Automotive radar interference problem between multiple radar sensors is investigated. Phase-coded frequency modulated continuous wave (PC-FMCW) radar structure with low sampling and processing power demands is introduced to blindly mitigate mutual interference. The interference resiliency of the proposed structure is evaluated and compared with the conventional frequency modulated continuous wave (FMCW) automotive radar. It is demonstrated that the proposed approach is more robust to both coherent and non-coherent interference types, allowing resilience to both external interference of radars but also the self-interference in the simultaneous multiple input multiple output (MIMO) transmission.

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Autonomous driving is a new emerging technology that will enhance traffic safety. Automotive radars are essential to attaining autonomous driving since they can function in adverse weather conditions and are used for detection, tracking, and classification in traffic settings. However, the dramatic growth in the number of radar sensors used for automotive radars has raised concerns about spectral congestion and the coexistence of radar sensors. The mutual interference between multiple radar sensors downgrades the sensing performance of automotive radar and needs to be mitigated. Moreover, automotive radars have limited processing power, preventing them from using computationally heavy techniques to countermeasure interference. This thesis aims at developing, evaluating and verifying a robust waveform with required processing steps suitable for automotive radars to boost the coexistence of multiple radar sensors. To achieve this task, phase-coded frequency modulated continuous wave (PC-FMCW) and necessary processing steps are studied. The first step is taken by investigating the sensing properties of the PC-FMCW waveforms and possible receiver strategies in Chapter 2. It is demonstrated that the ambiguity function of the code is sheared after frequency modulation. Moreover, different binary phase codes are examined with the PC-FMCW waveforms, and their sensing performance is compared in terms of integrated sidelobe level. Subsequently, two receiver approaches based on the dechirping process to decrease the sampling demands of the PC-FMCW waveforms are examined. The sensing performance of the investigated receiver approaches is compared, and the trade-offs between the sensing performance and the code bandwidth are analyzed. Moreover, the PC-FMCW waveform is applied to a real scenario, and the sensing performance of the investigated receiver structures is validated experimentally. Chapter 3 investigates the beat signal spectrum widening due to coding and explores the smoothed phase-coded frequency modulated continuous wave (SPC-FMCW) to improve the sensing performance in the limited receiver analogue bandwidth. The abrupt phase changes seen in binary phase-coded signal is analyzed, and a phase smoothing operation to reduce the spectral broadening of the coded beat signals is proposed. The introduced SPC-FMCW waveforms are analyzed in different domains and compared with the binary phase coding. It is shown that the proposed smoothing operation decreases the spectral broadening of the coded beat signal and improves the sensing performance of the waveform. In Chapter 4, the limitation in the group delay filter receiver approach is investigated, and the appropriate receiver strategy with low computational complexity is designed to process the PC-FMCW waveforms. The impact of the group delay filter on the coded beat signal is examined in detail, and a phase lag compensation is proposed to enhance decoding performance. It is demonstrated that performing phase lag compensation on the transmitted code eliminates the undesired effects of the group delay filter, and the beat signal is recovered properly after decoding. Then, the properties of the resulting waveforms are theoretically examined, and the sensing performance improvement over the existing approach is demonstrated. Moreover, both sensing and cross-isolation performance of the introduced waveforms with proposed processing steps are validated experimentally. Chapter 5 studies the PC-FMCW waveforms for a coherent multiple-input-multiple-output (MIMO) radar. To this end, the MIMO ambiguity functions of the PC-FMCW waveform with different code families are investigated for their separation capability and compared with the PMCW waveform. It is illustrated that the PC-FMCW ambiguity function outperforms the PMCW one in terms of range resolution, Doppler tolerance, and sidelobe level for the identical types of codes. Afterwards, the developed phase lag compensated waveform with a single transmitter-receiver approach is performed to a coherent MIMO radar, and a novel PC-FMCW MIMO structure is proposed in Chapter 5. The introduced MIMO structure jointly utilizes phase coding in both fast-time and slow-time to achieve low sidelobe levels in the range-Doppler-azimuth domains while maintaining high range resolution, unambiguous velocity, good Doppler tolerance and low sampling requirements. The sensing performance of the introduced MIMO structure is evaluated and compared with the state-of-the-art techniques. Moreover, the proposed MIMO structure's practical limitations are investigated and demonstrated. In addition, the sensing performance of the developed approach with the simultaneous transmission is verified experimentally. Finally, the interference resilience and communication capabilities of the developed PC-FMCW radar have been studied in Chapter 6. First, the automotive radar interference problem between various types of continuous waveforms is examined. The interference analysis formulation is extended to PC-FMCW waveforms, and a generalised radar-to-radar interference equation is proposed. The introduced equation can be utilised to quickly and accurately derive the numerous interference scenarios discussed in the literature. In addition, the proposed equation's validity to characterise the victim radar's time-frequency distribution is demonstrated experimentally using the commercially available off-the-shelf automotive radar transceivers. Afterwards, the robustness of the developed PC-FMCW radar against different types of FMCW interference cases is examined, and an improvement in the sensing performance over the conventional FMCW waveform is demonstrated. Moreover, the communication performance of the PC-FMCW with dechirping receivers is compared, and the trade-off between the bit error rate and the code bandwidth is investigated. This thesis shows that the developed PC-FMCW radar structure can provide high mutual orthogonality to enhance the functioning of multiple radars within the same frequency bandwidth while sustaining the low sampling demand and good sensing performance. Consequently, the introduced approach can be effectively utilized by automotive radars to mitigate mutual interference between multiple radar sensors and improve the sensing performance of simultaneous MIMO transmission. Although the focus is on the application in an automotive radar context, the developed approach can also be used in other radar fields.@en

