Browse Prior Art Database

Channel Access Protocol for coded light Disclosure Number: IPCOM000217011D
Publication Date: 2012-Apr-27
Document File: 4 page(s) / 55K

Publishing Venue

The Prior Art Database



This text was extracted from a PDF file.
This is the abbreviated version, containing approximately 48% of the total text.

Page 01 of 4

1 2012ID00771

Channel Access Protocol for coded light

    Communication systems that make use of data transmission through modulated light are currently in development. One such system is Coded Light, by Philips.

    A coded light transmitter is a lamp of which the light output can be modulated. A coded light receiver comprises a light-sensitive cell (photodiode) which converts incident light into electric current. A receiving 'antenna' for coded light is therefore a separate entity, different from the transmitting 'antenna'.

    For reasons of cost, and because some envisioned coded light applications do not need reception of data via the coded light by a lamp, it may be considered unnecessary to equip some lamps with a coded light receiver. Such lamps can transmit messages via coded light, buy they cannot receive them, nor are they able to sense the coded light channel for other ongoing transmissions.

    This is different from RF systems, a system that can transmit in the RF channel is usually required to be able to receive and sense the RF channel.

    Lamps with a receiver can employ the following strategy: when they have a packet to send via coded light, they shall first sense the optical channel. Only if the channel is found to be idle they are allowed to transmit; if they channel is found to be busy, they shall back-off for a randomly chosen amount of time and try again afterwards. Such a `listen-before-talk' protocol, also known as `Carrier Sense Multiple Access with Collision Avoidance' (CSMA- CA) will be used in the Philips coded light standard.

    Lamps without a coded light receiver cannot listen to the channel, so they must employ a different channel access algorithm. The only feasible algorithm for these lamps is `wait before transmission': when they have a packet to transmit, they shall wait for a random duration and only then send the packet. This procedure effectively lowers the rate of transmission of every lamp, and thus increases the probability that a transmission does not collide (overlap in time) with a transmission from another lamp. The he mean waiting time is a configurable parameter, and it is the task of the application builder to choose a value that is suitable for the application and the environment. A typical suitable value is in the order of `mean packet duration' times `number of lamps that the intended receiver of the coded light messages sees', and this is much longer than the typical back-off times in the CSMA protocol.

Problems or disadvantages overcome by the invention

    So now we have two different protocols, one for lamps with a receiver and one for lamps without receiver, with different typical waiting times.

    On the other hand, if lamps and luminaires are designed in a modular fashion, the receiver unit is simply a part that may or may not be present. This means that the coded light software stack must support both configurations.

    It would be much simpler if there were a single channel access protocol, with a sin...