WO2024098205A1

WO2024098205A1 - Frequency offset compensation in a network equipment

Info

Publication number: WO2024098205A1
Application number: PCT/CN2022/130361
Authority: WO
Inventors: Zhao Wang; Wei Zhou; Yipeng ZHANG
Original assignee: Telefonaktiebolaget Lm Ericsson (Publ); Wei Zhou
Priority date: 2022-11-07
Filing date: 2022-11-07
Publication date: 2024-05-16

Abstract

There is provided techniques for frequency offset compensation of a transmission signal to be transmitted by a transmitter over a channel. A method is performed by a network equipment. The method comprises calculating a phase-compensated frequency domain version of the transmission signal by using circular convolution between a frequency domain version of the transmission signal and a frequency domain phase rotation sequence, and by using knowledge of the channel. The frequency domain phase rotation sequence is determined as a function of an estimated frequency offset value for the transmitter.

Description

FREQUENCY OFFSET COMPENSATION IN A NETWORK EQUIPMENT

TECHNICAL FIELD

Embodiments presented herein relate to a method, a network equipment, a computer program, and a computer program product for frequency offset compensation.

BACKGROUND

In general terms, frequency division multiple access (FDMA) , where clusters of subcarriers are assigned to different user equipment (UEs) can be regarded as a basic multiple access scheme for orthogonal frequency division multiplexing (OFDM) . OFDM is based on using a number of subcarriers. FDMA based OFDM systems do not only require accurate receiver timing and a carrier frequency for each UE, but requires precise synchronization among all UEs to avoid intercarrier interference (ICI) .

For the downlink (i.e., for transmission from an access point at the network side to a UE at the user side) , synchronization between the access points and UEs is necessary and usually performed at the UE side. This increases the energy cost at the UE. From this point of view, downlink frequency offset pre-compensation can be used to prolong the battery life of UEs. Meanwhile, a large frequency offset might lead to timing synchronization failure in the downlink. Additionally, for some scenarios, such as multi access point scenarios where one UE is served by more than one access point, frequency offset compensation at the UE side is unfeasible.

Existing technology is mainly focused on frequency offset compensation at the receiver side. As noted above, this will inevitably increase the energy cost at the UE for processing of received downlink signals.

For multi access point scenarios, it can be challenging for the UE to estimate the frequency offsets from individual access points. Therefore, it can be challenging for the UE to perform frequency offset compensation. Instead, network-based techniques for frequency offset pre-compensation can be used.

Common downlink frequency offset pre-compensation is performed the in time domain. Since the frequency offsets associated with different UEs are different from each other, a frequency offsets compensation and inverse discrete Fourier transform (IDFT) and forward discrete Fourier transform (DFT) needs to be performed separately for each UE.

In the following, the basic principles of the state of art time domain frequency offset compensation techniques will be summarized. Fig. 1 is a schematic diagram illustrating a communication system 100 where embodiments presented herein can be applied. The communication system 100 comprises a network node 102 and a UE 104 configured to communicate with each other over a wireless channel 130. The network node 102 can be any of a (radio) access network node, a radio base station, a base transceiver station, a node B, an evolved node B, a gNB, a transmission and reception point, an access point, an access node, an integrated access and backhaul node, etc. The UE can be any of a portable wireless device, a mobile station, a mobile phone, a handset, a wireless local loop phone, a smartphone, a laptop computer, a tablet computer, a network equipped sensor, a network equipped vehicle, an Internet of Things device.

In Fig. 2 is schematically illustrated time domain frequency offset compensation at the transmitter side. The transmitter 110 is assumed to be located at a network node. Four UEs are frequency division multiplexed, and hence there are four frequency domain signals X1 (k) , X2 (k) , X3 (k) , X4 (k) . Either four IDFT blocks 112 (i.e., the same number as the number of UEs) are needed or one single IDFT block but with a higher latency (a factor four) . The time domain frequency offset compensation is performed in frequency offset compensation (FOC) blocks 114, one for each UE. The composite time domain signal composed of the frequency offset compensated time domain signals for all the UEs is then converter to the frequency domain by a DFT block 116 to form the frequency offset compensated frequency domain signal

In perspective of implementation, DFT/IDFT operations are comparatively resource consuming. If receiver time domain frequency offset compensation is considered, in addition to four IDFT blocks, also one DFT block is needed to transform the signal back to the frequency domain. With the increase of FDM UEs, more DFT/IDFT blocks are needed.

