A method for improving the transmission rate of LFM system

Abstract

The present invention relates to a method for improving the transmission rate of an LFM system, belonging to the field of wireless communication technology, including: assuming a binary bit stream, using Gary mapping, determining the frequency modulation slope μ from the index bit, and then obtaining the SSK-CSS index modulation sequence s(l). The received signal r(l) is obtained through an additive white Gaussian noise channel, and then a de-chirping operation is performed to obtain the de-chirped signal r0(l): r W‑1 (l), and then DFT operation, modulo operation, and maximum value operation are performed to obtain the maximum value and the index corresponding to the maximum value. By comparing the magnitudes of the maximum values, the frequency modulation slope at the transmitting end is determined, thereby decoding the value of the index bit. The present invention improves the transmission rate of the LFM system based on slant shift keying-chirp spread spectrum index modulation, and gives the mapping relationship between the index bit and the frequency modulation slope, with little performance loss while greatly improving the transmission capacity of the LFM system.

Description

Technical Field

[0001] The present invention belongs to the technical field of wireless communication, and specifically, relates to a method for improving the transmission rate of an LFM system. Background Art

[0002] As a wireless communication technology designed specifically for Internet of Things (IoT) applications, the Low Power Wide Area Network (LPWAN) technology has the advantages of low power consumption, low bandwidth, low cost, long distance, and large coverage capacity compared with traditional wireless communication technologies such as WiFi, ZigBee, and Bluetooth. It is suitable for battery-powered IoT devices with long-distance and small-data-volume transmission. The LFM system uses linear spread spectrum technology, which not only maintains the low-power consumption characteristic of Frequency Shift Keying (FSK) modulation but also improves network utilization and anti-interference ability. Terminal devices using different spreading factors can transmit simultaneously using the same frequency without interference.

[0003] Although the linear spread spectrum technology has the above advantages, its data transmission rate is very low, and its relatively low transmission rate is quite limited for many applications. Someone once proposed interleaving linear spread spectrum signals to expand the signal set to improve the transmission data rate of the LFM system. The results showed that the transmission data rate could be increased by up to 14% at most, with a performance loss of 0.8 dB. On this basis, someone further proposed to further expand the signal set by introducing a down-chirp signal to provide a higher data rate than traditional linear spread spectrum modulation schemes. This scheme can increase the data rate of the traditional LFM scheme by 28.6%, but it will also bring a greater performance loss. For the two methods of improving the transmission data rate mentioned above, as the number of bits transmitted in one symbol period increases, the number of frequency hops also increases accordingly, resulting in an increase in out-of-band emission, which is not desirable in the unlicensed ISM band. Therefore, the improvement of the transmission data rate by these schemes is limited. Summary of the Invention

[0004] The purpose of the present invention is to provide a method for improving the transmission rate of an LFM system. By utilizing the separable characteristics of chirp spread spectrum (CSS) signals between different slopes, at the sending end, the index bits introduced additionally are mapped one by one to the frequency modulation slopes. At the receiving end, the received signal is de-chirped, DFT operation, and maximum value operation are performed using down-chirp signals with all different slopes to obtain the maximum value and the index corresponding to the maximum value. By comparing the magnitudes of the maximum values, the frequency modulation slope at the sending end is determined, so as to decode the values of the index bits. While greatly improving the transmission capacity of the LFM system, the performance loss is not significant, and the problem that the application of the LFM system is limited due to its low transmission rate in the prior art is solved.

[0005] The object of the present invention can be achieved by the following technical solutions:

[0006] A method for improving the transmission rate of an LFM system, comprising:

[0007] Step 1: Obtain data bits b 0 ,..., b SF-1 and index bits b SF ,..., b SF+x-1 ;

[0008] Step 2: Determine the frequency modulation slope μ ∈ {μ 0 ,..., μ W-1} according to the mapping relationship between the index bits and the frequency modulation slope;

[0009] Step 3: Calculate the skew shift keying-chirp spread spectrum (SSK-CSS) index modulation sequence s(l) according to the frequency modulation slope μ;

