RSCP and Eb/No in WCDMA

RSCP is the CPICH chip power at receiver - it tells the path loss
EC/No is the Chip power to Noise ratio at receiver - it tells not only path loss, but also interference

On WCDMA - uplink and downlink has some essential difference:

Uplink:

For the W-CDMA up link, the interference is received at the Node B, and is thus the same for all links in that cell. Ideally then, the same received power at the Node B would be required for all links with the same SIR target. The received interference level will vary between the cells, but since the number of UEs can be expected to be large compared to the number of cells, and due to the similar placements of Node B antennas, this variation is relatively small. At low to medium loads, the noise rise at the Node B is typically between 0-5 dB, to be compared with the at least 50-60 dB range of the path loss. This means that the UE output power mainly will be determined by the path loss to the Node B.

Since uplink coverage is limited by the UE output power, the uplink quality will be strongly dependent on the path loss and hence on the CPICH RSCP.

downlink:

The W-CDMA downlink differs from the uplink in many ways.

• In the down link, the interference is received at the terminal, and is thus different for different locations within the network. The interference level is significantly higher at the cell border than at the cell centre, due to other cell interference. At the cell border, the other cell interference may be as much as three times larger than the interference from the home cell.
• Another aspect is that all the downlink power transmitted in the serving cell, except the own signal, will be recieved as interference. This interference will be subject to exactly the same coupling loss at every instant as the signal. The effect of this own cell interference will be reduced by the OVSF codes, which ideally cancel all own cell interference (except from P-SCH and S-SCH). Under realistic conditions the OVSF codes cancel about 50% of the interfering own cell power.
• Further, there is a floor of interference caused by the non-power controlled control channels, such as CPICH, CCPCH, P-SCH, S-SCH and so on. This means that even at low load there is a substantial amount of interference, many, many times larger than the receiver noise power.
• The total available transmission power is normally at least 100 times larger at the Node B than in the terminal.

As a consequence of these facts, the link power for the down link will not be proportional to the path loss between serving cell and terminal. Instead, the downlink power always has to match the interference level, which due to own cell interference is large even close to the cell centre.

Conclusion:

RSCP is suitable for Uplink quality evaluation, but EC/No is suitable for downlink quality.