Can't receive messages without internet connection

I am using an iPhone 4s running on ios 8.3. I am having trouble receiving messages when i don't have internet connection. i use imessage, but i dont always have internet connect or 3g. that means that when i am out i cannot receive messages from friends which is not very convenient. then i receive the messgae once i connect to internet which may be long after the person has sent me a message. is there anyway to fix this because i still want to use imessage.