Wifi is frequently disconnecting

The Wifi is frequently disconnecting in Ubuntu 15.04. My Wireless interface of Ubuntu is RT3290 Wireless 802.11n 1T/1R PCIe

When wifi is disconnected, on iwconfig I got the following

eth0      no wireless extensions.

wlan0     IEEE 802.11bgn  ESSID:off/any  
          Mode:Managed  Access Point: Not-Associated   Tx-Power=20 dBm   
          Retry short limit:7   RTS thr:off   Fragment thr:off
          Power Management:off

lo        no wireless extensions.

After each disconnection I have to restart network for reconnection by

sudo service network-manager restart

Check if in iwconfig Power Management is on.

If yes sudo iwconfig wlan0 power off and see if that helps.

To keep it upon restart do sudo touch /etc/pm/power.d/wireless