JP2019021964A - Communication system and communication method - Google Patents

Abstract

To provide a communication system and a communication method of such accuracy as can demultiplex a multiplexed signal, in a multi-hop network to which a CSS modulation system is applied.SOLUTION: A communication system and a communication method prepare respective matched filters outputting for all data modulation, while delaying by a mutually common specific time. When demodulating by using a large number of matched filters, output correlation peak of the matched filters can be separated, if the time difference arriving at a reception terminal station goes above the inverse number of bandwidth (8 μs if the bandwidth is 125 kHz). When setting a unique delay time (difference between terminals is the inverse number of bandwidth or more) for each terminal, a transfer destination terminal can receive signals with a time difference, and can separate them.SELECTED DRAWING: Figure 13

Description

  The present disclosure relates to a communication system and a communication method for simultaneously transmitting signals that are modulated by CSS (Chirp Spread Spectrum) modulation using a multi-hop transmission scheme.

  LPWA (Low Power Wide Area) has been attracting attention in the field of IoT (Internet of Things) as a wireless communication system that secures long-distance communication while suppressing power consumption (see, for example, Non-Patent Document 1). LPWA has a Cat. Standardized by 3GPP, a mobile phone standardization organization. M1, Cat. In addition to NB1 (for example, see Non-Patent Document 2), Wi-SUN (see Non-Patent Document 3) introduced in an automatic meter reading system or the like, and Sigfox (for example, Non-Patent Document 3) as a communication method considering IoT. Patent Document 4) and LoRa (for example, see Non-Patent Document 5).

  Among them, according to Non-Patent Document 6, it is assumed that LoRa uses CSS modulation and multi-level FM data modulation (M-ary). The outline is examined from Non-Patent Documents 7 and 8.

  The chirp signal itself can be satisfactorily detected under high noise using a compression gain by using a matched filter at the time of reception, and has a track record in radar or the like (see, for example, Non-Patent Documents 9 and 10). ). Further, the communication field using chirp signals is also being studied as a CSS system (see, for example, Non-Patent Documents 11 to 13).

  On the other hand, if the LPWA performs long-distance communication while suppressing power consumption, multi-hop communication is an effective means (see, for example, Non-Patent Document 1). In the IoT field, effective means has been proposed for a simultaneous transmission flooding type multi-hop network that can easily configure a network while removing a routing function while performing multi-hop communication (see, for example, Non-Patent Document 14).

[CSS modulation system]
The CSS data modulation will be described.
The chirp signal is a type of FM signal whose instantaneous frequency changes (generally monotonously increasing or monotonically decreasing) with time. As an example used for communication, there is SSK (Slope-ShiftKeying) for identifying data modulation by the difference between up or down chirp signals (see, for example, Non-Patent Document 11), and phase modulation (PSK) is performed for each chirp signal. In addition, there is an example in which a PSK system for receiving and demodulating a part of the bandwidth (subband) at the time of demodulation is shown (for example, see Non-Patent Document 12). Further, the inventors have proposed a demodulation method for demodulating binary difference in-phase coding (DBPSK) by obtaining a correlation with a matched filter for two periods of chirp signals (see, for example, Non-Patent Document 13). .)

In recent years, there is an example in which CSS data modulation is performed for LPWA applications. FIG. 1 shows an instantaneous frequency obtained by measuring an output signal of a LoRa module with a sampling oscilloscope. As shown in FIG. 1, a chirp signal whose instantaneous frequency changes with time is used, and data modulation and demodulation can be performed by a module (see, for example, Non-Patent Document 6). According to Non-Patent Documents 7 and 8, in FIG. 1, the bandwidth B is 125 kHz, and the time width (symbol length) T of the chirp signal is 1 / B × 4096. Since the time width T is 4096 times the 1 / B, the frequency difference of 1/44096 of the bandwidth B can be discriminated. In addition, the symbol length T can be selected to be 2 ^SF (SF = 7, 8, 9, 10, 11, 12) times 1 / B, where SF = 12 (2 ^SF = 4096). Describe the case.

First, the data modulation system will be outlined with reference to FIG. If the initial instantaneous frequency f (t = 0) of the Chirp signal is
(Equation 1)
f (t = 0) = fo + B × n / 4096 (1)
Here, t = 0 is the start time of one symbol, fo is the frequency at the lower end of the band, and n = 0 to 4095.

  Since each signal has a symbol length of 4096 times 1 / B, the frequency can be discriminated with a frequency resolution of 1/44096 of the bandwidth B, and the discriminable frequency difference is used as data modulation (M-Ary method). . At this time, 12-bit data can be transmitted / received with one symbol length (1 chirp signal length) in the correspondence relationship of n = 0 to 4095 in the equation (1).

In digital signal processing, when I / Q complex sampling is performed at a sampling frequency Bs of 125 kHz equal to the bandwidth B, the signals outside the upper and lower ends of the bandwidth are undersampling, and therefore can be handled as signals within the Bs band. Similarly, if the transmission chirp signal is used recursively in the band B, it can be handled as transmission / reception of signals in the Bs band. If the frequency waveform of the chirp signal is S (f) and the time waveform is s (t),
(Equation 2)
S (f + B) = S (f−B) = S (f) (2)
(Equation 3)
s (t + T ₀ ) = s (t−T ₀ ) = s (t) (3)
It becomes. Here, T ₀ is a symbol time length.

