- Transmitted by every cell in the network.
- Does not carry any information, only a predefined pilot sequence identical for all cells in the network.
- The 2 stage coding process is applied to this predefined pilot sequence.
- The CPICH acts as a kind of reference channel used by the UE to assess quality and signal strength of a cell.
The figure above shows the structure of the CPICH, the Common Pilot Channel. This physical channel is transmitted by every cell in the network, and acts as a kind of reference channel used by the UE to assess quality and signal strength of a cell.
The CPICH does not carry any information, but contains a predefined pilot sequence identical for all cells in the network. The two stage coding process is applied to this predefined pilot sequence, where the top code with SF 256 is used together with a cell specific Scrambling Code.
Since the first OVSF code with SF256 is used by the CPICH, all the OVSF codes on the way to the root are excluded for downlink usage as they do not provide orthogonality anymore. This is the reason why the top OVSF codes from the code tree cannot be used anymore to carry downlink traffic.