1. V. S. Hendre Department of E&TC, TCOER, Pune<br />1<br />UNIT-II<br />Baseband Demodulation/ Detection Techniques<br />9/27/2011<br />
2. UNIT-II: Baseband Demodulation/Detection Techniques<br />Signals & noise, <br />Data formats, <br />Synchronization <br />multiplexing, <br />Intersymbol interference, <br />Equalization,<br />Detection of binary signals in presence of Gaussian noise,<br />Matched and optimum filters.<br />9/27/2011<br />V. S. Hendre Department of E&TC, TCOER, Pune<br />2<br />
3. INTRODUCTION<br />In case of baseband signaling, the waveforms at Rx are in pulsed form, but these pulses are not in ideal form.<br />Due to such degradation & filtering at the transmitter, the problem of Intersymbol Interference occurred.<br />The goal of the demodulator are: <br /> 1) Recovered the baseband signal with less degradation<br /> 2)The SNR should be as high as possible<br /> 3) The received signal should be free from ISI.<br />9/27/2011<br />V. S. Hendre Department of E&TC, TCOER, Pune<br />3<br />Baseband Signaling Transmitter<br />Baseband Signaling Receiver<br />
4. INTRODUCTION<br />9/27/2011<br />4<br />Line Coding/ Data Formats<br />Signal Source <br />Channel encoder<br />Source encoder<br />Multiplexer<br />modulator<br />Communication channel<br />UNIT-II Baseband Demod/Detection Tech<br />Unit-III<br />Line codes<br />(Unipolar, polar)<br />-NRZ, RZ, AMI<br />-Manchester<br />Digital Mux<br /><ul><li>Synchronous</li></ul>-Asynchronous<br />-quasi-sync<br />Synchronization-bit, frame<br />Scrambling & unscrambling<br />V. S. Hendre Department of E&TC, TCOER, Pune<br />
5. V. S. Hendre Department of E&TC, TCOER, Pune<br />5<br />Binary line codes<br />Line coding: waveform pattern of voltage or current used to represent the 1s & 0s on a transmission link Line coding<br />Because of the ac coupling in the transformers & repeaters it is desirable to have a ‘0’ dc in the waveform generated by PCM<br />Line<br />9/27/2011<br />
6. V. S. Hendre Department of E&TC, TCOER, Pune<br />6<br />How DC component is generated???Reason 1<br />9/27/2011<br />
7. V. S. Hendre Department of E&TC, TCOER, Pune<br />7<br />Reason 2<br />Digital data representation<br />Well suited inside the machines (computers)<br />Not suitable for long distance-due to presence of stray capacitance in the transmission medium<br />For sufficient capacitance on line-adds DC component to data stream<br />5 V<br />0 V<br />9/27/2011<br />
8. V. S. Hendre Department of E&TC, TCOER, Pune<br />8<br />Conti…<br />Problem of synchronization<br />Receivers clock oscillator locks on the signal level<br />for long string of 1s & 0s-no level shift<br />receiver oscillator frequency drifts<br />Unsynchronise<br />9/27/2011<br />
9. V. S. Hendre Department of E&TC, TCOER, Pune<br />9<br />Figure 4.3 Effect of lack of synchronization<br />9/27/2011<br />
10. Line Coding Formats/ Data Formats/ Transmission Coding Formats<br />Binary signaling in the original form suffers degradation & ISI occurs.<br />To avoid these problems we are converting this digital pulses into another form of digital pulses which make this data suitable for line or channel.<br />Hence this digital to digital conversion is called as Line Coding or Data Formats.<br />If baseband data is itself in digital form, then it is necessary to convert that data into a PAM suitable format i.e. Data format<br />There are variety of Line coding or Data formats available but they are selected based upon different properties.<br />9/27/2011<br />V. S. Hendre Department of E&TC, TCOER, Pune<br />10<br />
11. V. S. Hendre Department of E&TC, TCOER, Pune<br />11<br />Requirements<br />1) Small BW: to send more signals in a communication channel<br />2) Enough Timing content: for receiver to extract the clock information & decode the signal<br />3) small probability of error: increases reliability of line codes<br />4) Good power efficiency: for a specific BW; transmitted power should be small<br />5)Transparency: the coded signal should be received correctly <br />9/27/2011<br />
12. Properties of Line Coding Formats<br />9/27/2011<br />Transmission Bandwidth: It should be as small as possible.<br />Power Efficiency: For a given bandwidth and specified detection error probability, it should be as small as possible<br />Error Detection & Correction Capability: eg in bipolar case single bit error will be indicated by polarity variation<br />Favorable PSD: PSD should be zero at w=0, i.e. D.C. component should be zero.<br />Adequate Timing Content: possible to extract timing or clock information from signal<br />Transparency: Possible to transmit digital signal correctly regardless of continuous ‘1’ & ‘0’<br />V. S. Hendre Department of E&TC, TCOER, Pune<br />12<br />
