How to check 3G modem RSSI Signal Quality

I used minicom and set it to 9600 baud rate and also must set the serial device to tty/USB on which your USB detected.

Below is the details.
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Signal quality

Command: AT+CSQ
Response: +CSQ: ,
Description: Returns signal quality.

Received Signal Strength Indicator

0
-113 dBm or less
1
-111 dBm
2 to 30
-109 to -53 dBm
31
-51 dBm or greater
99
not known or not detectable

Bit Error Rate, in percent 0..7.
99 not known or not detectable

A note on the RSSI (received signal strength), dBm is a decibel (logarithmic) scale with a reference of 1 milliwatt thus 0 dBm equals a received signal of 1 mW.
Signal strength is usually lower than 1 mW and therefore below 0, so the larger (closer to 0) the better signal strength.
You can convert the RSSI to dBM with
dBm = (rssi \times 2) - 113

Example

>AT+CSQ=?
+CSQ: (0-31,99),(99)
OK

>AT+CSQ
+CSQ: 14,99

Courtesy of Shapeshifter