The present invention relates in general to the field of emission/reception systems of the impulse UWB (Ultra Wide Band) or IR-UWB (Impulse Response UWB) type. It has a use in particular in the field of positioning via IR-UWB beacons and more particularly for indoor positioning.
The technique of IR-UWB transmission is well known in the prior art, whether for telecommunications systems (for example under the IEEE 802.15.4a or IEEE 802.15.16 standard) or for geolocation systems, and the very short duration of the impulses allows a high positioning resolution to be obtained.
One of the factors limiting systems for UWB positionings, particularly present in an indoor environment, is the presence of multiple propagation paths, also called multipath components or MPCs.
The estimation of the position of a receiver via an IR-UWB geolocation system requires in general the presence of a minimum number of emitter beacons (3 in the plane and 4 in space, but sometimes less in the presence of known reflective walls), also called bases, the positions of which are known to the receiver. Each propagation channel between a base and the receiver can be multipath in such a way that the receiver can receive a significant number of components. In such a case, the receiver can simply discriminate and use the LOS components. However, the multipath components can also be used to estimate the position of the receiver. Indeed, the presence of MPCs makes it possible, in particular if a direct path disappears (NLOS) when the receiver moves, to continue to estimate the position of the receiver without substantial degradation of the estimation error. More generally, the diversity introduced by the multipath components can be used (greater number of observables) to estimate the position of a mobile receiver in a navigation method.
A method for indoor navigation of an IR-UWB receiver, assisted by the tracking of multipath components, is described in the article by J. Maceraudi et al. titled “Multipath components tracking adapted to integrated IR-UWB receivers for improved indoor navigation” published in Proc. of 24th European Signal Processing Conference (EUSIPCO), 2016, pp. 753-757. The tracking algorithm, however, uses a complex set of multiple-hypothesis Kalman filters (MHKFs).
A method for obtaining the line-of-sight (LOS) arrival time aided by the determination of multipath components in an IR-UWB geolocation system is described in the article of J. Maceraudi et al. titled “Multipath-aided direct path ToA reconstruction for integrated UWB receivers in generalized NLoS, Proc. of 86th Vehicular Technology Conference (VTC), September 2017.
According to this method, the fading of the MPCs (caused by the collisions/interference) is partly avoided via an operation of low-pass filtering (moving average) before their detection, applied to the temporal channel response acquired by the receiver. This prefiltering, however, has the disadvantage of eliminating a portion of the useful signal and introducing a latency into the channel estimation, the system having to wait a certain number of samples before being able to return a filtered value.
In another field of use, a receiver of an IR-UWB telecommunications system uses an integration of the signal received in time intervals in order to recover the symbols transmitted. Such a receiver, further capable of operating in a plurality of possible bands, is described in the patent application FR-A-2 996 969. In such a receiver, it is necessary to determine the positions of the time intervals in which the impulses are located. In the presence of a multipath channel, it can be useful to use a rake filter (RAKE) to sum the contributions of the multipath components, which implies being able to determine them and, if necessary, track them when the receiver moves and/or when the environment changes.
In all cases, whether the receiver is part of a geolocation system or an IR-UWB telecommunications system, the determination of the multipath components must avoid, as much as possible, the interference situations mentioned above.
The object of the present invention is therefore to propose a method for determining multipath components of a propagation channel in a geolocation system or in an IR-UWB telecommunications system that allows the various multipath components to be resolved simply, even when there is interference.
The present invention is defined by a method for determining multipath components of a propagation channel in an IR-UWB system comprising an emitter and a receiver, characterised in that said emitter emits a plurality of UWB impulses at a plurality (N) of distinct central frequencies and in that the receiver translates the response of the channel to each of these impulses into the baseband, integrates it over time intervals in order to provide a plurality of complex samples, combines the squared moduli of the complex samples corresponding to the same time of flight and to the various central frequencies in order to obtain a composite sample at each time of flight, the multipath components of the channel being determined from the composite samples in successive times of flight, exceeding a predetermined threshold.
