AN ALL DIGITAL QAM MODULATOR WITH RADIO FREQUENCY OUTPUT

Transcription

1 AN ALL DIGITAL QAM MODULATOR WITH RADIO FREQUENCY OUTPUT Zhuan Ye (Motorola Labs, 131 E. Algonqun Rd., Schaumburg, IL 6196 John Grosspetsch (Motorola Labs, Schaumburg, IL 6196 ABSTRACT A software defned rado (SDR) termnal promotes programmable realzatons of the physcal layer functonaltes. A lot of research work has been done n applyng dgtal sgnal processors (DSP) and feld programmable gate arrays (FPGA) to mplement the baseband functonaltes of the physcal layer. In ths paper, the programmable solutons are extended to the rado frequency (RF) band for the transmtter. Dgtal pulse wdth modulaton () technque s used to generate bnary sgnals at rado frequency. A QAM modulator, combned wth, s mplemented usng off the shelf FPGA. The output of ths all dgtal transmtter has a center frequency of 8 MHz. 1. INTRODUCTION A software defned rado (SDR) s defned as a rado n whch the dgtzaton s performed at some stage downstream from the antenna. Then the rado can use flexble and reconfgurable functonal blocks for the mplementaton of dgtal sgnal processng algorthms. As technology advances, the dgtzaton mght be at, or very close to the antenna, such that almost all the rado functonaltes are realzed usng software usng hgh speed and reprogrammable dgtal sgnal processng engne [1]. The current rados consst of a mxture of analog and dgtal buldng blocks. The rado frequency (RF) functonaltes are most lkely beng mplemented usng analog crcuts, whle the baseband functonaltes are more sutable for DSP mplementatons. Dgtal front end (DFE) s often used to brdge between rado frequency and baseband processng [1]. The DFEs are normally capable of processng sgnals wth frequences at about tens of megahertz, often referred to as dgtal ntermedate frequences (IF). Therefore, there reman sgnfcant analog blocks between the RF and dgtal IF. Dgtal RF transcevers are studed to extend the software defned functonaltes nto the rado frequences [1]. The focus of ths paper s about all dgtal transmtter technology. The advantages of all dgtal transmtters are: hgh effcency power amplfcaton ([2], [3]) dgtal combnng of sgnals from multple channels software programmable, or reconfgurable Usng an all dgtal transmtter, the entre transmtter can be realzed usng DSP or FPGA, whch can take advantage of the rapd performance ncrease of CMOS technology. Besdes ts compatblty wth SDR s requrement, DSP based RF system can also be made to compensate for the mparments of the RF channel. Therefore dgtal generaton of sgnals drectly at rado frequency has drawn a lot of nterests among researchers and engneers. One of the more tradtonal methods s descrbed n [3], whch uses bandpass deltasgma modulaton to generate bnary sgnals at rado frequency. Bnary sgnalng can be used together wth swtch mode power amplfers (PA) to acheve hgher effcency comparng to other types PA technology. Ths transmtter archtecture s shown n Fgure 1. The drawback of ths archtecture s that the bandpass deltasgma (BPDS) modulator s runnng at 4 tmes of the center frequency, whch can be several ggahertz. In order to accommodate such hgh frequency of operatons, custom ntegrated crcuts have to be very carefully desgned, whch lacks reprogrammablty. Another method uses pulse wdth modulaton () [4] to synthesze bnary RF sgnals dgtally. was ntroduced as an analog modulaton long tme ago, but has ganed popularty recently, especally n the dgtal audo amplfcaton applcatons. ClassD audo PA, drven by modulated audo sgnals, can acheve effcency above 9%. Deltasgma type of modulaton can also be used n dgtal, but operates at much lower frequency compared wth aforementoned BPDS method. However the deltasgma loop tends to be more complcated than what s used n BPDS, because of the low oversamplng rato and the nonlnearty assocated wth. Both references [3] and [4] only presented smulaton and non realtme test results. The test setup uses sgnal sources computed offlne that are stored n pattern generator. In ths paper, a realtme system was desgned to demonstrate the capabltes of dgtal generaton of RF

