CN107454031A - A kind of OFDM MFSK water sound communication techniques based on packet signal to noise ratio confidence level - Google Patents

Abstract

The invention provides an OFDM-MFSK underwater acoustic communication technology based on the confidence degree of the packet signal-to-noise ratio. The transmitting end performs 2-diversity modulation on the information, and adds the beginning and end of a frame of data to the linear frequency modulation signal; the receiving end detects the preamble signal of the frame, completes data interception and Doppler factor estimation, and performs Doppler compensation on the received data ; Demodulate each data symbol, and combine the diversity signals according to the combination method of the group signal-to-noise ratio confidence; finally demodulate the data after the diversity combination. This method uses frequency diversity technology to effectively compensate for the performance loss of the OFDM-MFSK underwater acoustic communication system caused by the severe frequency selectivity of the channel, and proposes a diversity signal combination method based on the confidence level of the packet signal-to-noise ratio to improve the accuracy of the combined signal. SNR.

Description

A OFDM-MFSK Underwater Acoustic Communication Technology Based on Group SNR Confidence

technical field

The invention relates to the field of underwater acoustic channels, in particular to an OFDM-MFSK underwater acoustic communication technology based on the confidence level of the packet signal-to-noise ratio.

Background technique

OFDM-MFSK is a modulation technology that combines multi-carrier technology and MFSK. The transmitting end of OFDM-MFSK divides all subcarriers into groups of M elements, uses MFSK to map information, and the receiving end uses a non-coherent method. MFSK signal detection is performed without channel estimation and equalization process. OFDM-MFSK is compatible with the high communication rate of multi-carrier modulation technology, and retains the robust performance of MFSK modulation. Compared with Orthogonal Frequency Division Multiplexing (OFDM) modulation, it can better balance the transmission rate and robust performance.

The underwater acoustic channel is one of the most complex wireless channels so far, and it is a double-expanded underwater acoustic channel with serious multi-channel expansion and Doppler expansion. Severe multipath spreading in the time domain of the channel is manifested as severe frequency selective fading in the frequency domain. When the effective subcarrier of the multi-carrier system is located at the deep fading frequency of the channel, the signal-to-noise ratio of the subcarrier will be seriously lost , conventional equalization techniques still cannot achieve better results. Diversity technology refers to using the same information to transmit data through mutually independent channels to reduce interference and improve system performance. Different combining methods of diversity signals will obtain different processing gains. In the OFDM system, the combining methods of frequency diversity signals mainly include: selective combining, equal gain combining and maximum ratio combining, but the data needs to be channel estimated and equalized. For OFDM-MFSK systems with non-coherent demodulation, the combining method of diversity signals needs to be optimally designed according to the specific characteristics of symbols. The present invention applies the frequency diversity technology to the OFDM-MFSK system, and combines the characteristics of the symbol itself, and proposes a diversity signal combination method of grouping signal-to-noise ratio confidence to improve system performance.

Contents of the invention

The purpose of the present invention is to provide an OFDM-MFSK underwater acoustic communication method based on the confidence degree of packet signal-to-noise ratio, which realizes the improvement of system performance under fading underwater acoustic channels.

The purpose of the present invention is achieved like this:

A kind of OFDM-MFSK underwater acoustic communication technology based on packet signal-to-noise ratio confidence, is characterized in that, comprises the steps:

(1) The transmitter performs 2-diversity modulation on the information, and adds the end of a frame of data to the chirp signal;

(2) The receiving end detects the frame preamble signal, completes data interception and Doppler factor estimation, and performs Doppler compensation on the received data;

(3) Demodulate each data symbol, and combine the diversity signals according to the combination method of the group signal-to-noise ratio confidence;

(4) Finally, demodulate the data after diversity combining.

The 2-diversity modulation specifically includes:

The said 2-diversity modulation of information means that all N subcarriers are divided into two diversity branches, that is, diversity branch 1 is a subcarrier [1:N/2], and diversity branch 2 is a subcarrier [N/ 2+1:N], and the two diversity branch maps have exactly the same information.

The linear frequency modulation signal specifically includes:

The bandwidth of the chirp signal is the working bandwidth of the system, the time product of the bandwidth of the chirp signal and the chirp signal is greater than 100, and the guard interval length between the data and the two chirp signals is greater than the maximum multi-path extension length of the channel.

