《DSP（数值信号处理）芯片应用(1).pdf》由会员分享，可在线阅读，更多相关《DSP（数值信号处理）芯片应用(1).pdf（46页珍藏版）》请在taowenge.com淘文阁网|工程机械CAD图纸|机械工程制图|CAD装配图下载|SolidWorks_CaTia_CAD_UG_PROE_设计图分享下载上搜索。
1、DSP APPLICATIONS9.aSECTION 9DSP APPLICATIONSIHigh Performance Modems for Plain Old TelephoneService(POTS)IRemote Access Server(RAS)ModemsIADSL(Assymetric Digital Subscriber Line)IDigital Cellular TelephonesIGSM Handset Using SoftFone Baseband Processorand Othello RadioIAnalog Cellular BasestationsID
2、igital Cellular BasestationsIMotor ControlICodecs and DSPs in Voiceband and Audio ApplicationsIA Sigma-Delta ADC with Programmable Digital FilterDSP APPLICATIONS9.bDSP APPLICATIONS9.1SECTION 9DSP APPLICATIONSWalt KesterHIGH PERFORMANCE MODEMS FOR PLAIN OLDTELEPHONE SERVICE(POTS)Modems(Modulator/Demo
3、dulator)are widely used to transmit and receive digitaldata using analog modulation over the Plain Old Telephone Service(POTS)networkas well as private lines.Although the data to be transmitted is digital,the telephonechannel is designed to carry voice signals having a bandwidth of approximately 300
4、to 3300Hz.The telephone transmission channel suffers from delay distortion,noise,crosstalk,impedance mismatches,near-end and far-end echoes,and otherimperfections.While certain levels of these signal degradations are perfectlyacceptable for voice communication,they can cause high error rates in digi
5、tal datatransmission.The fundamental purpose of the transmitter portion of the modem isto prepare the digital data for transmission over the analog voice line.The purposeof the receiver portion of the modem is to receive the signal which contains theanalog representation of the data,and reconstruct
6、the original digital data at anacceptable error rate.High performance modems make use of digital techniques toperform such functions as modulation,demodulation,error detection and correction,equalization,and echo cancellation.A block diagram of an ordinary telephone channel(often referred to as plai
7、n oldtelephone service or POTS)is shown in Figure 9.1.Most voiceband telephoneconnections involve several connections through the telephone network.The 2-wiretwisted pair subscriber line available at most sites is generally converted to a 4-wiresignal at the telephone central office:two wires for tr
8、ansmit,and two wires forreceive.The signal is converted back to a 2-wire signal at the far-end subscriberline.The 2-to 4-wire interface is implemented with a circuit called a hybrid.Thehybrid intentionally inserts impedance mismatches to prevent oscillations on the 4-wire trunk line.The mismatch for
9、ces a portion of the transmitted signal to bereflected or echoed back to the transmitter.This echo can corrupt data thetransmitter receives from the far-end modem.Half-duplex modems are capable of passing signals in either direction on a 2-wireline,but not simultaneously.Full-duplex modems operate o
10、n a 2-wire line and cantransmit and receive data simultaneously.Full-duplex operation requires the abilityto separate a receive signal from the reflection(echo)of the transmitted signal.Thisis accomplished by assigning the signals in the two directions different frequencybands separated by filtering
11、,or by echo cancelling in which a locally synthesizedreplica of the reflected transmitted signal is subtracted from the composite receivesignal.DSP APPLICATIONS9.2Figure 9.1There are two types of echo in a typical voiceband telephone connection.The firstecho is the reflection from the near-end hybri
12、d,and the second echo is from the far-end hybrid.In long distance telephone transmissions,the transmitted signal ishetrodyned to and from a carrier frequency.Since local oscillators in the networkare not exactly matched,the carrier frequency of the far-end echo may be offsetfrom the frequency of the
