CN113067601B - System and method for improving initial synchronization performance of direct sequence spread system and direct sequence spread power meter reading system - Google Patents

Abstract

The invention relates to a system and a method for improving initial synchronization performance of a direct sequence spread spectrum system and application of the system in a direct sequence spread spectrum electric meter reading system, belonging to the field of communication of the Internet of things, wherein the system comprises a receiving baseband signal unit, a correlation calculation unit, a leading segment data correlation peak value search unit and a frequency deviation calculation unit, wherein a local one-bit leading chip generates a segment chip unit, and the segment chip generates a local baseband signal unit; receiving data from a radio frequency unit, completing automatic gain control and analog-to-digital conversion, and forming digital baseband data; generating a local preamble chip sequence according to the requirements of the spread spectrum code and the spread spectrum factor, and dividing the chip sequence into a plurality of sections of chip sequences; modulating the segmented chip sequence to generate corresponding baseband data; performing correlation calculation on the local leading segmented chip baseband data and the received baseband signal, and searching to obtain the starting position and the ending position of a leading bit; and carrying out frequency estimation by using the related likelihood value to obtain the frequency deviation.

Description

System and method for improving initial synchronization performance of direct expansion system and direct expansion power meter reading system

technical field

The invention belongs to the field of Internet of Things communication, and relates to a system and method for improving the initial synchronization performance of a direct expansion system and an application in a direct expansion power meter reading system.

Background technique

Direct Sequence Spread Spectrum (DSSS) technology, referred to as direct spread technology, encodes one bit of data into a multi-bit sequence, called a "chip". For example, the data bit "0" is encoded with the chip "00100111000", the data bit "1" is encoded with the chip "11011000111", and the data string "010" is encoded as "00100111000", "11011000111". "00100111000".

Direct sequence spread spectrum has the advantages of strong anti-interference ability, strong anti-multipath interference ability, strong anti-interception ability, can work on the same frequency, and is easy to realize multiple access communication. Direct sequence spread spectrum technology has been widely used in military communications and confidential industries. It is a reliable and safe industrial application solution.

Here is a scenario where direct sequence spread spectrum is used in power meter reading. The power meter reading system uses frame burst mode for communication. The power meter reading burst frame structure consists of three parts, namely the synchronization header (abbreviation: SHR), the physical layer frame header (abbreviation: PHR) and the physical layer service data unit (abbreviation: : PSDU), in which SHR is mainly used to complete burst frame data block search, frequency synchronization, timing synchronization and frame synchronization; PHR provides signaling for parsing PSDU; PSDU carries protocol signaling and service data content of power meter reading.

In the frame burst of power meter reading, the SHR is divided into two parts, namely the preamble (preamble for short) and the frame start delimiter (SFD). The preamble consists of multiple 0 bits. Under different PSDU transmission rates, it is recommended to Different lengths using Preamble bit 0. Preamble provides automatic gain control at the receiving end, frame burst detection, and preliminary adjustment of the frequency and timing of burst data blocks.

The frame start delimiter SFD is composed of a fixed bit sequence. Preamble provides the receiver to perform automatic gain control, frame burst detection, and preliminary adjustment of the frequency and timing of burst data blocks. The frame start delimiter SFD indicates the end position of the Preamble and the start position of the physical layer frame header PHR.

Since the power meter reading is transmitted in the direct spread spectrum mode, the Preamble and SFD data of SHR also need to be directly spread before they can be transmitted. Spread spectrum transmission, while SFD uses several fixed spreading factors for transmission.

From the theoretical analysis, in the direct spread spectrum sequence, the larger the spreading factor, the better the performance, so the spreading factor of the Preamble in the frame burst data structure adopts 256.

However, in actual engineering, after receiving frame burst data, the receiving end uses time domain correlation method to demodulate. Due to the increase of Preamble preamble spreading factor and the initial frequency difference between the transmitting end and the receiving end, this directly affects the receiving end. The research found that when there is no frequency deviation in the baseband at both ends of the transceiver, the correlation peak at the receiving end is very obvious. When there is a frequency deviation of 200 Hz in the baseband at both ends of the transceiver, the correlation peak at the receiving end is still obvious, but there is already interference. When there is a frequency deviation of 400 Hz in the baseband at both ends of the transceiver, there are already multiple correlation peaks at the receiving end, and the effective correlation peak cannot be clearly determined. However, in actual engineering, the baseband frequency deviation at both ends of the transceiver is generally above 1000Hz, and the limit can reach 2000Hz. Therefore, when the spreading factor is 256, 256 chips are directly used for related processing, and the effect is definitely not ideal. The crystal oscillator is too demanding.

SUMMARY OF THE INVENTION

In view of this, the purpose of the present invention is to provide a method for improving the synchronization accuracy of the direct spread spectrum communication technology.

For achieving the above object, the present invention provides the following technical solutions:

On the one hand, the present invention provides a system for improving the initial synchronization performance of a direct expansion system, including a baseband signal receiving unit, a correlation calculation unit, a preamble segment data correlation peak search unit, a frequency deviation calculation unit, and a local one-bit preamble chip generation component. a segment chip unit, the segmented chip generates a local baseband signal unit;

The receiving baseband signal unit receives data from the radio frequency unit, completes automatic gain control, performs analog-to-digital conversion, and forms digital baseband data;

The local one-bit preamble chip generating segmented chip unit generates a local preamble chip sequence according to the requirements of the spreading code and the spreading factor in the direct-spreading system, and divides the chip sequence into a multi-segment chip sequence as required;

The segmented chip generating local baseband signal unit performs modulation processing on the segmented chip sequence to generate corresponding baseband data;

The correlation calculation unit performs correlation calculation on the local preamble segmented chip baseband data and the received baseband signal, and performs a correlation calculation every time one chip symbol data is received;

The preamble segmented data correlation peak search unit is used to search the calculation result of the correlation calculation unit to obtain the start and end positions of the preamble bits;

The frequency deviation calculation unit performs frequency estimation using the correlation likelihood value obtained by the correlation calculation unit to obtain the frequency deviation.

