BACKGROUND

Life in the digital age can barely be imagined without wireless communication. While data rates of wireless communication grow exponentially with the announced 5G, users expect a good communication coverage in ever more challenging environments. Multipath signal propagation and Doppler effects due to mobility of the receivers are known to impact communication. The IEEE 802 standards specify the use of OFDM (Orthogonal frequency-division multiplexing) to efficiently overcome degrading effects of multipath.

With the guard interval, OFDM offers an elegant solution to not only filter out disturbances by delays, but even enhance the signal quality by including the delayed signals. In contrast to the temporal delay of echoes, Doppler effects introduce a shift in frequency domain by the Doppler frequency ƒ _D . This Doppler frequency is directly related to the speed of both communication partners and the velocity of the communication signal. For a transmitted carrier frequency ƒ ₀ , a signal velocity c, a velocity of transmitter v _Tx and a velocity of receiver v _Rx the observable Doppler Frequency for the receiver is given by and the received signal ƒ ^' is given by

The transmitter and receiver velocities v _Rx and v _Tx are understood to be positive when the communication partners move towards each other and negative when they move away from each other. Also, angular considerations are ignored here. As the magnitude of ƒ _D increases, the risk of inter carrier interference (ICI) also increases, until at certain velocities the carriers cannot be differentiated clearly any more.

However, approaches known in the art to estimate and equalize the degrading effects of Doppler perform poorly, are too complex for cost-efficient implementation and/or noticeably reduce the available channel capacity.

The objective of the invention is to reduce the degrading effects of Doppler in wireless communication.

SUMMARY

The objective of the invention is met by providing a method according to claim 1 and a receiver according to claim 8.

According to a first aspect, a method for compensating Doppler effects in received wireless communication signals is provided, comprising the steps of:

- receiving, in a receiver, of a first signal, that was transmitted by a transmitter at a first frequency ƒ ₁ and that was received at a doppler-shifted first frequency ƒ ₁ ' ;

- receiving, in said receiver, of a second signal, that was transmitted by said transmitter at a second frequency ƒ ₂ and that was received at a doppler-shifted second frequency ƒ ₂ ' ; wherein a frequency difference ƒ _s between the first frequency and the second frequency ƒ ₂ has a predetermined value;

- determining the first frequency based on said doppler-shifted first frequency ƒ ₁ ', said doppler-shifted second frequency ƒ ₂ ' and said frequency difference ƒ _s

- using said determined first frequency ƒ ₁ for pre-compensating Doppler effect in said received first signal. The first signal is transmitted at the first frequency ƒ ₁ by a transmitter moving with velocity v _Tx and received at a doppler-shifted first frequency ƒ ₁ ' by a receiver moving with a velocity v _Rx .

The doppler-shifted first frequency ƒ ₁ ' may then be calculated as:

Taking the frequency difference ƒ _s with a predetermined value, the second frequency ƒ ₂ is given by: ƒ ₂ = ƒ ₁ + ƒ _s (4)

When the second signal is transmitted at the second frequency ƒ ₂ by the same transmitter moving with the velocity v _Tx and received at a doppler-shifted first frequency ƒ ₂ ' by the same receiver moving with the velocity v _Rx, the doppler-shifted first frequency ƒ ₂ ' is given by:

The receiver observes both ƒ ₁ ' and ƒ ₂ ' . With the knowledge of relation (4) a linear equation can be solved to compute the Doppler factor D:

Now the first frequency ƒ ₁ may be determined based on said doppler-shifted first frequency ƒ ₁ ' , said doppler-shifted second frequency ƒ ₂ ' and said frequency difference ƒ _s :

In this, it is assumed that the velocity of the receiver and the transmitter is constant on the time scale of the signal communication.

