World Library  
Flag as Inappropriate
Email this Article
 

Ofdma

Orthogonal Frequency-Division Multiple Access (OFDMA) is a multi-user version of the popular orthogonal frequency-division multiplexing (OFDM) digital modulation scheme. Multiple access is achieved in OFDMA by assigning subsets of subcarriers to individual users as shown in the illustration below. This allows simultaneous low data rate transmission from several users.

Key features

The advantages and disadvantages summarized below are further discussed in the Characteristics and principles of operation section. See also the list of OFDM Key features.

Claimed advantages over CDMA

Claimed advantages over OFDM with time-domain statistical multiplexing

  • Allows simultaneous low-data-rate 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.
  • Combat narrow-band interference.

Claimed OFDMA Advantages

  • Flexibility of deployment across various frequency bands with little needed modification to the air interface.[1]
  • Averaging interferences from neighboring cells, by using different basic carrier permutations between users in different cells.
  • Interferences within the cell are averaged by using allocation with cyclic permutations.
  • Enables Single Frequency Network coverage, where coverage problem exists and gives excellent coverage.
  • Offers Frequency diversity by spreading the carriers all over the used spectrum.
  • Allows per channel or per subchannel power

Recognized disadvantages of OFDMA

  • Higher sensitivity to frequency offsets and phase noise.[1]
  • 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, are constantly active independent of the data rate, which is inefficient from power consumption point of view, while OFDM combined with data packet scheduling may allow FFT algorithm to hibernate 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.

Characteristics and principles of operation

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.

Different numbers 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.

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.

OFDMA can 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-data-rate users can send continuously with low transmission power instead of using a "pulsed" high-power carrier. Constant delay, and shorter delay, can be achieved.

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.

OFDMA is considered as highly suitable for broadband wireless networks, due to advantages including scalability and MIMO-friendliness, and ability to take advantage of channel frequency selectivity.[1]

In spectrum sensing cognitive radio, OFDMA is a possible approach to filling free radio frequency bands adaptively. Timo A. Weiss and Friedrich K. Jondral of the University of Karlsruhe proposed a spectrum pooling system in which free bands sensed by nodes were immediately filled by OFDMA subbands.

Usage

OFDMA is used in:

OFDMA is also a candidate access method for the IEEE 802.22 Wireless Regional Area Networks (WRAN). The project aims at designing the first cognitive radio based standard operating in the VHF-low UHF spectrum (TV spectrum).

Trademark and patents

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

See also

Notes

References

  • K. Fazel and S. Kaiser, Multi-Carrier and Spread Spectrum Systems: From OFDM and MC-CDMA to LTE and WiMAX, 2nd Edition, John Wiley & Sons, 2008, ISBN 978-0-470-99821-2.
  • Miriam Bank, M. Bank, M. Haridim, B. Hill "OFDMA in high-speed mobile systems, pilots and simulation problems Int'l. J. of Communications, 1(4), 2007 (173-179)

External links

  • Orthogonal Frequency Division Multiple Access: is it the multiple access system of the future?, S. Srikanth, V. Kumaran, C. Manikandan et al., AU-KBC Research Center, Anna University, India.
  • Short Introduction to OFDM - Tutorial written by Prof. Debbah, head of the Alcatel-Lucent Chair on flexible radio.
  • OFDMA Pioneer Runcom- Inventors and creators of OFDMA
  • http://www.naun.org/journals/communications/c-28.pdf - Miriam Bank, M. Bank, M. Haridim, B. Hill OFDMA in high-speed mobile systems, pilots and simulation problems
  • http://ieeexplore.ieee.org/xpl/login.jsp?tp=&arnumber=1304951&url=http%3A%2F%2Fieeexplore.ieee.org%2Fiel5%2F11%2F28985%2F01304951.pdf%3Farnumber%3D1304951 - M. Bank, "On Increasing OFDM Method Frequency Efficiency Opportunity " IEEE Transaction on Broadcasting, vol. 50 NO 2, June 2004.
This article was sourced from Creative Commons Attribution-ShareAlike License; additional terms may apply. World Heritage Encyclopedia content is assembled from numerous content providers, Open Access Publishing, and in compliance with The Fair Access to Science and Technology Research Act (FASTR), Wikimedia Foundation, Inc., Public Library of Science, The Encyclopedia of Life, Open Book Publishers (OBP), PubMed, U.S. National Library of Medicine, National Center for Biotechnology Information, U.S. National Library of Medicine, National Institutes of Health (NIH), U.S. Department of Health & Human Services, and USA.gov, which sources content from all federal, state, local, tribal, and territorial government publication portals (.gov, .mil, .edu). Funding for USA.gov and content contributors is made possible from the U.S. Congress, E-Government Act of 2002.
 
Crowd sourced content that is contributed to World Heritage Encyclopedia is peer reviewed and edited by our editorial staff to ensure quality scholarly research articles.
 
By using this site, you agree to the Terms of Use and Privacy Policy. World Heritage Encyclopedia™ is a registered trademark of the World Public Library Association, a non-profit organization.
 


Copyright © World Library Foundation. All rights reserved. eBooks from Project Gutenberg are sponsored by the World Library Foundation,
a 501c(4) Member's Support Non-Profit Organization, and is NOT affiliated with any governmental agency or department.