Further, the frequency deviation calculation unit first calculates the phase difference between two adjacent correlation peaks according to the determined multiple correlation values, then calculates the time interval between the two adjacent correlation peaks, and finally calculates the receiving end and the transmitting end. Baseband frequency deviation between basebands, and baseband frequency adjustment.

On the other hand, the present invention provides a method for improving the initial synchronization performance of a direct expansion system, comprising the following steps:

Step 1: The receiving end locally generates a chip sequence of preamble bits according to the spreading factor and the spreading code, so that the preamble spreading factor is N, then one bit of preamble data generates N chip sequence data;

Step 2: Divide the generated one-bit preamble chip sequence into K-segment chip data, also known as segmented chip sequence, the length of each segmented chip data is denoted as S, and S satisfies the relationship of S=(N/K) , and then modulate the segmented chip sequence to form a local preamble baseband signal; let the receiver use m times the chip rate to sample the received baseband signal, then the local segmented chip sequence modulation signal, the same m times Upsampling processing, that is, repeating each data in the local segmented chip sequence modulation signal m times;

Step 3: Receive frame burst data to form a baseband data stream, and the baseband data stream is m times up-sampling of the baseband signal;

Step 4: Correlation calculation is performed on the received baseband data stream and the local segmented chip sequence modulated signal respectively; each time the receiver receives a sampled data, a correlation calculation is performed with the local segmented chip sequence modulated signal, and K is obtained each time. correlation peaks;

Step 5: the receiving end searches for the correlation peak, and if there are K obvious correlation peaks, it indicates that the receiving end has searched for a leading baseband signal of bit 0 in the frame burst;

Step 6: The receiving end determines the start and end positions of the leading bits according to the position of the correlation peak, then uses the correlation value of the correlation peak to calculate the phase difference between two adjacent correlation peaks, and uses the position between the two correlation peaks The difference calculates the time length between the two correlation peaks, and calculates the baseband signal frequency difference between the receiving end and the transmitting end;

Step 7: The receiving end uses the baseband signal frequency difference calculated in step 6 to perform frequency adjustment on the received baseband signal.

In another aspect, the present invention provides a system for improving the initial synchronization performance of a direct expansion power meter reading system, the direct expansion power meter reading system uses frame burst mode for transmission, and each burst data block is composed of SHR, PHR and PSDU. It is composed of offset quadrature phase shift keying OQPSK modulation mode for modulation;

The power meter reading reception preamble is composed of a power meter reading signal receiving module, a local preamble chip data generation module, a local preamble baseband data generation module, a preamble correlation calculation module, a preamble segment data correlation peak search module and a frequency synchronization calculation module;

In the power meter reading signal receiving module, the signal received by the receiving end from the RF front-end is subjected to down-conversion processing, and then analog-to-digital conversion is performed. At the receiving end, the upsampling method of 8 times the chip rate is used to obtain 8 times the speed of the I/Q two. Road baseband digital signal;

Local preamble chip data generation module, the preamble adopts a fixed spreading factor of 256, and a leading bit of 0 is used to spread the spectrum into 256 chip sequences, and then use 16 or 8 segments to segment the chips to form two baseband chip segment data;

The local preamble baseband data generation module performs OQPSK modulation on the baseband chip segmented data locally at the receiving end to form I and Q two-way data, and the segmented preamble baseband data is upsampled by 8 times, and in the case of 16 segments, generates 16 groups of 16x8 segmented preamble baseband data; in the case of 8 segments, generate 8 groups of 32x8 segmented preamble baseband data;

The preamble correlation calculation module completes the correlation calculation between the received baseband digital signal and the segmented preamble modulated baseband data. Each time a baseband digital signal sample value is received, a correlation calculation is performed to obtain 16 or 8 correlation values;

The correlation peak search module of the leading segmented data, performs modulo calculation on the 16 or 8 correlation values obtained by the leading correlation module, and obtains 16 or 8 correlation peaks. If the 16 or 8 correlation peaks are greater than a certain threshold, then Determine that the receiving end searches for a valid leading 0-bit baseband signal;

The frequency synchronization calculation module, according to the determined 16 or 8 correlation values, first calculates the phase difference between two adjacent correlation peaks, then calculates the time interval between the two adjacent correlation peaks, and finally calculates the difference between the receiving end and the transmitting end. The frequency deviation between basebands is used for baseband frequency adjustment by the power meter reading signal receiving module.

Further, the local segmentation principle includes the following:

First, use the received 256×8 I/Q sampled data and the subsequent received 256×8 I/Q sampled data for correlation calculation. If there is an obvious correlation peak, it means that the receiver has searched for two chips with leading 0 bits. start and end positions;

The receiving end receives 256 × 8 I/Q data, locally generated 256-chip I/Q baseband data, the total length of the baseband data is 256 × 8, the first segment is divided into 16 local baseband signals, and the length of each baseband signal is is 16×8, used for coarse frequency adjustment;

The receiving end receives 256x8 I/Q data, locally generated 256-chip I/Q baseband data, the total length of the baseband data is 256x8, the second segment is divided into 8 local baseband signals, and the length of each local baseband signal is 32x8, For frequency fine tuning;

To complete the frequency adjustment, the 256x8 length preamble baseband signal and the received signal are used for direct correlation calculation in the process, the correlation peak is calculated, the first fuzzy phase adjustment is carried out, and the second phase adjustment is carried out by the same method.

