(frequency division multiple access)

What is FDMA ?

• FDMA (frequency division multiple access) is the division of the frequency band allocated for wireless cellular telephone communication into 30 channels, each of which can carry a voice conversation or, with digital service, carry digital data.

• FDMA is a basic technology in the analog Advanced Mobile Phone System (AMPS), the most widely-installed cellular phone system in North America.

• With FDMA, each channel can be assigned to only one user at a time.

• FDMA is a method that lets more than one user to share a single radio frequency spectrum. This is done by assigning active users a different frequency channel.

• FDMA puts each call on a separate frequency.• The Digital-Advanced Mobile Phone Service (D-AMPS) also

uses FDMA but adds time division multiple access (TDMA)

• Frequency division Multiple Access (FDMA) is a technology whereby the total amount of spectrum is divided in a number of channels.

• Between the different used frequency channels is a small amount of bandwidth not used. This space is called a guard band.

• This bandwidth is necessary to cater for instability of the sender, frequency shifts due to movement (the Doppler effect) and no-ideal filtering.

• FDMA separates the spectrum into distinct voice channels by splitting it into uniform chunks of bandwidth.

• While it is certainly capable of carrying digital information, FDMA is not considered to be an efficient method for digital transmission.

FDMA• All channels in a cell are available to all the mobiles. Channel

assignment is carried out on a first-come first- served basis.• The number of channels, given a frequency spectrum BT , depends

on the modulation technique (hence Bw or Bc ) and the guard bands between the channels 2Bguard . These guard bands allow for imperfect filters and oscillators and can be used to minimize adjacent channel interference.

• FDMA is usually implemented in narrowband systems.

Single-carrier FDMA

• Single-carrier FDMA (SCFDMA)- SC-FDMA can be interpreted as a linearly precoded OFDMA scheme, in the sense that it has an additional DFT processing preceding the conventional OFDMA processing

Main features• Continuous transmission : the channels, once assigned, are

used on a non-time-sharing basis

• Narrow bandwidth : Analog cellular systems use 25-30 kHz. Digital FDMA systems can make use of low bit rate speech coding techniques to reduce the channel band even more.

• Low overhead : Carry overhead messages for control, synchronization purposes

• Simple hardware at mobile unit and BS : (1) no digital processing needed to combat ISI (2) ease of framing and synchronization.

Main Features• In FDMA all users share the satellite transponder or frequency

channel simultaneously but each user transmits at single frequency.

• FDMA can be used with both analog and digital signal.

• FDMA requires high-performing filters in the radio hardware, in contrast to TDMA and CDMA.

• FDMA is not vulnerable to the timing problems that TDMA has.

• Due to the frequency filtering, FDMA is not sensitive to near-far problem which is pronounced for CDMA.

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