Method of monostatic sensing in terahertz band for range ambiguity mitigation

Abstract

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

Description

METHOD OF MONOSTATIC SENSING IN TERAHERTZ BAND FOR RANGE AMBIGUITY MITIGATION

[0001] TECHNICAL FIELD

[0002] A monostatic sensing method realized by a radar having a transmitting means suitable for transmitting a sensing signal in Terahertz band and having receiving means for receiving reflected signals in Terahertz band for sensing a target; wherein radar has a coverage area having non-uniform maximum unambiguous ranges depending on a direction between the radar and the target.

[0003] PRIOR ART

[0004] Wireless sensing has become a critical component for next-generation wireless communication networks, especially in the context of 6G. Envisioned 6G applications, such as intelligent transportation systems, autonomous vehicles, virtual reality (VR), extended 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 being capable of independent operation. The demand for high-resolution sensing information necessitates the use of higher frequency bands, particularly the terahertz (THz) band ranging from 0.1 to 10 THz.

[0005] THz pulsed radar transmits a series of narrow pulses and determines sensing parameters such as range, direction (azimuth and elevation angles), and velocity by processing received echo signals from potential targets. The maximum unambiguous range of a pulsed-Doppler radar is dependent on its pulse repetition frequency (PRF) or pulse repetition interval (PRI) [2]. Beyond this range, objects suffer from range ambiguity as the delay of returns from distant objects can exceed the PRI. This issue is exacerbated in the THz band due to lower transmit power compared to conventional microwave radars. Coherent integration of transmitted pulses are employed to enhance detection probability, though mobile targets can cause range ambiguity or migration through range cells, smearing the range-Doppler map and decreasing integration gain.

[0006] Various techniques have been proposed to address the issue of range ambiguity in pulsed-Doppler radars. Increasing the PRI can incorporate radar returns from distant objects, thus enhancing the maximum unambiguous range of the radar [3]. However, this approach can adversely affect Doppler precision when many objects with varying velocities are within the radar beam.

[0007] Pulse-agile waveform design using random frequency modulation (RFM) waveforms exhibits high spectral compactness, random sidelobes, quasi-orthogonality, and constant amplitude [4]. Quasi-orthogonal waveforms and pulse agility mitigate range ambiguity, with different filter banks used at the receiver for matching range intervals [5]. This method, however, increases receiver complexity and cost. Varying the pulse width in each PRI reduces the cross-correlation between subsequent pulses, thereby suppressing range ambiguity. This requires pulse width optimization to maintain a fixed range, which increases system complexity.

[0008] High-PRF synthetic aperture radars (SARs) use OFDM chirp diverse waveforms to mitigate range ambiguity [6]. This approach involves transmitting orthogonal waveforms in each PRI and employing multiple matched filters at the receiver, which increases processing complexity and cost. For THz band pulsed radar, coherent integration is used to improve detection performance, though range ambiguity or migration through range cells due to target mobility limits the integration time [7]. Transform techniques can reverse the effect of range-Doppler coupling, enhancing coherent integration gain and target detectability, but also increasing radar system complexity.

[0009] Apart from the shortcomings related to the state-of-the-art techniques discussed above, all of the existing techniques also do not incorporate the possibility of the presence of non-uniform coverage area for THz pulsed radar as described below.

[0010] Conventionally for a pulsed radar, a uniform coverage area is considered where the maximum distance (radius) from the transceiver is assumed to be same in all directions. Consequently, PRI is selected based on the uniform maximum distance from the transceiver. Apart in the directions whose maximum coverage distance is relatively shorter. If the PRI is chosen based on the minimum coverage distance, it will lead to range ambiguities from targets outside the coverage region due to non-uniformity. Additionally in the directions with relatively longer maximum coverage distance, ambiguities might arise even from the targets that lie inside the coverage region but beyond the distance used to specify PRI.

[0011] All the problems and shortcomings of the current solutions mentioned above have made it necessary to make an innovation in the relevant technical field as a result.

