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Đặng Thanh Bình
Chapter 3
Modulation Techniques
Contents
• Modulation Concept
– Definitions
– Wireless Modulation Components
– Bit/Baud Comparison
– Why Modulate?
– Advantages of Digital Modulation
Contents
• Modulation Techniques
– Analog Modulation
• AM, FM
– Digital Modulation
• ASK, FSK, PSK, QAM
– Spread Spectrum
• CSS, DSSS, FHSS, THSS
• Orthogonal Frequency-Division Multiplexing
MODULATION CONCEPT
Modulation for Wireless
• Modulation is the process of encodinginformation from a message source in a mannersuitable for transmission
• How does it work?
– In modulation, a message signal, which contains theinformation is used to control the parameters of acarrier signal, so as to impress the information ontothe carrier.
– In general it involves translating a baseband signal(source signal) to a modulated signal at a higherfrequency (the carrier frequency, fc)
Digital Modulation
• Digital modulation is the process by which asequence of pulses (message) of duration T istransformed into a sequence of sinusoidalwaveforms, s(t) of duration T.
• The general form of the modulated signal is:
• Digital modulation can then be defined as theprocess whereby the amplitude, frequency,phase or a combination of them is varied inaccordance with the information to betransmitted
Main Wireless Modulation Components
Main Wireless Modulation Components
• The Messages (Information source)
– The information source produces the contents of themessage to be transmitted over the link.
– Information sources fall into two basic categories:
• Analog: Information takes the form of a continuousfunction of time.
• Digital: Information takes the form of a sequence (or file)of discrete values – often 0’s and 1’s.
– The message signal could also be a multilevel signal,rather than binary; this is not considered here.
Main Wireless Modulation Components
• The Messages (Information source)
Main Wireless Modulation Components
• Media: Carrier
– The channel has certain types of signals that areeasily transmitted - known as carriers.
– Basically, the modulator works by putting the sourceinformation onto a carrier.
– For physical channels, sinusoidal signals are the mostsuitable carriers.
Main Wireless Modulation Components
• Media: Carrier
Main Wireless Modulation Components
• Modulator/Demodulator
– The modulator converts the source information into asignal that can be sent through the channel
– At the other end of the channel, the demodulatorreconverts the signal received through the channelinto its original form.
– For two-way (i.e., duplex) communication, both endsof the link have a modulator and a demodulator, acombination known as a modem.
• What is half-duplex? Full-duplex?
Main Wireless Modulation Components
• Review
– Full-Duplex (Song công toàn phần)
– Half-Duplex (Bán song công)
Bit Rate / Baud Rate
• Bit rate is the number of bits per second.
– Bit rate is important in computer efficiency
• Baud rate is the number of signal units persecond.
– Baud rate is less than or equal to the bit rate.
– Baud rate is important in data transmission.
• Baud rate determines the bandwidth required to sendsignal
– Baud rate = bit rate / # bits per signal unit
Bit Rate / Baud Rate- Example
• Example 1: An analog signal carries 4 bits in eachsignal unit. If 1000 signal units are sent persecond, find the baud rate and the bit rate
– Baud rate = 1000 bauds per second (baud/s)
– Bit rate = 1000 x 4 = 4000 bps
Bit Rate / Baud Rate- Example
• Example 2: The bit rate of a signal is 3000. If eachsignal unit carries 6 bits, what is the baud rate?
– Baud rate = 3000/6 =500 bauds/sec
Why modulate ?
• Ease of radiation– Reduce antenna size: the size of an antenna is
proportional to the signal wavelength. By increasingthe carrier frequency, the wavelength decreases.
