A narrowband modulation and demodulation method and system suitable for cross-border magnetic induction communication

Abstract

The application provides a narrowband modulation and demodulation method and system suitable for cross-border magnetic induction communication, and belongs to the field of magnetic induction communication. ‑2 The application provides a minimum shift keying (QM-VMSK / 2) encoding modulation method based on quadrature modulation, which realizes parallel transmission through quadrature modulation to double the data rate under the condition of keeping the same bandwidth as the minimum shift keying (VMSK) encoding modulation signal; and a decoding method based on double-phase code detection, which has better anti-noise interference ability compared with the detection method based on zero-crossing detection and double-phase code pulse width discrimination, and has a signal-to-noise ratio gain of more than 2.5 dB when the bit error rate is 10 ‑2 The QM-VMSK / 2 modulation and demodulation technology can provide a narrowband modulation method for underwater magnetic induction communication, reduce the distortion of high-rate signals in the underwater dispersion channel, realize high-rate cross-border magnetic induction communication under the condition that the carrier frequency and the data rate have the same order of magnitude, and improve the communication range x data rate performance limit of magnetic induction communication.

Description

Technical Field

[0001] This invention belongs to the field of magnetic induction communication technology, specifically relating to a narrowband modulation and demodulation method and system suitable for cross-border magnetic induction communication. Background Technology

[0002] In recent years, research on underwater communication based on magnetic induction (MI) coupling has attracted attention. MI communication has unique advantages, but MI signals attenuate rapidly with increasing distance and frequency, making it impossible for MI communication to simultaneously meet the requirements of communication range and data rate. Therefore, most research on underwater magnetic induction communication focuses on techniques to improve the communication range × data rate performance.

[0003] Currently, research on modulation in underwater MI communication is limited, with most cases employing conventional modulation techniques such as Frequency Shift Keying (FSK) and Quadrature Amplitude Modulation (QAM). However, modulation applied to underwater MI communication differs slightly from conventional modulation, requiring adaptation to the propagation characteristics of the MI channel. It is well known that regardless of whether single-carrier or multi-carrier modulation is used, signals transmitted at higher data rates typically occupy a larger bandwidth. Seawater is a dispersive medium for electromagnetic waves, meaning different frequency components may experience varying amplitude attenuation and phase velocities, leading to broadband signal distortion. Therefore, for underwater MI communication, the modulated signal should occupy a narrower bandwidth while transmitting at a higher data rate to reduce signal distortion; simultaneously, a lower carrier frequency should be used to minimize signal attenuation.

[0004] Minimum Shift Keying (VMSK) coding modulation is an ultra-narrowband modulation method proposed by HR Walker. One of its significant features is that the main energy of the signal is concentrated in a very narrow frequency band, which makes it potentially suitable for cross-domain magnetic induction communication.

[0005] We studied VMSK-based modulation techniques to find a narrowband modulation technique suitable for cross-air-water interface (MI) communication, achieving a high data rate within a narrow bandwidth and at a low carrier frequency. We first investigated VMSK / 2 coded modulation and a decoding method based on zero-crossing detection, and then verified the feasibility of this modulation in cross-air-water MI communication through sea trials. Sea trial data processing results show that at a transmit magnetic moment of approximately 100 Am... 2 At a carrier frequency of 500Hz, cross-domain magnetic induction communication using VMSK / 2 coding modulation can achieve error-free transmission from the air to a depth of 9m underwater at a data rate of 100bps. However, its communication data rate and decoding reliability still have the potential for further improvement. Summary of the Invention

[0006] To address the aforementioned problems, this invention proposes a narrowband modulation and demodulation method and system suitable for cross-boundary magnetic induction communication. This method proposes an orthogonal modulation VMSK / 2 (QM-VMSK / 2) scheme, which improves the data rate by transmitting two VMSK / 2 signals in parallel. Simultaneously, a VMSK / 2 signal decoding method based on biphase code detection is proposed to improve the reliability of VMSK / 2 signal decoding under low signal-to-noise ratio (SNR) conditions, achieving greater water depth. Numerical analysis shows that the proposed modulation and decoding scheme can effectively improve the range × data rate performance of cross-boundary MI communication. It also features narrow signal bandwidth, high cross-boundary data rate, and strong noise resistance.

