COMMUNICATION SYSTEM EECB353Chapter 5 Part V

DIGITAL TRANSMISSIONLINE ENCODING PROPERTIES

Intan Shafinaz MustafaDept of Electrical Engineering

Universiti Tenaga Nasionalhttp://metalab.uniten.edu.my/~shafinaz

Properties of Line Coding

1. Transmission voltages and DC component.

a) The positive voltage are not canceled by the negative voltages.

b) The positive voltages are canceled by any negative voltage.

Unipolar

Bipolar

Properties of Line Coding2. Duty cycle – NRZ maintained the binary pulse entire bit time,

while RZ the active time of the binary pulse is less than 100%.

BPNRZ – there are 2 non-zeroes voltage (+V = logic1 and – V = logic0) and 100% duty cycle is used.

BPRZ – shows that there are two non-zeroes voltages but each pulse is active only 50% of a bit time. Assuming equal-magnitude voltages for logic 1s and 0s, the average DC voltage of BPRZ is 0V.

Properties of Line Coding3. Bandwidth Requirements - To determine the minimum BW required to propagate a line-encoded digital signal, we must determine the highest fundamental freq associated with the signal.

The highest fundamental freq is determined from the worse case (fastest transition) binary bit sequence.

For UPNRZ, the worse case condition is an alternating 1/0 sequence; the period of the highest fundamental frequency takes the time of two bits i.e fb/2 It requires 1 signal changes to encode 1 bit, less BW

For UPRZ, the worse case condition occurs when the two successive logic 1s occurs. Therefore, the min BW is fb. It requires 2 signals change to encode 1 bit, more BW

UPNRZ

BPNRZ

UPRZ

BPRZ

BPRZ-AMI

BW = fb/2

BW = fb/2

BW = fb

BW = fb

BW = fb/2

Properties of Line Coding4. Clock and Framing Bit Recovery –To recover and maintain clock and framing bit synchronization from the received data, there must be sufficient transitions in

the data waveform. UPNRZ and BPNRZ encoding – a long string of 1s and 0s generates a data signal void/invalid of transitions and therefore, is inadequate for clock recovery. BPRZ – a transition occurs in each bit position regardless of whether the bit is a 1 or a 0. BPRZ-AMI encoding provides sufficient transition to ensure clock

synchronization.

Line Coding SummaryEncoding Format

Minimum BW

Average DC

Clock Recovery

UPNRZ fb/2* +V/2 Poor

BPNRZ fb/2* 0V* Poor

UPRZ fb +V/4 Good

BPRZ fb 0V* Best*

BPRZ-AMI fb/2* 0V* Good

*Denotes best performance or quality.