Visible Light Communications has been considered as a communications technology but you will note from my blog post on 18th March that one of my top 10 VLC applications is Location Based Services. This is a topic close to my heart since I was the founder and chairman of mobile location technology company, Trisent, prior to it being sold. Having worked on radio location technology for a number of years I thought it useful to consider the location techniques used in wireless radio and see how well they apply to optical wireless location.
So here are the main radio location techniques and my assessment of how well they are likely to perform in the optical domain.
Cell ID systems broadcast a unique identifier (ID) for that cell. The location of the ID source is stored in a database. When a receiver sees a particular ID, the location for that ID can be read from the database.
Radio: This technique is widely used in cellular systems; however the accuracy can be quite poor – many kilometres in error because of the large cell sizes in many places.
Optical: The Cell ID is identified via VLC transmitting the ID and/or location of the emitter. Very good accuracy of 2-5m is possible due to the small size of the illumination cell.
Received Signal Strength
Received signal strength (RSS) is a measure of the signal power detected at the receiver. The power diminishes with distance from the transmitter and so the distance of the receiver from the transmitter can be calculated.
Radio: The radio signal propagation is dependent on the physical environment and varies dramatically from location to location. Constructive and destructive interference adds further errors to any static RSS measurements. Even by averaging dynamic RSS measurement very large errors occur and so RSS is rarely used in cellular radio location.
Optical: Being mainly line of sight, the RSS quite predictably attenuates according to a square law in free space. The received signal also does not suffer from constructive and destructive interference. So the optical RSS can be used as a relatively accurate distance measurement if the emitter power and beam pattern is known.
Time of Arrival
Time of arrival (TOA) is based on the trilateration. Signals from multiple – say 3 sources are sent at exactly the same known time. The time they are each received is used to calculate the distance they will have propagated in that time (they all travel at the speed of light). If a circle with a radius equal to that distance if plotted for each source, S1, S2 & S3 then the point at which all circles intersect, X, is the location of the receiver. This is 2D trilateration. 3D trilateration can be performed using spheres instead of circles to give both the location and height.
Radio: TOA with 3D trilateration is the method used for GPS. It leads to high accuracy results if the transmitters are accurately synchronised and the receiver has a good clock.
Optical: TOA is equally difficult to implement in the optical domain since the synchronisation and accuracy issues remain.
Time Difference of Arrival
Time difference of arrival or TDOA differs from TOA in one important respect, the receiver clock does not need to be as accurate since it is the time difference between the signals from different sources which is important and not the absolute time of arrival. Accurate synchronisation of the signal sources is still required. Techniques called multilateration are used to solve a series of simultaneous equations to calculate the receiver position.
Radio: TDOA has been used in cellular systems with limited success due to the difficulty and expense of synchronising the base stations.
Optical: It is easy to synchronise emitters if they are in close proximity to each other since that can share the same clock. This is often the case for LED lighting applications. Imagine an array of LEDs in a luminaire or clusters of LEDs in a cars tail lights, these can be physically connected and synchronised.
Angle of Arrival
Angle of arrival (AoA) is widely used for location finding using triangulation. If you take your bearing to one know position (e.g. Mountain A) and do the same for another, Mountain B. The projection of the measured bearing from each mountain will intersect at your location.
Radio: It is difficult to measure the angle of arrival of a radio signal. Antenna arrays can be used for this purpose but they are expensive to implement. Aircraft navigation used this principal in reverse. A VOR system transmits different identifiable signals at different angles so the receiver knows the bearing relative to the transmitter of the received signal.
Optical: It is relatively simple to detect the angle of arrival of an optical signal. If two light sources A and B come from different locations (so from a different angle), their positions will be projected onto different positions A’ and B’ of an imaging sensor. This enables the angle from the sensor to the source to be calculate and then triangulation can be applied to find the actual location.
When I worked on mobile location technology at Trisent we adopted a hybrid approach in order to improve location accuracy and remove ambiguities. I believe the same approach would work well in the optical domain. In particular Cell-ID and RSS techniques are simple to apply but combined with angle-of-arrival if more than one cell (lamp) can be seen by a sensor array would lead to a highly accurate positioning system – more accurate than GPS and it would work indoors.
Locating and positioning is a great application for VLC. High accuracy positioning indoors would be extremely useful, but you would need a device with an imaging sensor and some processing. But hey, don’t you already have a smart phone with a camera and a processor? Okay it would need to be adapted a little for a higher frame rate, but essentially you have all of the hardware there already.
VLC for accurate indoor positioning on your smart phone. You heard it here first!