Code Division Multiple Access, or CDM, is a channel access method that allows multiple users to share the same frequency spectrum simultaneously. Unlike other methods that divide time or frequency, CDM uses unique codes to distinguish between different transmissions.
In CDM, each user is assigned a unique spreading code, also called a chip sequence. The original data bit is multiplied by this spreading code, which spreads the signal across a much wider bandwidth. The spreading codes are designed to be orthogonal, meaning they don't interfere with each other when properly synchronized.
The key advantage of CDM is that multiple users can transmit simultaneously on the same frequency band. Each user's signal is spread using their unique code. All signals are combined in the air, creating a composite signal. At the receiver, the desired signal is extracted by correlating with the appropriate spreading code, while other users' signals appear as noise.
At the receiver, signal recovery is achieved through correlation. The received composite signal is multiplied by the desired user's spreading code. Due to the orthogonal properties of the codes, signals from other users are suppressed, while the desired signal is enhanced. The correlator integrates the result to recover the original data bit.
CDM technology has found widespread applications in modern communication systems. It's the foundation of 3G cellular networks, GPS satellite systems, and various wireless technologies. The key benefits include high spectrum efficiency by allowing multiple users to share the same frequency, strong interference resistance due to spread spectrum properties, enhanced security through unique codes, and flexible capacity allocation based on demand.