[0007] The technical solution adopted in this invention is as follows:

[0008] A narrowband modulation and demodulation method suitable for cross-border magnetic induction communication, the modulation and demodulation method comprising:

[0009] Step 1: Encoding and modulation. The transmitted data b(t) at the transmitting end is encoded and modulated using QM-VMSK / 2 to output the signal x. QM-VMSk / 2 (t), the signal is transmitted through the antenna;

[0010] Step 2: Demodulation and decoding. After the receiver performs QM-VMSK / 2 demodulation and decoding on the received signal r(t), it outputs an estimate of the transmitted signal b(t).

[0011] In step 1 of the above technical solution, the specific process of QM-VMSK / 2 coding and modulation is as follows:

[0012] The transmitted data b(t) is first converted from serial to parallel to become two sub-data streams, which are then encoded using VMSK / 2 to obtain the encoded output a. I (t) and a Q (t); then for a I (t) and a Q (t) After performing in-phase and quadrature modulation respectively, the signals are added together to obtain the modulated output signal x. QM-VMSk / 2 (t).

[0013] In step 1 of the above technical solution, the specific process of QM-VMSK / 2 demodulation and decoding is as follows:

[0014] The receiver multiplies the received signal r(t) with the local in-phase and quadrature carriers to obtain the in-phase and quadrature demodulated signals. The demodulated signals are then sequentially subjected to biphase code detection, VMSK / 2 decoding, and parallel-to-serial conversion before being added together to output an estimate of the transmitted signal b(t).

[0015] A narrowband modulation and demodulation system suitable for cross-border magnetic induction communication, the modulation and demodulation system includes a transmitter and a receiver.

[0016] The sending end includes,

[0017] The serial-to-parallel conversion module converts the input data b(t) into two sub-data streams;

[0018] The first VMSK / 2 encoder receives one of the sub-data streams, encodes it, and obtains the encoded output a. I (t);

[0019] The second VMSK / 2 encoder receives another sub-data stream, encodes it, and obtains the encoded output a. Q (t);

[0020] In-phase modulation module, for data a I (t) Perform in-phase modulation;

[0021] The quadrature modulation module modulates data a Q (t) Perform quadrature modulation;

[0022] The aggregation module adds the two data streams after in-phase and quadrature modulation to obtain the modulated output signal x. QM-VMSk / 2 (t);

[0023] The receiver multiplies the received signal r(t) with the local in-phase and quadrature carriers to obtain the in-phase and quadrature demodulated signals. The demodulated signals are then sequentially subjected to biphase code detection, VMSK / 2 decoding, and parallel-to-serial conversion before being added together to output an estimate of the transmitted signal b(t).

[0024] In the above technical solution, the receiving end also includes

[0025] The in-phase demodulation module performs in-phase demodulation on the received signal r(t);

[0026] The quadrature demodulation module performs quadrature demodulation on the received signal r(t);

[0027] The first biphase code detection module performs biphase code detection on the in-phase demodulated signal and outputs an estimated VMSk / 2 encoded signal.

[0028] The second biphase code detection module performs biphase code detection on the quadrature demodulated signal and outputs another estimated VMSk / 2 encoded signal;

[0029] The first VMSK / 2 decoder performs VMSK / 2 decoding on the signal detected by the first biphase code detection module;

[0030] The second VMSK / 2 decoder performs VMSK / 2 decoding on the signal detected by the second biphase code detection module;

[0031] The parallel-to-serial conversion module converts the signals decoded by the first VMSK / 2 decoder and the second VMSK / 2 decoder into parallel and then adds them together to output an estimate of the transmitted information b(t).

[0032] The beneficial effects of this invention are:

[0033] 1. This invention proposes a QM-VMSK / 2 coding and modulation method, which doubles the data rate through parallel transmission without increasing bandwidth. Furthermore, numerical analysis shows that QM-VMSK / 2 modulation can achieve high-speed cross-domain magnetic induction communication under conditions of narrow bandwidth and carrier frequency on the same order of magnitude as the data rate, thus improving the range × data rate performance of magnetic induction communication.

[0034] 2. This invention also proposes a decoding method based on biphase code detection, which has better noise interference resistance than detection methods based on zero-crossing detection and biphase code pulse width discrimination, with a bit error rate of 10. -2 At that time, it has a signal-to-noise ratio gain of more than 2.5dB compared to conventional detection methods. Attached Figure Description

[0035] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0036] Figure 1 This is a schematic diagram of the QM-VMSK / 2 coding and modulation principle in this invention;

[0037] Figure 2 This is a schematic diagram of the demodulation and decoding principle based on biphase code detection in this invention;

[0038] Figure 3 This is a block diagram of the sending end module;

[0039] Figure 4 This is a block diagram of the receiving end module;

[0040] Figure 5 It is the power spectrum of the modulated signal;

[0041] Figure 6 It is a graph showing the bit error rate of narrowband modulation; Detailed Implementation

[0042] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings.

