Patent Description:
Embodiments of the present disclosure relate to, but are not limited to, orthogonal frequency division multiplexing (OFDM) wireless communication technology, for example, relate to a method and device for combating Doppler shift.

As shown in <FIG>, in a wireless communication system, when there are a line-of-sight channel and a relative speed between a transmitter and a receiver, a Doppler shiftfd will occur, and <MAT>. In the above equation, v is the relative speed between the transmitter and the receiver, c is a speed of light, fc is a carrier frequency, and θ is an angle between the transmitter and the receiver.

When a mobile client is in a high-speed moving scenario, the Doppler shift will cause serious interferences to signals.

At present, in a <NUM> or <NUM> high-speed rail communication system, a Vehicle-to-Vehicle (V2V) system, or a Vehicle-to-Everything (V2X) system, when a user-side equipment performs cell handover, there is no relevant technical solution to effectively combat the Doppler shift.

US Patent Application <CIT> discloses a wireless communication system that antenna nodes are controlled to maintain a respective radio cell, each cell having one and the same physical cell identity. The antenna nodes are further controlled to maintain the respective radio cell in a single direction substantially along a path such that each wireless communication device, during movement in a movement direction along the path, can connect either to consecutive antenna nodes towards which the wireless communication device is moving or connect to consecutive antenna nodes away from which the wireless communication device is moving, so as to direct the antenna cells in certain directions in order to solve the problem that the Doppler shift is reversed at a cell handover and that this leads to instability / high complexity in the carrier frequency offset compensation until the new Doppler shift is correctly estimated.

WO Patent Application <CIT> discloses a method for correcting the frequency shift, which includes correcting the frequency of an input signal of an apparatus for correcting the frequency shift and eliminating the Doppler frequency shift in the input signal of the apparatus when an UE is confirmed in a high speed motion by a frequency shift elimination module which is located at the front-end of the AFC circuit, and receiving the output signal of the frequency shift elimination module and performing automatic frequency control according to the output signal by a AFC circuit, however, it does not mention any particular handling during handover.

The embodiments of the present application provide a method for combating Doppler shift, including: obtaining, in a case where a user equipment (UE) performs cell handover, a Doppler shift estimation of the user equipment at a last moment in a source cell; and obtaining an additive inverse of the Doppler shift estimation, and using the additive inverse of the Doppler shift estimation for Carrier Frequency Offset (CFO) compensation in an Automatic Frequency Control (AFC).

The embodiments of the present application further provide a device for combating Doppler shift, including: an obtaining unit, configured to obtain, in a case where a user equipment performs cell handover, a Doppler shift estimation of the user equipment at a last moment in a source cell; and a processing unit, configured to obtain an additive inverse of the Doppler shift estimation, and use the additive inverse of the Doppler shift estimation for CFO compensation in an AFC.

The embodiments of the present application further provide a computer-readable storage medium having an information processing program stored thereon. When the information processing program is executed by a processor, the above-mentioned method for combating Doppler shift is realized.

Hereinafter, the embodiments of the present application will be described in detail with reference to the drawings. It should be noted that the embodiments in the present application and features in the embodiments can be combined with each other arbitrarily if there is no conflict.

Steps shown in a flowchart of the drawings may be executed in a computer system such as a set of computer-executable instructions. In addition, although a logical sequence is shown in the flowchart, in some cases, the steps shown or described may be performed in a sequence different from the logical sequence.

For example, as shown in <FIG>, in a <NUM> or <NUM> high-speed rail communication system, a Doppler shift will cause the following interferences on a mobile client.

For the user terminal, <NUM>) using the AFC loop can better combat the Doppler shift within a cell, but in the handover zone, a carrier frequency shift or a larger CFO estimation variance due to a low Signal to Interference plus Noise Ratio (SINR) causes a terminal to have a limited performance in tracking the Doppler shift mutation and thus leads to a decrease in a reception performance of the mobile terminal in the handover zone. <NUM>) Using the GPS signal to assist in the calculation of the terminal's moving speed and position can better assist the mobile terminal in combating a Doppler mutation in the handover zone. For example, as shown in <FIG>, when the mobile client enters the base station <NUM>, the Doppler shift is obtained through GPS-assisted calculation immediately, input to a CFO estimator and output to an AFC loop filter. However, for example, for a mobile terminal user in a high-speed rail car, the GPS is shielded by a carriage and cannot be used.

