Radar interference suppression

Abstract

The disclosure relates to suppression of interference in radar systems by dechirping, filtering and rechirping incoming signals. Example embodiments disclosed include a method of removing interference from a radar signal, comprising: receiving an incoming radar signal (y(t)) at an antenna (101); providing the incoming radar signal (y(t)) to an RF front end module (102); the RF front end module (102) amplifying and converting the incoming radar signal to a baseband signal (yl(t)); providing the baseband signal (yl(t)) to an analog- to-digital converter, ADC (103); the ADC (103) digitising the baseband signal (yl(t)) to generate a digital baseband signal (yl[u]); providing the digital baseband signal (yl[u] ) to a dechirping module (104, 105); the dechirping module (104, 105) estimating chirp parameters of the digital baseband signal (yl[u]) and dechirping the digital baseband signal (yl[u]) to generate a dechirped signal (yd[u]); providing the dechirped signal (yd[u]) to a filter module (106); the filter module (106) filtering out an interference signal from the dechirped signal (yd[uj) to generate a filtered signal (yf[u]); providing the filtered signal (yf[u]) to a rechirping module (107); the rechirping module (107) rechirping the filtered signal (yf[u]) with chirps generated from the estimated chirp parameters to generate a rechirped signal (yr[u]); providing the rechirped signal (yr[u]) to a radar signal processing module (108); and the radar signal processing module (108) processing the rechirped signal (yr[u]) to retrieve target information from the rechirped signal (yr[u])..

