Method of integrating FMCW and OFDM signals with minimum interference for ISAC applications

Abstract

This invention presents a novel approach to waveform design for Integrated Sensing and Communication (ISAC) systems, seamlessly merging Orthogonal Frequency Division Multiplexing (OFDM) with Frequency Modulated Continuous Wave (FMCW) signals. Key innovations include precise FMCW waveform extraction using the diagonal elements of the Discrete Fourier Transform (DFT) matrix, interference mitigation achieved by strategic multiplication and division of diagonal elements, seamless embedding of sensing information through element-wise multiplication with Inverse Fourier Transform (IFFT) output The method culminates in a unified composite signal, amalgamating the FMCW signal with embedded sensing data and the original OFDM signal, offering an efficient solution for simultaneous communication and sensing in wireless applications.

Description

[0001] METHOD OF INTEGRATING FMCW AND OFDM SIGNALS WITH MINIMUM INTERFERENCE FOR ISAC APPLICATIONS

[0002] Technical Field:

[0003] This invention relates to a method of integrating FMCW and OFDM signals with minimum interference for ISAC applications that can be applicable in industry and utilized in ISAC scenarios.

[0004] State of The Art:

[0005] The upcoming generation of wireless networks are poised to introduce a myriad of applications, with a key feature being the simultaneous execution of wireless communication and environmental sensing. The integration of communication and sensing has become a prominent area of focus, gaining momentum since the successful deployment of 4G, which promised dual functionalities for subsequent generations [1], Many various techniques were explored, with a central theme being the coexistence of two waveforms one optimized for communication, such as Orthogonal Frequency Division Multiplexing (OFDM), and the other tailored for sensing, like Frequency Modulated Continuous Wave (FMCW). However, a significant challenge lies in achieving the coexistence of these two waveforms, aiming to minimize or eliminate interwaveform interference. Some patent and studies conducted for this purpose are given below.

[0006] Application numerated US2009079620A1 describes "OFDM Frequency Scanning Radar". A radar system is disclosed in this application, which comprises an orthogonal frequency division multiplexing (OFDM) modem and a frequency scanning antenna. In transmit, the OFDM modem modulates radar waveforms, and the frequency scanning antenna radiates the OFDM modulated radio frequency (RF) energy. In receive mode, the frequency scanning antenna captures the echoes and the OFDM modem demodulates the echoes. Directionality of the frequency scanning antenna is dependent upon RF carrier frequency. In other features, the radar system further comprises a transmit / receive (T / R) module that up-converts and amplifies the OFDM modulation, and outputs the amplified signal to the frequency scanning antenna. The T / R module amplifies and down-converts a received RF echo from the frequency scanning antenna and outputs the down-converted echo to the OFDM modem. A plurality of scanning angles is measured simultaneously.

[0007] This patent application discusses various ISAC techniques, including non-orthogonal superposition of OFDM and FMCW waveforms.

[0008] In [2], the strategy for achieving orthogonality between overlapped radar and communication waveforms involves aligning the radar waveform in the time domain. This ensures that it only occupies specific subcarriers in the frequency domain through the utilization of Fourier transform properties.

[0009] In contrast, [3] adopts a method where Frequency Modulated Continuous Wave (FMCW) and Orthogonal Frequency Division Multiplexing (OFDM) are non-orthogonally superimposed with varying power levels over a common bandwidth. While this technique offers potential advantages, it introduces heightened complexity at the receiver.

[0010] The coexistence of the two waveforms involves two primary techniques: orthogonal and non-orthogonal.

[0011] 1. In the non-orthogonal scenario, a power domain separation technique is employed, utilizing the same bandwidth. The complexity of the approach in [3] arises from the need to effectively handle interference and separate signals, particularly when both waveforms share the same resources but exhibit different power levels. This challenge is intricately connected with power domain separation, underscoring the difficulty of achieving signal segregation in a scenario where resources are shared, and power levels vary. The intricate nature of these requirements highlights the demanding task of ensuring optimal system performance amidst the complexities associated with both power domain separation and the coexistence of waveforms featuring diverse power characteristics.

[0012] 2. On the other hand, in the orthogonal case, the system's design plays a pivotal role. In [2], a notable disadvantage is the occurrence of inter-waveform interference, accompanied by an increase in complexity for signal separability. The drawback of the approach in [2] is further compounded by the short chirp duration and bandwidth, resulting in diminished sensing capabilities while attempting to mitigate interference from both waveforms.

[0013] References:

[0014] [1] F. Liu et al., "Integrated Sensing and Communications: Toward Dual -Functional Wireless Networks for 6G and Beyond," in IEEE Journal on Selected Areas in Communications, vol. 40, no. 6, pp. 1728-1767, June 2022, doi: 10.1109 / JSAC.2022.3156632.

