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Application requirement: Remote basestation

General description:

An agency requires communications across remote coverage areas or corridors co-ordinated through a central control / dispatch centre.

Communication modes:

  • Dispatch voice communications to field personnel.
  • Voice communications between field personnel.
  • Calls from field personnel into the telephone system.
  • Monitoring of diagnostic data.
  • Network configuration:

    Basestations are located along a corridor or across a coverage area, with connection from each basestation back to a central control centre. The location of field personnel is monitored independently at the control centre.

    Radio sites:

    Existing sites are available with air-conditioned shelters, microwave link equipment and 110 or 240 VAC with 12 VDC backup from a battery bank.


    Solution

    Installation:

    One or two dual radio basestations are installed at each site, rack-mounted in the shelter. Normal antenna connections are made through the Rx and Tx connectors on the rear of the basestation, and AC and DC power are connected to the rear of the unit.

    Basestation configuration

    The basestation is configured with:

  • VHF or UHF Transceiver modules;
  • VHF or UHF power amplifiers;
  • AC power supply modules;
  • Battery revert;
  • Interface module with 2-wire and/or 4-wire interfaces.
  • Interconnection

    The basestation is connected to the analog microwave link through a multiplexer connected to:

  • each radio's 4-wire + E&M port for voice and remote control / diagnostics (to provide "de-vocoded" traffic if the console isn't capable of reverse-vocoding);
  • each radio's RS232 port via a modem for diagnostic data.
  • These signals are routed through the microwave link back to the central control centre. Any interconnection to PSTN or other networks is performed manually at the control centre via the PABX.

    Talkgroup configuration

    Dispatch Operation

    Talkgroups and frequencies are set up on subscriber units for each agency group requiring separate communication, with groups either operating in Project 25 digital or CTCSS/CDCSS analog mode, depending on the user terminals available within each group.

    In digital mode, the dispatcher can:

  • listen to all calls on a basestation radio by selecting "Monitor" mode on any channel having the required frequency pair;
  • listen to calls on a specific talkgroup by selecting "Selective squelch" mode on the channel corresponding to the talkgroup;
  • make calls on a specific talkgroup by keying the channel corresponding to the talkgroup;
  • make calls to all users by keying a channel set to the default "all user" talkgroup;
  • In analog mode, the dispatcher can:

  • listen to all calls with a valid CTCSS or CDCSS tone / code on a basestation radio by selecting "Monitor" mode on any channel having the required frequency pair;
  • make calls to a specific CTCSS or CDCSS group by keying the channel corresponding to the CTCSS or CDCSS tone / code.
  • Basestation programming

    Each radio is programmed with Tx and Rx frequencies, Rx and Tx network access codes, modulation scheme (digital or analog) and output power for up to 512 channels.

    The basestation will repeat any signal received on the selected channel's Rx frequency with the selected modulation scheme provided the signal has the correct NAC (in digital mode) or a valid CTCSS or CDCSS tone / code (in analog mode). The dispatcher can change the selected channel using any standard tone remote console.

    TGID or CTCSS / CDCSS codes /tones for each channel can also be programmed into the basestation, and these determine the addressing of any calls made on this channel by the dispatcher.

    Allocation of channels to individual basestation radios will depend on the amount of traffic expected on each channel and on the frequency plan.

    Wireline interfaces are allocated to each radio using the programming software, which may also allocate remote control tones to required functions such as channel selection.

    Subscriber unit operation

    Subscribers can:

  • communicate with the dispatcher on any talkgroup (providing the dispatcher is in "monitor" or "normal squelch" mode);
  • communicate with specific talkgroups by selecting the corresponding channel.
  • Variations

    Crossband Operation

    If field personnel in different frequency bands need to communicate, two radios must be used in the basestation. The DRB-25 is configured with UHF and VHF radios, and link radio software is activated. Crossband talkgroups are programmed into the basestation radios by entering the corresponding crossband channel in each channel table. The basestation then checks the Talkgroup ID or CTCSS code of all incoming calls and repeats on both radios for all talkgroups configured as crossband.

    Future Developments

    Subscriber unit operation

    Subscribers can send data to and retrieve data from the data network via a PC connected to a portable or mobile radio.

    Hot Standby

    If high reliability single channel operation is required in inaccessible locations (e.g. high mountaintops in winter), a second radio may be used as hot standby. The second radio, power amplifier and power supply are automatically switched in if the first radio / power amplifier / power supply fails.

    Voting network

    To eliminate the need for location tracking of field personnel, basestations are connected at nodes to "cluster controllers" which:

  • monitor received signal strength / BER to determine the return transmit site (transmitter steering);
  • interface to PABX / PSTN and data networks.