CN115913859B - Self-adaptive receiving method and system based on ZP-OFDM system - Google Patents
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Abstract
The invention belongs to the technical field of communication, and particularly relates to a self-adaptive receiving method and system based on a ZP-OFDM system, wherein the self-adaptive receiving method comprises the following steps: 1) In a ZP-OFDM system, carrying out autocorrelation on an acquired sampling signal according to a set window, and calculating to obtain an autocorrelation energy value; 2) Filtering the autocorrelation energy value to obtain a maximum energy value; determining a sampling interval according to the maximum energy value and a set threshold value, and determining a channel delay spread length L according to the length of the sampling interval ch The method comprises the steps of carrying out a first treatment on the surface of the 3) According to the length L of zero-filling suffix in OFDM symbol ZP And a channel delay spread length L ch Determining the length L of overlap and add OLA The method comprises the steps of carrying out a first treatment on the surface of the 4) Using overlap and add lengths L OLA Overlap-add is performed to remove the zero-padding suffix. Therefore, the invention solves the problems of reduced signal-to-noise ratio SNR and high system bit error rate caused by overlapping and adding to remove zero padding suffixes by using fixed length in the prior art.
Description
Technical Field
The invention belongs to the technical field of communication, and particularly relates to a self-adaptive receiving method and system based on a ZP-OFDM system.
Background
In an OFDM system, the most basic transmission unit is an OFDM symbol. Multipath fading channels can cause aliasing of two consecutive OFDM symbols, causing Inter-symbol interference (Inter-Symbol Interference, ISI). To ensure the performance of an OFDM system, a guard interval is inserted between two consecutive OFDM symbols to eliminate the intersymbol interference ISI effect caused by a multipath channel, where the length of the guard interval is greater than or equal to the maximum delay of the multipath channel. There are two different insertion methods for the guard interval of OFDM. One is Zero Padding (ZP), i.e. Padding zeros in the guard interval. Another is to implement cyclic extension of OFDM symbols using Cyclic Prefix (CP). The CP serves as a guard interval to avoid ISI and at the same time allows cyclic extension to eliminate inter-carrier interference (ICI).
However, CP has a problem of power loss, while ZP can circumvent this problem, but ZP breaks orthogonality between subcarriers, introducing ICI. In order to make ZP as well as CP capable of canceling ICI, a technique called overlap-and-add (OLA) is employed in ZP-OFDM systems to capture multipath energy of channels and maintain orthogonality of received data, thereby having the same properties as CP-OFDM signals. During transmission, the length of ZP is fixed. In a conventional OFDM receiver, the overlap-add procedure is done fixedly using the same length as ZP.
However, in the case of a small delay spread channel, the same sampling points as ZP length are collected and overlap-added, which in turn reduces system performance. For example, under an additive white gaussian channel (AWGN), OLA operation adds noise samples equal to ZP length to the FFT, degrading SNR by up to 1dB.
In summary, the prior art uses fixed length overlap-add to remove zero-padded suffixes, resulting in that in the case of small delay spread channels, some pure noise samples are taken to overlap during overlap-add, reducing SNR, which adversely affects the Bit Error Rate (BER) of the system.
Disclosure of Invention
The invention aims to provide a self-adaptive receiving method and a self-adaptive receiving system based on a ZP-OFDM system, which are used for solving the problems that the signal-to-noise ratio (SNR) is reduced and the bit error rate of the system is high due to the fact that zero padding suffixes are removed by overlap addition with fixed length in the prior art.
In order to solve the technical problems, the technical scheme and the corresponding beneficial effects of the technical scheme provided by the invention are as follows:
the invention discloses a self-adaptive receiving method based on a ZP-OFDM system, which comprises the following steps:
1) In a ZP-OFDM system, carrying out autocorrelation on an acquired sampling signal according to a set window, and calculating to obtain an autocorrelation energy value;
2) Filtering the autocorrelation energy value to obtain a maximum energy value; determining a sampling interval according to the maximum energy value and a set threshold value, and determining a channel delay spread length L according to the length of the sampling interval ch ；
3) According to the length L of zero-filling suffix in OFDM symbol ZP And a channel delay spread length L ch Determining the length L of overlap and add OLA ；
4) Using overlap and add lengths L OLA Overlap-add is performed to remove the zero-padding suffix.
The beneficial effects of the technical scheme are as follows: the invention firstly carries out autocorrelation calculation on the obtained sampling signal, then estimates the channel delay spread length according to the autocorrelation energy value, and determines the length L of overlap and addition according to the channel delay spread length OLA . Finally, overlap and add length L is used OLA Overlap-add is performed to remove the zero-padding suffix. The invention adjusts the overlap and add length L based on the measured channel delay spread OLA The size of the channel is suitable for different channel conditions, the receiving performance of the system is improved, and the problems of reduced signal-to-noise ratio and increased bit error rate of the system are solved.
