CN111917675A - Frame detection method suitable for frequency offset robustness in broadband non-line-of-sight communication - Google Patents
Frame detection method suitable for frequency offset robustness in broadband non-line-of-sight communication Download PDFInfo
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- H—ELECTRICITY
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Abstract
The invention provides a frame detection method for frequency offset robustness in broadband non-line-of-sight communication, which comprises the steps that firstly, a receiving end carries out differential operation on a received signal to obtain a molecule of a frame detection metric value; then, carrying out differential operation on the received signal, then carrying out modular squaring and integral operation in a certain time window range to obtain the denominator of the frame detection metric value; finally, dividing the frame detection metric value by the denominator of the frame detection metric value to form a frame detection metric value; and if the frame detection metric value is larger than the detection threshold, the frame is considered to arrive, otherwise, the frame does not arrive. The method improves the traditional frame detection method, carries out frame detection after carrying out differential operation on the received signal, can completely eliminate the influence of frequency deviation, and improves the system frame detection performance in a non-line-of-sight communication environment.
Description
Technical Field
The invention relates to a frame detection method, in particular to a frame detection method which is suitable for frequency offset robustness in broadband non-line-of-sight communication.
Background
The classic sliding cross-correlation frame detection method performs frame arrival detection through correlation between an ideal pilot and a received signal, and does not perform any processing on the received signal before detection. In an actual system, due to the fact that local oscillation frequencies at the transmitting end and the receiving end are different, the receiving end moves and the like, signals at the transmitting end and the receiving end have frequency offset. The classical sliding cross-correlation frame detection method is easily influenced by frequency deviation, and the false alarm probability MDP is very high in an environment with large frequency deviation.
Disclosure of Invention
The invention aims to provide a frame detection method which is suitable for frequency offset robustness in broadband non-line-of-sight communication.
In order to achieve the purpose, the technical scheme adopted by the invention is as follows:
a frame detection method for robustness to frequency deviation in broadband non-line-of-sight communication comprises the following steps:
the method comprises the following steps: the receiving end carries out differential operation on the received signal to obtain a molecule of a frame detection metric value;
step two: carrying out differential operation on a received signal, then carrying out modular squaring and integral operation in a certain time window range to obtain a denominator of a frame detection metric value;
step three: dividing the frame detection metric value by the denominator of the frame detection metric value to form a frame detection metric value; and if the frame detection metric value is larger than the detection threshold, the frame is considered to arrive, otherwise, the frame does not arrive.
The invention is further improved in that the specific process of the step one is as follows: without considering the noise, the received signal r (n) is the convolution of the transmitted sequence with the channel:
wherein j is an imaginary unit, n is an integer, phi is 2 pi/fsFor normalized frequency offset, for true frequency offset, fsFor the sampling rate, L is the channel length, L represents an integration variable, h (L) is the value of the channel h (n) at the integration variable L, x (n-L) is the roll-over shift value of the transmit sequence x (n) at the integration variable L, and x (n-L) is the roll-over shift value of the transmit sequence x (n) at the integration variable L;
the receiving end carries out differential operation on the received signals:
let xp(n)＝x*(n) x (n + p), then x*(n-l)x(n+1-k)＝xl-k+1(n-l), then formula (1) is represented as:
wherein k is an inner layer integral variable, and p is a signal mode;
after the difference operation, the received signal includes the following modes:
x-L+1(n)；
x-L+2(n),x-L+2(n-1)；
……
x1(n),x1(n-1),...,x1(n-L)；
x2(n-1),...,x2(n-L)；
……
xL+1(n-L)；
defining a pilot pattern matched to each signal pattern as sp(n)＝s*(N) s (N + p), N ═ 0,1,. and N-1; when N + p is less than 0 or N + p is more than or equal to N, making s (N) equal to 0, wherein s (N) is an ideal pilot frequency, N is an ideal pilot frequency length, p is a signal mode, and x is a conjugate symbol;
when the signal mode is x-L+1When (n), the pilot used is s-L+1(n); when the signal mode is x-L+2(n),x-L+2(n-1), the pilot used is s-L+2(n), and so on; the values calculated in all modes are then summed to obtain the frame detection metric numerator.
A further improvement of the invention is that the numerator P (d) of the frame detection metric is calculated as follows:
where d is the starting point of the sliding cross-correlation operation.
A further development of the invention is that the signal pattern p ranges from-L to L + 1.
