FDMA with example

Frequency Division Multiple Access (FDMA) is a channel access method in which the available bandwidth is divided into non-overlapping frequency bands, and each station (user) is given one band for its whole communication. All stations can transmit at the same time, each on its own band.

Features

• The bandwidth is split into N channels with guard bands between them.
• Each user has a dedicated channel; band-pass filters separate the users.
• Transmission is continuous, so no slot synchronisation is needed.
• Channels are allocated on demand (at call set-up) and freed when the call ends.
• It is simple, but it wastes bandwidth when a user is idle, and the guard bands waste spectrum.

 Freq
  f4 |==== User 4 ====|
  f3 |==== User 3 ====|
  f2 |==== User 2 ====|
  f1 |==== User 1 ====|
     +------------------> time
 (guard bands between bands)

Example

The first-generation cellular system AMPS uses 30 kHz channels in the 824–894 MHz band: one channel for the uplink (mobile to base) and one for the downlink, which gives FDD. AM/FM radio stations working in different frequency bands are another example.

Difference from FDM: FDM multiplexes several signals at one place, whereas FDMA gives access to stations located in different places.