Basics Modulation

8/8/2019 Basics Modulation

1/31

Wireless Systems:Modulation and Signal Bandwidth

Wireless Systems:Modulation and Signal Bandwidth

Chapter 2

102.2 - 1April, 2004 Technical Introduction to Wireless -- 102 -- (c) 2004 Scott Baxter - V1.5

fc

fc

UpperSideband

LowerSideband

fc

fc

1 0 1 0

1 0 1 0

1 0 1 0

I axis

Q axis

a

b

c

QPSK

I axis

Q axis

c

a

b

p

r

v

/4 shifted DQPSK


2/31

Characteristics of a Radio Signal

o The purpose of telecommunications is tosend information from one place to another

o Our civilization exploits the transmissiblenature of radio signals, using them in asense as our carrier pigeons

o To convey information, some characteristicof the radio signal must be altered (I.e.,modulated) to represent the information

o The sender and receiver must have a

consistent understanding of what thevariations mean to each other

o RF signal characteristics which can bevaried for information transmission:

Amplitude

Frequency

Phase

SIGNAL CHARACTERISTICS

S(t) =A cos

[ ct +

]

The complete, time-varying radio signal

Amplitude (strength)of the signal

Natural Frequencyof the signal

Phase of the signal

Compare these Signals:

DifferentAmplitudes

DifferentFrequencies

DifferentPhases



3/31

The Emergence of AM: A bit of History

o The early radio pioneers first used binary transmission, turning theircrude transmitters on and off to form the dots and dashes of Morsecode. The first successful demonstrations of radio occurred duringthe mid-1890s by experimenters in Italy, England, Kentucky, andelsewhere.

o Amplitude modulation was the first method used to transmit voiceover radio. The early experimenters couldnt foresee other methods(FM, etc.), or todays advanced digital devices and techniques.

o Commercial AM broadcasting to the public began in the early1920s.

o Despite its disadvantages and antiquity, AM is still alive:

AM broadcasting continues today in 540-1600 KHz.

AM modulation remains the international civil aviation standard,used by all commercial aircraft (108-132 MHz. band).

AM modulation is used for the visual portion of commercialtelevision signals (sound portion carried by FM modulation)

Citizens Band (CB) radios use AM modulation

Special variations of AM featuring single or independentsidebands, with carrier suppressed or attenuated, are used for

marine, commercial, military, and amateur communications

SSBLSB USB102.2 - 3April, 2004 Technical Introduction to Wireless -- 102 -- (c) 2004 Scott Baxter - V1.5


4/31

Amplitude Modulation (AM)

TIME-DOMAIN VIEWof AM MODULATOR

x(t) = [1 + amn(t)]cos c twhere:

a = modulation index (0 < a


5/31

An AM Modulator and Detector


TIME-DOMAIN VIEW:AM MODULATOR

x(t)

cos c

mn(t)

[1 + amn(t)]

Sat.

Lin.

o AM modulation can be simplyaccomplished in a saturatedamplifier

superimpose the modulatingwaveform on the supplyvoltage of the saturatedamplifier

o AM de-modulation (detection) can

be easily performed using asimple envelope detector example: half-wave rectifier this non-coherent detection

works well if S/N >10 dB.

o AM demodulation can also beperformed by coherent detectors

incoming signal is mixed(multiplied) with a locallygenerated carrier

enhances performance whenS/N ratio is poor (


6/31

Frequency Modulation (FM)

TIME-DOMAIN VIEW

sFM(t) =A cos [c t + mm(x)dx+0 ]t

t0

where:

A = signal amplitude (constant)c = radian carrier frequency

m= frequency deviation index

m(x) = modulating signal

0 = initial phase

o Frequency Modulation (FM) is a type ofangle modulation

in FM, the instantaneous frequencyof the signal is varied by themodulating waveform

o Advantages of FM

the amplitude is constant

simple saturated amplifiers canbe used

the signal is relatively immune

to external noise the signal is relatively robust;

required C/I values are typically17-18 dB. in wirelessapplications

o Disadvantages of FM relatively complex detectors are

required

a large number of sidebands areproduced, requiring even larger

bandwidth than AM

FREQUENCY-DOMAIN VIEW

Voltage

Frequency0 fc

SFM(t)