[0043] This invention specifically provides a narrowband modulation and demodulation method suitable for magnetic induction communication across air-water interfaces, the modulation and demodulation method comprising:

[0044] Step 1: Encoding and modulation. The transmitted data b(t) at the transmitting end is encoded and modulated using QM-VMSK / 2 to output the signal x. -VMSK / 2 (t), the signal is transmitted through the antenna;

[0045] Step 2: Demodulation and decoding. After the receiver performs QM-VMSK / 2 demodulation and decoding on the received signal r(t), it outputs an estimate of the transmitted signal b(t).

[0046] VMSK / 2 encoding rules

[0047] VMSK coding modulation, proposed by HR Walker, is an ultra-narrowband modulation method similar to Manchester coding. It uses non-return-to-zero (NRZ) biphase code, with each bit occupying a fixed number of clock cycles according to its value. VMSK signals transmit information by utilizing the change in zero-crossing times with the transmitted data. The basic idea is to make the carrier waveform representing the symbols "0" and "1" have slight differences or jitter. If this slight jitter is controlled by the transmitted information, the spectrum used for transmitting information can be compressed to the maximum extent, producing a modulated signal without spectral spread, achieving a modulation effect with extremely narrow bandwidth.

[0048] The VMSK baseband signal is divided into two parts, so that each period contains two bits of information. The resulting encoding is called VMSK / 2 encoding.

[0049] VMSK / 2 encoding inverts the encoded signal only once per bit cycle, with the inversion timing determined by the bit value. Assuming each bit occupies M clock cycles, when M is odd or even, for a bit "1", the baseband encoded signal inverts after (M+1) / 2 or M / 2+1 clock cycles, respectively; for a bit "0", the baseband encoded signal inverts after (M-1) / 2 or M / 2 clock cycles.

[0050] Therefore, the encoding rules for VMSK / 2 of length M can be given as follows: (1) When two adjacent bits are “10”, the biphase code output by the encoder is inverted after M-1 clock cycles; (2) When two adjacent bits are “01”, the biphase code is inverted after M+1 clock cycles; (3) When two adjacent bits are “11” or “00”, the biphase code is inverted after M clock cycles.

[0051] The decoding rule of VMSK / 2 is as follows: when the signal is detected to be inverted after M-1 clock cycles, the decoder outputs bit "0"; when the signal is detected to be inverted after M+1 clock cycles, it outputs bit "1"; when the signal is detected to be inverted after M clock cycles, it outputs the same bit as the previous one.

[0052] As can be seen from the above introduction, the VMSK / 2 encoding and decoding process is not complicated. The difficulty lies in how to accurately detect the subtle differences in the zero-crossing point of the signal during decoding and obtain the correct information code.

[0053] To improve the data rate, we propose a quadrature modulation VMSK / 2 (QM-VMSK / 2) coding and modulation scheme. The specific process of QM-VMSK / 2 coding and modulation in this scheme is as follows:

[0054] The transmitted data b(t) is first converted from serial to parallel to become two sub-data streams, which are then encoded using VMSK / 2 to obtain the encoded output a. I (t) and a Q (t); then for a I (t) and a Q (t) After performing in-phase and quadrature modulation respectively, the signals are added together to obtain the output signal x. QM-VMSK / 2 (t).

[0055] By utilizing quadrature modulation to simultaneously carry two signals, parallel transmission of the two signals is achieved. This doubles the data rate within the same bandwidth. The modulation principle of QM-VMSK / 2 is as follows: Figure 1 As shown.

[0056] VMSK signals transmit information using changes in zero-crossing times. Therefore, their decoding typically employs zero-crossing detection and biphase code pulse width discrimination. Correct decoding is only possible if the subtle differences in zero-crossing times are accurately detected and converted into changes in biphase code pulse width. However, under low signal-to-noise ratio conditions, the zero-crossing times of the demodulated signal are easily affected by noise, impacting biphase code pulse width discrimination and leading to decoding errors.

[0057] A decoding method based on biphase code detection is proposed to improve the noise immunity of the decoding. This method first uses amplitude discrimination to detect the biphase code in each clock cycle, and then decodes the bits according to the number of clock cycles occupied by different bits in the encoding rule. Since the signal frequency and data rate of cross-domain magnetic induction communication are both low, when the sampling frequency is much higher than the clock frequency, there will be enough sampling points within one clock cycle for biphase code discrimination, and its noise immunity is stronger than that of detection at zero crossing points.

