Line coding


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Line coding

  1. 1. Digital Communication System 7.1 and 7.2 (part) Source: sequence of digits Multiplexer: FDMA, TDMA, CDMA… Line Coder – Code chosen for use within a communications system for transmission purposes. – Baseband transmission – Twisted wire, cable, fiber communications Regenerative repeator – Detect incoming signals and regenerate new clean pulses EE 541/451 Fall 2006
  2. 2. Line coding and decodingEE 541/451 Fall 2006
  3. 3. Signal element versus data element EE 541/451 Fall 2006
  4. 4. Data Rate Vs. Signal Rate Data rate: the number of data elements (bits) sent in 1s (bps). It’s also called the bit rate Signal rate: the number of signal elements sent in 1s (baud). It’s also called the pulse rate, the modulation rate, or the baud rate. We wish to: – increase the data rate (increase the speed of transmission) – decrease the signal rate (decrease the bandwidth requirement) – worst case, best case, and average case of r – N bit rate – c is a constant that depends on different line codes. – S = c * N / r baud EE 541/451 Fall 2006
  5. 5. Example• A signal is carrying data in which one data element is encoded as one signal element ( r = 1). If the bit rate is 100 kbps, what is the average value of the baud rate if c is between 0 and 1? Solution – We assume that the average value of c is 1/2 . The baud rate is then• Although the actual bandwidth of a digital signal is infinite, the effective bandwidth is finite.• What is the relationship between baud rate, bit rate, and the required bandwidth? EE 541/451 Fall 2006
  6. 6. Self-synchronization Receiver Setting the clock matching the sender’s Effect of lack of synchronization EE 541/451 Fall 2006
  7. 7. Example• In a digital transmission, the receiver clock is 0.1 percent faster than the sender clock. How many extra bits per second does the receiver receive if the data rate is 1 kbps? How many if the data rate is 1 Mbps? Solution – At 1 kbps, the receiver receives 1001 bps instead of 1000 bps. – At 1 Mbps, the receiver receives 1,001,000 bps instead of 1,000,000 bps. EE 541/451 Fall 2006
  8. 8. Other properties DC components Transmission bandwidth Power efficiency Error detection and correction capability Favorable power spectral density Adequate timing content Transparency EE 541/451 Fall 2006
  9. 9. Line coding schemesEE 541/451 Fall 2006
  10. 10. Unipolar NRZ schemeEE 541/451 Fall 2006
  11. 11. Polar NRZ-L and NRZ-I schemes• In NRZ-L, the level of the voltage determines the value of the bit. RS232.• In NRZ-I, the inversion or the lack of inversion determines the value of the bit. USB, Compact CD, and Fast-Ethernet.• NRZ-L and NRZ-I both have an average signal rate of N/2 Bd. NRZ-L and NRZ-I both have a DC component problem. EE 541/451 Fall 2006
  12. 12. RZ scheme Return to zero Self clocking EE 541/451 Fall 2006
  13. 13. Polar biphase: Manchester and differential Manchester schemes In Manchester and differential Manchester encoding, the transition at the middle of the bit is used for synchronization. The minimum bandwidth of Manchester and differential Manchester is 2 times that of NRZ. 802.3 token bus and 802.4 Ethernet EE 541/451 Fall 2006
  14. 14. Bipolar schemes: AMI and pseudoternary In bipolar encoding, we use three levels: positive, zero, and negative. Pseudoternary: – 1 represented by absence of line signal – 0 represented by alternating positive and negative DS1, E1 EE 541/451 Fall 2006
  15. 15. Basic steps for spectrum analysis Figure 7.3, 7.4 – Basic pulse function and its spectrum P(w) x For example, rect. function is sinc – Input x is the pulse function with different amplitude as figure 7.3c x Carry different information with sign and amplitude x Auto correlation is the spectrum of Sx(w) Tb Rn = lim T →∞ T ∑a a k k k +n 1 ∞ 1 ∞  ∑Re S x ( w) = Tb n =−∞ n − jnwTb =  R0 + 2∑ Rn e − jnwTb  Tb  n =1  – Overall spectrum 2 S y ( w) = P ( w) S y ( w) EE 541/451 Fall 2006
