Visible Light Communications

This is a second part to the post ‘the shift to optical’ where I look at the current state and future trend for device interconnect technologies and see if there is a trend towards optical technologies.

The proliferation of devices that can display rich media has been causing data bottle neck problems.  Harald Haas recently highlighted the congestion on radio networks and how this is expected to get worse.  When the wireless network capacity is limited or unreliable we often reach for a cabled solution, e.g. a fixed line telephone, an Ethernet cable, or a USB interconnect.  There have been significant improvements in cabled systems over the years. The same is true of wireless systems although the data rates have always lagged behind.

Wired interconnect can transfer large data files (e.g. video) quickly, but it would be much nicer if this could all be done wirelessly.  The good news is that there are wireless technologies around the corner that promise very high transfer speeds.

In the table below I have highlighted 3 wired interconnect technologies that are at the top end of what is on the market today. i.e. FireWire, USB 3.0 and Thunderbolt.  All of these use wires rather than optical fibres although Thunderbolt considered fibre but reverted to conductors in order to carry power.

The table also contains the current wireless technologies that can be used for transferring data between devices today, i.e. Wi-Fi, Bluetooth and IrDA.  Only Wi-Fi currently offers very high data rates. The IEEE 802.11.n in most implementations provides up to 150Mbit/s (in theory the standard can go to 600Mbit/s) although in practice you receive considerably less than this. Note that one out of three of these is an optical technology.

Then looking ahead the table has 3 future wireless technologies; WiGig, GigaIR and Li-Fi. Note that 2 out of three of these technologies are optical.  Prior to WiGig, the new IEEE 802.11.ac standard at 5GHz promises up to 1Gbit/s in the future, although at MWC I saw a rate of 200Mbit/s over a short distance being proudly demonstrated by one of the leading players using a prototype system. WiGig has claims of up to 7Gbit/s but in practice we might expect maybe 1.2Gbit/s or perhaps 2Gbit/s .  GigaIR is being developed by the Infrared Data Association (IrDA) and is aiming for 1Gbit/s.  Li-Fi is close to1Gbit/s in lab demonstrators using commercial light bulbs with the promise of much greater rates with higher bandwidth LEDs.

We usually compare communications technology by looking at their maximum data rates.  However, this provides a maximum under ideal conditions (short distances with no interference). Data rates can diminish quite rapidly under normal non-ideal conditions for some technologies. Radio based communication in particular suffers if several radio interferers operate in the same band in the same area.  This spectral congestion is rapidly increasing with the proliferation of wireless radio systems.  Data density (measured in bps per unit of area) is a much more useful way to compare these technologies. So the table also includes a relative measure of data density for each technology.

Extremely high data densities can be achieved with wired systems since there is little cross-talk interference between cables. The next in line are the optical wireless systems since the interference is directional and is well contained. Li-Fi has the best data density since the optical output from the LED light bulbs may greatly exceed that of infra-red.  WiGig is short range and requires complex antenna configurations to maintain directionality.  Another interesting comparison will be that of cost.  The optical systems use direct modulation of the light carrier and so are simple to implement, whereas WiGig requires additional radio and antenna circuits.

Wired systems shifted to optical technology to deliver very high data rates.  The indicators are that wireless systems will be following a similar trend.