– The size of antenna ∝ λ/4 = c/4fe.g., 3 kHz50 km antenna
e.g., 3 GHz 5 cm antenna
• Simultaneous transmission of several signals– FDM (Frequency Division Modulation)
• Reduce the influence of interference– Frequency Hopping
Advantages of Digital Modulation
• Spectral efficiency – use of a narrow bandwidth tosend a large amount of data
• Good privacy and security features
– Digital encryption techniques may be employed
– greater noise immunity and robustness to channelimpairments
• Lower power consumption
• Repeatable, more easily produced, more flexibility
• Reduced device size
• Easier multiplexing
Hearing, Speech & Voice-band Channels
ANALOG MODULATION TECHNIQUES
Amplitude Modulation (AM)
• Amplitude of carrier signal is varied as themessage signal to be transmitted.
• Frequency of carrier signal is kept constant
Frequency Modulation (FM)
• FM integrates message signal with carrier signalby varying the instantaneous frequency.
• Amplitude of carrier signal is kept constant
DIGITAL MODULATION TECHNIQUES
Basic Digital Modulation Techniques
• Types of digital-to-analog modulation:
Amplitude Shift Keying (ASK)
• The strength of the carrier signal is varied torepresent binary 1 and 0.
• Frequency and phase remains the same.
Amplitude Shift Keying (ASK)
• Highly susceptible to noise interference.
Review: Narrowband vs Wideband
Frequency Shift Keying (FSK)
• Frequency of the carrier is varied to representdigital data (binary 0/1)
• Peak amplitude and phase remain constant.
• Avoid noise interference by looking atfrequencies (change of a signal) and ignoringamplitudes.
• Limitations of FSK is the physical capabilities ofthe carrier.
Frequency Shift Keying (FSK)
Phase Shift Keying (PSK)
• Phase of the carrier is varied to represent digitaldata (binary 0 or 1), i.e., Binary PSK (BPSK)
• Amplitude and frequency remains constant.
• Phases are separated by 180 degrees.– If phase 0 deg. to represent 0, 180 deg. to represent 1. (2-
PSK)
• PSK is not susceptible to noise degradation thataffects ASK or bandwidth limitations of FSK
• Simple to implement, inefficient use of bandwidth.
• Very robust, used extensively in satellitecommunication.
Phase Shift Keying (PSK)
Phase Shift Keying (PSK)
Quadrature Phase Shift Keying (QPSK)
• The essence of quadrature modulation methodsis the application of complementary pairs ofamplitude to two simultaneous sinusoidal wavesdiffering in phase by one-quarter of a cycle.
• Sinusoidal waves (of the same frequency) with aphase difference of a quarter (or three-quarters)of a cycle are said to be in a quadrature phaserelationship.
• It is customary to refer to one of these waves asthe I wave, or in-phase wave, and the other asthe Q wave, or quadrature wave.
Quadrature Phase Shift Keying (QPSK)
Quadrature Phase Shift Keying (QPSK)
• In fact, each symbol uses the addition of an Iwave and a Q wave, giving a total of four possiblesymbols.
• To each possible waveform is allocated one ofthe four, 2-bit binary combinations 00, 01, 10 or11, so any binary bit-stream can be transmittedby an appropriate sequence of sinusoidalsymbols.
Quadrature Phase Shift Keying (QPSK)
• QPSK can achieve twice the data rate of acomparable BPSK scheme for a given bandwidth
Quadrature Phase Shift Keying (QPSK)
Quadrature Phase Shift Keying (QPSK)
Constellation Diagrams
• A constellation diagram helps us to define theamplitude and phase of a signal when we areusing two carriers, one in quadrature of theother.
• The X-axis represents the in-phase carrier andthe Y-axis represents quadrature carrier
Constellation Diagrams
Constellation Diagrams
QPSK Constellation diagram
QPSK Constellation diagram
ASK, BPSK, QPSK Constellation Diagrams
π/4 QPSK
• Widely used in the majority of digital radio modems
• This variant of QPSK uses two identicalconstellations which are rotated by 45° (π/4 radians,hence the name) with respect to one another (twoQPSK constellations offset by ±π/4).– reduces the phase-shifts from a maximum of 180°, but
only to a maximum of 135°
– Eliminates Zero Crossings
• Usually, either the even or odd symbols are used toselect points from one of the constellations and theother symbols select points from the otherconstellation
π/4 QPSK Example
• The binary data that is conveyed by thiswaveform is: 1 1 0 0 0 1 1 0.
