Code Division Multiple Access (CDMA)

CDMA is a form of spread spectrum signalling where multiple users can share the same bandwidth simultaneously due to each user being assigned a unique sequence or code.

Above shows how two devices using the same code can communicate at the same time as another two devices due  each pair using their own unique code.

There are two types of CDMA:

• Direct Sequence (DS-CDMA)
• Frequency Hopping (FH-CDMA)

DS-CDMA

This is used in digital cellular systems like 3G. It works by assigning each user with a spreading code which is orthogonal to other codes.

FH-CDMA
The carrier frequencies of the individual users constantly changes with time.

Advantages of CDMA

• The security is better compared to the earlier generations which used FDMA or TDMA, which is Frequency Division and Time Division.
• Due to having individual codes for each user, it has immunity of interference from other users.
• It has the ability to cope with multipath channels.
• It also has good flexibility, so that a user can increase their rate without affecting other users.

Disadvantages of CDMA

• Due to the coding each device needs to be able to process the complex signal.
• If requires a large bandwidth.