Multicarrier underwater acoustic communication method

Technical Field

The invention relates to a communication method, in particular to a kind of underwater acoustic communication method.

Background Art

Underwater acoustic channel is a very complex-air-change channel, its main characteristic is that complexity, polytropic, strong multi-way and limited bandwidth. Acoustic propagation loss and sea water absorption losses so that the underwater acoustic channel bandwidth is extremely restricted, marine underwater acoustic channel in the presence of multipath effects of the received signal caused by the intersymbol interference distortion and serious, to the underwater acoustic communication system the design of the great difficulties.

OFDM (orthogonal frequency division multiplexing) technology is MCM (Multi-Carrier   Modulation, multi-carrier modulation) a. The main idea is that the channel is divided into a plurality of orthogonal channel, the high-speed data signal is converted into a parallel low-speed sub-data streams, modulated to in each of the sub-channel for transmission. Orthogonal signal may be passed through at the receiving end to separate the related art. Each sub-channel are the narrow-band signal of the signal, channel bandwidth is smaller than the related bandwidth, so each sub-channel can be viewed as the planarity of the fading, the inter-symbol interference can be eliminated.

OFDM technology have their disadvantages, the more sensitive to frequency offset and noise. OFDM technique distinguish each sub-channel is the method of utilizing the strictly between each sub-carrier orthogonality, each of its sub-carrier are single-frequency signal, the frequency offset and noise between each sub-carrier will be the orthogonal characteristic deterioration, only 1% of the frequency offset will make the signal-to-noise ratio drop 30dB. Therefore, the OFDM system are sensitive to frequency offset and noise.

After retrieval and the invention-related documents, including:

 Martone Massimiliano, "A   Multicarrier   System   Based   on   the   Fractional   Channels   Time-Frequency-Selective   for   Fourier Transform," IEEE   ON COMMUNICATIONS   TRANSACTIONS, VOL. 49, NO.6, JUNE   2001 (hereinafter referred to as literature 1).

chen Enqing ; carefree ; zhang Weiqiang ; Zhao Juan; meng Xiangyi , "fractional Fourier transform OFDM system self-adaptive equalization algorithm," electronic Journal, 2007, 35 (9): 1728-1733 (hereinafter referred to as literature 2).

carefree , such as chen Enqing , "based on fractional Fourier transform of the orthogonal frequency division multiplexing (OFDM) system," Chinese Patent CN public number   1859346A, 2006.01 (hereinafter referred to as Patent 1)

Literature 1 and Patent 1 on the basis of the fractional Fourier transform of the orthogonal frequency division multiplexing system examined (FRFT-OFDM), they mainly to the wireless-electrically communication system, according to different the relative movement speed of the of the different slope of the fractional Fourier transform, the need for a specialized transmission this slope of the channel. In high Doppler circumstances than the traditional OFDM has a better performance; however, in the two sides relatively static or low Doppler circumstances, system degradation into a traditional OFDM system, the traditional OFDM system performance. In general they are the traditional OFDM system in high Doppler an improved under the condition of. Literature 2 is similar to the previous two, only the frequency domain filter into the adaptive algorithm.

Content of the invention

The purpose of this invention is to provide an anti-noise capability, a high utilization rate of the frequency spectrum of the multi-carrier method for underwater acoustic communication.

The purpose of this invention is realized as follows:

In the transmitting terminal, the first is the need to transmit data for channel coding, and then carry on digital modulation, the modulated after the Serial-parallel conversion of the Serial symbol a, and the parallel symbols on distributed to the corresponding frequency, frequency distribution is symmetrical, half of the frequency modulation slope is, half of the frequency modulation slope is negative, the frequency domain symbols of the parallel distribution is good for discrete fractional Fourier transform, the above-mentioned base band modulation process is referred to as cross-modulation, the modulated time-domain symbol its sub-carrier pair, each pair is the same as the carrier center frequency, is equal to the frequency modulation slope, and symbol on the contrary, then, the time domain symbols of the real part and the imaginary part as quadrature modulation, modulation to the two orthogonal high-frequency carrier wave, the transmission of a real part and an imaginary part at the same time, the front of each symbol data by adding a guard interval;

