CN106878218A - A kind of high reliability demodulation method for IEEE802.15.4 - Google Patents

Abstract

A kind of high reliability BPSK demodulation methods for IEEE802.15.4, useful frequency offset information is extracted first with the corresponding channel received data of the lead code of 32 bits, then receiving sampled signal to the corresponding complex base band of PSDU carries out bit-level difference processing, obtains adjudicating observation, afterwards using extract frequency offset information to judgement observation compensate after carry out detection judgement, the PSDU data that will finally receive send MAC layer to and carry out CRC check.The present invention is based on influence negligible theory of the noise to useful signal when the Taylor series expansion and big signal to noise ratio of arcsin function, there is provided a kind of low-power consumption suitable for IEEE 802.15.4, highly reliable and inexpensive incoherent BPSK receivers.

Description

High-reliability demodulation method for IEEE802.15.4

Technical Field

The invention relates to the technical field of communication signal waveform detection, in particular to a high-reliability BPSK demodulation method for IEEE802.15.4.

Background

IEEE802.15.4 is the basis of the specifications of ZigBee, WirelessHART, etc., and describes the physical layer and media access control protocol of low-rate wireless personal area networks. It initially operates in the ISM band at 868/915MHz and 2.4GHz, with data transmission rates up to 250 kbps. The advantages of low power consumption and low cost enable the device to be widely applied to a plurality of fields such as data acquisition, processing and analysis, remote control precision farming agricultural automation, environmental protection and monitoring. In the latest standard provided in 2011, the working frequency bands of 314-.

As shown in fig. 1, the 802.15.4 protocol uses different modulation schemes and different data transmission rates on different carrier frequency bands. A total of 48 channels are provided in four typical frequency bands: 868MHz band 1 channel, 915MHz band 10 channels, 2450MHz band 16 channels, 950MHz band 21 channels. As shown in fig. 2, the signal processing is the same in the 868/915/950-MHz band, except for the data rate. The sender firstly differentially encodes binary data of a physical layer data protocol unit (PPDU), then converts each bit after differential encoding into a chip sequence with the length of 15, and finally modulates the chip sequence onto a channel by using BPSK. The differential encoding is to perform exclusive or operation on each original bit of data and a bit generated by the previous differential encoding:wherein E_nIs the result of differential encoding, R_nFor the original bits to be coded, E_n-1Is the result of the last differential encoding. For each transmitted data packet, R₁Is the first original bit, calculate E₁When assuming E₀0. The differential demodulation process is similar to the encoding process:for eachA received data packet, E₁For the first bit to be demodulated, E is calculated₁When assuming E₀0. As shown in fig. 3, each bit after differential encoding is converted into a slice sequence of length 15. The spread sequence is modulated on a carrier wave by using a BPSK modulation mode.

As shown in fig. 4, the first field of the physical layer data frame structure of the IEEE802.15.4 protocol is a four-byte all-zero preamble with 32 bits total, and during the reception of the preamble, the transceiver will complete chip synchronization and symbol synchronization according to the characteristics of the preamble sequence. The Start of Frame Delimiter (SFD) field is one byte in length and is fixed to a value of 0xA7 to indicate the start of a physical frame, and the transceiver can only synchronize the bits of data after receiving the preamble and can only synchronize to the bytes by searching the SFD field for a value of 0xA 7. The frame length is represented by the lower 7 bits of one byte, and the value is the length of the physical frame payload, so that the length of the physical frame payload does not exceed 127 bytes. The payload length of a physical frame, referred to as a physical layer service data unit (PSDU), is variable and is typically used to carry MAC frames.

There are two conventional demodulation methods for ieee802.15.4, one of which is a conventional typical complex baseband non-coherent demodulation method of the 868/915/950-MHz band as shown in fig. 5. By usingRepresents a complex baseband sampling signal received after transmission through a channel, wherein s (k) is transmitted data to be detected, s (k) ∈ { +1, -1}, ω is₀＝2πf₀,f₀And θ is a frequency offset and a phase offset, respectively, which remain constant throughout the data frame, T_cIndicating spreading code chip period, η₀(k) For complex baseband additive white gaussian noise, the demodulation process can be summarized as:

step one, calculating an observation value Y containing frequency offset information by using a complex baseband receiving sampling signal corresponding to a preamble of 32 bits:

wherein J represents the total number of bits of the preamble, J is 32, N represents the spreading length, N is 15, 1 ≦ m ≦ J-1,

0≤n≤N-1，p[n+Nm]a channel reception value representing an nth chip corresponding to an mth bit of the preamble (·)^*Indicating the conjugation operation, η₁Representing all noise terms.

