What’s the core difference?
It all has to do with the way your data is converted into the radio waves that your cellphone broadcasts and receives. GSM divides the frequency bands into multiple channels so that more than one user can place a call through a tower at the same time; CDMA networks layer digitized calls over one another, and unpack them on the back end with sequence codes.
The GSM Association is an international organization founded in 1987, dedicated to providing, developing, and overseeing the worldwide wireless standard of GSM, while CDMA was a standard designed and practically used by Qualcomm for the first time.
CDMA and GSM are both multiple access technologies. They’re ways for people to cram multiple phone calls or Internet connections into one radio channel.
GSM came first. It’s a “time division” system. Calls take turns. Your voice is transformed into digital data, which is given a channel and a time slot, so three calls on one channel look like this: 123123123123. On the other end, the receiver listens only to the assigned time slot and pieces the call back together.
The pulsing of the time division signal created the notorious “GSM buzz,” a buzzing sound whenever you put a GSM phone near a speaker. That’s mostly gone now, because 3G GSM isn’t a time division technology.
CDMA required a bit more processing power. It’s a “code division” system. Every call’s data is encoded with a unique key, then the calls are all transmitted at once; if you have calls 1, 2, and 3 in a channel, the channel would just say 66666666. The receivers each have the unique key to “divide” the combined signal into its individual calls.
Code division turned out to be a more powerful and flexible technology, so “3G GSM” is actually a CDMA technology, called WCDMA (wideband CDMA) or UMTS (Universal Mobile Telephone System). WCDMA requires wider channels than older CDMA systems, as the name implies, but it has more data capacity.
SIM(Subscriber Identity Module)
GSM phones use SIM cards. The removable SIM card allows phones to be instantly activated, interchanged, swapped out and upgraded, all without carrier intervention. The SIM itself is tied to the network, rather than the actual phone.
CDMA carriers require proprietary handsets that are linked to one carrier only and are not card-enabled. To upgrade a CDMA phone, the carrier must deactivate the old phone then activate the new one.
The Future is LTE(4G)
The CDMA vs. GSM gap will close eventually as everyone moves to 4G LTE, but that doesn’t mean everyone’s phones will be compatible. LTE, or “Long Term Evolution,” is the new globally accepted 4G wireless standard.
The distinction between the two technologies is largely irrelevant in 4G phones, since both can be used with the both of the main standards used with 4G phones, Long Term Evolution (LTE) and Worldwide Interoperability for Microwave Access (WiMax). Since GSM and CDMA work by means of radio waves, but 4G technologies work by means of an IP network, the radio signals from both have to be translated into electronic data for use on the network or the phone has to switch back to a 2G or 3G network to make voice calls.