In addition, in the <NUM> or <NUM> high-speed rail communication system, as shown in <FIG>, Doppler shifts of uplink transmitted signals of multiple mobile terminals in a cell, e.g., the transmitted signals at positions <NUM> and <NUM> in <FIG>, are inconsistent, which causes the base station to perform CFO estimation and compensation on uplink received signals one by one to combat possible ICI effects. A computational complexity of this combating method is relatively high, especially for a multiple UE uplink receiving node in a <NUM> or <NUM> V2V communication system.

Therefore, the embodiments of the present application provide a technical solution for combating Doppler shift, which can enable the user terminal to better combat the Doppler shift at smaller costs of the user terminal and the base station in different scenarios. Through an uplink CFO pre-compensation technology and a receiving-transmit AFC technology, a computational complexity of an uplink receiver is greatly reduced.

<FIG> is a schematic flowchart of a method for combating Doppler shift. As shown in <FIG>, the method includes step <NUM> and step <NUM>.

In step <NUM>, when a user equipment performs cell handover, a Doppler shift estimation of the user equipment at a last moment in a source cell is obtained.

In step <NUM>, the Doppler shift estimation is inversed, and the inversed Doppler shift estimation is used for CFO compensation in an AFC.

The step of obtaining the Doppler shift estimation of the user equipment at the last moment in the source cell, includes: determining the Doppler shift estimation at a moment when performing the cell handover according to the following equation: f̂d(n) = Δfc(n) - ΔfT(n).

Here, n is a moment when performing the cell handover, f̂d(n) is the Doppler shift estimation at the moment n, ΔfT(n) is an average value of Δfc in a time period of length T before the moment n while the user equipment is in a connected state, Δfc is an output value of an AFC loop filter, and Δfc(n) is an output value of the AFC loop filter at the moment n.

When the user equipment is in a PC5-based ad hoc network of a V2V/V2X system, a high-precision timing of a GPS is used to calibrate the ΔfT(n) within time T<NUM>, and a value of T<NUM> is any value from <NUM> to <NUM>.

A value of T is much larger than an average time of the user equipment in a high-speed moving state being in one cell.

The value of T is any value from <NUM> minutes to <NUM> minutes.

The step of inversing the Doppler shift estimation to be used and using the inversed Doppler shift estimation for the CFO compensation in the AFC includes: inversing the Doppler shift estimation for N times, and using the Doppler shift estimation that is inversed for N times for the CFO compensation in the AFC.

N is a natural number greater than or equal to <NUM>, and less than or equal to <NUM>.

The method further includes: enlarging a loop filter factor of the AFC in the user equipment.

The step of enlarging the loop filter factor of the AFC in the user equipment includes: enlarging the loop filter factor of the AFC in the user equipment by K times, for N times, i.e., enlarging the loop filter factor of the AFC in the user equipment by KN times.

When the user equipment is in a wireless mobile communication network, a value of K is any value from <NUM> to <NUM>.

When the user equipment is in a V2V/V2X system, the value of K is any value from <NUM> to <NUM>.

The method further includes: determining an uplink transmission carrier frequency fT , fT = f̂c - <NUM> * f̂d , f̂c = f<NUM> + f̂d , f<NUM> is a downlink carrier transmit frequency, f̂c is a frequency estimate of a current receiving carrier, f̂d is the Doppler shift estimation, f̂d is a Doppler shift, and f̂d = f̂c - f<NUM> , f̂<NUM> is a reference frequency estimation.

The method further includes: receiving Downlink Control Information (DCI), the DCI carrying a m-bit frequency adjustment value ΔfTxAFC(n), and <MAT>; and adjusting an uplink transmission carrier frequency fT(n) according to the frequency adjustment value ΔfTxAFC(n), fT(n) = fT(n - <NUM>) + ΔfTxAFC(n), Δfsubcarrier is a subcarrier spacing of an OFDM system, and α and β are preset coefficients.

