n, and { c ( } and { c j ( } are ndependent from each other. We make each column of the matrx F to have the same dstrbuton as h. The matrx F s known to the transmtters only. Then we calculate w ( by solvng h h w( =. (18) F a( For the authorzed user, the receved sgnal s stll (5) and (6). The key dea s to make the unauthorzed user unable to dscrmnate h from any column of F, even wth a brute-force search. Ths procedure s lsted below as Algorthm when the channel h s block fadng. 8

14 Algorthm. Desgn w ( for ntentonal ambguty 1. Generate random matrx F (for a block of symbols),. Generate random vector a ( (for each symbol), 3. Calculate w ( by solvng array equaton (18). The computatonal complexty of Algorthm s O ( J ). Note that 1 F s recalculated per symbol block, not per symbol. The power effcency of Algorthm can be made much hgher than Algorthm 1 because the problem of nvertng small h s gone. The lower bound of total transmsson power can be determned from ( ) ( ) [ ( ) ] h [ ] h h + a n a n E w n E h F = = 1. (19) ( ) tr([ ] [ ]) a n h F h F owever, the unt lower bound usually can not be obtaned Transmsson secrecy of Algorthm In the followng, we use P [x] to denote the probablty of a random varable X for notatonal smplcty. It equals the pdf f X (x) f X s contnuous, or the probablty mass functon p X when X s dscrete. Proposton. Even f the unauthorzed user knows ts channel u and works n noseless envronment, t can not dscrmnate h from any column f of F,.e., h { xu( }] = f { xu( }], 1 J 1, where { x u( } denotes the sequence ncludng all the avalable samples. Proof. Consderng the maxmum a posteror (MAP) detector for h, the unauthorzed user has h] h] h { xu( }] = { xu( } h] = { w( } h] { }] (0) { xu ( }] { xu ( }] Because of (6), one element of w ( s completely determned by others gven h. Wthout loss of T generalty, let w ( ) be determned by random varables z n ) = [ w (,, w J ( ]. Then 1 n [ { x }] { z ( }] { }] 1 ( L h] P h u( = 1. { x ( }] Smlarly, f the unauthorzed user consders f nstead of h, t has [ f { x }] { z ( }] { }] f ] P u( = 1. { xu ( }] Because f ] = h], the proposton s proved. Proposton shows that the unauthorzed user can not dscrmnate h from f. In other words, t can not dscrmnate b ( from c (. Ths s the ambguty created ntentonally by Algorthm. owever, f the number of vectors h and f that satsfy (18) s fnte, then the unauthorzed user can use c ( :1 J 1 s more brute-force search to determne whch sequence among { ( } u b and { } meanngful by recoverng them to message sequences. Therefore, we need to create sutably tme-varyng channels n order to make the brute-force search computatonally prohbtve. Tme-varyng channels can be ntentonally created by movng randomly transmttng antennas, or by choosng dfferent antenna subsets from a large array. Consderng the requrement of channel estmaton, channel tme-varyng rate should be slower than symbol rate. Each channel realzaton s used to transmt a short block of symbols wth a sutable F. As long as the determnaton of { b ( } requres a suffcently large number of blocks, computatonal secrecy can be acheved. 9

15 For example, f the symbols are suffcently nterleaved and transmtted n K blocks, the K 18 complexty of breakng secrecy s J. For J = 4 transmtters, K = 64 blocks gves a complexty. In addton, n practce, due to nose and the short block length, the unauthorzed user may not have suffcent statstc measures for determnng even h or f. ence computatonal secrecy can be guaranteed wth a moderate number of symbol blocks Perfect secrecy Accordng to the perfect secrecy defned by Shannon [9], f the unauthorzed user gets no nformaton on b ( from the receved sgnals { x u( } then perfect secrecy s guaranteed. One of the ways to show perfect secrecy s that gven the receved sgnals { x u( }, the probablty of detectng a symbol b (,.e., { x u( }], s ndependent of b (. Proposton 3. Assume that the unauthorzed user knows ts channel u but not h, and has noseless receved sgnals { x u( }. Then { x u( }] can be made ndependent of b ( f h s..d. for each symbol and the symbols have constant magntude,.e., b ( = 1. If the channel h s constant or slowly tme-varyng, or f b ( s not constant, then { x u( }] may not be ndependent of b ( snce the unauthorzed user can explot ts knowledge of (6). Proof. Snce w ( s randomly and ndependently generated n each symbol nterval, f the channel h s..d. for each symbol, then w ( s ndependent from x u (m) for any