GB2380881A

GB2380881A - Estimating the angle of arrival at a mobile terminal

Info

Publication number: GB2380881A
Application number: GB0124260A
Authority: GB
Inventors: Stephen William Wales
Original assignee: Roke Manor Research Ltd
Current assignee: Roke Manor Research Ltd
Priority date: 2001-10-10
Filing date: 2001-10-10
Publication date: 2003-04-16
Anticipated expiration: 2021-10-10
Also published as: GB2380881B; GB0124260D0; CN1326417C; CN1413056A

Abstract

A base station, with a plurality of antenna elements, radiates electromagnetic beams in distinct directions, where each beam has a distinct signature sequence corresponding to the radiated direction. A mobile terminal estimates the angle of arrival (AOA) by determining which of the received beams has the greater amplitude, or by interpolating the beam with the greatest amplitude with other neighbouring beams. This estimation is carried out through a defined relationship by associating the channel impulse response with the beam providing the greater amplitude and the midambles. This method can be used in wireless communication networks according to the UMTS Terrestrial Radio Access (UTRA) standard with code division multiple access (CDMA) or time duplex division (TDD). The mobile terminal can carry out self location by calculating the time of arrival (TOA) and/or the time difference of arrival (TDOA) from the intersection of beams from at least two base stations to estimate the angle of arrival.

Description

METHOD FOR ESTIMATING ANGLE OF ARRIVAL AT A MOBILE TERMINAL TECHNICAL FIELD The invention relates to a method for estimating the angle of arrival at a mobile terminal wherein a plurality of eictromagnetical beams are radiated from a base station through a plurality of antenna elements. The invention further relates to a method of locating a mobile terminal, in particular a cellular phone. The invention also relates to a mobile terminal, in particular a cellular phone.

BACKGROUND OF THE INVENTION In particular in mobile communication it is increasingly required to locate mobile terminals for most efficiently using the cellular networks. This also implies the needs for efficiently separating radiated signals and for estimating channel responses.

Various methods for locating a mobile terminal are known using angle of arrival (AOA), time of arrival (TOA) and time difference of arrival (TDOA) methods. Those methods are for example described in US-Patent 6,047, 192 to John E. Maloney at al.

The location of standard, wireless radio frequency (RF), communications transmitter/receivers ("transceivers") is determined without modifying standard communications devices in any way.

The communications transceivers most popularly used by the general public are the mobile units (i. e.,"telephones") of cellularized communications systems. Examples included the"cellular telephone"and"personal communications service" (PCS) systems. For advancing an AOA system matched-replica correlations are used to enhance the robustness of the AOA system and to extend the applicability of such fundamental concepts into the domain of severe co-channel interference. Co-channel interference is a particular problem and inherent with a type of digital communication system known as code-division-multiple-access (CDMA) communications. U. S.

Patent 6, 04if, 192 further enhances the utility of the correlative derivation of any

measurements by eliminating requirements for bursty, analog, control signals and for the back-haul communications of signal representations. It extends the applicability of the matched-replica processing to enable the processing of signals of"continuous"or opportunistic (rather than merely induced or transponded) transmissions as well as of transmissions of digital formats, such as of voice signals in CDMA systems. But although using matched-replica correlative processing to provide robust and efficient measures of AOAs, as well as TOAs or TDOAs, for all of the communications signal formats, the localisation is by using information gained at the base stations, so that still a central site connected to all base station is needed to determine the location of the mobile unit.

For the efficient operation of a wireless communication network various methods are know for joint detection and channel estimation for currently used transmission procedures like code division multiple access (CDMA), Frequency Division Multiple Access (FDMA), Time Division Multiple Access (TDMA) or Universal Mobile Telecommunication System (UMTS). WO 99/44314 Al refers to all of these wireless telecommunication methods using a common air interface for home telecommunication systems. German Patent DE 198 20 761 Cl refers to a method for channel estimation in particular for CDMA telecommunication systems, using a burst structure which includes between two data blocks a midamble for the channel estimation. German Patent DE 19623665 Cl, WO 99/40698 Al and WO 00/35129 Al, each of which provides another distinct method for channel estimation for CDMA telecommunication systems. Another efficient method for channel estimation in CDMA telecommunication systems is given by B. Steiner,"Optimum and SubOptimum Channel Estimation for the Uplink of CDMA Mobile Radio Systems with Joint Detection", European Transactions on Telecommunications, Vol. 5, No. 1, JanFeb, 1994, pp39-49.

