iphone 4 with wifi

ok i have a problem with the router wifi signal (billion 7800n) being not very strong up stairs, i have the router downstairs next to the phone socket in a window sill, and the signal is just bad on my iphone 4 up stairs. I know, move the router upstairs up but that would lower my sync to the exchange, anyway the router is not going to moved. So i wanna know is there away to boost the signal or could i just extent 1 of the 3 antennaes which the router as.
Any feed back welcome thanks.

I'd get a wireless acces point that can be used to extend the wireless signal from the unit downstairs.

Not exactly mobile broadband

Not the cheapest option but something along the lines of this and this would be my choice.

Have you tried tweeking the positions of the antennae on the router?