[0010] Step 4: Calculate the received signal r(l) through an additive white Gaussian noise (AWGN) channel according to the modulation sequence s(l);

[0011] Step 5: Perform de-chirping operations on the received signal r(l) respectively with the down-chirp signals having frequency modulation slopes of μ 0 and μ W-1 to obtain de-chirped signals r 0 (l) and r W-1 (l);

[0012] Step 6: Perform DFT operations, modulus operations, and maximum value operations on the de-chirped signals r 0 (l) and r W-1 (l) to obtain W maximum values M 0 ,..., M W-1 , and the corresponding indexes α 0 ,..., α W-1 , M 0 ,..., M W-1 The index of the maximum value of is marked as α β ,

[0013] Step 7: Convert α β into an SF-bit binary number to obtain Using inverse Gray mapping, convert β into an x-bit binary number to obtain For and perform serial-to-parallel conversion to obtain the output bit stream Take the finally obtained bit stream as the demodulated bit stream at the receiving end.

[0014] As a further solution of the present invention: the binary bit stream in step one is b 0 ,..., b SF-1 , b SF ,..., b SF+x-1 ;

[0015] where SF represents the spreading factor, x represents the number of bits transmitted more within one symbol period, and b SF ,..., b SF+x-1 are index bits. The index bits determine the frequency modulation slope m of the modulated signal. Therefore, W = 2 x different frequency modulation slopes μ 0 ,..., μ W-1 .

[0016] As a further solution of the present invention: within one symbol period, the shifted keying-chirp spread spectrum (SSK-CSS) index modulation sequence s(l) corresponding to the l-th chirp in the shifted keying-chirp spread spectrum (SSK-CSS) index modulation sequence s(l) in step three can be expressed as:

[0017]

[0018] where M = 2 SF is the number of orthogonal chirps, represents the decimal symbol converted from the binary data bit stream b 0 ,..., b SF-1 according to the natural mapping rule, μ ∈ {μ 0 ,..., μ W-1} is the frequency modulation slope, and its value is determined by the index bits b SF ,..., b SF+x-1 according to the Gary mapping.

[0019] As a further solution of the present invention: the formula of the received data signal r(l) in step four can be expressed as:

[0020]

[0021] where n(l) is a complex Gaussian random variable with a mean of 0 and a variance of σ 2 = N 0 / 2.

[0022] As a further solution of the present invention: the obtained de-chirped signals r 0 (l) and r W-1 (l) are to obtain W de-chirped signals r 0 (l) and r W-1 (l):

[0023]

[0024] where k = 0,..., W - 1.

[0025] As a further solution of the present invention: in step six, the de - chirping signals r 0 (l) and r W-1 (l) performing DFT operations and modulus operations include respectively performing M - point DFT operations and modulus operations on the de - chirping signals r 0 (l) and r W-1 (l) to obtain

[0026]

[0027] where

[0028] When μ = μ k ,

[0029]

[0030] It can be simplified to:

[0031]

[0032] As a further solution of the present invention: for the By performing a maximum - value operation, the maximum value M K and the index α k are obtained:

[0033]

[0034]

[0035] As a further solution of the present invention: for the M K performing an index operation corresponding to the maximum - value operation to obtain the index b of the maximum value in M K :

[0036]

[0037] Advantages of the present invention:

[0038] (1) The present invention proposes a method for improving the transmission rate of an LFM system based on Shift-Keying-Chirp Spread Spectrum (SSK-CSS) index modulation. By utilizing the separable characteristics of chirp spread spectrum (CSS) signals between different slopes, at the transmitter, the additional introduced index bits are mapped one-to-one with the frequency modulation slopes. At the receiver, the received signal is de-chirped with the down-chirp signals of all different slopes, followed by DFT operation and maximum value extraction operation to obtain the maximum value and the corresponding index. By comparing the magnitudes of the maximum values, the frequency modulation slope at the transmitter is determined, thereby decoding the value of the index bits. While significantly improving the transmission rate of the LFM system, the performance loss is not significant. Specifically, 2 more bits are transmitted within one symbol period, and when SF = 7, the transmission rate gain is 29%, and the performance loss is approximately 0.8 dB. When SF = 9, the transmission rate gain is 22%, and the performance loss is only 0.25 dB.