[0012] List of references mentioned above:[1] M. Giordani, M. Polese, M. Mezzavilla, S. Rangan and M. Zorzi, "Toward 6G Networks: Use Cases and Technologies," in IEEE Communications Magazine, vol. 58, no. 3, pp. 55-61, March 2020, doi: 10.1109 / MCOM.001.1900411.[2] Levanon, Nadav. "Mitigating range ambiguity in high PRF radar using inter-pulse binary coding." IEEE Transactions on aerospace and electronic systems 45.2 (2009): 687-697.[3] Doerry, Armin W. "A Study of Pulse-Doppler Radar Pulse Repetition Frequency." (2023).[4] Q. Xie, C. Liu, Z. Mo and W. Li, "A Novel Pulse-Agile Waveform Design Based on 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. 5110612, doi: 10.1109 / TGRS.2023.3326840.[5] Z. Liu, L. Ren, Y. Sun, H. Fan and E. Mao, "Waveform design of LFM pulse train based on pulse width agility," IET International Radar Conference (IET IRC 2020), Online Conference, 2020, pp. 1679-1684, doi: 10.1049 / icp.2021.0701.[6] W. -Q. Wang, "Mitigating Range Ambiguities in High-PRF SAR With OFDM Waveform Diversity," in IEEE Geoscience and Remote Sensing Letters, vol. 10, no. 1, pp. 101-105, Jan. 2013, doi: 10.1109 / LGRS.2012.2193870.[7] H. Li, C. Li, S. Wu and G. Fang, "Long-time coherent integration for target detection in terahertz radar," 2019 44th International Conference on Infrared, Millimeter, and Terahertz Waves (IRMMW-THz), Paris, France, 2019, pp. 1-2, doi: 10.1109 / IRMMW-THz.2019.8874229.

[0013] BRIEF DESCRIPTION OF THE INVENTION

[0014] The present invention relates to a method to eliminate the above-mentioned disadvantages and bring new advantages to the relevant technical field.

[0015] An object of the invention is to eliminate the echoes from any objects outside the designated coverage area of the radar, wherein the coverage area has non-uniform maximum unambiguity ranges.

[0016] Another object of the reducing the complexity because no alteration in the transmitted signal and / or in the radar signal processing is required.

[0017] To achieve all the objects mentioned above and that will emerge from the following detailed description, the present invention relates to A monostatic sensing method realized by a radar having a transmitting means suitable for transmitting a sensing signal in Terahertz band and having receiving means for receiving reflected signals in Terahertz band for sensing a target; wherein radar has a coverage area having non-uniform maximum unambiguous ranges depending on a direction between the radar and the target. Accordingly, determining the direction of the target; calculating maximum unambiguous range for the target using a predetermined formula using the determined direction (θ); estimating an optimum carrier frequency using an optimization algorithm which aims to minimize path loss for when the target distance is less than the maximum unambiguous range and maximizing path loss for when the target distance is larger than or equal to maximum unambiguous range using a path-loss formula.

[0018] The above mentioned techniques to suppress range ambiguity focused on altering the transmit signal structure or the signal processing techniques. However, none of the above methods exploited the inherent THz-band features to overcome this issue. Moreover, the prior art did not consider an adaptive PRI based on non-uniform coverage region.

[0019] In this invention, the concept of adapting PRI based on the maximum coverage distance in a given direction in non-uniform coverage area is proposed. By adaptively selecting the PRI, it is possible to achieve the maximum-possible Doppler resolution in each direction without compromising system immunity against range ambiguity within the coverage region. However, this does not ensure the mitigation of the range-ambiguity problem if the target moves beyond the maximum coverage distance in a given direction. We therefore leverage the molecular absorption phenomenon in THz band to mitigate the range-ambiguity problem that may raise due to objects outside the coverage region.

[0020] In THz bands, some of the frequencies align with the natural resonance frequencies of the atmospheric contents such as water vapor and Oxygen molecules. When excited at their resonance frequencies, these molecules absorb significant amount of energy from the signal, leading to high pathloss at these particular frequencies. This amount of incurred pathloss is also a function of the distance of separation between the transmitter and the target as the number of the absorbing molecules within the signal path increases with the distance.

