Orthogonal frequency-division multiple access
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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 figure below. This allows simultaneous low data rate transmission from several users.
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 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.
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 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-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.
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.
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]
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[edit] Key Features
See also the list of OFDM Key features.
[edit] Claimed advantages over CDMA
- OFDM can combat multipath with more robustness and less complexity.[1]
- OFDMA can achieve a higher MIMO spectral efficiency due to providing flatter frequency channels than a CDMA RAKE receiver can.[1]
[edit] 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.
[edit] 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 orthogonality in the uplink by synchronizing users in time and frequency.[1]
- Enables Single Frequency Network coverage, where coverage problem exists and gives excellent coverage.
- Enables adaptive carrier allocation in multiplication of 23 carriers = nX23 carriers up to 1587 carriers (all data carriers).
- Offers Frequency diversity by spreading the carriers all over the used spectrum.
- Offers Time diversity by optional interleaving of carrier groups in time.
- Using the cell capacity to the utmost by adaptively using the highest modulation a user can use, this is allowed by the gain added when less carriers are allocated (up to 18dB gain for 23 carrier allocation instead of 1587 carriers), therefore gaining in overall cell capacity.
[edit] Recognised 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, is 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 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.
[edit] Usage
OFDMA is used in the mobility mode of IEEE 802.16 WirelessMAN Air Interface standard, commonly referred to as WiMAX.
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).
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.
[edit] References
- ^ a b c d e f Hujun Yin and Siavash Alamouti (August 2007). "OFDMA: A Broadband Wireless Access Technology". IEEE Sarnoff Symposium, 2006 (IEEE): pp. 1-4. doi:.
[edit] Literature
- 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.
[edit] See also
- Code division multiple access
- Frequency division multiple access
- Time division multiple access
- Single carrier FDMA (SC-FDMA), a.k.a Linearly precoded OFDMA (LP-OFDMA)
- 3GPP Long Term Evolution
- WiMAX
- WiBro
[edit] 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.