Next, the preamble will be described.
From the LoRa signal measurement (FIG. 1), a packet is started with a symbol string as a preamble. A plurality of up chirps (chirp signals whose instantaneous frequency increases with time) and a plurality of down chirps (chirp signals whose instantaneous frequency decreases with time) are transmitted 1/4 times. A signal with no frequency offset (n = 0) is used for both up chirp and down chirp. In down chirp, fo is the frequency at the upper end of the band.

  Assuming LPWA applications themselves, IoT and other low power consumption, low cost wireless terminal stations, the frequency accuracy of the terminal stations is limited. If a specific low power in the 920 MHz band is assumed, it is not possible to expect a very high accuracy with respect to the frequency accuracy of the terminal station. Assuming that the frequency accuracy is about 30 ppm, the frequency deviation is 27.6 kHz, which is not negligible compared to the bandwidth 125 kHz and the channel interval 200 kHz. Therefore, it is necessary to identify the initial frequency transmitted as a preamble as n = 0 on the receiving station side.

  The down chirp signal can be detected by a matched filter for the down chirp signal, as will be described later. The down chirp matched filter (cross-correlation) of the up chirp signal is as shown in FIG. The average of the ratio to the autocorrelation is 36 dB, and the influence of the signal between the up / down chirp signals is small.

[CSS demodulation method]
The CSS demodulation by the matched filter will be described.
The CSS-modulated signal can be demodulated by using a matched filter (see, for example, Non-Patent Documents 11 and 13). Further, since the correlation peak (pulse width) is approximately the reciprocal of the band B (8 μs under the above examination conditions), this time accuracy is sufficiently short compared to the symbol length (32.768 ms). When importance is placed on sampling time, such as a sensor network, in IoT, CSS demodulation by a matched filter is an effective feature.

The demodulation performed by Non-Patent Documents 11 and 13 uses one matched filter.
The initial instantaneous frequency of the chirp signal is determined by the data value.
(Equation 4)
fn = fo + B × n / 4096 (4)
Then, since the frequency is swept cyclically within the bandwidth, different cross-correlation characteristics of n are characteristic. From this feature, correlation detection for n = 0 to 4095 can be performed with a matched filter of fo. Which chirp signal in the equation (4) comes is detected by the time shift amount of the correlation pulse with respect to the correlation pulse time of n = 0 (converted to pulse position modulation). The time origin is determined by the matched filter output time of the preamble. The frequency deviation between the transmitting and receiving radio stations generated in the apparatus mounting can be identified and calibrated because a chirp signal of n = 0 is transmitted a plurality of times in the preamble. As shown in FIG. 3, if the matched filter output of the down chirp signal is also used, the frequency difference between the transceivers can be detected.

  A configuration example of CSS demodulation using one matched filter is shown in FIG. The generation time of the correlation peak in the case of using one matched filter differs depending on the data modulation value n = 0 to 4095, and has two correlation outputs when n ≠ 0 depending on the instantaneous frequency component of the signal (see FIG. 6). . Since the relationship between the shift times of these two correlation peaks is known, if the time origin of the correlation peak (peak time when n = 0) is determined by the preamble, data demodulation (n = 0) is performed with the shift amount from the time origin. Discrimination of ˜4095) is possible.

[Simultaneous transmission type multi-hop method]
The constructive interference flooding method in the simultaneous transmission type multi-hop method is that a plurality of terminal stations transmit simultaneously by transferring the same packet at the same timing in low bit rate communication such as IEEE 802.15.4. This is a technique that focuses on the fact that no packet loss occurs even in such a situation (see, for example, Non-Patent Document 15). Adoption of the simultaneous transmission technique achieves both end-to-end reliability and power saving.

The characteristics of the simultaneous transmission type multi-hop method are as follows.
i) Since flooding by the simultaneous transmission technique is a basic communication method, rerouting or the like is not required even when the topology changes.
ii) With the simultaneous transmission technology, it is possible to perform power saving control by synchronizing at the packet level in the entire network, and it is possible to keep power consumption low although all communications are flooding operations. This is due to the fact that routing is not required and that idle listening and long preamble transmission are unnecessary because of the synchronous type.
iii) Rescheduling at the time of traffic fluctuation, which is a problem in the synchronous system, can be realized at a very low cost by using the simultaneous transmission technology.

