WO2023113734A1

WO2023113734A1 - Multiplexing in backscatter communication using orthogonal time frequency space (otfs) waveform for static and mobile backscatter devices

Info

Publication number: WO2023113734A1
Application number: PCT/TR2022/051443
Authority: WO
Inventors: Muhammad Bilal JANJUA; Hüseyin ARSLAN; Salah Eddine ZEGRAR
Original assignee: Istanbul Medipol Universitesi
Priority date: 2021-12-14
Filing date: 2022-12-07
Publication date: 2023-06-22
Also published as: TR2021019866A2

Abstract

This invention relates to a method for multiplexing multiple backscatter devices (i.e., Internet of Things (IoT) sensors/devices) with and without mobility using an orthogonal time-frequency space (OTFS) waveform. In the proposed method, the transmission point transmits an OTFS signal in the air, which is modulated and backscattered by backscatter devices. The information transmitted by backscatter devices is detected in the delay-Doppler domain.

Description

MULTIPLEXING IN BACKSCATTER COMMUNICATION USING ORTHOGONAL TIME FREQUENCY SPACE (OTFS) WAVEFORM FOR STATIC AND MOBILE BACKSCATTER DEVICES

Technical Field

This invention relates to a method for multiplexing multiple backscatter devices (i.e., Internet of Things (loT) sensors/devices) with and without mobility using an orthogonal time-frequency space (OTFS) waveform. In the proposed method, the transmission point transmits an OTFS signal in the air, which is modulated and backscattered by backscatter devices. The information transmitted by backscatter devices is detected in the delay-Doppler domain.

Prior Art

Wireless connectivity is an essential requirement of loT devices to share information and to transfer data to the cloud. Conventionally, these devices are energy-constrained and low data rate, and are expected to operate for a longer period. However, active signal transmission for communication consumes a significant amount of power and reduces battery life. Backscatter communication has been proposed as a potential candidate to solve this issue, which consumes very less amount of power than the conventional wireless systems proposed for loT e.g., long- range radio access (LoRA), narrowband loT (NB-IoT), Bluetooth low energy (BLE). Enabling backscatter communication for multiple loT devices located in the same environment is challenging due to inter-backscatter device interference and direct link interference in the case of ambient backscatter communication. The signal separation and interference problems become worse when there is mobility involved. Multiplexing a large number of mobile loT devices and separating the backscattered signal with reliability needs a waveform design that allows the detection and resolvability of multi-devices at the receiver.

There are very few works related to multiplexing in backscatter communication. One way to support backscatter communication of multiple devices is to multiplex them in the time domain or use of time division multiple access where different time slots are allocated to the different devices. Another way is to assign distinct orthogonal code to each backscatter device to avoid interference between the backscattered signals at the receiver. The backscatter devices are also multiplexed based on frequency switching between the tag’s impedances. Non-orthogonal multiple access (NOMA) based approaches are used in prior publications, where backscatter devices are multiplexed in spatial regions with different reflection coefficients. In another publication, a stochastic optimization receiver design is proposed to reduce the inter-backscatter device interference and direct link interference from the radio frequency source in multiple backscatter devices communication. The direct link interference from the ambient RF source is canceled using receive beamforming and inter-backscatter device interference is reduced through transmit beamforming. Similarly, beamforming at the large intelligent surface (LISA)/ reconfigurable intelligent surfaces (RIS) is also exploited to multiplex the data of multiple backscatter devices and reduce the interference issues.

The disadvantages of the previously proposed techniques are as follows:

1 - The time division-based multiplexing scheme lacks in combating the channel fading effect in practice even by increasing the number of time slots for a single device. Additionally as the number of devices increases, consequently the size of the time slot decreases. Thus, the data rate decreases with the increasing number of devices.

2- The orthogonal code-based multiplexing scheme requires the coordination between the backscatter devices and the performance of the scheme deteriorates with an increasing number of devices.

3- The limitation of frequency switching schemes is also related to coordination among the backscatter devices, which makes it unsuitable for power and processing limited devices.

4- The NOMA-based schemes work well with a small number of backscatter devices; however, as the number of devices increases the performance degrades drastically.

5- The proposed multiplexing schemes based on stochastic optimization and RIS reduce the interference for multiple backscatter device communication yet the mobility scenarios are not fully explored. The existing multiplexing techniques fail to detect the data of the multiple devices if mobility is involved and the number of devices is large. Additionally, the existing techniques can only multiplex a few backscatter devices.

