CN111585688B

CN111585688B - OCDM underwater acoustic communication method based on index modulation

Info

Publication number: CN111585688B
Application number: CN202010410679.7A
Authority: CN
Inventors: 何成兵; 张晓洋; 张阳; 史文涛; 戴祥飞; 吴新宇
Original assignee: Northwestern Polytechnical University; Shenzhen Institute of Northwestern Polytechnical University
Current assignee: Northwestern Polytechnical University; Shenzhen Institute of Northwestern Polytechnical University
Priority date: 2020-05-15
Filing date: 2020-05-15
Publication date: 2022-06-21
Anticipated expiration: 2040-05-15
Also published as: CN111585688A

Abstract

The invention provides an OCDM underwater acoustic communication method based on index modulation, which combines a spatial modulation technology with OCDM, and utilizes the index position of a silent subcarrier to carry information while activating the subcarrier to transmit phase modulation information. The invention makes the effective sub-carrier number in the OCDM communication system reduce to reduce the interference between the sub-carriers, and simultaneously, because the silent sub-carrier also bears the information, the data rate loss caused by the reduction of the factor carrier number is compensated.

Description

OCDM underwater acoustic communication method based on index modulation

Technical Field

The invention relates to the field of underwater acoustic communication, in particular to an OCDM (optical code division multiplexing) underwater acoustic communication method based on index modulation.

Background

With the development of the human marine industry, the development requirements of countries in the world on underwater information transmission technology are increasing day by day. As is known, electromagnetic waves cannot be transmitted underwater in a long distance, so that the underwater acoustic communication technology is the only reliable mode for the remote wireless underwater information transmission at present. However, compared to other communication channels, the underwater acoustic channel is considered to be one of the most challenging channels in the communication field due to its severe multipath spreading, time delay spreading and doppler effect, and various fading characteristics caused thereby.

The Orthogonal Frequency Division Multiplexing (OFDM) technique has a high Frequency band utilization ratio and can effectively reduce Inter Symbol Interference (ISI) caused by the multipath effect of a channel, but OFDM also has the disadvantages of being susceptible to channel time variation and Frequency attenuation, having a high peak-to-average power ratio, being sensitive to phase difference, and the like. These disadvantages result in poor performance of OFDM modulation in frequency offset and in underwater acoustic channels where multipath is significant.

Dawn red et al propose an Orthogonal Chirp Division Multiplexing (OCDM) underwater acoustic communication method (shenghong, marquis, "a mobile underwater acoustic communication method", chinese patent publication No. CN107682297A), which uses Discrete Fractional Fourier Transform (DFRT) to replace the Fourier Transform in the OFDM modulation scheme, so as to rotate the sub-carriers on the time-frequency plane, so that the sub-carrier signals become a set of Orthogonal LFM signals. Namely, the OCDM changes the narrow-band subcarrier of the OFDM into the broadband signal, so that the broadband signal can have the frequency band utilization rate of the multi-carrier modulation technology and simultaneously improve the frequency attenuation resistance of the system. While information transmission still uses phase modulation to carry information on the subcarriers as OFDM modulation does. In the method, the further improvement of the system performance is not only dependent on the improvement of the performance of a receiving end equalization algorithm, but also the most direct way is to increase the subcarrier interval and reduce the interference among the subcarriers, thereby improving the system performance. However, in the case of fixed subcarriers, increasing the subcarrier spacing means a decrease in the effective subcarriers, so that the performance of the communication system is improved at the expense of the data transmission rate.

Disclosure of Invention

In order to overcome the defects of the prior art, the invention provides an OCDM underwater acoustic communication method based on index modulation. In order to make up for the data rate sacrificed by improving the communication performance of the OCDM, the invention provides an index modulation-based OCDM underwater acoustic communication method, which combines a spatial modulation technology with the OCDM. And carrying information by utilizing the index position of the silent subcarrier while the active subcarrier transmits the phase modulation information. The invention reduces the effective sub-carrier number in OCDM communication system to reduce the interference between sub-carriers, and compensates the data rate loss caused by the reduction of factor carrier number because the silent sub-carrier also carries information.

