CN116112099A - Narrow-band modulation and demodulation method and system suitable for cross-boundary magnetic induction communication - Google Patents

Abstract

The invention provides a narrow-band modulation and demodulation method and a system suitable for cross-boundary magnetic induction communication, which belong to the field of magnetic induction communication, and provide a quadrature modulation-based minimum shift keying (QM-VMSK/2) code modulation method, wherein the data rate is improved by two times by quadrature modulation under the condition of keeping the same bandwidth with a minimum shift keying (VMSK) code modulation signal; the decoding method based on the double-phase code detection is provided, compared with the detection method based on zero crossing point detection and double-phase code pulse width discrimination, the decoding method has better anti-noise interference capability, and the bit error rate is 10 ^‑2 There is a signal to noise ratio gain of about 2.5dB or more. The QM-VMSK/2 modulation and demodulation technology can provide a narrow-band modulation method for underwater magnetic induction communication, and reduce distortion of high-rate signals in underwater dispersion channels; can realize the cross-boundary magnetic induction communication with higher speed under the condition that the carrier frequency and the data rate have the same order of magnitudeCommunication range x data rate performance limit for magnetic induction communication.

Description

Narrow-band modulation and demodulation method and system suitable for cross-boundary magnetic induction communication

Technical Field

The invention belongs to the technical field of magnetic induction communication, and particularly relates to a narrow-band modulation-demodulation method and system suitable for cross-boundary magnetic induction communication.

Background

In recent years, research on underwater communication based on Magnetic Induction (MI) coupling has been attracting attention. MI communication has unique advantages, but the MI signal decays rapidly with increasing distance and frequency, so that MI communication cannot meet the requirements of communication range and data rate at the same time. Therefore, underwater magnetic induction communication research is mostly put on technology for improving communication range×data rate performance.

Currently, there is little research on modulation in MI communication, and conventional modulation techniques such as Frequency Shift Keying (FSK), quadrature Amplitude Modulation (QAM), and the like are used in most cases. The modulation applied in underwater MI communication is slightly different from the conventional modulation and needs to be adapted to the propagation characteristics of the MI channel. It is well known that signals transmitted at higher rates typically occupy greater bandwidth, whether single carrier or multi-carrier modulation is employed. While seawater medium is a dispersive medium for electromagnetic waves, different frequency components may suffer from different amplitude attenuation and phase velocity, which may cause broadband signal distortion. Thus, for underwater MI communications, the modulated signal should occupy a narrower bandwidth while being transmitted at a higher data rate to reduce signal distortion; while using a lower carrier frequency to reduce signal attenuation.

Minimum shift keying (VMSK, veryMinimumShiftKeying) code modulation is an ultra-narrow band modulation method proposed by h.r.walker, one of the significant features of which is that the main energy of the signal is concentrated in a very narrow frequency band, which makes it potentially suitable for use in cross-domain magnetic induction communications.

The modulation technology based on VMSK coding is researched, and the purpose is to find a narrow-band modulation technology suitable for cross-air-water interface (cross-boundary for short) MI communication, and can realize higher data rate with lower carrier frequency in narrower bandwidth. Firstly, a decoding method of VMSK/2 coded modulation and zero crossing point detection discrimination is studied, and feasibility of the modulation in cross-boundary MI communication is verified through sea trial. Sea test data processing results show that in transmittingMagnetic moment is about 100Am ² When the carrier frequency is 500Hz, cross-domain magnetic induction communication adopting VMSK/2 code modulation can realize error-free transmission from air to underwater 9m with the data rate of 100 bps. But its communication data rate and decoding reliability have the potential to be further improved.

Disclosure of Invention

Aiming at the problems, the invention provides a narrow-band modulation and demodulation method and a narrow-band modulation and demodulation system suitable for cross-boundary magnetic induction communication, and the method provides a quadrature modulation VMSK/2 (QM-VMSK/2) modulation scheme, and improves the data rate through parallel transmission of two paths of VMSK/2 signals. Meanwhile, a VMSK/2 signal decoding method based on dual-phase code detection is also provided, so that the reliability of decoding of the VMSK/2 signal under the condition of low signal-to-noise ratio (SNR) is improved, and a larger water depth is achieved. Numerical analysis shows that the proposed modulation and decoding scheme can effectively improve range x data rate performance for cross-range MI communications. Meanwhile, the method has the characteristics of narrow signal bandwidth, high data rate of cross-boundary transmission and strong noise immunity.