m n. The same concluson holds for b (. Therefore, { x u( }] s equvalent to x u( ]. We have ] ] x u ( ] = xu ( ] = w( ]. (1) xu ( ] xu ( ] 1 w( The pdf of w ( gven b ( s f w ( ), where f w( ) denotes the jont pdf of w (. Because the channel coeffcents n h are jontly Gaussan wth zero mean, the pdf of h s phase jθ symmetrc (or phase nvarant),.e., the probablty of h e s the same as that of h for any θ [3]. Because w ( s obtaned from h, f w ( w( ) can also be phase symmetrc. Ths can be seen from the fact jθ jθ jθ that [ h e ] [ w( e ] = he. Ths equaton tells us that f there s a w ( obtaned from h wth certan jθ jθ probablty, then for any phase θ, w (e can be obtaned from h e wth the same probablty. Note jθ that dfferent h and h e do not share the same w (. Therefore, f b ( = 1, then w ( / and w ( have dentcal probablty, whch means that f w ( w( ) = f w( w( ). ence x u ( ] = w( ] ]/ xu ( ]. Snce P [ ] s P x u ( s ndependent of b (. owever, f the channel h s not..d. for each symbol, or f b ( are not constant, then constant, [ ] 1 w( f w( ) f w( w( ) n general. Some nformaton about b ( may be avalable gven { x u ( }. From Proposton 3, a necessary condton for perfect secrecy s that all symbols should have dentcal magntude, otherwse the dfferent power nformaton may be exploted. Such a concluson s smlar to that n [3], although the latter s obtaned under that assumpton that the unauthorzed user has no nformaton of the channel u, nor can t estmate u. Whle t s easy to realze b ( = 1, a more challengng task for realzng perfect secrecy n practce s to make the channel h random. The dffculty comes from the channel estmaton requrement at ether the transmtters or the authorzed user. On the other hand, snce t does not matter whether the 10

16 unauthorzed user knows ts channel u or not, tranng methods can be used for channel estmaton wth reduced complexty. A possble way for mplementng transmssons wth perfect secrecy s to ntentonally create channel varaton by movng antennas randomly, or by selectng randomly subsets of a large antenna arrays. The latter case stll requres tme-varyng channels, although the varaton rate can be slow. Wth each new channel realzaton, a tranng sequence can be transmtted for channel estmaton. After the transmtters know the channels from feedback, a symbol s transmtted wth a randomzed w (. The ntalzaton based on channel recprocty s stll requred. On the other hand, channel recprocty, f avalable durng normal data transmsson, can be exploted to remove feedback and thus enhance data rate Secure transmsson n dspersve channels As shown n Secton III.3.1 and [31], there are three possble channel models for cooperatve transmssons: synchronous flat-fadng channel model, synchronous dspersve channel model, and asynchronous dspersve channel model. The secure transmsson algorthms n Secton I.3 and I.4 can be extended to the dspersve channel models. To save space, detals of such extenson are not ncluded, but can be found n [31] Smulatons In ths secton, we show the performance of the proposed Algorthm 1 of Secton I.3 and Algorthm of Secton I.4. We use bt-error-rate (BER) to compare the recevng performance of the authorzed user and the unauthorzed user. We also examne the transmsson power of these two algorthms. For comparson purpose, we evaluate the performance of the optmal transmt beamformng [16] dscussed n Secton I.3., and gve the theoretcal BER curve of the Raylegh fadng channel wthout dversty [1]. For the unauthorzed user, blnd equalzers [18] are smulated. We frst study the performance of the Algorthm 1. Channels are assumed block Raylegh fadng,.e., they are constant durng transmsson of one packet, but randomly changng between packets. Each packet contans 00 QPSK symbols. We use 5000 runs to obtan each BER value. For Algorthm 1, we use α = 0. 5, σ = If there are less than two selectable channel coeffcents under (1), then we smply select h between the two strongest ones n order to make P n (10) random. Both flat-fadng channels and dspersve channels are smulated. For the dspersve channels, we use channel length L =. The smulaton results are shown n Fg. 4(a). Transmssons wth Algorthm 1 have smlar performance as the optmal transmt beamformng. The unauthorzed user can not ntercept symbols usng the blnd equalzaton wth 8 recevng antennas and suffcently good channels Bt Error Rate Adversary Lmt Fad Alg1 Dsps Beamform Alg1 Flat SNR E b /N 0 (db) Bt Error Rate Adversary Lmt Fad Alg Dsps Beamform Alg Flat SNR E b /N 0 (db) (a) (b) Fg. 4. Recevng performance comparson. (a) For Algorthm 1. (b) For Algorthm. J = 4. o :Algorthms 1 or wth flat-fadng channels. :Algorthms 1 or wth dspersve channels. + :transmt beamformng. :theoretcal BER curve wth Raylegh fadng channel. :blnd detector of unauthorzed user. 11

