CN108259092A - A kind of underwater sound modem and modulator approach based on the modulation of orthogonal differential phase-shift keying (PSK) - Google Patents
A kind of underwater sound modem and modulator approach based on the modulation of orthogonal differential phase-shift keying (PSK) Download PDFInfo
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Abstract
A kind of underwater sound modem and modulator approach based on the modulation of orthogonal differential phase-shift keying (PSK), the demodulator further include mode switch, emitting portion, receiving portion, USB interface and power supply module using microcontroller as core；The emitting portion is made up of the transmitting terminal signal amplification circuit, transmission energy converter and software section of hardware components program to complete the variable encoder of function, modulator and Waveform shaping device；The receiving portion is made up of reception energy converter, receiving terminal one stage signal amplifying circuit, bandwidth-limited circuit, receiving terminal second signal amplifying circuit and the software section of hardware components program to complete the demodulator and decoder of function；When the USB interface is detached from the water surface for the underwater sound modem information exchange is carried out with computer；The power supply module is used to power to the underwater sound modem, makes its normal work.The present invention has merged orthogonal differential phase-shift keying (PSK) technology, realizes the switching of transceiver and bistatic.
Description
Technical Field
The invention relates to the technical field of underwater acoustic communication, in particular to an underwater acoustic modem based on quadrature differential phase shift keying modulation and a modulation method.
Background
With the development of underwater sensor technology and modern communication technology, the application requirements of human beings on underwater space environments such as ocean and the like are continuously improved, and more countries begin to pay attention to the research of underwater acoustic sensor networks. Information interaction is carried out on the underwater acoustic sensing network in an acoustic communication mode, and in order to ensure stable and real-time data transmission, an underwater acoustic modem which is small in size, low in cost, low in power consumption and high in reliability needs to be designed urgently.
In the prior art, Chinese patent application No. 201620077806.5, named as: the invention relates to a self-generating underwater sound modem, which utilizes an ocean energy power generation module to supplement electric energy. The underwater acoustic modem is mainly designed for the power generation module, and the underwater acoustic transducer is supposed to be always in an information receiving and transmitting state, so that the network energy consumption is increased. In an actual underwater environment, the underwater acoustic modem power supply module has limited energy and is not easy to replace, and how to design a corresponding switching mechanism for the transducer on the premise of ensuring the underwater acoustic communication quality is very important for reducing the network energy consumption.
Further, chinese patent application No. 201220371949.9, entitled: a subsea modem. The invention adopts the orthogonal phase shift spread spectrum communication technology, the modulation scheme is absolute phase shift keying, and a coherent demodulation scheme is adopted during demodulation. Under the influence of complex environments such as underwater environment noise, underwater sediments and the like, a coherent demodulation scheme is easy to generate a phase ambiguity phenomenon during carrier recovery, so that demodulation judgment is wrong. The above defects prevent the effective transmission of information in the underwater environment, and further reduce the real-time performance and effectiveness of data transmission.
Further, it is found through search that the chinese patent application No. 201110224601.7, entitled: the invention relates to a wireless and underwater acoustic communication buoy, which takes a digital signal processing chip as a signal processing core, provides an underwater acoustic modem carrier with reliable performance, simple operation and high intelligent degree of maintenance and management and can be used for an underwater acoustic sensing network. However, the device has the advantages of large volume and weight, non-modular design, complex overall structure and high power consumption, so that the portability and the service life of the underwater sound modem are reduced, the underwater sound modem is easy to die due to exhaustion of electric quantity in the working process, and the requirement of large-scale networking is difficult to meet.
Therefore, how to consider different communication media in an underwater weak communication environment, it is important to design a low-cost and low-power-consumption underwater sound modem with transmit-receive mode switching capability by using a modular method, which can ensure reliable acquisition of underwater information and realize efficient return of water surface data.
Disclosure of Invention
The invention aims to overcome the defects in the prior art and provides a hydroacoustic modem based on quadrature differential phase shift keying modulation and a modulation method.