The technical scheme adopted by the invention for solving the technical problem comprises the following steps:

the method comprises the following steps: determining that the number of single symbol subcarriers is N, the index modulation order is M1, the phase modulation order is M, and the frequency domain interval is F;

step two: at the transmitting end of the communication system, serial-parallel conversion is carried out on the serial data stream after channel coding to obtain parallel data streams, and the length of each group of data streams is

A bit;

step three: modulating the parallel data obtained in the second step, log₂The (M) bit data is used for selecting the index position of the active subcarrier, namely, the part of data information is carried on the index position of the active subcarrier; another part log₂(M1) converting the bit data into phase information by phase modulation, and modulating the phase information on the active sub-carrier; meanwhile, in order to eliminate ISI to the maximum extent, a fractional order domain guard interval is introduced in the mapping process, so that each group of data is activated after all data mapping is finished

A subcarrier; respectively performing Inverse Fractional Fourier Transform (IDFRT) on the parallel data streams obtained in the second step, and obtaining a group of LFM signals with the same frequency modulation slope and different center frequencies after the IDFRT;

step four: respectively adding the parallel data streams obtained in the step three into a cyclic protection prefix;

step five: parallel data streams added with the cyclic prefixes in the step four are subjected to parallel-serial conversion and then transmitted through the energy converter;

step six: the receiving end carries out serial-to-parallel conversion on serial data received by the hydrophone after passing through the underwater acoustic channel;

step seven: respectively removing cyclic prefixes from the parallel data streams obtained through serial-parallel conversion in the step six;

step eight: respectively performing Fourier transform on the parallel data streams with the cyclic prefixes removed in the seventh step to convert time domain data into frequency domain data, equalizing the received data in the frequency domain by adopting Minimum Mean Square Error (MMSE), and converting the frequency domain data into time domain signals by inverse Fourier transform;

step nine: performing fractional order Fourier transform of corresponding orders on the time domain signals obtained in the step eight to obtain fractional order domain signals, and simultaneously removing the interval of sub-carriers of the fractional order domain;

step ten: searching the index position of the activated subcarrier for the fractional order domain signal obtained in the step nine so as to reflect the data information carried by the emission index position; then, phase modulation and demodulation are carried out on the phase information carried by the activated subcarrier to obtain the data information of the other part;

step eleven: and D, performing parallel-serial conversion on the demodulated parallel data information obtained in the step ten, and outputting the parallel data information.

The invention has the advantages that the information is carried by the index position of the activated subcarrier, so that the frequency shift resistance of the communication system is improved compared with the OCDM system due to the reduction of the effective subcarrier number, and the reduction of the data rate due to the reduction of the effective subcarrier number is compensated.

Drawings

FIG. 1 is a block diagram of an IM-OCDM underwater acoustic communication system of the present invention.

Fig. 2 is a schematic diagram of IM-OCDM subcarrier mapping according to the present invention.

Fig. 3 is a diagram of the fractional fourier transform energy focusing of the IM-OCDM subcarrier of the present invention.

FIG. 4 is a diagram of the error performance of IM-OCDM, OCDM and OFDM of the present invention.

Detailed Description

The invention is further illustrated with reference to the following figures and examples.

A bit;

step nine: performing fractional Fourier transform of corresponding orders on the time domain signals obtained in the step eight to obtain fractional domain signals, and simultaneously removing the interval of fractional domain subcarriers;

Example (b):

referring to fig. 1, the specific flow of the index modulation based OCDM underwater acoustic communication system provided by the present invention is as follows:

the method comprises the following steps: and determining that the number of the single symbol subcarriers is N, the index modulation order is M, the phase modulation order is M1, and the frequency domain interval is F.

Step two: serial-to-parallel conversion of serial data streams to be transmitted, each set of data having a length of

Bits

Step three: referring to FIG. 2, log₂M bits of data are used for selection of the index position of the active subcarriers, i.e. one subcarrier is activated from M subcarriers as the active subcarrier carrying the phase information, after which log₂The M1 bit data is phase modulated using MPSK to carry phase information on the activated subcarriers. Therefore, when each group of data is modulated, a single symbol activates the subcarrier number to be

And (4) respectively.