13. V. S. Hendre Department of E&TC, TCOER, Pune<br />13<br />Line codes: category<br />I) Level codes-<br /> -They are independent on past data. <br /> -They carry information on their voltage levels<br /> -two common formats-RZ & NRZ<br /> -NRZ:Pulse level remains constant during the bit duration <br /> -RZ:Pulse level zero for a portion of bit duration<br />II) Transition codes-<br /> -Current bit level depends on the previous levels<br /> -codes have memory<br />Ex: Miller code, Split phase (mark), Bi-phase (mark), Code mark inversion (CMI), Dou-Binary, Dicode<br />9/27/2011<br />
14. V. S. Hendre Department of E&TC, TCOER, Pune<br />14<br />Line Coding Formats<br />Split phase manchester<br />9/27/2011<br />
15. Mapping of Data Symbols into Signal Levels<br />9/27/2011<br />15<br />V. S. Hendre Department of E&TC, TCOER, Pune<br />
16. Unipolar<br />All signal levels are on one side of the time axis - either above or below<br />NRZ - Non Return to Zero scheme is an example of this code. The signal level does not return to zero during a symbol transmission.<br />Scheme is prone to baseline wandering and DC components. It has no synchronization or any error detection. It is simple but costly in power consumption.<br />9/27/2011<br />16<br />V. S. Hendre Department of E&TC, TCOER, Pune<br />
17. Unipolar NRZ scheme<br />Advantage:<br />1) only one power supply<br />2) Easy to generate<br />Disadvantage:<br /> 1) more power consumption<br />2) prone to DC component<br />3)spectrum is not approaching zero near DC<br />9/27/2011<br />17<br />V. S. Hendre Department of E&TC, TCOER, Pune<br />
18. V. S. Hendre Department of E&TC, TCOER, Pune<br />18<br />Spectrum: Unipolar NRZ<br />BW= R (Hz), data rate<br />Application: magnetic tape recording<br />9/27/2011<br />
19. V. S. Hendre Department of E&TC, TCOER, Pune<br />19<br />Unipolar RZ<br />Advantage:<br />1)relatively simple to implement<br /> 2)DC level is lower than NRZ<br />Disadvantage:<br />1)BW=2R (Hz)<br /> 2) needs 3 dB more power than polar signaling for the same probability of error<br />+A<br /> 0<br />9/27/2011<br />
20. V. S. Hendre Department of E&TC, TCOER, Pune<br />20<br />Spectrum: Unipolar RZ<br />Bandwidth: 2R<br />Application: In baseband data transmission & magnetic tape recording<br />9/27/2011<br />
21. Polar - NRZ<br />The voltages are on both sides of the time axis.<br />Polar NRZ scheme can be implemented with two voltages. E.g. +V for 1 and -V for 0.<br />There are two versions: <br />NRZ - Level (NRZ-L) - positive voltage for one symbol and negative for the other<br />NRZ - Inversion (NRZ-I) - the change or lack of change in polarity determines the value of a symbol. E.g. a “1” symbol inverts the polarity a “0” does not. <br />9/27/2011<br />21<br />V. S. Hendre Department of E&TC, TCOER, Pune<br />
22. Polar NRZ-L and NRZ-I schemes<br />Advt.:1)relatively easy to generate<br /> 2) better error probability<br />Disadvt.:1)requires two different voltages<br /> 2) Large PSD near zero <br />9/27/2011<br />22<br />V. S. Hendre Department of E&TC, TCOER, Pune<br />
23. V. S. Hendre Department of E&TC, TCOER, Pune<br />23<br />Spectrum: Polar NRZ<br />Bandwidth=R (Hz)<br />9/27/2011<br />
24. 4.24<br />Note<br />In NRZ-L the level of the voltage determines the value of the bit. In NRZ-I the inversion or the lack of inversion determines the value of the bit.<br />9/27/2011<br />V. S. Hendre Department of E&TC, TCOER, Pune<br />
25. 4.25<br />Note<br />NRZ-L and NRZ-I both have an average signal rate of N/2 Bd.<br />9/27/2011<br />V. S. Hendre Department of E&TC, TCOER, Pune<br />
26. 4.26<br />Note<br />NRZ-L and NRZ-I both have a DC component problem and baseline wandering, it is worse for NRZ-L. Both have no self synchronization &no error detection. Both are relatively simple to implement. <br />9/27/2011<br />V. S. Hendre Department of E&TC, TCOER, Pune<br />
27. Polar - RZ<br />The Return to Zero (RZ) scheme uses three voltage values. +, 0, -. <br />Each symbol has a transition in the middle. Either from high to zero or from low to zero.<br />This scheme has more signal transitions (two per symbol) and therefore requires a wider bandwidth.<br />No DC components or baseline wandering.<br />Self synchronization - transition indicates symbol value.<br />More complex as it uses three voltage level. It has no error detection capability.<br />9/27/2011<br />27<br />V. S. Hendre Department of E&TC, TCOER, Pune<br />
28. 4.28<br />Polar - RZ<br />9/27/2011<br />V. S. Hendre Department of E&TC, TCOER, Pune<br />
29. 4.29<br />Polar - Biphase: Manchester and Differential Manchester<br />Manchester coding consists of combining the NRZ-L and RZ schemes.<br />Every symbol has a level transition in the middle: from high to low or low to high. Uses only two voltage levels.<br />Differential Manchester coding consists of combining the NRZ-I and RZ schemes.<br />Every symbol has a level transition in the middle. But the level at the beginning of the symbol is determined by the symbol value. One symbol causes a level change the other does not. <br />9/27/2011<br />V. S. Hendre Department of E&TC, TCOER, Pune<br />
