High-performance space-time coding method and system for four-antenna backscattering tag

Abstract

The invention belongs to the technical field of backscatter communication, and discloses a high-performance space-time coding method and system for a four-antenna backscatter tag, and the method and system are better in energy harvesting (EH) performance. Only half of the antennas in each time slot are in an activated transmission state, the other half of the antennas are kept idle (in a matched state), and all the antennas participate in transmission in the activated state of traditional orthogonal space-time coding. As the energy harvesting efficiency of the antenna in the idle state is higher than that of the antenna in the activated state, the energy harvesting of the code in a linear energy harvesting model and a nonlinear energy harvesting model is obviously superior to that of an orthogonal space-time code, the autonomous endurance of the passive tag is facilitated, and the core problem of insufficient energy of the backscattering tag is solved. The symbol error rate (SER) is better; and energy efficiency and transmission performance are both considered.

Description

Technical Field

[0001] This invention belongs to the field of backscatter communication technology, and particularly relates to a high-performance space-time coding method and system for four-antenna backscatter tags. Background Technology

[0002] The Internet of Things (IoT) is one of the core technologies for building intelligent interconnection in future sixth-generation mobile communication systems, aiming to achieve low-cost, low-power interconnection of massive numbers of devices. However, traditional IoT devices heavily rely on battery power, and battery replacement or charging faces significant challenges when devices are deployed in special environments such as inside electrical appliances, in radioactive areas, or in high-pressure pipelines. To address this issue, backscatter communication, as an emerging low-cost, low-energy green communication mode, has received increasing attention from academia and industry. Backscatter communication achieves data transmission by reflecting radio frequency signals in the environment (such as unmodulated carriers from carrier transmitters, TV tower signals, Bluetooth signals, etc.), eliminating the need for built-in batteries and featuring low cost and microwatt-level power consumption. It has become one of the core solutions for passive IoT devices and has broad application prospects in fields such as smart logistics and environmental monitoring. With the development of multiple-input multiple-output (MIMO) technology, multi-antenna backscatter tags can effectively alleviate the "double fading" problem of backscatter channels (the channel is composed of cascaded forward and reverse links, resulting in more severe signal attenuation) through spatial diversity and multiplexing gain, further improving communication reliability and transmission efficiency.

[0003] In MIMO backscatter communication, space-time block code (STBC) is one of the key technologies for effectively improving transmission reliability. By jointly designing the coding matrix in both time and space dimensions, STBC can utilize the diversity gain of multiple antennas to offset channel fading. Furthermore, since backscatter tags are battery-free devices, energy harvesting (EH) is also an important performance indicator that must be considered when designing backscatter STBCs. However, existing STBCs have the following limitations:

[0004] Insufficient energy harvesting (EH) performance: Backscatter tags need to harvest energy from the incident carrier to drive the circuitry, and the energy harvesting efficiency of the antenna in the idle state (matched state) is much higher than in the active state (when transmitting data). Traditional schemes (such as Orthogonal Space-Time Coding OSTBC) involve all antennas in transmission when active, resulting in low energy harvesting efficiency.

[0005] Symbol error rate (SER) performance is limited: Traditional orthogonal STBC (OSTBC) has a low code rate in four-antenna systems (e.g., the code rate of 4×4 OSTBC is 3 / 4), which leads to a decrease in bit error rate performance when using high-order modulation to compensate for the insufficient data rate.

[0006] Insufficient multi-antenna adaptability: Existing backscatter STBC solutions (such as low-complexity STBC and zero-placing STBC) are mainly designed for dual-antenna tags and cannot be directly extended to four-antenna systems. Four-antenna tags are in urgent need in future high-density IoT scenarios due to their higher diversity potential.

[0007] An implementation scheme similar to this invention (Alamouti coding for stacked rotating four-antenna backscattering tags) is as follows:

[0008] 1) Traditional Orthogonal Space-Time Block Code (OSTBC)

[0009] Technical principle: OSTBC was the earliest coding scheme applied to multi-antenna backscatter tags, achieving transmit diversity through an orthogonal coding matrix. Taking a four-antenna tag with a 4×4 OSTBC as an example, its coding matrix is ​​as follows:

[0010]

[0011] Where c1, c2, and c3 are symbols to be transmitted, and * indicates conjugate. All antennas continuously transmit data when active, with no idle time.

[0012] Disadvantages: Low energy harvesting efficiency (all antennas are active, with zero idle time); the four-antenna system has a code rate of only 3 / 4, resulting in poor SER performance at the same data rate.

[0013] Source: Boyer C, Roy S. Space time coding for backscatter RFID[J]. IEEE Transactions on Wireless Communications, 2013, 12(5): 2272-2280.

