CDMA Orthogonal Spreading Codes
- an overview of the CDMA spreading codes and CDMA coding techniques used.
The CDMA orthogonal spreading codes are one of the major elements within the whole CDMA system. The CDMA orthogonal spreading codes are combined with the data stream to be transmitted in such a way that the bandwidth required is increased and the benefits of the spread spectrum system can be gained.
The CDMA codes are specific to each channel / user so that the different users can gain access to the system and communicate as required.
CDMA codes and correlation
The concept of CDMA is based around the fact that a data sequence is multiplied by a spreading code or sequence which increases the bandwidth of the signal. Then within the receiver the same spreading code or sequence is used to extract the required data. Only when the required code is used, does the required data appear from the signal.
The process of extracting the data is called correlation. When a code exactly the same as that used in the transmitter is used, then it is said to have a correlation of one and data is extracted. When a spreading code that does not correlate is used, then the data will not be extracted and a different set of data will appear. This means that it is necessary for the same spreading code to be used within the transmitter and receiver for the data to be extracted.
CDMA code types
There are several types of codes that can be used within a CDMA system for providing the spreading function:
- PN codes: Pseudo-random number codes (pseudo-noise or PN code) can be generated very easily. These codes will sum to zero over a period of time. Although the sequence is deterministic because of the limited length of the linear shift register used to generate the sequence, they provide a PN code that can be used within a CDMA system to provide the spreading code required. They are used within many systems as there is a very large number that can be used.
A feature of PN codes is that if the same versions of the PN code are time shifted, then they become almost orthogonal, and can be used as virtually orthogonal codes within a CDMA system.
- Truly orthogonal codes: Two codes are said to be orthogonal if when they are multiplied together 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. An example of an orthogonal code set is the Walsh codes used within the IS95 / CDMA2000 system.
By Ian Poole
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