Networking

Wi-Fi: Breakthroughs in optical networking may make it obsolete

Promises of high throughput could mean your future in-home network may be using light instead of radio signals.

Wi-Fi has less than adequate throughput bandwidth; yet, users overlook that. Why? Because being able to roam is more important. Still, business-savvy research and development types see opportunity and are working hard to provide both mobility and bandwidth.

Optical Wireless (OW) networking may be one such solution. That's because OW networks overcome the following weaknesses inherent to Radio Frequency (RF) networks as pointed out in an article written by Dr. Mohsen Kavehrad:

Transmission speed
  • RF: Power levels that do not hurt indoor occupants (think microwave) limits the maximum data-transfer rate to several hundred megabits per second.
  • OW: Data transfer speeds equivalent to wired links. Experiments have shown speeds in excess of one gigabit per second are possible.
Bandwidth Limitations
  • RF: Full duplex is not possible; radio signals sent at the same time and frequency will interfere with one another.
  • OW: Photons do not interfere with one another. Usable bandwidth is limited only by the efficiency of the receiver's photodiodes.
Security
  • RF: Radio waves pass through walls, opening up chances for eavesdroppers.
  • OW: Light waves cannot pass through walls, preventing interception of the signal.
Multipath fading
  • RF: Due to the physical environment, variations of the same RF burst may reach the receiver at different times. If sufficiently out of phase, the receiver will not process the signals correctly.
  • OW: Destructive interference is impossible with light waves. A point of clarification on this portion of the report. I should have used the complete statement, which includes the following: The sensors on the active area of a photodiode absorb the waves separately and then average out the incoming energy, so no canceling can result. This means out-of-phase/out-of-time signals arriving at the same moment will not adversely affect the fly-eye receiver. (I apologize for the confusion, 26 Apr 2010)

Sounds good to me, yet I remember trying to line up the IR sensors on two computers so they would sync. That wasn't any fun.

New approach using OW

OW networks are not new. Using Free Space Optics (FSO), a form of OW is quite common. In fact, awhile ago a Ham radio friend and I set up a point to point system using laser-tag equipment (don't tell my son). It worked and was interesting, but not as cool as what Penn State researchers Dr. Mohsen Kavehrad and Dr. Jarir Fadlullah have accomplished.

They designed and built an experimental optical system that has high bandwidth, and it's not location sensitive. Somehow they overcame the need to directly point communicating sensors at each other.

Network configuration

The researchers created an experimental test bed to prove that Non-Line-of-Sight (NLoS) OW links are possible. The experimental equipment consists of an optical transmitter and receiver. I will let the research team explain the two main components:

Spot-diffusing transmitter: Utilizes multiple narrow light beams pointed in different directions, as a replacement for the conventional diffuse transmitter, which utilizes a single broad light beam aimed at an extended reflecting surface.

While the diffuse transmitter provides considerable immunity against beam blockage near the receiver, it yields a high path loss. The spot-diffusing transmitter is expected to reduce path loss, because the narrow beams experience little path loss traveling from the transmitter to the reflective surfaces.

Fly-eye receiver: Consists of a single imaging optical concentrator (e.g., a lens) that forms an image of the received light on a collection of photo-detectors, thereby separating signals that arrive from different directions. Implementation of an angle-diversity receiver using imaging optics offers two advantages over a non-imaging implementation.
  • All photo-detectors share a common concentrator, reducing size and cost.
  • The photo-detectors can be laid out in a single planar array, facilitating the use of a large number of receiving elements or pixels.

Basically, there is a transmitter with multiple light beams that are bounced off of a reflective surface, such as the ceiling. The receiver has the ability to process multiple light beams into usable information. The following slide shows the actual test setup:

The next slide lists the basic components and an interpretation of how the system works:

Potential applications

I realize this technology is not quite ready for production, but the professors are already looking at the potential uses for OW networking. Here are some examples:

  • Large facilities could easily have complete high-speed coverage with just a few devices, allowing many more non-interfering data streams than an RF network.
  • Airplane manufacturers with their concern over RF interference could incorporate OW networking into passenger lighting.
  • White LED lighting is the future of home lighting. A Japanese research team suggests using the same white LEDs as light sources for OW networks and Ethernet over Powerline technology as the back haul.
Final thoughts

Some say OW networking will have too many problems. Maybe, but lots of people feel that way about Wi-Fi and look how popular it is. Besides, there is no reason why multiple methods of wireless networking couldn't be incorporated into devices.

It's a good thing to step outside the box. Otherwise, we may miss opportunities such as the research being done by Dr. Mohsen Kavehrad and Dr. Jarir Fadlullah. I also want to thank both of them for allowing me to use excerpts from their paper.

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