iPhone 5 unable to Join Network :(

i'm using iPhone 5 running iOS 6.1.2 , it was all good . 2 weeks ago i'm start getting weak wifi connection , and if i went a little far from the router ( to my room exmple ) it disconnect from the wifi and when i try to connect it back it says unable to Join network . try reset network , i try to restore it to 6.1.4 and i set it up as a new device , still the same . reset network setting again didn't work also , whenever i get close to the router it got connected by itself . any HELP ? Thx