[0014] 2) Zero-interpolated empty time block code (for dual-antenna tags)

[0015] Technical principle: For dual-antenna tags, idle time slots are created by inserting zero elements (zero-padding) into the coding matrix. For example, the transmission of one antenna is turned off in some time slots, putting it into an idle state to harvest energy. At the same time, the diversity characteristics of the coding are maintained by designing the position of the zero elements, thus balancing energy harvesting and SER performance.

[0016] Disadvantages: It only supports dual-antenna tags and cannot be extended to four-antenna tags; zero padding will reduce the bit rate.

[0017] Source: He C, Luan H, Wang Z J. Zero-Padding Space-Time Block Code for Dual-Antenna Backscatter Tag[J]. IEEE Internet of Things Journal, 2025.

[0018] 3) Joint design of block-level unitary query (BUTQ) and modified OSTBC (mOSTBC)

[0019] Technical principle: By employing block-level unitary query signals (such as orthogonal waveforms) at the query end, combined with a modified OSTBC (mOSTBC) at the tag end, the linear decoding problem of traditional OSTBC in backscattering is solved. Its core is to utilize the orthogonality of the unitary matrix, enabling the receiver to linearly separate the signal reflected from the tag, thus reducing decoding complexity.

[0020] Disadvantages: Not optimized for energy harvesting (the tag antenna remains active).

[0021] Source: He C, Wang ZJ, Miao C, et al. Block-level unitary query: Enablingorthogonal-like space-time code with query diversity for MIMO backscatterRFID[J]. IEEE Transactions on Wireless Communications, 2016, 15(3): 1937-1949.

[0022] Based on the above analysis, the problems and shortcomings of the existing technology are as follows:

[0023] (1) Backscatter communication has become a key candidate technology for future Internet of Things (IoT) applications due to its low cost and low power consumption. However, the performance degradation caused by its dual-channel fading and passive (battery-free) communication mechanism limits its further application.

[0024] (2) In the prior art, the traditional OSTBC is the main reference scheme for four-antenna backscatter tags, but it has performance defects in energy harvesting and code rate; although ZPSTBC optimizes the performance of dual-antenna scenarios, it cannot be directly extended to four-antenna tags.

[0025] (3) Insufficient Energy Harvesting (EH) Performance: In traditional OSTBCs, when the four-antenna backscatter tag is activated, all antennas are in data transmission state (no idle time slot), while the energy harvesting efficiency (η) of the antennas in the idle state (matching state) is insufficient. 空闲 =1) much higher than the active state (η) 激活 =1-|Γ|), where |Γ| is the reflection coefficient). This structure results in a significantly lower energy harvest in both linear and nonlinear EH models compared to this invention: in the linear model, the energy harvest of the traditional OSTBC is 4(1-|Γ|)P. 输入 The present invention increases the energy harvest to 4P by using a rotating idle time slot mechanism (half of the antennas are active, and the other half are idle). 输入 -2|Γ|P 输入 Under the nonlinear model, simulations also verified that the EH capability of the traditional OSTBC is also inferior to the proposed scheme (half of the antennas are active and the other half are idle).

[0026] (4) Poor Symbol Error Rate (SER) Performance: Traditional OSTBCs have a low code rate in four-antenna systems (e.g., the code rate of a 4×4 OSTBC is 3 / 4), while the present invention is a full-rate structure (code rate of 1). At the same data rate, due to the code rate limitation, the SER performance of traditional OSTBCs is significantly worse than that of the present invention. Simulations show that at the same data rate (3bps), the SER performance of traditional OSTBCs is 5-6dB worse than that of the present invention. Summary of the Invention

[0027] To address the problems existing in the prior art, this invention provides a high-performance space-time coding method and system for four-antenna backscatter tags.

[0028] This invention is implemented as follows: A high-performance space-time coding method for four-antenna backscatter tags includes:

[0029] Step 1, Encoding Construction -- Spatial Expansion Based on Traditional Low-Dimensional Alamouti Encoding;

[0030] Step 2, Encoding Rotation -- Rotating Sleep Slot Mechanism;

[0031] Step 3, Decoding Scheme -- Signal Spatial Domain Block Linear Decoding.

[0032] Furthermore, the encoding construction is as follows:

[0033] Based on the classic Alamouti code Extending to a four-antenna spatial dimension through spatial domain expansion, the formula is:

[0034]

[0035] in, For spatial expansion vector, This is the Kronecker product (tensor product); after expansion, the proposed encoding matrix is ​​specifically represented as follows:

[0036]

[0037] c1 and c2 are symbols to be transmitted, and * indicates complex conjugation.