The invention adopts the technical scheme that:

a narrowband modem method suitable for use in cross-border magnetic induction communications, the modem method comprising:

step 1: coded modulation, the transmission data b (t) of the transmitting end is subjected to QM-VMSK/2 coded modulation to output signal x _QM-VMSk/2 (t) transmitting the signal through the antenna;

step 2: and (3) demodulating and decoding, and outputting the estimation of the transmitted signal b (t) after the receiving end performs QM-VMSK/2 demodulating and decoding on the received signal r (t).

In the step 1 of the above technical scheme, the specific process of the QM-VMSK/2 code modulation is as follows:

the transmitted data b (t) is firstly converted into two sub-data streams after serial-parallel conversion, and VMSK/2 coding is respectively carried out to obtain a coding output a _I (t) and a _Q (t); then to a _I (t) and a _Q (t) respectively performing in-phase modulation and quadrature modulation, and adding to obtain a modulated output signal x _QM-VMSk/2 (t)。

In the step 1 of the above technical scheme, the specific process of QM-VMSK/2 demodulation decoding is:

the receiving end multiplies the received signal r (t) with the local in-phase and quadrature carriers to obtain in-phase and quadrature demodulation signals, and the demodulation signals are added after two-phase code detection, VMSK/2 decoding and parallel-serial conversion in sequence to output estimation of the transmitted signal b (t).

A narrow-band modem system suitable for cross-boundary magnetic induction communication comprises a transmitting end and a receiving end,

the transmitting end comprises a transmitting end and a receiving end,

the serial-parallel conversion module converts input transmission data b (t) into two sub-data streams;

a first VMSK/2 encoder for receiving one of the sub-data streams and encoding to obtain an encoded output a _I (t)；

A second VMSK/2 encoder for receiving another sub-data stream and encoding to obtain an encoded output a _Q (t)；

In-phase modulation module for data a _I (t) performing in-phase modulation;

quadrature modulation module for data a _Q (t) quadrature modulating;

the aggregation module is used for adding the two paths of data subjected to in-phase modulation and quadrature modulation to obtain a modulation output signal x _QM-VMSk/2 (t)；

The receiving end multiplies the received signal r (t) with the local in-phase and quadrature carriers to obtain in-phase and quadrature demodulation signals, and the demodulation signals are added after two-phase code detection, VMSK/2 decoding and parallel-serial conversion in sequence to output estimation of the transmitted signal b (t).

In the above technical solution, the receiving end further includes

The in-phase demodulation module carries out in-phase demodulation on the received signal r (t);

a quadrature demodulation module for performing quadrature demodulation on the received signal r (t);

the first biphase code detection module is used for carrying out biphase code detection on the in-phase demodulation signal and outputting an estimated VMSk/2 coding signal;

the second double-phase code detection module is used for carrying out double-phase code detection on the orthogonal demodulation signal and outputting another path of estimated VMSk/2 coding signal;

a first VMSK/2 decoder for performing VMSK/2 decoding on the signal detected by the first two-phase code detection module;

a second VMSK/2 decoder for performing VMSK/2 decoding on the signal detected by the second two-phase code detection module;

and the parallel-serial conversion module is used for carrying out parallel-serial conversion on the signals respectively decoded by the first VMSK/2 decoder and the second VMSK/2 decoder, adding the signals, and outputting estimation of the transmission information b (t).

The invention has the beneficial effects that:

1. the invention provides a QM-VMSK/2 coding modulation method, which improves the data rate by two times through parallel transmission under the condition of not increasing the bandwidth. And numerical analysis shows that QM-VMSK/2 modulation can realize higher-speed cross-boundary magnetic induction communication under the conditions of narrow bandwidth and the same order of magnitude of carrier frequency and data rate, and the range x data rate performance of magnetic induction communication is improved.

2. The invention also provides a decoding method based on the double-phase code detection, which has better anti-noise interference capability compared with the detection method based on zero crossing point detection and double-phase code pulse width discrimination, and has a bit error rate of 10 ^-2 And compared with the conventional detection method, the signal to noise ratio gain of more than about 2.5dB is achieved.

Drawings

In order to more clearly illustrate the embodiments of the invention or the technical solutions in the prior art, the drawings that are required in the embodiments or the description of the prior art will be briefly described, it being obvious that the drawings in the following description are only some embodiments of the invention, and that other drawings may be obtained according to these drawings without inventive effort for a person skilled in the art.

