CN102611657B - Method and device for detecting NFC (Near Field Communication) baseband symbols - Google Patents

Abstract

The present invention relates to a kind of NFC baseband symbol detection method and device, and it comprises the following steps: a, input I, Q passway data respectively into the first first-order difference module, the second first-order difference module; b, the first first-order difference module , the second first-order difference module calculates and stores the first-order difference value of the last three clock cycles respectively; c, the first second-order difference module obtains the second-order difference value DP1; the second second-order difference module obtains the second-order difference value DP2; d, the threshold calculation and judgment module obtains the first-order difference base value D2 and the second-order difference base value P2; e, the preset difference offset Derr and the judgment threshold Dmax and Dmin in the threshold calculation and judgment module, the threshold calculation and judgment module according to The absolute value of the first-order difference base value D2 judges the change of the NFC baseband symbol according to the relationship between the absolute value of the second-order difference base value P2 and the judgment threshold Dmax and Dmin at the same time, and adjusts the difference offset Derr and the judgment threshold Dmax and Dmin value. The invention has simple steps, high detection precision and improved decoding reliability.

Description

A kind of NFC baseband symbol detection method and device

technical field

The invention relates to a symbol detection method and device, in particular to an NFC baseband symbol detection method and device, belonging to the technical field of NFC data communication.

Background technique

NFC is a two-way short-range data communication system developed on the basis of RFID (Radio Frequency Identification), which has broad application prospects. At present, the main relevant standards are IEEE18092, IEEE14443, etc. Among them, IEEE 14443 defines the basic data rate and modulation mode, and the supplementary standard defines the extended data rate and modulation mode, up to 847kbps. To sum up, the modulation methods are OOK (on-off modulation) and BPSK (binary phase shift) modulation. NFC (Near Field Communication) is usually integrated on other mobile devices, such as mobile phones, etc., which has great limitations on the cost, power consumption, sensitivity, etc. of NFC. FIG. 1 is a structural diagram of an NFC system.

For the definition of basic data and modulation methods in IEEE14443, the carrier frequency of the system is 13.56Mhz, which is the local oscillator frequency. The operating object of the system can be another NFC device (in the way of "tag simulation"), or a passive tag. Load modulation of the RF signal. The modulated signal enters the digital signal processor for decoding through I/Q two-way mixing, low-pass filtering, and A/D conversion, so the decoder operates in a synchronous demodulation mode.

For BPSK, OOK decoding, the key point is how to detect the transition of the signal state, including the signal amplitude and the phase of the signal. In the case of low signal-to-noise ratio, the system must ensure a low bit error rate, and at the same time meet the constraints of cost and power consumption. That is to say, the implementation should not be too complicated, and the overall performance of the system should not be sacrificed.

Figure 2 shows the implementation of a typical BPSK/OOK decoder digital processor. The I/Q two-way input signal enters the digital processor after analog-to-digital conversion. First, the DC component is removed, and then the signal strength is estimated, and the timing recovery information fed back is used for correction. After gain control, it enters the symbol phase alignment. It mainly eliminates the influence of I/Q phase imbalance, and then adds the 2-way signals to make a hard decision.

It should be pointed out that if an Equalizer is introduced into the system and soft decisions are used, better performance will be achieved, but the system is more complex and requires more logic circuits to implement.

Figure 3 shows a decoding method that is simpler to implement. The I/Q signal first passes through the first order difference, and then enters the peak determination and limiting. At the same time, the difference also deletes the influence of the DC component. Since we only need to determine whether the signal amplitude (OOK) or phase (BPSK) has a sudden conversion, the difference suddenly reaches a peak value, which can be used as a basis for judgment.

Among the two solutions shown in Figure 2 and Figure 3 above, the method represented in Figure 2 has better performance, but the logic implementation is more complicated, which means higher cost and higher power consumption.

The solution represented in Figure 3 is simple in logic, but the system performance is subject to many restrictions, such as signal-to-noise ratio, modulation depth, distance, communication rate, and so on. Especially at high data rates, due to the relatively high inter-symbol interference (ISI), performance drops. Figure 4 and Figure 5 explain why simply using a differentiator can cause a performance impact.