UPPERSIDEBANDS

LOWERSIDEBANDS



7/31

An FM Modulator and Detector

o FM modulation can beaccomplished in tuned or voltage-controlled oscillator

the modulating signal varies areactance (varactor, etc.) orotherwise changes thefrequency of the oscillator

the modulation may beperformed at a low intermediatefrequency, then heterodyned toa desired communicationsfrequency

o FM de-modulation (detection) canbe performed by any of severaltypes of detectors

Phase-locked loop (PLL)

Pulse shaper and integrator

Ratio Detector

TIME-DOMAIN VIEW:FM MODULATOR

sFM(t)m(x) ~

VCOx

LO

HPA

TIME-DOMAIN VIEW:FM DETECTOR

x

LO

LNA PLLsFM(t)

m(x)



8/31

Phase Modulation (PM)

TIME-DOMAIN VIEW

sPM(t) =A cos [c t + mm(x) +0 ]

where:

A = signal amplitude (constant)c = radian carrier frequency

m= phase deviation index

m(x) = modulating signal

0 = initial phase

o Phase Modulation (PM) is a type of anglemodulation, a sister of FM

the instantaneousphase of thesignal is varied according to themodulating waveform

o Advantages of PM: similar to FM

the amplitude is constant

simple saturated amplifiers canbe used

the signal is relatively immune

to external noise the signal is relatively robust;

required C/I values are typically17-18 dB. in wirelessapplications

o Disadvantages of PM relatively complex detectors are

required

a large number of sidebands areproduced, requiring even larger

bandwidth than AM

FREQUENCY-DOMAIN VIEW

V

oltage

Frequency0 fc

SFM(t)

UPPERSIDEBANDS

LOWERSIDEBANDS



9/31

Generating and Detecting Phase Modulation

o PM and FM signals can be consideredidentical with only one exception: inFM, the analog modulating signal isinherently de-emphasized by 1/F

o Consequences of this realization:

the same types of circuitry can beused to generate and detect bothanalog PM or FM, determined byfiltering the modulating signal atbaseband

FM has poorer signal-to-noise

ratio than PM at high modulatingfrequencies. Therefore, pre-emphasis and de-emphasis areoften used in FM systems

TIME-DOMAIN VIEW:FM DETECTOR FOR PM

x

LO

LNA PLLsFM(t)

m(x)

Thephase of an FM signal is

proportional to the integral of theamplitude of the modulating signal.

Thephase of a PM signal is proportionalto the amplitude of the modulating

signal.

TIME-DOMAIN VIEW:PHASE MODULATOR

sFM(t)m(x)

~ Phase Shifterx

LO

HPA



10/31

Modulation and Occupied Bandwidth

o The bandwidth occupied by a signaldepends on:

input information bandwidth modulation method

o Information to be transmitted, calledinput or baseband

bandwidth usually is small, muchlower than frequency of carrier

o

Unmodulated carrier the carrier itself has Zero bandwidth!!o AM-modulated carrier

Notice the upper & lower sidebands total bandwidth = 2 x baseband

o FM-modulated carrier Many sidebands! bandwidth is a

complex mathematical functiono PM-modulated carrier

Many sidebands! bandwidth is a

complex mathematical function

Voltage

Time

Time-Domain(as viewed on an

Oscilloscope)

Frequency-Domain(as viewed on a

Spectrum Analyzer)

Voltage

Frequency0

fc

fc

UpperSideband

LowerSideband

fc

fc



11/31

Introduction to Digital Modulation

o The modulating signals shown in previousslides were all analog. It is also possible toquantize modulating signals, restricting themto discrete values, and use such signals to

perform digital modulation. Digitalmodulation has several advantages overanalog modulation:

o Digital signals can be more easilyregenerated than analog

in analogsystems, the effects of noise

and distortion are cumulative: eachdemodulation and remodulationintroduces new noise and distortion,added to the noise and distortion fromprevious demodulations/remodulations.

in digitalsystems, each demodulationand remodulation produces a cleanoutput signal free of past noise anddistortion

o Digital bit streams are ideally suited to manyflexible multiplexing schemes

transmission

demodulation-remodulation

transmission


transmission




12/31

Theory of Digital Modulation: Sampling

o Voice and other analog signals first mustbe sampled (converted to digital form) fordigital modulation and transmission

o The sampling theorem gives the criterianecessary for successful sampling,

digital modulation, and demodulation The analog signal must be band-

limited (low-pass filtered) to containno frequencies higher than fM

Sampling must occur at least twice

as fast as fM in the analog signal.This is called the Nyquist Rate

o Required Bandwidth for p(t)

