Apparatus, method and computer program for determining a frequency offset
Embodiments of the present invention relate to mobile communication systems, and, in
particular, to a estimation of a frequency offset in so-called direct air-to-ground (DA2G)
communication systems.
Background
Airlines are currently investigating solutions to provide broadband connectivity for their
passengers. Candidates are for instance commercial systems as Long-Term Evolution
(LTE), whic has been standardized as the successor o f the Universal Mobile Telecom¬
munications System (UMTS). For the downlink transmission, i.e. the direction from a
base station (BS, NodeB or eNodeB) to a mobile terminal or UE (User Equipment), LTE
utilizes Orthogonal Frequency Division Multiple Access (OFDMA) as the physical layer
technique which enables high data rate transmission, particularly in frequency selective
fading scenarios. LTE as the technology basis for a terrestrial cellular direct air-to-ground
(DA2G) communication system is a favorable option for the airlines' continental fleets
compared to satellite solutions due to the provision of higher bandwidth at lower cost.
The LTE air interface is optimized for terrestrial cellular networks. In the terrestrial environraent
there is a lot o f fading i a mobile communications channel and propagation loss
s often much heavier then free space loss due to the presence of buildings and other ob
stacles. In the direct air-to-ground scenario, wherein a terrestrial mobile communications
network is used for a communication between a mobile terminal located in an aircraft
and a ground-located base station, some partial fading may still occur but it will be typically
much less severe than the fading that a terrestrial UE on the ground may encounter.
Instead, the DA2G scenario is characterized by a wireless communications channel with
a dominating linc-of-sight (LOS) component. Reflected paths are cither negligible or - i f
observable - at u to 2 0 dB in power below the direct (LOS) path - with almost the same
Doppler shift as the LOS-componcnt. Due to the dominating LOS-component the DA2G
scenario is a Doppler shift scenario rather than a Doppler spread scenario as in terrestrial

the Doppler shifted base station carrier frequency (fc. + p r As the uplink signal
from the mobile terminal transmitter in the aircraft onboard unit also experiences the
Doppler shift f pp , it has a frequency offset of twice the Doppler shift when it reaches
the base station.
This frequency offset of twice the Doppler shift can be beyond the estimation capabilities
of the base station in th direct air-to-ground scenario. At the same time the base station
needs to receive uplink signals from multiple mobile terminals or aircraft onboard units
that may have frequency differences of four times the maximum Doppler shift, Four
times because one mobile terminal (aircraft) may move away from the base station and
another mobile terminal (aircraft) may move towards the base station at maximum al¬
lowed speed.
It is thus desirable to perform a Doppler pre-compensation by twice the Doppler shift at
the mobile terminals or the aircraft onboard units.
It is known to estimate the Doppler shi in a direct air-to-ground scenario by geometrical
calculations based on the knowledge of the ground-located base station positions, which
may be stored a database in the onboard terminal, and the heading and speed of the
aircraft obtained from an aircraft navigational system or a GPS (Global Positioning Sys¬
tem) receiver built into the onboard terminal. For that reason a mobility client entity m y
receive GPS information and base station position information and calculate the Doppler
shift, which can then be compensated in a specific DA2G processing entity.
This solution has two drawbacks. Firstly, the system relies on GPS or other navigational
data. This adds complexity as additional interfaces or components are required. Both the
required access to the aircraft data bus carrying navigational data or the required addi¬
tional GPS antenna limit installation positions inside the aircraft. If the GPS signal or
navigational data arc corrupted the system cannot be operated. Secondly, a up-to-date
database of base stations and their positions is required. If new base stations are added to
the communications system or single base station failures appear, the database becomes
inaccurate an the system cannot be operated properly at least in parts of the system's
coverage area.
Hence, it is desirable to provide improved estimation concepts for estimating speeds or
Doppler frequencies i a mobile communications network, in particular in a direct air-toground
scenario.