30. 4.30<br />Figure 4.8 Polar biphase: Manchester and differential Manchester schemes<br />9/27/2011<br />V. S. Hendre Department of E&TC, TCOER, Pune<br />
31. V. S. Hendre Department of E&TC, TCOER, Pune<br />31<br />Manchester NRZ/Bi--L/split phase/self clocking line code<br />Binary 1positive half bit period pulse followed by negative half bit period pulse<br />Binary 0 negative half bit period pulse followed by positive half bit period pulse<br />Advt:1)always 0 DC value regardless of data sequence<br />2)a string of 0’s will not cause a loss of clocking signal<br />3) In built ‘single error detection’ capacity<br />Disadvt: needs twice the BW of Unipolar NRZ/Polar NRZ codes<br />9/27/2011<br />
32. V. S. Hendre Department of E&TC, TCOER, Pune<br />32<br />spectrum<br />Applications:1) LAN like Ethernet & cheaper net 2) IEEE 802.3 baseband coaxial & twisted pair CSMA/CD bus LANS<br />9/27/2011<br />
33. 4.33<br />Note<br />In Manchester and differential Manchester encoding, the transition<br />at the middle of the bit is used for synchronization.<br />9/27/2011<br />V. S. Hendre Department of E&TC, TCOER, Pune<br />
34. 4.34<br />Note<br />The minimum bandwidth of Manchester and differential Manchester is 2 times that of NRZ. The is no DC component and no baseline wandering. None of these codes has error detection.<br />9/27/2011<br />V. S. Hendre Department of E&TC, TCOER, Pune<br />
35. 4.35<br />Bipolar - AMI and Pseudo ternary<br />Code uses 3 voltage levels: - +, 0, -, to represent the symbols (note not transitions to zero as in RZ).<br />Voltage level for one symbol is at “0” and the other alternates between + & -.<br />Bipolar Alternate Mark Inversion (AMI) - the “0” symbol is represented by zero voltage and the “1” symbol alternates between +V and -V.<br />Pseudoternary is the reverse of AMI.<br />9/27/2011<br />V. S. Hendre Department of E&TC, TCOER, Pune<br />
36. V. S. Hendre Department of E&TC, TCOER, Pune<br />36<br />Alternate Mark Inversion (AMI)/Bipolar RZ/Polar RZ<br />BW=R (Hz)<br />9/27/2011<br />
37. V. S. Hendre Department of E&TC, TCOER, Pune<br />37<br />Pseudo Ternary: 3 encoded signal levels for two level data<br />Advt: 1) Low BW, 2) zero DC value<br /> 3) In built ‘single error detection’ capacity<br /> 4) capable of recording clock information<br />Disadvt: 1) a long string of successive 0s will adversely affect the precision of synchronization<br /> 2)The receiver has to distinguish 3 different levels instead of just 2<br /> 3) Needs @ 3dB more signal power than polar signal for same probability of error<br />Application: Telephone systems<br />9/27/2011<br />
38. V. S. Hendre Department of E&TC, TCOER, Pune<br />38<br />All Line Codes<br />Unipolar NRZ <br />Polar NRZ<br />Unipolar RZ<br />Bipolar RZ<br />Split-phase or Manchester code<br />9/27/2011<br />
39. 9/27/2011<br />V. S. Hendre Department of E&TC, TCOER, Pune<br />39<br />
40. V. S. Hendre Department of E&TC, TCOER, Pune<br />40<br />PSD of All Line codes<br />9/27/2011<br />
41. V. S. Hendre Department of E&TC, TCOER, Pune<br />41<br />Comparison of all PSD’s <br />9/27/2011<br />
42. V. S. Hendre Department of E&TC, TCOER, Pune<br />42<br />MULTIPLEXING<br /><ul><li>Whenever the bandwidth of a medium linking two devices is greater than the bandwidth needs of the devices, the link can be shared.
43. Multiplexing is the set of techniques that allows the simultaneous transmission of multiple signals across a single data link.
44. As data and telecommunications use increases, so does traffic.</li></ul>9/27/2011<br />
45. V. S. Hendre Department of E&TC, TCOER, Pune<br />43<br />9/27/2011<br />
46. V. S. Hendre Department of E&TC, TCOER, Pune<br />44<br />Digital Multiplexing<br />Many base-band signals common channel<br />Analog system: 1) TDM, FDM. Digital system: interleaving ~TDM<br />9/27/2011<br />
47. V. S. Hendre Department of E&TC, TCOER, Pune<br />45<br />Digital Multiplexing<br />The MUX has to perform four functional operations:<br />1) Establish a frame as the smallest time interval containing at least one bit from every input.<br />2) Assign to each input a number of unique bit slots within the frame<br />3) Insert control bit for frame identification & synchronization<br />4) Make allowance for any variations of the bit rate.<br />Analog system: 1) TDM, FDM. Digital system: interleaving ~TDM<br />9/27/2011<br />
48. V. S. Hendre Department of E&TC, TCOER, Pune<br />46<br />Interleaving Process<br /> Ex. TRINITY COE PUNE………..<br />9/27/2011<br />Data Entry Row wise<br />Shift Register Bank<br />Read Column wise<br />Interleaved Sequence : TTEERY . I P.NCU.ION.<br />
