Wireless Access

I have a problem with client mobility, when a client is moving away from on AP the signal drops even though there is an AP with higher signal in range.

Is there anyway to tune the controller to force the client to disconnect when the signal is weak ??

What you describe is a "sticky client"; one that doesn't want to roam to a closer AP.   What version of code are you running?   Version 6.3.x of ArubaOS introduced ClientMatch.  When ClientMatch is enabled, it should do the very thing you are looking for; proactively move clients to a better AP selection based on a a variety of factors.  

http://www.arubanetworks.com/techdocs/ArubaOS%206_3_1_Web_Help/Web_Help_Index.htm#ArubaFrameStyles/ARM/Client_Match.htm

The OS is already 6.3.x

Is client match enables by default ?

It is.  How far apart are your access points and what is their transmit power?