DESC:FORM 2

THE PATENTS ACT 1970
[39 OF 1970]
&
THE PATENTS RULES, 2003

COMPLETE SPECIFICATION
[See section 10; rule 13]

TITLE: “A DIGITALLY RECONFIGURABLE METASURFACE FOR COMMUNICATION IN SCATTER-RICH MULTIPATH CHANNEL AND A METHOD THEREOF”

Name and address of the Applicant:
INDIAN INSTITUTE OF SCIENCE, Bangalore 560012, Karnataka, India.

Nationality: INDIA

The following specification particularly describes the invention and the manner in which it is to be performed.

TECHNICAL FIELD
The present disclosure relates to a Media Based Modulation (MBM) scheme for wireless communication networks. Particularly, but not exclusively, the present disclosure relates to a Digitally Reconfigurable Metasurface (DRM) for communication in scatter-rich multipath channel and a method thereof.

BACKGROUND
Wireless communication systems are extending from Internet of Things (IoT) to fifth generation (5G) cellular systems and beyond. Various innovative Radio Frequency (RF) designs have been proposed to improve the performance of the wireless communication systems and to miniaturize the system architecture. Further, in the wireless communication systems, an antenna is an essential part for transmitting/receiving a carrier tone or a modulated tone. From recent studies in wireless communication systems, it is found that to improve the performance of the communication systems and to reduce the complexity of the architecture, a beam steering of antenna or parasitic layers for antenna is used.

Further, in order to improve the performance of the wireless communication systems, reconfigurable antennas (or reconfigurable structures) are used. As an example, the reconfigurable structures may include a Metasurface structure. Particularly, the Metasurface structure is relatively a new class of periodic structures, which are considered as 2D equivalent of a conventional metamaterial used to improve the radiation characteristics of the antenna. The conventional Metasurface designs are reported as lens for gain enhancement, bandwidth enhancement, multibeam generation in an aperture, and for reduction of radar cross section of the antennas. Further, the conventional Metasurface layers are also used in various reconfigurable antenna designs. For example, by physically moving or rotating the Metasurface layers, an operating frequency and/or polarization of the antenna is modified.

Further, the antenna systems with Metasurface structures and reconfigurable Metasurfaces are reported in the context of Multiple Input and Multiple Output (MIMO) for reduction of mutual coupling at 60GHz of frequency implementing high gain antenna, and for millimetre wave imaging system. Although, the conventional Metasurface structures or layers have many applications, the conventional Metasurface structures or layers in an anechoic chamber do not facilitate a real-time reconfigurability to demonstrate the MBM.

Further, the reconfigurable structures are used for multi-antenna systems to address large number of users. The conventional multi-antenna systems of modern communication system require large spaces for placing or locating the multiple-antennas (for example, a spatial multiplexing MIMO system) at a transmitter. Consequently, multiple RF chains are required to drive multiple antennas, which are placed at the transmitter. Further, each RF chain may include mixers, filters, Power Amplifiers (PA) etc., which makes the RF hardware of Multiple Input and Multiple Output (MIMO) transmitters more complex.

To overcome the above problem, a single RF chain approach may be used in the multi-antenna systems. For instance, a Space Shift keying (SSK) and/or Spatial Modulation (SM) may be used. The SSK and SM are widely studied modulation schemes that adopt a single RF chain approach for MIMO transmission. Consequently, the SSK/SM provides a good performance while retaining low RF hardware complexity advantage. However, the SSK/SM has a drawback that it’s achieved rate, in bits per channel use (bpcu), does not scale well. The reason for the poor scalability is that the rate in SSK/SM increases only logarithmically with the number of transmit antennas (i.e., log2 nt bpcu is the achieved rate in SSK, where ‘nt’ is the number of antennas).

Further, multipath components of a signal in a scatter-rich environment limit the performance of a wireless link and results in a communication outage. The SSK, SM and a Generalized Spatial Modulation (GSM), MIMO, massive MIMO, and ultra-massive MIMO use spatial diversity techniques to operate in scattering environments, addressing multiple users simultaneously. In these techniques, multiple antennas are placed at different spatial locations to communicate using unique spatial signatures with a large radiating aperture, often with multiple RF chains. However, a scatter-rich multi-path channel augments these spatial signatures and affects the cardinality of constellation (i.e., set of spatial signatures) uniformly for all symbols. As a result, the MBM is inherently robust against the communication outage in a slow fade channel.

In view of the above, there is a need for a novel scheme that facilitates a real-time reconfigurability to demonstrate the MBM and improve the scalability of the achieved rate in the MBM.

The information disclosed in this background of the disclosure section is only for enhancement of understanding of the general background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art already known to a person skilled in the art.

SUMMARY
The present disclosure relates to a Digitally Reconfigurable Metasurface (DRM) for communication in scatter-rich multipath channel. Specifically, the present disclosure relates to a MBM transmitter for communication in scatter-rich multipath channels. The MBM transmitter comprises a single Radio Frequency (RF) chain and a transmitting directional antenna. The transmitting directional antenna comprises one or more dielectric sheets and one or more metallic geometries. The transmitting directional antenna is configured to emit a carrier tone. Further, the MBM transmitter comprises a DRM which is placed in the nearfield of the transmitting directional antenna and configured to generate unique sets of multiple spatial signatures by modulating the carrier tone. Further, the MBM transmitter provides a scheme to identify a sub-set of the multiple spatial signatures that forms a codebook for the modulate carrier tone shared between the transmitter and the receiver. Finally, a receiver is configured to receive the modulated carrier tone.

