OVSFOrthogonal Variable Spreading Factor
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The pilot information of CPICH is used because the receiver already knows the bit sequence of CPICH and the OVSF code of CPICH is always same.
And the fine symbol timing is obtained by the correlation between the interpolated reception signal and the CPICH chip spreaded by OVSF.
Since the final chip rate is constant, fewer OVSF code chips are accommodated within the bit period -- the SF is smaller.
The OVSF codes can be allocated using the code tree shown in Figure 2.
In this tree, OVSF codes of a certain SF are obtained by copying the mother-branch code of the previous SF, and repeating or inverting it.
If a high data rate channel using a code of SF = 4, for example, 1, 1, -1, -1 is selected, all lower data rate channels using longer OVSF codes that start with 1, 1, -1, -1 have to be inactive, because they are not orthogonal.
Careful choice of OVSF codes can lead to lower peak-to-average power ratios.
Even though the OVSF codes were selected to maximize the benefits of HPSK, the HPSK requirements would be completely fulfilled only for the first two DPDCHs.
The code domain power measurement helps not only to verify that each OVSF channel is operating at its proper amplitude, but also to identify problems throughout the transmitter design, from the coding to the RF section.