Data transmission method, corresponding transmitter and receiver, data transmission system and computer program product

Abstract

Data transmission method, corresponding transmitter and receiver, data transmission system and computer program product. The invention relates to a data transmission method using chirp-type spread spectrum modulation between a transmitter (3a, 3b, 3c, 3d) and a receiver (2), the method using frames comprising an alignment section portion of at least 2*M-1 chirps, and a payload portion composed of groups of M chirps. The invention also relates to a transmitter (3a, 3b, 3c, 3d), a receiver (2), a communication system (1) and a computer program product implementing the method. Figure to be published with the abstract: Figure 9

Description

Title of the invention: Data transmission method, corresponding transmitter and receiver, data transmission system and computer program product

[0001] The present invention relates to the field of communications and more particularly to a data transmission method, a corresponding transmitter and receiver, a data transmission system and a computer program product.

[0002] The LoRaWAN protocol is a radio communication protocol for the Internet of Things which uses a proprietary chirp spread spectrum modulation technique (CSS acronym) called LoRa and which defines how terminal equipment communicates wirelessly through gateways, thus constituting a low-power wide area network (LPWAN).

[0003] One of the strengths of the CSS-type spread spectrum modulation of the LoRa protocol is that transmissions at different data rates, corresponding to different spreading factors (SF), are almost independent of each other. This quality is only beneficial insofar as the communication range at low spreading factors allows for the coverage of a significant number of transmitters.

[0004] LoRa modulation exhibits a high spreading factor and therefore carries few bits per Hz of occupied bandwidth. Based on this observation, it is possible to imagine various modifications to CSS to carry more information per symbol.

[0005] With the modulation implemented in the data transmission method of the present invention, the aim is to design a modulation with lower spectral efficiency, based on a given spreading factor. While this proposal is of little interest for increasing short-range network capacity, it considerably increases the flexibility with which radio resources, particularly through the use of quasi-orthogonal modulations, are allocated around and at a distance from the gateway. At short range, using spreading factors (SF) smaller than SF7, which is generally used in the LoRa protocol, has an effect equivalent to increasing the number of bits per symbol, which is why the invention aims to achieve flexibility in the opposite direction.

[0006] By grouping chirp symbols two by two or four by four etc. to form new symbols corresponding to M (two or four etc.) times more time samples encoding 1 (or 2 etc.) additional bits, it is possible to construct new physical modulations quasi-orthogonal to each other in Adjusting radio resource allocation more flexibly on demand. This new use of CSS requires adding a physical preamble to align the receiver with the transmitter, without, however, requiring any modification to the radio hardware. Consequently, when numerous transmitters are located far from the gateway, the gains in aggregation capacity are very significant.

[0007] The present invention therefore relates to a data transmission method using a spread spectrum modulation of the chirp type, CSS, over a bandwidth BW, between a transmitter and a receiver separated by a distance d, the data transmission method using a plurality of distinct data rates D, i being a natural number greater than or equal to 7, with for each i >= 7, Di«2*Di+i, characterized in that the data transmission method uses frames comprising at least a preamble part of X chirps, X being a natural number greater than or equal to 1, an alignment section part of at least 2*M-1 chirps, and a payload part composed of groups of M chirps, with M=2q and q being a natural number greater than or equal to 1, such that each data rate D;corresponds to the use of CSS with the spreading factor SFi+q, a process in which if the distance d is such that the data cannot be transmitted between the transmitter and the receiver at a rate D; then the transmitter transmits the data to the receiver at a rate Dp, with p > i, the 2*M-1 chirps in the alignment section part being stepped in the bandwidth BW with respective starting frequencies shifted by n*(BW) / M, with n belonging to ne [«Q, n{) + 2M-1 ^modM^ with n0 an integer, the time representation PAn(t) of each chirp among the 2*M-1 chirps of the alignment section part being given by: ;

[0008] [Math.l]

[0009] where n corresponds to the ordering of the chirp among the 2*M-1 chirps in the alignment section part, t is the time, f0 is the base frequency used, a = BW2 / 2', j is the imaginary unit,

[0010] the chirps of the alignment section being demodulated at the receiver by M complex chirps conjugate with respect to the alignment section used by the transmitter, stepped in the BW bandwidth with respective starting frequencies shifted by n*(BW) / M, with n belonging to « e [[«q, h0+ 2M- V^modM, the demodulated signal for the m-th chirp during alignment for the receiver shifted by £ e K0, M -1 ^chirps being given by:

[0011] [Math.2]

[0012] It is thus possible, without changing the hardware (transmitters, receivers, gateways) to use the infrastructure of an existing LoRa network to allow remote transmitters to communicate with gateways, where an existing LoRa network would not have allowed the use of so many quasi-orthogonal modulations due to the excessive distance separating the transmitter from the receiver or gateway.

