Next year I plan to stay in Bangkok for several months, and I want to buy CAT usb cdma modem.

Where to buy it? Can it be use instantly after purchased? Is it any document necessary to buy it?

You can buy CAT modems at the CAT office in Bangkok afaik (was case last yr when we did so) (Google for address).

Keep in mind they only work outside Bangkok and central provinces. Go figure. TIT.

Thanks DigitalBanana, hmm so you mean they don't work inside Bangkok....so which provider I should choose if I want to use mobile internet in BKK

Depends where you'll live and cell towers. Welcome to Thailand!

CAT uses CDMA technology and covers north and south Thailand. Central regions and Bangkok covered by Hutch CDMA.

GSM 3G isn't official. So far only TOT offers 3G officially (?) in Bangkok areas. They are reportedly teaming up with Air Asia in January to promote themselves (at which they are pretty useless) with an MVNO as a 3G provider. In meantime True Move offers a very good albeit trial system across Bangkok, CM, Phuket and Pattaya.

AIS and DTAC have pretty much been screwed for time being with regards 3G but offer countrywide EDGE.