49. V. S. Hendre Department of E&TC, TCOER, Pune<br />47<br />DATA INTERLEAVING/INTERLEAVED CODES/INTERLACED CODE<br />Impulse noise-lightening & switching transients- <br /> burst of error.<br />Burst of error of length b<br /> -sequence of b bit error (1st & last bits-1)<br />-in between (b-2) digits-either errornous or correct<br />Bursty channels-1.channel causing multipath & <br /> feding <br /> 2.magnetic recording channel <br /> (tape/disks)<br />9/27/2011<br />
50. V. S. Hendre Department of E&TC, TCOER, Pune<br />48<br />Read out bits to modulator<br />data<br />m<br />rows<br />entry<br />(n-k)<br />Parity bits<br />K data bits<br />9/27/2011<br />
51. V. S. Hendre Department of E&TC, TCOER, Pune<br />49<br />Advantages over TDM<br />1) Free from compulsion of periodic sampling<br />2)waveform preservation-not necessary while multiplexing <br />Multiplexer used-”Binary Multiplexer”<br />Multiplexed signal-source digits interleaved <br /> -bit by bit/characters/words<br />For demultiplexing –constant bit rate at Tx<br />9/27/2011<br />
52. V. S. Hendre Department of E&TC, TCOER, Pune<br />50<br />Problem of bit rate variation-solved<br />A) Synchronous multiplexer<br /> -master clockgoverns all sources & eliminates bit rate variations<br /> -highest efficiency<br />Disadvantage: needs provision of distributing the master clock, design becomes complex. <br />B) Asynchronous Multiplexer<br /> -used for data source that operates in start/stop modewith burst of characters with variable spacing betn bursts<br /> -principle-buffering & interleaving<br /> -Application-computer networks<br />C) Quasi-synchronous multiplexer<br /> -used when input bit rates have the same nominal value butvaries within specified bound<br />9/27/2011<br />
53. V. S. Hendre Department of E&TC, TCOER, Pune<br />51<br />Multiplexing hierarchy for digital telecommunications:Increasing BIT RATES<br /><ul><li>Multiplexing patterns- 1) American Telephone & Telegraph Company (AT &T)hierarchy, 2) International Telegraph and Telephone Consultative Committee (CCIT) hierarchy</li></ul>-both 64 kbps voice PCM unit<br />Layout<br />Digital <br />Data<br />Channel Bank<br />Only<br />Multiplexing<br />Other MUX point to point transmission<br />9/27/2011<br />
54. V. S. Hendre Department of E&TC, TCOER, Pune<br />52<br />Parameters of AT&T and CCITT hierarchies<br />Out put bit rate>sum of input bit rates<br />Surplus-control bit+ stuff bits (for steady output rate)<br />9/27/2011<br />
55. V. S. Hendre Department of E&TC, TCOER, Pune<br />53<br />Illustrative configuration of the AT&T hierarchy<br />9/27/2011<br />
56. V. S. Hendre Department of E&TC, TCOER, Pune<br />54<br />Figure 6.23 Digital hierarchy<br />9/27/2011<br />
57. V. S. Hendre Department of E&TC, TCOER, Pune<br />55<br />Example-synchronous multiplexers<br />Basic TDM scheme<br />Wide Band co-axial cable<br />Framing &<br />Sequencial sampling<br />Local clock<br />repeaters<br />9/27/2011<br />
58. V. S. Hendre Department of E&TC, TCOER, Pune<br />56<br />Figure 6.24 T-1 line for multiplexing telephone lines<br />9/27/2011<br />
59. V. S. Hendre Department of E&TC, TCOER, Pune<br />57<br />Figure 6.25 T-1 frame structure<br />9/27/2011<br />
60. V. S. Hendre Department of E&TC, TCOER, Pune<br />58<br />Bits/frame<br />:commutator speed-8000 revolution/sec<br />:sampling rate-8000 samples/sec<br />: each sample 8 bits<br /><ul><li>no. of output bits=24x8=192 bits</li></ul>*frame synchronisation<br />:synchronisation information-extra bits/frame<br />Channel 1<br />Channel 23<br />Channel 24<br />Frame<br />bit<br />8 bits<br />8 bits<br />8 bits<br />193 bits, 125 s<br />9/27/2011<br />
61. V. S. Hendre Department of E&TC, TCOER, Pune<br />59<br />Figure 6.22 Framing bits<br />9/27/2011<br />
62. V. S. Hendre Department of E&TC, TCOER, Pune<br />60<br />Bit rate<br />-frame time Tp=1/8000=125 s<br />-Tp occupies 193 bits<br />-bit rate on T1 channel <br /> fb(T1)=no. of bits/Time=193/125 Mb/s=1.544 Mbps<br />Signaling information/supervisory information<br />-Dial pulses, busy signal with speech signal<br />-added to voice signal-method “BIT ROBBING”<br />-8th (LSB) bit of 6th sample- Voice transmission + signaling<br />-1st five samples-Eight bit, 6th sample- 7 bits + 8th bit for signaling<br />No. of bits in six frames=[5 frames x 8 bits]+[1 frame x 7 bits]<br /> =47 bits<br />Avg. bits/sample=47/6= 7 (5/6) bits<br />9/27/2011<br />
63. V. S. Hendre Department of E&TC, TCOER, Pune<br />61<br />Signalling bit frequecny=(1/6)xframe bit rate<br /> =(1/6) x 8000<br />fb(T1) signalling = 1333 Hz<br />Signalling Technique- <br />-“Channel Associated Signalling”<br />9/27/2011<br />
