PARCTAB ``Deathstar'' Basestation

The PARCTAB transceiver (aka ``deathstar'') pictured above, serves as a communication hub for any PARCTABs located in its particular cell. Typically its communication radius is about 20 feet---less if limited by the walls of an office. The transceiver hardware performs numerous functions in addition to transmission and reception, including: We designed the transceiver conservatively to ensure reliable communication. For transmission, two dozen IR emitters are placed at 15 degree intervals on a circular printed circuit board. For reception, two detectors provide a total viewing angle of 360 degrees (shown above). The transceiver is designed to be attached to a ceiling, preferably in the middle of a room as this usually gives an unobscured communication path over the required area. But since transceivers and PARCTABs can sense infrared light reflected from surfaces, it is not necessary that there be a line of sight between the two for them to communicate.

Limited space and power constrained our choice of a wireless communication technology to just two options: radio and infrared (IR). We chose 850nm IR to exploit the small, inexpensive IR components that are commercially available. These components offered low power consumption at the modest communication speeds of 9600 and 19200 baud. Because IR signals are contained by the walls of a room, this technology also made it easier to design a cellular system. Moreover, IR communication is unregulated. A radio link would have required more space, higher power equipment and potentially government operating licenses.

We decided that a cellular system would best handle the competition for bandwidth that inevitably would arise in a building-wide system supporting many users. By creating small, room-sized communication cells (nanocells), we could minimize the communication distance from the hub to the mobile user, reducing power needs concomitantly. Users are also less likely to interfere with one another's signals in a cellular system, although some situations---such as heavy tab use during a break in a large meeting---can still place large loads on the IR transceivers. Finally, small cells enable the system to pin down a user's location to the resolution of a room.

The tab infrared network thus consists of nanocells defined by the walls of a room surrounding an IR transceiver. Large open rooms and hallways may also support nanocells if transceivers are carefully placed out of communication range of each other. Transceivers connect to a LAN through the RS-232 ports of nearby workstations.

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