Optimised Wireless Network Using Smart Mobile Terminal Antenna (SMTA) System

Transcription

1 Optimised Wieless Netwok Using Smat Mobile Teminal Antenna (SMTA) System Autho Lu, Junwei, Sciven, Ian, Wate, Wayne Published 2010 Confeence Title Poceedings of IEEE APEMC2010 DOI Copyight Statement 2010 IEEE. Pesonal use of this mateial is pemitted. Howeve, pemission to epint/ epublish this mateial fo advetising o pomotional puposes o fo ceating new collective woks fo esale o edistibution to seves o lists, o to euse any copyighted component of this wok in othe woks must be obtained fom the IEEE. Downloaded fom Giffith Reseach Online

2 2010 Asia-Pacific Intenational Symposium on Electomagnetic Compatibility, Apil 12-16, 2010, Beijing, China Optimised Wieless Netwok Using Smat Mobile Teminal Antenna (SMTA) System Junwei Lu #1, Ian Sciven #2, Wayne Wate #3 # Cente of Wieless Monitoing and Applications School of Engineeing, Giffith Univesity, Qld, Austalia 1 j.lu@giffith.edu.au 3 Ian.Sciven@Giffith.edu.au 2 Wayne.Wate@Giffith.edu.au Abstact This pape pesents a complete novel smat mobile teminal antenna system (SMTA) fo optimising wieless netwok and secuity pefomance. This SMTA system uses a switched adiation beam appoach in an attempt to achieve optimal wieless netwok pefomance while inceasing netwok secuity fo indoo and outdoo applications, and signal tacking fo maine application. Test esults ae pesented that demonstate the ability of the SMTA system to maintain consistently high wieless signal levels in a dynamic and noisy envionment. I. INTRODUCTION Smat antennas ae becoming inceasingly popula fo mobile computing in WALN because they can give longe battey life, moe eliable connectivity, high signal-to-noise atio and bette secuity than conventional omni-diectional antennas. Unlike taditional omni-diectional antennas, whee adiation and infomation is tansmitted equally in all diections, smat antennas ae able to diect the majoity of the tansmission towads the intended eceive which is paticulaly impotant fo wieless secuity issues and maine application. Likewise, they ae able to alte thei diectional sensitivity in such a way that noise eception can be geatly educed. Lowe antenna gain o lowe tansmission powe can then be used, loweing the powe equiements of the system, and inceased noise esistance leads to moe eliable wieless netwok connections. Heavily utilized fequency spectum can be moe easily eused thanks to the spatial divesity intoduced though the use of such smat antennas. Pehaps most advantageous howeve is the inceased secuity that smat antennas can povide. One of the key secuity weaknesses of wieless netwoks is the possibility of eavesdoppes sniffing packets sent and eceived ove the wieless netwok. By diecting the adiation towads the intended taget and limiting tansmission in othe diections, it becomes much hade fo eavesdoppes to obtain the eceived signal stength equied to accuately intecept packets. II. WIRELESS NETWORK PERFORMANCE AND SECURITY The IEEE a/b/g/n potocol fo wieless netwoks is a obust potocol, and is able to scale connection speeds depending on the connection quality by alteing the modulation scheme [1]. In Fig. 1, the connections between the legitimate uses of the netwok will be made at a high speed. While the eavesdoppe will be able to eceive some of the tansmission, thei eceived signal powe will be much lowe, intoducing a lage numbe of eos in demodulation, geatly educing the eavesdoppes ability to accuately cack into the netwok o steal sensitive infomation. On the othe hand, the wieless netwok with SMTA can incease signal stength and impove the connectivity with the wieless netwok installed on the coast. (c ) Fig. 1. SMTA used in wieless communication and computing system, SMTA fo Desktop o notebook computes in BSS mode, SMTA fo wieless netwok in Ad-Hoc mode, (c) SMTA fo maine application. III. IMPLEMENTATION OF SMART MOBILE TERMINAL ANTENNA A. System Configuation of SMTA The smat mobile teminal antenna (SMTA) is a paasitic monopole aay consisting of a single, cental diven element suounded by six passive elements [2]. Figue 2 shows a concept diagam of the beam foming system whee the cental element is connected to RF souce and six paasitic elements ae contolled by RF switches and beam foming algoithm. The Radiation pattes at 60 intevals in azimuth plane fo six diections ae illustated by Figue 2. A /10/$ IEEE 98

