Communication System

Hello everyone, in last blog I told about EDC...

Now before beginning Communication Systems.. .give 2-4 hrs of time for revision of EDC..revise notes,solve questions which you won't able to solve in first attempt..and important topics which you find difficult to understand..so give a quick look to all these and also look for the diagrams ..

Now we will begin our next subject i.e Communication Systems,,

Study it from :-

• notes(self or coaching)
• Schaum's series
• Simon Haykin
• B.P Lathi
• Probability from Nptel lectures (random process from iit-g)
• few questions from Kannodia

Start with Analog communication..learn only basics ..as many question comes from Digital communication..

1. Start with basic diagram of communication system ..transmitter reciever ..wired

system..wireless..transducers(mic(1) and loudspeaker(8))..modulator..necessity of modulation(wrt antenna height,multiplexing..radio fm uses)..demodulation..

( above system:-first we have mechanical signal like I am speaking its speech or voice its because of change of pressure in throat so mechanical signal..transducer is suppose MIC..which converts mechanical to electrical..now what I speak is of low frequency and we can't transmit it directly as height of antenna is wavelength/4 ..so we increase frequency by modulator and decrease wavelength so the height of antenna..how what I speak is of low amplitude if i transmit it directly in space it will get attenuate with rain droplets,buildings and all so I need an amplifier to increase strength of signal ..now transmit the signal..after receiving we are at 5th point.now this is attenuated signal so we need amplifier again to increase strength then demodulation to decrease frequency as we earlier increased it(carrier signal is prior known to both transmitter and receiver) then we need transducer like loudspeaker to convert electrical signal back to mechanical so you can hear my voice as an audio)

2.Do revision of Fourier transform  from signal and system..

3,Note that whenever you are asked about bandwidth then first draw its frequency spectrum and from that calculate bandwidth..

4. Amplitude modulation :-DSB and SSB is important..few basics of VSB also..and Square Law modulator/demodulator..synchronous demodulator..depth of modulation

5. Frequency and Phase modulation and demodulation..depth of modulation..how to calculate power..truncation of frequency...PLL demodulator

6. Difference between Amplitude,frequency and phase modulation..how frequency modulator is converted into phase and vice versa...what is frequency discriminator..

7.Why frequency modulation is better than amplitude...and also why Foster Seeley discriminator is better than Ratio detector.

Note:-Read AM receiver and FM receiver.. Tuned Amplifier,Mixer,Why tuned is not good and we switched to superheterodyne receiver  ..IF amplifier..Image frequency.. preamplifier..why there is no image frequency in FM receiver.

Also Read what is selectivity,sensitivity,fidelity,Pre-emphasis and De-emphasis(how they are related to Phase lead and Lag compensator of control system or High pass or Low pass filter)..

8. Sampling of signals..frequency for sampling..frequencies at which samples will found..

9.How to convert continuous signal to digital.

10. What is Quantization?..PCM,DM,DPCM,ADM....In PCM the error was more as error is delta/2 and to reduce error we have to increase levels for that we have to increase n ..but as we increase n bandwidth gets increased..so we have to decrease numerator of delta that is Vm - Vl or 2Vm ..we can decrease it by passing it through capacitor but it will attenuate the signal so we use DPCM technique..
Like this study why new methods came...calculation of Amplitude , Noise and SNR of DM

Note:-Read Companding..why need of companding..µ-law and A-law..values of µ and A for which it is equal compression and expansion are 255 and 87.6

11. ASK(OOK),PSK and FSK..which is better and why..learn constellation diagram..calculation of prob of error and energy from constellation diagram..like if we square the distance of any signal from origin then we get signal energy ..if we square the distance between two signals like this minimum distance will give prob of error..Locus of ASK,PSK and FSK..also learn orthogonal signals

12, Learn ASK,PSK and FSK for binary and for M-ary both..and all formulas for both..also learn symbol rate..relation between symbol rate and bit rate..

13. Information Theory..what is information,entropy,mutual information,capacity..capacity for gaussian channel,erasure channel and binary symmetric channel ..SNR for gaussian channel..learn this chapter from Schaum's Series..


14..Random variable:-definition,probability density function,cumulative density function for discreet and continuous signal..pdf for few common signals like gaussian,exponential..mean,variance,standard deviation..from diagrams(not only definition but actual meaning)..relation between CDF and PDF..ESD,PSD,Auto correlation..calculation of ac power ,dc power,mean from PSD diagram..

15.Gaussian or Jhonsson or Thermal or White Noise..what is envelope of Gaussian Noise..general noise signal equation(combination of in-phase and quadrature phase)

16. Noise Figure,Figure of Merit for amplitude modulation,DSB,SSB ,frequency modulation and phase modulation...By FOM you will get to know that why envelope detector is worst than synchronous detector..as synchronous detector cancels noise..as its FOM is 1..and for envelope detector its is different..calculation of net Noise Figure formula from Gain of system..Equivalent temperature of system from Gain and Resistance from Gain of system..why pre-amplifier is at beginning why not at end..what is the change in Noise Figure..

17. Matched Filter..its necessity ..where it is use in whole communication system..SNR for minimum probability of error..difference between Transistion prob or cross over prob or bit error rate and Prob of error..change in PDF of gaussian noise when bit 0 or 1 is transmitted and then calculation of threshold (there is formula for it in Schaum Series)..calculation of optimum threshold from pdf also..like when P(0) is equal to P(1) then it is just the point of contact of  pdf of gaussian noise when 0 transmitted and when 1 is transmitted..
Relation of Prob of error and Signal Energy,..
Impulse signal (h(t)) for matched filter..in real signal ,imaginary signal and in complex signal..

18.Q function ,erfc and erf function..relation of Q with erfc and with CDF.

19.relation of rate and bandwidth...ISI ,Raised cosine filter..values of alpha to treat raised cosine as low pass filter..Bandwidth relation with alpha and Rate for raised cosine filter..

20.Kraft inequality,Shannon coding and Huffman coding for Source coding..Channel coding..Hamming distance...

21.QPSK,MSK..difference between QPSK and MSK

22. DPSK..why DPSK is better than PSK..

23.RZ,NRZ,Manchester ,AMI...which has zero dc value..

24.Strict and Wide sense stationary,auto correlation and PSD relation..if mean of input is given then how to calculate mean of output..if h(0) or H(0) is also given..PDF of one signal..multiple signal..

Read above mentioned topics from Schaum's series and Simon Haykin..

Note:- Read full Schaum's Series..solve all examples and few unsolved..Read few parts from Simon Haykin like in amplitude modulation what is the use of Ka(voltage modulation),eye diagram...also solve kannodia and previous year GATE/IES question..In GATE-2016 they repeated previous year GATE question with value change only..

Best of Luck ..prepare well..if have any doubt then do comment..

Next Subject we are going to take is Digital Electronics..

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