CD (Code Division)

Code Division (CD) refers to a digital communication technique used in wireless communication systems to transmit and receive data between devices. CD is based on a spread-spectrum modulation technique that uses a specific code to spread the signal over a wider frequency band than the signal itself, making it difficult for other signals to interfere with the transmission.

CD was initially developed for military applications, but it has since been widely adopted in commercial wireless communication systems such as cellular networks, wireless LANs, and satellite communication systems. CD offers several advantages over other digital communication techniques, including high security, robustness against interference and noise, and efficient use of bandwidth.

The following sections provide a detailed explanation of CD, including its history, working principle, types, applications, and advantages.

History of CD

The history of CD dates back to World War II when the military began using spread-spectrum technology to encrypt their communications to prevent interception by enemy forces. The technology was based on the concept of using a specific code to spread the signal over a wider frequency band than the signal itself, making it difficult for the enemy to intercept the transmission.

In the 1960s, the US military developed a digital communication system called Code Division Multiple Access (CDMA), which used CD as its modulation technique. CDMA was initially used for secure military communications, but it was later adopted by the commercial sector for cellular networks due to its ability to support multiple users on the same frequency band.

Working Principle of CD

The basic principle of CD is to use a specific code to spread the signal over a wider frequency band than the signal itself. The code is a sequence of bits that is unique to each user, and it is used to modulate the carrier signal. The modulated signal is then transmitted over the wireless channel.

At the receiver end, the receiver uses the same code to demodulate the signal and recover the original data. The receiver correlates the received signal with the same code that was used to modulate the signal. The correlation process extracts the original data from the modulated signal.

The code used in CD is called a spreading code or a sequence. The spreading code has two main characteristics: 1) it has a large number of bits, and 2) it has good correlation properties. The large number of bits ensures that the spread signal occupies a wider frequency band than the original signal, making it difficult for other signals to interfere with the transmission. The good correlation properties ensure that the receiver can extract the original data from the spread signal.

Types of CD

There are several types of CD, including Direct Sequence CD (DS-CD), Frequency Hopping CD (FH-CD), and Time Hopping CD (TH-CD).

Direct Sequence CD (DS-CD)

DS-CD is the most widely used type of CD in wireless communication systems. In DS-CD, the spreading code is applied directly to the data bits. The modulated signal is then transmitted over the wireless channel.

DS-CD has several advantages, including high security, robustness against interference and noise, and efficient use of bandwidth. DS-CD is used in cellular networks, wireless LANs, and satellite communication systems.

Frequency Hopping CD (FH-CD)

FH-CD is a type of CD that uses a spreading code to modulate the carrier signal, but instead of transmitting the modulated signal directly, it hops from one frequency band to another at a predetermined rate.

FH-CD has several advantages, including high security, robustness against interference and noise, and efficient use of bandwidth. FH-CD is used in Bluetooth wireless technology and some military communication systems.

Time Hopping CD (TH-CD)

TH-CD is a type of CD that uses a spreading code to modulate the carrier signal, but instead of transmitting the modulated signal continuously, it transmits it in short bursts or packets. The packets are transmitted at different times and on different frequency channels, making it difficult for other signals to interfere with the transmission.

TH-CD has several advantages, including high security, robustness against interference and noise, and efficient use of bandwidth. TH-CD is used in Ultra-Wideband (UWB) wireless technology and some military communication systems.

Applications of CD

CD is used in a wide range of wireless communication systems, including cellular networks, wireless LANs, satellite communication systems, Bluetooth technology, and military communication systems.

Cellular Networks

CDMA is the modulation technique used in cellular networks to transmit voice and data between mobile devices and base stations. CDMA allows multiple users to share the same frequency band by using unique spreading codes for each user.

Wireless LANs

CDMA is also used in wireless LANs to transmit data between devices. The spreading code used in wireless LANs is shorter than the one used in cellular networks, but it still ensures that the transmitted signal occupies a wider frequency band than the original signal.

Satellite Communication Systems

CD is used in satellite communication systems to transmit data between the satellite and the ground station. The spreading code used in satellite communication systems ensures that the transmitted signal can be received even in noisy and interference-prone environments.

Bluetooth Technology

FH-CD is used in Bluetooth technology to transmit data between devices. Bluetooth uses a hopping sequence that ensures that the transmitted signal can be received even in noisy and interference-prone environments.

Military Communication Systems

CD is widely used in military communication systems to transmit secure and encrypted data between devices. The spreading code used in military communication systems ensures that the transmitted signal cannot be intercepted by enemy forces.

Advantages of CD

CD offers several advantages over other digital communication techniques, including:

High Security

CD is highly secure because the spreading code used to modulate the carrier signal is unique to each user. The spreading code ensures that the transmitted signal cannot be intercepted or decoded by unauthorized users.

Robustness Against Interference and Noise

CD is robust against interference and noise because the spreading code ensures that the transmitted signal occupies a wider frequency band than the original signal. The wider frequency band ensures that the transmitted signal can be received even in noisy and interference-prone environments.

Efficient Use of Bandwidth

CD is efficient in its use of bandwidth because it allows multiple users to share the same frequency band without interfering with each other. The unique spreading code used by each user ensures that their transmitted signal can be separated from other users' signals at the receiver end.

Conclusion

CD is a digital communication technique that uses a specific code to spread the signal over a wider frequency band than the signal itself. CD is used in a wide range of wireless communication systems, including cellular networks, wireless LANs, satellite communication systems, Bluetooth technology, and military communication systems. CD offers several advantages over other digital communication techniques, including high security, robustness against interference and noise, and efficient use of bandwidth.