Method for eliminating inter-channel interference of magnetic induction MIMO (Multiple Input Multiple Output) system based on zero-forcing precoding

Abstract

The invention provides a method for eliminating interference between channels of a wireless magnetic induction MIMO (Multiple Input Multiple Output) system based on zero-forcing precoding. The method provided by the invention comprises the following steps: firstly, constructing a wireless magnetic induction MIMO communication system, and secondly, carrying out channel estimation on the transmission process of the wireless magnetic induction MIMO communication system to obtain a magnetic induction channel matrix; and performing zero-forcing precoding operation processing on the obtained magnetic induction channel matrix to eliminate mutual inductance interference generated by coupling of different receiving and transmitting coils. According to the method, mutual inductance interference generated by an induced magnetic field between different channels can be eliminated. And when the sending power is greater than 1, the data transmission rate is greatly improved compared with the mode of not adopting the zero-forcing precoding. When the sending power reaches 3, the data transmission rate can be increased by about 11%.

Description

Technical Field

[0001] The present invention belongs to the field of wireless communications, and mainly relates to a wireless magnetic induction communication transmission process, and specifically is a method for eliminating interference between channels of a magnetic induction MIMO system based on zero-forcing precoding. Background Art

[0002] As a new wireless communication technology, wireless magnetic induction communication has the advantages of stable channels and wide coverage, and has a wide range of application scenarios in underground and underwater communications.

[0003] In the wireless magnetic induction MIMO communication system, it is necessary to establish a system model of wireless magnetic induction MIMO communication, which includes: a serial-to-parallel / parallel-to-serial conversion module, a precoding module and a modulation and demodulation module connected in sequence at the transmitting end, a modulation and demodulation module and a serial-to-parallel / parallel-to-serial conversion module connected in sequence at the receiving end, and a transceiver coil pair (transmitting coil and receiving coil) is arranged between the two modulation and demodulation modules. When multiple transmitting coils at the transmitting end send information at the same time, the generated transmitting magnetic field will interfere with each other at the receiving end, which will reduce the transmission rate of effective data. The above communication system will encounter the problem of crosstalk between coils in actual transmission, resulting in a decrease in system performance.

[0004] In recent years, researchers have proposed some methods to eliminate the crosstalk problem between wireless magnetic induction communication coils. In the scheme of an underground magnetic induction wireless communication device with application number [CN201720390767.9], it is proposed that the transceiver antenna adopts a spherical structure, which is composed of 3 identical conductor coils. The coils are independent of each other and perpendicular to each other. This device overcomes the crosstalk problem between coils in the magnetic induction communication system, but the model places a maximum of three antennas at both ends of the transceiver, and has high requirements for the placement of the coils at both ends of the transceiver, which makes it difficult to apply in practice. In the document with application number [CN202310741126.3], a "MISO magnetic communication transmission method based on precoding within the transmitting coil array" is disclosed. This method analyzes the interference source within the MISO magnetic induction communication transmitting coil array, estimates the transmitting coil impedance matrix, and uses the zero-forcing precoding matrix operation method to reduce the mutual inductance impedance between different transmitting coils in the impedance matrix to zero, thereby eliminating the interference within the transmitting coil array. This method adopts a coaxial placement structure of the transmitting and receiving coils to avoid the problem of large differences in receiving power of different coils at the receiving end caused by vertical cross-polarization of the coils. However, this method only eliminates the mutual inductance interference between the transmitting coils, and there is still a crosstalk problem during the transmission process. In addition, the MISO system has only one coil for reception, and the system transmission rate is low, which is difficult to meet the needs of high-speed data transmission. Summary of the invention

[0005] In order to overcome the problem in the prior art that the system transmission rate is low and it is difficult to meet the high-speed data transmission requirements, the present invention provides a method for eliminating inter-channel interference in a magnetic induction MIMO system based on zero-forcing precoding.

[0006] In order to achieve the purpose of the present invention, the technical solution proposed by the present invention is as follows: a method for eliminating inter-channel interference in a magnetic induction MIMO system based on zero-forcing precoding, comprising the following steps:

[0007] Step 1: Construction of wireless magnetic induction MIMO communication system:

[0008] The precoding module is a zero-forcing precoding module, and the transceiver coil magnetic induction channel estimation module is arranged at the input side of the zero-forcing precoding module and the output side of the receiving coil.

[0009] Step 2: The transmitting end sends a preamble code to the transceiver coil magnetic induction channel estimation module to obtain a channel estimation matrix, which is expressed as:

[0010]

[0011] Where ω is the angular frequency of the input baseband modulation signal x(t), For the transmitting coil T n With receiving coil R m of mutual induction.