Described step (2) specifically comprises:

Use the linear frequency modulation signal at the beginning and end of the data to perform matched filtering, calculate the average Doppler factor of the entire frame of data, and use the linear interpolation method to complete the Doppler compensation of the received signal according to the average Doppler factor; specifically include the following steps:

(2.1) Correlate the received data with the synchronous signal as the reference signal, find the peak coordinate points of the pre-frame and post-frame synchronous signals, calculate the time interval T _r between the two signals, and calculate the multiplicity of the entire data block according to the following formula Puller factor

Wherein, T _t is the time interval between two chirp signals in the transmission frame signal;

(2.2) The amplitude of the Doppler-compensated signal is obtained by linear interpolation:

Let the function f(x) be f(x ₀ ) and f(x ₁ ) at the two endpoints of the interval [x ₀ ,x ₁ ], and use the linear function L(x)=ax+b to approximate the interval [x ₀ ,x ₁ ], by selecting parameters a and b, so that L(x ₀ )=f(x ₀ ), L(x ₁ )=f(x ₁ ), the two points of the straight line equation can be The expression to obtain L(x) is

Combining formula (2) and the new sampling point position is The Doppler compensation of the received signal can be completed.

The demodulation in the described step (3) specifically includes:

Perform serial-parallel conversion on the Doppler-compensated signal, and remove the cyclic prefix and cyclic suffix to obtain OFDM-MFSK symbols; perform DFT demodulation on each OFDM-MFSK data symbol to obtain the frequency domain data of the corresponding subcarrier.

The merging mode of the grouping SNR confidence in the described step (3) specifically includes:

Suppose an OFDM-MFSK data symbol is subjected to DFT operation to obtain N subcarrier frequency domain data R(f _i )i=1,2,...,N, then each diversity branch is composed of P groups of MFSK data, P=N /2M; Suppose the sets of effective carrier positions and empty carrier positions of each group of data are respectively Y _A (q, p) and Y _N (q, p), thus the noise estimates σ _{q, p} and Signal-to-noise ratio SNR _q,p , q represents different diversity branches, and the values here are 1 and 2; subtract the noise power σ _q,p from the current group data to get new data R′ _q,p (f _m ), and then follow The following formula completes the signal combination of group SNR confidence

in:

R′ _q,p (f _m )=R _q,p (f _m )-σ _q,p

Described step (4) specifically comprises:

The merging method of the confidence level of the grouping signal-to-noise ratio refers to performing noise and signal-to-noise ratio estimation on the data of different diversity branches according to M elements, each group of data subtracts the noise estimated by the group and divides the group The signal-to-noise ratio is multiplied as a confidence degree, and finally the data of the two diversity branches are correspondingly summed to complete the diversity combination.

The beneficial effects of the present invention are: the method adopts the frequency diversity technology to effectively improve the system performance loss caused by the severe frequency selectivity of the channel, and combines the own characteristics of the OFDM-MFSK symbol to propose a method of grouping SNR confidence The diversity signal combining method further improves the diversity signal gain. Compared with the conventional OFDM-MFSK communication system, the OFDM-MFSK system based on the confidence level of packet SNR has better application performance in fading underwater acoustic channel.

Description of drawings

Fig. 1 is a schematic diagram of the transmission frame structure of the OFDM-MFSK underwater acoustic communication system;

Fig. 2 is a flow chart of combining diversity signals based on group SNR confidence.

detailed description

The present invention will be described in more detail below in conjunction with the accompanying drawings.

The present invention mainly comprises the steps:

(1) The transmitter performs 2-diversity modulation on the information, and adds the end of a frame of data to the chirp signal;

(2) The receiving end detects the frame preamble signal, completes data interception and Doppler factor estimation, and performs Doppler compensation on the received data;

(3) Demodulate each data symbol, and combine the diversity signals according to the combination method of the group signal-to-noise ratio confidence;

(4) Finally, demodulate the data after diversity combining.