[0039] (2) In view of the characteristic that errors are more likely to occur between adjacent frequency modulation slopes in the present invention, Gray mapping instead of natural mapping is adopted between the index bits and the frequency modulation slopes, thereby further reducing the bit error rate. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings required for describing the embodiments will be briefly introduced below. Obviously, the accompanying drawings in the following description are only some embodiments of the present invention. For those of ordinary skill in the art, other drawings can be obtained based on these drawings without creative efforts.

[0041] Figure 1 is a schematic flowchart of a method for improving the transmission rate of an LFM system according to the present invention;

[0042] Figure 2 is a performance simulation diagram of the present invention when SF = 7;

[0043] Figure 3 is a performance simulation diagram of the present invention when SF = 9. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0044] The technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. All other embodiments obtained by those of ordinary skill in the art without creative efforts based on the embodiments of the present invention belong to the scope of protection of the present invention.

[0045] Please refer to Figure 1 As shown, the present invention is a method for improving the transmission rate of an LFM system. Taking x = 2 as an example, the present invention will be described in detail, including:

[0046] Step 1: Obtain the binary bit stream generated by the information source as b 0 ,...,b SF-1 ,b SF ,...,b SF+x-1 ;

[0047] where SF represents the spreading factor, x represents the number of bits transmitted more within one symbol period, and b SF ,...,b SF+x-1 are index bits. The index bits determine the frequency modulation slope m of the modulation signal and have a one-to-one correspondence with the frequency modulation slope m. Therefore, it is necessary to adopt W = 2 x = 4 different frequency modulation slopes μ 0 , μ 1 , μ 2 , μ 3 , where μ 0 = 1, μ 1 = 1.05, μ 2 = 1.1, μ 3 = 1.15;

[0048] Step 2: Through Gary mapping, uniquely determine the frequency modulation slope μ from the index bits b SF ,...,b SF+x-1 . The mapping relationship between the index bits b SF ,b SF+1 and the frequency modulation slope μ is shown in Table 1:

[0049] Table 1

[0050] <![CDATA[b SF ,b SF+1 > Frequency modulation slope μ [0,0] <![CDATA[μ 0 = 1.00]]> [0,1] <![CDATA[μ 1 = 1.05]]> [1,1] <![CDATA[μ 2 = 1.10]]> [1,0] <![CDATA[μ 3 = 1.15]]>

[0051] Step 3: Calculate the skew shift keying-chirp spread spectrum (SSK-CSS) index modulation sequence s(l) according to the frequency modulation slope μ;

[0052] Step 4: Calculate the received signal r(l) through the additive white Gaussian noise (AWGN) channel according to the modulation sequence s(l);

[0053] Step 5: Use the received signal r(l) to perform de-chirping operations with the lower chirp signals with frequency modulation slopes μ 0 and μ W-1 respectively, to obtain the de-chirped signals r 0 (l) and r W-1 (l);

[0054] Step 6: Use the de-chirped signals r 0 (l) and r W-1 (l) to perform DFT operation, modulo operation, and maximum value operation to obtain W maximum values M 0 ,...,MW-1 , and the index α 0 ,..., α W-1 , M 0 ,..., M W-1 The index of the maximum value of is denoted as α β ,

[0055] Step Seven: Convert α β into the binary of the SF bit, obtaining Using the inverse Gray mapping, convert β into the binary of x bits, obtaining For perform serial-to-parallel conversion to obtain the output bit stream Take the finally obtained bit stream as the demodulation bit stream at the receiving end.

[0056] In the LFM system, due to the very low transmission rate of the system, its application is limited. To address this problem, the present invention maps the index bits and the frequency modulation slopes one by one, and restores the index bits at the receiving end, thereby achieving the purpose of improving the transmission rate of the LFM system. According to a large number of simulation results, the difference between the frequency modulation coefficients should not be less than 0.05, otherwise it is difficult to distinguish at the receiving end, thus affecting the system performance. Therefore, the values of the frequency modulation slopes in the simulation are μ 0 = 1, μ 1 = 1.05, μ 2 = 1.1, μ 3 = 1.15.