The invention is further improved in that the denominator R (d) of the frame detection metric value is:
wherein, N is the ideal pilot frequency length, L is the channel length, N is an integer, and d is the starting point of the sliding cross-correlation operation.
The invention has the further improvement that the selection mode of the detection threshold is as follows: and under the condition of pure noise, circularly transmitting for 1000 times, and setting a threshold as a critical value of no missing report of the system.
Compared with the prior art, the invention has the beneficial effects that:
the invention firstly carries out differential operation on the received signal at the receiving end, and the differential operation can completely eliminate the influence of frequency deviation. After the difference operation, the frame arrival detection is performed by using the correlation between the pilot frequency and the received signal. The signals after the difference operation include a plurality of modes, so that the ideal pilot frequency needs to be converted into the corresponding mode to perform sliding cross correlation, and then the sliding cross correlation results under the plurality of modes are summed. Compared with the classic sliding cross-correlation frame detection method, the system adopting the method for frame detection has the advantages that the performance is not affected completely after the frequency deviation is introduced, the system performance is stable, and the system performance is better under the condition of larger frequency deviation, so that the stability and reliability of system frame detection in a non-line-of-sight communication environment are improved.
Drawings
Fig. 1 is a diagram of a non-line-of-sight communication scenario suitable for use with the present invention.
Fig. 2 is a schematic diagram of a signal frame structure adopted in the present invention.
Fig. 3 is a graph of a missed report probability MDP when the frame detection method and the classical sliding cross-correlation frame detection method of the present invention are used after frequency offset is introduced in a non-line-of-sight communication environment and under the condition that frequency offset is not introduced.
Detailed Description
The present invention will be described in detail with reference to the accompanying drawings.
The invention relates to a differential frame detection method which is applicable to frequency offset robustness in broadband non-line-of-sight communication. Setting a proper detection threshold, and considering that the frame arrives when the frame detection metric value is larger than the detection threshold, otherwise, the frame does not arrive. The frame detection measurement value is in a fractional form, and the calculation mode of the classic sliding cross-correlation frame detection method is to use a known pilot frequency sequence and a received signal to perform sliding cross-correlation and then perform modular squaring, and then perform integral operation in a certain time window range. However, in an actual system, the frequency offset between the transmitting and receiving nodes may affect the performance of the conventional frame detection method, and increase the false negative probability MDP of the system. Therefore, the invention improves the method, firstly carries out differential operation on the received signals, and then carries out processing, thereby fundamentally eliminating the adverse effect caused by the asynchronous frequency of the receiving and transmitting nodes. The signal after the difference operation contains a plurality of patterns, so the pilot used for frame detection must match the signal pattern. The calculating mode of the denominator of the frame detection metric value is to perform modular squaring after performing differential operation on a received signal, and then perform integral operation within a certain time window range. In non-line-of-sight communication, signals often arrive through multiple paths, energy is dispersed, and multipath energy can be concentrated through integration operation processing, so that frame detection is facilitated. The differential processing can completely eliminate the influence of frequency offset.
Specifically, the method comprises the following steps:
the method comprises the following steps: the numerator of the frame detection metric is calculated. The invention is similar to the classic sliding cross-correlation frame detection method in principle, the calculation method of the molecule of the frame detection metric value is to use the known pilot frequency to perform the sliding cross-correlation to the received signal, and then to perform the integral operation to the sliding cross-correlation result in a certain selected time window range. However, the present invention firstly performs a differential operation on the received signal to eliminate the influence of frequency offset, and therefore, the present invention is different from the metric numerator calculation method of the classical method.
The specific process is as follows:
without considering the noise, the received signal is the convolution of the transmitted sequence with the channel:
wherein j is an imaginary unit, n is an integer, n is 0,1sFor normalized frequency offset, for true frequency offset, fsFor the sampling rate, L is the channel length, L represents an integration variable, h (L) is the value of the channel h (n) at the integration variable L, x (n-L) is the inverse shift value of the transmitted sequence x (n) at the integration variable L, and x (n-L) is the inverse shift value of the transmitted sequence x (n) at L;
in order to eliminate the influence of frequency offset, the receiving end firstly performs differential operation on a received signal:
let xp(n)＝x*(n) x (n + p), then x*(n-l)x(n+1-k)＝xl-k+1(n-l), the above formula can be represented as:
where k is the inner layer integral variable and p is the signal pattern.