The transmitting end performs 2-diversity modulation on the information, and adds the end of a frame of data to the chirp signal. The 2-diversity modulation of information refers to dividing all N subcarriers into two diversity branches, namely subcarrier [1:N/2] (diversity branch 1) and subcarrier [N/2+1:N ] (diversity branch 2), and the two diversity branches map exactly the same information. The bandwidth of the chirp signal is the working bandwidth of the system, the time product of the bandwidth of the chirp signal and the chirp signal is greater than 100, and the guard interval length between the data and the two chirp signals is greater than the maximum multi-path extension length of the channel.

Using the linear frequency modulation signal at the beginning and end of the data to perform matched filtering, calculate the average Doppler factor of the whole frame of data, and use the method of linear interpolation to complete the Doppler compensation of the received signal according to the average Doppler factor.

Each data symbol is demodulated, and the diversity signals are combined according to the combination method of group SNR confidence. The merging method of the confidence level of the grouping signal-to-noise ratio refers to performing noise and signal-to-noise ratio estimation on the data of different diversity branches according to M elements, each group of data subtracts the noise estimated by the group and divides the group The signal-to-noise ratio is multiplied as a confidence degree, and finally the data of the two diversity branches are correspondingly summed to complete the diversity combination.

Specifically: 1. The transmitting end performs 2-diversity modulation on the information, and the transmitting end adds chirp signals at the beginning and end of a frame of data.

The transmitting end performs data mapping on all N data subcarriers according to 2 frequency diversity, that is, subcarriers [1:N/2] and subcarriers [N/2+1:N] carry exactly the same data information, and this data is The IDFT operation completes the data modulation, obtains OFDM-MFSK symbols in the time domain, and adds a cyclic prefix and cyclic suffix that are greater than the channel multi-path extension to obtain an OFDM-MFSK data block. The cyclic prefix is to copy the data behind the OFDM-MFSK symbol to before the symbol. , the cyclic suffix is to copy the data in front of the OFDM-MFSK symbol to the end of the symbol, see Figure 1 for details, so as to reduce the inter-symbol interference caused by channel multi-path extension.

The transmission frame structure of the OFDM-MFSK system is shown in Figure 1. Now, the chirp signal before the data is called a frame preamble signal, and the chirp signal after the data is called a frame postamble signal. The parameters of the two chirp signals are consistent, and the length of the guard interval between the data and the two chirp signals is greater than the maximum multipath extension length of the channel.

2. The receiving end detects the frame preamble signal, completes data interception and Doppler factor estimation, and performs Doppler compensation on the received signal.

Perform correlation calculation on the received data with the synchronization signal as the reference signal, find the peak coordinate points of the synchronization signal before and after the frame, calculate the time interval T _r between the two signals, and calculate the Doppler factor of the entire data block according to the following formula

Wherein, T _t is the time interval between two chirp signals in the transmitted frame signal.

Assuming that the original sampling point vector is x, according to the calculated Doppler factor, the position of the new sampling point after Doppler compensation is The amplitude of the signal after Doppler compensation is completed by linear interpolation.

Let the function f(x) be f(x ₀ ) and f(x ₁ ) at the two endpoints of the interval [x ₀ ,x ₁ ], and use the linear function L(x)=ax+b to approximate the interval [x ₀ ,x ₁ ], by selecting parameters a and b, so that L(x ₀ )=f(x ₀ ), L(x ₁ )=f(x ₁ ), the two points of the straight line equation can be The expression to obtain L(x) is

Combining formula (2) and the new sampling point position is The Doppler compensation of the received signal can be completed.

3. Demodulate each data symbol, and combine the diversity signals according to the combination method of group SNR confidence.

The Doppler-compensated signal is serial-to-parallel converted, and the cyclic prefix and cyclic suffix are removed to obtain OFDM-MFSK symbols. Perform DFT demodulation on each OFDM-MFSK data symbol to obtain the frequency domain data of the corresponding subcarrier, and perform diversity signal combination based on the confidence level of the packet SNR. The specific steps of signal combination are given below:

Suppose an OFDM-MFSK data symbol is subjected to DFT operation to obtain N subcarrier frequency domain data R(f _i )i=1,2,...,N, then each diversity branch is composed of P groups of MFSK data, P=N /2M. Assume that the sets of effective carrier positions and empty carrier positions of each group of data are Υ _A (q, p) and Υ _N (q, p) respectively, so that the noise estimation σ _{q, p} and the signal-to-noise ratio of the current group of data can be calculated SNR _q,p , where q represents different diversity branches, where the values are 1 and 2. Subtract the noise power σ _q,p from the current group data to obtain the new data R′ _q,p (f _m ), and then complete the signal combination of the group SNR confidence level according to the following formula. The specific signal combination flow chart is shown in Figure 2 Show.

in:

R′ _q,p (f _m )=R _q,p (f _m )-σ _q,p

Finally, the merged data is divided into groups of M elements to complete data demodulation.

