Ok, this is not going to be bandwidth related...
( so I'd change your thread title for more advice)
Note, I'm not claiming t be an expert on ps3 or gt5, so someone else may be able to help more ( I do however know networking very well

)
Anyway, It is much more likely to be related to the 'type' of connection you have ...
You need to check if you and your friend have nat type 1, 2 or 3
1 = direct connection to Internet, no firewall
2 = thru firewall/router, but 'open' to all ports required
3 = connected, but some ports closed , so limited capabilities
For zero issues either you or your friend need nat type 1 or 2
(gt5 does tell you you have 'limited' if you are using nat 3)
If you both have nat 3, then you will find that you can both join other sessions (as long as host nas 1/2) , but you won't be able to host yourself.
First you should determine if the above is really the issue...
(you don't say if both you and your friend are able to connect to other gt5 sessions, you need to test this)
Next,If it is, then you will need to search on here and google, on how to get rid of nat 3 for ps3... There is a huge amount of advice out there, and it's tricky so I'm not going to repeat it here
Hope this helps
Mods: looks like you moved this out of gt5 qa, I would change the title.. But it's is very much a gt5 related issue imho