Some GSM, UMTS and LTE Measurement Units

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:

GSM

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.

UMTS

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

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).

4 thoughts on “Some GSM, UMTS and LTE Measurement Units”

1. David Boettger says:

A couple of corrections. It is widely believed that Ec/N0 is an expression of signal-to-noise ratio and is therefore an indicator of signal quality. It is not. N0 consists of three things: interfering (non-orthogonal) power, thermal noise, and, crucially, non-interfering (orthogonal) power. In situations where N0 consists mainly of “in-cell” power (e.g., during HS-DSCH bursts) Ec/N0 can be “low”, whereas the objective signal quality is high.

Further, Ec is an expression of power in the CPICH, not in the downlink DPxCH. Though the dynamic range of downlink DPxCH power is often expressed relative to the CPICH, downlink power control will do its best to ensure that downlink SNR is whatever it needs to be to meet the transport channel quality target. The point being: The traffic channel SNR has almost nothing to do with pilot Ec/N0.

Regarding RSRQ: This measurement is more like SNR than Ec/N0, since the denominator does not include orthogonal/non-interfering power.

David Boettger

2. Hi David,

many thanks for that, it’s very much appreciated!

I’ve adapted the text of the post to take your comments into account.

Kind regards,
Martin

3. Mark says:

David’s comment is interesting, but the specs assume that Ec/N0 is a measure of signal quality. That is why handovers + reselections are usually triggered by low values of Ec/N0. The fact that perhaps the N0 is caused by non-interfering orthogonal noise, is irrelevant really as the UE will not make any distinction.

4. Vinh says:

Hi guys,

Could you plz explain a little bit more about the difference between Ec/I0 and Ec/N0.

Is that right that N0 = I0 + thermal noise?

Thanks a lot