Simulcast Radio systems

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Public safety personnel (police, fire, bus, rail, and emergency) rely on seamless wide area radio communications coverage to effectively perform their daily tasks. Typically a city/country will need to serve several hundred square miles or more with a handful of radio channels.

1. Description

2. Why

3. How

4. Future Trends

5. Related Links

Description 

A distributed radio system with multiple transceivers for simulcasting and selective processing of received signals is provided. The distributed radio system includes a plurality of processing elements and radio frequency transmitter elements interconnected by an Ethernet network.The system designates a number of radio frequency transmitter elements to be elements of a radio frequency simulcast set. The system utilizes relatively few frequency channels, and transmits at very low power levels, and thus causes minimal interference with an existing macro-cellular environment. 

  • The distributed processing utilized by the selection process may be synchronized.

  • A selection time window may be utilized for making the selection process for the upstream-received data traffic.

  • The system utilizes a centralized data link layer.

  • Transmissions are implemented by a two-level multicast technique.

  • The data transmission traffic is bundled.

  • Selective management of the Ethernet switches is performed by the system. 

The signals transmitted from a mobile unit are detected by multiple radio receivers and the system is able to select a desired radio receiver signal for processing. While selecting a received signal for processing, the system utilizes a distributed processing technique that performs the selection process throughout several levels. 

Why

A dispatcher may need to reach anyone in the serving area. But engineering wide area radio coverage with limited radio spectrum and limited number of radio channels is not easy. Generally many transmitter / receiver sites are required to get the widest possible coverage for mobiles and portables, for improved voice and data quality, and for better building penetration.

  • Cellular systems use many small cells, small towers, and reuse frequencies in non-adjacent cells to achieve wide area coverage.

  • This requires that individual radios be able to switch frequencies

  • The radio system be able to "handoff" from one base station to a neighboring base station as the radio user roams from one cell to the next.

  • This level of complexity is not always affordable or readily available to the public safety band.

How

"Talk-out" coverage can be greatly improved by simulcast transmission. Simulcast is the concatenation of two words "simultaneous" and "broadcast". It is the simultaneous transmission of audio on the same frequency from two or more sites. Wide area coverage is possible with multiple transmitters.Simulcast Radio Systems optimize the use of available spectrum by synchronizing transmission from multiple overlapping antennas.

  • Under this system, all base-band (VF) channels are assigned to identical RF frequencies at each transmitter.

  • This arrangement  makes best use of the available RF band and

  • It permits mobile units to leave radios set to the same frequency while traversing the entire service area.

The most important parameters that must be precisely matched among the transmitters are:

  • RF carrier frequency to within a 1 Hz

  • Audio phase to within 10 degrees

  • Amplitude response to within 0.1 dB

  • Audio delay to within 10 microseconds

Normally if two or more transmitters were to transmit the same signal, a receiver in the field would "lock" or "capture" the stronger of the signals. If the signals are within +/- 10 dB of each other, the signals either constructively or destructively combine in the receiver.In simulcast radio systems, VF channels need to be matched to provide clear  transmission in RF overlap areas. In these areas, received signal is the sum of signals coming from multiple transmitters. Therefore it is crucial that all voice frequencies be maintained near the same phase and amplitude so that the composite signal is a true replica of the parts. This is required for both human intelligibility and modem accuracy.To ensure that they "constructively" add together requires complex "synchronizing" transmitter equipment such that the signals are transmitted at exactly the same time (within microseconds) of each other.

Future Trends 

Accurate timing references are used to control the RF carrier frequency and to accurately set the delay compensation equipment. GPS makes this possible. In order to avoid overdependence on GPS (the military may limit access during wartime), rubidium oscillators are also used in parallel to GPS.In summary, simulcast systems provide wide geographic coverage with a uniform frequency plan, improved coverage in otherwise "dead spots", at the expense of a small degradation in overlap areas, improved building penetration for portables, frequency efficiency thru reuse, and simplicity of operation for dispatchers.

Keywords

RF overlap areas, multiple transceivers, simultaneous broadcast, dead spots.

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