How does a GSM phone measure RxLev (RSSI)?

In GSM phone, RxLev(RSSI) would be measured by the phone.
Can someone tell me what way the phone measure it ?
What circuit or what concept ?
Thanks a lot.

Between IQ demodulator and ADC is a Peak Detector that reports RSSI (RX_Lev in GSM). This signal is used also to control the AGC.
Below application note from AD is an example, but there are multiple topologies:
http://www.analog.com/UploadedFiles/...79804AN502.pdf

Hi vfone,
In GSM phone ,high RxLev means high input cell power.

If RxLev is detected by peak detector in transceiver,
when I do co-channel interference test, the Rxlev will increase even
though the BER has degraded?

Thanks.

-RxLev is reporting the in-channel input level.
-Co-channel Interferer it will make RxLev to report higher level. In GSM because the spec put the interferer 9dB below the desired, RxLev is not too much affected by this.
-DSP (that is located after Detector and ADC) has the duty to separate the co-channel signal from the desired signal, making BER to don?t be affected by the interferer.
-The Co-channel Rejection Test is a 100% DSP algorithm test.

Thanks, vfone.
I got another question and please advise.
In blocking test, RxLev decreased so it means there is in-band interference
to degrade BER(SNR).

Where this interference comes & how?
If in-band interference comes before peak detector, RxLev will increase.
But it is not, so the interference mixed after peak detector ?
Could you please breifly explain blocking test purpose ?
Thanks.

Out of channel interferer is a different story for RxLev. This is coming from the limited rejection of RF/IF filters. When the out-of-channel interferer is very strong, RxLev will report higher input level.

Blocking test is looking to find any out-of-channel interfering signal that block the receiver's ability to recover any useful GSM signals.

Receiver mixing products, LO Phase Noise, Front-end compression point, could be some of the receiver characteristics that can affect the Blocking test.