U. Raza, P.M. Kulkarni and M.M. Sooriyabandara, “Low Power Wide Area Networks: An Overview”, IEEE COMMUN SURV, vol. 19, no. 2, 2nd Quart. 2017. http: // www. 3 gpp. org / DynaReport / 36-series. htm https: // www. wi-sun. org / index. php / en / https: // www. sigfox. com / en https: // www. lora-alliance. org / SEMTECH, “AN1200.22 LoRatm modulation basics”, May 2015. “What is Lora?”, Https: // www. link-labs. com / blog / what-is-lora M.M. Knight and B.J. Seeber, “Decoding LoRa: Realizing a Modern LPWAN with SDR”, Proc. The GNU Radio Conference, vol. 1, no. 1, Sep. 2016. J. et al. R. Klauder, A.M. C. Price, S.M. Darlington, W.D. J. et al. Albefsheim, “The theory and design of chirp radars”, Bell Syst. Tech. 1, vol. 39, no. 4, pp. 745-808, Jul. 1960. G. Galati, G. Pavan and F.M. De Palo, "Waveforms design for modern radar: The chirp signal Fifty + years later", 2014 11th European Radar Conference, pp. 13-16, 2014. D. S. Dayton, “FM“ Chirp ”Communications: Multiple Access to Dispersive Channels”, IEEE Trans. Electromagn. Compat. , Vol. EMC-10 Issue: 2, pp. 296-297, 1968. H. Takai, Y .; Urabbe and H.M. Yamazaki, “Anti-multipath and anti-jamming modulation / demodulation scheme SR-chirp PSK for high-speed data transmission in dispersive in dispersive. 1994 IEEE VTC, pp. 1355-1359, Jun. 1994. Y. Takeuchi and K.K. Yamauchi, “A chirp spread spectrum DPSK modulator and demo-tor for a time shift multiple access system communication I S E device E 19 , Vol. 2, pp. 507-510, Jun. 1998. F. Ferrari, et al. "Efficient network flooding and time synchronization with Glossy", Proc. ACM / IEEE IPSN'11, pp. 73-84, Apr. 2011. Suzuki Makoto, Morikawa Hiroyuki, “Choco: Multipurpose Platform for Wireless Sensor Networks”, IEICE Technical Report MoNA2014-24, Jul. 2014. C. H. Liao, G.G. Zhu, D.D. Kuwabara, S .; Ohara and M.M. Suzuki and H.K. Morikawa, “Evaluating the Sub-GHz LoRa Receiver Performance Under Synchronized Packet Collations”, IEICE Technical Report RCS2017-27, Apr. 2017.

  In the case of CSS modulation, when there is no or very small frequency deviation between the terminal stations, signals from multiple terminals are combined within the accuracy of synchronization between the terminal stations at each symbol time, and the same construction as described above. Interferometric flooding is possible. On the other hand, when there is a frequency deviation that is potentially not negligible between the terminal stations, chirp signals from the terminals do not become signals of the same frequency (combining signals that differ only in phase within each symbol), The chirp signal is synthesized while maintaining the frequency interval. In this case, due to the frequency deviation between the terminal stations, the chirp signal has a different initial frequency, and the initial frequency of the synthesized wave cannot be specified.

  As a countermeasure, Non-Patent Document 16 implements a flooding method using the property of selectively receiving the strongest signal among a plurality of terminal stations using the capture effect of FM signals. ing.

  FIG. 12 shows a functional block diagram (A) on the transmission side and a functional block diagram (B) on the reception side of the CSS modulation system described in Non-Patent Document 16. The chirp signal on the receiving side is converted to a CW signal having a different slope by multiplying a signal having a reverse slope (down chirp if up chirp, up chirp if down chirp) by each symbol. It becomes. The matched filter for obtaining the maximum S / N at this time is a narrow band filter (frequency filter) corresponding to each multi-level FM narrow band signal, and since reproduction is required for each symbol, a time constant within approximately one symbol is required. Required and its bandwidth corresponds to the reciprocal of one symbol time. In addition, the time synchronization of the symbols is accurate to the symbol time length from the time constant of the narrow band filter (frequency filter).

  When such a CSS modulation method is applied to a multi-hop network as in Non-Patent Document 14, the same signal output from many terminals may reach one terminal of interest, and the configuration as shown in FIG. It is difficult to separate them with the accuracy of. That is, the CSS modulation demodulation configuration as shown in FIG. 4 has a problem that it is difficult to separate multiplexed signals. Accordingly, an object of the present invention is to provide an accurate communication system and communication method capable of separating multiplexed signals in a multi-hop network to which a CSS modulation scheme is applied in order to solve such problems. To do.

  In order to achieve the above object, in the communication system and communication method according to the present invention, each matched filter is prepared for all data modulations (all symbol types).

Specifically, in the communication system according to the present invention, a chirp signal having a bandwidth B and a symbol length T is modulated by a frequency modulation (FM modulation) corresponding to BT symbols and subjected to CSS (Chirp Spread Spectrum) modulation. A communication system for transmitting and receiving the same data in a multi-hop manner between a plurality of wireless terminals,
The wireless terminal is
BT matched filters each having a complex conjugate frequency response corresponding to each of the frequency modulations and outputting a correlation value corresponding to the received signal delayed by a common time in accordance with the instantaneous frequency of the modulated signal When,
An arithmetic unit for searching for a delayed signal having an amplitude peak within one symbol at a predetermined position (predetermined delay time amount) among the delayed signals output by the BT matched filters;
A synchronization unit that synchronizes a clock to the modulation signal using a peak position of the delayed signal searched by the arithmetic unit;
A transmission unit that is unique to the preset wireless terminal based on the clock and that transmits the modulated signal after a delay time of 1 / B or more and within one symbol that is different from the delay time of another wireless terminal; ,
It is characterized by providing.

In addition, the communication method according to the present invention uses a modulated signal obtained by subjecting a chirp signal having a bandwidth B and a symbol length T to frequency modulation (FM modulation) corresponding to BT symbols and performing CSS (Chirp Spread Spectrum) modulation. A communication method for transmitting and receiving the same data in a multi-hop manner between wireless terminals of
In the wireless terminal,
BT matched filters each having a complex conjugate frequency response corresponding to each of the frequency modulations and outputting a correlation value corresponding to the received signal delayed by a specific time common to each other in accordance with the extended binary frequency of the modulated signal Procedure and
An arithmetic procedure for searching for a delayed signal having one amplitude peak at a predetermined position within one symbol among the delayed signals output by the BT matched filters;
A synchronization procedure for synchronizing a clock to the modulation signal using a peak position of the delayed signal searched in the calculation procedure;
A transmission procedure that is specific to the wireless terminal set in advance based on the clock, and that transmits the modulated signal after a delay time of 1 / B or more and within one symbol that is different from the delay time of another wireless terminal; ,
It is characterized by performing.

