MIMO Optical Communications Breaks New Ground

By Jon Gabay, Mouser Electronics

Optical communications are one of the oldest pre-technology methods for signaling long distance. Reflective surfaces can reflect the sun’s rays and direct them to a specific location as a signal or as an alert. This directional reflection is pretty stealthy too since, typically, the only one who can see it is the one it is intended for.

Optical communications are still used today, mostly in fiber and TV remotes, but nowadays, RF is the preferred electromagnetic medium for high-speed unidirectional and omnidirectional communications. But don’t count optical out just yet. A relatively new form of parallel optical communications is gaining some attention as device makers look to expand the ways into and out of mobile and fixed-location devices.

Originally developed for RF, MIMO, which stands for multiple input multiple output, has been used by radio engineers to increase bandwidth and allow RF communications to take place with higher data rates than possible with a single band. Here, a signal is transmitted using many carrier signals at different frequencies to allow parallel data transfer instead of just serial transmission. Optical MIMO does this too, but with light.

Optical MIMO uses visible light to allow lighting systems to communicate with other equipment in one of three ways. One technique uses a single emitter composed of multiple color LEDs. Each LED is a transmitter, and by using optical filtration at the receiver end, each color carries data in parallel with the other colors. This technique is called Lambda MIMO.

An alternative approach is to place multiple emitters at various locations in a ceiling, for example. In this case, each emitter is the same type and color LED, and a parallel receiver—like a video camera—combines the spatially separated light rays, again, to form a parallel data transfer. This is called s-MIMO.

A third technique combines both approaches and uses multiple emitters, each a different color and placed at different locations. This is called h-MIMO and it also uses a parallel sensor such as a video camera to decode the spatially and color-separated light waves in parallel.

Speaking of decoding, unlike RF modulation techniques, LEDs are typically single color to keep costs low, so wavelength modulation is not a feasible approach. Pulse width and pulse frequency modulation techniques can be used instead. RF techniques like orthogonal frequency division multiplexing (OFDM) allow multiple users but limit data rates, so non-orthogonal multiple access (NOMA) techniques seem to be leading the crowd.

The key is controlling each color’s transmitting amplitude and each color’s receive gain. That is why normalized gain difference power allocation (NGDPA) is employed to reduce complexity and increase efficacy.

What is interesting is that experimental data shows channel data rates up to 55Mbit/s are achievable using both gain ratio power allocation (GRPA) and NGDPA. While both are effective, a slight advantage goes to NGDPA. Sum rates of 110Mbit/s are achievable with two sources using NOMA techniques.

With so many RF techniques and protocols effectively allowing our devices to communicate so well, why would anyone want to use an optical technique that is so dependent on proximity and line of sight? There are many reasons and applications for this smart lighting technique.

First, there are no pesky licensing and approvals required for optical communications. No FCC, TUV, or any expensive international standard hoops to jump through. Second, this technique is immune to EMI. Interference from other RF sources will not degrade performance and even very high EMP and spike levels—like turning on large motors—will not interfere with data integrity.

LEDs and optical receivers cost less than antennas RF front ends and filters, and the like, as well. True, LED-based divergent beam optical communications are relatively short-range, but there are still multiple applications that can take advantage of these characteristics.

For example, hospital beds using MIMO for transmission of vital statistics like heart rate, blood pressure, and so on, do not take up any RF bandwidth, have fixed locations for transmitters and receivers (meaning high reliability), and will not be affected by lightning strikes or other high-level impulse noise sources that can step on data.

Sensors inside engines and motors can be read without wires and with very high noise immunity. Even bidirectional communications can take place using different color LEDs and filtered optical receivers. For example, MIMO-based communications can be built into every airplane seat, enabling passengers to use their mobile devices while in flight without ever interfering with navigational RF.

Phone makers are aware of the many benefits of non-RF communications. Samsung, for example, already offers 4×4 MIMO arrays on the S10 series that boasts up to 2Gbit/s download speeds and 120Mbit/s upload speeds. Apple also offers a 4×4 MIMO array in its iPhone 12 models.

Laptops, tablets, watches, and other devices can also benefit from using this technology. It could even be used in cars to reduce the amount of RF energy we absorb. Supermarkets can use MIMO to communicate with price displays for each product, saving time and money when prices go up, as we know they will.

Initially introduced for RF, MIMO expands bandwidth and supports RF communications at higher data rates than possible with a single band. MIMO transmits signals using many carrier signals at different frequencies to enable parallel data transfer instead of serial only. Optical MIMO does this too, but with light. Low-cost LED-based multiple wavelength light-based communications are immune to RF noise and achieve good short-haul line-of-sight data rates by passing parallel data.