Design and Performance of a 24 GHz Band FM-CW Radar System and Its Application

Transcription

1 Frequeny Design and Performane of a 24 GHz Band FM-CW Radar System and Its Appliation Kazuhiro Yamaguhi, Mitsumasa Saito, Kohei Miyasaka and Hideaki Matsue Tokyo University of Siene, Suwa CQ-S net In., Japan yamaguhi@rs.tus.a.jp, matsue@rs.suwa.tus.a.jp, saitoh@kpe.biglobe.ne.jp Abstrat This paper desribes a design and performane of a FM-CW (Frequeny Modulated Continuous Wave) radar system using 24 GHz band. The priniple for measuring the distane and the small displaement of target objet is desribed, and the differential detetion method for deteting the only target is proposed under the environments whih multiple objets are loated. In omputer simulation, the basi performane of FM- CW radar system is analyzed about the distane resolution and error value aording to the various sampling time and sweep bandwidth. Furthermore, the FM-CW radar system with the proposed differential detetion method an learly detet only the target objet under the multiple objet environment, and the small displaement within 3.11 mm an be measured. In experiment, the performane about measuring the distane and displaement is desribed by using the designed 24 GHz FM- CW radar system. As the results, it is onfirmed that 24 GHz FM-CW radar system with the proposed differential detetion method is effetive for measuring target under the environments whih multiple objets are loated. I. INTRODUCTION Radar systems with 24 GHz band is based on ARIB standard T73 [1] as sensors for deteting or measuring mobile objets for speified low power radio station. And the 24 GHz band radar system an be applied in various field suh as seurity, medial imaging and so on under indoor and outdoor environments. There are various radar systems have been proposed [2], [3], [4], [5]. The pulsed radar system measures the period between the signal is transmitted and reeived. The pulsed radar an detet the distane in far field, however, the target in near field an not be deteted orretly. The Doppler radar system measures the frequeny differene between the refleted and transmitted signals. The Doppler radar an detet the moving veloity of the target, however, the distane of the target an not be deteted. The FM-CW (Frequeny-Modulated Continuous-Wave) radar system [6], [7] is the most widely used for deteting the distane of the target objet in near field and the small displaement of the target. In this paper, we used and developed the 24 GHz FM-CW radar system for measuring the distane and displaement of an objet when the objet is stati or moves very slowly. The basi performane of the 24 GHz FM-CW radar system for measuring a target objet is analyzed by using the omputer simulation. Moreover, we proposed the differential detetion method for signal proessing in the FM-CW radar system in order to detet only the target objet under the environments whih multiple objets are loated. Furthermore, an example of appliation with the 24 GHz FM-CW radar system is shown in experiment. Fig. 1. Transmitted signal Time Sawtooth frequeny modulation. Reeived signal This paper onsists of the following setions. Setion II desribes the priniple of a FM-CW radar system. Setion III desribes and analyses the basi performane and the proposed differential detetion method in omputer simulation. Setion IV shows the experimental results with 24 GHz FM-CW radar system. Finally, Setion V onludes this paper. II. PRINCIPLE FOR FMCW RADAR FM-CW (Frequeny-Modulated Continuous-Wave) radar is a radar transmitting a ontinuous arrier modulated by a periodi funtion suh as a sawtooth wave to provide range data shown in Fig. 1. Fig. 2 shows the blok diagram of a FM-CW radar system [8]. In the FM-CW radar system, frequeny modulated signal at the VCO is transmitted from the transmitter Tx, then signals refleted from the targets are reeived at the reeiver Rx. Transmitted and reeived signals are multiplied by a mixer, and beat signals are generated as multiplying the two signals. The beat signal pass through a low pass