SUMMARY OF THE INVENTION It is accordingly an object of the invention to provide an efficient method for determining the angle of arrival at a mobile terminal, in particular for locating this mobile terminal in a wireless telecommunication network using advanced

technologies like CDMA, UMTS or beyond. It is a further object of the invention to provide a mobile terminal allowing the self-determination of an angle of arrival and/or the self-location of the terminal.

With the foregoing and other objects in view there is provided, in accordance with the invention, a method for estimating the angle of arrival at a mobile terminal wherein a plurality of electromagnetical beams are radiated from a base station through a plurality of antenna elements, whereby each beam carries a distinct signature sequence and is radiated in a distinct direction. The method further comprising the steps of receiving a signal response at said mobile terminal comprising a composition of said signature sequences and determining the beam which is received with the highest energy at the mobile terminal and estimating the angle of arrival by associating it to the beam direction of said beam received with the highest energy at the mobile station.

The invention proceeds from the perception that in a mobile communication systems a mobile terminal could provide means to locate itself using signals received from one or more base station transmitters. Such a passive method of location would not require the mobile terminal to transmit to the base station. This method is of great advantage when the mobile may be irregularly transmitting to a base station. At the mobile terminal local scattering objects cause the received signal to arrive via a multitude of paths uniformly distributed in angle of arrival. At the base station it is usual to observe a similar phenomenon but the angular spread is concentrated along the direction to the mobile. If multiple receive beams are formed at the base station receiver by measuring the power in each of these beams it is possible to estimate the angular position of the mobile. Through reciprocity the mobile can estimate its angular position relative to the base station if the base station transmits different synchronisation patterns on each of a number of transmit beams by measuring the power received in each beam, despite the fact that the received signal arrives via a multitude of paths uniformly distributed in angle of arrival.

One method where a mobile terminal can locate itself would be using the signals transmitted by several base stations and to measure the observed time difference of

arrival between signals from different base stations. Time of arrival measurements are based upon the first arriving path detected by the receiver. The first arriving path due to multipath propagation may not be equal to the delay of the line of sight path, and an excess delay will be observed. The excess delay can cause a large error in the position estimate. So the invention does not rely on such a method at least not on such a method alone, but uses an angle of arrival determination.

If a base station uses a multi-element antenna or multiple antennas, for example smart antennas, to transmit on then it is possible for that antennas at each base station to form a number of beams in different known directions. By transmitting a different known signature sequence, also called transmission or synchronisation sequence, from a number of overlapping beams that provide coverage over the entire cell area it is possible for the mobile to determine the beam providing the strongest reception, which leads to the direction defining the angle of arrival. Data transmissions from each beam would be identical.

In accordance with another feature the step of estimating the angle of arrival comprises interpolating between the directions of the beam received with the highest energy and at least one other neighbouring beam. The resolution of the angle of arrival estimate can be improved by interpolation using the relative energies measured from beams adjacent to the beam producing the maximum energy. Averaging of multiple measurements may be applied to improve the accuracy of the estimate. Each beam may have an opening angle of about 10 , so that due to this the estimated value of the angle of arrival already has an uncertainty. By using interpolation techniques, which are known to those skilled in the art and working in the field, this uncertainty of the angle of arrival may be reduced to less than 5 , in particular about 30.

In accordance with a further feature the step of radiating comprises radiating signals having a data field and a midamble sequence, said midamble field being the signature sequence. Various midamble sequences are known, whereby the method is not limited to those midambles already used but it can also be applied to any midamble structure serving as a signature sequence.

In accordance with an added feature the step of determining the beam with the highest t energy comprises estimating the complex channel impulse response of each beam as a function of the received signature sequence signal, determining the beam which complex channel impulse response has the highest absolute value and selecting this beam as the one with the highest energy received at the mobile station. If are samples of the channel impulse response estimate pertaining to the m-th beam, then an estimate of the angle of arrival can be obtained through the following, by determining the channel response with maximum absolute value:

It is understood that for determining the beam with the highest energy received at the mobile terminal any other appropriate procedure may be applied.

