Monday, December 18, 2006

Mobile Phone: GSM & CDMA

What are the differences between two mobile communication technologies GSM and CDMA? The main difference lies in the manner in which users share the common resource. GSM does it by chopping up the channel into sequential time slices. Each user of the channel takes turns transmitting and receiving in a round-robin fashion. In reality, only one person is actually using the channel at any given moment, but he or she only uses it for short bursts. He then gives up the channel momentarily to allow the other users to have their turn. This is very similar to how a computer with just one processor can seem to run multiple applications simultaneously.

CDMA on the hand really does let everyone transmit at the same time. Conventional wisdom would lead you to believe that this is simply not possible. Using conventional modulation techniques, it most certainly is impossible. What makes CDMA work is a special type of digital modulation called "Spread Spectrum". This form of modulation takes the user's stream of bits and splatters them across a very wide channel in a pseudo-random fashion. The "pseudo" part is very important here, since the receiver must be able to undo the randomization in order to collect the bits together in a coherent order.
Advantages of GSM

  • GSM is mature, more stable network with robust features. Used since mid-80, well proved.
  • Less signal deterioration inside buildings.
  • Talktime is generally higher in GSM phones
  • The availability of Subscriber Identity Modules (SIM) allows users to switch networks or handsets.
  • International roaming permits subscribers to use one GSM phone in almost all part of world.

Disadvantages of GSM

  • Pulse nature of transmission interferes with some electronics, especially certain audio amplifiers.
  • 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.
Advantages of CDMA
  • Capacity is CDMA's biggest asset. meaning that the carrier can serve more subscribers.
  • Increased cellular communications security.
  • CDMA uses precise clocks that do not limit the distance a tower can cover.
  • CDMA consumes less power and covers large areas so cell size in CDMA is larger.
  • CDMA is able to produce a reasonable call with lower signal levels. Signal can reach extended area - beneficial to rural users situated far from cells.
  • 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.
  • Has a well-defined path to higher data rates.

Disadvantages of CDMA

  • Most technologies are patented.
  • A big problem facing CDMA systems is channel pollution. This occurs when signals from too many base stations are present at the subscriber's phone, but none are dominant. When this situation occurs the audio quality degrades rapidly, even when the signal seem otherwise very strong. Pollution occurs frequently in densely populated urban environments where service providers must build many sites in close proximity.Because CDMA towers interfere with themselves, they are normally installed on much shorter towers. Because of this, CDMA may not perform well in hilly terrain.
  • Since CDMA is available in lesser part of world, roaming facility may not be available while traveling to other countries.

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