[0013] Indeed, unlike conventional LoRa CSS modulation, in which, when transmitters are far from the receiver, only a portion of the signal strengths (the highest ones) are usable and the benefit of having various quasi-orthogonal modulations is lost, with SFi*M = SF(i+log2(M)) = SF(i+q) (i.e., the data transmission method according to the present invention), it is possible to have several modulations at a given robustness level. With SFi*M, it is possible to create as many virtual channels as there are available signal strengths, regardless of the required robustness level. More generally, SFi*M allows radio resources to be adapted as closely as possible to the observed demand around the gateway.

[0014] Each SFi*M symbol consists of 2M SFi robustness chirps. Thus, an SF7*2 modulation is equivalent to an SF8 transmission: the SF7*2 symbol carries 8 bits and corresponds to 256 time samples. Transmissions with SF7*2 are almost orthogonal to those with SF8, just as are those with SF7.

[0015] The present invention thus makes it possible to obtain a more robust CSS modulation by grouping chirps, and makes it possible to use quasi-orthogonal modulation at higher distances between transmitters and receivers.

[0016] According to one embodiment, X is equal to 10*M. The invention is not limited in this respect, however, and a person skilled in the art will understand that any number of consecutive identical chirps enabling the receiver or gateway to detect a frame preamble is conceivable according to the present invention.

[0017] According to one embodiment, n0=0.

[0018] According to one embodiment, the framework further comprises, after the preamble part, A synchronization word portion of 2*M chirps. This synchronization word portion ensures better synchronization of the receiver or gateway.

[0019] According to one embodiment, the frame further comprises, between the preamble part and the alignment section part, and where applicable between the synchronization word part and the alignment section part, a part comprising at least 2*M chirps Conjugate complexes, preferably 2.25*M conjugate complex chirps for frequency synchronization.

[0020] The invention also relates to a data transmitter in the ISM band, characterized in that it comprises circuit elements configured to implement the data transmission method as defined above.

[0021] According to one embodiment, the circuit elements are a radio transmitter / receiver associated with at least one of a processor, a microprocessor, a digital signal processor (DSP), a microcontroller, a field-programmable gate array (FPGA), an application-specific integrated circuit (ASIC), optionally associated with memory.

[0022] The invention also relates to a data receiver in the ISM band, characterized in that it comprises circuit elements configured to implement the data transmission method as defined above.

[0023] According to one embodiment, the circuit elements are a radio transmitter / receiver associated with at least one of a processor, a microprocessor, a digital signal processor (DSP), a microcontroller, a field-programmable gate array (FPGA), an application-specific integrated circuit (ASIC), optionally associated with memory.

[0024] The invention also relates to a communication system, characterized by the fact that it comprises at least one transmitter as defined above and at least one receiver as defined above implementing the data transmission method as defined above.

[0025] According to one embodiment, at least one receiver is configured to receive at least one frame from at least one transmitter, and to configure at least one transmitter to transmit according to a spreading factor SF; given to guarantee the reception by at least one receiver of any frame sent by at least one transmitter.

[0026] The invention also relates to a computer program product characterized by the fact that it includes instructions which, when executed by a computer, implement the process as defined above.

[0027] To better illustrate the object of the present invention, an embodiment will now be described in detail, with reference to the attached drawings.

[0028] On these drawings:

[0029] [Fig.1] represents a basic symbol SFi*M with M = 2.

[0030] [Fig.2] represents a LoRa frame with physical layer headers.

[0031] [Fig.3] represents a frame of the data transmission process according to the present invention with physical layer headers.

