Chapter 3 Modulation Techniques - Dang Thanh Binh's Page · PDF file · 2012-09-032012-09-03 · Contents •Modulation Techniques –Analog Modulation •AM, FM –Digital Modulation

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


• 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.


• The Messages (Information source)


• 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.


• Media: Carrier


• 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?


• 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.



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.



• 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.


• QPSK can achieve twice the data rate of acomparable BPSK scheme for a given bandwidth



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



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)





• 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.


• 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.



• 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 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.



• 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 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).


• 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 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

Documents

Chapter 3 Modulation Techniques - Dang Thanh Binh's Page · PDF file · 2012-09-032012-09-03 · Contents •Modulation Techniques –Analog Modulation •AM, FM –Digital Modulation