Step two, carrying out bit-level differential processing on the complex baseband receiving sampling signals corresponding to the PSDU to obtain a judgment observation value A [ m ]:

wherein r [ n + Nm]Indicating the channel reception value of the nth chip corresponding to the mth bit of the PSDU, η₂[m]Representing all noise terms, E [ m ]]Indicating the mth bit data transmitted.

Step three, utilizing Y in step one₀Extracting frequency offset information, and comparing A [ m ] in the second step]And (3) carrying out detection judgment after compensation:

wherein,represents the decision result for the mth bit data, q (-) is a quantization function, | · | represents a modulo operation, Re (-) represents an operation of the real part, and Im (-) represents an operation of the imaginary part. Bloch, M.R., Hayashi, M., and Thangaraj, A. in the article "IEEE transactions on Signal Processing" published in 9 2010, IEEE802.15.4BPSK receiver architecture base on a new effectiveness detection scheme "a method for calculating the quantization function of Y is providedThe body is provided with a plurality of grooves,is the phase of Y, also N ω₀T_cIs determined by the estimated value of (c),the calculation method of (2) is specifically described as:

the detection decision process of equation (3) requires extracting the frequency offset information N ω from Y in advance₀T_cIs estimated value ofThen pair of A [ m]Compensation is performed. As described above, the disadvantages of the conventional typical non-coherent demodulation method are: as shown in the formula (4), in the third step, the frequency offset information N ω needs to be obtained by division and complex arc tangent operation₀T_cIs estimated value ofFor 802.15.4 network terminals with strictly limited energy supply, the calculation complexity is high, the energy consumption is high, and the implementation cost is high.

Another demodulation method is to reduce the frequency offset compensation information Nomega extracted from Y₀T_cThe implementation complexity of (1) is improved by a simplified form of demodulation method proposed by Lee, s., Kwon, h., Jung, Y., and Kim, j.s. in "Electronics letters" published in 8 months 2007, in "Efficient non-coherent demodulation scheme for IEEE802.15.4 LR-wpans systems", and the quantization function of Y can be described as:

as shown in equation (5), in this simplified scheme, the frequency offset information N ω₀T_cIs estimated value ofCan be described specifically as:

it can be seen that this simplified form of demodulation method also requires that the frequency offset information N ω is extracted from Y beforehand₀T_cIs estimated value ofThen pair of A [ m]Compensation is performed. By the formula (5) pairThe approximation process of (6) to the frequency offset information N ω₀T_cThe estimation process of (a) has a more serious phenomenon of 'over-estimation' or 'under-estimation', which can cause great reduction of reliability, and does not achieve better balanced matching between complexity and performance. As shown in fig. 6, compared to the conventional typical complex baseband non-coherent demodulation method, the performance loss of the simplified demodulation method is serious, the carrier frequency used in the simulation is 924MHz, the frequency offset is 80ppm of the maximum value specified in the IEEE802.15.4 protocol, and the phase offset θ is (0, 2 pi) in the range]The internal servo is uniformly distributed, the data length of the PSDU is 20 bytes (160 bits), and at least 3000 frame errors are collected under each signal-to-noise ratio. The MAC layer of the 802.15.4 network uses Cyclic Redundancy Check (CRC) to determine the correctness of the transmission frame, and an automatic repeat request (ARQ) protocol determines whether the transmission frame needs to be retransmitted based on the CRC, but does not use a Forward Error Correction (FEC) mechanism, so the performance of the physical layer demodulation method has a great influence on energy consumption. When the channel condition is poor and the communication distance is long, the power loss of the received signal is large. This is achieved byIn time, if the simplified demodulation method is adopted, the same PSDU data frame may be retransmitted several times before being successfully received by the MAC layer. The communication process of multiple retransmissions also consumes a lot of energy if the amount of data is large, which reduces the lifetime of the 802.15.4 network with a poor energy supply.

