A Based Time-frequency Analysis Method for Selecting a Time-window of Low Noise

A method for avoiding cluttering influence by applying orthogonal demodulation and adaptive optimum kernel (AOK) is presented in this paper. This method is dissimilar to the traditional method of filtering clutter, applying time-frequency analysis to find out a time interval that has a little or without clutter to transmit a test signal. First, outdoor measurement was carried out in four measurement environment which including buildings, squares, car parks, and playgrounds. Then, the orthogonal demodulation is adopted to obtain the IQ data of clutter, and the time-frequency characteristic is then calculated by AOK based on the obtained IQ data. Meanwhile, the lengthy signal period is selected as the time window of time-frequency analysis in order to make the time window short. Through analyzing the results of four cases, it follows that the clutter near the building and car pack has a great influence on the test signal, but there a time interval of 400–800 ms can be transmitted a test signal with a special frequency of 60 MHz or frequency range 0–40MHz. There is a little clutter in the square and playground, so the time interval which can be utilized to transmit the test signal is longer than in other cases and the power of clutter is weak. The frequency range which can be selected to transmit a test signal is wider than in other measurement environment. The above analysis can demonstrate that the mentioned method can achieve the time interval of transmitting test signal has a negligible influence or without clutter. This can be utilized to increase the accuracy of measurement.