According to abbreviationfinder, CDMA or Code Division Multiple Access is a generic term for several methods of multiplexing or control access to the medium based on the technology of spread spectrum. The translation of the English spread spectrum is done with different adjectives according to the sources; Spread, spread, diffuse or scattered spectrum can be used interchangeably to refer to the same concept in all cases. It is commonly used in wireless communications (by radio frequency), although it can also be used in fiber opticor cable systems.
Introduction
One of the problems to be solved in data communications is how to distribute the use of a single communication channel or transmission medium among several users so that several communications can be managed at the same time. Without a method of organization, interference would appear that could either be annoying or directly impede communication. This concept is called multiplexing or media access control, depending on the context.
The name “multiplexed” is applied for cases in which a single device determines the distribution of the channel between different communications, such as a hub located at the end of a fiber optic cable; for end-user terminals, multiplexing is transparent. Instead, the term “media access control” is used when it is the users’ terminals, in communication with a device that acts as a network, which must use a certain communication scheme to avoid interference between them, such as For example, a group of mobile phones in communication with an operator’s antenna.
To solve this, CDMA uses spread spectrum technology and a special coding scheme, whereby each transmitter is assigned a unique code, chosen so that it is orthogonal with respect to the rest; the receiver captures the signals emitted by all the transmitters at the same time, but thanks to the coding scheme (which uses codes orthogonal to each other) you can select the signal of interest if you know the code used. Other multiplexing schemes use division in frequency (FDMA), in time (TDMA) or in space (SDMA) to achieve the same objective: the separation of the different communications that are taking place at all times, and avoid or suppress the interferences between them. Systems in real use (such as IS-95 or UMTS) often employ several of these strategies at the same time to ensure better communication.. A possible analogy for the multiple access problem would be a room (representing the channel) in which several people want to talk at the same time. If several people speak at the same time, interference will occur and understanding will be difficult. To avoid or reduce the problem, they could speak in turns (time division strategy), some speak in higher tones and others lower tones so that their voices are distinguished (frequency division), direct their voices in different directions of the pitch. room (spatial division) or speaking in different languages (division by code, the subject of this article): As in CDMA, only people who know the code (that is, the ” language “) can understand it.
Code division is used in multiple radio frequency communication systems, both mobile telephony (such as IS-95, CDMA2000, FOMA or UMTS), data transmission (WiFi) or satellite navigation (GPS).
Popular use of the term
The term CDMA, however, is often popularly used to refer to an interface wireless air mobile phone developed by the company Qualcomm, and later accepted as a standard by the American TIA under the name IS-95 (or, as trademark by Qualcomm, “cdmaONE” and its successor CDMA2000). Indeed, the systems developed by Qualcomm use CDMA technology, but they are not the only ones to do so.
Technical details
In CDMA, the signal is emitted with a bandwidth much greater than that required by the data to be transmitted; for this reason, code division is a spread spectrum multiple access technique. The data to be transmitted is simply XORed with the transmission code, which is unique to that user and is broadcast with a significantly higher bandwidth than the data.
The data signal, with a pulse duration Tb, is XORed with the transmission code, which has a pulse duration Tc. (Note: the bandwidth required by a signal is 1 / T, where T is the time taken to transmit one bit). Therefore, the bandwidth of the transmitted data is 1 / Tb and that of the spread spectrum signal is 1 / Tc. Since Tc is much smaller than Tb, the bandwidth of the emitted signal is much greater than that of the original signal, hence the name “spread spectrum”. Each user of a CDMA system uses a different (and unique) transmission code to modulate their signal. The selection of the code to be used for modulation is vital for the good performance of CDMA systems, because the selection of the signal of interest depends on it, which is done by cross-correlation of the signal captured with the code of the user of interest. as well as the rejection of the rest of the signals and of the multi-path interferences (produced by the different signal bounces).
The best case is when there is a good separation between the signal of the desired user (the signal of interest) and those of the rest; If the signal captured is the one sought, the result of the correlation will be very high, and the system will be able to extract the signal. On the other hand, if the received signal is not the one of interest, as the code used by each user is different, the correlation should be very small, ideally tending to zero (and therefore eliminating the rest of the signals). And furthermore, if the correlation occurs with any nonzero time lag, the correlation should also tend to zero. This is called autocorrelation and is used to reject multi-path interference. In general, there are two basic categories in code division:
Code Division Multiple Access (Synchronous CDMA)
Synchronous CDMA exploits the mathematical properties of orthogonality between vectors whose coordinates represent the data to be transmitted. For example, the binary string “1011” would be represented by the vector (1, 0, 1, 1). Two vectors can be multiplied by the scalar product (·), which adds the products of their respective coordinates. If the dot product of two vectors is 0, they are said to be orthogonal to each other. (Note: if two vectors are defined u = (a, b) and v = (c, d); their dot product will be u · v = a * c + b * d).