2 sgnals usng dgtal. The rest of ths paper wll be organzed as followng. Frst, the method of generaton of bnary RF sgnals s dscussed. Next, the dfferent nose shapng behavors between and tradtonal DAC are descrbed. Ths dfference wll lead to the dscusson on our noseshapng flter desgn method for the dgtal RF system. Then the detaled mplementaton of an all dgtal transmtter based on dgtal RF s presented. The fnal secton summarzes the paper and presents conclusons Dgtal 2. DIGITAL RF GENERATION Dgtal pulse wdth modulaton () was motvated by the dgtal power amplfcaton technque, by whch a dgtal sgnal can be converted drectly nto hgh power analog sgnals wthout ntermedate dgtal to analog converter (DAC) stages [5]. The dgtal power amplfcaton has ganed some popularty n the dgtal audo applcatons, due to the ncreasng nterest to develop all dgtal audo system. In dgtal, the pulse wdths are quantzed wth respect to a hgh speed clock. Therefore a smple counter can be used to generate the dgtal waveform based on the reference hgh speed clock. The general sgnal processng blocks consst of a dgtal system are shown n Fgure 2: Interpolator. The nterpolator ncreases the samplng frequency of the PCM nput to a frequency sutable for performng modulaton. In ths paper, ths frequency s referred to as pulse repetton frequency (PRF). Natural Sampler. The natural sampler calculates the naturally sampled sgnal values based on the unformly sampled dgtal sgnal. It has been shown that naturally sampled sgnal experences far less baseband dstorton comparng to unformly sampled sgnal, when s performed [6]. Quantzaton wth nose shapng. The quantzaton s necessary to make the hgh speed reference clock runnng at reasonable frequences for mplementaton purpose. For example, f the orgnal PCM nput s 44.1 KHz, a 16 tmes nterpolator wll result n the PRF equals to 75.6 KHz. The hgh speed reference clock needs to have the frequency of 46 GHz f the pulse wdth s quantzed to 16 bts. If only 8 bts are needed to quantze the pulse wdth, the frequency of the hgh speed reference clock can be reduced to 18 MHz, therefore makes t much easer to mplement usng moderate technology. Nose shapng technque, e.g., deltasgma modulaton, can be used to suppress the baseband nose ntroduced by quantzaton. One can also see that the processng ntensve blocks, natural sampler and quantzaton wth nose shapng, have the samplng frequency of PRF. Ths s the man motvaton of ths work: the man sgnal processng algorthms are executed at the lower PRF rather than the RF. In order to accommodate the bandwdth requrement n the QAM modulator n our demonstraton, ths dfference s 1 MHz versus several GHz All Dgtal RF The method of an all dgtal rado frequency pulse wdth modulaton RF was ntroduced n [4], called quadrature ntegral nose shapng (INS). INS s an algorthm used n the quantzaton and nose shapng block. Its man goal s to suppress the nose power n the baseband ntroduced by the pulse wdth quantzaton process. It dffers from other algorthm by ntroducng nonlnear terms nto the feedback loop. The detals of INS are provded n [7]. Wthout consderng the detals of the INS algorthm, the quadrature INS can be vewed as two ndvdual modulators for nphase (I) and quadrature (Q) paths of complex sgnals respectvely. These modulatons use the same archtecture as descrbed n subsecton 2.1. The outputs from these are baseband sgnals that need to be further mxed wth dgtal local oscllator sgnals to form bandpass sgnal at RF. If both the baseband sgnals and the dgtal local oscllator sgnals are bnary, ths mxng operaton s nothng more than a smple logc XOR operaton. Another method to smplfy ths mxng operaton s to make the nphase LO take these ternary values of {, +1,, 1}, whle the quardrature LO takng the values of { 1,, +1, }, therefore the dgtal mxer only outputs these values {Q, +I, +Q, I} n