Methods for Spatiotemporal Signal Coding in Non‑Stationary Frequency‑Selective MIMO‑OFDM Channels

The article investigates methods of spatio‑temporal encoding of signals in MIMO-OFDM systems during transmission in non‑stationary frequency‑selective channels. The limitations of traditional approaches (STBC, SFBC) at high levels of Doppler shift and frequency selection are considered. The relevance of the study is due to the need to increase the spectral efficiency and stability of wireless communication systems to intersymbol interference and Doppler distortion. Methods used. The article proposes the structure of a combined spatial encoding algorithm WHSTBC‑CC based on combined suppression and Walsh‑Hadamard transformation. Mathematical models of transmitting and receiving signals are constructed, taking into account the influence of channel distortions and additive white Gaussian noise. To analyze the effectiveness, modeling methods were applied taking into account the normalized values of the Doppler shift and the coefficients of time and frequency correlation. Results. It is shown that the use of WHSTBC‑CC provides an increase in transmission stability and a reduction in the probability of errors at fdT < 0.01. The energy gain coefficient of the method is 5-12 dB compared with the classical STBC and is practically independent of the frequency abundance of the channel. The modification of spatial coding developed in the work using the Walsh-Hadamard transform and the conjugate suppression algorithm partially compensates for these disadvantages, increasing the stability of the system and providing an energy gain of up to 12 dB compared with the classical STBC. Experimental measurements on a mobile SDR platform have confirmed the operability of the proposed approach in a real radio environment, including scenarios with and without direct visibility. The novelty of the work lies in the development of an adaptive spatial coding algorithm that integrates various schemes (STBC, SFBC) depending on the channel parameters. Practical significance. The results obtained can be used in the design and optimization of MIMO‑OFDM systems for mobile communications and broadband access networks, as well as in specialized radio networks operating in conditions of high instability and exposure to electronic jamming.