One of the most fundamental channel access methods in telecommunications. It works by dividing the total available bandwidth into several non-overlapping frequency sub-bands. Each user is assigned a specific frequency (a “channel”) for the entire duration of their communication.
How FDMA Works
In an FDMA system, sharing the network means carving up the radio spectrum like lanes on a highway.
- Dedicated Channels: Once a frequency is assigned to a user, no one else can use it until that user hangs up or disconnects.
- Guard Bands: To prevent interference between adjacent channels (where the signal from one “lane” spills into another), small unused portions of the spectrum called Guard Bands are placed between the frequencies.
- Continuous Transmission: Unlike TDMA, which sends data in bursts, FDMA allows continuous transmission because the user owns the frequency for the duration of the session.
FDMA Advantages
- Simplicity: The hardware requirements are simpler than those of time-synchronized systems because the base station doesn’t need to coordinate precise turns for users.
- Low Latency: Since the stream is continuous and not broken into time slots, there is no waiting for a turn to transmit.
- Reliability: It is highly effective for analog signals, which is why it was the backbone of the first-generation (1G) mobile phones.
FDMA Limitations
- Spectrum Waste: If a user isn’t talking, their assigned frequency remains idle and cannot be used by others, leading to inefficiency.
- Interference: High-quality filters are required to manage the Guard Bands and ensure signals don’t bleed into neighboring channels (Crosstalk).
- Limited Capacity: The number of users is strictly limited by the number of available frequency slices.
FDMA vs. TDMA: A Quick Comparison
| Feature | FDMA | TDMA |
| Division Logic | Divided by Frequency | Divided by Time |
| Synchronization | Low/Not critical | Very High (Strict timing) |
| Transmission | Continuous | Burst-like |
| Hardware | Simple, but requires filters | Complex digital processing |
Technical Formula
The number of channels (N) that can be supported in an FDMA system is determined by the total bandwidth (Btotal), the bandwidth allocated to each channel (Bc), and the guard band (Bg):
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Real-World Applications
- 1G Cellular (AMPS): The original analog mobile phone systems used FDMA exclusively.
- Satellite Broadcasting: Many older satellite links use FDMA to assign permanent transponder space to TV stations.
- Radio & TV Stations: Standard FM/AM radio uses FDMA—each station has its own frequency (e.g., 101.1 MHz) that it occupies 24/7.