Lens antenna adjustment for telecommunication and imaging modes in a sub-THz radio system

This article presents a concept of a dual-mode operated sub-THz frequency radio system by adjusting the lens-antenna distance for either telecommunication or imaging modes. The signal transmittance and imaging capabilities are demonstrated using a continuous wave operated transceiver with operation frequency 220 to 330 GHz where the 6G wireless radio system could be allocated. A high-gain bullet-shape lens antenna was investigated for imaging the natu-ral object (Bergenia leaf) in the short-range line-of-sight radio link. The bullet lens performed with the gain of 28 dB and the 1 ◦ beamwidth over the frequency band. For the demonstration of dual-mode operation, the leaf was placed into the middle of the radio link path and then its image was synthesized at three lenses to the antenna-distances by utilizing the inverse synthetic-aperture radar technique. For image synthesizing, the movement pattern area (100 mm × 100 mm) is defined with 5 mm steps to sweep the object area. Three 100 mm × 100 mm images by 5 mm steps from different lens positions in focal axis distances (4, 15, and 30 mm were synthesized. The comparison shows that 15 mm distance has the highest gain and 30 mm distance has the higher image quality. Therefore, there is the possibility to switch from the high gain low-resolution mode to the low gain high-resolution mode by changing the lens position in terms of the feed. The imaging feasibility can be applied at 6G radio systems if adjustable lens systems are used revealing the new potential features in future radios.

been proposed for the candidate of the 6G communication systems that allowing as a high speed as terabit per second.Current 5G telecommunication technology cannot meet previous technical requirements.
At sub-THz frequency, the antenna dimension will be decreased drastically which makes more much difficult to maintain large antenna aperture.If the aperture decreases, the power of the wave propagation is decreased.To overcome the challenge in the technology, lens antennas or phased array antennas have been applied as an alternative approach.Both options can provide higher gain and directivity, but however, the lens antenna is naturally enabling the focal length adjustment for either telecommunication or imaging purposes.The antenna beam is reflected in the lens and widen/reduced beam characteristics can be changed.
Several research works have been focused on dielectric lens antenna modification [1][2][3][4] and circuit components. 5Elliptical lens geometry could contribute to the gain of 36.5 dB for 110 GHz, 4 and hyperbolic horn lens model aimed by metal guide feeder could perform gain 39 dB at frequency 240 GHz, 3 respectively.The high gain antenna is a key requirement to support the high data rate radio link for the upcoming high frequency communication systems even for short range has been predicted about 100 dB gain 6 so gain enhancing methods with narrow beam properties still is a matter of research.Recently, experimental research has been carried out on the long-range radio link with frequency 300 GHz, the parabolic lens antenna around 55 dB was implemented to provide high bit rate communication for two long distances (500 m and 1 km). 7Lens antenna with a narrow beam and high directivity has been earlier presented to synthesize image 8 and accurate localization 9 already.Sub-THz frequency (or μwave), among wave to target interaction, the smaller target can be detected by smaller wavelength precisely. 10Consequently, higher resolution images can be synthesized.The reflected wave at sub-THz frequency is an alternative way to collect information from the target. 9,11,12Combined communications and sensing ideas are presented earlier in References 13-15.
THz imaging has a wide range of applications.7][18][19][20][21][22][23] Imaging at 300 GHz was presented in the short (50 cm) and long (2 m) range using fixed lens positions. 24In telecommunication, lenses are typically fixed to have maximum gain 25 between transmitter and receiver.This means that the object between transmitter and receiver is not in focus.Our research question is related on this phenomenon: If the lens is not fixed, then lens can be focused to the object for imaging or the high gain for communication.][28][29][30] In this article, a concept of a dual mode operated sub-THz frequency radio system by adjusting the lens-antenna distance for either telecommunication or imaging modes is presented.Bergenia leaf was used as an object for performance evaluation in the short-range continuous wave (CW) radio system.The image of the object was synthesized by aiming the narrow beam width bullet lens towards the leaf and receiver and moving the leaf with the pattern of a 2-dimensional matrix of (20 × 20 by 5 mm step) by utilizing the inverse synthetic-aperture radar technique.
The principle of the concept of the dual mode operated communication and imaging mode radio system is illustrated by schematic diagram in Figure 1.The bullet-shaped lens antenna is implemented at 15 mm antenna to the lens-distance to the transmitter (receiver) waveguide in the telecommunication mode and the radiated beam is collimated or focused to the receiver for the high gain characteristics.In the imaging mode, the bullet-shaped lens antenna is adjusted at 30 mm antenna to lens-distance to the transmitter (receiver) waveguide and the radiated beam is refracted for reduced beam width and the focal point is moved to the object for the high image quality characteristics.

F I G U R E 2 Simulated reflection co-efficient and gain of bullet shape lens antenna
The concept of the dual mode operated communication and imaging mode radio system is demonstrated, and the developed bullet-shaped lens antenna is characterized by simulation and measurements in the following sections.Section 2 will describe a setup for a 300 GHz radio link, a bullet lens antenna is designed, then the radio link is realized by integrating a setup of instruments.Section 3, the object will be placed into the middle radio link, the image of the object is synthesized by the aim of the bullet lens and its displacement in the focal axis direction.Finally, the applicability of the concept is discussed.

EXPERIMENTAL BULLET LENS DESING
To reach the narrow beamwidth and high gain the elliptical geometry was applied for lens 1,31 and the bullet-shaped lens antenna was designed by CST microwave Studio 32 at a center frequency of 300 GHz.The lens was fabricated using a room temperature fabrication (RTF) method. 33A low dielectric permittivity (ε r = 1.18, tan  = 0.003) ceramic composite (LMO-HGMS, the material recipe is presented in Reference 34, for sustainable 6G 30 ) was casted in a shape using a molding process.The fabricated lens size was 30 × 49.5 mm.The reflection coefficient of the antenna lens was simulated to below than −10.5 dB for the frequency band from 220 to 330 GHz in Figure 2 owing to the maximum gain of 28 dB with 1 • beamwidth (HPBW) at 300 GHz.

