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What Is Code Division Multiple Access?

What Is CDMA?

Code Division Multiple Access (CDMA) is a multiple-access technique that allows many users to transmit simultaneously on the same frequency and at the same time by assigning each user a unique spreading code. Instead of separating users by frequency, as in Frequency Division Multiple Access (FDMA), or by time, as in Time Division Multiple Access (TDMA), CDMA distinguishes users by means of their individual codes. It is widely used in cellular communications, satellite systems, military radios, and Global Navigation Satellite Systems (GNSS).

The basic principle is based on spread-spectrum communication. Before transmission, each user's data are multiplied by a high-rate pseudorandom spreading code. This spreads the signal over a much wider bandwidth than would otherwise be required. At the receiver, the same code is used to despread the desired signal, restoring it to its original bandwidth while leaving signals from other users spread across the spectrum as low-level interference.

A useful analogy is a crowded room in which several pairs of people are speaking simultaneously, each in a different language. Although everyone shares the same room at the same time, a listener who understands only one language can concentrate on the desired conversation while treating the others as background noise. CDMA operates in much the same way, with the spreading code acting as the "language" that distinguishes one user's signal from another.

One of the principal advantages of CDMA is its efficient use of radio spectrum. Since all users occupy the same frequency band simultaneously, system capacity can be increased simply by adding more users, provided the total interference remains acceptable. CDMA also provides inherent resistance to narrowband interference, multipath fading, and eavesdropping because of its spread-spectrum operation.

A key challenge in CDMA is the near-far effect. If one user's signal arrives at the receiver much stronger than another's, it can overwhelm the weaker signal, making it difficult or impossible to recover. For this reason, CDMA systems employ precise power control to ensure that signals from all users arrive at approximately equal power levels. This process, often referred to as power balancing, is essential to the successful operation of CDMA networks.

CDMA formed the basis of several important communication systems, including the IS-95 (cdmaOne) and CDMA2000 cellular standards. It is also used extensively in Global Positioning System (GPS) and other Global Navigation Satellite Systems (GNSS), where each satellite transmits using a unique spreading code, allowing receivers to distinguish signals from multiple satellites operating on the same frequency.

It is important to distinguish Code Division Multiple Access (CDMA) from Direct Sequence Spread Spectrum (DSSS). DSSS is the spreading technique that spreads a single signal using a pseudorandom code, whereas CDMA is the multiple-access technique that allows many users to share the same channel by assigning each a different spreading code. In practice, most CDMA systems employ DSSS to achieve multiple access.

Today, CDMA remains one of the most influential multiple-access techniques in communications engineering. Although modern cellular systems such as LTE and 5G primarily employ Orthogonal Frequency Division Multiple Access (OFDMA), CDMA continues to be used in satellite navigation, military communications, and specialist wireless systems. Its innovative use of spread-spectrum signalling transformed wireless communications and introduced concepts that continue to influence many modern communication technologies.