Description

[0001] RADAR INTERFERENCE SUPPRESSION Description Field The disclosure relates to suppression of interference in radar systems by dechirping, filtering and rechirping incoming signals. Background Mutual interference has become an important topic of interest in radar systems, in particular for automotive applications. Due to limitations of current analog radar systems, the introduction of digital radar could potentially take place in the field in the future. In such a scenario, it is expected that both analog and digital types of radar may need to coexist during a transition phase and may also need to share a common frequency band. Therefore, new techniques of interference suppression between the two types of radar will become necessary. In a particular example applicable to current radar systems, when FMCW (frequency modulated continuous wave) interference is received by an OFDM (orthogonal frequency division multiplexing) radar receiver, the interference causes an increase in the noise floor of the OFDM range-Doppler image, which in turn decreases the reliability of the radar. This can be potentially dangerous because it may result in actual targets to be detected becoming hidden. Various techniques proposed generally have a limitation linked to the relation between the FMCW chirp time and the OFDM symbol time, which are generally only valid when the former is greater than the latter. Alternative techniques are desirable that are not subject to such limitations, for example to reduce the noise floor to allow for further suppression of interference in a received signal. Summary According to a first aspect there is provided a method of removing interference from a radar signal, comprising: receiving an incoming radar signal at an antenna; providing the incoming radar signal to an RF front end module; the RF front end module amplifying and converting the incoming radar signal to a baseband signal; providing the baseband signal to an analog-to-digital converter, ADC; the ADC digitising the baseband signal to generate a digital baseband signal; providing the digital baseband signal to a dechirping module; the dechirping module estimating chirp parameters of the digital baseband signal and dechirping the digital baseband signal to generate a dechirped signal; providing the dechirped signal to a filter module; the filter module filtering out an interference signal from the dechirped signal to generate a filtered signal; providing the filtered signal to a rechirping module; the rechirping module rechirping the filtered signal with chirps generated from the estimated chirp parameters to generate a rechirped signal; providing the rechirped signal to a radar signal processing module; and the radar signal processing module processing the rechirped signal to retrieve target information from the rechirped signal. The dechirping module may iteratively processes the digital baseband signal using a range of chirp parameters to determine values for the chirp parameters at which a power spectral density of a dechirped signal is maximised. The chirp parameters may comprise a chirp time and a chirp bandwidth. The dechirping module may: determine the chirp parameters to a first resolution in a coarse dechirping step; and based on the chirp parameters from the coarse dechirping step, determine the chirp parameters to a second resolution in a fine dechirping step, wherein the second resolution is finer than the first resolution. The filter module may comprise a notch filter tuned to a frequency of the interference signal in the dechirped signal. The notch filter may have a centre frequency corresponding to a maximum in the power spectral density of the dechirped signal. The radar signal may be an orthogonal frequency division multiplexed, OFDM, radar signal and the interference a frequency modulated continuous wave, FMCW, interference signal. According to a second aspect there is provided a radar receiver, comprising: an antenna; an RF front end module configured to amplify a radar signal from the antenna and convert the radar signal to a baseband signal; an analog-to-digital converter, ADC, configured to receive the baseband signal from the RF front end module and digitise the baseband signal to generate a digital baseband signal; a dechirping module configured to receive the digital baseband signal from the ADC, estimate chirp parameters in the baseband signal and dechirp the digital baseband signal to generate a dechirped signal; a filter module configured to receive the dechirped signal and filter out an interference signal from the dechirped signal to generate a filtered signal; a rechirping module configured to receive the filtered signal and rechirp the filtered signal with chirps generated from the estimated chirp parameters to generate a rechirped signal; a radar signal processing module configured to receive the rechirped signal and process the rechirped signal to retrieve target information from the rechirped signal. The dechirping module may be configured to iteratively process the digital baseband signal using a range of chirp parameters to determine values for the chirp parameters at which a power spectral density of a dechirped signal is maximised. The chirp parameters may comprise a chirp time and a chirp bandwidth. The dechirping module may comprise: a coarse dechirping module configured to determine the chirp parameters to a first resolution; and a fine dechirping module configured to determine the chirp parameters to a second resolution based on the chirp parameters determined by the coarse dechirping module, wherein the second resolution is finer than the first resolution. The filter module may comprise a notch filter tuned to a frequency of the interference signal in the dechirped signal. The notch filter may have a centre frequency corresponding to a maximum in the power spectral density of the dechirped signal. The radar signal may be an orthogonal frequency division multiplexed, OFDM, radar signal and the interference a frequency modulated continuous wave, FMCW, interference signal. The radar receiver may be part of an automotive radar system. These and other aspects of the invention will be apparent from, and elucidated with reference to, the embodiments described hereinafter. Brief description of Drawings Embodiments will be described, by way of example only, with reference to the drawings, in which: Figure 1 is a schematic diagram of an example radar receiver; Figure 2 is a flow diagram illustrating an example coarse dechirping process; Figure 3 is a flow diagram illustrating an example fine dechirping process; Figure 4 is a flow diagram illustrating a process for filtering a dechirped signal; Figure 5 is an example plot of OFDM subcarriers against OFDM symbols prior to interference suppression; Figure 6 is an example plot of OFDM subcarriers against OFDM symbols after dechirping; Figure 7 is a plot of Error Vector Magnitude against frequency before and after interference