通信原理英文版課件：Ch6 Passband Data Transmission

1、Ch.6 Passband Data Transmission6.1 Introduction6.2 Passband Transmission Model6.3 Coherent Phase-Shift Keying6.4 Hybrid Amplitude/Phase Modulation Schemes6.5 Coherent Frequency-Shift Keying6.6 Detection of Signals with Unknown Phase6.7 Noncoherent Orthogonal Modulation6.8 Noncoherent Binary Frequenc

2、y-Shift Keying6.9 Differential Phase-Shift Keying6.10 Comparison of Digital Modulation Schemes Using a Single Carrier6.11 Voiceband Modulation6.12 Multichannel Modulation6.13 Discrete Multitone6.14 Synchronization6.15 Computer Experiments: Carrier Recovery and Symbol Timing6.16 Summary and Discussio

3、n16.1 IntroductionDigital Passband TransmissionHierarchy of Digital Modulation TechniquesSome Basic ConceptsProbability of errorPower spectraBandwidth efficiency2Digital Passband TransmissionData transmission by modulating a carrier (usually sinusoidal)“Keying” Switching the amplitude, frequency, or

4、 phase of a sinusoidal carrierASKPSKFSKFig. 1. Illustrative waveforms for the three basic forms of signaling binary information.(a) Amplitude-shift keying. (b) Phase-shift keying. (c) Frequency-shift keying with continuous phase.3Hierarchy of Digital Modulation TechniquesM-ary signaling schemesM-ary

5、 ASK (M-ASK), M-ary PSK (M-PSK), and M-ary FSK (M-FSK)M-ary QAM (M-QAM) a special case of M-ary APK (M-APK)Reduction in transmission bandwidth over binary schemesConstant and non-constant envelope modulation schemesCoherent and noncoherent systemsCoherent systems: synchronized in both frequency and

6、phase (M-PSK, M-QAM, M-FSK)Noncoherent systems (M-DPSK and M-FSK)4Some Basic ConceptsProbability of errorMinimizing average symbol error probability in AWGN channelsExact formulas or union boundsPower spectra5Some Basic Concepts (contd)Bandwidth efficiencyDefinition: The ratio of data rate to the ef

7、fectively utilized channel bandwidthObjectives:Maximizing Using minimum average power (or SNR)Two factors affecting Multilevel encodingSpectral shaping66.2 Passband Transmission ModelFig. 2. Functional model of passband data transmission system.76.3 Coherent Phase-Shift KeyingSchemes coveredBinary P

8、SK (BPSK)QPSK and its variantsM-PSKContentsSignal-space diagramError probabilityGeneration and detection of signalsPower spectra8BPSK Signal-Space DiagramFig. 3. Signal-space diagram for coherent BPSK system. The waveforms depicting the transmitted signals s1(t) and s2(t), displayed in the inserts,

9、assume nc 2.9BPSK Error ProbabilityThe observable elementThe decision region associated with symbol 1The conditional pdf of X1, given that symbol 0 was transmitted 10BPSK Error Probability (contd)The conditional probability of the receiver deciding in favor of symbol 1, given that symbol 0 was trans

10、mitted The overall average symbol error probability (in the case of BPSK, also the Bit Error Rate (BER)11BPSK Generation and Detection of Coherent BPSK SignalsFig. 4. Block diagrams for (a) binary PSK transmitter and (b) coherent binary PSK receiver.12BPSK Power SpectraFig. 5. Power spectra of binar

11、y PSK and FSK signals.13QPSK Signal-Space DiagramFig. 6. Signal-space diagram of coherent QPSK system.14Example 6.1Fig. 7. (a) Input binary sequence. (b) Odd-numbered bits of input sequence and associated binary PSK wave. (c) Even-numbered bits of input sequence and associated binary PSK wave. (d) Q

12、PSK waveform defined as s(t) si11(t) si22(t).15QPSK Error ProbabilityA QPSK system = Two BPSK systems( Pe can also be derived by using the Union Bound)16QPSK Generation and Detection of Coherent QPSK SignalsFig. 8. Block diagrams of (a) QPSK transmitter and (b) coherent QPSK receiver.17QPSK Power Sp