filter, then an output signal is obtained. In this proess, the frequeny of the input signal is varied with time at the VCO. The modulation waveform with a linear sawtooth pattern [9] as shown in Fig. 1. This figure illustrates frequeny-time relation in the FM-CW radar, and the red line denotes the transmitted signal and the blue line denotes the reeived signal. Here, f 0 denotes the enter frequeny, f w denotes the frequeny bandwidth for sweep, and t w denotes the period for sweep. We define that the transmitting signal V T (f, x) at the transmitter Tx in Fig. 2 is represented as V T (f, x) = Ae j 2πf x, (1) /14/$ IEEE 226

2 Signal D/A VCO BPF Tx Target 1 Target FFT Signal proessing A/D BPF Rx Target Fig. 2. Blok diagram of a FM-CW radar system. where f denotes a frequeny at a time, x denotes a distane between a target and the transmitter, A denotes an amplitude value and denotes the speed of light. The refleted signal V R (f, x) at the reeiver Rx in Fig. 2 is represented as V R (f, x) = Aα k γ k e jφ k e j 2πf (2d k x), (2) where γ k and φ k are the refletivity oeffiients for amplitude and phase on kth target, respetively. α k, denotes amplitude oeffiient for transmission loss from kth target, and d k is the distane between the transmitter and the kth target. Here, at the reeiver whose position is x = 0, Eq. (2) is rewritten as V R (f, 0) = Aα k γ k e jφ k e j 2πf (2d k). (3) The beat signal are generated as multiplying the transmitted signal in Eq. (1) and the reeived signal in Eq. (3) at the position x = 0. After LPF, the output signal V out (f, 0) is generated by V out (f, 0) = A 2 α k γ k e jφ k e j 4πfd k. (4) By using signal proessing, a distane and a displaement for the target are given from the generated output signal in Eq. (4). By using the Fourier transform, the distane spetrum of the output signal P (x) is alulated as follow. P (x) = = f0 + fw 2 f 0 f w 2 f0+ fw 2 f 0 f w 2 V out e j 4πf df K = A 2 α k γ k e jφ k = A 2 K A 2 α k γ k e jφ k e j 4πfd k e f0 + f w 2 f 0 fw 2 4πfx j e j 4πf(d k x) df α k γ k e jφ k e j 4πf 0(d k x) sin f w df 2πfw (d k x) 2πf w(d k x). (5) The amplitude value of the distane spetrum P (x) in Eq. (5) is given as P (x) = A 2 α k γ k e jφ k e j 4πf 0(d k x) sin 2πfw (d k x) f w 2πf w(d k x) K sin 2πfw (d k x) A 2 f w α k γ k, (6) 2πf w(d k x) and we have equality if and only if the phase omponents ϕ k + 4πf 0(d k x) about all of k are equal. Here, we assumed that the number of target is 1. The distane spetrum in Eq. (5) is rewritten as sin 2πfw (d 1 x) P (x) = A 2 α 1 γ 1 e jφ1 e j 4πf 0 (d 1 x) f w, 2πf w(d 1 x) and the amplitude value of distane spetrum is given as sin 2πfw(d 1 x) P (x) = A 2 α 1 γ 1 f w. (8) 2πf w (d 1 x) This equation indiates that the distane for the target is generated by the amplitude value of distane spetrum. as The phase value of distane spetrum P (x) = φ 1 + 4πf 0 (d 1 x) (7) P (x) is represented = θ 1 (x). (9) Here, θ 1 (x) satisfy π θ 1 (x) π, then the displaement for the target is ( π φ 1) 4πf 0 d 1 (π φ 1) 4πf 0. (10) If the phase value satisfies ϕ 1 = 0, Eq. (10) is rewritten as 3.11 [mm] d [mm] with f 0 = [GHz]. That is, the small displaement of the target within ±3.11 [mm] is generated by the phase value of distane spetrum. 227

3 PARAMETERS IN COMPUTER SIMULATIONS Parameters Value Center frequeny Bandwidth Sweep time Sampling time of sweep Number of FFT points Window funtion GHz 50, 100, 200, 400 MHz 1024 µs 0.1, 1, 10 µs 4096 hamming ௪ ʹͲͲ MHz ͲǤͳ ߤ ݏ ǡ ͳ ߤ ݏ ͳͳ ߤ ݏ ݔ TABLE I. ݐ ௦ ͳ ߤ ݏ ݔ 50 MHz Measured distane [m] 100 MHz Fig MHz Error value for distane spetrum aording to sampling interval. 