In one or more embodiments, the said first frequency ƒ ₁ and said second frequency ƒ ₂ are carrier frequencies of two respective communication channels. In wireless communication two different frequencies are observable: the data signal is transmitted with a signal frequency (exemplary for LTE the signal frequency has a magnitude of several MHz), and the carrier signal has a carrier frequency

(exemplary for LTE the carrier frequency has a magnitude of few GHz). While signal frequencies are dictated by the communication scheme or standard, carrier frequencies can be selected arbitrarily among the available frequency bands. Therefore, it may be advantageous to select the frequencies ƒ ₁ and ƒ ₂ as carrier frequencies of two communication channels, which are selected by the communication provider in such a way that equation (4) is fulfilled and frequency difference ƒ _s is known to both communication partners, i.e. the transmitter and the receiver. In one or more embodiments, the first frequency ƒ ₁ and the second frequency ƒ ₂ are in different frequency domains, for example one of said first and second frequency lies in the 2 GHz domain and the other of said first and second frequency lies in the 4 GHz domain. It can be assumed, that all disturbance on the channel, measurement imprecision and other factors impeding the communication is given by a Noise frequency. The noise has a direct exponential impact on the quality of the Doppler calculation. The larger the noise, the poorer the quality of the Doppler estimation. However, the degrading effect of noise on the communication channel can be counter-balanced by choosing ƒ _s as large as possible.

Using a second signal to counter Doppler effects presents a significant overhead. If the two signals would both be used for communication of the same information, this would indeed be true. However, since only the frequency of the carrier band is relevant, the data or information communicated over the second signal can be used for different purposes.

In one or more embodiments, said first signal and said second signal are both used for transmitting relevant data to said receiver. By transmitting relevant data or information using the second signal, the bandwidth used is increased or even doubled.

In one or more embodiments, said first signal is used for transmitting relevant data to said receiver and said second signal is used for transmitting relevant data to another receiver. By only using the second frequency of the second signal (and not the data transmitted by the second signal), the second signal can be used as for transmitting data to another receiver. In one or more embodiment, the other receiver is also arranged for executing the one or more embodiments of the method for compensating Doppler effects described in this document and the second signal may serve as a first signal for this other receiver when executing the one or more embodiments of the method.

In one or more embodiments, said first signal and said second signal are transmitted time-sliced. By transmitting relevant data or information using the first and the second signal in an alternating way, the bandwidth used remains the same with respect to using only one signal. However, energy consumption and frequency usage may be reduced by alternating between the first and second signal.

In one of more embodiments, information about said predetermined frequency difference ƒ _s is exchanged between said receiver and said transmitter, preferably during an initial handshake of a wireless communication setup procedure. This additional information may be considered as insignificant compared to the data transmitted during a regular communication session.

According to a second aspect, a receiver for a vehicle is provided that is arranged for executing a method according to any of embodiments of the method for compensating Doppler effect as described in this document. Also, a vehicle comprising such a receiver is provided.

According to a third aspect a system for compensating Doppler effects in received wireless communication signals is provided, comprising:

- a receiver according to claim 8; and,

- a transmitter arranged for transmitting, to said receiver, said first signal at said first requency ƒ ₁ , said second signal at said second frequency ƒ ₂ and, optionally, information about said frequency difference ƒ _s.

According to a fourth aspect a computer program is provided comprising instructions which, when the program is executed by a computer, cause the computer to carry out the steps of the method of described in this document. Also, a computer-readable medium having stored thereon said computer program is provided.

The working, advantages and embodiments of the receiver, the system, and the vehicle as well as the working, advantages and embodiments of the computer program and computer-readable medium, correspond with the working, advantages and embodiments of the method as described in this document, mutatis mutandis.

BRIEF DESCRIPTION OF THE DRAWINGS For a more complete understanding of the present invention, reference in the following description is made in to the accompanying drawings in which:

Figure 1 shows a schematic overview of a receiver, a vehicle and a system according to one or more embodiments of the invention; and,

Figure 2 shows a schematic overview of a method according to one or more embodiments of the invention.

DETAILED DESCRIPTION

Figure 1 shows a schematic overview of a receiver 100, a vehicle 110 and a system 120 according to one or more embodiments of the invention as described in this document. Furthermore, in figure 1 a transmitter 130 has been indicated.