Further, the present disclosure relates to a method of configuring a Media-based Modulation (MBM) transmitter for communication in scatter-rich multipath channels. The method comprising configuring at least one of a single Radio Frequency (RF) chain and a transmitting directional antenna which comprises one or more dielectric sheets and one or more metallic geometries to emit a carrier tone. Further, the method comprising configuring a Digitally Reconfigurable Metasurface (DRM), placed in the nearfield of the transmitting directional antenna to generate unique sets of multiple spatial signatures by modulating the carrier tone. Further, the method comprises a scheme to identify a sub-set of the multiple spatial signatures that forms codebook for the modulated carrier tone shared between the transmitter and a receiver. Finally, the method comprising configuring the receiver to receive the modulated carrier tone.

The foregoing summary is illustrative only and is not intended to be in any way limiting. In addition to the illustrative aspects, embodiments, and features described above, further aspects, embodiments, and features will become apparent by reference to the drawings and the following detailed description. For a better understanding of exemplary embodiments of the present invention, together with other and further features and advantages thereof, reference is made to the following description, taken in conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
The embodiments of the disclosure itself, as well as a preferred mode of use, further objectives and advantages thereof, will best be understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying drawings. One or more embodiments are now described, by way of example only, with reference to the accompanying drawings in which:

Figure. 1 illustrates a schematic illustration of a Media Based Modulation (MBM) transmitter in accordance with some embodiments of the present disclosure.

Figure. 2a illustrates a schematic diagram of the proposed subwavelength unit cell consisting of a PIN diode in the DRM in accordance with some embodiments of the present disclosure.

Figures. 2b-2c show a graph illustrating a simulation of a subwavelength unit cell at different switching conditions with and without biasing lines, and normalized surface current distribution, respectively, in accordance with some embodiments of the present disclosure.

Figures. 3a-3b illustrate a schematic diagram of a transmitting directional antenna and shows a graph illustrating a simulation of the antenna, respectively, in accordance with some embodiments of the present disclosure.

Figures 4a-4b show a graph illustrating a normalized maximum far field magnitude and ? Magnitude for different array sizes and number of switching combinations below different amplitude levels for different antenna-DRM distances in accordance with some embodiments of the present disclosure.

Figure. 4c shows a graph illustrating a comparison of distribution of constellation points in accordance with some embodiments of the present disclosure.

Figure. 5a illustrates fabricated prototype of DRM and antenna, DRM array, and current driver circuit in accordance with some embodiments of the present disclosure.

Figure. 5b illustrates a schematic illustration of measurement setup in accordance with some embodiments of the present disclosure.

Figure. 6 illustrates a schematic illustration of measurement arrangements inside and outside the anechoic chamber in accordance with some embodiments of the present disclosure.

Figures 7a-7b show a graph illustrating a measured constellations for different switching combinations when measurement arrangements are placed inside the anechoic chamber and outside the anechoic chamber respectively, in accordance with some embodiments of the present disclosure.

Figures 8a-8b show a graph illustrating a measured constellations and BER response in accordance with some embodiments of the present disclosure.

Figures 9a-9b show a graph illustrating measured constellations for Non-Line-of-Sight (NLOS) 3, 4, and 5 after normalization, and location of best selected symbols from NLOS 3, 4 and 5 and the location of those on other constellations in accordance with some embodiments of the present disclosure.

Figure 9c shows a graph illustrating BER response of the best selected symbols from NLOS 3, 4 and 5 compared to the same on other constellations in accordance with some embodiments of the present disclosure.

Figures 10a-10b show graphs illustrating measured constellations consisting of 14,848 symbols after normalization and their distribution of real and imaginary components in accordance with some embodiments of the present disclosure.

The figures depict embodiments of the disclosure for purposes of illustration only. One skilled in the art will readily recognize from the following description that alternative embodiments of the structures and methods illustrated herein may be employed without departing from the principles of the disclosure described herein.

DESCRIPTION OF THE DISCLOSURE
In the present document, the word "exemplary" is used herein to mean "serving as an example, instance, or illustration." Any embodiment or implementation of the present subject matter described herein as "exemplary" is not necessarily to be construed as preferred or advantageous over other embodiments.

While the disclosure is susceptible to various modifications and alternative forms, specific embodiment thereof has been shown by way of example in the drawings and will be described in detail below. It should be understood, however that it is not intended to limit the disclosure to the particular forms disclosed, but on the contrary, the disclosure is to cover all modifications, equivalents, and alternative falling within the spirit and the scope of the disclosure.

The terms “comprises”, “comprising”, or any other variations thereof, are intended to cover a non-exclusive inclusion, such that a setup, device or method that comprises a list of components or steps does not include only those components or steps but may include other components or steps not expressly listed or inherent to such setup or device or method. In other words, one or more elements in a device or system or apparatus proceeded by “comprises… a” does not, without more constraints, preclude the existence of other elements or additional elements in the device or system or apparatus.

In the following detailed description of the embodiments of the disclosure, reference is made to the accompanying drawings that form a part hereof, and in which are shown by way of illustration specific embodiments in which the disclosure may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the disclosure, and it is to be understood that other embodiments may be utilized and that changes may be made without departing from the scope of the present disclosure. The following description is, therefore, not to be taken in a limiting sense.

