Short-wave channel measurement clustering calculation method based on scattering function

Abstract

The invention belongs to the field of short-wave channel measurement, and particularly relates to a short-wave channel measurement clustering calculation method based on a scattering function, which comprises the following steps of: designing an LFM measurement signal frame structure aiming at time delay / Doppler resolution and a maximum measurement range; designing that a received signal is related to a local replica signal, and solving a channel scattering function by using fast Fourier transform; designing a composite calculation and clustering calculation measurement parameter definition, and calculating a single channel measurement result according to a channel scattering function; and screening single channel measurement results of the same link, time and frequency, and calculating statistical results of a mean value, a standard deviation and a median. According to the method, the LFM signal is taken as a measurement signal, the method has the characteristics of low peak-to-average ratio, high resolution and strong anti-interference capability, and meanwhile, LFM frame structures with different time frequency resolutions and maximum measurable ranges can be designed through theoretical analysis.

Description

Technical Field

[0001] The invention belongs to the field of shortwave channel measurement, and in particular relates to a shortwave channel measurement clustering calculation method based on a scattering function. Background Art

[0002] Shortwave propagation primarily occurs in two forms: ground wave and sky wave, with sky wave being the most dominant. Sky wave relies on single or multiple reflections from the ionosphere to achieve long-distance transmission. However, signal propagation is often affected by factors such as topography, climate, frequency, and time. These complex and variable channel characteristics make shortwave channel measurement extremely important.

[0003] In the early 1950s, a method for oblique ionospheric sounding was developed using the ionospheric backscatter propagation mechanism of sky waves. This oblique sounding system has been widely used for ionospheric sounding and shortwave channel measurement. In the early 1990s, a British defense research institute developed a shortwave channel measurement system called DAMSON. The measurement signals primarily consisted of pulses or pseudorandom sequences (m-sequences, Barker codes). Measurement objectives included ionospheric concentration, signal-to-noise ratio (SNR), delay and its spread, Doppler shift and its spread, and propagation time. In the early 21st century, a Spanish team launched the WHISPER project, conducting shortwave channel measurements for over a decade. The measurement signals primarily consisted of pseudorandom sequences (m-sequences, Barker codes). Measurement objectives included channel availability, delay spread, Doppler shift, and Doppler spread.

[0004] The basic principle of shortwave channel measurement is to transmit measurement signals into the ionosphere and use receivers and channel measurement instruments to collect and analyze these signals, thereby obtaining the signal's propagation characteristics under different conditions. Therefore, the main structure of shortwave channel measurement generally includes the following parts: ① Transmitter: A device used to generate shortwave signals. The transmitter usually has adjustable frequency and power to adapt to different propagation environments; ② Receiver: A device that receives the transmitted signal. The receiver must be designed to ensure that it can receive and analyze shortwave signals; ③ Antenna system: Transmitters and receivers are usually equipped with antennas. Shortwave signal propagation often depends on the quality of the antenna design and its performance at specific frequencies; ④ Channel measurement instruments: Data acquisition and analysis equipment used to record important parameters such as signal strength, signal quality, and bit error rate.

[0005] Channel measurement methods can be divided into direct pulse measurement method, swept frequency measurement method, sliding correlation measurement method, scattering function measurement method, etc.

[0006] 1) Direct pulse measurement method:

[0007] A narrow pulse signal is transmitted as the measurement signal, and the receiver uses a wideband filter to receive the signal. The main disadvantages of direct pulse measurement include: ① the measurement results are severely affected by noise; ② the power of long-distance transmission is limited; and ③ the pulse signal width has a significant impact on measurement accuracy.

[0008] 2) Sweep frequency measurement method:

[0009] The swept frequency measurement method operates in the channel frequency domain, measuring the channel response at each frequency point at a specific step size. It operates by exploiting the time-frequency correspondence of the channel response and obtaining the channel's time-domain impulse response through an inverse Fourier transform. The main disadvantages of the swept frequency measurement method include: ① The sweeping process is time-consuming, making real-time measurement difficult, especially over large bandwidths; ② The measurement results for time-varying channels are less reliable, making it unsuitable for scenarios with rapidly changing time.

