Method for OFDM based ambient backscatter communication utilizing pre-equalization

Abstract

Invention is a method realized by a Orthogonal Frequency Division Multiplexing (OFDM) based backscatter communication system wherein the system comprising at least a transmitter device (100) capable of transmitting signal to a receiver device (200) through a direct channel (410), at least the receiver device (200) and at least a backscatter device (300) which is capable of performing backscatter communication to receiver device (200) through a backscatter channel (430) using signal received from the transmitter device (100) through a forward channel (420) characterized in that comprising the steps of: receiving, by the transmitter device (100), data to be transmitted to the receiver device; (200) performing, by the transmitter device (100), channel sounding on the direct channel (410) and estimating channel impulse response (CIR) representing multipath behavior in channel as taps; performing, by the transmitter device (100), a partial equalization on the CIR by nullifying only some of the taps for leaving space for backscatter signals; generating OFDM signal using partially equalized CIR; transmitting OFDM signal.

Description

[0001] DESCRIPTION

[0002] METHOD FOR OFDM BASED AMBIENT BACKSCATTER COMMUNICATION UTILIZING PARTIAL PRE-EQUALIZATION

[0003] TECHNICAL FIELD

[0004] Invention relates to a method realized by a Orthogonal Frequency Division Multiplexing (OFDM) based backscatter communication system wherein the system comprising at least a transmitter device capable of transmitting signal to a receiver device through a direct channel, at least the receiver device and at least a backscatter device which is capable of performing backscatter communication to receiver device through a backscatter channel using signal received from the transmitter device through a forward channel.

[0005] PRIOR ART

[0006] In ambient backscatter communication, backscatter devices are distributed across the environment, leveraging ambient radio frequency (RF) sources such as cellular signals (e.g., orthogonal frequency division multiplexing (OFDM)) to transmit their information to a receiver. Compared to conventional communication systems, this approach offers significant benefits, including enhanced spectrum efficiency and energy efficiency, as passive devices operate without requiring active transmission. Instead, they achieve communication by modulating and reflecting existing RF signals. However, despite these advantages, ambient backscatter communication introduces unique challenges. Conventional communication systems, such as cellular and Wi-Fi technologies, were not designed to leverage the same time-frequency resources as the primary communication link for backscatter purposes. As a result, ambient backscatter is often treated as a side link in relation to the primary broadband communication. This paradigm creates several issues, including interference between the backscatter and primary links, which can degrade system performance. Moreover, the direct link is significantly stronger than the backscatter link, making it difficult to detect the weak backscatter signal at the receiver, particularly in the presence of strong multipath interference. Additionally, the modulation scheme employed by the backscatter device influences both the detectability of the backscatter signal and the interference introduced to the main link. These factors contribute to increased receiver complexity and a lower probability of detecting the weak backscatter signal, highlighting the need for innovative solutions to address these challenges associated between the co-existence of these asymmetric link. Considering state-of-the-art research on ambient backscatter communication leveraging OFDM waveforms, various techniques have been proposed from both transmitter and receiver perspectives to ensure reliable communication while maintaining coexistence between the two asymmetric links. In [1], the authors investigated the feasibility of integrating backscatter communication in the presence of a direct link at low frequencies using technologies such as LoRa and NB-loT under both LoS and NLoS conditions. They found that in bi-static schemes, the coverage area heavily depends on the receiver's sensitivity. The study highlighted the significant impact of direct path interference on backscatter communication, emphasizing its critical role in reliably detecting weak backscattered signals.

[0007] In [2], a notch filter bank was designed to process incoming OFDM signals at the backscatter receiver. The filters either null specific subcarriers or retain them depending on the modulated bits, effectively enabling OOK-based modulation. Detection at the receiver was achieved by first canceling direct link interference through known channel estimation between the transmitter and receiver. An energy detector employing maximum likelihood (ML) estimation over subcarriers was then used to decode the backscatter signal.

[0008] In [3], the authors proposed a framework to balance spectrum efficiency and backscatter reliability by optimizing the cyclic prefix (CP) duration and subcarrier spacing based on channel state information (CSI). By recommending a CP duration longer than the channel’s maximum excess delay, the framework preserved subcarrier orthogonality, mitigating inter-symbol interference (ISI) in multipath environments. This approach avoided direct link interference (DLI) and extended concepts discussed in [4], where the repeating structure of the CP was leveraged under the constraint of sufficient CP length to isolate the backscatter link from the legacy OFDM signal. Unlike [4], the authors in [5] proposed narrowband transmission within wideband transmission, compliant with standards using resource units (RUs). They leveraged differential coding to eliminate the need for channel estimation, enabling a non-coherent OFDM receiver and reducing complexity. In this case, direct link interference (DLI) was treated as background noise, enhancing spectrum efficiency.