EXPERIMENTAL MEASUREMENTS FOR DUAL MODE LENS ANTENNA
To realize the realistic short range over-the-air measurement system the Keysight PNA-X (operating frequency 67 GHz) was connected via cables to a pair of VDI extenders (operating frequency 220 to 330GHz), Tx (transmitter), and Rx (receiver) and it is illustrated into Figure 3. Tx and Rx are placed in reciprocal position and aligned on the same level and positions for the maximum transmitted power.The input power 0 dBm is adjusted to feed on PNA-X to VDI extender which Tx attached to a rectangular standard waveguide WR3.4 and the bullet-shaped lens.To collect the waves (as a receiver), Rx connected via cable to a standard horn antenna of 30 dB gain.After the PNA-X calibration in order to find the optimum antenna to lens-distance the lens was located at 21 mm leading to the transmitted signal in the system.Rx cable output as an S21 parameter was recorded in PNA-X.The measurement setup presented in Figure 3 was adjusted for lens characterization, whereas the leaf characterization is applied later in the same system.The previous measurement setup was used for demonstrating telecommunication and imaging modes by the measurements of three different sample distances (4, 15, and 30 mm) of Bergenia leaf.The imaging technique is called Inverse Synthetic Aperture Radar (ISAR) imaging, where instead of radar sweeping (or across the board) the target is moving.Regarding environment and facilities limitation, the measurement setup Tx and Rx are placed at 50 cm distance.For further investigation, the bullet lens beam waist size parameter ( = 1.2 mm) was calculated using Equation ( 1), which is function of wavelength  0 and focal length f 1 and lens D diameter 35 : Bergenia leaf was used as object in the measurements (Figure 4A) appearing substantial thickness of 100 μm in general and 2 mm in water veins.The leaf was placed in the middle of 50 cm distance.Then it is mounted over a plate that has moving capability into three directions (X, Y, and Z) with a controllable stepped motor.All setups were placed at the height of 1.5 m above the earth.
To acquire image of the object, a movement area pattern of 100 mm × 100 mm was defined for the plate (with leaf) into two Cartesian directions (Y and Z) with the step of 5 mm.It means that the leaf is moved and displaced into the width of radio link (Y and Z) path but not in focal axis direction (X).The image acquiring procedure is repeated for every lens displacement and images are synthesized by aim of MATLAB algorithm.Here, by displacing lens in three distances to Tx feeder, 4, 15, and 30 mm, three images are synthesized for frequency 300 GHz and displayed in Figure 4B-D, respectively.
The images are synthesized from the measured S21 parameter, the differences in the path loss (right side colorbar) are caused by the different thickness of the leaf and free space loss in the system.Recorded S21 was −45 dB when the lens was at 4 mm distance from the Tx, −40 dB at 15 mm distance and −48 dB at 30 mm distance.So, the system was optimized for maximum transmitted power at 15 mm distance.The synthesized image with the distance of 4 mm is presented in Figure 4B and the image with the distance of 15 mm in Figure 4C.The image resolution in terms of water veins is close to each other.However, the image with the distance of 30 mm (Figure 4D) had the best quality and the image resolution in terms of water veins was clearly improved.In fact, by changing distances, the focal point, and beam waist are changed which impacted the image quality, consequently.When the distance between lens to Tx (4 mm) is decreased, image (Figure 4B) is not clear, but when the distance is increased (30 mm), it means the leaf is in the focus and then the synthesized image quality is improved clearly.
Thus, the principle of the concept of the dual mode operated communication and imaging mode radio system was demonstrated by displacing the lens positions in focal axis direction (X), distance between Tx to lens meaning changing states between high gain and the image resolution, consequently.Briefly, changing the lens position it is possible to switch between the high gain low resolution mode and the low gain high resolution mode, which is demonstrated, experimentally.
The lens antenna implementation is helped to identify the image of assumed objects into the radio system, clearly, even the difference is distinguishable by the eye.Also, the leaf thick and thin part has appeared in the image with dark blue and light yellow based on colorbar indication, respectively.By inspiration of optical imaging, it demonstrated lens antenna performs properly with focal point distance.This high-quality image Figure 4D is thanks to bullet lens parameters include high gain 28 dB and narrow beamwidth 1 • .In addition, the capacity of transmitter lens's focal point adjustment can be used for increasing the radiated field strength in the location of receiver antenna or alternatively decreasing the field strength in the location of the adjacent user.

CONCLUSION
This work demonstrated the upcoming 6G radio system having the substantial potential to be implemented the short-range imaging applications in particular for an object that might appear in the radio channel.The bullet lens with performance parameters including gain 28 dBi and beamwidth 1 • was fabricated and implemented into the radio link.It aims to construct the image of the object.The impact of the distance, between lens to transmitter waveguide was varied: 4, 15, and 0 mm.Maximum transmitted power was at optimum 15 mm distance and image quality was at optimum distance of 30 mm, that are representing optimum lens displacement positions for telecommunication and imaging in this system.This is the main outcome of this research.Synthesized images are visible clearly even visually, and it shows by increasing lens distance higher resolution images are synthesized.This setup was successfully demonstrated the imaging mode radio system and confirmed that it has potential to be implement in the upcoming 6G telecommunication systems.

F I G U R E 3
Photo of lens measurement and imaging set up for 220-330 GHz sub-THz frequency including Tx and Rx units, waveguide, horn antenna, and fabricated bullet lens