400 MHz Measured distane [m] Fig. 3. Resolution for distane spetrum aording to sweep bandwidth. On the other hands, the maximum distane for measuring dmax is fw [Hz], tw /ts = [m], 4 f f = dmax Measured distane [m] (11) (a): 3D view. where tw denotes the sweep time, ts denotes the interval time for sampling. For example, in the ase with tw = 1024 [µs] and ts = 1 [µs], the maximum distane is dmax = 384 [m]. C OMPUTER S IMULATION A. Basi Performane At first, we desribes the basi performane about the FM-CW radar with 24 GHz band. Parameters for omputer simulation are listed in Table I. Center frequeny is GHz, bandwidth are 50, 100, 200, and 400 MHz. Note that the 400 MHz bandwidth is only used for the omputer simulation beause of standards in the Radio Law in Japan. Sweep time is 1024 µs, sampling times of sweep are 0.1, 1, 10 µs, number of FFT points is 4096, and the hamming windows is adapted as the window funtion in signal proessing. We assumed that a stati target is loated at 10 m from the transmitter and reeiver, and the distane spetrums are outputted with various parameters. Fig. 3 shows the amplitude value for distane spetrum versus measured distane with various sweep bandwidth. The result shows that the sweep bandwidth influenes the distane resolutions and widely bandwidth an improve the resolution. In the ase with ts = 1 µs, the distane resolutions with fw = 50, 100, 200, 400 MHz are ±5, ±1.5, ±1, ±0.5 m, respetively. Fig. 4 shows the amplitude value for distane spetrum versus measured distane with various sampling time. The result shows that 228 ௪ ʹͲͲ MHz, ݐ ௦ ͳ ߤ ݏ Measured distane [m] III. Target distane [m] Target distane [m] (b): 2D view. Fig. 5. Distane spetrum for measuring moving target. the sampling interval influenes the error about the measured distane and shortly sampling interval an redue the error value for distane. In the ase with fw = 200 MHz, the error values about the measured distane with ts = 10 µs is about 0.5 m. Fig. 5 shows the result for measuring a slowly moving target with fw = 200 MHz and ts = 1 µs. The target moved from 10 m to 20 m at intervals of 0.5 m. Fig. 5(a) shows

4 Measured distane [m] Measured displaement [mm] Measured distane [m] MHz, MHz, Spetrums of the other objets: Spetrum of the target objet Fig. 6. Measured displaement. Target displaement [mm] the amplitude value versus measured distane versus target distane with 3-dimensional viewing, and Fig. 5(b) shows measured distane versus target distane with 2-dimensional viewing. The olor in (b) is orresponding to the strength of the amplitude value in (a). From these figures, it is onfirmed that the distane an be measured orretly aording to the positions of the moving target. Fig. 6 shows the result for measuring a target with small displaement, and the measured displaement versus target displaement is outputted. The objet is loated at 10 m from the reeiver, and the objet moved from -5 mm to 5 mm at intervals of 0.1 mm. The small displaement an be measured by the phase value of distane spetrum, and the measured displaement is orresponding to the target displaement. Note that the measured displaement denotes the relative displaement and it is not orresponding to the absolute distane between the reeiver and the target objet. The small displaement within ± 3.11 mm is orretly measured with the parameters of the FM-CW radar system in this paper, however, the displaement more than ±3.11 mm has unertainty. B. Proposed target detetion As mentioned in the above setion, the FM-CW radar system an measure the distane and the small displaement for 1 target objet. However, it is a speial ase that only the refleted signal on a target an be reeived at the reeiver. In general, the reeiver may reeive the refleted signals from many objets. Therefore, when there is some objets for measuring the target distane, signal proessing for deteting the distane spetrum from the only target is required. The proposed method removes the signals from the other objets by using the differential detetion of distane spetrum. Fig. 7 shows the distane spetrum when the target objet moves from 10 m to 20 m and the other objets are loated at 15 m and 20 m. The transmitted signal is refleted on the target and the other objets, the reeiver reeives several refleted signals. Therefore, the distane spetrum of the other objets are also generated by the FM-CW radar system in Fig. 7(a), and the distane spetrum of the target an not be deteted learly. In partiular, when the refletion oeffiient of the target is lower