Six-Port Receiver for mm-wave Concept, Evaluation and Implementation

Transcription

1 S-Port Recever for mm-wave Concept, Evaluaton and mplementaton T. Erener, Member, EEE, and T. Müller Abstract A s-port recever uses smple power detectors to realze a drect converson or a zero-f recever. Ths paper reveals the theory about s-port recevers, whch are suted to realze a low cost archtecture for mm-wave applcatons. Further, a comparson between dfferent possble recever archtectures s presented. The RF characterstcs and boundary lmtatons show that the s-port recever s an nterestng alternatve to estng mer based archtectures. The mplementaton, especally the effort n the dgtal base band processng s presented, whch allowed a frst demonstrator to receve 0 4 error free symbols. nde Terms s-port, mult-port, drect converson, mult standard recever, addtve mng, software rado. NTRODUCTON HE requrements on recever archtectures are Tncreasng constantly. Recevers are epected to become more broadband, as well as the transmsson frequency s ncreasng steadly, e.g. 60 GHz n the WWGAM project [8]. Latest research n mm-wave has declared that actvtes at 60 GHz are just the opener for wreless applcaton up to 50 GHz []. A contrary trend clams to become smaller and cheaper, whch elmnates conventonal heterodyne concepts. Smaller and cheaper forces a reducton of monolthcally not ntegrable components, as well as the avodance of adjustment work. Further, the amount of dfferent communcaton standards that has to be mplemented s ncreasng enormously. Especally n the automotve area, a mult-standard recever platform has the potental to reduce sze and cost, whereas t offers update possblty to future standards. Conventonal heterodyne recevers are mostly workng wth real band pass sgnals n frequency regons far above the modulated base band sgnal. One ore more monolthcally not ntegrable flters are requred. The mplementaton consderng Manuscrpt receved February 6, 005. Ths work, supported n part by the German Mnstry of Educaton and Research (BMBF), contrbutes to the WGWAM (Wreless Ggabt Wth Advanced Multmeda Suppor project, whch s coordnated by Prof. Dr. Gerhard Fettwes (Dresden Unversty of Technology). Thomas Erener s wth Damler Chrysler Research and Technolgy, Ulm, Germany (e-mal: thomas.erener@ damlerchrysler.com). Dr. Thomas Müller s wth Damler Chrysler Research and Technolgy, Ulm, Germany (e-mal: thomas.mu.mueller@ damlerchrysler.com). fnancal aspects wth the focus on a mult-band/mode-recever archtecture s mpossble wth current and epectable technologcal progress. Homodyne or drect converson recevers, and zero-f or low-f recevers respectvely, are an alternatve and promsng realzaton possblty. Thereby, the analog sgnal processng s essentally moved down to low frequences and the number of monolthcally not ntegrable flters s reduced to a mnmum. An alternatve to conventonal archtectures s offered by the s-port technology. t promses to be cheap and etremely broadband and can be used at hghest frequences to realze a drect converson recever as well as a zero-f recever. Ths s acheved by the combnaton of a smple and cheap analog frontend actve mers are replaced by power measurements va dodes wth subsequent dgtal sgnal processng,.e. the boundary regardng upper frequency lmt wll be set by realzable dodes to detect the sgnal power.. THE SX-PORT THEORY A. Addtve Mng Contrary to conventonal multplcatve mer, the nput sgnals are added and subsequently squared. The squarng s done on a nonlnear element, e.g. a schottky dode. The local oscllator sgnal s ( t ) Fg..Prncple of addtve mng s = A cos( ω t + ϕ ) () wth ampltude A, frequencyω and ntal phase added to the receved RF bandpass sgnal s s = = Re = BP ( jωt { e cos( ωt + ϕ ) = cos( ω S ( S ( u sn( ω ϕ, s wth the carrer frequencyω and phase ϕ ( or nphase and { u( ()