Figure 4 shows the signal diagram of OOK, and the dashed line in the vertical direction represents the sampling time point. At time 0, a transition from 1 to 0 occurs. As can be seen in the figure, the carrier amplitude changes gradually, and there is a transition zone in the middle. The greater the intersymbol interference (ISI), the longer the transition zone. The black box on the figure is the first-order difference value. It can be seen in the figure that due to the existence of the transition zone, within 8 sampling points, the difference value 1 and the difference value 1 have reached or approximately reached a peak value, but in the ideal In the state, only the difference value 1 should reach the peak value, which means that at the time point of the difference value 1, the signal state transitions. The existence of the difference value 2 makes the decision device think that the signal jumps again, thus causing a bit error.

This failure principle also applies to BPSK, and Figure 5 shows the signal diagram of BPSK. For BPSK signals, intersymbol interference is manifested as a phase shift that is not 180° in an ideal state. The red dotted line in the figure represents the ideal signal. The modulated data jumps from 0 to 1 at time T0. Due to the existence of intersymbol interference, the phase offset of the actual signal has a certain delay, and there is a transition zone There is (the black dotted line box in the figure), the phase is gradually shifted between the transition bands, the larger the ISI, the wider the transition band, which may appear in multiple sampling periods. The absolute values of the first-order differential values 1 and 2 are close to a peak value, but in an ideal state, only the absolute value of the differential value 1 reaches the peak value, which marks the phase jump of the signal. Multiple sampling points falling in the peak interval cause misjudgment by the decision device, resulting in bit errors.

For the extended communication rate defined by IEEE14443, as the rate increases, the ISI increases and the sampling points of each symbol decrease. This failure mode will be very prominent and seriously affect the decoding performance.

Contents of the invention

The purpose of the present invention is to overcome the deficiencies in the prior art, to provide a method and device for detecting NFC baseband symbols, the steps are simple, and the reliability of decoding is improved while keeping the circuit relatively simple.

According to the technical scheme provided by the present invention, a kind of NFC baseband symbol detection method, described baseband symbol detection method comprises the steps:

a. Receive the NFC signal and demodulate it into I-channel data and Q-channel data, and input the I-channel data into the first first-order differential module, and input the Q-channel data into the second first-order differential module;

b. The first first-order differential module calculates the differential value of the I channel data, and stores the first-order differential values DI0, DI1 and DI2 of the last three clock cycles, wherein DI2 is the latest differential value; meanwhile, the second first-order differential module Calculate the differential value of the Q channel data, and store the first-order differential values DQ0, DQ1, and DQ2 of the last three clock cycles, where DQ2 is the latest differential value;

c. The first first-order difference module inputs the first-order difference values DI2 and DI1 into the first second-order difference module, and the first second-order difference module obtains the second-order difference value DP1 according to the first-order difference values DI2 and DI1; the second first-order difference The module inputs the first-order difference values DQ2 and DQ1 into the second second-order difference module, and the second second-order difference module obtains the second-order difference value DP2 according to the first-order difference values DQ2 and DQ1;

d. The threshold calculation and judgment module receives the first-order differential values DI0, DI1, DI2, DQ0, DQ1, and DQ2, and receives the second-order differential values DP1 and DP2; the threshold calculation and judgment module obtains the first-order differential base value D2 according to DI2 and DQ2 , and obtain the second-order difference base value P2 according to the second-order difference values DP1 and DP2;

e. The preset differential offset Derr and the judgment thresholds Dmax and Dmin in the threshold calculation and judgment module. The threshold calculation and judgment module is based on the absolute value of the first-order difference base value D2 and the absolute value of the second-order difference base value P2. The relationship between the thresholds Dmax and Dmin judges the change of the NFC baseband symbol to complete the detection of the NFC baseband symbol, and adjusts the difference offset Derr and the judgment threshold Dmax and Dmin.

In the step d, the first-order difference base value D2 is D2=|DI2|+|DQ2|, and the second-order difference base value P2 is P2=|DP1|+|DP2|.

