Wide band CDMA technology is selected for UTRAN air interface. WCDMA; TD- SCDMA. Base stations are referred to as Node-B and control equipment for. 3G CDMA – WCDMA and cdma W-CDMA Versus cdma  .  “ CDMA Evolution from IS, IS, to 1XTREME,” Technology Transfer Training. GSM Technology. GSM is now a worldwide standard. GSM uses Time Division Multiple Access or TDMA technology as their air interface standard. TDMA has.
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Code-division multiple access CDMA is a channel access method used by various radio communication technologies.
CDMA is an example of multiple accesswhere several transmitters can send information simultaneously over a single communication channel. This cdmw several users to share a band of frequencies see bandwidth.
To permit this without undue interference between the users, CDMA employs spread spectrum technology and a special coding scheme where each transmitter is assigned a code. CDMA is used as the access method in many mobile phone standards. The technology of code-division multiple access channels has long been known. The technology of CDMA was used inwhen the young etchnology radio engineer Leonid Kupriyanovich in Moscow made an experimental model of a wearable automatic mobile phone, called LK-1 by him, with a base station.
InKupriyanovich made the new experimental “pocket” model of mobile phone. This phone weighed 0. To serve more customers, Kupriyanovich proposed the device, which he called “correlator. It was placed in the trunk of the vehicles of high-ranking officials and used a standard handset in the passenger compartment. CDMA is a spread-spectrum multiple-access  technique.
A spread-spectrum technique spreads the bandwidth of the data uniformly for the same transmitted power. A spreading code is a pseudo-random code that has a filteype ambiguity functionunlike other narrow pulse codes. In CDMA a locally generated code runs at a much higher rate than the data to be transmitted. The figure shows how a spread-spectrum signal is generated. Each user in a CDMA system uses a different code to modulate their signal. Choosing the codes used to modulate the signal is very important in the performance of CDMA systems.
The separation of the signals is made by correlating the received filetyppe with the locally generated code of the desired user. If the signal matches the desired user’s code, then the correlation function will be high and the system can extract that signal. If the desired user’s code has nothing in common with the signal, the correlation should be as close to zero as possible thus eliminating the signal ; this is referred to as cross-correlation.
If the code is correlated with the signal at any time offset other than zero, the correlation should be as close to zero as possible. This is referred to as auto-correlation and is used to reject multi-path interference. An analogy to the problem of multiple access is a room channel in which people wish to talk to each other simultaneously. To avoid confusion, people could take turns speaking time divisionspeak at different pitches frequency divisionor speak in different languages code division.
CDMA is analogous to the last technologg where people speaking the same language can understand each other, but other languages are perceived as noise and rejected. Similarly, in radio CDMA, each group of users is given a shared code.
GSM and CDMA – How Cell Phones Work | HowStuffWorks
Many codes occupy the same channel, but only users associated with a particular code can communicate. In general, CDMA belongs to two basic categories: The digital modulation method is analogous to those used in simple radio transceivers. In the analog case, a low-frequency data signal is time-multiplied with a high-frequency pure sine-wave carrier and transmitted.
This is effectively a frequency convolution Wiener—Khinchin theorem of the two signals, resulting in a carrier with narrow sidebands. In the digital case, the sinusoidal carrier is replaced by Walsh functions. These are binary square waves that form a complete orthonormal set. The data signal is also binary and the time multiplication is achieved with a simple XOR function.
This is usually a Gilbert cell mixer in the circuitry. Synchronous CDMA exploits mathematical properties of orthogonality between vectors representing the data strings. For example, binary string is represented by the vector 1, 0, 1, 1. If the dot product is zero, the two vectors are said to be orthogonal to each other. Each user in synchronous CDMA uses a code orthogonal to the others’ codes to modulate their signal. An example of 4 mutually orthogonal digital signals is shown in the figure below.
Orthogonal codes have a cross-correlation equal to zero; in other words, they do not interfere with each other. In the case of IS, bit Walsh codes are used to encode the signal to separate different users. Since each of the 64 Walsh codes is orthogonal to all other, the signals are channelized into 64 orthogonal signals. The following example demonstrates how each user’s signal can be encoded and decoded.
Start with a set of vectors that are mutually orthogonal. Although mutual orthogonality is the only condition, these vectors are usually constructed for ease of decoding, for example columns or rows from Walsh matrices.
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An example of orthogonal functions is shown in the adjacent picture. These vectors will be assigned to individual users and are called the codechip codeor chipping code. In the interest of brevity, the rest of this example uses codes v with only two bits. Each user cxma associated with a different code, say v. Each sender has a different, unique vector v chosen from that set, but the construction method of the transmitted vector is identical.
Tecnology, due to physical properties of interference, if two signals at a point are in phase, they add to give twice the amplitude of each signal, but if they are out of phase, they subtract and give a signal that is the difference of the amplitudes.
Digitally, this behaviour can be modelled by the addition of the transmission vectors, component by component. Because signal0 and signal1 are transmitted at the same time into the air, they add to produce the raw signal. This raw signal is called an interference pattern.
