Lecture 12: Noise in Communication Systems

1 EECS 142 Lecture 12: Noise in Communication SystemsProf. Ali M. NiknejadUniversity of California, BerkeleyCopyrightc 2005 by Ali M. NiknejadA. M. NiknejadUniversity of California, BerkeleyEECS 142 Lecture 12 p. 1/31 p. 1/31 Degradation of Link QualityAs we have seen, Noise is an ever present part of allsystems. Any receiver must contend with analog Systems , Noise deteriorates the quality of thereceived signal, the appearance of snow on theTV screen, or static sounds during an digital Communication Systems , Noise degrades thethroughput because it requires retransmission of datapackets or extra coding to recover the data in thepresence of M.

2 NiknejadUniversity of California, BerkeleyEECS 142 Lecture 12 p. 2/31 p. 2/31 BER 11 SNR (dB)Bit Error RateIt s typical to plot the Bit-Error-Rate (BER) in a digitalcommunication shows the average rate of errors for a givensignal-to- Noise -ratio (SNR)A. M. NiknejadUniversity of California, BerkeleyEECS 142 Lecture 12 p. 3/31 p. 3/31 SNRIn general, then, we strive to maximize the signal tonoise ratio in a Communication system. If we receive asignal with average powerPsig, and the average noisepower level isPnoise, then theSN Ris simplySN R=SNSN R(dB) = 10 logPsigPnoiseWe distinguish between random Noise and Noise dueto interferers or distortion generated by the amplifierA.

3 M. NiknejadUniversity of California, BerkeleyEECS 142 Lecture 12 p. 4/31 p. 4/31 Spurious Free Dynamic Rangefunddistorti onspurSFDRSNR S( )The spurious free dynamic rangeSF DRmeasures theavailable dynamic range of a signal at a particular pointin a system. For instance, in an amplifier the largestsignal determines the distortion Noise floor and thenoise properties of the amplifier determine the noisefloor A. M. NiknejadUniversity of California, BerkeleyEECS 142 Lecture 12 p. 5/31 p. 5/31 Noise FigureTheNoise Figure(N F) of an amplifier is a block ( amplifier) is a measure of the degradation of theSN RF=SN RiSN RoN F= 10 log(F) (dB)The Noise figure is measured (or calculated) byspecifying a standard input Noise level through thesource resistanceRsand the temperatureFor RF Communication Systems , this is usually specifiedasRs= 50 andT= 293 M.

4 NiknejadUniversity of California, BerkeleyEECS 142 Lecture 12 p. 6/31 p. 6/31 Noise Figure of an AmplifierSuppose an amplifier has a gainGand apply thedefinition ofN FSN Ri=PsigNsSN Ro=GPsigGNs+Namp,oThe termNamp,ois the total output Noise due to theamplifier in absence of any input Ro=PsigNs+Namp,oGA. M. NiknejadUniversity of California, BerkeleyEECS 142 Lecture 12 p. 7/31 p. 7/31 Input Referred Noise (I)LetNamp,idenote the total input referred Noise of theamplifierSN Ro=PsigNs+Namp,iThe Noise figure is thereforeF=SN RiSN Ro=Psig Ns Ns+Namp,iPsig F= 1 +Namp,iNs 1 All amplifiers have a Noise figure 1.

5 Any real systemdegrades theSN Rsince all circuit blocks add M. NiknejadUniversity of California, BerkeleyEECS 142 Lecture 12 p. 8/31 p. 8/31 Input Referred Noise (II)+Pin+NsNamp,iGThe amount of Noise added by the amplifier isnormalized to the incoming noiseNsin the calculationofF. For RF Systems , this is the Noise of a50 sourceat293 any amplification degrades theSN R, why do anyamplification at all? Because often the incoming signalis too weak to be detected without M. NiknejadUniversity of California, BerkeleyEECS 142 Lecture 12 p.

6 9/31 p. 9/31 Noise Figure of Cascaded BlocksPin+NsG1G2F1F2G1G2 FIf two blocks are cascaded, we would like to derive thenoise figure of the total the blocks are impedance matched properly toresult in a gainG=G1G2. For each amplifier incascade, we haveFi= 1 +Namp,iNsA. M. NiknejadUniversity of California, BerkeleyEECS 142 Lecture 12 p. 10/31 pTotal Input Noise for CascadeBy definition, the Noise added by each amplifier to theinput is given byNamp,i=Ns(F 1)whereNsrepresents some standard input Noise . If wenow input refer all the Noise in the system we haveN amp,i=Ns(F1 1) +Ns(F2 1)G1 Which gives us the total Noise figure of the amplifierF= 1 +N amp,iNs= 1 + (F1 1) +F2 1G1=F1+F2 1G1A.

7 M. NiknejadUniversity of California, BerkeleyEECS 142 Lecture 12 p. 11/31 pGeneral Cascade FormulaApply the formula to the last two blocksF23=F2+F3 1G2F=F1+F23 1G1=F1+F2 1G1+F3 1G1G2 The general equation is written by inspection=F1+F2 1G1+F3 1G1G2+F4 1G1G2G3+ A. M. NiknejadUniversity of California, BerkeleyEECS 142 Lecture 12 p. 12/31 pCascade Formula InterpretationRest of Front-EndLNAWe see that in a cascade, the Noise contribution ofeach successive stage is smaller and Noise of thefirststage is the most important.

8 Thus,every Communication system employs alow noiseamplifier(LNA) at the front to relax the noiserequirementsA typical LNA might have aG= 20 dBof gain and anoise figureN F < dB. The Noise figure depends onthe M. NiknejadUniversity of California, BerkeleyEECS 142 Lecture 12 p. 13/31 pNF Cascade ExampleLNAVGALOG1G2F1F2G3F3 The LNA hasG= 15 dBandN F= dB. The mixerhas a conversion gain ofG= 10 dBandN F= 10 IF amplifier hasG= 70 dBandN F= 20 though the blocks operate at different frequencies,we can still apply the cascade formula if the blocks areimpedance matchedF= +10 160+100 160 10= dBA.

9 M. NiknejadUniversity of California, BerkeleyEECS 142 Lecture 12 p. 14/31 pMinimum Detectable SignalSay a system requires anSN Rof10 dBfor properdetection with a minimum voltage amplitude of1mV. If afront-end with sufficient gain hasN F= 10 dB, let scompute the minimum input power that can supportcommunication:SN Ro=SN RiF=PminNsF>10orPin>10 F Ns= 10 F kT Bwe see that the answer depends on the 10 dB +N F 174 dBm + 10 logBA. M. NiknejadUniversity of California, BerkeleyEECS 142 Lecture 12 p. 15/31 pMinimum Signal (cont)For wireless data,B 10 MHz:Pin= 10 dB + 10 dB 174 dB + 70 dB = 84 dBmWe see that the Noise figure has adBfordBimpact onthe minimum detectable input signal.

10 Since the receivedpower drops>20 dBper decade of distance, a fewdBimproved NF may dramatically improve thecoverage area of a Communication the transmitter has to boost the TX power,which requires excess power consumption due to theefficiency of the M. NiknejadUniversity of California, BerkeleyEECS 142 Lecture 12 p. 16/31 pEquivalent Noise GeneratorsNoisyTwo-PortNoiselessTwo-Port v2ni2nAny noisy two port can be replaced with anoiselesstwo-port and equivalent input Noise sourcesIn general, these Noise sources are correlated.