WO2002096148A1

WO2002096148A1 - Method for locating terminals within communication networks, system and terminal therefor

Info

Publication number: WO2002096148A1
Application number: PCT/IT2002/000316
Authority: WO
Inventors: Giorgio Grego
Original assignee: Telecom Italia S.P.A.
Priority date: 2001-05-23
Filing date: 2002-05-15
Publication date: 2002-11-28
Also published as: ITTO20010486A0; ITTO20010486A1

Abstract

A database (BD) is created that collects a first set of positions that can be occupied by at least one terminal (T) relative to a given point in the network (BS) as well as a second set of profiles in module and phase over time of wave forms that can be received in the aforesaid given point (BS) following the emission of an impulsive signal by the aforesaid terminal (T) when the terminal is located in a respective position relative to the given point (BS). To proceed with the locating action, an impulsive signal (P) is selectively generated, starting from the terminal (T), which signal is received in the given point (BS) with a determined time profile. The position occupied by the terminal (T) upon emitting the impulsive signal (P) is identified as the position that in the database (BD) identifies the corresponding profile, i.e. the profile that best approximates the profile of the received wave form.

Description

METHOD FOR LOCATING TERMINALS WITHIN COMMUNICATION NETWORKS_/

SYSTEM AND TERMINAL THEREFOR Technical Field

The present invention relates to communication networks and it was developed in view of possible use in mobile communication networks to identify the position of mobile terminals such as cellular telephones.

In particular, the present invention allows to identify the position of a cellular phone within a territory served by a mobile telephony network. Background Art

In the prior art, systems and methods are known for identifying the position of mobile terminals within a cellular communication network. For instance, international patent application O-A- 00/18148 discloses a method for locating cellular telephones (or, briefly, "cell phones") that is based on the solution of comparing the radio frequency information collected from the cell phone (RF fingerprint or RF measurements of the cell phone) with RF information contained in a reference database

(reference RF fingerprint) .

Each reference RF fingerprint thus bi-univocally identifies an elementary area or picocell of the territory served by the network, so that the locating operation can be accomplished assigning to the cell phone the position corresponding to the reference RF fingerprint whose values are closest to those of the cell phone.

A first problem linked to the practical application of said solution is given by the limited number of frequency channels (channels) used by cell phones to communicate.

Said number of channels is limited: for instance, in Italy for GSM (Global System for Mobile Communications) networks, 124 channels are available and of these only a part is allocated to each operator. It is therefore usual, within a given territory, for an operator to allocate identical channels to base radio stations that "cover" a given territory. Because of this practice (also known as "frequency reuse") the cell phone receives, for each channel, RF signals that correspond to the algebraic sum of the field values, in terms of power, received from the respective base radio stations that use the same channel. Deriving from a sum, the RF fingerprint of the cell phone for a given channel may therefore be identical even if generated in different points of the territory.

It is readily understandable that such problem would not exist if the channels were not reused: in this case the field value for each channel would depend only on electromagnetic attenuation factors due to the path to reach the cell phone.

A second practical problem is linked to the fact that cell phones generate an RF fingerprint comprising the field of a limited number of channels. For instance, in the GSM case cell phones are able to generate an RF fingerprint comprising at most seven values (value septuplet) , each corresponding to an RF field value of a different channel.

It is therefore very likely that cell phones positioned in different area elements or picocells of territory generate identical septuples, due, in fact, to the limited number of usable values. Naturally, if the cell phones could provide a number of values equal to the number of available channels, the error probabilities would be very limited.

Briefly, although the method described above is theoretically accurate, in practice, because i) of frequency reuse and ii) of the limited number of values handled by the cell phone to generate the RF fingerprint, it has been noted that the known method can entail even sizeable location errors .

For example, in GSM networks a systematic cell phone locating ambiguity has been experimentally observed. The reason is that the same septuplet is attributed, according to the known method, to points that are geographically very far from each other, even some kilometres apart. Disclosure of the Invention

The present invention thus aims to provide a solution usable, for instance, to locate individual cellular telephones within a mobile communication network, said solution being free from the drawbacks set out above .