[0015] [2] E. Memisoglu, M. M. Sahin and H. Arslan, "Orthogonal Coexistence of Overlapped Radar and Communication Waveforms," 2022 IEEE Wireless Communications and Networking Conference (WCNC), Austin, TX, USA, 2022, pp. 2190-2195, doi: 10.1109 / WCNC51071.2022.9771655.

[0016] [3] M. Mert §ahin and H. Arslan, "Multi-functional Coexistence of Radar-Sensing and Communication Waveforms," 2020 IEEE 92nd Vehicular Technology Conference (VTC2020-Fall), Victoria, BC, Canada, 2020, pp. 1-5, doi: 10.1109 / VTC2020- Fall49728.2020.9348582.

[0017] Description of The Invention:

[0018] This invention exploits the relationship between the DFT matrix elements and the discrete FMCW or chirp. Using this relationship, the proposed invention achieves low-complexity communication and sensing by slightly modifying the OFDM transmitter to generate OFDM+FMCW with minimum interference. The conventional joint sensing and communication techniques rely on coexisting the two waveforms by resource allocations or NOMA (Non Orthogonal Multiple Access) schemes, which results in degradation of spectral efficiency or high interference between the waveforms, respectively. The proposed invention doesn’t require any change in the structure of OFDM. The proposed invention can provide both communication and sensing without loss of data rate and with high range resolution.

[0019] The innovative transceiver design introduced in this invention signifies a substantial advancement beyond the methods prevalent in prior art. the advantages of this proposed invention are summarized as follows:

[0020] Minimized Interference in FMCW Generation from OFDM Signals: FMCW is generated directly from the diagonal elements of the Discrete Fourier Transform (DFT) matrix, ensuring the lowest possible interference. This approach restricts interference to only the neighboring samples, optimizing the integration of Frequency Modulated Continuous Wave (FMCW) with Orthogonal Frequency Division Multiplexing (OFDM) signals.

[0021] Backward Compatibility with Previous Cellular and Wi-Fi Generations: Leveraging a conventional OFDM waveform as a baseline, the integration of FMCW adds just a single step to the original design, ensuring seamless backward compatibility with earlier cellular and Wi-Fi generations. This implies a smooth coexistence with existing wireless infrastructure.

[0022] Minimal Increase in Signal Generation Complexity: The original system's complexity remains unaltered, with no additional intricacy introduced. The integration of FMCW seamlessly complements the original design, emphasizing a low-complexity approach to signal generation.

[0023] Streamlined Sensing Processing with Low Complexity: On the receiver side, a straightforward de-chirping process applied to the signal is sufficient for decoding sensing data. This simplicity in sensing processing underscores an efficient and uncomplicated approach to extracting meaningful information from the received signals.

[0024] The structural and characteristic features and all advantages of the method subject to the invention will be understood more clearly thanks to the figures given below and the detailed explanation written by referring to these figures, and therefore the evaluation should be made by taking these figures and detailed explanation into consideration.

[0025] Description of the Figures:

[0026] The invention will be described with reference to the accompanying figures, so that the features of the invention will be more clearly understood and appreciated, but the purpose of this is not to limit the invention to these certain regulations. On the contrary, it is intended to cover all alternatives, changes and equivalences that can be included in the area of the invention defined by the accompanying claims. The details shown should be understood that they are shown only for the purpose of describing the preferred embodiments of the present invention and are presented in order to provide the most convenient and easily understandable description of both the shaping of methods and the rules and conceptual features of the invention. In these drawings;

[0027] Figure 1 A graphical view of example that depicts the relationship between OFDM and FMCW (N=8).

[0028] Figure 2 A view of generalized diagram for the proposed system.

[0029] The figures to help understand the present invention are numbered as indicated in the attached image and are given below along with their names.

[0030] Disclosure of References: d. Unmodulated data

[0031] I. In-phase

[0032] Q. Quadrature-phase s. Data in time domain

[0033] S. Data in frequency domain

[0034] 100. Random Data Bits

[0035] 101. Serial to Parallel Conversion

[0036] 102. Data Mapping and Constellation Point Selection 103. Inverse Fourier Transform (IFFT)

[0037] 104. Diagonal Element Extraction and Frequency Response Generation

[0038] 105. Multiplication with (Ones) vector refer to equation (4)

[0039] 106. Embedding Sensing Information onto the OFDM Signal

[0040] 107. Signal Combination and Serialization

[0041] 108. Cyclic Prefix Addition and Transmission

[0042] Detail Description of The Invention:

[0043] Before describing the invention subject method, a disclose of some of the concepts mentioned in the description is given below:

[0044] FMCW (Frequency-Modulated Continuous Wave): A radar technique where the transmitted signal's frequency is continuously modulated over time, resulting in a chirp. This enables range resolution by measuring the time difference between the transmitted and received chirp signals.