FIG. 1 is a schematic diagram of QM-VMSK/2 code modulation according to the present invention;

FIG. 2 is a schematic diagram of demodulation decoding based on dual-phase code detection in the present invention;

FIG. 3 is a block diagram of a sender;

FIG. 4 is a block diagram of a receiver;

FIG. 5 is a power spectrum of a modulated signal;

FIG. 6 is a graph of the error rate for narrowband modulation;

Detailed Description

In order to make the objects, technical solutions and advantages of the embodiments of the present invention more clear, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention.

The invention particularly provides a narrow-band modulation-demodulation method suitable for magnetic induction communication across an air-water interface, which comprises the following steps:

step 1: coded modulation, the transmission data b (t) of the transmitting end is subjected to QM-VMSK/2 coded modulation to output signal x _-VMSK/2 (t) transmitting the signal through the antenna;

step 2: and (3) demodulating and decoding, and outputting the estimation of the transmitted signal b (t) after the receiving end performs QM-VMSK/2 demodulating and decoding on the received signal r (t).

VMSK/2 coding rules

VMSK code modulation the ultra-narrow band modulation method proposed by h.r.walker is a digital baseband modulation like manchester code, employing non-return-to-zero code bi-phase codes, each bit occupying a fixed number of clock cycles according to the bit value. The VMSK signal uses the change of zero crossing point moment along with the transmitted data to transfer information, the basic idea is to try to make the carrier wave waveforms representing the symbols 0 and l have tiny differences or jitters, if the tiny jitters of the waveforms are controlled by the transmitted information, the frequency spectrum used for transmitting information can be compressed to the maximum extent, a modulation signal without frequency spectrum expansion is generated, and a modulation effect with extremely narrow bandwidth is achieved.

Each bit period in the VMSK baseband signal is divided into two parts so that each period contains two bits of information, and the resulting code is referred to as VMSK/2 code.

VMSK/2 encoding encodes the signal only once in each bit period, the moment of inversion being determined by the bit value. Assuming that each bit occupies M clock cycles, when M is an odd or even number, for bit "1", the baseband encoded signal is inverted after (m+1)/2 or M/2+1 clock cycles, respectively; for bit "0", the baseband encoded signal is inverted after the (M-1)/2 or M/2 clock cycles.

The coding rule for VMSK/2 of length M can thus be given: (1) When the adjacent two bits are 10, the biphase code output by the encoder is inverted after M-1 clocks; (2) When the adjacent bit is '01', the two-phase code is inverted after M+1 clocks; (3) When the adjacent bit is "11" or "00", the two-phase code is inverted after M clocks.

The decoding rules of VMSK/2 are: when the signal is detected to be inverted after M-1 clocks, the decoder outputs a bit of '0'; when the signal is detected to be inverted after M+1 clocks, a bit '1' is output; when the signal is inverted after M clocks, the same bit as the previous one is output.

From the above description, the VMSK/2 encoding and decoding process is not complex, and the difficulty is how to accurately detect the nuances of the zero-crossing points of the signals during decoding, and obtain the correct information codes.

To increase the data rate, we propose a quadrature modulated VMSK/2 (QM-VMSK/2) code modulation scheme. In this scheme, the specific procedure of QM-VMSK/2 code modulation is:

the transmitted data b (t) is firstly converted into two sub-data streams after serial-parallel conversion, and VMSK/2 coding is respectively carried out to obtain a coding output a _I (t) and a _Q (t); then to a _I (t) and a _Q (t) respectively performing in-phase modulation and quadrature modulation, and adding to obtain an output signal x _QM-VMSK/2 (t)。

Two paths of signals are carried simultaneously by quadrature modulation, so that two paths of signals are transmitted in parallel. This can double the data rate over the same bandwidth. The modulation principle of QM-VMSK/2 is shown in FIG. 1.

VMSK signals use the change in zero crossing time to convey information, and therefore, decoding thereof is generally performed using zero crossing detection and two-phase code pulse width discrimination. Only the minute difference at the zero-crossing point is accurately detected and converted into a pulse width variation of the two-phase code for correct decoding. However, under the condition of low signal-to-noise ratio, the zero crossing point moment of the demodulation signal is easily interfered by noise, so that the judgment of the pulse width of the biphase code is affected, and decoding errors are caused.

A decoding method based on dual-phase code detection is provided to improve the noise immunity of decoding. The method firstly carries out the detection of the biphase code of each clock cycle by means of amplitude discrimination, and then carries out bit decoding according to the clock cycle number occupied by different bits in the coding rule. Because the signal frequency and the data rate of the cross-domain magnetic induction communication are low, when the sampling frequency is far greater than the clock frequency, enough sampling points are used for judging the double-phase code in one clock period, and the noise immunity is stronger than that of the detection at the moment of the zero crossing point.