[0038] Furthermore, the rotating sleep time slot mechanism:

[0039] Introducing rotation matrix Its left multiplier C 扩展 The encoding matrix is ​​then obtained.

[0040]

[0041] Taking BPSK modulation as an example, the codebook space of the rotated coding matrix can be represented as follows:

[0042] As can be seen from the codebook space above, the tag antenna alternately enters an "active / idle" state in two time slots (the active state reflects signals to transmit data, and the idle state maximizes energy harvesting), that is: Time slot 1: Antennas 1 and 3 are active (transmitting signals), while antennas 2 and 4 are idle (harvesting energy); Time slot 2: Antennas 2 and 4 are active, while antennas 1 and 3 are idle. This alternating sleep mechanism ensures that only two antennas are active in each time slot, while the other two antennas are idle, significantly improving energy harvesting efficiency.

[0043] Furthermore, the linear decoding corresponding to the encoding:

[0044] The receiver recovers the original symbols c1 and c2 from the received signal through the following steps:

[0045] Received signal modeling:

[0046] The received signal of the nth receiving antenna (n = 1, 2, ..., N) in the two time slots is:

[0047]

[0048] in, h i For the forward link between the RF source and the i-th tag antenna, g i,n Let ω be the reverse link between the i-th tag antenna and the n-th receiving antenna. n,t For noise; the received signal from the nth receiving antenna is subjected to maximum ratio combining (MRC) over two consecutive time slots. The linear decoder for the nth receiving antenna can then be constructed as follows:

[0049] Therefore, for the entire communication system, the decoding process can be represented as follows: The decoder is specifically in the form of

[0050]

[0051] Ultimately, c1 and c2 are recovered through a decision, achieving low-complexity decoding.

[0052] Another object of the present invention is to provide a system for a high-performance space-time coding method for four-antenna backscatter tags, comprising:

[0053] The encoding construction module is used for encoding construction;

[0054] The rotation module is used to rotate the sleep time slot mechanism;

[0055] The decoding module is used to encode the corresponding linear decoder.

[0056] Another object of the present invention is to provide a computer device including a memory and a processor, the memory storing a computer program that, when executed by the processor, causes the processor to perform the steps of the high-performance space-time coding method for four-antenna backscatter tags.

[0057] Another object of the present invention is to provide a computer-readable storage medium storing a computer program that, when executed by a processor, causes the processor to perform the steps of the high-performance space-time coding method for four-antenna backscatter tags.

[0058] Another objective of this invention is to provide an information data processing terminal for implementing the high-performance space-time coding system for four-antenna backscatter tags.

[0059] Based on the above technical solutions and the technical problems solved, the advantages and positive effects of the technical solution to be protected by this invention are as follows:

[0060] First, this invention addresses the problem of insufficient energy harvesting (EH) performance of traditional orthogonal space-time coding in four-antenna backscatter tags (poor performance in both linear and nonlinear models). It proposes a rotated stacked Alamouti coding and a corresponding linear decoding method. Compared to traditional orthogonal space-time coding, the proposed coding can be considered as spatial domain expansion followed by matrix rotation of traditional low-dimensional orthogonal space-time coding. Spatial domain expansion allows the proposed coding to fully exploit diversity gain while achieving full-rate transmission (code rate 1). Matrix rotation introduces a rotating sleep time slot mechanism to improve energy harvesting efficiency.

[0061] This invention is specifically designed for four-antenna tag scenarios. It utilizes the statistical characteristics of cascaded channels and a time-slot rotation sleep mechanism to address the aforementioned limitations by expanding and rotating the spatial domain of the classic Alamouti coding.

[0062] The purpose of this invention is to provide a space-time coding scheme suitable for four-antenna backscatter tags by comprehensively considering the passive mechanism of backscatter tags and the dual fading characteristics of backscatter channels. Specifically, it includes: improving energy harvesting performance: by designing alternating sleep time slots (half of the antennas are active for transmission and the other half are idle for energy harvesting), the energy harvesting performance of backscatter tags is improved to meet the self-powering requirements of passive IoT devices.

[0063] Optimize symbol error rate performance: Adopt a full diversity full rate coding structure (code rate 1) to reduce SER at the same data rate and improve communication reliability.

[0064] Ultimately, this achieves synergistic optimization of the energy harvesting and data transmission performance of the four-antenna backscatter tag, making it more suitable for the low power consumption and high reliability requirements of passive IoT devices.

[0065] The stacked rotating Alamouti coding proposed in this invention improves energy harvesting (EH) efficiency by constructing a rotating sleep time slot mechanism, so that only half of the antennas in each time slot are active to transmit signals while the other half remains idle.