In the step e, when the first-order differential base value D2 is less than the judgment threshold Dmin, the threshold calculation and judgment module outputs the NFC baseband signal state unchanged;

When the first-order difference base value D2 is less than the judgment threshold Dmax, the judgment threshold Dmin is set as Dmin=(D0+D1+D2)/3, and the differential offset Derr is set as Derr=Max((|D0|- Dmin), (|D1|-Dmin), (|D2|-Dmin)), the judgment threshold Dmax is set as Dmax=Dmin+Derr+D max*w; among them, D0=|DI0|+|DQ0|, D1 ＝|DI1|+|DQ1|, w is the weighting coefficient;

When the first-order difference value D2 is greater than the judgment threshold Dmax, and the second-order difference base value P2 is also greater than the judgment threshold Dmax, the threshold calculation and judgment module outputs the NFC baseband signal state to change, and the judgment threshold Dmax is set as Dmax=D2.

The threshold calculation and judgment module sets the judgment threshold Dmin to be zero at the initial preset time, and the judgment threshold Dmax is selected from the sum of the maximum values corresponding to the initial I data channel and Q data channel.

The weighting coefficient w is 0.75.

A kind of NFC baseband symbol detection device, comprises the first first-order difference module for receiving I channel data and the second first-order difference module for receiving Q channel data, the output terminal of described first first-order difference module is connected with respectively The first second-order difference module and the threshold value calculation are connected to the judgment module, the output terminals of the second first-order difference module are respectively connected to the second second-order difference module and the threshold value calculation and decision module, and the first second-order difference module and the second second-order difference module are connected to each other. The output terminals of the difference module are respectively connected with the threshold calculation and decision modules;

The first first-order difference module calculates and stores the I channel data difference values DI2, DI1 and DI0 of the last three clocks, and the second first-order difference module calculates and stores the Q channel data difference of the last three clocks to DQ2, DQ1 and DQ0, Among them, DQ2 and DI2 are the latest difference values; the first second-order difference module calculates and stores the second-order difference value DP1 according to the first-order difference values DI2 and DI1, and the second second-order difference module calculates and stores the second-order difference value according to the first-order difference values DQ2 and DQ1. Store the second-order differential value DP2; the threshold calculation and judgment module compares the first-order differential value and the second-order differential value of the I channel data with the first-order differential value and the second-order differential value of the Q channel data with the preset judgment threshold Dmax and the judgment threshold Dmin is compared to determine the change of the state of the NFC baseband symbol; and the values of the judgment threshold Dmax and the judgment threshold Dmin are adjusted according to the judgment of the state of the NFC baseband symbol.

The threshold calculation and judgment module obtains the first-order differential base value D2 according to DI2 and DQ2, and obtains the second-order differential base value P2 according to the second-order differential values DP1 and DP2; the first-order differential base value D2 is D2=|DI2|+| DQ2|, the second-order differential base value P2 is P2=|DP1|+|DP2|.

When the first-order differential base value D2 is less than the judgment threshold Dmin, the state of the NFC baseband signal output by the threshold calculation and judgment module has not changed;

When the first-order difference base value D2 is less than the judgment threshold Dmax, the judgment threshold Dmin is set as Dmin=(D0+D1+D2)/3, and the differential offset Derr is set as Derr=Max((|D0|- Dmin), (|D1|-Dmin), (|D2|-Dmin)), the judgment threshold Dmax is set as Dmax=Dmin+Derr+D max*w; among them, D0=|DI0|+|DQ0|, D1 ＝|DI1|+|DQ1|, w is the weighting coefficient;

When the first-order differential value D2 is greater than the judgment threshold Dmax, and the second-order differential base value P2 is also greater than the judgment threshold Dmax, the threshold calculation and judgment module outputs the state of the NFC baseband signal, and the judgment threshold Dmax is set as Dmax=D2 .

The threshold calculation and judgment module sets the judgment threshold Dmin to be zero at the initial preset time, and the judgment threshold Dmax is selected from the sum of the maximum values corresponding to the initial I data channel and Q data channel.

