Code Division Multiplexing : Working, Types & Its Applications

Multiplexing is a technique used to transmit multiple signals analog or digital over a communications link like a radio wave or fiber optic cable into a single composite signal. Once this composite signal reaches its destination, then it is demultiplexed. So demultiplexer divides the signal back to the original signals & outputs them into separate lines for the purpose of other operations. There are different types of multiplexing techniques like FDM, PDM, TDM, CDM, SDM & WDM.  This article discusses one of the types of multiplexing techniques; code division multiplexing or CDM – working with applications.

What is Code Division Multiplexing?

The term CDM stands for “Code division multiplexing” and it is a multiplexing technique where various data signals are merged for instantaneous transmission above a common frequency band. Once this multiplexing technique is utilized for allowing several users to transmit a single communications channel, then this technique is known as CDMA or code division multiple accesses.

Code Division Multiplexing Diagram

The code division multiplexing simply allots a unique code to each channel so that each channel can utilize a similar spectrum at the same time. CDM uses spread spectrum communication in which a narrowband signal is transmitted over a larger frequency band or across various channels through division. It does not restrict bandwidth frequencies or digital signals, so is less vulnerable to interference, and so provides better data communication capacity & more safe private line.

The code division multiplexing diagram is shown below. The following figure provides how all the channels utilize a similar frequency simultaneously for transmission. The CDM uses the spread spectrum technique in the wireless communication domain because every channel is coded so that its spectrum broadcasts over a much wider area than in use by the original signal.

Even though broadcasting of spectrum may appear faulty from a spectral point of view, so this is not the case since all users transmit the same spectrum. This CDM is frequently used for cell phones because it gives more flexibility within multiuser situations.

CDM uses spread spectrum technology to prevent enemies from intercepting as well as jamming transmissions. So, in a spread spectrum, a data signal is transmitted over a certain range of frequencies in an allotted frequency spectrum. Spread spectrum utilizes wideband, noise signals that are very hard to notice, intercept, or demodulate. In addition, spread-spectrum signals are very harder to jam as compared to narrowband signals. This multiplexing is also very secure because it is not easy to intercept or jam the signal in its coded nature view.

In the CDM system, the necessary components like the encoder & decoder are situated at the transmitter & receiver ends. The encoder at the transmitter transmits the signal spectrum above a much wider range than the least bandwidth required for transmission through a unique code. So, the decoder at the receiver uses a similar code for signal spectrum compression & data recovery.

There are many methods are used for encoding based on whether it is finished within the time domain, spectral domain, or otherwise both. The codes used are two-dimensional while both time & frequency are concerned. The time-domain codes comprise direct-sequence encoding as well as time hopping. The spectral codes are implemented with the phase or amplitude of different spectral components.

The operation of code division multiplexing is, a single bit can be transmitted by modulating a sequence of signal elements at various frequencies in some particular order. So the different frequencies for each bit are known as the chip rate. If single or multiple bits are transmitted at a similar frequency, then it is known as frequency hopping. So this will occur simply once the chip rate is below ‘1’ because it is the ratio of frequency & bit. The receiver at the receiving side decodes a zero or a one-bit by simply checking the frequencies in the right order.

How Code Division Multiplexing Works?

Code division multiplexing works by assigning a series of bits known as spreading code to every signal to differentiate one signal from another. This spreading code is merged with the original signal to generate a new flow of encoded data, after which it is transmitted over a shared medium. After that, a demux that knows the code can retrieve the original signals by simply subtracting out the spreading code which is called dispreading.

CDMA

CDMA stands for “Code-Division Multiple Access” and it is a type of multiplexing, which permits numerous signals to occupy a single transmission channel and it optimizes the utilization of accessible bandwidth.

CDMA system is extremely different as compared to frequency & time multiplexing. So in this type of system, an operator has the right of entry to the entire bandwidth for the whole period. The fundamental principle is that different CDMA codes are utilized to differentiate between the different users. This CDMA technology is used in UHF (ultra-high-frequency) cellular phone systems in the 800 MHz & 1.9 GHz bands.

The characteristics of CDMA mainly include the following.

