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1、83080 Receiver Architectures and Synchronization in Digital CommunicationsPractical RF Architectures for GSMand WCDMA Mobile Terminals1© NOKIATTKK00.PPT/18.10.2000 / Mikko PesolaNOKIAMain Drivers for RF Architecture DesignMobile Generations1st generation systems, analog NMT, AMPS, TACS2nd gener

2、ation systems, digital GSM, IS-136 TDMA, IS-95 CDMA, PDC3rd generation systems, digital wide band WCDMA standards have been prepared by ETSI (Europe), ARIB (Japan), TIA (USA), 3GPP1 is a joint project that combines all proposals into one standard Harmonized standard includes three modes Direct seque

3、nce mode based on 3GPP (UTRA 3GPP = 3rd Generation Partnership Project UTRA = Universal Terrestrial Radio Access/FDD) Multi carrier mode based on CDMA2000 (USA proposal)2© NOKIATTKKOO.PPT/18.10.2000 / Mikko PesolaNOKIA TDD mode based on 3GPP (UTRA/TDD)Cost, Size, Power consumptionSystem complex

4、ity, time to market3© NOKIAComponent count in GSM RFTTKKOO.PPT/18.10.2000 / Mikko PesolaNOKIADesign Process - time to marketReq Specfs| System DesMeeh. DesignCritical Interface:A/AS IC Desk sPrMCtrsw 二 _desi9n_ /ASIC DesignConceptsApplicationsfrom system design iodetailed design4© NOKIATTK

5、KOO.PPT/18.10.2000 / Mikko PesolaIntegration | Mfg ramp-upCritical Interface: from detailed desig to integrationNOKIAGSM Specifications for Receiver Bad frame indication performance Sensitivity Usable receiver input level range Co - channel rejection Adjacent channel rejection (selectivity) Intermod

6、ulation rejection Blocking and spurious response AM suppression5© NOKIATTKKOO.PPT/18.10.2000 / Mikko PesolaNOKIAExample of sensitivity specificationTable 1: Reference sensitivity performanceGSM 900Type of channelstaticPropagation conditionsHTI00TU50TU50RA250FACCH/H(FER)0.1 %(no FH) 6.9%(ideal F

7、H)6.9%(no FH) 5.7%(no FH) 10.0%FACCH/F(FER)0.1 %8.0%3.8%3.4%6.3%SDCCH(FER)0.1 %13%8%8%12%RACH(FER)0.5%13%13%12%13%SCH(FER)1 %16%16%15%16%TCH/F9.6 & H4.8(BER)IO-50.5%0.4%0.1 %0.7%TCH/F4.8(BER)-IO4io-410-4io-4TCH/F2.4(BER)一2 IO-410-5io-5io-5TCH/H2.4(BER)-IO4io-4io-4io-4TCH/FS(FER)0.1a%6a%3a%2a %7a

8、%class lb (RBER)0.4/a%0.4/a%0.3/a%0.2/a %0.5/a %class 11 (RBER)2%8%8%7%9%TCH/HS(FER)0.025 %4.1 %4.1 %4.1 %4.5%class lb (RBER. BF1=O)0.001 %0.36 %0.36 %0.28%0.56%class II (RBER. BF1=O)0.72%6.9%6.9%6.8%7.6%(U1R>0.048 %5.6%5.6%5.0%7.5%class lb (RBER.(BFI or UFl)=0)0.001 %0.24%0.24%0.21 %032%(EVS1DR)

9、0.06%6.8%6.8%6.0%9.2%(RBER. S1D=2 and (BFI or UFI)=0)0.001 %0.01 %0.01 %0.01 %0.02%(ES1DR)0.01 %3.0%3.0%3.2%3.4%(RBER.SID=1 or S1D=2)0.003%0.3%0.3%0.21 %0.42%6© NOKIATTKKOO.PPT/18.10.2000 / Mikko PesolaNOKIAHow to convert system specificationsto RF design parameters ? From the bit error rate (B

10、ER) requirement of the whole phone we can derive secondary specifications for the radio section (Gain, Signal to Noise ratio, Linearity) System specification and detector implementation define the required S/N ratio GSM needs 8.9 dB S/N (number includes some implementation margin 2.3dB)-15 dBm7©

11、; NOKIA-102 dBmTTKK00.PPT/18.10.2000 / Mikko PesolaNOKIASensitivitySignal must be 9 dB above noise floor, so requirement for receiver noise figure (NF) isNF = Ps+ 174 dBm -10 log B - S/N八PowerwhereB is equivalent noise bandwidth of the receiverPs is reference sensitivity levelFor GSM 900 MHz Ps = -1