[0010] 3) Sliding correlation measurement method:

[0011] The sliding correlation measurement method is based on pulse compression technology. Within the target measurement bandwidth, the carrier signal transmitted by the transmitter is spread by a pseudo-random sequence. The receiver uses correlation reception technology, which involves cross-correlating the received signal with a local pseudo-random sequence. The cross-correlation result is proportional to the channel being measured at a specific moment. A step-delay method is used to obtain the maximum correlation value, which corresponds to the channel being measured. The main drawbacks of the sliding correlation method are its high computational complexity and limited resolution.

[0012] 4) Channel scattering function measurement method:

[0013] During transmission from the transmitting antenna to the receiving antenna, radio signals traverse a variety of complex propagation paths, including direct, reflected, diffracted, scattered, and random combinations of these paths. The signal's changing relationship during propagation can be described by a scattering function. By modeling the channel scattering function, parameters such as the power, delay, Doppler shift, and phase of the measured signal along different paths can be determined. To capture time-frequency characteristics, channel scattering function measurement methods are even more complex. Traditional scattering function measurement methods typically define spread based on the overall variation in the delay and Doppler spectra, without distinguishing between spread within multiple clusters of paths. This mismatches the actual signal impact and hinders signal waveform design. Summary of the Invention

[0014] To solve the above technical problems, the present invention provides a shortwave channel measurement clustering calculation method based on a scattering function, comprising:

[0015] S1: Design the LFM measurement signal frame structure based on delay / Doppler resolution and maximum measurement range;

[0016] S2: Design the correlation between the received signal and the local replica signal, and use the fast Fourier transform to obtain the channel scattering function;

[0017] S3: Design the definition of composite calculation and cluster calculation measurement parameters, and calculate the single channel measurement results based on the channel scattering function;

[0018] S4: Filter the single channel measurement results of the same link, time, and frequency, and obtain the statistical results of the mean, standard deviation, and median.

[0019] Beneficial effects of the present invention:

[0020] The resolution and anti-interference ability of the pseudo-random sequence used in existing channel measurement are limited. The present invention uses LFM signals as measurement signals, which have the characteristics of low peak-to-average ratio, high resolution, and strong anti-interference ability. At the same time, through theoretical analysis, LFM frame structures with different time-frequency resolutions and maximum measurable ranges can be designed.

[0021] The present invention adopts a channel measurement method based on scattering function to obtain detailed delay-Doppler characteristics, and through composite calculation and cluster calculation methods, obtains the delay and spread within and between shortwave channel paths, thereby guiding waveform design;

[0022] By combining the software and hardware of SDR, the present invention can automatically, long-term and large-scale measure the characteristics of multi-link channels and obtain more reliable statistical results. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 Schematic diagram of the autocorrelation of the LFM signal of the present invention;

[0024] Figure 2 Schematic diagram of the channel measurement structure based on the channel scattering function of the present invention;

[0025] Figure 3 Schematic diagram of a typical LFM signal in the time domain of the present invention;

[0026] Figure 4 is the measured channel scattering function of the Luntai-Norway link of the present invention;

[0027] Figure 5 Schematic diagram of the delay domain power spectrum and Doppler domain power spectrum of the present invention;

[0028] Figure 6 It is a composite calculation schematic diagram of the present invention;

[0029] Figure 7 This is a schematic diagram of clustering calculation of the present invention;

[0030] Figure 8 Schematic diagram of the composite multipath extension calculation process of the present invention;

[0031] Figure 9 Schematic diagram of the intra-path delay spread calculation process of the present invention. DETAILED DESCRIPTION

[0032] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0033] A shortwave channel measurement clustering calculation method based on a scattering function, comprising:

[0034] S1: Design the LFM measurement signal frame structure based on delay / Doppler resolution and maximum measurement range;

[0035] S2: Design the correlation between the received signal and the local replica signal, and use the fast Fourier transform to obtain the channel scattering function;

[0036] S3: Design the definition of composite calculation and cluster calculation measurement parameters, and calculate the single channel measurement results based on the channel scattering function;

[0037] S4: Filter the single channel measurement results of the same link, time, and frequency, and obtain the statistical results of the mean, standard deviation, and median.