[0066] The full diversity full rate code structure proposed in this invention significantly improves the system symbol error rate (SER) performance compared with traditional orthogonal space-time block codes at the same data rate.

[0067] The low-complexity linear decoding scheme simplifies the processing of the received signal by decoding it in blocks in the spatial domain, thus avoiding the high computational complexity of maximum likelihood decoding.

[0068] Compared with the closest existing technology (traditional 4×4 orthogonal space-time block code OSTBC), the advantages of this invention are mainly reflected in the following aspects:

[0069] 1. Superior Energy Harvesting (EH) Performance: The proposed coding scheme constructs a rotating sleep time slot mechanism, ensuring that only half of the antennas are in active transmission mode in each time slot, while the other half remains idle (matched state). In contrast, traditional orthogonal space-time coding involves all antennas participating in transmission in the active state. Since the energy harvesting efficiency of the idle antennas is higher than that in the active state, the proposed coding scheme significantly outperforms orthogonal space-time coding in both linear and nonlinear energy harvesting models, which is more conducive to the autonomous endurance of passive tags and solves the core problem of insufficient energy in backscattered tags.

[0070] 2. Superior Symbol Error Rate (SER): The proposed code features a full diversity, full rate code structure, while the traditional orthogonal space-time coding rate is 3 / 4. At the same data rate, the proposed code offers better bit error rate performance due to its full rate characteristic. Compared with the traditional orthogonal space-time coding, its SER performance is improved by 5-6 dB, solving the problem of bit error rate degradation at higher data rates in traditional schemes.

[0071] 3. Balancing Energy Efficiency and Transmission Performance: The proposed coding optimizes both energy harvesting and system bit error rate, while traditional orthogonal space-time coding struggles to balance these two aspects. The proposed coding design precisely addresses the requirement of "low energy consumption and high-performance transmission" in backscatter communication, making it more suitable for passive IoT device scenarios.

[0072] Second, this invention is the first technology to combine a passive backscatter communication mechanism with the spatiotemporal coding design of a four-antenna backscatter tag. The invention simultaneously optimizes energy harvesting and system bit error rate, responding to the requirement of "low power consumption and high-performance transmission" in backscatter communication, and providing an effective technical solution and approach for high-performance transmission and energy self-sufficiency in passive devices. Attached Figure Description

[0073] Figure 1 This is a flowchart of a high-performance space-time coding method for four-antenna backscatter tags provided in an embodiment of the present invention.

[0074] Figure 2 This is a structural block diagram of a high-performance space-time coding method for four-antenna backscatter tags provided in an embodiment of the present invention.

[0075] Figure 3 This is a model diagram of a 1x4xN backscattering system provided in an embodiment of the present invention.

[0076] Figure 4 This is a schematic diagram of the circuit implementation of the proposed encoding scheme and the traditional encoding scheme on a 4-antenna tag provided in the embodiments of the present invention.

[0077] Figure 5 This is a performance comparison chart of the proposed coding method provided in this embodiment of the invention and the traditional orthogonal space-time coding method under linear energy harvesting models and nonlinear energy harvesting models.

[0078] Figure 6 This is a comparison chart of the bit error rate performance of the proposed coding method provided in the embodiments of the present invention and the traditional orthogonal space-time coding under different numbers of receiving antennas at the same data rate. Detailed Implementation

[0079] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0080] In existing backscatter communication systems, with the surge in the number of IoT terminals, the space-time modulation capabilities of single-antenna or dual-antenna tags are no longer sufficient to meet the high-performance transmission requirements under low-power conditions. Traditional methods rely on directly increasing the transmit power or increasing the complexity of the receiver to compensate for insufficient link performance, but this significantly increases energy consumption and violates the design principles of passive or low-power tags. Therefore, the industry urgently needs a coding mechanism that, under energy constraints, can improve link reliability while also considering energy harvesting efficiency, in order to support the stable operation of tag nodes in large-scale IoT application scenarios.

[0081] Against this backdrop, the proposed four-antenna space-time coding method maps the classical Alamouti code to a four-antenna backscattering tag after spatial domain expansion. Although this method breaks the orthogonality of the coding structure, due to the cascading characteristics of the backscattering communication channel, the expanded code can still fully utilize the spatial diversity gain of multiple antennas, achieving the same diversity gain as orthogonal space-time coding. This construction process directly solves the problem that existing four-antenna tag space-time orthogonal codebooks cannot be transmitted at full rate, avoiding the increased bit error rate caused by using high-order modulation to compensate for insufficient data rate. In industrial practice, this is beneficial for ensuring a transmission rate with high reliability.