  For all data modulations, the output of the corresponding matched filter has a correlation peak at the same time, and the level of the correlation peak is constant. In other words, a matched filter corresponding to all data modulations (for example, all 12-bit data) is prepared, and a matched filter having one peak is searched for from the output to know the modulated data. Can do.

  In the case of a configuration using one matched filter, the data is detected by the time difference from the time origin, so although only one matched filter is required, correlation calculation at a large number of times (4096 points for 12 bits). is necessary. On the other hand, in the case of using a large number (4096 if 12 bits) of matched filters, the correlation calculation is only at one point of the correlation peak, and the amount of calculation is not different from the case of using one matched filter.

  Further, when demodulating using a large number of matched filters as in the present invention, if the time difference reaching the receiving terminal station is equal to or greater than the reciprocal of the bandwidth (8 μs if bandwidth B is 125 kHz), the output of the matched filter Correlation peaks can be separated. In other words, the resolution can be increased by demodulating using a number of matched filters, and even if the same signal output from many terminals reaches one terminal, it can be separated. Therefore, the present invention can provide an accurate demodulator and demodulation method capable of separating multiplexed signals in a multi-hop network to which the CSS modulation method is applied.

In order to separate the multiplexed modulation signal, a preamble portion which is a known signal is used. Specifically, the modulation signal has a preamble portion that is CSS modulated with the spreading code corresponding to an arbitrary symbol,
When the delay signal of the preamble part does not have one amplitude peak in one symbol, the arithmetic part determines that the modulation signal is multiplexed shifted on the time axis,
When the modulation signal is multiplexed, the synchronization unit synchronizes the clock using the preamble portion of any one of the modulation signals,
The transmission unit transmits only the modulated signal with a clock synchronized.

  As described above, assuming the use of LPWA itself, a low-power wireless terminal station such as IoT, and the low-price wireless terminal station, the frequency accuracy of the terminal station is limited, and the center frequency of the received modulated signal may vary There is. The demodulator and the demodulation method according to the present invention recognize that the frequency fluctuation is generated as follows using the preamble part.

(Judgment method 1)
The modulated signal has a preamble portion that is CSS modulated by the frequency modulation corresponding to an arbitrary symbol;
The arithmetic unit shifts a center frequency of the modulation signal when a symbol assigned to the matched filter that outputs the delayed signal having one amplitude peak in one symbol of the preamble unit is not the arbitrary symbol. It is characterized by judging.

(Judgment method 2)
The modulated signal has a preamble portion that is CSS modulated by the frequency modulation corresponding to an arbitrary symbol;
The arithmetic unit shifts a center frequency of the modulation signal when a position of the peak of the delayed signal having one amplitude peak in one symbol of the preamble part is not in the predetermined position (predetermined delay amount). It is judged that it is.

  The present invention can provide a communication system and a communication method with high accuracy capable of separating multiplexed signals in a multi-hop network to which a CSS modulation method is applied.

It is a figure explaining the relationship between the time of a CSS modulation system, and an instantaneous frequency. It is a figure explaining the data modulation in CSS. It is a figure explaining the output of the matched filter with respect to a chirp signal. It is a figure explaining the structure of the CSS demodulator using one matched filter. It is a figure explaining the correlation output with respect to an Up / Down-chirp signal of a down-chirp matched filter. It is a figure explaining the signal which the matched filter of a CSS demodulator using one matched filter of n = 0 outputs. (A) is a block diagram explaining the demodulator with which the radio | wireless terminal of the communication system which concerns on this invention is provided. (B) is a figure explaining operation | movement of the matched filter of the demodulator with which the radio | wireless terminal of the communication system which concerns on this invention is provided. It is a figure explaining a time-sequential change about the output of each matched filter of the demodulator with which the radio | wireless terminal of the communication system which concerns on this invention is provided. It is a figure explaining the amplitude value about the output of each matched filter of the demodulator with which the radio | wireless terminal of the communication system which concerns on this invention is provided. (A) is a comparison at the first symbol, and (B) is a comparison at the second symbol (B). It is a figure explaining a time-sequential change about the output of each matched filter of the demodulator with which the radio | wireless terminal of the communication system which concerns on this invention is provided. It is a figure explaining the amplitude value about the output of each matched filter of the demodulator with which the radio | wireless terminal of the communication system which concerns on this invention is provided. It is a block diagram (A) explaining the function of the transmission side of a CSS modulation system, and a block diagram (B) explaining the function of the receiving side. It is a figure explaining operation | movement of the communication system which concerns on this invention. It is a figure explaining operation | movement of the communication system which concerns on this invention.

  Embodiments of the present invention will be described with reference to the accompanying drawings. The embodiments described below are examples of the present invention, and the present invention is not limited to the following embodiments. In the present specification and drawings, the same reference numerals denote the same components.