12、02 dBm -> NF = 10dB8© NOKIAtoTTKK00.PPT/18.10.2000 / Mikko PesolaNOKIASensitivity and SelectivityCascaded noise figure NF = 10 log F, where NF is noise figure and F is noise factor F = F1 + (F2-1 )/G1 + (F3-1 )/G-|G2 + . + (Fn-1)/ G1G2.Gn_1RF section must have enough gain to provide adequate

13、 signal level for A/D converterMinimum input level is -102 dBm -> 1.8 uV 50 ohm. If we need for example 100 mV at A/D converter input, voltage gain needs to be 20 log(100mV/1.8uV) = 95dB二FiF2G1G29© NOKIATTKK00.PPT/18.10.2000 / Mikko PesolaNOKIASelectivityBefore detector interfering signals n

14、eed to be filtered so that S/N is adequateThis attenuation is split between IF filter, analog BB filter and digital filter after A/DDigital filtering at baseband would be cost effective but then A/D coverter must be able to handle wider dynamic range (typical A/D resolution 10.12 bits)Distribution o

15、f gain and selectivity affects also linearity requirement of the analog partIf interfering signal is for example -43 dBm -> 5 mV 50 ohm and we amplify that 95 dB the signal level would be 280 V !Microsoft ExcelWorksheet10© NOKIATTKK00.PPT/ 18.10.2000 / Mikko PesolaNOKIAPhase noiseLocal oscil

16、lator (LO) phase noise has effect both on sensitivity and selectivityPhase noise may pass through mixer and degrade sensitivity different ways1) Noise at IF leaks directly into mixer IF port2) Noise at RF leaks directly into mixer RF port3) Noise at the distance of IF from the RX-frequency mixes wit

17、h RX and appears in the mixer IF port4) Noise at the distance of IF from the LO-frequency mixes with LO and appears in the mixer IF portTTKKOO.PPT/ 18.10.2000 / Mikko Pesola11© NOKIANOKIA12© NOKIATTKKOO.PPT/ 18.10.2000 / Mikko PesolaNOKIAUsually selectivity sets most difficult requirement

18、for VCO phase noiseExample GSM 900 MHz handportable: wanted signal -102 dBm + 3 dB = -99 dBm blocking signal -43 dBm 600 kHz offset typical values S/N = 9 dB, noise BW = 200 kHz Phase noise = -99 - (-43) - 10 log 200000-9 = -118 dBc/Hz (600 kHz)13© NOKIATTKK00.PPT/ 18.10.2000 / Mikko PesolaNOKI

19、AReceiver ArchitecturesHeterodyne+ flexibility, achievable component specifications+ standard components available-spurious responsesIF Sampling+ no need for l/Q mixer-requires better A/D converterHomodyne (direct conversion)+ simplest architecture+ no IF filters, advantage especially in multiband/m

20、ultisystem phones-very difficult to implement14© NOKIATTKKOO.PPT/ 18.10.2000 / Mikko PesolaNOKIAGSM Receiver Principles GMSK modulation has no AM component that carries information However the equalizer requires real time amplitude information to be able to correct the multipath propagation Mos

21、t common solution is linear receiver that provides in-phase (I) and quadrature (Q) signals for the basebandt Non linear (limiting) receiver possible, if it can provide amplitude information for the equalizer15© NOKIATTKKOO.PPT/ 18.10.2000 / Mikko PesolaNOKIALimiting Receiver in GSMRelaxes AGC a

22、nd A/D requirementsHigh level interferers must be low enough before limiter -> requires good IF filtersCan handle fast amplitude changes without saturation16© NOKIATTKKOO.PPT/ 18.10.2000 / Mikko PesolaNOKIASpurious response rejectionMixing products in nonlinear circuits can cause spurious re

23、sponses if they fall at IF or RX frequencyUsually low order products are the most importantSome examples of spurious responses in superheterodyne receiver with high side LO injectionImage: fspuR -九。=LfHalf IF: 2fLO - 2fSPUR2LO17© NOKIATTKKOO.PPT/ 18.10.2000 / Mikko PesolaNOKIASpurious response

24、rejectionIn full duplex systems the high level TX signal causes more spurious responsesIf TX leaks to RX mixer it mixes directly with spurious signal: fSPUR - fTX = fFTTKKOO.PPT/ 18.10.2000 / Mikko PesolaNOKIA18© NOKIAChoosing IF frequencySpurious response requirements Image should be at relati

