Bearing determination using signals transformed into frequency domain

An apparatus, a method and a computer program for determining a bearing. The apparatus may comprise: a first transformer configured to transform a first signal formed from a set of multiple orthogonal subcarriers and received via a first path, from a time domain to a frequency domain and produce for each of a plurality of the multiple orthogonal subcarriers a respective first coefficient; a second transformer configured to transform a second signal formed from the set of multiple orthogonal subcarriers and received via a second path, different from the first path, from a time domain to a frequency domain and produce for each of a plurality of the multiple orthogonal subcarriers a respective second coefficient; and processing circuitry configured to process the plurality of first coefficients and the plurality of second coefficients to determine a bearing for the apparatus.

FIELD OF THE INVENTION

Embodiments of the present invention relate to bearing determination. In particular; they relate to an apparatus, a method and a computer program for determining a bearing. Using, orthogonal frequency division multiplexed (OFDM) signals.

BACKGROUND TO THE INVENTION

There are known techniques for determining a tearing using radio frequency (RF) signals. For example, an RF signal that is transmitted by a transmission apparatus may fee received at an antenna arrangement that comprises multiple antennas. A bearing from the antenna arrangement to the transmitting apparatus may be determined, for example, by measuring the received signal strength intensity (RSSI) at each of the multiple antennas.

BRIEF DESCRIPTION OF VARIOUS EMBODIMENTS OF THE INVENTION

According to various, but not necessarily all, embodiments of the invention there is provided art apparatus comprising: at least a first transformer configured to transform a first signal formed from a set of multiple orthogonal subcarriers and received via a first path, from a time domain to a frequency domain and produce for each of a plurality of the multiple orthogonal subcarriers a respective first coefficient; and a second transformer configured to transform a second signal formed from the set of multiple orthogonal subcarriers and received via a second path, different from the first path, from a time domain to a frequency domain and produce for each of a plurality of the multiple orthogonal subcarriers a respective second coefficient; and first coefficients and the plurality of second coefficients to determine a bearing for the apparatus.

According to various, but not necessarily all, embodiments of the invention there is provided a method, comprising: transforming a first signal formed from a set of multiple orthogonal subcarriers and received via a first path, from a time domain to a frequency domain, to produce for each of a plurality of the multiple orthogonal subcarriers a respective first coefficient; transforming a second signal formed from the set of multiple orthogonal subcarriers and received via a second path, different from the first path, from a time domain to a frequency domain, to produce for each of a plurality of the multiple orthogonal subcarriers a respective second coefficient; and processing the plurality of first coefficients and the plurality of second coefficients to determine a bearing for the apparatus.

According to various, but not necessarily all, embodiments of the invention there is provided an apparatus, comprising: means for transforming a first signal formed from a set of multiple orthogonal subcarriers and received via a first path, from a time domain to a frequency domain to produce for each of a plurality of the multiple orthogonal subcarriers a respective first coefficient; means for transforming a second signal formed from the set of multiple orthogonal subcarriers and received via a second path, different from the first path, from a time domain to a frequency domain to produce for each of a plurality of the multiple orthogonal subcarriers a respective second coefficient; and means for processing the plurality of first coefficients and the plurality of second coefficients to determine a bearing for the apparatus.

According to various, but not necessarily all, embodiments of the invention there is provided an apparatus, comprising: a diversity antenna arrangement for receiving a signal comprising multiple orthogonal subcarriers from a transmitter and comprising a first antenna at a first position for receiving the signal via a first path and a second antenna at a second position for receiving the signal via a second path, different to the first path; a first transformer configured to transform the signal received by the first antenna from a time domain to a frequency domain and produce for each of a plurality of the multiple orthogonal subcarriers a respective first coefficient; a second transformer configured to transform the signal received by the second antenna from a time domain to a frequency domain and produce for each of a plurality of the multiple orthogonal subcarriers a respective second coefficient; and processing circuitry configured to process the plurality of first coefficients and the plurality of second coefficients to determine a bearing for the transmitter.

According to various, but not necessarily all, embodiments of the invention there is provided an apparatus, comprising: processing circuitry configured: to determine a plurality of phase values from one or more received signals, each of the determined phase values falling within a predetermined range defined by a maximum value and a minimum value, to apply a scaling factor to the determined phase values to produce scaled phase values failing within the range, to determine an average scaled phase value by averaging the scaled-phase values, and to determine an average phase value for the plurality of phase values by applying the scaling factor to the average scaled phase value.