2 quadratur ampltude. The added sgnal gets squared and low-pass fltered to the comple baseband bandwdth. { u( = = K A cos( + ϕ ϕ ) ( A + ( + ( ) + K + A [ ( cos( + ϕ ) ( sn( + ϕ )] + + A A lnear dependency of the lowpass sgnal power u( to (, ( and ( + ( can be seen. Therefore, the calculaton of the comple base band sgnal = + j by applyng a lnear equaton system requres at least three power measurements wth ndependent phase relatons. B. Mult-Port Recever ntroducng phase shftsψ n one nput path, allows creatng lnear ndependent power measurements. S ( Ψ S ( Fg..Prncple of a mult-port recever At least three ndependent paths are needed to calculate the comple baseband sgnal y( { u y + j c (4) = u { u (3) n order to elmnate the rectfed part ( + + A ). Assumng that the phase shfts are done by a smple delay lne an errorε s ntroduced by devatng from the desgn frequency. The ntal local oscllator phaseϕ, as well as the frequency dfference ω s unknown. o o o o [ 0, 90 + ε,80 + ε, ε ] ψ = 0 Wthψ and Θ ( follows: y = A c ) c sn( + ε ) c 3 + ε ) c 4 sn( + 3ε ) T c 0 sn( ) + ε ) sn( ( ) ) ( ) Θ t + ε t + 3ε ) (7) Equaton 7 reveals the possblty of calbraton by sendng a known tranng symbol sequence, presupposed the lnear equaton system s at least of rank three. C. Fve-Port vs. S-Port Equaton 3 shows that three output ports are enough to realze a mult-port recever. Wth respect to subsecton B, t can be shown that lnear ndependency can not be taken for granted, whch may be caused by errors of the phase shftsψ. by ntroducng a comple calbraton coeffcent c. A general mult-port recever equaton s formulated n matr notaton. T c c y( = A M c n cos( + ψ ) sn( + ψ ) + ψ Θ + ( ) ) sn( ψ ) t M M cos( + ψ ) sn( + ψ ) n n + + A + M (5) wth Θ = ω t + ϕ, ω = ω ω o o o o Choosng = [ 0, 90,80, 70 ] T to c = [ j j] ( ) ψ and assumng t ) = 0 leads A Equaton 5 shows that n any case c 0 (6) Fg. 3. nfluence of the phase error to fve/s-port Fg. 3 shows that a fve-port has only two lnear ndependent output ports already at 60 phase error, whereas a s-port shows ths behavor at phase error for the frst tme. Furthermore, a s-port shows a unform relatve phase

3 3 dstrbuton at multples of 80 relatng to the center frequency f 0. Thus, t possesses dentcal recevng characterstcs at the frequences f 0,.5f 0, f 0,.5f 0, assumng that the phase shfts are realzed by delay lnes. Ths corresponds to the requrement of broadband mult-mode and mult-standard recever. D. Realzaton of a S-Port A common method to realze the four ndependent phase shfts s usng -hybrd couplers as shown n Fg. 4 [5], wth the two nputs connected to the local oscllator () and the RF bandpass sgnal. -sgnal Dvder -3dB RF j 0.7 RF Fg. 4. Realzaton of a s-port j 0.5 RF j RF RF-sgnal j( RF) RF Fg. 5 shows a smple power detector crcut wth approprate lowpass flterng to detect the power of each sport output. RF-nput 50 Ohm real baseband ouput Fg. 5. Smple power detector crcut The layout for a 4 GHz s-port wth power detectors s shown n Fg. 6, where the rght pcture shows a magnfed depcton of the of the power detector realzaton. Fg. 6. S-port layout and power detector realzaton. SX-PORT RECEVER ARCHTECTURE N COMPARSON TO CONVENTONAL RECEVER ARCHTECTURES A. Overvew Recever Archtectures The ntroducton of s-port technology n mcrowave recevers seems to push the boundares regardng prce and upper frequency lmt. To enable a serous valdaton of sport technology we have to benchmark t wth estng recever archtectures. Lookng on applcatons ntended to use wth s-port technology the most mportant alternatve recever concepts are the drect converson recever and the F samplng recever. Both are depcted n Fg. 7 and Fg. 8, where s the local oscllator, a lowpass flter and BP a bandpass flter. The s-port recever s well descrbed n secton. All three recever concepts generate an / data stream, whch s further processed to recover the bnary data. Drect samplng recevers are actually not applcable wth state of the art technology for the heren addressed frequency range, so ths archtecture wll not be dscussed further. BP BP 0 Fg. 7 Drect converson recever f_f BP Analog- Dgtal Converter Clock n Bt Fg. 8 F samplng recever B. Comparson of RF Performance 0 dgtal sgnal processng Phase accuracy: n all recever archtectures phase nose of the local oscllator () wll be drectly transformed to phase nose n the comple baseband. n case of the F samplng recever, jtter of the samplng clock may lead to addtonal phase nose [6]. Ths knd of phase nose wll also lead to neghbor channel nterference, caused by recprocal mng and therefore reduce the selectvty of the recever. n all three cases channel selecton