Summary
An embodiment provides an apparatus for determining an estimate of a frequency offset
between a carrier frequency of a received signal and a carrier frequency of a transmitted
signal The apparatus comprises a processor for determining, based on the received sig¬
nal, an estimate of the carrier frequency of the received signal, A reference signal source
is provided for generating a reference signal having a reference frequency corresponding,
within a predefined tolerance range, to the carrier frequency o f the transmitted signal. An
estimator may estimate the frequency offset based on the estimated carrier frequency o f
the received signal and based o the reference frequency of the reference signal.
In a direct air-to-ground communication scenario, wherein a terrestrial mobile communi¬
cations network is used for a communication between a mobile terminal located in
aircraft and a ground-located base station, the frequency offset may b e a Doppler fre
quency offset resulting from a movement of the aircraft relative to ground. In such a sce¬
nario the received signal is a version o f the transmitted signal compromised or corrupted
by a wireless communications channel between a receiver o f the received signal, e.g. a
mobile aircraft onboard terminal, and a transmitter of the transmitted signal, e.g. a terrestrial
base station. The transmitted as well as the received signal may both be downlink
signals, wherein the transmitted downlink signal is sent towards the aircraft from a
ground-located base station of a terrestrial mobile communications system.
The transmitted and, hence, the received signal may be, depending on the used terrestrial
communications system, a code division multiplexing signal (CDMA) or an orthogonal
frequency division multiplexing signal (OFDM). CDMA signals are, e.g., used n 3rd
generation mobile communications systems, like the UMTS system. As has been ex
plained before, the downlink of the LTE system is based on OFDM/OFDMA. It is em
phasized that embodiments are not limited to CDMA or OFDM signals. Embodiments
may also be employed for other multiple access techniques like TDMA (Time Division
Multiple Access) or FDMA (Frequency Division Multiple Access), or combinations
thereof, like they are, e.g., used i GSM/EDGE communication systems.
According to embodiments the processor may be adapted to determine the estimate of the
frequency offset of the received signal carrier frequency based o a center frequency of
the received signal or its frequency spectrum. This is the frequency with which a timedomain
signal is transmitted fro a transmitter antenna device to a receiver antenna de¬
vice.
Embodiments may employ a highly accurate reference clock in a mobile terminal receiver.
This reference clock may run independently from other clocks used i F (Radio
Frequency) and digital processing terminal receiver parts for the received signal at the
known carrier frequency of the transmitter. According to embodiments, the transmitter
may be a base station of a mobile communications system. The frequency offset, which
may be caused by the Doppler effect, may be estimated to be a frequency difference bewee
the highly accurate reference clock and the estimated center or carrier frequency o f
the received signal, as the latter is the carrier frequency at the transmitter plus the experi
enced Doppler shift.
In embodiments the apparatus for determining the frequency offset estimate may be located
in an onboard terminal of an aircraft. Hence, embodiments also comprise an aircraft
comprising an apparatus for determining an estimate of a frequency offset. In this case,
power consumption, battery life and/or costs are issues that may be ess critical compared
to typical consumer mobile terminals, like e. g. cel phones. Hence, it is possible to em¬
ploy more accurate and/or stable reference clocks in such an aircraft onboard terminal. In
some embodiments the reference signal source may be as accurate as reference clocks
typically used in base stations, which means that the reference signal source may have an
accuracy of ±0.05 ppm (parts per million), wherein one part per million denotes one part
per 1,000,000 parts, one part in 106, and a value of 1 1For a exemplary nominal
reference frequency f = 2 GHz an accuracy of ±0.05 pp means that an actual fre¬
quency generated by the reference clock does not deviate from the nominal reference
frequency by more than ±100 Hz.
In one embodiment the reference signal source comprises a highly accurate reference
clock running at the same frequency as the transmitter, which may be a base station
transmitter. The estimator may comprise a frequency comparator to estimate the fre
quency offset or the Doppler shift by a comparison of the highly accurate reference clock
signal to a signal of a local oscillator which is tuned to the center frequency of the re¬
ceived signal or its frequency spectrum. In other words the reference signal source may
comprise a tunable local oscillator and the processor may be adapted to synchronize,
based on the received signal, a frequency of the tunable local oscillator to the carrier fre¬
quency of the received signal to obtain a synchronized frequency of the tunable local
oscillator as the estimate of the carrier frequency of the received signal. The estimator
may be adapted to estimate the frequency offset based on a difference between the syn
chronized frequency of the tunable local oscillator and the reference frequency of the
highly accurate reference signal. For that purpose the estimator may comprise fre¬
quency comparator to determine the frequency difference between the synchronized frequeiicy
and the reference frequency.
