Principles of transmission, applied to the eu transmission network 2011.12.06
Ch 7 Principles of Digital Data Transmission
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Transcript of Ch 7 Principles of Digital Data Transmission
Ch 7Principles of Digital Data Transmission
ENGR 4323/5323Digital and Analog Communication
Engineering and PhysicsUniversity of Central Oklahoma
Dr. Mohamed Bingabr
Chapter Outline
β’ Digital Communication Systems
β’ Line Coding
β’ Pulse Shaping
β’ Scrambling
β’ Digital Receiver and Regenerative Repeaters
β’ PAM: M-ARY Baseband Signaling for Higher Data
Rate
β’ Digital Carrier Systems
β’ M-ARY Digital Carrier Modulation 2
Digital Communication Systems
Line Coding
3
On-Off (RZ)
Polar (RZ)
Bipolar (RZ)
On-Off (NRZ)
Polar (NRZ)
Digital Communication Systems
Multiplexer
- Time Division
- Frequency Division
- Code Division
4
Digital Carrier Modulation
- Amplitude Modulation
- Frequency Modulation
- Phase Modulation
Digital Communication Systems
5
Regenerative Repeater- Used at regularly spaced interval.
- Timing information extracted from the received signal.
- Transparent line code does not effect the accuracy of the timing information.
Line Coding
6
Property of Line Code- Transmission Bandwidth
- Power Efficiency
- Error Detection and Correction Capacity
- Favorable Power Spectral Density
- Adequate Timing Content
- Transparency
PSD of Line Codes
7
π π¦ ( π )=|π ( π )|2ππ₯ ( π )
π¦ (π‘ )=β πππ (π‘βπππ )
The PSD will depend on the line code pattern x(t) and the pulse shape p(t).
PSD of Line Codes
8
We can express the impulse as a pulse with narrow width and large amplitude such that the strength of the pulse is the same as the impulse.
hπ=ππ
π
β οΏ½ΜοΏ½= limπβ β
1π β
πππ
2 (1 β ππ )
β οΏ½ΜοΏ½=π 0
ππ πΒΏΒΏ
π 0= limπβ β
1πβ
πππ
2=~ππ
2
|π|<π
PSD of Line Codes
9
βπ₯ (π )= 1π π
βπ=β β
β
π ππΏ (π βππ π)
π 1= limπβ β
1πβ
πππππ+1=~ππππ+1
π π= limπβ β
1π β
πππππ+π=~ππππ+π
To find , let Ξ΅0
The PSD is the FT of
PSD of Line Codes
10
ππ₯( π )= 1π π [π 0+2β
π=1
β
π ππππ (π2π π ππ) ]π π¦ ( π )=|π ( π )|2ππ₯( π )
π π¦( π )=|π ( π )|2
ππ [π 0+2βπ=1
β
π ππππ (π2π π π π )]
π π= limπβ β
1π β
πππππ+π=~ππππ+π
Again Rn is
PSD of Polar Signaling
11
π π¦( π )=|π ( π )|2
ππ
π 0= limπβ β
1πβ
πππ
2= limπβ β
1π β
π1=1
π π= limπβ β
1π β
πππππ+π=0 1 or -1 with equal
probability
For rectangular pulse shape π (π‘ )=Ξ ( 2π‘ππ )
π ( π )=π π
2π πππ( π π ππ
2 )π π¦ ( π )=
ππ
4π πππ 2( π π π π
2 )
PSD of Polar Signaling
12
π π¦ ( π )=ππ
4π πππ 2( π π π π
2 )- Essential Bandwidth 2Rb Hz
- No capability for error detection or correction
- Nonzero PSD at dc ( f = 0)
- For a given power, Polar signaling has the lowest error detection probability.
- Transparent
- Rectification of polar signal can help in extracting clock timing.
Constructing a DC Null in PSD by Pulse Shaping
13
Split-phase (Manchester or twinned-binary) signal. Fig. a: Basic pulse p(t) for Manchester signaling.Fig. b: Transmitted waveform for binary data sequence using Manchester signaling.
π ( π )=β«β β
β
π(π‘ )πβ π2π ππ‘ ππ‘
π (0)=β«β β
β
π (π‘ )ππ‘=0
Read On-Off Signaling
PSD of Bipolar Signaling
14
π 0= limπβ β
1πβ
πππ
2
π 1= limπβ β
1π [π4 (β1 )+ 3π
4(0 )]=β 1
4
Half the time aK equals 0 and the other half time equals either 1 or -1.
