Higher Baud Rates vs. Higher Order Modulation: Which is Best?
Paul Momtahan, Director of Solution Marketing, Coriant
Optical equipment vendors are evolving coherent technology with support for higher capacity wavelengths in order to meet network operator demands for increased bandwidth at a lower cost and to support new client interfaces such as 400 GbE and FlexE. To do this, they have two primary levers: the baud rate, the number of symbols per second, and the modulation, the number of bits per symbol. Each has its advantages, as shown in Table 1 below.
Table 1 - Higher Order Modulation vs. Higher Baud Rates
To understand their relative advantages, let us look at what happens if we double the wavelength capacity, starting with a 100G wavelength based on QPSK modulation and 34.5 Gbaud. If we do this by doubling the bits per symbol by going from QPSK (2 bits per symbol) to 16QAM (4 bits per symbol), this increases the number of constellation points by a factor of four, making the wavelength approximately four times more sensitive to noise and nonlinearities. As shown in Figure 1, the reach is decreased to approximately 25%, but we have doubled the spectral efficiency.
Figure 1 - The Impact of Doubling Wavelength Capacity
If instead, we double the baud rate to 69 Gbaud, we double the sensitivity to noise and nonlinearities, which by itself would reduce the reach to 50%. However, we get most of this reach back as we can pump more power into the wavelength with fewer nonlinear penalties as the power is spread over 69 GHz rather than 34.5 GHz. This typically puts the reach of the 69 Gbaud 200G at around 90% of the original 100G wavelength. The price to be paid for this is the wavelength now has half the spectral efficiency of the 16QAM 200G alternative.
Figure 2 - Baud Rate Use Cases
But is a higher baud rate always better? While for flexi-grid networks, as a general rule, higher is better, there will be boundary scenarios where lower baud rates can provide the better result in terms of spectral efficiency. And while 50 GHz point-to-point networks can support baud rates up to 46 Gbaud, 50 GHz mesh ROADM networks will be typically limited to under 35 Gbaud. This is why the recently announced Coriant CloudWave™ T Optics provides the ability to select from a wide range of baud rate options between 28 Gbaud and 69 Gbaud.
For more information on this topic, see the new Coriant white paper, The Role of Higher Baud Rates in Evolving Coherent Transport.