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Issues facing free-space optical test

Posted: 23 Jul 2015 ?? ?Print Version ?Bookmark and Share

Keywords:pureLiFi? optical test? WiFi? LiFi? IoT?

The progress of WiFi technology may soon allow optical links to boost its performance. However, with higher data rates, security and RFI resistance, are engineers up to the challenge of advanced optical test and measurement?

The dramatic proliferation of wireless devices (WiFi, IoT, handheld products) has forced engineers who don't have much RF background to become somewhat RF-knowledgeable and meet performance standards. Fortunately, in many cases, the required tests are well-defined, the needed equipment and test suites are available, and there's often a specialist with experience on the team or one within reach.

Optical test, however, is a foreign area to most electronic engineers, and with good reason. Unless they are designing the display itself or the light-source drivers, most EE's usually don't need to know much about optical issues. Many of the issues are fairly basic, such as LED brightness and colour. In contrast, advanced optical specialists are viewed somewhat the way RF engineers were perceived twenty or thirty years ago: a small, esoteric subculture involved with hard-to-grasp rituals and mystical idiosyncrasies. In-depth optical test is very different world, with parameters such as optical power, photon counting, strange spectrum issues, dispersion, radiance, efficiency, efficacy, and many others that are either alien to the RF world or have different meanings and measurements (even though both are based on Maxwell's equations, of course).

Soon, though, there may be a need for more optical expertise, just as RF went from being niche to mainstream. There is a proposal for a WiFi-like equivalent, often called LiFi, for short-range mobile use in an office, meeting room, large hall, or retail store, but based on free-space (non-fibre) optical links.

The idea is to take advantage of the installation of LED-based lighting, by modulating the LEDs at roughly 100Mb/s data rates (obviously well beyond what the eye can perceive) so the LED serves two roles: area illumination and data transmitter. The marketing pitch from pureLiFi, a major innovator and proponent of the technology, is simple: "wireless data from every light bulb."

LiFi system

A LiFi system uses optical links rather than RF for wireless connectivity, but is not limited to fixed, point-to-point use (image from pureLiFi).

Will visible light communications (VLC) be a success? I'll give my usual answers: a) I certainly don't know, nor does anyone else, and b) it depends on the situation. It may not become widespread, but it may well be a good solution in well-defined cases, such as offices, meeting halls, or classrooms where RF links are being bogged down by too much traffic. Also, the mostly line-of-sight performance of optical inks (reflections from walls do work, to a lesser extent than a direct link) is both a limitation and a virtue, if you are concerned about sniffing and security, as is their inherent RFI immunity.

Implementing an optical link that supports mobile PCs and smartphones, and is also as easy to use as WiFi has become, will take lots of work beyond the just deploying the optical emitters and receivers. Even if the LED-based lamps that provide the major nodes are simply screw-in replacements for illumination-only bulbs (as some proponents presume they can be), there's the issue of creating the backhaul network to link all these dual-purpose bulbs into a high-speed system. Providing that part of the total system may be as big a challenge as establishing the optical link between the user and the in-ceiling lamp itself.

If LiFi does get some traction, even in niche situations, both designers of these systems and especially those who install them will have to learn about the realities of sophisticated high-speed, free-space optical test and measurement. There's a whole new world of instrumentation, test set-ups and criteria, all very different from RF or even constrained optical-fibre situations.

Do you think that LiFi optical links have a future? Are engineers ready for the optical test and measurement challenges?

- Bill Schweber
??EE Times

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