Even regardless of implementation complexity, time domain frequency offset pre-compensation at the transmitter side faces a challenge to consider the impact of the channel 130 because channel fading is involved in the transmission together with frequency offset. Ideally, channel state information is available at the transmitter side, either obtained by the transmitter 110 itself or from the receiver side, which poses stringent constraints for the system.

Further aspects of the basic principles of the state of art time domain frequency offset compensation techniques will be disclosed next with reference to Fig. 3 for time domain frequency offset compensation at the receiver side and to Fig. 4 for time domain frequency offset compensation at the transmitter side. The frequency offset is caused by the asynchronization between the transmitter 110 and the receiver 120, and behaves as a phase rotation function over time. Thus, the frequency offset can be modelled as a multiplication after the time domain channel, as shown in Fig. 3 and Fig. 4. In Fig. 3 and Fig. 4, x denotes the time domain transmission signal, h denotes the time domain channel, p denotes the phase rotation due to frequency offset,

denotes the estimated phase rotation due to frequency offset,

is the convolution operation, and “z ^*” denotes the conjugate of z.

As illustrated in Fig. 3, a signal transmitted by the transmitter 110 is subjected to the channel, including frequency offset, before reaching the receiver 120, where the signal is subjected to frequency offset estimation and frequency offset compensation before equalization. When the transmitter side frequency offset pre-compensation relies on feedback, or frequency offset estimation, to be provided from the receiver side, frequency offset still persists at the receiver side due to the frequency offset introduced by the channel 130. As illustrated in Fig. 4, a signal to be transmitted by the transmitter 110 is first subjected to time domain frequency offset pre-compensation before being transmitted. The transmitted signal is then subjected to the channel, including frequency offset, before reaching the receiver 120, where the signal is subjected to equalization.

According to Fig. 3 and Fig. 4, frequency offset pre-compensation at the transmitter side is not equivalent to frequency offset compensation at the receiver side.

Specifically, frequency offset pre-compensation at the transmitter side cannot be used to completely remove the phase rotation.

Hence, there is still a need for improved frequency offset compensation techniques in general, and in particular for improved frequency offset pre-compensation techniques at the transmitter side.

SUMMARY

An object of embodiments herein is to address the above issues so as to provide frequency offset pre-compensation at the transmitter side that does not suffer from the above identified issues.

According to a first aspect there is presented a method for frequency offset compensation of a transmission signal to be transmitted by a transmitter over a channel. The method is performed by a network equipment. The method comprises calculating a phase-compensated frequency domain version

of the transmission signal by using circular convolution between a frequency domain version X (k) of the transmission signal and a frequency domain phase rotation sequence P (k) , and by using knowledge of the channel. The frequency domain phase rotation sequence P (k) is determined as a function of an estimated frequency offset value f _o for the transmitter.

According to a second aspect there is presented a network equipment for frequency offset compensation of a transmission signal to be transmitted by a transmitter over a channel. The network equipment comprising processing circuitry. The processing circuitry is configured to cause the network equipment to calculate a phase-compensated frequency domain version

According to a third aspect there is presented a network equipment for frequency offset compensation of a transmission signal to be transmitted by a transmitter over a channel. The network equipment comprises a calculate module configured to calculate a phase-compensated frequency domain version

According to a fourth aspect there is presented a computer program for frequency offset compensation of a transmission signal to be transmitted by a transmitter, the computer program comprising computer program code which, when run on a network equipment, causes the network equipment to perform a method according to the first aspect.

