ENERGY EFFICIENT MILLIMETER WAVE RADIO LINK ESTABLISHMENT WITH SMART ARRAY ANTENNAS

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1 ENERGY EFFICIENT MILLIMETER WVE RDIO LINK ESTLISHMENT WITH SMRT RRY NTENNS ehnam Neekzad, John S. aras Insttute for Systems Research and Electrcal and Computer Engneerng Department Unversty of Maryland College Park College Park, MD, 074 Kamran Sayrafan-Pour dvance Network Technology Dvson Natonal Insttute of Standard and Technology Gathersburg, MD, 0899 STRCT Consder a system of two mllmeter wave transcever nodes and. We assume that each node s equpped wth a crcular array antenna that has beamformng capablty. We are nterested n usng the beamformng capablty to fnd the best possble drectons for transmsson and recepton such that the communcaton lnk between two nodes provdes maxmal SNR at the recever. protocol that n most cases acheves the optmal soluton s developed and smulaton results are provded to show the effectveness of the algorthm. 1. INTRODUCTION Mllmeter wave technology s becomng ncreasngly mportant n many mltary and commercal applcatons. Remote sensng, rado astronomy, plasma dagnoss, passve magng (e.g. magng to dsplay hdden contraband, weapons and nonmetal obects), radar and ggabt data rate communcaton are among these applcatons. To establsh a hgh data rate communcaton lnk between two moble nodes, acceptable Sgnal to Nose Rato (SNR) at the recever s requred. However, due to the hgh propagaton loss and sgnal attenuaton that s assocated wth the mllmeter wave sgnal, the communcaton range wll be extremely lmted. rray antennas are an effectve tool to manage the spatal transmsson (or recepton) of sgnals between two nodes. Usng such array antennas, one would be able to ncrease the transmsson range of mllmeter wave systems (Huang et al., 006). n mportant property of array antennas for mllmeter wave transcevers s ts small physcal sze. For example, at 60 GHz carrer frequency, an 8-element cylndrcal array antenna requres a radus of only 3.3 mm (Ramanathan, et al., 001). Ths property makes array antennas an attractve practcal soluton for establshng hgh range and relable communcaton lnks between nodes n a network. Utlzng approprate sgnal processng technques (such as beamformng), an antenna array wll enable a transcever to pont ts man lobe toward a desred drecton. y focusng the radaton (or recepton) pattern of an array antenna, hgher communcaton range can be acheved. lternatvely, for a gven communcaton range, lower power consumpton can be expected. lso, chances of eavesdroppng are sgnfcantly reduced (low probablty of ntercept) snce nformaton s not broadcast n all unnecessary drectons. The collecton of all these benefts, ponts to usng smart array antennas as a tool to control the spatal transmsson and recepton of sgnals between nodes. In ths paper, we are nterested n usng ths beamformng capablty to fnd the best possble drectons for transmsson and recepton such that the communcaton lnk between two nodes provdes maxmal SNR at the recever. To acheve ths, an ntellgent algorthm s requred n order to choose the best (.e. optmal) drectons for the recever-transmtter par. For example, n urban or ndoor envronments where the rado lne of sght s not necessarly the best propagaton path between the two communcatng nodes, ths ntellgent algorthm has to be able to smultaneously fnd the optmal drectons for transmsson and recepton of the nformaton. Smlar technques have been used for ndoor moble postonng and source drecton estmaton (Sayrafan et al., 006a, 006b). Knowledge of the characterstcs of the propagaton channel could also be exploted n order to select the best strategy for steerng the beam pattern of the array antennas at the recever and transmtter nodes (Sayrafan et al., 006). Problem descrpton and formulaton wll be descrbed n secton. In secton 3, we wll outlne an algorthm that fnds a soluton for the stated problem. Smulaton results are provded n secton 4. Fnally, 1