[0012] Step 3: Perform zero-forcing interference elimination on the channel estimation matrix:

[0013] The zero-forcing interference adopts the zero-forcing matrix in the zero-forcing precoding algorithm in the zero-forcing precoding processing module:

[0014]

[0015] is the magnetic induction channel matrix The pseudo-inverse matrix of for The complex conjugate transposed matrix of .

[0016] The channel matrix after the zero-forcing precoding algorithm eliminates mutual inductance interference is as follows:

[0017]

[0018] The zero-forcing precoding algorithm eliminates the crosstalk between different transmission channels, and the receiving coil only receives the signal from the transmitting coil in the same channel.

[0019] Compared with the existing method, the beneficial effects of the present invention are:

[0020] The present invention takes wireless magnetic induction MIMO communication as a system model, proposes a method for eliminating interference between channels of a wireless magnetic induction MIMO system based on zero-forcing precoding, performs channel estimation on the transmission process, and uses zero-forcing precoding to perform matrix operation processing on the obtained magnetic induction channel, thereby eliminating the mutual induction interference generated by the induced magnetic field between different channels during the transmission process, and improving the useful data transmission rate of the wireless magnetic induction MIMO communication system.

[0021] (1) The present invention combines MIMO technology with wireless magnetic induction, and compared with the MISO wireless magnetic induction communication system, there are multiple magnetic induction transmission paths between multiple groups of transceiver coils in the present invention, which improves the reliability of communication and increases the overall data transmission rate.

[0022] (2) The present invention fully considers the transmission process of the actual wireless magnetic induction MIMO communication system, estimates the magnetic induction communication channel, and obtains a channel estimation matrix. Since the signal received by the receiving coil of the wireless magnetic induction MISO system is the superposition of all transmitting coils, there is no need to perform channel estimation on the magnetic induction communication channel. The channel estimation matrix of the present invention clearly describes the crosstalk between multiple channels. The obtained magnetic induction channel estimation matrix provides the necessary premise for eliminating the interference of non-corresponding transceiver coils, clearly describes the directional coupling relationship of the induced magnetic field of the corresponding transceiver coils, and can identify interference from different transmitting coils, thereby optimizing the directionality of the signal, improving the transmission rate of useful information, and enhancing the reliability of the communication system.

[0023] (3) The present invention introduces zero-forcing precoding technology into the wireless magnetic induction MIMO communication system and eliminates the interference terms in the obtained wireless magnetic induction channel estimation matrix. Therefore, this method eliminates the mutual inductance interference of the induced magnetic field generated by the coupling between different channels during the transmission process, and improves the system transmission performance. The simulation results show that when the transmission power is greater than 1mW, the transmission rate is significantly superior to that of the MIMO magnetic induction communication without zero-forcing precoding processing, especially when the transmission power reaches 3mW, the data transmission rate is increased by about 11%. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 This is a block diagram of the principle of precoding MIMO magnetic induction communication during transmission;

[0025] Figure 2 This is a diagram of the application scenario of MIMO magnetic induction communication;

[0026] Figure 3 Schematic diagram of the MIMO magnetic induction communication channel estimation process;

[0027] Figure 4 This is a simulation diagram of the wireless magnetic induction MIMO communication transmission rate based on the elimination of mutual interference between different channels. DETAILED DESCRIPTION

[0028] The following is a clear and complete description of the technical solution of the present invention in combination with the specific implementation methods and illustrations of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0029] The design idea of ​​the present invention is: the system includes multiple transceiver coils, which are coaxial and placed in parallel. The system uses electromagnetic coupling between transceiver coils to achieve wireless transmission of information. Since the system of the present invention has multiple transceiver coils, there are multiple transmission channels during the transmission process, and there will be crosstalk problems between different channels. Therefore, the method first estimates the magnetic induction transmission channel, and uses a zero-forcing precoding algorithm to process the obtained magnetic induction channel estimation matrix, and optimizes the channel matrix to a unit matrix, so that the signal obtained by the receiving coil only comes from the corresponding transmitting coil, thereby achieving the purpose of eliminating the crosstalk caused by the induced magnetic field between different transceiver coils during the transmission process.

[0030] See also Figure 1 In the wireless magnetic induction MIMO communication system, the transmitted signal I(t) is converted from parallel to serial and then the magnetic induction channel is estimated to obtain the magnetic induction channel matrix between the transceiver coils. The magnetic induction channel matrix is ​​subjected to zero-forcing precoding processing, so that the mutual inductance interference between different coils in the magnetic induction channel matrix of the transceiver coil is zero, thereby suppressing the mutual inductance interference between different channels in the transmission process. The signal x(t) after the modulation module is input to the transmitting coil array. At the receiving end, the receiving coil generates an induced voltage according to the change of the magnetic field, and after the demodulation module and serial-to-parallel conversion, the signal is received.