According to a first embodiment, said plurality of impulses is emitted in the form of a frame of successive impulses, the squared moduli of the complex samples corresponding to the same central frequency and to the various successive times of flight being stored in a buffer before the combination step.
Advantageously, the duration of the frame is chosen as less than the coherence time of the channel.
Said receiver can be for example a double-quadrature multiband receiver.
According to one embodiment, said plurality of impulses at the various central frequencies is emitted simultaneously by the emitter.
The central frequencies are for example chosen as equal to fc1=3.5 GHz, fc2=4 GHz, fc3=4.5 GHz and the bandwidth of the impulses is chosen as equal to 500 MHz.
According to an example of use, said multipath components thus determined are used to estimate the position of the receiver.
The invention also relates to a receiver suitable for implementing the method for determining multipath components of a propagation channel according to the first embodiment, said receiver comprising:
Finally, the invention relates to a receiver suitable for implementing the method for determining multipath components of a propagation channel according to the second embodiment, said receiver comprising:
The IR-UWB receiver modules can be for example of the double-quadrature type.
Other features and advantages of the invention will be clear upon reading a preferred embodiment of the invention, made in reference to the appended drawings among which:
An IR-UWB system comprising at least one emitter Tx and a receiver Rx is considered below. This system can be a telecommunications system, a geolocation system as mentioned above, or even a simple system for measuring distance between the emitter and the receiver.
It is supposed that the emitter Tx is suitable for transmitting in a plurality of frequency bands, more precisely UWB impulses at a plurality of distinct central frequencies, or:
where Ae is the amplitude of the impulse emitter,
is the central frequency of the impulse, which can have a plurality of values fc1, . . . , fcN, tc is the time of emission corresponding to the centre of the impulse, φ is the phase upon emission, unknown and modelled as a random variable uniformly distributed over [0,2π[, TBW is a duration representative of the Gaussian envelope and inversely proportional to the width of the bandwidth BW. In order to distinguish the emissions of impulses at the various temporal instances k=1, . . . , K of the propagation channel, the notation se(k)(t,ωe) is used to indicate that the impulse was emitted at the instance k:
where tc(k) and φ(k) are, respectively, the time of emission and the phase upon emission related to the instance k of the channel.
If it is supposed that the propagation channel comprises a plurality P of multipath components (considered independently of k here in order to simplify the notation) and the same origin of the times of flight is taken (or equivalently, of the arrival times, the latter being deduced from the former via knowledge of the emission time), tc(k)=0 at each instance, the signal received by the receiver Rx for the instance k of the channel, is written as:
where αi(k), θi(k) and τi(k) are, respectively, the extinction coefficient and the phase shift introduced by the path associated with the MPC i of the channel temporal instance k.
In a multiband IR-UWB receiver like that described in the application FR-A-2 996 969, the combination of the four outputs of the double-quadrature receiver module allows the signal to be acquired in various sub-bands. For a given sub-band, the signal is decomposed over an orthogonal basis consisting of two sinusoids offset by 90° at a central frequency, then integrated over a time interval
having a width W centred on the sampling time ts, in order to provide a sample of intensity:
where bin(k)(ts,ωc) represents the sample of intensity obtained via integration over the time interval Wt
If the case of interference between two multipath components of the channel (for example two indirect paths and absence of a direct path) is now examined, the sample at the output of the receiver bin(k)(ts,ωc) can be expressed in the following form:
bin(k)(ts,ωc)=(A1(k))2+(A2(k))2+I(k)(ts,ωc) (5-1)
with
and where I(k) (ts,ωc) is an interference term defined by:
I(k)(ts,ωt)=2A1(k)A2(k) cos(ωc(τ1(k)−τ2(k))−(θ1(k)−θ2(k))) (5-2)
It is clear that in the expression (5-1), the terms (A1(k))2 and (A2(k))2 are the respective contributions of the first and of the second multipath component in the absence of interference and that the interference term I(k)(ts,ωc) varies sinusoidally according to the difference in time of flight between the two paths of the channel.