13、ectraFig. 9. Power spectra of QPSK and MSK signals.18Offset QPSKPhase transitions are confined to 90Compared to QPSK, amplitude fluctuations due to filtering is reducedThe same error probability with QPSKFig. 10. Possible paths for switching between the message points in (a) QPSK and (b) offset QPSK

14、.19/4-Shifted QPSKHaving 8 possible phase states and possible phase changes are /4, 3/4, -3/4, and -/4Compared to QPSK, envelope variations due to filtering are significantly reducedCompared to offset QPSK, can be noncoherently detectedFig. 11. Eight possible phase states for the /4-shifted QPSK mod

15、ulator.20/4-Shifted QPSK (contd)Fig. 12. Block diagram of the /4-shifted DQPSK detector.21M-PSK Signal-Space Diagram and Error ProbabilityFig. 13. (a) Signal-space diagram for 8-PSK. The decision boundaries are shown as dashed lines. (b) Signal-space diagram illustrating the application of the union

16、 bound for 8-PSK.22M-PSK Power SpectraFig. 14. Power spectra of M-ary PSK signals for M 2, 4, 8.23M-PSK Bandwidth EfficiencyM248163264r (bits/s/Hz)0.511.522.53Table 1. Bandwidth efficiency of M-PSK signalsMrError performance(Require more Eb/N0)246.4 Hybrid Amplitude/Phase Modulation SchemesPhase mod

17、ulation schemes (M-PSK)The envelop is constantHybrid amplitude/phase modulation schemes (QAM and CAP)The in-phase and quadrature component are independent, so that the envelop will not be constantQAM and CAP have the same signal constellations and performance25M-QAMM-QAM stands for “M-ary quadrature

18、 amplitude modulation” is a two dimensional generalization of M-PAMTwo types of QAM constellations (assuming M = 2n)Square constellation (n is an even number)Cross constellation (n is an odd number)26QAM Square Constellation Signal-Space DiagramFig. 15. Signal-space diagram of (a) 16-QAM with Gray-e

19、ncoding (b) the corresponding 4-PAM signal.27QAM Square Constellation Error Probability28QAM Cross ConstellationFig. 16. Illustrating how a square QAM constellation can be expanded to form a QAM cross-constellation.Perfect Gray coding is not possible.29CAP - Definition30CAP Definition (contd)31CAP P

20、ropertiesProperty 1Property 2Property 3Makes it possible for a CAP receiver to separate the transmitted real and imaginary symbols from the channel output32CAP Bandwidth-Efficient Spectral ShapingFig. 17. The baseband pulse g(t).rolloff factor a 0.2.Fig. 18. (a) In-phase pulse p(t), and (b) quadratu

21、re pulse .Using baseband raised-cosine shaping filter33CAP Generation and Detectionof CAP SignalsFig. 19. Block diagram of CAP transmitter.34CAP Generation and Detectionof CAP Signals (contd)Fig. 20. Block diagram of CAP receivers (a) for an AWGN channel and (b) for a noisy and dispersive channel.(a

22、)(b)35CAP Generation and Detectionof CAP Signals (contd)Fig. 21. Digital implementation of the CAP receiver. (a) Using an A/D converter. (b) Replacement of the matched filter/equalizer pairs with equivalent (digitally implemented) FIR filters.366.5 Coherent Frequency-Shift KeyingCoherent FSK is a no

23、nlinear modulation schemeM-PSK and M-QAM are linear modulation schemesSchemes covered in this sectionBinary FSK (BFSK)Minimum shift keying (MSK)Gaussian-filtered MSK (GMSK)M-ary FSK (M-FSK)37BFSK Signal-Space DiagramFig. 22. Signal-space diagram for binary FSK system. The diagram also includes two i