Further, relevant calculations include the following:

The 256 chips generated by the leading bit 0 and the generated I/Q two-way data are represented by complex numbers. The real part represents the I channel and the imaginary part represents the Q channel. The baseband data sent by the sender is expressed as:

(a ₁ +b ₁ j),(a ₂ +b ₂ j),...,(a ₂₅₆ +b ₂₅₆ j)

The baseband signal sent by the sender is expressed in polar coordinates as:

The locally generated baseband signal is represented as:

The correlation calculation is recorded as the conjugate correlation of the two signals, and then accumulated and processed, which is expressed as:

Further, the frequency deviation calculation method includes:

According to the xCorr value obtained by the correlation calculation, be ^jΨ is obtained in each calculation, the absolute value abs(be ^jΨ ) is the correlation peak value, Ψ is the correlation phase, and the correlation difference between two adjacent correlation peaks ΔΨ is the phase deviation between the two ends of the transceiver, Δt represents the time length of two adjacent correlation peaks;

2πΔfΔt=ΔΨ

That is, the frequency deviation Δf of the baseband signal at both ends of the transceiver is expressed as:

Δf=ΔΨ/(2πΔt).

After the frequency offset is calculated, the phase ambiguity is also eliminated, that is, the received signal is multiplied by _e ^-jΨ .

Further, the frame preamble search process for power meter reading includes:

In the power meter reading system, frame search or frame header search is completed at the receiving end. The receiving end continuously receives the frame burst data from the transmitting end, goes through the RF front-end, down-converts the frequency, and finally obtains the I/Q two data by sampling at 8 times the chip rate. road baseband signal;

The first local preamble chip segmentation method is: the local preamble is a 0-bit, according to the requirements of the direct expansion power meter reading specification, 256 chips are generated, and the 256-chip sequence data is divided into 16 segments, each segment is 16 segmented codes with a length of 16. Chip sequence; according to the requirements of the direct expansion power meter reading specification, OQPSK is used for modulation to form 16 I/Q two-channel local baseband data; finally, the 16 I/Q local baseband data is continuously processed by 8 times up sampling;

Use the 16-segment segmented baseband data signal and the received baseband signal to perform correlation processing, and each time a sample value is received, a 16-segment correlation calculation is calculated to obtain 16 correlation peaks;

Using the calculated 16 correlation values, initially calculate the frequency deviation of the baseband signal, and then use the frequency deviation to perform frequency compensation on the received baseband signal;

The second local preamble chip segmentation method is: the local preamble is a 0-bit, generating 256 chips, dividing the 256-chip sequence data into 8 segments, and each segment has a length of 32. The segmented chip sequence is modulated by OQPSK to form The local baseband data of 8 I/Q two channels; finally, the 8 I/Q local baseband data will continue to be sampled by 8 times upsampling;

After completing the correlation calculation and frequency adjustment of the two local baseband signals in the first and second modes, the non-segmented baseband signal is used for the correlation calculation to obtain a correlation value, and the phase of the correlation value is used for phase compensation.

The beneficial effects of the present invention are: in the direct expansion system, and the frame burst mode is used for transmission, because there is a clock frequency deviation at the two ends of the transceiver, the baseband signal at the two ends of the transceiver also has a frequency deviation. Using a long spread spectrum sequence to directly perform correlation calculation on the received signal, the performance is relatively poor, and the frequency deviation can not be used when the frequency deviation reaches a certain range, so that the receiving end cannot search for the frame burst signal from the transmitting end.

In view of this, the present invention divides the longer spread spectrum chip sequence into shorter chip sequences, and uses the shorter chip sequence to generate the local baseband signal and the received baseband signal for correlation calculation, thereby suppressing the transmission and reception There is a problem that the frequency deviation affects the related performance at the terminal. The invention makes the transceiver and receiver do not use high-precision crystal oscillators, reduces equipment cost, and can still achieve good performance through software methods.

Other advantages, objects, and features of the present invention will be set forth in the description that follows, and will be apparent to those skilled in the art based on a study of the following, to the extent that is taught in the practice of the present invention. The objectives and other advantages of the present invention may be realized and attained by the following description.

Description of drawings

In order to make the objectives, technical solutions and advantages of the present invention clearer, the present invention will be preferably described in detail below with reference to the accompanying drawings, wherein:

Fig. 1 is the working principle diagram of segmented chip;

Fig. 2 is a schematic diagram of a direct spreading sequence preamble segment;

Fig. 3 is a segmented chip workflow;

Fig. 4 is the synchronous head sending process of direct expansion system;

Fig. 5 is a schematic diagram of OQPSK modulation;

Figure 6 is a schematic diagram of the initial synchronization of power meter reading;

Fig. 7 is the relevant schematic diagram of the local Preamble subsection of power meter reading;

Fig. 8 is the relevant calculation principle of the receiving end;

Fig. 9 is the preamble synchronization process of electric meter reading;

Fig. 10 is a scene without frequency deviation, wherein (a) is a graph of correlation results without segmentation, (b) is a graph of correlation results with 16 segments, and (c) is a graph of correlation results with 8 segments;

Fig. 11 is a scene of 200Hz frequency deviation in the baseband signal receiving and transmitting, wherein (a) is a graph of the correlation result without segmentation, (b) is a graph of the correlation result of 16 segments, and (c) is a graph of the correlation result of 8 segments;

Fig. 12 is the scene that 400Hz frequency deviation exists in transmitting and receiving baseband signals, wherein (a) is the result of correlation without segmentation, (b) is the result of correlation with 16 segments, and (c) is the result of correlation with 8 segments;

Figure 13 shows a scenario where the baseband signal is received and received with a frequency deviation of 2500 Hz, where (a) is the result of correlation without segmentation, (b) is the result of correlation with 16 segments, and (c) is the result of correlation with 8 segments.

