CN102752253A - Method for inhibiting inter-carrier interference of orthogonal frequency division multiplexing (OFDM) system by time-frequency domain combined processing - Google Patents
Method for inhibiting inter-carrier interference of orthogonal frequency division multiplexing (OFDM) system by time-frequency domain combined processing Download PDFInfo
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Abstract
The invention relates to a method for inhibiting inter-carrier interference of an orthogonal frequency division multiplexing (OFDM) system by time-frequency domain combined processing. The method includes (1) coding user data at a transmitting terminal of an OFDM signal transmission device and performing constellation mapping after grouping and segmenting; (2) subjecting the data to precoding and relevant coding after the constellation mapping to achieve frequency domain partial response; (3) subjecting time domain partial response signals to serial-to-parallel conversion and modulating the signals to a plurality of subcarriers by means of inverse fast Fourier transform (IFFT); (4) adding a window function to the formed OFDM time domain signals, wherein obtained signals are results of the time-frequency domain combined processing; (6) subjecting the received OFDM time domain signals to digital to analog conversion and down-conversion; (7) removing cycling prefixes after the serial-to-parallel conversion and using the IFFT to demodulate the OFDM signals on the subcarriers; (8) finally, using inverse operation of relevant codes to resolve partial responses; and (9) performing decoding to obtain transmission signals.
Description
Technical Field
The present invention relates generally to Orthogonal Frequency Division Multiplexing (OFDM) systems, and more particularly, to designing an inter-subcarrier interference (ICI) cancellation technique for OFDM systems that performs well in time-varying channels.
Background
The OFDM system is a high-speed transmission technology in a wireless environment, and its main idea is to divide a given channel into a plurality of orthogonal sub-channels in the frequency domain, and the signal is transmitted in parallel using one sub-carrier on each sub-channel, so that the symbol interval becomes long to resist the frequency selective fading in the wireless environment, which is a great advantage in the wireless communication field, and thus, the OFDM technology will become one of the core technologies of the fourth generation mobile communication (4G).
The reliability of an OFDM system is based on the orthogonality of the individual subcarriers. However, in practice the effects of time-varying fading channels can destroy the orthogonality between the channels. One of the main problems faced by OFDM systems is therefore the sensitivity to frequency offsets which will lead to inter-subcarrier interference, thereby significantly reducing system performance. To solve this problem, measures must be taken to suppress inter-subcarrier interference (ICI) of the system.
For the purpose of suppressing inter-subcarrier interference, there are generally two solutions. An effective way is to compensate at the receiving end by estimating the frequency offset due to channel time variations in the design of the OFDM receiver. However, this method necessarily requires timing synchronization, and inaccuracies in timing will affect the estimation of the frequency offset. Another way is to design a suitable OFDM transmitter to process the OFDM signal to reduce the sensitivity of the signal to frequency offsets. For example, time domain windowing is adopted, and attenuation of sidelobes of each subcarrier is accelerated by multiplying a designed low-interference window shape by a time domain OFDM signal, so that interference is suppressed. However, this method has certain disadvantages, especially when the frequency offset is large, interference cannot be suppressed well by only using time domain windowing.
The second criterion of nyquist tells us that: artificially, intersymbol interference is regularly introduced at the sampling time of a code element and eliminated before judgment at a receiving end, so that the aims of improving the spectrum efficiency, compressing a transmission frequency band, improving the frequency band utilization rate to the theoretical maximum value, accelerating the attenuation of a transmission waveform tail and reducing the requirement on timing precision can be achieved. Such a waveform is commonly referred to as a partial response waveform. Initially, partial responses are used in single carrier systems to reduce inter-symbol interference caused by time domain errors, referred to as time domain coherently encoded signals. The OFDM system can be regarded as the extension of a single carrier system, can also artificially introduce the correlation among all subcarriers and eliminate the correlation at a receiving end, so that the tailing of each subcarrier can be reduced, the influence of the interference among the subcarriers can be reduced, the interference among the subcarriers of the OFDM system can be reduced, and the design method is flexible. The idea is the frequency domain partial response.