24、nserts showing example waveforms of the two modulated signals s1(t) and s2(t).38BFSK Error ProbabilityCompared to coherent BPSK, coherent B-FSK requires double SNR for the same performance.39BFSK Generation and Detection of Coherent BFSK SignalsFig. 23. Block diagrams for (a) binary FSK transmitter

25、and (b) coherent binary FSK receiver.40BFSK Power SpectraFig. 5. Power spectra of binary PSK and FSK signals.41MSK - DefinitionContinuous Phase FSK (CPFSK)h is defined as “deviation ratio.” The minimum value of h that allows two FSK signals for symbols 1 and 0 to be coherently orthogonal is 1/2. A C

26、PFSK signal with h = 1/2 is referred to as Minimum Shift Keying (MSK).42MSK Phase TrellisThe transitions of phase across interval boundaries of bits form a phase tree.At odd or even bit duration Tb, the phase is an odd or even multiple of hAt Sundes FSK, h = 1, there is no memoryWhen h = 1/2, the ph

27、ase can only take values of /2 at odd multiples of Tb, and 0 and at even multiples of Tbh = 1/2 is the minimum value to ensure orthogonality of the two FSK signals representing symbols 1 and 0.43MSK Phase Trellis (Contd)Fig. 24. Phase tree of CPFSK.Fig. 25. Phase trellis of MSK; boldfaced path repre

28、sents the sequence 1101000.44MSK Signal-Space Diagram45MSK Signal-Space Diagram (contd)Fig. 26. Signal-space diagram for MSK system.46MSK An ExampleFig. 27. (a) Input binary sequence. (b) Waveform of scaled time function s1f1(t). (c) Waveform of scaled time function s2f2(t). (d) Waveform of the MSK

29、signal s(t) obtained by adding s1f1(t) and s2f2(t) on a bit-by-bit basis.47MSK Error ProbabilityMSK has the same error probability with BPSK and QPSKWith MSK, decision is made based on observations over 2Tb intervals48MSK Generation and Detectionof MSK SignalsFig. 28. Block diagrams for (a) MSK tran

30、smitter and (b) coherent MSK receiver.49MSK Power SpectraFig. 9. Power spectra of QPSK and MSK signals.50GMSK - DefinitionGMSK is MSK with a baseband pulse-shaping filter, whose impulse response is defined by a Gaussian function.GMSK has better out-of-band spectral characteristics than MSK.The impul

31、se responseThe transfer function51GMSK The Frequency Shaping PulseFig. 29. Frequency-shaping pulse g(t) shifted in time by 2.5Tb and truncated at 2.5Tb for varying time-bandwidth product WTb.The time-bandwidth product WTb is a design factor.52GMSK Power SpectraFig. 30. Power spectra of MSK and GMSK

32、signals for varying time-bandwidth product. (Reproduced with permission from Dr. Gordon Stber, Georgia Tech.)53GMSK Error PerformanceFig. 31. Theoretical Eb/N0 degradation of GMSK for varying time-bandwidth product. (Taken from Murata and Hirade, 1981, with permission of the IEEE.)54GMSK Application

33、 in GSMFig. 32. Power spectrum of GMSK signal for GSM wireless communications.WTb = 0.3Bandwidth for each channel is 200 KHzData rate is 271 kb/s99% of the radio frequency power is confined to a bandwidth of 250 KHzCo-channel interference is negligible.55M-FSK56M-FSK Power SpectraFig. 33. Power spec

34、tra of M-ary PSK signals for M 2, 4, 8.57M-FSK Bandwidth EfficiencyM 2481632 64r (bits/s/Hz) 110.750.50.3125 0.1875Table 2. Bandwidth efficiency of M-FSK signalsMr586.6 Detection of Signals with Unknown PhaseIn some cases, carrier phase recovery is difficultVariable multipath channelsRapidly varying

35、 delaysA digital receiver without carrier phase recovery is said to be “noncoherent”An optimum quadratic receiver and its two equivalent forms59Optimum Quadratic ReceiverI0(x) is the modified Bessel function of zero order.60Optimum Quadratic Receiver (contd)Fig. 34. Quadrature receiver using correla