FIG. 13A is a diagram illustrating the configuration of the communication system according to the present embodiment. This communication system uses a modulated signal obtained by CSS-modulating a chirp signal with a bandwidth B and a symbol length T by frequency modulation (FM modulation) corresponding to BT symbols, and the same data is transmitted between a plurality of wireless terminals. Send and receive in a multi-hop manner. A terminal that transmits data among the wireless terminals is referred to as “initiator I”. A plurality of wireless terminals _An exist in the area 21 where the transmission signal from the initiator I can reach. And, on the outside of the area 21, there are areas 22 which can reach the transmission signal from the wireless terminal A _n, there is a transmission destination radio terminal B in the area 22. The data transmitted by the initiator I is relayed by a plurality of wireless terminals _An (for example, wireless terminals A ₁ and A ₂ ) and arrives at the wireless terminal B (see FIG. 13B). That is, there are a plurality of transmission routes from the initiator I to the wireless terminal B.

First, a method for demodulating a CSS-modulated modulation signal performed by the radio terminals _An and B will be described.
Figure 7 (A) is a block diagram illustrating a wireless terminal _{A n.} The wireless terminal _An is
An antenna 11 for receiving a CSS modulated signal;
An amplifier 12 for amplifying the received modulated signal;
A converter 13 for converting the frequency of the amplified modulated signal;
A specific time having a complex conjugate frequency response corresponding to each of the frequency modulations and a correlation value corresponding to the received signal corresponding to the instantaneous frequency of the modulation signal with respect to the modulation signal whose frequency is converted BT matched filters 14 that are output with a delay of
An arithmetic unit 15 for searching for a delayed signal having one amplitude peak at a predetermined position within one symbol among the delayed signals output from the BT matched filters 14;
A synchronization unit 16 that synchronizes a clock to the modulation signal using a peak position of the delayed signal searched by the calculation unit 15;
Based on the clock, a transmission unit 17 that is unique to the preset wireless terminal and that transmits the modulated signal after a delay time of 1 / B or more and within one symbol has elapsed from the delay time of another wireless terminal. When,
Is provided.

  The computing unit 15 of the wireless terminal B searches for a delayed signal whose amplitude peak is one at a predetermined position (predetermined delay time) within one symbol among the delayed signals output from the BT matched filters 14. A symbol assigned to the matched filter that outputs the delayed signal is output.

In this embodiment, the bandwidth B is 125 kHz, the symbol length T is described in ^{2 12} times the 32.768msec of CSS modulated signal 1 / B as an example. The CSS modulation signal is composed of a preamble part and a data part as shown in FIG. The preamble part is composed of 10 symbols for up chirp and 2 symbols for down chirp and 1/4 symbol. In the data part, up chirp is modulated with 12-bit data for each symbol. Specifically, the frequency at the start of up chirp is changed by Δf (= 1 / T = 30.5 Hz) by 12-bit data.

FIG. 7B is a diagram for explaining the operation of the matched filter 14. FIG. 7B (a) shows the operation of the matched filter 14 (# 0), and FIG. 7B (b) shows the operation of the matched filter 14 (# 1024). The matched filter 14 (# 0) is assigned data [0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0].
Data [0, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0] is assigned to the matched filter 14 (# 1024).

A specific example will be described in which the modulation signal (# 0) and the modulation signal (# 1024) are received. (I) illustrates the waveforms of the modulation signal (# 0) and the modulation signal (# 1024). (Ii) is a delay characteristic set for each matched filter 14. The matched filter 14 (# 0) has a fixed delay characteristic in the complex conjugate of the frequency response of the corresponding signal on the frequency axis, and has a delay characteristic in which the delay time decreases as the frequency increases on the time axis. Matched filter 14 (# 0), when the modulated signal (# 0) is input and output at the same time _{t 0} all frequencies as in (iii). That is accumulated signal strength of all frequencies, the output of the matched filter 14 (# 0) is a signal one peak appears in a time t ₀ as (iv).

  On the other hand, since the delay characteristic of the matched filter 14 (# 0) and the modulated signal (# 1024) do not match when the modulated signal (# 1024) is input, the matched filter 14 (# 0) has a frequency as shown in (iii). The output time will be separated. That is, the signal intensities of the frequencies that are separated and output are accumulated, and the output of the matched filter 14 (# 0) becomes a signal in which two peaks appear as in (iv).

  The matched filter 14 (# 1024) has a delay characteristic that matches the modulation signal (# 1024). That is, the frequency delay time response decreases as the frequency becomes higher as in (ii), and the initial delay time decreases cyclically when it exceeds a certain frequency, from which the delay time decreases toward the initial delay time. This is a delay characteristic. Since the delay characteristic of the matched filter 14 (# 1024) and the modulated signal (# 0) do not match when the modulated signal (# 0) is input, the matched filter 14 (# 1024) is output according to the frequency as in (iii). Time to separate. That is, the signal intensities of the frequencies that are separated and output are integrated, and the output of the matched filter 14 (# 1024) becomes a signal in which two peaks appear as in (iv).

On the other hand, the matched filter 14 (# 1024) is when the modulated signal (# 1024) is input and output at the same time _{t 0} all frequencies as in (iii). That is accumulated signal strength of all frequencies, the output of the matched filter 14 (# 1024) is a signal one peak appears in a time t ₀ as (iv).

