Multiple-input, multiple-output orthogonal frequency-division multiplexing (MIMO-OFDM) is the dominant air interface for 4G and 5G broadband wireless communications.
It combines multiple-input, multiple-output (MIMO) technology, which multiplies capacity by transmitting different signals over multiple antennas, and orthogonal frequency-division multiplexing (OFDM), which divides a radio channel into a large number of closely spaced subchannels to provide more reliable communications at high speeds.
Space–time codes are employed to ensure that the signals transmitted over the different antennas are orthogonal to each other, making it easier for the receiver to distinguish one from another.
OFDM enables reliable broadband communications by distributing user data across a number of closely spaced, narrowband subchannels.
Though the CP slightly reduces spectral capacity by consuming a small percentage of the available bandwidth, the elimination of ISI makes it an exceedingly worthwhile tradeoff.
DFTs may be applied to composite OFDM signals, avoiding the need for the banks of oscillators and demodulators associated with individual subcarriers.
MIMO-OFDM is a particularly powerful combination because MIMO does not attempt to mitigate multipath propagation and OFDM avoids the need for signal equalization.
CSI may be used, for example, to allocate different size signal constellations to the individual subcarriers, making optimal use of the communications channel at any given moment of time.
The maximum data rate assumes use of the optional 160 MHz channel in the 5 GHz band and 256 QAM (quadrature amplitude modulation).
Again, MU-MIMO beamforming is used to enable the base station to transmit independent data streams to multiple handsets on the same channel at the same time.
This can be done using macro diversity for improved reception of signals from handsets or multi-cell multiplexing to achieve higher downlink data rates.
In a theoretical paper, he proved that with the proper type of MIMO system—multiple, co-located antennas transmitting and receiving multiple information streams using multidimensional coding and encoding—multipath propagation could be exploited to multiply the capacity of a wireless link.
The models showed that for a MIMO system using QAM with an equalizer or DSSS with a rake receiver, computational complexity grows quadratically as data rate is increased.
Clarity developed specifications in the Broadband Wireless Internet Forum (BWIF) that led to the IEEE 802.16 (commercialized as WiMAX) and LTE standards, both of which support MIMO.
Several early papers on multi-user MIMO were authored by Ross Murch et al. at Hong Kong University of Science and Technology.
Prominent researchers to date include Jakob Hoydis (of Alcatel-Lucent), Robert W. Heath (at the University of Texas at Austin), Helmut Bölcskei (at ETH Zurich), and David Gesbert (at EURECOM).
[18] Japanese operator NTT DoCoMo plans to trial 5G technology in collaboration with Alcatel-Lucent, Ericsson, Fujitsu, NEC, Nokia, and Samsung.