Disclosure of Invention

In order to solve the defects in the prior art, the incoherent BPSK demodulation method is based on the theory that the Taylor series expansion of an arcsine function and the influence of noise on a useful signal in a large signal-to-noise ratio can be ignored, and is suitable for IEEE802.15.4 and low in computation complexity, power consumption, reliability and cost.

In order to achieve the purpose, the invention adopts the specific scheme that:

a high reliability BPSK demodulation method used in IEEE802.15.4, the data frame of the physical layer of the sending end is transmitted to the receiving end through the signal channel after spreading and BPSK modulation, the data frame includes the lead code of 32 bits and physical layer service data unit PSDU; the complex baseband sampling signal received at the receiving end is represented asWhere s (k) is the transmitted data to be detected, s (k) ∈ { +1, -1}, ω₀＝2πf₀,f₀And θ is the frequency offset and phase offset, respectively, and remains constant throughout the data frame, T_cIndicating spreading code chip period, η₀(k) Complex base band additive white gaussian noise; the specific demodulation steps are as follows:

step one, extracting a frequency offset observation value Y containing frequency offset information by using channel receiving data corresponding to a preamble of 32 bits:

wherein J represents the total number of bits of the preamble, J is 32, N represents the spreading length, N is 15, 1 ≦ m ≦ J-1, 0 ≦ N ≦ N-1, p [ N + Nm ≦ N-1]A channel reception value representing an nth chip corresponding to an mth bit of the preamble (·)^*Indicating the conjugation operation, η₁Represents all noise terms;

step two, carrying out bit-level differential processing on the complex baseband sampling signals corresponding to the PSDU to obtain a judgment observation value A [ m ]:

wherein r [ n + Nm]Indicating the channel reception value of the nth chip corresponding to the mth bit of the PSDU, η₂[m]Representing all noise terms, E [ m ]]Represents the mth bit data transmitted;

step three, extracting frequency offset information by using the frequency offset observation value Y in the step one, compensating the A [ m ] in the step two, and then performing detection judgment:

wherein,the mth bit data obtained by detection decision is represented, q (-) is a quantization function, and q (y) is specifically represented as:

wherein, | - | represents a modulo operation,is represented by A [ m ]]Medium frequency offset N omega₀T_cAn estimated value of (d);

after the detection is finished, transmitting the received PSDU data to an MAC layer for CRC;

in the third step, the first step is that,the calculation formula of (2) is as follows:

where Re (·) represents the real part operation, and Im (·) represents the imaginary part operation.

As a preferable scheme, for the step threeThe calculation method of (a) is simplified to obtain:

has the advantages that:

1. the demodulation method and the further simplified form provided by the invention can completely meet the requirements of the IEEE802.15.4 protocol on performance, and the IEEE802.15.4 protocol specifies that when the signal-to-noise ratio is 5-6 dB and the PSDU is 20 bytes (160 bits), the packet error rate is lower than 1 percent, namely the PER is less than 1 × 10^-2The requirements are fully met especially when the signal-to-noise ratio is about 1 dB;

2. compared with the traditional typical complex baseband non-coherent demodulation method, the method has lower calculation complexity, lower energy consumption and lower implementation cost, and the traditional typical complex baseband non-coherent demodulation method carries out one division and one arc tangent operation on the frequency offset information Nomega₀T_cAs can be seen from equation (10), the frequency offset information N ω of the present invention₀T_cIs estimatedThe counting method needs three times of comparison, two times of multiplication, one division and two times of addition operation at most; further simplified form of frequency offset information N ω₀T_cThe estimation method needs three times of comparison, one multiplication, one division and two times of addition operation at most, thereby having lower calculation complexity, lower energy consumption and lower realization cost;

3. the invention has higher reliability than the conventional simplified demodulation method which uses the formula (6) to the N omega₀T_cAn approximate estimate is made, i.e. 0,-a andfour phase pairs N ω₀T_cThe estimation is carried out, the error is larger, and the invention adopts the formula (10) to carry out the estimation on the frequency offset information Nomega₀T_cAnd the phenomenon of 'over-estimation' or 'under-estimation' existing in the estimation process is weaker, the performance is better, and the reliability is higher.