If each sample p(t) is expressed asan n-bit binary number, thebandwidth required to convey p(t) asa digital signal is at least N*2* fM

this follows Shannons Theorem: atleast one Hertz of bandwidth isrequired to convey one bit per

second of data

The Sampling Theorem: Two PartsIf the signal contains no frequency higherthan fMHz., it is comletely described byspecifying its samples taken at instants oftime spaced 1/2 fMs.The signal can be completely recovered

from its samples taken at the rate of 2 fMsamples per second or higher.

m(t)

Sampling

Recoverym(t)

p(t)



13/31

Sampling Example: the 64 kb/s DS-0

o Telephony has adopted a world-wide PCMstandard digital signal employing a 64 kb/sstream derived from sampled voice data

o Voice waveforms are band-limited

upper cutoff between 3500-4000 Hz. to

avoid aliasing rolloff below 300 Hz. to minimize

vulnerability to hum from AC power mains

o Voice waveforms sampled at 8000/second rate

8000 samples x 1 byte = 64,000 bits/second

A>D conversion is non-linear, one byte persample, thus 256 quantized levels arepossible

Levels are defined logarithmically ratherthan linearly to accommodate a wider range

of audio levels with minimum distortion -law companding (popular in North

America & Japan)

A-law companding (used in most othercountries)

o A>D and D>A functions are performed in aCODEC (coder-decoder) (see following figure)

-10dB

-20dB

-30dB

-40dB

0 dB

100 300 1000 3000 10000Frequency, Hz

C-Message Weighting

t

0

1

234568791011121314

15

16

4

16

1

3

15

8

34

8

A-LAWy= sgn(x)

A|x|ln(1+A)

for0x1

A

(where A = 87.6)

y= sgn(x)ln(1+A|x)|ln(1+A)

for1

A


14/31

CODEC Block Diagram

Sample andHold, or PulseStretcher(Boxcar)Circuit

8 kHz clockpulse train

analoginput

analoginput

filtered(smoothed)analogsignal

stair step(PulseAmplitudeModulation-PAM) signal

Analog-Digital

Convertor(A or -law)

Digital-Analog

Convertor(A or -law)

Digitaloutput(serial orparallel)Pulse Code

Modulation(PCM)

Digitalinput(serial orparallel)

CODER

DECODERLow-pass Filters



15/31

Digital Signals: the Bandwidth Penalty


o Although digital modulation has many advantages, it requiressubstantially more bandwidth than corresponding analog methods

o Various techniques are used to minimize and compensate for the

bandwidth-appetite of digital

Advanced modulation techniques: maximizing the number ofbits carried per hertz of bandwidth

QPSK, DQPSK, GMSK, and other advanced forms Compression of the content of digital signals: reducing the

number of bits required to carry the message

for voice information content: Vocoding techniques

(VSELP, RLP-LTP, CELP, etc.) for data content: various compression techniques


16/31

Vocoders: Compression vs. Distortion


o Objective: to significantly reduce the number of bits which must betransmitted, but without creating objectionable levels of distortion

o We are concerned mainly with telephone applications, with voice signalalready band-limited to 4 kHz. max. and sampled at 8 kHz.

o The objective is toll-qualityvoice reproduction

o General Categories of Speech Coders

Waveform Coders

attempt to re-create the input waveform

good speech quality but at relatively high bit rates Vocoders

attempt to re-create the sound as perceived by humans

quantize and mimic speech-parameter-defined properties

lower bit rates but at some penalty in speech quality Hybrid Coders

mixed approach, using elements of Waveform Coders &Vocoders

use vector quantization against a codebook reference low bit rates and good quality speech


17/31

Symbol Rate, bit/s/Hz and Constant

Envelope PM


o Bit rate= (symbols/sec)*(bits/symbol)

o Use of a rapid symbol rate requires increased bandwidth in a non-

bandlimited channel Unless phase transitions are synchronized with carrier zero

voltage crossings, the resulting waveform discontinuities willrequire large bandwidth

o Using a rapid symbol rate together with narrow band channelfiltering causes the envelope of the resulting signal to fluctuate

