AT+CSQ without Disconnecting PPP Connection in ARCH Linux
Try the following:

Suspend the PPP connection by sending the +++ escape sequence:

(wait one second) +++ (wait one second)

If the modem comes back with OK, you are back in command mode.  If it comes back with NO CARRIER, then this method will not work as it has hung up.

Assuming it comes back with OK, you can now send your AT+CSQ command (or any others).

When done, use this to re-enter the PPP data state:

AT+CGDATA=”PPP”,1