The advantages of the present invention are: the I channel data and the Q channel data are respectively input into the first first-order difference module and the second first-order difference module, and after performing the first-order difference through the first first-order difference module and the second first-order difference module, a nearly The first-order difference value of three clock cycles, and input the corresponding first-order difference value into the first second-order difference module and the second second-order difference module in the next clock cycle; the threshold calculation and decision module simultaneously use the first-order difference base value , the second-order differential base value is compared with the preset threshold Dmax and the preset threshold Dmin, so as to accurately determine whether the NFC baseband symbol changes, and adjust the preset threshold Dmax, Dmin and the differential offset Derr according to the judgment result, In order to be able to accurately judge the subsequent NFC baseband symbols, thereby providing a necessary basis for subsequent NFC baseband signal decoding, the steps are simple, the detection accuracy is high, and the reliability of decoding is improved.

Description of drawings

FIG. 1 is a structural block diagram of an existing NFC system.

Fig. 2 is an implementation block diagram of existing NFC baseband symbol detection.

Fig. 3 is another implementation block diagram of existing NFC baseband symbol detection.

FIG. 4 is an error analysis diagram of sampling baseband decoding of an existing OOK signal.

FIG. 5 is an error analysis diagram of baseband decoding of an existing BPSK signal.

Fig. 6 is a structural block diagram of the present invention.

Fig. 7 is a specific implementation principle diagram of the first-order difference module, the second-order difference module, and the threshold calculation and judgment module of the present invention.

Fig. 8 is a flow chart of making a judgment in the present invention.

Detailed ways

The present invention will be further described below in conjunction with specific drawings and embodiments.

As shown in Fig. 6, Fig. 7 and Fig. 8: in order to be able to effectively detect the symbol state of the NFC baseband signal, and avoid the error problem caused by the signal interference caused by ISI in the existing symbol detection and decoding, the present invention is based on the NFC baseband The method for symbol detection includes the following steps:

a. Receive the NFC signal and demodulate it into I-channel data and Q-channel data, and input the I-channel data into the first first-order differential module, and input the Q-channel data into the second first-order differential module;

In order to avoid the bit error problem caused by ISI to signal interference, first order differential calculation is performed on the demodulated I channel data and Q data in the present invention; at the same time, the NFC signal is received and demodulated into I channel data and Q data. The channel data belongs to the conventional demodulation mode in NFC data communication, and will not increase the complexity and cost of baseband symbol detection in the present invention;

b. The first first-order differential module calculates the differential value of the I channel data, and stores the first-order differential values DI0, DI1 and DI2 of the last three clock cycles, wherein DI2 is the latest differential value; meanwhile, the second first-order differential module Calculate the differential value of the Q channel data, and store the first-order differential values DQ0, DQ1, and DQ2 of the last three clock cycles, where DQ2 is the latest differential value;

Fig. 7 is the differential implementation structure of the first first-order difference module and the second first-order difference module; at the rising edge of each sampling clock, the first first-order difference module and the second first-order difference module respectively receive corresponding new data S1 , the first-order difference value obtained by making a difference between the new data S1 and the data S0 stored in the first-order difference module; and the first-order difference value of the last three clock cycles is stored in the first first-order difference module, and DI1 is the previous sampling The difference value obtained by the clock, DI0 is the difference value obtained by the previous two sampling clocks; the second first-order difference module and the first first-order difference module are calculated synchronously, and perform the same storage operation; when the first first-order difference module and the first After the second first-order difference module has stored three first-order difference values, when the latest first-order difference value is obtained, the data with the longest storage time is removed to keep the storage of the first first-order difference module and the second first-order difference module state;

c. The first first-order difference module inputs the first-order difference values DI2 and DI1 into the first second-order difference module, and the first second-order difference module obtains the second-order difference value DP1 according to the first-order difference values DI2 and DI1; the second first-order difference The module inputs the first-order difference values DQ2 and DQ1 into the second second-order difference module, and the second second-order difference module obtains the second-order difference value DP2 according to the first-order difference values DQ2 and DQ1;

