Analog Communications Unit 5

Lecture Notes 38

T. Kalpana,ECE,GMRIT

Noise in analog communication system model

Accordingly, for the performance analysis of different modulation types in the

presence of noise, we will be limiting the transmitter to the modulator only and the

receiver to a filter followed by the demodulator. To accommodate the channel noise, the

new communication system model is shown in Fig 1

Let ST be the transmitted signal power. We assume that the noise corrupting the

signal in the channel is additive and, unless otherwise specified, is white.

Fig 1 communication system model

We assume that the channel is allpass and of unity gain so that the input signal to

the bandpass filter at the receiver is the transmitted (modulated) signal plus channel

noise. The receiver bandpass filter is also assumed to be of unity gain and having a pass

band corresponding to the frequency band of the received modulated signal. Hence, the

modulated signal passes through the filter unchanged while the channel noise gets

converted to bandpass noise.

Let us denote

Si = useful signal power at the input of the demodulator

So = useful signal power at the output of the demodulator

Ni = noise power at the input of the demodulator and

N0 = noise power at the output of the demodulator

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Analog Communications Unit 5

Lecture Notes 38

T. Kalpana,ECE,GMRIT

In analog communication systems, the quality of the received signal is determined

by the output SNR, o

o

N

S for a given transmitted power, ST. However, since Si is more

accessible, and also given the fact that it is proportional to ST, it is common to determine

o

o

N

Sfor a given Si. A useful of merit is the difference (in dB) between

o

o

N

Sand

i

i

N

S

The simplest communication system one can think of is the baseband system.

This serves as the basis for comparing the performance of different systems. In baseband

communication, no modulation is involved. Both the transmitter and the receiver are

simple ideal lowpass filters with bandwidth fm. Again, the channel is assumed to be

distortionless and having unity gain. The baseband system is shown in Fig 2

Fig 2 The baseband system

Since channel gain is unity, we have: Si = ST

To find the output noise power No, we observe that white noise from the channel passes

through an ideal lowpass filter (receiver) to yield an output noise with a lowpass PSD as

given in Fig 3

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Analog Communications Unit 5

Lecture Notes 38

T. Kalpana,ECE,GMRIT

Fig 3 lowpass PSD

The output noise power, No, is simply the area under the PSD. Hence,

( )∫ ∫−

===M

M

M

o

f

f

f

mnO nfdfn

dffSN0

22

Therefore,

γ==m

i

o

o

nf

S

N

S

The parameter m

i

nf

S=γ denotes the baseband signal-t-noise ratio.In this system, the out-

of-band noise has been removed by filtering. The result will serve as the standard for

comparing the performance of the other systems

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