Aim of the Invention

Applications of loT are spreading enormously in health, agriculture, transportation, industrial automation, remote monitoring, etc. Usually, loT devices and sensors are used to collect and transfer data and are expected to operate for a longer period. Two major application scenarios of loT are healthcare and transportation. In healthcare applications, multiple loT devices may be implanted into the body to perform multiple functions such as monitoring the health statistics or releasing the dose, etc. If the devices have a shorter life span then the devices need to be replaced regularly, which is inconvenient for the patients as well as the medical staff. On the other hand in transportation, autonomous driving and drones need to interact with the infrastructure and collect the data to support their mobility functions from low to high speed. In particular, small drones or flying machines for the environment or traffic monitoring and control have limited power and high mobility. To reduce the power consumption and increase the operation period of such devices backscatter communication is considered as a potential candidate. However, providing communication to the multiple backscatter devices is challenging.

The invention aims to;

1- Provide low power and interference-free communication to multiple loT devices through OTFS modulated signal-based backscatter communication.

2- Enable the backscatter communication for loT devices with high mobility.

3- Support different configurations of the backscatter communication such as monostatic, bistatic, and ambient through the disclosure.

4- Multiplex large number of backscatter devices.

5- Exploiting diversity gains provided by OTFS waveform to improve the reliability of backscatter communication. Brief Description of the Invention

The present invention relates to a method for multiplexing multiple backscatter devices wherein said method comprises the steps of; i. Training phase wherein the delays and Doppler shifts corresponding to all the backscatter devices located in the environment are identified ii. Transmission of OTFS modulated signal iii. Signal modulation at backscatter devices iv. Detection of backscatter devices’ signals

The method of the invention can multiplex multiple mobile/static loT devices in backscatter communication over the OTFS signal. The backscatter devices modulate and backscatter the OTFS modulated signal transmitted by the RF source, where the data of each backscatter device is separated in the delay domain if the mobility is not involved. Additionally, if the backscatter devices are mobile then their signals may be separated both in delay and Doppler domains.

In an aspect, the present invention maybe applicable to all the configurations of the backscatter communication system such as monostatic, bistatic, and ambient.

Backscatter communication has been considered as a potential candidate to provide low power communication compared to active communication systems, particularly for loT devices. Three different approaches are used in the backscatter communication based on the transmitter and receiver location, which are monostatic, bistatic, and ambient. In the monostatic and bistatic configurations, the carrier signal design is controlled according to the requirements; however, in the case of the ambient backscatter communication, there is no control over the carrier signal as it is transmitted for primary communication purposes than to support the backscatter communication. Nonetheless, a symbiotic radio allows the ambient backscatter communication to not only share the signal but also share the infrastructure such as joint transmitter and receiver design with the primary system (i.e., a conventional wireless communication system e.g., WiFi, Cellular). The design of the carrier signal or the waveform of the carrier signal is critical to detect the signal at the receiver. Unlike other waveforms, the OTFS waveform has the unique property of modulating and detecting the signals in the delay-Doppler domain. By utilizing the OTFS waveform, the method proposed in the invention enables the communication of multiple backscatter devices with and without mobility in different system configurations.

For instance, in the monostatic configuration, the signal source and the receiver are located in the same device; however, in the bistatic configuration, the signal source and transmitter are located separately. Besides, the ambient backscatter communication system is a type of bistatic communication system but instead of deploying a dedicated signal source, the signals transmitted by of transmission point of ambient wireless systems e.g., TV towers, cellular base stations, and WiFi access points, are used for backscatter communication. We also consider a symbiotic radio system, where the primary wireless system cooperate with the ambient backscatter communication through a joint transmitter and/or receiver design. Figure 1 shows the steps of the proposed method in the invention,

Explanation of Figures

Figure 1: The flow chart of the basic operations by certain aspects of the present disclosure.

110: Training phase

120: OTFS modulated signal transmission

130: Signal modulation at backscatter device

140: Backscatter devices’ signals detection

Figure 2: An illustration of the bistatic backscatter communication system model with the transmitted signal 220 in the delay 206-Doppler 207 domain, and the received signal 230, when no backscatter device 203 is transmitting.

201: Transmission Point that OTFS modulated Signal

202: Receiver 203: Backscatter device equipped with multiple antennas

204: Channel between transmission point 201 and backscatter device 203205 Channel between transmission point 201 and receiver 202.206: Delay

207: Doppler

208: Pilot Symbol

209: Null bin

210: Occupied bin

211: Backscatter device’s signal bin

220: Transmitted signal in delay 206-Doppler 207 domain

230: Received signal in delay 206-Doppler 207 domain

Figure 3: an illustration of the bistatic backscatter communication system model with the transmitted signal 220 in the delay 206-Doppler 207 domain and the received signal 230 when one backscatter device 203 is transmitting.

