CDM (code division multiplexing)

Code Division Multiplexing, or CDM, is a technique used in telecommunications to allow multiple users to share the same frequency band. It is a form of spread spectrum communication, which means that it spreads the signal over a wider frequency range than is strictly necessary, which helps to improve the signal's resistance to interference and jamming.

CDM works by assigning each user a unique code that is used to modulate their signal. These codes are carefully designed so that they are orthogonal to each other, meaning that they have no overlap and do not interfere with each other. When multiple users transmit at the same time, their signals are combined into a single waveform that is transmitted over the same frequency band. At the receiver, each user's signal is demodulated by multiplying it with their unique code, which separates it from the other users' signals.

CDM has a number of advantages over other multiplexing techniques, such as Frequency Division Multiplexing (FDM) and Time Division Multiplexing (TDM). One of the main advantages is that it allows multiple users to share the same frequency band without interfering with each other, which makes more efficient use of the available bandwidth. This is particularly useful in wireless communications, where the available frequency bands are limited and in high demand.

Another advantage of CDM is that it provides a degree of security against eavesdropping and jamming. Since each user's signal is modulated with a unique code, it is difficult for an attacker to intercept or interfere with the signal without knowing the code. This makes CDM a popular technique in military and other sensitive communications applications.

There are two main types of CDM: Direct Sequence Spread Spectrum (DSSS) and Frequency Hopping Spread Spectrum (FHSS). In DSSS, the signal is spread over a wider frequency range by directly modulating it with a spreading code. The spreading code is a binary sequence that is much longer than the data being transmitted, and is chosen to have good autocorrelation properties, meaning that it has a low cross-correlation with other spreading codes. The resulting spread signal is then transmitted over a wideband channel, which helps to reduce the effects of interference and jamming.

In FHSS, the signal is spread over a wider frequency range by rapidly switching between a set of predetermined carrier frequencies. The switching is controlled by a hopping sequence, which is also designed to have good autocorrelation properties. The advantage of FHSS is that it can be used in environments with narrowband interference, as the signal hops to a different frequency when interference is detected.

Both DSSS and FHSS have their own advantages and disadvantages, and the choice of which to use depends on the specific application and the characteristics of the communication channel. In general, DSSS is better suited for applications with high data rates, while FHSS is better suited for applications with narrowband interference.

CDM is used in a variety of applications, including wireless communications, satellite communications, and local area networks (LANs). It is also used in GPS (Global Positioning System) receivers, where the spread signal helps to improve the accuracy of the location measurement by reducing the effects of multipath interference.

In conclusion, Code Division Multiplexing (CDM) is a powerful technique that allows multiple users to share the same frequency band without interfering with each other. It provides a degree of security against eavesdropping and jamming, and is used in a variety of applications, including wireless communications, satellite communications, and GPS receivers. CDM is also used in cellular networks, where it is the basis of the popular 3G and 4G wireless standards. In these networks, CDM is used to allow multiple users to share the same frequency band in the same cell, which helps to increase the capacity of the network and improve the quality of service. In 3G networks, the CDM technique is used in a variant called CDMA (Code Division Multiple Access), which uses a combination of DSSS and FHSS techniques to allow multiple users to share the same frequency band.