Cellular Network Basics Frequency Division Multiple Access (FDMA); Time Division Multiple Access (TDMA); Code Division Multiple Access (CDMA). 3G CDMA – WCDMA and cdma Rodger E. Ziemer. IEEE Communications Society. Distinguished Lecturer Program. May June1, 2. Rules for. Preface Code division multiple access (CDMA) has proven itself to be a viable technique for enabling the simultaneous transmission and reception of data over .
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CDMA is based around the use of direct sequence spread spectrum techniques. Essentially CDMA is a form of spread spectrum transmission which uses spreading codes to spread the signal out over a wider bandwidth then would normally be required. By using CDMA spread spectrum technology, many users are able to use the same channel and gain access to the system without causing undue interference to each other. Although as the number of fdma increases care has to be taken to ensure that interference levels do not rise to the extent that performance falls, it is still possible to provide access to a large number of different users and allow them access.
Direct sequence spread spectrum is a form of transmission that looks very similar to white noise over the bandwidth of the transmission. However once received and processed with the correct descrambling codes, it is possible to extract the required data. When transmitting a CDMA spread spectrum signal, the required data signal is multiplied with what is known as basicz spreading or chip code data stream.
The resulting data stream has a higher data rate than the data itself. Each bit in the spreading sequence is called a chip, and this is much shorter than each information bit. The spreading sequence or filetpe sequence has the same data rate as the final output from filettype spreading multiplier. The baseband data stream is then modulated onto a carrier and in this way the overall the overall signal is spread over a much wider bandwidth flietype if the data had been simply modulated onto the carrier.
CDMA Tutorial | Code Division Multiple Access Basics |
This is because, signals with high data rates occupy wider signal bandwidths than those with low data rates. To decode the signal and receive the original data, the CDMA signal is first demodulated from the carrier to reconstitute the high speed data stream.
This is multiplied with the spreading code to regenerate the original data. When this is done, then only the data with that was generated with the same spreading code is regenerated, all the other data that is generated from different spreading code streams is ignored. The use of CDMA spread spectrum is a powerful principle and using this CDMA technique, it is possible to transmit several sets of data independently on the same carrier and then reconstitute them at the receiver without mutual interference.
In this way a base station can communicate with several mobiles on a single channel. Similarly several mobiles can communicate with a single base station, provided that in each case an independent spreading code is used.
In order to visualise how the CDMA spread spectrum process operates, the easiest method is to show an example of how the system actually operates in terms of data bits, and how the data is recovered from the CDMA spread spectrum signal. The first part of the process is to generate the CDMA spread spectrum signal. Take as an example that the data riletype be transmitted isand the chip or spreading code riletype For each data bit, the complete spreading code is used to multiple the data, and in this way, for each data bits, the spread or expanded signal consists of four bits.
In this way it can be seen that the original data is recovered exactly by using the same spreading or chip code. Had another code been used to regenerate the CDMA spread spectrum signal, then it would have resulted in a random sequence after de-spreading.
This would have appeared as noise in the system. The spreading code used in this example was only four bits long.
CDMA technology basics tutorial
This enabled the process to be visualised more easily. Commonly spreading codes may be 64 bits, or even bits long to provide the required performance. The bandwidth of the CDMA spread spectrum signal will be much wider than the original data stream.
To quantify the increase in bandwidth, a term known as the spreading gain is used. It is found that the larger the spreading gain of the CDMA spread spectrum signal, the more effective the performance of the system is.
This is because the wanted signal becomes larger. In the example shown above, the spreading gain is four, as seen by the fact that four “1”s are generated for each required data bit.
Data produced by other dispreading codes would appear as noise and can be discarded as it would fletype lower in value.
The principle behind CDMA spread spectrum communications is relatively straightforward. The same code must be sued within generation and decoding of the CDMA spread spectrum signal to enable the data to pass unchanged through the system.
The use of a different code in transmission and reception results in a signal similar in character to noise being generated and this can be discarded. Latest news LoRa technology integrated in utility metering solution Four-in-one environmental sensor saves space and power MACOM debuts new ultra low phase noise amplifier Ericsson and Panasonic Avionics bringing connectivity to the skies Beeline chooses Broadpeak Solutions for TV Everywhere service.
Build More-Effective Smart Devices: Mark Patrick Mouser Electronics Long-range low-power wireless network have the potential to create the Internet of Agricultural Things The application of technology has always driven an increase in productivity, the two are inextricably linked because it is often the filefype for higher productivity that csma the motivation for achieving technological breakthroughs; necessity is the mother of invention, after all.
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