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Code Division Multiplexing (CDM) is a technique in which multiple signals are simultaneously transmitted over a single communication channel by assigning a unique code to each signal. The most widely recognized application of CDM is in Code Division Multiple Access (CDMA), a digital cellular technology.

Here’s an overview of CDM:

How it Works:

1. Each user signal is multiplied by a unique spreading code (a sequence of chips, where a chip is a bit). This process spreads the spectrum of the signal.
2. All user signals are transmitted simultaneously over the same wide frequency band.
3. At the receiver, the desired user signal is extracted from the combined signals using the same unique spreading code. The receiver correlates the received signal with the spreading code, which separates out the intended signal from all the others.

Key Characteristics:

1. Unique Spreading Code: Each user or channel has its own distinct code, allowing the receiver to distinguish between different signals.
2. Wideband Signal: The original signal is spread over a wider frequency band, making it resistant to interference and fading.

Advantages:

1. Simultaneous Transmission: Multiple signals can be transmitted over the same channel simultaneously without interference.
2. Privacy: Since each transmission is encoded with a unique code, eavesdropping is challenging.
3. Resistance to Multipath Fading: The wideband nature of CDMA makes it resistant to multipath fading and interference.
4. Flexibility: New users can be added to the system without needing to allocate specific frequencies or time slots.

Disadvantages:

1. Complexity: The system requires complex signal processing for encoding and decoding.
2. Near-Far Problem: If one user transmits at a significantly higher power level than others, it can drown out the other signals. This problem is addressed using power control techniques.

Applications:

1. CDMA Cellular Systems: One of the primary technologies for 2G and 3G cellular systems.
2. Satellite Communication: The GPS system uses a form of CDM where each satellite transmits a unique coded signal, allowing multiple satellites to be tracked simultaneously.

In essence, CDM allows the transmission of multiple signals over a single communication channel by spreading each signal across a wide frequency band using a unique code. This ensures that signals can be transmitted simultaneously without interfering with each other. The receiver, knowing the unique code, can extract the intended signal from the combined signals.

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