CN111600820A

CN111600820A - Physical layer transmission signal generation method based on MFSK modulation and Chirp spread spectrum technology

Info

Publication number: CN111600820A
Application number: CN201910799258.5A
Authority: CN
Inventors: 赵锋; 李帆; 李东敏; 李健勋; 毕扬帆; 江升辉
Original assignee: Qingdao Topscomm Communication Co Ltd
Current assignee: Qingdao Topscomm Communication Co Ltd
Priority date: 2019-08-28
Filing date: 2019-08-28
Publication date: 2020-08-28
Anticipated expiration: 2039-08-28
Also published as: CN111600820B

Abstract

The invention provides a physical layer transmission signal generation method based on MFSK modulation and Chirp spread spectrum technology. The implementation scheme is as follows: the physical layer, upon receiving input from the Medium Access Control (MAC) sublayer, performs cyclic encoding of information from the MAC layer. The data information is interleaved after being circularly encoded. And then mapping the information of the interleaved data. After mapping, signal modulation is performed. And meanwhile, generating preamble information for modulation. And finally, adding a preamble signal, forming a frame signal with the frame control symbol and the load symbol, and entering a radio frequency link. The signal waveform generated by the invention has the constant envelope characteristic, and the modulation technology has the advantages of low power consumption, high sensitivity, high reliability and the like.

Description

Physical layer transmission signal generation method based on MFSK modulation and Chirp spread spectrum technology

Technical Field

The invention belongs to the technical field of communication of the Internet of things, and particularly relates to a physical layer transmission signal generation method based on MFSK modulation and Chirp spread spectrum technology.

Background

Nowadays, the internet of things has entered into our daily life, such as power consumption information acquisition, automatic device sensing, interconnection and control systems in power applications, remote meter reading systems in the field of water-gas-heat metering, remote energy environment monitoring systems, and the like. The low-power consumption wide area technology is provided, so that the Internet of things is more on the first floor. In the low-power-consumption wide-area technology, a modulation technology with the characteristics of long distance, expandability, low cost and the like, a high-performance physical layer modulation scheme and a communication algorithm of a self-adaptive communication rate are the basis of the communication system of the Internet of things.

In conventional Direct Sequence Spread Spectrum (DSSS), a pseudo-random code is used as a spreading code to perform despreading to obtain a spreading gain. If a high gain is desired, the spreading code needs to be long and the complexity is high. Also, frequency hopping spread spectrum requires a frequency hopping pattern. For the modulation technology applied to the Internet of things, the modulation technology is simple, high in gain, low in complexity and particularly important.

The Chirp spread spectrum modulation (CSS) technology does not need a pseudo-random code as a spread spectrum code or a frequency hopping pattern, but utilizes the matched filtering and pulse compression characteristics of a Chirp signal to realize spread spectrum communication, and has the advantages of simplicity and low complexity. The method comprises the steps of firstly carrying out MFSK modulation, coding information by using the modulation frequency of the MFSK, then carrying out Chirp spread spectrum modulation, obtaining spread spectrum gain by using the pulse compression characteristic, having high sensitivity and the characteristic of constant envelope, and meeting the low power consumption requirement of the Internet of things. In the field of wireless communication, the MFSK modulation is combined with chirp spread spectrum modulation technology, and the technology is a relatively novel technology.

Disclosure of Invention

The invention mainly provides a physical layer transmission signal generation method based on MFSK modulation and Chirp spread spectrum technology, and the generation flow of a transmitter signal is shown in figure 1.

The frame structure of the physical layer of the present invention is composed of preamble, frame control and payload, and the frame structure of one frame signal is shown in fig. 2. Wherein the preamble is composed of unmodulated a, modulated a, and unmodulated B. The specific format of the pre-derivative data is shown in fig. 3. Without loss of generality, can be provided withThe number of the A is 8 or more than 8. A of the modulation is 2 and is respectively set as a₁And a₂If a₁Is set to be 4, a₂Can be set as M-4, M is the modulation order of FSK modulation; a can also be set₁Is 4, a₂Is 8, a₁，a₂The specific value of (A) can be flexibly changed according to actual requirements. B is the reverse spreading of a.

The invention also provides a cyclic coding method, which divides data into 4-bit groups, adds a supervision bit to each group, and the length of a code word of each group is 7 or 8.

In another aspect, the present invention provides an interleaving method. The interleaving method comprises the following steps: determining the size of an interleaving matrix according to the size of the load data block; inputting the data into the interleaver according to columns; then, circularly shifting the data of each row in the interleaver, and circularly shifting the ith row by i data without loss of generality; and finally, reading data according to rows to complete interleaving.