17 Total Transmsson Power Alg1 Alg Beamform Number of Transmtters J Power of One Transmtter Alg1 Alg Beamform Number of Transmtters J (a) (b) Fg. 5. Transmsson power and standard devaton. Standard devaton s shown by above the power value. (a) Total transmsson power. (b) Power of a sngle transmtter. :Algorthm 1. o :Algorthm. :transmt beamformng. Then we study the performance of Algorthm wth the smlar smulaton parameters. For Algorthm, we let the transmtters to fnd the best w ( from J dfferent F matrces n order to reduce transmsson power and to avod ll-condtoned matrces. The results are shown n Fg. 4(b), from whch the concluson smlar to Algorthm 1 can be drawn. One of the major dfferences between Algorthm 1 and Algorthm s ther transmsson power, whch s compared n Fg. 5(a) and (b) Realzng Physcal-layer Secured WLAN The man purpose of ths research topc s to realze the physcal-layer securty technques n WLAN wthout a complete overhaul of exstng physcal-layer hardware. Frst, as can be seen, physcal antenna arrays not only ncrease system cost but also requre new hardware desgn (because multple parallel sgnal processors are requred n the same board. The concept of cooperatve transmssons may be more advantageous for cost reducton and for explotng exstng redundant (but separate) hardware. For example, multple access ponts, or multple WLAN cards, each of whch may have only a sngle antenna, can be used to transmt/receve the same data packet n a collaboratve manner. Therefore, our objectve s to use multple access ponts (AP) to jontly transmt a packet to the authorzed user (clent), whle at the same tme to make the recepton at other unauthorzed users mpossble. Besdes showng the dea of secure wreless networks, such a demonstratve testbed can also be used to verfy the practcablty of cooperatve communcatons. Cooperatve communcatons are a new area wth many challenges nvolved n the applcaton and mplementaton such as synchronzaton and collaboraton. A testbed, especally f constructed usng WLAN COTS devces, wll be an effectve way to show the feasblty of cooperatve communcatons and the potental of cooperatve communcatons as a way to enhance the performance and functon of ether exstng or future systems. In the followng, we propose two ways for constructng such a testbed: channel or tme based approaches. 1

18 Channel-based approach The channel-based approach depends on the theores descrbed n Sectons I.3 and I.4,.e., Algorthm 1 and, where the dfference between the propagaton channels of the authorzed user and the unauthorzed user s exploted. The random ntersymbol nterference (ISI) created ntentonally by the randomzaton procedure may stop most of the WLAN recevers from workng, n partcular those currently on market. Snce current WLAN recevers do not have equalzers (because of the flat fadng channel models used n ndoor WLAN envronment), even a trvally ntroduced ISI may acheve certan degree of securty. The major problems are relatve to channel estmaton and the synchronzaton among the cooperatng APs. For the channel estmaton, we may depend on the feedback from the authorzed recever. Ths can be realzed f the authorzed recever knows the channel. Another way s to ask the authorzed recever to feedback some receved samples drectly, from whch the APs can estmate the channels. In order to acheve ths objectve, one way s to reprogram the frmware of the authorzed user to ask hm to transmt the receved samples. For the APs, a channel estmaton algorthm needs to be mplemented. Ths can be realzed by programmng nstead of new hardware desgn. For synchronzaton, smlarly frmware needs to be reprogrammed so that we can ask the physcal-layer to mantan synchronzaton clock. The synchronzaton can not be done n the MAC or above layer only snce the clock accuracy of these layers are n the unts of mcrosecond, not accurate enough for transmsson. Another problem that we have skpped s whether the carrer frequency f c s dentcal among all APs. owever, ths may not a bg ssue n WLAN snce the carrer frequency drftng s at most 5 ppm, whch s suffcently small Tmng-based approach Compared wth the channel-based