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

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

ECE 4710: Lecture #12 3 Unipolar RZ Normalized A = 2 Advantages: 1) Discrete impulse 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 T b 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 =

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 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) e n-1 d n e n Encode Decode

ECE 4710: Lecture #12 7 Differential Coding  Binary “1” encoded if the present input bit, d n, and past encoded bit, e n- 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 T b  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” Polar NRZ

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

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 T b Comparator: high value (“1”) when sample > V T ; low value (“0”) when sample < V T ; 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 V T »BER determined by S/N ratio, V T, 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 P me  mP e assuming good operating conditions such that P e << 1