According to a further embodiment the processor may be adapted to determine the esti¬
mate of the carrier frequency of the received signal based on the reference signal and a
down -converted signal, which is obtained by mixing the reference signal with the received
signal. In this embodiment the frequency offset, which may result from a Doppler
shift plus any additional offset coming e.g. from an inaccuracy of the reference signal
source, is not compensated by tuning a local oscillator. Instead, the frequency offset may
be fully compensated in the digital domain by appropriate signal processing algorithms.
The output from the carrier frequency estimator may be directly compared to th f i c -
quency of the reference signal source to obtain the estimate of the frequency offset, i.e.
the Doppler shift plus any other inaccuracy-offset. Preferably an accuracy o f the refer¬
ence signal source, e.g. a fixed local oscillator, is high enough, such that there is negligible
frequency offset resulting from its own inaccuracy. I .e., also in this embodiment the
reference signal source may be adapted to generate the reference signal set such that its
reference frequency corresponds to the carrier frequency of the transmitted signal within
the range of ±0.05 ppm.
Note that in embodiments the accuracy that needs to be achieved by the independently
running highly accurate reference signal source in the mobile terminal should be in a
dimension such that unwanted frequency offsets due to its inaccuracy are tolerable within
defined performance bounds in the processing chain o f the mobile terminal's downlink
receiver, its uplink transmitter and the base station's uplink receiver. Furthermore, the
accuracy of the local oscillator at t e transmitter (base station) used to generate the
downlink signal is assumed to be high enough such that any frequency offsets are negli¬
gible.
According to some embodiments, the apparatus further comprises a transmitter for
transmitting a radio signal via a reverse communication link (e.g. uplink) to an origin of
the transmitted signal, e.g. a ground-located base station. A frequency offset compensator
may be foreseen to configure a carrier frequency of the radio signal based on the esti
mated frequency offset. I.e., before transmitting the radio signal in the reverse communi¬
cation link, e.g. the uplink from the aircraft's on-board terminal to the base station, the
carrier frequency of the uplink signal may be adjusted based on the estimated (Doppler)
frequency offset. The compensated uplink carrier frequency fc,upiink,comp ay then deviate
from a nominal uplink carrier frequency fc, p i ,n m by the negative frequency offset esti
mate, i.e. fc,upiink,com = c , p in ,no - f pi r - this case the uplink signal transmitted
from the moving aircraft reaches the base station at approximately the nominal uplink
carrier frequency f m
Embodiments may further comprise a method for determining an estimate of a frequency
offset between a carrier frequency of a received signal and a carrier frequency o a
transmitted signal. The method comprises steps of determining, based on the received
signal, an estimate of the carrier frequency of the received signal, generating a reference
signal having a reference frequency corresponding, within a predefined tolerance range,
to the carrier frequency of the transmitted signal, and estimating the frequency offset
based on the estimated carrier frequency of the received signal and the reference fre¬
quency of the received signal.
Moreover, embodiments may comprise a computer program having a program code for
performing one of the above methods when the computer program is executed on a com¬
puter or processor.
Here and i the remainder, information can typically be exchanged using signaling. Exchanging
a signal may comprise writing to and/or reading from a memory, transmitting
the signal electronically, optically, or by any other appropriate means.
Embodiments may allow for an efficient and robust implementation of Doppler estima¬
tion required for Doppler pre-compensation for uplink transmission in an aircraft's LTE
onboard unit. Embodiments may lead to self-containment of an LTE DA2G onboard unit
with respect to the Doppler compensation, i.e., no interfaces are required to additional
systems like GPS or other navigational information systems. This may reduce the prob¬
ability for failures and may ease installation processes inside the aircraft. Furthermore, no
up-to-date base station database may need to be maintained within the LTE DA2G onboard
unit.