2 conclusons and future work wll be dscussed n Secton 5.. PROLEM DESCRIPTION ND FORMULTION Consder a system of two transcever nodes and. We assume that each node s equpped wth a crcular array antenna that has beamformng capablty. In other words, t s able to electroncally steer ts man lobe n the two-dmensonal space (.e. azmuth) toward any desred drecton (see Fg. 1). In addton, assume that each antenna s also capable of generatng an omn-drectonal gan pattern as well. The Receved Sgnal Strength (RSS) s clearly a functon of the drectons where the man lobes of both antennas are pontng at. The drectons that maxmze the RSS are functons of both antenna patterns and the channel response (.e. the envronment). For envronments wth severe multpath propagaton, t s mportant to locate these optmal drectons n order to maxmze the sgnal to nose rato at the recever. Ths s especally nterestng n scenaros where the lne of sght path has been blocked by an obect wth large attenuaton. Sde lobe α Man lobe Fg 1: eam steerng n array antennas ssume that x ( θ, α) denotes the transmtter s antenna gan at angle θ when the man lobe s pontng at drectonα. Smlarly, y ( φ, ) denotes the recever s antenna gan at angle φ when the man lobe s pontng at drecton. If h( θ, φ ) s the angular channel response for a path that starts from angle θ of the transmtter s antenna and ends at angle φ of the recever s antenna, then the receved sgnal R can be expressed by: = R( α, ) x( θ, α) h( θ, φ) y( φ, ) dθ dφ θφ, Here, for smplcty, we are also assumng that the propagaton process s statonary. Our obectve n ths paper s to fnd the optmal drectons ( α, ) that maxmze the receved power as defned below: ( α, ) = rg maxα, R( α, ) smple method to fnd the soluton of ths problem s to conduct an exhaustve search over the space of all possble ( α, ). However, dependng on the resoluton of ths space, ths methodology mght not be feasble for practcal purposes. In ths paper, we propose a two step approach to fnd a sub-optmal value for the par ( α, ). In the next sectons, we wll outlne our approach and nvestgate ts performance wth smulatons for dfferent scenaros. 3. POROTOCOL DESCRIPTION Step 1: Node s n the transmt mode wth an omndrectonal gan pattern. Ths means that deally, the transmtter s antenna gan for all azmuth angels s c where c s a constant. t the same tme, node s n the receve mode wth beamformng capablty. Intally, the man lobe of the recever s antenna pattern s pontng toward a startng angle e.g. 0. Now, the beamformer rotates the man lobe around the 360 degrees feld of vew wth a step sze of. (.e. = 0 + ). t each step, the recever calculates the receved sgnal strength and fnds the drecton where the RSS s maxmal. If denotes ths angle, mathematcally ths step can be expressed by the followng equatons. R( ) π C x( θ) h( θ, φ) y( φ, ) dθdφ = Step : Now that φθ = rg max R( ) has been obtaned, node goes nto transmt mode and notfes node that t has found ts desred drecton. Then, node turns nto a recever wth beamformng capablty and the man lobe toward the startng angle α 0. Next, the beamformer at node rotates the man lobe around the 360 degrees feld of vew wth a step sze of (.e. α = α 0 + ). t each step, the recever at node calculates the receved sgnal strength and fnds the drecton α where the RSS s maxmal. If α denotes ths angle, then mathematcally ths step can be expressed by the followng equatons.