The key of the design of the invention is to suppress the ICI in a time-frequency domain combined processing mode so as to improve the performance of the system.
Disclosure of Invention
The technical problem is as follows: the invention aims to provide an effective mode for reducing the sensitivity of an OFDM system to frequency offset and a method for reducing the interference between system subcarriers. The method reduces the sensitivity of the OFDM system to frequency offset, overcomes the defects of high requirement on frequency offset estimation precision and bandwidth effectiveness reduction, and therefore, the invention provides a method for effectively inhibiting inter-subcarrier interference in the OFDM system.
The technical scheme is as follows: the invention relates to a method for inhibiting inter-carrier interference of an orthogonal frequency division multiplexing system by time-frequency domain combined processing, which adopts the method for reducing the inter-carrier interference ICI of a received orthogonal frequency division multiplexing OFDM signal by time-frequency domain combined processing, and specifically comprises the following steps:
an OFDM signal transmitting device is adopted to carry out code modulation, precoding, related coding, serial-parallel transformation, Inverse Fast Fourier Transform (IFFT), parallel-serial transformation, time domain windowing, digital-to-analog transformation and up-conversion processing on a transmitting signal in sequence,
1) at an OFDM transmitting end, encoding, grouping and segmenting user data and then mapping a planet seat;
2) pre-coding and relevant coding are carried out on data after constellation mapping to realize partial response of a frequency domain, specifically, relevant coding is that a current signal code element after coding modulation and n code elements before are overlapped according to an optimal weighting coefficient which enables the frequency spectrum tailing to be minimum, and the relevance between adjacent subcarriers is regularly introduced to form a new signal code element to be sent, wherein n is called a relevant coding series;
3) the time domain partial response signals are subjected to serial-parallel transformation, are modulated onto a plurality of subcarriers by utilizing Inverse Fast Fourier Transform (IFFT) to form OFDM time domain signals, are guided by adopting cyclic prefixes, and are subjected to parallel-serial transformation;
4) adding a window function to the OFDM time domain signal formed in the step 3), wherein the obtained signal is a result of frequency-time domain joint processing, and specifically, the OFDM time domain signal is multiplied by the window function, so that side lobe fading is accelerated;
5) carrying out digital-to-analog conversion and up-conversion processing on the signals subjected to frequency-time domain combined processing, and sending out the signals;
the OFDM signal receiving device comprises an analog-to-digital conversion circuit, a down-conversion circuit, a serial-to-parallel conversion circuit, a Fast Fourier Transform (FFT) circuit, a parallel-to-serial conversion circuit and a decoding and demodulation circuit which are connected in sequence,
6) carrying out digital-to-analog conversion and down-conversion on the received OFDM time domain signal;
7) after serial-to-parallel conversion, removing cyclic prefixes, and respectively demodulating OFDM signals on each subcarrier by adopting Fast Fourier Transform (FFT);
8) finally, the inverse operation solution part response of the relevant coding is adopted, and when the relevant coding is solved, the currently received code element and the n code elements received before are subtracted to obtain a required signal;
9) the decoding obtains a sending signal.
The step 1) encodes the user data, the constellation mapping and OFDM modulation are performed on each segment of data after grouping and segmenting, the encoded user data are grouped and segmented, the constellation mapping is adopted on each segment of data, the length of the segmented data depends on the constellation mapping mode, and each group of data is modulated on each subcarrier in the step 3) respectively.
The related coding weighting coefficient in the step 2) is a symmetric matrix RnIs given, wherein R is the eigenvector corresponding to the smallest eigenvalue ofnIn the form of: <math>
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</math> n is the relative coding order.
The data after constellation mapping is pre-coded and relevant coded, namely the data after constellation mapping is differentially coded, and the error propagation of decoding caused by the correlation among symbols introduced by adopting partial response is resisted.
