CN109347778B - Improved companding transformation method for reducing peak-to-average ratio of orthogonal frequency division multiplexing underwater acoustic communication system - Google Patents
Improved companding transformation method for reducing peak-to-average ratio of orthogonal frequency division multiplexing underwater acoustic communication system Download PDFInfo
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- H04L27/00—Modulated-carrier systems
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Abstract
The invention relates to an improved companding conversion method for reducing the peak-to-average power ratio (PAPR) of an Orthogonal Frequency Division Multiplexing (OFDM) underwater acoustic communication system, belonging to the field of underwater acoustic communication and relating to a companding conversion method for reducing the PAPR of the OFDM underwater acoustic communication system, aiming at avoiding the problem that the orthogonality among subcarriers of the OFDM system is influenced by the clipping distortion generated when an underwater acoustic power amplifier works in a nonlinear area due to the overhigh signal peak power and further influencing the error rate of the whole communication system. The invention improves the companding conversion method at the transmitting end and the receiving end respectively, reduces the influence of the companding conversion method on the error code performance of the system, and the simulation result aiming at the underwater environment shows that when the signal to noise ratio is 20dB, the error rate of the underwater sound system adopting the improved companding conversion method is 10 lower than that of the underwater sound system adopting the traditional companding conversion method‑1And the improved method is proved to be more suitable for the underwater sound field of the complex channel environment.
Description
Technical Field
The invention relates to an improved companding transformation method for reducing the peak-to-average power ratio (PAPR) of an Orthogonal Frequency Division Multiplexing (OFDM) underwater acoustic communication system, belonging to the field of underwater acoustic communication.
Background
At present, two major development directions in the field of underwater acoustic communication are short-range high-speed underwater acoustic communication and long-range low-speed underwater acoustic communication, wherein the short-range high-speed underwater acoustic communication mainly depends on a multi-carrier modulation technology. In the multicarrier modulation technology, the OFDM technology has been widely used in recent years in underwater acoustic high-speed communication with limited bandwidth resources due to its advantage of high spectrum utilization. However, the OFDM technology has a drawback of a high peak-to-average power ratio (PAPR), which becomes an obstacle to its wide application in the field of underwater acoustic communications, and the PAPR increases as the number of subcarriers increases. The high PAPR is restricted by the linear dynamic range of the power amplifier of the underwater acoustic transmitter, generates clipping distortion, influences the error rate performance of the whole system, increases the complexity of an analog-to-digital converter, reduces the accuracy of the analog-to-digital converter, causes signal distortion, and destroys the orthogonality among OFDM subcarriers, so that the method for reducing the OFDM peak-to-average ratio is particularly important.
The research on the technology for reducing the peak-to-average power ratio at home and abroad is mainly divided into the following categories, namely a coding technology, a probability technology and a signal predistortion technology. Among the three technologies, the signal predistortion technology has the advantages of simple algorithm and no increase of calculation complexity along with the increase of the number of subcarriers, and is more suitable for OFDM underwater acoustic communication with limited channel bandwidth and more subcarriers. The companding method in the signal predistortion type technique is one of the peak-to-average ratio reduction algorithms commonly used in radio. The main idea of the conventional companding method is to raise the average power of a signal by raising a low amplitude value in the signal and maintaining the peak amplitude, thereby achieving the purpose of reducing the PAPR. The calculation complexity of the companding conversion method is much smaller than that of other methods, and the calculation complexity is not increased along with the increase of the number of the subcarriers because only the transmitted or received data is subjected to numerical conversion and is independent of the number of the subcarriers. However, this method increases the average transmission power of the system, and makes the power value of the symbol closer to the nonlinear transformation region of the power amplifier, thereby easily causing signal distortion. There are also many studies on companding algorithms by the scholars in the industry, wherein the study focuses mainly on the optimized selection of the companding function to improve the companding method to reduce the PAPR performance. The companding function is mainly divided into a linear companding function and a nonlinear companding function. In the radio field, many scholars at home and abroad make intensive research on reduction of the peak-to-average power ratio of the OFDM, but in the underwater acoustic field, the technology is slowly developed, and due to the fact that the performance of an underwater acoustic communication system is affected in various aspects such as complexity of an underwater acoustic channel, complexity of an algorithm, selection of parameters and the like, the peak-to-average power ratio reduction method which is simple and easy to implement is more suitable for the underwater acoustic communication field. When the companding parameters are selected by the conventional radio companding conversion method, the peak value of a signal is generally selected as an important parameter, but in underwater acoustic communication, due to the severe multipath effect of an underwater acoustic channel, the underwater acoustic signal received by a receiving end is influenced by marine organisms, passing ships and the like, and sudden noise exists. The invention improves the traditional companding transformation method at the transmitting end, selects the average value of the time domain signal as the parameter of companding transformation, and avoids the problem of improving the integral average peak value of the signal. In the process of channel transmission, the parameters and the transmitting signals are transmitted to the underwater sound channel, and decompression and expansion operation is carried out on the receiving signals by using the parameters at the receiving end, so that further expansion of decompression and expansion noise is avoided.
