使用GSM或者3G模块时，都会接触到信号强度CSQ。通过指令AT+CSQ，模块返回当前的信号质量，例如：
AT+CSQ
+CSQ: 28,0
其中28就是信号强度CSQ，但它不是真实的CSQ，他应该叫RSSI，Received Signal Strength Indication接收的信号强度指示。
那么好了，RSSI和CSQ之间的对应关系式什么呢？OK，就是：CSQ值=(接收信号强度dBm+113)/2。
参考值如下图：
note：CSQ取值范围是【0，1，2~31，99】，没有其他值。如果rssi值大于-51，CSQ一律取31。
+CSQ: 28,0
其中0代表误码率，Ber。
在通话的时候误码率不能太高，否则会听到嘶嘶沙沙的声音。
2013-6-30编辑
现在android手机比较火，其中信号方面也有个描述，叫ASU，实际上就是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