In order to solve the technical problems and achieve the purpose of the invention, the invention adopts the following technical scheme:
a underwater sound modem based on quadrature differential phase shift keying modulation takes a microcontroller as a core, and also comprises a mode switcher, a transmitting part, a receiving part, a USB interface and a power supply module;
the transmitting part consists of a transmitting end signal amplifying circuit of a hardware part, a transmitting transducer, a variable encoder, a modulator and a wave-form former, wherein the software part completes the function through a program; when data is sent, in the microcontroller, the variable encoder calls a corresponding encoding algorithm to carry out source encoding on information collected underwater, the modulator calls a digital modulation algorithm to carry out digital modulation on the encoded signal, and then the waveform former calls a waveform forming algorithm to convert the modulated signal into an analog signal and transmits the analog signal to a signal amplifying circuit of a sending end; the transmitting end signal amplifying circuit is used for receiving the analog signal of the microcontroller and finishing signal amplification; the transmitting transducer is connected with a signal amplifying circuit of the transmitting end to complete the conversion from the analog signal to the underwater sound signal;
the receiving part consists of a receiving transducer of a hardware part, a first-stage signal amplifying circuit of a receiving end, a band-pass filter circuit, a second-stage signal amplifying circuit of the receiving end and a demodulator and a decoder of a software part which complete functional action through programs; the receiving transducer, the receiving end primary signal amplifying circuit, the band-pass filter circuit and the receiving end secondary signal amplifying circuit are sequentially connected; when receiving data, the receiving transducer converts the underwater sound signal into an analog signal and inputs the analog signal to the receiving end primary signal amplifying circuit, the receiving end primary signal amplifying circuit completes signal amplification and transmits the amplified signal to the band-pass filter circuit; the band-pass filter circuit performs band-pass filtering on the received primary amplified signal and then inputs the signal to a secondary signal amplification circuit at a receiving end; the receiving end secondary signal amplifying circuit amplifies the received signal, realizes the level conversion of the signal and transmits the signal to the microcontroller; in the microcontroller, firstly calling a corresponding algorithm to convert an analog signal into a digital signal, then calling a digital demodulation algorithm by the demodulator to carry out judgment and digital demodulation, and finally calling a decoding algorithm by the decoder to finish channel decoding;
the USB interface is used for performing information interaction with a computer when the underwater sound modem is separated from the water surface;
the power supply module is used for supplying power to the underwater sound modem so as to enable the underwater sound modem to work normally;
the mode switcher provides two modes of transmitting and receiving separately and transmitting and receiving integrally, when the mode switcher is in the transmitting and receiving separately mode, the transmitting transducer and the receiving transducer are respectively independent transducers, when the mode switcher is in the transmitting and receiving integrally mode, the transmitting transducer and the receiving transducer are the same transducer, namely the transmitting transducer is called when transmitting information, and the receiving transducer is called when receiving information, and in the two modes, the working mechanisms of the mode switcher are the same;
the mode switcher consists of two rectifier tubes, two power resistors and two switching tubes; one end of the first rectifying tube and one end of the second rectifying tube are connected with the transmitting end circuit in parallel in the reverse direction, and the other end of the first rectifying tube is connected with the transmitting transducer and one end of the first power resistor; one end of the first switch tube and one end of the second switch tube after being connected in parallel in the reverse direction are grounded, the other end of the first switch tube is connected with the other end of the first power resistor and one end of the second power resistor, and the other end of the second power resistor is connected with the receiving transducer and the receiving end circuit.
The modulation method of the underwater acoustic modem based on the quadrature differential phase shift keying modulation comprises the following steps:
step 1, the mode switcher can automatically sense the mode, and then compares the inherent threshold value of the mode switcher with a sending signal or a signal collected by a receiving transducer to determine the receiving and sending states of the underwater sound modem, so as to select the information transmission direction;
step 2, when the underwater sound modem is in a transmitting state, a modulation algorithm is called to modulate a transmission signal, and the transmission signal is transmitted out through a transmitting end signal amplifying circuit;
when the signal is transmitted, the microcontroller calls corresponding code systems to carry out adaptive coding according to different requests of a user, and the character signal is converted into a binary signal; then packing into data frames to form effective baseband digital signals m (t):
⑴, where t is time, probability p is random value, and is different with different signals, PSK modulation is a modulation form for transmitting digital information by controlling the change of carrier phase by baseband digital signal, and the specific modulation is:
the input serial binary information sequence is divided into two paths of sequences a with half rate after serial-parallel conversionnAnd bnThen, level conversion g (t) generates bipolar two-level pulse signals i (t) and q (t), and the mapping rules are "1" → "+ 1" and "0" → "-1". Then to cos omegact and sin ωct, modulating to obtain a quadrature phase shift keying signal; after the quadrature phase shift keying modulation is finished, adding the two paths of signals, and carrying out differential phase shift keying modulation, namely representing digital information by using the phase difference of front and back code elements to modulate final information s (t);
in the modulation process, the pulse shaping modes of the i (t), q (t) branches are the same, wherein the signals of the i (t) branches can be represented as:
