Method for determining multipath components of a UWB impulse channel

Abstract

A method for determining the multipath components of a propagation channel in a geolocation system or an IR-UWB telecommunications system. The IR-UWB emitter emits a plurality of UWB impulses at a plurality of central frequencies, sequentially or in parallel. The receiver translates the response of the channel to each of these impulses into the baseband, integrates it over a plurality of time intervals in order to provide intensity samples related to successive times of flight. The intensity samples related to the same time of flight and to the various frequencies are combined in order to provide a composite sample at the output of a multiband IR-UWB receiver module. The multipath components are determined from the composite samples exceeding a predetermined threshold value.

Claims

1. A method for determining multipath components of a propagation channel in an Impulse Radio Ultra-Wideband (IR-UWB) system comprising an emitter and a receiver, wherein said emitter emits a plurality of UWB impulses at a plurality of distinct central frequencies and wherein the receiver translates a response of the channel to each of these impulses into baseband, integrates it over time intervals in order to provide a plurality of complex samples, combines squared moduli of the complex samples corresponding to a same time of flight and to the 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.

2. The method for determining multipath components of a propagation channel in an IR-UWB system according to claim 1, wherein said plurality of UWB impulses is emitted in a form of a frame of successive impulses, the squared moduli of the complex samples corresponding to a same central frequency and to various successive times of flight being stored in a buffer before the combination of the squared moduli of the complex samples.

3. The method for determining multipath components of a propagation channel in an IR-UWB system according to claim 1, wherein duration of the frame is chosen as less than a coherence time of the channel.

4. The method for determining multipath components of a propagation channel in an IR-UWB system, according to claim 1, wherein said receiver is a double-quadrature multiband receiver.

5. The method for determining multipath components of a propagation channel in an IR-UWB system according to claim 1, wherein said plurality of UWB impulses at the central frequencies is emitted simultaneously by the emitter.

6. The method for determining multipath components of a propagation channel in an IR-UWB system, according to claim 1, wherein the central frequencies are f.sub.c.sup.1=3.5 GHz, f.sub.c.sup.2=4 GHz, f.sub.c.sup.3=4.5 GHz and a bandwidth of the impulses is 500 MHz.

7. The method for determining multipath components of a propagation channel in an IR-UWB system, according to claim 2, wherein said multipath components thus determined are used to estimate a position of the receiver.

8. A receiver suitable for implementing the method for determining multipath components of a propagation channel according to claim 1, comprising: a multiband IR-UWB receiving circuit configured to receive the response of said channel to a frame of successive UWB impulses emitted at a plurality of distinct central frequencies, to translate the response of said channel to each of the impulses into the baseband, to integrate a signal thus translated over successive time intervals in order to provide a plurality of complex samples corresponding to successive times of flight; a calculation circuit configured to calculate the squared moduli of the complex samples thus obtained; a combiner configured to combine the complex samples corresponding to the same time of flight and to the frequencies in order to provide a composite sample for said time of flight; a comparison circuit configured to determine the multipath components from the composite samples at the successive times of flight exceeding a predetermined threshold.

9. The receiver suitable for implementing the method for determining multipath components of a propagation channel according to claim 8, comprising: a plurality of IR-UWB receiving circuits in parallel, each IR-UWB receiving circuit being configured to translate into the baseband, the response of said channel at a central frequency out of a plurality of distinct central frequencies, to integrate a signal thus translated over successive time intervals in order to provide a plurality of complex samples corresponding to successive times of flight; a plurality of calculating circuits configured to calculate the squared moduli of the complex samples respectively provided by the receiving circuits; a combiner configured to combine the complex samples corresponding to the same time of flight and to the distinct central frequencies in order to provide a composite sample for said time of flight; a comparison circuit configured to determine the multipath components from the composite samples at the successive times of flight exceeding a predetermined threshold.