The method for forming the OFDM time domain signal by utilizing IFFT modulation to a plurality of subcarriers comprises the following steps: dividing the signals after the relevant coding into N groups, and independently transmitting the N groups on N mutually orthogonal sub-channels; the OFDM is realized by IFFT, the lowest subcarrier frequency of the OFDM is 0, and in order to obtain the frequency position of the required final modulated signal, the frequency spectrum of the obtained OFDM signal is moved to a specified high frequency by an up-conversion method.
The OFDM signal guided by the cyclic prefix is: the cyclic prefix is implemented by estimating the maximum time dispersion of the symbols, adding the last bit of each data symbol repeatedly to the front of each symbol, and the number of bits of the repeated symbols must be larger than the maximum time dispersion of the symbols to resist the interference between the signal symbols.
Multiplying the OFDM time domain signal by a window function such that the window shape meets the nyquist criterion, the window function being greater than N, the window selection possibly complying with one of a number of alternatives, the window function being a first order step function which starts with a step function, said step function increasing from a minimum value to an intermediate value, the window function remaining at the intermediate value for a first period of time and then increasing from the intermediate value to a maximum value with a further step function, the window function remaining at the maximum value for a second period of time; then, reducing the maximum value to an intermediate value by adopting a step function, keeping the intermediate value in a third time period, and then reducing the intermediate value to the minimum value by the step function; the size of the window roll-off coefficient is determined according to actual requirements, or the size of the window is selected according to the shape of the window.
Has the advantages that: according to the technical scheme, the invention has the following beneficial effects:
1. the method for eliminating the inter-subcarrier interference based on the OFDM system reduces the system performance loss caused by ICI interference caused by a time-varying fading channel.
2. The method for eliminating the interference between the subcarriers based on the OFDM system realizes the time-frequency domain joint processing to improve the sensitivity of the OFDM system to the frequency offset.
Drawings
The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description taken in conjunction with the accompanying drawings, in which;
fig. 1 shows a block diagram for implementing the cancellation of inter-subcarrier interference in an OFDM-based system.
Figure 2 shows a partial response implementation.
Fig. 3 shows the frequency spectrum of a conventional OFDM system and the herein proposed time-frequency domain joint processing OFDM system.
Fig. 4 compares the carrier-to-interference ratio of a conventional OFDM system and the herein proposed time-frequency domain joint processing OFDM system.
Fig. 5 compares the bit error rates of a conventional OFDM system and the herein proposed time-frequency domain joint processing OFDM system.
Detailed Description
The invention aims to provide a method for inhibiting interference between subcarriers of an OFDM system by time-frequency domain joint processing, which is a method for quickly attenuating the side lobe of the frequency spectrum of an OFDM transmission signal and reducing the interference to adjacent subcarriers by time-frequency domain joint processing and comprises the following steps:
the method for reducing the inter-carrier interference ICI of the received OFDM signals by using time-frequency domain joint processing specifically comprises the following steps:
an OFDM signal transmitting device is adopted to carry out code modulation, precoding, related coding, serial-parallel transformation, Inverse Fast Fourier Transform (IFFT), parallel-serial transformation, time domain windowing, digital-to-analog transformation and up-conversion processing on a transmitting signal in sequence,
1) at an OFDM transmitting end, encoding, grouping and segmenting user data and then mapping a planet seat;
2) pre-coding and relevant coding are carried out on data after constellation mapping to realize partial response of a frequency domain, specifically, relevant coding is that a current signal code element after coding modulation and n code elements before are overlapped according to an optimal weighting coefficient which enables the frequency spectrum tailing to be minimum, and the relevance between adjacent subcarriers is regularly introduced to form a new signal code element to be sent, wherein n is called a relevant coding series;
3) the time domain partial response signals are subjected to serial-parallel transformation, are modulated onto a plurality of subcarriers by utilizing Inverse Fast Fourier Transform (IFFT) to form OFDM time domain signals, are guided by adopting cyclic prefixes, and are subjected to parallel-serial transformation;