• The odd bits, highlighted here, contribute to thein-phase component: 1 1 0 0 0 1 1 0
• The even bits, highlighted here, contribute to thequadrature-phase component: 1 1 0 0 0 1 1 0
• Thus, the first symbol (1 1) is taken from the'blue' constellation and the second symbol (0 0)is taken from the 'green' constellation
π/4 QPSK Example
Quadrature Amplitude Modulation
• PSK is limited by the ability of the equipment todistinguish between small differences in phases.– Limits the potential data rate.
• If multiple pairs of Q and I amplitude (say 1 and -1;and 3 and -3) are allowed, then more symbolsbecome available.
• This is the principle of quadrature amplitudemodulation, or QAM, which you can think of as theapplication of ASK to QPSK (or PSK).
– We can have x variations in phase and y variations ofamplitude
– x • y possible variation (greater data rates)
Quadrature Amplitude Modulation
Quadrature Amplitude Modulation
Quadrature Amplitude Modulation
Quadrature Amplitude Modulation
• 64-QAM– 5 bits per
symbol
Why Not Just Keep Going?
• Errors in IQ modulation create symbolerrors in transmission
• Noise in the transmission channel createsymbol errors
• Inaccuracies in the receiver creates errors
• Signal-to-noise ratio (SNR) requirementsincrease with higher order modulations
Why Not Just Keep Going?
SPREAD SPECTRUM
Spread Spectrum Principles
• The signal occupies a bandwidth much largerthan is needed for the information signal
• The spread spectrum modulation is done using aspreading code, which is independent of the datain the signal
• Despreading at the receiver is done by correlatingthe received signal with a synchronized copy ofthe spreading code
• Developed initially for military applications
• Basis for CDMA
Spread Spectrum Principles
• In spread spectrum (SS), we combine signals fromdifferent sources to fit into a larger bandwidth,but our goals are to prevent eavesdropping andjamming. To achieve these goals, spreadspectrum techniques add redundancy.
Spread Spectrum Procedure
• Input fed into channel encoder– Produces narrow bandwidth analog signal around central
frequency
• Signal modulated using sequence of digits– Spreading code/sequence
– Typically generated by pseudonoise/pseudorandomnumber generator
• Increase bandwidth significantly
• Receiver uses the same sequence to demodulatesignal
• Demodulated signal fed in to channel decoder
Spread Spectrum Advantages
• Anti-jamming
• Interfrence rejection
• Message security and privacy
• Low probability of interception
Spread Spectrum Types
• Frequency-hopping spread spectrum (FHSS)
• Direct-sequence spread spectrum (DSSS)
• Chirp spread spectrum (CSS)
• Time-hopping spread spectrum (THSS)
FHSS
• Rapidly switching a carrier among manyfrequency channels, using a pseudorandomsequence known to both transmitter and receiver
FHSS
Frequency selection in FHSS
FHSS cycles
Bandwidth sharing
DSSS
• Each bit in the original signal is represented bymultiple bits (chip code) in the transmitted signal
• The chipping code spreads the signal across awider frequency band in direct proportion to thenumber of bits used
DSSS
DSSS example
ORTHOGONAL FREQUENCY-DIVISION MULTIPLEXING (OFDM)
OFDM and Multicarrier Transmission
• Single carrier transmission– The concept of single-carrier is that each user
transmits and receives data stream with only onecarrier at any time.
• Multicarrier transmission– The concept of multi-carrier transmission is that a
user can employ a number of carriers to transmitdata simultaneously.
OFDM and Multicarrier Transmission
• Single carrier transmission– The concept of single-carrier is that each user
transmits and receives data stream with only onecarrier at any time.
• Multicarrier transmission– The concept of multi-carrier transmission is that a
user can employ a number of carriers to transmitdata simultaneously.