suppression; Figure 8 is a magnified plot of Error Vector Magnitude against frequency after interference suppression; Figure 9 is a plot of various signal spectra at different points in signal processing; Figure 10 is an OFDM Radar Range-Doppler map prior to interference suppression; Figure 11 is an OFDM Radar Range-Doppler map after interference suppression; and Figure 12 is a plot of SINR difference as a function of radar receiver SNR. It should be noted that the Figures are diagrammatic and not drawn to scale. Relative dimensions and proportions of parts of these Figures have been shown exaggerated or reduced in size, for the sake of clarity and convenience in the drawings. The same reference signs are generally used to refer to corresponding or similar feature in modified and different embodiments. Detailed description of embodiments Disclosed herein is a method for estimating the parameters of an FMCW interference signal using an iterative process of dechirping. This allows the interference energy to be concentrated into a small bandwidth (or, in an ideal case, a single beat frequency). The interference can then be suppressed using a digital filter. An advantage of this approach is that FMCW interference can be estimated and suppressed regardless of its chirp time compared to the OFDM symbol time. According to the present disclosure, the term ‘dechirping’ is the act of demodulating a signal with a linear time-varying frequency signal. A dechirped signal is to be interpreted accordingly. The term ‘rechirping’ is the act of modulating a dechirped signal with a linear time-varying frequency signal. A rechirped signal is to be interpreted accordingly. In the examples disclosed herein, a Frequency Modulated Continuous Wave (FMCW) interference suppression technique is proposed based on the impact of an FMCW radar on an Orthogonal Frequency Division Multiplexing (OFDM) radar. This allows interference to be attenuated by a technique termed grid dechirping, which consists of estimating the FMCW interference by consecutive dechirping steps, followed by its suppression using a digital filter. This allows the increase in the noise floor of the radar image due to the interference to be reduced. An advantage of this method is that it does not impose any constraint on the parameters of the radars, such as the relation between the FMCW chirp time and the OFDM symbol time. The interference is estimated through an iterative process of dechirping, resulting in the energy of the interference being concentrated into a beat frequency, or narrow bandwidth. This narrowband signal can then be removed, thereby suppressing the presence of interference in the received signal. Consequently, the increase in noise floor caused by the FMCW interference is reduced. Figure 1 illustrates an example radar receiver 100 comprising an antenna 101, an RF front end 102, an analog-to-digital converter (ADC) 103 and baseband processing modules 104-108. An incoming signal ^^^^^^ is received via the antenna 101. The incoming signal ^^^^^^ is composed of a superposition of FMCW interference with an echo from a transmitted OFDM radar signal. The incoming signal ^^^^^^ passes to the RF front-end module 102, which amplifies and down-converts the incoming signal ^^^^^^ to generate a baseband signal ^^^^^^^. The baseband signal ^^^^^^^ is then sampled by the ADC 103 and converted to a digital baseband signal ^^^^^^^. Thedigital baseband signal ^^^^^^^ is then processed by baseband processingmodules 104-107. Baseband processing of incoming signals may be triggered only when interference is detected. An FMCW interference detector (not shown) may provide a signal indicating the presence of such interference. Otherwise, the digital baseband signal ^^^^^^^ may be passed from the ADC 103 straight to the OFDM radar processing module 108, resulting in the radar receiver 100 operating as a conventional radar receiver. FMCW interference may be identified by its signature pattern, for example in terms of sharp ridges, observed in time-frequency representations, as described by C. Knill, B. Schweizer, P. Hügler, and C. Waldschmidt, “Impact of an Automotive Chirp-Sequence Interferer on a Wideband OFDM Radar,” in 2018 15th European Radar Conference (EuRAD), Sep. 2018, pp. 34–37. FMCW interference may alternatively be detected by monitoring the spectrum of the received signal, such interference appearing as a plateau in the OFDM bandwidth. After identifying a plateau, the time-domain signal can be checked for the presence of a time- varying frequency signal. Since the OFDM signal has a noise-like shape and the interference power is much stronger than the OFDM power, a time-domain signal would be expected that appears as a noisy FMCW signal, assuming a radar receiver with a sufficiently high dynamic range. To estimate the interference in the received signal, the digital baseband signal ^^^^^^^ is first subjected to a dechirping process by a dechirping module, which is shown in Figure 1 carried out by a coarse dechirping module 104 followed by a fine dechirping module 105. The coarse dechirping module 104 allows a rough estimation of the interference to be obtained, which is then refined by the fine dechirping module 105. In the resulting dechirped signal ^^ௗ^^^^, the interference in the signal is concentrated in either a single frequency (if the estimation of interference is perfect) or in a narrow bandwidth (with a non-perfect estimation). The dechirped signal ^^ௗ^^^^ is provided by the dechirping module 104, 105 to a filter module 106, which suppresses the interference by filtering it out using a notch filter tuned to the frequency at which the interference is concentrated. The resulting filtered signal ^^^^^^^ is provided by the filter module 106 to a rechirping module 107, in which the signal is rechirped with chirps generated using the estimated chirp parameters found by the dechirping module 104, 105. Theresulting rechirped signal ^^^^^^^ is provided to the OFDM radar processingmodule 108 for the signal to be processed as normal to retrieve target information from the signal. The algorithm described below defines an example method of dechirping the digital baseband signal performed by the dechirping module 104, 105. The algorithm is divided into a coarse dechirping stage in which a coarse estimate of the chirp parameters is obtained by determining a maximum power spectral density (PSD), following by a fine dechirping stage in which a more accurate determination of the chirp parameters is made, starting from the coarse estimate obtained in the coarse dechirping stage. The result is a dechirped signal in which the interference signal is concentrated in a narrow frequency band. In a general aspect, the dechirping method iteratively processes the digital baseband signal using a range of chirp parameters to determine values for the chirp parameters at which a power spectral density of a dechirped signal is maximised. The chirp parameters