Patent GB-1459041-A (GB)
Title: Radio communication system
IPC: H04W12/02
Filed: 1974-03-07
Abstract: 1459041 Radio communication system PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd 7 March 1974 [9 March 1973] 10281/74 Heading H4L In a radio communication system comprising a number of radio stations in which enciphered digited signals representing sampled speech signals are transmitted on carrier frequencies selected from a number of frequencies each defining a channel, a control unit is incorporated in each station switchable between (a) a call/search mode of operation in which a free channel is selected by the unit and a calling code transmitted thereon for a distant station with which it is desired to communicate, or the calling code associated with the home station is searched for and a reply transmitted in response to reception of this code and (b) a transmission/ reception mode in which the unit enciphers the output of a speech signal sampler and encoder and supplies the result to a modem in the station, or to decipher the output of the modem. The control unit is automatically switched to mode (b) in response to the reply from a distant station subsequent to transmission of the calling code for that station, or in response to transmission of a reply signal from the home station. The arrangement enables circuitry provided for channel selection to be used for ciphering/deciphering, provided the connection pattern thereof is switchable and not fixed. To establish communication between a home and distant station, a code for the distant station is selected by an actuation unit 13 at the home station. This code is applied to a control unit 12 where it is converted to pulses of particular length and thence to a modulator/ demodulator 14 where it is modulated on to a particular one of a number of carriers selected by the control unit. The output from the modem 14 is passed to a transceiver 15 which transmits the signal. The distant station is of similar construction and is continuously scanning the possible carriers. When it detects its own code, it transmits a reply signal on the selected carrier. After reception and demodulation, this signal is used to activate a control device 18 which causes the unit 12 to switch to a second operating mode such that coded speech signals can be transmitted in cipher. The digitized speech signals are mixed with pseudo random pulses. To transmit speech, the transceiver switch 20 is operated, linking the microphone/ telephone 10 to a speech coder 11 which may be delta modulator. The output for the coder 11 passes to the ciphering stage. The ciphering stage may consist of shift register assembly for generating the pseudo random pulses. The particular pattern of pulses may be changed from day to day by appropriately changing settings at the actuator unit. The particular start position in the pattern for each transmission is selected at random by the unit 21 when the speech key 20 is operated, and information about this setting passed to the transceiver 15. To avoid confusion between calling code signals and possible speech patterns, the calling code pulses are larger than the digitized speech and ciphering pulses. To synchronize home and distant station clocks, an alternating series of ones and zeros is transmitted immediately the transceiver switch 20 is operated, the distant station detecting and receiving this series. Having achieved bit synchronization, a special code is transmitted which indicates the start position in the pseudo random pulse pattern.