13、 transmitted carrier signal.In modern applications thisshift can affect the degree to which the echo signal can be canceled.It is thereforedesirable for the echo canceller to compensate for this frequency offset.For transmission over the telephone voice network,the digital signal is modulatedonto an
14、 audio sinewave carrier,producing a modulated tone signal.The frequency ofthe carrier is chosen to be well within the telephone band.The transmitting modemmodulates the audio carrier with the transmit data signal,and the receiving modemdemodulates the tone to recover the receive data signal.The base
15、band data signal may be used to modulate the amplitude,the frequency,orthe phase of the audio carrier,depending on the data rate required.These threetypes of modulation are known as amplitude shift keying(ASK),frequency shiftkeying(FSK),and phase shift keying(PSK).In its simplest form the modulatedc
16、arrier takes on one of two states-that is,one of two amplitudes,one of twofrequencies,or one of two phases.The two states represent a logic 0 or a logic 1.Low-to medium-speed data links usually use FSK up to 1,200 bits/s.MultiphasePSK are used for 2,400 bits/s and 4,800 bits/s links.PSK utilizes ban
17、dwidth moreefficiently than FSK but is more costly to implement.ASK is least efficient and isANALOG MODEM USING PLAIN OLD TELEPHONESERVICE (POTS)ANALOG CHANNELHYBRIDHYBRID+TRANSMITCHANNELRECEIVECHANNEL+TRANSMITTERRECEIVERRECEIVERTRANSMITTERNOISEFREQUENCYSHIFTNOISEFREQUENCYSHIFTFAR-ENDECHONEAR-ENDECH
18、OFAR-ENDMODEMNEAR-ENDMODEMFOUR-WIRE TRUNK222222DSP APPLICATIONS9.3used only for very low speed links(less than 100 bits/s).For 9,600 bits/s up to33,600 bits/s a combination of PSK and ASK is used,known as QuadratureAmplitude Modulation(QAM).The International Telegraph and Telephone Consultative Comm
19、ittee(CCITT inFrance)has established standards and recommendations for modems which aregiven in Figure 9.2.Figure 9.2The goal in designing high performance modems is to achieve the highest datatransfer rate possible over the POTS network and avoid the expense of usingdedicated conditioned private te
20、lephone lines.The V.90 recommendation describes afull-duplex(simultaneous transmission and reception)modem that operates on thePOTS network.The V.90 modem communicates downstream from the central officeto the subscriber modem at a rate of 56,000 bits/s using Pulse Code Modulation(PCM).Upstream commu
21、nication from the subscriber to the central office is at theV.34 rate of up to 33,600 bits/s(QAM).A simplified block diagram for a V.90 analog modem is shown in Figure 9.3.Thediagram shows that the bulk of the signal processing is done digitally.Both thetransmit and receive portions of the modem sub
22、ject the digital signals to a numberof DSP algorithms which can be efficiently run on modern processors.SOME MODEM STANDARDSCCITTRec.ApproximateDateSpeed(bits/s)maximumHalf Duplex/Full Duplex/EchoCancelModulationMethodV.211964300FDXFSKV.221200FDXPSKV.22bis2400FDX16QAMV.231200HDXFSKV.26 bis2400HDXPSK
23、V.26 ter2400FDX(EC)PSKV.27 ter4800HDX8PSKV.329600FDX(EC)32QAMV.32 bis14400FDX(EC)QAMV.3433600FDX(EC)QAMV.90199856000*FDX(EC)PCMV.92200156000*FDX(EC)PCM*DOWNSTREAM ONLY,UPSTREAM IS V.34*UPSTREAM AND DOWNSTREAMDSP APPLICATIONS9.4Figure 9.3The TX input serial bit stream is first scrambled and encoded.S
24、crambling takes theinput bit stream and produces a pseudo-random sequence.The purpose of thescrambler is to whiten the spectrum of the transmitted data.Without thescrambler,a long series of identical symbols could cause the receiver to lose carrierlock.Scrambling makes the transmitted spectrum resem
25、ble white noise,to utilizethe bandwidth of the channel more efficiently,makes carrier recovery and timingsynchronization easy,and makes adaptive equalization and echo cancellationpossible.The scrambled bit stream is divided into groups of bits,and the groups of bits arefirst differentially encoded a