3 Pototype SMTA system fo indoo and outdoo o maine applications is pesented in Figue 3 and espectively, whee Figue 3 is designed fo maine application. Fig. 4. Antenna elements ae embedded in the high dielectic medium. Fig. 2. concept diagam of the beam foming system, Radiation pattes at 60 intevals in azimuth plane fo six diections. Fig. 3. Pototype SMTA system, indoo application and outdoo o maine application B. SMTA Design Consideation In mobile teminals the size of antenna is an impotant issue as potability is a key equiement. Phased aay antennas equie an inte-element spacing of geate than 0.5λ 0. In wie SMTA, the sepaation distance can be educed to 0.25λ 0. Despite this, antennas designed fo the IEEE standad at 2.4 GHz ae still consideed too lage to be integated into the compute and potable devices. If the antenna elements ae embedded in the high dielectic medium (non-magnetic mateial) as shown in Figue 4, the wavelength will become λ = λ0 / ε and the eduction facto F fo the height of antenna elements will be expessed as F = ε. Futhe size eduction based on optimised design techniques was achieved by embedding an aay of wie monopoles in a dielectic mateial [3]. 1) The dielectically embedded SMTA (DE-SMTA): The wie SMTA antennas designed to opeate at the 2.4GHz fequency ange equied fo a/b/g wieless compute netwoks ae geneally too lage to be pactical. Lu et al ecently developed a novel new SMTA design, the dielectically embedded SMTA (DE-SMTA) [4-5]. By embedding the antenna dipole elements in a ceamic mateial with a elative pemittivity of 4.5, Lu et al wee able to educe the volume of the antenna by 80% and its footpint by 50%. The DE-SMTA is shown in Figue 4. As with the SMTA, the diectivity of the DE-SMTA is detemined by the impedances of the six passive antenna elements. As stated peviously, DE-SMTA adiation diectionality is achieved by setting the impedances of the passive antenna elements. A wide ange of impedances can be set though the use of vaactos (voltage contolled vaiable capacitos) howeve these equie a wide ange of contol voltages (up to twenty volts) in ode to obtain a full ange of impedances [2]. A simple and less expensive method is to contol adio fequency switch connections between the passive antenna elements and the gound plane [6]. Using this set up, it is possible to obtain a numbe of useful adiation pattens with a bette diectivity and gain. Simulations have shown that the best adiation patten (the best compomise between gain and adiation beamwidth) was achieved by connecting thee adjacent passive elements to gound pefomed as eflectos, while leaving the opposite thee elements disconnected (that is, open cicuited) pefomed as diectos. This configuation can be eplicated utilising the symmetical natue of the DE- SMTA to povide six possible adiation beams with 3dB beamwidths of appoximately 80 degees to completely cove the 360 degee azimuthal plane with minimal ovelapping, as can be seen in Figue 2. 2) A beam switching system: A beam switching system has been developed to select the most suitable beam fom this goup of six possibilities afte pefoming a scan whee the pefomance of each beam is measued. This method has a geat advantage in that it is emakably simple, esulting in vey low powe equiements, in tems of both computational powe and electical powe. The beam switching method is not without its disadvantages, howeve. As the beam positions ae fixed, unifom gain cannot be maintained exactly in all diections. The vaiation in gain can be educed by adding moe beam positions, howeve this would esult in a linea incease in the amount of time equied to scan all beams. It would also make it necessay to incease the fequency of scanning and beam switching opeations in ode to maintain the optimum beam position. 3) The DE-SMTA contol system softwae: The softwae fo this contol system utilises Micosoft s Netwok Dive Inteface Specification (NDIS, cuently vesion 5.1). NDIS is an Application Pogamming Inteface (API) fo netwok inteface cads unde Micosoft Windows. Vesion 5.1 in Windows XP suppots wieless netwok cads, and povides access to a wide ange of infomation and statistics on wieless netwok connectivity unde Windows. Windows Management Instumentation (WMI) povides an easy to use 99