According to various, but not necessarily all, embodiments of the invention, there is provided a method, comprising: determining a plurality of phase values from one or more received signals, each of the determined phase values failing within a predetermined range defined by a maximum value and a minimum value; applying a scaling factor to the determined phase values to produce scaled phase values failing within the range; determining an average scaled phase value by averaging the scaled phase values; and determining an average phase value for the plurality of phase values by applying the scaling factor to the average scaled phase value.

FIG. 1schematically illustrates an apparatus2. The apparatus2is a receiver apparatus. Each of N received signals siwhere i=1, 2 . . . N is input to a respective transformer Ti. N is equal to or greater than 2 or 3.

The respective transformers Timay, for example, be provided by different distinct hardware circuits arranged in parallel to operate simultaneously or may be provided by a single hardware circuit that is used in a time multiplexed manner so that it sequentially operates as each of the respective transformers Tiin turn.

Each of the N input signals sihas traveled along a different signal path pito arrive at the apparatus2. A different path may result from having multiple receiver locations at the apparatus (receiver diversity) and/or may result from having multiple transmission locations for the received signals si(transmitter diversity).

FIG. 2illustrates a system14comprising the apparatus2, a first transmission point101for sending a first transmitted signal121that is received as received signal s1by the apparatus2and a second transmission point102for sending a second transmitted signal122that is received as received signal s2by the apparatus2. The first transmission point101is at a different location to the second transmission point102and transmitted signals121,122are received as received signal s1, s2at a single point by receiver circuitry comprising an antenna20. Thus the received signal s1is received via a first signal path p1and the received signal s2is received via a second signal path p2that have different lengths. The difference in length between the signal paths depends upon the relative displacement of the first transmission point101and the second transmission point102and a bearing θ of the apparatus2. In this simplified two-dimensional figure, the bearing is reduced to a single angle. It should, however, be appreciated that in three dimensional use the bearing will comprise two angles.

FIG. 3illustrates a system14comprising the apparatus2and a transmission point101for sending a first transmitted signal121that is received as received signal s1by the apparatus2and for sending a second transmitted signal122that is received as received signal s2by the apparatus2. The first transmitted signal121and second transmitted signal121may in different implementations be the same signal at a single point in time or the same signal at different points in time or different signals at the same time or different signals at different times. The apparatus2, in this example, has a diversity antenna arrangement comprising multiple antennas at different positions, in this example, two antennas201,202are illustrated but in other implementation more antennas may be used. A first antenna202receives the signal s2via a first signal path p1and a second antenna202receives the signal s2via a second signal path p2that has a different length to the first signal path p1. The difference in length between the signal paths in this example depends upon the relative displacement of the first antenna201and the second antenna202and a bearing θ of the apparatus2, in this simplified two-dimensional figure, the bearing is reduced to a single angle, it should, however, be appreciated that in normal three dimensional use more than two antennas are used which are not arranged in a straight line and the bearing will comprise two angles. The magnitude of the relative displacement of the first antenna201and the second antenna202is typically very small compared to the length of the signal paths.

Referring back toFIG. 1, a received signal sican be represented as a weighted linear combination of a plurality of sub-carrier frequencies ωj, where j=1, 2 . . . M. Typically the received signals are orthogonal frequency division multiplexed (OFDM) signals

For example, a received signal simay be represented by:

si=∑j=1M⁢aij⁢exp⁡(jωj⁢t)
where the frequencies ωjare mutually orthogonal sub-carrier frequencies, and aijis a complex weighting coefficient that modulates the frequency ωje.g. a received symbol.

The transmitted signal12ioriginally transmitted along signal path pi(which is received as signal si) may be represented as

The complex weighting coefficient aijmay be modelled as a multiplication of the original transmitted coefficient bijand a path-sensitive complex value xij.

The path-sensitive complex value xijcan be represented as a magnitude Rijand a phase φij.
aij=bij*Rijexp(jφij)
where the frequencies ωjare mutually orthogonal sub-carrier frequencies, bijis a complex weighting coefficient e.g. a transmitted symbol and −π≦φij≦π.