[0031] The present invention provides a method for eliminating interference between MIMO magnetic induction communication channels based on zero-forcing precoding, which specifically comprises the following steps:

[0032] Step 1: Construction of wireless magnetic induction MIMO communication system:

[0033] like Figure 1 As shown, the transmitting end includes a serial-parallel / parallel-serial conversion module, a precoding module and a modulation and demodulation module connected in sequence, and the receiving end includes a modulation and demodulation module and a serial-parallel / parallel-serial conversion module connected in sequence, and a transceiver coil pair (a transmitting coil and a receiving coil) is arranged between the two modulation and demodulation modules. The precoding module is a zero-forcing precoding module, and the transceiver coil magnetic induction channel estimation module is arranged at the input side of the zero-forcing precoding module and the output side of the receiving coil.

[0034] like Figure 2As shown, at the transmitting end, each transmitting coil sends the same modulated signal, and the receiving end is composed of the same number of receiving coils. The received signal is received after the demodulation module and serial-to-parallel conversion. T =N R =N c , radius r T =r R = r,N T For each transmitting coil turns, N R For each receiving coil turns, r T is the radius of the transmitting coil, r R is the radius of the receiving coil. The transmitting and receiving coils are coaxial and placed in parallel, the transmitting coil and the receiving coil are connected in parallel, and the distance between adjacent coils at the transmitting and receiving ends is a (a≥2r).

[0035] Interference analysis between different channels: When the transmitting and receiving coils are parallel but not on the same axis, the mutual inductance between the coils induced by the induced magnetic field This leads to mutual interference and mutual inductance between signals in different channels. The calculation formula is:

[0036]

[0037] Where μ is the magnetic permeability of the medium, d is the straight-line distance between the centers of the two coils, and θ (n,m) For the transmitting coil T n With receiving coil R m The angle between the radial direction and the line connecting the center of the coil, G is the additional loss factor caused by eddy current.

[0038] Step 2: The transmitter sends a preamble code to the transceiver coil magnetic induction channel estimation module to obtain a channel estimation matrix:

[0039] From step 1, we can see that the receiving coil R m Received from different transmitting coils T n Mutual inductance between coils induced by induced magnetic field is the interference source of the magnetic induction channel matrix. Figure 3 As shown, at the transmitting end, each transmitting coil T n ,n={1,2,…,N} respectively send a preamble code, that is, input a modulated alternating current signal, and measure the mutual inductance impedance of N receiving coils at the receiving end. Feed the measured mutual inductance impedance value back to the sending end to obtain the channel estimation matrix The channel estimation matrix is ​​expressed as:

[0040]

[0041] Where ω is the angular frequency of the input baseband modulation signal x(t), For the transmitting coil T n With receiving coil R m The mutual inductance impedance.

[0042] Mutual inductance between coils induced by induced magnetic field This leads to mutual interference and mutual inductance between signals in different channels. Calculation formula:

[0043]

[0044] Where μ is the magnetic permeability of the medium, d is the straight-line distance between the centers of the two coils, and θ (n,m) For the transmitting coil T n With receiving coil R m The angle between the radial direction and the line connecting the center of the coil, G is the additional loss factor caused by eddy current.

[0045] Step 3: Perform zero-forcing interference elimination on the channel estimation matrix:

[0046] When multiple transmitting coils at the transmitting end send information simultaneously, the generated transmitting magnetic fields will cause mutual crosstalk at the receiving end. This step uses a zero-forcing precoding processing module to perform zero-forcing precoding calculations on the magnetic induction channel estimation matrix obtained in step 102 to eliminate mutual inductance interference caused by the induced magnetic field between different channels, thereby improving the transmission performance of the wireless magnetic induction MIMO communication system.

[0047] In order to eliminate interference, the zero-forcing precoding algorithm makes is the identity matrix, and the zero-forcing matrix is:

[0048]

[0049] is the magnetic induction channel matrix The pseudo-inverse matrix of for The complex conjugate transposed matrix of .

[0050] This design performs zero-forcing precoding on the obtained magnetic induction communication channel matrix. Therefore, each channel in the wireless magnetic induction MIMO communication system is independent of each other. There is no induced magnetic field generated by coupling between different channels. Each receiving coil can only receive the signal from the corresponding transmitting coil. The channel state of the zero-forcing precoding system can be expressed as a diagonal matrix Description, the original channel estimation matrix The mutual inductance interference value between different channels caused by the induced magnetic field is reduced to zero, thereby eliminating the interference between different channels during the communication transmission process of the wireless magnetic induction MIMO system.