[0045] Chirp: A signal whose frequency continuously changes over time. In this invention, the FMCW signal is a chirp.

[0046] OFDM (Orthogonal Frequency Division Multiplexing): A modulation technique that divides the available bandwidth into multiple subcarriers, each carrying a separate data stream. Orthogonality ensures that the subcarriers do not interfere with each other, allowing for high spectral efficiency.

[0047] DFT (Discrete Fourier Transform): A mathematical operation that converts a signal from the time domain to the frequency domain. In this invention, it is used to generate the FMCW signal by extracting the diagonal elements of the DFT matrix.

[0048] Diagonal Elements of the DFT Matrix: The diagonal elements of a matrix are those where the row index and column index are the same. In this invention, these elements are used to generate the FMCW signal.

[0049] Peak-to- Average Power Ratio (PAPR): The ratio of the peak power to the average power of a signal. A high PAPR can lead to signal distortion and problems in power amplifiers. Sensing Processing: The process of extracting information about the surrounding environment from the received signal. In this invention, the de-chirping process is used for sensing processing. De-chirping: The process of reversing the frequency modulation of a chirp signal to extract the range information.

[0050] Power Domain Separation: A technique used in ISAC systems to separate the communication and sensing signals based on their power levels.

[0051] Non-Orthogonal Superposition: A technique used in ISAC systems where the communication and sensing signals are superimposed on the same spectrum without being orthogonal.

[0052] 3GPP: The 3rd Generation Partnership Project, a global organization responsible for standardizing mobile telecommunications technologies.

[0053] The proposed OFDM chirp waveform is designed by generating a chirp like signal from the diagonal elements of the DFT matrix. The process steps of the invention are as follows:

[0054] OFDM Chirp generation:

[0055] In the conventional OFDM, the use of DFT is the key step to generate the orthogonal subcarriers. For a system with N subcarriers, the transmitter modulates data into N orthogonal subcarriers. The N-point DFT is applied to map the time domain sequence into the frequency domain.

[0056] Where Xkis the complex amplitude of k-th subcarrier, xnis the input signal in time domain, and N is the total number of subcarriers. Over one period the complex exponentials e7 71« for different values of k are orthogonal.

[0057] The signal in the time domain is as follows: s[n] = [s[0],s[l], ... ,s[A — 1]] The A -point DFT of s[n] is given by

[0058] The N * N DFT matrix F is defined as

[0059] To extract the diagonal elements, it is set m=n:

[0060] On the other hand, the discrete Chirp signal is given by: cn=eJn(an2) (2)

[0061] As seen from equation (1) and (2), the diagonal elements of the DFT matrix are indeed a discrete chirp (FMCW) which is depicted in Figure 1.

[0062] In the disclosed invention, this relationship is used to generate chirp along with OFDM directly from the IDFT matrix ensuring the lowest interference and the smoothest coexistence between the two waveforms. For that it was proposed the following modulator instead of IDFT at the transmitter:

[0063] Where s is the time domain signal, S denotes the data symbols in frequency domain and T is the proposed modulator given as follows: The transmitter in equation (3) can be further simplified and written in terms of OFDM transmitter so that there is no need to modify the conventional OFDM system as follows:

[0064] = FHS + C(-S + 1) (4)

[0065] Where C = diag(c) and l(n) = 1 is the ones vector.

[0066] Figure 2 provides an overall diagram of how the disclosed invention is generated.

[0067] Random Data Bits (100):

[0068] This step represents the initial input to the system, consisting of the binary information to be transmitted. These bits could be text, audio, video, or any other digital data format.

[0069] Serial to Parallel Conversion (101):

[0070] Here, the input data stream is divided into multiple parallel streams, typically corresponding to the number of subcarriers used in the OFDM system. This allows for parallel processing and transmission of the data across different frequency channels.

[0071] Data Mapping and Constellation Point Selection (102):

[0072] Each parallel data stream is mapped to a specific constellation point based on the chosen modulation scheme. This translates the binary data into complex symbols for each subcarrier, defining the amplitude and phase of the transmitted signal. Popular modulation schemes include QPSK, 16QAM, and 64QAM.

[0073] Inverse Fourier Transform (IFFT) (103):

[0074] The mapped subcarrier symbols are transformed from the frequency domain back to the time domain using the IFFT operation. This generates the individual OFDM symbols, which are then combined to form the overall OFDM signal.

[0075] Diagonal Element Extraction and Frequency Response Generation (104):

[0076] This is where the invention's key innovation comes into play. Instead of using the entire IFFT output, the system extracts the diagonal elements from the DFT matrix of a desired size. These elements, when plotted in the frequency domain, represent a discrete chirp signal, essentially the desired FMCW waveform.

