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Noise in Communication System


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Noise in Communication System

  1. 1. Noise in Communication SystemNoise in Communication System
  2. 2. 2IntroductionIntroduction“How do you want to send data/information tosomeone who is far from you?”“If the information that you want to send is yourvoice, how to make sure that what your wordsare understood by your friend?”“What is the source and technology availablesurround you that can help?”
  3. 3. 3NoiseNoise• Practically, we cannot avoid the existence ofunwanted signal together with the modulated signaltransmitted by the transmitter.• This unwanted signal is called noise.• Noise is a random signal that exists in acommunication system.• Random signal cannot be represented with a simpleequation.• The existence of noise will degrade the level ofquality of the received signal at the receiver.
  4. 4. 4Types of noiseTypes of noiseNoiseInternal Noise External NoiseDue to random movement ofelectrons in electronic circuit• Thermal noise/Johnson noise• Shot noiseMan-made noise andnatural resources• Lightning• Solar noise• Ignition• Crosstalkany phenomenon by which a signal transmitted on one frequencycreates an undesired effect on another undesired coupling(energy transfer) fromone circuit or medium to another
  5. 5. 5Noise EffectNoise Effect• Degrade system performance for both analog anddigital systems.• The receiver cannot understand the sender.• The receiver cannot function as it should be.• Reduce the efficiency of communication system.
  6. 6. 6Thermal NoiseThermal Noise• Johnson–Nyquist noise (thermal noise, Johnson noise, orNyquist noise) is the Electronic noise - generated by thethermal agitation of the charge carriers (the electrons) insidean electrical conductor in equilibrium, which happensregardless of any applied voltage.• Movement of the electrons will forms kinetic energy in the conductorrelated to the temperature of the conductor.• When the temperature increases, the movement of free electrons willincrease and the current flows through the conductor.• Current flows due to the free electrons will create noise voltage, n(t).• Noise voltage, n(t) is influenced by the temperature and therefore it iscalled thermal noise.• Also known as Johnson noise or white noise.
  7. 7. 7kTBPTBPnn=∝WattwherePn = noise power (Watt)k = Boltzmann’s constant (1.38 x 10-23J/K)T = Temperature (K)B = BW spectrum system (Hz)In 1928, J. B. Johnson have proven that noise power generated isproportional to the temperature and the BW.Noise power can be modeled using voltage equivalent circuit (Theveninequivalent circuit) or current equivalent circuit (Norton equivalent circuit)This type of noise was first measured by John B. Johnson at Bell Labs in 1928. He described hisfindings to Harry Nyquist, also at Bell Labs, who was able to explain the results.
  8. 8. 8• Noise source will be connected to a system with the inputresistance RL.• Therefore, total noise power is Pn.• With the concept of maximum power transfer ie when Rn = RL , allthe power will be transferred to the load.• Also called as impedance matching.Rn, NoisesourceVn, Noisevoltage sourceRn, noisefree=(b) Thevenin equivalent circuit(a) Noise source circuitIt can be modeled by a voltage source representing the noise of the non-idealresistor in series with an ideal noise free resistor.revise on circuit theory
  9. 9. 9RL, system inputresistanceVn, Noisevoltage sourceRn, Noisefree(c) Thevenin equivalent circuit with the loadVLRRR Ln ==RVRVRVP nnLL42 222===kTBPP Ln ==2nnLnLLVVRRRV=+=Note: Vn = VrmskTBRVkTBRVkTBRVnnn44422===GivenVoltage across RL :=>and=>voltage divider rule
  10. 10. 10Example 1One operational amplifier with a frequency range of (18-20) MHz hasinput resistance 10 kΩ. Calculate noise voltage at the input if theamplifier operate at ambient temperature of 270C.Vn2= 4KTBR= 4 x 1.38 x 10-23x (273+ 27) x 2 x 106x 104Vn= 18 µvoltBW = fh – fl = (20-18) MHz= 2 MHz
  11. 11. 11How to determine noise level inHow to determine noise level incommunication system?communication system?• Noise effect can be determined by measuring:- Signal to Noise Ratio, SNR for analog system- probability of error or bit error rate, BER for digitalsystem• To determine the quality of received signal at thereceiver or an antenna, SNRiis used.• SNR o is always less than SNRi, due to the facts thatthe existence of noise in the receiver itself.• Another parameters that can be used is NoiseFactor, F and Noise Temperature, Te.output=receiver
  12. 12. 12Noise CalculationNoise Calculation• SNR is a ratio of signal power, S to noise power, N.• Noise Figure, F• Noise factor, NFdBNSSNR log10=dBNSNSFNFooiilog10log10==ooiiNSNSF = dB
  13. 13. 13Noise calculation in AmplifierNoise calculation in Amplifier• To simplify the analysis, two types of noise model are used.• - Amplifier with noise• - Amplifier without noiseaio NGNN +=GNaNi NoGNiNoNai(a) Amplifier with noise (b) Amplifier without noise( )aiio NNGN +=whereGNN aai = BkTNP iin ==andinput noise, Niartificial noise source, NaiGain
  14. 14. 14Analysis Amplifier with NoiseAnalysis Amplifier with Noiseio GSS =( )aiiaiaioNNGGNNGNGNN+=+=+=GNaModel Amplifier with noiseNiSoNoSi(1)( )iaiiaiiaiiiiioiNNNNNNNGGSNSSNRSNR+=+=+=1iaiNNF +=1io SNRSNR <<(2)( ) ie TFT 1−=ieTTF +=1Noise Figure: Noise Temperature:BkTBkTFie+=1We have:BkTN ii = BkTN eai =and=>(3)Noise Figure, F
  15. 15. 15Analysis Amplifier Without NoiseAnalysis Amplifier Without NoiseGSi SoNoNi+NaiModel Amplifier without noise( )aiioioNNGNGSS+==( )iaiiaiiaiiiiioiNNNNNNNGGSNSSNRSNR+=+=+=1iaiNNF +=1io SNRSNR <<(2)( ) ie TFT 1−=ieTTF +=1Noise Figure: Noise Temperature:BkTBkTFie+=1We have:BkTN ii = BkTN eai =and=>(3)(1)
  16. 16. 16Example 2Noise generated in amplifier of 5 MHz bandwidth is represented byamplifier input noise power of 0.082 pW. Calculate noise factor andnoise figure if the amplifier was fed with the(a) source input signal match the temperature of 300 K(b) source input signal match the temperature of 100 KNoNiNe = 0.082PW(a) Noise power from the source input = KTiB= 1.38 x 10-23x 300 x 5 x 106= 0.021 pW9.4021.0103.0021.0082.0021.0FigureNoise ==+=+=NiNeNiNoise Factor = 10log104.9 = 6.9 dBNi
  17. 17. 17(b) Noise power from the source input = KTiB= 1.38 x 10-23x 100 x 5 x 106= 0.007 pW7.12007.0103.0007.0082.0007.0ureFaktor Fig ==+=+=NiNeNiNoise Factor = 10log1012.7 = 11.04 dBNoise factor and noise figure were less when operated at roomtemperature.NoNiNe = 0.082PW
  18. 18. 18Example 3An antenna is connected to an amplifier with noise temperature, Te= 125 oK,gain, G = 108. Given the bandwidth, B = 10 MHz and output receiver noise, No= 10 µW. Determine the antenna temperature, Ti and noise figure, F of thereceiver.( )( )( )( )KTTGTTKBGBKTBKTGNNNiieieieioo86236001012510101038.110=∴+×××=+=+=+=−µ2.160012511 =+=+=ieTTF or 2.18.82100==+=ieiNNNFThanks to Samura(600+125)/600