⑵ wherein in the formula (I), the amino acid sequence,g (t) is a signal envelope waveform which is a rectangular wave and has the amplitude of 1; t isSIs the time width; omegacIs the carrier angular frequency; a isnThe value of the modulated information is +1 or-1; g (t) is an analog signal which is compared with anAnd cos omegact is multiplied, and modulation is finished; when the waveform of g (t) is designed, the whole response of the communication system is designed to only respond to the current symbol at each sampling moment of a receiving end, and the response to other symbols is all zero, so that the discontinuous jump phenomenon of the modulation waveform when the signal phase is reversed when g (t) is designed into rectangular pulse is avoided, the problem of intersymbol interference is eliminated, and the error rate is reduced;
wherein, s (t) the modulation signal can be expressed as:
⑶ formula, where g (T) is signal envelope waveform, which is rectangular wave with amplitude of 1, TSIs the time width; omegacIs the carrier angular frequency;the phase corresponding to the nth code element; i.e. the modulation signal s (t) is represented in quadrature form:
⑷ wherein in the formula (I), the amino acid sequence,
⑷ shows that the power spectrum main lobe of the quadrature phase shift keying is narrow, and the frequency band utilization rate is high;
after the signal modulation, the signal enters a signal amplifying circuit of a sending end; it is a secondary signal amplifying circuit; the first-stage amplifying circuit adopts a common collector amplifying mode, completely asks for signal voltage and avoids distortion of signal transmission; the second-stage amplifying circuit adopts an amplifying mode which is also a common collector mode, the amplifying mode has small output resistance, almost all voltage can be loaded on the transmitting transducer at an output stage, and the transmitting transducer is driven to convert signals from analog signals into underwater sound signals by utilizing ultrasonic waves and send the signals out;
step 3, when the underwater sound modem is in a receiving state, utilizing a receiving end primary signal amplifying circuit, a band-pass filter circuit and a receiving end secondary signal amplifying circuit to realize signal amplification, filtering and level conversion and transmit the signals to a microcontroller, and finally calling a corresponding algorithm by the microcontroller to convert analog signals into digital signals, and performing decision demodulation and digital demodulation and finishing channel decoding; the concrete content is as follows:
when the underwater sound modem receives signals, the receiving transducer utilizes the voltage generated by the ultrasonic waves at the two ends of the crystal slice to form weak analog electric signals, so that the conversion from the underwater sound signals to the analog electric signals is completed, the weak analog signals are sent to a primary signal amplifying circuit at a receiving end, and the signal current is amplified; in an underwater acoustic channel, the environment is complex, the noise is numerous, the frequencies of analog electric signals converted by a transducer are different, and before the analog electric signals are demodulated and decoded by a microcontroller, the underwater acoustic modem performs band-pass filtering after primary amplification signals so as to extract effective frequency signals, eliminate unnecessary burrs in the signals and reduce the error rate of information transmission; after band-pass filtering, the signals pass through a secondary signal amplification circuit at a receiving end to complete the adjustment of voltage being stably amplified and current being correspondingly reduced, so that the microcontroller can conveniently and completely acquire the signals;
the receiving signal collected by the microcontroller is a quadrature phase shift keying signal code element r (t):
r (t) multiplied by two paths of orthogonal coherent carriers are respectively output as:
whereinIs twice the carrier frequency, andis a pure baseband signal; using a low-pass filter to suppress the high-frequency part and keeping the baseband signal; the output of the low pass filter is:
according toThe value of (2) is different, and the signal voltage can be positive or negative and is bipolar voltage; provision is made during the modulation process for: "1" → "+ 1", "0" → "-1", and therefore, at the time of decision, the positive voltage is judged as "1", and the negative voltage is judged as "0"; the function of inverse code conversion is opposite to that of the sending end, and the relative code output by the judger is restored into absolute code; the absolute code enters a data buffer area for caching in a data frame mode, and the variable coding table is subjected to table look-up decoding to complete information receiving;
step 4, if data transmission is needed to be continued, the step 1 is executed again to perform mode sensing; when the USB interface is needed to be transmitted, the microcontroller transfers the memory information to the USB interface to wait for the transmission of the USB interface and exchange information with the computer; if not, enabling the underwater sound modem to enter a dormant state, waiting for waking up the equipment for next data transmission, and ending the data transmission; and repeating the steps 2-3 when underwater sound transmission is required.
Compared with the existing underwater sound modem, the underwater sound modem based on the quadrature differential phase shift keying modulation and the modulation method provided by the invention have the following beneficial effects:
1. the invention adopts two modulation and demodulation modes of transmitting and receiving separately and transmitting and receiving integrally, and the transmitting and receiving modes can be configured according to the requirements of information transmission, power consumption, service life and the like, thereby not only reducing the generation of faults and lowering the cost of maintenance, manufacture and the like, but also fully playing the hardware efficiency of the invention;
2. the invention uses the variable coding method to map the information, simply and quickly realizes the coding of the information, and realizes the steady transmission of the underwater sound information;
3. the invention adopts a modulation and demodulation mode of quadrature differential phase shift keying, and has strong reliability; the theory is mature and easy to realize;
4. compared with the traditional modem, the invention reduces the use of hardware, for example, only a sending end signal amplifying circuit is used at a sending end, and a filter circuit designed for avoiding noise interference is omitted.
