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ECE 4371, Fall, 2017
Introduction to Telecommunication Engineering/Telecommunication Laboratory
Zhu Han
Department of Electrical and Computer Engineering
Class 9
Oct. 2nd, 2017
OutlineOutline Digital Communication System Line Coding
– NRZ and its variance– AMI and its variance– Multilevel– Spectrum
Scrambler
Digital Communication SystemDigital 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
Line coding and decodingLine coding and decoding
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
ExampleExample• 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?
Self-synchronization Receiver Setting the clock matching the sender’s Effect of lack of synchronization
ExampleExample• 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.
Other properties DC components Transmission bandwidth Power efficiency Error detection and correction capability Favorable power spectral density Adequate timing content Transparency
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.
ExampleExample 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.
RZ scheme Return to zero Self clocking
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
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
HDB3 (High Density Bipolar of order 3 code) 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
HDB3 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.
Bipolar 8-Zero Substitution (B8ZS)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
Amplitude
Time
0 0 0 0 0 0 0 01 0 1
Violation Violation
10000000001 +000+-0-+01 in general 00000000000V(-V)0(-V)V
Coded Mark Inversion (CMI)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
Multilevel: 2B1Q scheme Integrated
Services
Digital
Network
ISDN
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
8B6T code table (partial)8B6T code table (partial)
PSD of various line codesPSD of various line codes Details in next class
Clock RecoveryClock 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.
Clock RecoveryClock Recovery Simple Circuit
PLL
Summary of line coding schemesSummary of line coding schemes
Plus HDB3 and B8ZS
ScramblingScrambling Make the data more random by removing long strings of 1’s or 0’s.
Improve timing The simplest form of scrambling is to add a long pseudo-noise (PN)
sequence to the data sequence and subtract it at the receiver (via modulo 2 addition); a PN sequence is produced by a Linear Shift Feedback Register (LSFR).
In receiver, descrambling using the same PN. Secure: what is the PN and what is the initial
data
scrambleddata
PN sequence length2m – 1 = 26 – 1 = 63
ScramblingScrambling Exercise: 100000000000
Scrambling ExampleScrambling Example Scrambler
Descrambler
Homework 3Homework 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 10/16/17