In one or more embodiments, the receiver 100 may be a communication unit or an Electronic Control Unit (ECU) of a vehicle. In one or more embodiments, the vehicle 110 may be a car, a motorbike, a van, a truck, a bicycle or a scooter. In one or more embodiments, the system 120 comprises (i) the transmitter 130 and (ii) the receiver 100 or the vehicle 110 with the receiver 100. In one or more embodiments, the transmitter may be a base station of cellular network (5G, UMTS, etc.) or a WLAN access point. The receiver and the transmitter are arranged for exchanging data using a wireless communication protocol, such as 5G, UMTS or WLAN. According to one or more embodiments, the transmitter transmits a first signal 140 at first frequency ƒ ₁ and a second signal 150 at a second frequency ƒ ₂ . The receiver 100 receives both signals, but because vehicle 110 is moving, the signals are received at a doppler-shifted first f frequency ƒ ₁ ' and a doppler-shifted second frequency ƒ ₂ ' respectively.

A frequency difference ƒ _s between the first frequency ƒ ₁ and the second frequency ƒ ₂ has a predetermined value. In one or more embodiments, information about the frequency difference ƒ _s is stored in receiver 100. In other embodiments, information about the frequency difference ƒ _s is transmitted from transmitter 130 to receiver 100.

As explained above, the first frequency ƒ ₁ may be determined or calculated on the basis of the doppler-shifted first frequency ƒ ₁ ', the doppler-shifted second frequency ƒ ₂ ' and the frequency difference ƒ _s , preferably in the receiver 100.

The determined first frequency ƒ ₁ is used for a pre-compensation of the Doppler effect on the first signal, when receiving the first signal at the doppler-shifted first frequency ƒ ₁ ',

In one or more embodiments, a Doppler frequency shift is used or calculated for the pre-compensation. Pre-compensation on the receiver side with a known doppler frequency shift is considered technically trivial and well known in the art.

In one or more embodiments, the second frequency ƒ ₂ is determined or calculated on the basis of the doppler-shifted first frequency ƒ ₂ ' _, the doppler-shifted second frequency ƒ ₂ ' and the frequency difference ƒ _s , preferably in the receiver 100. The determined second frequency ƒ ₂ may be used for a pre-compensation of the Doppler effect on the second signal.

In wireless communication two different frequencies are observable: the data signal is transmitted with a signal frequency (when LTE is used, the signal frequency has a magnitude of several MHz), and the carrier signal has a carrier frequency (when LTE is used, the carrier frequency has a magnitude of few GHz). While signal frequencies are dictated by the communication standard such as LTE, carrier frequencies can be selected arbitrarily among the available frequency bands. The available frequency bands are determined by political and regulatory authorities as well as economical competition among the communication providers. However, apart from authorities’ constraints there is no technical reason not to select suitable carrier signals and corresponding frequencies for communication. Therefore, in one or more embodiments, the frequencies ƒ ₁ and ƒ ₂ are carrier frequencies of two communication channels, which are selected by the communication provider in such a way that equation f ₂ = f ₁ + f _s and the frequency difference ƒ _s is known to both communication partners, that is the transmitter and the receiver.

It can be assumed, that all disturbance on the channel, measurement imprecision and other factors impeding the communication is given by the Noise frequency ƒ _N, such that and hence

The noise ƒ _N has a direct exponential impact on the quality of the Doppler calculation. The larger the noise, the poorer the quality of the Doppler estimation. However, the degrading effect of noise on the communication channel can be counter-balanced by the choice of the frequency difference ƒ _s. The quality of the estimation may be improved by choosing the frequency difference ƒ _s as large as possible. If OFDM subcarrier signals are used, the frequency difference ƒ _s is determined by the OFDM channel width, the number of subcarriers and the width of those individual carriers. Second, the quality of the observed carrier frequencies ƒ ₁ and ƒ ₂ (or, respectively, the avoidance of noises ƒ _N1 and ƒ _N2 has a direct impact on the quality of the estimated Doppler. Measurements of the carrier frequencies of OFDM subchannels are more vulnerable to noise than e.g. measuring carrier frequencies of all subcarriers and calculating a “central OFDM carrier frequency”, thus reducing non-systematic disturbances. Therefore, in one or more embodiments, two distinct, and far apart, channels are used for best results. Those two channels can be taken from separate frequency domains. Exemplarily, the upcoming 5G New Radio standard permits transmission of data in the existing LTE frequency range (600MHz to 6GHz) and extends available frequencies by millimeter wave bands (24GHz to 86GHz). Due to coverage short term, focus will be on the bands between 600MHz and 5GHz, but already coupling channels in the range of 2GHz with channels in the range of 4GHz offers the frequency difference ƒ _s of 2GHz, which is significantly better than the 20MHz spacing of outer LTE (4G) OFDM subcarriers. Using an additional channel to counter Doppler presents a significant overhead. If two channels would be used for communication of the same information this would indeed be true. Flowever, since only the frequency of the carrier band is relevant, the data communicated over this coupled channel can be used arbitrarily. In one or more embodiments, the first signal and the second signal are both used for transmitting relevant data to said receiver. Together the two bands can transmit twice as much information, thus doubling the bandwidth between the transmitter and receiver, at the cost of overall available channels. In one or more embodiments, the first signal is used for transmitting relevant data to said receiver and said second signal is used for transmitting relevant data to another receiver. The second channel can provide its carrier frequency as a reference only and can be used to transmit information for a different receiver - which in turn could use the carrier frequency of the first prior channel as a reference.