4) adding a window function to the OFDM time domain signal formed in the step 3), wherein the obtained signal is a result of frequency-time domain joint processing, and specifically, the OFDM time domain signal is multiplied by the window function, so that side lobe fading is accelerated;
5) carrying out digital-to-analog conversion and up-conversion processing on the signals subjected to frequency-time domain combined processing, and sending out the signals;
the OFDM signal receiving device comprises an analog-to-digital conversion circuit, a down-conversion circuit, a serial-to-parallel conversion circuit, a Fast Fourier Transform (FFT) circuit, a parallel-to-serial conversion circuit and a decoding and demodulation circuit which are connected in sequence,
6) carrying out digital-to-analog conversion and down-conversion on the received OFDM time domain signal;
7) after serial-to-parallel conversion, removing cyclic prefixes, and respectively demodulating OFDM signals on each subcarrier by adopting Fast Fourier Transform (FFT);
8) finally, the inverse operation solution part response of the relevant coding is adopted, and when the relevant coding is solved, the currently received code element and the n code elements received before are subtracted, so that the required signal can be obtained;
9) the decoding obtains a sending signal.
Performing constellation mapping and OFDM modulation on each segment of data, namely grouping and segmenting the user data coded in the step 1), wherein each segment of data adopts constellation mapping, the length of the segmented data depends on the constellation mapping mode, and each group of data is modulated on each subcarrier in the step 3).
Step 2) the relevant coding weighting coefficient is a symmetric matrix RnIs given, wherein R is the eigenvector corresponding to the smallest eigenvalue ofnIn the form of: <math>
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</math> n is the relative coding order.
The use of a precoding operation prior to correlation coding, and in particular, differential coding of constellation mapped data, resists error propagation in decoding due to inter-symbol correlation introduced by the use of partial responses.
Modulating each group of signals to a plurality of subcarriers by using Inverse Fast Fourier Transform (IFFT) to form OFDM time domain signals, wherein the method comprises the following steps: the signals after the relevant coding are divided into N groups and independently transmitted on N mutually orthogonal sub-channels. In order to implement OFDM by IFFT, the lowest subcarrier frequency of OFDM can be set to 0, and in order to obtain the frequency position of the final modulated signal, the frequency spectrum of the obtained OFDM signal can be shifted to a specified high frequency by using an up-conversion method.
The OFDM signal is guided by a cyclic prefix. The cyclic prefix is implemented by estimating the maximum time dispersion of the symbols, adding the last bit of each data symbol repeatedly to the front of each symbol, and the number of bits of the repeated symbols must be larger than the maximum time dispersion of the symbols to resist the interference between the signal symbols.
The OFDM time domain signal is added with a window function, and the realization method is to multiply the OFDM time domain signal and the window function. The window shape satisfies the Nyquist criterion and the window function is greater than N. The window selection may correspond to one of a number of alternatives, the window function being a first order step function starting with a step function that increases from a minimum value to an intermediate value, the window function remaining at the intermediate value for a first period of time, then increasing from the intermediate value to a maximum value with another step function, and remaining at the maximum value for a second period of time. Then using a step function to reduce from the maximum value to the middle value, keeping the middle value in a third time period, and then reducing from the middle value to the minimum value by the step function. The size of the window roll-off coefficient is determined according to actual requirements, or the size of the window is selected according to the shape of the window.
(1) At the transmitting end of the OFDM system, binary data is coded and mapped.
(2) The signal is processed in partial response, that is, the signal is encoded according to a certain rule.
(3) The related coded signals are modulated to each subcarrier respectively, and more specifically, a time domain OFDM signal is obtained by IFFT.
(4) The time domain OFDM signal is windowed, e.g., raised cosine window, modified raised cosine window, etc.
(5) And (4) transmitting the signals obtained in the step (4) after parallel-serial frequency domain transformation.
