Orthogonal Frequency Division Multiple Access (OFDMA)
It 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.
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 rate by assigning different code spreading factor or 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.
History of MIMO
1975,1976
A.R. Kaye and D.A. George and W. van van Etten
created earliest ideas
1984,1986
Jack Winters and Jack Salz
published several papers on beamforming
1993
Arogyaswami Paulraj and Thomas Kailath
proposed the concept of Spatial Multiplexing using MIMO
1994
Patent No. 5,345,599 issued 1994 on Spatial Multiplexing
1996
Greg Raleigh and Gerard J. Foschini refine new approaches to MIMO
technology
1998
Bell Labs was the first to demonstrate a laboratory prototype of SM
2006
MIMO-OFDMA based solutions for IEEE 802.16e WIMAX broadband mobile
standard.
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