Further, in order to improve the accuracy of the channel delay spread length, in step 2), a moving average filter with a length of J is used to filter the autocorrelation energy value to obtain a maximum energy value, and the value of the length J should be greater than the theoretical channel delay spread length; searching two corresponding sampling points in the autocorrelation energy value, wherein the two sampling points are corresponding to the maximum energy value multiplied by a first set threshold value, determining a sampling interval according to the two sampling points, and the difference between the length J and the length of the sampling interval is the channel delay expansion length L ch 。
Further, in order to improve the accuracy of the channel delay spread length, in step 2), a maximum energy value is found among the autocorrelation energy values, all autocorrelation energy values larger than the maximum energy value multiplied by a second set threshold value are determined on both sides of the sampling point corresponding to the maximum energy value, and according to the first one of the sampling points corresponding to the all autocorrelation energy valuesThe sampling point and the last sampling point determine a sampling interval, and the length of the sampling interval is the length L of the delay spread of the channel ch 。
Further, to improve the signal-to-noise ratio, the overlap-and-add length L is determined in step 3) using the following formula OLA ：
L OLA ＝min(L ch ,L ZP )。
Further, in order to improve the signal-to-noise ratio, the sampled signal in step 1) is a signal after being subjected to matched filtering.
Further, the following formula is used for matched filtering:
wherein B (N) is a matched filtering output signal, N is a sampling point data index, k is more than or equal to 0 and less than or equal to N-1, N is FFT point number, and C * (k) For the conjugation of C (k), a (n) is the symbol sequence of the received baseband sampling signal r (n), and C (k) is the symbol sequence of the local PS sequence p (k).
Further, in order to improve the signal-to-noise ratio, the autocorrelation energy value D (n) is calculated in step 1) by using the following formula:
D(n)＝B(n)·B * (n-M·L) 2
wherein L is the number of time domain points of one preamble OFDM symbol, M is the number of interval OFDM symbols, B * (n) is the conjugate of B (n), and B (n) is the matched filtered output signal.
Further, to improve the accuracy of the channel delay spread length, the first set threshold is a preset percentage threshold T ratio1 ，85％≤T ratio1 ≤95％。
Further, in order to improve the accuracy of the channel delay spread length, the second set threshold value is 10 ThdB20 ThdB is a preset power threshold value which is more than or equal to-15 and less than or equal to-10.
The invention provides an adaptive receiving system based on a ZP-OFDM system, which comprises a processor, wherein the processor is used for executing a computer program to realize the adaptive receiving method based on the ZP-OFDM system.
The beneficial effects of the technical scheme are as follows: the invention firstly carries out autocorrelation calculation on the obtained sampling signal, then estimates the channel delay spread length according to the autocorrelation energy value, and determines the length L of overlap and addition according to the channel delay spread length OLA . Finally, overlap and add length L is used OLA Overlap-add is performed to remove the zero-padding suffix. The invention adjusts the overlap and add length L based on the measured channel delay spread OLA The size of the channel is suitable for different channel conditions, the receiving performance of the system is improved, and the problems of reduced signal-to-noise ratio and increased bit error rate of the system are solved.
Drawings
FIG. 1 is a block diagram of a receiver system in an embodiment of the method of the present invention;
FIG. 2 is a schematic diagram of an overlap and add method in a method embodiment of the present invention;
FIG. 3 is a block flow diagram of an adaptive receiving method based on a ZP-OFDM system of the present invention;
fig. 4 is a functional block diagram of an adaptive receiving system based on ZP-OFDM system according to the present invention;
fig. 5 is a block diagram of an adaptive receiving system based on ZP-OFDM system according to the present invention.
Detailed Description
Method embodiment:
the invention discloses an adaptive receiving method embodiment based on a ZP-OFDM system, which mainly aims to solve the problem that in the prior art, zero padding suffixes are removed by overlap addition with fixed length, so that under the condition of small delay spread channels, a plurality of pure noise sampling points are taken for superposition in the overlap addition process, SNR is reduced, and the error bit rate (BER) of the system is adversely affected. The received sampled signal is first matched filtered, then the matched filtered output is auto-correlated and an auto-correlation energy value is obtained, and finally the channel delay spread length is estimated based on the auto-correlation energy value to determine the overlap and add length.
