Blind channel evaluation using OFDM-FHSS technology
Authors: Kartashevsky V.G. , Semenov E.S. , Cymbal V.A. 
Published: October 2024

Pseudo-random tuning of the operating frequency (PRFC) is one of the effective methods of spectrum expansion, in which the signal occupies a frequency band significantly wider than the minimum required for transmitting information. Тhe operating frequency of the signal is hopped over a wide range of the frequency range allocated for communication in accordance with a pseudo-random code, known only on the receiving side and unknown to anyone trying to intercept a radio transmission or organize jamming. The main disadvantage of HFPR is the low data transfer rate. Therefore, recently there have been ideas of using the hopping method with broadband signals, for example OFDM-hopping where the frequency changes within a set of orthogonal carriers. This approach makes it possible to preserve the advantages of the frequency converter method, supplementing them with the ability to implement high-speed digital communications. The paper considers the situation when one OFDM symbol is transmitted in the time interval of one hop, formed without a prefix and postfix. In this case, the transmission signal in the time interval of one step was a set of sequentially transmitted complex samples of the OFDM symbol envelope. Samples of the OFDM symbol envelope, after they are generated during transmission in the OBPF block, must be sequentially transmitted over a communication channel whose properties are unknown. In the case when there is temporary dissipation of the energy of the transmitted signal in the channel (channel memory), during the sequential transmission of complex samples of the OFDM symbol envelope, due to temporary dissipation, each of the previous samples will have an interference effect on any transmitted sample at reception. Assuming that the channel parameters are constant over the analysis interval at the receiving location, it is necessary, with an unknown impulse response of the channel, to generate estimates of the OFDM symbol envelope samples, which, in order to solve the demodulation problem, must be subjected to the FFT calculation operation. The constancy of the channel parameters over the analysis interval allows us to solve the problem of estimating samples of the channel impulse response using blind identification based on the maximum likelihood method. In this case, it is necessary to use diversity reception and impose some non-burdensome restrictions on the statistical properties of the transmitted message. After solving the problem of identifying the impulse response of a channel with memory, the problem of estimating the components of the sample vector of the envelope of the received OFDM symbol is solved by the regularization method. Using the “reception “as a whole” with element-by-element assessment generation” (PCPFO) algorithm and using “assessment feedback” (EFE), minimization of the regularizing functional leads to the solution of a system of linear algebraic equations, the order of which is determined by the number of samples of the impulse response of the communication channel with memory. By modeling, the upper limit of noise immunity for receiving OFDM-QAM-4 signals was obtained with a Gaussian distribution of errors in estimating OFDM-symbol envelope samples. The proposed procedure for normalizing envelope samples after solving systems of linear equations eliminates the appearance of outliers characteristic of the generated noise sequence.