Brief description of the Figures
Some embodiments of apparatuses and/or methods will be described in the following by
way of example only, and with reference to the accompanying figures, in which
Fig. shows a schematic block diagram of an apparatus for determining an estimate of a
frequency offset according to an embodiment;
Fig. 2 shows a more detailed block diagram of an apparatus for determining an estimate
of a frequency offset according to a further embodiment;
Fig. 3 shows a block diagram of a apparatus for determining a frequency-offsetestimate
according to yet a further embodiment; and
Fig, 4 shows a schematic flowchart illustrating a method for determining a frequencyoffset-
estimate according to an embodiment.
Description of Embodiments
Fig. 1 shows a schematic block diagram of an apparatus 10 for determining an estimate
17 of a frequency offset f between a carrier frequency fc>ra of a received signal 12 and a
carrier frequency f , of a transmitted signal.
The apparatus 10 comprises a processor 11 for determining, based on the received signal
12, an estimate 13 of the carrier frequency fc, of the received signal 12. The apparatus
further comprises a reference signal source 14 for generating a reference signal 15
having a reference frequency fr r corresponding, within a predefined tolerance range fref
to the carrier frequency ¾ of the transmitted signal. The frequency offset 17 may be
estimated by an estimator 16 based on the estimate 3 of the carrier frequency fc,rx of the
received signal 1 and the reference frequency f r of the reference signal 15.
For example, the apparatus 10 may be coupled with or built into a mobile onboard termi
nal of an aircraft for a direct air-to-ground communication (DA2G) between the aircraft
and at least one base station of a terrestrial mobile communications network. In such a
embodiment, the apparatus 10 may be employed in order to determine an estimate of a
Doppler shift f pp as the frequency offset f . The movement of the aircraft introduces
a Doppler frequency shift. Since the direct air-to-ground communication between the
aircraft and a base station is characterized by a dominant line-of-sight channel compo¬
nent between the moving aircraft and the terrestrial base station one may assume a rather
discrete Doppler shift instead of a Doppler spectrum, which is more common for nonline-
of-sight mobile fading channels.
The received signal 12 may e.g. be interpreted as a downlink signal stemming from the
terrestrial base station and transmitted towards the moving aircraft. For its reception the
apparatus 10 may be coupled to an antenna or an antenna array 18. In a line-of-sight
(LOS) scenario, like the DA2G scenario, an antenna array can be particularly advantageous
since receive- as well as transmit-beamforming algorithms may be effectively em
ployed i such LOS-scenarios.
The usage of embodiments of the apparatus 10 is generally not limited to a processing of
OFDM signals. However, the received signal 2 as well as the transmitted signal may be
regarded as such OFDM signals, which are used in the downlink of 4 h generation mobile
communication systems such as LTE. Since LTE is capable of delivering broadband ser
vices also to aircraft passengers, some embodiments of the present invention are directed
towards LTE-OFDM/OFDMA. As has been explained before, an uncompensated fre¬
quency offset f is particularly critical for OFDM based signals, since this modulation
technique relies on mutually orthogonal sub-carriers. For that reason, and in order to
avoid severe performance degradations, a frequency offset compensation should be per
formed before converting a received time-domain OFDM signal into the frequency do
main for further processing. In the direct air-to-ground scenario scenario the frequency
offset compensation may be performed by adjusting an uplink carrier frequency based on
the estimated Doppler shift of the received downlink signal, since a frequency offset o f
twice the Doppler shift may be beyond the estimation capabilities of the base station i
the DA2G scenario, as has been explained before.
According to embodiments the carrier frequency fc of the received signal (as well as
the transmitted signal) may be understood as the center frequency of a used communica¬
tion band. Hence, the center frequency of the wireless transmitted and/or received signal
will depend on an available spectrum, which may differ between different operators
and/or different countries. The processor 11, hence, may be adapted to determine the
estimate 7 of the carrier frequency fc, of the received signal 2 based on the center
frequency of a received signal frequency spectrum. The bandwidth o received signal
frequency spectrum is dependent on a mode of operation of the wireless communications
system. For example, if UMTS/WCDMA was used as the underlying communications
system, the received (transmitted) signal bandwidth would be 5 MHz. In LTE-systems a
scalable signal bandwidth may vary between 5 MHz, 10 MHz, 15 MHz and 20 MHz.