3 = R α x θα h θφ y φ dθdφ ( ) (, ) (, ) (, ) θφ, α = rg max R( α ) If the beamwdths of the man lobes created by the array antennas at Nodes and are γ and γ respectvely, then the rotaton step sze n the above protocol should obey the followng equatons α Sde lobe α Man lobe γ γ Fg. : Smulaton layout The beamwdths γ and γ depend on the sze (.e. number of elements) of the array antennas at the Nodes and. The choces for and clearly affect the speed wth whch the algorthm runs. Therefore, other hardware mplementaton ssues mght need to be taken nto account. Havng such an antenna wth beamformng capablty enables each node to steer the drecton of ts man beam toward any desred angle (.e. 360-degree feld of vew). Sample beam patterns of such an antenna for varous array szes (.e. number of elements) are shown n Fg. 3. In the next secton, we provde smulatons of ths algorthm and show the effectveness of the results. 4- SIMULTION RESULTS To nvestgate the effectveness of the proposed algorthm, a smulaton platform was mplemented. The man dffculty n smulatng a wreless channel s the strong dependence of the receved sgnal on the surroundng envronment (e.g. multpath channel). In partcular, all obstacles such as walls, that affect the propagaton of RF waves, wll drectly mpact the sgnal strength and more mportantly the drectons from whch RF sgnal energy s receved. Emprcal, statstcal and determnstc models have been used to descrbe the behavor of such multpath channels (Hashem 1993; Spencer et al., 000; Ertel et al., 1998). In ths study, we have elected to use a sophstcated ray-tracng tool to accurately predct the receved sgnal strength n a multpath RF channel. Wreless System Engneerng (WSE) s a ray-tracng tool that has been developed and verfed by ell Laboratores (Fortune et al., 1995; Valenzuela et al., 1997). We realze that even such models have lmtatons n ther accuracy and are also subect to errors when there are changes n the envronment, however, ths approach wll gve us the opportunty to smulate the performance of the proposed protocol n a multpath channel and at the same tme provde a geometrc nterpretaton of the results. We frst consder a very smple layout and two transcever nodes ( and ) as shown n Fg.. It s assumed that each node s equpped wth a crcular array antenna of a gven sze. (a) (b) Fg. 3: eam pattern of a crcular array wth (a) 8 elements (b) 3 elements In general, the strength of the receved sgnal heavly depends on the envronment layout, delectrc propertes of the surroundng materal, transmtter-recever locatons, operatng frequency and array szes of the recever and transmtter antennas. For the example gven n Fg., the receved power s a functon of the azmuth angles of the man lobes of both transmtter and recever. Fg. 4 dsplays ths receved power as a result of smulaton wth a par of 3 element crcular array antennas at the recever-transmtter par and a transmsson frequency of 60GHz. 3

4 The effectveness of the lnk establshment algorthm was also nvestgated n a varety of other scenaros and some examples demonstratng the fnal drecton, chosen for the man lobes of the antenna s beam pattern, are shown n Fg. 8. These nclude sngle-reflected and double reflected paths as they created the hghest sgnal strength at the recever. Fg. 4: Normalzed RSS as a functon of the drecton of the transmtter/recever s man lobes s observed, 1 dfferent locatons on ths plot (.e. pars of ( α, ) ) exhbt receved sgnal strength that are relatvely stronger than other drectons. These peaks correspond to the stuatons where the man lobes of the transmtter-recever par are postoned n a way to capture the lne of sght, sngle-reflected and doublereflected sgnals respectvely. Exstence of a clear LOS path n ths example sgnfes the strong RSS component that s vsble at α = 90 and = 70. The lnk establshment algorthm proposed n the prevous secton acheves these optmal drectons (.e. α = 90 and = 70 ) as symbolcally shown n Fg. 5. Fg. 6: α, selected by the lnk establshment algorthm Fg. 7: RSS as a functon of the drecton of the transmtter/recever man lobes Fg. 5: α, selected by the lnk establshment algorthm If ths LOS path dd not exst, for example due to an obstacle that blocks ths path, then one of the propagaton paths that contans a sngle-reflected sgnal wll yeld the best sgnal strength for the recever. We also smulated ths stuaton and Fg. 6 shows the resultng lnk that our proposed algorthm has chosen. These are ndeed the optmal drectons for the man lobes of the transmtter and recever nodes as the smulaton shows that the hghest value of the RSS occurs at α = 40 and = 30 (see Fg. 7). 