[0009] Different from conventional modulations such as OOK, FSK, and PSK, the work in [6] introduced Differential Code Keying (DCK) with OFDM at the backscatter device level. This scheme delays symbols in the time domain at the backscatter transmitter, creating phase variations in the frequency domain at the receiver. Using the Wi-Fi standard frame structure and coherent detection, this approach enables the receiver to identify backscatter signals via phase variations in diagonal channel elements. The design also incorporated M-DSK levels at the tag to enhance resolution, achieving performance gains compared to PSK.

[0010] In [7], the authors explored the use of MIMO systems for backscatter communication and proposed an I M-M I MO-based approach with techniques such as space shift keying (SSK) and spatial modulation (SM). This enabled multiplexing multiple backscatter devices while providing diversity gains.

[0011] A novel pilot design, termed coupled channel estimation, was introduced to mitigate direct link and multipath interference. Detection was achieved by identifying activated antenna indices and constellation symbols corresponding to backscattered signals under various schemes. Similar pilot-based approaches for handling DLI and multipath interference were extended in [8]. Another method for eliminating DLI, proposed in [9], involves nullifying the transmitted signal by leveraging CSI at the transmitter. Using MIMO, a pre-coded signal is generated such that destructive interference occurs at the receiver after convolution with the channel, effectively nullifying the direct link. This enables more relaxed detection of the backscattered signal without interference from the direct path. Considering the channel effects on the tag signal, which is subject to double fading, the literature leads to the following observations:

[0012] Utilizing a notch filter at the backscatter level in OFDM systems is inefficient because the notch does not have an ideal cutoff frequency at the tag level. In this case, the transmitter needs to leave guard bands between subcarriers to prevent leakage that affects detection at the receiver. This problem becomes even more challenging in the presence of a direct link, where overlapping signals make estimating the weak signal from the strong one difficult, necessitating channel estimation to track variations and mitigate the interference.

[0013] Using a CP longer than the maximum excess delay to maintain orthogonality between the tags and the transmitter mitigates multipath effects and simplifies detection, removing the need for channel estimation under the assumption that CSI is available at the transmitter. However, this scheme is not spectrum efficient because more resources are used to accommodate the backscatter link, conflicting with the goal of ambient backscatter systems, which aim for spectrum efficiency by reusing the same time-frequency resources. Although not consuming all resources, this approach partially sacrifices spectrum efficiency to simplify detection and mitigate multipath and direct link interference. Narrowband (NB) transmission within a wideband system is less spectrum efficient than the CP-based solution because the transmitter does not fully exploit its capacity. While this approach simplifies detection by allowing a non-coherent receiver to detect the backscatter signal, it treats the direct link as noise, leading to significant SNR degradation due to increased noise variance.

[0014] Differential Code Keying (DSK) introduces time-domain manipulation at the tag level. While assuming coherent channels where the transceiver channel is known and phase tracking is performed, backscatter signal delays within the CP duration expose the signal to strong multipath components. Even with known channels, residual interference persists, degrading performance. Increasing the modulation order to M-DSK enhances resolution to mitigate this residual but increases complexity with marginal detection gains.

[0015] Leveraging MIMO at the tag level adds a degree of freedom for tag modulation by exploiting spatial multiplexing and spatial modulation. Additionally, diversity in the spatial domain mitigates multipath and direct link interference. However, from a standards perspective, tags typically cannot support multiple antennas due to energy constraints, making this solution impractical despite its benefits.

[0016] Eliminating the impact of direct link interference via Ml MO-based pre-coding allows easier detection of the backscatter signal. However, this compromises the simultaneous communication of the legacy transmitter and backscatter system. Sacrificing the main link for backscatter communication is not feasible, as it directly impacts conventional communication.

[0017] References:

[0018] [1] Biswas, R., Sheikh, M. U., Yigitler, H., Lempiainen, J., & Jantti, R. (2021 , April). Direct path interference suppression requirements for bistatic backscatter communication systems. In 2021 IEEE 93rd Vehicular Technology Conference (VTC2021 -Spring) (pp. 1-5). IEEE.

