Adaptive cancellation of nonstationary interference in HF antenna arrays zyxw G.A.Fabrizio Y.I.Abramovich S.J.Anderson D.A. Gray M . D. Tu r I ey zyxwvutsrqp Indexing terms: Adaptive arrays, Interference rejection zyxwvutsrq Abstract: zyxwvutsrqp The paper statistically analyses and compares tempora 1 variability in the spatial structure of two HF interference signals propagated from known locations via ionospheric channels with different spatial and temporal characteristics. The impact of the particular ionospheric paths propagating H F interference on the cancellation performance of various adaptive beamforming algorithms is investigated using data collected by the receiving antenna array of the Jindalee Facility Alice Springs OTH radar in Alice Springs, Australia. Measurements and analysis statistically confirm that the spatial properties of H F interference signals, and the actual performance improvements gained through the use of adaptive beamforming techniques, are highly dependent on the spatio-temporal characteristics of the prevailing ionospheric circuits linking the particular source location to the receiving array at the selected operating frequency. 1 Introduction Much of the time, user congestion in the high-fre- quency (HF) band (3-30MHz) significantly reduces the number of suitable clear frequency channels available for the operation of wireless networks [l]. This poten- tially leads to severe diegradations in the quality of H F communication links and the performance of both sky- wave and H F surface wave radars. In such situations the spatial diversity of antenna arrays may be utilised to adaptively cancel unwanted radio frequency interfer- ence (RFI) whose spatial structure differs from that of the desired signals. Since RFI sources are arbitrarily located with respect to the particular area considered zyxwvu 0 IEE, 1998 IEE Proceedings online no. 19981779 Paper first received 28th April and in revised form 4th November 1997 G.A. Fabrizio, Y.I. Abramovich and D.A. Gray are with the Cooperative Research Centre for Sensor Signal Information Processing (CSSIP), SPRI Building, Technology Park, ‘Be Levels, South Australia 5095, Australia (G.A. Fabrizio is also zyxwvutsrq with DSTO, Salisbury) S.J. Anderson and M.D. Tilrley are with the High Frequency Radar Division, Defence Science and Technology Organisation, PO Box 1500, Salisbury, South Australia, 51~08, Australia for optimum frequency selection, the ionospheric paths propagating H F interference may have characteristics quite different from those which propagate the desired signals. Severe signal distortion may result if the unwanted transmission is propagated via a highly per- turbed ionosphere, as commonly occurs in the equato- rial F-region or the auroral zone. In these circumstances, the spatial characteristics of the received H F interference signals may exhibit significant tempo- ral variability over the required coherent integration time (CIT). Despite the significance of this phenomenon to the H F interference rejection problem, its existence within typical CIT has as yet not been rigorously verified and insufficient attention has been paid to the cancellation performance of adaptive beamforming algorithms in highly nonstationary HF interference environments. It has been observed [2] that temporal variations in mean adaptive processor response were necessary for efficient cancellation of RFI signals; these variations were attributed to the convergence properties of the adaptive algorithm used and also to statistical changes in the received data. The consequences of the ‘spatial nonsta- tionarity’ phenomenon were observed differently in [3], where a significant degradation in rejection perform- ance was noted when the adaptive beamforming weights were held constant to process the entire CIT. This paper statistically analyses temporal variations in the spatial structure of two ionospherically propagated HF interference signals and examines their impact on the rejection performance of various adaptive beam- forming approaches. zyxw 2 The Jindalee Facility Alice Springs (JFAS) receiving system used to collect the RFI data is a narrowband uniform linear array (ULA) located near Mt. Everard, approximately 40 km north-west of Alice Springs. At the receiver site we utilise the outputs of 16 overlapped subarrays, oriented in the north-west direction, with a total aperture spanning 1.4km. Each subarray consists of 28 dual-fan vertically polarised elements spaced 6 m apart, the outputs of which were hardware beam- ware beamformed outputs of each subarray were then demodulated by 16 identical mixers driven by a com- mon local oscillator generating a linear sawtooth sweep-frequency modulated continuous waveform Experimental facility and data collection formed using a delay-line steering method. The hard- 19 IEE Proc.-Radar, Sonar Navig., Vol. zyxwvutsrqpo 145, No. I, February 1998