The abscissae represent the successive positions of the receiver (channel instances) over time and the ordinates represent the sampling times ts (in other words, the delays in the channel response). It was supposed that the times of flight τi(k), i=1, 2, are linearly dependent on the position of the receiver (case of a receiver moving with a constant velocity). The impulses have a central frequency of 4 GHz and a bandwidth of 500 MHz. The receiver uses integration intervals having a width of 2 ns, offset by ins. It is noted that in
The idea on which the invention is based is to use IR-UWB impulses at various central frequencies and to carry out, for the same channel temporal instance, a combination of the samples relating (to the same time of flight and) to various central frequencies. Indeed, it is understood that, for a given channel temporal instance, the situation of interference between two MPCs only affects a single central frequency. In other words, instead of considering a channel temporal instance at a given central frequency, the responses of this channel are combined at the various frequencies in order to be freed from the interference.
More precisely, the receiver Rx, 300, comprises an antenna 310 and a multiband IR-UWB receiver module, 320. The receiver module 320 has a double-quadrature architecture and is advantageously configured to be able to operate in a plurality of sub-bands, as described in the application FR-A-2 996 969. In the present case, the module 320 is configured in order to be synchronised with the IR-UWB impulses at various central frequencies fc1, . . . , fcN and translate them into baseband.
It is supposed in this first embodiment that for each channel temporal instance k, the emitter Tx emits a sequence of N impulses respectively centred on the aforementioned central frequencies. The emitted signal comprises at least one frame consisting of N intervals having a duration T, an IR-UWB impulse having a central frequency fcN being emitted during the ith interval. In correlation, the receiver module 320 is centred on fc1 during a first reception interval, on fc2 during a second reception interval and so on until the Nth reception interval. It is also supposed hereinafter that NT<Tcoh, that is to say, the frame length is less than the coherence time of the channel. The frame can be itself repeat at the repetition frequency 1/NT.
At each integration interval, the complex samples (dI,dQ) at the output of the multiband receiver module 320 are subjected to a calculation of the squared modulus, 325. The intensity samples thus obtained at the output of 325 are none other than bin(k)(ts,2πfcn),
where fcn is the central frequency of the selected sub-band in the receiver, and ts=sW,
are successive times of flight from the emitter.
These samples are demultiplexed by the demultiplexer 330 into the buffers 3401, . . . ,340n. More precisely, the samples obtained during a reception interval corresponding to the central frequency fcn are stored in a corresponding buffer, 340n. Thus, after reception of a frame during the instance k of the channels at the various frequencies, the buffer 340n contains the samples bin(k)(sW,2πfcn),
corresponding to the successive times of flight bin(k)(ts,2πfcn),
The samples relating to the same time of flight and to the various central frequencies are then combined in a combination module 350 to give a composite sample:
where λn, n=1, . . . , N are predetermined weighting coefficients. For example,
can be taken in order to calculate a simple average or λn∝SNRn(ts) where SNRn(ts) is the signal-to-noise ratio at the frequency fcn and at the time ts.
By carrying out the combination of the samples bin(k)(ts,2πfcn) according to the expression (6) for each time of flight ts=sW,
the various MPCs components are brought to light, even if some of them disappear at certain frequencies for certain channel instances. More precisely, the comparator 360 detects the MPCs components from the composite samples bin(k)(ts), ts=sW,
exceeding a predetermined threshold, Th.
This receiver module is described in detail in the application FR-A-2 966 969 incorporated here by reference.
The architecture of this receiver module is recalled below.