[0082] On the other hand, this method employs a rotating sleep time slot mechanism, ensuring that only two antennas are in reflective activation mode within any given time slot, while the other two enter energy harvesting mode. This design not only maintains an effective signal transmission path but also allows the tag to accumulate sufficient energy during its operational cycle for long-term operation, thus overcoming the problem of excessive reliance on external power supply in existing technologies, which limits tag lifespan and coverage. This mechanism demonstrates significant advantages in industrial applications requiring long-term deployment, such as wireless sensor networks, warehousing and logistics, and smart healthcare.

[0083] At the receiver, this scheme establishes a received signal model based on dual-channel fading of the forward and reverse links, and constructs a linear decoder through maximum ratio combining, effectively offsetting the effects of multipath fading and link asymmetry. Unlike traditional algorithms that rely on iterative detection or complex matrix inversion, this decoding method has low computational complexity and can be implemented in real time on embedded receiving platforms, which is particularly crucial for the industrial deployment of large-scale, low-cost readers.

[0084] From a system perspective, the coding construction module, rotation module, and decoding module work together to extend the lifespan of the backscatter tag while ensuring communication reliability. Especially under low-order modulation such as BPSK, the introduction of the rotation matrix further expands the codebook space and enhances the Euclidean distance between codewords, thereby improving anti-interference and error performance. In industrial practice, this advantage can be directly translated into stable communication in complex indoor environments, reducing data loss caused by link failures.

[0085] This technology provides a scalable solution for backscatter communication within the existing industry chain through the organic integration of "encoding, energy management, and low-complexity decoding." It breaks away from the traditional path of relying on hardware enhancement, achieving both high performance and low power consumption through the synergistic optimization of encoding theory and energy harvesting mechanisms. This not only directly promotes the design of existing IoT nodes but also lays the methodological foundation for the industrialization of future high-density tag networks and battery-free sensing nodes.

[0086] like Figure 1 As shown in the figure, a high-performance space-time coding method for four-antenna backscattering tags provided by an embodiment of the present invention includes the following steps:

[0087] S101, Encoding Construction -- Spatial Expansion Based on Traditional Low-Dimensional Encoding;

[0088] S102, Encoding Rotation - Alternating Sleep Time Slot Mechanism;

[0089] S103, Decoding Scheme -- Spatial Block Linear Decoding.

[0090] The working principle of this method is first reflected in the coding construction stage. Traditional Alamouti coding can achieve perfect space-time orthogonality in dual-antenna scenarios, but it cannot be directly applied to four-antenna backscatter tags. Therefore, this invention extends the two-dimensional orthogonal structure to four-dimensional space through tensor product, enabling the original symbols to form a strict algebraic orthogonal relationship between local antennas. The cascaded nature of the backscatter channel allows backscatter space-time block codes (STBCs) to escape the stringent requirement of overall coding orthogonality; that is, even if the coding is not orthogonal overall, its local orthogonality can still achieve diversity performance comparable to orthogonal space-time coding. This enables the receiver to achieve stable transmission in low signal-to-noise ratio environments, laying the theoretical foundation for reliable communication of backscatter tags in low-power links.

[0091] During the encoding rotation phase, this method introduces a rotating sleep time slot mechanism. Specifically, in two consecutive transmission time slots, the four antennas are divided into alternating active and idle groups. The active antennas perform reflection modulation, while the idle antennas enter energy harvesting mode. Through the mapping of the rotation matrix, the coded words are rearranged between the antennas and time slots, ensuring that each symbol is distributed with equal probability between the active and sleep states of different antennas. This mechanism significantly improves the tag's energy self-sufficiency and avoids the energy depletion problem caused by continuous reflection in existing technologies.

[0092] In the decoding phase, the receiver constructs a linear decoder based on spatial domain segmentation. This decoder first jointly models the received signals from two consecutive time slots, incorporating the cascaded fading of the forward and reverse links into the channel matrix. Subsequently, a maximum ratio combining strategy is used to weighted superimpose the segmented signals, thereby maximizing the received signal-to-noise ratio. Finally, a linear decision unit is used to demodulate the recovered symbols with low complexity. Since this encoding is based on the classic Alamouti coding, the decoding process can be completed without complex matrix inversion or iterative optimization, ensuring the real-time performance and scalability of the hardware implementation and providing a feasible path for the deployment of large-scale IoT readers.

[0093] The encoding structure provided in this embodiment of the invention:

[0094] Based on the classic Alamouti code Extending to a four-antenna spatial dimension through spatial domain expansion, the formula is:

[0095]

[0096] in, For spatial expansion vector, This is the Kronecker product (tensor product); after expansion, the proposed encoding matrix is ​​specifically represented as follows:

[0097]

[0098] c1 and c2 are symbols to be transmitted, and * indicates complex conjugation.