[0019] [2] Nemati, M., Soltani, M., Ding, J., & Choi, J. (2020). Subcarrier-wise backscatter communications over ambient OFDM for low-power loT. IEEE Transactions on Vehicular Technology, 69(11 ), 13229-13242.

[0020] [3] Nguyen, T. L., Shin, Y., Kim, J. Y., & Kim, D. I. (2019). Signal detection for ambient backscatter communication with OFDM carriers. Sensors, 19(3), 517.

[0021] [4] Yang, G., Liang, Y. C., Zhang, R., & Pei, Y. (2017). Modulation in the air: Backscatter communication over ambient OFDM carrier. IEEE Transactions on Communications, 66(3), 1219-1233. [5] Wang, G., Gao, F., Fan, R., & Tellambura, C. (2016). Ambient backscatter communication systems: Detection and performance analysis. IEEE Transactions on Communications, 64(11 ), 4836-4846.

[0022] [6] Hara, T., Takahashi, R., & Ishibashi, K. (2021 ). Ambient OFDM pilot-aided backscatter communications: Concept and design. IEEE Access, 9, 89210-89221.

[0023] [7] Niu, Z., Xiao, L., Kan, J., He, S., & Jiang, T. (2023). A unified implementation framework for index modulation assisted MIMO backscatter communication. IEEE Communications Letters, 27(5), 1422-1426.

[0024] [8] Niu, Z., Xiao, L., Ma, W., Peng, M., & Jiang, T. (2023). Generalized space-time architecture for ambient backscatter communication. IEEE Transactions on Communications, 71 (4), 1912- 1925.

[0025] [9] Ali Abedi and Omid Abari-Salehi, Method and System for Long Range Wi-Fi Backscatter Communication, US20230115786A1 , filed on October 12, 2022, published on April 13, 2023. Status: Pending.

[0026] All the problems mentioned above have made it necessary to make an innovation in the relevant technical field as a result.

[0027] BRIEF DESCRIPTION OF THE INVENTION

[0028] The present invention relates to a method to eliminate the above-mentioned disadvantages and bring new advantages to the relevant technical field.

[0029] An object of the invention is achieve joint communication between users and backscatter devices within the same time-frequency resources with significantly reduced interference from the direct link.

[0030] Another object of the invention is to increase spectrum efficiency.

[0031] To achieve all the objects mentioned above and that will emerge from the following detailed description, the present invention relates to A method realized by a Orthogonal Frequency Division Multiplexing (OFDM) based backscatter communication system wherein the system comprising at least a transmitter device capable of transmitting signal to a receiver device through a direct channel, at least the receiver device and at least a backscatter device which is capable of performing backscatter communication to receiver device through a backscatter channel using signal received from the transmitter device through a forward channel. Accordingly the method is characterized in that comprising the steps of:

[0032] - receiving, by the transmitter, data to be transmitted to the receiver device;

[0033] - performing, by the transmitter device, channel sounding on the direct channel and estimating channel impulse response (CIR) representing multipath behavior in channel as taps;

[0034] - performing, by the transmitter device, a partial equalization on the CIR by nullifying only some of the taps for leaving space for backscatter signals;

[0035] - generating OFDM signal using partially equalized CIR;

[0036] - transmitting OFDM signal. Thus, receiver receives transmitting signal and backscatter signal thanks to nullified taps. A legacy (generic) receiver can equalize the channel using since nullified taps are filled with the backscatter signal.

[0037] A possible embodiment of the invention is characterized in that ...

[0038] Another possible embodiment of the invention is characterized in that ...

[0039] BRIEF DESCRIPTION OF THE DRAWINGS

[0040] Figure 1 is a drawing illustrating top schematic view of the system.

[0041] Figure 2 is a drawing illustrating the transmitter and the receiver in an embodiment of the system.

[0042] Figure 3 is a drawing illustrating the transmitter and the receiver in another embodiment of the system.

[0043] Figure 4 is a drawing illustrating the steps realized in the method.

[0044] Figure 5 is a drawing illustrating the overall effect of the pre-equalization in the both transmitter, receiver and backscatter communication.

[0045] Figure 6 illustrates graphics showing effects of pre-equahzation in the frequency domain.

[0046] Figure 7 illustrates graphics showing effects of pre-equahzation in the time domain. REFERENCE NUMBERS GIVEN IN THE FIGURE

[0047] 100 T ransmitter device

[0048] 110 OFDM modulator

[0049] 120 Pre-equalization unit

[0050] 130 CP adder

[0051] 140 Transmit antenna

[0052] 200 Receiver device

[0053] 210 OFDM demodulator

[0054] 220 Equalization unit

[0055] 230 CP remover

[0056] 240 Receive antenna

[0057] 300 Backscatter device

[0058] 410 Direct channel

[0059] 420 Forward channel

[0060] 430 Backscatter channel

[0061] DETAILED DESCRIPTION OF THE INVENTION

[0062] In this detailed description, the subject matter is explained with references to examples without forming any restrictive effect only in order to make the subject more understandable.