comprise a chirp time and a chirp bandwidth, which are expressed in the algorithm below as a vector of chirp times and a vector of chirp bandwidths, each having a number of elements. As described below, the dechirping method may comprise a first coarse dechirping stage in which the chirp parameters are determined to a first resolution, following by a second fine dechirping stage in which the chirp parameters are determined to a second resolution based on the chirp parameters obtained by the first coarse dechirping stage. Using coarse and fine dechirping stages allows the dechirping process to be carried out substantially faster than a single dechirping stage with a single fine resolution. The dechirping process may alternatively involve more than two stages if a finer final resolution is required, but at the expense of more computational effort. As described above, prior to starting the dechirping process the vectors of chirp times and chirp bandwidths are each initialised with a number Nt1 of elements and a global power spectral density is initialised to zero. The coarse dechirping step involves iterating the chirp parameters, in this example in two nested loops for the time and bandwidth parameters. In each iteration, a spectrum of the dechirped signal is estimated and, if this is greater than the current global PSD value, the global PSD value is updated. Following completion of the iterative process, the chirp parameters at which the global PSD is maximised are stored and passed on to the subsequent fine dechirping step. In the fine dechirping step, the chirp time and chirp bandwidth parameters are initialised according to the chirp parameters determined from the coarse dechirping step. A similar iterative process is then carried out on this narrower range of parameters, resulting in a determination of chirp parameters to a finer resolution at which the global PSD is a maximum. Figure 2 illustrates a flow chart of the coarse dechirping process as outlined above. In a first step 201, the process is started with a signal being received at the radar receiver 100 (Figure 1). At step 202, a check is made whether FMCW interference is detected. If no interference is detected, or if a level of interference is below a minimum threshold, the process proceeds to step 203 where the received signal is processed normally. If, at step 202, an interference signal is detected, at step 204 the coarse dechirping process is initialised, in which vectors of the chirp parameters to be swept are defined, including chirp time and chirp bandwidth. At each iteration of the following process steps, one element of each vector is chosen. A check is made at step 205 as to whether the chirp parameters have been entirely swept. If not, at step 206 a digital chirp is defined with the current parameters ^^ a^^^ nd ^^^. At step 207, the signal is dechirped using the generated chirp and at step 208 the power spectrum of the resulting dechirped signal is estimated. At step 209 a maximum value of the obtained spectrum, PSDmax, is determined for the estimated spectrum. If, at step 210, the value PSDmax is greater than the current stored global value PSDglobal, the current stored value is updated at step 211 to equal PSDmax. The current chirp parameters are saved at step 212 and the process returns to step 205. Otherwise, i.e. if PDSmax is not greater than PSDglobal, the process returns to step 205 without updating PSDglobal. The process repeats until all values for the chirp parameters have been swept. The coarse estimations of the chirp parameters are then obtained at step 213 based on the stored value for PSDglobal. Figure 3 illustrates a flow diagram of the subsequent fine dechirping process. The process starts at step 301 with the coarse estimations for the chirp parameters resulting from the above coarse estimation process. The fine dechirping process then starts at step 302 with the vectors of chirp parameters to be swept being initialised as these coarse estimated parameters. The process then follows a similar series of steps to the coarse estimation process. A check is made at step 303 as to whether the chirp parameters have been entirely swept. If not, at step 304 a digital chirp is defined with the current parameters and ^^^ଶ^^ . At step 307, the signal ^^^^^^^is dechirped using the generated chirp and at step 306 the power spectrum of the resulting dechirped signal is estimated. At step 307 a maximum value of the obtained spectrum, PSDmax, is determined for the estimated spectrum. If, at step 308, the value PSDmaxis greater than the current stored global value PSDglobal, the current stored value is updated at step 309 to equal PSDmax. The current chirp parameters are saved at step 310 and the process returns to step 303. Otherwise, i.e. if PDSmaxis not greater than PSDglobal, the process returns to step 303 without updating PSDglobal. The process repeats until all values for the chirp parameters have been swept. The signal ^^^^^^^ is then dechirped with the generated chirp leading to the greatest PSDglobalvalue, which is the one with the parameters most recently stored at step 310. The process then finishes at step 312 by proceeding to filtering at step 312. Once the interference is estimated and the dechirping step is performed, thedechirped signal ^^ௗ^^^^ contains the interference energy concentrated in a narrow bandwidth. Ideally, when the interference estimation coincides with the interference, the energy of this latter is concentrated in a single frequency (or beat frequency) after the dechirping step. The goal is then to suppress this frequency component using a notch filter. An example filtering process following the dechirping process described above is illustrated in the flow diagram in Figure 4. In a first step 401, the PSD of the dechirped signal ^^ௗ^^^^ is computed, resulting in ^^^^^^ௗ^^^^. In a second step 402,the frequency ^^^ is found at which the PSD is a maximum, i.e. ^^^^^^ௗ^^^^^ ൌmax ^^^^^^^ௗ^^^^^. If, at step 403, the frequency ^^^belongs to a beat frequency, this needs to be suppressed. At step 404 therefore, the specifications are set fora notch filter, comprising the filter order (O), center frequency (^^^ ൌ ^^^), qualityfactor (Q) and stop-band attenuation (Ast). At step 405, the dechirped signal ^^ௗ^^^^ is filtered using the notch filter. The method described above has been verified using simulations at the system level. In the definition of the FMCW interference, only the chirp time ^^^and the chirp bandwidth ^^^were considered as parameters. When defining the vector parameters for the grid dechirping process, minimum and maximum values were selected in the coarse dechirping stage. These values were chosen according to values commonly used in current state-of-the-art FMCW radars. For ^^^, this ranges from tens to hundreds of ^^^^, and ^^^ranges from hundreds of MHz to a few GHz. Typical OFDM radar parameters are provided in Table 1 below and typical FMCW parameters in Table 2 below.

