Interestingly the next generation of cellular technology called Long Term Evolution (LTE) is a wideband version of CDMA. Unfortunately carriers are using different frequency bands with different 3G backup systems. In the near-term that means truly world phones will be more difficult to come by.
Not exactly. 4g LTE supports a variety of formats, but most (including all the major carriers in the U.S.) are going with OFDMA. To explain how OFDMA works simply, each phone communicates on a set of frequencies assigned only to it. Depending on which frequencies have or didn't have a broadcast, the tower can tell which phone(s) transmitted. Even though two phones may transmit on the same frequency, based on which other frequencies also saw a transmission, the tower can figure out which of the two phones (or both) transmitted at any given time.
CDMA (Sprint and Verizon for voice) works on a similar concept, except it assigns a unique signaling code to each phone. Like writing vertically and horizontally on a sheet of paper, you can tell the signals apart even though they occasionally overlap. Both CDMA and OFDMA work very well for saturating bandwidth, since the bandwidth limit is determined by the noise floor. The more phones are communicating with a tower, the higher the noise floor (broadcasts from other phones are interpreted as noise when processing a particular phone's signal), and the bandwidth to each phone gets scaled back automatically. So when you have few phones transmitting, each phone gets a lot of bandwidth. When you have a lot of phones transmitting, each phone gets a little bandwidth. Automatically.
GSM (AT&T and T-Mobile in the U.S.) uses TDMA for voice. This doesn't scale at all. Each phone is assigned a timeslice to communicate with the tower, and it gets that full timeslice even if it doesn't transmit anything. Consequently, the tower's bandwidth is split evenly across all active phones, even if they aren't actually transmitting. It's so bad that GSM took nearly 2 years to catch up with CDMA data speeds when 2g rolled out. By the time 3g rolled out, it was pretty clear CDMA was the better technology. GSM threw in the towel and added CDMA to their spec specifically for data (HSDPA is wideband CDMA). That's why you can talk and use data at the same time on GSM phones - they have a TDMA radio for voice, and a CDMA radio for data, and both can be used simultaneously. The Verizon and Sprint 3g phones just have a single CDMA radio, which is switched between voice and data modes depending on the current need (data can suffer latency, voice cannot). Thus the limitation of voice or data but not simultaneously.
Because TDMA uses timeslices, the speed of light becomes a factor. There is a hard limit to the distance a GSM phone can be from a tower and still make a voice call. Beyond that limit, the phone's signal reaches the tower after the phone's timeslice is over, so you can't make a voice call period. Theoretically, because no other phones are transmitting during your phone's timeslice, there is less noise and the range should be greater with GSM. But because of the speed of light limit, your range is actually less with GSM. I don't remember exactly, but I believe it was around 30 miles. With a CDMA phone, if you can mount an external antenna/amplifier high enough and crank up the gain, you could conceivably make voice calls with a tower a hundred miles away. Signal degredation will begin more quickly than with TDMA however, because of the noise from other phones.
OFDMA is so good it's also used for 802.11ac. It's only coming into use now because it requires a lot more signal processing than CDMA. Electronics have finally gotten powerful and efficient enough that you can do this on a phone or laptop without killing the battery in an hour. All phones require a separate OFDMA radio for LTE, so they can now all make calls and use data at the same time (unless your CDMA phone has to fall back to 3g data).
The LTE standard also mandated a SIM card just like GSM, so even Verizon's and Sprint's LTE phones use a SIM card. If you're careful to make sure the phone you buy supports LTE bands used in the countries you'll be visiting and the carrier unlocks it for international use, you can just pop in a local SIM card and use data there. The North American version of the Google Nexus 5 is particularly good for this, as it supports a wide variety of frequencies on TDMA (GSM), CDMA (Sprint but not Verizon), WCDMA (data for 3g GSM), and LTE, and comes unlocked if you buy from Google.
Phones (Phone Scoop)