[0051] A method for eliminating interference between MIMO magnetic induction communication channels based on zero-forcing precoding provided by the present invention is applied to a wireless magnetic induction MIMO communication system, and the receiving and sending process is as follows:

[0052] Step 1: Wireless magnetic induction MIMO communication system signal transmission:

[0053] In the wireless magnetic induction MIMO communication system of the present invention, the receiving and transmitting coil arrays are composed of multiple identical metal coils and the number of coils at the receiving end is the same as the number of coils at the transmitting end. The receiving and transmitting coils are coaxial and placed in parallel. The data sent at the transmitting end is modulated after parallel-to-serial conversion and sent out simultaneously through multiple transmitting coils. After modulation, the transmission signal of each coil at the transmitting end is:

[0054] x(t)=I(t)cos(2πf c t)(5)

[0055] Where I(t) is the source of information transmission, f c is the baseband modulation frequency.

[0056] Step 2: Wireless magnetic induction MIMO communication system signal reception:

[0057] At the receiving end, multiple receiving coils receive signals simultaneously. The received signals are demodulated and converted from serial to parallel to obtain the correct received data. The changing magnetic field induces electromotive force and current in the receiving coil. The induced voltage generated by each receiving coil is:

[0058]

[0059] Among them, h ij , i∈[1,N], j∈[1,N] is the channel gain of magnetic induction communication, expressed as

[0060]

[0061] Where α is the transmitting coil T n The angle between the line connecting the center of the receiving coil and the normal line, d is the plane distance between the transmitting and receiving coils, is the skin depth, σ is the dielectric constant of the medium. The noise of the magnetic induction communication system is n(t). The main source of this noise is the thermal effect of the resistor device. The noise power is expressed as

[0062] N t =k B Tf c ×1000(8)

[0063] k B is the Boltzmann constant and T is the Kelvin temperature.

[0064] After eliminating the mutual inductance interference between different channels, the receiving signal-to-noise ratio of the wireless magnetic induction MIMO communication system is:

[0065]

[0066] Information transmission rate

[0067] C=Wlog 2 (1+SNR)(10)

[0068] W is the system bandwidth.

[0069] The present invention adopts ASK modulation, and the coil parameters and simulation parameters are shown in the following table:

[0070] Table 1 Simulation parameter setting table

[0071]

[0072]

[0073] The simulation results are as follows Figure 4 As shown in the figure, when the transmission power is greater than 1mW, compared with the MIMO magnetic induction communication without mutual induction interference elimination between different channels, as the transmission power increases, the system transmission rate advantage after zero forcing processing becomes increasingly obvious, increasing by about 6% to 11%. When the transmission power reaches 3mW, the data transmission rate can be increased to about 152bit / s. This fully reflects that the transmission rate performance of MIMO magnetic induction communication has been significantly improved after adopting the method proposed in the present invention.

[0074] The above is an explanation of the specific implementation of the present invention, rather than a limitation of the present invention. Those skilled in the relevant technical field can also make various equivalent technical solutions without departing from the scope of the present invention, so all equivalent technical solutions should be included in the patent protection scope of the present invention.

Claims

1. A method for eliminating inter-channel interference in a magnetic induction MIMO system based on zero-forcing precoding, comprising the following steps: Step 1: Construction of wireless magnetic induction MIMO communication system: The precoding module is a zero-forcing precoding module, and the transceiver coil magnetic induction channel estimation module is arranged at the input side of the zero-forcing precoding module and the output side of the receiving coil; Step 2: The transmitting end sends a preamble code to the transceiver coil magnetic induction channel estimation module to obtain a channel estimation matrix, which is expressed as: Where ω is the angular frequency of the input baseband modulation signal x(t), For the transmitting coil T n With receiving coil R m mutual induction; Step 3: Perform zero-forcing interference elimination on the channel estimation matrix: The zero-forcing interference adopts the zero-forcing matrix in the zero-forcing precoding algorithm in the zero-forcing precoding processing module: W ZF =H + ==H H (HH H ) -1 H + is the pseudo-inverse matrix of the magnetic induction channel matrix H, H H is the complex conjugate transposed matrix of H; The channel matrix after the zero-forcing precoding algorithm eliminates mutual inductance interference is as follows: The zero-forcing precoding algorithm eliminates the crosstalk between different transmission channels, and the receiving coil only receives the signal from the transmitting coil in the same channel.

Images