Abdel,
Ok, I was curious about the H-pol antenna in GSM.
So to come back to the initial point : Yes, it’s just a convention : if you do the opposite (TX on -45) it will not change anything in terms of coverage or quality.
It’s not even possible to argue about this : it’s like holding a torchlight a certain way or another way. As long as you’re pointing it in the right direction, the light will go there. The positions of the hands around the torchlight will not change the path of the light 🙂