For R1, the combination of akak+1 = 11, 10, 01, 00. For bipolar rule the product is zero for the last three combination and -1 for the first combination.
π 0=12
for
PSD of Bipolar Signaling
15
π π¦ ( π )=ππ
4π πππ 2( π π π π
2 ) π ππ2 (π π π π )
π π¦( π )=|π ( π )|2
ππ [π 0+2βπ=1
β
π ππππ (π2π π π π )]π π¦ ( π )=
|π ( π )|2
2ππ[1 βπππ (2π π ππ ) ]
π π¦( π )=|π ( π )|2
πππ ππ2 ( π π ππ )
PSD of Bipolar Signaling
16
π π¦ ( π )=ππ
4π πππ 2( π π π π
2 ) π ππ2 (π π π π )
- Essential Bandwidth Rb Hz.
- Single error detection capability.
- Zero PSD at dc ( f =0).
- Disadvantage require twice the power as a polar signal needs.
- It is not transparent.
High-Density Bipolar (HDB) Signaling
17
The HDB scheme is an ITU standard. In this scheme the problem of nontransparency in bipolar signaling is eliminated by adding pulses when the number of consecutive 0s exceeds N.
(a) HDB3 signal and (b) its PSD.
Pulse Shaping
18
The pulse shape p(t) effect the PSD Sy( f ) more than the choice of line code.
Intersymbol Interference (ISI): Spreading of a pulse beyond its allocated time interval Tb will cause it to interfere with neighboring pulses.
Nyquist 1st criteria for Pulse Shaping
19
Nyquist criteria for pulse shaping to eliminate ISI:
Pulse shape that has a nonzero amplitude at its center and zero amplitudes at t = nTb (n =1, 2, 3, β¦)
π (π‘ )={ 1 π‘=00π‘=Β±ππ π
π π=1π π
Nyquist 1st criteria for Pulse Shaping
π ( π )={ 1|π |< π π
2β π
π₯
12 [1βπ πππ ( π βπ π /2
2 π π₯)]|π β
π π
2 |< π π₯
0|π |>π π
2+ π
π₯
Nyquist 2nd criteria for Pulse Shaping
Pulse broadening in the time domain leads to reduction of its bandwidth. Pulse satisfying second criteria is also knowing as the duobinary pulse.
π (πππ )={ 1π=0 ,10 for all other π
Information Sequence
Samples y(kTb)
Detected sequence
1 1 0 1 1 0 0 0 1 0 1 1 1
1 1 0 1 1 0 0 0 1 0 1 1 1
1 2 0 0 2 0 -2 -2 0 0 0 2 2
Nyquist 2nd criteria Duobinary Pulse
The minimum bandwidth pulse that satisfiesthe duobinary pulse criterion and (b) its spectrum.
π (π‘ )=π ππ (π π π π‘ )
π π ππ‘ (1 βπ π π‘ )
π ( π )= 2π π
πππ ( π ππ π ) Ξ ( π
π π )πβ π π π /π π
Scrambling
Scrambler tends to make the data more random by removing long strings of 1s and 0s. Removing long 0s or 1s help in timing extraction. However, the main purpose of scrambling is to prevent unauthorized access to the data.
π=πβ¨π·3πβ¨π·5π π=πβ¨ (π·3πβ¨π·5π )
Scrambling Example
The data stream 101010100000111 is fed to the scrambler. Find the scrambler output T, assuming the initial content of the registers to be zero.
Scrambling Example
The data stream 101010100000111 is fed to the scrambler. Find the scrambler output T, assuming the initial content of the registers to be zero. S 1 2 3 4 5 T
1 0 0 0 0 0 1 0 1 0 0 0 0 01 0 1 0 0 0 10 1 0 1 0 0 11 1 1 0 1 0 10 1 1 1 0 1 01 0 1 1 1 0 00 0 0 1 1 1 00 0 0 0 1 1 1 0 1 0 0 0 1 10 1 1 0 0 0 00 0 1 1 0 0 1T=101110001101001
Digital Receivers and Regenerative Repeaters
Tasks of Receivers or repeaters:
1. Reshaping incoming pulses by means of an equalizer.
2. Extracting the timing information required to sample incoming pulses.
3. Making symbol detection decisions based on the pulse samples.
Time Extraction
Three general methods of synchronization
1- Derivation from a primary or a secondary standard (transmitter and receiver slaved to a master timing source).