than that of the other objets, the distane spetrum of the other objet has higher amplitude value than that of the target. All of spetrums: Target distane [m] (a): without differential detetion. Target distane [m] MHz, Spetrum of the target objet (b): with differential detetion (Proposed). Fig. 7. Distane spetrum for measuring moving target distane with / without the differential detetion under the environments whih multiple objets are loated. In the proposed differential detetion, at first, the distane spetrum of the other objets P 0 is generated beforehand in Fig. 7(a). Then, the distane spetrum of the target and the other objet P is subtrated by P 0. By using the differential detetion, distane spetrum removed the distane spetrum of the other targets is generated as P P 0. Therefore, the distane spetrum of the desired target is only deteted. Fig. 7(b) shows the distane spetrum by using the proposed differential detetion method, and the distane spetrum of the target is orretly measured. As ompared with the measured distane spetrums in Fig. 7(a) and (b), it is learly onfirmed that the proposed method an detet target distane by using the differene detetion. The proposed differential detetion an effetively detet the moving or stati target distane from multiple refletions of the bakground stati objets. IV. EXPERIMENTS In order to evaluate the effetiveness of the proposed method for deteting the target distane and displaement, we develop a FM-CW radar system and arried out the experiments with the radar system in atual environment. Table II lists the parameters, and the developed FM-CW radar system get a ertifiate of onformity with tehnial regulations in 229

5 PARAMETERS IN EXPERIMENTS Value Center frequeny f0 Sweep bandwidth fw Sweep time tw Sampling time of sweep ts Transmitter power output Antenna gain Range of distane Range of relative displaement GHz 200 MHz 1024 µs 1 µs W 11 dbi m ±3.11 mm 5 se. Time [s] Fig. 9. Measured distane [m] Parameters Measured small displaement [mm] TABLE II. Fig. 10. Displaement for measuring the movement of human breathing. Setup of FM-CW Radar for deteting human breathing. 1. Without any person 2. Refletion on the human body Time [s] (a): without differential detetion. Measured small displaement [mm] Measured distane [m] Measured distane [m] 3. Breathing detetion Time Time Fig. 11. Time [s] (b): with differential detetion (Proposed). Fig. 8. Distane spetrum for measuring moving target distane with / without the differential detetion. Example of appliation. detetion method, the distane spetrum without the person is measured beforehand. By generating the distane spetrum of the bakground objets beforehand, the distane spetrum of the moving person is orretly deteted in Fig. 8(b) with the proposed differential detetion. Therefore, the FM-CW radar system an measure movement of the target person effetively. Artile 38-6 Paragraph 1 of the Radio Law in Japan, and developed FM-CW radar system is aommodate to ARIB standard T73 in Japan [1]. Fig. 9 shows the result of measuring the small displaement for human breathing. The human s hest movement is measured within the range of relative small displaement. In Fig. 9, it is deteted that the period of breathing is about 4 [s] and the breathing movement is about within ±2 [mm]. A. Distane Spetrum B. Example for appliation Fig. 8 shows the distane spetrum of a moving target. A person walked away from the FM-CW radar and then ame lose between 2 [m] to 10 [m]. In Fig. 8(a), several distane spetrums of the person and the bakground objets are outputted. The distane spetrum of the moving person is not learly deteted in Fig. 8(a). In order to detet the distane spetrum of the moving person with the differential Finally, we show an example of appliation with 24 GHz FM-CW radar system. Fig. 10 shows a setup of the FMCW radar system for deteting human breathing in atual environments. The FM-CW radar satisfies the safety guideline, and the details of the safety guideline is desribed in Appendix. 230 Fig. 11 shows the example for deteting human breathing.

6 The distane spetrum in this example is measured as following flow. 1) Measuring distane spetrum without any person. 2) A person omes to the bed. The radar reeived signals from human s body. 3) The person lies asleep on the bed. The radar detets the person s breathing movement. By generating the distane spetrum of the bakground objets without the person, the distane spetrum of the person is only deteted. When the person omes within the range of radar, the radar system an detet refleted signals from the person, and the distane spetrums of the human s body are deteted. After the person lies on the bed, the radar system an detet the small displaement for the person s breathing movement. By using the differential detetion method, the distane and small displaement of the moving objet is learly deteted. V. CONCLUSION In this paper, design and performane of a FM-CW radar system with 24 GHz band is desribed. In omputer simulations, basi performanes of FM-CW radar system is analyzed about the distane resolution and error value aording to the sweep time and the sampling interval, respetively. Moreover, the differential detetion method for deteting only the target objet is proposed for measuring the distane and the displaement of the target under the environments whih multiple objets are loated. In experiments, the distane spetrum of the target objet is learly deteted by using the differential detetion method under the environments whih multiple objets are loated. Furthermore, an example of appliation for deteting human s breathing movement is shown. As the result, the 24 GHz FM-CW radar with the proposed differential detetion method effetively detet the distane and the small displaement under the environments whih multiple objets are loated. ACKNOWLEDGMENT A part of this work was supported by Ashita wo Ninau Kanagawa Venture Projet of Kanagawa in Japan. The authors appreiate Prof. Toshio Nojima at Hokkaido University in Japan getting the valuable advies for analyzing the safety properties of the developed FM-CW radar system aording to the safety guideline. [6] T. SAITO, T. NINOMIYA, O. ISAJI, T. WATANABE, H. SUZUKI, and N. OKUBO, Automotive fm-w radar with heterodyne reeiver, IEICE transations on ommuniations, vol. 79, no. 12, pp , de [7] W. Butler, P. Poitevin, and J. Bjomholt, Benefits of wide area intrusion detetion systems using fmw radar, in Seurity Tehnology, st Annual IEEE International Carnahan Conferene on, Ot 2007, pp [8] M. Skolnik, Radar Handbook, Third Edition. MGraw-Hill Eduation, [9] W. Sediono and A. Lestari, 2d image reonstrution of radar indera, in Mehatronis (ICOM), th International Conferene On, May 2011, pp [10] C , IEEE Standard for Safety Levels with Respet to Human Exposure to Radio Frequeny Eletromagneti Fields, 3 khz to 300 GHz, IEEE Std. [11] Ministry of Internal Affairs and Communiations. [Online]. Available: APPENDIX In general, eletromagneti wave must be satisfied the guidelines on human exposure to eletromagneti fields, where it have been instituted in various organizations. IEEE C95.1 in USA [10] and ICNIRP in Europe are the guidelines, and MIC also have instituted the guideline in Japan [11]. Developed 24 GHz FM-CW radar in this paper have the properties as follow. The power of the transmitter is 7 [mw], the transmitting antenna gain is 11 [dbi], the effetive radiated power is 88 [mw], the radiation angle of the transmitting wave is about 50 [degree], and the distane between the transmitter and the human is 2.5 [m]. Aording to the radar equation, the eletri field strength E and the power density P on the human body is alulated as E = = 0.65 [V/m], 2.5 P = E2 z 0 = π = [mw/m 2 ]. Aording to the guideline [11], these parameters must be satisfied as E 61.4 [V/m], P 1 [mw/m 2 ]. Therefore, the developed 24 GHz FM-CW radar system in this paper suffiiently satisfies the onditions in the guideline. REFERENCES [1] ARIB STD-T73 Rev. 1.1, Sensors for Deteting or Measureing Mobile Objets for Speified Low Power Radio Station, Assoiation of Radio Industries and Businesses Std. [2] S. MIYAKE and Y. MAKINO, Appliation of millimeter-wave heating to materials proessing( speial issue reent trends on mirowave and millimeter wave appliation tehnology), IEICE transations on eletronis, vol. 86, no. 12, pp , de [3] M. Skolnik, Introdution to Radar Systems. MGraw Hill, [4] S. Fujimori, T. Uebo, and T. Iritani, Short-range high-resolution radar utilizing standing wave for measuring of distane and veloity of a moving target, ELECTRONICS AND COMMUNICATIONS IN JAPAN PART I-COMMUNICATIONS, vol. 89, no. 5, pp , [5] T. Uebo, Y. Okubo, and T. Iritani, Standing wave radar apable of measuring distanes down to zero meters, IEICE TRANSACTIONS ON COMMUNICATIONS, vol. 88, no. 6, pp , jun