The method further includes: when the user equipment is in the PC5-based ad hoc network of the V2X system, determining the transmission carrier frequency fT of a Physical Sidelink Share Channel (PSSCH), a Physical Sidelink Control Channel (PSCCH) and a Physical Downlink Shared Channel (PSDCH); where fT = f̂c - <NUM> * f̂d , f̂c = f<NUM> + fd , f<NUM> is the downlink carrier transmit frequency, f̂c is a frequency estimate of a current receiving carrier, fd is a Doppler shift, f̂d is the Doppler shift estimation, and f̂d = f̂c - f̂<NUM> , f̂<NUM> is a reference frequency estimation; and determining a carrier frequency fT,PSBCH / SLSS(n) of transmitted signals of a Physical Sidelink Broadcast Channel (PSBCH) and Sidelink Synchronization Signals; where fT,PSBCH / SLSS(n) = f̂c(n) - f̂d(n); n is a moment when performing the cell handover, f̂d(n) is the Doppler shift estimation at the moment n, f̂c(n) is a frequency estimate of a current receiving carrier at the moment n.

The technical solution provided by the present application will be described in detail below through several embodiments.

<FIG> is a schematic structural diagram of a user-side equipment. As shown in <FIG>, the user-side equipment (a receiver) includes: an AFC including a mixer, a Phase Locked Loop (PLL) local oscillator circuit, a Voltage Controlled Oscillator (VCTCXO) and a Loop Filter (LF), a Digital to Analog Converter (DAC), an Analog to Digital Converter (ADC), a CFO estimator, and a TX port, etc. The mixer receives a signal with a certain frequency from a transmitter, detects a frequency error after comparing the signal with a local oscillator signal output by the PLL local oscillator circuit and outputs it to the CFO estimator for CFO estimation. The CFO outputs an estimated CFO to the LF for compensation. The LF filters out a voltage control signal and controls the VCTCXO to adjust the frequency to approximate the frequency of the received signal. The VCTCXO uses the compensated frequency signal as a local oscillator signal and feeds it back to the mixer through the PLL for frequency error detection. Then the above processes are repeated, thereby gradually reducing the frequency error between the local oscillator signal and the received signal, and realizing automatic frequency control.

Based on the user-side equipment provided in <FIG>, <FIG> is a schematic flowchart of a method for combating Doppler shift. As shown in <FIG>, the method includes step <NUM> to step <NUM>.

In step <NUM>, a reference frequency of a user terminal is obtained.

The reference frequency is a downlink carrier transmit frequency f<NUM>, and a Doppler shift fd cannot be included in a reference frequency estimation f̂<NUM> obtained by a mobile terminal.

In step <NUM>, the Doppler shift at the user terminal is estimated.

The user terminal may obtain a frequency estimate of a current receiving carrier f̂c = f<NUM> + fd through a downlink pilot signal, and the mobile terminal uses f̂c to complete a reception for the downlink signal. It can be seen that the mobile terminal estimates the Doppler shift as f̂d = f̂c - f̂<NUM>.

In step <NUM>, - f̂d transformed from the Doppler shift estimation f̂d of a source cell is used as CFO compensation in an AFC, when the user terminal performs a cell handover.

In the high-speed rail channel, before and after performing the handover of the mobile terminal, the Doppler shift fd will inevitably change from a negative maximum value to a positive maximum value, and a mobile client can know exactly when it enters a new cell. By using these two pieces of known information, when the user terminal enters a target cell, the Doppler shift estimation f̂d of the source cell is transformed to - f̂d, and - f̂d is input into the CFO estimator and output to the loop filter of the AFC for the CFO compensation.

In step <NUM>, an AFC loop filter factor is increased.

By increasing the AFC loop filter factor, the AFC of the mobile terminal can quickly converge to a new mutation frequency, so as to reduce degradation for a reception performance after performing handover.

In step <NUM>, an uplink transmission carrier frequency fT is determined.

Since Doppler shift estimations f̂d of different mobile terminals already contain their respective position and speed information, for a <NUM> or <NUM> communication system using an OFDM system, the CFO compensation is generally performed in a time domain and requires a large amount of computation. In step <NUM>, the Doppler shift of the uplink transmission carrier frequency is pre-compensated, so that transmission carrier frequencies of multiple UEs received by an uplink receiver can be substantially the same and equal to its downlink transmit frequency fo, thereby omitting the calculation of CFO compensation for each UE.

If there is uplink data to be sent at this moment, the uplink data may be sent according to the determined uplink transmission carrier frequency fT.

In step <NUM>, control information of the transmitter is received, and the uplink transmission carrier frequency is adjusted according to the control information.

