how to measure the RSSI strength signal between peers using wifi direct?
You can get this information inside wpa_cli. Open wpa_cli in wifi-direct interface and do a scan and scan_results as below.

adb shell wpa_cli -i&lt;wifi-direct interface name&gt; -p&lt;socket path&gt; scan
adb shell wpa_cli -i&lt;wifi-direct interface name&gt; -p&lt;socket path&gt; scan_results


You will get small info about connected peer(device address, channel,rssi etc). Hope this works!