[0002] Table 1: Typical OFDM radar parameters. Table 2: Typical FMCW radar parameters. Figure 5 is a plot of OFDM subcarrier against OFDM symbol, prior to the dechirping process. The FMCW chirps 501, 502, 503 are visible in the plot as high power lines varying with frequency (corresponding to OFDM carrier) and time (corresponding to OFDM symbol). These chirps impact multiple OFDM subcarriers. Figure 6 is a plot of OFDM subcarrier against OFDM symbol after dechirping, in which it can be see that most of the interference energy is concentrated in a single OFDM subcarrier 601. This interference can therefore be more readily removed. As described above, once the interference is dechirped a notch filter can be applied to the signal to suppress the beat frequency component. The Error Vector Magnitude (EVM) of the modulation symbols in each subcarrier can be used to evaluate how much interference is present in each subcarrier. The reference signal is the received OFDM signal without interference. Figure 7 illustrates a plot of EVM against frequency between transmitted and received modulation symbols before interference suppression 701 and after interference suppression 702. The frequency band 703 initially impacted by FMCW interference, in this example up to around 170 MHz, is substantially reduced. Figure 8 is a plot illustrating the EVM 702 with interference suppression with the y-axis magnified so the magnitude of suppression can be seen. Although not totally suppressed, the error contained in the band initially interfered is substantially reduced, in this example by around 15 times, compared to the EVM without interference suppression. Figure 9 is a plot of signal magnitude spectra at different stages of signal processing. Spectrum 901 is the received OFDM signal. Spectrum 902 is the received FMCW signal. Spectrum 903 is the received signal including the OFDM and FMCW interference. Spectrum 904 is the signal after dechirping and before filtering. Spectrum 905 is the signal after filtering. Spectrum 906 is the signal after rechirping, in which is can be seen that the interference component is almost entirely suppressed, leaving behind only a small peak. After the interference is suppressed and the processed signal is rechirped to compensate the effect of the dechirping step on the unaffected subcarriers, conventional radar processing is performed. The Signal-to-Interference-plus- Noise Ratio (SINR) and Signal-to-Spur Ratio (SSR) can be used to evaluate the effect of the proposed invention on the radar results, as illustrated in Figures 10, 11 and 12. Figure 10 illustrates an OFDM Radar Range-Doppler map of power as a function of velocity and range without interference suppression, in which the SINR is 60.54 dB and the SSR 42.31 dB. Figure 11 illustrates a corresponding OFDM Radar Range-Doppler map with interference suppression, in which the SINR is increased to 65.10 dB, an increase of 4.56 dB. In both examples, a target 1001 is present at around 10 dBsm of RCS with a range of 10 m and velocity of 10 m / s, with a receiver SNR of 25 dB. Figure 12 illustrates the changes in SINR before and after interference suppression as a function of OFDM radar receiver SNR, showing that the performance of interference suppression varies according to the OFDM receiver SNR, with better results being obtained in less noisy conditions. From reading the present disclosure, other variations and modifications will be apparent to the skilled person. Such variations and modifications may involve equivalent and other features which are already known in the art of radar systems, and which may be used instead of, or in addition to, features already described herein. Although the appended claims are directed to particular combinations of features, it should be understood that the scope of the disclosure of the present invention also includes any novel feature or any novel combination of features disclosed herein either explicitly or implicitly or any generalisation thereof, whether or not it relates to the same invention as presently claimed in any claim and whether or not it mitigates any or all of the same technical problems as does the present invention. Features which are described in the context of separate embodiments may also be provided in combination in a single embodiment. Conversely, various features which are, for brevity, described in the context of a single embodiment, may also be provided separately or in any suitable sub-combination. The applicant hereby gives notice that new claims may be formulated to such features and / or combinations of such features during the prosecution of the present application or of any further application derived therefrom. For the sake of completeness it is also stated that the term "comprising" does not exclude other elements or steps, the term "a" or "an" does not exclude a plurality, a single processor or other unit may fulfil the functions of several means recited in the claims and reference signs in the claims shall not be construed as limiting the scope of the claims.