Mutual interference in the frequency modulated continuous wave (FMCW) radar is studied, and the influence of the FMCW interference on the beat frequency is analyzed. An analytical expression for the victim radar received signal spectrum is derived. Different interference scenarios are investigated by means of interference impact on the range-Doppler profile. It is shown that coherent interference concentrates within multiple range cells while non-coherent interference spreads the interference power over the whole range-Doppler plane.@en

Phase-coded frequency modulated continuous wave (PC-FMCW) radars for joint sensing and communication are considered. The sensing and communication performance of the two signal processing methods, phase lag compensated group delay filter and filter bank receivers, are compared. It is demonstrated that the phase lag compensated group delay receiver provides better sensing performance and requires less computational complexity than the filter bank receiver. The application of the former receiver is, however, limited by the bit error rate degradation with the communication signal bandwidth.@en

The phase-coded linear-frequency-modulated continuous-wave (PC-FMCW) waveform with a low sampling processing strategy is studied for coherent multiple-input multiple-output (MIMO) radar. The PC-FMCW MIMO structure, which jointly uses both fast-time and slow-time coding, is proposed to reduce sidelobe levels while preserving high range resolution, unambiguous velocity, good Doppler tolerance, and low sampling needs. The sensing performance and practical aspects of the introduced PC-FMCW MIMO structure are evaluated theoretically and verified experimentally. The numerical simulations and experiments demonstrate that the proposed MIMO keeps the advantages of the linear-frequency-modulated continuous-wave (LFMCW) waveform, including computational efficiency and low sampling demands, while having the ability to provide low sidelobe levels with simultaneous transmission.@en

Smoothed phase-coded frequency modulated continuous waveform (SPC-FMCW), which is aimed to improve the coexistence of multiple radars operating within the same frequency bandwidth, is studied, and the receiving strategy with a low analog-to-digital converter sampling requirement is investigated. The Gaussian filter is applied to obtain smooth waveform phase transitions, and then, quadratic phase lag compensation is performed before waveform transmission to enhance decoding. The proposed waveform is examined in different domains, and its waveform properties are analyzed theoretically and demonstrated experimentally. Both simulation and experimental results show that the introduced waveform with the investigated processing steps helps combine all advantages of the FMCW waveform, including hardware simplicity and small operational bandwidth of the receiver, with the advantages of phase coding.

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Two receiver structures of phase modulated FMCW signals with low ADC sampling requirement are investigated, namely the matched filter of the dechirped signal and the group delay filter approach. The sensing performance of the investigated receiver strategies are analyzed in application to BPSK modulated chirp. Numerical simulations demonstrate that both techniques provide comparable performance for low to moderate bandwidth of the modulation signal. Matched filter outperforms the group delay receiver for the modulation waveform with large bandwidth, hence with the price of larger computational complexity.@en

The sensing properties of the binary phase codes are investigated with their application to phase-coded (linearly) frequency modulated continuous waveform (PC-FMCW). It is shown that the ambiguity function of FMCW signal modulated with a binary phase code corresponds to sheared ambiguity function of the code itself. The range profiles of PC-FMCW with different code families are analysed and compared in terms of integrated sidelobe level (ISL).

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Mutual interference between different radar waveforms used in automotive radar applications is studied. The existing interference analysis is extended to a generalised radar-to-radar interference equation that covers most of the common interference scenarios for automotive radar systems. The outcome of the generalised equation is demonstrated for a number of typical scenarios where radars with different continuously transmitting waveforms are involved. The proposed equation can be used to characterise the received interference and its features by analysing the instantaneous beat frequency of the victim radar. Moreover, an interference analysis of phase-coded frequency-modulated continuous waveforms is performed and demonstrated experimentally by using real-time automotive radars for the first time in the literature. The experimental results corroborate the interference analysis of different waveforms and validate the proposed generalised interference equation under various conditions.

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The phase coded FMCW and its properties for joint sensing and communication are studied. Two different receiver structures for the sensing properties of this waveform are compared theoretically and experimentally. It is shown both by simulations and experiments that the phased coded FMCW combines communication capabilities of PMCW and sensing capabilities of FMCW while using a realizable hardware complexity for an automotive radar.@en