[0038] 1) Scattering function measurement principle

[0039] Assume that the transmitter sends a signal x(t) with ideal autocorrelation characteristics (i.e., the autocorrelation function is the impulse function R x,x (τ)=δ(τ)), the impulse response of the channel is expressed as h(t,τ), then the signal received by the receiver is the convolution of the two, that is,

[0040]

[0041] Now, the cross-correlation function of the received signal with x(τ) can be obtained

[0042]

[0043] Combining the two equations, we get

[0044]

[0045] The physical meaning of the above formula is: when the transmitter sends a signal with ideal autocorrelation characteristics, the cross-correlation function between the received signal and the transmitted signal is exactly the impulse response of the channel. LFM signal has good autocorrelation characteristics, such as Figure 1 shown.

[0046] Let C r,x The autocorrelation function of (t,τ) is

[0047]

[0048] The channel scattering function is the Fourier transform of the channel impulse response. Therefore, the above formula can be combined to obtain the channel scattering function:

[0049]

[0050] The physical meaning of the above formula is: when the transmitter sends a signal with ideal autocorrelation characteristics, the channel scattering function is exactly the Fourier transform of the autocorrelation function of the cross-correlation function of the receiver's received signal and the transmitted signal.

[0051] Based on the channel scattering function, the delay power function is

[0052]

[0053] The Doppler power function is

[0054]

[0055] Figure 2 A block diagram of a method for calculating key channel parameters based on the channel scattering function is presented. The signal generated by an LFM signal generator is sent to the transmitter, modulated, and then transmitted. After propagating through the channel, it reaches the receiver for demodulation. It is then cross-correlated with the sequence generated by the LFM signal generator, followed by an autocorrelation operation. After Fourier transform, the channel scattering function is obtained. The marginal functions with respect to frequency and delay are calculated along the frequency and delay axes, respectively. The results are the delay power spectrum and Doppler power spectrum, respectively. Finally, the delay and Doppler parameters are calculated.

[0056] 2) LFM frame structure design:

[0057] LFM signal is used for channel measurement. Typical LFM signal time domain is as follows: Figure 3 shown.

[0058] Time-domain and frequency-domain measurement capabilities are mutually constrained. To meet measurement requirements, the LFM frame structure must be accurately designed. Based on the measurement requirements, the LFM frame structure is designed, including the sampling rate, bandwidth, frame length, subframe length, and number of subframes. The sampling rate determines the time resolution: time resolution = 1 / sampling rate, and maximum latency = subframe length. The frame length determines the frequency resolution: frequency resolution = 1 / frame length, frame length = subframe length × number of subframes, and maximum frequency deviation = 1 / subframe length.

[0059] 3) Channel measurement calculation method:

[0060] The received signal obtained by the receiver generates discrete values of the scattering function, which can be visualized as follows: Figure 4 shown.

[0061] Based on the scattering function method, the channel measurement parameters for each channel are defined in Table 1. Compared to composite expansion, the addition of the maximum intra-path delay / Doppler spread definition allows for independent measurement of each path's channel characteristics, more clearly reflecting the channel's impact on communications.

[0062] Table 1 Definition of channel measurement parameters

[0063]

[0064]

[0065] Channel measurement parameters can also be obtained by Figure 5 The delay domain power spectrum and Doppler domain power spectrum are described in detail.

[0066] The delay power spectrum and Doppler power spectrum can be obtained through the scattering function, and the delay / Doppler parameters can be calculated. Figure 6 、 7 As shown in Figure 2, the composite calculation method treats the power spectrum as a whole, considering the first rising edge as the start of the signal and the last falling edge as the end, and then calculates the offset and spread of the entire power spectrum. Alternatively, the clustered calculation method treats the signal's multipath as multiple clusters and fits the power spectrum curve for each cluster separately. This allows calculation of the offset and spread of each path and analysis of the relationships between paths.