The set of path-sensitive complex values xijrecord for each pairing of signal path piand frequency ωjthe effect that signal path pihas had in changing the originally transmitted data, the coefficient bij.

The transformer Tiin this example is configured to transform a received signal sifrom a time domain to a frequency domain and produce for each of the M multiple orthogonal sub-carriers ωja respective coefficient aijto create a set Aiof coefficients {ai1, ai2, . . . aiM}. The coefficients are provided to processing circuitry4.

The transform that is performed fey the transformer Timay be, for example, an inverse discrete Fourier transform. A fast Fourier transform algorithm may in some embodiments be used to perform the discrete Fourier transform.

The set of coefficients {ai1, ai2, . . . aiM} are associated with a particular signal path pi. Each of the complex valued coefficients comprises a path-sensitive complex value xijthat includes phase information φijand amplitude information Rij. This information is dependent upon the length of the signal path pi. For example, the difference between φijfor path p1and p2depends upon the difference in position and orientation of the first transmission point101and the second transmission point101when there is transmitter diversity. For example, the difference between φijfor path p1and p2depends upon the difference in position and orientation of the first antenna201and the second antenna202when there is receiver diversity. The information may also be subject to dispersion effects and have some dependence upon the dispersive properties of the signal path piat the time the received signal sitraveled along that path. The set of coefficients {ai1, ai2, . . . aiM} share in common that they have traveled along the same signal path pibut are each associated with a different frequency ωjand therefore potentially different dispersion effects.

The processing circuitry4may obtain phase information φijand amplitude information Rijusing a known stored value of bij(e.g. the transmitted symbol) and the determined coefficient aij. For example, each of the determined coefficients aijmay be compared with a known stored value of bij.

The processing circuitry4, as illustrated by block6, combines phase information φijand amplitude information Rijfor the set of coefficients {ai1, ai2, . . . aiM} to obtain the representative phase information φiand representative amplitude information Rifor the signal path pi. This may compensate for dispersive effects and random noise effects.

The representative phase information φiand representative amplitude information Rifor the signal path pimay be the average phase information φiand average amplitude information Rifor the signal path pi.

For example, the average amplitude information Rimay be determined using the following equation:

The phase information φijthat is determined using known stored values of bijand the determined coefficients aijrelates to phase values that fall within a predetermined range defined by maximum and minimum values. For example, it may be that −π≦φij≦π.

In the event that the phase values φjfor a particular signal path piare situated close to the minimum and maximum boundaries of the range, it may not be possible to determine the average phase information φicorrectly by simply calculating the arithmetic mean of the phase values φj.

A first method for determining the average phase information φiinvolves determining phase values φjfor a particular signal path pi, and treating each phase value φjas a vector of arbitrary length. The vectors are summed and the angle of the resulting vector provides the average phase information φi.

A second method100for determining the average phase information φi, is illustrated inFIG. 4. Advantageously, the second method100generally requires less computational power than the first method.

At step101ofFIG. 4, phase values φiare determined using known stored values of bjand the determined coefficients ajfor a particular signal path pi.

At step102, a scaling factor φsis applied to each of the phase values φj. In this particular example, the scaling factor φsis subtracted from the phase values φjto produce a plurality of scaled phase values φj′:
φ′j=φj−φs

The scaling factor φsmay be determined from the phase values φi. For example, the scaling factor φsmay be set as:
φs=φ1−φm
where φmis the midpoint of the range for φijand φ1is the first phase value that is determined for a particular signal path pi. For example, if −π≦φij≦π, then φmis zero, so φs=φ1.

If the variance of the phase values φjis small and the first phase value φ1is located close to either the minimum or maximum boundary of the range, setting the scaling factor φsin accordance with the above equation will produce a plurality of scaled phase values φj′ that are located away from the minimum and maximum boundaries of the range.

In some embodiments of the Invention, the processing circuitry4determines whether the first phase value φ1appears to be erroneous before it is used to set the scaling value φs. For example, it may do this by calculating the difference between the first phase value φ1and some or all of the other phase values φifor that signal path pi. In the event that the average difference is above a threshold, the processing circuitry4may determine that the first phase value φ1is erroneous, it may perform the same procedure for other phase values φjuntil an appropriate value is found for use in setting the scaling factor φs.