[0032] [Fig.4] represents the target reception rate (PDR) of 60%, for a density of 4 nodes / km2 for a gateway, for LoRa (a) and the transmission method of the invention (b).

[0033] [Fig.5] represents the target reception rate (PDR) of 60%, for a density of 4 nodes / km2 for two gateways, for LoRa (a) and the transmission method of the invention (b).

[0034] [Fig.6] represents the synchronization procedure of the transmission process SFi*M data of the invention.

[0035] [Fig.7] represents the alignment step at the receiver for a modulation SFi*M, with M=2.

[0036] [Fig.8] represents the alignment step at the receiver for a modulation SFi*M, with M=4.

[0037] [Fig.9] represents a communication system implementing the process of data transmission of the invention.

[0038] Since the method of the invention is intended to be used with existing LoRa equipment, it will be described in connection with the known LoRa protocol.

[0039] In the existing LoRa protocol, each symbol is modulated according to a spreading factor SFi, with i between 7 and 12.

[0040] The protocol of the present invention is equivalent to using the LoRa protocol with an SFi*M symbol, each SFi*M symbol being the combination of M chirps with a spreading factor of SFi. Like the LoRa protocol, the data transmission protocol of the invention uses the ISM (industrial, scientific, and medical) frequency bands, defined for Europe by standard EN 55011.

[0041] As illustrated in [Fig. 1], which represents a basic SFi*M symbol with M = 2, composed of two identical frequency ramps, for a bandwidth BW. The symbol is followed by another, carrying different data; for i = 7 and M = 2, for example, each SFi*M symbol corresponds to 256 samples. The dashed lines represent the two SFi*M symbols used for demodulation.

[0042] The symbol lasts M * 21 time samples at the sampling frequency BW and allows the encoding of i+log2(M) = (i+q) bits (whereas M chirps normally carry M * i bits). Indeed, after multiplication at the receiver by M chirps (conjugate complexes of those used for transmission), sampling and then FFT produce M*2' samples in the frequency domain, among which only one peak is expected. The modulation and demodulation steps are analogous to those in SF(i+log2(M)) = SF(i+q), when M = 2q.

[0043] LoRa reception takes place in several phases: first the preamble, consisting of a certain number of identical basic chirps, is detected by the receiver. Next, after receiving the synchronization word, the transmission of two inverted chirps allows us to estimate the frequency offset between the transmitter and the receiver, which may be due to the Doppler effect or imperfections in the oscillators.

[0044] A classic LoRa frame is thus represented in [Fig. 2], and comprises a 10-chirp preamble, a two-chirp synchronization word, a 2.25-chirp frequency alignment, and a payload. A cyclic redundancy check (CRC) control (not shown) may also be included at the end of the payload.

[0045] By contrast, the data frame of the data transmission protocol according to the present invention is shown in [Fig. 3], and comprises, for an SFi*M type frame, a preamble portion of 10*M chirps, a synchronization word portion of 2*M chirps, a frequency calibration portion of 2.25*M chirps, and a payload portion. A control portion (not shown) of the cyclic redundancy check (CRC) type may also be provided at the end of the payload portion. It should be noted that the frequency calibration portion and the synchronization word portion are optional.

[0046] In order to be able to align the M identical symbols transmitted according to SFi*M instead of a single symbol transmitted according to SFi, an alignment section part of at least 2*M-1 chirps is added before the payload part in the present invention.

[0047] The idea with SFi*M is to maintain the same steps, with the same number of SFi*M symbols as LoRa chirps. Reception of the preamble, the synchronization word, and the inverted frequency-lock symbols will therefore be similar, with the same robustness and the same overhead. However, a difficulty arises: at this stage, the receiver has detected the chirp boundary but may have synchronized to any one of the chirps in the SFi*M symbol.