Claims

CLAIMS 1. A method of removing interference from a radar signal, comprising: receiving an incoming radar signal (^^^^^^) at an antenna (101); providing the incoming radar signal (^^^^^^) to an RF front end module (102); the RF front end module (102) amplifying and converting the incoming radar signal to a baseband signal (^^^^^^^); providing the baseband signal (^^^^^^^) to an analog-to-digital converter, ADC (103); the ADC (103) digitising the baseband signal (^^^^^^^) to generate a digital baseband signal (^^^^^^^); providing the digital baseband signal (^^^^^^^) to a dechirping module (104, 105); the dechirping module (104, 105) estimating chirp parameters of the digital baseband signal (^^^^^^^) and dechirping the digital baseband signal (^^^^^^^) to generate a dechirped signal (^^ௗ^^^^); providing the dechirped signal (^^ௗ^^^^) to a filter module (106); the filter module (106) filtering out an interference signal from the dechirped signal (^^ௗ^^^^) to generate a filtered signal (^^^^^^^); providing the filtered signal (^^^^^^^) to a rechirping module (107); the rechirping module (107) rechirping the filtered signal (^^^^^^^) with chirps generated from the estimated chirp parameters to generate a rechirped signal (^^^^^^^); providing the rechirped signal (^^^^^^^) to a radar signal processing module (108); and the radar signal processing module (108) processing the rechirped signal(^^^^^^^) to retrieve target information from the rechirped signal (^^^^^^^).

