CDMA or Code Division Multiple Access is now in widespread use for mobile or cell phone (cellular telecommunications) systems around the world. It was first used for the IS-95 mobile phone system also known by the trade name cdmaOne, and in its later 3G developments as CDMA2000. CDMA is also being used in the other major 3G cell phone system, Wideband-CDMA system originally called UMTS.
CDMA technology is based on a form of transmission known as Direct Sequence Spread Spectrum (DSSS). This form of transmission originally used for military and police communications because the transmissions were difficult to detect in many instances, and even if they were received they were very difficult to decipher without the correct codes. However the possibilities of using this technology to provide a multiple access scheme for mobile telecommunications and have now been exploited in a major way.
Previous cellular telecommunications technologies used either frequency division multiple access (FDMA) where different users were allocated different frequencies, or time division multiple access (TDMA) where they were allotted different time slots on a channel. CDMA is different. Using the CDMA system, different users are allocated different codes to provide access to the system. It can be likened to many different people standing in a room talking to others in many different languages. Although the ambient noise level is very high, it is nevertheless still possible to pick out someone speaking in the same language as yourself.
DSSS basics
The key element of code division multiple access CDMA is its use of DSSS. In essence the required data signal is multiplied with what is known as a spreading or chip code data stream. This has a higher data rate than the data itself and it enables the overall signal to be spread over a much wider bandwidth. 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 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
This is a powerful principle and using code division multiple access 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.
The CDMA spreading codes can either be a random number (or pseudo random), or more usually orthogonal codes are used. Two codes are said to be orthogonal if when they are multiplied together and then the result is added over a period of time they sum to zero. For example a codes 1 -1 -1 1 and 1 -1 1 -1 when multiplied together give 1 1 -1 -1 which gives the sum zero. Although pseudo random number codes can be used there is possibility of data errors being introduced into the system.
Advantages
There are several advantages to using code division multiple access CDMA. The main reason for its acceptance is that it enables more users to use a given amount of spectrum. Its use also enables adjacent base stations to operate on the same channel, allowing more efficient use of the spectrum and it provides for an easier handover.
In view of these advantages CDMA has been adopted for all the 3G technologies and will be around for many years to come.
CDMA technology is based on a form of transmission known as Direct Sequence Spread Spectrum (DSSS). This form of transmission originally used for military and police communications because the transmissions were difficult to detect in many instances, and even if they were received they were very difficult to decipher without the correct codes. However the possibilities of using this technology to provide a multiple access scheme for mobile telecommunications and have now been exploited in a major way.
Previous cellular telecommunications technologies used either frequency division multiple access (FDMA) where different users were allocated different frequencies, or time division multiple access (TDMA) where they were allotted different time slots on a channel. CDMA is different. Using the CDMA system, different users are allocated different codes to provide access to the system. It can be likened to many different people standing in a room talking to others in many different languages. Although the ambient noise level is very high, it is nevertheless still possible to pick out someone speaking in the same language as yourself.
DSSS basics
The key element of code division multiple access CDMA is its use of DSSS. In essence the required data signal is multiplied with what is known as a spreading or chip code data stream. This has a higher data rate than the data itself and it enables the overall signal to be spread over a much wider bandwidth. 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 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
This is a powerful principle and using code division multiple access 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.
The CDMA spreading codes can either be a random number (or pseudo random), or more usually orthogonal codes are used. Two codes are said to be orthogonal if when they are multiplied together and then the result is added over a period of time they sum to zero. For example a codes 1 -1 -1 1 and 1 -1 1 -1 when multiplied together give 1 1 -1 -1 which gives the sum zero. Although pseudo random number codes can be used there is possibility of data errors being introduced into the system.
Advantages
There are several advantages to using code division multiple access CDMA. The main reason for its acceptance is that it enables more users to use a given amount of spectrum. Its use also enables adjacent base stations to operate on the same channel, allowing more efficient use of the spectrum and it provides for an easier handover.
In view of these advantages CDMA has been adopted for all the 3G technologies and will be around for many years to come.
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