sequence. When both I and Q are bnary sgnals, the mxer s output wll be bnary too. In order to make the LO sgnals have one of these two formats, the samplng frequency needs to be at 4 tmes of the LO frequency. The sgnals from two dgtal mxers are then combned to form the desred sgnal at the rado frequency, referred to as RF sgnal n ths paper. For a sgnal modulated usng, the sgnal nformaton s carred n the wdth of the pulse. Snce pulse wdth s defned by the duraton tme from the rsng edge to the fallng edge, the transton edges should be preserved after the baseband. However there exsts 9 degree phase dfference between the nphase LO sgnal and quadrature LO sgnal, extra attenton needs to be pad to ensure that the baseband s are synchronzed wth ther LO sgnals respectvely. Snce the samplng rate s set at 4 tmes of the LO frequency, the 9 degree phase dfference s equvalent to one quarter cycle tme dfference. The baseband waveforms for nphase and quadrature are

3 constructed dfferently: there s an ntentonal quarter cycle dfference beng ntroduced to compensate for the phase dfference between the nphase LO sgnal and the quadrature LO sngal. The spectrum plots after each processng steps n the RF are llustrated n [4]. Fgure 3 shows the tmng waveforms of baseband s and RFs. In ths fgure, the LO sgnals take the ternary format, and t can be seen that the combned output RF sgnal s bnary. Addtonally, both baseband s are synchronzed to the rsng edges of ther LO sgnals respectvely. The QAM modulator presented n ths paper uses the quadrature archtecture, whle INS algorthm s not chosen due to ts hgh computatonal requrement. A nonrecursve nose shapng method s used nstead [8]. 3. NOISE SHAPING IN DIGITAL Nose shapng has been wdely adopted n oversampled data converters. The purpose of nose shapng s to generate coarsely quantzed sgnals nstead of fnely quantzed sgnals, whle preservng the SNR performance wthn a lmted bandwdth. In the dgtal system, nose shapng s necessary snce the reference clock would be have nhbtly hgh frequency had the pulse wdth been fnely quantzed, e.g., usng 16 bts nstead of 8 bts when PRF s 75.6 KHz. The nose shapng flters used n tradtonal data converters have been well studed, and most methods can be used n the dgtal as well. But the nose shapng behavor n dgtal dffers from tradtonal DtoA converter, due to the nonlnear effects ntroduced by. A smulaton model was created to compare the nose shapng performance between and tradtonal DAC as shown n Fgure 4. The dgtal and the tradtonal DAC use the same nose shapng flter and the same quantzer. The nose transfer functons (NTF) used n ths smulaton are specfed by H(z)=(1z 1 ) N, where N ranges from 1 to 5. Ths type of nose shapng flter s not optmal as far as the nband nose suppresson performance s consdered, but t s suffcent to demonstrate the dfferent behavors between the and tradtonal DAC. The effects of analog components n the DAC are not ncluded n the smulaton, only the all dgtal nose shapng loop s consdered. The followng parameters were selected for the smulaton Frequency of the nput sngle tone sgnal = 11 KHz Baseband bandwdth = 2 KHz Samplng frequency = 75.6 KHz Input sgnal level = 6 dbfs Quantzaton levels = 64 For tradtonal DAC, the quantzaton levels ndcate the number of bts used n the fnal DAC, whle for, ths means the number of hgh speed clock edges n each pulse cycle. The nband SNR performance from and tradtonal DAC s summarzed n Table 1. Table 1 Comparson of nose shapng between and DAC NTF order SNR (db) DAC SNR (db) It can be seen that the SNR measurements are almost dentcal between and tradtonal DAC when the lower order NTFs are used,.e., N=1 and N=2. When the order of NTF s ncreased, the SNR trend for dffers from tradtonal DAC: 1. The SNR may not be mproved as rapdly as tradtonal DAC when N s ncreased from 2 to 3 2. The SNR deterorates when N s further ncreased to 4 and 5 The 2 nd observaton was ponted out n [5] that the quantzaton nose mght be folded