Method and apparatus for cordless infrared communication

Abstract

An infrared (IR) communication system is described in which a base unit for a cell can communicate with a plurality of infrared portable devices through distributed infrared receiver/transmitter (RT) modules over a plurality of channels using IR carrier signals. A call processor, referred to as a radio exchange unit, and controlling communication of a cell, communicates with the base unit to place or receive calls with the IR portable devices. The communication occurs in standard communication frames divided into transmission and receiving segments with each segment further divided into slots and with the slots containing digital data, with each communication channel formed by a slot. Each transmission segment to an RT module is immediately followed by a responsive receiving segment. As an IR portable device moves from the vicinity of one RT module to another, the base unit automatically and seamlessly and in a robust manner hands over control to the nearer RT module by monitoring signal strength signals from various RT modules coupled to the base unit. When an IR portable device moves from one cell to another, the call processor hands over control to another base unit using standard protocols. Since an IR portable device may receive IR communication signals from several RT modules, care is taken to avoid signal interference by effectively controlling signal propagation lengths between the base unit and RT modules so that signals arriving from nearby RT modules at a common IR portable device do not have a phase difference more than a predetermined amount. The path lengths can be controlled by selecting cable lengths or by insertion of delays between the base unit and the RT modules to assure that IR signals arrive at portable devices with a minimum amount of interference.