The calculation unit 15 confirms the output of each matched filter 14 and detects the matched filter 14 having an output in which only one peak appears. The calculation unit 15 outputs data assigned to the output matched filter 14 in which only one peak appears as data of the symbol. Thus, the radio terminals _An and B can demodulate the received CSS modulated signal.

Subsequently, advantages of the wireless terminals _An and B will be described.
When one matched filter is used as in Non-Patent Documents 11 and 13, two correlation peaks appear depending on the data, and when the peak levels are linearly added, they are the same for all data modulations. Becomes complicated.

If each matched filter is prepared for all data modulations (4096) as in the wireless terminals _An and B, the output of the corresponding matched filter for all the data modulations is a correlation peak at the same time. And the level of the correlation peak is constant.

Data information is included only in the correlation peak detection time. Although it is necessary to prepare 4096 matched filters, the calculation for data determination can be performed only at one point of correlation peak time per symbol (t _{0 in} FIG. 7B). The correlation peak for the data appears in only one of all correlators. Further, since 4096 correlation calculations are performed on the same input signal, data demodulation can be performed with maximum likelihood estimation based on a level difference between correlator outputs.

  In the case of the configuration example using one matched filter as in Non-Patent Documents 11 and 13, since the data is detected by the time difference from the time origin, one matched filter is sufficient, but at the time of 4096 points. Correlation calculation is required. On the other hand, when 4096 matched filters are used as in this demodulator, the correlation calculation is only at one point of the correlation peak, and the amount of calculation is not different from the case of using one matched filter.

  In FIG. 8, n = 0 for the first symbol, and n = 1023 for the second symbol, the output values (relative values where the peak is 0 dB) of each matched filter for the time (symbol length is 1 unit) The result calculated by time is shown. FIG. 9 shows the matched filter numbers at the time 0 and the time 1, which are data determination points, with the horizontal axis representing the matched filter number and the relative output value representing the vertical axis (normalized with the peak as 0 dB). The matched filter of n = 0 and n = 1023 has a correlation peak.

  If the preamble can be accurately symbol-synchronized and the preamble signal can be identified as a chirp signal with n = 0, the data portion can be demodulated. If the required accuracy at this time is 2 sec per packet, the width of the correlation peak is 1 / B = 8 μs. Therefore, in the normal standard, the deviation accuracy is 4 ppm or less because it is within 2 sec per packet. The individual clock accuracy between the terminal stations is the offset at the time of data identification in the preamble, and the frequency short-term stability in the time width of about 2 seconds at the frequency source of the terminal station (transmitting terminal) is the Doppler frequency during movement. It can be demodulated if it is several ppm or less including the transition (the required frequency stability which is not a problem when using normal LPWA).

  In addition, when the frequency of the transmitting terminal is greatly shifted (for example, 30 ppm, 27.6 kHz), the wireless terminal determines the frequency shift using the preamble portion. For example, if 10 symbols of the preamble part are modulation signals (# 0), a signal with one amplitude peak is output from the matched filter 14 (# 0) if the center frequency of the modulation signal is not shifted. The On the other hand, when a signal having two amplitude peaks is output from the matched filter 14 (# 0) and a signal having one amplitude peak is output from the other matched filter 14 (#X), the calculation unit 15 can be determined that the center frequency of the modulation signal is shifted. Moreover, the calculating part 15 can also judge the deviation | shift amount of the center frequency of a modulation signal from the matched filter 14 (#X). For this reason, the calculation unit 15 outputs data in consideration of the shift amount of the center frequency of the modulation signal when demodulating the data part.

Further, when the signal having one amplitude peak is output from the matched filter 14 (# 0) when the center frequency of the modulation signal is shifted, the peak position is changed from a predetermined position (t _{0 in} FIG. 7B). Therefore, the calculation unit 15 can determine that the center frequency of the modulation signal is shifted from this shift.

(Other embodiments)
In the above description, it has been described that a plurality of matched filters 14 are physically prepared, but processing may be performed in software. In other words, the received modulation signal is subjected to arithmetic processing corresponding to the matched filter 14 (# 0 to # 4095) by a computer, and a delay characteristic having a constant delay time (one peak) at all frequencies is found. The data assigned to may be output.

[Multiplexed modulation signal]
The signal waveform of the LPWA CSS modulation system shown in FIG. 1 shows that, in the case of data modulation shown in FIG. 2, one of n = 0 to 4095 in the equation (4) per symbol with respect to the frequency bandwidth (125 kHz). Only sent on the street. If only one chirp signal is transmitted within one symbol, only a part (1/4096) of one symbol time is used at a specific frequency point. Further, when viewed at a specific time point, only a signal having a specific instantaneous frequency (1/4096 of the bandwidth) is used. The frequency utilization efficiency is low in terms of both time and frequency.

  According to Non-Patent Document 13, when differential binary modulation (DBPSK) is performed between successive chirp signals, even if the chirp signal itself is multiplexed, the time of the correlation peak for each multiplexed signal differs (mutually). Can be separated and demodulated. If frequency weighting is not performed using a linear chirp signal, the correlation output becomes an amplitude sinx / x type response having a pulse width that is the reciprocal of the bandwidth. 4096 multiplexing is possible under the same conditions as the report. This can be regarded as a form in which the frequency and time of a general OFDM are exchanged by passing a matched filter on the chirp signal.