In an implementation, for a base station or the transmitter, if there is a reverse channel, e.g., a downlink channel in a high-speed rail communication system, the system may add an AFC control word ΔfTxAFC(n) for its uplink transmit frequency in a reverse UE dedicated signaling so that an uplink receiving carrier frequency of each UE converges near fo. Therefore, carrier interference (ICI) effects between UEs are further reduced. The CFO of the uplink receiving frequency of each UE needs to be estimated one by one on the uplink receiver.

If there is uplink data to be sent at this moment, the uplink data may be sent according to the determined uplink transmission carrier frequency.

The Example <NUM> of the present application is applied to an application scenario where a user-side equipment performs a cell handover in a <NUM> or <NUM> high-speed rail communication system.

<FIG> is a schematic flowchart of a method for combating Doppler shift. As shown in <FIG>, the method includes step <NUM> to step <NUM>.

A reference frequency estimation f̂<NUM> of a certain cell is a nominal frequency of the cell frequency obtained by the user terminal through high-level signaling plus ΔfT(n), ΔfT(n) is an average value of Δfc in a time period of length T before a current moment n while the user terminal is in a connected state, and Δfc is an output value of an AFC loop filter. T must be much larger than an average time of the user terminal in a high-speed moving state being in one cell, and generally ranges from <NUM> to <NUM> minutes.

The moment n is a moment when performing cell handover.

A Doppler shift estimation of the user terminal at the current moment n is f̂d(n) = ΔfT(n) - ΔfT(n) , and Δfc(n) is an output value of the AFC loop filter at the moment n.

In step <NUM>, the Doppler shift estimation f̂d of a source cell is inversed for N times, and the inversed Doppler shift estimation is used as an output of a CFO estimator.

In an implementation, at a moment when the user terminal enters a target cell, immediately the output of the CFO estimator is forced as the last Doppler shift estimation of a previous serving cell after being inversed for N times. In this way, a AFC performs CFO compensation based on the output of the CFO estimator.

In step <NUM>, an AFC loop filter factor of the user terminal is enlarged by K times, for N times, i.e., enlarging the loop filter factor of the AFC in the user equipment by KN times.

K is an empirical value, and generally ranges from <NUM> to <NUM>. N is an empirical value, and generally ranges from <NUM> to <NUM>. By increasing the AFC loop filter factor, the AFC of the mobile terminal can quickly converge to a new mutation frequency, so as to reduce degradation for a reception performance after performing the handover.

In step <NUM>, a Doppler shift of a carrier frequency of an uplink transmit signal of the user terminal is pre-compensated.

When the user terminal is in the connected state, the carrier frequency of the transmit signal at the current moment n is fT = f̂c - <NUM>* f̂d. The compensation is generally performed in a digital domain.

In step <NUM>, an uplink transmit frequency is adjusted according to received Downlink Control Information (DCI) information.

When the user terminal is in the connected state, each downlink subframe may have a DCI scheduling for a certain user terminal, and m-bit information may be added to the DCI. When the user terminal receives its own DCI information, it adjusts the uplink transmit frequency fT(n) = fT(n - <NUM>) + ΔfTxAFC(n) according to ΔfTxAFC(n) carried in the m-bit information. For example, m may be <NUM>.

Δfsubcarrier is a subcarrier spacing of an OFDM system, and α and β are empirical values, for example, α=<NUM>, and β=<NUM>. The compensation is generally performed in the digital domain.

The Example <NUM> of the present application is applied to an application scenario where a user-side equipment performs a cell handover in a V2V/V2X system of <NUM> or <NUM>.

The V2V/V2X system has both a uu interface-based networking method that is not much different from an ordinary Long Term Evolution (LTE) system, and an ad hoc networking method based on PC5 interfaces and using sidelink channels.

Regardless of whether a current user terminal is interacting with a base station or a certain forwarding node, in order to estimate a Doppler shift through GPS information, the user terminal must be knowledge of a current position, a movement speed and a movement direction of the user terminal, as while as being knowledge of a current position, a movement speed and a movement direction of the base station or the certain forwarding node, which leads to excessive system signaling overhead and complicated calculations of user terminal. Therefore, The Example <NUM> of the present application provides a solution to combat multiple Doppler shifts, which can avoid the above situation.

<FIG> is a schematic structural diagram of a user-side equipment. <FIG> has the same structure as <FIG>, and will not be repeated herein.