At the receiving end, the guard interval removed at first, the orthogonal demodulated to baseband complex signal, the channel of each frame for learning sequence for the channel estimation, channel transmission matrix obtained, time domain equalization of the signal, multipath diversity receiving, application discrete fractional Fourier transform to the signal make sub-carrier demodulation, the symbols receive mark step territory , finally in the fractional step territory , based on maximum likelihood criteria for the decision feedback equalization algorithm, the signal for frequency-domain equalization, digital demodulation and decision at the same time form a data output stream.

The invention may also include:

1, of a transmitting end of the parallel frequency domain symbols sub-assembled as discrete fractional Fourier transformation of the formula is:

s=F_-α · x

Wherein s= [s (0), s (1), ...s (N-1)]^T for the data vector and receiving vector of treats the demodulation , F_-α to a discrete inverse fractional Fourier transform matrix.

2, the application of the used discrete fractional Fourier transform to the signal to make sub-carrier demodulation of the formula:

y=F_α · r

Wherein r= [r (0), r (1), ...r (N-1)]^T for the data vector and receiving vector of treats the demodulation , F_α into discrete fractional Fourier transform matrix.

3, sub-carrier for the positive and negative frequency modulation slope of the sub-carrier to, a 2K sub-carrier, is capable of sub-carrier K, for K negative accent frequency sub-carriers, each sub-carrier frequency is:

&# X003C9; &# X003B1; , n = n 2 &# X003C0; T Symbol - t Cos ( [! PlusMinus! ] &# X003B1; ) , n = 0, 1 , . . . . . . , K - 1

The angle α is the transformation of the IDFrFT, T_symbol to symbol duration, t   ∈ (-T_symbol/2, T_symbol/2), n to the subcarrier number.

4, the orthogonal modulation modulation formula is:

u (t) =real (s (t)) ·cos (2 π f_c t) +imag (s (t)) ·sin (2 π f_c t)

Wherein s (t) for the emitting end of the baseband complex signal, u (t) for the emitting end of the high-frequency transmission signal of quadrature modulation, f_c the carrier frequency of the quadrature modulation, and real(.)imag(.)indicated that fetching the real part and takes the imaginary component , t∈ (-T_symbol/2, T_symbol/2).

5, the quadrature demodulation of the demodulating formula is:

r (t) = LP[v (t) ·cos (2 π f_c t) +j·v (t) ·sin (2 π f_c t)]

R of them (t) the baseband complex signal for the receiving end, v (t) in order to receive the end of the high-frequency transmission signal of quadrature modulation, f_c the carrier frequency of the quadrature modulation, said LP low-pass filter [.], t∈ (-T_symbol/2, T_symbol/2).

The invention according to the characteristics of the underwater acoustic communication, provides a high efficiency on the basis of the fractional Fourier transform of the orthogonal frequency division multiplexing (OFDM) communication system, the fractional-Fourier transform of the slope, or stationary relative to the two sides under the condition of low Doppler, sub-carrier remains is a linear frequency-modulated signal, with 5logBT processing gain, characteristic is superior to the traditional OFDM system, and for underwater acoustic communication characteristic for the improvement and innovation. The of the present invention is different from the systems of prior are as follows:

1. Because the underwater acoustic channel bandwidth is far less than that of the radio channel, the invention provides on the basis of cross-modulation, with a positive and negative frequency modulation leads carrier baseband modulation technique, the utilization factor of the frequency band can be doubled.

2. In order to ensure orthogonality between sub-carriers, the invention uses the complex signal modulation and transmission.

3. In order to improve the transmission efficiency, the invention adopts the quadrature modulation method, at the same time transmitting the composite signal of the real and the imaginary part.

