Adaptive MIMO communication systems
- In future mobile communication systems, the valuable resource of bandwidth has to be used more
efficiently than today. Therefore, it is necessary to adapt transmission techniques to the propagation
conditions in a better way. For conventional radio communication systems, the temporal and spatial
properties of the radio channel lead to degradations because of time dispersion and fading effects.
Time dispersion causes intersymbol interference which is avoided by multicarrier transmission. The effects
of fading are reduced by adaptive transmission techniques as well as multi-antenna concepts (MIMO,
Parameters of a multi-antenna multicarrier system which can be adapted:
- Type of modulation per subchannel,
- Transmit power per subchannel,
- Channel coding,
- MIMO techniques: diversity methods, beamforming, spatial multiplexing.
Fig. 1: Throughput of an adaptive OFDM-based 2x2 transmission system in indoor (left) and outdoor (right) scenario
Implementing adaptive techniques (e.g. bit-loading, adaptive modulation and coding, and adapting the MIMO technique)
results in significant gains in throughput and reliability over non-adaptive systems. As an example, Fig. 1 illustrates the
advantage of an adaptive MIMO system in terms of its throughput. In this setup, both the MIMO scheme and the modulation
orders on each subchannel have been optimally adapted to the current state of the radio channel.