The augmented reality system consists of a positioning system and wearable device that plays contextually appropriate sounds.
Users of the system carry a simple wearable computer that receives location information from transmitters placed in the environment. Each transmitter sends out a unique ID over a limited range creating a small cell. The wearable then uses the cell's ID to infer location. The current location is used in conjunction with the list of previously visited locations and the order in which they are visited to play contextually appropriate sounds from the augmented reality.
The users carry a simple wearable computer that is responsible for
tracking the user's location and controlling the playback of sounds.
The wearable consists of an off-the-shelf MPEG 1 Layer 3
(MP3) Diamond Rio 
digital audio player, a Microchip PIC
microcontroller, and a RF receiver (Figure 
Left).
The PIC decodes information from the RF receiver into location
information and controls the MP3 player, including selecting the
correct track and starting and stopping the device at appropriate times.
The PIC is responsible for tracking the user's position over time and
updates the user's state in the augmented
reality. That information is then used to select and play appropriate
sounds stored on the MP3 player.
The location beacon consists of a PIC and RF transmitter (Figure
Right). Each beacon transmits a unique ID
approximately once every second. The range of the transmitters was
deliberately limited to create a small cell with a diameter of between
5 to 30 feet that largely depends on antenna configuration. The
beacons are small, low powered and inexpensive. Currently each
beacon should last approximately one year on a 9V battery but
eventually we hope to environmentally power the beacons to avoid
batteries altogether. We plan to deploy the beacons as an economical
infrastructure for general use to gather location information for a
variety of wearable devices [6].
The use of the the beacon system greatly reduces the complexity of gathering the position of the user for the augmented reality. Other techniques such as vision or magnetic tracking devices typically require much more computational power, are more expensive and cover a smaller range. Other beacon systems have been created using infrared (IR), but they require either line of sight, limiting the area covered by a beacon or flood an entire area increasing the cell size to a whole room. Using RF we eliminate both problems and we are able to create a relatively small cell.
The largest problem with RF beacons from a technical standpoint is the error caused by noise and interference. It was found that the MP3 player generated a lot of interference while playing a sound, but very little performing its other functions. Collisions of transmissions between beacons is another source of error. Although the system is designed such that transmitters are spaced apart into cells, there are bound to be areas of overlap when trying to get complete location coverage. To minimize this type of interference the beacons use the transmitter for as little time as possible. Currently a single byte provides more than enough unique IDs. Most error detection or correction techniques are designed for systems that send much more data (on the order of kilobytes) and typically they either double the amount of data sent, or add on extra bytes with error detection information. In either case using these techniques would greatly increase the amount of data being sent and correspondingly increase the chance of a collision between transmissions. In our system we found that the simple heuristic of waiting to update position until the same ID was received twice was sufficient to block most errors. Once we deploy this system for general use we will have to increase the ID space and then more general error detection/correction schemes may become a better option.