A reference frequency estimation f̂<NUM> of a certain base station or node is a nominal frequency of the cell frequency obtained by the user terminal through high-level signaling plus ΔfT(n), ΔfT(n) is an average value of Δfc in a time period of length T before a current moment n while the user terminal is in a connected state, and Δfc is an output value of an AFC loop filter. When the user terminal is connected to a base station through a uu interface, GPS may not be needed to assist in status indication. T must be much larger than an average time of the user terminal in a high-speed moving state being in one cell, and generally ranges from <NUM> to <NUM> minutes. When the user terminal is in a PC5-based ad hoc network, if the user terminal still has a reference frequency connected to the uu interface, the reference frequency is used; otherwise, a high-precision timing of the GPS is used by the user terminal to calibrate a free oscillating VCTCXO within T<NUM> time, and the calibration value is converted to the output of the AFC loop filter to form ΔfT(n). T<NUM> generally ranges from <NUM> to <NUM>.

A Doppler shift estimation of the user terminal at the current moment n is <MAT>.

At a moment when the user terminal enters a target cell, immediately the output of the CFO estimator is forced as the last Doppler shift estimation of a previous serving cell that is inversed for N times. N is an empirical value, and generally ranges from <NUM> to <NUM>.

K is an empirical value, and considering an uncertainty of the position and the movement direction of the user terminal relative to a base station of an original serving cell and a base station of the target cell under the V2V network, K generally ranges from <NUM> to <NUM>.

When the user terminal uses the uu interface to connect, the carrier frequency of the transmit signal at the current moment n is fT = f̂c - <NUM> * f̂d.

When the user terminal is in the PC5-based ad hoc network, carrier frequencies of transmit signals PSSCH, PSCCH and PSDCH at the current moment n are fT = f̂c - <NUM> * f̂d ; carrier frequencies of transmit signals PSBCH and Sidelink Synchronization Signal are fT,PSBCH / SLSS(n) = f̂c(n) - f̂d(n). The compensation is generally performed in a digital domain.

In step <NUM>, an uplink transmit frequency is adjusted according to received DCI information.

Only when the user terminal is connected through the uu interface, each downlink subframe may have a DCI scheduling for a certain user terminal, and m-bit information can be added to the DCI. When the user terminal receives its own DCI information, it adjusts the uplink transmit frequency fT(n) = fT(n - <NUM>) + ΔfTxAFC(n) according to ΔfTxAFC(n) carried in the m-bit information. For example, m may be <NUM>.

Δfsubcarrier is a subcarrier spacing of an OFDM system, and α and βare empirical values. For example, α=<NUM>, and β=<NUM>. The compensation is generally performed in the digital domain.

The technical solution provided by the embodiments of the present application can enable a user terminal to better combat the Doppler shift at smaller costs of user terminal and base station in different scenarios.

<FIG> is a schematic structural diagram of a device for combating Doppler shift. As shown in <FIG>, the device includes an obtaining unit and a processing unit.

The obtaining unit is configured to obtain, in a case where a user equipment performs cell handover, a Doppler shift estimation of the user equipment at a last moment in a source cell.

The processing unit is configured to inverse the Doppler shift estimation, and use the inversed Doppler shift estimation for CFO compensation in an AFC.

The obtaining unit is configured to determine that f̂d(n) = Δfc(n) - ΔfT(n).

Here, n is a moment of when performing cell handover, f̂d(n) is the Doppler shift estimation at the moment n, ΔfT(n) is an average value of Δfc in a time period of length T before the moment n while the user equipment is in a connected state, Δfc is an output value of an AFC loop filter, and Δfc(n) is an output value of the AFC loop filter at the moment n.

When the user equipment is in a PC5-based ad hoc network of a V2V/V2X system, a high-precision timing of the GPS is used to calibrate the ΔfT(n) within time T<NUM>, and a value of T<NUM> is any value from <NUM> to <NUM>.

The processing unit is configured to inverse the Doppler shift estimation for N times, and use the Doppler shift estimation that is inversed for N times for the CFO compensation in the AFC.

The processing unit is further configured to enlarge a loop filter factor of the AFC in the user equipment.

The processing unit is configured to enlarge the loop filter factor of the AFC in the user equipment by K times, for N times, i.e., enlarging the loop filter factor of the AFC in the user equipment by KN times.