In one or more embodiments, the first signal and the second signal are transmitted time-sliced. In that way the channels can be shared flexibly between different communication pairs.

In one or more embodiments, information about said predetermined frequency difference ƒ _s is exchanged between said receiver and said transmitter, preferably during an initial handshake of a wireless communication setup procedure. Alternatively, the information about said predetermined frequency difference ƒ _s is exchanged using characteristics of the carrier frequencies. E.g. Phase Keying could be used to ensure the correct baseband channels are used, even when the communicated factor is incorrect due to Doppler.

Figure 2 shows a schematic overview of a method 200 according to one or more embodiments of the invention. The receiver 100, the vehicle 110 and/or the system 120 of figure 1 may be arranged for executing the steps of this method.

The method 200 may comprise the following steps:

Step 210: receiving in a receiver of a first signal, that was transmitted by a transmitter at a first frequency and that was received at a doppler-shifted first frequnency ƒ ₁ ^' ;

Step 220: receiving in said receiver of a second signal, that was transmitted by said transmitter at a second frequency ƒ ₂ and that was received at a doppler-shifted second frequency ƒ ₂ ' _, wherein a frequency difference ƒ _s between the first frequency and the second frequency ƒ ₂ has a predetermined value

Step 230: determining the first frequency ƒ ₁ based on said doppler-shifted first frequency ƒ ₁ ', said doppler-shifted second frequency ƒ ₂ ' and said frequency difference ƒ _s

Step 240: using said determined first frequency for pre-compensating Doppler effects in said received first signal.

A computer program may be provided comprising instructions which, when the program is executed by a computer, cause the computer to carry out the steps of one or more embodiments of the method as described in this document. A computer-readable medium may be provided, having stored thereon this computer program.

Furthermore, one or more embodiments may be described by the following: A method, a transmitter, a vehicle and a system for compensating Doppler effects in received wireless communication signals are provided. The method comprises the following steps. In a receiver a first signal is received, that was transmitted by a transmitter at a first frequency and that was received at a doppler-shifted first frequency ƒ ₁ ' and a second signal, that was transmitted by said transmitter at a second frequency ƒ ₂ and that was received at a doppler-shifted second frequency ƒ ₂ ' is also received. A frequency difference ƒ _s between the first frequency and the second frequency ƒ ₂ has a predetermined value. Based on said doppler-shifted first frequency ƒ ₁ ', said doppler-shifted second frequency ƒ ₂ ' and said frequency difference ƒ _s, the first frequency is determined for pre-compensating Doppler effects in the received first signal.

As will be appreciated by one skilled in the art, aspects of the present invention may be embodied as a system, device, method or computer program product. Accordingly, aspects of the present invention may take the form of an entirely hardware embodiment, an entirely software embodiment (including firmware, resident software, micro-code, etc.) or an embodiment combining software and hardware aspects that may all generally be referred to herein as a "unit", "module", "system", “device” or "element".

Functions or steps described in this document may be implemented as an algorithm executed by a microprocessor of a computer. Furthermore, aspects of the present invention may take the form of a computer program product embodied in one or more computer readable medium(s) having computer readable program code embodied, e.g., stored, thereon.

It is to be understood that the above description is included to illustrate the operation of the preferred embodiments and is not meant to limit the scope of the invention. From the above discussion, many variations will be apparent to one skilled in the art that would yet be encompassed by the scope of the present invention.