25、vely quiet band Usually IF/2 spurious should not be in RX band -> first IF > 2 * operating band TX + IF should not fall in RX band (on the other hand we can use TX+ IF =RX)IF filter availability, size and cost 71 MHz is most popular GSM IF at the moment Higher frequency SAW filter is smaller b

26、ut stopband attenuation is not as goodLO frequency lower frequency is slightly easier to implement. On the other hand in mobile TX band is below RX and LO may be too close to TX band with low side injectionAmount of oscillators Use same oscillator for RX and TX if possible harmonics of the oscillato

27、rs should not fall in RX band19© NOKIATTKKOO.PPT/ 18.10.2000 / Mikko PesolaNOKIADuplexing Techniques Uplink and downlink signals can be separated using different frequency or different time slot RX and TX use same frequency but different time slot -> Time Division Duplex (TDD) RX and TX are

28、on simultaneously but use different frequency -> Frequency Division Duplex (FDD) GSM can use either switch or duplexer WCDMA must use a duplexer (option for TDD in the spec.)TTKK00.PPT/ 18.10.2000 / Mikko Pesola20© NOKIANOKIADuplexing IssuesUsing a duplex filter in GSM may be feasible if RF

29、archtecture is such that also TX needs good filteringIf a simple high pass filter is adquate in TX, lower cost and size can be achieved with a switch and separate filtersTypically swithch and filters are combined in a ceramic moduleIn GSM system receive and transmit functions occur in different time

30、 slots Isolation between TX and RX is not a concern High level circuit integration possible Common blocks for RX and TX possible, e.g. synthesizer (frequency can be shifted between RX and TX time slots) Less spurious responses in the RX21© NOKIATTKKOO.PPT/ 18.10.2000 / Mikko PesolaNOKIAAutomati

31、c Gain Control (AGC)With AGC the average signal level at the A/D converter input is kept almost constant Trade off between AGC and A/D converter dynamic rangeIn GSM phone reports received signal strength and +1 dB relative measurement accuracy is required from -110 dBm to -48 dBmIn IS-95 CDMA the TX

32、 power must follow received signal strength: Pout =- Pin -76 dBm at the input range -104 dBm to -25 dBm (open loop power control, accuracy requirement +9.5 dB)In CDMA RX and TX gain controls should have similar temperature behavior22© NOKIATTKK00.PPT/ 18.10.2000 / Mikko PesolaNOKIAAGO Implement

33、ationAccuracy and temperature compensation are easier to implement at IF and basebandBy adjusting front-end gain, AGC also provides a measure against intermodulation distortion at high signal levelsNOKIAOften AGC is split between front-end and IF, for example 20.30 dB step in front-end and 6080 dB a

34、t IF23© NOKIA TTKK00.PPT/ 18.10.2000 / Mikko PesolaExample of a GSM Block Diagram24© NOKIATTKKOO.PPT/ 18.10.2000 / Mikko PesolaNOKIA25© NOKIASuperheterodyne GSM 900 MHzTTKK00.PPT/ 18.10.2000 / Mikko PesolaNOKIA26© NOKIATTKKOO.PPT/ 18.10.2000 / Mikko PesolaNOKIADirect Conversion I

35、mplementationLO leakage to the RX input causes DC- offsetLO leakage can not be filtered out and it may cause too high spurious emissions (limit in GSM specification is -57 dBm)High level interference signals can couple to LO an cause LO pullingRF part must be very linear because strong adjacent chan

36、nel signals are not filtered out until basebandTo meet all requirements mixer linearity and balance are essential27© NOKIATTKKOO.PPT/ 18.10.2000 / Mikko PesolaNOKIADirect Conversion ImplementationCan be much larger than a weak signalDC offset is first measured without signal before RX burstThis

37、 information is then used to cancel offset just before detectionDirect conversion implemented in GSM by Alcatel and NokiaRFIC vendors are developing direct conversion chipsets (Analog Devices)28© NOKIATTKKOO.PPT/ 18.10.2000 / Mikko PesolaNOKIABalanced circuits and double LO frequency reduce int

38、erference couplingBBMIX6OP 06CLDSPLO29© NOKIATTKKOO.PPT/ 18.10.2000 / Mikko PesolaNOKIAGSM Data Services, HSCSD and GPRSHSCSD & GPRS HSCSD = High Speed Circuit Switched Data GPRS = General Packet Radio Service Different coding 9.6 kbit/s -> 14.4 kbit/s More slots, for example 2 slots -&g

39、t; 28.8 kbit/s Dynamic use of slots in GPRS In multislot case receiver has less time to change channel -> synthesizer must be faster Adjacent slots may have different amplitude -> more dynamic range needed because gain control has no time to adapt30© NOKIATTKK00.PPT/ 18.10.2000 / Mikko Pe