After the difference operation, the influence of the frequency offset is completely eliminated, but the received signal contains the following modes:
x-L+1(n)；
x-L+2(n),x-L+2(n-1)；
……
x1(n),x1(n-1),...,x1(n-L)；
x2(n-1),...,x2(n-L)；
……
xL+1(n-L)。
therefore, when frame detection is performed on the signal after the difference operation, the used pilot sequence needs to be changed according to different signal modes. Defining a pilot pattern matched to each signal pattern as sp(n)＝s*(N) s (N + p), N ═ 0, 1. And when N + p is less than 0 or N + p is more than or equal to N, making s (N) equal to 0. Where s (N) is the ideal pilot, N is the pilot sequence length, p is the signal pattern, and p ranges from-L to L +1, where is the conjugate sign.
For example, the following steps are carried out: when the signal mode is x-L+1When (n), the pilot used is s-L+1(n); when the signal mode is x-L+2(n),x-L+2(n-1), the pilot used is s-L+2(n), and so on. The values calculated in all modes are then summed, which is the frame detection metric numerator after the difference operation.
The frame detection metric numerator P (d) can thus be calculated as follows:
where d is the starting point of the sliding cross-correlation operation.
Step two: the denominator of the frame detection metric value is calculated. After carrying out differential operation on the received signal, performing modular squaring and performing integral operation within a certain time window range, wherein a denominator of a frame detection metric value can be expressed as:
step three: and (4) dividing the numerator calculated in the step one and the denominator calculated in the step two to form a frame detection metric value, selecting a proper detection threshold, and if the frame detection metric value is greater than the detection threshold, determining that the frame arrives, otherwise, determining that the frame does not arrive. The frame detection threshold is selected in the following way: and under the condition of pure noise, circularly transmitting for 1000 times, and setting a threshold as a critical value of the system without missing report, namely setting the threshold to ensure that the probability of the system without missing report is less than 1 per thousand.
Fig. 1 is a non-line-of-sight communication scene diagram applicable to the present invention, where NLOS transmission causes a received signal at a receiving end to be a superposition of a large number of non-direct waves (mainly reflected waves), and multipath effects form severe rayleigh fading. The receiving end is an antenna array composed of M antennas, the wireless channel model is a multipath delay channel obeying Rayleigh fading, and the channel matrix between the transmitting end and the receiving end is H ═ h1,h2,…hL]∈CM×L. Wherein the content of the first and second substances,
is formed by superposing P multipath channels. A guide vector a (theta)l,p)∈CM×1Having the form:
here, the
n is the array element spacing and λ is the signal wavelength.
Fig. 2 shows the structure of a signal frame used in the present invention, and the signal frame is composed of a pilot sequence and data. And a guard interval UW is also added to the pilot sequence, the load of a signal frame is formed by splicing a data block and the guard interval UW block by block, the data is divided into blocks at a transmitting end, and the UW is added at the tail. In the subsequent simulation, UW is 64 in length, the pilot sequence is composed of m-sequences of length 128, and one data block is 512 in length.
Fig. 3 is an MDP curve of the false negative probability under the condition that no frequency offset is introduced and the frame detection method and the classical sliding cross-correlation frame detection method of the present invention are used after the frequency offset is introduced in the non-line-of-sight communication environment.
