Signal processing methods in the OFDM radar

Abstract

This paper presents novel methods for solving several problems of orthogonal frequency division multiplexing (OFDM) radar. First, a method to cancel mutual interference between OFDM radar systems is proposed. The method is implemented by inserting pre-assigned pilot signals into OFDM radar waveforms. This allows the received interfering signal to be reconstructed, after which it is subtracted from the received signals to cancel the interfering signal. The proposed method not only makes it possible to detect undetectable targets due to the interfering signal but also increases the signal-to-noise-plus-interference ratio (SNIR) of the detected targets. Second, a time-frequency phase-coded sub-Nyquist sampling orthogonal frequency division multiplexing (PC-SNS-OFDM) radar system is also introduced to reduce the analog-to-digital converter (ADC) sampling rate without any additional hardware or signal processing. The proposed radar divides the transmitted OFDM signal into multiple sub-bands along the frequency axis and provides orthogonality to these sub-bands by multiplying phase codes in both the time and frequency domains. The proposed PC-SNS-OFDM radar effectively eliminates symbol-mismatch noise while introducing trade-offs in the range and Doppler ambiguities. The utilization of phase codes in both the frequency and time domains provides flexible control of the range and Doppler ambiguities. Finally, a sensing-aided compensation method of nonlinear distortion in an OFDM radar is presented. The proposed method assumes that the transmitter of the OFDM radar operates in a nonlinear region to have high power efficiency, which introduces strong distortions to the transmitted radar waveform. This distorted signal is transmitted and reflected by targets and received by the radar. The method initially performs radar processing on the distorted received signals. The target detected through the radar processing is removed from the received signal. The resultant signal is then considered as estimated symbols which contains the distortion of the OFDM radar signal due to the nonlinearity of the efficient transmitter. Subsequently, the received signal is demodulated by utilizing the estimated symbols during radar processing. The proposed sensing-aided approach allows the estimate of distorted OFDM signals without any additional hardware such as a directional coupler and an auxiliary receiver chain, which are conventionally required to obtain the distorted signals at the output of a power amplifier (PA). Also, it effectively reduces the sidelobe level caused by the nonlinear distortion of the radar transmitter, enhancing the dynamic range of the radar. It is demonstrated through simulations and measurements that the sidelobe level becomes comparable to that achieved in the absence of nonlinear distortions.