Viscore

Coherent 40g/100g technology is currently a hot topic in the industry. We often hear about 40g signals being carried on a 10g optical link. What does that really mean?

We could ignore the complicated physical layer details relating to the coherent transmitter and receiver. The “40g on 10g” can be understood by distinguishing between the baud rate and the bit rate.

The baud rate is the number of symbol changes (waveform changes or signaling events) made to the transmission medium per second using a digitally modulated signal or a line code. The symbol rate is measured in baud (Bd) or symbols/second. Each symbol can represent or convey one or several bits of data.

With traditional modulation, the signal is either 0 or 1, i.e. one bit per symbol.  However, by increasing the number of states that the carrier signal can take, the number of bits encoded in each symbol can be greater than one. The bit rate can then be greater than the symbol rate>

In QPSK (Quadrature Phase Shift Keying), the signal state can be in one of four different phases.  We can denote that using four points on the constellation diagram, equi-spaced around a circle. With four phases, QPSK can encode two bits per symbol.

In the DP-QPSK (Dual Polarization QPSK) coding, smart engineers leverage the polarization feature of photonics, i,e. there are two orthogonal polarizations (e.g. horizontal and vertical) which can be used to multiplex signals. Each polarization carries TWO (2) bits per symbol. Totally, a DP-QPSK encoding scheme carries FOUR (4) bits per symbol.