4 inteface though with applications can obtain a wide ange of system management infomation, including NDIS statistics [7]. The DE-SMTA contol system softwae uses WMI to access NDIS wieless infomation and obtain details of all available wieless netwoks, access points and the eceived signal level of each access point, allowing the use to specify which wieless netwok o access point they connect to and tack. C. The Chaacteistics of DE-SMTA The complex impedances of the six passive elements detemine the diectionality of the DE-SMTA. The DE-SMTA antenna functions on the pinciple of mutual coupling, a fundamental pinciple in multi-element antenna systems descibing the intechange of electomagnetic adiation between the adiating elements of the antenna aay. Figue 5 shows measued esults of adiation patten fo H field and E field with the gain of 4.5dBi and eflection coefficient is at -13.4dB. The simulation esult of eflection coefficient compaed with measued esults fo an optimized dielectic embedded DE-SMTA ae shown in Figue 6, whee the esonant fequency moved to 2.5GHz and second esonant fequency appeaed at 2.7GHz due to the effect of coupling between active element and PCB gound plane. SMTA TABLE I FONT SIZES FOR PAPERS Reflection coefficients (S11) db Testing esults of Antenna Gain (max.)(dbi) Fequency Bandwidth at -10dB DE SMTA dbi 320MHz Wie SMTA dbi 150 MHz IV. TESTING RESULTS OF SMTA An extenal application, Netwok Stumble [8] (o moe commonly NetStumble) was used in duing testing to povide egula, accuate eceived wieless signal stength measuements. Two tests wee pefomed, both involving moving a laptop using the smat antenna system aound in an envionment containing a wieless access point. In the fist test, the smat antenna contol application was unning and actively tacking the wieless access point, pefoming scans at ten second intevals. The esults, taken fom NetStumble (and showing eceived signal stength on the Y axis and time on the X axis), can be seen in Figue 7, and show that the system does a good job of maintaining the best possible connection when tacking a wieless signal. In some cases small (appoximately 5 dbm) dips in signal stength can be obseved when scanning is taking place, o when the laptop has moved a significant distance befoe the system has escanned, howeve these dops ae small in both magnitude and duation, and as such pose no poblem to connection integity. Fig. 5 H-Field (theta = 90) pola (Gain ~4dBi), S 11 = -13.4dB and E Field (phi = 0), Gain ~4.5dBi Fig. 6 Measued eflection coefficient S 11 fo a 7-Element Dielectic Embedded SMTA, measued (solid line); simulated (dot line). Table I pesents the compaison of expeiment esults of SMTAs at the esonant fequency of 2.5GHz. The pototype DE-SMTA has a bette fequency bandwidth aound 380MHz while the wie SMTA only achieved 150 MHz. Fig. 7 Measued eceived signal stength, contol application actively tacking. A second contol test was pefomed to povide a compaison fo the above esults. In this test, a fixed beam position was maintained, while the laptop was moved aound in the same manne as befoe. The esults fom NetStumble, as shown in Figue 8, show that this time thee wee significant dops in eceived signal stength, in some cases appoaching 20 dbm. Obviously, aeas with highe eceived signal stength occu when the position of the laptop in the testing space esults in the fixed beam position being close to 100

5 the optimal position, while the lowe egions occu when the fixed beam position is close to opposite of the optimal diection. While in the majoity of cases these changes did not esult in the connection being dopped, polonged dops in signal stength cause the wieless system to change modulation schemes and scale down the connection speed. [4] J. Lu, D. Ieland, and R. Schlub, Dielectic embedded ESPAR (DE- ESPAR) antenna aay fo wieless communications, IEEE Tansactions on Antennas and Popagation, vol. 53, no. 8, pp , Aug [5] Junwei Lu, David Ieland and Andew Lewis, Multi-objective Optimization in High Fequency Electomagnetics an Effective Technique fo Smat Mobile Antenna IEEE Tansaction On Magnetics, Vol. 45, No. 3, Mach 2009, pp [6] R. Schlub, J. Lu and T. Ohia, Seven element gound skit monopole ESPAR antenna design fom a genetic algoithm and the finite element method, IEEE Tansactions on Antennas and Popagation, vol. 51, no. 11, pp , Nov [7] Micosoft, "WMI: Intoduction to Windows Management Instumentation," 23/05/06; [8] M. Milne, "stumble dot net," Mach; Fig. 8 Measued eceived signal stength with a fixed beam position. V. CONCLUSIONS The smat mobile teminal antenna system pesented in this pape has been shown to have consideable benefits in egads to wieless netwok pefomance and secuity. By using a steeable diectional antenna it is possible to maintain a consistently high wieless connection stength in a noisy ealwold envionment, even whilst moving. By diecting wieless tansmissions towads thei intended ecipient instead of boadcasting them (and stongly) in all diections. Testing esults have been pesented which demonstate the effectiveness of the DE-SMTA system in this egad. The DE-SMTA system also allows fo inceased noise immunity, as pe Loentz ecipocity, the smat mobile teminal antenna s diectivity is the same fo eceiving as it is fo tansmitting. This means that the effect of elatively poweful (and common) noise signals in the 2.4 GHz band can be minimized poviding the noise souce is not in the same diection as the taget of communications. Finally, any potential eavesdoppes who ae not located diectly between the DE-SMTA tansmitte/eceive and its countepat (whethe anothe mobile compute o a wieless access point) will have poblems demodulating and tansmissions due to thei lowe eceived signal stength. REFERENCES [1] Infomation technology-- Telecommunications and infomation exchange between systems-- Local and metopolitan aea netwoks-- Specific equiements-- Pat11: Wieless LAN Medium Access Contol (MAC) and Physical Laye (PHY) Specifications (1999) [2] K.Gyoda and T. Ohia, Design of Electonically Steeable Passive Aay Radiato (ESPAR) Antennas, IEEE AP-S Intenational Symposium, 69P.7, pp , Salt Lake City, July [3] J. Lu, D. Thiel and S. Saaio, FD-TD Analysis of Dielectic Embedded Electonically Switched Multiple-Beam Antenna Aay, IEEE Tans. on Magnetics, Vol. 38, No. 2, Mach 2002, pp