Figure. 1 illustrates a schematic illustration of Media Based Modulation (MBM) transmitter in accordance with some embodiments of the present disclosure.

As shown in Figure 1, the MBM transmitter 100 comprises a single Radio Frequency (RF) chain 101, a transmitting directional antenna 111, a Digitally Reconfigurable Metasurface, DRM, (i.e., a set of RF windows or RF mirrors) 113, a receiver 115, a scheme, and scatterers 117. In an embodiment, a single RF chain 101 may include, without liming to, an oscillator 103 and a Power Amplifier 105. In an embodiment, the transmitting directional antenna 111 comprises one or more dielectric sheets and one or more metallic geometries. The transmitting directional antenna 111 is configured to emit a carrier tone. In an embodiment, the DRM 113 is placed in a nearfield of the transmitting directional antenna 111 and it may include an array of subwavelength unit cells, and each subwavelength unit cell comprises a meander line and a high frequency Positive-Intrinsic-Negative (PIN) diode. As an example, the PIN diode is a diode with a wide, undoped intrinsic semiconductor region between a p-type semiconductor and an n-type semiconductor region.

Further, as an example, the DRM 113 is configured to generate unique sets of multiple spatial signatures by modulating the carrier tone. Further, the PIN diode in each array of the subwavelength unit cell switches between, OFF and ON states, which causes the subwavelength unit cell to switch between a transmitting state and a blocking state in the DRM 113.

In an embodiment the receiver 115 is configured to estimate channel fade coefficients for all switching patterns of the DRM 113. In an embodiment, the receiver 115 is placed in Line-of-sight (LoS) or Non-Line-of-Sight (NLoS). In an embodiment, a scheme identifies a sub-set of the multiple spatial signatures that forms a codebook for the modulated carrier tone shared between the transmitter and the receiver. In an implementation, the carrier tone at the transmitting directional antenna input does not embed information bits, and the carrier tone, after passing through the DRM 113, embeds information bits conveyed by the DRM 113 to the receiver 115.

In an embodiment, the scatterers 117 diversify the received symbols across a constellation to help the wireless link to overcome poor channel conditions (such as slow fade channel) without sacrificing the spectral efficiency of the system.

As an example, the MBM transmitter 100 uses different sets of subwavelength unit cells to modulate a transmitting tone to generate different transmitting symbols (i.e., magnitude and phase) 107. The MBM transmitter 100 is designed by extensive EM simulations using Microwave studio. The subwavelength unit cells designed in the DRM are arranged as a planar array in front of the transmitting directional antenna 111 and the field radiated at boresight is simulated by switching the subwavelength unit cells between transmitting and blocking states.

Further, for an array with n×n unit cells, 2n2, switching states are possible leading to as many complex field values, which may be used as constellation points for the MBM transmitter 100. Since the array is in nearfield of the transmitting directional antenna 111, the constellation points vary with the distance of the array and the transmitting directional antenna 111. In some cases, the loading effect of the array may affect the impedance characteristics and deteriorate the transmitting directional antenna 111 matching. The small array may have less impact on the transmitting directional antenna 111 performance but it has lesser options for constellation points. On the other hand, a large array may produce more constellation points, but with less parasitic effects.

Further, the arrangement with transmitting directional antenna 111 and the DRM 113 shown in Figure. 1, is used to create a channel modulation alphabet and use it for the communication. As an example, the channel modulation alphabet may be a set of complex channel fade coefficients between the MBM transmitter 100 and a receiver 115 corresponding to all the ON/OFF patterns of the DRM or RF windows 113.

In an implementation, to create the channel modulation alphabet, the DRM or RF windows 113 are designed to exhibit different RF radiation characteristics for excitation by ‘0’ state and ‘1’ state and use a set of DRMs 113 as digitally controlled scatterers in the nearfield of the transmitting directional antenna 111. Further, in the MBM transmitter 100, if ‘m’ DRM or RF windows 113 are placed near the transmitting directional antenna 111, each controlled by one information bit, then the near field of the transmitting directional antenna 111 is controlled or perturbed by ‘m’ information bits. Since a small perturbation in the near field causes many random reflections in a rich scattering environment, this leads to different complex field values in the far field for different ON/OFF patterns of the ‘m’ RF windows 113. This process results in 2m complex fade coefficients between the transmit and receive antennas, one for each of the ON/OFF pattern of the ‘m’ information bits. The collection of these 2m fade coefficients form the channel modulation alphabet for MBM. Therefore, by using this modulation alphabet MBM may convey ‘m’ information bits in each signalling interval. That is, the achieved rate in MBM is ‘m’ bits per channel use (bpcu). This linear scaling of rate with the number of subwavelength unit cells in DRM 113 is an advantage of MBM over SSK/SM where the rate scales only logarithmically with the number of the transmitting directional antennas 111. As an example, bpcu is the number of information bits that the system can convey in every channel use or transmission, which is given by log2 |Ss|.

Further, at the receiver 115, the channel fade coefficients are estimated apriori for all the possible ON/OFF patterns of the RF windows or DRM cells 113, and the channel knowledge is then used to detect the conveyed information bits at the receiver 115. In an embodiment, the channel estimation task to learn the modulation alphabet at the receiver 115 in MBM increases exponentially in ‘m’. Therefore, MBM is well suited for applications where the channel coherence time is large, and MBM provides significant performance gains compared to the conventional multiantenna systems in simulation.