Embodiments rely on a highly accurate reference signal source in the mobile terminal
operating at the known carrier frequency fe.t of the base station transmitter. Thereby the
reference signal source 14 operates independently from other signal sources used in RF
and digital signal processing terminal receiver parts for the received signal 12. The car¬
rier frequencies fc. of the base stations may, e.g., be stored i a dedicated digital storage
or database comprised by the apparatus . Typically, LTE uses a frequency reuse factor
of one, which means that adjacent or neighboring cells or base stations will use the same
frequency band and, hence, the same transmit carrier or center frequency c. . However,
the used carrier frequencies of the base stations may vary for different operators of wire
less communications systems, depending on available spectral resources. Hence, the
storage in the apparatus 10 may provide different transmit signal carrier frequencies for
different network providers .
The frequency offset f caused by the Doppler effect ca be estimated by the estimator 1
to be the frequency difference between the highly accurate reference signal 5 having the
reference frequency f f and the estimate 13 of carrier frequency fc, of the received
downlink signal 12, as the latter corresponds to the carrier frequency fc,tx at the base Sta¬
tion transmitter plus the experienced Doppler shift f . o r-
Turning now to Fig. 2, a further embodiment of an apparatus 20 for determining a fre¬
quency-offset-estimate will be described. The same reference numerals as in Fig. 1 indicate
similar functional components and/or signals.
Here the processor 1 comprises a RF (Radio Frequency) processing part 1 , a digital
baseband processing part 1 2 and a tunable local oscillator 13. The radio frequency
processing part 1 may be coupled to the receive antenna device 1 such that the received
signal is input from the receive antenna device 1 to the RF-processing part
111, which may be an analog RF front-end receiver. Hence, the RF-processing part 1 1
may comprise electrical circuitry to down-convert the received analog signal 1 from the
analog RF-signal domain to an intermediate frequency domain or to a analog or digital
baseband domain. A down-converted baseband signal 121 is fed from the RF-processing
block 1 1 to the digital baseband processing block 112. The down-conversion of the re¬
ceived signal 12, having the center or carrier frequency fc,re corresponding to +
o , into the intermediate frequency or baseband domain may be achieved by mixing
the received signal 12 with an output signal 122 o f the tunable local oscillator 1 13. In
embodiments the tunable local oscillator 13 may be used for a direct down-conversion
of the received signal 12 to the baseband domain.
The R -front-end 11 , the digital baseband processor 1 2 including a carrier frequency
estimator 123, and the tunable local oscillator 113 together form a control loop for syn¬
chronizing the frequency of the local oscillator's output signal 122 to the center fre¬
quency c of the received signal 12. For this reason the LO-signal 122 or frequency
information thereof may be provided to the carrier frequency estimator 123, which may
be implemented in the baseband processing part 112 in some embodiments. The local
oscillator's 13 output signal 122 or the frequency information thereof may be used in the
baseband processor 1 2 and/or the carrier frequency estimator 123 for clearing out am¬
biguous Doppler frequency offset estimates. In other embodiments the carrier frequency
estimator 1 3 may also be realized b analog or digital circuitry comprised by the RFfront-
end 111.
According to some embodiments the carrier frequency estimator 23 may perform a cell
search procedure in order to derive a first estimate for the carrier frequency fc of the
received signal 12. In LTE the cell search procedure is based on the use of primary and
secondary synchronization signals. For the cell search the carrier frequency estimator 1 3
may b e adapted to search for the primary synchronization signal at the center frequencies
fc.tx possible at the frequency band in question. For this purpose a control signal 124 may
be used for controlling the tunable local oscillator 13 to the possible center frequencies
fc t - There exist three different possibilities for the primary synchronization signal as the
primary synchronization signal may point to one of three Physical-layer Cell Identities
(PCI). Once the primary synchronization signal has been detected, the mobile terminal
may look for the secondary synchronization signal pointing at one of 168 PCI groups.