The invention also provides an information mapping method, which maps the interleaved data bit into the data to be modulated in a certain mapping mode.

The invention provides a modulation method, which is based on MFSK modulation to obtain initial frequency, and then performs chirp spread spectrum to obtain a linear frequency modulation signal.

The technical scheme of the invention comprises the following steps:

(1) the physical layer receives input from a Medium Access Control (MAC) sublayer;

(2) the physical layer encodes data from the MAC layer;

(3) interleaving the encoded data;

(4) carrying out information mapping on the interleaved data;

(5) modulating the data after information mapping;

(6) generating a preamble signal;

(7) adding the preamble to generate a frame signal to enter the radio frequency link.

The invention has the following advantages:

the invention utilizes MFSK modulation and chirp spread spectrum technology, thus having many advantages. For example, the modulation technique of the present invention has the characteristics of low power consumption, high sensitivity, high reliability, etc. The method can normally communicate in a severe noise environment, and has strong channel fading resistance and strong Doppler frequency shift tolerance. The invention reduces the error rate of data and improves the robustness of the system by the modes of cyclic coding, interleaving, information mapping and the like. Reasonable selection among rate and robustness can be made through different parameter configurations.

Drawings

FIG. 1 is a block diagram of transmitter signal generation of the present invention;

fig. 2 is a frame structure diagram of a frame signal of the present invention; the frame structure of the present invention consists of preamble, frame control and data load.

FIG. 3 is a diagram of a preamble structure; the preamble consists of unmodulated a, modulated a, and unmodulated B.

FIG. 4 is a schematic diagram of the operation of a [7,4] cyclic code encoder;

FIG. 5 is a diagram illustrating a specific input method of inputting data into an interleaver;

FIG. 6 is a diagram illustrating cyclic shifting of data in an interleaver;

FIG. 7 is a graph of frequency f versus M

FIG. 8 is a time-frequency, time-domain waveform diagram of unmodulated A

FIG. 9 shows modulation a₁(a₁4) time-frequency, time-domain waveform diagram

FIG. 10 is a time-frequency, time-domain waveform diagram of modulation 38

Detailed Description

The following description of the exemplary embodiments of the present invention is intended to be taken in conjunction with the accompanying drawings, but is not intended to represent the only embodiments in which the present invention may be practiced. The term "exemplary" used throughout this description means "serving as an example, instance, or illustration," and should not necessarily be construed as preferred or advantageous over other exemplary embodiments. The detailed description includes specific details for the purpose of providing a thorough understanding of exemplary embodiments of the present invention. It will be apparent to one skilled in the art that the exemplary embodiments of the invention may be practiced without these specific details.

Referring to fig. 1, the specific implementation steps of the present invention are as follows:

step one, a physical layer receives input from a MAC layer.

Step two, the physical layer encodes the data from the MAC layer:

fig. 4 shows the working principle of the [7,4] cyclic code encoder, in which information bits are grouped into 4-bit groups, 4-bit information is sequentially sent into a shift register, and after shifting for 3 times by performing shift operation and modulo-2 addition operation, 3-bit supervisory bits are all output. Together with the 4bit information bits, form a complete code group.

[7,4]The cyclic code coding may also be according to the primitive polynomial g (x) x³And encoding the generator matrix corresponding to + x + 1.

[7,4] the generator matrix corresponding to the cyclic code is:

the [8,4] cyclic code is formed by adding a parity bit of one bit behind the [7,4] cyclic code, so that the checking capability is enhanced.

Step three, interleaving the coded data:

the interleaving process comprises the following steps: determining the size of an interleaving matrix according to the number PB of bytes sent and the number of effective information bits infoBitPerSymbol sent by each symbol; inputting the data into the interleaver according to columns; then, circularly shifting the data of each line in the interleaver; and finally, reading data according to rows to complete interleaving.

3a) Determining the size of an interleaving matrix;

the data size of the payload is defined from PB4 to PB 508. The number of valid information bits transmitted per symbol during modulation is denoted as infobitperssymbol.

The interleaver column number nColumn is shown in table 1:

table 1: number of interleaver columns

inforBitPerSymbol/PB	PB<16	16＝<PB<64	64＝<PB<128	PB>＝128
					1	8	16	32	64
2	8	16	32	64
					3	6	12	24	48
4	8	16	32	64
					5	5	20	40	80
6	6	24	48	96
					7	7	14	28	56
8	8	16	32	64
					9	9	18	36	72
10	10	20	40	80
					11	11	22	44	88
12	12	24	48	96

The interleaver row number is calculated as follows:

where nRow represents the number of interleaving rows, PB represents the number of bytes of a transmission signal, codeRate represents the coding rate, and nColumn represents the number of columns of the interleaver.