[0048] The 2*M-1 chirps of the alignment section part are therefore staggered in the BW bandwidth with respective starting frequencies shifted by n*(BW) / M, with n belonging to ne [«q, nQ + 2M- Y^modM, with n0 an integer, the time representation PAn(t) of each chirp among the 2*M-1 chirps of the alignment section part being given by:

[0049] [Math.3]

[0050] where n corresponds to the ordering of the chirp among the 2*M-1 chirps in the alignment section part, t is the time, f0 is the base frequency used, a = BW2 / 2', j is the imaginary unit,

[0051] the chirps of the alignment section portion being demodulated at the receiver by M complex chirps conjugated with respect to the alignment section portion used by the transmitter, stepped in the BW bandwidth with respective starting frequencies shifted by n*(BW) / M, with n belonging to ng [«g, n0+ 2M- IJmodM, the demodulated signal for the m-th chirp during alignment for the receiver shifted by ks [[0, M -1 Jchirps given by:

[0052] [Math.4]

[0053] Thus, all the chirps in the alignment phase produce the same frequency peak, which depends only on

[0054] of k.

[0055] It can be seen that with the data transmission method according to the invention, the reception of an SFi*M symbol is analogous to that of a chirp with spreading factor SF(i+log2 (M)) = SF(i+q), with a significant distance between the possible values, and this step must not introduce a weak point in the reception process.

[0056] When the density of emitters is low or their distribution is inhomogeneous and they are rather

[0057] away from the gateway, the use of SFi*M leads to a very significant increase in capacity.

[0058] Indeed, if all the nodes could only be covered with SF12 or equivalent, the use of SF7x32,

[0059] SF8xl6, SF9x8, SF10x4, SF11x2 and SF12 lead to an increase in capacity of approximately 500% (exactly 500% without the surcharge due to the alignment section). Furthermore, there is nothing preventing the use of i values ​​lower than 7.

[0060] Of course, in reality, the gain is more moderate, unless using i<7. One way to estimate it is to consider a mathematical model of the channel and access, with physical capture during collisions.

[0061] Figures 4 and 5 respectively represent the target reception rate (PDR) of 60%, for a density of 4 nodes / km² for LoRa (a) and for the transmission method of the invention (b) for one gateway or for two LoRa gateways. The load is the frame generation intensity multiplied by their duration, the dashed curve is the frame reception rate, and the unbroken line represents the collision-free success rate.

[0062] Thus, for example, for 4 nodes / km2 and a target PDR (reception rate) of 60% (which allows 90% of application layer data to be received with just three repetitions of each packet), LoRa can cover 505 nodes, with a range of 6.3 km; with SFi*M, the gateway collects for 642 nodes (+27%) at a range of

[0063] 7.1 km excluding the SF12 zone (part (a) of [Fig. 4]). The parameters used For SFi*M, the following are listed here: SF7x4,

[0064] SF8x4, SF9x4, SF10x4, SF11x2, SF12. In all cases, here, the SF12 zone is assumed to have infinite range, and is therefore saturated, so that the Packet Delivery Ratio (PDR) in this zone is zero. The boundaries between the different SF spreading factors are constructed by bisection moving away from the center, and the choice of M values ​​is ad hoc.

[0065] With two gateways (GW) placed close to each other, receive diversity allows two chances to receive each frame, due to the re-collection of the multipath channel gain. For any network at capacity limits, and especially with a multipath channel, the benefits of having this elementary form of diversity are well known (specifically, it explains the difference between parts (a) of the

[0066] Figures 4 and 5). In an LPWAN, this diversity allows for the reception of traffic from distant nodes or, in the event of a collision, for the reception of each frame due to different physical capture conditions on each antenna. The results are shown in [Fig. 5]: the gain is 37% more nodes covered with the SFi*M protocol of the invention. Here, the parameters used are SF7x8, SF8x8, SF9x8, SF10x4, SF11x2, and SF12. The capacity is more than doubled between LoRa with a single gateway and SFi*M with diversity. It can be noted that with SFi*M, the balance between losses due to collisions and attenuation is approximately the same throughout the cell. The load (represented by the thin dashed line) is much more homogeneous than for LoRa, where areas near the gateway are much less loaded than the periphery.

[0067] Figure 6 represents the synchronization procedure at a receiver for SFi*M modulation. The dashed lines represent the SFixM demodulation symbols (with M = 2) used by receivers Rec. 1 and Rec. 2, which are offset from each other in time by a chirp duration of SFi.

[0068] At this stage, there is an ambiguity regarding the chirp of the SFixM symbol to which the receiver is aligned: in [Fig. 6], at steps A or B, receivers Recep. 1 and Recep. 2 are in exactly the same state, but if they used the same time offset for symbol alignment and data reception started immediately, receiver Recep. 2 would suffer from inter-symbol interference at step C. Therefore, an additional step in the reception process is needed to find the boundary of the SFixM symbol after the frequency correction step, instead of the first data symbols in the normal LoRa system.