(6) And demodulating and decoding the OFDM receiving end to obtain transmitted data.
In step (1), the signal may be modulated in a variety of ways, such as BPSK, QPSK, M-QAM, and so on.
In step (2), the partial response processing of the modulation signal means that correlation is introduced between signals so that the current transmission signal is a linear combination of the current transmission signal and the previous signals. Can be expressed as:
wherein a iskFor modulating the signal, the sequence b is obtained after the relevant codingk，ciIs a weighting factor of the response. The greater the number of combinations, the higher the number of partial response stages, the better the system performance but the higher the complexity. The level of the partial response series is determined by considering the compromise between the system performance and the complexity requirement. On the other hand, since correlation is introduced between signals, in order to prevent error propagation at the time of decoding at the receiving end, the signals may be precoded before the signal modulation (step (1) described above).
In step (3), the encoded signals are grouped, so that the signals are divided into N groups and transmitted independently on N mutually orthogonal subchannels. In order to implement OFDM by IFFT, the lowest subcarrier frequency of OFDM can be set to 0, and in order to obtain the frequency position of the final modulated signal, the frequency spectrum of the obtained OFDM signal can be shifted to a specified high frequency by using an up-conversion method.
In step (4), the time domain OFDM signal is windowed. The window may be one of a number of alternatives. For example, in some alternative embodiments, the window function is a first order step function that begins with a step function that increases from a minimum value to an intermediate value, the window function remaining at the intermediate value for a first period of time, then increasing from the intermediate value to a maximum value with another step function, and remaining at the maximum value for a second period of time. Then using a step function to reduce from the maximum value to the middle value, keeping the middle value in a third time period, and then reducing from the middle value to the minimum value by the step function.
By combining the two processing modes (2) and (4), various design schemes of the baseband transmitter of the OFDM system can be obtained, and various performance requirements can be met.
Therefore, the transmitting signal processing circuit for improving the interference between the subcarriers of the OFDM system comprises a code modulation circuit, a related coding circuit, a serial-parallel conversion circuit, an IFFT conversion circuit, a parallel-serial circuit, a time domain windowing circuit, a digital-to-analog conversion circuit and an up-conversion circuit which are sequentially connected. The method is characterized in that a relevant coding circuit is added before IFFT, and a time domain windowing circuit is added after IFFT. The related coding circuit carries out related coding on the baseband modulation signal, and the side lobe attenuation of the signal spectrum is increased by artificially adding the interference between subcarriers. The time domain windowing circuit windows the entire OFDM time domain signal. The window shape satisfies the Nyquist criterion, and the side lobe of the window shape spectrum attenuates as fast as possible. E.g. the side lobe of a rectangular window in f-1Attenuating, raising the side lobe of the cosine window by f-2Attenuation, the side lobe of the advanced raised cosine window is expressed by f-3The faster the attenuation of the window-shaped spectrum sidelobe is, the better the suppression effect of the window-shaped spectrum sidelobe on the interference between the subcarriers of the OFDM system is. Meanwhile, the received signal processing circuit for improving the interference between the subcarriers of the OFDM system comprises an analog-to-digital conversion circuit, a down-conversion circuit, an FFT (fast Fourier transform) circuit and a decoding and demodulating circuit which are sequentially connected. The decoding circuit described above needs to demodulate the associated code and baseband modulation separately.
For the purpose of promoting a better understanding of the objects, features and advantages of the invention, reference is made to the following detailed description taken in conjunction with the accompanying drawings.
The method for eliminating the interference between the subcarriers of the OFDM system is carried out by theoretical analysis and simulation combined with the OFDM system. The method combines the time domain and the frequency domain to eliminate and suppress the inter-subcarrier interference so as to effectively reduce the deterioration of the system performance caused by the frequency offset.
