Common ground serial communication trouble SIM800L
I finally fixed my issue.
I disable the auto baud rate of SIM800L trough FTDI232, with these at commands:

AT + IPR = 9600
AT &amp; W


AT + IPR: Fix the baud rate at 9600
AT &amp; W: Save in SIM800L

Then when i use with SoftwareSerial i have no issue anymore. When i reset the module with short connect RST with GND i have a good reply from SIM800L : 

RDY

+CFUN: 1

+CPIN: READY

Call Ready

SMS Ready


Thanks for your help.

Note : And be sure you are a good power supply and good wire.