[0057] In addition, in view of the characteristic that errors are more likely to occur between adjacent frequency modulation slopes, the present invention adopts the Gary mapping instead of the natural mapping between the index bits and the frequency modulation slopes, thereby further reducing the bit error rate.

[0058] In a preferred embodiment of the present invention, within one symbol period, the shifted keying-chirp spread spectrum (SSK-CSS) index modulation sequence s(l) corresponding to the l-th chirp can be expressed as:

[0059]

[0060] where M = 2 SF is the number of orthogonal chirps, represents the decimal symbol converted from the binary data bit stream b 0 ,..., b SF-1 according to the natural mapping rule, μ ∈ {μ 0 , μ 1 , μ 2 , μ 3} is the frequency modulation slope, and its value is determined by the index bit bSF , b SF+1 Determined according to the mapping relationship in Table 1.

[0061] Through this step, we map the information of the index bit to the frequency modulation slope and the information bit to the initial frequency, enabling the modulated signal to carry more information and thus improving the transmission rate of the system.

[0062] In a preferred embodiment of the present invention, the formula of the received data signal r(l) can be expressed as:

[0063]

[0064] where n(l) is a complex Gaussian random variable with a mean of 0 and a variance of σ 2 = N 0 / 2.

[0065] In a preferred embodiment of the present invention, Step 5 is specifically: respectively perform de-chirping operations on the received signal r(l) through down-chirping signals with frequency modulation slopes of μ K (k = 0, 1, 2, 3) to obtain the de-chirped signal r k (l):

[0066]

[0067] In a preferred embodiment of the present invention, Step 6 is specifically: perform DFT operations, modulus operations, and maximum value operations on the de-chirped signal r k (l) to obtain the maximum value M K , and the index α k corresponding to the maximum value, M 0 : M 3 The index of the maximum value of is denoted as β;

[0068] Perform DFT operations and modulus operations on the de-chirped signal r k (l) respectively through M-point DFT operations to obtain

[0069] where k = 0,..., 3,

[0070] Particularly, when μ = μ k , the above formula can be simplified to:

[0071]

[0072] In a preferred case of this embodiment, for the Through the maximum value operation, the maximum value M Kand index α k :

[0073]

[0074] The maximum value M k The corresponding index is:

[0075] M 0 :M 3 The index corresponding to the maximum value is:

[0076] According to the inverse Gray mapping, convert β into a two-bit binary number to obtain That is:

[0077] If β = 0,

[0078] If β = 1,

[0079] If β = 2,

[0080] If β = 3,

[0081] To further illustrate the beneficial effects of the present invention, a comparative description will be given through simulation experiments below.

[0082] Simulation 1

[0083] 1.1 Simulation conditions

[0084] The bit error performance of a method for improving the transmission rate of an LFM system, and the simulation parameters are shown in Table 2.

[0085] Table 2

[0086]

[0087] 1.2 Simulation results and analysis

[0088] Table 2 lists the simulation parameters used in the transmission scheme proposed by the present invention. Correspondingly, Figure 2-3 the bit error performance of the transmission scheme proposed by the present invention is given.

[0089] Figure 2 The black curve marked with a star in [the figure] represents the bit error performance of LFM modulation with SF = 7 in an AWGN channel.

[0090] Figure 2 The red curve marked with a cross in [the figure] represents the bit error performance of the present invention with x = 2 and SF = 7 in an AWGN channel.

[0091] Figure 3The black curve marked with a star in the figure represents the bit error performance of LFM modulation with SF = 9 in an AWGN channel.

[0092] Figure 3 The red curve marked with a circle in the figure represents the bit error performance of the present invention with x = 2 and SF = 9 in an AWGN channel.