The invention provides a system structure chart for eliminating interference between subcarriers of an OFDM system, which comprises the following steps: partial response is adopted in a frequency domain, correlation among subcarriers is artificially introduced and eliminated before judgment at a receiving end, and therefore attenuation of a transmission waveform tail is accelerated to inhibit interference among the subcarriers. And simultaneously, selecting a window shape with small tail oscillation amplitude and fast convergence on a frequency domain to further optimize the system performance.
Referring now to fig. 1, the OFDM signal partial response processing is studied on the basis of the selected window shape. The signal processing process comprises the following steps:
1. the input signal is code modulated.
2. The invention uses a frequency domain partial response, that is to say a correlation coding of the modulated signal.
Therefore, a class of related coding types needs to be selected. Assume that the signal before the correlation coding is denoted as akN, where k is the serial number of the subcarriers and N is the total number of the subcarriers. Forming a signal b after the correlation codingkObtaining:
when decoding according to
A responsive signal can be obtained. But the partial response introduces correlation between the signals, and in order to avoid error propagation during decoding, the invention provides for the binary signal before modulationPrecoding is performed. As an example, the fourth type partial response is taken as an example. Precoding may be employed by combining akBecoming a differential code. Thus decoding becomes a modulo two operation.
Fig. 2 is a method of generating a partial response signal. Discrete time series akPassing through a discrete-time filter whose coefficients are ciN-1, with the output sequence of the filter being b (i/2w), i being 0, 1k。
In the above-mentioned description,
for the sequence akThe result of the filtering is such that bkProducing a correlation, in fact, bkThe autocorrelation function of (a) is:
when the input sequence is zero mean and white, E (a)k-iak+m-l)＝δm+i-lIf E (a)k 2)＝1，bkHas an autocorrelation function ofThe spectrum is represented as:
and T-1/2 w. This partial response design thus provides a transmission signal spectrum for the second inter-subcarrier interference.
3. Like the traditional OFDM system, the related coded signals are modulated to each subcarrier for transmission.
The invention adopts IFFT transformation to obtain OFDM time domain signals. The encoded signals are grouped such that the signals are divided into N groups for independent transmission on N mutually orthogonal subchannels. In order to implement OFDM by IFFT, the lowest subcarrier frequency of OFDM can be set to 0, and in order to obtain the frequency position of the final modulated signal, the frequency spectrum of the obtained OFDM signal can be shifted to a specified high frequency by using an up-conversion method.
In addition, the OFDM signal is preceded by a cyclic prefix, which includes: the amount of time dispersion of the subcarrier k through the channel is estimated and adjusted to a window size, described below, such that it is larger than the number of IFFT points N but not so large as to be within a portion of the cyclic prefix affected by the time dispersion of the channel.
4. Windowing the time domain OFDM signal.
Obtaining a term representing the signal received on the kth subcarrier at time t by applying a windowing function to the multiple received OFDM signal to produce a set of selected weighted samples, wherein: the symbols transmitted on the OFDM signal are represented by N samples, the window function size is not smaller than N, and the window function is a nyquist function. The window is selected by one person who may meet a large number of alternatives, the size of the window roll-off coefficient can be determined according to actual requirements, and the size of the window can also be selected according to the shape of the window.
As an example, the present invention adopts the following two window shapes as comparison.
The first window is a conventional rectangular window, specifically, no additional window is added after the IFFT. The second window shape is an improvement over raised cosine windows, referred to herein as BTRC windows. Both window shapes are nyquist windows, and the time domain and frequency domain forms can be expressed as:
(1) rectangular window
Pr(t)＝1/T，|t|≤T/2 Pr(f)＝sin c(fT)
(2) Improved raised cosine window, referred to herein as (BTRC window)
The improved raised cosine window has small tail oscillation amplitude, fast convergence and better performance in inhibiting the interference between subcarriers.
It should be noted that this window shape is only one embodiment of the present invention, and in practice, different window shapes may be used according to the requirements for system performance.
The subsequent operation of the transmitting end is similar to that of the conventional OFDM system.