The invention takes a microprocessor as a core, integrates the orthogonal differential phase shift keying technology and realizes the switching of the receiving and transmitting integration and the receiving and transmitting separation.
Drawings
FIG. 1 is a schematic diagram of an underwater acoustic modem based on quadrature differential phase shift keying modulation according to the present invention;
FIG. 2 is a schematic diagram of a mode switcher based on orthogonal differential phase shift keying according to the present invention;
FIG. 3 is a block diagram of the overall modulation and demodulation process based on the quadrature differential phase shift keying modulation technique according to the present invention;
fig. 4 is a block diagram of the modulation principle of the present invention based on the quadrature differential phase shift keying modulation technique.
Detailed Description
The invention is described in further detail below with reference to the accompanying drawings:
the invention relates to a hydroacoustic modem based on quadrature differential phase shift keying modulation, which takes a microcontroller as a core, and also comprises a mode switcher, a transmitting part, a receiving part, a USB interface and a power supply module, wherein the mode switcher is used for switching a water channel between a receiving part and a USB interface;
the transmitting part consists of a transmitting end signal amplifying circuit of a hardware part, a transmitting transducer, a variable encoder, a modulator and a wave-form former, wherein the software part completes the function through a program; when data is sent, in the microcontroller, the variable encoder calls a corresponding encoding algorithm to carry out information source encoding on information collected underwater, the modulator calls a specific digital modulation algorithm to carry out digital modulation on the encoded signal, and then the waveform former calls a corresponding waveform forming algorithm to convert the modulated signal into an analog signal and transmits the analog signal to the signal amplification circuit of the sending end; the transmitting end signal amplifying circuit is used for receiving the analog signal of the microcontroller and finishing signal amplification; the transmitting transducer is connected with a signal amplifying circuit of the transmitting end to complete the conversion from the analog signal to the underwater sound signal;
the receiving part consists of a receiving transducer of a hardware part, a first-stage signal amplifying circuit of a receiving end, a band-pass filter circuit, a second-stage signal amplifying circuit of the receiving end and a demodulator and a decoder of a software part which complete functional action through programs; the receiving transducer, the receiving end primary signal amplifying circuit, the band-pass filter circuit and the receiving end secondary signal amplifying circuit are sequentially connected; when receiving data, the receiving transducer converts the underwater sound signal into an analog signal and inputs the analog signal to the receiving end primary signal amplifying circuit, the receiving end primary signal amplifying circuit completes signal amplification and transmits the amplified signal to the band-pass filter circuit; the band-pass filter circuit performs band-pass filtering on the received primary amplified signal and then inputs the signal to a secondary signal amplification circuit at a receiving end; the receiving end secondary signal amplifying circuit amplifies the received signal, realizes the level conversion of the signal and transmits the signal to the microcontroller; in the microcontroller, firstly calling a corresponding algorithm to convert an analog signal into a digital signal, then calling a specific digital demodulation algorithm by the demodulator to carry out judgment and digital demodulation, and finally calling a corresponding decoding algorithm by the decoder to finish channel decoding;
the USB interface is used for performing information interaction with a computer when the underwater sound modem is separated from the water surface;
the power supply module is used for supplying power to the underwater sound modem so as to enable the underwater sound modem to work normally;
the mode switcher provides two modes of transmitting and receiving separately and transmitting and receiving integrally, when the mode switcher is in the transmitting and receiving separately mode, the transmitting transducer and the receiving transducer are respectively independent transducers, when the mode switcher is in the transmitting and receiving integrally mode, the transmitting transducer and the receiving transducer are the same transducer, namely the transmitting transducer is called when transmitting information, and the receiving transducer is called when receiving information, and in the two modes, the working mechanisms of the mode switcher are the same;
the mode switcher, as shown in fig. 2, is composed of two rectifier tubes, two power resistors and two switch tubes; one end of the first rectifying tube 1 and the second rectifying tube 2 which are connected in parallel in the reverse direction is connected with a transmitting end circuit, and the other end is connected with a transmitting transducer and one end of a first power resistor 3; one end of the first switch tube 5 and the second switch tube 6 which are connected in parallel in the reverse direction is grounded, the other end of the first switch tube is connected with the other end of the first power resistor 3 and one end of the second power resistor 4, and the other end of the second power resistor 4 is connected with the receiving transducer and the receiving end circuit.