[0099] The rotating sleep time slot mechanism provided in this embodiment of the invention:

[0100] Introducing rotation matrix Its left multiplier C 扩展 The encoding matrix is ​​then obtained.

[0101]

[0102] Taking BPSK modulation as an example, the codebook space of the rotated coding matrix can be represented as follows:

[0103] As can be seen from the codebook space above, the tag antenna alternately enters an "active / idle" state in two time slots (the active state reflects signals to transmit data, and the idle state maximizes energy harvesting), that is: Time slot 1: Antennas 1 and 3 are active (transmitting signals), while antennas 2 and 4 are idle (harvesting energy); Time slot 2: Antennas 2 and 4 are active, while antennas 1 and 3 are idle. This alternating sleep mechanism ensures that only two antennas are active in each time slot, while the other two antennas are idle, significantly improving energy harvesting efficiency.

[0104] The linear decoding corresponding to the encoding provided in this embodiment of the invention:

[0105] The receiver recovers the original symbols c1 and c2 from the received signal through the following steps:

[0106] Received signal modeling:

[0107] The received signal of the nth receiving antenna (n = 1, 2, ..., N) in the two time slots is:

[0108]

[0109] in, (h i For the forward link between the RF source and the i-th tag antenna, g i,n (where ω is the reverse link between the i-th tag antenna and the n-th receiving antenna) n,t (for noise); the received signal from the nth receiving antenna is subjected to maximum ratio combining (MRC) over two consecutive time slots. The linear decoder for the nth receiving antenna can then be constructed as follows: Therefore, for the entire communication system, the decoding process can be represented as follows: The decoder is specifically in the form of

[0110]

[0111] Ultimately, c1 and c2 are recovered through a decision, achieving low-complexity decoding.

[0112] like Figure 2 As shown, an embodiment of the present invention provides a high-performance space-time coding system for four-antenna backscatter tags, comprising:

[0113] The encoding construction module is used for encoding construction;

[0114] The rotation module is used to rotate the sleep time slot mechanism;

[0115] The decoding module is used to encode the corresponding linear decoder.

[0116] The working principle of this system is first reflected in the coding construction module. Traditional dual-antenna backscatter tags can usually only achieve low-dimensional space-time coding, which suffers from high bit error rate in multipath fading environments. This embodiment of the invention introduces a spatial domain extension method based on Alamouti orthogonal codes into the coding construction module, extending the two-dimensional orthogonal structure to four-dimensional space, and constructing locally orthogonal coding.

[0117] After the coding matrix is ​​generated, the rotation module is responsible for the dynamic allocation of antenna activation and idle states. This module uses a left-multiplication of the rotation matrix to ensure that the coded words are alternately distributed across different antenna groups in two time slots. Specifically, in the first time slot, the first and third antennas are activated, while the second and fourth antennas remain idle to perform energy harvesting; in the second time slot, the second and fourth antennas reflect the signal, and the first and third antennas return to an idle state. Through this mechanism, the system achieves a coordinated balance between communication and energy.

[0118] The core design principle of the rotation module lies in ensuring the integrity of the codebook structure after rotation mapping. Since the energy supply of backscatter tags is highly dependent on an external radio frequency source, continuous full antenna activation leads to rapid energy consumption. This module addresses this by using an alternating sleep mode, allowing the tag to periodically replenish its energy reserves while completing signal modulation, thus solving the bottleneck of long-term stable operation in existing technologies. In application scenarios, this mechanism enables tags to maintain communication latency several times longer even without batteries.

[0119] The decoding module is another key part of the system, and its working principle is based on a linear decoding structure constructed using maximum ratio combining (MRC). During the modeling process at the receiver, both the forward link from the RF source to the tag antenna and the reverse link from the tag antenna to the receiving antenna are considered, forming a cascaded fading channel matrix. The decoding module processes the received signal in blocks over two consecutive time slots, maximizing the received signal-to-noise ratio using the combining gain. Because the encoding maintains local orthogonality, the decoding process does not require complex matrix inversion; symbol recovery can be completed simply through linear weighting.

[0120] Furthermore, the decoding module employs a low-complexity hardware architecture, avoiding the computational overhead of multiple iterations required in traditional space-time decoding schemes. This is particularly important for the industrial deployment of large-scale IoT receivers, as it ensures near-optimal detection performance even with limited computing resources. Simultaneously, the decision-making process in linear decoding recovers the original symbol using the minimum Euclidean distance criterion, further enhancing the system's robustness under low signal-to-noise ratio conditions.

