Last week, Infinera announced a “breakthrough” in long distance optical transmission that includes several advances, one in particular that they are calling Nyquist capabilities. Read a little more closely, and you’ll see that this is a version of subcarrier mutliplexing coming back for another round. Not that there’s anything wrong with that.

In the late 1990s, I worked for a company called Kestrel Solutions that was based on a technology known as subcarrier multiplexing (SCM). The story that we told analysts and customers was that SCM was a technique used in satellite communications that we were transitioning to optical fiber to allow better communications over longer distances – which was correct.

Subcarrier multiplexing is, in short, a method of breaking up a large, fast signal into smaller, slower signals that operate in frequency bands very close to each other. Whereas a 10Gbps signal might have a single carrier operating within a 25nm frequency band, a SCM version might have four 2.5Gbps subcarrier signals operating in that same band. The major benefit of subcarrier multiplexing is that the symbol rate – the speed of information in each subcarrier – is lower than the speed of the original signal. Lower symbol rates mean better performance over long distances. Faster speeds (symbol rates) mean shorter pulses which have a greater chance of being distorted by fiber properties like dispersion and intersymbol interference (one signal impacting another one). By using lower speeds, these distortions are greatly reduced.

In the 1990s, 10Gbps transmission was bulky, expensive, and very susceptible to fiber properties. Kestrel’s answer was to use SCM to break up 10Gbps signals into 64 subcarriers operating at 1.55Mbps. Subcarriers could be combined to make larger pipes (e.g. 2.5Gbps pipes), but the symbol rate remained a puny 1.55Mbps per subcarrier in all cases. It was an innovative  idea, but one that would have eventually lost out to advances in digital technologies even if the telecom bust hadn’t taken away the market. 10Gbps digital transmission is now cheap, pluggable, and reliable, and no one is talking about SCM at 10Gbps seriously anymore.

People are talking about SCM for higher rates, however. At one point, several companies were showing SCM optics at 40Gbps. (The decline of 40Gbps as a long-distance format is a subject for another discussion.) A few companies and research facilities have used SCM for 100Gbps and above transmission. However, most SCM development at 100Gbps was for “direct detect” systems as opposed to “coherent” systems. Direct detect means a detector interprets the received signal directly. Coherent systems use advanced technology and local oscillators to deconstruct the received information from the noisy signal received. Direct detect is low cost, coherent is high cost. Direct detect is simple, coherent is complex. Generally, direct detect works over short distances while coherent is ideal for longer distances. Yet, when the industry coalesced around coherent as the standard for single carrier transmission, most SCM development faded away.

What is interesting about the Infinera announcement is that they seem to have resurrected SCM in coordination with coherent optics for ultra-long haul transmission. The use cases sites are over extremely long distances (e.g. undersea). In that market, there are only a few companies playing, many of which are doing very interesting and complex things with optics, and Infinera has wanted a larger piece of that market. This could be a very intriguing next step.

Of note is that the announcement is a technology announcement and not a product announcement. While Infinera claims to be first in this particular application of the technology, they aren’t the only ones who have spent time working on the concept. It will be very interesting to see if SCM has finally found its permanent niche in the optical transmission space.