After the first-order differential value is obtained, on the falling edge of the next clock cycle, the first-order differential values DI2 and DI1 are respectively input into the first and second-order differential module to obtain the second-order differential value DP1=DI2-DI1; at the same time, the first-order differential value Values DQ2 and DQ1 are input into the second second-order difference module to obtain the second-order difference value DP2=DQ2-DQ1; generally, three second-order difference values can be obtained according to three first-order difference values, but in the NFC baseband symbol detection Generally, the first-order difference value of the last two clock cycles can be used;

d. The threshold calculation and judgment module receives the first-order differential values DI0, DI1, DI2, DQ0, DQ1, and DQ2, and receives the second-order differential values DP1 and DP2; the threshold calculation and judgment module obtains the first-order differential base value D2 according to DI2 and DQ2 , and obtain the second-order difference base value P2 according to the second-order difference values DP1 and DP2;

On the rising edge of the next clock, the corresponding first-order difference value and second-order difference value will be input into the threshold calculation and judgment module; wherein, the first-order difference base value D2 is D2=|DI2|+|DQ2|, the second-order difference base value The value P2 is P2=|DP1|+|DP2|; the first-order difference base value D2 and the second-order difference base value P2 are obtained as the judgment basis of the threshold calculation and judgment module;

e. The preset differential offset Derr and the judgment thresholds Dmax and Dmin in the threshold calculation and judgment module. The threshold calculation and judgment module is based on the absolute value of the first-order difference base value D2 and the absolute value of the second-order difference base value P2. The relationship between the thresholds Dmax and Dmin judges the change of the NFC baseband symbol to complete the detection of the NFC baseband symbol, and adjusts the difference offset Derr and the judgment threshold Dmax and Dmin.

After judging the change of the NFC baseband symbol in step e, the present invention can indicate the subsequent decoding under the condition of determining the initial symbol state of the NFC, avoiding the error situation caused by ISI, and improving the decoding reliability of the NFC communication. In the digital discrete state, the inversion of the signal state can be judged by the differential value, and the inversion here includes inversion from high level to low level, or from low level to high level.

Specifically, when the first-order differential base value D2 is less than the judgment threshold Dmin, the state of the threshold calculation and judgment module output NFC baseband signal has not changed;

When the first-order difference base value D2 is less than the judgment threshold Dmax, the judgment threshold Dmin is set as Dmin=(D0+D1+D2)/3, and the differential offset Derr is set as Derr=Max((D0|-Dmin ), (|D1|-Dmin), (|D2|-Dmin)), the judgment threshold Dmax is set as Dmax=Dmin+Derr+Dmax*w; wherein, D0=|DI0|+|DQ0|, D1=| DI1|+|DQ1|, w is the weighting coefficient;

When the first-order difference value D2 is greater than the judgment threshold Dmax, and the second-order difference base value P2 is also greater than the judgment threshold Dmax, the threshold calculation and judgment module outputs the NFC baseband signal state to change, and the judgment threshold Dmax is set as Dmax=D2. Since it is easy to cause ISI when only the first-order difference value is used for judgment, the second-order difference value is required for secondary judgment.

The threshold calculation and judgment module judges that the threshold Dmin is zero at the initial preset time, and according to the analog front end, assuming that the modulation depth is 100%, after the signal is A/D converted, the maximum value in the I channel data and the Q channel data is taken and the initial value of the most judgment threshold Dmax. The weighting coefficient w in the present invention is generally taken as 0.75.

As shown in Fig. 6 and Fig. 7: the above-mentioned NFC baseband symbol detection method can be completed by using the following NFC baseband symbol detection device. Specifically, it includes a first first-order difference module for receiving I-channel data and a second first-order difference module for receiving Q-channel data, and the output terminals of the first first-order difference module are respectively connected to the first second-order difference module. The module and the threshold calculation are connected to the judgment module, the output terminals of the second first-order difference module are respectively connected to the second second-order difference module and the threshold calculation and judgment module, and the output terminals of the first second-order difference module and the second second-order difference module respectively connected to the threshold calculation and decision modules;

