4.3 GSM Multiple Access Techniques

Multiple Access Techniiques

• Purpose: to allow several users to share the resources of the
air interface in one cell
• Methods:
• FDMA - Frequency Division Multiple Access
• TDMA - Time Division Multiple Access
• CDMA - Code Division Multiple Access

Multiple access techniques are essential to allow more efficient use of the radio spectrum. 1st
generation systems used only FDMA so that a complete radio carrier was allocated to a user
throughout their call. This made poor use of the spectrum, but was all that was possible with
an analogue system.

Frequency Division Multiple Access (FDMA)

• Divide available frequency spectrum
into channels each of the same bandwidth
• Channel separation achieved by filters:
• Good selectivity
• Guard bands between channels
• Signalling channel required to allocate a traffic
channel to a user
• Only one user per frequency channel at any
time
• Used in analog systems, such as AMPS, TACS
• Limitations on:
• frequency re-use
• number of subscribers per area

Time Diiviisiion Mullttiipllee Acceessss ((TDMA))

• Access to available spectrum is limited to timeslots
• User is allocated the spectrum for the duration of one timeslot
• Timeslots are repeated in frames

TDMA became possible with digital systems such as GSM in which the data stream could be
divided into bursts and allocated to a timeslot. By sharing access to the spectrum, the traffic
capacity of the system is enhanced. GSM uses both FDMA to provide carriers and TDMA to
share access to the carriers.