Once one alternative for 168 possible secondary synchronization signals has been de¬
tected, the UE has figured out the PCI value from an address space of 504 IDs. From the
PCI the UE may derive information about the parameters used for downlink reference
signals and thus the UE may decode the PBCH (physical broadcast channel) carrying
system information needed to access the mobile communications system.
After initial carrier frequency estimation the carrier frequency estimator 123 may b e
adapted, in one embodiment, to output a non- vanishing control signal 24 in response to
a detected a non-vanishing residual frequency offset in the down-converted digital baseband
signal 121 . In order to avoid ambiguities when estimating the residual frequency
offset, frequency information of the LO-signal 1 2 may be used in the baseband part 12,
123. Thereby the residual frequency offset may e.g. be obtained by covarianee methods,
level-crossing rate methods or power-spectrum measurements. Based on the residual fre¬
quency offset the control signal 124 may be used for controlling or tuning the local oscillator
113 to the shifted center frequency fc, of the received signal 12. In other words the
processor 1 is adapted to synchronize, based o the received signal 12, the frequency of
the tunable local oscillator 113 to the carrier frequency f , of the received signal 12 to
obtain a synchronized frequency o the tunable local oscillator 113, which may then also
be used as the estimate 17 of the received carrier frequency. As shown in Fig. 2 , the synchronization
may be achieved with a control loop similar to a phase-locked-loop (PLL),
wherein the control loop comprises the RF front-end 1 11, the digital baseband processor
12 and the tunable local oscillator 113.
In the embodiment depicted in Fig. 2 the carrier frequency estimator 123 resides in the
digital baseband part 112 of the processor 11. The carrier frequency estimator 123 may,
however, also be implemented by analog circuits residing in the radio frequency process
ing circuit 11. The carrier frequency estimator 123 is adapted to control the local oscil
lator 113 of the processor 11 in such a way that its output frequency coincides with the
center or carrier frequency f ,n of the frequency-shifted received signal 12. The center
frequency ¾, o f the received signal is the base station transmitter's carrier frequency
fc,re plus the frequency offset ,D caused by the Doppler effect.

an atomic frequency standard, such as a rubidium standard, caesium standard, or hydro¬
gen maser. Another cheaper alternative is to discipline a crystal oscillator with a GPS
time signal, creating a so-called GPS Disciplined oscillator (GPSDO). Using an onboard
GPS receiver of the aircraft that can generate accurate time signals (down to within 30
ns of UTC), a GPSDO can maintain oscillation accuracy of 10 13 for extended periods of
time.
Turning now to Fig. 3 a further embodiment of an apparatus 30 for determining an esti
mate 17 of a frequency offset f between a carrier or center frequency fc,ra of the received
signal 1 and a carrier or center frequency ¾ of a transmitted signal will be explained.
Again, ilie same reference numerals as in Fig. 1 and/or Fig. 2 indicate similar functional
components and/or signals.
As well as the apparatus 10 and the apparatus 20, the apparatus 30 may be incorporated
in an aircraft onboard terminal for a DA2G communication with a base station of a ter
restrial mobile communications network. The apparatus 30 differs from the apparatus 20
in that the down-conversion of the received (uplink) signal to the baseband domain is
done by mixing the received signal 12 with a fixed-frequency reference signal 5 instead
of mixing the received signal 12 with a variable output of a tunable local oscillator. the
embodiment of Fig. 3 the independent reference signal source 14 comprises a local oscil¬
lator with a fixed reference frequency f f. The fixed reference frequency f .f may corre¬
spond to the transmitted carrier or center frequency c t - at least within a predefined
tolerance range, i.e., f f = fc,t + Af . Slight variations Af r from the nominal transmit
carrier frequency fc, within a range of ±0.05 ppm are hardly avoidable - even with higliprecision
reference signal sources 14, like the above-mentioned TCXOs, MCXOs,
OCXOs, and GPSDOs. Again the reference signal source 14 may be adapted to generate
the reference signal 15 independently from other signal or clock sources used to process
the received signal 12.