The receiver then extracts an intelligible signal for any known sender by combining the ciletype code with the interference pattern. The following table explains how this works and shows that the signals do not interfere with one another:. Further, after decoding, all values greater than 0 are interpreted as 1, while all values less than zero are interpreted as 0.
Values of exactly 0 means that the sender did not transmit any data, as in the following example:. The following table shows the decode at the receiver:.
When the receiver attempts to decode the signal using sender1’s code, the data is all zeros, therefore the cross-correlation is equal to zero and it is clear that sender1 did not transmit any data. When mobile-to-base links cannot be precisely coordinated, particularly due to the mobility of the handsets, a different approach is required.
Since it is not mathematically possible to create signature sequences that are both orthogonal for arbitrarily random starting points and which make full use of the code space, unique “pseudo-random” or “pseudo-noise” PN sequences are used in asynchronous CDMA systems. A PN code is a binary sequence that appears random but can be reproduced in a deterministic manner by intended receivers.
These PN sequences are statistically uncorrelated, and the sum of technopogy large number of PN sequences results in multiple access interference MAI that is approximated by a Gaussian noise process following the central limit theorem in statistics. Gold codes are an example of a PN suitable for this purpose, as there is fileyype correlation between the codes.
If all of the users are received with the same power level, then the variance e. In other words, unlike synchronous CDMA, the signals of other users will appear as noise to the signal of interest and interfere slightly with the desired signal in proportion to number of users.
All forms of CDMA use spread-spectrum process gain to allow receivers to partially discriminate against riletype signals. Signals filegype with the specified PN sequence code are received, while signals with different codes or the same code but a different timing offset appear as wideband noise cda by the process gain. This is not true for asynchronous CDMA; rejection of unwanted signals is only partial. If any or all of the unwanted signals are much stronger than the desired signal, they cdka overwhelm it.
This leads to a general requirement in any asynchronous CDMA system to approximately match the various signal power levels as seen at the receiver. In CDMA cellular, the base station uses a fast closed-loop power-control scheme to tightly control each mobile’s transmit power.
TDMA systems must carefully synchronize the transmission times of all the users to ensure that they are received in the correct time slot and do not cause interference.
Since this cannot be perfectly controlled in a mobile environment, each time slot must have a guard time, which reduces the probability that users will interfere, but decreases the spectral tedhnology.
How Cell Phones Work
Similarly, FDMA systems must use a guard band between adjacent channels, due to the unpredictable Doppler shift of the signal spectrum because of user mobility. The guard bands will reduce the probability that adjacent channels will interfere, but decrease the utilization of the spectrum. Asynchronous CDMA offers a key advantage in the flexible allocation of resources i.
There are a fixed number of orthogonal codes, time slots or frequency bands that can be allocated for CDM, TDMA, and FDMA systems, which remain underutilized due to the bursty nature of telephony and packetized data transmissions. There is no strict limit to the number of users that can be supported in an asynchronous CDMA system, only a practical limit governed by the desired bit error probability since the SIR signal-to-interference ratio varies inversely with the number of users.
In a bursty traffic environment like mobile telephony, the advantage afforded by asynchronous CDMA is that the performance bit error rate is allowed to fluctuate randomly, with an average value determined by the number of users times the percentage of utilization.
Suppose there are 2 N users that only talk half of the time, then 2 N users can be accommodated with the same average bit error probability as N users that talk all of the time. The key difference here is that the bit error probability for N users talking all of the time is constant, whereas it is a random quantity with the same mean for 2 N users talking half of the time.
In other words, asynchronous CDMA is ideally suited to a mobile network where large numbers of transmitters each generate a relatively small amount of traffic at irregular intervals. For instance, if there are N time slots in a TDMA system and 2 N users that talk half of the time, then half of the time there will be more than N users needing to use more than N time slots.
Furthermore, it would require significant overhead to continually allocate and deallocate the orthogonal-code, time-slot or frequency-channel resources. By comparison, asynchronous CDMA transmitters simply send when they have something to say and go off the air when they don’t, keeping the same PN signature sequence as long as they are connected to the system.
Most modulation schemes try to minimize the bandwidth of this signal since bandwidth is a limited resource. However, spread-spectrum techniques use a transmission bandwidth that is several orders of magnitude greater than the minimum required signal bandwidth.
One of the initial reasons for doing this was military applications including guidance and communication systems. These systems were designed using spread spectrum because of its security and resistance to jamming. Asynchronous CDMA has some level of privacy built in because the signal is spread using a pseudo-random code; this code makes the spread-spectrum signals appear random or have noise-like properties.
A receiver cannot demodulate this transmission without knowledge of the pseudo-random sequence used to encode the data.
CDMA is also resistant to jamming. A jamming flietype only has a finite amount of power available to jam the signal. The jammer can either spread its energy over the entire bandwidth of the signal or jam only part of the entire signal. CDMA can also effectively reject narrow-band interference.