The idea of employing an electromagnetic system as an aid for visually impaired people is quite recent. In^[1], a vector network analyzer and a double-ridge horn antenna were used for transmitting and receiving a sequence of pulses from 1 to 6 GHz. Then, an inverse Fourier transform was applied to the frequency-domain measurements in order to recreate a very short time-domain pulse. The measured delay of the received pulse corresponds to the round-trip time-of-flight (TOF) of the electromagnetic wave from the transmitting antenna to the obstacle. Thereafter, the range of the object could be easily computed^[2]. The diagram of the system and the echo pulse reflected by a 25 × 25 cm² metal plate at a distance of 2 m are shown in Figure 1.

Figure 1. Diagram of the system.

The proposed approach was compared with an ultrasonic system, making the two experimental set-ups as equivalent as possible. The proposed system showed better precision and the ability to detect obstacles that are not detectable by ultrasonic systems (e.g., a partially open door). Although the system is very bulky and expensive, its future miniaturization has been claimed.

An enhanced version of the system proposed in^[1] has been reported in^[3]. A portable vector network analyzer was employed for testing the effectiveness of the methodology in both indoor and outdoor scenarios. The maximum range was fixed to 3 m, whereas the operating frequencies from 4 to 6 GHz achieved a spatial resolution of 12 cm. The tests were carried out by a blind volunteer during different test conditions (i.e., obstacles located in different arrangements). The test set-up was arranged in order to have the obstacles aligned, shaped as a cage, and with an obstacle at chest level—as depicted in Figure 2. The system could correctly detect the presence of one or more obstacles even at the level of the head or chest (e.g., an open window or a tree branch).

Figure 2. Test set-up with (a) aligned obstacles, (b) obstacles shaped as a cage, and (c) obstacles at chest level.

A peculiar application of an electromagnetic aid for visually impaired people is reported in^[4], where a system capable of guiding a blind athlete during running and physical activities was designed, realized, and tested. This arose from the need to enhance the runners’ freedom during competitions or training. As a matter of fact, athletes currently need  to be guided by a non-extendible cable carried by another runner, thus limiting the possibility of free movement^[5].

The proposed system is composed of: (a) a mobile unit preceding the runner that generates two “electromagnetic walls” (i.e., two electromagnetic waves radiated on the left and right sides of the athlete) and (b) a receiving unit worn by the runner equipped with vibro-tactile devices.

A representation of the guiding system is shown in Figure 3.

Figure 3. Representation of the guiding system^[4].

The mobile unit generates a vibro-tactile warning when the athlete is leaving the area delineated by the electromagnetic walls, to guide them towards the center of the path.

In this circumstance, the use of electromagnetic waves ensures confinement of the wall to a narrow and well-defined area. The operating bandwidth of the system is inside the X-band, thus reducing the dimension and weight of the antenna in the receiving unit worn by the blind runner. To achieve this, the antenna was designed with lightweight and compact microstrip technology. The system was tested by a blind athlete using four different paths as reported in Figure 4, demonstrating the proper operation of the prototype. A video of the tests can also be watched online^[5].

Figure 4. Paths traveled by the athlete in a rectangular room (20 × 17 m²): (a) square, (b) spiral, and (c,d) complex paths^[4].

In [19], the challenge of avoiding collision with obstacles for visually impaired users was achieved by creating a radar-based navigation device. The system employs a frequency-modulated continuous-wave signal. Although the authors plan to have a main sensor working at the center frequency of 80 GHz with a bandwidth of 24 GHz, a bandwidth of 4 GHz—from 80 GHz to 84 GHz—was adopted for the tests. With this frequency bandwidth, a spatial resolution equal to 6 cm can be obtained. The radar sensor was attached on the head of the blind person, whose position was continuously tracked by an inertial sensor. This sensor scanned the surrounding environment and detected the position of obstacles up to 5 m. The intention of the authors is to scan the scenario using a beam-steering antenna. Its design is shown in^[6], but since it is still under development, the rotation was mechanically achieved by a stepper motor. Due to the size of the setup, the radar was placed on a large mounting. A virtual map of the environment was created and mapped into a 3D audio signal. The authors assert that a long training process is not required.

A superior system performance compared to camera-based systems is claimed in terms of independence from light conditions, ability to detect transparent obstacles such as glass doors, and better visual range in rain or fog^[7][8].

The challenge of tailoring a suitable electromagnetic aid is shown in^[9], highlighting the effectiveness of the authors’ solution. A clinical investigation was performed with 25 visually impaired people wearing a radar-based assistive device.

The working frequency of the system was 24 GHz, selected to facilitate a lightweight wearable device, because at this frequency the signal can penetrate textiles, making it possible to wear the device under clothing. The system automatically compensates small tilt errors or notifies the person if the incline is excessive. It is able to detect obstacles up to 3.5 m, within a horizontal sector of 25° and a vertical sector of 70°.

The maximum usage time of the prototype was measured as between 4 and 5 h, but this can be improved in a future designs. The warning about the presence of obstacles is fed back to the user by means of acoustic or haptic feedback. In Figure 5, the operating principle of the system and a picture of the device is shown.

