Digital Modulation Techniques

download Digital Modulation Techniques

of 21

  • date post

    18-Jul-2016
  • Category

    Documents

  • view

    8
  • download

    0

Embed Size (px)

description

digital modulation for telecom copamanies.

Transcript of Digital Modulation Techniques

  • page1Iwao Sasase, Keio University

    Digital Modulation Techniques

    Iwao Sasase Department of Information and

    Computer Science, Keio Universityhttp://www.sasase.ics.keio.ac.jpEmail:sasase@ics.keio.ac.jp

  • page2Iwao Sasase, Keio University

    Digital Modulation Techniques

    Amplitude Shift Keying (ASK)Frequency Shift Keying (FSK)Phase Shift Keying (QPSK)M (Multi)-ASK ,M-FSK, M-PSKQPSK, /4shift QPSKQuadrature Amplitude Modulation (QAM)Minimum Shift Keying (MSK) and GMSK

  • page3Iwao Sasase, Keio University

    Waveform and SpectrumBaseband pulse waveform has low frequency components

    By modulating the carrier depending on the input data, we can transmit the data in RF frequency

    -T/2 T/20

    V

    T

    time

    Frequency1/T 2/T 3/T0

    Am

    plitu

    de

    Spec

    trum

  • page4Iwao Sasase, Keio University

    Principle of Modulation

    Depending on the information of the baseband signal, amplitude, frequency or phase of the carrier is changed. tccos

    Modulation

    ctAtS cos)()( =amplitudefrequency tttS c )}(cos{)( +=phase )}({cos)( tttS c +=

    tccos

  • page5Iwao Sasase, Keio University

    Classification of Modulation Techniques

    Analog Modulation

    Digital Modulation

    AM (Amplitude Modulation)

    FM (Frequency Modulation)

    PM (Phase Modulation)

    ASK (Amplitude Shift Keying)

    FSK (Frequency Shift Keying)

    PSK (Phase Shift Keying)QAM (Quadrature Amplitude Modulation )

    )(tA

    )(t)(t)(tA

    )(tA)(t

    )(t

    )(t

  • page6Iwao Sasase, Keio University

    tf a(t)cos2(t)S cASK = )(ta amplitudecf carrier frequency

    ASK (Amplitude Shift Keying)Accomplished by changing an amplitude of carrier,

    depending on the input digital signalASK is also called as On-Off-Keying

    In binary signal case (2ASK),

    a(t) is either 1 or 0.As shown in Fig. 1, carrier becomes on and off

    In binary signal case (2ASK), a(t) is either 1 or 0.

    As shown in Fig. 1, carrier becomes on and off

    Binary OOK modulation waveform

  • page7Iwao Sasase, Keio University

    Non-Coherent DetectionEnvelope Detection of ASK

    Bandpass filter Rectifier

    Lowpass Filter Decision

    Th

    Threshold Voltage

  • page8Iwao Sasase, Keio University

    Coherent detectionCoherent Detection of ASK

    BandpassFilter

    Carrier Recovery

    Circuit

    Lowpass Filter Decision

    Threshold Voltage

    tcos

  • page9Iwao Sasase, Keio University

    FSK (Frequency Shift Keying)Accomplished by changing an instantaneous Frequency, depending on the input digital signal

    )2cos()2cos(

    {)(22

    11

    ++=

    tfAtfA

    tS FSK

    Input waveform

    FSK

    Binary FSK modulation waveform

    Carrier frequency fand f2 respond to 1 and 0, respectively

  • page10Iwao Sasase, Keio University

    PSK (Phase Shift Keying)Accomplished by changing a phase of carrier, Depending on the input digital signal

    PSK

    Binary PSK modulation waveform

    )tf cos(2 (t)S cPSK iA += A amplitudei phase

  • page11Iwao Sasase, Keio University

    SNR (Signal-to-noise ratio in dB)

    B

    E

    R

    (

    b

    i

    t

    e

    r

    r

    o

    r

    r

    a

    t

    e

    )

    P

    e PSKcoherent

    ASKcoherent

    FSKcoherent

    PSKdifferential FSKnoncoherent

    ASKnoncoherent

    Error Performance

    2/10 )4/(2

    1 NEerfcPe =ASK2/1

    0 )/(21 NEerfcPe =PSK

    2/1

    0

    )2

    (21

    NEerfcPe =FSK

    Bit error probability Bit error probability

    erfc complementary error function

    E energy per bit

    N power spectrum density (psd) of noiseBER Performance

  • page12Iwao Sasase, Keio University

    PSK has better power and frequency efficiencies compared to ASK and FSK

    PSK achieves small bit error rate (BER) for the same C/Ncarrier-noise ratio)

    PSK has constant envelope (no information in amplitude ), and is robust to time-varying fading channel.