Figure. 2a illustrates a schematic diagram of the proposed subwavelength unit cell consisting of a PIN diode in the DRM in accordance with some embodiments of the present disclosure.

As shown in Figure.2a, the subwavelength unit cell or DRM unit cell 201 has the dimension 10×10 mm2 making it ?0/7.5 in size, where ?0 is the free space wavelength at the design frequency of 4GHz. In an embodiment, the subwavelength unit cell or the DRM unit cell 201 has a planar beam-Lead Positive-Intrinsic-Negative (PIN) diode 203 for switching between two different states of different transmission characteristics. The dimensions used in the design, i.e., wm=7.1 mm, lm=5.59 mm, sm=1 mm, gm=0.53 mm, wst=1 mm, gmst=1 mm and gu=0.2 mm) produce transmission zeros at 4 GHz and at 5.3 GHz when the PIN diode 203 is turned ON and OFF respectively. In an implementation, the subwavelength unit cell 201 has a high inductance due to the unfurled length making it more compact compared to the conventional work. Further, the gap between traces accounts for the capacitance required to make the subwavelength unit cell 201 resonate at the desired frequency.

In an embodiment, a three-turn meander is placed at the center with two horizontal strips on an FR-4 substrate of thickness 0.5 mm. The electromagnetic windowing operation is achieved by reconfiguring the surface impedance of the subwavelength unit cell 201 with digital inputs from a controller. As an example, the control signal trace is printed at the opposite side of the substrate and connected to the meander line. The top horizontal strip is used as the common ground for the PIN diode 203. In an implementation, the subwavelength unit cell 201 is designed with the objective to maximize the difference between the transmitted powers of the two distinct switching states. Further, the subwavelength unit cell 201 is simulated with periodic boundary condition using the frequency domain solver in CST Microwave Studio.

Figures. 2b-2c show a graph illustrating a simulation of a subwavelength unit cell at different switching conditions with and without biasing lines, and normalized surface current distributions, respectively, in accordance with some embodiments of the present disclosure.

The simulated transmission coefficient of the subwavelength unit cell 201 of ON and OFF states are shown in Figure 2b, where the effects of incorporating the biasing lines are also explained. In an implementation, when the transmission zero is at 4 GHz, then the transmission coefficient is -20.8 dB, making the subwavelength unit cell blocking the transmitted signals. Further, when the PIN diode 203 is turned OFF, the transmission zero is shifted to 5.3 GHz and the transmission coefficient at 4 GHz becomes -1.5 dB, and the subwavelength unit cell 201 is considered as transmitting. Moreover, the difference in the transmitted powers of the subwavelength unit cell 201 is found to be more than 10 dB over a band of 700 MHz.

Further, the normalized surface current distributions at the resonant frequencies are shown in Figure 2c, which gives more physical insight to the operation of the subwavelength unit cell 201 in the two states, i.e., transmitting state and blocking state. In the transmitting state, the current is restricted to the meander alone as the PIN diode 203 is OFF. In the blocking state, the current is distributed over the meander as well as the horizontal strip above. Hence, the longer current path and lower resonant frequency is observed in the latter state.

Figures. 3a-3b illustrate a schematic diagram of a transmitting directional antenna and shows a graph illustrating a simulation of the antenna, respectively, in accordance with some embodiments of the present disclosure.

As shown in Figure 3a, since the concept of MBM transmitter 100 requires only a single tone to be transmitted, a simple probe fed rectangular microstrip patch (transmitting directional) antenna 111 is used. Further, the patch dimensions are la = 34 mm and wa = 21:85 mm to operate at a frequency of 4 GHz, as shown in Figure. 3b. An Arlon AD-250 substrate (er = 2.5, tand = 0.0018) with a thickness of 1.57 mm is used. The overall dimension of the dielectric slab is kept lsub = 120 mm and wsub = 60 mm to align with the Metasurface screen dimensions. The main design parameters are antenna dimensions, a structure of the unit cell, a size of the array, and distance of the array from the antenna.

Figures 4a-4b show a graph illustrating a normalized maximum far field magnitude and ? Magnitude for different array sizes and number of switching combinations below different amplitude levels for different antenna-DRM distances in accordance with some embodiments of the present disclosure.

As shown in Figure 4a, in the first simulation, the magnitudes (i.e., field values at boresight) corresponding to all subwavelength unit cells 201 of the array in their transmitting (i.e., all ON) and blocking (i.e., all OFF) states are analyzed. In Figure 4a, the blue bars indicate the magnitudes of field values for different array sizes in their transmitting states. It is observed to be maximum for 8×8 array size. The normalization of magnitudes for other arrays are performed by setting the global maximum of 8×8 array as the reference. Further, the green bars indicate the difference between the field magnitudes between the transmitting and blocking states (? Magnitude) for different array sizes. It is observed that ? Magnitude increases with the array size, as the aperture of blocking increases. Although the hardware complexity increases with a larger array, it facilitates a better MBM implementation by generating a higher number of constellation points. Therefore, the 8×8 array is considered for the DRM 113.

In an implementation, the distance is kept around 40mm between the DRM 113 and the transmitting directional antenna 111. The interactions between the subwavelength unit cells 201 of array and the transmitting directional antenna 111 affect the impedance matching of the latter for certain switching combinations. To analyze this condition, the DRM 113 is placed at different distances away from the transmitting directional antenna 111 and the total number of switching combinations below different |S11| levels are shown in Figure 4b.