In accordance with an additional feature the channel impulse response is estimated as h = (SHS) S"r, with r being the received signature sequence signal. For example by denoting the midamble chips by s, (m) for the i-th midamble chip and m-th beam, the

midamble sequence transmitted by the n-th element may be given by : t

Where w are a set of weights defining the beam pattern for the m-th beam and g (T) is the response of a pulse shaping filter. If c, denote the chips of the data part of the waveform then the signal transmitted by the n-th element is given by:

At the receiver, the received version of the midamble sequence can be represented by:

Where h is the coefficient of the channel impulse response at delay 7) for the n-th transmission path. An impulse response relating to the m-th beam can be obtained at the receiver by various methods. A particular efficient method is described in B.

Steiner,"Optimum and Sub-Optimum Channel Estimation for the Uplink of CDMA Mobile Radio Systems with Joint Detection", European Transactions on Telecommunications, Vol. 5, No. 1, Jan-Feb, 1994, pp39-49, which is being incorporated herewith by reference. According to this method at the receiver, the mobile terminal, the received signal is sampled. P samples of the midamble sequence are located in the received signal, these samples form the vector r and the channel impulse response estimate for all M beams can be obtained from the following equation:

Where the matrix S=ts, S < '\.. Sjwith =, , "i = Smod ( (M-m) W+r-j) p i E [0, P -1 ], j E [0, W -1] are the chips of the midamble sequence.

The matrix inversion and multiplication involved in forming the matrix (SHSr1SH need only be performed once. Consequently the channel estimation process involves only a matrix multiplication.

By estimating the coefficients of the channel impulse response for each of the M beams using the above algorithm and calculating the energy of each estimated channel impulse response an angle of arrival estimate can be obtained by associating the channel impulse response providing the maximum energy with a particular beam through a defined relationship between beams and midambles.

In accordance with yet another feature the step of radiating comprises radiating signals according to the UMTS Terrestrial Radio Access (UTRA) standard or a

similar proposal to this standard. In accordance with yet a further feature the signals are generated by time duplex division (TDD), in particular when using UMTS standard. In accordance with yet an added feature the signature sequence comprises P + W-1 chips, with W being the maximum duration in chip intervals of the channel dispersion and P the number of samples of signature sequence located in the received signal. For example in an UTRA TDD (UMTS Terrestrial Radio Access with Time Division Duplex) system the base station comprises a transmit antenna consisting of N antenna elements through which different signals can be transmitted. By altering the relative phasing of transmissions to each antenna element different beams can be formed in different directions. It is possible to form M beams simultaneously such that coverage of the entire cell area is possible. For the broadcast channel the data transmitted by each beam is identical, but a different synchronisation sequence is used. In the UTRA TDD system this synchronisation sequence is referred to as a midamble and comprises a sequence ofP+W-1 chips. Cyclic shifts of the basic midamble sequence of length P are used to generate different sequences. Different midamble sequences can be transmitted simultaneously and at the receiver (the mobile terminal) a joint channel estimation algorithm can be used to obtain different channel impulse response estimates that pertain to different transmission paths, which in this case relate to transmissions via different beams. The construction of the midamble sequences based on cyclic shifts of a particular sequence enables an efficient implementation of the joint channel estimation to be realised. By measuring the power received in each beam it is possible to estimate the angle of arrival of the signal to an accuracy equal to the angular separation of the beams. In practice multipath propagation will limit the accuracy to which angle of arrival can be measured.

According to another object of the invention there is provided a method for selflocating of a mobile terminal, wherein in said mobile terminal the angle of arrival (AOA) from at least two base stations is determined by using a received signature sequence of a number of beams-with each beam having a distinct direction-for estimating which beam is received with the highest energy at the mobile terminal and defining the direction associated with that beam as representation of the angle of arrival, whereby the location is estimated as the intersection of the at least two determined trajectories to said base stations. In an alternative method for self-location

of a mobile terminal, in said mobile terminal at least one angle of arrival (AOA) from at least one base stations is determined as mentioned above and the time of arrival (TOA) and/or the time difference of arrival (TDOA) from at least two base stations is determined in the mobile terminal and the times of arrival (TOA) and/or time differences of arrival (TDOA) are used in conjunction with the angle of arrival (AOA) to determine the location of said mobile terminal. To those skilled in the art there are known various methods for determining the time of arrival and the time difference of arrival.

