Proposal of multiple access FMCW radar for inter‐radar interference avoidance
2019; Institution of Engineering and Technology; Volume: 2019; Issue: 21 Linguagem: Inglês
10.1049/joe.2019.0166
ISSN2051-3305
AutoresMikihiro Kurosawa, Takuya Nozawa, Masahiro Umehira, Xiaoyan Wang, Shigeki Takeda, Hiroshi Kuroda,
Tópico(s)Microwave Imaging and Scattering Analysis
ResumoThe Journal of EngineeringVolume 2019, Issue 21 p. 7304-7308 IET International Radar Conference (IRC 2018)Open Access Proposal of multiple access FMCW radar for inter-radar interference avoidance Mikihiro Kurosawa, Mikihiro Kurosawa Graduate School of Science and Engineering, Ibaraki University Hitachi, JapanSearch for more papers by this authorTakuya Nozawa, Takuya Nozawa Graduate School of Science and Engineering, Ibaraki University Hitachi, JapanSearch for more papers by this authorMasahiro Umehira, Corresponding Author Masahiro Umehira masahiro.umehira.dr@vc.ibaraki.ac.jp Graduate School of Science and Engineering, Ibaraki University Hitachi, JapanSearch for more papers by this authorXiaoyan Wang, Xiaoyan Wang Graduate School of Science and Engineering, Ibaraki University Hitachi, JapanSearch for more papers by this authorShigeki Takeda, Shigeki Takeda Graduate School of Science and Engineering, Ibaraki University Hitachi, JapanSearch for more papers by this authorHiroshi Kuroda, Hiroshi Kuroda Hitachi Automotive Systems, Ltd., Hitachinaka, 319-1295 JapanSearch for more papers by this author Mikihiro Kurosawa, Mikihiro Kurosawa Graduate School of Science and Engineering, Ibaraki University Hitachi, JapanSearch for more papers by this authorTakuya Nozawa, Takuya Nozawa Graduate School of Science and Engineering, Ibaraki University Hitachi, JapanSearch for more papers by this authorMasahiro Umehira, Corresponding Author Masahiro Umehira masahiro.umehira.dr@vc.ibaraki.ac.jp Graduate School of Science and Engineering, Ibaraki University Hitachi, JapanSearch for more papers by this authorXiaoyan Wang, Xiaoyan Wang Graduate School of Science and Engineering, Ibaraki University Hitachi, JapanSearch for more papers by this authorShigeki Takeda, Shigeki Takeda Graduate School of Science and Engineering, Ibaraki University Hitachi, JapanSearch for more papers by this authorHiroshi Kuroda, Hiroshi Kuroda Hitachi Automotive Systems, Ltd., Hitachinaka, 319-1295 JapanSearch for more papers by this author First published: 15 October 2019 https://doi.org/10.1049/joe.2019.0166Citations: 2AboutSectionsPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat Abstract Inter-radar interference will be a crucial problem when a number of frequency modulated continuous wave (FMCW) radars are densely deployed in future, since it can cause miss detection and false detection of the target due to wideband interference and narrowband interference. One of the approaches to solve this problem is to introduce a multiple access technique for automotive radars, e.g. carrier sense multiple access used in wireless LAN to avoid packet collision. This study proposes multiple access FMCW radar which can avoid both narrow-band and wideband interference assuming all of the FMCW radars have the same design parameters. It also, proposes a carrier sense technique as a key technique for multiple access FMCW radar and evaluates the carrier sense performance by computer simulations. 1 Introduction Research and development of automated driving car has been actively conducted aiming at safer and efficient mobility [1]. As an automated driving car must be able to have 360° view surrounding, it needs various sensors such as optical camera, millimeter-wave radar, and laser imaging detection and ranging for surrounding environment recognition [2]. Millimeter-wave frequency modulated continuous wave (FMCW) radar is one of the promising solutions since it can simultaneously detect the relative distance and moving speed of the target with sufficiently high-distance resolution, and it has better detection performance in rainy, foggy or dark environments than the others [3]. However, when a number of automated driving cars equipped with multiple FMCW radars are densely deployed in future, inter-radar interference can cause miss-detection of the target due to wideband interference, and false-detection due to narrowband interference [4, 5]. Wideband interference causes pulse-like interference in time domain, thus noise floor increases, i.e. signal-to-noise ratio (SNR) in frequency spectrum is degraded. As the target is detect as the peak signal in frequency spectrum. SNR degradation results in increase of miss detection probability of the target. On one hand, narrowband interference occurs when the interference radar signal has the same chirp direction and chirp rate as the observation radar signal and it is received almost the same timing as the observation radar signal reflected from the target. This interference radar signal is regarded as a ghost target and it causes false detection. There are several reports