Data Center 800G: PAM4 or Coherent?
PAM4 is a pulse-amplitude modulation technique that uses four different signal levels for signal transmission. 4 represents four different amplitude states corresponding to 00, 01, 11 and 10, with each symbol representing 2 bits of information. PAM4 can carry a total of 106Gbps of data when operating at a 53G baud rate.
The main use of coherent modulation and outlier detection technology in coherent optical communication is to use the signal to be transmitted to change the frequency, phase and amplitude of the optical carrier (unlike intensity detection which only changes the intensity), which requires the optical signal to have a defined frequency and phase (unlike natural light which does not have a defined frequency and phase), i.e. it should be coherent light. It is mainly used for high speed rates and long distance transmission.
PAM4 is generally used for high rate, short and medium distance transmission and is extremely well suited to the new generation of data center internal connections.
Take NADDOD 400G optical transceiver as an example, 400G QSFP-DD SR8 adopts 50Gbps PAM4 modulation, transmission distance can reach up to 100m, widely used in data center 400G direct connection, 400G-2x200G interconnection and 400G-8x50G interconnection; The 400G QSFP-DD DR4 is modulated with 100Gbps PAM4 and has a transmission distance of up to 500m, and can be used for data center 400G direct connection and 400G-4x100G interconnection. The NADDOD 400G QSFP-DD FR4/LR4 is modulated with 100Gbps PAM4 and has a transmission distance of up to 2km and 10km respectively.
- SR8 Transmission Range 100m
8-channel, 16 core MTP multimode with a channel rate of 50Gbps.
- DR4 Transmission Range 500m
4-channel, 12 core MTP single mode with a channel rate of 100Gbps.
- FR4 Transmission Range 2km
4-channel, LC dual core MTP single mode with a channel rate of 100Gbps.
In DCI long-range interconnection scenarios, PAM4 is somewhat overwhelmed by the coherent modulation based on the 400ZR protocol, which can operate at baud rates of around 60Gbaud with dual-polarised 16QAM (DP-16QAM) modulation (the optical signal is encoded in both phase and amplitude), supporting single wavelengths carrying rates of 400Gbps or higher. Therefore, higher performance requirements are required for optical transceiver lasers, which require ultra-narrow linewidth lasers, I/Q modulators and coherent receivers. Compared with the direct modulation adopted by PAM4, this allows for much longer transmission distances.
In fact, the baud rates are similar for both technologies, but coherent transmission using higher power DSP allows for single wavelength encoding of more data than can be modulated with PAM4, which compensates by using multiple wavelengths and simple lasers.
However, as data center rates have grown, there has been an overlap between the two technologies. In the 800G era, the gap between PAM4 and coherent technologies will become even smaller. The decision of whether a technology is competitive is based on cost and power consumption.
The easiest way to double the data rate, while keeping the baud rate the same, is to upgrade the hardware. For example, PAM4 can use 4 or 8 100G/200G wavelengths and coherent modulation can use two 400G wavelengths.
Another way is to increase the baud rate, for example by doubling the baud rate to approximately 110G baud, to achieve an overall rate increase from 400 to 800Gbps. For coherent technology, this becomes a question of whether the I/Q modulator and receiver should be InP or silicon photonic. Silicon photonics is relatively low cost, but its performance is also significantly lower. It can be argued that silicon photons have high peak voltages and poor bandwidth, whereas InP has low peak voltages and good bandwidth, but its cost is higher.
For PAM4, an indirectly modulated EML with a laser with built-in indium phosphide (InP) can be used. Alternatively, an integrated array of silicon photon modulators and an array of InP lasers can be used. As with coherent solutions, high peak voltages and poorer bandwidths put a damper on silicon photonics compared to EML solutions, but silicon photonics is cheaper.
For both PAM4 and coherent technologies, InP transceivers are more costly, while silicon light is cheaper.
In terms of power consumption, as chip technology evolves from 7nm to 5nm and even 3nm, it is not only the DSP processing rate that increases but also the performance in terms of power reduction that becomes increasingly superior. As the graph below shows, 100G coherent is nearly 10 times more power efficient than 100G PAM4, but this difference is significantly reduced in 5nm node based 800G applications. The graph below shows the power performance of Coherent and PAM4 DSPs at different CMOS nodes.
These schemes have all been experimentally proven by different companies. NADDOD believes that as yields increase and costs decrease, the fact that the coherent scheme requires only one laser, modulator and receiver will allow it to achieve cost competitiveness comparable to PAM4, even if the optical devices become more complex. The greater flexibility and performance that can be achieved with coherent solutions can then come into play, and PAM4 with four simple lasers, modulators and receivers, even if they are not so simple at 800G, is enough to quickly reduce costs and remain competitive ahead of coherent. Overall, the competition between coherent and PAM4 transmission has begun, and future results will have to wait.