We hear a lot about the emerging 5G standard for wireless communications. It will require new everything: components, technologies, topologies, antenna arrays, algorithms, coding and error-correction techniques…the list is fairly long, and much of it is in flux. However, sooner or (more likely) later, many of the issue will be resolved, technical challenges will be overcome, and 5G will at least start to become a reality. Then as time goes on, it will undoubtedly improve and mature.
This 5G confidence is possible because we now are so good at implementing wired and wireless high-speed data links that we accept their performance and progress as “normal.” Certainly, using the air around us or the vacuum of space as the link medium can be difficult and frustrating at times, but in most cases the smart systems, advanced coding and modulation, and other techniques have been fairly successful at giving us reliable, speedy wireless links using RF or optical approaches.
But there’s one place where we have had severe difficulties in overcoming a dramatically unforgiving communications medium: untethered, roaming underwater links. Here, the traditional trio of performance and cost tradeoffs among power, distance, and data rate are nasty and harsh, to be blunt. Even if you are willing to expend considerable effort and expense to make that link work better, it may not really matter, as everything about the underwater environment works against you.
This was all made very clear in a well-written, readable article in Laser Focus World, which was both impressive and depressing. The article, “Free-space Optical Communications: Building a 'deeper' understanding of underwater optical communications” discussed some of the options, challenges, impediments, and physics-based limits and options, Figure 1, and was written by a leading hands-on researcher at the University of North Carolina. It almost makes doing deep-space links look easy (which they are not, of course).
This chart of data rate versus distance for various modes of underwater communication just hints at the difficulties; it also does not show the third dimension of “power” needed for modest success. (Source: Laser Focus World)
There are three options for underwater communications links, each of them in use to some extent:
- acoustic, using low-frequency audio energy, which can reach kilometers but does poorly in shallow waters (and think of the natural and man-made noise sources); off-the-shelf modems using this approach are available, Reference 1.
- RF, using VLF (very low frequency) electromagnetic waves in the 3 to 30 kHz range; it takes megawatts to reach submarines even in shallow waters and signaling rates are in the 100 bps (yes, that’s bits per second) range;
- optical, which is sometimes viable for short distances, and where there are many developments due to innovations in both sources and receivers with varying wavelengths, output levels, and sensitivities (this is what the bulk of the article is about, after the review on audio and RF reality and status). Free-space optical links are already in widespread use for air and space, References 2 and 3.
One of the big issues with underwater links, in addition to their fundamental limits as defined by physics and briefly discussed in the article, is that the “channel” is not only hostile, it is constantly changing. There are variations in salinity, opacity, water depth, noise, currents, and more, all of which impede the channel initially and also make it very hard to compensate for the medium’s shortcoming via various techniques. In addition, there are difficult, non-static multipath conditions (and thus intersymbol interference) as signals bounce off underwater objects, ships, air/water interfaces and even boundaries between different layers of water (temperature, motion, salinity) which are flowing alongside each other, Figure 2. (Note that communication through Earth is also very difficult, but that’s another situation and has fewer options to try.)
Acoustic links are already in widespread use over short distances, but they are subject to many sources of multipath, most of which are poorly predictable and time-varying. (Source: Reference 1 via Slideshare.net)
Time-varying channel conditions are not a new phenomenon. Increased power output, frequency/time/space diversity topologies, coding, formatting, and advanced ECC are among the standard ways to deal with interference, noise, fading and unpredictable propagation issues in air/vacuum links. Nonetheless, water environments and the ocean are far more difficult, unpredictable, and unyielding. The article notes the realities, and explain that new LED/laser colors and control may offer additional options for better underwater links, but it is also honest. It does not say that there is a clear path to success (no pun intended) which just need some modest refinement.
Have you ever tried to develop an underwater link? For what distance and data rate? What unexpected surprises – both good and bad – did you uncover?
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- G. Raviteja and P.V. Manikanta, Pondicherry University, “High Speed Underwater Acoustic Communication Based on OFDM”
- Planet Analog, May 28, 2017, “When a Sensor Is Truly In “The Twilight Zone”
- EDN, March 16, 2000, “Optical link forgoes constricting fiber, finds bliss in free space”