Orthogonal Frequency Division Multiple Access (OFDMA) is a multi-user version of the popular OFDM digital modulation scheme. Multiple access is achieved in OFDMA by assigning subsets of subcarriers to individual users as shown in the figure below. This allows simultaneous low data rate transmission from several users. Statistical multiplexing is similar to time-division multiplexing (TDM), except that, rather than arbitrarily assigning a time slot to each signal, each signal is assigned a slot according to priority and need. ... Wikipedia does not yet have an article with this exact name. ... Time division multiple access (TDMA) is a channel access method for shared medium (usually radio) networks. ... Frequency-hopping spread spectrum (FHSS) is a spread-spectrum method of transmitting radio signals by rapidly switching a carrier among many frequency channels, using a pseudorandom sequence known to both transmitter and receiver. ... In telecommunications, direct-sequence spread spectrum (DSSS) is a modulation technique. ... In communications, a channel access method is used to share a communications channel or physical communications medium between multiple users. ... The Media Access Control (MAC) data communication protocol sub-layer, also known as the Medium Access Control, is a part of the data link layer specified in the seven-layer OSI model (layer 2). ... Orthogonal Frequency-Division Multiplexing (OFDM) — essentially identical to Coded OFDM (COFDM) — is a digital multi-carrier modulation scheme, which uses a large number of closely-spaced orthogonal sub-carriers. ... In telecommunication, the term multiple access has the following meanings: The connection of a user to two or more switching centers by separate access lines using a single message routing indicator or telephone number. ...

Based on feedback information about the channel conditions, adaptive user-to-subcarrier assignment can be achieved. If the assignment is done sufficiently fast, this further improves the OFDM robustness to fast fading and narrow-band cochannel interference, and makes it possible to achieve even better system spectral efficiency. Fading (or fading channels) are mathematical models for the distortion that a carrier-modulated telecommunication signal experiences over certain propagation media. ... Spectral efficiency or Spectrum efficiency is the amount of information that can be transmitted over a given bandwidth in a specific digital communication system. ...

Different number of sub-carriers can be assigned to different users, in view to support differentiated Quality of Service (QoS), i.e. to control the data rate and error probability individually for each user. In the fields of packet-switched networks and computer networking, the traffic engineering term Quality of Service (QoS) refers to control mechanisms that can provide different priority to different users or data flows, or guarantee a certain level of performance to a data flow in accordance with requests from the...

OFDMA resembles code division multiple access (CDMA) spread spectrum, where users can achieve different data rates by assigning a different code spreading factor or a different number of spreading codes to each user. Code division multiple access (CDMA) is a form of multiplexing and a method of multiple access to a physical medium such as a radio channel, where different users use the medium at the same time thanks to using different code sequences. ...

OFDMA can also be seen as an alternative to combining OFDM with time division multiple access (TDMA) or time-domain statistical multiplexing, i.e. packet mode communication. Low datarate users can send continuously with low transmission power instead of using a "pulsed" high-power carrier. Constant delay, and shorter delay, can be achieved. Time division multiple access (TDMA) is a channel access method for shared medium (usually radio) networks. ... Statistical multiplexing is similar to time-division multiplexing (TDM), except that, rather than arbitrarily assigning a time slot to each signal, each signal is assigned a slot according to priority and need. ... Wikipedia does not yet have an article with this exact name. ...

However, OFDMA can also be described as a combination of frequency domain and time domain multiple access, where the resources are partitioned in the time-frequency space, and slots are assigned along the OFDM symbol index as well as OFDM sub-carrier index.

Image File history File links Size of this preview: 800 × 282 pixelsFull resolution (816 × 288 pixel, file size: 15 KB, MIME type: image/png) Author : Mohammad A. Kanso & Johnny N. Maalouf I, the copyright holder of this work, hereby release it into the public domain. ...

Key features

See also the list of OFDM Key features. Orthogonal frequency division modulation (OFDM, also called orthogonal frequency division multiplexing) is a technique for the modulation of digital information onto an analog carrier electromagnetic (e. ...

Advantages over CDMA

• OFDM can combat multipath with less complexity.
• OFDMA can achieve higher spectral efficiency.

Advantages over OFDM with time-domain statistical multiplexing

• Allows simultaneous low datarate transmission from several users.
• Pulsed carrier can be avoided.
• Lower maximum transmission power for low data rate users.
• Shorter delay, and constant delay.
• Contention based multiple access (collision avoidance) is simplified.
• Further improves OFDM robustness to fading and interference.