4 The drecton par dsplayed n ths fgure s also the optmal drectons that could have been selected by exhaustve search of all possble drectons. 5- CONCLUSIONS ND FUTURE WORK The underlyng phlosophy n ths paper s that explotng the nformaton n the spatal dstrbuton of RF energy around both transmtter and recever results n better qualty communcaton lnks. The proposed protocol outlnes a methodology to obtan the drectons where the beam steerng algorthms at the recever and transmtter should use. These drectons acheve hgher sgnal strength at the recever; and therefore, a more relable lnk can be expected. lthough, all our smulatons have shown that the selected drectons are optmal, t s stll concevable that specal scenaros could exst where the optmal

5 drectons are not acheved by the proposed algorthm. However, n order to captalze on the capablty of such antenna arrays, t s mportant to have an algorthm that can quckly converge to optmal (or near optmal) drectons. The speed of ths convergence s even more mportant when one or both of the communcatng nodes are moble. n adaptve verson of the proposed algorthm wll be an attractve soluton for applcatons that nvolve moble nodes. fact that the large bandwdth avalable at mllmeter wave frequences results n very hgh data transmsson rate; also helps to mnmze the amount of tme that a node needs to stay n transmsson mode; and therefore, mnmzes the possblty of ts transmsson beng detected. In ths paper, we have only consdered scenaros nvolvng two nodes. In the future, we would lke to nvestgate the extenson of the algorthm when multple nodes, equpped wth mult-beam array antennas, are communcatng. Further studes and more mportantly expermental data are requred to assess the performance of the lnk establshment algorthm n practce. Node REFERENCES Node (a) R.. Ertel, P. Carder, K. W. Sowerby, T. S. Rappaport, J. H. Reed, Overvew of spatal channel models for antenna array communcaton systems, IEEE Personal Communcatons, Vol. 5, Issue: 1, Pages: 10, Feb S. J. Fortune, D. M. Gay,. W. Kernghan, O. Landron, R.. Valenzuela, M. H. Wrght, WISE desgn of ndoor wreless systems: practcal computaton and optmzaton, IEEE Computatonal Scence and Engneerng, Vol., Issue: 1, Pages: 58 68, Sprng Node Node H. Hashem, The Indoor Rado Propagaton Channel, Proceedngs of the IEEE, Vol. 81, No. 7, July (b) Fg. 8: Optmal drectons of the transmtter/recever man lobes (a) Sngle reflected path (b) Double reflected path Once the lnk s establshed, a power control mechansm can be mplemented to reduce transmsson power n order to conserve energy and prolong battery lfetme for moble nodes. Mnmzng the necessary transmsson power also contrbutes to lowerng the nterference on spectrally co-exstng systems. Therefore, multple nodes can have smultaneous communcaton wthout affectng the relablty of each other s lnk. Fnally, n many mltary and defense-related applcatons, where low probablty of ntercept s of paramount mportance, avodng wreless transmsson n all drectons sgnfcantly reduces the chance of the RF sgnal to be detected by the enemy. Ths along wth the K. Huang, Z. Wang Mllmeter-Wave Crcular Polarzed eam-steerng ntenna rray for Ggabt Wreless Communcatons IEEE Trans. on ntennas and Propagaton, Vol. 54, Feb 006. J. C. Lbert and T. S. Rappaport, Smart ntennas for Wreless Communcatons, Prentce Hall, R. Ramanathan, On the Performance of d Hoc Networks wth eamformng ntennas, n Proceedngs of CM MOIHOC, 001. K. Sayrafan,. Neekzad, J. Perez, J. aras, Ray- Tracng Smulaton of the NICT Channel Measurements, IEEE Standard c mm-wave channel modelng subgroup doc. # IEEE c, July 006. K. Sayrafan, D. Kaspar, Source-sssted Drecton Estmaton Insde uldngs, Proceedngs of IEEE INFOCOM 06, prl 3-9, 006a, arcelona, Span. 5

6 K. Sayrafan-Pour, D. Kspar, pplcaton of eamformng n Wreless Locaton Estmaton, EURSIP Journal on ppled Sgnal Processng, Volume 006 (006b), rtcle ID Q. H. Spencer,. D. Jeffs, M.. Jensen,. L. Swndlehurst, Modelng the statstcal tme and angle of arrval characterstcs of an ndoor multpath channel, IEEE Journal on Selected reas n Communcatons, Vol. 18, Issue: 3, Pages: , March 000. R.. Valenzuela, O. Landron, D. L. Jacobs, EstmatngLocal Mean Sgnal Strength of Indoor Multpath Propagaton, IEEE Transactons on Vehcular Technology, Vol. 46, No.1, Feb