C4FM
C4FM is a digital modulation technique that is used to transmit digital voice and data information over a radio channel . C4FM is an acronym for English Continuous 4-level frequency modulation , according to be used in the frequency shift using four frequencies. These are in frequency ranges such as the ultra-short wave and the decimeter wave below 1 GHz . The modulation method is used, among other things, in APCO P25 ( Radio Land Mobile Communications, Project 25 ), a higher-level transmission network for digital authority radio for police and rescue services in North America, and worldwide in amateur radio . C4FM is specified for this application by the Telecommunications Industry Association (TIA), an association of government agencies in the United States, in the ANSI / TIA-102.CAAB-C standard .
realization
C4FM is specified in ANSI / TIA-102 for a bandwidth of 12.5 kHz, with a symbol rate of 4800 baud . Four different transmission symbols are used for data transmission, each symbol having a fixed assignment and carrying the information of two bits and being represented by four different frequencies. This results in a bit rate of 9600 bps gross. In relation to the carrier frequency f c , which also represents the center frequency in the radio channel, the four frequencies are defined in the standard as a frequency deviation from the carrier frequency relative to the transmission frequency:
| Transmission symbols at C4FM | ||
|---|---|---|
| symbol | Bit combination | Frequency deviation |
| +1 | 00 | f c + 900 Hz |
| +3 | 01 | f c + 2700 Hz |
| −1 | 10 | f c - 900 Hz |
| −3 | 11 | f c - 2700 Hz |
For the data transmission, the user data in the transmission layers superordinate to the modulation are summarized in so-called frames in the ANSI / TIA-102 standard and, depending on the operating mode, provided with a forward error correction (FEC). Both voice and data can be transmitted simultaneously and in multiplexed form, or the transmission channel can be used exclusively for data or only for voice transmission. The voice data is reduced in the data volume to 4400 bps independently of C4FM with a vocoder defined in TIA-102 . The entire digital signal processing with all transmission layers can be implemented with a signal processor .
Relay stations
The method is also suitable for use in radio networks with relay radio stations . Relay radio stations are unmanned radio stations that are usually located in elevated positions, e.g. B. be installed in mountain stations of cable cars . Thus, with a comparatively small number of relay stations, extensive radio networks, e.g. B. across several counties.
Web links
- TIA-102 Series Documents
- Information about C4FM amateur radio Switzerland
- Information on C4FM in the amateur radio area from the ÖVSV
- C4FM relay card
Individual evidence
- ↑ a b Land Mobile Radio Transceiver Performance - Recommendations - Project 25 - Digital Radio Technology C4FM / CQPSK Modulation. TIA-102.CAAB-C. Telecommunications Industry Association (TIA), 2010, accessed May 29, 2019 .
- ↑ Land Mobile Radio Transceiver Performance - Recommendations - Project 25 - Vocoder Description. TIA-102.BABA. Telecommunications Industry Association (TIA), 2010, accessed May 29, 2019 .
- ↑ New "System Fusion" repeater on the Säntis in operation