In order to ensure that all of the scaled phase values φj′ fall within the range −π≦φij′≦π, the scaling factor φsmay be applied to the phase values φiusing modular arithmetic (also known as modulo arithmetic). In this particular case, type of the modular arithmetic used is “modulo 2π” because the phase values φijare measured in radians and the range is 2π. If the phase values φijwere measured in degrees, the type of modular arithmetic used would be “modulo 360”.

At step103, an average scaled phase value φi′ is determined by calculating the arithmetic mean of the scaled phase values φi:

At step104, the sealing factor φsis applied to the average scaled phase value φi′ to determine an average phase value φi. In this particular example, the scaling factor φsis added to the average scaled phase value φi′, because the scaling factor φswas subtracted from the phase values φjto produce a plurality of the scaled phase values φj′. Therefore:
φi=φ′i+φs

In order to ensure that the average phase value φifalls within the range −π≦φi≦π, the scaling factor φsmay be applied to the average scaled phase value φi′ using modular arithmetic. As indicated above, in this particular case, the type of modular arithmetic used is “modulo 2π” because the phase values φijare measured in radians and the range is 2π.

The combination, for a signal path pi, of the phase information φijand amplitude information Rijfor each of the sub-carrier frequencies ωjresults in a fast and accurate assessment of the phase information φiand amplitude information Rifor the signal path piwithout the need for frequency hopping.

If the transmitted coefficients bijare the same value b for all values of j then the set of coefficients ai1, ai2, . . . aiM} can be averaged and the average compared at block8to b to obtain the phase information φiand amplitude information Rifor the signal path pi.

If the transmitted coefficient bijare different for values of j then the set of coefficients ai1, ai2, . . . aiM} are compared at block8to the respective transmitted coefficient bijto obtain a set of path-sensitive complex values xij. The set of path-sensitive complex values xijare then averaged at block6to obtain the average phase information φiand amplitude information Rifor the signal path pi.

The transmitted coefficients bijmay have a value b that is constant for all paths and frequencies, alternatively the transmitted coefficients bijmay have a value bithat is constant for all frequencies of a particular signal path pibut changes to a different constant for different signal paths, alternatively the transmitted coefficients bijmay have a value bijthat may be different for different frequencies of a path and/or may be different for different paths.

FIG. 5schematically illustrates the process200for determining a bearing at the apparatus2. Although the process is illustrated as an algorithm this is merely for illustrative purposes and does not imply that software must be used. The process determines for each signal path pithe representative phase information φiand amplitude information Rifor the signal path piand then uses the representative phase information φiand amplitude information Rifor the signal paths to determine the bearing. The phase information φiand amplitude information Rifor the different signal paths may foe determined in parallel if parallel hardware is proved or sequentially. The Figure illustrates the sequential implementation.

At block202a received signal siis transformed from a time domain to a frequency domain. This produces for each of the M multiple orthogonal sub-carriers ωja respective coefficient aijto create a set Aiof coefficients {ai1, ai2, . . . aiM}.

The set of coefficients {ai1, ai2, . . . aiM} are associated with a particular signal path pi. Each of the complex valued coefficients in a set comprise a path-sensitive complex value xijthat includes phase information φijand amplitude information Rij. At block204, phase information φijand amplitude information Rijfor the set of coefficients {ai1, ai2, . . . aiM} is combined to obtain the representative phase information φjand representative amplitude information Rifor the signal path pi.

The combination, for a signal path pi, of the phase information φijand amplitude information Rijfor each of the subcarrier frequencies ωjresults in a fast and accurate assessment of the phase information φiand amplitude information Rifor the signal path piwithout the need for frequency hopping.

Blocks206and208, result in the repetition of blocks202and204for each of the N signal paths pi.

At block208, the phase information φiand amplitude information Rifor all or some of the N signal paths is processed to determine the bearing of the apparatus2.

At block208, the processing circuitry may renormaiize the phase information φiand amplitude information Rifor the signal paths pi. For example, a particular signal path prmay be designated a reference path and phase information φrand amplitude information Rrfor this path are used as a reference for the phase information φiand amplitude information Riof the other paths.