Detailed ways

The embodiments of the present invention are described below through specific specific examples, and those skilled in the art can easily understand other advantages and effects of the present invention from the contents disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments, and various details in this specification can also be modified or changed based on different viewpoints and applications without departing from the spirit of the present invention. It should be noted that the drawings provided in the following embodiments are only used to illustrate the basic idea of the present invention in a schematic manner, and the following embodiments and features in the embodiments can be combined with each other without conflict.

Among them, the accompanying drawings are only used for exemplary description, and represent only schematic diagrams, not physical drawings, and should not be construed as limitations of the present invention; in order to better illustrate the embodiments of the present invention, some parts of the accompanying drawings will be omitted, The enlargement or reduction does not represent the size of the actual product; it is understandable to those skilled in the art that some well-known structures and their descriptions in the accompanying drawings may be omitted.

The same or similar numbers in the drawings of the embodiments of the present invention correspond to the same or similar components; in the description of the present invention, it should be understood that if there are terms “upper”, “lower”, “left” and “right” , "front", "rear" and other indicated orientations or positional relationships are based on the orientations or positional relationships shown in the accompanying drawings, and are only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the indicated device or element must be It has a specific orientation, is constructed and operated in a specific orientation, so the terms describing the positional relationship in the accompanying drawings are only used for exemplary illustration, and should not be construed as a limitation of the present invention. situation to understand the specific meaning of the above terms.

Please refer to FIG. 1 to FIG. 13. According to the problems in practical engineering, the present invention proposes a method for analyzing the preamble in the direct sequence spread spectrum system. The basic method is to divide the local one-bit preamble spread spectrum chip into multiple segments. The chip sequence, abbreviated as segmented chip sequence, uses each segmented chip sequence to form the local baseband signal and the received baseband signal for correlation calculation, and then uses the phase difference between the correlation peaks to calculate the frequency deviation, using this The frequency offset performs frequency compensation on the received signal.

The invention consists of a receiving baseband signal unit, a correlation calculation unit, a preamble segmented data correlation peak search unit, a frequency deviation calculation unit, a local one-bit preamble chip generating a segmented chip unit, and a segmented chip generating a local baseband signal unit. As shown in Figure 1.

The baseband signal receiving unit receives data from the radio frequency unit, completes automatic gain control, performs analog-to-digital conversion, and forms digital baseband data; the local one-bit preamble chip generates segmented chip units according to the spreading code and spreading code in the direct spread system. According to the requirements of the frequency factor, the local preamble chip sequence is generated, and the chip sequence is divided into multi-segment chip sequences according to the needs; the segmented chip generation local baseband signal unit modulates the segmented chip sequence to generate the corresponding baseband data; the correlation calculation unit completes the correlation calculation between the local preamble segmented chip baseband data and the received baseband signal, and performs a correlation calculation each time a chip symbol data is received; the preamble segmented data correlation peak search unit completes the correlation calculation unit calculation result , the start and end positions of the leading bits are obtained by searching; the frequency deviation calculation unit uses the correlation likelihood value obtained by the correlation calculation unit to perform frequency estimation to obtain the frequency deviation.

In order to further illustrate the method for segmenting a preamble bit data chip in the present invention, it is specifically shown in FIG. 2 . Assuming that in this direct spreading system, the preamble spreading factor is N, then one preamble bit will form baseband data of N chips after direct spreading. The N chip data is divided into K sub-chip sequences, each sub-chip sequence is composed of S chips. Among them, N, K and S satisfy the relationship of N=KxS.

Implementation process of the present invention

Step 1: The receiving end locally generates a chip sequence of preamble bits according to the spreading factor and the spreading code. Assuming that the preamble spreading factor is N, one bit of preamble data generates N chip sequence data. Step 1 as shown in Figure 3.

Step 2: Divide the generated one-bit preamble chip sequence into K-segment chip data, also known as segmented chip sequence, the length of each segmented chip data is denoted as S, and S satisfies the relationship of S=(N/K) , and then modulate the segmented chip sequence to form a local preamble baseband signal. Assuming that the receiving end uses m times the chip rate to sample the received baseband signal, then the local segmented chip sequence modulation signal is also subjected to m times up-sampling processing, that is, each data in the local segmented chip sequence modulated signal is processed. Perform m repetitions. Step 2 as shown in Figure 3.

Step 3: Receive frame burst data to form a baseband data stream, and the baseband data stream is m times up-sampling of the baseband signal. Step 3 as shown in Figure 3.

Step 4: Correlation calculation is performed on the received baseband data stream and the local segmented chip sequence modulation signal respectively. Each time the receiving end receives a sampled data, a correlation calculation is performed with the local segmented chip sequence modulated signal, and K correlation peaks are obtained each time. Step 4 as shown in Figure 3.

Step 5: The receiving end searches for correlation peaks. If there are K obvious correlation peaks, it indicates that the receiving end has searched for a preamble baseband signal with a bit of 0 in the frame burst. Step 5 as shown in Figure 3.

Step 6: The receiving end determines the start and end positions of the leading bits according to the position of the correlation peak, then uses the correlation value of the correlation peak to calculate the phase difference between two adjacent correlation peaks, and uses the position between the two correlation peaks The difference calculates the time length between the two correlation peaks, and calculates the baseband signal frequency difference between the receiver and the transmitter. Step 6 in Figure 3.

Step 7: The receiving end uses the baseband signal frequency difference calculated in step 6 to perform frequency adjustment on the received baseband signal. Step 7 in Figure 3.

In order to make it clearer that the present invention is applied in practical engineering, it will be described in the direct expansion power meter reading system. As introduced in the technical background, the direct expansion power meter reading system adopts frame burst mode for transmission, and each burst data block is composed of SHR, PHR and PSDU. The system adopts offset quadrature phase shift keying (abbreviation: OQPSK) modulation mode for modulation. The transmission chain of the SHR part is shown in Figure 4.