s usually done by settng the frst local oscllator, therefore the requrements and the phase nose wll be about the same. Only the F samplng recever makes t prncpally possble to desgn the frst wth fed frequency by takng a wde bandpass flter BP and perform the channel selecton n the dgtal doman. Ths could enhance the performance sgnfcantly. n case of the s-port recever and the drect converson recever addtonal phase naccuracy wll be ntroduced by naccurate calbraton. Because of always estng phase mbalance n drect converson recevers, phase dstorton has also to be reduced by calbraton. Nose fgure: All dscussed recever archtectures approve

4 4 the use of a low nose amplfer (LNA) stage. n ths case the nose fgure wll be defned by the LNA. For frequences where no LNA s avalable we have to compare the nose of a mer to the nose of a power detector recever. For frequences beyond 50 GHz both are usually bult up usng Schottky dodes. The nose fgure for dode mers [] [7] s comparable to ts converson loss that s typcally about 7dB. Smulatons on mcrowave power detectors usng a beam lead GaAs Schottky dode have shown that the nose fgure goes down to 4.8dB for nput powers smaller than -0dBm. power: To obtan a good converson gan, the power of the local oscllator should be n range of 0 to 0dBm for most mers []. Best workng condton for s port recevers s a power n the area of the recever nput power whch s much less. n-band dynamc range: When havng a strong nterferer drectly besde the wanted sgnal or for hgh order modulaton schemes, the recever has to cope wth the resultng dynamc range. For s-port recevers ths dynamc range s manly gven by the accuracy of the calbraton. Frst trals have shown a dynamc range of 30 to 40dB [4]. Calbraton wll also lmt the lnearty of the recever. For mer based recevers the dynamc range s lmted by the lnearty and a db compresson pont of -0dBm to +0dBm can be obtaned. Ths results n a dynamc range of about 90 to 00dB for a sgnal bandwdth of 0MHz. The nfluence of out-of-band nterferers has to be verfed n further smulatons. Self mng effects: Drect couplng and eternal reflectons lead to a DC offset for drect converson recever [3] and s-port recever. n s-port archtectures ths topc s nherently handled by the calbraton procedure. Drect converson recevers usually contan some sgnal processng unt to cope wth ths effect. C. Comparson of Boundary Lmtatons Sze: The sze of mer based archtectures s manly gven by actve components and flters, but no wavelength λ correlated lnes are necessary. For F samplng recevers an addtonal F flter s necessary, whch s usually large n sze and can not be ntegrated on the chp. The usual setup of a sport s about ¾ λ n square. For V, W and D band applcatons t s well possble to ntegrate the s-port on chp. Cost: The cost of drect converson recever and F samplng recever depend very much on the frequency range. Especally for frequences n the W and D band, mers become rare and epensve, whereas power detectors are avalable for hgher frequences. For mm-wave sgnal generaton usually a lower frequency s followed by a frequency multpler, whch produces a small power. The s-port recever wll have a cost advantage by reduced power requrements. The effort n dgtal sgnal processng s actually much hgher for s-port recevers; ths porton of costs wll become neglgble n the near future. D. Revew Recever Archtectures S-port recevers are an nterestng alternatve to estng mer based recever archtectures especally n the mm-wave and sub-mm-wave range. n ths frequency range we usually do not have to cope wth strong neghbor channel nterference so the reduced dynamc range wll be acceptable. V. SX-PORT RECEVER ARCHTECTURE A. Overvew The s-port recever archtecture can be separated n an analog and a dgtal part of the frontend. The analog frontend mes addtvely the amplfed and bandpass fltered nput sgnal under four dfferent phase condtons. The measured sgnal power s fed to the after correspondng matchng,.e. low pass flterng to the baseband bandwdth and amplfcaton to match the, whch feeds the converted data nto an FPGA. The FPGA contans the necessary dgtal sgnal processng n order to calculate the comple base band sgnal. RF sgnal BPF Analog Frontend LNA SX-PORT VCO Fg. 9. S-port