[0121] This invention, through the high-dimensional design of the encoding construction module, the energy harvesting mechanism of the rotation module, and the low-complexity linear detection of the decoding module, forms a complete high-performance space-time coding system. This system not only improves the reliability of backscatter communication but also significantly enhances the self-sufficiency of tags under energy-constrained conditions, meeting the practical needs of low-power, high-reliability, and long-life communication nodes in smart logistics, passive sensing, and large-scale IoT environments.

[0122] Specific implementation of the present invention:

[0123] System model (corresponding) Figure 3 (1×4×N backscatter channel model)

[0124] This invention relates to a communication system (denoted as a 1×4×N system) consisting of a single-antenna carrier transmitter, a four-antenna backscatter tag, and an N-antenna receiver. The specific structure and signal flow are as follows:

[0125] 1. Components:

[0126] Carrier transmitter: A single antenna that transmits an unmodulated radio frequency carrier signal (such as a continuous sine wave) as the energy source and signal carrier of the tag.

[0127] Four-antenna backscatter tag: Equipped with four antennas, it embeds information into the incident unmodulated carrier and reflects it to the receiver, while simultaneously harvesting energy from the incident carrier to drive internal circuitry.

[0128] Receiver: N antennas (N≥1) receive and decode the modulated signal reflected by the tag.

[0129] 2. Signal transmission process:

[0130] The signal from the carrier transmitter reaches the four-antenna tag via the forward link (transmitter → tag). The tag embeds data into the incident signal through encoding, and then transmits it to the receiver via the reverse link (tag → receiver). The received signal matrix can be represented as:

[0131]

[0132] Where s is the transmitter signal, h T Let C be the forward link channel vector (1×4), C be the coding matrix, G be the reverse link channel matrix (4×N), W be additive white Gaussian noise, and ° be the Hadamard product (element-wise multiplication).

[0133] II. Proposed Coding Scheme (Core Innovation)

[0134] This invention improves energy harvesting efficiency and bit error rate performance by spatially expanding the classic Alamouti coding and then rotating the resulting code to construct a rotating sleep time slot mechanism, as detailed below:

[0135] 1. Encoding Construction:

[0136] Based on the classic Alamouti code After spatial domain expansion, the formula is:

[0137]

[0138] in, For spatial expansion vector, This is the Kronecker product (tensor product); after expansion, the proposed encoding matrix is ​​specifically represented as follows:

[0139]

[0140] c1 and c2 are symbols to be transmitted, and * indicates complex conjugation.

[0141] 2. Core Mechanism: Rotating Sleep Slot Mechanism ( Figure 4 )

[0142] Introducing rotation matrix Its left multiplier C 扩展 The encoding matrix is ​​then obtained.

[0143]

[0144] Taking BPSK modulation as an example, the codebook space of the rotated coding matrix can be represented as follows:

[0145] As can be seen from the codebook space above, the tag antenna alternately enters an "active / idle" state in two time slots (the active state reflects signals to transmit data, and the idle state maximizes energy harvesting), that is: Time slot 1: Antennas 1 and 3 are active (transmitting signals), while antennas 2 and 4 are idle (harvesting energy); Time slot 2: Antennas 2 and 4 are active, while antennas 1 and 3 are idle. This alternating sleep mechanism ensures that only two antennas are active in each time slot, while the other two antennas are idle, significantly improving energy harvesting efficiency.

[0146] Its working principle is as follows Figure 4 As shown.

[0147] 3. The linear decoding scheme corresponding to the proposed encoding.

[0148] The receiver recovers the original symbols c1 and c2 from the received signal through the following steps:

[0149] Received signal modeling:

[0150] The received signal of the nth receiving antenna (n = 1, 2, ..., N) in the two time slots is:

[0151]

[0152] in, (h i For the forward link between the RF source and the i-th tag antenna, g i,n (where ω is the reverse link between the i-th tag antenna and the n-th receiving antenna) n,t (for noise); the received signal from the nth receiving antenna is subjected to maximum ratio combining (MRC) over two consecutive time slots. The linear decoder for the nth receiving antenna can then be constructed as follows: Therefore, for the entire communication system, the decoding process can be represented as follows: The decoder is specifically in the form of

[0153]

[0154] Ultimately, c1 and c2 are recovered through a decision, achieving low-complexity decoding.

[0155] III. Key Performance Optimization Mechanism

[0156] 1. Energy Harvesting (EH) Optimization: Rotating Sleep Slots

[0157] In traditional orthogonal space-time coding, all four antennas are in transmission mode (no idle time) during activation, resulting in an energy gain of [missing information]. (|Γ| is the reflection coefficient, P) 入射 (Incident energy), and the proposed encoding uses a rotating sleep time slot mechanism, in which only 2 antennas are active (transmission) in each time slot, and the other 2 antennas are idle (matched state, EH efficiency η) 空闲 =1), the energy harvest is Compared to traditional OSTBC, energy harvest is increased by 2|Γ|P 入射 >0, maintaining its advantage even under nonlinear EH models (including nonlinear components such as diodes), as shown in the performance comparison results. Figure 5 As shown.