The leading link of the direct expansion power meter reading system is the same as the power meter reading synchronization head link. After the synchronization data is ready, it undergoes differential coding, DSSS spread spectrum, framing, OQPSK modulation, and then sent out through radio frequency. Among them, for the leading Preamble bits, the leading bits are composed of 0 bits, so the differential coding has no effect on the leading Preamble bits. DSSS spread spectrum uses the spread spectrum code with the spreading factor 256 determined by the power meter reading system to spread the spectrum. After the spreading code sequence is modulated by OQPSK, the RF front end sends out the burst data containing the preamble frame. Among them, the OQPSK modulation mode and the mapping rules of the chip sequence to the I and Q channels are shown in Figure 5.

In this embodiment, the preamble receiving and processing module is shown in Figure 6, that is, the schematic diagram of the initial synchronization of the power meter reading. The power meter reading reception preamble consists of a power meter reading signal receiving module, a local preamble chip data generation module, a local preamble baseband data generation module, a preamble correlation calculation module, a preamble segment data correlation peak search module and a frequency synchronization calculation module.

Among them, in the power meter reading signal receiving module, the signal received by the receiving end from the RF front-end is subjected to down-conversion processing, and then analog-to-digital conversion (A/D change) is performed. In order to improve the demodulation performance, the receiving end adopts 8 times the chip rate. Up-sampling processing, that is, the frame burst data received from the radio frequency is sampled by 8 times the chip clock, so as to ensure the signal quality of the signal received by the receiving end, and finally obtain 8 times the speed of the I/Q two-way baseband digital signal.

The local preamble chip data generation module, according to the requirements of the power meter reading system, uses the spreading code defined by the 0 bit in the preamble to form a spreading sequence of bit 0. In this embodiment, a fixed spreading factor of 256 is used for the preamble. Therefore, a leading bit of 0 is spread into a sequence of 256 chips. Then, the chips are segmented. In this embodiment, two segmentation methods, ie, 16-segment or 8-segment methods, are used to form two types of baseband chip segment data.

The local preamble baseband data generation module performs OQPSK modulation on the baseband chip segment data locally at the receiving end to form I and Q two-way data, also called segmented preamble baseband data, as shown in Figure 5. In this module, in the power meter reading signal receiving module, the chip rate is 8 times for sampling, so the segmented preamble baseband data is also up-sampled by 8 times. In the case of 16 segments, 16 sets of 16x8 segment preamble baseband data are generated. In the case of 8 segments, 8 sets of 32x8 segmented preamble baseband data are generated.

The preamble correlation calculation module completes the correlation calculation between the received baseband digital signal and the segmented preamble modulation baseband data. Each time a baseband digital signal sample value is received, a correlation calculation is performed to obtain 16 or 8 correlation values.

The correlation peak search module of the leading segmented data, performs modulo calculation on the 16 or 8 correlation values obtained by the leading correlation module, and obtains 16 or 8 correlation peaks. If the 16 or 8 correlation peaks are greater than a certain threshold, then Make sure that the receiving end searches for a valid leading 0-bit baseband signal.

The frequency synchronization calculation module, according to the determined 16 or 8 correlation values, first calculates the phase difference between two adjacent correlation peaks, then calculates the time interval between the two adjacent correlation peaks, and finally calculates the difference between the receiving end and the transmitting end. The frequency deviation between basebands is used for baseband frequency adjustment by the power meter reading signal receiving module.

In this embodiment, two kinds of subsections are used for correlation. In the direct-spreading power meter reading system, since the preamble preamble adopts a spreading factor of 256, when the baseband frequency synchronization has not been performed at both ends of the transceiver, it is directly used. The OQPSK baseband signal formed by the 256-chip sequence is processed for correlation. Because in the Internet of Things, due to the consideration of equipment cost, it is basically impossible to choose a high-precision crystal oscillator, so the frequency deviation between the transceivers is relatively large. As a result, it is not possible to directly use 256 chips to form a baseband signal for direct correlation calculation during engineering implementation.

First: In this embodiment, since there are at least 8 0 bits as the preamble data, the correlation calculation is performed before and after the baseband data is received, that is, the received 256x8 I/Q sampling data and the subsequent 256x8 I/Q sampling data are used. Q-sampled data for correlation calculations. If there is an obvious correlation peak, it indicates that the receiver has searched for the start and end positions of two leading 0-bit chips. (1) marked in Figure 7.

Second: the receiving end receives 256x8 I/Q data (8 times the chip clock rate sampling), locally generated 256-chip I/Q baseband data, the total length of the baseband data is 256x8, the first segment is divided into 16 local segments Baseband signal, that is, the length of each baseband signal is 16x8. (2) in Figure 7. Used for coarse frequency adjustment.

Third: the receiving end receives 256x8 I/Q data (8 times the chip clock rate sampling), locally generated 256-chip I/Q baseband data, the total length of the baseband data is 256x8, the second segment is divided into 8 local segments The baseband signal, that is, the length of each segment of the local baseband signal is 32x8. (3) marked in Figure 7. Used for frequency fine tuning.

Fourth: The frequency adjustment is completed through the first three processes. In this process, the 256x8 length preamble baseband signal and the received signal can be directly correlated to calculate the correlation peak value and perform the first fuzzy phase adjustment. Use the same method for a second phase adjustment. (4) and (5) are marked in Figure 7.

