RSCP & Ec/Io
Posted February 3, 2011on:
The “Received Signal Code Power” (RSCP) is the collected RF energy after the correlation / descrambling process, usually given in dBm. Because this process already “filters out” the signal with the correct code (the code meant for the specific UE), the RSCP can not be calculated back to the total received RF power that a normal monitoring receiver or spectrum analyzer measures. Instead, a correlation receiver has to be used and the RSCP has to be measured for the specific code only, in the code domain. Only this code power is of interest for the following receiver stages when judging on the quality of the reception.
A commercial UMTS receiver has to know the code that is transmitted for it in order to perform the correlation process. With monitoring equipment, however, we want to measure UMTS emissions with any code. Therefore, special measurement receivers and equipment are necessary for UMTS measurements. These receivers have to try and correlate the received pseudo-noise-like signal with every possible code. This process is called PN scanning. Only after the receiver has found a match, the descrambling can take place, followed by measurement of the RSCP in the code domain.
This is the ratio of the received energy per chip (= code bit) and the interference level, usually given in dB. In case no true interference is present, the interference level is equal to the noise level. However, in a UMTS network the UE normally receives signals from multiple base stations, all transmitting on the same frequency. Therefore it is possible that even at a location close to a base station, with a high RSCP, no logon is possible, due to high interference levels from a second nearby base station. This effect is called “pilot pollution” and network planners try to avoid too close spacing of base stations to minimize regions where it can occur.
Because also the chip energy can only be measured after the descrambling in the code domain, the same special measurement equipment is needed as described above.
The Received Signal Strength Indicator (RSSI) is a value that takes into account both RSCP and Ec/I0. It is usually given in dBm and can be calculated as follows: RSSI [dBm] = RSCP [dBm] – Ec/I0 [dB]
As with RSCP and Ec/I0, it can only be measured in the code domain and needs the special monitoring equipment as described above.
Looking at the definitions of the three parameters RSCP, Ec/I0 and RSSI above, the RSSI seems to be the most sensitive parameter to observe. However, UMTS network planners mostly design their network to provide certain RSCP and Ec/I0 values.
The “signal-to-interference-ratio” ( Ec/I0) in UMTS is a fixed value and set by physical constraints of the correlation process. Inside well covered regions, the interference part is dominated by the receive level of nearby other UMTS stations. Therefore, the minimum RSCP can be much higher as the receiver sensitivity in these areas. However, at the rim of the coverage area it can be assumed that the serving base station is the last in reach of the mobile and therefore the only emission on the frequency. In this case, the receiver noise takes over the part of “interference” and the Ec/I0 becomes equal to the signal-to-noise ratio. Consequently, from the measurements shown above it can be assumed that the minimum RSSI of -106 dBm is the true receiver sensitivity.
It has been found that the following values just allow to log onto the network:
- Ec/I0 > -9 (maximum value in Fig. 4)
- RSCP > -114 dBm (minimum value at Ec/I0 of -9 dB in Figure 5, including a fading margin of 1 dB between actual login and measurement)
- RSSI > -106 dBm (minimum value at Ec/I0 of -9 dB in Figure 5)
Note that all values are levels at the receiver input (or after the descrambling process) and not field strength. In UMTS network planning this is an agreed practice and all prediction tools are also based on input level. It is assumed that a standard matched monopole antenna is used. The height of the receiving antenna is assumed to be at 2 m above ground (car roof level).