Envelope oscillation occurs when symbol rate exceedschannel bandwidth

Such a non-constant envelope requires use of a linear RFpower amplifier, which is more complex and less efficient thanconstant envelope waveform with a Class C power amplifier


18/31

Digital ModulationDigital Modulation


Cl d Sh


19/31

Claude Shannon:The Einstein of Information Theory and Signal Science


o The core idea that makes CDMApossible was first explained byClaude Shannon, a Bell Labs

research mathematician

o Shannon's work relates amountof information carried, channelbandwidth, signal-to-noise-ratio,

and detection error probability It shows the theoretical

upper limit attainable

In 1948 Claude Shannon published his landmark paper on information theory,A Mathematical Theory of Communication. He observed that "thefundamental problem of communication is that of reproducing at one pointeither exactly or approximately a message selected at another point." Hispaper so clearly established the foundations of information theory that hisframework and terminology are standard today.Shannon died Feb. 24, 2001, at age 84.


20/31

Digital Modulation Systems


ModulationScheme

Shannon Limit,BitsHz

BPSK 1 b/s/hz

QPSK 2 b/s/hz

8PSK 3 b/s/hz16 QAM 4 b/s/hz

32 QAM 5 b/s/hz

64 QAM 6 b/s/hz

256 QAM 8 b/s/hz

o Each symbol of a digitallymodulated RF signal conveysa number of bits of information

determined by the numberof degrees of modulationfreedom

o More complex modulationschemes can carry more bitsper symbol in a givenbandwidth, but require bettersignal-to-noise ratios

o The actual number of bits per

second which can beconveyed in a given bandwidthunder given signal-to-noiseconditions is described byShannons equations

SHANNONSCAPACITY EQUATION

C = B log2 [ 1 + ]SNB = bandwidth in HertzC = channel capacity in bits/secondS = signal powerN = noise power

Mod lation b Digital Inp ts


21/31

Modulation by Digital Inputs

o For example, modulate a signal with thisdigital waveform. No more continuous

analog variations, now were shiftingbetween discrete levels. We call this shiftkeying.

The user gets to decide what levelsmean 0 and 1 -- there are no

inherent valueso Steady Carrier without modulation

o Amplitude Shift Keying

ASK applications: digital microwave

o

Frequency Shift KeyingFSK applications: control messages in

AMPS cellular; TDMA cellular

o Phase Shift Keying

PSK applications: TDMA cellular,GSM & PCS-1900

Our previous modulation examples used continuously-variable

analog inputs. If we quantize the inputs, restricting them todigital values, we will produce digital modulation.

Voltage

Time1 0 1 0

1 0 1 0

1 0 1 0

1 0 1 0102.2 - 21April, 2004 Technical Introduction to Wireless -- 102 -- (c) 2004 Scott Baxter - V1.5


22/31

Digital Modulation Schemes


o There are many different schemes for digital modulation, each a

compromise between complexity, immunity to errors in transmission,required channel bandwidth, and possible requirement for linear amplifiers

o Linear Modulation Techniques BPSK Binary Phase Shift Keying

DPSK Differential Phase Shift Keying QPSK Quadrature Phase Shift Keying IS-95 CDMA forward link Offset QPSK IS-95 CDMA reverse link Pi/4 DQPSK IS-54, IS-136 control and traffic channels

o Constant Envelope Modulation Schemes

BFSK Binary Frequency Shift Keying AMPS control channels MSK Minimum Shift Keying GMSK Gaussian Minimum Shift Keying GSM systems, CDPD

o Hybrid Combinations of Linear and Constant Envelope Modulation

MPSK M-ary Phase Shift Keying QAM M-ary Quadrature Amplitude Modulation MFSK M-ary Frequency Shift Keying FLEX paging protocol

o Spread Spectrum Multiple Access Techniques

DSSS Direct-Sequence Spread SpectrumIS-95 CDMA

FHSS Frequency-Hopping Spread Spectrum


23/31

Phase-Plane (Argand) Diagrams for

BPSK, QPSK, /4 DQPSK

I axis

Q axis

ca

b

Binary (bipolar)

phase shift keying

I axis

Q axis

a

b

c

QPSK

I axis

Q axis

c

a

b

p

r

v

/4 shifted DQPSK

The I axis is in-phase with a carrier reference signal. Each dotrepresents a digital code value. The decision area is bounded bya sector (180 or 90 deg) around the point. QPSK may use absoluteor differential coding. Phase change sequences shown by greenlines may occur. Transitions from a to p,r,or v are permitted, others are

not. Phasor ab represents additive interference, the resulting

phase angle.