The processing circuitry4may, for example, calculate φre=φi−φrfor each i=1, 2 . . . N and calculate Rre=Ri/Rrfor each i=1, 2 . . . N. φrerepresents the phase introduced by signal path pirelative to signal path pi. Rrerepresents the gain introduced by signal path pirelative to signal path pr. The set of pairs φre, Rrefor each of the signal paths pidefines a bearing for the apparatus2.

The processing circuitry4may access a lookup table that has measured reference pairs φre, Rrefor each possible bearing (each possible combination of azimuth angle and elevation angle). A particular set of pairs φre, Rremay be used to look-up a bearing.

The lookup table may have measured reference pairs φre, Rrefor a limited number of the possible bearings. A correlation may be performed between the measured pair φre, Rreand the set of reference pairs φre, Rreto identify a closest matching reference pair φre, Rre. The closest matching reference pair φre, Rreis then used to look-up a bearing in the lookup table.

The look-up table may be created by calibrating the apparatus2. The values in the look-up table depend upon the relative displacement of the transmission points101,102. . . if transmission diversity is used and depend upon the relative displacement of the antennas201,202. . . if receiver diversity is used.

There are other mechanisms for calculating a bearing. For example, if for example, receiver diversity is used and the antennas201,202. . . are arranged; along three orthogonal axis then the average phase information φifor each path in combination with straightforward trigonometry may be used by the processing circuitry4to determine the bearing.

FIG. 8illustrates one example of the apparatus10. The apparatus10may, in some embodiments of the invention, be a hand portable electronic device.

In this example, the functionality of the transformers Tiare carried out in hardware by a single transform hardware circuit40and the functionality of the processing circuitry4is carried out in a processing hardware circuit42. In some embodiments, the transform hardware circuit and the processing hardware circuit may be provided in a single component whereas in other embodiments they may fee provided as separate components. The transform hardware circuit sequentially operates as each of the transformers Ti. Switching circuitry44upstream of the transform hardware circuit40provides the received signals siin a time division multiplexed manner.

The transform hardware circuit40and the switching circuitry44may be part of a typical OFDM receiver. Although such a typical OFDM receiver may also have processing hardware circuitry it will not perform the operations of processing circuitry4described, for example, in relation toFIGS. 1 and 5and may not have receiver diversity. Thus in implementations of the invention existing components of an OFDM receiver may be refused.

Furthermore, it should be appreciated that the functionality of the transformers Tiand/or the processing circuitry4could, alternatively be implemented using a processor50and a memory52storing a computer program54as illustrated inFIG. 7.

The processor50is configured to read from and write to the memory52. The processor50may also comprise an output interface via which data and/or commands are output by the processor50and an input interface via which data and/or commands are input to the processor50.

The memory52stores a computer program54comprising computer program instructions that control the operation of the apparatus2when loaded into the processor50. The computer program instructions54provide the logic and routines that enables the apparatus2to perform one or more of the blocks illustrated inFIG. 4andFIG. 5. The processor50by reading the memory52is able to load and execute the computer program54.

The computer program54may arrive at the apparatus2via any suitable delivery mechanism56. The delivery mechanism58may be, for example, a computer-readable storage medium, a computer program product, a memory device, a record medium such as a CD-ROM or DVD, an article of manufacture that tangibly embodies the computer program. The delivery mechanism may be a signal configured to reliably transfer the computer program. The apparatus2may in some implementations propagate or transmit the computer program54as a computer data signal.

Although the memory52is illustrated as a single component it may be implemented as one or more separate components some or all of which may be integrated/removable and/or may provide permanent/semi-permanent/dynamic/cached storage.

Although embodiments of the present invention have been described in the preceding paragraphs with reference to various examples, if should be appreciated that modifications to the examples given can be made without departing from the scope of the invention as claimed. For example, the method of determining an average phase value illustrated inFIG. 4has been described in relation to an OFDM signal. However, it will be appreciated that the method could be used to determine an average phase value for phase values determined from more than one signal. Also, the signals need not be OFDM signals. For example, they could be Bluetooth signals.

The description of the method ofFIG. 4above describes a scaling factor φsbeing deducted from the determined phase values φjand then being added to the average scaled phase value φi′, in order to determine the average phase value φi. Alternatively, the scaling factor φsmay be added to the determined phase values φiand then deducted from the average scaled phase value φi′ to determine the average phase value φi.