In measuring the time of arrival of transmissions the mobile would detect a known transmission from each of the base stations in order to determine the channel impulse response to each base station. The known transmission might comprise a known pattern of data symbols. Correlation with this known pattern of data symbols would enable the mobile to determine the channel impulse response, the first detectable path being used to compute the time of arrival. The base station will continually transmit a broadcast channel that covers the entire cell area and this transmission is most likely to be used to determine the time of arrival.

According to a further object of the invention there is provided a mobile terminal for self-estimating the angle of arrival (AOA) at the mobile terminal comprising a receiving device for receiving a signal response from a plurality of electromagnetical beams which are radiated from a base station through a plurality of antenna elements, whereby each beam carries a distinct signature sequence and is radiated in a distinct direction and said response comprising a composition of said signature sequences.

The mobile terminal further comprises an estimating device for determining the beam which is received with the highest energy at the mobile terminal and estimating the angle of arrival by associating it to the beam direction of said beam received with the highest energy at the mobile station. Those receiving devices for receiving the signals radiated by the base station are well known, so that no further explanation is needed for those skilled in the art. The estimating device may be implemented as a microprocessor, a software application in an already existing processing device of the mobile terminal or any other suitable device.

In accordance with yet an additional feature the mobile terminal is a cellular phone, a personal communication service (PCS), a portable computer or an in-vehicle communication system.

Also the invention is illustrated and described herein as embodied in a communication system comprising a mobile terminal, in particular a cellular phone, and a base station it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of disclosures of the claims.

The method of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a schematic block diagram of a multi-beam base station and a mobile terminal Figure 2 is a schematic representation of two base stations and a localisation area of a mobile wireless communication unit.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the figures of the drawings, components corresponding to one another of the respectively shown exemplary embodiments in each case have the same reference numerals. The drawing is partially simplified in order to emphasis certain features of the invention.

Referring now to the figures of the drawing in detail and first, particularly to Figure 1 thereof, there is shown in a schematic block diagram an exemplary embodiment of a base station 10 sending radio beams 19. 1-19. M to a mobile terminal 1, a cellular phone. The mobile terminal 1 has a receiving device 2, which may be a standard

device used within cellular phones comprising an antenna. The receiving device 2 being connected within the mobile terminal 1 to an estimating device 3 for estimating an angle of arrival with the base station 10.

The base station 10 comprises M signature sequence generators 12.1 to 12. M, which generate a signature sequence to distinguish between different beams 19.1 to 19. M radiated by the base station 10. The base station 10 comprises a data generator 13, which generates those data representing the information to be transmitted to the mobile terminal 1. In M data combiners 14.1 to 14. M the data from the data generator 13 is combined with the signature sequence generated by an respective signature sequence generator 12.1 to 12. M. To each data combiner 14.1 to 14. M a respective beam forming network 15.1 to 15. M is connected. Each beam forming network 15.1 to 15. M is connected through a plurality of data providers 16.1 to 16. M with one single translator 17. The translator 17 translates from baseband signals into radio frequency signals. The translator 17 is connected to M antenna elements 18.1 to 18. M, for example smart antennas.

In Figure 1 the situation is depicted where the base station 10 transmits a broadcast channel via M beams 19.1 to 19. M that cover an entire cell area. Beams 19.1 to 19. M are formed by each of the M beam forming networks 15.1 to 15. M. The signals applied to the beam forming networks 15.1 to 15. M comprise a combined data field and a signature sequence, in particular a midamble sequence. For example in the UTRA TDD system each burst comprises two data fields separated by a known synchronisation sequence or midamble. Each data field comprises data symbols spread by a known spreading sequence. The midamble is constructed according to the method described in B. Steiner (see above) and comprises P+W-1 chips, where W is the maximum duration in chip intervals of the channel time dispersion.

The received midamble signal allows as described above the estimation of the channel response for each beam 19.1 to 19. M at the mobile terminal 1, which is performed in the estimating device 3. The channel response having the highest energy at the mobile terminal 1 serves for selecting the direction of the respective beam (s. Fig. 1 beams 19. K; 19. L) as the direction which defines the angle of arrival between the base

station 10 and the mobile terminal 1.