[0021] Therefore, propagation ranges of an RF signals transmitted at these particular frequencies are highly limited.

[0022] Exploiting this frequency- and distance-dependent property of the molecular absorption in THz, we intelligently adapt a carrier frequency based on the maximum coverage distance in a given direction to limit the coverage of the THz radar and hence any object beyond this coverage region will not be able to receive the radar probing signal due to significant molecular absorption. Therefore any object moving outside the radar's maximum unambiguous range, will not be able to reflect the probing signal and create range ambiguity. Hence, without tailoring the transmit signal or employing any transform / signal processing technique, the issue of range ambiguity can be resolved in the THz band by appropriately selecting the carrier frequency.

[0023] At certain frequencies in the THz band, molecular absorption is significant leading to greater pathloss peaks where reliable communication is not possible. These molecular absorption peaks also vary with the distance as the signal travels. In this invention, we estimate a carrier frequency and limit the coverage of the THz radar and hence any object beyond this coverage region will not be able to receive the radar probing signal due to significant molecular absorption. Therefore any object moving outside the radar's maximum unambiguous range, will not be able to reflect the probing signal and create range ambiguity. Hence, without tailoring the transmit signal or employing any transform / signal processing technique, the issue of range ambiguity can be resolved in the THz band by appropriately selecting the carrier frequency.

[0024] The advantages of the proposed invention can be summarized as follows: Direction dependent enhanced Doppler resolution within the coverage area; mitigation of range ambiguity resulting from target(s) that may lie inside or outside the specified coverage area; reducing complexity because no alteration in the transmitted signal and / or in the radar signal processing is required. Improving the sensing parameters' (range and Doppler) accuracy; improving the coherent integration gain in low-SNR scenarios by mitigating the range ambiguity. The proposed scheme best fits the practical scenarios where the targeted coverage area is non-uniform.

[0025] BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 is a drawing illustrating the radar, a target, which is inside the maximum unambiguous range and another target that is mobile and outside of the ambiguity range.

[0027] Figure 2 is illustrating range estimation when the target is inside the coverage area i.e. dtarget (θ) < dmax(θ).

[0028] Figure 3 is illustrating range estimation when sensing target is outside the coverage area i.e. dtarget (θ) > dmax(θ).

[0029] Figure 4 and figure 5 are illustrating the range estimation with the proposed method when sensing target is inside the coverage area and an interfering object is outside the coverage area.

[0030] REFERENCE NUMBERS GIVEN IN THE FIGURE

[0031] 100 Radar200 Target300 Sensing signal310 Reflected signal320 Maximum unambiguous range321 First maximum unambiguous range322 Second maximum unambiguous range330 Target range340 Coverage area410 First direction420 Second direction

[0032] DETAILED DESCRIPTION OF THE INVENTION

[0033] In this detailed description, the subject matter is explained with references to examples without forming any restrictive effect only in order to make the subject more understandable.

[0034] Referring to figure 1, invention is a radar (100) which is capable of realizing monostatic sensing and a method thereof. Said radar (100) comprises transmitting means suitable for transmitting a sensing signal (300) in Terahertz (THz) band and having receiving means for receiving reflected signals (310) in THz band. In other words, the radar (100) is a sensing transceiver. Receiving means and transmitting means comprises single or multiple antennas. Radar (100) comprises components for providing signal to transmitting antenna, processing signals received from receiving means. Monostatic sensing radars (100) are well known in the art, so

[0035] further technical details are not disclosed herein. Radar (100) may be implemented in a or be a base station, road side unit (RSU), node-B, gNode-B.

[0036] Radar (100) transmits a sensing signal (300) in series of pulses and determines the range of the target (200) based on reflected signals (310) from the target (200). The radar (100) has a coverage area (340). Coverage area (340) is defined by the maximum unambiguous range (320) of the radar (100). In this invention, radar (100) has non-uniform maximum unambiguous ranges (320) as depicted in figure 1. In other words, radar (100) has different maximum unambiguous ranges (340) depending on the direction between the radar (100) and the target (200). For instance, in figure 1, a first maximum unambiguous range (321) of a first direction (410) is larger than a second maximum unambiguous range of a second direction (420).