Step four: in step three, in order to effectively cope with multipath interference, a fractional order domain guard interval is added in the frequency point mapping process, namely, an interval is inserted between effective carriers in the mapping process.

Step five: and respectively carrying out N-point IDFRT on the parallel data added with the guard interval in the step four to obtain a group of orthogonal LFM signals with equal intervals. The DFRT algorithm adopted by the invention is an Ozaktas decomposition type algorithm. The continuous fractional fourier transform is defined as:

in the formula

p is the order of the fractional Fourier transform. F^αOperator for fractional Fourier transform, F^α[f(t)]I.e. to perform a fractional fourier transform on the function f (t).

The Ozaktas algorithm decomposes the formula (1) correspondingly, converts a complex integral form in continuous fractional Fourier transform into a simple form, combines a Shannon interpolation formula, and finally obtains a DFRT result by FFT calculation. Therefore, the computational complexity of the method is basically equivalent to that of FFT, so that the calculation cost of OCDM is almost the same as that of OFDM.

And (3) performing fractional Fourier inverse transformation on the signal delta (u-nsin alpha/T) mapped in the step (2) by using a formula 1 to obtain:

after IDFRT is carried out on delta (u-nsin alpha/T), a group of LFM signals with the same frequency modulation slope, different central frequencies and central frequency interval of 2 pi/T are obtained. The transmitted waveforms of the present invention are thus a set of orthogonal LFM waveforms, where the frequency of each LFM waveform is:

meanwhile, the following formula (2) can be derived:

as can be seen from the equation (4), the LFM sub-carriers obtained after IDFRT are orthogonal to each other as well as the OFDM sub-carriers.

Step six: at the receiving end, the invention estimates the channel by using the block pilot frequency, then transfers the received data to the frequency domain, and then removes the channel action by using Minimum Mean Square Error (MMSE). And after equalization is completed, data is transferred to a time domain again through inverse Fourier transform, and subcarrier energy focusing is completed through fractional Fourier transform. Wherein the equalizer coefficients of the MMSE equalization are

In the above formula, H is channel impulse response, N₀/(2E_s) Is the signal to noise ratio.

Step seven: removing the subcarrier intervals of the signals converted into the fractional order domain in the sixth step;

step eight: after the seventh step, referring to the IM-OCDM subcarrier fractional fourier transform energy focusing diagram of fig. 3, by detecting the position of the activated subcarrier after energy focusing, corresponding index information can be obtained, and then phase demodulation is performed on the activated subcarrier to obtain phase information.

Step nine: and finally, performing parallel-serial conversion on the demodulated data and outputting the data.

The invention is further described in detail by utilizing a specific experimental example in combination with a flow chart of an IM-OCDM system:

simulation parameter setting of the embodiment of the invention: the bandwidth is 4000Hz, the central carrier frequency is 6000Hz, the total carrier number is 64, the CP length is 16, the index modulation order is 4, and the fractional Fourier transform order is 1.01. Meanwhile, the fractional order domain interval of the IM-OCDM is 1, and the fractional order domain guard interval of the OCDM and the frequency domain interval of the OFDM are both 2, so that the effective carrier number of the IM-OCDM is 16, and the effective carrier number of the OCDM and the OFDM is 32. The parameter setting ensures that the data rates of IM-OCDM, OCDM and OFDM are 3047 bits/s. Therefore, the experiment can carry out performance comparison experiments under the condition that the communication rates of the three modulation modes are the same.

Referring to FIG. 4, it is a diagram of error code performance of IM-OCDM, OFDM under the condition of underwater acoustic channel with multipath number of 10 and white Gaussian noise. Through comparison of error code performance curves, compared with OCDM and OFDM, the IM-OCDM has the same communication rate as other two modulation modes while improving the system performance due to the increase of the effective subcarrier spacing.

Claims

1. An OCDM underwater acoustic communication method based on index modulation is characterized by comprising the following steps:

the method comprises the following steps: determining the number of single symbol subcarriers as N, the index modulation order as M1, the phase modulation order as M and the frequency domain interval as F;

A bit;