[0058] The specific process of QM-VMSK / 2 demodulation and decoding is as follows:

[0059] The received signal r(t) is multiplied by the local in-phase and quadrature carriers respectively to obtain the in-phase and quadrature demodulated signals. Then, after biphase code detection, VMSK / 2 decoding, parallel-to-serial conversion, and addition, the estimated transmitted information b(t) is output. Figure 2 This is a block diagram illustrating the demodulation and decoding principle of narrowband modulated signals.

[0060] In practical applications, a synchronization signal can be added before the signal to be sent to ensure synchronization of the received signal detection.

[0061] The present invention also provides a narrowband modulation and demodulation system suitable for cross-border magnetic induction communication, the modulation and demodulation system comprising a transmitter and a receiver.

[0062] like Figure 3 As shown, the sending end includes,

[0063] The serial-to-parallel conversion module converts the input data b(t) into two sub-data streams;

[0064] The first VMSK / 2 encoder receives one of the sub-data streams, encodes it, and obtains the encoded output a. I (t);

[0065] The second VMSK / 2 encoder receives another sub-data stream, encodes it, and obtains the encoded output a. Q (t);

[0066] In-phase modulation module, for data a I (t) Perform in-phase modulation;

[0067] The quadrature modulation module modulates data a Q (t) Perform quadrature modulation;

[0068] The aggregation module adds the two data streams after in-phase and quadrature modulation to obtain the output signal x. QM-VMSk / 2 (t).

[0069] The receiver multiplies the received signal r(t) with the local in-phase and quadrature carriers to obtain the in-phase and quadrature demodulated signals. The demodulated signals are then sequentially subjected to biphase code detection, VMSK / 2 decoding, and parallel-to-serial conversion before being summed to output an estimate of the transmitted signal b(t). For example... Figure 4 As shown, the receiving end includes

[0070] The in-phase demodulation module performs in-phase demodulation on the received signal r(t);

[0071] The quadrature demodulation module performs quadrature demodulation on the received signal r(t);

[0072] The first biphase code detection module performs biphase code detection on the in-phase demodulated signal and outputs an estimated VMSk / 2 encoded signal.

[0073] The second biphase code detection module performs biphase code detection on the quadrature demodulated signal and outputs another estimated VMSk / 2 encoded signal;

[0074] The first VMSK / 2 decoder performs VMSK / 2 decoding on the signal detected by the first biphase code detection module;

[0075] The second VMSK / 2 decoder performs VMSK / 2 decoding on the signal detected by the second biphase code detection module;

[0076] The parallel-to-serial conversion module converts the signals decoded by the first VMSK / 2 decoder and the second VMSK / 2 decoder into parallel and then adds them together to output an estimate of the transmitted information b(t).

[0077] 1. Numerical Analysis of Narrowband Modulation

[0078] Let the carrier frequency f c =500Hz, bit rate f b =100bps, sampling frequency f s =25kHz. During VSMK / 2 encoding, each bit occupies M = 4 clock cycles. The cross-domain MI channel can be modeled using an additive white Gaussian noise channel. Due to the narrow bandwidth of the VMSK / 2 modulated signal, it is assumed that all frequency components in the signal are subject to the same channel attenuation.

[0079] Figure 5 Power spectra of conventional phase-shift keying (PSK) and VMSK / 2 type narrowband modulation signals. Figure 3 As can be seen, the spectrum of QM-VMSK / 2 is very close to that of VMSK / 2, and its bandwidth is much smaller than that of the PSK signal. Therefore, modulation like QM-VMSK / 2 is well-suited for MI channels.

[0080] The bit error rate curve of the narrowband modulation obtained from the simulation is as follows: Figure 6 As shown, decoding methods using biphase code detection (BPCD) and zero-crossing detection (ZCD) are employed respectively.

[0081] From Figure 6 The following conclusions were drawn.

[0082] (1) When the given data rate f b and sampling frequency f s At this time, the carrier frequency f can be reduced by decreasing the number of sampling points per bit. c . Figure 6 f is given in c =500Hz and f c The bit error rate curve for narrowband modulation at 200Hz. As can be seen from the figure, using f...c The bit error rate at 200Hz is slightly higher than f. c The bit error rate at 500Hz is significant because reducing the number of sampling points affects the noise immunity of signal detection. However, the relatively small difference in bit error rate suggests that we can reduce the carrier frequency without significantly increasing the bit error rate, thereby reducing the transmission attenuation of the MI signal and achieving greater depth of penetration. The key to narrowband modulation allowing for lower carrier frequencies lies in VMSK / 2 encoding, which allows the coded signal to have consecutive "1" or "0" bits, thus tolerating a lower number of sampling points than a single bit symbol.