[0063] Invention is a method for orthogonal frequency division multiplexing (OFDM) based ambient backscatter communication. It realizes a partial pre-equalization on channel impulse response in order to create space for backscatter device’s (300) backscatter signal.

[0064] Referring the figure 1 , system comprises at least a transmitter device (100) capable of transmitting signal to a receiver device (200) through a direct channel (410), at least the receiver device (200) and at least a backscatter device (300) which is capable of performing backscatter communication to receiver device (200) through a backscatter channel (430) using signal received from the transmitter device (100) through a forward forward (420).

[0065] The transmitter is capable of performing OFDM communication. The transmitter for instance may be an access point or a base station. Receiver device (200) is capable of performing OFDM communication. Receiver device (200) may be a user equipment. A backscatter device (300) may comprise an antenna for capturing ambient OFDM signals and reflecting modulated signals; a modulation circuit for altering the reflected signal's phase, amplitude, or frequency to encode data; a controller for managing the modulation process based on input data or preconfigured logic; and an energy harvesting unit for collecting power from ambient RF signals to enable operation without an external power source. Backscatter device (300) for instance may be a tag.

[0066] Referring to figure 2, an embodiment of the invention is given. The transmitter device (100) comprises an input means (not shown) for receiving data to be sent to receiving device. The transmitter device (100) a the transmitter comprising a pre-equalizer unit where the preequalizer unit is configured to perform partial equalization on the channel impulse response of the direct channel (410), representing multipath behavior in channel as taps, wherein the CIR is acquired by performing channel sounding on the direct channel (410) and estimating, such that partial equalization is performed by nullifying only some of the taps for leaving space for backscatter signals. Transmitter comprises an OFDM modulator (110) for modulating the signal for generating an OFDM signal. Transmitter comprises a transmit antenna (140) for transmitting OFDM signal. In this embodiment pre-equalizer unit performs pre-equalization in frequency domain.

[0067] In a possible embodiment it comprises a CP adder (130) for adding cyclic prefix to OFDM signal.

[0068] Receiving device comprises a receive antenna (240) for receiving OFDM signals. Receiving device comprises an OFDM demodulator (210) for demodulating OFDM signal. Receiver comprises an equalization unit for equalizing the demodulated signal. Then an output means outputs signal from backscatter and user data.

[0069] Pre-equalization unit (120) provides empty spaces for backscatter’s backscatter signals. Receiver device (200) realizes normal receiving operation and normal equalization operation since nullified taps are filled with backscatter signal. Thus, receiver device (200) experiences as the transmission was realized considering all taps.

[0070] In a possible embodiment receiver comprises a CP remover (230) for removing cyclic prefix from OFDM signal. Referring to figure 3, pre-equalization unit (120) is configured to perform pre-equalization after OFDM modulation (110) in time domain.

[0071] Referring to figure 4, the method comprises following steps:

[0072] - receiving, by the transmitter device (100), data to be transmitted to the receiver device; (200)

[0073] - performing, by the transmitter device (100), channel sounding on the direct channel (410) and estimating channel impulse response (CIR) representing multipath behavior in channel as taps;

[0074] - performing, by the transmitter device (100), a partial equalization on the CIR by nullifying only some of the taps for leaving space for backscatter signals;

[0075] - generating OFDM signal using partially equalized CIR;

[0076] - transmitting OFDM signal.

[0077] In more detail, the transmitter device (100) performs an initial channel sounding process to estimate the channel impulse response (CIR) at the receiver device (200).

[0078] The CIR, may be defined as: where < rmax> represents the multipath characteristics. The estimated CIR is fed back to the transmitter device (100) for further processing.

[0079] The transmitter device (100) applies partial pre-equalization based on the feedback. Unlike conventional zero-forcing, this method selectively nullifies certain channel taps, leaving space for backscatter signals. The filtered CIR is defined as: H H, ensuring partial rather than complete channel compensation.