The principle of performing correlation calculation between the received baseband signal and the local baseband signal in the present invention

First: According to the OQPSK modulation definition of the direct-spreading power system, the 256 chips generated by the leading bit 0 and the generated I/Q two-way data are represented by complex numbers for the convenience of analysis and processing. The real part represents the I channel and the imaginary part Represents the Q-way. Then the baseband data sent by the sender can be expressed as: (a ₁ +b ₁ j),(a ₂ +b ₂ j),...,(a ₂₅₆ +b ₂₅₆ j), as marked by (1) in Figure 8 .

采用同样的方法，则本地生成的基带信号表示为：

如图8中(2)标注。Second: For intuitive analysis, the baseband signal sent by the sender is expressed in polar coordinates as:

Using the same method, the locally generated baseband signal is expressed as:

(2) in Figure 8.

The third: correlation calculation, recorded as the conjugate correlation of the two signals, and then accumulated processing, using the formula to express as:

It is marked as (3) in Figure 8.

Assuming that the preamble baseband sequence received at the receiving end and the locally generated preamble baseband sequence, if the operating frequency of the transmitting end and the receiving end are the same, Ψ=Φ _i -θ _i in the xCorr calculation formula should be a fixed value. If the baseband operating frequencies at both ends of the transceiver are different, the shorter the correlation sequence, the more stable the ΔΨ value.

Therefore, in the invention, 16 shorter local baseband signals are used first, then 8 longer local baseband signals are used, and finally the spread spectrum baseband signal corresponding to the entire bit is used for correlation calculation.

Fourth: Frequency deviation calculation method

According to the xCorr value obtained by the correlation calculation, be ^jΨ can be obtained in each calculation, where the absolute value of b abs(be ^jΨ ) is the correlation peak value, and Ψ is the correlation phase. The correlation difference ΔΨ between two adjacent correlation peaks can be regarded as the phase deviation between the two ends of the transceiver. Δt represents the time length of two adjacent correlation peaks.

2πΔfΔt=ΔΨ

That is, the frequency deviation Δf of the baseband signal at both ends of the transceiver is expressed as:

Δf=ΔΨ/(2πΔt)

Fifth: About Phase Ambiguity (Phase Synchronization)

Phase ambiguity at the receiving end, even in the case of frequency synchronization between the sending end and the receiving end, there is also a phase ambiguity problem. Theoretically, it is manifested in the correlation calculation. Due to the phase deviation between the sending end and the receiving end, there is a fixed Ψ value to eliminate the phase. Blurring is simply multiplying the received signal by an e ^-jΨ . Then the result of correlating the sending and receiving signals is a real number. It should be noted that in the process of synchronization search and frequency compensation, only the correlation peak value and the phase difference value are concerned. And in actual engineering, the basebands at both ends of the transceiver cannot be completely synchronized, and phase compensation needs to be performed in real time.

The present invention completes the frame preamble search process for power meter reading

Step 1: The frame search or frame header search in the power meter reading system is completed at the receiving end. The receiving end continuously receives frame burst data from the transmitting end, goes through the RF front-end, down-converts the frequency, and finally conducts sampling at 8 times the chip rate. Obtain I/Q two-way baseband signal. Step 1 as shown in Figure 9.

Step 2: Local preamble chip segmentation mode 1. There is a 0 bit in the local leading, according to the requirements of the direct expansion power meter reading specification, 256 chips are generated, and the 256-chip sequence data is divided into 16 segments, each segment is a segmented chip sequence with a length of 16. According to the requirements of the direct expansion power meter reading specification, OQPSK is used for modulation to form local baseband data of 16 I/Q channels. Finally, the 16 I/Q local baseband data continues to be processed by 8 times upsampling. The essence is to repeat each I/Q data 8 times to form 16 local I/Q baseband signals, and the length of each signal is 16x8. Step 2 as shown in Figure 9.

Step 3: Use the 16-segment segmented baseband data signal and the received baseband signal to perform correlation processing, and calculate the 16-segment correlation calculation once every time a sample value is received. 16 correlation peaks are obtained. Step 3 as shown in Figure 9.

Using the calculated 16 correlation values, the frequency deviation of the baseband signal is preliminarily calculated, and then the frequency deviation is used to perform frequency compensation on the received baseband signal.

Step 4: Local preamble chip segmentation mode 2. The local leading one is 0 bit to generate 256 chips, and the 256-chip sequence data is divided into 8 segments, each segment is a segmented chip sequence with a length of 32. According to the requirements of the direct expansion power meter reading specification, OQPSK is used for modulation to form local baseband data of 8 I/Q channels. Finally, the 8 I/Q local baseband data is continuously processed by 8 times upsampling. The essence is to repeat each I/Q data 8 times to form 8 local I/Q baseband signals, and the length of each signal is 32. Steps 4 and 5 in Figure 9.

Step 5: After completing the correlation calculation and frequency adjustment of the two local baseband signals in the first and second modes, finally use the non-segmented baseband signal to perform the correlation calculation to obtain a correlation value. After steps 3 and 4, the non-segmented baseband signal performs correlation calculation to obtain a correlation value, and uses the phase of the correlation value to perform phase compensation. Steps 6, 7, and 8 in Figure 9.

The performance of the present invention in an actual direct expansion power meter reading system will be described below using the scenario where there are different frequency deviations at both ends of the transceiver. According to the description of this embodiment, it is divided into (a) no segmentation to directly perform the correlation result, (b) 16 segmentation to perform the correlation result, (c) 8 segmentation to perform the correlation result.

First: in a scenario where there is no frequency deviation, as shown in Figure 10. There is good correlation performance with no segmentation and with 16 and 8 segmentations.

Second: In a scene with a frequency deviation of 200 Hz, as shown in Figure 11. There is good correlation performance with no segmentation and with 16 and 8 segmentations. It shows that there is a frequency deviation of 200Hz in the baseband signals at both ends of the transceiver, which does not affect the receiving performance of the receiving end, that is, it can receive normally without sampling.