[0067] For composite extension, the signal multipath delay range [τ1, τ2] and Doppler range [v1, v2] are set according to the maximum multipath and Doppler offset. Combined with the delay power spectrum S h (τ) and Doppler power spectrum S H (v), calculate the power spectrum after denoising:

[0068]

[0069] Where T is the total duration of the delay power spectrum, and V is the total bandwidth of the Doppler power spectrum. Therefore, the total power of the delay power spectrum in the multipath delay range [τ1, τ2] after denoising is:

[0070]

[0071] The total power of the Doppler power spectrum in the Doppler range [v1, v2] is:

[0072]

[0073] Take the composite multipath expansion as an example (composite Doppler expansion is similar), Figure 8 As shown, the algorithm flow for calculating the composite extension is as follows:

[0074] Initialization: start time t1 = τ1, delay spread t d =τ2-τ1;

[0075] cycle:

[0076] ①Calculation end time

[0077] ② Judgment: If t2 does not exist, exit the loop and execute step (3); if t2 exists, continue the loop;

[0078] ③Calculate temporary value like Update delay extension

[0079] ④ Update start time t1 = t1 + Δt, where Δt is the delay search step;

[0080] Output: Delay spread t d .

[0081] For intra-path spread, we use clustering calculations. Based on the ITS shortwave channel model, the delay / Doppler power spectrum can be considered a Gaussian spectrum. Therefore, we can perform a Gaussian fit on the discrete values of the scattering function for each path signal to obtain the delay / Doppler spread of a single path. Taking intra-path delay spread calculation as an example (intra-path Doppler spread calculation is similar), the process is as follows: Figure 9 shown.

[0082] First, search for the peak of the scattering function to obtain the multipath number (peak number) and the single-path delay and frequency shift corresponding to the peak position. Second, based on the peak position, the single-path delay / Doppler power spectrum profile can be obtained.

[0083] S hp (τ)=S(τ,v p )

[0084] S Hp(v) = S(τ p ,v)

[0085] Among them, τ p and v p is the time delay and frequency deviation corresponding to the peak position. Then, the Gaussian function is used to fit the discrete values of the delay / Doppler power spectrum profile. The Gaussian function fitting of the delay / Doppler power spectrum profile is described as

[0086]

[0087] in, and σ τ / σ v are the estimated parameters, representing the peak value, peak position, and half-width of the delay / Doppler Gaussian function. Taking the logarithm of the above equation and transforming it into a quadratic polynomial function, it can be solved according to the least squares principle.

[0088] Combined with SDR technology, a shortwave channel measurement system is implemented, including:

[0089] 1) Generate a local wav file based on the LFM frame structure for signal transmission and reception matching.

[0090] 2) Set the SDR receiving parameters, including receiving frequency, sampling rate, bandwidth, etc., and transmit the received data to the back-end channel measurement calculation program.

[0091] 3) Set the transmission parameters, including transmission frequency, sampling rate, transmission power, etc., read the local LFM file, and then transmit through the antenna.

[0092] 4) When an LFM signal is detected, the channel measurement program calculates and saves the measurement results.

[0093] While embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions, and variations may be made to these embodiments without departing from the principles and spirit of the invention, and that the scope of the invention is defined by the appended claims and their equivalents.

Claims

1. A shortwave channel measurement clustering calculation method based on scattering function, characterized in that: include: S1: Design the LFM measurement signal frame structure based on delay / Doppler resolution and maximum measurement range; S2: Design the correlation between the received signal and the local replica signal, and use the fast Fourier transform to obtain the channel scattering function; S3: Design the definition of composite calculation and cluster calculation measurement parameters, and calculate the single channel measurement results based on the channel scattering function; S4: Filter the single channel measurement results of the same link, time, and frequency, and obtain the statistical results of the mean, standard deviation, and median.

2. The shortwave channel measurement clustering calculation method based on scattering function according to claim 1 is characterized in that: Based on the delay / Doppler resolution and maximum measurement range, the LFM measurement signal frame structure is designed, including: Based on the measurement indicators, the LFM frame structure is designed, including: sampling rate, bandwidth, frame length, subframe length, and number of subframes. Among them, the sampling rate determines the time resolution, that is, time resolution = 1 / sampling rate; maximum delay = subframe length; frame length determines the frequency resolution, that is, frequency resolution = 1 / frame length; frame length = subframe length × number of subframes; maximum frequency deviation = 1 / subframe length.

