Line Coding: Different Techniques and MATLAB Implementation, Summaries of Matlab skills

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Lab No. 8

Data Communication and Networks

Line Coding

Line Coding

• In telecommunication, a line code is a code chosen for use within a

communications system for transmitting a digital signal down a line.

• Line coding is often used for digital data transport.
• Line coding consists of representing the digital signal to be transported, by a

waveform that is optimally tuned for the specific properties of the physical

channel (and of the receiving equipment).

Unipolar Scheme:

• In a unipolar scheme, all the signal levels are on one side of the time axis, either above or below.
• NRZ (Non-Return-to-Zero): Traditionally, a unipolar scheme was designed as a non-return-to- zero

(NRZ) scheme in which the positive voltage defines bit 1 and the zero voltage defines bit O. It is

called NRZ because the signal does not return to zero at the middle of the bit. The following figure

shows a unipolar NRZ scheme.

Polar Schemes

• In polar schemes, the voltages are on the both sides of the time axis. For example, the voltage level for 0 can be positive and the voltage level for I can be negative.

Return to Zero (RZ):

• The main problem with NRZ encoding occurs when the sender and receiver clocks

are not synchronized. The receiver does not know when one bit has ended and the

next bit is starting. One solution is the return-to-zero (RZ) scheme, which uses

three values: positive, negative, and zero. In RZ, the signal changes not between

bits but during the bit. In the following figure, we see that the signal goes to 0 in

the middle of each bit. It remains there until the beginning of the next bit.

Biphase Manchester and Differential Manchester:

• The idea of RZ (transition at the middle of the bit) and the idea of NRZ-L are combined into the

Manchester scheme.

• In Manchester encoding, the duration of the bit is divided into two halves. The voltage remains at

one level during the first half and moves to the other level in the second half. The transition at the

middle of the bit provides synchronization.

• Differential Manchester, on the other hand, combines the ideas of RZ and NRZ-I. There is always a

transition at the middle of the bit, but the bit values are determined at the beginning of the bit. If

the next bit is 0, there is a transition; if the next bit is 1, there is none. The following figure shows

both Manchester and differential Manchester encoding.

Line Coding in MATLAB Function For Biphase Manchester

function [t,x] = manchester(bits, bitrate)

% MANCHESTER Encode bit string using Manchester code.

% [T, X] = MANCHESTER(BITS, BITRATE) encodes BITS array using Manchester

% code with given BITRATE. Outputs are time T and encoded signal

% values X.

T = length(bits)/bitrate; % full time of bit sequence

n = 200;

N = n*length(bits);

dt = T/N;

t = 0:dt:T;

x = zeros(1,length(t)); % output signal

Function For Biphase Manchester (Cont…) for i = 0:length(bits)- 1 if bits(i+1) == 1 x(in+1:(i+0.5)n) = 1; x((i+0.5)n+1:(i+1)n) = - 1; else x(in+1:(i+0.5)n) = - 1; x((i+0.5)n+1:(i+1)n) = 1; end end

Function For Polar RZ (cont…) for i = 0:length(bits)- 1 if bits(i+1) == 1 x(in+1:(i+0.5)n) = 1; x((i+0.5)n+1:(i+1)n) = 0; else x(in+1:(i+0.5)n) = - 1; x((i+0.5)n+1:(i+1)n) = 0; end end

Function For Unipolar NRZ

function [t,x] = unrz(bits, bitrate)

% UNRZ Encode bit string using unipolar NRZ code.

% [T, X] = UNRZ(BITS, BITRATE) encodes BITS array using unipolar NRZ

% code with given BITRATE. Outputs are time T and encoded signal

% values X.

T = length(bits)/bitrate; % full time of bit sequence

n = 200;

N = n*length(bits);

dt = T/N;

t = 0:dt:T;

x = zeros(1,length(t)); % output signal

Function For Unipolar RZ

function [t,x] = urz(bits, bitrate)

% URZ Encode bit string using unipolar RZ code.

% [T, X] = URZ(BITS, BITRATE) encodes BITS array using unipolar RZ

% code with given BITRATE. Outputs are time T and encoded signal

% values X.

T = length(bits)/bitrate; % full time of bit sequence

n = 200;

N = n*length(bits);

dt = T/N;

t = 0:dt:T;

x = zeros(1,length(t)); % output signal

Function For Unipolar RZ (Cont…) for i = 0:length(bits)- 1 if bits(i+1) == 1 x(in+1:(i+0.5)n) = 1; x((i+0.5)n+1:(i+1)n) = 0; else x(in+1:(i+1)n) = 0; end end

Calling the functions (Cont…)

figure;

[t,s] = prz(bits,bitrate);

plot(t,s,'LineWidth',3);

axis([0 t(end) - 1.1 1.1])

grid on;

title(['Polar RZ: [' num2str(bits) ']']);

figure;

[t,s] = manchester(bits,bitrate);

plot(t,s,'LineWidth',3);

axis([0 t(end) - 1.1 1.1])

grid on;

title(['Manchester: [' num2str(bits) ']']);