[0083] To achieve BER=10 -2 Using QM-VMSK / 2 modulation requires approximately 2 dB more signal-to-noise ratio than using VMSK / 2 modulation, but its data transmission rate is 2f. b =200bps, which is twice that of VMSK / 2. If VMSK / 2 modulation is to achieve a data rate of 200bps while maintaining the same number of sampling points within a single bit symbol, its carrier frequency must be increased to f. c =1000Hz. Calculated according to the magnetic field in a layered medium. [1] f c The magnetic field strength at 1000Hz is greater than f c At 500Hz, the attenuation increases by approximately 2.6 times. This means that compared to using VMSK / 2 modulation, at the same water depth, cross-domain communication using QM-VMSK / 2 modulation can achieve a 2x increase in data rate at the cost of 2dB more signal power; if the same data rate is maintained, cross-domain communication can reach even greater water depths.

[0084] (2) Compared to the zero-crossing detection method, the biphase code detection method is more effective at BER=10. -2 At the same time, a signal-to-noise ratio gain of approximately 3dB can be obtained. This indicates that the biphase code detection method has better noise immunity and can achieve a greater water penetration depth under the same signal power and BER conditions.

[0085] The contents not described in detail in this specification are existing technologies known to those skilled in the art.

Claims

1. A narrowband modulation method suitable for use in cross-border magnetic induction communication, characterized by, The modulation and demodulation method includes: Step 1: encoding modulation, sending data at the sending end b t ) output signal after QM-VMSK / 2 encoding modulation x QM-VMSk / 2 t sending the signal out through an antenna;​​ Step 2: Demodulation and decoding, the receiving end demodulates and decodes the received signal r t to output the transmitted signal b t the estimation of​​ The specific process of QM-VMSK / 2 coding modulation is as follows: Transmitting data b t Firstly, the data is converted into two sub-data streams after serial-parallel conversion, and then encoded by VMSK / 2 respectively to obtain the encoded output a I t a Q t a I t a Q t After the in-phase and quadrature modulation of and respectively, the modulated output signal is obtained x QM-VMSk / 2 t ​​​​​​​​​​ The specific process of QM-VMSK / 2 demodulation and decoding is as follows: The receiving end multiplies the received signal with local in-phase and quadrature carrier signals to obtain in-phase and quadrature demodulation signals r ( t ) and then adds the signals after double-phase code detection, VMSK / 2 decoding and parallel-serial conversion to output a transmission signal b ( t ) estimation 2. A narrowband modulation and demodulation system suitable for cross-border magnetic induction communication, the modulation and demodulation system comprising a transmitter and a receiver, characterized in that, The sending end includes, a serial-to-parallel conversion module converts the input transmission data b ( t ) into two sub-data streams; The first VMSK / 2 encoder receives one of the sub-data streams, encodes it, and obtains the encoded output. a I ( t ); a second VMSK / 2 encoder receives another sub-stream for encoding to obtain an encoded output a Q ( t ) a phase modulation module modulates the data a I t )​ a quadrature modulation module to quadrature modulate the data a Q ( t ) The collection module adds the two data after in-phase and quadrature modulation to obtain a modulated output signal x QM-VMSk / 2 ( t ) The receiving end multiplies the received signal with local in-phase and quadrature carrier signals to obtain in-phase and quadrature demodulation signals r ( t ) and then adds the signals after double-phase code detection, VMSK / 2 decoding and parallel-serial conversion to output a transmission signal b ( t ) estimation.

3. A narrowband modem system suitable for use in cross-border magnetic induction communication according to claim 2, characterized in that, The receiver also includes a phase demodulation module demodulates the received signal in phase r t )​ a quadrature demodulation module demodulates the received signal r t in quadrature​ The first biphase code detection module performs biphase code detection on the in-phase demodulated signal and outputs an estimated VMSk / 2 encoded signal. The second biphase code detection module performs biphase code detection on the quadrature demodulated signal and outputs another estimated VMSk / 2 encoded signal; The first VMSK / 2 decoder performs VMSK / 2 decoding on the signal detected by the first biphase code detection module; The second VMSK / 2 decoder performs VMSK / 2 decoding on the signal detected by the second biphase code detection module; The parallel-to-serial conversion module adds the signals decoded by the first VMSK / 2 decoder and the second VMSK / 2 decoder after parallel-to-serial conversion, and outputs the transmission information b ( t ) of the estimate.

Images