[0069] To resolve the ambiguity, the method of the invention proposes adding to the preamble an alignment step in which, exceptionally, the chirps are grouped into M identical chirps. Consider [Fig. 7]: for M = 2, the alignment step would consist of a base chirp and a chirp shifted by BW / 2 (where BW is the channel bandwidth), followed by a base chirp. Using the corresponding chirp pairs at the receiver for demodulation produces a pair of spectral rays in the same position for both chirps, the position of which depends on its initial time shift. It should be noted that the receiver can either perform a pair of chirp-length FFT calculations for this step (as in Figures 7 and 8), or a single FFT with a length of SFixM, since the desired symbol is constant over M chirps.

[0070] From the perspective of a LoRa receiver, SFi*M modulations are analogous to LoRa transmissions with the SFi spreading spectrum. Consequently, the near-orthogonality between the different data rates is maintained provided that chirps with the same frequency slope are used only once near the gateway.

[0071] In practice, a given gateway can receive both LoRa and SFi*M traffic: the preamble is similar, although more extensive for SFi*M. It is possible to distinguish one modulation from the other by using different synchronization words. Thus, the two modulations can coexist, even if this is not necessarily desirable from a resource allocation perspective.

[0072] To use SFi*M, the gateway hardware does not need to be modified; there are no new functionality requirements. However, the Adaptive Data Rate (ADR) must handle the additional M parameter.

[0073] The data transmission protocol of the invention, designated SFi*M, is a relatively direct extension of LoRa modulations, which allows for the distribution of radio resources, here in the sense of quasi-orthogonal codes, with greater flexibility. In cases where the nodes are far apart and conventional modulations are of little use, SFi*M can prove very useful. In denser networks, other dimensions remain to be explored, such as using smaller signal drivers (if necessary with SFi*M).

[0074] Fig. 9 represents a data transmission system 1 according to the invention, implementing the data transmission method according to the invention.

[0075] The data transmission system 1 comprises a receiver 2 and several transmitters 3a, 3b, 3c, 3d.

[0076] Receiver 2 can be any existing LoRaWAN network receiver or even an existing LoRaWAN gateway; the advantage of the data transmission method is the invention being that it is possible to use the equipment of an existing LoRaWAN network using the LoRa protocol.

[0077] Similarly, each transmitter 3a, 3b, 3c, 3d can be an existing LoRaWAN network transmitter.

[0078] Although only one receiver 2 is shown in [Fig. 9], it is understood that the invention is not limited in this respect and that a data transmission system 1 according to the invention implementing the data transmission method of the invention can use any number of receivers with any number of transmitters, the receivers 2 being able to be of the same type or of different types, and the transmitters 3a, 3b, 3c, 3d being able to be of the same type or of different types, the transmitters 3a, 3b, 3c, 3d being able to be at the same distance or at different distances from the receiver(s) 2. In [Fig. 9], the transmitters 3a, 3b, 3c, 3d are at different distances from the receiver 2.

[0079] The method of the invention is implemented in the receiver(s) 2 and the transmitter(s) 3a, 3b, 3c, 3d in a software manner, the program being integrated into at least one of a processor, a microprocessor, a digital signal processor (DSP), a microcontroller, a field-programmable gate array (FPGA), an application-specific integrated circuit (ASIC), optionally associated with memory integrated into the receiver(s) 2 and the transmitter(s) 3a, 3b, 3c, 3d.

[0080] The receiver(s) 2 and the transmitter(s) 3a, 3b, 3c, 3d communicate bidirectionally.

[0081] In practice, as in the LoRa protocol, each transmitter 3a, 3b, 3c, 3d transmits a frame to the receiver 2. When the receiver 2 receives the frame from a given transmitter 3a, 3b, 3c, 3d, it indicates to the transmitter 3a, 3b, 3c, 3d in question according to which SFi*M protocol it must transmit the frame to guarantee good reception by the receiver 2.

[0082] It is understood that the particular embodiment just described has been given by way of example and not limitation, and that modifications may be made without departing from the present invention.