of interference mitigation techniques for wideband interference [6, 7]; however, just a few reports on narrowband interference mitigation technique since occurrence probability of narrowband interference have been considered to be relatively low. However, as the number of cars equipped with millimeter-wave FMCW radar increases, it will become a serious problem. For an example of narrowband interference mitigation technique, frequency hopping random chirp (FHRC) – FMCW technique was proposed to avoid narrowband interference of FMCW radar [7], where the centre frequency, chirp bandwidth and chirp duration of FMCW radar signal are changed at random chirp by chirp. With this randomisation, each FMCW radar signal is expected to have different chirp rates, which is defined by chirp bandwidth divided by chirp duration; thus narrowband interference can be avoided. However, wideband interference increases instead of narrowband interference when FHRC is used. Furthermore, measurement performance changes for each chirp of FMCW radar signal transmission. In order to solve the narrowband interference problem, this paper proposes multiple access FMCW radar which can avoid both narrowband and wideband interference assuming that each FMCW radar has the same centre frequency, chirp bandwidth and chirp duration. The proposed multiple access technique is a distributed approach based on carrier sense multiple access with collision avoidance (CSMA/CA), which is widely used in IEEE802.11 wireless LAN. Therefore, it is well suited to automotive FMCW radar applications. Furthermore, this paper proposes a carrier sense technique for the proposed multiple access FMCW radar, which is a key technology to detect the transmission frequency and transmission timing of the transmitted FMCW radar signal prior to transmitting the observation FMCW radar signal. It also shows the carrier sense performance to confirm the feasibility of carrier sense for the multiple access FMCW radar. 2 Multiple access FMCW radar 2.1 Principle of multiple access FMCW radar Let us suppose each FMCW radar is so-called fast chirp FMCW radar using saw-tooth modulation where T U is up chirp time duration, T D is down chirp time duration and Δf is chirp frequency bandwidth, and each employs the same parameters. Fig. 1 illustrates wideband and narrowband interference in the fast chirp FMCW radar. Asshown in Fig. 1 a, wideband interference occurs when beat frequency is lower than the cutoff frequency of low-pass filter (LPF) in FMCW radar, f LPF, and time duration of the interference radar signal at the LPF output, ΔT is given by (1) Therefore, if , wideband interference among the fast chirp FMCW radars can be negligibly small. Fig. 1Open in figure viewerPowerPoint Wideband and narrowband interference in fast chirp FMCW radar (a) Wideband interference, (b) Narrowband interference On the other hand, as illustrated in Fig. 1 b, narrowband interference occurs at the LPF output when the interference radar signal is received with a small time delay τ D, and beat frequency caused by the interference FMCW radar is <f LPF. This means if τ D is large enough to make the beat frequency is larger than f LPF, narrowband interference between FMCW radars can be avoided. This is the principle of the proposed multiple access FMCW radar which transmits the chirp signals while making τD satisfy the following condition: (2) The occurrence probability, Pni of narrowband interference for the two fast chirp FMCW radars is given by (3) As T U is short in fast chirp FMCW radar, f LPF needs to be relatively large, thus Pni will not be negligibly small although it has been said that Pni is extremely low. Frequent false detection of a ghost target becomes a serious problem when a number of fast chirp FMCW radars are deployed in future. Thus, we need multiple access FMCW radar which can avoid both wideband and narrowband interference, and it can be realised by satisfying and (3). 2.2 CSMA operation of multiple access FMCW radar Fig. 2 shows a block diagram of the proposed multiple access FMCW radar. As shown here, multiple access FMCW radar needs ON/OFF switch to control radar signal transmission, carrier sense function to detect whether FMCW radar signal is transmitted, and timing control to controls the chirp initiation timing of fast chirp FMCW radar signal. The carrier sense function needs to detect not only the existence of FMCW radar signal but also the chirp initiation timing of fast chirp FMCW to control the transmission timing while satisfying (3). This is unlike carrier sense function of CSMA in LAN, i.e. power detection. The timing control sets the chirp initiation timing of fast chirp FMCW radar signal according to the carrier sense results. Fig. 2Open in figure viewerPowerPoint Block diagram of the proposed multiple access FMCW radar Fig. 3 illustrates CSMA operation of the proposed multiple access FMCW radar. The multiple access FMCW radar performs carrier sense prior to transmitting radar