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 [2] . [4] “ 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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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. Each user in a CDMA system uses a different code to modulate their signal. In other projects Wikimedia Commons. A spreading code is a pseudo-random code that has a narrow ambiguity functionunlike other narrow pulse codes. Furthermore, it would require significant overhead to continually allocate and deallocate the orthogonal-code, time-slot or frequency-channel resources.

It was placed in the trunk of the vehicles of high-ranking officials and used a standard handset in the passenger compartment. InKupriyanovich made the new experimental “pocket” model of mobile phone.

GSM and CDMA – How Cell Phones Work | HowStuffWorks

In contrast, CDMA systems use the soft hand-off, which is undetectable and provides a more reliable and higher-quality signal. CDMA is also resistant to jamming. Channel access methods Media access control.

The digital modulation method is technnology to those used in simple radio transceivers. The figure shows how a spread-spectrum signal is generated.

This article is about a channel access method. Channel access methods Code division multiple access Multiplexing Radio resource management Media access control. 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. This is referred to as auto-correlation and is used to reject multi-path interference. These systems were designed using spread spectrum because of its security and resistance to jamming.

How Cell Phones Work

In a recent study, a novel collaborative multi-user transmission and detection scheme called collaborative CDMA [14] has been investigated for the uplink that exploits the differences between users’ fading channel signatures to increase the user capacity well beyond the spreading length in the MAI-limited environment.

CDMA is used as the access method in many mobile phone standards. A rake receiver combines the information from several correlators, each one tuned to a different path delay, producing a stronger version of the signal than a simple receiver with a single correlation tuned to the path delay of the strongest signal. Retrieved from ” https: Like the narrow-band interference, this will result in only a small loss of data and can be overcome.

This is effectively a frequency convolution Wiener—Khinchin theorem of the two signals, resulting in a carrier with narrow sidebands. Asynchronous CDMA offers a key advantage in the flexible allocation of resources i. CDMA can also effectively reject narrow-band interference.

A receiver cannot demodulate this transmission without knowledge of the pseudo-random sequence used to encode the data. Filetupe receiver then extracts an intelligible signal for any known sender by combining the sender’s code with the interference pattern.

Orthogonal codes have a cross-correlation equal to zero; in other words, they do not interfere with each other. Further, after decoding, all values greater than 0 are interpreted as 1, while all values less than zero are interpreted as 0.

These are binary square waves that form a complete orthonormal set. The key difference here is that the bit error probability for Ccdma 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. 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.

Many codes occupy the same channel, but only users associated with a particular code can communicate.

In CDMA cellular, the base station uses a fast closed-loop power-control scheme to tightly control each mobile’s transmit power. Each user is associated with a different code, say v. 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. These vectors will be assigned to individual users and are called the fieltypechip codeor chipping code.

The guard bands will reduce the probability that adjacent channels will interfere, but decrease the utilization of the spectrum.

Code-division multiple access

An analogy to the problem of multiple access is a room channel in which people wish to talk to each filletype simultaneously. Soft hand-offs allow the mobile telephone to communicate simultaneously with two or more cells.

In general, CDMA belongs to two basic categories: By comparison, asynchronous CDMA transmitters simply send when they have something to say etchnology go off the air when they don’t, keeping the same PN signature sequence as long as they are connected to the system.

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. Frequency reuse is the ability to reuse the same radio channel frequency at other cell sites within a cellular filletype. One of the initial reasons for doing this was military applications including guidance and communication systems.

Code-division multiple access – Wikipedia

Another reason CDMA is resistant to multipath interference is because the delayed versions of the transmitted pseudo-random codes will have poor correlation with the original pseudo-random code, and will thus appear as another user, which is ignored 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.

The authors show that it is possible to achieve this increase at a low complexity and high bit error rate performance in flat fading channels, which is a major research challenge for overloaded CDMA systems. Proceedings of the Leningrad Experimental Institute of Communication: This phone weighed 0. CDMA is a spread-spectrum multiple-access [8] technique. Choosing the codes used to modulate the signal is very important in the performance of CDMA systems. Since adjacent cells use the same frequencies, CDMA systems have the ability to perform soft hand-offs.