Figure 2 provides a schematic representation of two base stations 10 and a localisation area 5 of a mobile wireless communication unit 1. Each base station 10 has a plurality of M antennas. For sake of clarity only three antennas (18. K, 18. K+1, 18. K-1 ; 18. L, 18. L+1, 18. L-1) for each base station 10 are shown. The plurality of antennas 18 cover the entire cell area, transmitting signals over 360 , i. e. a full circle. Each beam 19. L; 19. K of the antenna may have an opening angle ot, from some degrees, for example about 100. According to the method described above the angle of arrival for both base stations 10 is determined in the mobile terminal 1, so that the location of the mobile terminal 1 can estimated to be in the intersection area of the widened beams 19. K and 19. L, which define in its intersection a localisation area 4. A refined localisation area 5 can be obtained by using interpolation methods, including the channel responses of neighbouring beams 18. K-1 and 18. K+1 as well as 18. L-1 and 18. L+1. Taking more beams into account may even improve the accuracy of the refined localisation area 4. It is also possible to use more than 2 base stations and/or use time of arrival or time difference of arrival methods in addition to one or more angle of arrival determinations.

Claims

CLAIMS 1. Method for estimating the angle of arrival (AOA) at a mobile terminal (1) wherein - a plurality of electromagnetical beams (19) are radiated from a base station (10) through a plurality of antenna elements (18), whereby each beam (19) carries a distinct signature sequence and is radiated in a distinct direction - receiving a signal response at said mobile terminal (1) comprising a composition of said signature sequences - determining the beam (19. K; 19. L) which is received with the highest energy at the mobile terminal (1) - estimating the angle of arrival (AOA) by associating it to the beam direction of said beam (19. K; 19. L) received with the highest energy at the mobile station (1).
2. Method according to claim 1, wherein said step of estimating the angle of arrival (AOA) comprises interpolating between the directions of the beam received with the highest energy and at least one other neighbouring beam (19. K; 19. K+1, 19. K-1).
3. Method according to claims 1 or 2, wherein said step of radiating comprises radiating t : l signals having a data field and a midamble sequence, said midamble field being the signature sequence.
4. Method according to anyone of the preceding claims, wherein said step of determining the beam (19. K; 19. L) with the highest energy comprises - estimating the complex channel impulse response of each beam (19) as a function of the received signature sequence signal - determining the beam which complex channel impulse response has the highest absolute value and selecting this beam as the one with the highest energy received at the mobile station.
5. Method according to claim 4, wherein the channel impulse response is estimated as fi = (S H S t S H r, with r being the received signature sequence signal.
6. Method according to anyone of the preceding claims, wherein said step of radiating

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comprises radiating signals according to the UMTS Terrestrial Radio Access standard (UTRA) or a proposal to this standard.
7. Method according to claim 6, wherein said signals are generated by time duplex division (TDD).
8. Method according to anyone of the preceding claims with the signature sequence ZD comprising P + W-1 chips, with W being the maximum duration in chip intervals of the channel dispersion and P the number of samples of signature sequence located in the received signal.
9. Method for self-locating of a mobile terminal (1), wherein in said mobile terminal (1) the angle of arrival (AOA) from at least two base stations (10) is determined according to the method of anyone of the preceding claims and the location is estimated as the intersection (5) of the at least two determined trajectories to said base stations (10).
10. Method for self-locating of a mobile terminal (1), wherein in said mobile terminal (1) at least one angle of arrival (AOA) from at least one base station (10) is determined according to the method of anyone of claims 1 to 8 and the time of arrival (TOA) and lor the time difference of arrival (TDOA) from at least two base stations (10) is determined in the mobile terminal (1) and the times of arrival (TOA) and/or time differences of arrival (TDOA) are used in conjunction with the angle of arrival (AOA) to determine the location (4,5) of said mobile terminal (1).
11. Mobile terminal (1) for self-estimating the angle of arrival (AOA) at the mobile terminal (1) comprising - a receiving device (2) for receiving a signal response from a plurality of electromagnetical beams (19. 1-19. M) which are radiated from a base station (10) through a plurality of antenna elements (18. 1-18. M), whereby each beam (19. 1-19. M) carries a distinct signature sequence and is radiated in a distinct direction and said response comprising a composition of said signature sequences; and

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- an estimating device (3) for determining the beam (19. K; 19. L) which is received with the highest energy at the mobile terminal (1) and estimating the angle of arrival (AOA) by associating it to the beam direction of said beam (19. K; 19. L) received with the highest energy at the mobile station (1).
12. Mobile terminal (1) according to claim 11, which is a cellular phone, a personal communication service (PCS), a portable computer or an in-vehicle communication system.