approach, the tmng-based approach may be more feasble. By tmng-based approach, we adjust the transmsson delays nstead of the transmsson weghts of the APs, as shown n Fg.. Ths s somewhat smlar to wreless locaton usng tme-of-arrval (TOA). The most promsng aspect s that the energy-of-arrval and thus the RSSI value n WLAN may be drectly used for dervng the tmng nformaton. As llustrated n Fg., the APs can purposely adjust the delay of ther transmsson tme nstant (.e., the tme nstant that they begn transmsso. Though the APs need to know all delays, such delays can n fact be obtaned from ther receved sgnals from the desred user, especally through the RSSI nformaton. Wth such nformaton, the APs can adjust ther delay. The effectve of ths approach depends on the symbol nterval T. In 80.11b, T s 1 / 11 mcro-seconds, whch gves suffcent adjustment range for the delays. The lkelhood that such a delay dfference among the desred user and the undesred user s large depends on the dstance between the desred user and the undesred user. Ths s smlar to the accuracy of the wreless locaton problem. As long as dstance between the two users are larger than 1/(T ), then such a lkelhood s hgh. The potental of the approach s that we do not have to change anythng n the authorzed user. We need only to reprogram the APs. owever, the freware of APs s stll subject to change because the transmsson delay needs to be synchronzed A smple testbed for demonstratng the concepts The major challenge for the above two methods s the synchronzaton n transmsson tmng, whch requres sophstcated reprogrammng work n the frmware. We need to study the frmware programmng of some real mplementatons. The programmng work may be tme-consumng, especally snce such programmng needs to be compatble wth the entre networkng. 13

19 owever, we have a much qucker way to setup a demonstratve testbed. Instead of workng on the real WLAN network, we work on separate transmtters and recevers wthout consderng the entre network. For example, we can use the standard transcever blocks (see Comblock.com) to buld the cooperatve transmtters, and mplement the secure transmsson algorthms n general purpose PCs. Ths way, we can sample and analyze the sgnals to obtan certan performance benchmark. Part Applcaton of STBC-encoded Cooperatve Transmssons n Wreless Sensor Networks.1. Introducton In wreless sensor networks, energy effcency s a domnatng desgn crteron. Transmsson energy effcency s especally mportant because wreless transcevers usually consume a major porton of battery energy. Transmsson energy effcency can be enhanced by dversty technques wth antenna arrays, among whch space-tme block codes (STBC) are attractve because of ther lnear complexty [4]. For moble users wthout antenna arrays, STBC wth cooperatve transmsson schemes have been proposed [5]-[7]. owever, the requrement of extreme energy effcency n wreless sensor networks makes the applcaton of cooperatve transmsson questonable. Frst, when sensors schedule jont transmssons, the overhead of cooperaton ncurs extra energy consumpton. Second, t s not an easy task to synchronze cooperatng transmtters n terms of carrer frequency, carrer phase, symbol tmng (symbol rate) and tmng phase (samplng tme nstant). Wthout perfect synchronzaton, STBC-encoded transmsson becomes more complex, sometmes even not applcable [7], [8]. Fnally, although cooperatve dversty enhances transmsson energy effcency, the nvolvement of more than one transmttng sensor ncreases electronc energy consumpton [9]. So far, cooperatve transmsson has been studed mostly under the assumpton of perfect synchronzaton. The overhead, synchronzaton, complexty and energy effcency are to be justfed. To address ths task, wthout loss of generalty we consder a typcal networkng/communcaton protocol for wreless sensor networks,.e., low-energy adaptve clusterng herarchy (LEAC) [30]. We propose ways to ncorporate cooperatve transmsson n LEAC and study the assocated overhead, synchronzaton and energy effcency... LEAC wth cooperatve transmsson We consder a wreless sensor network where sensors need to transmt ther data to a remote data collector. LEAC s an nterestng networkng/communcaton protocol for sensors to form herarchcal clusters and to schedule TDMA channel access. The operaton of LEAC s broken up nto rounds, and each round conssts of four phases: advertsement, cluster setup, transmsson schedulng, and data transmsson. Advertsement. In