[0037] Figure 2 and 3 shows the sensing signal (300) and the reflected signals (310) in prior art systems. Figure 2 depicts the sensing signal (300) and reflected signal (310) received from the target (200), the reflected signal (310) of a pulse is received before the next pulse is transmitted. Thus, reflection signal of the target (200) remains between two pulses.

[0038] Figure 3 depicts the sensing signal (300) and reflected signal (310) received from the target (200), which is outside the maximum unambiguous range (320). Reflected signal (310) of a pulse is received after next pulse is transmitted. Thus creating an unambiguity.

[0039] Subject matter method utilizes a specific THz band selected for the target (200) in order to mitigate range unambiguity of the target (200). The objects beyond maximum unambiguous range does not reflect the signal since any object beyond this coverage region will not be able to receive the radar probing signal due to significant molecular absorption. Therefore any object moving outside the radar's maximum unambiguous range, will not be able to reflect the probing signal and create range ambiguity. Hence, without tailoring the transmit signal or employing any transform / signal processing technique, the issue of range ambiguity can be resolved in the THz band by appropriately selecting the carrier frequency.

[0040] The radar transmits a series of pulses. A conventional THz monostatic radar transmits a half-duplex pulsed waveform, alternating between the transmission of pulses and receiving back the reflections from the target(s). During the transmission time, the radar receiver is switched off to avoid interference. The THz transmit pulse may be expressed asx(t) = Σn=0 to N-1 xn(t-nτ), 0 ≤ t ≤ Ντ,

[0041] Where τ » 1 / B is the pulse repetition interval (PRI), N is the number of PRIs within a single coherent processing interval (CPI). Additionally, xn(t) represent the waveform transmitted over the nth PRI with the bandwidth B and can be expressed as:xn(t) = {√Pt*p(t), 0 ≤ t ≤ Tp ; 0, Tp<t≤T}

[0042] Here, Tp < T represents the radar pulse duration, Pt denotes radar transmit power and p(t) is the radar pulse having the normalized power 1 / Tp ∫ from 0 to Tp |p(t)|^2 dt = 1. The received radar reflection at the receiver after interaction with the target within one CPI is written asy(t) = αx(t − τd)e^j2πfd(t-τd) + n(t), 0 ≤ t ≤ Ντ',

[0043] Where α denotes the complex channel coefficient of the reflected path by the target. Moreover, τd represents the two-way propagation delay that translates into the range of the target through the relation r = cτd / 2, fd is the Doppler frequency shift caused due to the motion of the target and is given by fd = 2vd / λ, where vd is the radial velocity of the target and λ is the wavelength of the THz carrier frequency. Additionally, n(t) denotes the additive white Gaussian noise (AWGN), which is assumed as a circularly symmetric complex Gaussian (CSCG) random process where n(t) ~ CN (0, No), with No being the noise power spectrum density (PSD).

[0044] The maximum unambiguous range dmax upto which the radar can detect the range of a target reliably may be given by the following model:dmax(θ) = c(τd,max(θ) - Tp) / 2

[0045] Where the maximum unambiguous range of a radar changes as a function of target direction θ of the target (200), i.e. dmax varies with the direction of the target (200) within a specified coverage area (340). Additionally, τd,max is the round trip delay corresponding to the maximum coverage distance (maximum unambiguous range (320)) dmax(θ). However, given that radar pulse duration Tp is much short in comparison to the PRI τ, the maximum unambiguous range (320) can be rewritten asdmax (θ) = c*τd,max(θ) / 2

[0046] Subject matter method comprises following steps:

[0047] Determining the direction θ of the target (200).

[0048] Calculating maximum unambiguous range (320) for the target (200) from a predetermined model of the coverage area (340) using the determined direction (θ) as input. Maximum unambiguous range (320) may be calculated using one of the above mentioned models.

