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:

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


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


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


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


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


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


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)


Differential Coding


Differential Coding

Channel polarity has no effect on decoded sequence!!


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


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


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


Distorted Eye Pattern

ISI

ISI


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



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



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

Documents

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: