TR2024011240A2 - MONOSTATIC DETECTION METHOD IN TERAHERTZ BAND FOR RANGE UNCERTAINTY REDUCTION - Google Patents

Abstract

A monostatic detection method performed by a radar (100) having a transmitting means suitable for transmitting a detection signal (300) in the Terahertz band to detect a target (200) and receiving means for receiving reflected signals (310) in the Terahertz band, wherein the radar has a coverage area with non-uniform maximum definite ranges depending on a direction between the radar (100) and the target (200). The method determines the direction, then calculates the most suitable carrier frequency using a path loss formula.

Description

DESCRIPTION MONOSTATIC SENSING METHOD IN THE TERAHERTZ BAND FOR RANGE UNCERTAINTY REDUCTION TECHNICAL DOMAIN A monostatic sensing method performed by a radar having a transmitting means suitable for sending a sensing signal in the Teraheitz band to detect a target and receiving means for receiving the reflected signals in the Teraheitz band, wherein the radar has a coverage area with maximum definitive ranges that are not uniform depending on a direction between the radar and the target. BACKGROUND ART Wireless sensing has become a critical component for next-generation wireless communication networks, especially in the context of 6G. Envisioned 6G applications such as smart transportation systems, autonomous vehicles, virtual reality (VR), augmented reality (XR), holographic communication and smart cities require precise and high-resolution sensing information [1]. Both academic and industrial research is focused on developing sensing prototypes that can seamlessly integrate with existing communication networks while also operating autonomously. The demand for high-resolution sensing information necessitates the use of higher frequency bands, particularly the terahertz (THz) band, which ranges from 0.1 to 10 THz. A THz pulsed radar emits a series of narrow pulses and processes echo signals received from potential targets to determine sensing parameters such as range, bearing (azimuth and elevation angles), and speed. The maximum absolute range of a pulse-Doppler radar depends on the pulse repetition frequency (PRF) or pulse repetition interval (PRI) [2]. Beyond this range, objects suffer from range uncertainty because the delay of returns from distant objects can exceed the PRI. This problem is exacerbated in the THz band due to the lower transmit power compared to conventional microwave radars. Coherent integration of transmitted pulses is used to increase detection probability, but moving targets can cause range ambiguity or migration between range cells, blurring the range-Doppler map and reducing integration gain. Various techniques have been proposed to address the range ambiguity problem in pulsed-Doppler radars. Increasing the PRI value can include radar returns from distant objects and thus increase the radar's maximum absolute range [3]. However, this approach can negatively impact Doppler sensitivity when multiple objects with different speeds are present in the radar beam. Pulse agile waveform designs using random frequency modulation (RFM) waveforms exhibit high spectral compactness, random sidelobes, quasi-orthogonality, and constant amplitude [4]. The quasi-orthogonal waveforms and pulse agile design reduce range ambiguity by using different filter banks for matching range ranges at the receiver [5]. However, this method increases the complexity and cost of the receiver. Varying the pulse width at each PRI reduces the cross-correlation between subsequent pulses, thereby suppressing range ambiguity. This requires pulse width optimization to maintain a constant range, which increases system complexity. High-frequency synthetic aperture radars (SARs) use OFDM chirp waveforms to reduce range ambiguity [6]. This approach involves sending orthogonal waveforms for each PRI and using multiple matched filters at the receiver, which increases processing complexity and cost. Coherent integration is used to increase detection performance in pulsed radars operating in the THz band, but the transition between range cells due to range uncertainty or target mobility limits the integration time [7]. Transform techniques can reverse the effect of range-Doppler coupling, increasing coherent integration gain and target detectability, but also increasing radar system complexity. In addition to the shortcomings of the state-of-the-art techniques discussed above, all existing techniques suffer from non-uniform coverage for THz pulsed radar, as described below. Traditionally, a pulsed radar has been considered to have uniform coverage, assuming the maximum distance (radius) from the transceiver is the same in all directions. Consequently, the PRI is chosen based on the uniform maximum distance from the transceiver, except in directions where the maximum coverage distance is relatively short. If the PRI is chosen based on the minimum coverage distance, this non-uniformity will lead to range uncertainties from targets outside the coverage area. Furthermore, in directions where the maximum coverage distance is relatively long, uncertainties can arise even from targets within the coverage area but beyond the distance used to determine the PRI. All the problems mentioned above and the shortcomings of existing solutions have ultimately necessitated an innovation in the relevant technical field. List of references cited above: Use Cases and Technologies," in IEEE Communications Magazine, vol. 58, no. 3, pp. 55-61, Random FM Waveforms for Range Sidelobe Suppression and Range Ambiguity Mitigation," in IEEE Transactions on Geoscience and Remote Sensing, vol. 61, pp. 1-12, 2023, Art no. on pulse Width agility," IET International Radar Conference (IET IRC 2020), Online Diversity," in IEEE Geoscience and Remote Sensing Letters, vol. 10, no. 1, pp. 101-105, Jan. terahertz radar,"20]9 44th International Conference on Infrared, Millimeter, and Terahertz BRIEF DESCRIPTION OF THE INVENTION The present invention relates to a method that will eliminate the above-mentioned disadvantages and bring new advantages to the relevant technical field. One object of the invention is to eliminate echoes from any object outside the designated coverage area of the radar, where the coverage area has non-uniform maximum absolute ranges. Another object of reducing complexity is that no changes are required in the transmitted signal and/or radar signal processing. In order to achieve all the objectives stated above and which will emerge from the detailed description below, the present invention is provided by a radar having a transmitting means suitable for transmitting a detection signal in the Teraheitz band and receiving means for receiving the reflected signals in the Teraheitz band to detect a target The present study relates to a monostatic detection method, where the radar has a coverage area with non-uniform maximum definite ranges depending on a direction between the radar and the target. Accordingly, determining the direction of the target 9; calculating the maximum definite range for the target using a predetermined formula using the determined direction (9); estimating an optimum carrier frequency using an optimization algorithm that aims to minimize the path loss for cases where the target distance is less than the maximum definite range and to maximize the path loss for cases where the target distance is equal to or greater than the maximum definite range using a path loss formula. To suppress range uncertainty, the above-mentioned techniques have focused on modifying the transmission signal structure or signal processing techniques. However, none of the above methods have taken advantage of the inherent properties of the THz-band to overcome this problem. Furthermore, previous techniques have not considered an adaptive PRI based on the non-uniform coverage area. This In this invention, the concept of adapting the PRI based on the maximum coverage distance in a specific direction within a non-uniform coverage area is proposed. By adaptively selecting the PRI, it is possible to achieve the maximum possible Doppler resolution in all directions without compromising the system's immunity to range uncertainty within the coverage area. However, this does not reduce the range uncertainty problem when the target moves beyond the maximum coverage distance in a specific direction. Therefore, we exploit the molecular absorption phenomenon in the THz band to reduce the range uncertainty problem that may arise due to objects outside the coverage area. In the THz bands, certain frequencies align with the natural resonance frequencies of atmospheric constituents such as water vapor and oxygen molecules. When excited at their resonance frequencies, these molecules absorb significant amounts of energy from the signal, resulting in high path loss at these specific frequencies. This amount of path loss is also a function of the distance between the transmitter and the target because the number of absorbing molecules in the signal path increases. Therefore, by exploiting this frequency- and distance-dependent property of molecular absorption in THz, the propagation of an RF signal transmitted at these specific frequencies is intelligently adapted to a carrier frequency based on the maximum coverage distance in a specific direction to limit the coverage area of a THz radar. Therefore, any object beyond this coverage area will not be able to receive the radar probing signal due to significant molecular absorption. Therefore, any object moving outside the maximum absolute range of the radar will not be able to reflect the probing signal, creating range uncertainty. Therefore, the range uncertainty problem in the THz band can be solved by appropriate selection of the carrier frequency without adapting the transmit signal or using any conversion/signal processing techniques. At certain frequencies in the THz band, molecular absorption is significant and leads to larger path loss peaks, without which reliable communication is not possible. These molecular absorption peaks also change with distance as the signal propagates. In this invention, we estimate a carrier frequency and limit the coverage area of the THz radar, and therefore any object beyond this coverage area will not be able to receive the radar probing signal due to significant molecular absorption. Therefore, any object moving outside the maximum absolute range of the radar will not be able to reflect the probing signal and will create range uncertainty. Therefore, the range uncertainty problem in the THz band can be solved by appropriate selection of the carrier frequency without adapting the transmit signal or using any conversion/signal processing techniques. The advantages of the proposed invention can be summarized as follows: Improved Doppler resolution depending on the direction within the coverage area; reduction of range uncertainty originating from target(s) that may be located inside or outside the specified coverage area; reduction of complexity since no changes are required in the transmitted signal and/or radar signal processing. Improvement of the accuracy of detection parameters (range and Doppler) improvement; improving coherent integration gain by reducing range uncertainty in low SNR scenarios. The proposed scheme best suits practical scenarios where the target coverage area is not uniform. BRIEF DESCRIPTION OF THE FIGURES Figure 1 is a plot showing the radar, a target within the maximum positive range, and another target which is moving and outside the positive range. Figure 2 shows the range estimation when the target is within the coverage area, i.e., target (3). Figure 3 shows the range estimation when the detection target is outside the coverage area, i.e., target (3). Figure 4 and Figure 5 show the range estimation with the proposed method when the detection target is within the coverage area and a cluttering object is outside the coverage area. REFERENCE NUMBERS GIVEN IN THE FIGURE 100 Radar 200 Target 300 Detection Signal 310 Reflected Signal 320 Maximum precise range 321 First maximum precise range 322 Second maximum precise range 330 Target range 340 Coverage area 410 First direction 420 Second direction DETAILED DESCRIPTION OF THE INVENTION In this detailed description, the subject is explained by referring only to examples without creating any limiting effect in order to make the subject more understandable. Referring to Figure 1, the invention is a radar (100) capable of performing monostatic detection and a method thereof. The radar (100) has transmitting means suitable for transmitting a detection signal (300) in the Terahertz (THz) band and receiving means for receiving reflected signals (310) in the THz band. In other words, the radar (100) is a is a detection transceiver. The receiving means and the sending means include single or multiple antennas. The radar (100) includes components that provide signals to the sending antenna and process the signals received from the receiving means. Monostatic detection radars (100) are well known in the art, therefore, no further technical details are explained here. The radar (100) can be implemented in a base station, roadside unit (RSU), node-B, gNode-B. The radar (100) sends a detection signal (300) in a series of pulses and determines the range of the target (200) based on the signals (310) reflected from the target (200). The radar range is defined by (320). In this invention, the radar (100) has non-uniform maximum absolute ranges (320) as shown in Figure 1. In other words, the radar (100) and the target The radar 100 has different maximum precise ranges 340 depending on the direction between 200. For example, in Figure 1, the first maximum precise range 321 in a first direction 410 is greater than the second maximum precise range in a second direction 420. Figures 2 and 3 show the detection signal 300 and the reflected signals 310 in prior art systems. Figure 2 shows the detection signal 300 and the reflected signal 310 received from the target 200, the reflected signal 310 of one pulse is received before the next pulse is transmitted. Thus, the reflection signal of the target 200 falls between the two pulses. Figure 3 shows the detection signal 300 and the reflected signal 310 received from the target 200 which is outside the maximum precise range 320. The reflected signal of the pulse (310) is received after the next pulse is transmitted. Thus, a certainty is created. The method in question uses a specific THz band selected for the target (200) in order to reduce the range uncertainty of the target (200). Since any object beyond this coverage area cannot receive the radar probing signal due to significant molecular absorption, objects beyond the maximum absolute range do not reflect the signal. Therefore, any object moving outside the maximum absolute range of the radar will not be able to reflect the probing signal and will create a range uncertainty. Therefore, the range uncertainty problem can be solved in the THz band by appropriately selecting the carrier frequency without adapting the transmission signal or using any conversion/signal processing techniques. The radar sends a series of pulses. A conventional THz monostatic radar sends a half-duplex pulsed waveform, alternating between sending pulses and receiving reflections from the target(s). During the transmission period, the radar receiver is turned off to prevent interference. The THz transmitted pulse can be expressed as follows: (lift) Elif âxnft III] 0 IE: NI: where r 1 /B is the pulse repetition interval (PRI), N is the number of PRIs in a single coherent processing interval (CPI). In addition, ;cn t represents the waveform transmitted over the 11th PRI with a bandwidth of B and can be expressed as: xn(t) = FEN& 0 5 t 5 TP, k: Ü,---,K- 1, where, Tu < T is the radar pulse duration, R. is the radar transmit power, and mn is the radar pulse with normalized power LIJPIFÜIIIZ dt: 1. The radar reflection received at the receiver after interaction with the target within a CPI is written as: yt [IFII :deî'znfdtw nt D I NI' Here 5; represents the complex channel coefficient of the path reflected by the target. Furthermore, T, , represents the two-way propagation delay that is converted into the range of the target through the relationship ;i- = CT, ,:2, f; is the Doppler frequency shift due to the motion of the target and is given by fa = &, where v.? is the radial velocity of the target and ;1 THz is the wavelength of the carrier frequency. Furthermore, mn denotes the additive white Gaussian noise (AWGN) in which mn N warm, Ji-,m is assumed to be a circularly symmetric complex Gaussian (CSCG) random process and ;d is the noise power spectrum density (PSD). The maximum absolute range d at which the radar can reliably detect the range of a target can be given by the following model: f: Td max 5 TI? In cases where the maximum precise range of a radar varies as a function of the target 200's direction 0, that is, d_max varies with the direction of the target 200 within a given coverage area 340. In addition, ramax is the round-trip delay corresponding to the maximum coverage distance (maximum precise range 320)) d (6). However, considering that the radar pulse duration T, , is very short compared to PRI 1', the maximum precise range 320 can be rewritten as d (6) : Hawks (9). The method involves the following steps: Determining the direction 0 of the target 200. Calculating the maximum precise range 320 for the target 200 from a predetermined model of the coverage area 340 using the determined direction 0 as input. The maximum absolute range (320) can be calculated using one of the models mentioned above. The optimum carrier frequency fopt, target (9) :1 dmax (match PL (fani target (6)))y01 is estimated using an optimization algorithm that aims to minimize the loss and maximize the PL (fapt, target (6)) yo ay for target (6) 3, dmax (ß); where dtaret is the distance between the target and the radar and dmax is the maximum clear range in the 0 direction (320); and the path loss formula is ) ekabsmtardef (9) where, km (1". is the frequency-dependent molecular absorption coefficient, c is the speed of light; f is the optimum carrier frequency. Any suitable optimization algorithm known in the art can be used for the estimation. Transmission of the detection signal (300) using the estimated carrier frequency. In one possible configuration, the method is: - The method includes the steps of calculating a pulse repetition interval (PRI) for direction 0 and transmitting the detection signal 300 using the calculated PRI. The PRI can be calculated using the following formula: 2 dmax a where c is the speed of light. In another possible configuration, the method can include the steps of calculating the PRI and the optimum carrier frequency for each direction and storing them in a search table to be used for the next target and updating the search table at predetermined periods. Thus, the resources consumed during the process for each direction are reduced. Such a search table is given below as an example. Figure 4 and Figure 5 show the range estimation with the proposed method when the detection target 200 is within the coverage area 340 and an object is outside the coverage area. The object does not interact with the target. The method in question is not applicable to any system such as Integrated Sensing and Communication (ISAC) systems. It can be applied to THz detection systems. The scope of protection of the invention is specified in the appended claims and cannot be limited to those explained in this detailed description for illustrative purposes. It is clear that a person skilled in the art can demonstrate similar configurations in light of the facts stated above without departing from the main theme of the invention.