36、tors.Since I0(x) is a monotonically increasing function, comparison between L(si) is equivalent to comparison between li, and also to that of li2.The decision rule:61Two Equivalent forms of theQuadratic ReceiverFig. 35. Noncoherent receivers. (a) Quadrature receiver using matched filters. (b) Noncoh

37、erent matched filter.Fig. 36. Output of matched filter for a rectangular RF wave: (a) q 0, and (b) q 180.(a)(b)626.7 Noncoherent Orthogonal ModulationNoncoherent orthogonal modulationThe signals s1(t) and s2(t) are orthogonal and have the same energy.The channel shifts the carrier phase of the signa

38、ls, and the shifted signals g1(t) and g2(t) are also orthogonal and have the same energy.Generalized binary receivers and average error probabilityNoncoherent BFSK and DPSK63Generalized Binary ReceiversFig. 37. (a) Generalized binary receiver for noncoherent orthogonal modulation. (b) Quadrature rec

39、eiver equivalent to either one of the two matched filters in part (a); the index i 1, 2.646.8 Noncoherent BFSKFig. 38. Noncoherent receiver for the detection of BFSK signals.The transmitted signal:Bit error rate:656.9 Differential Phase-Shift Keying (DPSK)DPSK is a noncoherent version of PSKAt the t

40、ransmitter: differential encoding + phase-shift keyingAt the receiver: retrieving data from relative phase difference between two successive bit intervalsAn example of noncoherent orthogonal modulation with T = 2Tb and E = 2EbBit error rate:66Generation of DPSK Signalsbk 1 0 0 1 0 0 1 1 dk-1 1 1 0 1

41、 1 0 1 1 dk 1 1 0 1 1 0 1 1 1Transmitted phase 0 0 p 0 0 p 0 0 0Table 3. Illustrating the generation of DPSK signalsFig. 39. Block diagram of DPSK transmitter.67Detection of DPSK SignalsFig. 40. Block diagram of DPSK receiver.Fig. 41. Signal-space diagram of received DPSK signal.686.10 Comparison of

42、 Digital Modulation Schemes Using a Single CarrierProbability of error (BER) ofCoherent schemes including BPSK, QPSK, BFSK, and MSK andNoncoherent schemes including noncoherent BFSK and DPSKBandwidth efficiency of M-PSK, M-QAM, and M-FSK69Comparison of BER over AWGN ChannelsFig. 42. Comparison of th

43、e noise performance of different PSK and FSK schemes.Coherent BPSK, QPSK, and MSKCoherent BFSKDPSKNoncoherent BFSK70Comparison of Bandwidth EfficiencyHigher order M-PSK schemes have higher bandwidth efficiency, but require more power.Among the M-PSK schemes, QPSK offers the best trade-off between po

44、wer and bandwidth requirements, and thus widely used in practice.M-QAM outperforms M-PSK in error performance for M 4M-QAM requires that the channel is free of nonlinearities.M-FSK schemes behave in an opposite manner to that of M-PSK schemes716.11 Voiceband ModemsModemA contraction of modulator-dem

45、odulatorA conversion device for transmission and reception of data over the Public Switched Telephone Network (PSTN)We focus on modems providing communications between a user and an Internet Service Provider (ISP)Two classes of configurations: symmetric and asymmetric72Two Classes of Modem Configura

46、tionsFig. 43. (a) Environmental overview of symmetric modem configuration: the upstream and downstream data rates are equal. (b) Environmental overview of “asymmetric” modem configuration: data rate downstream is higher than upstream.73Symmetric Modem ConfigurationsThe configuration is symmetric in

47、that:Modems at both the user end and the ISP end are the same (the analog modem)The downstream (from the ISP to the user) and upstream (from the user to the ISP) data rates are the sameIncluding a large number of modem types with data rates ranging from 300 b/s to 36600 b/sIn the following, taking t