According to the present invention, said aim is achieved thanks to a method having the characteristics set out in the claims that follow. The invention further relates to the associated system as well as the associated terminal.

In particular, the present invention exploits, for purposes of determining the location, a phenomenon that has traditionally been considered a negative or antagonist phenomenon opposing the execution of a correct communication.

Within cellular networks, one of the main problems

^'linked to the propagation and correct reception of signals, especially in urban areas, derives from the establishment of multiple propagation paths (multipath or MP) . The phenomenon in question arises both in communications from base stations to mobile terminals and in communications from mobile terminals to base stations. It is linked to a set of phenomena (wave reflection on obstacles such as buildings, etc.) and it manifests itself essentially by the fact that a signal transmitted with an impulsive wave form is received in the form of a signal with a complex shape that lost the purity or cleanliness of the transmitted impulse. The phenomenon has a clearly perturbing effect on the transmission of digital signals, especially for their decoding .

For this reason, over the years a very high number of techniques aimed at contrasting this phenomenon have been developed. Some of these techniques are used in current mobile telecommunication networks.

However, the phenomena described above are also the basis for techniques that allow to retransmit a signal received in phase conjugation relationship.

For instance, in the field of optics (but the principle is generally applicable to any signal that propagates in the form of waves, and in particular in the form of electromagnetic waves in general) the devices currently called phase conjugation mirrors are known.

For a general description of the concept of phase conjugation and of its possible applications to the propagation of electromagnetic waves, reference can be made, for instance, to pages 2 to 11 of the volume "Optical Phase Conjugation" edited by Robert A. Fisher, Academic Press 1983. While a normal reflecting mirror inverts the component of the propagation vector k normal to its surface, a Phase Conjugation Mirror (PCM) is able to invert the vector entity k as a whole. For example, consider a monochromatic electromagnetic field with pulsation ω propagating in a direction z, i.e. E_α(r, t) = ψ(r) exp [i(ωt - kz)] + c.c. (1) The effect of a phase conjugation mirror on such an electromagnetic field is to give rise to a conjugated field which can be expressed as

E₂(r, t) = f(r) exp [i(ωt + kz)] + c.c. (2) The formulas set out above are excerpted from page 342 of Vol. 4 of the Laser Handbook edited by M.L. Stitch and M. Bass, Worth Holland, 1985.

A possible simplified application of the conjugation principles described above is given by the possible time reversal of a signal according to the criteria constituting the basis for the operation of so-called time-reversal mirrors or TRM.

For a general description of the criteria constituting the basis for time reverse acoustics, reference can be made to the work by M. Fink, "Time-reverse Acoustics", in Physics Today, 50, n. 3, March 1997 or to the paper by M. Fink, "Acustica a inversione temporale" ("Time-reverse Acoustics) published at pages 79 through 86 of the March 2000 issue of the weekly journal "Le Scienze" .

This latter paper describes an ultrasound propagation experiment through a so-called forest of 2000 parallel steel bars immersed at random in a tank filled with water. The wave emitted by a small transducer in the form of a pulse with a duration of one microsecond was transmitted through the bar forest towards an array of 96 transducers. The signals received by the transducers were subjected to time reversal and sent back towards the source, making them propagate backwards through the bar forest, then measuring, with a hydrophone, the incoming wave in correspondence with the source. Although the array sent back a signal with a duration of 200 microseconds through the chaotic dispersion produced by the forest, a regenerated pulse of about one microsecond was measured at the source. The same paper reports on experiments conducted in sea waters in which a sound pulse was sent from a departure area and recorded to a distance of 30 kilometres by an array of receivers/ transmitters, distorted due to refraction and to the multiple reflections produced by the surface and by the sea bottom. After undergoing a time reversal (phase conjugation) , the signal was sent back by the array towards the target, where it was found to be well focused. It has already been proposed to exploit the physical principle described above to reconstruct, in correspondence with a terminal of a mobile communication network, a signal constituting a replica, that is as faithful as possible, of the individual pulse used for transmission starting from the transmitter itself.