  On the other hand, when data modulation is performed with M-Ary using the above-described frequency difference and a plurality of chirp signals having different times within the same symbol time are transmitted, a plurality of chirp signals may overlap with each other (multiplexing of CSS modulation signals). ). However, if the preamble of each modulated signal to be multiplexed is used, the time of the data correlation peak can be estimated by the reciprocal of the bandwidth (1 / B), so that the multiplexed signals having different correlation peak times can be separated and demodulated. It becomes possible.

FIG. 14 is a conceptual diagram illustrating a technique for separating and demodulating a multiplexed CSS modulated signal by a wireless terminal of the communication system. FIG. 14A shows a multiplexed CSS modulated signal received by the wireless terminal. The modulation signal has a preamble portion composed of a known chirp signal (up chirp signal starting from the frequency f _{0 in the} figure). For this reason, the computing unit 15 of the wireless terminal confirms the output of the matched filter # ₀ and performs time synchronization so that the correlation peak time becomes t0. At this time, since two peaks occur (FIG. 14B), the calculation unit 15 can recognize that the received modulation signal is multiplexed, and can synchronize time according to each peak (FIG. 14 ( C) and (D)). That is, the arithmetic unit 15 can separate the signal α and the signal β using the preamble part. Then, in the data portion, each data can be extracted from the matched filter number that generates the correlation peak at each time t ₀ synchronized in time.

  FIG. 10 shows a case where the signal at the time of writing “Add” is added to the signal shown in FIG. 8 (data is n = 0). In FIG. 10, when the data value is the data indicated by x at the determination point (Add point) of the added signals, the multiplexed chirp signals overlap each other. However, if a certain amount of error is allowed, the probability is not necessarily high in the number of cases in which two types of errors occur in consideration of the symbols before and after the multiplexed signal among 4096 patterns. Furthermore, since a peak is detected at the same frequency at a point one symbol time before and after multiplexing, separation is aided by examining the same time peak in the preceding and following symbols. Further, since the level of the correlation peak is proportional to the time width in which the chirp signals overlap in the correlation time, separation from the multiplexed signal becomes easier if the signal level is measured.

  Looking at the time series change of each correlation output of CSS demodulation by the matched filter of FIG. 8, since the chirp signal corresponding to each data is a frequency-shifted signal (see FIG. 2), the matched filter output for that signal is These are frequency-shifted and time-shifted in time series, and it cannot be said that each cross-correlation is independent. However, the time series change of the correlation output is characteristic and common to all the signals for each data (see FIG. 8). It is possible to separate and demodulate the multiplexed signal by paying attention to this feature point.

[Simultaneous transmission multi-hop]
As shown in FIG. 13A, this communication system propagates data by the simultaneous transmission multi-hop method. For this reason, there are a plurality of paths from the initiator I to the wireless terminal B, and the data arrival time differs depending on the transmission distance. Since this communication system is CSS modulated, the signal received by the wireless terminal B can be regarded as the multiplexed modulated signal described above. Better be transferred data each wireless terminal A _n is awaiting delay time specific to actively put the time difference (Fig. 13 (C)).

Specifically, the wireless terminals A ₁ and A ₂ receive the modulated signal from the initiator I, and the wireless terminals A ₁ and A ₂ synchronize time using the preamble portion of the modulated signal. Then, the wireless terminals A ₁ and A ₂ transmit the modulated signals after different inherent delay times. Note that the wireless terminals A ₁ and A ₂ may check whether or not there is an error in the received signal, and if there is no error, the hop count and the delay amount may be rewritten to transmit the modulated signal. On the other hand, the radio terminals A ₁ and A ₂ do not transmit the modulation signal when there is an error in the signal or when the number of hops exceeds the upper limit.

In addition, the wireless terminal _An may also receive modulated signals from a plurality of wireless terminals. In this case, the radio terminal _An determines that the arithmetic unit 15 determines that the modulated signal is multiplexed with a shift on the time axis because the delayed signal of the preamble unit does not have one amplitude peak in one symbol. In this case, the synchronization unit 16 may synchronize the clock using the preamble portion of any one of the modulation signals, and the transmission unit 17 may transmit only the modulation signal with the clock synchronized. In the subsequent stage, the number of multiplexed modulation signals can be reduced.

The wireless terminal B receives a modulated signal having a delay time difference and a transmission time difference from the wireless terminals A ₁ and A ₂ . Since the wireless terminal B demodulates using a number of matched filters, the output correlation peak of the matched filter can be separated if the time difference of arrival of each modulated signal is equal to or greater than the reciprocal of the bandwidth (1 / B = 8 μs). Yes (time filter). Wireless terminal B, if handle received signal by utilizing this characteristic as a multiplexed CSS modulated signal, the signal sent at time intervals of the same symbols from a plurality of radio terminals A _n also are each separable demodulation.

Further, the wireless terminal B, that you composite in consideration of the offset of the demodulated values once separated demodulated signal due to the frequency deviation of the radio terminal A _n, redundancy than relay transmission from first terminal by statistical multiplexing effect Can be increased.

[effect]
In the present invention, in order to improve the frequency utilization efficiency of multi-level FM data modulation of M-Ary, each signal can be separated by adding a time difference more than the reciprocal of the bandwidth when multiplexing a plurality of chirp signals. This is a communication system that employs a simultaneous transmission type multi-hop transmission system in the CSS system.
As described above, there are cases where chirp signals overlap and it is difficult to separate multiplexed signals depending on the data due to the nature of the cross-correlation, but the effect of improving the deterioration due to the statistical multiplexing effect of the constructive interference flooding method is I can expect. This communication system can be expected to have equivalent or better performance than the selective reception of strong signals by the FM capture effect of Non-Patent Document 16.

