Telecommunications – Transmitter and receiver at separate stations – Plural transmitters or receivers
Reexamination Certificate
1999-09-20
2003-02-11
Maung, Nay (Department: 2681)
Telecommunications
Transmitter and receiver at separate stations
Plural transmitters or receivers
C455S520000, C455S521000
Reexamination Certificate
active
06519472
ABSTRACT:
TECHNICAL FIELD
The present invention relates to methods and apparatus for monitoring trunked radio communications.
BACKGROUND
In conventional radio frequency communications systems, communication between two parties typically occurs over a preselected channel corresponding to a single operating frequency. For example, a conventional police radio communication system may include multiple mobile transceivers and a stationary central unit at a control site. Each mobile transceiver includes circuitry for transmitting and receiving over a single channel corresponding to a single operating frequency. Alternatively, each mobile transceiver may include a channel selector switch for selecting one of several channels, i.e., frequencies, for communication. However, each communication conventionally occurs over a single frequency.
In a typical communication, a police officer may select a channel on a mobile transceiver (if channel selection is provided), for example 800 MHz, and transmit voice signals at 800 MHz to a dispatcher at the control site. The dispatcher receives the communication and responds by transmitting at 800 MHz or at another fixed frequency to which the mobile transceiver is tuned for receiving. Since the mobile transceiver is tuned to the control site transmission frequency, the police officer hears the response from the dispatcher. The police officer may then reply to the communication from the dispatcher on the selected transmitting frequency. In this manner, communication occurs over a single channel corresponding to a single operating frequency. Alternatively, transmissions from the mobile transceiver and the control site transceiver may occur on two respective frequencies in a duplex operation. Listening to these single frequency or dual frequency transmissions is easily accomplished with a conventional fixed frequency receiver.
In past years, frequency scanning receivers have permitted monitoring of multiple frequency transmissions, particularly on public safety and public service channels. The typical scanning receiver has multiple tuners or an electronically driven tuner so that each locally used frequency can be received, one transmission at a time. The receiver automatically steps from frequency to frequency, i.e., scans, stopping or dwelling on a channel in use until the transmissions on that channel cease. Then, the frequency scanning resumes. Alternatively, the listener may skip or modify scanning to listen to a particular channel even if there are interruptions in transmission on that channel.
In trunked radio communications systems, communications between two parties may occur on multiple channels, i.e., on differing frequencies, for respective transmissions. For example, in a trunked system, a police officer may transmit voice information from a mobile transceiver to the dispatcher at the control site at a first frequency. The dispatcher may respond on a second frequency. The police officer may reply on a third frequency, and so on. Digital signaling between the mobile transceiver and the control site on a separate control channel supplies each unit with the next transmission frequency for tuning both the transmitter and receiver sections of the transceivers.
Conventional scanning radio receivers are unable to monitor communication in a trunked system coherently. Scanning of frequencies in a fixed pattern produces reception of only some of the transmissions of a trunked system. Dwelling on one channel may result in reception of only one transmission of a multiple transmission communication. Thus, a conventional scanning receiver dwelling on a channel cannot reliably indicate the source of the transmission received, e.g., police, fire, rescue, etc. Moreover, since the conventional scanning receiver does not have the ability to receive, decode, and use the digital signaling information, it cannot follow the frequency changes of a particular group of transmissions, i.e., talk group. Therefore, a listener using a conventional scanning receiver can almost never hear a complete conversation in a trunked radio system.
If a conventional scanning receiver tries to monitor a trunked communication, the scanning receiver may receive a first transmission in the communication. However, the scanning receiver is unlikely to receive subsequent transmissions in communications occurring at different frequencies. For example, a scanning receiver may receive a first transmission in the trunked communication at a first frequency. When the first transmission ends, the scanning receiver may resume frequency scanning. The second transmission in the trunked communication occurs at a second frequency. However, instead of receiving that subsequent transmission, the scanning receiver may dwell on a transmission at a third frequency, most likely a transmission from another trunked or non-trunked communication system. Even if the conventional scanning receiver begins frequency scanning after receiving a first transmission and, by accident, dwells on a second frequency where the first communication continues, the scanner may be late in tuning to the second frequency so that part of the second transmission is missed. Thus, a conventional scanning receiver monitoring a trunked transmission may receive interleaved fragments of several unrelated communications from different sources or fragments of one communication.
U.S. Pat. No. 4,905,302 to Childress et al. (hereinafter Childress), the disclosure of which is incorporated by reference, describes an example of a trunked radio communications system. In the Childress system, radios send and receive digital control signals over a dedicated control channel for designating operating frequencies, i.e., working channels, for voice communications. In the Childress system, a radio initiating a communication is referred to as a calling unit. A radio to which a communication is addressed is referred to as a called unit. In order to initiate a communication, a calling unit sends a digital signal referred to as a channel request over the control channel to a control site. The control site responds by transmitting another digital signal referred to here as a channel assignment message over the control channel to the calling unit.
A channel assignment message comprises bit codes that include a channel identification code and a talk group identification code. The channel identification code indicates an assigned working channel over which a voice transmission will occur. The talk group identification code indicates the group of users intended to receive the transmission (the called units).
Once the calling unit receives the channel assignment message with a talk group identification code that matches the talk group identification code of the calling unit, the calling unit tunes to that assigned working channel for transmission and reception. The control site also sends a confirmation message on the assigned working channel. The calling unit responds by sending a verification message to the central unit over the assigned working channel. The central unit responds to the verification message by sending a command over the assigned working channel to “unsquelch” the radios of the called units, i.e., to permit reception of the voice communication. The user of the calling unit then transmits voice information. All of these exchanges are completed before voice communication and are initiated, for example, when the system user first presses the push-to-talk button on the calling unit.
Only the called units having the transmitted talk group identification code are actuated by the “unsquelch”, i.e., unmuting, command and receive the transmission. Radios within the system but outside of the called talk group remain muted or tuned to the control channel for receiving other communications. Thus, the only way for a radio within the system to receive a transmission is to have a talk group identification code matching the talk group identification code in the channel assignment message of the digital control signal.
During communications in a trunked system between a
Brennan Terence
Sullivan Terence Sean
Leydig , Voit & Mayer, Ltd.
Maung Nay
Persino Raymond
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