OVSFOrthogonal Variable Spreading Factor
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OVSF codes are orthogonal codes similar to the Walsh codes used in 2G CDMA (IS-95) BTS.
The combination of OVSF codes and scrambling codes provides the signal spreading, and therefore the spreading gain needed for the receiver correlators to pull the desired signal out of the noise.
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.
The division of downlink channel is conducted by OVSF codes and the division of cell is conducted by long scrambling codes.
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.
In the case of OVSF codes, LO instability results in higher code domain noise for channels with the same codes as the active code channels but with different I/Q path.
The OVSF codes are generated using the code tree shown in Fig.
The multiplexed data is then sent over the 3GPP physical layer, where the resulting bits are first mapped onto a QPSK signal, spread using an OVSF code, and scrambled using a Gold code [5].