10. The receiver suitable for implementing the method for determining multipath components of a propagation channel according to claim 9, wherein the IR-UWB receiving circuits are of double-quadrature type.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) 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:

(2) FIG. 1A, already described, shows a diagram of a multipath propagation channel between an emitter and an IR-UWB receiver;

(3) FIG. 1B, already described, shows the signal received by the receiver of FIG. 1A for various positions of the latter;

(4) FIG. 2 shows a case of interference between two multipath components at an IR-UWB receiver;

(5) FIG. 3 shows a diagram of an IR-UWB receiver implementing a method for determining multipath components according to a first embodiment of the invention;

(6) FIG. 4 shows a diagram of the structure of a receiver module that can be used in the IR-UWB receiver of FIG. 3;

(7) FIG. 5 shows a diagram of an IR-UWB receiver implementing a method for determining multipath components according to a second embodiment of the invention;

(8) FIG. 6A is an enlargement of FIG. 2 around the point at which the interference between the two multipath components appears;

(9) FIG. 6B shows the same multipath components after processing of the interference in the receiver of FIG. 3.

DETAILED DISCLOSURE OF SPECIFIC EMBODIMENTS

(10) 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.

(11) 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:

(12) $\begin{array}{cc}{s}_e\left(t,{}_c\right)=A_e\cos \left({}_c\left(t-t_c\right)+\right)\exp \left(-\frac{{\left(t-t_c\right)}^2}{2T_{\mathrm{BW}}^2}\right)& \left(1\right)\end{array}$
where A.sub.e is the amplitude of the impulse emitter,

(13) $f_c=\frac{{}_c}{2}$
is the central frequency of the impulse, which can have a plurality of values f.sub.c.sup.1, . . . , f.sub.c.sup.N, t.sub.c is the time of emission corresponding to the centre of the impulse, is the phase upon emission, unknown and modelled as a random variable uniformly distributed over [0,2[, T.sub.BW 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 s.sub.e.sup.(k)(t,.sub.e) is used to indicate that the impulse was emitted at the instance k:

(14) $\begin{array}{cc}{s}_e^{\left(k\right)}\left(t,{}_c\right)=A_e\cos \left({}_c\left(t-t_c^{\left(k\right)}\right)+{}^{\left(k\right)}\right)\exp \left(-\frac{{\left(t-t_c^{\left(k\right)}\right)}^2}{2T_{\mathrm{BW}}^2}\right)& \left(2\right)\end{array}$
where t.sub.c.sup.(k) and .sup.(k) are, respectively, the time of emission and the phase upon emission related to the instance k of the channel.

(15) 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), t.sub.c.sup.(k)=0 at each instance, the signal received by the receiver Rx for the instance k of the channel, is written as:

(16) $\begin{array}{cc}{s}_r^{\left(k\right)}\left(t,{}_c\right)=\underset{i=1}{\overset{P}{.Math.}}{A}_e{}_i^{\left(k\right)}\cos \left({}_c\left(t-{}_i^{\left(k\right)}\right)+{}_i^{\left(k\right)}+{}^{\left(k\right)}\right)\exp \left(-\frac{{\left(t-{}_i^{\left(k\right)}\right)}^2}{2T_{\mathrm{BW}}^2}\right)& \left(3\right)\end{array}$
where .sub.i.sup.(k), .sub.i.sup.(k) and .sub.i.sup.(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.

(17) 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

(18) $W_{t_s}=\left[t_s-\frac{W}{2},t_s+\frac{W}{2}\right],$
having a width W centred on the sampling time t.sub.s, in order to provide a sample of intensity:

(19) $\begin{array}{cc}{\mathrm{bin}}^{\left(k\right)}\left(t_s,{}_c\right)={\left(\underset{W_{\mathrm{ts}}}{}{s}_r^{\left(k\right)}\left(t,{}_c\right)\cos \left({}_{c}t\right)\mathrm{dt}\right)}^2+{\left(\underset{W_{\mathrm{ts}}}{}{s}_r^{\left(k\right)}\left(t,{}_c\right)\sin \left({}_{c}t\right)\mathrm{dt}\right)}^2& \left(4\right)\end{array}$
where bin.sup.(k)(t.sub.s,.sub.c) represents the sample of intensity obtained via integration over the time interval W.sub.t.sub.z.

