Estimation of signal correlation at spaced antennas for multi-moded ionospherically reflected signals and its effect on the capacity of SIMO and MIMO HF links

MIMO (Multiple Output Multiple Input) systems have been shown to be capable of providing significant capacity improvement for UHF communications in a multipath environment. MIMO utilizes antenna arrays at both transmitting and receiving locations and relies on there being limited correlation between the different propagation paths from any one transmit antenna and any one receiving antenna, a condition which can arise in a multipath propagation environment. It would be very desirable if such capacity increase could be implemented at HF, permitting much higher data rates. To achieve this, it is important that the receiving (or transmitting) antennas are sufficiently far apart that the fading is relatively independent for each transmitter receiver path as otherwise little advantage is gained. Thus in this paper the correlation between receiving spaced antennas for a multipath HF link is investigated. In accomplishing this it is important to include the effect of the time-varying small scale structure in the ionosphere which will gives rises to such reduced correlation. This is achieved using a physically based model of the HF channel incorporating a realistic background ionosphere model (IRI) together with a stochastic model for the embedded time-varying irregularities and determining the propagation on the basis of the complex phase method.