40、solaNOKIAGSM Data Services, EDGEIn EDGE also modulation changes GMSK -> 8PSKNot constant amplitude anymore, only linear receiver possibleEDGE has several coding schemes and fastest data speeds can only be achieved with very good signal to noise conditionsTheoretical maximum 69 kbit/s per timeslot

41、AGC gain distribution important LNA gain step should not be used as low levels as in basic GSM31© NOKIATTKKOO.PPT/ 18.10.2000 / Mikko PesolaNOKIAGSM Radio Access in the Time Domain32© NOKIA4.615 msAdjacent cell BCCH max. 6 pcs.TTKKOO.PPT/ 18.10.2000 / Mikko PesolaNOKIAHSCSD RF 2 + 2 slots

42、or 3 +1 slots still leave some settling time between the slots In dual slot operation with maximum timing advance the time between RX and TX slots is 577 jus - 233(lis = 344 jus It becomes difficult to achieve this with conventional synthesizer Separate synthesizers for RX and TX Fractional-N synthe

43、sizer Use double frequency (synthesizer step 400kHz) and divide for LO 3 + 3 slots and higher the RX and TX overlap and whole RF system design must be different including two synthesizers, duplex filter and high isolation between TX and RX33© NOKIATTKK00.PPT/ 18.10.2000 / Mikko PesolaNOKIASprea

44、d-Spectrum Basics Spectrum is spread using a code that is independent of the information signal Bandwidth used in spread spectrum transmission is typically at least 100 times wider than the information signal bandwidth WCDMA uses direct sequence spreading where a pseudo-random code is directly combi

45、ned to the signal Users are separated by different codes Bits in the spreading code are called chips/frequencyTTKK00.PPT/ 18.10.2000 / Mikko PesolaWCDMA34© NOKIAfrequencyNOKIAMobile Radio ChannelMultipath propagation (frequency selective fading)ShadowingDoppler shiftlinear time-variant channelT

46、TKKOO.PPT/ 18.10.2000 / Mikko Pesola35© NOKIANOKIAMobile ReceiverIn GSM training sequence is used to estimate the impulse response of the radio channelThis information is necessary to equalize the user bits on a burst by burst basisCDMA chip rate is typically greater than flat fading BW of the

47、channel and multipath components appear like uncorrelated noise -> no equalizer is neededRAKE receiver combines time shifted versions of the original signal One finger is assigned for the direct signal, another for the reflected signalFingers in RAKE receiver can also be used for soft handoff One

48、 finger is assigned for new base station while another still listens the old BSRAKE receiver in implemented in digital baseband and it does not require any special functionality in RF part -> Receiver can be implemented with similar blocks as current GSM receiver36© NOKIATTKKOO.PPT/ 18.10.20

49、00 / Mikko PesolaNOKIACDMA System ParametersIS-95 CDMA Frequency bands Cellular band (AMPS): TX 824.849, RX 869.894 MHz PCS1900: TX 1850.1910, RX 1930.1990 MHz Carrier spacing 1.25 MHz, 64 Walsh codes Chip rate 1.2288 Mchip/sWCDMA Frequency bands UTRA FDD (Europe & Asia):TX 1920. 1980 MHz , RX 2

50、110.2170 MHz Carrier spacing 5.00 MHz Chip rate 3.84 Mchips/s (first proposal 4.096 Mchips/s)37© NOKIATTKK00.PPT/ 18.10.2000 / Mikko PesolaNOKIAReceiver requirementsSensitivity requirement is -117 dBm 12.2 kbps user data rateChanging user bit rate changes processing gain (PG) and also sensitivi

51、ty levelPG = 10 log (3.84Mcps/user symbol rate)Selectivity requirement is defined as RX filter attenuation at adjacentchannel compared to wanted channel, spec is 33 dB, interferer crest factorNoise floorTTKK00.PPT/ 18.10.2000 / Mikko Pesola38© NOKIANOKIARX Design issuesFull duplex operation TX

52、noise at RX band must be attenuated A good duplexer is relatively large and expensive componentHigh chiprate Wide BW in baseband -> higher current consumption Wide band IF filters -> higher loss39© NOKIATTKKOO.PPT/ 18.10.2000 / Mikko PesolaNOKIAWCDMA RX Design IssuesLinear modulation ->

53、; limitting receiver not possibleIn full duplex system TX noise at RX band may desense receiver If we require that TX is not decreasing RX sensitivity, noise at RX input must below thermal noise floor (-174 dBm/Hz) Even if TX chain is relatively low noise, about 40dB attenuation is needed in duplexerWider band