  A communication method according to the present invention is a method combining CSS modulation and multi-level FM data modulation (M-ary), which is one of LPWA communication methods, and performs demodulation using a plurality of (for M-ary) matched filters. Yes. Since the demodulation of this communication method has one data determination point per symbol, there is no difference in the amount of calculation compared to the demodulation of one matched filter.

In addition, this communication method can synchronize time between transmission and reception with a sufficiently short time accuracy (1/2 ^SF, for example, SF = 7 to 12) compared to the symbol time by using a plurality of matched filters for demodulation. This is an effective feature for sensor networks that require simultaneous sampling.

11: Antenna 12: Amplifier 13: Converter 14: Matched filter 15: Calculation unit 16: Synchronization unit 17: Transmission unit I: Initiator A _n : Wireless terminal (n is a natural number)
B: Receiving wireless terminal

Claims (8)

The chirp signal with bandwidth B and symbol length T is modulated using frequency modulation (FM modulation) corresponding to BT symbols and modulated with CSS (Chirp Spread Spectrum), and the same data is transmitted between a plurality of wireless terminals. A communication system that transmits and receives in a multi-hop manner,
The wireless terminal is
BT matched filters each having a complex conjugate frequency response corresponding to each of the frequency modulations and outputting a correlation value corresponding to the received signal delayed by a common time in accordance with the instantaneous frequency of the modulated signal When,
An arithmetic unit for searching for a delayed signal having an amplitude peak at one predetermined position (predetermined delay amount) within one symbol among the delayed signals output from the BT matched filters;
A synchronization unit that synchronizes a clock to the modulation signal using a peak position of the delayed signal searched by the arithmetic unit;
A transmission unit that is unique to the preset wireless terminal based on the clock and that transmits the modulated signal after a delay time of 1 / B or more and within one symbol that is different from the delay time of another wireless terminal; ,
A communication system comprising: The modulation signal has a preamble portion that is CSS modulated with the spreading code corresponding to an arbitrary symbol;
When the delay signal of the preamble part does not have one amplitude peak in one symbol, the arithmetic part determines that the modulation signal is multiplexed shifted on the time axis,
When the modulation signal is multiplexed, the synchronization unit synchronizes the clock using the preamble portion of any one of the modulation signals,
The communication system according to claim 1, wherein the transmission unit transmits only the modulated signal with a clock synchronized. The modulated signal has a preamble portion that is CSS modulated by the frequency modulation corresponding to an arbitrary symbol;
The arithmetic unit shifts a center frequency of the modulation signal when a symbol assigned to the matched filter that outputs the delayed signal having one amplitude peak in one symbol of the preamble unit is not the arbitrary symbol. The communication system according to claim 1, wherein the communication system is determined. The modulated signal has a preamble portion that is CSS modulated by the frequency modulation corresponding to an arbitrary symbol;
The arithmetic unit determines that the center frequency of the modulation signal is shifted when the position of the peak of the delayed signal having one amplitude peak in one symbol of the preamble part is not in the predetermined position. The communication system according to claim 1 or 2. A chirp signal having a bandwidth B and a symbol length T is modulated using a signal modulated by CSS (Chirp Spread Spectrum) with a frequency modulation signal corresponding to BT symbols. A communication method for transmitting and receiving at
In the wireless terminal,
BT matched filter procedures having a complex conjugate frequency response corresponding to each of the spreading codes, and outputting a correlation value corresponding to the instantaneous frequency of the modulated signal delayed by a specific time common to each other;
An arithmetic procedure for searching for a delayed signal having one amplitude peak at a predetermined position within one symbol among the delayed signals output by the BT matched filters;
A synchronization procedure for synchronizing a clock to the modulation signal using a peak position of the delayed signal searched in the calculation procedure;
A transmission procedure that is specific to the wireless terminal set in advance based on the clock, and that transmits the modulated signal after a delay time of 1 / B or more and within one symbol that is different from the delay time of another wireless terminal; ,
A communication method characterized by: The modulation signal has a preamble portion that is CSS modulated with the spreading code corresponding to an arbitrary symbol;
In the calculation procedure, when the delayed signal of the preamble part does not have one amplitude peak in one symbol, it is determined that the modulated signal is multiplexed shifted on the time axis,
When the modulation signal is multiplexed in the synchronization procedure, the clock is synchronized using the preamble portion of any one of the modulation signals,
6. The communication method according to claim 5, wherein in the transmission procedure, only the modulated signal with a clock synchronized is transmitted. The modulated signal has a preamble portion that is CSS modulated by the frequency modulation corresponding to an arbitrary symbol;
If the symbol assigned to the matched filter that has output the delayed signal having one amplitude peak in one symbol of the preamble part is not the arbitrary symbol in the calculation procedure, the center frequency of the modulation signal is shifted. The communication method according to claim 5, wherein the communication method is determined. The modulated signal has a preamble portion that is CSS modulated by the frequency modulation corresponding to an arbitrary symbol;
In the calculation procedure, when the peak position of the delayed signal having one amplitude peak within one symbol of the preamble portion is not at the predetermined position, it is determined that the center frequency of the modulation signal is shifted. The communication system according to claim 5 or 6.