(20) 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.sup.(k)(t.sub.s,.sub.c) can be expressed in the following form:
bin.sup.(k)(t.sub.s,.sub.c)=(A.sub.1.sup.(k)).sup.2+(A.sub.2.sup.(k)).sup.2+I.sup.(k)(t.sub.s,.sub.c)(5-1)
with

(21) $A_1^{\left(k\right)}=A_e{}_1^{\left(k\right)}{}_{W_{\mathrm{ts}}}\exp \left(-\frac{{\left(t-{}_1^{\left(k\right)}\right)}^2}{2T_{\mathrm{BW}}^2}\right)\mathrm{dt},A_2^{\left(k\right)}=A_e{}_2^{\left(k\right)}{}_{W_{\mathrm{ts}}}\exp \left(-\frac{{\left(t-{}_2^{\left(k\right)}\right)}^2}{2T_{\mathrm{BW}}^2}\right)\mathrm{dt}$
and where I.sup.(k) (t.sub.s,.sub.c) is an interference term defined by:
I.sup.(k)(t.sub.s,.sub.t)=2A.sub.1.sup.(k)A.sub.2.sup.(k) cos(.sub.c(.sub.1.sup.(k).sub.2.sup.(k))(.sub.1.sup.(k).sub.2.sup.(k)))(5-2)

(22) It is clear that in the expression (5-1), the terms (A.sub.1.sup.(k)).sup.2 and (A.sub.2.sup.(k)).sup.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.sup.(k)(t.sub.s,.sub.c) varies sinusoidally according to the difference in time of flight between the two paths of the channel.

(23) FIG. 2 illustrates the case studied and mentioned above (simplified channel with two MPCs) by giving the intensity of the signal estimated by the IR-UWB receiver for successive instances k of the transmission channel.

(24) The abscissae represent the successive positions of the receiver (channel instances) over time and the ordinates represent the sampling times t.sub.s (in other words, the delays in the channel response). It was supposed that the times of flight i.sup.(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 FIG. 2, the two components give constructive interference for certain positions of the receiver and destructive interference in others. In particular, at the positions identified by 210 and 220, a total absence of a signal is noted, corresponding to an exact superposition of the impulses received with a phase opposition between them. In these positions 210 and 220, the IR-UWB receiver does not allow the MPC components to de determined.

(25) 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.

(26) FIG. 3 shows a diagram of an IR-UWB receiver implementing a method for determining multipath components of a channel, according to a first embodiment of the invention.

(27) 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 f.sub.c.sup.1, . . . , f.sub.c.sup.N and translate them into baseband.

(28) 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 f.sub.c.sup.N being emitted during the i.sup.th interval. In correlation, the receiver module 320 is centred on f.sub.c.sup.1 during a first reception interval, on f.sub.c.sup.2 during a second reception interval and so on until the N.sup.th reception interval. It is also supposed hereinafter that NT<T.sub.coh, 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.

(29) At each integration interval, the complex samples (d.sub.I,d.sub.Q) 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.sup.(k)(t.sub.s,2f.sub.c.sup.n),

(30) $s=0,\mathrm{.Math.},.Math.\frac{T}{W}.Math.,$
where f.sub.c.sup.n is the central frequency of the selected sub-band in the receiver, and t.sub.s=sW,

(31) $s=0,\mathrm{.Math.},.Math.\frac{T}{W}.Math.$
are successive times of flight from the emitter.