Fig. 3 is a flow chart showing the operation of the present invention, the underwater acoustic communication can be performed by placing the underwater acoustic modem of the present invention in water, and the modulation method comprises the following steps:
step 1, the mode switcher can automatically sense the mode, and then compares the inherent threshold value of the mode switcher with a sending signal or a signal collected by a receiving transducer to determine the receiving and sending states of the underwater sound modem, so as to select the information transmission direction;
the specific process of selecting the information transmission direction is as follows:
when a transmitting end transmits a signal, namely in a transmitting mode, the signal flows through the transmitting end and passes through an amplifying circuit, two rectifying tubes which are connected in parallel in a reverse direction are met, the signal can pass through the first rectifying tube of the mode switcher without distortion because the signal voltage is far greater than the forward conducting voltage of the first rectifying tube, and the signal drives a transmitting transducer to transmit information to an underwater sound channel under the lifting of a first power resistor; because of the isolation of the first power resistor, the signal flowing through the transmitting transducer is an amplified voltage signal, and therefore, the signal is immediately conducted after the signal reaches the first switching tube through the first power resistor, so that the signal voltage is directly grounded and cannot flow into the receiving transducer and a receiving end circuit; when the receiving transducer receives signals, the second power resistor is isolated, the generated current is small, and the switching tube and the rectifying tube cannot be conducted, so that the transmitting transducer cannot be driven, and the signals can well enter the receiving end circuit for demodulation.
Step 2, when the underwater sound modem is in a transmitting state, a modulation algorithm is called to modulate a transmission signal, and the transmission signal is transmitted out through a transmitting end signal amplifying circuit;
when data is sent, the microcontroller calls a corresponding algorithm to carry out information source coding and digital modulation on the information collected underwater, and then converts a digital signal into an analog signal to be output to the microcontroller; the hardware of the transmitting part consists of a transmitting end signal amplifying circuit and a transmitting transducer, the transmitting end signal amplifying circuit is used for receiving the analog signal output by the microcontroller and finishing the amplification of the signal, and the transmitting transducer is connected with the transmitting end signal amplifying circuit and finishes the conversion from the analog signal to the underwater sound signal;
because the quality of the communication system is closely related to the adopted modulation and demodulation mode, and the modulation enables the signal to be matched with the channel characteristic, the quadrature differential phase shift keying modulation is used as the modulation and demodulation scheme of the invention according to the channel characteristic of the underwater acoustic sensing network.
When the signal is transmitted, the microcontroller can call corresponding code systems to carry out adaptive coding according to different requests of a user, and the character signal is converted into a binary signal; then packing into data frames to form effective baseband digital signals m (t):
in the formula, t is time, and the probability p is a random value and is different with different signals; the phase shift keying modulation of the invention is a modulation form for transmitting digital information by controlling the change of carrier phase by using baseband digital signals; the specific modulation is as follows:
as shown in FIG. 4, the modulation scheme of the present invention is that the input serial binary information sequence is divided into two paths of sequences a with half rate after serial-parallel conversionnAnd bnThen, level conversion g (t) generates bipolar two-level pulse signals i (t) and q (t), and the mapping rules are "1" → "+ 1" and "0" → "-1". Then to cos omegact and sin ωct, modulating to obtain a quadrature phase shift keying signal; after the quadrature phase shift keying modulation is finished, the two paths of signals are added to carry out the difference of the next stepSub-phase shift keying modulation, namely representing digital information by using the phase difference of front and back code elements to modulate final information s (t);
in the modulation process, the pulse shaping modes of the I (t) branch and the Q (t) branch are the same, wherein the signals of the I (t) branch can be expressed as
Wherein,g (t) is a signal envelope waveform which is a rectangular wave and has the amplitude of 1; t isSIs the time width; omegacIs the carrier angular frequency; a isnThe value of the modulated information is +1 or-1; g (t) is an analog signal which is compared with anAnd cos omegact is multiplied, and modulation is finished; when the waveform of g (t) is designed, the whole response of the communication system is designed to only respond to the current symbol at each sampling moment of a receiving end, and the response to other symbols is all zero, so that the discontinuous jump phenomenon of the modulation waveform of the signal phase when the signal phase is turned over when g (t) is designed into rectangular pulse is avoided, the problem of intersymbol interference is eliminated, and the error rate is reduced;
s (t) the modulation signal can be expressed as
In the formula,the phase corresponding to the nth symbol. The modulation signal s (t) is represented in quadrature form as
In the formula,
after the signal modulation, the signal enters a signal amplifying circuit of a sending end; it is a secondary signal amplifying circuit; the first-stage amplifying circuit adopts a common collector amplifying mode, completely asks for signal voltage and avoids distortion of signal transmission; the second stage amplifying circuit adopts an amplifying mode which is also a common collector mode, the amplifying mode has small output resistance, almost all voltage can be loaded on the transmitting transducer at an output stage, and the transmitting transducer is driven to convert signals from analog signals into underwater sound signals by utilizing ultrasonic waves and send the signals out.
