使用GSM或者3G模塊時，都會接觸到信號強度CSQ。通過指令AT+CSQ，模塊返回當前的信號質量，例如：
AT+CSQ
+CSQ: 28,0
其中28就是信號強度CSQ，但它不是真實的CSQ，他應該叫RSSI，Received Signal Strength Indication接收的信號強度指示。
那么好了，RSSI和CSQ之間的對應關系式什么呢？OK，就是：CSQ值=(接收信號強度dBm+113)/2。
參考值如下圖：
note：CSQ取值范圍是【0，1，2~31，99】，沒有其他值。如果rssi值大於-51，CSQ一律取31。
+CSQ: 28,0
其中0代表誤碼率，Ber。
在通話的時候誤碼率不能太高，否則會聽到嘶嘶沙沙的聲音。
2013-6-30編輯
現在android手機比較火，其中信號方面也有個描述，叫ASU，實際上就是RSSI
ASU
ASU or "Arbitrary Strength Unit" is an integer value proportional to the received signal strength measured by the mobile phone.
It is possible to calculate the real signal strength measured in dBm (and thereby power in Watts) by a formula. However, there are different formulas for 2G and 3G networks.
In GSM networks, ASU is equal to the RSSI (received signal strength indicator, see TS 27.007).
dBm = 2*ASU - 113, ASU in the range of 0..31 and 99
In UMTS networks, ASU is equal the RSCP level (received signal code power, see TS 25.125)
dBm = ASU - 116, ASU in the range of -5..91