    PSK is popularly used in many communication systems such as satellite and mobile communication systems.

    Characteristic of PSK

  • page13Iwao Sasase, Keio University

    MASK refers to ASK that transmit more bits by using multilevel amplitude a(t) of baseband waveform. MASK is usually called as Multilevel ASK, or MASK, where k =log 2 M bits of information are transmitted by using M-level amplitude.

    Multilevel Amplitude Shift KeyingM-ASK

    In MASK, band, 1/k of occupancy bandwidth can be conserved while transmitting k bit information. That is, by using multilevel amplitude, the bit rate can be increased without the bandwidthexpansion. Larger transmission power is required.

    Waveform of MASK signal

  • page14Iwao Sasase, Keio University

    Q

    I

    Q

    I

    Q

    I

    -ary PSK -ary PSK -ary PSK

    Phase allocation of MPSK signal

    In MPSK, signal amplitude is constant. Each signal constellation point is placed on the circumference where the center equals to origin.

    Multi-ary Phase Shift Keying (MPSK)

  • page15Iwao Sasase, Keio University

    Quadrature PSK (QPSK)

    Serial/Parallel

    cost

    sintQPSK modulator

    The transmitter consists of two quadrature modulation components cost and sint to transmit 2 bits/symbol

    Input data +

    +

  • page16Iwao Sasase, Keio University

    QPSK DemodulationQuadrature Detection

    BPF

    LPF

    cost

    In-phase component

    LPF

    sint

    Quadrate component

    QPSK receiver

    Signal phase

    LPF input

    Signal phase LPF input

  • page17Iwao Sasase, Keio University

    Quadrature DetectionIn quadrature detection, for symbol detection, plus and minus of in-phase and quadrate components can be decided independently.

    Quadrate Q

    In-phase I

    QPSK

    Quadrate Q

    In-phase I

    Quadrature detection of cos component

    Decision boundary

  • page18Iwao Sasase, Keio University

    /4 shift QPSK

    adopted in mobile phonePDC2bits/symbol8 signal points are allocated

    on the circumferenceIn each symbol transmission, QPSK

    signal points are shifted by /4. Since there are no phase transition

    crossing origin, the signal envelope does not become zero, and thus,envelope fluctuation is smaller compared to the conventional QPSK.

    Q

    I

  • page19Iwao Sasase, Keio University

    Quadrature Amplitude Modulation (QAM )QAM changes both amplitude and phase.Quadrature multilevel ASK using two orthogonal carrierssin and cos of the same frequency

    )2sin(2)2cos(2)( minmin tfbTEtfa

    TEts ci

    sci

    sQAM +=

    MiTt ,.....,2,10 =

    16QAM signal points

    16QAM

    4bits/symbol

    Compared to QPSK, frequency efficiency is improved to be double.

  • page20Iwao Sasase, Keio University

    MSK

    TffTff cc 4/1,4/1 21 =+=

    MSK is continuous phase FSK with minimum frequency shift

    += kb

    Qcb

    bMSK T

    ttmtmtfTEts

    2)()(2cos2)(

    Feature of MSK

    continuous phase FSK with constant envelope digital FM with modulation index 0.5 good spectral efficiency

  • page21Iwao Sasase, Keio University

    Input NRZ pulse sequence is band-limited by Gaussian LPF.

    GMSK

    GaussianLow pass

    filter

    FM transmitter with modulation index 0.5

    GMSK Output NRZ Data

    GMSK transmitter

    GMSK has advantages of MSK, and achieves narrow spectrum.

    GSMGlobal System for Mobile Communication DECT (Digital European Cordless Telecommunication) CT2