The first first-order difference module calculates and stores the I channel data difference values DI2, DI1 and DI0 of the last three clocks, and the second first-order difference module calculates and stores the Q channel data difference of the last three clocks to DQ2, DQ1 and DQ0, Among them, DQ2 and DI2 are the latest difference values; the first second-order difference module calculates and stores the second-order difference value DP1 according to the first-order difference values DI2 and DI1, and the second second-order difference module calculates and stores the second-order difference value according to the first-order difference values DQ2 and DQ1. Store the second-order differential value DP2; the threshold calculation and judgment module compares the first-order differential value and the second-order differential value of the I channel data with the first-order differential value and the second-order differential value of the Q channel data with the preset judgment threshold Dmax and the judgment threshold Dmin is compared to determine the change of the state of the NFC baseband symbol; and the values of the judgment threshold Dmax and the judgment threshold Dmin are adjusted according to the judgment of the state of the NFC baseband symbol.

For the working principles and states of the first first-order difference module, the first second-order difference module, the second first-order difference module, the second second-order difference module, and the threshold calculation and judgment module, refer to the above description.

As shown in Figures 6 to 8: When working, input the I channel data and Q channel data into the first first-order difference module and the second first-order difference module respectively, and pass through the first first-order difference module and the second first-order difference module After the first-order difference, the first-order difference value of nearly three clock cycles is obtained, and the corresponding first-order difference value is input into the first second-order difference module and the second second-order difference module in the next clock cycle; the threshold calculation and judgment module At the same time, the base value of the first-order difference and the base value of the second-order difference are compared with the preset threshold Dmax and the preset threshold Dmin to accurately determine whether the NFC baseband symbol changes, and adjust the preset threshold Dmax and Dmin according to the judgment result and the differential offset Derr to accurately judge the subsequent NFC baseband symbols, thereby providing the necessary basis for subsequent NFC baseband signal decoding. The steps are simple, the detection accuracy is high, and the reliability of decoding is improved.

Claims (5)

