Does is normal over 3G my phone to heats up

Well sometimes , but sometimes, when i am on 3g, my iphone 5, heats, i check the battery temp with battsaver and its 38-39 celsius.
But that happens sometimes,not always, does the reason is that maybe when iam in low signal area so, the phone is forcing hard to get 3g network and heats up or?

and also while iam od 3g,my battery always is draining alot faster comapring with when i am on wifi...

and over wifi is stone cold.. i.e normal..

and also which battery working temp is normal for iphone batteries?

thanks in advance