301: Channel between backscatter device 203 to receiver 202.

Figure 4: An illustration of the the bistatic backscatter communication system model with the transmitted signal 220 in the delay 206-Doppler 207 domain and the received signal 230 when all the backscatter devices 203 are transmitting.

Figure 5: An illustration of the pilot and data symbol placement transmitted signal 220 in the delay 206-Doppler 207 domain and the received signal 230 when all the backscatter devices 203 are transmitting in a symbiotic ambient backscatter communication system.

501: Data symbol Detailed Description of the Invention

The present invention relates to a method for multiplexing multiple backscatter devices wherein said method comprises the steps of;

Training phase wherein the delays and Doppler shifts corresponding to all the backscatter devices located in the environment are identified

Transmission of OTFS modulated signal

Signal modulation at backscatter device and

Detection of backscatter devices’ signal

The method of the invention is novel in terms of; (i) Multiplexing backscatter devices data over OTFS modulated signal, (ii) Signal separation of backscatter devices in the delay-Doppler domain at the receiver, (iii) An OTFS waveform-based symbiotic radio system with joint transmitter and joint receiver to support ambient backscatter communication and (iv) Exploiting diversity gains provided by OTFS waveform to improve the reliability of backscatter communication.

Now, a detailed description of each of the steps of the method according to the present invention is given below.

Training Phase (110):

In the training phase, we identify the delays and Doppler shifts corresponding to all the backscatter devices 203 located in the environment. One way to obtain the delay 206-Doppler 207 information related to the backscatter devices 203 (not necessarily the only way), initially all backscatter devices are turned off and they do not backscatter the signal (i.e., non-reflecting state) as shown in 230 of Figure 2. Afterward, each backscatter device 203 is turned on to backscatter the signal (i.e., reflecting state), and its delay 206-Doppler 207 information is obtained at the receiver 202 as shown in Figure 2The bins with the line pattern 211 at 202 in Figure 2 illustrate the delays 206 and Doppler 207 shifts that occurred due to backscatter devices 203. OTFS modulated signal transmission (120):

In this step, the data 501 and/or pilot symbols 208 arranged in the delay 206-Doppler 207 grid are modulated to the time-frequency domain using inverse symplectic fast Fourier transform (ISFFT). Then, these modulated time-frequency symbols are converted to the time domain signal using Heisenberg transform and are transmitted over the channel. Two types of symbols arrangement in the delay 206-Doppler 207 grid is used according to system configurations mentioned earlier, which are described as follows:

1. In the case of monostatic and bistatic communication, the transmission point 201 sends only OTFS modulated pilot symbols 208 over the channel as illustrated in Figures 2, 3 and 4. Figure 2 shows the system model, where the bin in the delay 206-Doppler 207 grid with the pilot symbol is represented by x, and null bins are denoted by o 209. The delay 206-Doppler 207 grid consists of M delay bins and N Doppler bins.

2. In the case of the symbiotic radio system, a joint transmitter is designed for primary wireless system and ambient backscatter communication, one way is that the 201 sends pilot symbols 208 and data symbols 501 of primay wireless system, and guard symbols 209 arranged in OTFS frame as illustrated in Figure 5, given by Raviteja, Patchava, Khoa T. Phan, and Yi Hong. "Embedded pilot-aided channel estimation for OTFS in delay-Doppler channels." IEEE Transactions on Vehicular Technology 68.5 (2019): 4906-4917.

where x_p denote the pilot symbols 208, x_d denotes the data symbols 501, , 0 < I < M — 1 denotes the delay 206 bins, 0 < k < N — 1 denotes the Doppler 207 bins, and l_T and k_v denote the delay 206 taps and Doppler 207 taps, respectively. The pilot symbols 208 can be placed in the delay 206 l_p and Doppler 207 k_p bins in the delay 206-Doppler 207 grid with M delay bins and N Doppler bins. Null bins 209 are left as guard periods between the data symbols 501 and pilot symbols 208 to avoid interference at the receiver. The pilot symbols 208 is used for both the channel estimation and backscatter communication.