First, in the prior art, CP-OFDM based systems are less sensitive to symbol timing synchronization errors. The equalizer can correct the phase offset due to the timing estimation error as long as the estimated FFT window start point is within the CP range. However, ZP-OFDM based systems require a more accurate estimation of the starting point of the FFT window. This is because in CP-OFDM based systems, cyclic convolution is a natural phenomenon. The physical propagation of the OFDM signal through the multipath channel results in a linear convolution, which appears to be a cyclic convolution, since the CP addition artificially preserves the cyclic characteristics. However, for ZP-OFDM based systems, the linear convolution does not naturally appear as a cyclic convolution. The overlap-add (OLA) operation ensures the occurrence of such a manual cyclic convolution, depending on the actual starting point of the FFT window. Therefore, the ZP-OFDM based system is more sensitive to time synchronization estimation errors than the CP-OFDM based system.
In ZP-OFDM based systems, the ZP is typically removed by overlap-add with a fixed length equal to ZP. For small delay spread channels, some pure noise samples will be taken to overlap during overlap-add, which reduces the SNR and adversely affects the Bit Error Rate (BER) of the system. In fact, for a length L OLA The noise power is increased by (1+L) in the frequency domain OLA /N FFT ) Multiple of N FFT Number FFT points. In the extreme case of an AWGN channel (l=0), L OLA =0 is optimal because of L OLA >OLA of 0 reduces SNR by adding only pure noise samples. This means that an adaptive algorithm should be used to estimate the channel delay spread and select the minimum necessary length sample number to maintain the cyclic convolution characteristics, i.e. the length variable L is used in the overlap-add procedure OLA To increase the receiver SNR and thus the data transmission range.
Conventional OFDM-based transmission systems use a Cyclic Prefix (CP) in OFDM symbols to maintain orthogonality of data transmission. High-speed Ultra Wideband (UWB) systems use multi-band OFDM (MB-OFDM) technology for transmission in Wireless Personal Area Networks (WPAN) and the like. To avoid interference with other existing systems, the Federal Communications Commission (FCC) in the united states has placed corresponding restrictions on UWB radiated power, requiring that the maximum equivalent omni-directional radiated Power Spectral Density (PSD) not exceed-41.3 dBm/MHz. CP introduces correlation in the transmitted data sequence and thus ripple in the Power Spectral Density (PSD) of the transmitted data. While any fluctuations in power spectral density require power backoff at the transmitter. In fact, the required power back-off is equal to the peak-to-average ratio of the power spectral density, which in MB-OFDM systems may be as high as 1.5dB, which will greatly reduce the range of data transmission. Whereas OFDM symbols using zero-filled suffixes (ZPS) have smaller in-band ripple in terms of Power Spectral Density (PSD), this will allow more power to be used for transmission and have a fixed peak transmit power, resulting in a larger data transmission range.
In a ZP-OFDM UWB system, as shown in fig. 1, to obtain the same properties as a CP-OFDM signal, the receiver removes the ZP by overlap-add (OLA). Let overlap-add length be L OLA The OLA method can be described as last L after timing the OFDM symbol OLA Sum of front L OLA The samples are added one by one as shown in fig. 2, so that the ZP-OFDM symbol is converted into an equivalent CP-OFDM symbol. L (L) OLA Has great influence on the system performance. ZP length (L) ZP ) And the maximum delay spread of the multipath channel is larger than or equal to. If the overlap-add procedure employs L OLA ＝L ZP Then for small delay spread channels, the pure noise samples will be superimposed, reducing the received SNR, affecting the system received bit error rate.
The invention proposes to adjust the size of the overlap-add (OLA) window according to the measured channel delay spread to adapt to different channel conditions and improve the system reception performance.
The present invention will be described in further detail with reference to the drawings and examples, in order to make the objects, technical solutions and advantages of the present invention more apparent.
The method comprises the following steps, as shown in fig. 3:
the first step: the receiver receives the baseband sampling signal, quantizes the sampling signal, and performs matched filtering on the quantized baseband sampling signal.
First, the received baseband sampling signal r (n) is quantized to obtain a symbol sequence a (n) (or quantized sequence). A (n) is specifically represented as follows:
where n is the sample point data index.
The local PS (packet synchronization) sequence p (k) is quantized to obtain a symbol sequence C (k) (or quantized sequence), which is specifically expressed as follows:
wherein k is more than or equal to 0 and less than or equal to N-1, and N is FFT point number.
By the quantization, the high-cost multiplier in the filtering calculation process can be simplified into a simple counter. I.e., the multipliers and adders are considered as "+1" and "-1" counters, thereby significantly reducing area and power consumption.