In addition to overcoming the drawbacks linked to multipath transmission, such a solution allows, at least in virtual terms, to do without a network of base stations susceptible to provide a cellular coverage of the territory: the mechanism for reconstructing the pulse by effect of the transmission in phase conjugation relationship is independent from the distance between the fixed station and the mobile terminal. All this, naturally, without ignoring the need to take into account the attenuation phenomena the signal undergoes by effect of the absorption of energy by the medium through which the signal propagates .

Specifically, the solution according to the invention is based on the acknowledgement of the fact that a given signal transmitted by a mobile terminal, once it is received by the base station, has an amplitude and phase profile over time that is univocally (or, rather, bi-univocally) associated to the position of the mobile terminal itself.

The information about this signal therefore constitutes a unique "marker" of that precise point in the territory. If geo-referenced (by establishing what kind of information corresponds to what point of the covered territory: result achievable, for instance, through a measurement campaign and/or a software simulating the propagation of electromagnetic waves) and stored in a system for managing the mobile network, this information allows to identify the geographic co-ordinates (i.e. the location) of any terminal that contacts a given base station. In other words, every time the base station receives a given known signal (for instance a "1", a "0" and/or a certain - known - string of symbols) and acquires therefrom the characteristics of module and phase over time, the base station itself can correlate the signal received with the tracks (module and phase over time) geo-referenced and stored in the network database. The base station (and the system in general) can thus identify the position of the mobile terminal on the territory, proceeding with locating it. Brief Description of Drawings The invention will now be described, purely by way of a non limiting example, with reference to the accompanying drawings, in which:

- Figure 1 schematically shows the manifestation of the multiple path propagation mechanism, - Figures 2 and 3 illustrate the possible effect which multiple path propagation may have on a signal transmitted in the form of a concentrated pulse,

- Figure 4 shows, in the form of a block diagram, a possible architecture for embodying the invention within a base station of a radio mobile communication system. Best mode for Carrying Out the Invention

Figure 1 schematically shows the effect of propagation over multiple paths (multipath or MP) in the communication between any mobile terminal T of a cellular mobile communication network and a base station BS whereto the terminal T in question is currently connected.

In practice, the effect of multiple path propagation is to cause a signal emitted by the terminal T in the form of a pulse P of duration D, as shown in Figure 2, to be received at the base station BS in the form of a signal S of complex shape, which has lost the purity or cleanliness of the transmitted pulse P. The base station BS shown in Figure 4 comprises an antenna 4 constituted by a plurality of antenna elements 101, ..., lOn. The antenna is therefore typically identifiable as an array or synthetic aperture antenna. The antenna 10 is thus configured in such a way as to receive signals starting from one or more mobile terminals T obtaining the space-time information (hence, the module and phase profile over time) relating to the propagation thereof.

In particular, each of the antenna elements 101, ..., lOn bears associated therewith a respective dedicated transmitter/receiver group.

A receiver 12 is present which, in order to receive the signals coming from the elements 101, ..., lOn of the antenna 10, comprises a plurality of groups 121, ..., 12n each dedicated to a respective antenna element 101, ..., lOn. The signals thus received are subjected to demodulation in a demodulator 14, itself comprising a plurality of groups or modules 141, ... , 14n, in view of the forwarding of said signals towards the communication network N.

In dual fashion, the same base station comprises a modulator 16 that receives the signals starting from the network N to proceed, through a transmitter 18, with the transmission of the signals themselves to the mobile terminal T through the elements 101, ..., lOn of the antenna 10.

The modulator 16 as well as the transmitter 18 also comprise a plurality of modules 161, ..., 16n and, respectively, 181, ..., 18 each dedicated to a respective antenna element 101, ..., lOn. The reference 20 indicates a set of bi-directional devices (for instance a set of so-called circulators) that allows to use the elements 101, ..., lOn of the antenna 10 both in reception and in transmission. The representation of the reception chain 12 , 14 and of the transmission chain 16, 18 of the base station BS is intentionally schematic. The elements constituting such a station can be considered widely known in the art, also in regard to the possibility of employing different solutions according to the transmission standards adopted.

