Enhancing the Dynamic Range of OFDM Radars Using Non-Linear Operation and Symbol-Based Equalization

Abstract

Due to the increased flexibility in terms of waveform generation, digitally-modulated radar systems are gaining increasing popularity among research and academia. The ability to dynamically allocate frequency and time slots, as well as to employ different coding, enables novel multiplexing techniques for multiple-input-multiple-output radars and sensor networks, compared to state-of-the-art chirp-modulated systems. Another significant benefit is the possibility for joint radar and communication with the same hardware. A popular modulation for digital radar is orthogonal frequency-division multiplexing (OFDM), due to its well-known properties from communication applications. It offers all the above-mentioned possibilities, however OFDM radar has two main drawbacks: high peak-to-average power ratio (PAPR) due to the varying signal envelope which significantly increases the linearity requirements on the analog hardware and reduces the power efficiency, as well as a decoupling of target range and frequency-shift, which eliminates the possibility of using a high-pass filter to suppress close targets and thus decreases the sensor's dynamic range in comparison to conventional frequency-modulated continuous-wave (FMCW). The latter weakness is sometimes also viewed as a positive, due to the fact that range-frequency coupling can lead to undesired effects in some cases. This work addresses both of these problems together, which exist in every real-world OFDM radar sensor, and presents detailed analysis, simulation, characterization and measurement validation results that show how these effects negatively influence the signal quality and the obtained range-Doppler map of an automotive radar. Furthermore, a non-linear operation mode with receiver-sided healing that does not require any additional processing steps or computation power during radar operation is proposed. Its high potential is verified by measurements with a 78 GHz prototype setup in an anechoic chamber with static reflectors, as well as an automotive radar echo generator for dynamic targets and extended ranges.