64. V. S. Hendre Department of E&TC, TCOER, Pune<br />62<br />Synchronization Techniques<br />Types: 1) Symbol / Bit synchronization<br /> 2) Frame Synchronization<br /> 3) Carrier synchronization<br />Synchronization in a Binary Receiver <br />y(t)<br />Bit Synchronization:-Open loop bit synchronization<br /> -closed loop bit synchronization<br /> -Early Late Synchronization<br />9/27/2011<br />Output Message<br />LPF<br />Regenerator<br />Bit Sync.<br />Frame Sync.<br />CLK.<br />Frame Identification<br />
65. V. S. Hendre Department of E&TC, TCOER, Pune<br />63<br />Carrier Sync.:1)Mth power Law carrier synchronization<br /> 2)Costas Loop<br />Bit Synchronization:-Open loop bit synchronization<br />Used when y(t)=unipolar RZ format<br />y(t) from polar to unipolar conversion<br />9/27/2011<br />
66. V. S. Hendre Department of E&TC, TCOER, Pune<br />64<br />9/27/2011<br />
67. V. S. Hendre Department of E&TC, TCOER, Pune<br />65<br />Closed loop bit synchronization<br />9/27/2011<br />
68. V. S. Hendre Department of E&TC, TCOER, Pune<br />66<br />Disadvantages<br />Sync will suffer from timing jitter when 1) zero crossing of y(t) are not spaced by integer multiples of Tb<br />2)message includes a long string of 1’s & 0’s<br />Problem is solved by message scrambling <br />9/27/2011<br />
69. V. S. Hendre Department of E&TC, TCOER, Pune<br />67<br />Early-late bit synchronization (a) waveform (b) block diagram<br />Filtered digital signal<br /><ul><li>Independent of zero crossings
70. Properly filtered digital signal has peaks at the optimum sampling times & symmetric on either side
71. tksinchronised & <Tb/2</li></ul>|y(tk-)|~ |y(tk+)|<|y(tk|<br /><ul><li> Early synchronisation</li></ul>|y(tk-)|<|y(tk+)|<br />v(t)=|y(tk-)|-|y(tk+)|>0<br />speeds up clock<br />Late sync<br />|y(tk-)|>|y(tk+)|<br />9/27/2011<br />
72. V. S. Hendre Department of E&TC, TCOER, Pune<br />68<br />Scrambler & message sequence generator<br />Coding technique at transmitter—long string of likely bits occure —randomized<br />Eliminates periodic bit patterns-undesired discrete frequency components in the power spectrum<br />Tapped shift registers are used<br />9/27/2011<br />
73. V. S. Hendre Department of E&TC, TCOER, Pune<br />69<br />Binary scrambler (b) un-scrambler at receiver <br />tap gains 1=2=0, <br />& 3=4=1<br />a)<br />M”k=M’k-3 M’k-4<br />M’k=Mk M”k<br />9/27/2011<br />
74. V. S. Hendre Department of E&TC, TCOER, Pune<br />70<br />Input sequence:10 11 00 00 00 00 01<br />9/27/2011<br />
75. V. S. Hendre Department of E&TC, TCOER, Pune<br />71<br />Frame synchronization<br />Receiver should know-when signal is present<br />Aspects of frame sync.<br /> 1) Identify start of frame<br /> 2) Identify subdivisions/subframes within the message<br />For frame sync-special N bit sync word <br />prefix-time for bit-sync acquisation<br />Start of message<br />Message bits<br />t<br />N bit sync. word<br />9/27/2011<br />
76. V. S. Hendre Department of E&TC, TCOER, Pune<br />72<br />Start of message-different codeword followed by Prefix<br />Frames are labeled by sync words-inserted periodically in the bit stream<br />Frame synchronizer<br />o/p bits with polar format<br />~sync word bits in polar form<br />V<br />9/27/2011<br />
77. V. S. Hendre Department of E&TC, TCOER, Pune<br />73<br />Phase locked loop<br />Phase<br />Compa-<br />rator<br />LPF<br />VCO<br />Narrow band F<br />()M<br />Divide by M<br />BPF<br />Received signal<br />Carrier Synchronizer-1) Mth power carrier recovery circuit<br />Recovered carrier<br />Removes noise outside the Band<br />Spectral components + Mfc<br />Mfc<br />fc<br /><ul><li>Due to jitter-carrier at transmitter is unstable
78. Oscillator frequency drifts: f & larger
79. Narrowband filter cannot be made as narrow as our requirements
80. Replace narrowband filter by PLL
81. PLL-filter whose bandpass is determined by LPF
82. PLL will follow oscillator jitter</li></ul>9/27/2011<br />
83. V. S. Hendre Department of E&TC, TCOER, Pune<br />74<br />Phase comparator<br />LPF, fc<br />900 phase <br />shift<br />VCO<br />Loop filter<br />Vm<br />LPF, fc<br />Phase comparator<br />b) The Costas Loop<br />Involves two PLL’s with common VCO & loop filter<br />9/27/2011<br />
84. V. S. Hendre Department of E&TC, TCOER, Pune<br />75<br />If VCO frequency differs form carrier frequency progressive change in phase difference -<br />This change change in Vm—increases/decreases VCO frequency<br />9/27/2011<br />
85. V. S. Hendre Department of E&TC, TCOER, Pune<br />76<br />Signal & Noise<br />Error Performance Degradation in Digital Communication System: The task of the detector is to retrieve the bit stream from the received waveform as error free as possible, notwithstanding the impairments to which the signal have been subjected.<br />Two Causes of error:<br />1) The effect of filtering at the transmitter, channel, and receiver which causes ISI.<br />2) Electrical noise and Interference produced by a variety of sources, such as atmospheric noise switching transients, as well as interfering signals from other sources.<br />9/27/2011<br />
