FDMA is a channel access method used in telecommunications. It allows multiple users to share a common communication medium by dividing the available frequency bandwidth into smaller sub-channels.
Each sub-channel is allocated to a specific user or device, ensuring that they can transmit data simultaneously without interfering with each other.
How FDMA Works:
Frequency Division: The total available frequency spectrum is divided into non-overlapping frequency channels.
Allocation to Users: Each user or station is assigned one or more of these frequency channels for their exclusive use.
Reserved Bands: Once allocated, a specific frequency band remains reserved for a particular station at all times.
Fixed Bandwidth: The bandwidth assigned to each user remains constant, ensuring predictable communication.
Two Main Techniques within FDMA:
Multi-Channel-Per-Carrier (MCPC) Transmission:
In satellite communication, analog multiplexing is initially used at earth stations.
Telephone channels are combined into a single baseband signal, which is then modulated onto a single RF carrier.
Up to 1800 telephone channels can be multiplexed using Frequency Division Multiplexing (FDM), resulting in a wide baseband occupying an 8 MHz bandwidth.
Different RF carriers are used for frequency modulation, and a common transponder is shared among various earth stations.
This technique is known as FDM-FM-FDMA.
Single-Channel-Per-Carrier (SCPC) Transmission:
When a single signal is transmitted via an earth station, it’s referred to as SCPC transmission.
SCPC-FDMA systems are used in scenarios with many small earth stations (e.g., mobile phones) accessing a single transponder.
These systems can be reconfigured based on traffic conditions, making them compatible with demand assignment systems.
Only the carrier for the active SCPS (Single Channel Per Station) channel is transmitted, reducing transponder power consumption.
Comparison with Other Multiple Access Schemes:
FDMA vs. TDMA (Time Division Multiple Access):
FDMA allocates frequency bands, while TDMA divides time into slots.
In FDMA, each user has a dedicated frequency band, whereas in TDMA, users share the same frequency but transmit in different time slots.
FDMA vs. CDMA (Code Division Multiple Access):
CDMA uses unique codes to differentiate users, allowing simultaneous transmission over the same frequency.
FDMA, on the other hand, assigns separate frequency bands to each user.
