Code Division Multiple Access (CDMA) is a digital cellular technology that allows multiple users to share the same frequency band simultaneously. Unlike Time Division Multiple Access (TDMA) or Frequency Division Multiple Access (FDMA), which divide the frequency band into time slots or channels, CDMA uses a spread spectrum technique.
In CDMA, each user is assigned a unique code, known as a spreading code. These codes are used to modulate the user’s signal before transmission. All users in the system share the same frequency band, but their signals are distinguished by the unique codes.
Key characteristics include:
- Spread Spectrum: CDMA uses spread spectrum technology, which spreads the signal across a wider frequency band. This provides advantages in terms of resistance to interference and improved security.
- Soft Capacity: the systems exhibit a concept called “soft capacity,” meaning that the system can support a higher number of users than the number of available orthogonal codes. This is because users can be distinguished not only by their unique codes but also by the strength of their received signals.
- Interference Rejection: it is known for its ability to handle interference well. Users can share the same frequency band, and the receiver can separate and recover the individual signals based on their unique spreading codes.
CDMA has been widely used in 2G (second generation) and 3G (third generation) cellular networks. CDMA2000 and WCDMA (Wideband CDMA) are examples of CDMA-based 3G technologies. However, in recent years, many mobile networks have transitioned to LTE (Long-Term Evolution) and 5G technologies, which use different modulation schemes.