Baud rate in optical transceiver
In a coherent transmission system, you control two parameters: the Quadrature Amplitude Modulation (QAM) and the moderate. However, the QAM is limited. As you increase your QAM density from Quadrature Phase Shift Keying (QPSK) to 8 QAM, then 16 QAM, and 32 QAM, you reduce the tolerance to noise, which affects the reachable distance. For this reason, we need another knob to adjust instead of the QAM, which gives you the baud rate. The baud rate is the number of symbols or pulses that the optical transceiver sends every second. This used to be fixed at 30 to 35 billion symbols, but the Fujitsu 1FINITY™ T900 optical transceiver now has the capability to transmit up to 150 gigabaud, which is five times the old rate.
If you want to transmit at 400 gigs per second, with both the QAM density and the baud rate as available adjustments, you have more flexibility to adjust the optical transceiver setting to your specific distance requirement. For example, you can use 16 QAM at 65 gigabaud, or you can use QPSK at 150 gigabaud. Both of them achieve the same capacity, but the QPSK mode of operation will provide you with at least twice the reachable distance. You have the flexibility built into the optical transceiver itself, and that flexibility can be used to program the optimal setting for your distance and line rate application.
However, a higher baud rate is not always better. A higher baud rate requires more spectrum, and it reduces the number of channels that you have within the C-band. If you have a short distance requirement, you are better served by keeping the baud rate low in order to be more spectrally efficient.
Programmability is the key here. You have a single optical transceiver, which can be programed with multiple combinations of modulations, baud rates, and error correction to optimize your transmission for the specific line rate, distance, and available spectrum. The larger number of combinations in the most recent optical transceiver generations provides the service provider with transmission flexibility, so they have combinations ranging from 100 gig to 1.2 terabit, with distances between 50 to 6,000 kilometers.
Programmable 1.2 terabit 150GBd optical transponder
The new Fujitsu 1FINITY T900 is a next-generation optical transponder that support baud rates up to 150 gigabaud. It also has the flexibility to be programed at lower baud rates to support shorter distance applications. The Fujitsu 1FINITY T900 provides the ability to transmit wavelengths operating at 1.2 terabit over the existing ROADM infrastructure, which is a first. This enables 800 gigs to reach distances, making most metro, regional, and even some long-distance applications possible. It provides additional QAM modulation and baud rate combinations, enabling longer distance application or very high-rate transmission over short distances. The 1FINITY T900 has many capabilities in a single unit, which can be deployed for multiple applications. With the 1FINITY T900, you get programmability in the optical network to deploy a single high baud rate capable unit and select the optimum mode of operation for your application. The Fujitsu 1FINITY T900 provides flexibility, programmability, and repeatability over an application range, which is the broadest in the industry. If you’re interested in learning more, we have a video series on how the Fujitsu 1IFNITY T900 improves efficiency in optical transceivers.
