Codes within is-95 cdma
A CDMA system is based around Direct Sequence Spread Spectrum techniques. Here the function of the system revolves around the use of the orthogonal chip or spreading codes.
Forms of codes:
There are two forms of codes used within IS-95 which are used in slightly different ways:
- Walsh codes: The Walsh Codes used within IS-95 are a fixed 64 bit length. This provides for a maximum of 64 Individual physical channels to be carried by the system. However the number of usable channels is more usually limited by the level of interference that results from the number of active channels and their data within the cell, and also interference from adjacent cells.
PN codes: The pseudo-random or PN code is used in assembling the IS-95 air interface signal. They consist of a series of bits that have random properties. Three PN codes are used within IS-95, two short codes and one long one.
These codes are used because it is found that if one code is time shifted then the code becomes uncorrelated with the same upshifted code. This means that the same code can be used twice, but time shifted to provide two uncorrelated sequences. In fact when used in the network, the codes are synchronized by GPS timing signals.
In order to utilize the PN codes in IS-95, an approach using a mask is adopted. Codes transmitted at a different time offset are uncorrelated, and therefore by adopting a known given set of offsets it is possible to create a set of "codes" that can be used. The short codes are 32768 bits long and offset by 64 bits. This gives a total of 512 offsets that can be used within IS-95. As the short code is transmitted at the CDMA data rate of 1.2288 Mbps this equates to a full cycle time of 26.667 mS.