Signal modulation at backscatter devices (130):

There are i = {1,2, ••• , MN — 1} backscatter devices 203 located in the environment, where the ith backscatter device is represented as BDj 203. For instance, each backscatter device 203 modulates the incident OTFS modulated signal by switching its antennas into reflecting and nonreflecting states to transmit 1 and 0, respectively. Figure 2 indicates the scenario where all the backscatter devices 203 are in non-reflecting state; thus, backscatter devices 203 signals are not received at 202. In Figure 3 one backscatter device 203 is in a reflecting state and others are in a non-reflecting state. Figure 4 indicates the scenario where all the backscatter devices 203 are in the reflecting state and backscattered signals are received at 202. Furthermore, these backscattered devices 203 are multiplexed in the delay 206-Doppler 207 domain and create different delays 206 and Doppler 207 shifts in the incident OTFS modulated signal according to their distance and mobility, respectively. The proposed method can be used in all scenarios and other modulation schemes with higher modulation order can also be considered for higher data rates.

Backscatter devices signal detection (140):

To detect the modulated backscatter signals at 202 , the time-domain received signal is converted into the time-frequency domain by applying the Wigner transform and sampling. Afterward, symplectic fast Fourier transform is applied to the time-frequency signal and converted into the delay 206-Doppler 207 domain. The backscatter devices’ 203 signals are received with different delays 206 and Doppler 207 shifts according to their distance and mobility 230, respectively, which are known from the training phase. If the backscatter devices 203 are not moving then they are separated in delays. Otherwise, the backscatter devices 203 are separated in both the delays 206 and Doppler 207 shifts. Figure 2 illustrates the pilot symbols received in the delay 206- Doppler 207 domain when all the backscatter devices 203 are in a non-reflecting state. The line pattern filled 211 delay 206-Doppler 207 bins in the delay 206-Doppler 207 grid denote the delays 206 and Doppler 207 shifts related to the backscatter devices 203 (see Figure 2). When the line pattern filled bins 211 contain the received pilot symbols in 230 210 that means the backscatter devices 203 are in a reflecting state (i.e., 1 is transmitted), and when the line pattern filled bins 211 contain the null bins 209 that means the backscatter devices 203 are in a nonreflecting state (i.e., 1 is transmitted). However, the received pilot symbols 208 in bins other than line pattern filled bins 211 that indicates the signals coming from multipath components other than backscatter devices 203. Figure 3 illustrates the received signal 230 in the delay 206- Doppler 207 grid with the received pilots 210 when only one backscatter device 203 is in a reflecting state. Wherein Figure 4, the received signal is illustrated in delay 206-Doppler 207 grid when all the backscatter devices 203 are backscattering the OTFS signal. Figure 5 shows the received signal 230 in the case of a symbiotic radio system, where the pilot symbols 210 are used to detect the backscatter communication system and channel estimation at 202 and backscatter signal detection is performed using the aforementioned steps in the bistatic configuration.

Industrial Applicability of the Invention

The invention is a novel method for multiplexing backscatter communication for loT devices and supports mobility scenarios. This is very critical for the industry which is related to loT devices and sensors and develops RFID applications.

Any wireless technology not limited to communication and/or sensing can utilize this invention for backscatter signal transmission and/or reception.

Standards like 3GPP-based cellular, IEEE 802.11 based LAN standards, IEEE 802.15 based wireless personal area network standards, RFID related standards (e.g., ISO/IEC, ASTM) are particularly relevant due to the support of backscatter communication in one way or the other.

Furthermore, the proposed technique in the invention can be implemented on any device, system, or network capable of supporting any of the aforementioned standards such as 3GPP-based cellular, IEEE 802.11 based LAN standards, IEEE 802.15 based wireless personal area network standards, RFID related standards (e.g., ISO/IEC, ASTM).

Definitions of some of the terms used within this description are given below; The term “Backscatter communication” refers to a way of communication in which the transmitter modulates and backscatter the signal transmitted by the transmission point to transmit its information to the receiver.

The term “Monostatic backscatter communication” refers to a type of backscatter communication in which the signal source and receiver are located in the same device.

The term “Bistatic backscatter communication” refers to a type of backscatter communication in which the signal source and the receiver are separated from each other.

The term “Ambient backscatter communication” refers to backscatter communication in which the backscatter device utilizes the signals of ambient sources e.g., WiFi access point, TV tower, cellular base station, etc.

The term “Symbiotic Radio” Symbiotic radio refers to a type of radio system in which two or more radio systems have mutual coexistence to support each other e.g., cooperative ambient backscatter communication.

The term “Internet of things (IoT)” refers to a network of devices that are connected to other devices through the internet to collect and share data about the environment or the way they are used.

The term “Symplectic fast Fourier transfrom (SFFT)” refers to two-dimensional (symplectic) Fourier transform between a grid in the reciprocal time-frequency plane and a grid in the delay - Doppler plane.