3b) Inputting data into an interleaver in columns;

before data is input into the interleaver, the size requirement of the interleaver may not be met, zero padding is needed first, and the number of the zero padding is as follows:

where Npad indicates the number of zero padding required, PB indicates the number of bytes of a transmission signal, codeRate indicates the coding rate, and nColumn indicates the number of interleaver columns.

Then, the data is input into the interleaver column by column, and assuming that the input data is 7 and then is sized without loss of generality, the specific input method is as shown in fig. 5.

3c) Circularly shifting each row of data in the interleaver;

without loss of generality, assuming that the interleaver size is 7-interleaver, the cyclic shift method is as shown in fig. 6 below. The cyclic shift interval can be flexibly set according to specific situations, and in the embodiment, it is assumed that the cyclic shift parameter of the ith row is mod (i, bitPerSymbol), and can also be set as the cyclic shift mod (2i, bitPerSymbol) of the ith row according to actual situations.

3d) Reading data by row

And reading the data after cyclic shift according to rows, and finishing interleaving.

Step four, carrying out information mapping on the interleaved data:

the input data stream is grouped in infoBitPerSymbol bits, mapped once per infoBitPerSymbol information bit. The input data flow is carried out according to the sequence of low order advanced and high order backward.

The information mapping is to add redundant bits to the infoBitPerSymbol bits in a certain mapping mode and map the bits into bitPerSymbol bits. Finally, the bitPerSymbol bits are remapped to decimal positions.

Let the number of bits actually sent by each symbol be bitPerSymbol, and the information bit before mapping be (I)₁I₂…I_{inforBitPerSymbol-1}I_{inforBitPerSymbol})

Wherein, I_{inforBitPerSymbol}The information bit indicating the position of the first infoBitPerSymbol before mapping is 0 or 1.

The mapped information bit is (m)₁m₂… m_{bitPerSymbol-1}m_bitPerSymbol)

Wherein m is_bitPerSymbolAnd the information bit for indicating the position of the mapped bit PerSymbol is 0 or 1.

The mapping method is as follows:

m₁＝0；

…

m_{bitPerSymbol-inforBitPerSymbol}＝1；

m_{bitPerSymbol-inforBitPerSymbol+1}＝mod(I₁+I₂+…+I_{inforBitPerSymbol},2)；

m_{bitPerSymbol-inforBitPerSymbol+2}＝mod(I₂+…+I_{inforBitPerSymbol},2)；

…

m_{bitPerSymbol-1}＝mod(I_{inforBitPerSymbol-1}+I_{inforBitPerSymbol},2)；

m_bitPerSymbol＝I_{inforBitPerSymbol}；

the way in which the bitPerSymbol bits are mapped into decimal positions is as follows:

L_i＝2^{bitPerSymbol-inforBitPerSymbol-1}

+m_{bitPerSymbol-inforBitPerSymbol+1}×2^{bitPerSymbol-inforBitPerSymbol}

+m_{bitPerSymbol-inforBitPerSymbol+2}×2^{bitPerSymbol-inforBitPerSymbol+1}

+m_{bitPerSymbol-inforBitPerSymbol+3}×2^{bitPerSymbol-inforBitPerSymbol+2}

+…

+m_{bitPerSymbol-1}×2^{bitPerSymbol-2}+m_bitPerSymbol×2^{bitPerSymbol-1}

wherein L is_iThe decimal position of the bit PerSymbol bit information after mapping is represented, and L is more than or equal to 0_i≤2^bitPerSymbol-1。L_iAlso indicates the corresponding position of the MFSK mapped frequency.

Without loss of generality, let infobibitpsersymbol be 5, bitPerSymbol be 7, information bit before mapping be (10110), information bit after mapping be (0110010), and 7bit is remapped to decimal place be loc be 38.

Step five, modulating the data after information mapping:

5a) MFSK modulation

Through the information mapping, each infoBitPerSymbol information bit is mapped to obtain a decimal position L_iL of the compound_iCorresponding to the modulation frequency of the MFSK.

MFSK modulation frequency and L_iThe correspondence of (a) is as follows:

wherein M is 2^bitPerSymbol,i＝1,2,…2^{inforBitPerSymbol}，

Is expressed log₂Frequency, T, corresponding to Mbit information_sIndicates the length of time of one modulation symbol,

to represent

Without loss of generality, the starting frequency f₀And may be any frequency including, but not limited to, -B/2.