Step 3, when the underwater sound modem is in a receiving state, receiving and processing signals are completed by utilizing a receiving end primary signal amplifying circuit, a band-pass filter circuit, a receiving end secondary signal amplifying circuit and a demodulation algorithm; the concrete content is as follows:
when receiving data, the hardware of the receiving part consists of a receiving transducer, a receiving end primary signal amplifying circuit, a band-pass filter circuit and a receiving end secondary signal amplifying circuit, wherein the receiving transducer converts the underwater sound signal into an analog signal in the receiving end primary signal amplifying circuit, the receiving end primary signal amplifying circuit completes the amplification of the analog signal, the band-pass filter circuit realizes the band-pass filtering of the amplifying signal of the receiving end primary signal amplifying circuit, the receiving end secondary signal amplifying circuit is connected with the band-pass filter circuit to realize the level conversion of the signal and transmit the signal to the microcontroller, and finally the microcontroller calls a corresponding algorithm to convert the analog signal into a digital signal to perform decision demodulation and digital demodulation and complete channel decoding;
when the underwater sound modem receives signals, the receiving transducer utilizes the voltage generated by the ultrasonic waves at the two ends of the crystal slice to form weak analog electric signals, so that the conversion from the underwater sound signals to the analog electric signals is completed, the weak analog signals are sent to a primary signal amplifying circuit at a receiving end, and the signal current is amplified; in an underwater acoustic channel, the environment is complex, the noise is numerous, the frequencies of analog electric signals converted by a transducer are different, and before the analog electric signals are demodulated and decoded by a microcontroller, the invention performs band-pass filtering after primary amplification signals so as to extract effective frequency signals, eliminate unnecessary burrs in the signals and reduce the error rate of information transmission; after band-pass filtering, the signals pass through a secondary signal amplification circuit at a receiving end to complete the adjustment of voltage being stably amplified and current being correspondingly reduced, so that the microcontroller can conveniently and completely acquire the signals;
the demodulation algorithm corresponds to quadrature differential phase shift keying modulation, and the demodulation scheme adopts a coherent demodulation and inverse code conversion technology; the coherent demodulation comprises the following steps: separating two paths of orthogonal baseband signals by two paths of orthogonal coherent carriers, obtaining code elements of I (t) and Q (t) paths after the two paths of baseband signals are judged, and outputting the code elements as serial data after inverse code conversion and parallel-serial conversion; the implementation of coherent demodulation is:
after the received signal is band-pass filtered, the symbol r (t) of the quadrature phase shift keying signal input to the demodulator is:
r (t) multiplied by two paths of orthogonal coherent carriers are respectively output as:
each branch has two branches, whereinIs twice the carrier frequency, andis a pure baseband signal, i.e. requiresThe resulting fraction; then, a low-pass filter is used to suppress the high frequency part, and the baseband signal is reserved for subsequent processing;
the output of the low-pass filter is
According toThe value of (2) is different, and the signal voltage can be positive or negative, so the voltage is bipolar voltage. During the modulation process, it was specified that: "1" → "+ 1" and "0" → "-1", and therefore, the positive voltage is also judged as "1" and the negative voltage is judged as "0" at the time of judgment. The function of inverse code conversion is opposite to that of the sending end, and the relative code output by the judger is restored into absolute code;
the absolute code enters a data buffer area for buffering in a data frame mode; the decoder performs table lookup on the variable coding table to restore ASCII codes of the characters, further restores the characters and restores original information;
step 4, after the data is demodulated, when the data does not need to be transmitted, the underwater sound modem stores the information and is in a dormant state, and waits for the next step of awakening of the host; if the information needs to be sent, the underwater sound modem can select a sending mode; the present invention provides two transmission mode options: underwater sound transmission and USB interface transmission; repeating the steps 1-3 when underwater sound transmission is needed; when the USB interface is needed to be transmitted, the microcontroller transfers the memory information to the USB interface to wait for the transmission of the USB interface, and then the information interaction with the computer is carried out.
The foregoing illustrates and describes the principles, general features, and advantages of the present invention. It will be understood by those skilled in the art that the present invention is not limited to the embodiments described above, which are described in the specification and illustrated only to illustrate the principle of the present invention, but that various changes and modifications may be made therein without departing from the spirit and scope of the present invention, which fall within the scope of the invention as claimed. The scope of the invention is defined by the appended claims and equivalents thereof.