According to a fifth aspect there is presented a computer program product comprising a computer program according to the fourth aspect and a computer readable storage medium on which the computer program is stored. The computer readable storage medium could be a non-transitory computer readable storage medium.

Advantageously, these aspects provide frequency offset pre-compensation at the transmitter side without suffering from the above identified issues.

Advantages achieved over performing frequency offset compensation at the receiver side will be disclosed next.

Firstly, for some scenarios (such as multi access point scenarios, etc. ) , frequency offset compensation at the receiver side is unfeasible. Downlink synchronization cannot even be performed successfully. Frequency offset pre-compensation at the transmitter side as herein disclosed decreases the risk of timing synchronization failures in the receiver.

Secondly, for other scenarios, frequency offset compensation at the transmitter side decreases the energy cost at the receiver side which is beneficial for power saving reasons.

Advantages achieved over performing frequency offset compensation in the time domain generally relate to complexity reductions, performance improvement, and the requirements for channel information at the transmitter side. Details relating thereto will be disclosed next.

With respect to complexity reductions, in contrast to time domain frequency offset compensation whose complexity increases proportional to the number of receivers, the computational load of the herein disclosed embodiments decreases as the number of receivers increases.

The herein disclosed embodiments are simpler to implement than time domain frequency offset compensation when the number of receivers is greater than two. Unlike time domain frequency offset compensation, the herein disclosed embodiments do not affect other receiver’s data after compensation for a specific receiver, hence bringing more flexibility for the transmitter.

With respect to performance improvement, as will be demonstrated below, the herein disclosed embodiments outperform traditional time domain frequency offset compensation schemes.

With respect to requirements for channel information at the transmitter side, channel information at the transmitter side is only needed for some embodiments, depending on assumptions of frequency properties of the channel; for a certain set of scenarios, no channel information at the transmitter side is required.

Other objectives, features and advantages of the enclosed embodiments will be apparent from the following detailed disclosure, from the attached dependent claims as well as from the drawings.

Generally, all terms used in the claims are to be interpreted according to their ordinary meaning in the technical field, unless explicitly defined otherwise herein. All references to "a/an/the element, apparatus, component, means, module, step, etc. " are to be interpreted openly as referring to at least one instance of the element, apparatus, component, means, module, step, etc., unless explicitly stated otherwise. The steps of any method disclosed herein do not have to be performed in the exact order disclosed, unless explicitly stated.

BRIEF DESCRIPTION OF THE DRAWINGS

The inventive concept is now described, by way of example, with reference to the accompanying drawings, in which:

Fig. 1 is a schematic diagram illustrating a communication system according to embodiments;

Fig. 2 schematically illustrates a transmitter according to an example;

Fig. 3 schematically illustrates a transmitter and a receiver according to an example where frequency offset compensation is performed at the receiver side;

Fig. 4 schematically illustrates a transmitter and a receiver according to an example where frequency offset compensation is performed at the transmitter side;

Fig. 5 is a flowchart of methods according to embodiments;

Fig. 6 schematically illustrates a transmitter according to embodiments;

Figs. 7 to 15 show simulation results according to embodiments;

Fig. 16 is a schematic diagram showing functional units of a network equipment according to an embodiment;

Fig. 17 is a schematic diagram showing functional modules of a network equipment according to an embodiment; and

Fig. 18 shows one example of a computer program product comprising computer readable storage medium according to an embodiment.

DETAILED DESCRIPTION

The inventive concept will now be described more fully hereinafter with reference to the accompanying drawings, in which certain embodiments of the inventive concept are shown. This inventive concept may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided by way of example so that this disclosure will be thorough and complete, and will fully convey the scope of the inventive concept to those skilled in the art. Like numbers refer to like elements throughout the description. Any step or feature illustrated by dashed lines should be regarded as optional.

As noted above there is still a need for improved frequency offset compensation techniques in general, and in particular for improved frequency offset pre-compensation techniques at the transmitter side.