Claims (7)

1. A OFDM-MFSK underwater acoustic communication technology based on grouping SNR confidence, specifically comprising the steps: (1) The transmitter performs 2-diversity modulation on the information, and adds the end of a frame of data to the chirp signal; (2) The receiving end detects the frame preamble signal, completes data interception and Doppler factor estimation, and performs Doppler compensation on the received data; (3) Demodulate each data symbol, and combine the diversity signals according to the combination method of the group signal-to-noise ratio confidence; (4) Finally, demodulate the data after diversity combining. 2. a kind of OFDM-MFSK underwater acoustic communication technology based on packet signal-to-noise ratio confidence degree according to claim 1, is characterized in that, described 2 diversity modulations specifically comprise: The said 2-diversity modulation of information means that all N subcarriers are divided into two diversity branches, that is, diversity branch 1 is a subcarrier [1:N/2], and diversity branch 2 is a subcarrier [N/ 2+1:N], and the two diversity branch maps have exactly the same information. 3. a kind of OFDM-MFSK underwater acoustic communication technology based on grouping SNR confidence degree according to claim 1, is characterized in that, described chirp signal specifically comprises: The bandwidth of the chirp signal is the working bandwidth of the system, the time product of the bandwidth of the chirp signal and the chirp signal is greater than 100, and the guard interval length between the data and the two chirp signals is greater than the maximum multi-path extension length of the channel. 4. a kind of OFDM-MFSK underwater acoustic communication technology based on packet SNR confidence degree according to claim 1, is characterized in that, described step (2) specifically comprises: Use the linear frequency modulation signal at the beginning and end of the data to perform matched filtering, calculate the average Doppler factor of the entire frame of data, and use the linear interpolation method to complete the Doppler compensation of the received signal according to the average Doppler factor; specifically include the following steps: (2.1) Correlate the received data with the synchronous signal as the reference signal, find the peak coordinate points of the pre-frame and post-frame synchronous signals, calculate the time interval T _r between the two signals, and calculate the multiplicity of the entire data block according to the following formula Puller factor <mrow><mover><mi>&amp;epsiv;</mi><mo>^</mo></mover><mo>=</mo><mfrac><msub><mi>T</mi><mi>r</mi></msub><msub><mi>T</mi><mi>t</mi></msub></mfrac><mo>-</mo><mn>1</mn><mo>-</mo><mo>-</mo><mo>-</mo><mrow><mo>(</mo><mn>1</mn><mo>)</mo></mrow></mrow> Wherein, T _t is the time interval between two chirp signals in the transmission frame signal; (2.2) The amplitude of the Doppler-compensated signal is obtained by linear interpolation: Let the function f(x) be f(x ₀ ) and f(x ₁ ) at the two endpoints of the interval [x ₀ ,x ₁ ], and use the linear function L(x)=ax+b to approximate the interval [x ₀ ,x ₁ ], by selecting parameters a and b, so that L(x ₀ )=f(x ₀ ), L(x ₁ )=f(x ₁ ), the two points of the straight line equation can be The expression to obtain L(x) is <mrow><mi>L</mi><mrow><mo>(</mo><mi>x</mi><mo>)</mo></mrow><mo>=</mo><mfrac><mrow><mi>x</mi><mo>-</mo><msub><mi>x</mi><mn>1</mn></msub></mrow><mrow><msub><mi>x</mi><mn>0</mn></msub><mo>-</mo><msub><mi>x</mi><mn>1</mn></msub></mrow></mfrac><mi>f</mi><mrow><mo>(</mo><msub><mi>x</mi><mn>0</mn></msub><mo>)</mo></mrow><mo>+</mo><mfrac><mrow><mi>x</mi><mo>-</mo><msub><mi>x</mi><mn>0</mn></msub></mrow><mrow><msub><mi>x</mi><mn>1</mn></msub><mo>-</mo><msub><mi>x</mi><mn>0</mn></msub></mrow></mfrac><mi>f</mi><mrow><mo>(</mo><msub><mi>x</mi><mn>1</mn></msub><mo>)</mo></mrow><mo>-</mo><mo>-</mo><mo>-</mo><mrow><mo>(</mo><mn>2</mn><mo>)</mo></mrow></mrow> Combining formula (2) and the new sampling point position is The Doppler compensation of the received signal can be completed. 