signals. As shown in Fig. 3 a, if the multiple access FMCW radar detects that no FMCW radar signal exists, i.e. channel is idle, it turns on the ON/OFF switch, and starts to transmit radar signals. After transmitting radar signals, it turns off the switch just like packet transmission in wireless LAN using CSMA/CA. On the other hand, if channel is busy, it defers the radar signal transmission, and after random back-off time, it performs the carrier sense again and if it detects that channel is idle, it starts to transmit radar signals. In this way, we can keep the initiation timing of chirp longer than Δt, which is given by (4) where CR is chirp rate, i.e. . Fig. 3Open in figure viewerPowerPoint CSMA operation of multiple access FMCW radar (a) In the case that channel is idle, (b) In the case that channel is busy 3 Carrier sense technique for multiple access FMCW radar 3.1 Principle of carrier sense of FMCW radar signal As described in the previous section, multiple access FMCW radar needs to detect not only the existence of FMCW radar signal but also the chirp initiation timing of fast chirp FMCW radar signal assuming all of the fast chirp FMCW radar have the same design parameters such as chirp frequency bandwidth, chirp duration and f LPF. In this case, if we detect the transmission of a specific frequency, f s, during the carrier sensing period around the carrier sense timing, T s, we can avoid the narrowband interference by shifting the chirp initiation timing as shown in Fig. 3 b. On the other hand, if we detect the channel is idle, i.e. the transmission of a specific frequency, f s, is not detected during the carrier sense period around T s, we can determine the chirp initiation timing, To, as shown in Fig. 3 a. Therefore, the carrier sense function is required to detect the frequency, f s of the transmitted FMCW radar signals when the channel is busy. This is achieved by setting the output of VCOof the FMCW radar at f s, i.e. unmodulated carrier signal with frequency, f s. Fig. 4 shows the output signal of LPF when fast chirp FMCW signals are received and the output frequency of VCO is set at f s. The transmission of the multiple access FMCW radar is turned off by ON/OFF switch during the carrier sense period. As shown here, the pulse-like signal appears as a beat signal at the LPF output when the frequency of the received FMCW radar signal is around f s. It is something like the inter-radar interference of wideband interference. Even though frequency of beat signal changes during carrier sensing period, it can be rectified and converted to the detection pulse signal. If this carrier sense signal is detected during the carrier sense period, the radar signal transmission is deferred, and it is not detected, the radar signal transmission is initiated by determining the initiation timing of frequency chirp. Fig. 4Open in figure viewerPowerPoint LPF output signals during carrier sense period 3.2 Algorithm of carrier sense of FMCW radar signal As described above, the beat signal appears at the LPF output when the frequency f of the received signal satisfies the following condition: (5) The time duration of the beat signal is given by 2Δt, where Δt is given by (4). As the beat signal is a pulse-like beat signal whose frequency changes from to . As indicated by this observation, carrier sense period need to be longer than 2Δt. Fig. 5 shows flowchart of the proposed carrier sense algorithm for multiple access FMCW radar, where m (n) is beat signal sampled by analogue-to-digital converter. Then, detection result of , d (n) is obtained, i.e. d (n) = 1 if r th and d (n) = 0 if where r th is pre-determined threshold. However, as amplitude of is not constant, can be zero even if there is no thermal noise. Therefore, majority voting is made to avoid the miss/false detection, where d (m) for m = n to n + M is summed up to obtain D (n), and majority voting of D (n) is made to obtain R (n), i.e. R (n) = 1 if D (n) ≥ (M + 1)/2 and R (n) = 0 if D (n)<(M + 1)/2. M is assumed to be odd number for majority voting here. R (n) is like a rectangular pulse with the pulse width of 2Δt, and its edge timing is detected, i.e. n U = n if R (n) = 0 and R(n + 1) = 1, and n D = n if R (n) = 1 and R (n + 1) = 0. Finally, the detection timing of carrier sense of the FMCW radar signal can be obtained by n s = (n U + n U)/2. The channel state is detected as 'busy' if this detected timing, n s is within the carrier sense period, T CS, and it is detected as 'idle' if not. Fig. 5Open in figure viewerPowerPoint Flowchart of the proposed carrier sense algorithm for multiple access FMCW radar 4 Performance evaluation of the proposed carrier sense technique 4.1 Simulation conditions As the carrier sense is a key function to realise the proposed multiple access FMCW radar, performance of the proposed carrier sense technique is evaluated by computer simulations, in terms of miss-detection probability, false detection probability and detection timing error. Their