[0077] Vector ones multiplication Streamlining the transmitter involves a multiplication operation on the diagonal elements of matrix C (denoted as 'C') by the inverse of our symbol set, summed with a vector of ones. This results in a simplified version of the transmitter, expressed in the form of a traditional OFDM transmitter. Importantly, this approach eliminates the necessity for modifications to the standard OFDM system. Please refer to Equation (4) for a detailed representation of this process.

[0078] Embedding Sensing Information onto the OFDM Signal (106):

[0079] The extracted diagonal elements are element-wise multiplied with the IFFT output (OFDM symbol). This essentially embeds the sensing information, encoded within the chirp signal, onto the OFDM signal carrying the communication data. Additionally, a complex exponential signal with a frequency chirp further modulates the multiplied elements, shaping the final FMCW waveform with embedded sensing capabilities.

[0080] Signal Combination and Serialization (107):

[0081] The generated FMCW signal with embedded sensing data is combined with the original OFDM signal, resulting in a single composite signal carrying both communication and sensing information. This combined signal is then converted back to a serial data stream for transmission.

[0082] Cyclic Prefix Addition and Transmission (108):

[0083] To combat channel impairments and symbol interferences, a cyclic prefix is added to the beginning of the combined signal. This prefix consists of a copied portion of the signal's ending, ensuring smoother transitions between symbols and improving signal robustness during transmission. Finally, the cyclic prefix-extended signal is transmitted over the wireless communication channel.

[0084] The foregoing descriptions of specific embodiments of the present technology have been presented for the purposes of illustration and description. They are not intended to be exhaustive or to limit the present technology to the precise forms disclosed, and obviously many modifications and variations are possible considering the above teaching. The embodiments were chosen and described to best explain the principles of the present technology and its practical software, to thereby enable others skilled in the art to best utilize the present technology and various embodiments with various modifications as are suited to the particular use contemplated. It is understood that various omissions and substitutions of equivalents are contemplated as circumstance may suggest or render expedient, but such are intended to cover the software or implementation without departing from the spirit or scope of the claims of the present technology.

[0085] In a case that no conflict occurs, the embodiments in the present disclosure and the features in the embodiments may be mutually combined. The foregoing descriptions are merely specific implementations of the present disclosure but are not intended to limit the protection scope of the present disclosure. Any variation or replacement readily figured out by a person skilled in the art within the technical scope disclosed in the present disclosure shall fail within the protection scope of the present disclosure. Therefore, the protection scope of the present disclosure shall be subject to the protection scope of the claims.

Claims

CLAIMS1- The invention relates to method of integrating FMCW and OFDM signals with minimum interference for ISAC applications, its feature is; i. input of digital data such as text, audio, video into the system in the form of random data bits (100), ii. dividing the input data stream entering the system into multiple parallel streams corresponding to the number of subcarriers used in the OFDM system, iii. mapping of each parallel data stream to a specific constellation point based on the chosen modulation scheme, iv. the mapping subcarrier symbols are transformed from the frequency domain back to the time domain using the inverse Fourier transform (IFFT) (103) operation, v. using the DFT matrix diagonal elements for extracting an FMCW waveform, vi. multiplying the diagonal elements with a vector of ones and dividing by the data symbols to have minimum interference between the FMCW formulated by the diagonal elements and the modulated data symbols which is the OFDM modulated data, vii. Multiplying the extracted diagonal elements by the IFFT output on an element- wise to embed the sensing information encoded within the chirp signal onto the OFDM signal carrying the communication data, viii. a complex exponential signal with frequency chirps, modulating the multiplied elements to shape the final FMCW waveform with embedded sensing capabilities, ix. the generating FMCW signal with embedded sensing data is combined with the original OFDM signal, resulting in a single composite signal carrying a communication and sensing information, x. including process steps of adding a cyclic prefix consisting of a copied portion of the signal's end at the beginning of the combined signal to combat channel impairments and symbol interference.2- The method according to claim 1, characterized in that the Diagonal elements of the Discrete Fourier Transform (DFT) matrix are utilized to extract an FMCW waveform.3- The method according to claim 1, characterized in that the diagonal elements are multiplied by a vector of ones and divided by the data symbols to minimize interference between the FMCW waveform and the modulated OFDM data (step (105), equation (4)).4- The method according to claim 1, characterized in that the extracted diagonal elements are multiplied elementwise by the IFFT output to embed sensing information within the chirp signal onto the OFDM signal carrying communication data.5- The method according to claim 1, characterized in that the generated FMCW signal with embedded sensing data is combined with the original OFDM signal, resulting in a single composite signal carrying both communication and sensing information.6- The method according to claim 4, characterized in that the overall OFDM transmitter structure remains unchanged, with the operation in step (105) facilitating this continuity.

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