Further, in Figure 4b, a good matching (|S11| < -10dB) is observed for maximum number of cases with a gap of 40 mm. Further, it may be noted that this spacing is close to ?/2 at 4 GHz and is also consistent with the observations. In an implementation, the analysis is restricted to 28 distinct cases by switching all elements in one column of the array together. Since only 26 switching states are worse than the -10dB threshold, 8×8 array kept at 40 mm from the antenna is used for further investigations. Further, it may be noted that if individual subwavelength unit cells 201 are separately controlled, a total of 264 (18×1018) switching combinations and as many symbols are possible to obtain. However, in practical applications such a large number may become impractical.

Figure. 4c shows a graph illustrating a comparison of distribution of constellation points in accordance with some embodiments of the present disclosure.

As shown in Figure 4c, the number of possibilities are increased by choosing different sets of subwavelength unit cells 201. The real and imaginary parts of the complex field values at boresight are plotted as constellation points in a constellation diagram. As an example, the constellation diagram is the two-dimensional xy-plane scatter plot representation (with real and imaginary parts of a complex number on x and y axes, respectively) of these modulation symbols (or alphabet). The constellation diagram is useful in examining the distinguishability of the symbols. Further, the increase in the spread is seen to be marginal when other possible switching combinations are appended with the column wise control. Further, in a practical system, only a few symbols are required for data communications (2, 4 or 8 in cases that are studied below) with the minimum Euclidean distance among then maximized. Therefore, a comparison of Euclidean distances after selecting 2, 4, and 8 symbols from sets of 256 (column wise) and 2048 (all considered) constellation points as mentioned in Table 1, which shows that the minimum Euclidean distance does not increase significantly even if smaller group of subwavelength unit cells 201 are controlled separately.

On the other hand, controlling individual subwavelength unit cells 201 separately, increases the hardware complexity significantly. Hence, the DRM 113 and its controller are designed to control each column of the array as one unit, resulting in a maximum of 28 constellation points. As an example, the minimum Euclidean distance is the minimum of all possible pairwise Euclidean Distances (EDs) between symbols of the modulation alphabet. The minimum Euclidean Distance can be evaluated using the equation below. i.e.,

Where hi is the channel fade coefficient corresponding to the ith ON/OFF pattern of the RF windows or DRM unit cell 113. The reliability of the scheme is proportional to the minimum ED, i.e., higher the dmin higher is the reliability.

2 symbols 4 symbols 8 symbols
All symbols 0.70 0.42 0.19
256 symbols for column wise control
0.65
0.41
0.18

Table 1: Minimum Euclidean Distances

Further, the PIN diodes 203 in subwavelength unit cells 201 are controlled using an Arduino microcontroller interfaced to an interface. These diodes 203 draw a current of approximately 15 mA when turned ON. Since eight diodes are connected in parallel in one column, a Metal Oxide Semi-conductor Field Effect Transistor (MOSFET) based high speed current driver (IXDN602SIA) with current limiting resistors are used for its control. This driver circuit has eight separate connections to the DRM.

Figure. 5a illustrates fabricated prototype of DRM and antenna, DRM array, and current driver circuit in accordance with some embodiments of the present disclosure.

A prototype of the reconfigurable Metasurface structure (DRM) 113 is fabricated, as shown in Figure 5a. The 8×8 array is printed on one side of the substrate, where the control lines are patterned on the other side.As shown in Figure 5a, a DRM 113 in the near field of a microstrip patch antenna or transmitting directional antenna 111 is described. The DRM 113 consists of an array of unit cells 113. Each cell has a meandered line and an information bit, which can control the nearfield performance of the antenna-metasurface structure 113. The metasurface 113 is kept around half wavelength away from the antenna (radiative near field) at its operating frequency to utilize maximum number of switching conditions without degradation in the input as reported. The fabricated prototype of the current driving circuit 205 with the inputs (i.e., binary symbols from microcontroller (Adruino (Sym.2))), outputs (CS. Control signal to the array, as shown in Figure 5a.

Further, a schematic of the measurement setup is shown in Figure 5b. In an implementation, a python interface generates the symbols to be transmitted (Sym.10) that are fed to a micro-controller 507 board that generates the decoded eight-bit data (Sym.2) 109 corresponding to the input symbol. As an example, the microcontroller 507 may include an Adruino micro controller to generate decoded eight-bit data 109. Further, the current driver circuit 205 produces the required currents (CS) for the diodes 203 in the Metasurface array 113 according to the Adruino output. A 4 GHz tone is transmitted by the antenna assembly and is received by the receiving horn 501 and the S21 is continuously measured by a Keysight N5230A PNA series Vector Network Analyzer, VNA, 503. The VNA 503 is interfaced with the interface 505 to download the data, to synchronize (Sync.) the entire process from symbol generation to data acquisition. As an example, the interface 505 may include, without limiting to, a laptop, a desktop, a smartphone, a Personal Digital Assistant (PDA) and the like.

Figure. 6 illustrates a schematic illustration of measurement arrangements inside and outside the anechoic chamber in accordance with some embodiments of the present disclosure.

