What is CDMA (Code-Division Multiple Access)?

CDMA (Code-Division Multiple Access) is a general term for various multiplexing or media access control methods based on spread spectrum technology.

What is the CDMA Mobile Telephone Network?

Spread, diffuse, or diffuse spectrum can be used interchangeably to refer to the same concept in all cases. This technology is commonly used in wireless communication but can also be used in fiber optic or cable systems.

One of the issues that need to be improved in data communication is how a single communication channel or transmission medium will be distributed among several users so that several communications can be managed simultaneously.

Without an organizational method, this concept is called multiplexing or media access control, as direct communication can be blocked.

The name multiplexed is applied for situations where a single device determines the distribution of the channel between different communications, such as a hub at the end of a fiber optic cable, and multiplexing is transparent for end-user terminals.

Instead, media access control is used when user terminals are communicating with a device operating as a network, such as a group of mobile phones communicating, and a specific communication scheme must be used to avoid interference between them.

To address this problem, the CDMA method uses spread spectrum technology and a particular encoding scheme where a unique code is assigned to each transmitter and selected to be orthogonal to the others.

The receiver captures the signals emitted by all transmitters simultaneously and can receive the corresponding signal thanks to the coding scheme.

Other multiplexing schemes use division in frequency (FDMA), time (TDMA), or space (SDMA) to achieve the same purpose.

Thus, separation of different communications takes place at any time, and prevention or suppression of interference between them takes place.

Real-use systems often use one or more of these strategies to provide better communication. However, there will be problems if several people want to talk at the same time.

To avoid this problem, people can take turns speaking. One person can say in a higher tone, while the other can speak in a lower tone. Only then can the voices be distinguished. However, they can direct the voices in different directions in the room or talk in other languages.

Thus, as with CDMA technology, only people who know the code can communicate efficiently.

Code division is used in multiple radio frequency communication systems such as mobile telephony, data transmission (WiFi), or satellite navigation (GPS).

Also, the term CDMA is used for a cell phone wireless air data communication developed by Qualcomm and later adopted as a standard by the North American TIA under the name IS-95.

Features

In CDMA, the signal is broadcast in a much larger bandwidth than required by the data to be transmitted, and code splitting serves as a spread spectrum multiple access technique.

The data to be transmitted is unique to the user and is XORed with the broadcast transmission code with significantly higher bandwidth than the data.

The data signal with vibration duration Tb is XORed with transmission code with a jittered time Tc. Therefore, the bandwidth of the transmitted data is 1/Tb, and the bandwidth of the spread-spectrum signal is 1/Tc.

Since Tc is much smaller than Tb, the bandwidth of the emitted signal is much larger, so this is called a spread spectrum.

Each user of a CDMA system uses a different transmission code to modulate their signals. Choosing the code to be used for modulation is very important for CDMA systems to ensure good performance.

The relevant signal is selected by cross-correlating the captured signal with the user’s code.

The healthiest communication situation is when there is a good separation between the desired user’s signal and the rest’s signal. If the captured signal is being searched for, the result of the correlation will be very high, and the system will be able to output the signal.

If the received signal is not required, the correlation should be very small, ideally prone to zero, as the code used by each user is different.

If the correlation occurs with any non-zero time delay, the correlation should also tend to zero. This is called autocorrelation, and it is used to reject multipath interference. Generally, the code section is divided into synchronous and asynchronous CDMA.

1. Simultaneous CDMA

Simultaneous CDMA uses the mathematical properties of orthogonality between vectors whose coordinates represent the data to be transmitted. Two vectors can be multiplied by the scalar product that adds the products of their respective coordinates, and if the dot product of the two vectors is 0, it is expressed orthogonal to each other.

Some features of the dot product help to understand how CDMA works and if the vectors A and b are orthogonal, represent the codes of the two synchronized CDMA users A and B.

Therefore, even if the receiver receives linear combinations of vectors a and b, and the user of interest knows the transmission code, it can always isolate its data from those of other users only through the scalar product of the received signal.

Since the user code is orthogonal to the others, the product will isolate the signal of interest and cancel the rest.

For two users, this result can be extended to any number of users as long as there is enough orthogonal code for the desired number of users, and this is achieved by increasing the length of the code.

The CDMA user uses a unique code to modulate the signal, and the codes of users in the same domain are orthogonal to each other.

Since their inner products are 0, orthogonal codes have zero cross-correlation, and in other words, they do not interfere with each other.

As a result, it is not necessary to use a frequency filtering or switching circuit according to a time scheme to isolate the signal of interest, and the signals from all users are simultaneously received and sent to digital processing.

In the case of IS-95, 64-bit orthogonal Walsh codes are used to encode the signals and separate their different users.

2. Asynchronous CDMA

Synchronous CDMA systems work well as long as there is no excessive delay in the arrival of signals, but the radio links between cell phones and base stations can be very precisely coordinated.

Because the terminals can move, they may encounter obstructions in the signal path, resulting in some variation in arrival delays.

Due to the mobility of the terminals, different signals have a variable arrival delay.

Pseudo-random sequences are used in asynchronous CDMA systems since, in theory, it is impossible to create coding sequences that are orthogonal at all random times the signal can reach.

The PN code is a binary sequence that appears random but can be determined deterministically if the receiver desires it.

These sequences are used to encode and decode signals to which asynchronous CDMA users are interested, as orthogonal codes are used in the synchronous system.

Since PN sequences show no statistical correlation, the sum of multiple PN sequences is referred to as a numerous access attempt defined as a Gaussian noise process following the central theorem of the statistical boundary.

If signals from all users are received with equal strength, the variance of the MAI increases in direct proportion to the number of users.

The rest of the users’ signals will appear as noise relative to the signal of interest and will interact with the signal of interest. Thus, the higher the number of concurrent users, the greater the interference.

The fact that the sequences are seemingly random and of distributed power over a relatively wide bandwidth has an additional advantage. These will be harder to detect since they are confused by background noise if someone tries to catch them.

This feature was used in military communication in the 20th century. All CDMA types can take advantage of the processing gain provided by spread spectrum systems. This gain allows buyers to distinguish unwanted signals partially.

Signals encoded with the specified PN code are received, and the rest of the signals are processed as broadband noise that is either reduced or eliminated by the processing gain.

Since all users produce MAIs, it is essential to control the emission power. Therefore, synchronous CDMA, TDMA, or FDMA systems can eliminate unwanted signals due to the orthogonality of these media access schemes.

However, this statement is not valid for asynchronous CDMA because the rejection of unwanted signals is only relative. If some of the unwanted signals are received with much greater power than the desired signal, it cannot be separated from the remaining signal.

To avoid this problem, a general requirement in the design of these systems is that the power of all emitters is controlled. Tt approximately synchronizes the energy captured by the receiver for all incoming signals.

In cellular telephone systems, the base station uses a closed-loop power control scheme to control the emission power of each phone precisely.