When the difference operation is not carried out on the received signal, frame detection is carried out in a non-line-of-sight environment, and the pilot frequency sequence of N points is utilized to carry out normalized correlation calculation on the received signal. Let s ═ s (0) s (1.. s (N-1))]T∈CN×1Indicating the pilot sequence. Assuming the normalization of the transmitted symbol power, the receiving end receives the signal sequence with N points starting from dThe following are listed as:
r(d)＝[r(d)r(d+1)...r(d+N-1)]T∈CN×1
considering the maximum possible channel delay spread as LchFor a received signal sequence starting with d, the following correlation coefficients are taken as frame detection metric values:
the simulation compares the false negative probability MDP of frame detection under various frequency offsets by using the frame detection method of the invention and the classic sliding cross-correlation frame detection method. The simulated pilot frequency sequence adopts an m sequence with the length of 128, the adopted constellation mapping mode is BPSK mapping, the receiver adopts one antenna for receiving, each data block has 512 symbols, wherein the UW length is 64, the maximum time delay expansion of a channel, namely the window length is 50, and the window length is internally provided with 6 paths which are randomly distributed. The detection threshold selected by simulation is to make the false positive probabilities under the detection of the two methods basically the same, and then the false negative probabilities MDP are compared. The threshold set when the method of the invention is used for frame detection is 1.3; the threshold is set to be 1 when the classical sliding cross-correlation method is used for detection, so that the false alarm probabilities of the two detection methods are basically kept consistent and are below 1 per thousand. The magnitude of frequency deviation in simulation is embodied in the magnitude of phase deviation, and comparison is carried out under the condition of adding larger frequency deviation (2 pi x 0.01) and smaller frequency deviation (2 pi x 0.006) when a classical sliding cross-correlation method is used for detection; when the method is used for detection, comparison is carried out under the conditions of adding no frequency deviation and adding larger frequency deviation (2 pi x 0.01). As can be seen from fig. 3, the performance of the classical sliding cross-correlation frame detection method without differential operation after the frequency offset is increased is greatly affected, and the probability of missing report MDP is very high; but the system adopting the method to detect the frame has no influence on the performance after adding the frequency deviation, the system has stable performance and the system has better performance under the condition of larger frequency deviation.
The above-mentioned contents are only for illustrating the technical idea of the present invention, and the protection scope of the present invention is not limited thereby, and any modification made on the basis of the technical idea of the present invention falls within the protection scope of the claims of the present invention.
Claims (6)
1. A frame detection method for robustness to frequency deviation in broadband non-line-of-sight communication is characterized by comprising the following steps:
the method comprises the following steps: the receiving end carries out differential operation on the received signal to obtain a molecule of a frame detection metric value;
step two: carrying out differential operation on a received signal, then carrying out modular squaring and integral operation in a certain time window range to obtain a denominator of a frame detection metric value;
step three: dividing the frame detection metric value by the denominator of the frame detection metric value to form a frame detection metric value; and if the frame detection metric value is larger than the detection threshold, the frame is considered to arrive, otherwise, the frame does not arrive.
2. The method for detecting frame of robustness to frequency offset in wideband non-line-of-sight communication according to claim 1, wherein the specific process of step one is as follows: without considering the noise, the received signal r (n) is the convolution of the transmitted sequence with the channel:
wherein j is an imaginary unit, n is an integer, phi is 2 pi/fsFor normalized frequency offset, for true frequency offset, fsFor the sampling rate, L is the channel length, L represents an integration variable, h (L) is the value of the channel h (n) at the integration variable L, x (n-L) is the roll-over shift value of the transmit sequence x (n) at the integration variable L, and x (n-L) is the roll-over shift value of the transmit sequence x (n) at the integration variable L;
the receiving end carries out differential operation on the received signals:
let xp(n)＝x*(n) x (n + p), then x*(n-l)x(n+1-k)＝xl-k+1(n-l), then formula (1) is represented as:
wherein k is an inner layer integral variable, and p is a signal mode;
after the difference operation, the received signal includes the following modes:
x-L+1(n)；
x-L+2(n),x-L+2(n-1)；
……
x1(n),x1(n-1),...,x1(n-L)；
x2(n-1),...,x2(n-L)；
……
xL+1(n-L)；
defining a pilot pattern matched to each signal pattern as sp(n)＝s*(N) s (N + p), N ═ 0,1,. and N-1; when n + p<When 0 or N + p is more than or equal to N, making s (N) equal to 0, wherein s (N) is an ideal pilot frequency, N is an ideal pilot frequency length, p is a signal mode, and x is a conjugate symbol;
when the signal mode is x-L+1When (n), the pilot used is s-L+1(n); when the signal mode is x-L+2(n),x-L+2(n-1), the pilot used is s-L+2(n), and so on; the values calculated in all modes are then summed to obtain the frame detection metric numerator.
3. The method of claim 2, wherein the numerator P (d) of the frame detection metric is calculated as follows:
where d is the starting point of the sliding cross-correlation operation.
4. The method of claim 2, wherein the signal pattern p is in a range of-L to L + 1.
5. The method of claim 2, wherein the denominator R (d) of the frame detection metric value is:
wherein, N is the ideal pilot frequency length, L is the channel length, N is an integer, and d is the starting point of the sliding cross-correlation operation.
6. The method of claim 1, wherein the detection threshold is selected by the following method: and under the condition of pure noise, circularly transmitting for 1000 times, and setting a threshold as a critical value of no missing report of the system.
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