  16. 16. Line coding schemes 10 points in the finalsEE 541/451 Fall 2006
  17. 17. NRZ R0=1, Rn=0, n>0 Figure 7.5 pulse width Tb/2 P(w)=Tb sinc(wTb/2) Bandwidth Rb for pulse width Tb EE 541/451 Fall 2006
  18. 18. RZ scheme DC Nulling ωT sin 2 Split phase r ( t ) ↔ R( ω ) = T 4 ωT Figure 7.6(a) 4 EE 541/451 Fall 2006
  19. 19. Polar biphase: Manchester and differential Manchester schemes In Manchester and differential Manchester encoding, the transition at the middle of the bit is used for synchronization. The minimum bandwidth of Manchester and differential Manchester is 2 times that of NRZ. 802.3 token bus and 802.4 Ethernet EE 541/451 Fall 2006
  20. 20. Bipolar schemes: AMI and pseudoternary R0=1/2, R1=-1/4, Rn=0,n>1, page 307 for reasons Figure 7.8 2 P ( w) Tb  wT  2  wTb  S y ( w) = [ 1 − cos wTb ] = sin c 2  b  sin  2  2Tb 4  4    Reason: the phase changes slower EE 541/451 Fall 2006
  21. 21. Multilevel: 2B1Q scheme NRZ withamplituderepresenting morebits EE 541/451 Fall 2006
  22. 22. HDB3 (High Density Bipolar of order 3 code) Replacing series of four bits that are to equal to "0" with a code word "000V" or "B00V", where "V" is a pulse that violates the AMI law of alternate polarity and is rectangular or some other shape. The rules for using "000V" or "B00V" are as follows: – "B00V" is used when up to the previous pulse, the coded signal presents a DC component that is not null (the number of positive pulses is not compensated for by the number of negative pulses). – "000V" is used under the same conditions as above when up to the previous pulse the DC component is null. – The pulse "B" ("B" for balancing), which respects the AMI alternancy rule, has positive or negative polarity, ensuring that two successive V pulses will have different polarity. Used in E1 EE 541/451 Fall 2006
  23. 23. HDB3 The timing information is preserved by embedding it in the line signal even when long sequences of zeros are transmitted, which allows the clock to be recovered properly on reception. The DC component of a signal that is coded in HDB3 is null. EE 541/451 Fall 2006
  24. 24. Bipolar 8-Zero Substitution (B8ZS) Adds synchronization for long strings of 0s North American system Same working principle as AMI except for eight consecutive 0s 10000000001  +000+-0-+01 in general 00000000000V(-V)0(-V)V 1 0 0 0 0 0 0 0 0 0 1Amplitude Time Violation Violation Evaluation – Adds synchronization without changing the DC balance – Error detection possible Used in T1/DS1 EE 541/451 Fall 2006
  25. 25. Coded Mark Inversion (CMI) Another modification from AMI: Binary 0 is represented by a half period of negative voltage followed by a half period of positive voltage Advantages: – good clock recovery and no d.c. offset – simple circuitry for encoder and decoder − compared with HDB3 Disadvantages: high bandwidth EE 541/451 Fall 2006
  26. 26. Multilevel: 2B1Q scheme IntegratedServicesDigitalNetworkISDN EE 541/451 Fall 2006
  27. 27. mBnL schemes• In mBnL schemes, a pattern of m data elements is encoded as a pattern of n signal elements in which 2^m ≤ L^n.• Multilevel: 8B6T scheme, T4 EE 541/451 Fall 2006
  28. 28. 8B6T code table (partial)EE 541/451 Fall 2006
  29. 29. Multilevel: 4D-PAM5 schemeEE 541/451 Fall 2006
  30. 30. Multitransition: MLT-3 schemeEE 541/451 Fall 2006
  31. 31. PSD of various line codesEE 541/451 Fall 2006
  32. 32. Clock Recovery A timing reference signal can be extracted from the received signal by differentiation and full-wave rectification − provided that the signal carries sufficient transitions. This timing reference signal is then used to fine tune the frequency and phase of a local oscillator. The receiver clock is then derived (e.g. add a phase shift) from this local oscillator. EE 541/451 Fall 2006
  33. 33. Clock Recovery Simple Circuit PLL EE 541/451 Fall 2006
  34. 34. Summary of line coding schemes Plus HDB3 and B8ZS EE 541/451 Fall 2006