As shown in Figure 6, the measurements were performed in the laboratory environment with scattering objects such as chairs, tables, storage shelves and exposed walls (i.e., scatter-rich channel). In an implementation, exploitation of multipath fading is one of the major features of MBM scheme. To analyze the effects of multipath on the wireless link, constellations for selected switching states are measured inside and outside the anechoic chamber with the modulator facing the receiving horn antenna in an LOS configuration. The observed field values are spatial signatures embedded by the DRM 113 to the transmitted tone. These cases are repeated with different distances between the transmitter-receiver pair by placing the transmitter 100 on a rack and pinion arrangement atop a mast. This arrangement allows movement of the transmitter by tuning a screw.

Figures 7a-7b show a graph illustrating a measured constellations for different switching combinations when measurement arrangements are placed inside the anechoic chamber and outside the anechoic chamber respectively, in accordance with some embodiments of the present disclosure.

As shown in Figure 7a, constellations of ten states for four different screw positions (moving the transmitter by around ?/4 successively) inside the chamber are plotted and are labelled as din-1 to din-4. The rotation (by 900) of centroids of these clusters is marginal as the overall distance between the antennas are very close to one another. Further, an identical set of switching states are evaluated outside the anechoic chamber at similar distances in a scattering environment depicted. The resulting constellations are plotted as shown in Figure 7b with different colors representing measurements at distances dout-1 to dout-4, where no distinct clusters are observed. However, these constellation points are distributed over an annular region. Therefore, it is clear that multipath components affect both amplitude and phase corresponding to each point. These are significantly dispersed due to the random scattering environment. Further, it may be noted that the received field values are the superposition of the spatial signatures received directly as well as by multipath components from the surrounding scatterers.

In an implementation, the constellation points are more spread out in a scattering environment, the experiments are extended for different configurations to evaluate the performances in different multipath environments. These include cases with and without a direct paths (Line-of-Sight (LOS) and Non-Line-of-Sight (NLOS)) of propagation between transmitting directional antenna and receiver antenna.

In an implementation, the transmitter 100 and the horn antenna 501 are placed at different pairs of locations in the environment to evaluate the performance of the MBM transmitter 100 in different LOS and NLOS scenarios. Further, in the following configurations, (a) LOS-1 (R1-T1): both transmitter and receiver within the lab, and 9b) LOS-2 (R2-T2): transmitter inside and receiver directly outside the door, (c) NLOS-1 (R3-T3): transmitter and receiver inside the lab with a shielded surface blocking the line of sight, and (d) NLOS-2 (R4-T3): transmitter inside and receiver outside the lab with a stone wall blocking each other.

Figures 8a-8b show a graph illustrating a measured constellations and BER response in accordance with some embodiments of the present disclosure.

The measured constellations for LOS-1, LOS-2 and BER response for LOS configurations are summarized in Figure 8a. In Figure 8a, despite the identical transmission in each configuration, the scatter is different in these cases, indicating diverseness of multipath components in the arrangements. Due to the absence of any direct path, the received power are significantly reduced in NLOS configurations. From each scatterplot with 64 constellation points, a Euclidean distance measure can be used to identify 2 or 4 symbols for communication. These symbols are selected using an optimization algorithm that maximizes the minimum Euclidean distance in the complex number space of pairs of symbols. In an embodiment, the symbol subset selection is a popularly used technique to improve the reliability of the system, where a subset of modulation symbols is chosen from all the possible symbols based on some criteria. The Euclidean distance is based on the symbol subset selection, which chooses a subset with the higher minimum Euclidean distance.

The symbol subset selection can be evaluated using the equation below. i.e.,

Where St, S, and Ss denote the entire set of modulation symbols, a possible subset, and the selected subset, respectively. The ON/OFF patterns of the selected subset are used to convey the information bits.

Further, the calculated BER performances of LOS and NLOS configurations are plotted in Figures 8a and 8b respectively. For all the four cases, SNR requirements for 2 symbols are less than those for the 4 symbols. The relationship is established as selecting more symbols from a fixed scatter effectively reduces the Euclidean distances among the symbols, leading to degraded error rates for fixed SNRs. As an example, BER is a popular performance metric used in communication systems. BER to the probability of a transmitted information bit being decoded wrongly (i.e., 0 as 1 and 1 as 0) receiver, which is given by

The above equation denote the transmitted information bit (in a channel use) and detected information bits, respectively. It is numerically evaluated using Monte Carlo simulations. Further, the BER is inversely proportional to the minimum ED, i.e., higher the minimum ED, lower is the BER. At a given SNR, a lower BER is the desired requirement for any communication system.

From Figure 8a, it is observed that the error rates for LOS-1 and LOS-2 are nearly overlapped. As the direct path powers are significantly more than the multipath powers, the LOS is unable to augment the Euclidean distances among the symbols even though scatterers 117 are present. On the other hand, the Figure 8b, shows significant improvements in the error rates as the symbols are augmented well across the constellation. For example, in a 2-symbol case, LOS-1 and LOS-2 require around 20dB of SNR of the BER of 10-3, whereas the same with NLOS-1 is only 10dB. This implies that the same error rate is achieved in NLOS with reduced transmit power by utilizing the multipath components. These measurements confirm the numerical predictions of performance improvement in MBM when scatterers are present. Further, to reduce the number of experimental cases, the outer columns of the DRM 113 are kept in the transmit case to generate the above scatterplot measurements. It must be called that communication in NLOS arrangements depend primarily on signal components reflected from the surroundings.

Further, it may be noted that since transmitting symbols 107 are encoded as control sequences, which operate switches in the array to embed different spatial signatures to this tone, it is clear that this system will not be impacted by nonlinearity of the power amplifier 105 and hence, the overall efficiency of the MBM transmitter 100 is observed to be high.

