Types Of Line Coding

Some Types Of Line Coding:

1 Polar Signaling

In polar signaling NRZ, 1 is transmitted by a pulse p(t) for a duration of T_b seconds and 0 is transmitted by  - p(t) for a duration of Tb seconds.

In polar signaling RZ, 1 is transmitted by a pulse p(t) for a duration of   T_b /2 seconds followed by zero voltage for a duration of T_b /2 seconds and 0 is transmitted by  - p(t) for a duration of T_b/2 seconds followed by zero voltage for a duration of T_b /2 seconds.

Properties Of Polar Signaling

1.Transmission bandwidth : For a half width pulse ( RZ ) The essential bandwidth of the signal is 2R_b Hz (where R_b is the clock frequency). This is four times the theoretical bandwidth (Nyquist bandwidth) required to transmit R_b pulses per second. Increasing the pulse width reduces the bandwidth (expansion in the time domain results in compression in the frequency domain). For a full width pulse (NRZ), the essential bandwidth is half, that is, R_b Hz. This is still twice the theoretical bandwidth. Thus, polar signaling is not bandwidth efficient.

2. Error detection and correction capability : polar signaling has no error detection or error correction capability.

3. power spectral density : It has nonzero PSD at dc (w = 0). This will rule out the use of ac coupling during transmission. The ac coupling, which permits transformers and blocking capacitors to aid in impedance matching, and which allows dc powering of the line repeaters over the cable pairs, is very important in practice.

4. Power efficiency : polar signaling is the most efficient scheme from the power requirement viewpoint. For a given power, it can be shown that the detection error probability for a polar scheme is the smallest possible.

 5. Transparency : Polar signaling is also transparent because there is always some pulse (positive or negative) regardless of the bit sequence.    6. Adequate timing content : Rectification of the polar signal, yields a periodic signal of the clock frequency and can be used to extract timing.