CPICH Ec/No and RSCP
One way of estimating the pathloss is the
received signal code power (RSCP) of the CPICH. However the CPICH RSCP does not
take any interference into consideration. The Ec/No of the CPICH is a measurement that
takes both the pathloss and the interference situation into consideration.
The CPICH Received Signal Code Power (CPICH RSCP) is dependent on the CPICH
transmitted code power, the pathloss. Since the transmitted power on the CPICH is
constant the CPICH RSCP will primarily be affected by the pathloss. Hence a decrease in
the CPICH RSCP will mean that the pathloss in both up and downlink has increased. The
absolute accuracy requirement for the CPICH RSCP measurements is 6-8 dB .
The CPICH Ec/No is defined as the energy per chip divided by the total in-band
interference. Theoretically it is defined as the CPICH RSCP divided by the RSSI (received
signal strength indicator).
CPICH Ec/No = CPICH RSCP / RSSI
Both the CPICH RSCP and the CPICH Ec/No will increase with a decrease in the pathloss.
However the Ec/No is both proportional to the pathloss and inversely proportional to the
RSSI. Thus, the relation between CPICH Ec/No and pathloss is not linear as in the RSCP
case. Particularly at the cell border in cases where there is mainly one dominant server the
behaviour of the CPICH Ec/No is not completely straightforward.
The figure above describes the principle of the scenario previously mentioned. As can be
seen the CPICH Ec/No remains relatively constant since both the pilot signal and the
interfering signals are more or less subject to the same pathloss. Once the thermal noise
becomes the dominant part of the RSSI the CPICH Ec/No also starts to degrade but not as
steep as the RSCP degradation. When the signal finally becomes lower than the thermal
noise floor the Ec/No will drop sharply. This description of the scenario is highly simplified.
The advantage with the CPICH Ec/No measurement is that it takes into consideration the
downlink interference situation in the network through its dependency on the RSSI.
The absolute accuracy requirement for the CPICH Ec/No measurements is 1.5-3 dB,
which is considerably higher than the requirements for the RSCP