86. Intersymbol Interference<br /><ul><li> No channel has infinite bandwidth
87. Most transmission schemes require higher bandwidth than available in the channel.
88. Square wave requires infinite bandwidth.
89. Synch function is not possible due to causality violation.
90. Modified synch function to satisfy the causality requires higher bandwidth.
91. Each symbol may be smeared into adjacent time slots.
92. Intersymbol Interference (ISI) is the spreading of symbol pulses from </li></ul> one slot into adjacent slots.<br />9/27/2011<br />77<br />V. S. Hendre Department of E&TC, TCOER, Pune<br />
93. V. S. Hendre Department of E&TC, TCOER, Pune<br />78<br />ISI & EYE Pattern<br />Fundamental limitation (digital transmission)<br />relationship betn ISI, BW & signaling rate <br />“Given an ideal Low pass channel of BW ‘B’, it is possible to transmit independent symbols at a rate r≤2B boud without Inter- Symbol Interference”<br />‘No transmission at r>2B’<br />Practically, no channel-ideal freqn response<br />Linear distortion=amplitude & delay<br />Solution-channel+equalisationideal freqn response<br />9/27/2011<br />
96. V. S. Hendre Department of E&TC, TCOER, Pune<br />81<br /> Hc(f)<br /> Heq(f)<br />x(t)<br />y(t)<br />channel<br />Equalizer<br />Overall T.F.=Hc(f).Heq(f)filtering characteristic<br />If Heq(f)-chosen to minimize ISIfilter at Rx ‘Equalizing filter’<br />Final o/p - distortion less (minimum ISI) if<br />Time delay<br />Gain factor<br />9/27/2011<br />
97. V. S. Hendre Department of E&TC, TCOER, Pune<br />82<br />Methods to eliminate ISI<br />a) Nyquist First Method:Zero ISI<br />b) Nyquist Second Method:control of ISI<br />c) Nyquist Third Method<br />Eye-Pattern<br />:effect of channel filtering & channel noise-seen by observing received line codes on an Analog Oscilloscope-displayeye-pattern<br />Eye pattern provides excellent way of assessing the quality of the received line code & the ability of the receiver to combat bit errors<br />General eye pattern<br />9/27/2011<br />
98. V. S. Hendre Department of E&TC, TCOER, Pune<br />83<br /><ul><li>Under normal operating conditions (No detected bit error)</li></ul>eye –opened<br /><ul><li>For noise/ISI eye closedbit error at receiver</li></ul>9/27/2011<br />
99. V. S. Hendre Department of E&TC, TCOER, Pune<br />84<br />9/27/2011<br /> Eye Pattern Seen in oscilloscopeThe Cleaner, the betterGood indication of transmission quality<br />
100. V. S. Hendre Department of E&TC, TCOER, Pune<br />85<br />(a) Distorted polar binary signal (b) eye pattern<br />9/27/2011<br />
101. V. S. Hendre Department of E&TC, TCOER, Pune<br />86<br />Binary Baseband Demodulation / Detection<br />9/27/2011<br />
102. V. S. Hendre Department of E&TC, TCOER, Pune<br />87<br />Demodulation / Detection<br />9/27/2011<br /><ul><li> For any binary channel, the transmitted signal over a symbol interval (0, T) is represented by
103. The received signal r(t) degraded by noise n(t) and possibly degraded by the pulse response of the channel hc(t) was described</li></li></ul><li>V. S. Hendre Department of E&TC, TCOER, Pune<br />88<br />Demodulation & Detection<br />9/27/2011<br /><ul><li> Demodulation: Demodulation is a recovery of a waveform (to an undistorted baseband pulse),
104. Detection: To mean the decision-making process of selecting the digital meaning of that waveform.
105. Frequency down-conversion block:performs frequency translation for band pass signals operating at some radio frequency (RF). It may take place within the front end of the receiver, within the demodulator, shared between the two locations, or not at all.</li></li></ul><li>V. S. Hendre Department of E&TC, TCOER, Pune<br />89<br />Demodulation & Detection<br />9/27/2011<br /><ul><li>The Receiving filter: which performs waveform recovery in preparation of the next important step-detection.
106. The goal of the receiving filter is to recover a baseband pulse with the best possible signal-to-noise ratio (SNR), free of any ISI.
