wireless connection

Hello

I am an Iphone user and i have problem with wifi connection

At the first time I couldn't connect to wireless then after waiting 20 minutes and restarting my iphone it connect but the problem is that you have to stay near the modem of wifi for full connection . If i move just 1 meter away it has just one "line" connection or it disconnect.

Can you tell me how to fix this problem ?

Thank you