Patent GB-1287607-A (GB)
Title: Apparatus for channel selection in address-coded radio telephone system
Inventor: GUANELLA GUSTAV
IPC: H04W99/00
Filed: 1969-09-26
Abstract: 1287607 Radio-telephone systems PATEL. HOLD PATENTVERWERTUNGS - UND ELEKTRO-HOLDING AG 26 Sept 1969 [30 Sept 1968] 47498/69 Heading H4K In a radio-telephone system idle stations hunt for and dwell on idle receive channels, and hunt for and dwell on further idle channels if the one on which a station dwells is made unavailable, a calling station stepping over all channels in search of a wanted station by modulating the channels successively with the called station identity and the identity of the calling station receive channel, the called station, when it recognizes its own identity on the receive channel on which it happens to dwell, sending an acknowledgement over the indicated calling station receive channel and indicating the identity of the receiver channel over which it may be reached. Channels are seized by use of a pilot signal present during the connection. As shown in Fig. 1, where the so-called channelnumber is plotted as ordinate against time, a calling station stands on channel EA1 to receive channel EB1. The calling station transmitter sends the caller station identity aB and the calling station receive channel number eA1 on each of the channel carriers in turn and in order of ascending channel number as indicated by SAO. When the called station receive channel is so modulated (as at K) the called station sets its transmitter to channel eA1 and sends the calling station its station identity aB and its receive channel number eB1. The calling station now suspends its search over channels and sets its transmitter to the called station receive channel eB1. Duplex or simplex transmission may now be instituted, the simplex working being such that only one channel is held, a channel in the reverse direction being reseized or newly set up whenever it is needed. True simplex working can be employed if the calling and called station happen to be standing on the same receive channel, Fig. 3, not shown. If, as shown in Fig. 1, the calling station receive channel is interrupted at time NA, the calling station switches to another channel, such as EA2 as shown, and sends the necessary data to the called station to adapt itself. The same process of restoration is applied if the caller station receive channel is interrupted. Such interruptions and readaptions may be deliberately induced to switch the channels to make eaves dropping and jamming more difficult. If bothreceive channels are interrupted at the same time, Fig. 2, not shown, the attempts by both stations to signal a new receive channel cannot be successful; accordingly with no response to such attempts both stations behave like calling stations by sending receive channel and wanted station codes on channel after channel in succession until one station finds the other. The idle/busy conditions of receive channels at a calling station do not necessarily correspond to the idle/busy conditions of receive channels available to the called station. In a system of setting up simplex connections the calling station therefore sends, over each channel carrier of its scan, an indication of whether that channel is or is not available to the calling station. If the idle channel on which a called station dwells, when it is called, is not available also at the sending station, the called station responds to its code when the calling station scan reaches its channel and steps with the calling station channel by channel in a continuation of the scan, each station reporting to the other whether a channel is idle or busy to itself. When a channel is reached which is idle to both stations it is seized for simplex working. To reduce the number of signals sent in this procedure the called station may report idle conditions only, busy conditions being presumed from absence of signals received at the calling station. A conference of stations on a single channel is obtainable by a calling station first establishing a connection with one station termed a "main partner", a "group call" code indicating that a conference is being convened and holding the channel seized for the connections for the conference. An address which identifies all the parties to the conference is then transmitted by the calling station in all the channels in sequence starting from the channel one step beyond the seized conference channel and ending on the conference channel. For all channels except the conference channel at the end of the complete channel scan, the calling station gives a channel busy indication so that all stations to the conference step with the calling station from the channel on which they are picked up to the conference channel at the end of the scan which is indicated as free. If this channel is in fact not free for one or more of the conference group, as evidenced by lack of response, the conference so far convened may be directed to another channel and the missing stations are looked for on this other channel.