The MFSK-modulated signal is represented as

5b) Chirp spread spectrum

Through MFSK modulation, the frequency is obtained

Of the signal of (1). The signal is Chirp spread, and the Chirp spread signal can be expressed as

Where y represents the time domain signal, B represents the signal bandwidth, T_sRepresenting one symbol time, M representing the number of sample points of one symbol, M-2^bitPerSymbol。

The slope k represents how fast the frequency varies linearly with time. The frequency f varies with M as shown in fig. 7.

Firstly, MFSK modulation is performed to generate the starting frequency

Then Chirp spread spectrum is carried out, and the signals after MFSK + Chirp spread spectrum mixed modulation can be representedIs composed of

Wherein, x represents the time domain signal after MFSK + Chirp spread spectrum, B represents the signal bandwidth, M represents the number of sampling points of a symbol, and M is 2^bitPerSymbol，

Indicating the starting frequency.

Without loss of generality, assuming loc is 38, the time-frequency plot after modulation is shown in fig. 10 below.

Step six, generating a leading signal,

FIG. 3 shows a preamble data structure, the preamble being modulated by 8 unmodulated A, 1 into a₁One modulation is a₂2.25 unmodulated B. A is the basic waveform and B is the reverse spreading of A. Modulated a₁And a₂Can be flexibly changed according to actual requirements. Without loss of generality, if a₁Is set to be 4, a₂Can be set to M-4, M being the modulation order of FSK modulation, or a₁Is set to be 4, a₂Set to 8.

Here the starting frequency f of the MFSK is set₀is-B/2, the decimal position loc corresponding to the unmodulated A is 0, and the frequency range is-B/2 to B/2. Modulation is a₁Has a decimal position loc corresponding to the symbol of₁In the frequency range-B/2 + a₁/T_s～B/2，-B/2～-B/2+a₁/T_s. Modulation is a₂Has a decimal position loc corresponding to the symbol of₂Frequency range-B/2 + a₂/T_s～B/2，-B/2～-B/2+a₂/T_s. The unmodulated B is the inverse modulation of the unmodulated A, and the frequency range is B/2 to-B/2. Specific time-frequency curves and time-domain waveforms are shown in fig. 8 and 9.

And step seven, adding the preamble to generate a frame signal to enter the radio frequency link.

The frame structure of one frame signal is shown in fig. 2. The frame structure of the physical layer consists of a preamble, a frame control and a data payload. As shown in fig. 2, each symbol has a length of nLength, where the number of leading symbols is 12.25 and the length is 12.25 × nLength. The number of symbols of the frame control and data payload is variable and is related to the infobitperssymbol of the information map.

Claims

1. A physical layer transmission signal generation method based on MFSK modulation and Chirp spread spectrum technology comprises the following steps:

(2) the physical layer encodes data from the MAC layer;

(3) interleaving the encoded data;

(4) carrying out information mapping on the interleaved data;

(5) modulating the data after information mapping;

(6) generating a preamble signal;

2. The method as claimed in claim 1, wherein in step (2), when encoding the data from the MAC layer, the [7,4] cyclic code encoder operates according to the principle shown in fig. 4, and the information bits are grouped into 4-bit groups, and the 4-bit information is sequentially sent to the shift register, and after shifting 3 times by performing shift operation and modulo 2 addition operation, the 3-bit parity bits are all output. Together with the 4bit information bits, form a complete code group.

[7,4] the generator matrix corresponding to the cyclic code is:

3. The method according to claim 1, wherein in step (3), when interleaving the encoded data, determining the size of the interleaving matrix according to the number PB of bytes sent and the number of bits infobi tpersymbol sent by each symbol; inputting the data into the interleaver according to columns; then carrying out cyclic shift on the data of each row in the interleaver; and finally, reading data according to rows to complete interleaving.

4. The method of claim 1, wherein in the step (3a), when interleaving the encoded data, the interleaver size is determined as follows:

the interleaver column number nColumn is shown in table 1:

table 1: number of interleaver columns

The interleaver row number is calculated as follows:

5. The method as claimed in claim 1, wherein in the step (3b), the encoded data is input to the interleaver in columns when being interleaved.

6. The method of claim 1, wherein in step (3c), each row of data in the interleaver is cyclically shifted while interleaving the encoded data. Without loss of generality, assuming an interleaver size of 7 × 7, the cyclic shift method is as shown in fig. 6 below. The cyclic shift interval can be flexibly set according to specific situations, and in the embodiment, it is assumed that the cyclic shift parameter of the ith row is mod (i, bitPerSymbol), and can also be set as the cyclic shift mod (2i, bitPerSymbol) of the ith row according to actual situations.

