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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Now, 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.
Although mutual orthogonality is the only condition, these vectors are usually constructed for ease of decoding, for example columns or rows from Walsh matrices. 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.
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. Views Read Edit View history.
technooogy A receiver cannot demodulate this transmission without knowledge of the pseudo-random sequence used to encode the data. Since the spread-spectrum signal occupies a large bandwidth, only a small portion of this will undergo fading due to multipath at any given time. A spreading code is a pseudo-random code that has a narrow ambiguity functionunlike other narrow pulse codes.
Code-division multiple access
CDMA can also effectively reject narrow-band interference. For instance, if there are N time slots filetyle 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. Fundamentals and Applications Second ed. This phone weighed 0. In cdka projects Wikimedia Commons. Channel access methods and media access control.
Proceedings of the Leningrad Experimental Institute of Communication: If the dot product is zero, the two vectors are said to be orthogonal to each other.
These vectors will be assigned to individual users and are called the codechip codeor chipping code. The frequencies used in different cells must be planned carefully to ensure signals from different cells do not interfere with each other. An analogy to the problem of multiple access is a room channel in which people wish to talk to each other simultaneously.
Direct-sequence spread spectrum and near-far problem.
The best signal quality is selected until the hand-off is complete. An example of orthogonal functions is shown in the adjacent picture.
See also Digital communication Modulation Statistical multiplexing Waveform. Soft hand-offs allow the mobile telephone teechnology communicate simultaneously with two or more cells.
The following table explains how this works and shows that the signals do not interfere with one another:. For example, binary string is represented by the vector 1, 0, 1, 1. In CDMA a locally generated code runs technoolgy a much higher rate than the data to be transmitted.
The following example demonstrates how each user’s signal can be encoded and decoded. Synchronous CDMA exploits mathematical properties of orthogonality between vectors representing the data strings. When the receiver attempts to decode the signal using sender1’s code, the data is all zeros, therefore the cross-correlation cdmw equal to zero and it is clear that sender1 did filetgpe transmit any data.
Similarly, in radio CDMA, each group of users is given a shared code. Channel access methods Code division multiple access Multiplexing Radio resource management Media access control.
Code-division multiple access – Wikipedia
CDMA is a spread-spectrum multiple-access  technique. Since narrow-band interference affects only a small portion of the spread-spectrum signal, it can easily be removed through notch filtering without much loss of information. Since each of the 64 Walsh codes is orthogonal to all other, the signals are channelized into 64 orthogonal signals. These systems were designed using spread spectrum because of its security and resistance to jamming.
In a hard-hand-off situation, as the mobile telephone approaches a hand-off, signal strength may vary abruptly. Technnology encoding and interleaving can be used filetypw assist in recovering this lost data.
Many codes occupy the same channel, but only users associated with a particular code can communicate. This leads to a general requirement in any asynchronous CDMA system to approximately match the various signal power levels as seen at the receiver.
Gold codes are an example of a PN suitable for this purpose, as there is low correlation between the codes. The guard bands will reduce the probability that adjacent channels will interfere, but decrease the utilization of the spectrum. In general, CDMA belongs to two basic categories: CDMA tedhnology used as the access method in many mobile phone standards. Cdmx codes have a cross-correlation equal to zero; in other words, they do not interfere with each other.
Circuit mode constant bandwidth. Values of exactly 0 means that the sender did not transmit any data, as in the following example:. Reusing the same frequency in every cell eliminates the need for frequency planning in a CDMA system; however, planning of the different pseudo-random sequences must be done to ensure that the received signal from one cell does not correlate with the signal from a nearby cell.
The following table shows the decode at the receiver:. This is not true for asynchronous CDMA; rejection of unwanted signals is only partial. Since adjacent cells use the same frequencies, CDMA systems have the ability to perform soft hand-offs.
A spread-spectrum technique spreads the bandwidth of the data uniformly for the same transmitted power.