Figure 5. Operating principle of the system and picture of the device^[9].

The clinical investigation was implemented during an observation period of two weeks, using the electromagnetic aid during everyday activities.

The results of an interview after usage of the device are shown in Figure 6a in response to the question “Does the assistive device help you to perceive your environment better?”, whereas in Figure 6b the question was “Does the assistive device increase your confidence in independent walking?”.

For both graphs in Figure 6, the answers recorded on the X-axes are: (1) not at all, (2) not much, (3) to some extent, (4) significantly, (5) very significantly.

In the results, 92% of the participants stated that the perception of the surrounding environment was enhanced by the electromagnetic aid, whereas 80% noticed an increased confidence in independent mobility.

Figure 6. Results of the interview questions: (a) “Does the assistive device help you to perceive your environment better?” and (b) “Does the assistive device increase your confidence in independent walking?” The answers reported on the X-axes are: (1) not at all, (2) not much, (3) to some extent, (4) significantly, (5) very significantly^[9].

Solutions employing 24 GHz radars based on FMCW modulation are presented in^[10][11][12], but different choices were made concerning the way the users employ the devices.

Despite the extensive range of available ETAs, the most widespread aid is still the traditional white-cane, due to its ease of use and cost effectiveness^[13][14].

Therefore, the authors decided to create a device so small and lightweight that it can be integrated into a white-cane—a kind of “microwave cane”. The authors based their design on feedback from users during interviews and the remarks of visually impaired and blind users, considering the positive reaction to a new device as the main task. Volunteers testing ETAs usually suggest the simultaneous use the white cane.

Since the self-confidence that the traditional white cane is reported to give users is absent in ETAs, the authors chose to go down this route.

Of course, this new device must overcome the limitations of the traditional cane (i.e., it does not provide any protection against obstacles at head or chest height and the range is confined to the length of the cane). At the same time, the system has to be cost effective and easy to use without extensive training.

In^[10], great attention is paid to the design of a customized antenna. It was designed by employing microstrip technology in order to have a compact system that can be attached to the cane. They highlight the advantages of the system compared to ones based on ultrasonic or optical technology, overcoming the limitations of limited range and poor performance operating within a low incidence angle on smooth surfaces. The system was tested up to 5 m, with a spatial resolution of 60 cm (24.0–24.25 GHz).

In^[11], a feasibility study was undertaken with the aim of accurately identifying the technical specification of the microwave cane. The behavior of the entire system was simulated using computer-aided design (CAD), with preliminary results obtained from newly designed transmitting and receiving antennas.  The first experimental activity using the microwave cane is outlined in^[12].

The authors obtained system spatial resolution close to 14 cm, reduced energy consumption, a maximum range of 5 m, and horizontal and vertical antenna patterns close to 12° and 40°, respectively.

A picture of the prototype is shown in Figure 7.

Figure 7. Picture of the radar prototype^[12].

The radar system was tested with different obstacles commonly used for daily tasks, that is, a wooden chair, a chest of drawers, and a human subject. The targets, characterized by different materials, dimensions, and shapes, were located at the range of 1.5 m and successfully detected, as shown in Figure 8.

Figure 8. Intermediate frequency (IF) signals obtained from obstacles at the range of 1.5 m. From the top to bottom, results are shown for a wooden chair, a chest of drawers, and a human target^[12].

An interesting solution is illustrated in^[15][16], where the radar technology was employed in conjunction with an RGB-D sensor. The radar operated from 77 to 81 GHz, implementing an FMCW signal. This 4 GHz bandwidth gives a spatial resolution of 6 cm. The purpose of the exercise was to enhance the features of object detection and localization. Whereas mm-wave radar is capable of providing speed information and a relatively high spatial resolution in the radial direction, the RGB-D sensor accomplishes the task of distinguishing two targets at the same distance. A block diagram of the system is shown in Figure 9.

Figure 9. Schematic diagram of the system in^[15]. FMCW: frequency-modulated continuous wave.

Obstacles at the same distance are recognized using color information and the application of a single deep neural network or object instance segmentation^[17][18].

In Figure 10, results relating to tests performed in an indoor environment are shown. In Figure 10a1,a2 the mask R-CNN was used for detecting and recognizing the targets, whereas in (b1,b2) the SSD network was used. In (c1,c2) the image results are shown with, detection results enclosed in a red box, whereas in (d1,d2) the data detected by the radar are illustrated. Finally, in (e1,e2), the data are fused using a particle filter.

Figure 10. Tests performed in an indoor environment and related results [24]. (a1,a2) Detection performed by R-CNN, (b1,b2) detection performed by SSD network, (c1,c2) image results, (d1,d2) data detected by the radar, (e1,e2), data fused using a particle filter.

The potential of this technology is also highlighted by the Integrated Smart Spatial Exploration System (INSPEX H2020 project), where the main goal is to take advantage of the automotive techniques concerning spatial exploration and obstacle detection in the design of portable sensors^[19]. Potential applications employing radar-based sensors range from safer human navigation in reduced visibility to navigation for visually impaired people. The authors intend to integrate the INSPEX system into a smart white cane.