Comparison of mobile phone standards

Issues

• Time-division multiple access (TDMA) provides multiuser access 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.
• Frequency-division multiple access (FDMA) provides multiuser access by separating the used frequencies. This is used in GSM to separate cells, which then use TDMA to separate users within the cell.
• Code-division multiple access (CDMA) This uses a digital modulation called spread spectrum which spreads the voice data over a very wide channel in pseudorandom fashion using a user or cell specific pseudorandom code. The receiver undoes the randomization to collect the bits together and produce the original data. As the codes are pseudorandom and selected in such a way as to cause minimal interference to one another, multiple users can talk at the same time and multiple cells can share the same frequency. This causes an added signal noise forcing all users to use more power, which in exchange decreases cell range and battery life.
• Orthogonal frequency-division multiple access uses bundling of multiple small frequency bands that are orthogonal to one another to provide for separation of users. The users are multiplexed in the frequency domain by allocating specific sub-bands to individual users. This is often enhanced by also performing TDMA and changing the allocation periodically so that different users get different sub-bands at different times.

Comparison

Strengths and weaknesses of IS-95 and GSM

Advantages of GSM

• 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, aside from a subsidy lock.
• GSM List of [mobile network operators|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.

  Disadvantages of GSM

• 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. Situation is however worse in CDMA-based systems like IS-95, where Qualcomm is the major IP holder.
• GSM has a fixed maximum cell site range of 120 km, which is imposed by technical limitations. This is expanded from the old limit of 35 km.

  Advantages of IS-95

• Capacity is IS-95's biggest asset; it can accommodate more users per MHz of bandwidth than any other technology.
• Has no built-in limit to the number of concurrent users.
• Uses precise clocks that do not limit the distance a tower can cover.
• Consumes less power and covers large areas so cell size in IS-95 is larger.
• Able to produce a reasonable call with lower signal levels.
• 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.

  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 each other, they are normally installed on much shorter towers. Because of this, IS-95 may not perform well in hilly terrain.
• USSD, PTT, concatenated/E-sms are not supported by IS-95/CDMA
• 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.
• Even barring subsidy locks, CDMA phones are linked by ESN to a specific network, thus phones are typically not portable across providers.

  Development of the market share of mobile standards

Comparison of wireless Internet standards