This applies, for instance, for the generation of the signals destined to be sent towards the network N starting from the demodulator: these signals are obtained starting from the individual signals processed by the groups or modules 141, ..., 14n whereof the demodulator 14 is composed.

In any case, the specific constructive details of the station BS are not relevant in themselves for purposes of understanding the invention.

The same station BS comprises an analysis block 22 destined to measure the broadening effect (in terms of module and phase profile over time) that the individual transmission pulse emitted by the mobile transmitter T undergoes, primarily by effect of multipath propagation, during transmission towards the base station BS. To allow this analysis, the mobile transmitter T periodically emits (with fixed periodic cadence, i.e. every time it transmits a certain message unit, for instance a time slot, towards the base station BS) a fixed encoded sequence (probe or sample sequence) constituted, for instance, by a sequence comprising a logic "one" associated to one or more "zero", with associated, preferably, a time synchronisation sequence. All this in such a way as to allow the base station BS, and in particular the analysis block 22 thereof, to ready itself to analyse the signals corresponding to the aforesaid logic "one" as the latter is received at the base station BS after the perturbing effect due to propagation.

Persons skilled in the art will appreciate that - to facilitate the understanding of what has been described heretofore - this example was formulated in an intentionally simplified manner, assuming that the transmission from the mobile transmitter T towards the base station BS takes place with a very simple modulation scheme in which to a logic "one" corresponds the transmission of pulse P of the type shown in Figure 2, whilst to the logic "zero" does not correspond the emission of a signal (carrier frequency not modulated) .

However, as those skilled in the art know well, the encoding procedures which may be adopted to transmit digital signals are practically infinite. This applies in particular for the possibility of positively transmitting signals both in correspondence with a logic "one" and in correspondence with a logic "zero". Returning to the simplified example referenced herein for the sake of facilitating illustration, the block 22 reveals the make up (in terms of module and phase profile over time) of the wave forms S received by the base station BS when the mobile transmitter T transmits a pulse like the pulse P of Figure 2.

Referring, for the sake of simplicity, to the action based on the individual electrical signal (examination, by each module of the block 22, of the respective signal output by the receiver 12 ) , the set of data corresponding to the analysis is typically constituted by a sampling of the wave form S represented in Figure 3 with a sufficiently fine sampling resolution Tc, typically chosen according to the general criteria that regulate the conduct of signal sampling actions (Nyquist criterion) .

The result of the analysis is sent to an additional block 24. In the block 24 the aforesaid sequence of samples is subjected to a time reversal, i.e. to a processing operation as a result whereof the wave form S shown in Figure 3 is, so to speak, flipped over in time, making the first time samples become the last and vice versa.

Naturally, the aforesaid time reversal action is performed distinctly on the signals received through the antenna elements 101, ..., lOn and the corresponding groups 121, ..., 12n of the receiver 1. For this reason, both the block 22 and the block 24 have a corresponding organisation on multiple groups or channels 221, ..., 22n and 241, ..., 24n, each destined to operate on a respective signal received through a corresponding antenna element 101, ..., lOn. The conjugated wave form that results from the time reversal operation generally has a profile that is comparable to the one shown schematically with dashed lines and indicated as S* in Figure 3.

The phase conjugated wave form S* thereby obtained is transferred towards the modulator 16 in such a way that said conjugated wave form is used by the base station B to transmit the signal corresponding to the logic " one " value to the terminal T.

The aforesaid time reversal operation is performed in distinct fashion for each of the aforesaid signals, in order to allow the individual groups or modules 161, ... , lβn and 181, ..., 18n of the modulator 16 and of the transmitter 18 to use a phase conjugated wave form for purposes of transmission to the terminal T, through the corresponding antenna elements 101, ... , lOn. An impulsive wave form, constituting a faithful replica of the pulse P that the transmitter T used previously in transmission is thereby originated, in reception at the level of the transmitter T. The receiving organs of the mobile terminal T thus perform their reception operation on a very "clean" wave form, purified from the negative effects of multiple path propagation. This is achieved, indeed, by effect of the phase conjugation action accomplished at the BS station level. It is in fact possible to equip the terminal T with an inertial sensor (for example, the component ADXL 202. manufactured by Analog Devices) that provides the measure of any movements. In addition to a certain movement size, the mobile terminal T asks the base station BS to effect the update by sending the probe or sample sequence to the BS station. This is the sequence that allows the block 22 to process the module and phase information that enables block 24 to carry out the time reversal, i.e. phase conjugation, operation. It should be stated, in any case, that the specific details whereby the phase conjugation operation is achieved are not, in themselves, relevant for purposes of understanding and implementing the present invention.