recever archtecture FPGA Calbraton -Calculaton Frequency Offset Cancelaton Dgtal Frontend B. Analog Frontend The antenna nput sgnal s band pass fltered and amplfed (LNA) n order to lmt the nose power and suppress near band nterferer. Propagaton propertes and near range S transmsson n the upper mm-wave regon may allow to dsmss the bandpass flter and LNA. The antenna sgnal and the VCO sgnal, whch s set to the recepton channel by the dgtal frontend, are fed nto the s-port. The powers of the four ndependent outputs are measured by power detectors, shown n Fg. 6. The power detector contans the baseband flter and an addtonal DC amplfer to match the nput range of the. C. Dgtal Frontend ) Archtecture The comple base band sgnal s calculated by a multplcaton of the measured power values wth the calbraton coeffcents c, see equatons 4 and 5. The

5 5 calbraton coeffcents are determned by solvng a lnear equaton system, generated by sendng a known tranng sequence. Research has shown that the Gaussan elmnaton method s best suted for hardware mplementaton. X Matched Flter Decder CORDC j + j ϕ *e X*C 80Ms/s Frequency Detecton DPLL fsym Calbraton (Gauß) C A FFT ϕ Phase and Ampltude Offset FPGA Fg. 0. Dgtal frontend archtecture An eact calbraton s only possble, f the frequency offset between local oscllator and comple baseband sgnal s elmnated. Further, a calbraton s necessary to determne the eact frequency offset, whch leads to an teratve calbraton procedure. The calbraton by a known symbol sequence forces the symbol detecton and matched flterng to be as early as shown n the presented archtecture, see Fg. 0. ) Frequency Offset Elmnaton A frst estmaton of the frequency offset s done by a comple FFT analyss of two orthogonal nput power streams. The offset estmaton allows a rough calbraton, whch s suted to determne the eact frequency offset by the control loop structure. After recalbraton the comple baseband sgnal can be calculated and subsequently demodulated, see Fg.. A result of a real mcrowave transmsson s shown n Fg.. An error free transmsson of 0 4 symbols was possble by calbratng, determnng the frequency offset and processng the comple base band output sgnal wth the presented archtecture. Fg.. Calbraton and correcton of the frequency offset (6-AM, f = 0 khz, f sym = 0 MHz, SNR = 30 db) Fg.. Real mcrowave transmsson of 0 4 symbols (AM, f 0 = 4 GHz, f = 0 khz, f s = 0 MHz) V. CONCLUSON The presented paper reveals the basc theory on s-port recevers. t has been shown that prncpally fve-ports are enough, but the sth port offers the possblty of realzng a broadband mult-mode/band recever. The comparson to conventonal recever archtectures has shown that the s-port recever s an nterestng alternatve to estng mer based archtectures especally n mm- and sub-mm-wave range. A s-port archtecture has been mplemented for 4 GHz and a dgtal frontend mplementaton for a frst demonstrator has been shown. Frst measurements allowed an error free transmsson of 0 4 symbols. ACKNOWLEDGMENT The authors specally thank Konrad Böhm and Matthas Wetz for ther contrbuton to the analog frontend. Further thanks to Aleander Kölpn and Sebastan Wnter, who are wth the Fredrch-Aleander-Unversty of Erlangen. REFERENCES [] B. Gaucher, M. Soyeur, and M. Oprysko, Slcon Mlmeter Wave ntegrated Crcuts for Wreless Applcatons, BM Research, New York, 0 th October, 004 [] Hewlett Packard, The Schottky Dode Mer, Applcaton Note 995 [3] H.-J. Jentschel, H. Berndt, U. Pusche, Drect Converson Recevers Epectatons and Eperences, Workshop RF Front End Archtectures, EEE MTT-S nternat. Mcrowave Symposum, June 000, Boston, Workshop Proc. WMH [4] J.-U. Jürgensen, D. Krupezevc, M. Ratn, Z. Wang, Baseband Aspects of a Drect Converson Recever Concept usng Fve-Port Technology, nd Karlsruhe Workshop on Software Rado, Karlsruhe, March 00 Proc. pp. 63ff [5] J L, R. G. Bosso, Ke Wu, A S-Port Drect Dgtal Mllmeter Wave Recever, Dgest of EEE Mtt-Symposum, pp , San Dego, 994 [6] T. Müller, S. Herter, J.-F. Luy, Clock Generator Phase Nose n RF Samplng Recevers, nd Karlsruhe Workshop on Software Rado, Karlsruhe, March 00 Proc. pp.3ff [7] Rck Poore, Nose n Rng Topology Mers, Aglent EEsof EDA, [8] WGWAM Wreless Ggabt Wth Advanced Multmeda Support,