4. This invention introduces technical time-domain equalization of the receiver Rake, anti-multipath diversity reception.

5. The invention proposes new fractional Fourier-domain equalization method, based on the maximum likelihood criterion of decision feedback equalization technology.

In addition to the defect in the prior art, this invention has the following improvements:

1. The cyclic prefix is not adopted, and is directly inserted into the zero intersymbol interference as a protection interval.

2. Does not adopt the inserted pilot estimated channel method, data of each frame is added to the learning signal before channel.

Description of drawings

Figure 1 is block diagram the underwater sound are many -carrier communication system;

Figure 2 the carrier the underwater sound are many -carrier communication system is time-frequency map;

Figure 3 is a plurality of signal and the actual signal transmission compared with the Figure;

Fig. 4 is a communication data frame chart;

Figure 5 is based on the maximum likelihood criterion of decision feedback equalization algorithm flow chart;

Figure 6 are based on fractional conversion OFDM bit error rate comparison with traditional OFDM;

Fig. 7 is based on the fractional conversion OFDM bit error rate comparison with traditional OFDM.

Mode of execution

In conjuction with the examples below of the present invention is used as described in more detail:

For base scallop -carrier modulation, baseband multi-carrier modulation/demodulation formula is:

s=F_-α · x     (1)

and

y=F_α · r     (2)

Wherein s= [s (0), s (1), ...s (N-1)]^T and r= [r (0), r (1), ...r (N-1)]^T for the data vector and receiving vector of treats the demodulation , F_-α and F_α are respectively a discrete inverse fractional Fourier transform matrix and discrete fractional Fourier transform matrix.

For underwater acoustic channel of limited bandwidth, in order to improve the utilization factor of the frequency band, the invention provides a sub-carrier for the positive and negative frequency modulation slope of the sub-carriers. With 2K sub-carrier, is capable of sub-carrier K, for K negative accent frequency sub-carriers, each sub-carrier frequency is:

&# X003C9; &# X003B1; , n = n 2 &# X003C0; T Symbol - t Cos ( [! PlusMinus! ] &# X003B1; ) , n = 0, 1 , . . . . . . , K - 1 - - - ( 3 )

Each of the sub-carrier is:

<!---->

<!---->

The angle α is the transformation of the IDFrFT, T_symbol to symbol duration, t∈ (-T_symbol/2, T_symbol/2), n to the subcarrier number, sub-carrier pair, each sub-carrier center frequency the same, frequency modulation slope equivalent symbol on the contrary, such a modulation referred to as cross-modulation. Can prove that all sub-carriers of positive and negative slope between the two orthogonal (prove slightly).

As shown in Figure 2, the time-frequency distribution for the carrier, the frequency of the transverse, axis of ordinates is taken as a time. Wherein based on the Fourier transform of the OFDM system, each sub-carrier is a single frequency signal, a total of 10 sub-carrier; to the original in on the basis of the fractional Fourier transform of the OFDM system, each sub-carrier is a linear FM signal, only one kind of frequency modulation slope and 7 of the sub-carrier; for the communication system of this invention, each sub-carrier is a linear FM signal, capable of a total of both positive and negative is divided into 14 sub-carrier. As can be seen from the chart, in having the same bandwidth, the same as under the condition of the sub-carrier interval, the original on the basis of the fractional Fourier transform of the OFDM sub-carrier is a broadband signal, itself, it will take a certain bandwidth, at least the number of sub-carriers. The present invention uses cross-modulation technique, available sub-carriers is two times of the former, than OFDM system based on Fourier transform of sub-carriers but also, with the highest spectrum utilization.