Then, the formula for performing matched filtering on the baseband sampling signal is as follows:
wherein B (N) is a matched filtering output signal, N is a sampling point data index, N is more than 0, k is more than or equal to 0 and less than or equal to N-1, and N is FFT point number. C (C) * (k) Let r (n) be the received baseband sampling signal, and a (n) be the symbol sequence (quantized sequence) of the received sampling signal r (n), for the conjugation of C (k).
And a second step of: the matched filtered output signal is auto-correlated and a correlation energy value (also known as an autocorrelation energy value) is calculated.
The autocorrelation and calculation correlation energy value D (n) uses the following formula:
D(n)＝B(n)·B * (n-M·L) 2 (4)
where L is the number of time domain points of one preamble OFDM symbol, preferably l=165. M is the number of the spaced OFDM symbols, the value of M is determined according to the frequency hopping receiving conditions of the system in different frequency bands, and M takes 3 when the time frequency code tfc=1 or 2 in the MB-OFDM UWB system; when tfc=3 or 4, M takes 1; when tfc=5 or 6 or 7, M takes 1; when tfc=8 or 9 or 10, M takes 2.B (B) * And (n) is the conjugate of B (n).
And a third step of: the channel delay spread length is estimated based on the correlation energy values to determine the length of overlap and add during zero suffix removal.
Methods of estimating the channel delay spread length include, but are not limited to, any of the following two methods, described in detail below:
the first method is as follows: moving average filtering is carried out on the related energy value, and the length of a platform formed by filtering output is measured; thereby determining the channel delay spread. The method specifically comprises the following steps:
a1 A moving average filter of length J is used to filter the correlation energy value, J should be greater than the theoretical channel delay spread length, where the range of values may be [32, 37], and the corresponding expression is as follows:
a2 Finding the maximum value Q of Q (n) max ；
a3 Finding a Q (n) value equal to Q max *T rati o 1 Rise point n of (2) r And a drop point n f The method comprises the steps of carrying out a first treatment on the surface of the Here T rati o 1 Is a preset percentage threshold value; the value range can be 85%,95%]Preferably T ratio1 The removable value is 90%;
a4 Calculating the rising point n r And a drop point n f The interval between the two is obtained, namely the length of the filtering output platform is obtained
L p ＝n f -n r (6)
a5 Obtaining a channel delay spread length estimate L based on the moving average filter output platform length ch Channel delay spread estimate is also known as channel delay spread, i.e
L ch ＝J-L p (7)
The second method is as follows: and obtaining the maximum value of the correlation energy values, taking the energy value within a plurality of dB of the maximum value, and calculating the length of the interval between the first energy value and the last energy value in the energy values meeting the condition, thereby determining the channel delay spread. The method specifically comprises the following steps:
b1 Finding the maximum value D of the correlation energy value D (n) max ；
b2 Finding D) max Nearby is greater than D max *T ThdB Let the first and last points satisfying the condition be denoted as n respectively s And n e The method comprises the steps of carrying out a first treatment on the surface of the Wherein,
T ThdB ＝10 ThdB20 (8)
wherein, thdB is a preset power threshold value, the unit is dB, the range of the value is [ -15, -10], and preferably, the value of ThdB is-10 dB.
b3 According to said n) s And n e Obtaining a channel length estimation value L ch I.e.
L ch ＝n e -n s (9)
The overlap and add length is determined based on the channel delay spread length by the following method:
L OLA ＝min(L ch ,L ZP ) (10)
wherein L is OLA For the length of overlap and add, L ZP The length of the suffix is filled with zeros in one OFDM symbol.
The invention can adjust the size of an overlap-add (OLA) window according to the measured channel delay spread under different channel conditions, has low realization complexity and has high application value in an OFDM system.
System embodiment:
an embodiment of an adaptive receiving system based on a ZP-OFDM system of the present invention, as shown in fig. 5, includes a memory, a processor, and an internal bus, where the processor and the memory complete communication and data interaction with each other through the internal bus. The memory includes at least one software functional module stored in the memory, and the processor executes various functional applications and data processing by running the software programs and modules stored in the memory, so as to implement the adaptive receiving method based on the ZP-OFDM system in the method embodiment of the present invention.
The processor may be a microprocessor MCU, a programmable logic device FPGA, or other processing device.
The memory may be various memories for storing information by using electric energy, such as RAM, ROM, etc.; the magnetic storage device can also be various memories for storing information by utilizing a magnetic energy mode, such as a hard disk, a floppy disk, a magnetic tape, a magnetic core memory, a bubble memory, a U disk and the like; various memories for optically storing information, such as CDs, DVDs, etc.; of course, other types of memory are also possible, such as quantum memory, graphene memory, etc.