48、he popular V.32 standard as an example (carrier frequency = 1800 Hz, modulation rate = 2400 bauds, and data rate is up to 9600 b/s)74The V.32 StandardTwo modulation schemesNonredundant coding (16-QAM)Trellis coding (Convolutional encoding + 32-QAM)4 dB effective coding gainSNR must be high enoughEmp

49、loying differential encoding in both schemes90rotational invariance is mandatedFig. 44. Gray encoding of the four quadrants and dibits in each quadrant for the V.32 modem. The dashed arrows illustrate the 90rotational invariance.75The V.32 Standard (contd)Fig. 45. Block diagrams of V.32 modem. (a) N

50、onredundant coding. (b) Trellis coding.76The V.32 Standard (contd)Fig. 46. Signal constellation of V.32 modem using (a) nonredundant coding. (b) trellis coding.77Asymmetric Modem ConfigurationsThe PSTN is almost digital except the local loop, which is analogA digital PSTN uses PCM for voice signal t

51、ransmission64 kb/s data signaling rate (8 kHz sampling rate and 8-bit coding for each sample)Analog modem at the user end and digital modem at the ISP endNo ADC between the ISPs digital modem and the digital PSTNDownstream data rate can be higher than upstream data rate78Digital and Analog ModemsDig

52、ital modemBandwidth of the baseband filter is about 3.5 kHzAchievable data rate is up to 56 kb/sAnalog modemThe same baseband filter as that of digital modemPerformance is limited by quantization noiseThe V.34 modem achieves data rate up to 33.6 kb/sV.90 modemIncludes both digital and analog modems7

53、96.12 Multichannel ModulationMultichannel modulationDividing a wideband channel with severe intersymbol interference into a large number of subchannels, each may be viewed effectively as an AWGN channelThis subsection introducesCapacity of AWGN channelContinuous-time channel partitioningThe concept

54、of geometric SNRLoading80Capacity of AWGN ChannelThe channel capacity:The realistic data rate: signal-to-noise ratio gapP: transmitted signal powers2: channel noise variance measured over the bandwidth B(Ref. 9.10.)81Continuous-Time Channel PartitioningFig. 47. Staircase approximation of an arbitrar

55、y magnitude response H(f); only positive-frequency portion of the response is shown.82Continuous-Time Channel Partitioning (contd)Fig. 48. Block diagram of multichannel data transmission system.The passband basis functions:83Continuous-Time Channel Partitioning (contd)Property 1Property 2Property 3P

56、roperties of the passband basis functions:84Geometric Signal-to-Noise Ratio85LoadingTaking into account the effect of channel:With the constraint that86Loading (contd)Fig. 49. Water-filling interpretation of the loading problem.876.13 Discrete MultitoneDiscrete MultiTone (DTM)Transforming a wideband

57、 channel into a set of N subchannels operating in parallelThe transformation is discrete in both time and frequencyThe communication system has a linear matrix representation, and Discrete Fourier Transform (DFT) (as well as Fast Fourier Transform (FFT) can be used for system implementationHaving ap

58、plications in Asymmetric Digital Subscriber Lines (ADSL) and Very-high-rate Digital Subscriber Lines (VDSL)A closely relative technology is Orthogonal Frequency Division Multiplexing (OFDM)88Matrix Representation ofa DMT systemFig. 50. Discrete-time representation of multichannel data transmission s

59、ystem.A circulant matrix, spectral decomposition (and therefore DFT) is possible89DFT/IDFTX is the DFT of x, and x is the IDFT (Inverse DFT) of X.The analysis functionThe synthesis function90DFT/IDFT (contd)Spectral decompositionQ is an orthonormal matrix or unitary matrix91Frequency-Domain Descript

60、ion of the ChannelTransmitter input in frequency domainIDFT of SDFT of x92DFT-Based DMT SystemFig. 51. Block diagram of the DMT data-transmission system.93Applications of DMTADSLTransmission over twisted pairsDate rates: 1.544 Mb/s downstream and 160 kb/s upstreamVDSLTransmission over twisted pairs