Tema 8 Modulación Digital v0

SISTEMAS DE COMUNICACIN 2

Universidad de CuencaIngeniera Electrnica y Telecomunicaciones

Ciclo Marzo 2015 a Agosto 2015

SISTEMAS DE COMUNICACIN 2TEMA 8: MODULACIN DIGITAL

Sub-Tema 1 :Conmutador de desplazamiento de amplitud (ASK)Sub-Tema 2 :Conmutador de desplazamiento de frecuencia (FSK)Sub-Tema 3 :Conmutador de desplazamiento de fase (PSK)Sub-Tema 4 :Comparacin de sistemas de modulacin binaria digitalSub-Tema 5 :AM de cuadratura (QAM) y PSK cuaternaria (QPSK)Sub-Tema 6 :FSK ortogonal M-nariaSub-Tema 7 :PSK M-ariaSub-Tema 8 :Comparacin de mtodos de modulacin digital

TEMA 8: MODULACIN DIGITAL

MODULACIN DIGITAL


Sub-Tema 1 :Conmutador de desplazamiento deamplitud (ASK)

AMPLITUDE-SHIFT KEYING (ASK)Sub-Tema 1 :Conmutador de desplazamiento de amplitud (ASK)

In amplitude-shift keying, the amplitude of a high-frequency carrier signal isswitched between two or more values in response lo the PCM code. For thebinary case, the usual choice is on-off keying (sometimes abbreviated as OOK).


In amplitude-shift keying, the amplitude of a high-frequency carrier signal isswitched between two or more values in response lo the PCM code. For thebinary case, the usual choice is on-off keying (sometimes abbreviated as OOK).

Time-autocorrelation function

Optimum decision time is for t = T

Optimum decision threshold E/2


ASK system is equivalent to an on-offbase band system in terms of theaverage signal-to-noise ratio requiredfor a given probability of error.


Sub-Tema 2 :Conmutador de desplazamiento defrecuencia (FSK)

FREQUENCY-SHIFT KEYING (FSK)Sub-Tema 2 :Conmutador de desplazamiento de frecuencia (FSK)

In frequency-shift keying, the instantaneous frequency of the carrier signal isswitched between two (or more) values in response to the PCM code


On an average bit energy-to-noise basis, for the same peak-power requirements FSK has a3-dB advantage over ASKbecause the latter is offapproximately one-half of thetime


What is the optimum frequency separation for the detection of FSK?

Orthogonality condition


Sub-Tema 3 :Conmutador de desplazamiento defase (PSK)

PHASE-SHIFT KEYING (PSK)Sub-Tema 3 :Conmutador de desplazamiento de fase (PSK)

In phase-shift keying, the phase of the carrier signal is switched between two(or more) values in response to the PCM code.

For binary PCM, a 180 phase shift is a convenient choice because it simplifiesthe modulator design and is often used. This particular choice is known asphase-reversal keying (PRK)

The maximum peak signal-to-noise ratio for a givensignal energy is then obtained for the condition

Antipodal


PRK has the same probability of-errorperformance as polar-baseband systems.

To attain a given error probability theaverage power required for FSK and ASK istwice that required for PRK.


Sub-Tema 4 :Comparacin de sistemas demodulacin binaria digital

COMPARISON OF BINARY DIGITAL MODULATION SYSTEMSSub-Tema 4 :Comparacin de sistemas de modulacin binaria digital


Sub-Tema 5 :AM de cuadratura (QAM) y PSKcuaternaria (QPSK)

QUADRATURE AM (QAM) AND QUATERNARY PSK (QPSK)Sub-Tema 5 :AM de cuadratura (QAM) y PSK cuaternaria (QPSK)

In tbe QAM systemTo restrict the bandwidthand provide the desiredspectral shaping.

I and Q data signals aremodulated using DSB-SCmodulation


QAM system

Bandwidtb efficiency ofQAM is 2 bps/Hz


QAM system

QAM can be viewed as phase modulation if the I and Q signals have identical magnitudes.Such systems are popularly known as quadrature PSK (QPSK) systems.


In the QPSK system, all of the information is conveyed by the phase, and aconstant envelope is desirable. To restrict the bandwidth of the data

stream to less than one-half the carrierfrequency

I and Q data signals aremodulated using DSB-SCmodulation

Spectral shaping

To maintain a constant envelope


QPSK system

Bandwidtb efficiency ofQPSK is 2 bps/Hz


Phase shifts of 0, 90, 180, and 270

Sub-Tema 5 :AM de cuadratura (QAM) y PSK cuaternaria (QPSK)


The probability of error for QPSK is greater than that for BPSK (PRK). If, however, weaccount for the fact that twice as much data is being transmitted, the probability of errorperformance of both systems is the same.

Carrier recoverycircuits must beused in QPSKdemodulators.


Sub-Tema 6 :FSK ortogonal M-naria

M -ARY ORTHOGONAL FSKSub-Tema 6 :FSK ortogonal M-naria

M-ary orthogonal signaling, useful in power-limited channels to conservetransmitted power at the expense of added bandwidth and equipmentcomplexity.

M orthogonal signals

Minimum frequencyseparation


Average power in M-ary orthogonal signalingdecrease as M is increased.

Larger values of M tend to increase thecomplexity of the transmitting and receivingequipment


Sub-Tema 7 :PSK M-aria

MARY PSKSub-Tema 7 :PSK M-aria

A convenient set of signals for M-ary PSK is the set

Bandwidth effciency


Signal constellation


8 PSK constellation


16 PSK constellation


Error Probability

M> 8 are seldom used in M-ary PSK signaling as a resultof the excessive additionalpower requirements.


Error Probability


Sub-Tema 8 :Comparacin de mtodos demodulacin digital

COMPARISON OF DIGITAL MODULATION SYSTEMSSub-Tema 8 :Comparacin de mtodos de modulacin digital

Choice of digital modulation methods Bandwidth efficiency (in bps/Hz), Error performance (Pe versus S/N) Equipment complexity (cost).

COMPARISON OF DIGITAL MODULATION SYSTEMSSub-Tema 8 :Comparacin de mtodos de modulacin digital


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Tema 8 Modulación Digital v0

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