Drawings

FIG. 1 is a diagram illustrating the basic characteristics of four frequency bands of the physical layer of the IEEE802.15.4 protocol;

FIG. 2 is a diagram of a physical layer data transmission process in the 868/915/950-MHz band of the IEEE802.15.4 protocol;

FIG. 3 is a diagram of an IEEE802.15.4 protocol 868/915/950-MHz band spreading mapping scheme;

FIG. 4 is a diagram of an IEEE802.15.4 protocol physical layer frame structure;

FIG. 5 is a block diagram of a typical conventional non-coherent demodulation method suitable for the 868/915/950-MHz band;

FIG. 6 is a graph comparing the performance of a conventional demodulation method and a simplified version demodulation method for the 868/915/950-MHz band;

FIG. 7 is a graph showing a comparison of the performance of two demodulation methods provided by the present invention and two exemplary demodulation methods;

FIG. 8 shows a frequency offset f₀Probability distribution map of (2).

Detailed Description

Embodiments of the present invention will be specifically described below with reference to the accompanying drawings.

A high reliability BPSK demodulation method used in IEEE802.15.4, the data frame of the physical layer of the sending end is transmitted to the receiving end through the signal channel after spreading and BPSK modulation, the data frame includes the lead code of 32 bits and physical layer service data unit PSDU; the complex baseband sampling signal received at the receiving end is represented asWhere s (k) is the transmitted data to be detected, s (k) ∈ { +1, -1}, ω₀＝2πf₀,f₀And θ is the frequency offset and phase offset, respectively, and remains constant throughout the data frame, T_cIndicating spreading code chip period, η₀(k) Complex base band additive white gaussian noise; the specific demodulation steps are as follows:

step one, extracting a frequency offset observation value Y containing frequency offset information by using channel receiving data corresponding to a preamble of 32 bits:

wherein J represents the total number of bits of the preamble, J is 32, N represents the spreading length, N is 15, 1 ≦ m ≦ J-1,

0≤n≤N-1，p[n+Nm]a channel reception value representing an nth chip corresponding to an mth bit of the preamble (·)^*Indicating the conjugation operation, η₁Means all ofA noise term;

step two, carrying out bit-level differential processing on the complex baseband sampling signals corresponding to the PSDU to obtain a judgment observation value A [ m ]:

wherein r [ n + Nm]Indicating the channel reception value of the nth chip corresponding to the mth bit of the PSDU, η₂[m]Representing all noise terms, E [ m ]]Represents the mth bit data transmitted;

step three, extracting frequency offset information by using the frequency offset observation value Y in the step one, compensating the A [ m ] in the step two, and then performing detection judgment:

wherein,the mth bit data obtained by detection decision is represented, q (-) is a quantization function, and q (y) is specifically represented as:

wherein, | - | represents a modulo operation,is represented by A [ m ]]Medium frequency offset N omega₀T_cAn estimated value of (d);

after the detection is finished, transmitting the received PSDU data to an MAC layer for CRC;

in the third step, the first step is that,the calculation formula of (2) is as follows:

as a further simplified form, the quantization function of step three isWherein:

the theoretical basis of the present invention is set forth below.

First, tan is subtended at x ═ 0^-1x is subjected to Taylor series expansion to obtain:

when | x | is small, there is an approximate relationship tan^-1x≈x。

When the frequency is offset by N omega₀T_cOn the premise of smaller signal-to-noise ratio,the value of (c) is small. Can directly tan^{- 1}x ≈ x the simplified treatment of formula (4) gives:

but when the frequency is offset by an amount N ω₀T_cIn the case of a large or low SNR,the value of (a) is large. tan (r) is^-1The approximate calculation of x ≈ x for equation (4) will introduce large errors, resulting in a great loss of final performance. That is, equation (16) is only applicable to the detection process under two constraints of small frequency offset and large signal-to-noise ratio.

