Even when a mobile device just sits there and seemingly doesn't doesn't do a whole lot it is nevertheless quite busy every now and then internally to check the signal strength and quality of the current cell and those around it to make sure it never looses contact to the network even if the user roams around. I've recently came across a good description of which values are actually measured in this book and thought I'd put together an ultra brief summary:
Here, the world is still quite simple. Once the mobile has found a cell it keeps monitoring it's received signal power and computes the Received Signal Strength Indicator (RSSI) over the complete carrier frequency (200 kHz). Neighboring cells use different frequencies and the same value is calculated for those as well.
Here, things get a bit more complicated as all cells use the same frequency. As a result not only the received signal strength is important but also the general noise level on the carrier. In the end, what good does it do to you when the overall signal level is strong but most of it is interference? That's why there are three values:
- RSCP: The Received Signal Code Power: That's the power level the pilot channel of a cell is received with and usually expressed in dBm (mW on a logarithmic scale). With this parameter, different cells using the same carrier can be compared and handover or cell reselection decisions can be taken.
- RSSI: In UMTS that's the signal power over the complete 5 MHz carrier which includes all components received, including the signals from the current and neighboring cells on the same frequency.
- EcNo: That's the received energy per chip (Ec) of the pilot channel divided by the total noise power density (No). In other words the EcNo is the RSCP divided by the RSSI. And again in other words: The better this value the better can a signal of a cell be distinguished from the overall noise. The EcNo is usually expressed in dB as it's a relative value. The value is negative as the RSCP is smaller than the total received power. As the RSCP this value can be used to compare different cells on the same carrier and handover or cell reselection decisions can be taken.
LTE networks also use the same carrier frequency in neighboring cells so again the signal level and interference are important:
- RSRP: That's the Received Power of the Reference Signal.
- RSSI: That's the total received power including Interference from neighboring cells and other sources.
- RSRQ: That's the Reference Signal Received Quality and equals the RSRP divided by the RSSI. The better this value the better can the signal of the cell be received against the interference generated by other cells.
Quite a lot of similarities between UMTS and LTE from a high level point of view even though the values are measured in quite a different way due to the different air interface natures (WCDMA vs. OFDMA).