Mobile Phone Technology

Multiplexing

Frequency and Time Division Multiplexing

image ©LINK-PP


Mobile phone operators have limited bandwidth, so they need to divide the available space among different users. The available bandwidth is split into channels, which are then allocated to users. One of their requirements is to share these channels between users. Often, the entire bandwidth is divided into these channels. However, the operators need to make the best use of these channels; they cannot afford to allocate a single channel to each user. There are two basic methods of sharing the available frequency:

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

Frequency Division Multiple Access In FDMA, the available bandwidth is split into channels, each of which is separated from its neighbours. These separate channels allow different users to access the bandwidth simultaneously, albeit with more limited bandwidth. However, this can waste resources due to interference between adjoining channels. Thus, space is made between the channels to prevent interference. In a pure FDMA environment, not all users have the same demands. For example, one channel may be busy, restricting one user, while another channel may have spare capacity and not be well used. There is no ability to 'share'.

Time Division Multiple Access The available time is split into time slots, giving one user the entire spectrum for a limited time. After that, the bandwidth is given to the next user. This method of multiple access allows sharing, and the demand for time on the channel is fairly distributed among users. However, it does rely on precise synchronisation for channel usage.

Time and Frequency Division Multiple Access Together The best use of the scarce frequency resource is to combine frequency division multiple access (FDMA) and TDMA. The frequency/time space is divided into blocks, which are then allocated fairly. Although this is a complex channel allocation process, it allows for the efficient use of this precious resource in a 'fair' way.

deepl.com [AI] has been used to improve the text in this description