Bandpass Modulation by saad randhawa

8/7/2019 Bandpass Modulation by saad randhawa

1/14

Bandpass ModulationBandpass Modulation


2/14


3/14

WhyModulate?WhyModulate?Digital modulation is the process by which digital

symbols are transformed into waveforms that are

compatible with the characteristics of the channel

Modulation shifts the spectrum of a baseband signal tosome high frequency

In bandpass modulation the shaped pulses modulatea sinusoid called a carrier wave, or simply a carrier


4/14

ExampleExample In radio transmission carrier is converted to an EM field

for propagation

Size of antenna depends upon wavelength

=c/f

c= 3x108 m/s , f=3000 Hz For cellular telephones, antennas are typically /4 in size

= 3x108 / 3000 = 100000

/4=2.5x104 m or 15 miles

15 miles span antenna is required without carrier wavemodulation


5/14

But if the baseband information is first modulated on

higher frequency carrier e.g. 900 MHz carrier, diameter

of antenna would be about 8cm

=3x108 / 900x106 = 1/3 or 0.33

/4=0.333/4=8 cm

15 miles

8 cm


6/14

Bandpass ModulationBandpass Modulation Bandpass modulation is the process by which an

information signal is transformed into a sinusoidal

waveform

Sinusoid has three features:

i. Amplitude

ii. Phase

iii. Frequency

Process by which the amplitude, phase, frequency of anRF carrier, or a combination of them, is varied in

accordance with the information to be transmitted


7/14

WaveformWaveform The signal has the form:

s(t) =A(t) cos (t) =A(t) cos[ot + (t)]

The average power is

= =A2

/2 The average power of the signal may also be expressed

as the energy in timeT divided byT

A2/2= E/T

orA= (2E/T)1/2

Therefore the signal can be expressed as:

s(t) = (2E/T)1/2 cos[ot + (t)]


8/14

Basic Digital Communication TransformationBasic Digital Communication Transformation


9/14

BandpassBandpass Modulation & DeModulation & De--ModulationModulation


10/14

Coherent DetectionCoherent Detection

The receiver uses the phase of the carrier todetect the signal

The receiver has a prototype of each of thesignals in the symbol and uses these replicasto correlate with the incoming signal

The receiver is phase locked to the incoming

signal so that these prototype waveformsduplicate all characteristics including phaseof the transmitted signal set


11/14

NonNon--coherent Detectioncoherent Detection The receiver does not use such phase reference

information

The demodulators are designed to operate withoutknowledge of the phase reference of the signal

It results in reduced complexity because it does not do

phase estimation, but it also results in increaseprobability of error Pe


12/14

Amplitude Shift Keying (ASK)Amplitude Shift Keying (ASK)

ASK is a form of modulation that represents digital data

variations in the amplitude of a carrier wave

For BASK (also called ON-OFF Keying (OOK)), one

and zero are represented by two amplitude levels


13/14

Phase Shift Keying (PSK)Phase Shift Keying (PSK) PSK is a digital modulation scheme that conveys data by

changing, or modulating, the phase of a reference signal

or the carrier wave


14/14

FrequencyShift Keying (FSK)FrequencyShift Keying (FSK) FSK is a frequency modulation scheme in which digital

information is transmitted through discrete frequency

changes of a carrier wave

FSK conveys the data using distinct carrier frequencies

to represent symbol states

Bandpass Modulation by saad randhawa

Documents

Transcript of Bandpass Modulation by saad randhawa