It optimizes the use of available bandwidth. The technology is commonly used in ultra-high-frequency UHF cellular telephone systems, bands ranging between the MHz and 1. Code Division Multiple Access system is very different from time and frequency multiplexing. In this system, a user has access to the whole bandwidth for the entire duration.
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It optimizes the use of available bandwidth. The technology is commonly used in ultra-high-frequency UHF cellular telephone systems, bands ranging between the MHz and 1. Code Division Multiple Access system is very different from time and frequency multiplexing.
In this system, a user has access to the whole bandwidth for the entire duration. The basic principle is that different CDMA codes are used to distinguish among the different users. Here, a signal is generated which extends over a wide bandwidth. A code called spreading code is used to perform this action.
Using a group of codes, which are orthogonal to each other, it is possible to select a signal with a given code in the presence of many other signals with different orthogonal codes. CDMA allows up to 61 concurrent users in a 1. There are 64 Walsh codes available to differentiate between calls and theoretical limits.
Operational limits and quality issues will reduce the maximum number of calls somewhat lower than this value. In fact, many different "signals" baseband with different spreading codes can be modulated on the same carrier to allow many different users to be supported.
Using different orthogonal codes, interference between the signals is minimal. Conversely, when signals are received from several mobile stations, the base station is capable of isolating each as they have different orthogonal spreading codes. The following figure shows the technicality of the CDMA system. During the propagation, we mixed the signals of all users, but by that you use the same code as the code that was used at the time of sending the receiving side.
You can take out only the signal of each user. This means, CDMA operates in the presence of noise and interference. In addition, neighboring cells use the same frequencies, which means no re-use.
So, CDMA capacity calculations should be very simple. No code channel in a cell, multiplied by no cell. But it is not that simple. Although not available code channels are 64, it may not be possible to use a single time, since the CDMA frequency is the same. CDMA is a spread spectrum technique. Each data bit is spread by a code sequence. This means, energy per bit is also increased.
This means that we get a gain of this. This is a gain factor and the actual data propagation rate. On an average, a typical transmission condition requires a signal to the noise ratio of 7 dB for the adequate quality of voice. Use of same frequency in surrounding cells causes some additional interference. CDMA has a soft capacity. The greater the number of codes, the more the number of users. CDMA requires a tight power control, as it suffers from near-far effect.
In other words, a user near the base station transmitting with the same power will drown the signal latter. All signals must have more or less equal power at the receiver. Rake receivers can be used to improve signal reception. Delayed versions of time a chip or later of the signal multipath signals can be collected and used to make decisions at the bit level. Flexible transfer may be used. Mobile base stations can switch without changing operator.
Two base stations receive mobile signal and the mobile receives signals from the two base stations. The code length must be carefully selected. A large code length can induce delay or may cause interference. As the sum of the power received and transmitted from a base station needs constant tight power control.
This can result in several handovers. Previous Page. Next Page. Previous Page Print Page.
Code-division multiple access
Code-division multiple access CDMA is a channel access method used by various radio communication technologies. CDMA is an example of multiple access , where several transmitters can send information simultaneously over a single communication channel. This allows 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 in , when the young military 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.
CDMA (Code-Division Multiple Access)
CDMA - Technology