The reconfigurable intelligent surface (RIS) fundamentally changes the existing antijamming paradigm by reconstructing the wireless propagation environment. However, the existing RIS-based anti-jamming work mainly considers narrow-band frequency-flat fading channels. As an extension of existing work, the use of RIS to enhance the antijamming capability of broadband orthogonal frequency division multiplexing (OFDM) system in frequency-selective fading channels is first studied. In the multipath frequencyselective fading channels, the anti-jamming design needs to satisfy the channel gains of all signal paths with different delays. To this end, the authors propose a joint design scheme of active precoding at the transmitter and passive precoding at the RIS to attenuate the jamming signal and enhance the legitimate signal, and formulate the joint design objective to maximize the average rate of all subcarriers. An alternating optimization algorithm is developed to address this non-convex problem due to the complex coupling of active and passive precoding. In detail, the active and passive precoding are solved by the Cauchy-Schwarz inequality and power water-filling method and the alternating direction method of multipliers method assisted by multidimensional complex quadratic transform, respectively. Simulation results demonstrate that the proposed method can significantly improve the anti-jamming communication performance of the OFDM system compared with the scheme without RIS. In addition, the impact of the number of RIS elements and deployment location on system performance is revealed.

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