Common downlink frequency offset pre-compensation is performed in the time domain. This tends to require heavy computation at the transmitter side and might cause some performance degradation.

According to at least some of the herein disclosed embodiments there is therefore proposed to perform frequency offset compensation at the transmitter side. This can be used, for example, for OFDM-FDMA systems. Accordingly, frequency offsets can be compensated for directly in the frequency domain (via circular convolution) . This is in contrast to time domain frequency offset compensation where the complex exponents of the offset estimates are computed after the IDFT. Some of the embodiments rely on utilizing channel information for the compensation, which enables different channel types to be taken into consideration.

The embodiments disclosed herein in particular relate to techniques for frequency offset compensation of a transmission signal to be transmitted by a transmitter 110. In order to obtain such techniques there is provided a network equipment 200, a method performed by the network equipment 200, a computer program product comprising code, for example in the form of a computer program, that when run on a network equipment 200, causes the network equipment 200 to perform the method.

The following notation will be used in the rest of this disclosure.

x (n) represents a transmitted time domain signal of length N samples, with a sampling interval T _s.

X (k) represents a transmitted frequency domain signal of length N samples, with a subcarrier spacing f _k.

h (n) represents the time domain channel of length N samples, with a sampling interval T _s.

H (k) represents the frequency domain channel of length N samples, with a subcarrier spacing f _k.

y (n) represents a received time domain signal of length N samples, with a sampling interval T _s.

Y (k) represents a received frequency domain signal of length N samples, with a subcarrier spacing f _k.

represents phase rotation of length N samples, with sampling interval T _s, as caused by frequency offset f _o in the time domain.

y _fo (n) represents a received time domain signal of length N samples, with a sampling interval T _s, as subjected to the phase rotation.

Y _fo (k) represents a received frequency domain signal of length N samples, with a subcarrier spacing f _k, as subjected to the phase rotation.

This yields the following system model:

Y (k) =H (k) ·X (k) ,

y _fo (n) =y (n) ·p (n) ,

Here

represents circular convolution operation. It then follows that

Only if the channel 130 is single path, i.e., if

it follows that

According to properties of Fourier transformation, circular convolution can be used instead of linear convolution. That is:

This can be expressed in matrix form as follows:

where

The phase rotated transmitted signal can thus in the frequency domain be expressed as:

Let

It can be observed that P _mat is a unitary matrix. In order to compensate the frequency offset at the transmitter side, the unitary matrix property can be exploited as follows:

Here:

By this, the frequency offset is compensated. That is,

represents a phase-compensated frequency domain version of the frequency domain transmission signal X (k) .

Fig. 5 is a flowchart illustrating embodiments of methods for frequency offset compensation of a transmission signal to be transmitted by a transmitter 110 over a channel 130. The methods are performed by the network equipment 200. The methods are advantageously provided as computer programs 1820.

The frequency offset is compensated for directly in the frequency domain via circular convolution, taking the frequency offset into account, as in S106

S106: The network equipment 200 calculates a phase-compensated frequency domain version

of the transmission signal by using circular convolution between a frequency domain version X (k) of the transmission signal and a frequency domain phase rotation sequence P (k) . The phase-compensated frequency domain version

of the transmission signal is also calculated using knowledge of the channel 130.

The frequency domain phase rotation sequence F (k) is determined as a function of an estimated frequency offset value f _o for the transmitter 110.

Embodiments relating to further details of the frequency offset compensation as performed by the network equipment 200 will now be disclosed with continued reference to Fig. 5.

The network equipment 200 might be part of a network node 102 and/or a user equipment 104.

The transmission signal might be transmitted in an OFDM-FDMA operation based system.

There might be different ways for the network equipment 200 to estimates the frequency offset value f _o.

In some aspects, f _o is determined based on a received reference signal. For example, when the network equipment 200 is part of a network node 102, the network equipment 200 might estimate f _o based on an uplink reference signal, such as a sounding reference signal (SRS) . For example, when the network equipment 200 is part of a user equipment 104, the network equipment 200 might estimate f _o based on a downlink reference signal, such as a channel state information reference signal (CSI-RS) , or a synchronization signal burst (SSB) . Therefore, in some embodiments, the network equipment 200 is configured to perform (optional) steps S102a, S104a:

S102a: The network equipment 200 receives a reference signal.