Because of its mobility, the terminal T moves relative to the base station BS . For the same impulsive wave form P used in transmission, the received wave forms change, in particular because the reflection mechanisms that constitute the basis for multiple path propagation change.

In any case (thus also in the case in which it is not directly used for transmission from the base station BS to the terminal T in the manner described above) , the information of module and phase profile over time which, collected by the block 22, enables the block 24 to carry out the time reversal, i.e. phase conjugation, operation, includes an item of information that identifies in a univocal (bi-univocal) manner the location of the terminal T, i.e. the fix or geographic position - in two or three dimensions - in which the terminal T was located when it sent the probe or sample sequence to the station BS.

Other conditions being equal, the probe sequence transmitted by a terminal T that is in a given geographic position relative to the base station BS is received at the base station BS itself in the form of a signal S that is unique and characteristic of - and hence identifies - the aforesaid geographic position.

In dual fashion, given a determined form of pulse P transmitted by a terminal T starting from a certain position relative to the base station BS (given point) , the module and phase characteristics of the signal S corresponding to said signal as received by the base station identify one and only one possible geographic position of the terminal T relative to the base station BS. This characteristic, intrinsic to the manifestation of multiple path propagation, can be used to locate the terminals T, providing at the base station BS (or - in general - in any other position of the communication network whereof said station BS is a part - memory means (of a known kind) destined to house a database BD that allows to "geo- reference" the individual wave forms S that may be received by the base station BS, establishing a correspondence between said wave forms and the position occupied by a terminal T which, having emitted a probe sequence constituted by or comprising a pulse such as the pulse P shown in Figure 1, gave rise, upon reception at the base station BS, to a signal S having a particular profile in terms of module and phase over time. The aforesaid database BD can be constituted, for instance, by means of a measurement campaign (which can be easily conducted, exploiting the mobility of the terminals T) aimed at originating a sort of ideal table in which, for each geographic position included in the coverage of the base station BS, in the database BD is stored the corresponding profile (in module and phase over time) of the corresponding signal S received from said geographic position.

All this giving thus rise to a first set of geographic positions relative to a base station BS (given location) and a second set of stored wave form profiles, the aforesaid two sets being mutually linked by a bi-univocal correspondence relationship .

The database BD is constituted according to determined resolution values, depending on the precision to be achieved in the aforesaid locating action.

Obviously, the database BD is the richer (i.e. with a higher number of stored wave form S profiles) the greater is the discrimination ability to be achieved in performing the locating operation.

For instance, individual measurements can correspond to wave form S profiles measured each in correspondence with a respective sub-cell within the scope of coverage of the base station BS having dimensions in the order of, for example, 5x5 m or 10x10 m.

All this according to the locating needs to be met, taking into account the reasons for performing said locating action.

It can take place, for instance, with the purpose of allowing the transmission to the terminals T of information

(possibly of a promotional and/or commercial nature) inherent to commercial venues, various services, entertainment locales, etc. that are situated in the area where the user of the individual mobile terminal T involved is currently located.

It will therefore be appreciated that in many cases the spatial resolution requirements are not extremely stringent. In symmetric fashion, one should also take into account the fact that, for the same wave form transmitted by the mobile terminal T, the characteristics of the received wave form S can also change for very small movements of the terminal T. Consequently, in organising the database BD it is necessary to take into account (according to known criteria) the possible variations of the wave form S corresponding to the resolution grid defined in carrying out the locating action: this is feasible by executing electromagnetic wave propagation simulations by means of simulation tools currently available on the market.