(32) These samples are demultiplexed by the demultiplexer 330 into the buffers 340.sub.1, . . . ,340.sub.n. More precisely, the samples obtained during a reception interval corresponding to the central frequency f.sub.c.sup.n are stored in a corresponding buffer, 340.sub.n. Thus, after reception of a frame during the instance k of the channels at the various frequencies, the buffer 340.sub.n contains the samples bin.sup.(k)(sW,2f.sub.c.sup.n),

(33) 0$s=0,\mathrm{.Math.},.Math.\frac{T}{W}.Math.,$
corresponding to the successive times of flight bin.sup.(k)(t.sub.s,2f.sub.c.sup.n),

(34) $s=0,\mathrm{.Math.},.Math.\frac{T}{W}.Math..$

(35) 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:

(36) $\begin{array}{cc}{\mathrm{bin}}^{\left(k\right)}\left(t_s\right)=\underset{n=1}{\overset{N}{.Math.}}{}_n{\mathrm{bin}}^{\left(k\right)}\left(t_s,2f_c^n\right)& \left(6\right)\end{array}$
where .sub.n, n=1, . . . , N are predetermined weighting coefficients. For example,

(37) ${}_n=\frac{1}{N}$
can be taken in order to calculate a simple average or .sub.nSNR.sub.n(t.sub.s) where SNR.sub.n(t.sub.s) is the signal-to-noise ratio at the frequency f.sub.c.sup.n and at the time t.sub.s.

(38) By carrying out the combination of the samples bin.sup.(k)(t.sub.s,2f.sub.c.sup.n) according to the expression (6) for each time of flight t.sub.s=sW,

(39) $s=0,\mathrm{.Math.},.Math.\frac{T}{W}.Math.,$
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.sup.(k)(t.sub.s), t.sub.s=sW,

(40) $s=0,\mathrm{.Math.},.Math.\frac{T}{W}.Math.,$
exceeding a predetermined threshold, Th.

(41) FIG. 4 shows a diagram of the structure of the multiband receiver module 320 of FIG. 3.

(42) This receiver module is described in detail in the application FR-A-2 966 969 incorporated here by reference.

(43) The architecture of this receiver module is recalled below.

(44) 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 LO.sub.1. 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, s.sub.I,s.sub.Q, 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 LO.sub.2. The in-phase and quadrature outputs of the second quadrature mixer, s.sub.II,s.sub.IQ, are filtered by low-pass filters 436. Likewise, the in-phase and quadrature outputs of the third quadrature mixer, s.sub.QI,s.sub.QQ, 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 (d.sub.I,d.sub.Q):

(45) $\begin{array}{cc}\left(\begin{array}{c}{d}_I\\ d_Q\end{array}\right)=\left(\begin{array}{cccc}{\mathrm{.Math.}}_{\mathrm{II}}^I& {\mathrm{.Math.}}_{\mathrm{IQ}}^I& {\mathrm{.Math.}}_{\mathrm{QI}}^I& {\mathrm{.Math.}}_{\mathrm{QQ}}^I\\ {\mathrm{.Math.}}_{\mathrm{II}}^Q& {\mathrm{.Math.}}_{\mathrm{IQ}}^Q& {\mathrm{.Math.}}_{\mathrm{QI}}^Q& {\mathrm{.Math.}}_{\mathrm{QQ}}^Q\end{array}\right)\left(\begin{array}{c}{r}_{\mathrm{II}}\\ r_{\mathrm{IQ}}\\ r_{\mathrm{QI}}\\ r_{\mathrm{QQ}}\end{array}\right)& \left(7\right)\end{array}$
where the elements of the matrix

(46) $E=\left(\begin{array}{cccc}{\mathrm{.Math.}}_{\mathrm{II}}^I& {\mathrm{.Math.}}_{\mathrm{IQ}}^I& {\mathrm{.Math.}}_{\mathrm{QI}}^I& {\mathrm{.Math.}}_{\mathrm{QQ}}^I\\ {\mathrm{.Math.}}_{\mathrm{II}}^Q& {\mathrm{.Math.}}_{\mathrm{IQ}}^Q& {\mathrm{.Math.}}_{\mathrm{QI}}^Q& {\mathrm{.Math.}}_{\mathrm{QQ}}^Q\end{array}\right)$
are chosen from {1,0,+1} according to the desired sub-band and r.sub.II,r.sub.IQ,r.sub.QI,r.sub.QQ 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, f.sub.c.sup.1, . . . , f.sub.c.sup.N (for example via a pilot sequence).