Disclosure of Invention
The invention aims to provide an improved companding transformation method for reducing the peak-to-average power ratio of an orthogonal frequency division multiplexing underwater acoustic communication system in order to avoid the problem that the orthogonality among subcarriers of an OFDM system is influenced by the clipping distortion generated when an underwater acoustic power amplifier works in a nonlinear region due to overhigh signal peak power and further influence the error rate of the whole communication system.
The purpose of the invention is realized as follows: the method comprises the following steps:
the method comprises the following steps: at a transmitting end, a transmitting signal is subjected to series-parallel transformation and then is subjected to an inverse fast Fourier transform module, and a frequency domain signal is converted into a time domain signal;
step two: at a transmitting end, carrying out improved companding conversion processing on the time domain signal;
step three: at the transmitting end, transmitting the signals after the improved companding conversion and the improved companding conversion information to an underwater sound channel;
step four: at a receiving end, carrying out synchronous operation on the receiving domain signals;
step five: at a receiving end, performing cyclic prefix removing operation on the synchronized signal;
step six: at a receiving end, carrying out improved decompression transformation operation on the signal without the cyclic prefix;
step seven: at a receiving end, carrying out operations such as channel estimation and the like on the signals after the improved decompression and expansion transformation to obtain required signals;
the invention also includes such features:
1. in the second step, at the transmitting end, the improved companding transformation adopts companding transformation of improved companding parameters, and the function expression of the improved companding transformation is as follows:
wherein s isnFor signals before companding, scnFor companded signals, V mean(s)n) Representing the mean of the signal.
2. In step three, at the transmitting end, the information transmitted to the receiving end is added with the improved companding transformation parameter, i.e. V, in step two.
3. In the sixth step, at the receiving end, the improved companding transformation parameters of the transmitting end are adopted for the improved companding transformation, wherein:
the receiving end receives the signal from rnIs represented byn＝scn+qn+wnWherein q isnIs quantization noise, wnIf the additive noise generated by the channel is inverse transformed at the receiving end, then:
will r isnSubstituting the formula to obtain a signal after decompression, expansion and transformation of the receiving end as follows:
compared with the prior art, the invention has the beneficial effects that: the invention verifies the feasibility of the method through the simulation of an underwater environment, can effectively reduce the influence of a quantization process in companding conversion on a system on the premise of ensuring the effect of reducing the peak-to-average ratio in the simulation, and when the signal-to-noise ratio is 20dB, the error rate of an underwater sound system adopting the improved companding conversion method is 10 dB lower than that of an underwater sound system adopting the traditional companding conversion method-1And the magnitude achieves the effect of effectively improving the error code performance of the communication system.
Drawings
FIG. 1 is a diagram of a companding transformation process;
FIG. 2 is a flow chart of the transmitter side improved companding transformation;
FIG. 3 is an overall flow chart of an improved companding transformation method of an OFDM underwater acoustic communication system;
FIG. 4 is a graph comparing complementary cumulative distribution functions;
FIG. 5 is a time domain comparison diagram before and after the OFDM underwater acoustic signal companding transform processing;
Detailed Description
The present invention will be described in further detail with reference to the accompanying drawings and specific embodiments.
The invention comprises the following steps:
the method comprises the following steps: at a transmitting end, a transmitting signal is subjected to series-parallel transformation and then is subjected to an inverse fast Fourier transform module, and a frequency domain signal is converted into a time domain signal;
step two: at a transmitting end, carrying out improved companding conversion processing on the time domain signal;
step three: at the transmitting end, transmitting the signals after the improved companding conversion and the improved companding conversion information to an underwater sound channel;
step four: at a receiving end, carrying out synchronous operation on the receiving domain signals;
step five: at a receiving end, performing cyclic prefix removing operation on the synchronized signal;
step six: at a receiving end, carrying out improved decompression transformation operation on the signal without the cyclic prefix;
step seven: at a receiving end, carrying out operations such as channel estimation and the like on the signals after the improved decompression and expansion transformation to obtain required signals;
the invention relates to an improved companding transformation method for reducing the peak-to-average power ratio of an orthogonal frequency division multiplexing underwater acoustic communication system, which is described in more detail as follows:
at the transmitting end, the improved companding function is expressed as:
wherein s isnFor signals before companding, scnFor companded signals, V mean(s)n) Representing the mean of the signal.