7. The method as claimed in claim 1, wherein in step (4), when mapping the interleaved data, the input data stream is grouped by infoBitPerSymbol bits, and the mapping is performed once every infoBitPerSymbol information bit. The input data flow is carried out according to the sequence of low order advanced and high order backward.

Let the number of bits actually sent by each symbol be bitPerSymbol, and the information bit before mapping be

(I₁I₂… I_{inforBitPerSymbol-1}I_{inforBitPerSymbol})

The mapped information bit is (m)₁m₂… m_{bitPerSymbol-1}m_bitPerSymbol)

The mapping method is as follows:

m₁＝0；

…

m_{bitPerSymbol-inforBitPerSymbol}＝1；

m_{bitPerSymbol-inforBitPerSymbol+2}＝mod(I₂+…+I_{inforBitPerSymbol},2)；

…

m_{bitPerSymbol-1}＝mod(I_{inforBitPerSymbol-1}+I_{inforBitPerSymbol},2)；

m_bitPerSymbol＝I_{inforBitPerSymbol}；

L_i＝2^{bitPerSymbol-inforBitPerSymbol-1}

+m_{bitPerSymbol-inforBitPerSymbol+1}×2^{bitPerSymbol-inforBitPerSymbol}

+m_{bitPerSymbol-inforBitPerSymbol+2}×2^{bitPerSymbol-inforBitPerSymbol+1}

+m_{bitPerSymbol-inforBitPerSymbol+3}×2^{bitPerSymbol-inforBitPerSymbol+2}

+…

+m_{bitPerSymbol-1}×2^{bitPerSymbol-2}+m_bitPerSymbol×2^{bitPerSymbol-1}

wherein L is_iThe decimal position of the bit PerSymbol bit information after mapping is represented, and L is more than or equal to 0_i≤2^bitPerSymbo^l-1。L_iAlso indicates the corresponding position of the MFSK mapped frequency.

8. The method as claimed in claim 1, wherein in step (5), when modulating the information mapped data, MFSK modulation is performed, the start frequency is determined, and then chirp spreading is performed.

9. The method according to claim 1, wherein in step (5a), when modulating the information mapped data, performing MFSK modulation.

Through the aboveThe information mapping of (1) obtains a decimal position L by mapping each infoBitPerSymbol information bit_iL of the compound_iCorresponding to the modulation frequency of the MFSK.

MFSK modulation frequency and L_iThe corresponding relationship is as follows

Wherein M is 2^bitPerSymbol,i＝1,2,…2^{inforBitPerSymbol}，

to represent

The MFSK-modulated signal is represented as

。

10. The method of claim 1, wherein in step (5b), Chirp spreading is performed while modulating the information-mapped data.

Through MFSK modulation, the frequency is obtained

Firstly, MFSK modulation is performed to generate the starting frequency

Then Chirp spread spectrum is carried out, and the signals after MFSK + Chirp spread spectrum mixed modulation can be expressed as

Indicating the starting frequency.

11. The method as claimed in claim 1, wherein in step (6), when generating the preamble signal, the preamble data structure is as shown in fig. 3, the preamble is modulated by 8 unmodulated a, 1 modulated as a₁One modulation is a₂2.25 unmodulated B. A is the basic waveform and B is the reverse spreading of A. Modulated a₁And a₂Can be flexibly changed according to actual requirements. Without loss of generality, if a1 is set to 4, a2 can be set to M-4, M being the modulation order of FSK modulation, or a₁Is set to be 4, a₂Set to 8.

Here the starting frequency f of the MFSK is set₀for-B/2, unmodulated A corresponds toThe decimal position loc is 0, and the frequency range is-B/2. Modulation is a₁Has a decimal position loc corresponding to the symbol of₁Frequency range-B/2 + a₁/T_s～B/2，-B/2～-B/2+a₁/T_s. Modulation is a₂Has a decimal position loc corresponding to the symbol of₂Frequency range-B/2 + a₂/T_s～B/2，-B/2～-B/2+a₂/T_s. The unmodulated B is the inverse modulation of the unmodulated A, and the frequency range is B/2 to-B/2. Specific time-frequency curves and time-domain waveforms are shown in fig. 8 and 9.

12. The method according to claim 1, wherein in step (7), when the preamble is added to generate a frame signal into the radio frequency link, the frame structure of the frame signal is as shown in fig. 2. The frame structure of the physical layer consists of a preamble, a frame control and a data bearer. As shown in fig. 2, each symbol has a length of nLength, where the number of leading symbols is 12.25 and the length is 12.25 × nLength. The number of symbols of the frame control and data payload is variable and is related to the infobitperssymbol of the information map.