hotspot doesn't reconnect automatically ipad 4g to iphone 5s !!!

Hello..

There’s something bothers me a lot about the “hotspot” connection. When I try to open the hotspot from my ipad 4g so I can use internet in my iphone 5s I keep losing the connection! for example I’m sitting in my office surfing the internet in my iphone 5s using the hotspot from my ipad, then when I leave the area I’m sure gonna lose the connection but the problem is when I come back to my office where my ipad is, the connection doesn’t re-connect automatically ? I have to go to wifi an search all over again for the hotspot.. This is totally annoying me how can I make it re-connect automatically..