5. a kind of OFDM-MFSK underwater acoustic communication technology based on grouping SNR confidence degree according to claim 1, is characterized in that, the demodulation in the described step (3) specifically comprises: Perform serial-parallel conversion on the Doppler-compensated signal, and remove the cyclic prefix and cyclic suffix to obtain OFDM-MFSK symbols; perform DFT demodulation on each OFDM-MFSK data symbol to obtain the frequency domain data of the corresponding subcarrier. 6. a kind of OFDM-MFSK underwater acoustic communication technology based on grouping SNR confidence degree according to claim 1, is characterized in that, the merging mode of the grouping SNR confidence degree in described step (3) is specific include: Suppose an OFDM-MFSK data symbol is subjected to DFT operation to obtain N subcarrier frequency domain data R(f _i )i=1,2,...,N, then each diversity branch is composed of P groups of MFSK data, P=N /2M; Suppose the sets of effective carrier positions and empty carrier positions of each group of data are respectively Y _A (q, p) and Y _N (q, p), thus the noise estimates σ _{q, p} and Signal-to-noise ratio SNR _q,p , q represents different diversity branches, and the values here are 1 and 2; subtract the noise power σ _q,p from the current group data to get new data R′ _q,p (f _m ), and then follow The following formula completes the signal combination of group SNR confidence <mrow><mtable><mtr><mtd><mrow><msub><mi>R</mi><mi>p</mi></msub><mrow><mo>(</mo><msub><mi>f</mi><mi>m</mi></msub><mo>)</mo></mrow><mo>=</mo><munderover><mo>&amp;Sigma;</mo><mrow><mi>q</mi><mo>=</mo><mn>1</mn></mrow><mn>2</mn></munderover><msub><mi>SNR</mi><mrow><mi>q</mi><mo>,</mo><mi>p</mi></mrow></msub><msubsup><mi>R</mi><mrow><mi>q</mi><mo>,</mo><mi>p</mi></mrow><mo>&amp;prime;</mo></msubsup><mrow><mo>(</mo><msub><mi>f</mi><mi>m</mi></msub><mo>)</mo></mrow><msubsup><mi>R</mi><mrow><mi>q</mi><mo>,</mo><mi>p</mi></mrow><mrow><mo>&amp;prime;</mo><mo>*</mo></mrow></msubsup><mrow><mo>(</mo><msub><mi>f</mi><mi>m</mi></msub><mo>)</mo></mrow></mrow></mtd><mtd><mrow><mi>p</mi><mo>=</mo><mn>1</mn><mo>,</mo><mn>2</mn><mo>,</mo><mo>...</mo><mo>,</mo><mi>N</mi><mo>/</mo><mrow><mo>(</mo><mn>2</mn><mi>M</mi><mo>)</mo></mrow></mrow></mtd><mtd><mrow><mi>m</mi><mo>=</mo><mn>1</mn><mo>,</mo><mn>2</mn><mo>,</mo><mo>...</mo><mo>,</mo><mi>M</mi></mrow></mtd></mtr></mtable><mo>-</mo><mo>-</mo><mo>-</mo><mrow><mo>(</mo><mn>3</mn><mo>)</mo></mrow></mrow> in: R′ _q,p (f _m )=R _q,p (f _m )-σ _q,p 7. a kind of OFDM-MFSK underwater acoustic communication technology based on packet signal-to-noise ratio confidence degree according to claim 1, is characterized in that, described step (4) specifically comprises: The merging method of the confidence level of the grouping signal-to-noise ratio refers to performing noise and signal-to-noise ratio estimation on the data of different diversity branches according to M elements, each group of data subtracts the noise estimated by the group and divides the group The signal-to-noise ratio is multiplied as a confidence degree, and finally the data of the two diversity branches are correspondingly summed to complete the diversity combination.