DCN LOC 4.3 Digitala Modulation

7/28/2019 DCN LOC 4.3 Digitala Modulation

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Digital Modulation

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Linear Modulation Techniques:

Digital modulation can be broadly classified as:

1. Linear (change Amplitude or phase)

2. Non linear modulation techniques (changefrequency).

Linear Modulation Techniques:

The amplitude/phase of the transmitted signal s(t),varies linearly with the modulating digital signal, m(t).

These are bandwidth efficient(because it doesntchange frequency) and hence are very attractive foruse in wireless communication systems where thereis an increasing demand to accommodate more and

more users within a limited spectrum.


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Linear Modulation schemes havevery good spectral efficiency,

However, they must be transmittedusing linear RF amplifiers which

have poor power efficiency.

Pros & Cons


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Note

Phase modulation can be regarded asamplitude modulation because it can

really change envelope;

Thus both of them belong to linear

modulation!


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Change which part of theCarrier?

Carrier:Asin[t +] A = const

= const

= const

Amplitude modulation(AM)

A = A(t) carriesinformation

= const

= const

Frequency modulation(FM)

A = const

= (t) carries information

= const

Phase modulation (PM)

A = const

= const

= (t) carriesinformation

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Amplitude Shift Keying (ASK)

Pulse shaping can be employed to remove spectral spreadingASK demonstrates poor performance, as it is heavily affected bynoise, fading, and interference

Baseband

Data

ASKmodulated

signal

1 10 0 0

Acos(t) Acos(t)

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Frequency Shift Keying (FSK)

Example: The ITU-T V.21 modem standard uses FSKFSK can be expanded to a M-ary scheme, employing multiple frequenciesas different states

Baseband

Data

BFSKmodulated

signal

1 10 0

where f0 =Acos(c-)t and f1 =Acos(c+)t

f0 f0f1 f1

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Phase Shift Keying (PSK)

Major drawbackrapid amplitude change between symbols due to phasediscontinuity, which requires infinite bandwidth. Binary Phase Shift Keying(BPSK) demonstrates better performance than ASK and BFSKBPSK can be expanded to a M-ary scheme, employing multiple phases andamplitudes as different states

Baseband

Data

BPSKmodulated

signal

1 10 0

where s0 =-Acos(ct) and s1 =Acos(ct)

s0 s0s1 s1

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Differential Modulation

In the transmitter, each symbol is modulatedrelative to the previous symbol andmodulating signal, for instance in BPSK 0= no change, 1 = +1800

In the receiver, the current symbol isdemodulated using the previous symbol as areference. The previous symbol serves as anestimate of the channel. A no-changecondition causes the modulated signal toremain at the same 0 or 1 state of theprevious symbol.

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Let {dk} denote the differentially encoded sequence withthis added reference bit. We now introduce the followingdefinitions in the generation of this sequence:

If the incoming binary symbol bk is 1, leave the symboldkunchanged with respect to the previous bit.

If the incoming binary symbol bkis 0, change the symbol

dkwith respect to the previous bit.

DPSK


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to send symbol 0, we advance the phase of thecurrent signal waveform by 180 degrees,

to send symbol 1, we leave the phase of the currentsignal waveform unchanged.

Generation of DPSK: The differential encoding process at the transmitter

input starts with an arbitrary first bit, serving asreference.

DPSK


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DifferentialPhase Shift Keying (DPSK):

DPSK is a non coherent form of phase shift keying whichavoids the need for a coherent reference signal at the

receiver.

Advantage:

Non coherent receivers are easy and cheap to build,hence widely used in wireless communications.

DPSKeliminates the need for a coherent reference signalat the receiver by combining two basic operations at the

transmitter:


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Pulse Carrier

Carrier:

A train of identicalpulses regularlyspaced in time

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Pulse-Amplitude Modulation(PAM)

Modulation in whichthe amplitude ofpulses is varied in

accordance with themodulating signal.

Used e.g. intelephone switching

equipment such as aprivate branchexchange (PBX)

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Pulse-Duration Modulation (PDM)

Modulation inwhich the durationof pulses is variedin accordance withthe modulatingsignal.

Deprecated synonyms:pulse-length modulation,pulse-width modulation.

Used e.g. in telephone switchingequipment such as a privatebranch exchange (PBX)

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Demodulation & Detection

Demodulation

Is process of removing the carrier signal to

obtain the original signal waveformDetection extracts the symbols fromthe waveform

Coherent detection Non-coherent detection

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Coherent Detection

An estimate of the channel phase andattenuation is recovered. It is then possible toreproduce the transmitted signal and

demodulate.

Requires a replica carrier wave of the samefrequency and phase at the receiver.