Some properties of the dot product help to understand how CDMA works. If the vectors a and b are orthogonal, and represent the codes of two synchronous CDMA users A and B, then: Therefore, even if the receiver picks up linear combinations of vectors a and b (that is, the signals coming from A and B at the same time, summed over the air), knowing the transmission code of the user of interest can always isolate its data from those of the rest of the users, simply by means of the scalar product of the signal received with the user’s code; As the user code is orthogonal with respect to all the others, the product will isolate the signal of interest and cancel the rest. This result for two users is extensible to as many users as desired, as long as there are sufficient orthogonal codes for the desired number of users, which is achieved by increasing the length of the code.
Each synchronous CDMA user uses a unique code to modulate the signal, and the codes of the users in the same area must be orthogonal to each other. Four mutually orthogonal codes are shown in the image. Since their dot product is 0, the orthogonal codes have a zero cross-correlation, and, in other words, they do not interfere with each other.
This result implies that it is not necessary to use frequency filtering circuitry (as it would be used in FDMA), or switching according to some time scheme (as it would be used in TDMA) to isolate the signal of interest; signals from all users are received at the same time and separated by digital processing. In the case of IS-95, 64-bit orthogonal Walsh codes are used to encode the signals and separate its different users.
Asynchronous CDMA
Synchronous CDMA systems work well as long as there is not an excessive delay in the arrival of the signals; however, the radio links between mobile phones and their bases cannot be coordinated very precisely. As the terminals can move, the signal can encounter obstacles in its path, which will give rise to some variability in the arrival delays (due to the different bounces of the signal, the Doppler effect and other factors). Therefore, a somewhat different approach becomes advisable.
Due to the mobility of the terminals, the different signals have a variable arrival delay. Since, mathematically, it is impossible to create coding sequences that are orthogonal at all random times the signal could arrive, in asynchronous CDMA systems unique “pseudo-random” or “pseudo-noise” sequences are used., PN sequences). A PN code is a binary sequence that appears random, but can be reproduced deterministically if required by the receiver. These sequences are used to encode and decode the signals of interest of asynchronous CDMA users in the same way that orthogonal codes were used in the synchronous system.
PN sequences do not present statistical correlation, and the sum of a large number of PN sequences results in what is called multiple access interference (MAI), which can be estimated as a Gaussian process of noise that follows the central theorem of the statistical limit. If signals from all users are received with equal power, the variance (ie, noise power) of the MAI increases in direct proportion to the number of users. In other words, unlike what happens in synchronous CDMA, the signals of the rest of the users will appear as noise in relation to the signal of interest, and will cause interference with the signal of interest: the more simultaneous users, the greater the interference.
On the other hand, the fact that the sequences are apparently random and of distributed power in a relatively wide bandwidth carries an additional advantage: they are more difficult to detect in case someone tries to capture them, because they are confused by the background noise . This property has been used during the 20th century in military communications.
All types of CDMA take advantage of the processing gain that spread spectrum systems introduce; this gain allows receivers to partially discriminate unwanted signals. The signals encoded with the specified PN code are received, and the rest of the signals (or those with the same code but different delay, due to different arrival paths) are presented as broadband noise that is reduced or eliminated thanks to the processing gain.
As all users generate MAI, it is very important to control the emission power. Synchronous CDMA, TDMA, or FDMA systems can, in theory at least, completely reject unwanted signals (using different codes, time slots, or frequency channels) because of the orthogonality of these media access schemes. But this is not true for asynchronous CDMA; rejection of unwanted signals is only partial. If part (or all) of the unwanted signals are received with much greater power than the desired signal, it cannot be separated from the rest. To avoid this problem, a general requirement in the design of these systems is that the power of all emitters be controlled; it seeks to ensure that the power captured by the receiver is approximately the same for all incoming signals. In systems Cellular telephony, the base station employs a closed-loop power control scheme (fast closed-loop power control, in English) to strictly control the emission power of each telephone.
