Method and apparatus for determining availability of a public base unit

Abstract

A public cordless telephone system includes an arrangement for informing a public handset unit when it is within the reception range of a public base unit even though all communication channels are then occupied or all public base units in the reception area are then busy communicating with other handset units. In operation, each public base unit while communicating with any public handset unit inserts in the communication channel it is then occupying spectrum energy such as is provided either by a pilot signal or a frequency shift keying (FSK) signal. By providing this additional spectrum energy along with normal voice signal energy in the communication channel, a public handset unit while searching for an available channel and interrogating this channel will be able to determine, even though the channel is occupied, that there is a public base unit within its reception range. If no available communication channel is found and a signal from one or more public base units is detected, the handset unit generates a first alerting signal for its user thereby informing the user of the one or more public base units in the area even though a telephone call may not be initiated due to all of the communication channels then being occupied. If an available communication channel is found, an attempt to establish communication with a public base unit is initiated by the handset unit over this available channel. If no response is received from a public base unit and if a signal from one or more public base units is detected, the handset unit generates a second alerting signal which thereby informs the user of the one or more public base units in the area even though a telephone call may not be initiated due to all of these public base units then being busy.