[0158] 2. Improved Bit Error Rate Performance: Traditional orthogonal space-time coding, limited by its orthogonal algebraic structure, can achieve a code rate of 3 / 4. The proposed coding scheme, however, is derived from Alamouti coding through spatial domain extension, resulting in a code rate of 1. At the same data rate, i.e., when traditional Alamouti coding uses higher-order modulation, the proposed coding scheme exhibits superior bit error rate performance. Performance comparison results are shown below. Figure 6 As shown.

[0159] Figure 5 The performance comparison between the proposed scheme and traditional orthogonal space-time block codes is presented under linear and nonlinear energy harvesting models. For linear energy harvesting models (with energy harvesting efficiencies of 0.2 and 0.4), the passive device using the proposed coding scheme can harvest more energy compared to the traditional orthogonal space-time coding scheme. For nonlinear energy harvesting models, the proposed coding scheme still exhibits superior energy harvesting capability. These results demonstrate that the proposed technical solution is more suitable for energy-constrained passive devices.

[0160] Figure 6 This paper compares the bit error rate performance of the proposed scheme with that of traditional orthogonal space-time coding at the same data rate (3 bps), considering 1x4x1, 1x4x2, and 1x4x3 channels, and assuming that the channels follow quasi-static flat Rayleigh fading. Figure 6 As can be seen, the bit error rate curve of the proposed code is parallel to that of the traditional OSTBC under the same channel configuration, indicating that the proposed code has the same diversity performance as the traditional OSTBC. Furthermore, due to the full-rate structure of the proposed code, it exhibits a 5-6 dB performance improvement in bit error rate compared to the traditional OSTBC at the same data rate.

[0161] The above description is merely a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any modifications, equivalent substitutions, and improvements made by those skilled in the art within the scope of the technology disclosed in the present invention, and within the spirit and principles of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A high-performance space-time coding method for four-antenna backscattering tags, characterized in that, Includes the following steps: Step 1: Construct the encoding for the symbols to be transmitted; Step 2: Energy harvesting and data transmission are achieved based on the antenna's rotating sleep time slot mechanism; Step 3: Recover the symbol to be transmitted using a linear decoding method.

2. The high-performance space-time coding method for four-antenna backscattering tags as described in claim 1, characterized in that, The encoding structure is based on Alamouti codes and forms a four-antenna encoding matrix through spatial domain expansion. The elements of the encoding matrix consist of the symbols to be transmitted and their complex conjugates.

3. The high-performance space-time coding method for four-antenna backscattering tags as described in claim 1, characterized in that, The alternating sleep time slot mechanism is as follows: in two time slots, two antennas are alternately kept in an active state while the other two antennas are kept in an idle state. The active state is used for signal reflection and transmission, and the idle state is used for energy harvesting.

4. The high-performance space-time coding method for four-antenna backscattering tags as described in claim 3, characterized in that, In the first time slot, the first and third antennas are active, while the second and fourth antennas are idle; in the second time slot, the second and fourth antennas are active, while the first and third antennas are idle.

5. The high-performance space-time coding method for four-antenna backscattering tags as described in claim 1, characterized in that, The linear decoding includes the following steps: performing maximum ratio combining on the received signal over two consecutive time slots to construct a linear decoder to recover the original symbol.

6. A high-performance space-time coding method for four-antenna backscattering tags as described in claim 5, characterized in that, The received signal is composed of the forward link between the radio frequency source and the tag antenna, the reverse link between the tag antenna and the receiving antenna, and noise.

7. A high-performance space-time coding system for four-antenna backscatter tags, characterized in that, include: The coding construction module is used to generate a four-antenna coding matrix based on the symbols to be transmitted. The rotation module is used to implement the antenna's rotation sleep time slot mechanism; A decoding module is used to recover the symbol to be transmitted through linear decoding.

8. The high-performance space-time coding system for four-antenna backscatter tags as described in claim 7, characterized in that, The encoding construction module is used to perform spatial domain expansion of Alamouti codes.

9. The high-performance space-time coding system for four-antenna backscatter tags as described in claim 7, characterized in that, The rotation module controls that only two antennas are active in each time slot, while the other two antennas are idle.

10. The high-performance space-time coding system for four-antenna backscatter tags as described in claim 7, characterized in that, The decoding module uses the maximum ratio combining method to decode and decide the received signals from two consecutive time slots.

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