For instance, this can be accomplished by identifying the position occupied by the terminal T at the time the probe signal is emitted as the position of the aforesaid first set corresponding to the profile of the second set of wave form profiles stored in the database BD in order to approximate better, according to a given criterion, (for instance, mean square error, etc.) the profile of the received wave form (S) .

Indeed, depending on the possible manners of using the information about the location of the terminals T, the database BD is preferably constructed in such a way that it can be interrogated remotely through the network N.

This can be done, for example, in such a way as to cause the location information to be readable solely by an entity that is duly authorised by the user of the terminal T (for instance to receive the aforesaid information relating to services available or accessible within the location area) . It will be appreciated that the communication technique described with reference to Figure 4 allows in itself to the base station BS to obtain the reconstitution of very sharp and pure transmission pulses in correspondence with the terminal T involved on each occasion without the base station BS having to locate the terminal T itself.

The possibility of achieving the transmission from the base station BS towards the terminal T exploiting the phase conjugation mechanism can be used to verify the precision of the locating action. For this purpose, having completed the action of locating the terminal T involved on each occasion, in the manner described above, the base station BS can send to said terminal T - instead of the signal obtained directly by means of phase conjugation (block 24) - a signal based on the wave form (amplitude and phase profile over time) that in the database BD is stored as identifying the geographic position occupied by the terminal T.

The signal reconstructed at the terminal T will therefore have characteristics (essentially, power level) that are indicative of the actual correspondence between the real position of the terminal T and the determined position of the base station BS.

The power level of the pulse reconstructed at the terminal T will in general be greatest when the wave form that the same station BS receives at the moment from the terminal T corresponds exactly to the wave form stored in the database .

The more the aforesaid wave forms (the one received at the moment and the stored one) differ from each other, the lower the power level of the reconstructed pulse in correspondence with the terminal T.

The aforesaid level information (which the terminal T can process by direct comparison with the power level of the reconstructed pulse following the transmission of the phase conjugated signal by the station BS) can be retransmitted by the terminal T itself towards the base station BS. Said information thus constitute an indication of the deviation of the actual position of the terminal relative to the reference position of the location grid defined by the data base BD.

The criteria constituting the basis for the solution according to the invention (previously described with specific reference to a situation in which the phase conjugation action is carried out at the level of the base station BS in view of the transmission to the mobile terminals T) can naturally be applied in dual fashion to allow a mobile terminal to locate a base station, possibly providing for the information collected in the data base of the geographic references to be selectively transferred to the mobile terminal through the communication network.

In this sense, it will be appreciated that the terms "given point" and "terminal", as they are used in the claims that follow, apply each, indifferently, both to a base station BS (which is, in any case, a terminal of the network) and to a terminal T (which, mobile or fixed, represents a given point of the network) .

Naturally, without any modification to the principle of the invention, the construction details and the embodiments may be widely varied with respect to what is disclosed and illustrated herein, without thereby departing from the scope of the present invention.

This applies in particular for the possible application to networks in which the described transmission criteria are employed for transmission between base stations BS and terminals T that are configured, at least in part, as fixed stations (homes, offices, etc.). In this case, the completion of the locating operation is made once or "una tantum" since each terminal involved in the locating action is "marked" in a determined position, which does not change except in case of a move.

Claims

1. Method for locating at least one terminal (T) relative to a given point (BS) of a communication network, characterised in that it comprises the steps of: - creating a database (BD) that collects a first set of positions which can be occupied by said at least one terminal (T) relative to said given point (BS) and a second set of module and phase profiles over time of wave forms which can be received in said given point (BS) following the emission of an impulsive signal (P) by said at least one terminal (T) located in a respective position of said first set of positions; said first and second set being mutually in a bi- univocal correspondence relationship, so that each of the profiles of said second set univocally identifies a respective position of said first set of positions,

- selectively generating, starting from said at least one terminal (T) , said impulsive signal (P) ,

- measuring the wave form (S) received in said given point (BS) following the emission of said impulsive signal (P) , and

- identifying the position occupied by said at least one terminal (T) as the respective position of said first set identified by the profile of said second set corresponding to the profile of said wave form (S) received.

2. Method as claimed in claim 1, characterised in that said respective positions of said first set are identified as portions of a given coverage area of said communication network (N) .