5. A signal is received over a channel. After passing through the channel, the received signal is frequency-shifted and inter-subcarrier interference (ICI) exists, and the simplified received signal can be expressed as
First part C of the above formulakFor useful signals, a second part IkIs inter-subcarrier interference. The inter-subcarrier interference power reduced by the partial response may be expressed in the form of:
analyzing the above formula, the finally obtained weighting coefficient of the optimal frequency domain partial response is a real symmetric matrix RnThe minimum eigenvalue of (2) corresponds to the eigenvector. RnExpressed as:
for example:
on the other hand, ICI cancellation in embodiments of the present invention is determined by the following equation:
to determine the above-mentioned middle min (lambda)n) Is a matrix RnThe minimum eigenvalue of (c). It can be seen that the choice of the shape of the visible window and the choice of the associated coding coefficients to achieve the partial response can affect the inter-subcarrier interference performance of the system.
6. And decoding by the receiving end.
After the down-conversion of the receiving end, the FFT demodulation is still adopted, and the obtained signal is obtained according to the formulaThe signal before the relevant encoding can be obtained.
In addition, error propagation is difficult to avoid at the time of reception with the above-described method because correlation coding introduces correlation between symbols. According to different related coding modes, precoding can be adopted to avoid such errors, and precoding operation can be introduced before step 1. As a specific embodiment, the fourth type partial response precoding is differential coding, so that when decoding, only the signal after FFT needs to be processed in a modulo two mode to replace the above-mentioned signal
And (4) processing. The influence of error propagation is effectively suppressed.
Fig. 3, fig. 4 and fig. 5 are simulation comparison results of the embodiments of the present invention (OFDM system, the number of subcarriers is 200, and the number of FFT points is 512): fig. 3 shows the frequency spectrum of an OFDM signal with normalized frequency on the abscissa and power on the ordinate. Wherein (a) is a traditional OFDM signal, and (b) is an OFDM signal adopting time-frequency domain joint processing. Fig. 4 compares the carrier-to-interference ratio characteristics of a conventional OFDM system and a time-frequency domain joint processing system. Observing fig. 3, it is found that the CIR of the conventional OFDM system is the lowest under different frequency offsets, and the carrier-to-interference ratio of the OFDM system using the time-frequency domain joint processing studied in this document is significantly improved, which proves the advantages of the time-frequency domain joint processing again. Meanwhile, as the number of frequency domain partial response subcarriers (i.e., the number n of partial response levels) increases, the CIR gradually becomes larger. Fig. 5 compares the bit error rate of the system of the conventional OFDM and the time-frequency joint processing in this text by the simulation of the OFDM system by the computer. It is proved again that the time-frequency domain joint processing mode adopted by the method can effectively inhibit ICI of the OFDM system and reduce the sensitivity of the system to frequency offset.
The above-mentioned embodiments are intended to illustrate the objects, technical solutions and advantages of the present invention, and it should be understood that the above-mentioned embodiments are only exemplary embodiments of the present invention, and are not intended to limit the present invention, and any modifications, equivalent substitutions and the like made within the spirit and principle of the present invention should be included in the scope of the present invention.