Figures 9a-9b show a graph illustrating measured constellations for Non-Line-of-Sight (NLOS) 3, 4, and 5 after normalization, and location of best selected symbols from NLOS 3, 4 and 5 and the location of those on other constellations in accordance with some embodiments of the present disclosure.
As shown in Figure 9a, the measured constellations for these scenarios consisting of 256 (28) points each, after normalization to zero mean and unit variance. These show significant differences among them, even though the scattering environment remain static. In an implementation, a unique set of four symbols are identified based on Euclidean distance measure, from each of the sets of measurements. These sets of symbols are unique to the measurement scenario as illustrated in Figure 9b. For example, if the symbols are identified from the measurement NLOS-3, positions of these symbols in the complex plane (in C-33) change with the measurement scenario, as indicated by C-43 and C-53 of Figure 9b for NLOS-4 and NLOS-5, respectively. C-ij represents the set of selected symbols from the constellation of the jth NLOS experimental campaign, superimposed on the constellation of ith NLOS measurement. These results demonstrate that the selected symbols in a new environment does not perform as well, as at the original location. Similarly, the second row of Figure 9b shows deviations observed for symbols identified from NLOS-4 in the other two scenarios. The randomness of deviations is observed in the third case as well. The impact of these deviations is quantified in the Bit-Error-Rate (BER).

Figure 9c shows a graph illustrating BER response of the best selected symbols from NLOS 3, 4 and 5 compared to the same on other constellations in accordance with some embodiments of the present disclosure.

As shown in Figure 9c, the randomness of deviations is quantified in the BER plot, which shows that performance achieved degrade as the scenario is changed. Further, it is clear that the minimum Euclidean distance within the set of four symbols indicate the Signal-to-Noise (SNR) ratio of the MBM system and it is the highest for the set of symbols corresponding to the set of symbols in each of the measurement sets are uncorrelated, receivers 115 at these positions can function independently with different codebooks.

As seen from the above, significant variation of the received symbol are observed due to strong multipath components even for small changes in the receiver locations. Apart from the benefits of multiple received codebooks with multipath components, the MBM offers an enhanced security to the wireless link by spatial coding. That is, the users only at a certain spatial location can decode the symbol belonging to his codebook being oblivious about the messages of the neighbors and vice versa. It prevents an eavesdropper to decode the channel content which can further be secured by sharing the mutual phases between the transmit-receive pairs. Further, in MBM, the received complex vector of length ‘D’ represents ‘D’ parallel channels mimicking the behaviour of a D×D Multiple Input and Multiple Output-Source Based Modulation (MIMO-SBM) system. In other words, ‘D’ different messages may be received in a single transmission using the same transmitted energy and bandwidth, leading to “D times energy harvesting”. In contrast, in a conventional Single Input and Multiple Output-Source Based Modulation (SIMO-SBM) system with D receivers, the received vector of length D indicates a single complex point in the constellation.

Figures 10a-10b show a graph illustrating measured constellations consisting of 14,848 symbols after normalization and their distribution of real and imaginary components in accordance with some embodiments of the present disclosure.

For a conventional SBM, the effect of a low gain channel can be severe and transmission outage can happen indefinitely for a slow fade channel. In SBM, this outage can only be overcome by reducing the rate of transmission or by waiting till the channel condition improves. On the other hand, the channel realizations (i.e., spatial signatures) in MBM are controlled by DRM 113, where both good and bad channels may contribute to the cardinality of the constellation. Hence it may be possible to alleviate transmission outages without sacrificing the transmission rate or the spectral efficiency.

To investigate this feature of MBM with a large constellation, experiments are performed under different NLOS environments by placing the transmitter and receiver at dozens of random locations in the laboratory environment in different orientations ensuring no direct path between them. All sets of scatter points are normalized to zero mean and unit variance and are plotted in Figure 10a. The near and far points from the center indicate low and high-power levels of the symbols, respectively. The real and imaginary components of these are plotted separately in Figure 10b. it can be observed that both real and imaginary components are Gaussian distributed. Therefore, MBM may achieve the capacity of a non-fade Gaussian channel as it can employ several multipath components for communication. In other words, the scatterers help diversifying the received symbols across the constellation to help the wireless link overcome poor channel conditions (such as slow fade channel) without sacrificing the spectral efficiency of the system.

ADVANTAGES OF THE INVENTION
• The present disclosure proposes Media Based Modulation (MBM) with the spectrum efficiency. Consequently, the hardware cost is reduced and a bandwidth and power efficient multiuser system can be implemented with a high physical layer security.
• The present disclosure proposes a DRM with a compact resonant unit cell with an embedded PIN diode and biasing lines. Consequently, the proposed design optimizes the performance of the DRM geometry and its distance from the radiator by numerical simulations to maximize the modulator performance.
• The present disclosure proposes a MBM with linear scaling or higher scalability of achieved rate with the number of subwavelength unit cells in DRM, which is an advantage of MBM over conventional modulation schemes.
• The present disclosure proposes the DRM, which generates unique spatial signatures, which further improve the wireless link in wireless communication system.
• The present disclosure proposes the MBM where, in a single transmission, multiple users may receive different messages without increasing the bandwidth or the transmitted power.
• The present disclosure proposes multipath components, which help in diversifying the transmitted symbols across a constellation. This leads to an improved Signal-to-Noise (SNR) ratio response. Consequently, the high spectral efficiency, energy efficiency, and enhanced security, and reduced outage probability in a slow fade channel is achieved.
• The present disclosure proposes an electromagnetic window with the DRM which facilitates a real-time reconfigurability to demonstrate the MBM.
• The proposed scheme uses a channel-based modulation which overcomes the need for complex RF chains and beam steering network required in a modern multi-user digital communication scheme.
• The present disclosure proposes a codebook used by different users may be specific to their location and therefore all users may be geo-coded by the transmitter to ensure secure multi-user communication.

