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- 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. Line coding and decodingEE 541/451 Fall 2006
- 3. Signal element versus data element EE 541/451 Fall 2006
- 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. 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. Self-synchronization Receiver Setting the clock matching the sender’s Effect of lack of synchronization EE 541/451 Fall 2006
- 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. 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. Line coding schemesEE 541/451 Fall 2006
- 10. Unipolar NRZ schemeEE 541/451 Fall 2006
- 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. RZ scheme Return to zero Self clocking EE 541/451 Fall 2006
- 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. 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. 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. Line coding schemes 10 points in the finalsEE 541/451 Fall 2006
- 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. 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. 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. 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. Multilevel: 2B1Q scheme NRZ withamplituderepresenting morebits EE 541/451 Fall 2006
- 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. 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. 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 00000000000V(-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. 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. Multilevel: 2B1Q scheme IntegratedServicesDigitalNetworkISDN EE 541/451 Fall 2006
- 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. 8B6T code table (partial)EE 541/451 Fall 2006
- 29. Multilevel: 4D-PAM5 schemeEE 541/451 Fall 2006
- 30. Multitransition: MLT-3 schemeEE 541/451 Fall 2006
- 31. PSD of various line codesEE 541/451 Fall 2006
- 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. Clock Recovery Simple Circuit PLL EE 541/451 Fall 2006
- 34. Summary of line coding schemes Plus HDB3 and B8ZS EE 541/451 Fall 2006

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