CDMA vs TDMA vs GSM
GSM has slightly better voice quality. CDMA has slightly better data rates. Everyone agrees that TDMA is inferior to the two.
GSM is the international standard. It is used in over 184 countries. GSM works by chopping up a conversation into timed bursts and seperates callers by time index and frequency (these are refered to as FDMA - frequency division multiple access - and TDMA - time division multiple access). Although its method of seperating calls is the same as that of TDMA, these are very different technologies. GSM currently uses 2 different codecs for encoding calls: EFR (enhances full rate) and AMR (advanced multi rate). These codecs were developed from the ISDN landline standard and offer very high call quality. GPRS (general packet radio service - the 2.5G data solution for GSM - offers data rates of up to 115kbps according to the International Telecommunications Union, but in reality will never exceed 64kbps. These networks exist in the US today and power networks by Cingular, ATT, T-Mobile and I'm sure some other small ones. The next iteration of GSM is called EDGE (enhanced data rates for GSM evolution) may or may not become a reality. It offers data rates up to 2Mbps according to the ITU which will translate into speeds of 384kbps for customers. The reason it may not become a reality is because as time goes on wCDMA - the stage after EDGE - gets closer and closer. wCDMA is a departure from the TDMA/FDMA way of splitting calls that GSM currently uses. wCDMA is a spread spectrum technology similar to CDMA, but over a wider frequency band than CDMA uses. wCDMA is sometimes refered to as UMTS (universal mobile telecommunications service), but they are acutally different - sort of. UMTS refers to wCDMA in a certain frequency band (2.1 or 2.4GHz I believe) not the technology itself, but many people use the UMTS name so that people realise that it isn't owned by Qualcomm. wCDMA does use one Qualcomm patent.
CDMA is currently the dominant technology in the United States. CDMA works by having all of the calls transmitted on the same frequency at the same time. It identifies different calls by code. This, theoretically, gives CDMA an unlimited capacity under perfect conditions, but of course CDMA does have a limited capacity. There is only a certain ammount of noise that a channel can accomodate before the system and your phone can't recognise one signal from another. If you doubt the limitations of CDMA ask some angry Sprint customers in these forums complaining of network busy signals. Getting back to CDMA, it works to limit this noise by having a phone transmit at the lowest possible setting that will get the call through. One of the factors that determines how much power is need to transmit is distance from the tower. All phones have to take this into consideration. The other factor that only CDMA-type systems have to account for is the number of callers connected to the system. This is an example that one of the mods here (bobolito I believe) used: Say one person is 1 mi. from the tower using 1 watt to transmit. He is the only one connected to the system. Then another person joins the system from 1/2 mi. away transmitting at 1/2 watt. The person further away would have to up his transmition power so that the closer person's signal didn't totally cover his. As more people join the system the current people have to continue to up their power so that their call isn't drowned out by the new ones (think of people at a party talking. The more people talking the louder you're gonna have to talk). Also, the more people who are connected to the system means that there are more distances and more levels of transmitter power and the closer people are starting higher. Now there are 1000s of people connected and the people at 1/4mi. are using 1 watt, the people at 1/2 mi. are using 2 watts, and the people at 3/4 mi. are using 3 watts and the person at 1 mi. looses his connection because it is drowned out by the closer voices and he can't transmit more powerfully (talk louder) than 3 watts. In this way, CDMA coverage is variable in a way that TDMA/FDMA (and systems based on those principals) aren't. Beyond calling, that oversimplification shows how CDMA does have limitations even if everyone is at the same distance using the same transmitting power. 1xRTT - the 2.5G voice/data solution for CDMA - is currently deployed by Sprint and Verizon in certain areas. A lot of the times the carrier has implimented 1xRTT data, but not voice since the data is immiedately apparent. 1xRTT voice does offer some improvement in call quality, but not much and is still a tiny bit behind GSM's EFR codec. 1xRTT data improves data rates to a theoretical 144kbps, but again, in reality, it will never exceed 64kbps. After 1xRTT comes 1xEV-DO (data only/optimised) and 1xEV-DV (data/voice). 1xEV-DO has a theoretical 2Mbps, but will be 384kbps in the real world just like EDGE and wCDMA.