Also known as synchronous detection (I.e.carrier recovery)

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Coherent Detection 2

Carrier recovery methods include Pilot Tone (such as Transparent Tone in Band)

Less power in the information bearing signal, High peak-

to-mean power ratio Carrier recovery from the information signal

E.g. Costas loop

Applicable to

Phase Shift Keying (PSK) Frequency Shift Keying (FSK)


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Non-Coherent Detection

Requires no reference wave; does not exploitphase reference information (envelopedetection)

Differential Phase Shift Keying (DPSK)

Frequency Shift Keying (FSK)


Non coherent detection is less complex thancoherent detection (easier to implement), but hasworse performance.

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QPSK

Quadrature Phase Shift Keying (QPSK)can be interpreted as two independent

BPSK systems (one on the I-channeland one on Q-channel), and thus thesame performance but twice the

bandwidth (spectrum) efficiency.

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QPSK Constellation Diagram

Quadrature Phase Shift Keying has twice thebandwidth efficiency of BPSKsince 2 bits aretransmitted in a single modulation symbol

Carrier phases

{0, /2, , 3/2}Carrier phases

{/4, 3/4, 5/4, 7/4}

Q

I I

Q

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Types of QPSK

Conventional QPSKhas transitions through zero (i.e. 1800

phasetransition). Highly linear amplifiers required.In Offset QPSK, the phase transitions are limited to 900, the transitions onthe I and Q channels are staggered.In /4 QPSKthe set of constellation points are toggled each symbol, sotransitions through zero cannot occur. This scheme produces the lowest

envelope variations.All QPSK schemes require linear power amplifiers

I

Q

I

Q

I

Q

Conventional QPSK /4 QPSKOffset QPSK

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Quadrature Phase Shift Keying (QPSK):

Also a type of linear modulation schemeQuadrature Phase Shift Keying (QPSK) has twice thebandwidth efficiency of BPSK, since 2 bits are transmittedin a single modulation symbol.

The phase of the carrier takes on 1 of 4 equally spacedvalues, such as where each valueof phase corresponds to a unique pair of message bits.

The QPSK signal for this set of symbol states may be

defined as:


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The striking result is that the bit error probability ofQPSK is identical to BPSK, but twice as much data can besent in the same bandwidth. Thus, when compared toBPSK, QPSK provides twice the spectral efficiency with

exactly the same energy efficiency.

Similar to BPSK, QPSK can also be differentially encodedto allow non-coherent detection.

QPSK


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Multi-level (M-ary) Phase andAmplitude Modulation

Amplitude and phase shift keying can be combined to transmit severalbits per symbol.

Often referred to as linearas they require linear amplification. More bandwidth-efficient, but more susceptible to noise.

For M=4, 16QAM has the largest distance between points, but requiresvery linear amplification. 16PSKhas less stringent linearityrequirements, but has less spacing between constellation points, and istherefore more affected by noise.

16 QAM16 APSK16 PSK

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Distortions

Perfect channel White noise Phase jitter

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Comparison of Modulation Types

ModulationFormat

Bandwidthefficiency C/B

Log2(C/B) Error-freeEb/N0

16 PSK 4 2 18dB

16 QAM 4 2 15dB8 PSK 3 1.6 14.5dB

4 PSK 2 1 10dB

4 QAM 2 1 10dBBFSK 1 0 13dB

BPSK 1 0 10.5dB

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S l Effi i i


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Spectral Efficiencies -Examples

GSM Europe Digital Cellular

Data Rate = 270kb/s; Bandwidth = 200kHz

Bandwidth efficiency = 270/200 =1.35bits/sec/Hz

IS-95 North American Digital Cellular

Data Rate = 48kb/s; Bandwidth = 30kHz Bandwidth efficiency = 48/30 =

1.6bits/sec/Hz

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FSK Spectrum


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Minimum Shift Keying (MSK)

MSK is a continuous phase-frequency shift keying;

Why MSK?

-- Exploitation ofPhase Information besides frequency.


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M-ary

Combined Linear and nonlinear (Constant Envelope)

Modulation Techniques


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Topics :

What is M-ary modulation?

Various M-ary modulation Techniques:

M-ary Phase Shift Keying (MPSK)

M-ary Quadrature Amplitude Modulation

(QAM)

M-ary Frequency Shift Keying (MFSK)


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Definition:

In this modulation Technique the digital data is sent byvarying both the envelope and phase(or frequency) of an RFcarrier.

These modulation techniques map base band data into fouror more possible RF carrier signals. Hence, thesemodulation techniques are called M-ary modulation.

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DCN LOC 4.3 Digitala Modulation