OFDM and Multicarrier Transmission
OFDM and Multicarrier Transmission
• Orthogonal frequency division multiplexing(OFDM) technique is widely used inwireless communication nowadays.
• OFDM is a special case of a multi-carriertransmission technique, which :– divides the available spectrum into manysubcarriers, each one being modulated by a low datarate stream. modulated by a low data rate stream.
– splits data stream into N parallel streams of reduceddata rate and transmit each on a separate subcarrier.
OFDM and Multicarrier Transmission
• OFDM carriers are frequency spaced by amultiple of 1/T, where T is the modulationperiod, and it is characterized by an overlapof the spectrum of the signals transmittedon different carriers.
OFDM and Multicarrier Transmission
OFDM and Multicarrier Transmission
• In radio wave bands, the carriers are typicallyseparated by several kilohertz or more
• In OFDM, the subcarriers are typically only a fewtens of hertz apart.
• Also:
– Conventional FDMA: each modulated carrier hasdata from a separate source
– OFDM: the modulated subcarriers usually carry datafrom a single source.
OFDM and Multicarrier Transmission
• OFDM modulation cannot be implemented viahardware with analog oscillators:
– it would be too much expensive and theimperfections of the oscillators (frequency drift,phase noise) would cause critical malfunctions.
– But it can easily implemented via software, in atotally digital way, using the FFT (Fast FourierTransform).
OFDM and Multicarrier Transmission
• No N RF radio in both transmitter and receiver
• OFDM uses an efficient computational technique– Discrete Fourier Transform (DFT) and itscounterpart, the Inverse Discrete FourierTransform (IDFT)- to replace sinusoidal generator.
– Implemented through Fast Fourier Transform (FFT)routines – highly optimized
– Fast Fourier transform (FFT) is an efficient algorithmto compute the Discrete Fourier Transform (DFT) andits inverse.
OFDM and Multicarrier Transmission
OFDM and Multicarrier Transmission
OFDM Carriers
• OFDM = Orthogonal FDM
• OFDM symbol forms Rectangular Window ofduration T
• Has a sinc(x)/x-spectrum with zeros at 1/ T
• Other carriers are put in these zeros
• peak value of f1 is at zeroes of others sub-carriers are orthogonal
• Difference between successive subcarrier is justone cycle
OFDM Carriers
OFDM Carriers
OFDM Spectrum
• Total Power spectrum is almost square shape
• Band width (W)= NΔf
OFDM Spectrum
OFDM In Time and Frequency Domain
OFDM: Multiplex or Modulation?
• OFDM can be viewed as either a modulationtechnique or a multiplex technique.
–Modulation technique
• Viewed by the relation between input and output signals
–Multiplex technique
• Viewed by the output signal which is the linear sum of themodulated signals
OFDM: Multiplex or Modulation?
• OFDM in its primary form is considered as adigital modulation technique, and not a multi-user channel access method, since it is utilizedfor transferring one bit stream over onecommunication channel using onesequence ofOFDM symbols.
• However, OFDM can be combined with multipleaccess using time, frequency or codingseparation of the users.
• Example OFDMA = OFDM + TDMA
Advantages of OFDM
• Allows carriers to overlap (no guard band as inFDMA), resulting in lesser wasted bandwidthwithout any Inter Carrier Interference (ICI)
• High data rate distributed over multiple carriersresulting in lower symbol rate (more immune toISI)
• Permits higher data rate as compared to FDM
• Increased security and bandwidth efficiencypossible using CDMA – OFDM (MC-CDMA)
• Simple guard intervals make the system morerobust to multipath effects.
OFDM Transmitter
OFDM Summary
• OFDM – a multi-carrier modulation scheme
– Basic transmission unit – OFDM symbol of a certaintime duration
– An OFDM symbol consisting of multiple subcarriers
– Each subcarrier transports an information symbol(e.g., QPSK)
• The subcarriers are orthogonal
– Integral in time domain vanishing:
– Sampling at frequency domain: no mutualinterference between subcarriers