Noise in Communication System BHAGALPUR COLLEGE OF ENGINEERING

Noise in Communication System BHAGALPUR COLLEGE OF ENGINEERING, BHAGALPUR Department of Electronics and Communication Presented by : Anshu Kumari , Assistant Professor 1

Noise • The unwanted signal with the modulated signal is called Noise. • Noise is random in nature. • Noise degrade the level of quality of the received signal at the receiver. • Due to the presence of noise efficiency of communication system reduces. 2

Types of Noise Internal Noise Ex-Thermal noise/ White noise/ Gaussian noise/ Johnson noise External Noise • Solar noise • Crosstalk • Lightening 3

Thermal Noise • Due to thermal agitation, charge carriers(the electrons) in the electrical conductor gain energy and move in random fashion. • This movement of electrons generates heat, and this heat corresponds to Thermal noise. • Due to this movement of electrons current flows in the conductor and this current creates Noise Voltage, n(t). • Noise voltage, n(t) is influenced by the temperature and therefore it is called Thermal Noise. • Also known as Johnson noise or white noise. 4

Thermal Noise • Thermal noise is also referred as ‘white noise’ because it has a uniform ‘spectral density’. • In 1928, J. B. Johnson had proven that noise power generated is proportional to the temperature and the bandwidth. Watt 5

Thermal Noise Where 6

How to determine noise level in communication system? • Noise effect can be determined by measuring: o Signal to Noise ratio, SNR for analog system o Probability of error or bit error rate, BER for digital system • To determine the quality of received signal at the receiver i. e. at an antenna , and at the output of the receiver, are used. • is always less than because of the existence of noise in the receiver itself. 7

Noise Calculation • SNR is a ratio of signal power, S to noise power, N. • Noise Figure, F is given as • Noise factor, • F equals to 1 for noiseless network and in general F>1. • The lower the value of F, the better the network/system 8

Additive White Gaussian Noise • Additive Noise is usually additive in nature and it is added to the signal as shown below • White 9

• Gaussian We generally assume that noise voltage amplitudes have Gaussian or Normal distribution. v Narrow band noise: • When additive white Gaussian noise signal passed through band pass filter, the resulting is called as Narrow band noise. • To find the effect of noise signal on AM, DSB and SSB, noise should be considered as o Additive White Gaussian Noise(AWGN) o Narrow band Noise • Time domain representation of narrowband AWGN noise is given as 10

Where v Power spectral density(PSD) of AWGN noise affecting AM &DSB: Watt 11

v. PSD of AWGN affecting SSB signal: Watt NOTE: The total effect of AWGN signal on AM, DSB and SSB is due to its in-phase component as the effect of quadrature component is zero. 12

v. PSD of AWGN affecting message signal: Total white noise power affecting message signal, Watt 13

Noise Figure of DSB Receiver SDSB(t)+n(t) SDSB(t) Tx (SNR)i Rx (SNR)o Where P = Power of message signal, m(t) If 14

• Here, we consider channel as distortionless channel. • Therefore total AWGN power affecting message signal is given by Watt • In the receiver, Rx 15

SDSB(t) + Multiplier LPF n(t) L. O. signal Fig: Synchronous detector noise • The above expression confirms that only in-phase component affects the signal as quadrature component of noise is filtered at the output. 16

Hence, Or, It confirms that synchronous detector is very effective in nullifying the effect of AWGN noise which is interfered with transmitted DSB signal. 17

Noise Figure of SSB Receiver • General expression for SSB signal is given as 18

SSSB(t) and signal noise 19

Hence Noise Figure of AM Receiver • General expression for AM signal is given as and 20

SAM(t) Thus, FOMAM is always inferior to FOMDSB-SC. 21

Noise Figure of FM Receiver • Figure of merit of FM receiver is given by • Noise figure, F=1/FOM 22

1. Thermal Noise is also known as a) b) c) d) Johnson Noise Partition Noise Flicker Noise Solar Noise 23

Ans. (a) Johnson Noise 2. Random Signal is a) b) c) d) May be specified in time Occurrence is random Repeat over a period None of the above 24

Ans. : (b) Occurrence is random 3. Low frequency noise is a) b) c) d) Transit time noise Flicker noise Shot noise None of the above 25

Ans. : (b) Flicker noise is generated due to fluctuations in the density of the carrier that increases the conductivity of the material. 4. Noise power at the resistor is affected by the value of the resistor as a) b) c) d) Directly proportional to the value of the resistor Inversely proportional to the value of the resistor Unaffected by the value of the resistor Becomes half as the resistance value is doubled 26

Ans. (c)Unaffected by the value of the resistor 5. Noise is added to a signal in a communication system a) b) c) d) At the receiver end At transmitting antenna In the channel During regeneration of the information 27

Ans. (c) In the channel 6. The noise temperature at a resistor depends upon a) Resistance Value b) Noise Power c) Both a and b d) None of the above 28

Ans. (b) Noise Power 7. Figure of merit is a) Ratio of output signal to noise ratio to input signal to noise ratio b) Ratio of input signal to noise ratio to output signal to noise ratio c) Ratio of output signal to input signal to a system d) Ratio of input signal to output signal to a system 29

Ans. : (a)Ratio of output signal to noise ratio to input signal to noise ratio 8. Transit time noise is a) b) c) d) Low frequency noise High frequency noise Due to random behaviour of carrier charges Due to increase in reverse current in the device 30

Ans. : (b) High frequency noise Transit time noise is the noise caused due to increase in the conductance i. e. due to propagation of the carrier through the junction or from input to the output. 9. The noise due to random behaviour of charge carrier is a) b) c) d) Shot noise Partition noise Industrial noise Flicker noise 31

Ans. : (a) Shot noise 10. Noise a) b) c) d) has Infinite energy Infinite power Infinite energy and power None of the mentioned 32

Ans. : (a) Infinite energy 11. Thermal noise is a) b) c) d) Energy signal Power signal Energy and power signal None of the mentioned 33

Ans. : (b) Power signal 12. The RMS value of thermal noise voltage is related to Boltzmann’s constant k as a) b) c) d) Directly proportional to K 2 Directly proportional to K 1/2 Directly proportional to K 3 34

Ans. : Directly proportional to K 1/2 35

Thank You 36