Orthogonal frequency-division multiple access
From Wikipedia, the free encyclopedia
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 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.
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.
Contents |
[edit] Key Features
See also the list of OFDM Key features.
[edit] Advantages over CDMA
- OFDM can combat multipath with less complexity.
- OFDMA can achieve higher spectral efficiency.
[edit] 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] OFDMA Advantages
- Averaging interference's from neighboring cells, by using different basic carrier permutations between users in different cells.
- Interference’s within the cell are averaged by using allocation with cyclic permutations.
- Enables orthogonality in the uplink by synchronizing users in time and frequency.
- Enables Multipath mitigation without using Equalizers and training sequences.
- Enables Single Frequency Network coverage, where coverage problem exists and gives excellent coverage.
- Enables spatial diversity by using antenna diversity at the Base Station and possible at the Subscriber Unit.
- Enables adaptive modulation for every user QPSK, 16QAM, 64QAM and 256QAM.
- 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.
- The power gain can be translated to distance - 3 times the distance for R4 and 8 time for R2 for LOS conditions.
- Enabling the usage of Indoor Omni Directional antennas for the users.
- MAC complexity is the same as for TDMA systems.
- Allocating carrier by OFDMA/TDMA strategy.
- Minimal delay per OFDMA symbol of 300 usec.
- Using Small burst per user of about 100 symbols for better statistical multiplexing and smaller jitter.
- User symbol is several times longer then for TDMA systems.
- Using the FEC to the outmost by error detection of disturbed frequencies.
[edit] 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 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] 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)
- WiMax
- Wibro
[edit] References
- K. Fazel and S. Kaiser, Multi-Carrier and Spread Spectrum Systems, John Wiley & Sons, 2003, ISBN 0-470-84899-5