Claims (2)
1. The underwater sound modem takes a microcontroller as a core, and also comprises a mode switcher, a transmitting part, a receiving part, a USB interface and a power supply module;
the transmitting part consists of a transmitting end signal amplifying circuit of a hardware part, a transmitting transducer, a variable encoder, a modulator and a wave-form former, wherein the software part completes the function through a program; when data is sent, in the microcontroller, the variable encoder calls a corresponding encoding algorithm to carry out source encoding on information collected underwater, the modulator calls a digital modulation algorithm to carry out digital modulation on the encoded signal, and then the waveform former calls a waveform forming algorithm to convert the modulated signal into an analog signal and transmits the analog signal to a signal amplifying circuit of a sending end; the transmitting end signal amplifying circuit is used for receiving the analog signal of the microcontroller and finishing signal amplification; the transmitting transducer is connected with a signal amplifying circuit of the transmitting end to complete the conversion from the analog signal to the underwater sound signal;
the receiving part consists of a receiving transducer of a hardware part, a first-stage signal amplifying circuit of a receiving end, a band-pass filter circuit, a second-stage signal amplifying circuit of the receiving end and a demodulator and a decoder of a software part which complete functional action through programs; the receiving transducer, the receiving end primary signal amplifying circuit, the band-pass filter circuit and the receiving end secondary signal amplifying circuit are sequentially connected; when receiving data, the receiving transducer converts the underwater sound signal into an analog signal and inputs the analog signal to the receiving end primary signal amplifying circuit, the receiving end primary signal amplifying circuit completes signal amplification and transmits the amplified signal to the band-pass filter circuit; the band-pass filter circuit performs band-pass filtering on the received primary amplified signal and then inputs the signal to a secondary signal amplification circuit at a receiving end; the receiving end secondary signal amplifying circuit amplifies the received signal, realizes the level conversion of the signal and transmits the signal to the microcontroller; in the microcontroller, firstly calling a corresponding algorithm to convert an analog signal into a digital signal, then calling a digital demodulation algorithm by the demodulator to carry out judgment and digital demodulation, and finally calling a decoding algorithm by the decoder to finish channel decoding;
the USB interface is used for performing information interaction with a computer when the underwater sound modem is separated from the water surface;
the power supply module is used for supplying power to the underwater sound modem so as to enable the underwater sound modem to work normally;
the mode switcher provides two modes of transmitting and receiving separately and transmitting and receiving integrally, when the mode switcher is in the transmitting and receiving separately mode, the transmitting transducer and the receiving transducer are respectively independent transducers, when the mode switcher is in the transmitting and receiving integrally mode, the transmitting transducer and the receiving transducer are the same transducer, namely the transmitting transducer is called when transmitting information, and the receiving transducer is called when receiving information, and in the two modes, the working mechanisms of the mode switcher are the same;
the mode switcher consists of two rectifier tubes, two power resistors and two switching tubes; one end of the first rectifying tube and one end of the second rectifying tube are connected with the transmitting end circuit in parallel in the reverse direction, and the other end of the first rectifying tube is connected with the transmitting transducer and one end of the first power resistor; one end of the first switch tube and one end of the second switch tube after being connected in parallel in the reverse direction are grounded, the other end of the first switch tube is connected with the other end of the first power resistor and one end of the second power resistor, and the other end of the second power resistor is connected with the receiving transducer and the receiving end circuit.