Error Vulnerabilities of


24/31

Error Vulnerabilities ofHigher-Order Modulation Schemes

Q

I

Distortion(Gain Compression)o Higher-Order Modulation

Schemes (16PSK, 32QAM,64QAM...) are morevulnerable to transmissionerrors than the simpler, morerugged schemes (BPSK,QPSK)

Closely-packedconstellations leave littleroom for vector error

o Non-linearities (gaincompression, clipping,reflections within antennasystem) warp theconstellation

o Noise and long-delayedechoes cause scatteraround constellation points

o Interference blursconstellation points intorings of error

Q

I

Normal 64QAM

Q

I

Noise Q

I

Interference



25/31

Error Vector Magnitude and (Rho)


o A common measurement ofoverall error is Error VectorMagnitude EVM

usually a small fraction oftotal vector amplitude, ~0.1

o EVM is usually averaged overa large number of symbols

Root-mean-square (RMS)o Commercial test equipment

for BTS maintenancemeasures EVM

o Signal quality is oftenexpressed as 1-EVM

normally called (Rho)

typically 0.89-0.96


26/31

Digital Modulation Schemes: Binary FSK


o Binary Frequency Shift Keying is the modulation scheme used tocarry digital information on the AMPS analog cellular controlchannel

o The constant-amplitude carrier signal is switched between twofrequencies according to the binary value of the message bits

o In AMPS control channels, the two FSK frequencies are 8 kHz.above and below the channel center frequency and the bit rate is

10 kb/s.o Required bandwidth: Carsons Rule gives the bandwidth required:

BT = 2f + 2B, where:

BT = total bandwidth of BFSK signalf = difference between the two frequencies employed

B = bandwidth of the digital baseband signal

o Binary FSK signals can be detected non-coherently or coherently


27/31

Digital Modulation: GMSK for GSM and CDPD


o MSK (Minimum Shift Keying) is a version of FSK in which the peakfrequency deviation is set equal to half the bit rate. This is theminimum frequency separation that allows orthogonal detection of

the two binary stateso Advantages of MSK:

constant envelope, spectral efficiency, good BER performance,self-synchronizing capabilities

o GMSK is a derivative of MSK

before modulation, the message waveform (in NRZ format) isfed through a Gaussian filter to accomplish pulse shaping

this greatly reduces the sidelobes in the signals spectrum this introduces a small penalty in BER performance, but it has

been shown that the mobile channel introduces an irreducibleerror rate larger than the GMSK penalty anyway. Thus there is

no effective penalty for using GMSK

M d l i d i IS 95 CDMA S


28/31

Modulation used in IS-95 CDMA Systems


Base Stations: QPSKQ Axis

I Axis

Short

PN Q

cos t

sin t

Userschips

ShortPN I

Mobiles: OQPSKQ Axis

I Axis

ShortPN Q

cos t

sin t

Userschips

1/2chip

ShortPN I

o CDMA mobiles use offsetQPSK modulation

the Q-sequence isdelayed half a chip, sothat I and Q neverchange simultaneouslyand the mobile TX never

passes through (0,0)o CDMA base stations use

QPSK modulation

every signal (voice, pilot,sync, paging) has its ownamplitude, so thetransmitter is unavoidablygoing through (0,0)sometimes; no reason to

include 1/2 chip delay

CDMA B St ti M d l ti Vi


29/31

CDMA Base Station Modulation Views


o The view at top right shows theactual measured QPSK phaseconstellation of a CDMA base

station in normal serviceo The view at bottom right shows

the measured power in the codedomain for each walsh code on a

CDMA BTS in actual service Notice that not all walsh codes

are active

Pilot, Sync, Paging, and

certain traffic channels are inuse


30/31


1 EV DO and 1 EV DV Constellations


31/31

1xEV DO and 1xEV DV Constellations

16-QAM 64-QAM


o Dynamic selection of modulation type, coding scheme, and datarate squeeze the best performance out of each moment

o Although complex modulation schemes pack large amounts ofdata into a relatively small bandwidth, they are very vulnerable tonoise and distortion during transmission

Documents

Basics Modulation