The term “Inverse symplectic fast Fourier transform (ISFFT)” refers to the inverse operation of SFFT.

The term “Heisenberg Transform” refers to a transformation between a grid in the reciprocal time-frequency plane and the time domain.

The term “Wigner Transform” refers to the inverse of the Heisenberg Transform. Around these basic concepts, it is possible to develop several embodiments regarding the subject matter of the invention; therefore, the invention cannot be limited to the examples disclosed herein, and the invention is essentially as defined in the claims.

It is obvious that a person skilled in the art can convey the novelty of the invention using similar embodiments and/or that such embodiments can be applied to other fields like those used in the related art. Therefore, it is also obvious that these kinds of embodiments are void of the novelty criteria and the criteria of exceeding the known state of the art.

Claims

CLAIMS A method for multiplexing multiple mobile or static backscatter devices in backscatter communication over the orthogonal time- frequency space (OTFS) signal, then the backscatter devices modulate and backscatter the OTFS signal transmitted by a radio frequency (RF) source where;

If mobility is involved; the signals of backscatter devices are separated both in delay and Doppler domains at the receiver or

If mobility is not involved; the data of each bacscatter device is separated in delay domain at the receiver. A method for multiplexing multiple backscatter devices according to claim 1 wherein said method comprises the steps of; i. Training phase wherein the delays and Doppler shifts corresponding to all the backscatter devices located in the environment are identified ii. Transmission of OTFS modulated signal iii. Signal modulation at backscatter device and iv. Detection of backscatter devices’ signal A method according to claim 1 characterized in that in step (i) identification of delays and Doppler shifts corresponding to all the backscatter devices located in the environment is made by turning off all the backscatter devices, such that they do not backscatter the signal and then turning on all backscatter devices to backscatter the signal and its delay and/or Doppler information is obtained at the receiver. A method according to any one of claims 1-3 characterized in that in step (ii) the data and/or pilot symbols arranged in the delay-Doppler grid are modulated to the time-frequency domain using inverse symplectic fast Fourier transform (ISFFT) and then, these modulated timefrequency symbols are converted to the time domain signal using Heisenberg transform and are transmitted over the channel. A method according to claim 4 characterized in that in step (ii) symbols arrangement in the delay-Doppler grid depends on the system configuration being (a) monostatic and bistatic communication or (b) symbiotic radio system.

6. A method according to claim 5 characterized in that in monostatic and bistatic communication; the transmission point sends only OTFS modulated pilot symbols over the channel.

7. A method according to claim 5, characterized in that in symbiotic radio system a joint transmitter is designed for ambient wireless system and backscatter communication, for example transmission point sends pilot and data symbols of ambient wireless system, and guard symbols arranged in OTFS frame

wherein x_p is the pilot symbols, x_d is the data symbols, 0 < k < N — l is the Doppler bins, , 0 < I < M — 1 is the delay bins, l_T and k_v are the delay taps and Doppler taps, respectively.

8. A method according to claim 7, characterized in that the pilot can be placed in the delay l_p and Doppler k_p bins in the delay-Doppler grid with M delay bins and N Doppler bins and Null bins o are left as guard periods between the data symbols and pilot to avoid interference at the receiver and the pilot is used for both the channel estimation and backscatter communication.

9. A method according to any one of claims 1-8 characterized in that in step (iii) there are i = {1,2, -, MN — 1} backscatter devices located in the environment, where the ith backscatter device is represented as BDj 203 each backscatter device modulates the incident OTFS modulated signal by switching its antennas into reflecting and non-reflecting states to transmit 1 and 0, respectively.

10. A method according to any one of claims 1-9 characterized in that in step (iv) the timedomain received signal is converted into the time-frequency domain by applying the Wigner transform and sampling to detect the modulated backscatter signals at receiver 202 and then symplectic fast Fourier transform is applied to the time-frequency signal and converted into the delay-Doppler domain wherein he backscatter devices’ signals are received with different delays and Doppler shifts according to their distance and mobility as determined by the step (i) of the method.

11. A method according to claim 10 characterized in that where the backscatter devices are not mobile then they are separated in delays.

12. A method according to claim 10 characterized in that where the backscatter devices are mobile then they are separated in both the delays and Doppler shifts

13. A method according to any of the preceding claims chracterized in that the backscatter device is monostatic, bistatic, ambient or symbiotic.

14. Use of a method according to any one of claims 1-13 for for multiplexing backscatter communication for loT devices and supporting mobility scenarios.

15. Use of a method according to claim 14 characterized in that the method of claims 1-13 are for loT devices and sensors and RFID applications.

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