back to the baseband f the gan of the NTF s hgh at the hgh frequency. We thnk the 1 st observaton can be explaned by the nonlnearty nature of the modulaton. However ths nonlnearty effect can be neglected f only moderate baseband SNR performance s requred. Through ths smulaton model, we formulated our crtera to desgn a nonrecursve NTF flter used for dgtal system: 1. Mnmze the rato between baseband energy and total energy when the flter s nput s whte nose. 2. The flter coeffcents should satsfy mnmum phase, 1norm requrement, and ts 1 st coeffcent should be unty. These conclusons are provded n [8] wth detals. 3. The flter gan at hgh frequency (ω=π) should be lmted. Ths requrement s derved from the above smulaton results. These crtera can then be formulated mathematcally as follows. Assume an Ntap FIR flter wth coeffcents h, where h = {h, h 1,, h N1 } T, the baseband energy can be calculated by E b ω b jω T = H ( e ) dω = h Rh (1) wheren R s a matrx, and ωb, m = n rm. n = sn( n m) ωb (2), m n n m The total energy can be calculated usng Parseval s theorem,.e., 2 2 E t = h (3)

4 Therefore the optmzed NTF should try to mnmze α ( E ) J = b β (4) ( Et ) The parameter α and β are used as weghtng factors when performng the optmzaton. The object functon should be mnmzed subject to these condtons: The 1norm of h should be bounded, whch s dependent on the number of levels n the quantzer,.e., N 1 = h q (5) The flter should be mnmum phase,.e., all the zeros of the flter should be wthn the unt crcle The gan at hgh frequency should be bounded,.e. N 1 = ( 1) h b (6) The all dgtal transmtter to be demonstrated s specfed to have 1 MHz baseband bandwdth, and the PRF s chosen to be 1 MHz. The center frequency s 8 MHz; therefore the frequency of the hgh speed reference clock s 3.2 GHz. The prototype mplementaton uses an 8 tap fnte mpulse response (FIR) NTF desgned usng ths method. 4. HARDWARE PROTOTYPING AND MEASUREMENTS As descrbed n secton 2.1, the entre sgnal processng algorthms requred by dgtal s runnng at the frequency of PRF. The only hgh speed crcut requred s the fnal waveform generaton. Therefore t s feasble to prototype an all dgtal QAM modulator on an off the shelf FPGA devce. The archtecture of the dgtal QAM modulator s shown n Fgure 5. Only the block dagram of the nphase path s shown n detal; however the quadrature path should consst of almost dentcal archtectural blocks. The QAM modulator conssts of one QAM symbols generator whose symbol rate s 5.57 MHz, one nterpolaton flter to upsample the samplng rate to 16 tmes of the symbol rate, and one sample rate converter that converts the samplng rate to the 1 MHz PRF. The detals of ths QAM modulator are provded n [9]. The dgtal conssts of one natural sampler, one quantzaton wth nose shapng block, and one waveform generator. The natural samplng algorthm [6] conssts of only feedforward datapaths; therefore t can be ppelned farly easly. The quantzaton wth nose shapng block has feedback paths, therefore s more dffcult to mplement even though the PRF s merely 1 MHz. The FIR NTF flter s realzed usng the transposed struccture, combned wth retmng technque and canonc sgned dgt (CSD) converson of some coeffcents. The mxng between LO sgnals and all the possble baseband sgnals were precomputed and stored n a ROM. Ths ROM s addressed by the quantzaton output,.e., the quantzed pulse wdth. The selected RF waveforms from nphase path and quadrature path wll be combned before the hgh speed seralzer generates the 1 bt RF sgnal. The FPGA devce chosen for the prototype s Xlnx s Vrtex2pro: XC2VPX2FF896, speed grade 7. Ths FPGA has on chp multggabt transcever (MGT) whch s used as the hgh speed parallel to seral converter to generate bnary sgnal at 3.2 GHz. In Table 2, the logc resource allocaton s broken down for the QAM modulator mplemented on ths devce. Table 2 Logc resource allocaton for QAM modulator Logc Elements QAM REGISTER 787(26.1%) 2221(73.5%) LUT 2447(47.7%) 1833(35.7%) MUXCY 