Referral Numerals:
Reference Number Description
100 MBM transmitter
101 Single RF chain
103 Oscillator
105 Power amplifier
107 Transmitting symbols
109 Decoded control bits
111 Transmitting directional antenna
113 Digitally reconfigurable metasurface
201 Subwavelength unit cell
203 PIN diode
205 Control circuitry/Driving circuit
501 Horn
503 Vector network analyzer
505 Interface
507 Microcontroller

,CLAIMS:WE CLAIM:

1. A Media-Based Modulation (MBM) transmitter (100) for communication in scatter-rich multipath channels, the MBM transmitter (100) comprising:
a single Radio Frequency (RF) chain (101);
a transmitting directional antenna (111) comprising one or more dielectric sheets and one or more metallic geometries, and configured to emit a carrier tone;
a Digitally Reconfigurable Metasurface, DRM, (113), placed in the nearfield of the transmitting directional antenna (111), and configured to generate unique sets of multiple spatial signatures by modulating the carrier tone;
a receiver (115) configured to receive the modulated carrier tone; and
a scheme to identify a sub-set of the multiple spatial signatures that forms a codebook for the modulated carrier tone shared between the transmitter and the receiver.

2. The MBM transmitter (100) as claimed in claim 1, wherein the single RF chain (101) comprises at least one of an oscillator (103) and a power amplifier (105).

3. The MBM transmitter (100) as claimed in claim 1, wherein the DRM (113) comprises an array of subwavelength unit cells (201), and each subwavelength unit cell (201) comprises a meander line and a high frequency Positive-Intrinsic-Negative (PIN) diode (203).

4. The MBM transmitter (100) as claimed in claim 3, wherein the PIN diode (203) in each array of the subwavelength unit cells (201) is controlled by a control circuitry (205), wherein each PIN diode (203) of the subwavelength unit cell (201) switches between OFF and ON states which causes the subwavelength unit cell (201) to switch between a transmitting state and a blocking state in the DRM.

5. The MBM transmitter (100) as claimed in claim 1, wherein the carrier tone at the transmitting directional antenna (111) input does not embed information bits.

6. The MBM transmitter (100) as claimed in claim 1, wherein the carrier tone, after passing through the DRM (113), embeds information bits conveyed by the DRM (113) to the receiver (115).

7. The MBM transmitter (100) as claimed in claim 1, wherein the receiver (115) is further configured to estimate channel fade coefficients for all switching patterns of the DRM (113).

8. The MBM transmitter (100) as claimed in claim 1, wherein the receiver (115) is placed in Line-of-Sight (LoS) or Non-Line-of-Sight (NLoS) configuration with respect to the transmitting directional antenna (111).

9. A method of configuring a Media-Based Modulation (MBM) transmitter (100) for communication in scatter-rich multipath channels, the method comprising configuring at least one of:
a single Radio Frequency (RF) chain (101);
a transmitting directional antenna (111) comprising one or more dielectric sheets and one or more metallic geometries to emit a carrier tone;
a Digitally Reconfigurable Metasurface (DRM) (113), placed in the nearfield of the transmitting directional antenna (111) to generate unique sets of multiple spatial signatures by modulating the carrier tone;
a receiver (115) to receive the modulated carrier tone; and
a scheme to identify a sub-set of the multiple spatial signatures that forms a codebook for the modulated carrier tone shared between the transmitter and the receiver.

10. The method as claimed in claim 9, wherein the single RF chain (101) comprises at least one of an oscillator (103) and a power amplifier (105).

11. The method as claimed in claim 9, wherein the DRM (113) comprises an array of subwavelength unit cells (201), and each subwavelength unit cell (201) comprises a meander line and a high frequency Positive-Intrinsic-Negative (PIN) diode (203).

12. The method as claimed in claim 11, wherein the PIN diode (203) in each array of the subwavelength unit cells (201) is controlled by a control circuitry (205), wherein each PIN diode (203) of the subwavelength unit cell (201) switches between OFF and ON states which causes the subwavelength unit cell (201) to switch between a transmitting state and a blocking state in the DRM (113).

13. The method as claimed in claim 9, wherein the carrier tone at the transmitting directional antenna (111) input does not embed information bits.

14. The method as claimed in claim 9, wherein the carrier tone, after passing through the DRM (113), embeds information bits conveyed by the DRM (113) to the receiver (115).

15. The method as claimed in claim 9 comprises estimating channel fade coefficients for all switching patterns of the DRM (113).

16. The method as claimed in claim 9 comprises placing the receiver (115) in Line-of-Sight (LoS) or Non-Line-of-Sight (NLoS) configuration with respect to the transmitting directional antenna (111).

Dated this 13th Day of September, 2022

Sandeep N P
Of K&S Partners
Attorney for the Applicant
IN/PA 2851