[0049] Estimating an optimum carrier frequency fopt using an optimization algorithm which aims to minimize path loss PL (fopt, dtarget(θ)), for dtarget(θ) ≤ dmax(θ) while aiming to maximizes path loss PL (fopt, dtarget(θ)) for dtarget(θ) > dmax(θ) wherein where dtarget is the distance between the target and the radar and dmax is the maximum unambiguous range (320) on direction θ; and wherein the path loss formula isPL(f, d) = ( (4πf dtarget(θ)) / c )^2 * e^(kabs(f)dtarget(θ))where, kabs (f) is the frequency dependent molecular absorption coefficient, c the speed of light; f is the optimal carrier frequency. Any suitable optimization algorithm known in the art may be used for the estimation.

[0050] Transmitting sensing signal (300) using estimated carrier frequency.

[0051] In a possible embodiment the method comprises the steps of:- calculating a pulse repetition interval (PRI) for the direction θ.- transmitting sensing signal (300) using calculated PRI.

[0052] PRI may be calculated using below formula:TPRI(θ) = 2dmax(θ) / cwhere c is speed of light.

[0053] In another possible embodiment the method may comprise the steps of calculating PRI and optimal carrier frequency for each direction and recording on a look-up table to be used for next target; and updating the lookup table on predetermined periods. Thus, reducing the resources consumed during processing for each direction.

[0054] Such look-up table is given below as example.| θ | PRI(θ) | fopt(θ) ||---|---|---|| 0 | : | : || : | : | : || 2π | : | : |Table 1

[0055] Figure 4 and figure 5 are illustrating the range estimation with the proposed method when sensing target (200) is inside the coverage area (340) and an object is outside the coverage area. Object does not interfere with the target.

[0056] Subject matter method is applicable to any THz sensing system such as Integrated Sensing and Communication (ISAC) systems.

[0057] The scope of protection of the invention is specified in the attached claims and cannot be limited to those explained for sampling purposes in this detailed description. It is evident that a person skilled in the art may exhibit similar embodiments in light of the above-mentioned facts without drifting apart from the main theme of the invention.

Claims

1. A monostatic sensing method realized by a radar (100) having a transmitting means suitable for transmitting a sensing signal (300) in Terahertz band and having receiving means for receiving reflected signals (310) in Terahertz band for sensing a target (200); wherein radar has a coverage area (340) having non-uniform maximum unambiguous ranges (320) depending on a direction between the radar (100) and the target (200) characterized in that comprising steps of:- determining the direction θ of the target (200);- calculating maximum unambiguous range (320) for the target (200) from a predetermined model of the coverage area (340) using the determined direction (θ) as input;- estimating an optimum carrier frequency fopt using an optimization algorithm which aims to minimize path loss PL (fopt, drarget(0)), for dtarget(θ) ≤ dmax(θ) while aiming to maximizes path loss PL(fopt, dtarget(0)) for dtarget(θ) > dmax(θ) wherein where dtarget is the distance between the target (200) and the radar (100) and dmax is the maximum unambiguous range on direction θ; and wherein the path loss formula isPL(f, d) = ( (4πf dtarget (0)) / c )^2 * e^(kabs(f)dtarget(θ))where, kabs (f) is the frequency dependent molecular absorption coefficient, c the speed of light; f is the optimal carrier frequency;- transmitting sensing signal (300) using estimated carrier frequency.

2. The method according to claim 1, characterized in that comprising the steps of:- calculating a pulse repetition interval (PRI) for the direction θ,- transmitting sensing signal (300) using calculated PRI .

3. The method according to claim 2, characterized in that calculating PRI using below formula;TPRI(θ) = 2dmax(θ) / cwhere c is speed of light.

4. The method according to claim 2, characterized in that comprising the steps of:calculating PRI and optimal carrier frequency for each direction and recording on a look-up table to be used for next target (200); updating the lookup table on predetermined periods.

5. A system comprising a radar (100) having a transmitting means suitable for transmitting a sensing signal (300) in Terahertz band and having receiving means for receiving reflected signals (310) in Terahertz band for sensing a target (200); wherein radar has a coverage area (340) having non-uniform maximum unambiguous ranges (320) depending on a direction between the radar (100) and the target (200) characterized that wherein said radar (100) is configured to execute one of the method in claim 1 to 4.

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