Basically - without a UA you've no chance.

Generally, it's not going to very easy to do.
You can, technically, still do it (using the TAC id of the handset) but in order to even get this information you'll need a special agreement in place with carriers. And even then, you'll only be able to detect those devices that come to your site on those carriers.

Where are you based?

Thanks Hunt sir.

I am in China. But I need to detect the devices all over the world.

From your experience, how much percent of devices can be detected by UA?

What is the TAC id of the handset? How it works? What technology can parse TAD id?

Besides UA, what other header or request parameters can be used to detect device? How it works?

Unfortunately, I think you're stuck Darthin.
It's a well known fact that Chinese mobile carriers filter out UA strings of their devices :(

What about the URL to the device's UA Profile RDF file? Is that filtered out too?

Perhaps, if that UA Profile URL is supplied, and not stripped, there might be some help (assuming, of course, the phone supplies it too :).

As far as I know, in China all headers are stripped, regardless of what you're requesting and where you're requesting it from.
It *is* possible to have your website added to a whitelist internally, so that you can receive the full device header information, but I'm guessing that this isn't a trivial matter...

Suppose I can receive the full device header information, what header parameter is useful for device detection besides UA and UA Profile URL?

I am in China, but our product runs in US.

Thanks.