2. The method of claim 1, wherein the dechirping module (104, 105) iteratively processes the digital baseband signal (^^^^^^^) using a range of chirp parameters to determine values for the chirp parameters at which a power spectral density of a dechirped signal is maximised.

3. The method of claim 2, wherein the chirp parameters comprise a chirp time and a chirp bandwidth.

4. The method of claim 2 or claim 3, wherein the dechirping module (104, 105): determines the chirp parameters to a first resolution in a coarse dechirping step; and based on the chirp parameters from the coarse dechirping step, determines the chirp parameters to a second resolution in a fine dechirping step, wherein the second resolution is finer than the first resolution.

5. The method of any preceding claim, wherein the filter module (106) comprises a notch filter tuned to a frequency of the interference signal in the dechirped signal (^^ௗ^^^^).

6. The method of claim 5, wherein the notch filter has a centre frequency (^^^) corresponding to a maximum in the power spectral density of the dechirped signal (^^ௗ^^^^).

7. The method of any preceding claim, wherein the radar signal is an orthogonal frequency division multiplexed, OFDM, radar signal and the interference is a frequency modulated continuous wave, FMCW, interference signal.

8. A radar receiver (100), comprising: an antenna (101); an RF front end module (102) configured to amplify a radar signal (^^^^^^) from the antenna (101) and convert the radar signal (^^^^^^) to a baseband signal (^^^^^^^); an analog-to-digital converter, ADC (103) configured to receive the baseband signal (^^^^^^^) from the RF front end module (102) and digitise the baseband signal (^^^^^^^) to generate a digital baseband signal (^^^^^^^); a dechirping module (104, 105) configured to receive the digital baseband signal (^^^^^^^) from the ADC (103), estimate chirp parameters in thebaseband signal (^^^^^^^ ) and dechirp the digital baseband signal (^^^^^^^ ) togenerate a dechirped signal (^^ௗ^^^^); a filter module (106) configured to receive the dechirped signal (^^ௗ^^^^) and filter out an interference signal from the dechirped signal (^^ௗ^^^^) to generate a filtered signal (^^^^^^^); a rechirping module (107) configured to receive the filtered signal (^^^^^^^) and rechirp the filtered signal (^^^^^^^) with chirps generated from the estimated chirp parameters to generate a rechirped signal (^^^^^^^); a radar signal processing module (108) configured to receive the rechirped signal (^^^^^^^) and process the rechirped signal (^^^^^^^) to retrieve target information from the rechirped signal (^^^^^^^).

9. The radar receiver of claim 8, wherein the dechirping module (104, 105) is configured to iteratively process the digital baseband signal (^^^^^^^) using a range of chirp parameters to determine values for the chirp parameters at which a power spectral density of a dechirped signal is maximised.

10. The radar receiver of claim 9, wherein the chirp parameters comprise a chirp time and a chirp bandwidth.

11. The radar receiver of claim 9 or claim 10, wherein the dechirping module (104, 105) comprises: a coarse dechirping module (104) configured to determine the chirp parameters to a first resolution; and a fine dechirping module (105) configured to determine the chirp parameters to a second resolution based on the chirp parameters determined by the coarse dechirping module (104), wherein the second resolution is finer than the first resolution.

12. The radar receiver (100) of any one of claims 8 to 11, wherein the filter module (106) comprises a notch filter tuned to a frequency of the interference signal in the dechirped signal (^^ௗ^^^^).

13. The radar receiver (100) of claim 12, wherein the notch filter has a centre frequency (^^^) corresponding to a maximum in the power spectral density of the dechirped signal (^^ௗ^^^^).

14. The radar receiver (100) of any one of claims 8 to 13, wherein the radar signal is an orthogonal frequency division multiplexed, OFDM, radar signal and the interference is a frequency modulated continuous wave, FMCW, interference signal.

15. The radar receiver (100) of any one of claims 8 to 14, wherein the radar receiver is part of an automotive radar system.

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