Cross-modulation between sub-carrier signals of two orthogonal to each other is the result of the complex domain, the sub-carrier to the real signal does not can be completely orthogonal. Figure 3 is a fractional step territory bandmap of, at the as to cross a plurality of signal modulation, after sterilization to cross-modulation to the real signal. Building decoration can see real signal transmission will produce very strong sub-inter-channel interference, therefore, the invention adopts the transmission mode of the complex signal, at the same time in the signal channel transmission complex signal of the real and the imaginary part.

Because of the need to transmit a real part and an imaginary part of the complex signal, in order not to reduce the transmission data rate, this invention adopts the orthogonal modulation technique. The baseband complex signal of the real and imaginary parts are respectively two orthogonal carrier modulation to the high-frequency, at the same time transmission; in the receiving end thereof is used for quadrature demodulation, reconfigurable baseband complex signal. Quadrature modulation/demodulation formula is:

u (t) =real (s (t)) ·cos (2 π f_c t) +imag (s (t)) ·sin (2 π f_c t)     (5)

and

r (t) = LP[v (t) ·cos (2 π f_c t) +j·v (t) ·sin (2 π f_c t)]     (6)

Wherein s (t) and r (t) to a transmitting end and a receiving end of the base-band signal, u (t) and v (t) to a transmitting end and a receiving end through the high-frequency transmission signal of quadrature modulation, f_c the carrier frequency of the quadrature modulation, and real(.) takes the imaginary component that fetching the real part and said imag (.), LP [.] expressed the low-pass filter, t∈ (-T_symbol/2, T_symbol/2). In this way, can transmit the real part and the imaginary part signal at the same time, the transmission time is not increased, thereby improving the transmission efficiency.

Traditional OFDM system for each subcarrier single-frequency signal, therefore, by adopting the cyclic prefix, as long as the multi-way time delay does not exceed the length of the cyclic prefix can be ensure that each sub-carrier has a complete cycle, so can guarantee orthogonality between sub-carriers, will not produce sub-inter-channel interference (ICI).

Unfortunately the cyclic prefix is applied to the technology can not be based on fractional Fourier transform in OFDM. Because for single-frequency signal, the translation of the on time will only produce phase change, will not change the signal frequency, the orthogonal still between sub-carriers. In the fractional step territory the translation of the time not only can produce phase change, but also changing the frequency, the score on the translation of the spectral lines step territory , after the superposition thus the plurality of channels, the sub-carrier can not be guaranteed between the orthogonal. Therefore, the invention is given up and used in the system of the cyclic prefix of technical, directly in between each of the symbols by adding a guard interval, then using follow-up method of a plurality of interference caused by ICI. For the same reason the pilot channel estimation method will also greatly affected and not available, the invention adopts the data of each frame adding a special channel before method of learning signal to estimate the channel, the data frame structure is shown in Figure 4. The following algorithm assuming that channel known, and assuming that more extends the lengthwhen way smaller than the guard interval, (ISI) therefore not consider inter-symbol interference.

To multi-way interference, this invention adopts the time domain and fractional step territory two-step balancing method, time domain the receiver Rake, using decision feedback equalization in the frequency domain algorithm.

At the receiving end to introduce Rake receiver technology, the multi-path diversity reception, increasing the signal-to-noise, reduce sub-inter-channel interference. Assume that the received signal is:

R=Hs+n     (7)

R the received signal which, s to transmit a signal, for the channel matrix H, n is the noise vector. Its algorithm formula is:

s ^ = H H r = H H Hs + H H n - - - ( 8 )

Wherein H^H H guan Zhen is the channel each other, and H^H n is colored complex Gaussian noise. The receiver technology Rake channel impulse response to the received signal with a matched filtering, interference suppressing multiple transit.

Attention usually H^H H is non-diagonal matrix, therefore the performance of the receiver technology Rake of the is limited. That is to say, even if the signal-to-noise ratio is infinitely large, it cannot be guaranteed that no ICI interference, therefore we also want to join fraction step territory equalization technology.