Disadvantages

• Asynchronous data communication services such as web access are characterized by short communication bursts at high data rate. Few users in a base station cell are transferring data simultaneously at low constant data rate.
• The complex OFDM electronics, including the FFT algorithm and forward error correction, is constantly active independent on the data rate, which is inefficient from power consumption point of view, while OFDM combined with data packet scheduling may allow that the FFT algorithm hibernates during certain time intervals.
• The OFDM diversity gain, and resistance to frequency-selective fading, may partly be lost if very few sub-carriers are assigned to each user, and if the same carrier is used in every OFDM symbol. Adaptive sub-carrier assignment based on fast feedback information about the channel, or sub-carrier frequency hopping, is therefore desirable.
• Dealing with co-channel interference from nearby cells is more complex in OFDM than in CDMA. It would require dynamic channel allocation with advanced coordination among adjacent base stations.
• The fast channel feedback information and adaptive sub-carrier assignment is more complex than CDMA fast power control.

A cleverly mounted base station on top of a Electricity pylon. ... FFT may be: Fast Fourier transform Finite Fourier transform, another name for the discrete Fourier transform US Navy hull classification symbol for Reserve Training Frigates Final Fantasy Tactics, a video game. ... In telecommunication, forward error correction (FEC) is a system of error control for data transmission, whereby the sender adds redundant data to its messages, which allows the receiver to detect and correct errors (within some bound) without the need to ask the sender for additional data. ... Selective fading causes a cloudy pattern to appear on a spectrogram display. ... In cellular radio systems and wireless networks based on FDMA, perhaps in combination with TDMA, channel allocation schemes are required to allocate frequency channels to base stations and access points, and to avoid co-channel interference among nearby Cell site. ...

Usage

OFDMA is used in the mobility mode of IEEE 802.16 WirelessMAN Air Interface standard, commonly referred to as WiMAX. WiMAX, the Worldwide Interoperability for Microwave Access, is a telecommunications technology aimed at providing wireless data over long distances in a variety of ways, from point-to-point links to full mobile cellular type access. ...

OFDMA is currently a working assumption in 3GPP Long Term Evolution downlink, named High Speed OFDM Packet Access (HSOPA). Also, OFDMA is the candidate access method for the IEEE 802.22 "Wireless Regional Area Networks". The project aims at designing the first cognitive radio based standard operating in the VHF-low UHF spectrum (TV spectrum). 3GPP LTE (Long Term Evolution) is the name given to a project within the Third Generation Partnership Project to improve the UMTS mobile phone standard to cope with future requirements. ... High Speed OFDM Packet Access (HSOPA) is a proposed part of 3GPPs Long Term Evolution (LTE) upgrade path for UMTS systems. ... IEEE 802. ...

The term "OFDMA" is claimed to be a registered trademark by [1]Runcom Technologies Ltd., with various other claimants to the underlying technologies through patents.

See Also

• Code division multiple access
• Frequency-division multiple access
• Time division multiple access
• WiMax
• Wibro

Code division multiple access (CDMA) is a form of multiplexing and a method of multiple access to a physical medium such as a radio channel, where different users use the medium at the same time thanks to using different code sequences. ... This article is in need of attention from an expert on the subject. ... Time division multiple access (TDMA) is a channel access method for shared medium (usually radio) networks. ... WiMAX, the Worldwide Interoperability for Microwave Access, is a telecommunications technology aimed at providing wireless data over long distances in a variety of ways, from point-to-point links to full mobile cellular type access. ... WiBro (Wireless Broadband, Korean: 와이브로) is a wireless broadband Internet technology being developed by the Korean telecoms industry. ...

References

• K. Fazel and S. Kaiser, Multi-Carrier and Spread Spectrum Systems, John Wiley & Sons, 2003, ISBN 0-470-84899-5

Links

• 3GPP Long Term Evolution
• Single Carrier FDMA (SC-FDMA)

3GPP LTE (Long Term Evolution) is the name given to a project within the Third Generation Partnership Project to improve the UMTS mobile phone standard to cope with future requirements. ... There are very few or no other articles that link to this one. ...

External links

• [2] for more details on OFDM
• [3] for more details on FFT and IFFT.