Claims

1. Demands A data transmission method using chirp spread spectrum modulation, CSS, over a bandwidth BW, between a transmitter (3a, 3b, 3c, 3d) and a receiver (2) separated by a distance d, the data transmission method using a plurality of distinct data rates D; i being a natural number greater than or equal to 7, with for each i >= 7, D; «2*Di+i, characterized in that the data transmission method uses frames comprising at least a preamble portion of X chirps, X being a natural number greater than or equal to 1, an alignment section portion of at least 2*M-1 chirps, and a payload portion composed of groups of M chirps, with M=2q and q being a natural number greater than or equal to 1, such that each data rate D;corresponds to the use of CSS with the spreading factor SFi+q, a process in which if the distance d is such that the data cannot be transmitted between the transmitter (3a, 3b, 3c, 3d) and the receiver (2) at the rate D; then the transmitter (3a, 3b, 3c, 3d) transmits the data to the receiver at a rate Dp, with p > i, the 2*M-1 chirps in the alignment section part being stepped in the bandwidth BW with respective starting frequencies offset by n*(BW) / M, with n belonging to n0+2M- with n0 an integer,; the temporal representation PAn(t) of each chirp among the 2*M-1 chirps of the alignment section part being given by: [Math.5] where n corresponds to the chirp ordering among the 2*M-1 chirps in the alignment section part, t is time, f0 is the base frequency used, a = BW2 / 2', j is the imaginary unit, the chirps in the alignment section part being demodulated at the receiver (2) by M complex chirps conjugate with respect to the alignment section part used by the transmitter (3a, 3b, 3c, 3d), stepped in the BW bandwidth with respective starting frequencies shifted by n*(BW) / M, with n belonging to n₀ + 2M⁻¹ ] modM. The demodulated signal for the m-th chirp during alignment for the receptor (2) shifted by ke [[ 0, M- IJchirps being given by: [Math.6]

2. X¢: Data transmission method according to claim 1, characterized in that X is equal to 10*M.

3. Data transmission method according to claim 1 or claim 2, characterized in that no=O.

4. Data transmission method according to any one of claims 1 to 3, characterized in that the frame further comprises, after the preamble part, a synchronization word part of 2*M chirps.

5. Data transmission method according to any one of claims 1 to 4, characterized in that the frame further comprises, between the preamble part and the alignment section part, where appropriate between the synchronization word part and the alignment section part, a part comprising at least 2*M conjugate complex chirps, preferably 2.25*M conjugate complex chirps.

6. Data transmitter (3a, 3b, 3c, 3d) in the ISM band, characterized in that it comprises circuit elements configured to implement the data transmission method according to any one of claims 1 to 5.

7. Data transmitter (3a, 3b, 3c, 3d) according to claim 6, characterized in that the circuit elements are a radio transmitter / receiver associated with at least one of a processor, a microprocessor, a digital signal processor, DSP, a microcontroller, a programmable pre-diffused array, FPGA, an application-specific integrated circuit, ASIC, optionally associated with memory.

8. Data receiver (2) in the ISM band, characterized in that it comprises circuit elements configured to implement the data transmission method according to any one of claims 1 to 5.

9. Data receiver (2) according to claim 8 characterized in that the circuit elements are a radio transmitter / receiver associated with at least one of a processor, a microprocessor, a digital signal processor (DSP), a microcontroller, a field-programmable gate array (FPGA), an application-specific integrated circuit (ASIC), optionally associated with memory.

10. Communication system (1), characterized in that it comprises at least one transmitter (3a, 3b, 3c, 3d) according to any one of claims 6 and 7 and at least one receiver (2) according to any one of claims 8 and 9 implementing the data transmission method according to any one of claims 1 to 5.

11. Communication system (1) according to claim 10, characterized in that at least one receiver (2) is configured to receive at least one frame from at least one transmitter, and to configure at least one transmitter (3a, 3b, 3c, 3d) to transmit according to a spreading factor SF; given to guarantee the reception by at least one receiver (2) of any frame sent by at least one transmitter (3a, 3b, 3c, 3d).

12. Product computer program characterized in that it includes instructions which, when executed by a computer, implement the method according to any one of claims 1 to 5.

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