Area Defending System (ADS) by Enigx   Area Defending System (ADS) is a simple and stable script for mission makers to generate a cycle of random infantry attacks on area positioned on map from editor. You can specify the number of attacks and the time between each one.
It is sufficient to define two concentric areas from editor. Enemies will be created between the two areas so that the inner one is free from enemies and used as a defense zone against attacks. Enemy spawning position is random. They will attack the center of the internal area, with a range of 25m, following random directional waypoints.
You can start, stop and restart the cycle at will. You can also start two at the same time.     Armaholic Area Defending System (ADS)   Hey, it seems Armaholic has been shut down!! 
Alternatively, I added the file to this link which uses a personal external website (completely unrelated to the Arma game).
Features: Selection of infantry units, type and numbers Selection of different shapes and orientation of the defending area Random or fixed number of units per fire team Random AI spawning positions with random routes to central position You can start, stop and restart the cycle at will Detailed guide step by step with addons included Possibility to activate the system whenever you want from ACE self interaction menu
Installation / Usage: For instructions and information of how to use the ADS please refer to the included documentation and ADS_OpenMe.sqf.   Notes: The system can be activated/stopped by triggers. 
Alternatively you can launch it whenever you want from ACE self interaction menu (code and details included). In this case, of course, it needs the ACE mod loaded.
Credits & Thanks:
Special thanks to Shuko for sharing his SHK_pos - Random position generator system.
Thanks to Armaholic Community.
Changelog:
First official version 1.0