The output signal 15 of the reference signal source 14, i.e. the fixed local oscillator, has
the reference frequency f r= f . + fr f, wherein Af denote oscillator frequency varia¬
tions with the predefined tolerance range. The output signal 15 of the reference signal

Fig. 2 and Fig. 3, this may be done with the processor 1 which may have RF- and base¬
band processing parts 111, 112 and 13. Further, the method 40 comprises a step 42 of
generating a reference signal 15 having a reference frequency fi f corresponding, within a
predefined tolerance range Af to the carrier frequency f of the transmitted signal.
Thereby, the reference signal 15 is generated with a highly accurate reference signal
source 14 comprising, e.g., highly stable oscillators with frequency stability which is
comparable to high-accuracy reference signal sources commonly used in base stations. In
a further step 43 the frequency offset f0 is estimated based on the estimated carrier fre¬
quency fc. of the received signal 12 and the reference frequency f f of the generated
reference signal 15. Possible physical realizations of said estimation step have been ex¬
plained with reference to Figs. 2 and 3.
A person of skill in the art would readily recognize that steps of various above-described
methods can be also performed by programmed computers or signal processors. Herein,
some embodiments are also intended to cover program storage devices, e.g., digital data
storage media, which are machine or computer readable and encode machine -executable
or computer-executable programs of instructions, wherein said instructions perform some
or all of the steps of said above-described methods. The program storage devices may be,
e.g., digital memories, magnetic storage media such as a magnetic disks and magnetic
tapes, hard drives, or optically readable digital dala storage media. The embodiments are
also intended to cover computers programmed to perform said steps of the abovedescribed
methods.
The description and drawings merely illustrate the principles of the invention. It will thus
he appreciated that those skilled in the art will be able to devise various arrangements
that, although not explicitly described or shown herein, embody the principles of the in¬
vention and are included within its spirit and scope. Furthermore, all examples recited
herein are principally intended expressly to be only for pedagogical purposes to aid the
reader in understanding the principles of the invention and th concepts contributed by
the inventor(s) to furthering the art, and are to be construed as being without limitation to
such specifically recited examples an conditions. Moreover, all statements herein reciting
principles, aspects, and embodiments of the invention, as well as specific examples
thereof, are intended to encompass equivalents thereof.
The functions of the various elements shown in the figures, including any functional
blocks labeled as "processor", "signal source" or "estimator", may be provided through
the use of dedicated hardware, as e.g. a processor, as well as hardware capable of execut¬
ing software in association with appropriate software. When provided by a processor, the
functions may be provided by a single dedicated processor, by a single shared processor,
or by a plurality of individual processors, some of which may be shared. Moreover, explicit
use of the term "processor" or "controller" should not be construed to refer exclu¬
sively to hardware capable of executing software, and ma implicitly include, without
limitation, digital signal processor (DSP) hardware, network processor, application spe¬
cific integrated circuit (ASIC), field programmable gate array (FPGA), read only mem¬
ory (ROM) for storing software, random access memory (RAM), and non-volatile storage.
Other hardware, conventional and/or custom, may also be included.
It should be appreciated by those skilled in the art that any block diagrams herein repre¬
sent conceptual views of illustrative circuitry embodying the principles of the invention.
Similarly, it will be appreciated that any flow charts, flow diagrams, state transition diagrams,
pscudo code, and the like represent various processes which may be substantially
represented in computer readable medium and so executed by a computer or processor,
whether or not such computer or processor is explicitly shown.
Claims
A mobile terminal apparatus (10; 20; 30) for determining an estimate (17) of a fr e
quency offset between a carrier frequency of a received signal (12) and a carrier
frequency of a transmitted signal, the apparatus comprising:
a processor ( 11) for determining, based on the received signal (12), an estimate
(13) of the carrier frequency of the received signal (12);
a highly accurate reference signal source (14) for generating a reference signal ( 5)
having a fi ed reference frequency corresponding, with a frequency stability which
is comparable to high-accuracy reference signal sources used i base stations, to
the carrier frequency of the transmitted signal; and
an estimator (16) for estimating the frequency offset (17) based on the estimated
carrier frequency (13) of the received signal (12) and the fixed reference frequency
of the reference signal (15).
