I found that the 2.4G wifi on my Lenovo Y1 router got weak receive sensitivity since 15.05, and the problem still exists in 15.05.1.

Below is my native test for the issue, because I have less idea about the linux wireless site survey tools.

ROUND1 - flash openwrt 15.05.1-y1 firmware from downloads.openwrt.org - < BAD RESULT!  >

  * Connect an android phone to the router
  * Open the Luci/Wireless UI and check the signal, the value is about 90%
  * Move the phone to 5 meters away, and the signal seems dropped dramatically to 60%
  * Move to another room and the signal will decreased to about 15%, and the ping start reporting some packet lost

- ROUND 2 - flash some close-source firmware - <GOOD RESULT!>
  * I then re-flash the device with another firmware called pandorabox openwrt
  * repeat the above test steps and the signal is 100% at 20 cm away/ 100% at 1meter / 90% at 5 meter / 70+% in another room
  * no ping packet lost during the test. (both with 2.4/5g)

I found the difference of the 2 firmwares is the ra0 driver.  The pandorabox firmware is using a driver named RT2860v2 for the ra0 device, and found a possible copy of the driver source at https://github.com/wuqiong/rt2860v2-for-openwrt-mt7620 but I have no idea about why it works.   
I believe the pandorabox guys do some tweaks on the driver but they never released the source code from the dmesg info.

Also, I opened the my device case and checked the PCB. There is no external PA/LNA, so I'm guessing if the official openwrt driver disabled the internal LNA by default ?

Could you guys please take a look into those issues?