[0093] From Figure 2 and Figure 3 the simulation results, it can be seen that the present invention can significantly improve the transmission data rate of the LFM system without sacrificing too much system performance. Specifically, when SF = 7, the transmission data rate is increased by 29% and the performance loss is 0.9 dB; when SF = 9, the transmission data rate is increased by 22% and the performance loss is 0.2 dB.

[0094] In the description of the specification, the descriptions referring to terms such as "one embodiment", "example", "specific example", etc. mean that the specific features, structures, materials or characteristics described in connection with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic representations of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described can be combined in a suitable manner in any one or more embodiments or examples.

[0095] The above content is only an example and illustration of the concept of the present invention. Those skilled in the art of this technology can make various modifications or supplements to the described specific embodiments or use similar ways to replace them, as long as they do not deviate from the concept of the invention or exceed the scope defined by the claims of this patent, they should all fall within the protection scope of the present invention.

Claims

1. A method for improving the transmission rate of an LFM system, characterized in that: include: Step 1: Obtain the data bits of the binary bit stream generated by the preset source and index bit ; Where SF represents the spreading factor, and x represents the number of bits transmitted in one symbol period; Step 2: Determine the frequency modulation slope according to the Gary mapping relationship between the index bit and the frequency modulation slope ; Step 3: Calculate the skew-shift keying-chirp spread spectrum (SSK-CSS) index modulation sequence s(l) according to the frequency modulation slope μ; The skew-shift keying-chirp spread spectrum (SSK-CSS) index modulation sequence s(l) in one symbol period, the skew-shift keying-chirp spread spectrum (SSK-CSS) index modulation sequence s(l) corresponding to the l-th chirp is expressed as: ； in, is the orthogonal chirp number, Represents a binary data bit stream According to the natural mapping rule, it is converted into decimal symbols. is the frequency modulation slope, whose value is determined by the index bit According to Gary's mapping decision; Step 4: Calculate the received signal r(l) through the additive white Gaussian noise (AWGN) channel according to the modulation sequence s(l); Step 5: Compare the received signal r(l) with the frequency modulation slopes μ0 and μ W-1 The down-chirp signal is dechirped to obtain the dechirp signals r0(l) and r W-1 (l); Step 6: According to the dechirped signal r k (l) Perform DFT operation, modulus operation and maximum value operation to obtain the maximum value M K , and the index corresponding to the maximum value , the index of the maximum value of M0:M3 is denoted as β; for the dechirped signal r k (l) Through the DFT calculation and modulo operation of M points, we can get ; By taking the maximum value operation, we get the maximum value M K and index ; ; Maximum The corresponding indexes are: ; The index corresponding to the maximum value of M0:M3 is: ; Step 7: Convert to binary of SF bit, get , using the inverse Gray mapping, convert β into x-bit binary, and get ;right Perform serial-to-parallel conversion to obtain the output bit stream , and the final bit stream is used as the demodulated bit stream at the receiving end.

2. A method for improving the transmission rate of an LFM system according to claim 1, characterized in that: The binary bit stream in step 1 is ; Where SF represents the spreading factor, x represents the number of bits transmitted in one symbol period, Index bit, which determines the frequency modulation slope of the modulated signal , so we need W=2 x Different FM slopes .

3. A method for improving the transmission rate of an LFM system according to claim 1, characterized in that: The formula of the received data signal r(l) in step 4 is expressed as: ； Among them, n(l) is a number with a mean of 0 and a variance of A complex Gaussian random variable, N0 is the noise unilateral power spectrum density; is the frequency modulation slope.

4. A method for improving the transmission rate of an LFM system according to claim 1, characterized in that: The dechirped signals r0(l) and r W-1 (l) To obtain W dechirped signals r0(l) and r W-1 (l): ； in, ; .

5. A method for improving the transmission rate of an LFM system according to claim 1, characterized in that: Step 6 describes the dechirp signals r0(l) and r W-1 (l) Perform DFT operation and modulus operation on the dechirped signals r0(l) and r W-1 (l) is obtained by DFT calculation and modulo operation of M points respectively. : ； Where, q=0,...,m-1; ; ; When μ=μ k , when q=m, Can be simplified to: 。

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