3. Method as claimed in claim 1 or claim 2, characterised in that it comprises the step of identifying the position occupied by said at least one terminal (T) upon emitting said impulsive signal (P) as the position of said first set identified by the profile of said second set that best approximates according to a given criteria the profile of said wave form (S) received.

4. Method as claimed in any of the previous claims, characterised in that said at least one impulsive signal corresponds to the signal used by said at least one terminal (T) to transmit to said given point (BS) at least one level associated to a digital signal.

5. Method as claimed in any of the previous claims, characterised in that said at least a terminal (T) is a mobile terminal.

6. Method as claimed in any of the previous claims, characterised in that it comprises the step of measuring said wave form profiles in said given point (BS) by means of a plurality of receiving elements (101, ... , lOn) .

7. Method as claimed in any of the previous claims, characterised in that it comprises the steps of: subjecting said received wave form (S) to phase conjugation, in order to generate at least a respective phase conjugated wave form (S*), and - using said phase conjugated wave form (S*) for transmission from said given point (BS) to said at least one terminal (T) .

8. Method as claimed in claim 7, characterised in that it further comprises the steps of: - using, for the transmission from said given point (BS) to said at least one terminal (T) whose position was identified as a given position of said first set, the wave form having said corresponding profile of said second set,

- measuring, in said at least one terminal (T) , the level of the signal thus received, said level being indicative of the precision of the location of said terminal

(T) relative to said given point.

9. System for locating at least one terminal (T) relative to a given point (BS) of a communication network, characterised in that it comprises:

- memory means for storing a database (BD) that collects a first set of positions that can be occupied by said at least one terminal (T) relative to said given point (BS) and a second set of profiles in module and phase over time of wave forms that can be received in said given point (BS) following the emission of an impulsive signal (P) by said at least one terminal (T) located in a respective position of said first set; said first and second set being mutually in a bi-univocal correspondence relationship, so that each of the profiles of said second set univocally identifies a respective position of said first set, - receiver means (BS) located in said given point to detect a wave form (S) received in said given point (BS) following an emission of said impulsive signal (P) by said at least one terminal (T) , the position occupied by said at least one terminal (T) being identified as the position of said first set identified by the profile of said second set corresponding to the profile of said wave form (S) received.

10. System as claimed in claim 9, characterised in that said respective positions of said first set are identified as a given coverage area of said communication network (N) .

11. System as claimed in claim 9 or claim 10, characterised in that said receiving means (BS) are configured to identify the position occupied by said at least one terminal (T) upon emitting said impulsive signal (P) as the position of said first set identified by the profile of said second set that best approximates the profile of said received wave form (S) received according to a given criterion.

12. System as claimed in any of the claims 9 through 10, characterised in that said at least one impulsive signal corresponds to the signal used by said at least one terminal (T) to transmit to said given point (BS) at least one level associated to a digital signal.

13. System as claimed in any of the claims 9 through 12, characterised in that said at least one terminal (T) is a mobile terminal.

14. System as claimed in any of the claims 9 through 13, characterised in that said receiving means (BS) comprise a plurality of receiving elements (101, ..., lOn) to measure said wave form profile in said given point (BS) .

15. System as claimed in any of the claims 9 through 14, characterised in that said receiving means (BS) are configured to: subject said received wave form (S) to phase conjugation (24), in order to generate at least a respective phase conjugated wave form (S*) and use said phase conjugated wave form (S*) for transmission from said given point (BS) to said at least one terminal (T) .

16. System as claimed in claim 15, characterised in that said at least one terminal (T) and said received means (BS) are configured for: - using, for the transmission from said given point (BS) to said terminal whose position was identified as a given position of said first set, the wave form having said corresponding profile of said second set,

- measuring, in said terminal (T) , the level of the signal thus received, said level being indicative of the precision of the location of said terminal (T) relative to said given point.

17. Terminal (T) for communication networks, configured to operate based on the method as claimed in any of the claims 1 through 8.

18. Terminal (T) for communication networks, configured to operate within the system as claimed in any of the claims 9 to 16.