3. The shortwave channel measurement clustering calculation method based on scattering function according to claim 1 is characterized in that: The received signal is correlated with the local replica signal and the channel scattering function is obtained using the fast Fourier transform, including: The signal generated by the LFM signal generator is sent to the transmitter, modulated and transmitted, and then reaches the receiver for demodulation after propagation through the channel. It is then cross-correlated with the sequence generated by the LFM signal generator, and then autocorrelated. After Fourier transform, the channel scattering function is obtained. The sequence generated by the LFM signal generator is correlated with the local replica signal.

4. The shortwave channel measurement clustering calculation method based on scattering function according to claim 3 is characterized in that: The demodulated signal of the receiver is cross-correlated with the sequence generated by the LFM signal generator, including: Among them, C r,x (t,τ) represents the cross-correlation function between the receiver's received signal and the transmitted signal, h represents the impulse response of the channel, x represents the transmitter's transmitted signal, t represents time, and τ represents the delay. represents the mathematical transformation of the first integration time, ξ represents the mathematical transformation of the second integration time, and η represents the mathematical transformation of the third integration time.

5. The shortwave channel measurement clustering calculation method based on scattering function according to claim 3 is characterized in that: Perform autocorrelation operations, including: Among them, R C (τ, Δt) represents the autocorrelation function of the channel impulse response, τ represents the delay, Δt represents the time offset, E represents the mean operation, C * (t,τ) represents the conjugate of the cross-correlation function, and C(t+Δt,τ) represents the cross-correlation function.

6. The shortwave channel measurement clustering calculation method based on scattering function according to claim 3, characterized in that: After Fourier transform, the channel scattering function is obtained, including: Among them, S(τ,v) represents the channel scattering function, τ represents the delay, v represents the Doppler, R C (τ,Δt) represents the autocorrelation function of the channel impulse response.

7. The shortwave channel measurement clustering calculation method based on scattering function according to claim 1, characterized in that: Design the definition of composite and clustered calculation measurement parameters, and calculate the single channel measurement results based on the channel scattering function, including: Obtain the delay power spectrum and Doppler power spectrum through the channel scattering function, and calculate the delay / Doppler spread through composite calculation and cluster calculation methods; The composite calculation method considers the power spectrum as a whole, considers the first rising edge as the start of the signal and the last falling edge as the end of the signal, and calculates the expansion of the entire power spectrum; The clustering calculation method regards the multipath of the signal as multiple clusters, fits the power spectrum curve of each cluster respectively, calculates the expansion of each path, and analyzes the relationship between paths.

8. The shortwave channel measurement clustering calculation method based on scattering function according to claim 7, characterized in that: Obtain the delay power spectrum and Doppler power spectrum through the channel scattering function, including: Among them, S h (τ) represents the delay power spectrum, τ represents the delay, S(τ,v) represents the channel scattering function, and v represents the Doppler; S H (v) represents the Doppler power spectrum.

9. The shortwave channel measurement clustering calculation method based on scattering function according to claim 7, characterized in that: Delay / Doppler spread is calculated using a composite calculation method, including: According to the maximum multipath and Doppler offset, set the signal multipath delay range [τ1,τ2] and Doppler range [v1,v2]; Combined with the time-delay power spectrum S h (τ) and Doppler power spectrum S H (v) Calculate the denoised delay and Doppler power spectrum, and obtain the total power of the delay power spectrum in the denoised multipath delay range [τ1,τ2] and the total power of the Doppler power spectrum in the Doppler range [v1,v2]; The delay spread and Doppler spread are calculated based on the total power of the delay power spectrum and the total power of the Doppler power spectrum.

10. The shortwave channel measurement clustering calculation method based on scattering function according to claim 7, characterized in that: Delay / Doppler spread calculations are performed using clustering methods, including: Search for the peak of the channel scattering function to obtain the multipath number and the single-path delay corresponding to the peak position. Based on the peak position, the delay / Doppler power spectrum profile of the single path can be obtained. The discrete values of the delay / Doppler power spectrum profile are fitted using a Gaussian function, and after taking the logarithm, they are converted into a quadratic polynomial function. The quadratic polynomial function is solved according to the least squares principle to obtain the delay / Doppler spread.

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