TDMA is more accurately refered to as D-AMPS because that is what it is: digital analog. TDMA takes the old analog system and puts a bandaid on it by converting the signal to digital and adding a time division to it (analog is only divided by frequency). It is not much more advanced than that and is the only system that can handoff from digital to analog.
In the end, TDMA won't exist and the evolutions of GSM and CDMA will have the same capabilities.
Some related stuff...
CDMA's soft handoff. This is something that people always bring up, but it really isn't so amazing. People claim that the signal spiders so that you are connected to 10 towers at a time, but it doesn't. Soft hand-offs are still hard (difficult to do). You are never talking through two towers at the same time (people often think well tower 1 will get part of what I'm saying and the rest will all get other parts and in real time it will all be compared with overlapping sections thrown out and the rest pieced together). The reality is that it would be impossible with the current technolgy to do that (plus, what happens when two of the same pieces are different - which is the correct signal). As previously brought up, edge signals like that are drowned out so that even if you are close enough to be transmitting to 10 towers, other signals would drown your's out in a CDMA system. The difference between a soft hand off and a hard hand off is make before break and break before make. CDMA uses a make before break meaning it connects to a new tower and the tower verifies the connection is better before it breaks the connection with the current tower. The new tower doesn't handle any parts of the call except analyzing the signal strenght until the connection with the previous tower has been terminated. TDMA and GSM use break before make systems (hard handoffs). This means that the tower you are currently connected to directs your phone and you disconnect from the current tower before connecting to the new one. While the CDMA system makes logical sense and is techncally superior, it hasn't proven that way in real life. Part of the downfall of soft hand offs is the fact that they are more difficult to engineer. This leads to soft and hard hand offs having about the same success rate. People always try to determine which is better from their personal experience of dropped calls, but that is faulty. People can't tell if their call was lost due to a gap in coverage or an ill-executed handoff - both drop your call in exactly the same way.
TD-SCDMA is the new Chinese standard that shows a lot of promise. It is still in its infancy and is developed by Siemens and one of China's universities. It basically seperates signals by time and code. It is unknown how efficient this network is or even if it is viable at this time.
People always mention analog roaming as an advantage to CDMA. This isn't true. I'm sure we've all seen the single band phones that Sprint sells. CDMA is not inherently compatable with analog. The only truely backward compatable technology is TDMA. Most CDMA phones have had analog capabilities added to them because Verizon wanted to slowly upgrade its network (way back when) and still haven't gotten to some areas where there is no reason to upgrade since they aren't taxing the bandwidth there and Sprint found that it couldn't get customers to overcome their fears of no coverage off their network. GSM phone manufacturers could have done this, but it wasn't important to them. CDMA handsets were bound almost entirely for the US where analog still existed en masse. In Europe, the destination for most GSM phones, providers needed to guarentee 99% population coverage for most licences. With GSM literally everywhere, there wouldn't be analog coverage where GSM wasn't so why bother. At the time, the US only had GSM from the companies that became T-Mobile and there wasn't a push to cater to them. Now that Cingular and ATT (as well as some others) are moving to GSM from TDMA/analog companies like Nokia and SonyEricsson (Siemens as well) have seen a larger US market for their phones and have begun to experiment with TDMA/GSM/analog hybrids that are just as compatable with analog as TDMA.
The future...
GSM will have coverage pretty equal to CDMA in the US when Cingular and ATT finish their upgrades and Western Wireless adds it to their network.
wCDMA is GSM's ace. It allows CDMA and GSM carriers to upgrade to it easily. This would be the place that any CDMA carriers would jump ship since it would involve the same hassle to upgrade to it as it would to the continuation of CDMA. People will mention things like "Verizon has said it's happy with CDMA" not realising that corporations don't want to talk about any future plans and won't say they are unhappy with what they have to make sure that customers and potential customers think that they are having second thoughts about their own technology.
The real world (or at least the real US)...
CDMA and TDMA currently have coverage anywhere there is digital. Once Cingular and ATT finish their upgrades, GSM will be the same. From a user's standpoint GSM and CDMA offer nearly identical features and quality. GSM providers do seem to offer way more minutes though (Cingular, ATT, and T-Mobile offer 1000 anytime for $40/mo.). GSM does offer worldwide roaming and CDMA does have the most phones are analog compatable advantage, but those two difference may be short lived as Qualcomm tries to make a GSM CDMA hybrid and GSM phones are quickly gaining analog capabilities.