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, multiple-input multiple-output).

     

  • 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.