26、nd then convolutionally encoded.The symbols are then mapped into the signal space using QAM as defined in theV.34 standard.The signal space mapping produces two coordinates,one for the realpart of the QAM modulator and one for the imaginary part.As an example,adiagram of a 16-QAM signal constellatio
27、n is shown in Figure 9.4.Largerconstellations are used in V.90 modems,and the actual size of the constellation isadaptive and determined during the training,or“handshake”interval when themodems synchronize with each other for upstream or downstream signaling.V.90 ANALOG MODEM SIMPLIFIED BLOCK DIAGRA
28、MSIGNALENCODINGMODULATIONANDFILTERING SIGNALENCODINGDACHYBRIDANALOGLPFANALOGLPFADCSIGNALDECODING DE-MODULATIONANDFILTERINGSIGNALDECODINGECHO CANCELLINGADAPTIVEFILTER sin tcos tsin tcos tPOTSLINETXDATARXDATAfsfsMIXED-SIGNAL FUNCTIONSDSP FUNCTIONSDSP APPLICATIONS9.5Figure 9.4Used prior to modulation,d
29、igital pulse shaping filters attenuate frequencies abovethe Nyquist frequency that are generated in the signal mapping process.Thesefilters are designed to have zero crossings at the appropriate frequencies to cancelintersymbol interference.QAM is easily implemented in modern DSP processors.The proc
30、ess of modulationrequires the access of a sine or cosine value,the access of an input symbol(x or ycoordinate)and a multiplication.The parallel architecture of the ADSP-21xx-familypermits all three operations to be performed in a single instruction cycle.The output of the digital modulator drives a
31、DAC.The output of the DAC is passedthrough an analog lowpass filter and to the 2-wire telephone line for transmissionover the POTS network.The receiver is made up of several functional blocks:the input antialiasing filterand ADC,a demodulator,an adaptive equalizer,a Viterbi decoder,an echocanceller,
32、a differential decoder,and a descrambler.The receiver DSP algorithmsare both memory-intensive and computation-intensive.The ADSP-218x-familyaddresses both needs,providing sufficient program memory RAM(for both code anddata)on chip,data memory RAM on chip,and an instruction execution rate of up to75M
33、IPS.The antialiasing filter and ADC in the receiver need to have a dynamic range fromthe largest echo signal to the smallest.The received signal can be as low as-40dBm,QUADRATURE AMPLITUDE MODULATED(QAM)SIGNALTRANSMITS 4 BITS PER SYMBOL(16-QAM)00001111QI4-BITS/SYMBOLI OR Q CHANNELSAMPLINGCLOCKtDSP A
34、PPLICATIONS9.6while the near-end echo can be as high as-6dBm.In order to ensure that the analogfront end of the receiver does not contribute any significant impairment to thechannel under these conditions,an instantaneous dynamic range of 84dB and anSNR of 72dB is required.In order to compensate for
35、 amplitude and phase distortion in the telephone channel,equalization is required to recover the transmitted data at an acceptably low biterror rate.In order to respond to rapidly changing conditions on the telephone line,adaptive equalization is required for the V.90 modem receiver.An adaptiveequal
36、izer can be implemented digitally in an FIR filter whose coefficients arecontinuously updated based on current line conditions.Separation between the transmit and receive signal in the V.90 modem isaccomplished using echo cancellation.Both near-end and far-end echo must becancelled in order to yield
37、 reliable communication.Echo cancellation is achieved bysubtracting an estimate of the echo return signal from the actual received signal.The predicted echo is determined by feeding the transmitted signal into an adaptivefilter with a transfer function that approximates the telephone channel.Theadap