detection performance depends on SNR of the observation radar signal when it is received with one or more interference radar signals. Fig. 6 illustrates definition of miss-detection, false-detection, and detection timing error. The upper figure shows frequency of the transmitted FMCW radar signal according to time where the chirp period of FMCW radar is T, the chirp frequency bandwidth is Δf and the carrier sense frequency is f s. Note that we assume all of the FMCW radars have the same chirp period, T and the chirp frequency bandwidth, Δf. Carrier sense is performed during the carrier sense period, TCS and detect the timing, when the frequency of the transmitted the FMCW radar is f s, as described before. In this case, 'miss-detection' is defined as the case that the majority voting filter (MVF) outputs '0' at the timing, within the carrier sense period, T CS when the frequency of the transmitted FMCW radar is . Thus, 'detection' is defined as the case that MVF outputs '1' at the timing, . False detection is defined as the case that MVF outputs '1' when there is no FMCW radar signal within T CS or the frequency of the transmitted FMCW radar is not f s. The detection timing error, is defined as the time deference between the detected timing, and the actual timing of when the frequency of the transmitted FMCW radar is . The detected timing, is given by ( where is the timing when MVF output changes from '0' to '1' and is the timing when MVF output changes from '1' to '0' as explained in the previous section. Fig. 6Open in figure viewerPowerPoint Evaluation item definition Major simulation parameters are shown in Table 1. MVF length, M is an important design parameter in the proposed carrier sense technique, thus carrier sense performance with M = 1, 5, 9, 13, 25 is evaluated according to SNR of the FMCW radar signals. The threshold, r th used in Fig. 5 is set to achieve the false-detection of 0.001 when the received signal is only noise. Number of trials is 1000. Table 1. Major simulation parameters Parameter Notation Value unit sweep frequency 500 MHz sweep time T 3 ms sampling rate — 2.5 MHz lPF passband 1 MHz number of trials — 1000 — 4.2 Performance evaluation results Fig. 7 shows miss-detection probability and false detection probability performance of the proposed carrier sense technique when M = 1, 5, 9, 13, 25. Fig. 7 a shows the miss-detection probability according to SNR when M = 1, 5, 9, 13, 25. As SNR increases, miss-detection probability decreases. As M increases, carrier sense performance is improved, i.e. the required SNR to achieve miss-detection probability decreases compared with M = 1. When M = 9, the required SNR to achieve miss-detection probability of 0.1 is ∼10 dB lower than that with M = 1. However, when M increases further, the miss-detection probability begins to increase, i.e. carrier sense performance is degraded. Therefore, we can conclude that M has an optimum value according to the FMCW radar parameters. Fig. 7 b shows false-detection probability rate according to SNR. As shown here, false-detection rate is 0% for any SNR and any MVF length, M. Fig. 7Open in figure viewerPowerPoint Detection performance according to SNR (a) Miss-detection probability, (b) False-detection probability Fig. 8 shows miss-detection probability as a function of MVF length, M when SNR = 4, 6 and 8 dB. As shown here, miss-detection probability decreases as M increases and begins to increase when M exceeds 9. M has an optimum value to achieve the least miss detection probability. Fig. 8Open in figure viewerPowerPoint Miss-detection probability as a function of MVF length, M Fig. 9 shows standard deviation of detection timing error, as a function of SNR. The standard deviation of decreases as the SNR increases; however, it has a floor of ∼0.5 sample time for any MVF length M. The standard deviation of becomes smaller as M increases, however, miss-detection probability increases if M > 9. Fig. 9Open in figure viewerPowerPoint Standard deviation of detected timing error as a function of SNR 5 Conclusion This paper has proposed a novel multiple access FMCW radar concept and also proposed a carrier sense technique as a key technology to realise the proposed multiple access FMCW radar, where we assume all of the FMCW radar are fast chirp FMCW radars with the same chirp frequency bandwidth, chirp period and centre frequency. Carrier sense performance of the proposed carrier sense technique has been evaluated by computer simulations and its feasibility has been confirmed. Thus, we can conclude the proposed multiple FMCW radar will be feasible from the technical point of view. Further studies are needed for the proposed multiple access FMCW radar including CSMA protocol design, design optimisation of the carrier sense technique and so on. In addition, prototype development of the proposed multiple access FMCW radar is required to demonstrate its feasibility by experiments. 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