The mobile terminal apparatus ( 10; 20; 30) according to claim 1, wherein the proc¬
essor ( 11) s adapted to determine the estimate (13) of the carrier frequency of the
received signal (12) based on a center frequency of a received signal frequency
spectrum,
The mobile terminal apparatus ( 0; 20) according to claim 1, wherein the processor
( 11) is adapted to synchronize, based on the received signal (12), a frequency of a
tunable local oscillator ( 1 13) to the carrier frequency of the received signal (12) to
obtain a synchronized frequency of the tunable local oscillator ( 1 13) as the estimate
(13) of the carrier frequency.
The mobile terminal apparatus (10; 20) according to claim 3, wherein the estimator
( 6) is adapted to estimate the frequency offset based on a difference between the
synchronized frequency of the tunable local oscillator ( 113) and the fixed reference
frequency of the reference signal ( ).
5 . The mobile terminal apparatus (10; 20) according to claim 4 , wherein the estimator
(16) comprises a frequency comparator to determine the difference between the
synchronized frequency and the reference frequency.
6. The mobile terminal apparatus (30) according to claim 1, wherein the processor
( 1) is adapted to determine the estimate (13) of the earner frequency of the re
ceived signal (12) based on the reference signal (15) and a down-converted signal
(121), which is obtained based on mixing the reference signal (15) with the re¬
ceived signal (12).
7. The apparatus ( 10; 20; 30) according to claim 1, wherein the highly accurate refer
ence signal source (14) is adapted to generate the reference signal ( 15) such that its
reference frequency corresponds to the carrier frequency of the transmitted signal
within a range of 0. 1 ppm, in particular within a range of ± 0.05 ppm.
8. The mobile terminal apparatus (10; 20; 30) according to claim 1, wherein the high¬
ly accurate reference signal source (14) is adapted to generate the reference signal
(15) independently from other signal sources used to process the received signal
(12) ,
9. The mobile terminal apparatus (10; 20; 30) according to claim 1, further compris
ing
a transmitter for transmitting a radio signal via a reverse communication link to an
origin of the transmitted signal; and
a frequency offset compensator (13 1) bein adapted to configure a carrier fre
quency of the radio signal based on the estimated frequency offset (17).
10. The mobile tenranal apparatus ( 10; 20; 30) according to claim 1, being adapted to
determine, as the frequency offset, an estimate of a Doppler frequency offset for a
direct air-to-ground communication using a terrestrial mobile communications
network between a mobile terminal located in an aircraft and a ground-located base
station.
The mobile terminal apparatus ( 10; 20; 30) according to claim 1, wherein the re
ceived signal (12) is a version of the transmitted signal compromised by a wireless
Communications channel between a receiver of the received signal and a transmitter
of the transmitted signal.
The apparatus (10; 20; 30) according to claim , wherein the transmitted signal is a
code division multiplexing signal or a orthogonal frequency division multiplexing
signal.
An aircraft comprising the mobile terminal apparatus (10; 20; 30) according to
claim 1 for a direct air-to-ground communication using a terrestrial mobile com¬
munications network,
A method (40) for determining an estimate (17) of a frequency offset between a
carrier frequency of a signal (12) received at a mobile terminal and a carrier fre
quency of a transmitted signal, the method comprising:
determining (41), based on the received signal (12), an estimate (13) of the carrier
frequency of the received signal (12);
generating (42) a highly accurate reference signal (15) having a fixed reference fre
quency corresponding, with a frequency stability which is comparable to highaccuracy
reference signal sources used in base stations, to the carrier frequency of
the transmitted signal; and
estimating (43) the frequency offset based o the estimated carrier frequency (13)
of the received signal (12) and the fixed reference frequency of the reference signal
(15).
15. A computer program having a program code for performing the method of claim 1
when the computer program is executed on a computer or processor.