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

the received signal r (t) is multiplied by the local in-phase and quadrature carriers respectively to obtain in-phase and quadrature demodulation signals, and then the in-phase and quadrature demodulation signals are added after double-phase code detection, VMSK/2 decoding and parallel-serial conversion, and the estimation of the transmitted information b (t) is output. Fig. 2 is a schematic block diagram of demodulation and decoding of a narrowband modulated signal.

When practical, a synchronizing signal can be added before the signal to be transmitted to ensure the synchronization of the detection of the received signal.

The invention also provides a narrow-band modem system suitable for the cross-boundary magnetic induction communication, which comprises a transmitting end and a receiving end.

As shown in fig. 3, the transmitting end includes,

the serial-parallel conversion module converts input transmission data b (t) into two sub-data streams;

a first VMSK/2 encoder for receiving one of the sub-data streams and encoding to obtain an encoded output a _I (t)；

A second VMSK/2 encoder for receiving another sub-data stream and encoding to obtain an encoded output a _Q (t)；

In-phase modulation module for data a _I (t) performing in-phase modulation;

quadrature modulation module for data a _Q (t) quadrature modulating;

the aggregation module is used for modulating two paths of in-phase and quadratureThe data are added to obtain an output signal x _QM-VMSk/2 (t)。

The receiving end multiplies the received signal r (t) with the local in-phase and quadrature carriers to obtain in-phase and quadrature demodulation signals, and the demodulation signals are added after two-phase code detection, VMSK/2 decoding and parallel-serial conversion in sequence to output estimation of the transmitted signal b (t). As shown in fig. 4, the receiving end includes

The in-phase demodulation module carries out in-phase demodulation on the received signal r (t);

a quadrature demodulation module for performing quadrature demodulation on the received signal r (t);

the first biphase code detection module is used for carrying out biphase code detection on the in-phase demodulation signal and outputting an estimated VMSk/2 coding signal;

the second double-phase code detection module is used for carrying out double-phase code detection on the orthogonal demodulation signal and outputting another path of estimated VMSk/2 coding signal;

a first VMSK/2 decoder for performing VMSK/2 decoding on the signal detected by the first two-phase code detection module;

a second VMSK/2 decoder for performing VMSK/2 decoding on the signal detected by the second two-phase code detection module;

and the parallel-serial conversion module is used for carrying out parallel-serial conversion on the signals respectively decoded by the first VMSK/2 decoder and the second VMSK/2 decoder, adding the signals, and outputting estimation of the transmission information b (t).

1. Numerical analysis of narrow-band modulation

Setting carrier frequency f _c =500 Hz, bit rate f _b =100 bps, sampling frequency f _s =25 kHz. In VSMK/2 encoding, each bit takes m=4 clock cycles. The cross-domain MI channel can be modeled with an additive white gaussian noise channel. Because of the narrow bandwidth of the VMSK/2 modulated signal, it is assumed that each frequency component in the signal is subject to the same channel attenuation.

Fig. 5 is a power spectrum of conventional Phase Shift Keying (PSK) and VMSK/class 2 narrowband modulated signals. As can be seen from fig. 3, the spectrum of QM-VMSK/2 is very close to VMSK/2, occupying a much smaller frequency band than PSK signals. Thus, modulation such as QM-VMSK/2 is well suited for MI channels.

The simulated bit error rate curves of the narrowband modulation are shown in fig. 6, and decoding methods adopting double-phase code detection (BPCD) and Zero Crossing Detection (ZCD) are adopted respectively.

From fig. 6, the following conclusion is drawn.

(1) When the data rate f is given _b And sampling frequency f _s In this case, the carrier frequency f can be reduced by reducing the number of sampling points per bit _c . F is given in FIG. 6 _c =500 Hz and f _c A narrow-band modulated bit error rate curve at 200 Hz. As can be seen, use f _c The bit error rate at 200Hz is slightly higher than f _c Bit error rate at 500Hz because a reduction in the number of sampling points affects the anti-noise performance of signal detection. However, the result of small error rate difference implies that the carrier frequency can be reduced under the condition of not obviously increasing the error rate, so that the transmission attenuation of MI signals is reduced, and a larger water inlet depth is achieved. The key to allowing lower carrier frequencies is that the VMSK/2 encoding can allow the encoded signal to have consecutive "1" or "0" bit symbols and thus tolerate lower sample numbers than single bit symbols.