• CDMA allows a number of users for connecting at a specified time and so provides enhanced data as well as voice communication capacity.
• In a CDMA system, there is no limit to the number of users although when the number of users increases then the performance will degrade.
• A CDMA system removes noise & interference and improves the quality of the network.
• The user transmissions can be encoded into distinct & unique codes by CDMA to protect its signals.
• In CDMA, a full spectrum is utilized through all the channels.
• All the cells within CDMA systems can utilize a similar frequency.

Advantages and Disadvantages

The advantages of code division multiplexing include the following.

• The signal quality is better.
• It protects from interference & tapping because the sender & the receiver only knows the spreading code.
• It is much protected from hackers.
• The addition of users is simple & with no boundary for the number of users.
• Large signal bandwidth decreases the multipath fading.
• Efficient use of particular frequency spectrum.
• Resources distribution is flexible.
• It is efficient highly.
• It does not need any synchronization.
• In this multiplexing, a number of users can split the same bandwidth.
• CDM is scalable.
• It is compatible with other types of cellular technologies.
• It uses a fixed frequency spectrum efficiently.
• Interference is decreased due to different code words assigned to each user.
• The improved security, resistance to interference and jamming, and efficient use of bandwidth. CDMA’s spread spectrum technique makes it more difficult for an eavesdropper to intercept the signal, and the unique spreading codes make it resistant to interference and jamming.

    The disadvantages of code division multiplexing include the following.

• When the number of users increases then the overall service quality will be decreased.
• The problem of near-far occurs.
• It needs time synchronization.
• In CDM, the transmitted bandwidth of each user is enlarged than the digital data speed of the source.
• The rate of data transmission is low.
• CDM is complex.

Applications

The applications of code division multiplexing include the following.

• CDM is widely used in so-called second-generation (2G) and third-generation 3G wireless communications. The technology is used in ultra-high-frequency (UHF) cellular telephone systems in the 800-MHz and 1.9-GHz bands. This is a combination of analog-to-digital conversion and spread spectrum technology.
• CDM networking technique is used to combine several data signals for simultaneous transmission above a common frequency band.
• This multiplexing is extensively used in second-generation & third-generation wireless communications.
• It is used in UHF (ultra-high-frequency) cellular telephone systems within the 800-MHz & 1.9-GHz bands. So this is a combination of both analog-to-digital conversion & spread spectrum technology.

Q: How is CDMA used in cellular networks?

A: CDMA is widely used in 3G and 4G cellular networks, as well as in wireless local area networks (WLANs). The technology allows multiple users to share the same frequency band, increasing the capacity of the network and providing better call quality.

Q: Can CDMA be used in satellite communications?

A: Yes, CDMA can be used in satellite communications as it allows multiple signals to be transmitted simultaneously over limited bandwidth. This makes it a popular choice in situations where a large number of signals need to be transmitted simultaneously, such as in satellite communications.

Q: What is the difference between direct sequence CDMA and frequency hopping CDMA?

A: Direct sequence CDMA (DS-CDMA) modulates the signal’s carrier wave using a pseudorandom binary sequence as a spreading code while frequency hopping CDMA (FH-CDMA) transmit the signal on a different frequency at different times, and the receiver uses the hopping pattern to reconstruct the original signal.

Q: How is CDMA used in cellular networks?

A: CDMA is widely used in 3G and 4G cellular networks, as well as in wireless local area networks (WLANs). The technology allows multiple users to share the same frequency band, increasing the capacity of the network and providing better call quality.

Q: Can CDMA be used in satellite communications?

A: Yes, CDMA can be used in satellite communications as it allows multiple signals to be transmitted simultaneously over limited bandwidth. This makes it a popular choice in situations where a large number of signals need to be transmitted simultaneously, such as in satellite communications.

Q: What is the difference between direct sequence CDMA and frequency hopping CDMA?

A: Direct sequence CDMA (DS-CDMA) modulates the signal’s carrier wave using a pseudorandom binary sequence as a spreading code while frequency hopping CDMA (FH-CDMA) transmit the signal on a different frequency at different times, and the receiver uses the hopping pattern to reconstruct the original signal.

Thus, this is all about an overview of code division multiplexing – working with advantages, disadvantages & applications. In CDM, various data signals are merged for transmission above a common frequency band simultaneously. Once this CDM networking technique is used to let many users transmit a single communications channel, then this technology is known as CDMA or code division multiple access (CDMA). Here is a question for you, what is FDM?