252(67.1%) 18(32.9%) XORCY 1958(66.8) 973(33.2%) The logc elements allocaton results are extracted from Synplcty synthess results. The fnal place and route tool reports that the entre QAM desgn utlzes 18 multplers (2%), 4 RAM16s (4%), and 3911 slces (39%). The percentage number n the parenthess s the percentage wth respect to the total avalable resources on ths FPGA. The spectrum and EVM measurements are shown n Fgure 6. Whle the passband nose floor s about 45 db down from the sgnal, more aggressve nose shapng technque can be used to acheve better passband nose performance. However the computaton requrement for that case mght prevent t beng mplemented, at least usng the off the shelf FPGA devce wll be very dffcult. The EVM measurement s less than 1%, whch s almost dentcal to the EVM measured at the output of QAM [9]. 5. SUMMARY An all dgtal QAM modulator s presented n ths paper. To the best of the authors knowledge, ths work s the frst demonstraton of real tme dgtal RF sgnal synthess, especally usng an off the shelf FPGA devce. The fundamental operatons for dgtal are brefly dscussed, ncludng natural samplng and quantzaton wth nose shapng. The nose shapng of dgtal dffers from tradtonal DAC converter based on smulaton results; therefore the NTF for dgtal needs to be desgned usng addtonal restrctons. A desgn method s formulated based on the observaton from smulaton, and one desgn example s presented based on our QAM modulator

5 requrements. Ths type of NTF s chosen manly because of ts less computaton requrement. The QAM modulator s archtecture and performance measurements are presented at the end. DSP N F s=4f o 6. ACKNLEDGEMENT The authors would lke to thank Curts Wllams n Motorola Labs for sharng hs experence of usng MGT on the Xlnx s FPGA devces. DSP I Q Swtchng PA Dgtal upconverter Analog flter BPDS modulator 7. REFERENCES Analog [1] Walter Tuttlebee (edtor), Software Defned Rado: Enablng Technologes, John Wley and Sons, England, 22 [2] P. Wagh, et. al., An All Dgtal Unversal RF Transmtter, Proceedngs of IEEE 24 Custom Integrated Crcuts Conference, pp [3] K. Keyzer, et. al., Dgtal Generaton of RF Sgnals for Wreless Communcatons wth Bandpass DeltaSgma Modulaton, Mcrowave Symposum Dgest, 21 IEEE MTTS Internatonal, pp [4] P. Mdya, P. Wagh, and P. Rakers, Quadrature Integral Nose Shapng for Generaton of Modulated RF Sgnals, Proceedngs of the 45 th Mdwest Symposum on crcuts and systems, Vol. 2, pp [5] J.M. Goldberg and M.B. Sandler, New Hgh Effcency Pulse Wdth Modulaton Based DgtaltoAnalogue Converter/Power Amplfer, IEE Proceedngs on Crcuts, Devces and Systems, Vol. 141, No. 4, pp [6] B. Gwee, J. S. Chang, and H. L, A Mcropower Low Dstorton Dgtal Pulsewdth Modulator for a Dgtal Class D Amplfer, IEEE Trans. On Crcuts and Systems II, Vol. 49, No. 4, pp [7] P. Mdya, M. Mller, and M. Sandler, Integral Nose Shapng for Quantzaton of Pulse Wdth Modulaton, 19 th Conventon of the Audo Engneerng Socety, Fall 2 [8] S. R. Norsworthy, Optmal Nonrecursve Nose Shapng Flters for Oversamplng Data Converters Part I: Theory, Proceedngs of ICASSP 1993, pp [9] Z. Ye and J. Grosspetsch, Sample Rate Converter for Dgtal Modulator, Proceedngs of the 24 SDR Techncal Conference, Vol. B, pp.2934 PCM nput I Q I Q Fgure 1 Dgtal transmtter usng BPDS Interpolator Dgtal modulator Hgh speed clock Natural Sampler Dgtal F s = PRF Quantzaon wth nose shapng Power stage Fgure 2 All dgtal transmtter usng LO waveforms Baseband waveforms 1 1 RF waveforms Fgure 3 Dgtal RF tmng dagram

6 PCM data Natural samplng + Quantzer + 1H(z) generator Wndowng, FFT, Inband SNR calculaton + + Quantzer Fgure 4 Smulaton to compare nose shapng behavors of and tradtonal DAC QAM 5.57 MHz 1 MHz QAM symbols generator 16x nterpolator Sample rate converter 14 Natrual sampler quantzer + 1H(z) RF waveform ROM 32 1 MHz Inphase + 32 P/S from Quadrature path 3.2 GHz Fgure 5 Archtectural dagrams for all dgtal RF prototype Fgure 6 Measurements of RF: wdeband spectrum and EVM