(47) FIG. 5 shows a diagram of an IR-UWB receiver implementing a method for determining the multipath components of a channel, according to a second embodiment of the invention.

(48) 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 f.sub.c.sup.1, . . . , f.sub.c.sup.N. This emission in parallel can be repeated during the same channel instance at a frequency greater than 1/T.sub.coh.

(49) The signal received by the antenna 510 is provided to a plurality of N IR-UWB receivers, 520.sub.1, . . . , 520.sub.N, respectively operating at the central frequencies f.sub.c.sup.1, . . . , f.sub.c.sup.N, 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.

(50) At each integration interval having an index s, the complex samples (d.sub.I,d.sub.Q) at the output of the multiband receiver module 520.sub.n, are subjected to a calculation of the squared modulus in the quadratic module 525.sub.n. The samples thus obtained at the output of 525.sub.n are none other than bin.sup.(k)(t.sub.s,2f.sub.c.sup.n),

(51) $s=0,\mathrm{.Math.},.Math.\frac{T}{W}.Math.$
where t.sub.s=sW,

(52) $s=0,\mathrm{.Math.},.Math.\frac{T}{W}.Math.$
are successive times of flight from the emitter.

(53) The samples bin.sup.(k)(t.sub.s,2f.sub.c.sup.n) related to the same sampling time t.sub.s and to the various central frequencies f.sub.c.sup.1, . . . , f.sub.c.sup.N 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.sup.(k) (t.sub.s), sW,

(54) 0$s=0,\mathrm{.Math.},.Math.\frac{T}{W}.Math..$

(55) Finally, the comparator 560 detects the MPCs components from the composite samples exceeding a predetermined threshold, Th.

(56) FIG. 6A illustrates a situation of interference between two paths of an IR-UWB channel. It corresponds to an enlargement of FIG. 2 around the point at which the interference appears. It is clear that the two multipath components disappear in the zones 610, 620, 630, 640 corresponding to a temporal overlap and a phase opposition of the two impulses. It is recalled that the central frequency is 4 GHz, that the bandwidth of the impulses is 500 MHz and that the signal received is integrated over intervals 2 ns wide, offset by ins.

(57) FIG. 6B shows the intensity of the signal after processing of the interference in the receiver of FIG. 3. The channel has the same two paths as in FIG. 6A. In this example, the frequency of the local oscillator, LO.sub.1, of the first quadrature stage of the multiband receiver module was chosen as equal to 4 GHz, and the frequency of the local oscillator, LO.sub.2, of the second quadrature stage was chosen as equal to 500 MHz. It is thus possible, by combining the outputs of the double-quadrature paths, II,IQ,QI,QQ in the combination stage 450, to select the reception sub-bands centred around the central frequencies f.sub.c.sup.1=3.5 GHz, f.sub.c.sup.2=4 GHz, f.sub.c.sup.3=4.5 GHz.

(58) The combination module 350 of the receiver then calculates the average:

(59) $\begin{array}{cc}{\mathrm{bin}}^{\left(k\right)}\left(t_s\right)=\frac{1}{3}\underset{n=1}{\overset{3}{.Math.}}{\mathrm{bin}}^{\left(k\right)}\left(t_s,2f_c^n\right)& \left(8\right)\end{array}$

(60) It is noted than in FIG. 6B, the composite samples no longer have the phenomenon of extinction in the zones 610, 620, 630, 640 and the two paths of the channel can still be identified therein by the determination method. It is thus possible to track the two multipath components, including in these zones, for example in order to assist an IR-UWB geolocation method.