The receiving end receives the signal from rnIs represented byn＝scn+qn+wnWherein q isnIs quantization noise, wnIf the additive noise generated by the channel is inverse transformed at the receiving end, then:
will r isnSubstituting the formula to obtain:
further, the specific steps of the error rate and parameter analysis are as follows:
study of the impact of companding transformation on the performance of the error rate of the systemThe effect of companded signals on system performance is discussed. The companded signal is represented by the above formula, the variance of quantization noise sigmaq：
Where Q is the quantization interval and L is the number of quantization bits. The value of a in the quantization function is typically the peak value.
Decompressed signal:
will scnThe expression of (a) is substituted into the above formula,
wherein the content of the first and second substances,
the exponential function is subjected to a taylor series expansion,
quantization noise tends to be small, so the high order quantity in the equation is negligible and the de-companding function can be approximated as:
the decompressed signal is sent to a receiving end FFT module, and the source data D of the kth subcarrierl(k)：
The second term and the third term in equation (24) represent the quantization noise component and the channel noise component after the FFT at the receiving end, respectively. Noise variance σ of gaussian channelkwComprises the following steps:
quantization noise variance σkqComprises the following steps:
if the equation holds, then:
the QPSK modulation mode is adopted, and the error rate is satisfied
Wherein σsAs signal variance, σnThe noise variance is composed of quantization noise and channel noise, and as can be seen from equations (25) and (26), the change in the μ value has an effect on both the channel noise and the quantization noise. Q (x) is a Q function. The bit error rate of the system is:
where V is the mean of the transmitted signals.
The above embodiments are only preferred embodiments of the present invention, and are not intended to limit the present invention in any way, and all simple modifications and equivalent variations of the above embodiments according to the technical spirit of the present invention are within the scope of the present invention.
Claims (4)
1. An improved companding transform method for reducing the peak-to-average ratio of an orthogonal frequency division multiplexing underwater acoustic communication system is characterized in that: firstly, the time domain signal is processed by improved companding transform at the transmitting terminal of the underwater acoustic communication system, secondly, the corresponding signal is inverse transformed at the receiving terminal, namely, the companding transform, and the method is verified to be more suitable for being applied to the field of the underwater acoustic with complex propagation environment,
wherein, the improved companding transformation adopts companding transformation of improved companding parameters, and the function of the improved companding transformation is expressed as:
wherein s isnFor signals before companding, scnFor companded signals, V mean(s)n) Representing the mean of the signal.
2. The improved companding transform method for reducing the peak-to-average ratio of the orthogonal frequency division multiplexing underwater acoustic communication system as claimed in claim 1, wherein: comprises the following steps:
the method comprises the following steps: at a transmitting end, a transmitting signal is subjected to series-parallel transformation and then is subjected to an inverse fast Fourier transform module, and a frequency domain signal is converted into a time domain signal;
step two: at the transmitting end, the improved companding conversion processing is carried out on the time domain signals;
step three: at the transmitting end, transmitting the signals after the improved companding conversion and the improved companding conversion information to an underwater sound channel;
step four: at a receiving end, carrying out synchronous operation on the receiving domain signals;
step five: at a receiving end, performing cyclic prefix removing operation on the synchronized signal;
step six: at a receiving end, carrying out improved decompression transformation operation on the signal without the cyclic prefix;
step seven: at the receiving end, the channel estimation operation is carried out on the signal after the improved decompression and expansion transformation, and a required signal is obtained.
3. The improved companding transform method for reducing the peak-to-average ratio of the orthogonal frequency division multiplexing underwater acoustic communication system as claimed in claim 2, wherein: in step three, at the transmitting end, the information transmitted to the receiving end is added with the improved companding transformation parameter, i.e. V, in step two.
4. The improved companding transform method for reducing the peak-to-average ratio of the orthogonal frequency division multiplexing underwater acoustic communication system as claimed in claim 2, wherein: in the sixth step, at the receiving end, the improved companding transformation parameters of the transmitting end are adopted for the improved companding transformation, wherein:
the receiving end receives the signal from rnIs represented byn＝scn+qn+wnWherein s iscnFor companded signals, qnIs quantization noise, wnIf the additive noise generated by the channel is inverse transformed at the receiving end, then:
will r isnSubstituting the formula to obtain a signal after decompression, expansion and transformation of the receiving end as follows:
wherein, V is mean(s)n) Representing the mean of the signal.
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