1 Digital intensity modulations: Alternate Mark Inversion, Duobinary Modulation, Carrier Suppressed Return to Zero
1.5 Duobinary modulation

The principle

Duobinary modulation (DB) is implemented in optical communications by using three logical states, where:

A binary zero is represented by the absence of a laser pulse; binary 1s can be represented by a laser pulse with altered phase, based on the previous symbols in the following manner.

Phase of a binary symbol is shifted by π if there is an odd number of binary 0 between two binary 1.

It can be combined with RZ or NRZ rule.

Making the pulse length shorter than the duration of a logical symbol 1 causes the power falling to zero between two (or more) logical 1s. For data containing long chains of binary 1, it is a practical solution for synchronization issues.

Main benefits

Advantages of DB:

  • High tolerance to chromatic dispersion (CD)
  • Easy narrow-band filtering , (DB's narrow bandwidth could handle similar to the performance of Differential Quadrature Phase Shift Keying (DQPSK) format also a 12.5 GHz DWDM grid
  • DB is the only intensity format which is still stable even for a 130 km long transmission and its system performance is quite similar to that of phase modulations, e.g. DQPSK.
  • DB can be even more efficient than NRZ-DQPSK and CSRZ-DQPSK in terms of transmitter's design and implementation cost.

Transceiver’s construction:

The DB transmitter consists of an amplitude dual-arm MZM with two electrical inputs:

  • the first one is the standard electrical signal achieved by pseudorandom binary sequence signals passing through a NRZ driver a low-pass filter
  • the second input by performing a bitwise logical NOT operation on the logical input which again travels through a driver and an electrical filter
  • Similarly as for NRZ, a continuous wave laser is used as a light source.

In the following figure, +1 corresponds to phase 0 and -1 corresponds to phase π or its odd multiples.

image
The principle of DB modulation – symbol’s phase is shifted by π radians if there is odd number of binary zeros (1, 3, 5...).