Yu.K. Sveshnikov, S.Yu. Lobacheva, A.A. Mukhametova, A.A. Vasenina

The task of constructing the frequency response in HF channel as a part of the communication complex has been formulated in the course of path testing based on passive sounding. ONIIP has developed and tested an algorithm for constructing HF path frequency response based on the deciphered convolution - chirp ionogram. The feature of the proposed algorithm is that due to the addition in quadratures the instantaneous frequency response is constructed in complex form at particular time with 5-minute intervals while evaluating the propagation over the entire HF frequency range 2-30 MHz. The essence of the algorithm is as follows. The algorithm input is an array of secondary data from the ionogram decipherer, which contains time delay values vector TK, frequency values vector FK, and amplitude values vector AK in a matrix form. The length of these vectors depends on the decipherer FFT base and dynamically on the number of peaks detected in the chirp signal convolution exceeding the detection threshold relative to the average interference level. Output data in each algorithm cycle are as follows: f- output frequency vector, a - real quadrature vector, b - imaginary quadrature vector. The algorithm core includes operations for calculating in the complex form the total instantaneous frequency response for each current frequency over all detected beams of the chirp signal. The algorithm scheme and detailed description in writing are given. For displaying the selected single-beam or multibeam fragments of the frequency response the decipher output data are formed as separate files in compliance with the number of beams on the fragment or the numbers of hops (tracks) of multihop paths. In the program developed using this algorithm by pressing the manipulator left button on the field of the frequency response plot in the cursor area close to one of the frequency response points the accurate numerical value of the carrier frequency and HF path transmission ratio at this point in dB are displayed in the window. The figures show 2 typical HF path frequency responses calculated by the program developed using the proposed algorithm. The chirp convolution source audiofile (ionogram), identification number 08270913.wav, was obtained by testing the chirp sounder on the path Cyprus-Omsk in August 2005, ionogram 09111035 was obtained for the path Cyprus-Moscow in September 2006. To evaluate the validity of the results obtained by the processing program based on the proposed algorithm the paper as an example uses the comparison with the chirp signal convolution envelope observed on the oscilloscope or its computer analog in *.wav audiofile of the chirp convolution. The qualitative evaluation reveals a good agreement in time of the convolution envelope and the channel frequency response and confirms the correct concept of constructing the algorithm. The frequency response fine structure in close vicinity to MUF is of interest. As an example, the figure shows the frequency response of ionogram 10231000.wav for Irkutsk-Omsk path dated October 23, 2008. The ionogram is artificially limited in the local frequency domain and is deciphered with a higher frequency resolution. Due to the addition of multiple beams including Pedersen beam the amplitude "beat" is observed which is especially important close to MUF as in the adaptive radio links the operating frequency is automatically selected by the short-term prediction within the range of the best propagation frequencies, i.e. just close to MUF and the propagation fluctuations are in the range 15-28 dB. Existing techniques of constructing HF channel frequency response by the averaged model by the definition cannot reflect the frequency response current fine structure. The more so as the averaged model is constructed in the frequency-amplitude plane not taking into account phases of multiple beams. The proposed technique takes into account 3-dimensional frequency-amplitude-delay space and multiple beams and therefore the frequency response fine structure is revealed. This is the difference between the proposed algorithm and the technique of fitting the MUF plot to the radio wave propagation model averaged over longer intervals. If necessary by adding the results of deciphering several ionograms, for example 3 per 15-minute interval, it is possible to obtain the averaged frequency response that reduces the dispersion of the instantaneous frequency response. Developed and tested on a set of real ionograms the algorithm for constructing the HF path instantaneous frequency response after time and frequency averaging is recommended for the assignment of optimal radio frequencies for radio communication systems.