[0080] The pre-equalization is subject to the constraint: Where pt ;is the pre-equalization coefficient. The effective channel observed at the receiver device becomes:

[0081] HH = Heff

[0082] For instance, with L=7 and M=4, the receiver device (200) observes three taps for the direct link, leaving the remaining taps for backscatter. Then the overall channel of the direct link and the backscatter link satisfies the orginal taps from CIR:

[0083] HBD + HDL= H

[0084] This allow the receiver to effectively separate the backscatter signal from the direct link knowing the delays and thus we can equalize the direct link as the conventional communication where as the backscatter link we can estimate the detection in delay domain since they delays are known and separable from the direct link or in frequency domain since these delays are phase terms.

[0085] Referring to figure 5, the pre-equalization process creates a "clean region" in the time domain, allowing backscatter devices (300) to utilize this region for communication. The transmitter devce performs partial filtering in the frequency or time domain to ensure separation. At the receiver device (200), the direct link and backscatter signals are separated using the known delays of the channel. Equalization is performed on the direct link, while the backscatter signal is estimated without the need for channel estimation since the backscatter signal delays are more then the multipath of the direct link, thus the detection of delays with respect to symbols or phases with respect to symbols is performed, this operation can be done in both time or frequency domain. In time domain the range Doppler map can be used in this case to detect the delays while the Doppler axis is not used in the case of single backscatter device (300) only.

[0086] Figure 6 and 7 gives simulation results of frequency responses of the filtered (partly preequalized) channels.

[0087] The scope of protection of the invention is specified in the attached claims and cannot be limited to those explained for sampling purposes in this detailed description. It is evident that a person skilled in the art may exhibit similar embodiments in light of the above-mentioned facts without drifting apart from the main theme of the invention.

Claims

CLAIMS1. A method realized by a Orthogonal Frequency Division Multiplexing (OFDM) based backscatter communication system wherein the system comprising at least a transmitter device (100) capable of transmitting signal to a receiver device (200) through a direct channel (410), at least the receiver device (200) and at least a backscatter device (300) which is capable of performing backscatter communication to receiver device (200) through a backscatter channel (430) using signal received from the transmitter device (100) through a forward channel (420) characterized in that comprising the steps of:- receiving, by the transmitter device (100), data to be transmitted to the receiver device; (200)- performing, by the transmitter device (100), channel sounding on the direct channel (410) and estimating channel impulse response (CIR) representing multipath behavior in channel as taps;- performing, by the transmitter device (100), a partial equalization on the CIR by nullifying only some of the taps for leaving space for backscatter signals;- generating OFDM signal using partially equalized CIR;- transmitting OFDM signal.

2. The method according to claim 1 , characterized in that comprising the steps of;- performing, OFDM modulation (110) to the data to be transmitted;- performing partial equalization in time domain after OFDM modulation (110).

3. The method according to claim 1 , characterized in that comprising the steps of;- performing partial equalization in frequency domain to the data to be transmitted;- performing, OFDM modulation (110) to the data to be transmitted after it is subjected to partial equalization.

4. The method according any one of the preceding claims, characterized in that comprising the steps of;- performing, by the receiver device (200), OFDM demodulation (210) to received OFDM signal;- performing, by the receiver device (200), equalization to demodulated signal for acquiring backscatter data and the data to be transmitted.

5. The method according any one of the preceding claims characterized in that comprising the steps of;- adding, by the transmitter device (100), cyclic prefix to the OFDM signal before transmission;- removing, by the receiver device (200), cyclic prefix from the OFDM signal after receiving.

6. An Orthogonal Frequency Division Multiplexing (OFDM) based backscatter communication system comprising at least a transmitter device (100) capable of transmitting signal to a receiver device (200) through a direct channel (410), at least the receiver device (200) and at least a backscatter device (300) which is capable of performing backscatter communication to receiver device (200) through a backscatter channel (430) using signal received from the transmitter device (100) through a forward channel (420) characterized in that; the transmitter comprising a pre-equalizer unit where the pre-equalizer unit is configured to perform partial equalization on the channel impulse response of the direct channel (410), representing multipath behavior in channel as taps, wherein the CIR is acquired by performing channel sounding on the direct channel (410) and estimating, such that partial equalization is performed by nullifying only some of the taps for leaving space for backscatter signals; an OFDM modulator (110) for generating OFDM signal, and a transmit antenna (140) for transmitting the signal.

7. The system according to claim 6, wherein the partial equalization unit is configured to partially equalize output of the OFDM modulator (110) in time domain.

8. The system according to claim 6, wherein the partial equalization unit is configured to partially equalize data in frequency domain before it is fed to OFDM OFDM (110).

Images