The receiver module comprises, at the input, a low noise amplifier, 410, followed by a first stage of frequency translation 420. This first stage comprises a first quadrature mixer 421, the signals of which that are offset by 90° are generated by a first local oscillator LO1. This first stage, which can be shunted, allows the signal to be translated into the baseband or offset to an intermediate frequency. The signals that are in phase and offset by 90°, sI,sQ, are filtered via the low-pass filters 422. The signals thus filtered pass into a second quadrature stage 430, comprising a second quadrature mixer 432 on the in-phase path and a third quadrature mix 433 on the quadrature path. The signals offset by 90° are generated by a second local oscillator LO2. The in-phase and quadrature outputs of the second quadrature mixer, sII,sIQ, are filtered by low-pass filters 436. Likewise, the in-phase and quadrature outputs of the third quadrature mixer, sQI,sQQ, are filtered by low-pass filters 436. The signals at the output of the second stage (which can also be shunted) are integrated over successive time intervals W having a width W in the integrators 441. The results of integration for the various paths are then digitised in the analogue/digital converters 443 and combined in the combination stage 450, in order to provide complex samples (dI,dQ):
where the elements of the matrix
are chosen from {−1,0,+1} according to the desired sub-band and rII,rIQ,rQI,rQQ are the results of integration respectively provided by the paths II,IQ,QI,QQ. The sets of combination coefficients (elements of the matrix E) are provided to the combination stage 450 at the frequency 1/T, the sequence of the sets of coefficients having been previously synchronised with the sequence of central frequencies, fc1, . . . , fcN (for example via a pilot sequence).
It is supposed in this second embodiment that for each channel temporal instance k, the emitter Tx emits N impulses in parallel respectively centred on the central frequencies fc1, . . . , fcN. This emission in parallel can be repeated during the same channel instance at a frequency greater than 1/Tcoh.
The signal received by the antenna 510 is provided to a plurality of N IR-UWB receivers, 5201, . . . , 520N, respectively operating at the central frequencies fc1, . . . , fcN, for example double-quadrature receivers as described in the application EP-A-1 580 901, the first stage of these receivers respectively translating the N impulses into a baseband. Advantageously, the low noise amplifier (LNA) upstream of the first stage of these receivers can be shared by them.
At each integration interval having an index s, the complex samples (dI,dQ) at the output of the multiband receiver module 520n, are subjected to a calculation of the squared modulus in the quadratic module 525n. The samples thus obtained at the output of 525n are none other than bin(k)(ts,2fcn),
where ts=sW,
are successive times of flight from the emitter.
The samples bin(k)(ts,2πfcn) related to the same sampling time ts and to the various central frequencies fc1, . . . , fcN are combined via a combination module 550, identical to the combination module 350 described above, carrying out a combination according to the expression (6) with the same variants. The result of the combination is a sequence of composite samples bin(k) (ts), sW,
Finally, the comparator 560 detects the MPCs components from the composite samples exceeding a predetermined threshold, Th.
The combination module 350 of the receiver then calculates the average:
It is noted than in
Number | Date | Country | Kind |
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17 59611 | Oct 2017 | FR | national |
Number | Name | Date | Kind |
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20030058971 | Langford | Mar 2003 | A1 |
20080042845 | Richards | Feb 2008 | A1 |
Number | Date | Country |
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1 580 901 | Sep 2005 | EP |
2 996 969 | Apr 2014 | FR |
WO 2008156909 | Dec 2008 | WO |
WO 2015055522 | Apr 2015 | WO |
Entry |
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French Preliminary Search Report dated Jun. 19, 2018 in French Application 17 59611, filed on Oct. 13, 2017 (with English Translation of Categories of cited documents). |
Maceraudi, J., et al. “Multipath Components Tracking Adapted to Integrated IR-UWB Receivers for Improved Indoor Navigation”, 2016 24th European Signal Processing Conference (EUSIPCO), 2016, 5 pages. |
Maceraudi, J., et al. “Multipath-Aided Direct Path ToA Reconstruction for Integrated UWB Receivers in Generalized NLoS”, 2017 IEEE 86th Vehicular Technology Conference (VTC-Fall), 2017, 5 pages. |
Number | Date | Country | |
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20190113599 A1 | Apr 2019 | US |