Comparison of mobile phone standards

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Global System for Mobile Communications (aka GSM) and IS-95 (aka cdmaOne) are the two most prevalent mobile communication technologies. Both technologies have to solve the same problem: to divide the finite RF spectrum among multiple users.

TDMA (Time Division Multiple Access - underlying technology used in GSM's 2G) 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 - underlying technology used in GSM's 3G and IS-95's 2G) on the other hand, uses a special type of digital modulation called spread spectrum which spreads the voice data over a very wide channel in pseudorandom fashion. The receiver undoes the randomization to collect the bits together and produce the sound.

As a trivial comparison imagine a cocktail party, where couples are talking to each other in a single room. The room represents the available bandwidth. In GSM, a speaker takes turns talking to a listener. The speaker talks for a short time and then stops to let another pair talk. There is never more than one speaker talking in the room, no one has to worry about two conversations mixing. In CDMA, any speaker can talk at any time; however each uses a different language. Each listener can only understand the language of their partner. As more and more couples talk, the background noise (representing the noise floor) gets louder, but because of the difference in languages, conversations do not mix.

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[edit] Advantages of 2G GSM

  • GSM is mature; this maturity means a more stable network with robust features.
  • Less signal deterioration inside buildings.
  • Ability to use repeaters.
  • Talktime is generally higher in GSM phones due to the pulse nature of transmission.
  • The availability of Subscriber Identity Modules allows users to switch networks and handsets at will.
  • GSM covers virtually all parts of the world so international roaming is not a problem.
  • The much bigger number of subscribers globally creates a better network effect for GSM handset makers, carriers and end users.
  • Tools for unlocking GSM phones are widely available. Meaning it is easier to keep the same phone and go with another carrier using the same technology.

[edit] Disadvantages of 2G GSM

  • Pulse nature of TDMA transmission used in 2G interferes with some electronics, especially certain audio amplifiers. 3G uses W-CDMA now.
  • Intellectual property is concentrated among a few industry participants, creating barriers to entry for new entrants and limiting competition among phone manufacturers.
  • GSM has a fixed maximum cell site range of 35 km, which is imposed by technical limitations. [1]

[edit] Advantages of IS-95

  • Capacity is IS-95's biggest asset. It can accommodate more users per MHz of bandwidth than any other technology.
  • IS-95 has no built-in limit to the number of concurrent users.
  • IS-95 uses precise clocks that do not limit the distance a tower can cover. [2]
  • IS-95 consumes less power and covers large areas so cell size in IS-95 is larger.
  • IS-95 is able to produce a reasonable call with lower signal (cell phone reception) levels.
  • IS-95 uses soft handoff, reducing the likelihood of dropped calls.
  • IS-95's variable rate voice coders reduce the rate being transmitted when speaker is not talking, which allows the channel to be packed more efficiently.
  • Has a well-defined path to higher data rates.

[edit] Disadvantages of IS-95

  • Most technologies are patented and must be licensed from Qualcomm.
  • Breathing of base stations, where coverage area shrinks under load. As the number of subscribers using a particular site goes up, the range of that site goes down.
  • Because IS-95 towers interfere with themselves, they are normally installed on much shorter towers. Because of this, IS-95 may not perform well in hilly terrain.
  • IS-95 covers a smaller portion of the world, and IS-95 phones are generally unable to roam internationally.
  • Manufacturers are often hesitant to release IS-95 devices due to the smaller market, so features are sometimes late in coming to IS-95 devices.
  • Tools for unlocking CDMA phones are not common. Meaning every time you subscribe with a new provider you must buy a new phone.
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