no GPS signal when inside an expressway tunnel

Why on earth do you need a GPS within a tunnel? Tunnels generally don't have any junctions or turn offs. Mostly you can only travel forwards or backwards, you'd really have to try very hard to get lost.

The satellite signals you need for GPS of course can't penetrate into tunnels.

You haven't been to Boston have you @Rindi...

The tunnels in and out of the city have multiple exits

I said they "generally" don't have turn offs or junctions. Those few that do you should easily be able to navigate the traditional way, by reading the signs and maybe planning your route a little in advance so you know the important points.

to funny rindi.........

At my place   the tunnels have several exits;  yestday   I came across 2 exits within 5km apart in the same tunnel, both labelled as Exit 2A

>  5km apart
actually 15km apart.

is there sort of antenna which I could connect to my Android phone
to enhance the signal ?

https://www.google.com.sg/?gfe_rd=cr&ei=YeYcV-G5OsSJ8Qez1oHQCA&gws_rd=ssl#q=gps+booster+antenna+for+android

http://www.ebay.com/sch/Car-GPS-Antennas/73348/bn_887030/i.html

https://play.google.com/store/apps/details?id=com.octy.gps.booster&hl=en

https://play.google.com/store/apps/details?id=tisinadev.activegps&hl=en

Would something like the above work in tunnels or at least give a glimpse
of signal when my car passes exits of tunnels?

Those GPS boosters would be worth trying. It's hard to say how they would perform without actually getting in the tunnel and testing them.