FDMA

Anumber of two-way radio networks still just use FDMA to divide users within a given
frequency band onto individual narrowband RF channels. Examples are the European
ETSI 300/230 digital PMR (Private Mobile Radio) standard in which users have access
to an individual digitally modulated 12.5 kHz or 6.25 kHz channel, the French
TETRAPOL standard in which users have access to an individual digitally modulated
12.5, 10, or 6.25 kHz channel, and the US APCO 25 standard in which users have access
to an individual digitally modulated 12.5 kHz or 6.25 kHz RF channel.
Narrowband RF channels increase the need for RF filtering and an accurate frequency
reference (typically better than 1 ppm long-term stability). They do, however,
allow for a narrowband IF implementation that helps minimize the noise floor of the
receiver. The result is that narrowband two-way radios work well and have good sensitivity
and good range in noise-limited environments, including VHF applications
where atmospheric noise makes a significant contribution to the noise floor. The only
disadvantage, apart from additional RF component costs, is that maximum data rates
are constrained by the RF channel bandwidth, typically to 9.6 kbps.