ths phase, each sensor determnes by tself whether t becomes a cluster head durng ths round. Each self-selected cluster head then broadcasts an advertsement message. We do not need to make changes n ths phase for cooperatve transmsson, though we rename the cluster head as prmary head. Cluster setup. In ths phase, each sensor transmts a cluster-jonng packet to ts desrable prmary head. For J-sensor cooperatve transmsson, besdes the prmary head, we need to choose J-1 secondary heads n each cluster. In our scheme, they wll be selected by the prmary head n the next phase. Meanwhle, when a sensor transmts cluster-jonng packet, t should pggyback nformaton about ts capablty of beng a secondary head, e.g., ts current energy status. The overhead of ths procedure can be as small as just transmttng one extra byte along wth the relatvely long cluster-jonng packet. Schedule creaton. Ths phase s for each prmary head to create TDMA channel access schedule, and to nform each sensor the assgned slot. For cooperatve transmsson, each prmary head frst selects 14

20 the secondary heads based on both the reported energy status and the receved sgnal power. The power can be used as an estmaton of the sensor dstance. Then the prmary head nforms the selected secondary heads about ther roles n cooperatve transmsson, whch can be mplemented by pggybackng one extra byte n the orgnal schedulng packet. The overhead ncludes the selecton of secondary heads n the prmary head, and one byte more transmsson to each of the J 1 secondary heads. Such overhead s stll neglgbly small. Data transmsson. In ths phase, each cluster head receves data packets from the other sensors n the cluster, fuses these packets, and transmt the fuson result to the data collector. In cooperatve transmsson mode, t s stll the prmary head that receves and fuses data packets. owever, after that, the prmary head frst broadcasts the fused data to the secondary heads, and all J heads then transmt the data to the data collector cooperatvely n the followng slot. Ths procedure s llustrated n Fg. 6(a). The overhead n ths phase, whch s the major one for the proposed scheme, ncludes the broadcastng procedure and the added electronc energy consumpton. The mpact of such overhead on energy effcency wll be analyzed n Secton II Synchronzaton among cooperatng sensors.3.1. Synchronzaton and channel models Before cooperatve transmsson, the secondary heads can synchronze ther carrer frequency and symbol tmng to ther receved sgnals when the prmary head broadcasts the fused data. The remanng ssue s then relatve to carrer phase and tmng phase synchronzaton. We have to omt the transmsson delays from the prmary head to the secondary heads snce they are dffcult to estmate and compensate. Therefore, f the maxmum dstance between the prmary head and the secondary heads s d max, then the begnnng tme of cooperatve transmsson at the prmary head s up to d max / c earler than the secondary heads, where c s the speed of lght. Among the sgnals transmtted by the cooperatng sensors, the maxmum (worst case) relatve delay s d max / c when they arrve at the data collector. These delays cause synchronzaton error n both carrer phase and tmng phase. Let the passband sgnal from a head sensor be ( ) Re[ j πf ct s t = ρ b ( l) ( l ) ] l= p t T e where Re[.] stands for real part, ρ s a transmsson power adjustor, b (l) s the complex symbol at symbol nterval [ l T, ( l + 1) T ) s the baseband pulse shapng flter, and f c s the carrer frequency. The receved sgnal at the data collector s then J j(πf c t θ ) x p ( x) = Re[ ρ ab( l) p( t lt τ ) e + v p ( t)], () = 1 l= where a and θ are gan and phase of the propagaton channel, and τ s the delay. We use v p (t) to denote passband nose. Flat fadng propagaton s assumed, and wth same ρ, the transmsson power s evenly dstrbuted among cooperatng head sensors. Because sgnals from head sensors have dfferent θ and τ, t s mpossble to acheve synchronzaton n carrer phase and tmng phase. Therefore, wthout loss of generalty, we demodulate jπf ct () wth local carrer e and then perform samplng at tme nstants t n = nt + τ (for arbtrary τ ). The baseband samples x( = xb ( nt + τ ) are x( = ρ J j ae [ p( τ τ ) b ( + = 1 l n θ p(( n l) T + τ τ ) b ( l)] + v(, (3) where v ( s baseband nose. Obvously, resdual nter-symbol nterference (ISI) s nevtable. In flat fadng envronment, we would prefer that sngle-tap channel model stll be used n cooperatve 15