Based on the maximum likelihood criterion of decision feedback equalization algorithm flow as shown in Figure 5, wherein the score is received from r step territory signal, a fraction of the d step territory signal after equalization, an estimate of the information code is s, r 'virtual transmission is the fraction of the received signal after step territory , with r r to e' error. The first fraction of the received judgment step territory signal, to obtain an estimate of the information; and then the estimated value obtained after the virtual transmission r ', transmission formula is:

r &# X02032; = F &# X003B1; H H H F - &# X003B1; s ^ - - - ( 9 )

R r to the ' difference of e, according to the e d by the feedback algorithm for judgment, so repeated iteration until the e to a minimum. At this time, s is the output. The invention adopts the feedback algorithm is as follows:

D=r-e     (10)

This invention adopts the constant frequency modulation slope of the linear frequency-modulated signal as the sub carrier, each sub-carrier is a broadband signal, than the traditional OFDM system has more high processing gain, therefore anti-noise capability is stronger; and uses the baseband cross-modulation method of quadrature modulation, has more high frequency spectrum utilization rate; the (Rake receiver) in the time domain and frequency domain (decision feedback equalizer) two balanced algorithm anti-ICI interference. To sum up, this invention has a more excellent than the conventional the performance of the OFDM system.

In the transmitting terminal, the first is the need to transmit data for channel coding, and then carry on digital modulation (for example, QPSK). The modulated after the Serial-parallel conversion of the Serial symbol a, and the parallel symbols on distributed to the corresponding frequency, frequency distribution is symmetrical, half of the frequency modulation slope is, half of the frequency modulation slope is negative. Push-button (1) the distribution is shown by the parallel frequency domain symbol as the discrete fractional Fourier transform, the above-mentioned base band modulation process is referred to as cross-modulation. The modulated time-domain symbol its sub-carrier pair, each pair is the same as the carrier center frequency, is equal to the frequency modulation slope, and symbol on the contrary, the carrier formula formula (4), wherein each of the carrier frequency formula (3). Furthermore, the time domain symbols of the real part and the imaginary part as quadrature modulation, modulation to the two orthogonal high-frequency carrier wave, the transmission of a real part and an imaginary part at the same time, see formula (5). Finally, in front of each symbol data by adding a guard interval, preventing ISI interference, a guard interval depends on the size of the channel multi-path expansion.

At the receiving end, the guard interval removed at first, the orthogonal demodulated to baseband complex signal, see formula (6). The channel of each frame for learning sequence for the channel estimation, channel transmission matrix is obtained, the specific method can reference other literature. Formula (8) of the receiver to signal Rake time-domain equalization, multipath diversity receiving, increasing the signal-to-noise, reduce sub-inter-channel interference. Then according to the formula (2) as shown, the application of discrete fractional Fourier transform to the signal make sub-carrier demodulation, the symbols receive mark step territory. Finally in the fractional step territory , based on maximum likelihood criteria for the decision feedback equalization algorithm, the signal for frequency-domain equalization, digital demodulation and decision at the same time form a data output stream. In order to illustrate the system of the present invention and the effectiveness of the algorithm, a simulation experiment. Simulation conditions are as follows, assume that channel is a generalized smooth non-correlated scattering multi-path channel, the multi-path number 3 strip, one of the direct sound to the, the other two are the seabed and the surface reflection acoustic, relative to their direct link sound for the attenuation of the 3dB. Each symbol of the sampling sequence is 512 points, guard interval length is 128 points, for QPSK digital modulation mode, each frame includes 80 a one symbol, system bandwidth is 2.2KHz, carrier frequency to 8KHz. Figure 6 on the basis of the fractional Fourier transform of the OFDM bit error rate comparison with traditional OFDM, the same two system parameters, the sub-carrier number is 64 a, it can be seen the FRFT-based OFDM performance is obviously superior to that of conventional OFDM. The reducing of the sub-carriers, as shown in Figure 7, the sub-carrier number is 32, based on the fractional Fourier transform is more excellent performance OFDM.