Think Spectral Efficiency and Wavelength Capacity-reach are the Same Thing? Think Again.

Spectral efficiency describes how many bits per second of effective data payload can be transmitted in a given amount of spectrum for a given set of reach conditions. As shown in Figure 1, spectral efficiency is determined by three things.

First, is the number of raw bits per symbol, which is directly proportional to the number of raw bits per second per Hz.

Second, is overhead efficiency, which is the percentage of these bits that can be used for the data payload as opposed to the overhead for functions such as forward error correction (FEC), framing, performance monitoring, and in-band management.

Third, is the amount of spectrum that is “lost” or wasted, which includes both guard bands between wavelengths and the occupied bandwidth of the wavelength due to its relative shape based upon its roll-off vs. an ideal wavelength where the symbol rate in Gbaud exactly matches its spectrum in GHz (i.e., 0% roll-off).

Figure 1 – The three factors that determine spectral efficiency

Spectral Efficiency and Wavelength Capacity-reach

Wavelength capacity-reach describes how much capacity you can get out of the optical interface for a given set of reach conditions. For the same reach requirement, it is possible to have better wavelength capacity but worse spectral efficiency, as shown on the left of Figure 2, or lower capacity for individual wavelengths but higher spectral efficiency, as shown on the right of Figure 2.

Figure 2 –Wavelength capacity-reach vs. spectral efficiency (for the same reach)

Performance Priorities and Applications

While optical performance is not the only purchasing criteria, spectral efficiency is typically the most important performance criteria in submarine networks and a key performance criterion in long-haul networks, as shown in Table 1. In metro networks, ROADM cascadability and wavelength capacity-reach tend to take priority as performance criterion unless the fiber availability is constrained, and bandwidth requirements are very high. In metro DCI applications, wavelength capacity-reach also tends to be the top performance criteria unless fiber availability is constrained.

Table 1 – Typical optical engine performance priorities by application

No Longer Moving in Tandem

In the past, for example, going from 2.5 to 10G or from 10G to 100G, wavelength capacity and spectral efficiency moved in tandem: when you increase the wavelength’s capacity you also increased the spectral efficiency. Increases in spectral efficiency came not from increasing the bits per symbol of the wavelength but from wasting less spectrum.

However, with higher baud rates and flexible grid ROADMs the relationship between wavelength capacity-reach and spectral efficiency has changed. For example, increasing the wavelength data rate from 200 Gb/s with 30 Gbaud (~35 GHz) to 800 Gbps with 84 Gbaud (~90 GHz), increases the spectral efficiency by approximately 50% rather than the 4x as with the data rate. This is because the 800 Gb/s wavelength requires a little under 3x the spectrum and we can no longer count on big reductions in spectral waste to boost spectral efficiency, with more bits per symbol now the primary source of spectral efficiency gains.

Wavelength Capacity-Reach and Spectral Efficiency Trade-offs

With higher baud rates, there is also some trade-off between wavelength capacity reach and spectral efficiency. Higher baud rates indirectly enable better wavelength capacity-reach as fewer bits per symbol are required for the same data rate. The wider spectrum of the high baud rate wavelength also enables higher transmit power and therefore improved noise tolerance without increasing the power spectral density of the wavelength and therefore the nonlinear penalties. These two factors outweigh the increased sensitivity to noise of the increased baud rate itself. But to the extent that the higher baud rate itself reduces the reach, we reduce the spectral efficiency when compared with a lower baud rate and higher modulation (i.e., more bits per symbol) that could deliver the same reach. This is illustrated by the example shown in Figure 3, where two 600G wavelengths in 225 GHz can deliver the same reach as two 550 Gb/s wavelengths in 200 GHz, with the former having better wavelength capacity-reach and the latter having better spectral efficiency.

Figure 3 – Wavelength capacity-reach vs. spectral efficiency (same reach): baud rate

Figure 4 provides another example of this trade-off. A larger spectral gap between wavelengths typically enables higher transmit power for the same nonlinear penalties, and a higher transmit power enables a higher OSNR with greater tolerance of amplified spontaneous emission (ASE) noise from the amplifiers in the wavelength’s path. In this example, the choice is between maximizing wavelength capacity-reach with two widely spaced 800 Gb/s wavelengths or getting better spectral efficiency with three 700 Gb/s wavelengths crammed closer together.

Figure 4 – Wavelength capacity reach vs. spectral efficiency (same reach): channel spacing

To summarize, spectral efficiency and wavelength capacity-reach are not the same thing and no longer move in tandem. Different applications prioritize these two metrics differently. In addition to having a bigger or smaller gap between the wavelengths, it is also possible to prioritize wavelength capacity-reach or spectral efficiency by optimizing the baud rate and bits per symbol. However, this requires a highly programable optical engine such as Infinera’s ICE6 which currently supports over 200 combinations of bits per symbol and baud rate.

In a second blog on this topic, I will further discuss the additional features that enable ICE6 to deliver superior spectral efficiency, up to 8.833 bits/s/Hz (up to 42.4 Tb/s in the 4.8 THz of the extended C-band). Features include long codeword probabilistic constellation shaping (LC-PCS) , second generation Nyquist subcarriers, dynamic bandwidth allocation (DBA), an overhead-efficient Ethernet mode, and a shared wavelocker. In the meantime, for more information on this important topic, download the new Infinera white paper, Maximizing Spectral Efficiency with Optical Engine and Line System Innovations.