The system may also be implemented in a manner that includes a matched filtering module, an autocorrelation module, a channel delay spread estimation module, and an overlap-add length determination module, as shown in fig. 4. The matched filtering module is used for carrying out matched filtering on the received sampling signals. And the autocorrelation module is used for carrying out autocorrelation according to the matched filtering output result and calculating a correlation energy value. The channel delay spread estimation module is used for estimating the channel delay spread length according to the correlation energy value. The channel delay spread length estimation method includes, but is not limited to, any one of the following two methods:
a) Moving average filtering is carried out on the related energy value, and the length of a platform formed by filtering output is measured; determining a channel delay spread length according to the length of the platform;
b) And obtaining the maximum value of the correlation energy values, taking the energy value within a plurality of dB of the maximum value, calculating the length of the interval between the first energy value and the last energy value in the energy values meeting the condition, and determining the delay expansion length of the channel according to the length of the interval. The overlap-add-length determination module is configured to determine a final overlap-add-length based on the channel delay spread length estimate and the zero suffix length value.
Claims (10)
1. An adaptive receiving method based on a ZP-OFDM system is characterized in that: the method comprises the following steps:
1) In a ZP-OFDM system, carrying out autocorrelation on an acquired sampling signal according to a set window, and calculating to obtain an autocorrelation energy value;
2) Filtering the autocorrelation energy value to obtain a maximum energy value; determining a sampling interval according to the maximum energy value and a set threshold value, and determining a channel delay spread length L according to the length of the sampling interval ch ；
3) According to the length L of zero-filling suffix in OFDM symbol ZP And a channel delay spread length L ch Determining the length L of overlap and add OLA ；
4) Using overlap and add lengths L OLA Overlap-add is performed to remove the zero-padding suffix.
2. The ZP-OFDM system-based adaptive receiving method according to claim 1, wherein: in the step 2), a moving average filter with the length of J is adopted to filter the autocorrelation energy value to obtain the maximum energy value, and the value of the length J is larger than the delay expansion length of a theoretical channel; searching two corresponding sampling points in the autocorrelation energy value, wherein the two sampling points are corresponding to the maximum energy value multiplied by a first set threshold value, determining a sampling interval according to the two sampling points, and the difference between the length J and the length of the sampling interval is the channel delay expansion length L ch 。
3. The ZP-OFDM system-based adaptive receiving method according to claim 1, wherein: step 2) finding the maximum energy value in the autocorrelation energy values, determining all autocorrelation energy values larger than the maximum energy value multiplied by a second set threshold value at both sides of the sampling point corresponding to the maximum energy value, and according to the all autocorrelation energy valuesThe first sampling point and the last sampling point in the sampling points corresponding to the energy value determine a sampling interval, and the length of the sampling interval is the length L of the delay spread of the channel ch 。
4. The ZP-OFDM system-based adaptive receiving method according to claim 1, wherein: determining the overlap and add length L in step 3) using the following formula OLA ：
L OLA ＝min(L ch ,L ZP )。
5. The ZP-OFDM system-based adaptive receiving method according to claim 1, wherein: the sampling signal in the step 1) is a signal subjected to matched filtering processing.
6. The adaptive receiving method based on ZP-OFDM system according to claim 5, wherein: the following formula is used for matched filtering:
wherein B (N) is a matched filtering output signal, N is a sampling point data index, k is more than or equal to 0 and less than or equal to N-1, N is FFT point number, and C * (k) For the conjugation of C (k), a (n) is the symbol sequence of the received baseband sampling signal r (n), and C (k) is the symbol sequence of the local PS sequence p (k).
7. The ZP-OFDM system-based adaptive reception method according to any one of claims 1 to 6, characterized in that: in step 1), the autocorrelation energy value D (n) is calculated using the following formula:
D(n)＝|B(n)·B * (n-M·L)| 2
wherein L is the number of time domain points of one preamble OFDM symbol, M is the number of interval OFDM symbols, B * (n) is the conjugate of B (n), and B (n) is the matched filtered output signal.
8. The adaptive receiving method based on ZP-OFDM system according to claim 2, wherein: the first set threshold value is a preset percentage threshold value T ratio1 ，85％≤T ratio1 ≤95％。
9. The adaptive reception method based on ZP-OFDM system according to claim 3, characterized in that: the second set threshold value is 10 ThdB/20 ThdB is a preset power threshold value which is more than or equal to-15 and less than or equal to-10.
10. An adaptive receiving system based on a ZP-OFDM system, characterized in that: the system comprising a processor for executing a computer program for implementing the ZP-OFDM system based adaptive reception method according to any of the preceding claims 1-9.
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