What is a multicast radio system?

The origin of the same frequency simulcast

Conventional wireless communication is mostly a single-base station communication system, which uses a pair of intermediate frequency points to meet the communication needs of a smaller area. Co-frequency simulcast network refers to the deployment of multiple repeaters of the same frequency and networking in the same area. Each repeater covers an area, thereby increasing the coverage of the wireless communication network and forming a large-scale wireless coverage network. The same frequency simulcast system is a professional wireless communication system used to realize long-distance walkie-talkie coverage without blind area coverage.

The same-frequency simulcast technology originated in the paging system in the mid-1990s and was used to improve the coverage of low-speed digital paging. With the development of wireless communication services, conventional wireless communication cannot meet the needs of wide-coverage services. In the 1990s, analog conventional co-channel simulcast systems began to appear, and analog cluster co-simulcast systems appeared later. With the digitization of wireless communications, digital conventional and trunking co-channel simulcast systems have emerged around 2012.

The basic structure and technical characteristics of the same-frequency simulcast system

(1) Simultaneous frequency simulcast system consists of full-duplex base station, simulcast base station controller, link machine, GPS receiver board, remote control telemetry unit, antenna feed system, simulcast center, dispatch center, dispatch software, dedicated remote telemetry Software and other components;

(2) In the same-frequency simulcast system, there are three main key technologies:

1. Downlink transmit co-frequency technology

Each simulcast base station is equipped with a GPS receiver, which uses the GPS reference time signal to lock the transmitter frequency to ensure that the transmitter frequency of each base station is synchronized. That is, through frequency correction, the center frequency deviation of the transmission carrier frequency of each simulcast base station is controlled to a level of a few hertz to a dozen hertz, so as to avoid annoying "howling" due to the center carrier frequency deviation in co-frequency interference.

2. Downlink transmission synchronization technology

That is, through timing synchronization, the phase deviation of the same voice signal of each channel when the same channel of the voice signal is forwarded through different simulcast base stations and received by the same mobile station is controlled within a certain range, and the downlink signal strength of adjacent simulcast base stations is similarly improved. Received voice quality at the time.

3. Upstream receiving and judging technology

When the transmission on the same road is received by multiple simulcast base stations at the same time, the one with the best signal quality is selected for relay forwarding to improve the upstream voice quality. That is, multiple base stations in the coverage area receive the signal of the mobile station at the same time. The base station controller automatically optimizes and judges the received signal quality, and then sends it to each base station transmitter through the link to transmit, to ensure that the voice received by the other mobile station is clear.

From a deeper technical principle, the communication quality guarantee of the co-channel simulcast system mainly relies on the co-channel interference tolerance (typically 8dB) of the narrow-band FM system itself. The difference in signal strength is below the co-frequency interference tolerance (that is, less than 8dB), which improves the communication quality of adjacent simulcast base stations to a certain extent.