1. a kind of NFC baseband symbol detection method, it is characterized in that, described baseband symbol detection method comprises the steps: (a), receiving the NFC signal, and demodulating it into I channel data and Q channel data, and inputting the I channel data into the first first-order differential module, and inputting the Q channel data into the second first-order differential module; (b), the first first-order differential module calculates the differential value of the I channel data, and stores the first-order differential values DI0, DI1 and DI2 of the last three clock cycles, wherein DI2 is the latest differential value; meanwhile, the second first-order The differential module calculates the differential value of the Q channel data, and stores the first-order differential values DQ0, DQ1, and DQ2 of the last three clock cycles, where DQ2 is the latest differential value; (c), the first first-order difference module inputs the first-order difference values DI2 and DI1 into the first second-order difference module, and the first second-order difference module obtains the second-order difference value DP1 according to the first-order difference values DI2 and DI1; the second one The first-order difference module inputs the first-order difference values DQ2 and DQ1 into the second second-order difference module, and the second second-order difference module obtains the second-order difference value DP2 according to the first-order difference values DQ2 and DQ1; (d), the threshold calculation and judgment module receives the first-order difference values DI0, DI1, DI2, DQ0, DQ1 and DQ2, and receives the second-order difference values DP1 and DP2; the threshold calculation and judgment module obtains the first-order difference base according to DI2 and DQ2 value D2, and obtain the second-order difference base value P2 according to the second-order difference values DP1 and DP2; In the step (d), the first-order difference base value D2 is D2=|DI2|+|DQ2|, and the second-order difference base value P2 is P2=|DP1|+|DP2|; (e), preset differential offset Derr and judgment thresholds Dmax and Dmin in the threshold calculation and judgment module, the threshold calculation and judgment module is based on the absolute value of the first-order difference base value D2 and the absolute value of the second-order difference base value P2 at the same time The relationship between the judgment threshold Dmax and Dmin determines the change of the NFC baseband symbol to complete the detection of the NFC baseband symbol, and adjusts the value of the differential offset Derr and the judgment threshold Dmax and Dmin; In the step (e), when the first-order differential base value D2 is less than the judgment threshold Dmin, the threshold calculation and judgment module output NFC baseband signal state does not change; When the first-order difference base value D2 is less than the judgment threshold Dmax, the judgment threshold Dmin is set as Dmin=(D0+D1+D2)/3, and the differential offset Derr is set as Derr=Max((|D0|- Dmin),(|D1|-Dmin),(|D2|-Dmin)), the judgment threshold Dmax is set as Dmax=Dmin+Derr+Dmax*w; where, D0=|DI0|+|DQ0|, D1= |DI1|+|DQ1|, w is the weighting coefficient; When the first-order difference value D2 is greater than the judgment threshold Dmax, and the second-order difference base value P2 is also greater than the judgment threshold Dmax, the threshold calculation and judgment module outputs the NFC baseband signal state to change, and the judgment threshold Dmax is set as Dmax=D2. 2. NFC baseband symbol detection method according to claim 1, it is characterized in that: described threshold calculation and judging module judge threshold Dmin is zero at initial preset time, and judging threshold Dmax selects initial I data channel and Q data channel correspondence The sum of the maximum value of . 3. NFC baseband symbol detection method according to claim 1, is characterized in that: described weighting factor w is 0.75. 4. A kind of NFC baseband symbol detection device is characterized in that: comprise the first first-order difference module for receiving I channel data and the second first-order difference module for receiving Q channel data, described first first-order difference module The output terminals of the modules are respectively connected to the first second-order difference module and the threshold calculation and decision module, the output terminals of the second first-order difference module are connected to the second second-order difference module and the threshold calculation and decision module respectively, and the first and second-order difference modules are respectively connected to each other. The output terminals of the module and the second second-order difference module are respectively connected with the threshold calculation and judgment module; The first first-order difference module calculates and stores the I channel data difference values DI2, DI1 and DI0 of the last three clocks, and the second first-order difference module calculates and stores the Q channel data difference of the last three clocks to DQ2, DQ1 and DQ0, Among them, DQ2 and DI2 are the latest difference values; the first second-order difference module calculates and stores the second-order difference value DP1 according to the first-order difference values DI2 and DI1, and the second second-order difference module calculates and stores the second-order difference value according to the first-order difference values DQ2 and DQ1. Store the second-order differential value DP2; the threshold calculation and judgment module compares the first-order differential value and the second-order differential value of the I channel data with the first-order differential value and the second-order differential value of the Q channel data with the preset judgment threshold Dmax and the judgment threshold Dmin is compared to judge the change of the NFC baseband symbol state; and adjusts the value of the judgment threshold Dmax and the judgment threshold Dmin according to judging the NFC baseband symbol state; The threshold calculation and judgment module obtains the first-order differential base value D2 according to DI2 and DQ2, and obtains the second-order differential base value P2 according to the second-order differential values DP1 and DP2; the first-order differential base value D2 is D2=|DI2|+| DQ2|, the second-order differential base value P2 is P2=|DP1|+|DP2|; When the first-order differential base value D2 is less than the judgment threshold Dmin, the state of the NFC baseband signal output by the threshold calculation and judgment module has not changed; When the first-order difference base value D2 is less than the judgment threshold Dmax, the judgment threshold Dmin is set as Dmin=(D0+D1+D2)/3, and the differential offset Derr is set as Derr=Max((|D0|- Dmin),(|D1|-Dmin),(|D2|-Dmin)), the judgment threshold Dmax is set as Dmax=Dmin+Derr+Dmax*w; where, D0=|DI0|+|DQ0|, D1= |DI1|+|DQ1|, w is the weighting coefficient; When the first-order difference value D2 is greater than the judgment threshold Dmax, and the second-order difference base value P2 is also greater than the judgment threshold Dmax, the threshold calculation and judgment module outputs the NFC baseband signal state to change, and the judgment threshold Dmax is set as Dmax=D2. 5. NFC baseband symbol detection device according to claim 4, is characterized in that: described threshold calculation and judging module are zero at initial preset time judgment threshold Dmin, and judging threshold Dmax selects initial I data channel and Q data channel correspondence The sum of the maximum value of .