Comparison of mobile phone standards

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GSM and CDMA are the two most prevalent mobile communication technologies. Both technologies have the same goal: to divide the finite RF spectrum between multiple users.

GSM (Time Division Multiple Access - underlying technology used in GSM) does it by chopping up the channel into sequential time slices. Each user of the channel takes turns to transmit and receive signals. In reality, only one person is actually using the channel at a specific moment. This is analogous to time-sharing on a large computer server.

CDMA (Code Division Multiple Access) on the other hand, uses special digital modulation Spread Spectrum which spreads the voice over very wide channel in pseudo random fashion. The receiver undoes the randomization to collect the bits together and produce the sound.

Imagine that you go to a UN party where couples from different countries are invited. In GSM, each couple takes turns talking. They talk for a short time and then stop to let another couple talk. As there is never more than one person speaking in the room, no one has to worry about being heard over the background noise. In CDMA, each couple talks at the same time; however they all use different languages. Because (we assume) that none of the listeners understand any other language than that of their partner, the background noise does not create any problem.

Advantages of GSM

1. GSM is mature, this maturity means a more stable network with robust features.
2. No signal deterioration inside buildings.
3. Talktime is generally higher in GSM phones due to pulse nature of transmission.
4. The availability of subscriber Identity Modules allows to user's to switch networks and handsets at will.
5. GSM covers virtually all parts of world so international roaming is not a problem.
6. GSM is a standard so operator doesn't have to pay royalty to use it.

Disadvantages of GSM

Pulse nature of transmission interferes with some electronics, especially certain audio amplifiers.

Advantages of CDMA

1. Capacity is CDMA's biggest asset. It can accommodate more users per MHz of bandwidth than any other technology.
2. CDMA consumes less power and covers large areas so cell size in CDMA is larger.
3. CDMA is able to produce a reasonable call with lower signal levels.
4. CDMA uses Soft Handoff, reducing the likelihood of dropped calls.
5. CDMA's variable rate voice coders reduce the rate being transmitted when speaker is not talking, which allows the channel to be packed more efficiently.
6. Has a well-defined path to higher data rates.

Disadvantages of CDMA

1. Breathing of base stations, where coverage area shrinks under load.
2. Most technologies are patented and must be licensed from Qualcomm.