SDH Modulation Techniques: NRZ and RZ

This page covers ON OFF Keying modulation techniques used in SDH/SONET optical communication networks.

The basic modulation technique used in fiber optics is On-Off Keying (OOK). In this method, light is turned off when a binary zero is to be transmitted and turned on when a binary one is to be transmitted.

The simplest way to implement this is to emit light for the duration of a “one” bit time and stop light emission for the duration of a “zero” bit time. This method is referred to as Non-Return-to-Zero (NRZ).

NRZ Modulation

Optical NRZ is different from electrical NRZ. In the optical domain, there’s no “negative light” like there is negative voltage in electrical signals.

In NRZ modulation, a continuous long string of ones leads to the light being on for an extended period. This can cause the receiver to lose timing synchronization, ultimately leading to incorrect decoding of the bits.

RZ Modulation

To avoid the synchronization problem associated with NRZ, the Return-to-Zero (RZ) technique can be used. However, RZ modulation allows for long strings of zeros and effectively reduces the number of photons seen by the photodetector by half for each bit period. Because of this reduction in signal strength, RZ is generally not used for long-distance communication.

DC Balance and Long-Distance Communication

DC balance refers to maintaining a constant average transmitted power in the optical communication system.

For long-distance communication, NRZ is preferred to minimize long strings of ones and zeros to maintain DC balance. The most effective way to prevent long strings of identical bits is through line coding and scrambling techniques. These methods ensure sufficient transitions in the data stream, helping maintain synchronization and reliable communication.