2. The method for modulating the underwater acoustic modem based on the quadrature differential phase shift keying modulation as claimed in claim 1, wherein: the method comprises the following steps:
step 1, the mode switcher can automatically sense the mode, and then compares the inherent threshold value of the mode switcher with a sending signal or a signal collected by a receiving transducer to determine the receiving and sending states of the underwater sound modem, so as to select the information transmission direction;
step 2, when the underwater sound modem is in a transmitting state, a modulation algorithm is called to modulate a transmission signal, and the transmission signal is transmitted out through a transmitting end signal amplifying circuit;
when the signal is transmitted, the microcontroller calls corresponding code systems to carry out adaptive coding according to different requests of a user, and the character signal is converted into a binary signal; then packing into data frames to form effective baseband digital signals m (t):
⑴, where t is time, probability p is random value, and is different with different signals, PSK modulation is a modulation form for transmitting digital information by controlling the change of carrier phase by baseband digital signal, and the specific modulation is:
the input serial binary information sequence is divided into two paths of sequences with half rate after serial-parallel conversionanAnd bnThen, level conversion g (t) generates bipolar two-level pulse signals i (t) and q (t), and the mapping rules are "1" → "+ 1" and "0" → "-1". Then to cos omegact and sin ωct, modulating to obtain a quadrature phase shift keying signal; after the quadrature phase shift keying modulation is finished, adding the two paths of signals, and carrying out differential phase shift keying modulation, namely representing digital information by using the phase difference of front and back code elements to modulate final information s (t);
in the modulation process, the pulse shaping modes of the i (t), q (t) branches are the same, wherein the signals of the i (t) branches can be represented as:
⑵ wherein in the formula (I), the amino acid sequence,g (t) is a signal envelope waveform which is a rectangular wave and has the amplitude of 1; t isSIs the time width; omegacIs the carrier angular frequency; a isnThe value of the modulated information is +1 or-1; g (t) is an analog signal which is compared with anAnd cos omegact is multiplied, and modulation is finished; when the waveform of g (t) is designed, the whole response of the communication system is designed to only respond to the current symbol at each sampling moment of a receiving end, and the response to other symbols is all zero, so that the discontinuous jump phenomenon of the modulation waveform when the signal phase is reversed when g (t) is designed into rectangular pulse is avoided, the problem of intersymbol interference is eliminated, and the error rate is reduced;
wherein, s (t) the modulation signal can be expressed as:
⑶ formula, where g (T) is signal envelope waveform, which is rectangular wave with amplitude of 1, TSIs the time width; omegacIs the carrier angular frequency;the phase corresponding to the nth code element; i.e. the modulation signal s (t) is represented in quadrature form:
⑷ wherein in the formula (I), the amino acid sequence,
⑷ shows that the power spectrum main lobe of the quadrature phase shift keying is narrow, and the frequency band utilization rate is high;
after the signal modulation, the signal enters a signal amplifying circuit of a sending end; it is a secondary signal amplifying circuit; the first-stage amplifying circuit adopts a common collector amplifying mode, completely asks for signal voltage and avoids distortion of signal transmission; the second-stage amplifying circuit adopts an amplifying mode which is also a common collector mode, the amplifying mode has small output resistance, almost all voltage can be loaded on the transmitting transducer at an output stage, and the transmitting transducer is driven to convert signals from analog signals into underwater sound signals by utilizing ultrasonic waves and send the signals out;
step 3, when the underwater sound modem is in a receiving state, utilizing a receiving end primary signal amplifying circuit, a band-pass filter circuit and a receiving end secondary signal amplifying circuit to realize signal amplification, filtering and level conversion and transmit the signals to a microcontroller, and finally calling a corresponding algorithm by the microcontroller to convert analog signals into digital signals, and performing decision demodulation and digital demodulation and finishing channel decoding; the concrete content is as follows:
when the underwater sound modem receives signals, the receiving transducer utilizes the voltage generated by the ultrasonic waves at the two ends of the crystal slice to form weak analog electric signals, so that the conversion from the underwater sound signals to the analog electric signals is completed, the weak analog signals are sent to a primary signal amplifying circuit at a receiving end, and the signal current is amplified; in an underwater acoustic channel, the environment is complex, the noise is numerous, the frequencies of analog electric signals converted by a transducer are different, and before the analog electric signals are demodulated and decoded by a microcontroller, the underwater acoustic modem performs band-pass filtering after primary amplification signals so as to extract effective frequency signals, eliminate unnecessary burrs in the signals and reduce the error rate of information transmission; after band-pass filtering, the signals pass through a secondary signal amplification circuit at a receiving end to complete the adjustment of voltage being stably amplified and current being correspondingly reduced, so that the microcontroller can conveniently and completely acquire the signals;
the receiving signal collected by the microcontroller is a quadrature phase shift keying signal code element r (t):
r (t) multiplied by two paths of orthogonal coherent carriers are respectively output as:
whereinIs twice the carrier frequency, andis a pure baseband signal;
using a low-pass filter to suppress the high-frequency part and keeping the baseband signal; the output of the low pass filter is:
according toThe value of (2) is different, and the signal voltage can be positive or negative and is bipolar voltage; in thatThe modulation process is stipulated as follows: "1" → "+ 1", "0" → "-1", and therefore, at the time of decision, the positive voltage is judged as "1", and the negative voltage is judged as "0"; the function of inverse code conversion is opposite to that of the sending end, and the relative code output by the judger is restored into absolute code; the absolute code enters a data buffer area for caching in a data frame mode, and the variable coding table is subjected to table look-up decoding to complete information receiving;
step 4, if data transmission is needed to be continued, the step 1 is executed again to perform mode sensing; when the USB interface is needed to be transmitted, the microcontroller transfers the memory information to the USB interface to wait for the transmission of the USB interface and exchange information with the computer; if not, enabling the underwater sound modem to enter a dormant state, waiting for waking up the equipment for next data transmission, and ending the data transmission; and repeating the steps 2-3 when underwater sound transmission is required.
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