38、tive filter commonly used in echo cancellers is the FIR filter(chosen for itsstability and linear phase response).The taps are determined using the least-mean-square(LMS)algorithm during a training sequence executed prior to full-duplexcommunications.The most common technique for decoding the receiv
39、ed data is Viterbi decoding.Named after its inventor,the Viterbi algorithm is a general-purpose technique formaking an error-corrected decision.Viterbi decoding provides a certain degree oferror correction by examining the received bit pattern over time to deduce the valuethat was the most likely to
40、 have been transmitted at a particular time.Viterbidecoding is computation-intensive.A history for each of the possible symbols sent ateach symbol interval has to be maintained.At each symbol interval,the length ofthe path backward in time from each possible received symbol to a symbol sentsome time
41、 ago is calculated.The symbol that has the shortest path back to theoriginal signal is chosen to be the current decoded symbol.A complete description ofViterbi decoding and its implementation on the ADSP-21xx-family of processors isgiven in documentation available from Analog Devices(Reference 2).Fi
42、gure 9.5 shows a comparison between V.34 and V.90 modems.Note that in thecase of V.34(Figure 9.5A),the communication is between two analog modems.Thisrequires and ADC/DAC in both the transmit and receive path as shown in thediagram.The V.90 system requires an all digital network and a V.90 digital m
43、odemas shown in Figure 9.5B.Note that the second ADC/DAC combination is eliminated,thereby allowing the faster downstream data rate of 56Kbits/s.Downstreamcommunication to the V.90 analog modem uses 64Kbits/s PCM data,which isstandard for all digital telephone networks.This serial data is converted
44、into apulse amplitude modulated(PAM)signal(8-bits,8kSPS)using an 8-bit DAC.Thesignal from the DAC to the analog modem is therefore a 256K constellation with noimaginary component,i.e.,the analog modem receiver must detect which one of the256 levels is being sent during the symbol interval.DSP APPLIC
45、ATIONS9.7The V.90 standard allows downstream data rates of up to slightly less than56Kbits/s,and upstream data rates of up to 33.6Kbits/s(V.34).The V.92 standardwill allow 56Kbits/s transmission in both directions.Figure 9.5REMOTE ACCESS SERVER(RAS)MODEMSRapid growth and use of the Internet has crea
46、ted a problem in that there are moreusers trying to get on the Internet than there is equipment to accommodate allthese users.Internet Service Providers(ISPs)like America On Line purchasemodem equipment so their customers(referred to as subscribers)can remotelyaccess a network(like the Internet from
47、 home).This application of accessing anetwork from a remote location is called Remote Network Access.The equipmentused in this application is called a Remote Access Server(RAS)as shown in Figure9.6.The Remote Access Server is made up of many modem ports;each modem portcan connect to a different user
48、.The RAS can use analog modems,which connects toa POTS line,or digital modems which are compatible with T1,E1,PRI,or BRI lines.Digital modems are used in most RAS systems since they are more efficient for 8ports or more.Network access equipment enables individuals,small offices,and travelingemployee
49、s to connect to corporate networks(Intranets)and the Internet.InternetService Providers use devices called concentrators to connect their telephone accesslines to their networks.These concentrators are also referred to as Remote AccessServers.The rapid growth in the use of the Internet and Intranets
50、 has created atremendous demand for modem equipment.V.34 VERSUS V.90 MODEMSV.34ANALOGMODEMDACADCPSTN-ANALOG ORDIGITALADCDACV.34ANALOGMODEM33.6K bps33.6K bpsV.90ANALOGMODEMDACADCPSTN-DIGITAL56K bps33.6K bpsV.90DIGITALMODEM8-BIT,8KSPSPAM DATAQAMDATAQAMDATA(V.34)8-BIT,8KSPS PCM=64K bpsQAMDATAABDSP APPL