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ECE 4371, Fall, 2013 Introduction to Telecommunication Engineering/Telecommunication Laboratory

ECE 4371, Fall, 2013 Introduction to Telecommunication Engineering/Telecommunication Laboratory. Zhu Han Department of Electrical and Computer Engineering Class 10 Sep. 30 th , 2013. Outline. Digital Communication System Line Coding NRZ and its variance AMI and its variance Multilevel

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ECE 4371, Fall, 2013 Introduction to Telecommunication Engineering/Telecommunication Laboratory

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  1. ECE 4371, Fall, 2013Introduction to Telecommunication Engineering/Telecommunication Laboratory Zhu Han Department of Electrical and Computer Engineering Class 10 Sep. 30th, 2013

  2. Outline • Digital Communication System • Line Coding • NRZ and its variance • AMI and its variance • Multilevel • Spectrum

  3. Digital Communication System • 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

  4. Line coding and decoding

  5. Signal element versus data element

  6. 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

  7. 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?

  8. Self-synchronization • Receiver Setting the clock matching the sender’s • Effect of lack of synchronization

  9. 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.

  10. Other properties • DC components • Transmission bandwidth • Power efficiency • Error detection and correction capability • Favorable power spectral density • Adequate timing content • Transparency

  11. Line coding schemes

  12. Unipolar NRZ scheme

  13. 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.

  14. Example • A system is using NRZ-I to transfer 1-Mbps data. What are the average signal rate and minimum bandwidth? • Solution • The average signal rate is S = N/2 = 500 kbaud. The minimum bandwidth for this average baud rate is Bmin = S = 500 kHz.

  15. RZ scheme • Return to zero • Self clocking

  16. 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

  17. 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

  18. 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

  19. 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.

  20. 0 0 0 0 0 0 0 1 0 0 1 Amplitude Time Violation Violation 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 • Evaluation • Adds synchronization without changing the DC balance • Error detection possible • Used in T1/DS1 10000000001  +000+-0-+01 in general00000000000V(-V)0(-V)V

  21. 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

  22. Multilevel: 2B1Q scheme • Integrated Services Digital Network ISDN

  23. 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

  24. 8B6T code table (partial)

  25. Multilevel: 4D-PAM5 scheme

  26. Multitransition: MLT-3 scheme

  27. PSD of various line codes • Details in next class

  28. 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.

  29. Clock Recovery • Simple Circuit • PLL

  30. Summary of line coding schemes Plus HDB3 and B8ZS

  31. Homework 3 • Draw line codes for 1010 0000 0000 1011 0000 1011 0000 • NRZ • NRZ-L, NRZ-I • AMI, Pseudoternary, HDB3, B8ZS, CMI • Manchester and differential Manchester schemes • 2B1Q, MLT-3 • If the bit rate is 1Kbps, what are the baud rates for the above line codes. • Matlab plot of spectrum • Due 10/28/13

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