Claims (7)
1. A time-frequency joint processing method for suppressing the inter-carrier interference of an orthogonal frequency division multiplexing system is characterized in that the inter-carrier interference ICI of a received orthogonal frequency division multiplexing OFDM signal is reduced by adopting time-frequency domain joint processing;
the specific implementation method comprises the following steps:
1) at the transmitting end of OFDM signal transmitting equipment, user data is encoded, grouped and segmented and then mapped by a planet seat;
2) pre-coding and relevant coding are carried out on data after constellation mapping to realize partial response of a frequency domain, specifically, relevant coding is to superpose a current signal code element after coded modulation and n code elements before according to an optimal weighting coefficient which enables the tail of a frequency spectrum to be minimum, and the relevance between adjacent subcarriers is introduced to form a new signal code element to be sent, wherein n is called a relevant coding series;
3) the time domain partial response signals are subjected to serial-parallel transformation, are modulated onto a plurality of subcarriers by utilizing Inverse Fast Fourier Transform (IFFT) to form OFDM time domain signals, are guided by adopting cyclic prefixes, and are subjected to parallel-serial transformation;
4) adding a window function to the OFDM time domain signal formed in the step 3), wherein the obtained signal is a result of frequency-time domain joint processing, and specifically, the OFDM time domain signal is multiplied by the window function, so that side lobe fading is accelerated;
5) carrying out digital-to-analog conversion and up-conversion processing on the signals subjected to frequency-time domain combined processing, and sending out the signals;
6) carrying out digital-to-analog conversion and down-conversion on the received OFDM time domain signal;
7) after serial-to-parallel conversion, removing cyclic prefixes, and respectively demodulating OFDM signals on each subcarrier by adopting Fast Fourier Transform (FFT);
8) finally, the inverse operation solution part response of the relevant coding is adopted, and when the relevant coding is solved, the currently received code element and the n code elements received before are subtracted to obtain a required signal;
9) the decoding obtains a sending signal.
2. The method for suppressing the interference between the subcarriers of the OFDM system by the time-frequency joint processing according to claim 1, wherein the step 1) encodes the user data, the constellation mapping after grouping and segmenting is to perform the constellation mapping and OFDM modulation on each segment of data, the encoded user data is grouped and segmented, each segment of data adopts the constellation mapping, the length of the segmented data depends on the constellation mapping mode, and each group of data is modulated on each subcarrier of the step 3) respectively.
3. The time-frequency joint processing suppression orthogonal frequency division multiplexing system subcarriers of claim 1Method for interference between waves, characterized in that the weighting coefficients in step 2) are a symmetric matrix RnIs given, wherein R is the eigenvector corresponding to the smallest eigenvalue ofnIn the form of: <math>
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</math> n is the relative coding order.
4. The method for suppressing inter-subcarrier interference in an Orthogonal Frequency Division Multiplexing (OFDM) system according to claim 1, wherein the pre-coding and the correlation coding are performed on the constellation mapped data by differentially coding the constellation mapped data to resist error propagation of decoding due to inter-symbol correlation introduced by using partial response.
5. The method for suppressing inter-subcarrier interference of an orthogonal frequency division multiplexing system according to claim 1, wherein the method for forming the OFDM time domain signal by IFFT modulation using inverse fast fourier transform is as follows: dividing the signals after the relevant coding into N groups, and independently transmitting the N groups on N mutually orthogonal sub-channels; the OFDM is realized by IFFT, the lowest subcarrier frequency of the OFDM is 0, and in order to obtain the frequency position of the required final modulated signal, the frequency spectrum of the obtained OFDM signal is moved to a specified high frequency by an up-conversion method.
6. The method for suppressing inter-subcarrier interference in an orthogonal frequency division multiplexing system as claimed in claim 1 wherein the OFDM signal is guided by a cyclic prefix: the cyclic prefix is implemented by estimating the maximum time dispersion of the symbols, adding the last bit of each data symbol repeatedly to the front of each symbol, and the number of bits of the repeated symbols must be larger than the maximum time dispersion of the symbols to resist the interference between the signal symbols.
7. The method of claim 1, wherein the step of multiplying the OFDM time domain signal by a window function satisfies the nyquist criterion, the window function is greater than N, the window selection may be one of a number of alternatives, the window function is a first order step function that begins with a step function that increases from a minimum value to an intermediate value, the window function remains at the intermediate value for a first period of time and then increases from the intermediate value to a maximum value with another step function, and the maximum value remains at a second period of time; then, reducing the maximum value to an intermediate value by adopting a step function, keeping the intermediate value in a third time period, and then reducing the intermediate value to the minimum value by the step function; the size of the window roll-off coefficient is determined according to actual requirements, or the size of the window is selected according to the shape of the window.
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