107. The optimum receiving filter for accomplishing this function is called a matched filter or correlator.
108. An optional equalizing filter follows the receiving filter; it is only needed for those systems where channel induced ISI can distort the signals.</li></li></ul><li>V. S. Hendre Department of E&TC, TCOER, Pune<br />90<br />Baseband Signal Detection: Integrate & Dump Switch<br />9/27/2011<br />
109. V. S. Hendre Department of E&TC, TCOER, Pune<br />91<br />Baseband Signal Detection: Integrate & Dump Switch<br />9/27/2011<br />
110. V. S. Hendre Department of E&TC, TCOER, Pune<br />92<br />Baseband Signal Detection: Integrate & Dump Switch<br />9/27/2011<br />
111. V. S. Hendre Department of E&TC, TCOER, Pune<br />93<br />Baseband Signal Detection: Integrate & Dump Switch<br />9/27/2011<br /><ul><li>We will be interested in a quantity called Peak pulse signal-to-noise ratio at the output which is given by,</li></ul>To find , we need transfer function of integrator which is given by,<br />
112. V. S. Hendre Department of E&TC, TCOER, Pune<br />94<br />Baseband Signal Detection: Integrate & Dump Switch<br />9/27/2011<br />
113. V. S. Hendre Department of E&TC, TCOER, Pune<br />95<br />Baseband Signal Detection: Integrate & Dump Switch<br />9/27/2011<br />Thus. we can see from above equation that-<br />The signal-to-noise ratio at the output of integrate-and-dump circuit increases with bit duration T.<br />It also depends on A2Twhich is normalized energy of the bit (symbol).<br />Since and the signal voltage increases linearly with T and the noise voltage increases slowly with. Hence we can say that integrator enhances the signal more than the noise.<br />
114. V. S. Hendre Department of E&TC, TCOER, Pune<br />96<br />Probability of Error<br />9/27/2011<br />Probability density function (pdf) of the Gaussian random noise no can be represented as<br />The conditional PDF:<br />
115. V. S. Hendre Department of E&TC, TCOER, Pune<br />97<br />Probability of Error<br />9/27/2011<br />
116. V. S. Hendre Department of E&TC, TCOER, Pune<br />98<br />Example<br />9/27/2011<br />Find the error probability of a binary baseband receiver with the binary pulse S(t) = +0.5 V and -0.5 V with bit rate 1 kbps. The noise power spectral density Is 10-5 W/Hz. What is the probability of error if the transmitted amplitudes are reduced by<br />50%? Given erf (3.535)=0.999999445<br />
117. V. S. Hendre Department of E&TC, TCOER, Pune<br />99<br />Example<br />9/27/2011<br />
118. V. S. Hendre Department of E&TC, TCOER, Pune<br />100<br />Optimum Filter<br />9/27/2011<br />The integrate-and-dump circuit emphasizes signal output in comparison with the noise voltage. The error probability of this circuit is dependent on Eb/No ratio. But then, is it an optimum value of probability that we get?<br />
119. V. S. Hendre Department of E&TC, TCOER, Pune<br />101<br />Error Probability of Optimum Filter<br />9/27/2011<br />In order to find error probability in this receiver, consider that S2 (t) was transmitted.<br />• Let no(T) be positive and SOl (T) > SO2 (T). If no(T) is larger than error will be made in decision-making i.e. we will be deciding in favor of Sl(t).Thus, error in detection occurs when,<br />
120. V. S. Hendre Department of E&TC, TCOER, Pune<br />102<br />Error Probability of Optimum Filter<br />9/27/2011<br />
121. V. S. Hendre Department of E&TC, TCOER, Pune<br />103<br />Error Probability of Optimum Filter<br />9/27/2011<br />Let<br />
122. V. S. Hendre Department of E&TC, TCOER, Pune<br />104<br />Transfer function of Optimum Filter<br />9/27/2011<br />
123. V. S. Hendre Department of E&TC, TCOER, Pune<br />105<br />Matched Filter<br />9/27/2011<br />The optimum filter is considered with generalized Gaussian noise. An optimum filter which gives a maximum ratio<br />when input noise is white Gaussian noise, is called a matched filter<br />
124. V. S. Hendre Department of E&TC, TCOER, Pune<br />106<br />Properties of Matched Filter<br />9/27/2011<br />1. The spectrum of output signal of a matched filter with matched signal as input is proportional to energy density of input signal.<br />Hence, spectrum of output signal [Y(t)] is proportional to its Energy Spectral Density <br />
125. V. S. Hendre Department of E&TC, TCOER, Pune<br />107<br />Properties of Matched Filter<br />9/27/2011<br />2. The output signal of a matched filter is proportional to a shifted version of autocorrelation function of input signal to which the filter is matched.<br />3. The output signal-to-noise ratio of matched filter depends only on the ratio of the signal energy to P.S.D. of white noise at filter input.<br />
126. Conclusion : Baseband Demodulation/Detection Techniques<br />Signals & noise, <br />Data formats, <br />Synchronization <br />multiplexing, <br />Intersymbol interference, <br />Equalization,<br />Detection of binary signals in presence of Gaussian noise,<br />Matched and optimum filters.<br />9/27/2011<br />V. S. Hendre Department of E&TC, TCOER, Pune<br />108<br />
127. V. S. Hendre Department of E&TC, TCOER, Pune<br />109<br />Maximum length /Pseudo noise sequence Generator<br />Shift resister-non-zero state & output is fed back to input<br />Unit acts-periodic sequence generator<br />Ex:5 stage shift resister with [5,2] configurations with initial non zero states<br />periodic<br />1111100110100100001010 111 011 000……<br />9/27/2011<br />