In order to obtain a low-complexity frequency offset estimation method under an unconstrained condition, an equivalence relation between an arcsine function and an arctan function is utilized to obtain:

if Y falls in the first and fourth quadrants of the complex planar coordinate system, i.e. if Y falls in the first and fourth quadrants of the complex planar coordinate systemThen there are:

if Y falls in the second and third quadrants of the complex planar coordinate system, i.e.Then there are:

from equations (17) and (18), equation (4) becomes equivalently:

secondly, sin is paired at x ═ 0^-1x is subjected to Taylor series expansion to obtain:

then when | x | is smaller, there is an approximate relationship sin^-1x≈x。

Due to the fact thatIs always true under any frequency offset and signal-to-noise ratio conditions, and sin can therefore be directly utilized^-1x ≈ x pairs of (19)The term simplification can result in:

thirdly, the calculation process of the partial expression of the expression (20) can be considered to be further simplified. When the signal-to-noise ratio is large, we have the following approximate relationship:

fourth, only the numerator im (Y) of the partial formula (20) is approximated by the formula (21) to obtain:

as shown in fig. 7, the demodulation method provided by the present invention has superior performance and higher reliability compared to the conventional exemplary simplified form demodulation method. The carrier frequency used in the simulation was 924MHz, frequency offset f₀Following a triangular distribution as shown in FIG. 8, the phase shift θ is at (0, 2 π]The internal servo is uniformly distributed, the data length of the PSDU is 20 bytes (160 bits), and at least 3000 frame errors are collected under each signal-to-noise ratio. It can be seen that the performance of the two demodulation methods provided by the present invention lies between the two conventional demodulation methods. Compared with the traditional simplified demodulation method, the packet error rate is1 × 10-3, a gain of no less than 1.3dB can be achieved, so the two schemes provided by the present invention achieve a better balanced match between implementation complexity and performance.

The theoretical basis for further simplification is: the simultaneous approximation of the numerator Re (Y) and the denominator im (Y) in the numerator of formula (20) by formula (21) yields:

the calculation process can be further simplified, and the energy consumption can be reduced.

Claims (2)

1. A high reliability BPSK demodulation method used in IEEE802.15.4, the data frame of the physical layer of the sending end is transmitted to the receiving end through the signal channel after spreading and BPSK modulation, the data frame includes the lead code of 32 bits and physical layer service data unit PSDU; the complex baseband sampling signal received at the receiving end is represented asWhere s (k) is the transmitted data to be detected, s (k) ∈ { +1, -1}, ω₀＝2πf₀,f₀And theta is eachFrequency offset and phase offset and remain constant throughout the data frame, T_cIndicating spreading code chip period, η₀(k) Complex base band additive white gaussian noise; the specific demodulation steps are as follows: step one, extracting a frequency offset observation value Y containing frequency offset information by using channel receiving data corresponding to a preamble of 32 bits:

wherein J represents the total number of bits of the preamble, J is 32, N represents the spreading length, N is 15, 1 ≦ m ≦ J-1, 0 ≦ N ≦ N-1, p [ N + Nm ≦ N-1]Denotes a channel reception value of the nth chip corresponding to the mth bit of the preamble, (· denotes a conjugation operation, η₁Represents all noise terms;

step two, carrying out bit-level differential processing on the complex baseband sampling signals corresponding to the PSDU to obtain a judgment observation value A [ m ]:

wherein r [ n + Nm]Indicating the channel reception value of the nth chip corresponding to the mth bit of the PSDU, η₂[m]Representing all noise terms, E [ m ]]Represents the mth bit data transmitted;

step three, extracting frequency offset information by using the frequency offset observation value Y in the step one, compensating the A [ m ] in the step two, and then performing detection judgment:

wherein,the mth bit data obtained by detection decision is represented, q (-) is a quantization function, and q (y) is specifically represented as:

wherein, | - | represents a modulo operation,is represented by A [ m ]]Medium frequency offset N omega₀T_cAn estimated value of (d);

after the detection is finished, transmitting the received PSDU data to an MAC layer for CRC;

the method is characterized in that:

in the third step, the first step is that,the calculation formula of (2) is as follows:

where Re (·) represents the real part operation, and Im (·) represents the imaginary part operation.

2. The high reliability BPSK demodulation method for ieee802.15.4 of claim 1, wherein: for the third stepThe calculation method of (a) is simplified to obtain:

。