What are the differences between FDMA, TDMA, and CDMA?

Multiple users need to share a communication channel efficiently. FDMA (Frequency Division Multiple Access), TDMA (Time Division Multiple Access), and CDMA (Code Division Multiple Access) are three fundamental techniques that enable multiple users to access the same communication medium without interference.

FDMA (Frequency Division Multiple Access)

FDMA is an analog technique that divides the available frequency spectrum into separate frequency bands. Each user is assigned a unique frequency band for the entire duration of their communication session.

In FDMA, the total bandwidth of the channel is divided into logical channels, with each transmitting device given exclusive control of one frequency band. At the receiver end, a demultiplexer separates the composite signal back into individual frequency components.

TDMA (Time Division Multiple Access)

TDMA is a digital technique where users share the entire frequency spectrum by taking turns in time. Each user gets exclusive access to the full bandwidth for a specific time slot, and control rotates among users in a round-robin fashion.

A common example is GSM mobile networks, where multiple users share the same frequency channel by transmitting in different time slots within a frame.

Variants of TDMA

• STDM (Statistical Time Division Multiplexing) − Dynamically allocates time slots based on actual data transmission needs, skipping idle users.

• ATDM (Asynchronous Time Division Multiplexing) − Users transmit data asynchronously without fixed time slot assignments.

CDMA (Code Division Multiple Access)

CDMA allows multiple users to transmit simultaneously over the same frequency spectrum by assigning each user a unique spreading code or chip sequence. Unlike FDMA and TDMA, CDMA users can transmit at the same time and frequency.

Each user's data is multiplied by their unique code before transmission. At the receiver, the same code is used to extract the specific user's signal while treating other users' signals as noise. This technique is based on orthogonal vector spaces from mathematics.

CDMA provides better capacity utilization and lower delay compared to TDMA, especially in heavily loaded networks, since users don't have to wait for their turn to transmit.

Comparison of Multiple Access Techniques

Conclusion

FDMA, TDMA, and CDMA represent three distinct approaches to multiple access: frequency division, time division, and code division respectively. Each technique has unique advantages, with CDMA offering the highest capacity and FDMA being the simplest to implement.