The device further includes a determining unit configured to determine an uplink transmission carrier frequency fT.

Here, fT = f̂c - <NUM> * fd, f̂c = f<NUM> + fd , f<NUM> is a downlink carrier transmit frequency, fe is a frequency estimate of a current receiving carrier, fd is a Doppler shift, f̂d is the Doppler shift estimation, and f̂d = f̂c - f̂<NUM>, f̂<NUM> is a reference frequency estimation.

The device further includes a receiving unit configured to receive Downlink Control Information (DCI), the DCI carrying a m-bit frequency adjustment value ΔfTxAFC(n), and <MAT>.

The device further includes an adjusting unit configured to adjust an uplink transmission carrier frequency fT(n), according to the frequency adjustment value ΔfTxAFC(n), fT(n) = fT(n - <NUM>) + ΔfTxAFC(n), Δfsubcarrier is a subcarrier spacing of an OFDM system, and α and βare preset coefficients.

The device further includes a determining unit.

The determining unit is configured to: determine a transmission carrier frequency fT of a PSSCH, a PSCCH and a PSDCH, when the user equipment is in a PC5-based ad hoc network of a V2X system; where fT = f̂c - <NUM> * fd, f̂c = f<NUM> + fd , f<NUM> is a downlink carrier transmit frequency, f̂c is a frequency estimate of a current receiving carrier, f̂d is a Doppler shift, f̂d is the Doppler shift estimation, and f̂d = f̂c - f̂<NUM>, f̂<NUM> is a reference frequency estimation; and determine the carrier frequency fT,PSBCH / SLSS(n) of transmitted signals of a PSBCH and Sidelink Synchronization Signals; where fT,PSBCH / SLSS(n) = f̂c(n) - f̂d(n); n is a moment when performing the cell handover, f̂d(n) is the Doppler shift estimation at the moment n, f̂c(n) is a frequency estimate of a current receiving carrier at the moment n.

The embodiments of the present application further provide a user equipment, including a memory, a processor, and a computer program stored on the memory and capable of running on the processor. The computer program is executed by the processor to realize any one of the above methods for combating Doppler shift.

The embodiments of the present application further provide a computer-readable storage medium, characterized in that an information processing program is stored on the computer-readable storage medium, and when the information processing program is executed by a processor, steps of any one of the above methods for combating Doppler shift are realized.

It should be understood by those having ordinary skill in the art that all or some of the steps in the method disclosed above, and functional modules/units in systems, and devices may be implemented as software, firmware, hardware, or suitable combinations thereof. If implemented as hardware, divisions among the functional modules/units stated above do not necessarily correspond to divisions of physical components. For example, one physical component may have a plurality of functions, or one function or step may be performed through cooperation of several physical components. Some or all of the physical components may be implemented as software executed by a processor, such as a central processing unit, a digital signal processor, or a microprocessor, or may be implemented as hardware, or may be implemented as an integrated circuit, such as an application specific integrated circuit. Such software may be distributed on computer-readable media, which may include computer storage media (or non-transitory media) and communication media (or transitory media). As well known by those having ordinary skill in the art, the term "computer storage media" includes volatile/nonvolatile and removable/non-removable media used in any method or technology for storing information (such as computer-readable instructions, data structures, program modules and other data). The computer storage media include, but are not limited to, a Random Access Memory (RAM), a Read-Only Memory (ROM), an Electrically Erasable Programmable Read-Only Memory (EEPROM), a flash memory or other memory techniques, a Compact Disc Read-Only Memory (CD-ROM), a digital versatile disk (DVD) or other optical discs, a magnetic cassette, a magnetic tape, a magnetic disk or other magnetic storage devices, or any other media which can be used to store the desired information and can be accessed by a computer. In addition, it is well known by those having ordinary skill in the art that the communication media generally include computer-readable instructions, data structures, program modules or other data in a modulated data signal, such as a carrier wave or other transmission mechanism, and may include any information delivery media.

Claim 1:
A method for combating Doppler shift performed by a user equipment, comprising:
obtaining (<NUM>), in a case where a user equipment performs cell handover, a Doppler shift estimation of the user equipment at a last moment in a source cell; and
obtaining (<NUM>) an additive inverse of the Doppler shift estimation, and using the additive inverse of the Doppler shift estimation for Carrier Frequency Offset, CFO, compensation in an Automatic Frequency Control, AFC.