Third: In a scene with a frequency deviation of 400 Hz, as shown in Figure 12. Both 16 and 8 segments have good correlation performance, but there is no segmented direct correlation calculation, and there is no obvious correlation peak. It indicates that there is a frequency deviation of 400 Hz in the baseband signals at both ends of the transceiver. If the preamble is not sampled for segment processing, the preamble cannot be searched normally.

Fourth: In a scene with a frequency deviation of 2500 Hz, as shown in Figure 13. The correlation calculation is performed directly with the non-segmented preamble, and there is no obvious correlation peak. When the 16-segment segmented preamble is used for correlation, although the correlation peak can be identified, there is already interference. There should be only 16 correlation peaks, but 19 have been identified. However, these 19 correlation peaks are still valid. After using the 19 correlation peaks for frequency adjustment, 8-segment segmentation is performed again, and 8 better correlation peaks can still be obtained.

From the above simulation, if the present invention is not used, the receiving end can only suppress the baseband frequency deviation of 200 Hz, but after using the method of the present invention, the receiving end can suppress the baseband frequency deviation of 2500 Hz. In this embodiment, even if a low-cost crystal is selected, an algorithm can still be used to suppress the baseband frequency deviation existing at both ends of the transceiver.

Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit them. Although the present invention has been described in detail with reference to the preferred embodiments, those of ordinary skill in the art should understand that the technical solutions of the present invention can be Modifications or equivalent replacements, without departing from the spirit and scope of the technical solution, should all be included in the scope of the claims of the present invention.

Claims (8)

1. a system that improves the initial synchronization performance of the direct-spreading system, is characterized in that: comprising receiving baseband signal unit, correlation calculating unit, leading segment data correlation peak searching unit, frequency deviation calculating unit, local one-bit leading chip generation component. a segment chip unit, the segmented chip generates a local baseband signal unit; The receiving baseband signal unit receives data from the radio frequency unit, completes automatic gain control, performs analog-to-digital conversion, and forms digital baseband data; The local one-bit preamble chip generating segmented chip unit generates a local preamble chip sequence according to the requirements of the spreading code and the spreading factor in the direct-spreading system, and divides the chip sequence into a multi-segment chip sequence as required; The segmented chip generating local baseband signal unit modulates the segmented chip sequence to generate corresponding baseband data; The correlation calculation unit is used to perform correlation calculation on the local preamble segmented chip baseband data and the received baseband signal, and perform a correlation calculation every time one chip symbol data is received; The preamble segmented data correlation peak search unit is used to search the calculation result of the correlation calculation unit to obtain the start and end positions of the preamble bits; The frequency deviation calculation unit performs frequency estimation using the correlation likelihood value obtained by the correlation calculation unit to obtain the frequency deviation. 2. The system for improving the initial synchronization performance of the direct expansion system according to claim 1, wherein the frequency deviation calculation unit first calculates the phase difference of two adjacent correlation peaks according to the determined multiple correlation values, Then, the time interval between two adjacent correlation peaks is calculated, and finally the baseband frequency deviation between the receiving end and the transmitting end is calculated, and the baseband frequency is adjusted. 3. a method for improving the initial synchronization performance of a direct expansion system, characterized in that: comprising the following steps: Step 1: The receiving end locally generates a chip sequence of preamble bits according to the spreading factor and the spreading code, so that the preamble spreading factor is N, then one bit of preamble data generates N chip sequence data; Step 2: Divide the generated one-bit preamble chip sequence into K-segment chip data, also known as segmented chip sequence, the length of each segmented chip data is denoted as S, and S satisfies the relationship of S=(N/K) , and then modulate the segmented chip sequence to form a local preamble baseband signal; let the receiving end sample the received baseband signal at m times the chip rate, then the local segmented chip sequence modulation signal, the same m Upsampling processing, that is, repeating each data in the local segmented chip sequence modulation signal m times; Step 3: receiving a baseband signal to form a baseband data stream, and the baseband data stream is m times up-sampling of the baseband signal; Step 4: Correlation calculation is performed on the received baseband data stream and the local segmented chip sequence modulation signal respectively; each time the receiving end receives and moves one sample data, the correlation calculation is performed with the local segmented chip sequence modulation signal, and each time it is obtained. K correlation peaks; Step 5: the receiving end searches for the correlation peak, and if there are K obvious correlation peaks, it indicates that the receiving end has searched for a leading baseband signal of bit 0 in the frame burst; Step 6: The receiving end determines the start and end positions of the leading bits according to the position of the correlation peak, then uses the correlation value of the correlation peak to calculate the phase difference between two adjacent correlation peaks, and uses the position between the two correlation peaks The difference calculates the time length between the two correlation peaks, and calculates the baseband signal frequency difference between the receiving end and the transmitting end; Step 7: The receiving end uses the baseband signal frequency difference calculated in step 6 to perform frequency adjustment on the received baseband signal. 4. A system for improving the initial synchronization performance of a direct expansion power meter reading system, characterized in that: comprising a power meter reading signal receiving module, a local leading chip data generation module, a local leading baseband data generating module, a leading correlation computing module, a leading The segmented data correlation peak search module and the frequency synchronization calculation module are used to improve the initial synchronization performance of the direct expansion power meter reading system; The direct expansion power meter reading system adopts frame burst mode for transmission, and each burst data block is composed of SHR, PHR and PSDU, and adopts offset quadrature phase shift keying OQPSK modulation mode for modulation; In the power meter reading signal receiving module, the signal received by the receiving end from the RF front-end is subjected to down-conversion processing, and then analog-to-digital conversion is performed. At the receiving end, the upsampling method of 8 times the chip rate is used to obtain 8 times the speed of the I/Q two. Road baseband digital signal; Local preamble chip data generation module, the preamble adopts a fixed spreading factor of 256, and a leading bit of 0 is used to spread the spectrum into 256 chip sequences, and then use 16 or 8 segments to segment the chips to form two baseband chip segment data; The local preamble baseband data generation module performs OQPSK modulation on the baseband chip segmented data locally at the receiving end to form I and Q two-way data, and the segmented preamble baseband data is upsampled by 8 times, and in the case of 16 segments, generates 16 groups of 16x8 segmented preamble baseband data; in the case of 8 segments, generate 8 groups of 32x8 segmented preamble baseband data; The preamble correlation calculation module is used to perform correlation calculation between the received baseband digital signal and the segmented preamble modulation baseband data, and each time a baseband digital signal sample value is received, a correlation calculation is performed to obtain 16 or 8 correlation values; The correlation peak search module of the leading segmented data, performs modulo calculation on the 16 or 8 correlation values obtained by the leading correlation module, and obtains 16 or 8 correlation peaks. If the 16 or 8 correlation peaks are greater than a certain threshold, then Determine that the receiving end searches for a valid leading 0-bit baseband signal; The frequency synchronization calculation module, according to the determined 16 or 8 correlation values, first calculates the phase difference between two adjacent correlation peaks, then calculates the time interval between the two adjacent correlation peaks, and finally calculates the difference between the receiving end and the transmitting end. The frequency deviation between basebands is used for baseband frequency adjustment by the power meter reading signal receiving module. 5. the system of improving the initial synchronization performance of the direct expansion power meter reading system according to claim 4, is characterized in that: the local segmentation principle comprises the following content: First, use the received 256×8 I/Q sampled data and the subsequent received 256×8 I/Q sampled data for correlation calculation. If there is an obvious correlation peak, it means that the receiver has searched for two chips with leading 0 bits. start and end positions; The receiving end receives 256 × 8 I/Q data, locally generated 256-chip I/Q baseband data, the total length of the baseband data is 256 × 8, the first segment is divided into 16 local baseband signals, and the length of each baseband signal is is 16×8, used for coarse frequency adjustment; The receiving end receives 256x8 I/Q data, locally generated 256-chip I/Q baseband data, the total length of the baseband data is 256x8, the second segment is divided into 8 local baseband signals, and the length of each local baseband signal is 32x8, For frequency fine tuning; To complete the frequency adjustment, the 256x8 length preamble baseband signal and the received signal are used for direct correlation calculation in the process, the correlation peak is calculated, the first fuzzy phase adjustment is carried out, and the second phase adjustment is carried out by the same method. 6. The system for improving the initial synchronization performance of a direct expansion power meter reading system according to claim 5, wherein the relevant calculation includes the following content: The 256 chips generated by the leading bit 0 and the generated I/Q two-way data are represented by complex numbers. The real part represents the I channel and the imaginary part represents the Q channel. The baseband data sent by the sender is expressed as: (a ₁ +b ₁ j),(a ₂ +b ₂ j),...,(a ₂₅₆ +b ₂₅₆ j) The baseband signal sent by the sender is expressed in polar coordinates as:

