Why are we moving to Higher Baud Rate?

Coherent systems designers are making use of new higher baud rate components to both increase the total data rate and to improve the reach of coherent systems.

For example, doubling the baud rate from the standard 35 Gbaud currently used in 100G systems to 64 Gbaud immediately doubles the amount of information the system can transmit, and since it requires the same number of optical components, nearly cuts the cost per bit in half.

There are two technique to increase the total transmission capacity of coherent transceiver:

-         Higher baud rate

-         Higher modulation order

Higher baud rate:

Baud rate also known as symbol rate is the rate at which symbols are transmitted and received. Note, that a symbol may contain multiple bits of information. The baud rates for 100G transmission vary from 28 to 35 Gbaud or 28 – 35 billion symbols/second with higher baud rate needed to support more aggressive error correction

Higher baud rate is easier to increase the transceiver’s capacity and lower the signal-to-noise ratio (SNR)

For example, we observe for the same 200Gb/s net data rate, 65Gbaud/DP-QPSK can still transmit in a long-haul system, while 35Gbaud/DP-16QAM can only be used in a metro system.

The total transmitted bit rate is determined by three transmission system parameters:

 Bit rate = baud rate (symbols/sec) X polarization (typically = 2) X coding (bits/symbol)

Higher modulation order:

There is another technique to increase the data capacity of a coherent link, and that is to use higher order modulation (HOM) like 16 QAM or 64 QAM. In this technique, both the phase and the amplitude are varied to pack more different states representing more bits into a single “symbol”.

The picture below shows QPSK (2 bits), 16 QAM (4 bits) and 64 QAM (6 bits).

- It is easily seen that as you move to higher order QAM the different states get closer together and harder to distinguish.

- Higher modulation rate limit the reach and required higher signal-to-noise ratio (SNR).

- For example, a single wavelength 400Gbps system utilizing 65 Gbaud and 16 QAM has a reach of approximately 200 km, while an equivalent 400Gbps system utilizing 35 Gbaud and 64 QAM has a practical reach of less than 40 km.

- In real systems, of course, both techniques will be used, depending on the application and the required reach.