Frequency-division multiple access
Encyclopedia
Frequency Division Multiple Access or FDMA is a channel access method
used in multiple-access protocols as a channelization protocol. FDMA gives users an individual allocation of one or several frequency bands, or channel
s. It is particularly commonplace in satellite communication. FDMA, like other Multiple Access systems, coordinates access between multiple users. Alternatives include TDMA
, CDMA, or SDMA
. These protocols are utilized differently, at different levels of the theoretical OSI model
.
Disadvantage: Crosstalk may cause interference among frequencies and disrupt the transmission.
FDMA is distinct from frequency division duplexing (FDD). While FDMA allows multiple users simultaneous access to a transmission system, FDD refers to how the radio channel is shared between the uplink
and downlink (for instance, the traffic going back and forth between a mobile-phone and a mobile phone base station). Frequency-division multiplexing
(FDM) is also distinct from FDMA. FDM is a physical layer technique that combines and transmits low-bandwidth channels through a high-bandwidth channel. FDMA, on the other hand, is an access method in the data link layer
.
FDMA also supports demand assignment
in addition to fixed assignment. Demand assignment allows all users apparently continuous access of the radio spectrum
by assigning carrier frequencies on a temporary basis using a statistical assignment process. The first FDMA demand-assignment system for satellite was developed by COMSAT
for use on the Intelsat
series IVA and V satellites.
There are two main techniques:
Channel access method
In telecommunications and computer networks, a channel access method or multiple access method allows several terminals connected to the same multi-point transmission medium to transmit over it and to share its capacity...
used in multiple-access protocols as a channelization protocol. FDMA gives users an individual allocation of one or several frequency bands, or channel
Channel (communications)
In telecommunications and computer networking, a communication channel, or channel, refers either to a physical transmission medium such as a wire, or to a logical connection over a multiplexed medium such as a radio channel...
s. It is particularly commonplace in satellite communication. FDMA, like other Multiple Access systems, coordinates access between multiple users. Alternatives include TDMA
Time division multiple access
Time division multiple access is a channel access method for shared medium networks. It allows several users to share the same frequency channel by dividing the signal into different time slots. The users transmit in rapid succession, one after the other, each using its own time slot. This...
, CDMA, or SDMA
Space-division multiple access
Space-Division Multiple Access is a channel access method based on creating parallel spatial pipes next to higher capacity pipes through spatial multiplexing and/or diversity, by which it is able to offer superior performance in radio multiple access communication systems...
. These protocols are utilized differently, at different levels of the theoretical OSI model
OSI model
The Open Systems Interconnection model is a product of the Open Systems Interconnection effort at the International Organization for Standardization. It is a prescription of characterizing and standardizing the functions of a communications system in terms of abstraction layers. Similar...
.
Disadvantage: Crosstalk may cause interference among frequencies and disrupt the transmission.
Features
- In FDMA all users share the satellite 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 TDMATime division multiple accessTime division multiple access is a channel access method for shared medium networks. It allows several users to share the same frequency channel by dividing the signal into different time slots. The users transmit in rapid succession, one after the other, each using its own time slot. This...
and CDMA. - FDMA is not vulnerable to the timing problems that TDMATime division multiple accessTime division multiple access is a channel access method for shared medium networks. It allows several users to share the same frequency channel by dividing the signal into different time slots. The users transmit in rapid succession, one after the other, each using its own time slot. This...
has. Since a predetermined frequency band is available for the entire period of communication, stream data (a continuous flow of data that may not be packetized) can easily be used with FDMA. - Due to the frequency filtering, FDMA is not sensitive to near-far problemNear-far problemThe near-far problem or hearability problem is a situation that is common in wireless communication systems, in particular, CDMA. In some signal jamming techniques, the near-far problem is exploited to disrupt communications.-Technical explanation:...
which is pronounced for CDMA. - Each user transmits and receives at different frequencies as each user gets a unique frequency slot
FDMA is distinct from frequency division duplexing (FDD). While FDMA allows multiple users simultaneous access to a transmission system, FDD refers to how the radio channel is shared between the uplink
Uplink
A telecommunications link is generally one of several types of information transmission paths such as those provided by communication satellites to connect two points on earth.-Uplink:...
and downlink (for instance, the traffic going back and forth between a mobile-phone and a mobile phone base station). Frequency-division multiplexing
Frequency-division multiplexing
Frequency-division multiplexing is a form of signal multiplexing which involves assigning non-overlapping frequency ranges to different signals or to each "user" of a medium.- Telephone :...
(FDM) is also distinct from FDMA. FDM is a physical layer technique that combines and transmits low-bandwidth channels through a high-bandwidth channel. FDMA, on the other hand, is an access method in the data link layer
Data link layer
The data link layer is layer 2 of the seven-layer OSI model of computer networking. It corresponds to, or is part of the link layer of the TCP/IP reference model....
.
FDMA also supports demand assignment
Demand assignment
In telecommunication, a demand assignment is a method which several users share access to a communications channel on a real-time basis, i.e., a user needing to communicate with another user on the same network requests the required circuit, uses it, and when the call is finished, the circuit is...
in addition to fixed assignment. Demand assignment allows all users apparently continuous access of the radio spectrum
Radio spectrum
Radio spectrum refers to the part of the electromagnetic spectrum corresponding to radio frequencies – that is, frequencies lower than around 300 GHz ....
by assigning carrier frequencies on a temporary basis using a statistical assignment process. The first FDMA demand-assignment system for satellite was developed by COMSAT
COMSAT
The Communications Satellite Corporation is a global telecommunications company, based in the USA, and with branches in Brazil, Argentina, Colombia, Mexico, Peru, Venezuela and several other countries in the Americas. It is present also in Turkey...
for use on the Intelsat
Intelsat
Intelsat, Ltd. is a communications satellite services provider.Originally formed as International Telecommunications Satellite Organization , it was—from 1964 to 2001—an intergovernmental consortium owning and managing a constellation of communications satellites providing international broadcast...
series IVA and V satellites.
There are two main techniques:
- Multi-channel per-carrier (MCPC)
- Single-channel per-carrier (SCPC)