The locally generated baseband signal is represented as:

The correlation calculation is recorded as the conjugate correlation of the two signals, and then accumulated and processed, which is expressed as:

7. The system for improving the initial synchronization performance of a direct expansion power meter reading system according to claim 6, wherein the frequency deviation calculation method comprises: According to the xCorr value obtained by the correlation calculation, be ^jΨ is obtained in each calculation, the absolute value abs(be ^jΨ ) is the correlation peak value, Ψ is the correlation phase, and the correlation difference between two adjacent correlation peaks ΔΨ is the phase deviation between the two ends of the transceiver, Δt represents the time length of two adjacent correlation peaks; 2πΔfΔt=ΔΨ That is, the frequency deviation Δf of the baseband signal at both ends of the transceiver is expressed as: Δf=ΔΨ/(2πΔt) After the frequency offset is calculated, the phase ambiguity is also eliminated, that is, the received signal is multiplied by _e ^-jΨ . 8. the system of improving the initial synchronization performance of the direct expansion power meter reading system according to claim 4, is characterized in that: the frame leading search process of power meter reading comprises: In the power meter reading system, frame search or frame header search is completed at the receiving end. The receiving end continuously receives the frame burst data from the transmitting end, goes through the RF front-end, down-converts the frequency, and finally obtains the I/Q two data by sampling at 8 times the chip rate. road baseband signal; The first local preamble chip segmentation method is: the local preamble is a 0-bit, according to the requirements of the direct expansion power meter reading specification, 256 chips are generated, and the 256-chip sequence data is divided into 16 segments, each segment is 16 segmented codes with a length of 16. Chip sequence; according to the requirements of the direct expansion power meter reading specification, OQPSK is used for modulation to form 16 I/Q two-channel local baseband data; finally, the 16 I/Q local baseband data is continuously processed by 8 times up sampling; Use the 16-segment segmented baseband data signal and the received baseband signal to perform correlation processing, and each time a sample value is received, a 16-segment correlation calculation is calculated to obtain 16 correlation peaks; Using the calculated 16 correlation values, initially calculate the frequency deviation of the baseband signal, and then use the frequency deviation to perform frequency compensation on the received baseband signal; The second local preamble chip segmentation method is: the local preamble is a 0-bit, generating 256 chips, dividing the 256-chip sequence data into 8 segments, and each segment has a length of 32. The segmented chip sequence is modulated by OQPSK to form The local baseband data of 8 I/Q two channels; finally, the 8 I/Q local baseband data will continue to be sampled by 8 times upsampling; After completing the correlation calculation and frequency adjustment of the two local baseband signals in the first or second mode, the non-segmented baseband signal is used for the correlation calculation to obtain a correlation value, and the phase of the correlation value is used for phase compensation.

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