Mobile Phone Technology

Multiplexing

Frequency and Time Division Multiplexing

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The mobile phone operators have limited bandwidth and they need to split the available space between different users. There is a limited amount of bandwidth available to the mobile phone operators which are split into channels. One of their requirements is to share their available channels between users. Often the whole bandwidth is split into different channels. But the operators need to make the best use of these channels, they cannot afford to allocate a single channel per user. There are two basic methods of sharing the available frequency with:

  • FDMA (Frequency Division Multiple Access)
  • TDMA (Time Division multiple Access)

Frequency Division Multiple Access The available bandwidth is split into channels each separated from their neighbour. These separate channels allow for different users to access the bandwidth at the same time albeit with more limited (channel) bandwidth. This however can be wasteful in resources due to interference between adjoining channels. Thus space is made between the channels to stop the interference. Also within a pure FDMA environment, not all users have the same demands, and hence one channel may be busy and restricting one user, whereas another channel may not be well used, with spare capacity. There is no ability to "share".

Time Division Multiple Access This is where the available time is split into time slots giving one user the whole of the spectrum, but for a limited time. After that the bandwith is given to the next user. This method of multiple access does allow sharing, the demand for the precious time on the channel is fairly shared between users. It does however rely on precise synchronisation for the user of the channel.

Time and Frequency Division Multiple Access Together The best use of the scare frequency resource is to mix FDMA and TDMA. The frequency / time space is split into block and these are allocated in a fair manner. Whilst this is a complex channel allocation, it does allow for an efficient use of the precious resource in a 'fair' way.