http://developer-blog.net/hard…..ew-7811un/

The solution to the issue consists of two parts.
First we adjust the responsible 8192cu driver options by creating a new configuration file:

sudo nano /etc/modprobe.d/8192cu.conf

the file should contain the following line:

options 8192cu rtw_power_mgnt=0 rtw_enusbss=0

This should disable the sleep-mode of the Edimax EW-7811Un Wi-Fi stick.
To make this really sure, we can also create a cronjob pinging the router in regular intervals.
For this we open crontab:

sudo nano /etc/crontab

and add the line:

*/1 * * * * ping -c 1 192.168.1.1

adjusting "192.168.1.1" with the IP of your router.