ece 4710: lecture #12 1 normalized a = 2 unipolar nrz advantages: 1) easy to generate for ttl (0,...
TRANSCRIPT
ECE 4710: Lecture #12 1
Normalized A = 2
Unipolar NRZ
Advantages:1) Easy to generate for TTL (0,
+5V)
2) Single supply voltage
3) Best FNBW
Disadvantages:1) Must have DC coupled circuit
2) Power “wasted” on DC
3) Poorer S/N vs. BER performance
compared to polar NRZ
1 1 0 1 0 0 1
ECE 4710: Lecture #12 2
Polar NRZ
Normalized A = 1
Advantages:
1) Fairly easy to generate
2) Good S/N vs. BER compared to
unipolar NRZ
3) Best FNBW
Disadvantages:1) Large PSD at DC need
frequent 1/0 data toggles for AC
coupled channel not 100%
transparent
2) Dual supply voltages ±V
1 1 0 1 0 0 1
ECE 4710: Lecture #12 3
Unipolar RZ
1 1 0 1 0 0 1
Normalized A = 2
Advantages:
1) Discrete impulse term @ f = R
filter and use for clock recovery in Rx!
2) Single supply voltage
Disadvantages:1) Larger FNBW relative to NRZ codes
2) Some power wasted on DC
3) Poor S/N vs. BER performance
compared to unipolar NRZ 3 dB
more signal power b/c of 0.5 Tb
duration
ECE 4710: Lecture #12 4
Bipolar RZ (AMI)
Advantages:1) No energy at DC AC coupling OK
2) Can be converted to unipolar RZ
using full-wave rectifier clock signal
3) Single error detection bipolar line
rule violated for “1” errors
Disadvantages:1) String of 0’s loss of clock signal
and not 100% transparent
2) OK BW not as good as unipolar
or polar NRZ b/c first sidelobe is larger
3) Rx must distinguish 3 levels (not 2)
4) 3 dB more power for same S/N
Normalized A = 2
1 1 0 1 0 0 1
ECE 4710: Lecture #12 5
Manchester NRZ
Advantages:1) Always has DC value = 0 for any
data stream on bit-by-bit basis
2) One zero crossing per bit provides
good recovery of clock signal
3) Excellent synchronization since
string of 0’s won’t cause loss of clock
Disadvantages:1) Double FNBW relative to NRZ codes
2) Dual power supply for ±V
1 1 0 1 0 0 1
Normalized A = 1
ECE 4710: Lecture #12 6
Differential Coding
Serial data stream can be unintentionally inverted (complemented) when passing thru many circuits along a long-distance communication channel (e.g. landline telephony) Inversion all “1”s become “0”s and vice versa Twisted pair transmission line (phone) with inverted leads
Differential encoding ( = XOR)1 nnn ede
en-1 dn en
0 0 0 0 1 1 1 0 1 1 1 0
1~~~
nnn eed
Encode
Decodedata received ~ and encoded input nn ed
ECE 4710: Lecture #12 7
Differential Coding
Binary “1” encoded if the present input bit, dn , and past encoded bit, en-1 , are opposite (0/1, 1/0) and binary “0” if states are the same (0/0, 1/1)
At Rx the encoded signal is decoded by comparing states of adjacent (sequential) bits Decoding “0” = 0/0 or 1/1, “1” = 0/1 or 1/0
Advantages Channel polarity inversion does not affect data Pass encoded signal thru thousands of circuits/systems Doesn’t require phase information of bit when decoding modulated
signals where symbol phase represents data» Binary Phase Shift Keying (BPSK)
ECE 4710: Lecture #12 8
Differential Coding
ECE 4710: Lecture #12 9
Differential Coding
Channel polarity has no effect on decoded sequence!!
ECE 4710: Lecture #12 10
Line Codes
Effect of channel noise, filtering, and ISI on received line code can be observed on digital oscilloscope in the form of an “Eye Pattern” Specialized communication O-scopes have this
functionality built in along with other useful diagnostics Eye pattern generated by multiple sweeps of
received signal Synchronized clock signal used so that bit periods
precisely overlap on multiple sweeps» Sweep width is a little larger than Tb
Received 1’s and 0’s from multiple sweeps produce eye pattern
ECE 4710: Lecture #12 11
Eye Pattern
Eye pattern provides excellent way of visually assessing the Quality of received line code Ability of Rx to combat bit errors
Under good operating conditions the eye will be fully open:
“Ideal”PolarNRZ
ECE 4710: Lecture #12 12
Eye Pattern
Distortions in eye pattern can be used to visualize effects of: Channel noise & interference Imperfect baseband filtering Channel bandwidth limitations
Measurements on eye pattern can quantify effects of: Allowed timing error width of open eye Sensitivity to timing error slope of open eye evaluated at zero-
crossing point (symbol edge) Noise margin height of the eye opening Amount of ISI height difference between open eye and partially
closed eye
ECE 4710: Lecture #12 13
Distorted Eye Pattern
ISI
ISI
ECE 4710: Lecture #12 14
Regenerative Repeaters
Line code signal can easily be corrupted when being transmitted over a long-distance twisted pair telephone line Signal is attenuated, filtered, and corrupted by noise Data cannot be recovered unless repeaters are placed at multiple
points along the line Regenerative Repeaters:
Amplify and clean up signal distortions by detecting correct line code and regenerating it non-linear processing
Not practical with analog information signal» Requires linear amplifiers only since amplitude contains information» In-band noise/distortion accumulates from repeater to repeater
Greatly improved S/N performance compared to analog methods» In band noise/distortion does not accumulate over long-distance link» Small amount of bit errors can be introduced by regenerators
ECE 4710: Lecture #12 15
Regenerative Repeaters
Amplifier/Filter: increases weak input signal and minimizes channel noise and ISI equalizing filter
Bit Synchronizer: generates clock signal so sample circuit will sample line code at time where eye opening is maximum
Sample/Hold: produces single amplitude value and holds for Tb
Comparator: high value (“1”) when sample > VT ; low value (“0”) when sample < VT; functions as non-linear decision maker
ECE 4710: Lecture #12 16
Regenerative Repeaters
Regenerated signal Noise free “clean” due to non-linear processing Bit errors introduced when noise and ISI alter input signal
substantially so that sample value is pushed beyond VT
» BER determined by S/N ratio,VT, and statistics of signal and noise
Long-distance communication system Spacing between repeaters determined by attenuation (path loss) of
the channel and amount of added noise
Repeater required when S/N ratio falls below a threshold required for acceptable BER
Overall probability for bit error for m repeaters is Pme mPe assuming
good operating conditions such that Pe << 1