128. V. S. Hendre Department of E&TC, TCOER, Pune<br />110<br />Longest possible sequence for n stage shift resister<br /> L=2n -1, output MLS/PN sequence<br />Pseudo noise-correlation properties of PN sequence<br />PN signal –acts like-white noise with small DC component<br />Application of PN sequence:<br />1) in test instruments<br />2)radar ranging<br />3)spread spectrum communication<br />4) Digital framing<br />9/27/2011<br />
129. V. S. Hendre Department of E&TC, TCOER, Pune<br />111<br />Pseudorandom Numbers<br />Generated by algorithm using initial seed<br />Deterministic algorithm<br />Not actually random<br />If algorithm good, results pass reasonable tests of randomness<br />Need to know algorithm and seed to predict sequence<br />9/27/2011<br />
130. V. S. Hendre Department of E&TC, TCOER, Pune<br />112<br />Properties of PN sequence<br />PN sequence is periodic<br />In each period-number of 1’s >0’s by 1<br />Among the runs of consecutive 1’s & 0’s<br /> -(1/2)-of the runs of each kind are of length 1<br /> -(1/4)-are of length 2<br /> -(1/8)-are of length 3 etc.<br />Ex: calculate PN sequence for 4 bit shift resister with [3,2] feedback connections<br />Autocorrelation of a PN sequence<br />9/27/2011<br />
131. V. S. Hendre Department of E&TC, TCOER, Pune<br />113<br />9/27/2011<br />
132. V. S. Hendre Department of E&TC, TCOER, Pune<br />114<br />[1]<br />[2,1]<br />[3,2]<br />[4,3]<br />[5,3]<br />[6,5]<br />[7,6]<br />[8,4,3,2]<br />[9,5]<br />[10,7]<br />Unit-III-ENDS<br />9/27/2011<br />
133. V. S. Hendre Department of E&TC, TCOER, Pune<br />115<br />Maximum length shift register codes /PN sequence<br />Class of cyclic codes with (n,k)=(2 m-1,m) , where m= +ve integer<br />M stage digital shift register –feedback based on parity polynomial<br />Ex. 3 stage(m=3) shift register with feedback<br />Source<br />m bits<br />2<br />1<br />Out<br />put <br />Flip-flop<br />9/27/2011<br />
134. V. S. Hendre Department of E&TC, TCOER, Pune<br />116<br />For each code word- m info. Bits ->SR<br />Switch –from 1 to 2<br />Shift register –shifted 1 bit left for 2m-1 shifts<br />Systematic code –length n= 2m-1 <br />Data undergo cyclical shift for 2m-1 shifts<br />SR-original state –in 2m-1 shifts<br />9/27/2011<br />
135. V. S. Hendre Department of E&TC, TCOER, Pune<br />117<br />Consider example with I/p: 0 0 1<br />0 0 1<br />2<br />1<br />Flip-flop<br />0<br />0<br />0<br />1<br />9/27/2011<br />
136. V. S. Hendre Department of E&TC, TCOER, Pune<br />118<br />0<br />0<br />1<br />1<br />1<br />1<br />1<br />1<br />1<br />1<br />0<br />1<br />1<br />1<br />0<br />1<br />0<br />0<br />1<br />0<br />1<br />1<br />0<br />0<br />9/27/2011<br />
137. V. S. Hendre Department of E&TC, TCOER, Pune<br />119<br />9/27/2011<br />
138. V. S. Hendre Department of E&TC, TCOER, Pune<br />120<br />Output seqn-periodic & length n= 2m-1 <br />Length-largest possible period<br />Hence , 2m-1 codewords –different cyclic-shift of a single codeword<br />Not all f/b arrangements- MLS<br />To check-polynomial f(x)= 0+ 1x+ 2x2 +-------+(n-1)x(n-1)+xn<br />Check if irreducible.<br />9/27/2011<br />
139. V. S. Hendre Department of E&TC, TCOER, Pune<br />121<br />Shift register connection – MLSR code (for 2<=m<=34)<br />9/27/2011<br />
140. V. S. Hendre Department of E&TC, TCOER, Pune<br />122<br />Properties of MLSR code<br />Sequence –periodic<br />Each codeword (except all zero)- 2m-1 ones & 2m-1 -1 zeros<br />All codewords-identical weights w = 2m-1 =dmin<br />Codeword compared-cyclical shift of itself no of agreements differ from no of <br /> disagreements - by one<br />9/27/2011<br />
141. V. S. Hendre Department of E&TC, TCOER, Pune<br />123<br />Same shift register arrangements- a periodic binary sequence –period n= 2m-1<br />Seqn-periodic autocorrelation(m) =n for m=0,+-n,+-2n,--- (m) =-1 for all other shifts<br />->impulse like autocorrelation =>power spectrum ~white<br />Seqn resembles white noise<br />Maximum length sequence-Pseudo noise sequence -used-for data scrambling -SS generation<br />All zero state-prohibited<br />9/27/2011<br />
142. V. S. Hendre Department of E&TC, TCOER, Pune<br />124<br />PN sequence<br />mls –long string of likely bits- at receiver -disturbs synchronisation<br />Randomized at transmitter<br />->PN sequence <br />Device-scrambler -eliminates periodic bit pattern<br />At receiver- Unscrambler<br />9/27/2011<br />
143. V. S. Hendre Department of E&TC, TCOER, Pune<br />125<br />Scrambler & unscrambler –4 stage SRtap gains 1= 2=0 & 3= 4=1<br />Scrambler unscrambler<br />m”k<br />m”k<br />mk<br />m’k<br />mk<br />m’k<br />9/27/2011<br />
144. V. S. Hendre Department of E&TC, TCOER, Pune<br />126<br />m’’k=m’k-3 m’k-4 & m’k=mk m’’k (a)<br /> m’k m’’k = [mk m’’k] m’’k = mk[ m’’k m’’k] = mk 0 = mk<br />9/27/2011<br />
145. V. S. Hendre Department of E&TC, TCOER, Pune<br />127<br />I/p seqn: 1011 0000 0000 01*SR-initially- zeros<br />9/27/2011<br />
146. V. S. Hendre Department of E&TC, TCOER, Pune<br />128<br />Application of PN sequence<br />In Test equipments<br />Radar ranging<br />Spread spectrum communication<br />Digital framing <br />9/27/2011<br />
147. V. S. Hendre Department of E&TC, TCOER, Pune<br />129<br />Mean square value of r.v. x<br />Putting II) into I), mean square value of noise voltage <br /> =<br />At R=1, noise power is normalized<br /> Normalized noise power/ quantization noise power = <br />9/27/2011<br />
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