2- Transmitting a separate synchronizing signal (pilot clock)
3- Self-synchronization, where the timing information is extracted from the received signal itself.
Eye Diagrams: An Important Tool
Three general methods of synchronization
Eye diagrams of a polar signaling system using a raised cosine pulse with roll-off factor 0.5: over 2 symbol periods 2Tb with a time shift Tb/2;
PAM: M-ARY Baseband Signaling for Higher Data Rate
The information IM transmitted by an M-ary symbol is
πΌπ= log2 π bits
The transmitted power increases as M2.
Example
Determine the PSD of the quaternary (4-ary) baseband signaling when the message bits 1 and 0 are equally likely.
Digital Carrier Systems
In transmitting and receiving digital carrier signals, we need a modulator and demodulator to transmit and receive data. The two devices, modulator and demodulator are usually packaged in one unit called a modem for two-way (duplex) communication.
Amplitude Shift Keying (ASK)(a) The carrier cos Οct. (b) The modulating signal m(t). (c) ASK: the modulated signal m(t) cos Οct.
Digital Carrier Systems (Modulator)
Phase Shift Keying (PSK)
Frequency Shift Keying (FSK)
Spectrum of Modulated Digital Signals
PSD of PSK
PSD of FSK
PSD of ASK
Digital Carrier Systems (Demodulator)Noncoherent detection of FSK
Coherent detection of FSK
Coherent binary PSK detector
Differential PSK (DPSK)
DPSK allows noncoherent demodulation at the receiver. The transmitter encodes the information data into the phase difference ΞΈk - ΞΈk-1. For example a phase difference of zero represent 0 whereas a phase difference of signifies 1.
Transmitter Encoding
Receiver Decoding
Differential PSK (DPSK)
Transmitter Encoding
Receiver Decoding
M-Ary Digital Carrier Modulation
Higher bit rate transmission can be achieved by either reducing Tb or by applying M-ary signaling; the first option requires more bandwidth; the second requires more power to keep the error bit rate within acceptable level.
M-ary ASK and noncoherent Detection
M-ary shift keying can send Log2 M bits each time by transmitting any one of M signals.
M-ary FSK and noncoherent Detection
where
and
Choice of the Frequencies for FSK
Large leads to bandwidth waste, whereas small is prone to detection error due to transmission noise interference.
To minimize error detection the choice of should be large enough to make the FSK modulating signals orthogonal over the period Tb.
The choice of will determine the performance and bandwidth of the FSK modulation.
β π =π πβ π 1
2=1
2(πβ1 )πΏ π
β«0
π π
π΄πππ (2π π π π‘ ) π΄πππ (2π π ππ‘ ) ππ‘=0 πΏ π =1
2π ππ»π§
Comparison between ASK and FSK
ASK does not require increase in bandwidth but the power increase linearly with M.
FSK does not require increase in power but the bandwidth increase linearly with M (compared with binary FSK or M-ary ASK).
M-ary PSK
ππππΎ (π‘ )=π΄πππ (πππ‘+ππ ) π=1 ,2 , β¦,π
ππ=π0+2ππ (πβ1 )
M-ary PSK symbols in the orthogonal signal space: (a) M = 2; (b) M = 4; (c) M = 8.
π0=2ππ
π0=180π0=90 π0=45
M-ary PSK
ππππΎ (π‘ )=ππβ 2ππ
πππ πππ‘+ππβ 2π π
π πππππ‘ 0 β€ π‘<ππ
π 1 (π‘ )=β 2ππ
πππ πππ‘ π 2 (π‘ )=β 2π π
π πππππ‘
ππππΎ (π‘ )=πππ 1 (π‘ )+πππ 2 (π‘ )
M-ary PSK symbols in the orthogonal signal space: (a) M = 2; (b) M = 4; (c) M = 8.
Quadrature Amplitude Modulation (QAM)
ππ (π‘ )=πππ(π‘ )πππ πππ‘+πππ (π‘)π πππππ‘ 0 β€ π‘<ππ
π π=βππ2+ππ
2 ππ=π‘ππβ1 ππ
ππ
p(t) is a properly shaped baseband pulse.A simple choice is a rectangular.
ππ (π‘ )=π ππ (π‘ )πππ (πΒΏΒΏππ‘βπ π)ΒΏ
16-point QAM (M = 16).
QAM or Multiplexing