To achieve ber=10 ^-2 The use of QM-VMSK/2 modulation requires an increase in signal-to-noise ratio of about 2dB over the use of VMSK/2 modulation, but with a transmission data rate of 2f _b =200 bps, twice as much as VMSK/2. If the VMSK/2 modulation is to achieve a data rate of 200bps and the same number of samples is maintained within a bit symbol, its carrier frequency is increased to f _c =1000 Hz. From calculation of magnetic induction fields in layered media ^[1] ，f _c Magnetic induction of =1000 Hz is higher than f _c Attenuation increases by about 2.6 times when =500 Hz. This means that cross-domain communication using QM-VMSK/2 modulation can achieve a 2-fold increase in data rate at the cost of 2db more signal power over VMSK/2 modulation at the same water depth; if the same data rate is maintained, the cross-domain communication can reach a larger water depth.

(2) Compared with the zero-crossing detection method, the two-phase code detection method is adopted when BER=10 ^-2 When the signal-to-noise ratio gains of about 3dB are obtained, respectively. This illustrates the use of two-phase code detectionThe method has better anti-noise performance, and can reach larger water depth under the same signal power and BER condition.

What is not described in detail in this specification is prior art known to those skilled in the art.

Claims (5)

1. The narrow-band modulation-demodulation method suitable for cross-boundary magnetic induction communication is characterized by comprising the following steps:

step 1: coded modulation, the transmission data b (t) of the transmitting end is subjected to QM-VMSK/2 coded modulation to output signal x _QM-VMSk/2 (t) transmitting the signal through the antenna;

step 2: and (3) demodulating and decoding, and outputting the estimation of the transmitted signal b (t) after the receiving end performs QM-VMSK/2 demodulating and decoding on the received signal r (t).

2. The narrowband modem method according to claim 1, wherein in the step 1, the specific procedure of QM-VMSK/2 code modulation is:

the transmitted data b (t) is firstly converted into two sub-data streams after serial-parallel conversion, and VMSK/2 coding is respectively carried out to obtain a coding output a _I (t) and a _Q (t); then to a _I (t) and a _Q (t) respectively performing in-phase modulation and quadrature modulation, and adding to obtain a modulated output signal x _QM-VMSk/2 (t)。

3. The narrowband modem method according to claim 1, wherein in the step 2, the specific process of QM-VMSK/2 demodulation and decoding is:

the receiving end multiplies the received signal r (t) with the local in-phase and quadrature carriers to obtain in-phase and quadrature demodulation signals, and the demodulation signals are added after two-phase code detection, VMSK/2 decoding and parallel-serial conversion in sequence to output estimation of the transmitted signal b (t).

4. A narrow-band modem system suitable for cross-boundary magnetic induction communication, the modem system comprises a transmitting end and a receiving end, characterized in that,

the transmitting end comprises a transmitting end and a receiving end,

the serial-parallel conversion module converts input transmission data b (t) into two sub-data streams;

a first VMSK/2 encoder for receiving one of the sub-data streams and encoding to obtain an encoded output a _I (t)；

A second VMSK/2 encoder for receiving another sub-data stream and encoding to obtain an encoded output a _Q (t)；

In-phase modulation module for data a _I (t) performing in-phase modulation;

quadrature modulation module for data a _Q (t) quadrature modulating;

the aggregation module is used for adding the two paths of data subjected to in-phase modulation and quadrature modulation to obtain a modulation output signal x _QM-VMSk/2 (t)；

The receiving end multiplies the received signal r (t) with the local in-phase and quadrature carriers to obtain in-phase and quadrature demodulation signals, and the demodulation signals are added after two-phase code detection, VMSK/2 decoding and parallel-serial conversion in sequence to output estimation of the transmitted signal b (t).

5. The narrowband modem system for use in cross-border magnetic induction communications as claimed in claim 4, wherein the receiver further comprises

The in-phase demodulation module carries out in-phase demodulation on the received signal r (t);

a quadrature demodulation module for performing quadrature demodulation on the received signal r (t);

the first biphase code detection module is used for carrying out biphase code detection on the in-phase demodulation signal and outputting an estimated VMSk/2 coding signal

A second two-phase code detection module is provided,

performing double-phase code detection on the orthogonal demodulation signal, and outputting another path of estimated VMSk/2 coding signal;

a first VMSK/2 decoder for performing VMSK/2 decoding on the signal detected by the first two-phase code detection module;

a second VMSK/2 decoder for performing VMSK/2 decoding on the signal detected by the second two-phase code detection module;

and the parallel-serial conversion module is used for carrying out parallel-serial conversion on the signals respectively decoded by the first VMSK/2 decoder and the second VMSK/2 decoder, adding the signals, and outputting estimation of the transmission information b (t).

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