SPREADING CODES
There are two broad categories of code used, namely channelisation codes and scrambling
codes :
-channelisation codes are used to distinguish between the different physical channels on the uplink and downlink paths.
-Scrambling codes are used to distinguish between different Node Bs on the downlink and different UEs on the uplink.
distinguish between different Node Bs on the downlink and different UEs on the uplink.
The channelisation codes used in UTRA FDD are known as orthogonal variable spreading factor
(OVSF) codes and they are essentially a set of Walsh codes of different lengths. Walsh codes have the
attractive property that, when they are synchronised in time, they are orthogonal. In other words, if you take two different Walsh codes of equal length and multiply them together and sum over the length of the Walsh code, the result will be zero, provided the two codes are time aligned. To demonstrate this property, let us take two Walsh codes each of eight chips in length and multiply them together chip-by-chip, If we sum the resulting chip sequence we get a value of zero.
Visit our Facebook Page
The implication of this orthogonality property is that if we assign two users different Walsh codes as
their spreading codes, then we can completely remove the signal (or interference) from one user as we de-spread the signal from the other user. This is the reason why Walsh codes are used to differentiate between different channels in the UTRA FDD system. In the UTRA FDD system, the different user data rates are supported by using Walsh codes of different lengths and these are formed based on a simple tree structure whereby each code of length N can be used to form two further codes of length 2N.
where cch x y represents the y-th channelisation code of length x chips, or spreading factor (SF) of x. The process essentially consists of taking the base channelisation code and copying it twice to form one new channelisation code of twice the length and performing the same process, but inverting the second copy of the code to get the second new channelisation code.
Visit our Facebook Page
in the above chart as an example ,We start with cch 8 4 (i.e. the fourth channelisation code that is eight chips in length). This is copied twice to produce cch 16 8 (i.e. the eighth channelisation code that is 16 chips in length). The code is copied once, inverted and copied again to produce cch 16 9.
In the UTRA FDD system, spreading codes with lengths of four chips up to 512 chips can be used
in powers of two and, therefore, the code ‘tree’ has eight sets of branches with the first set of branches consisting of four four-chip codes, the second set of branches consisting of eight eight-chip codes and so on until we reach the eighth set of branches with 512 512-chip codes
This means that we cannot use every code in the code tree at the same time and we have a set of
rules that govern which codes can be used simultaneously. A code can only be used if none of the
codes in the path from the code to the root of the code tree are already in use and none of the codes
in the sub-tree below the code are in use.
Visit our Facebook Page