S104a: The network equipment 200 estimates the frequency offset value f _o from measurements made on the reference signal.

In some aspects, f _o is determined from a feedback signal received from the receiver side 120. Therefore, in some embodiments, the network equipment 200 is configured to perform (optional) steps S102b, S104b:

S102b: The network equipment 200 receives a feedback signal.

S104b: The network equipment 200 estimates the frequency offset value f _o from information obtained from the feedback signal.

Further aspects of the frequency domain phase rotation sequence P (k) will be disclosed next.

In some aspects, the network equipment 200 first obtains a time domain phase rotation sequence and then convert the time domain phase rotation sequence to the frequency domain. In particular, in some embodiments, the frequency domain phase rotation sequence P (k) is determined by determining a time domain phase rotation sequence p (n) as a function of the frequency offset value f _o, and then converting the time domain phase rotation sequence p (n) into frequency domain.

Further in this respect, the time domain phase rotation sequence p (n) might be determined as:

where T _s denotes sampling time interval, and where n≥0 denotes the sample index within an OFDM symbol.

The frequency domain phase rotation sequence P (k) might then be determined from a discrete Fourier transform (DFT) of size N from p (n) as:

Here, N is not smaller the number of samples within the OFDM symbol.

Further aspects of how the network equipment 200 might calculates the phase-compensated frequency domain version

of the transmission signal will be disclosed next.

In some aspects, the phase-compensated frequency domain signal

for each subcarrier k considers the joint impact of phase rotation for all subcarriers caused by the frequency offset, in which channel information over all subcarriers is taken into account to obtain a point-wise scaling factor. In particular, in a first embodiment, the phase-compensated frequency domain version

is composed of subcarriers to be transmitted over the channel130, and the phase-compensated frequency domain version

for the subcarrier with index k is dependent on the channel H (n) for a non-phase-compensated frequency domain version X (n) of the transmission signal at sample index n as scaled with the channel H (k) for the subcarrier with index k. The knowledge of the channel 130 is then the channel H (m) and the channel H (k) for the subcarrier with index k For example, for a frequency domain channel estimate H (k) , the phase-compensated frequency domain version

for the subcarrier with index k might be determined as:

where ‘mod’ denotes the modulus operation. The factor H (n) /H (k) represents the point-wise scaling factor.

In some aspects, for some deployment scenarios, such as high-speed relative movement between the transmitter 110 and the receiver 120, suburban scenarios, or air-to-ground/air communications, the channel 130 is likely to be frequency-flat. The delay spread for the above scenarios is comparatively small, meaning that the channel is frequency-flat. In such scenarios, the phase-compensated frequency domain version

may take an approximated form such that the impact of the channel information is not required in the compensation. The compensated signal for each subcarrier then becomes a linear sum of point-wise phase compensations for the original frequency domain transmission signal X (k) . In particular, in a second embodiment, the phase-compensated frequency domain version

is composed of subcarriers to be transmitted over a channel 130 that is assumed to be frequency flat or quasi-flat. The knowledge of the channel 130 is then the assumption that the channel 130 is frequency flat or quasi-flat. For example, the phase-compensated frequency domain version

for the subcarrier with index k might be determined as:

One precondition for this alternative is that channel is frequency flat or quasi-flat.

Then the factor H (n) /H (k) =1 for all the subcarriers k. This means that the phase-compensated frequency domain transmission signal

can be directly derived by performing a circular convolution between the original frequency domain transmission signal X (k) and the frequency domain phase rotation sequence.

In some aspects, the phase-compensated frequency domain signal

is approximated by the partial sum of the point-wise phase compensation of the original frequency domain transmission signal X (k) such that a lower computation complexity is achieved. In particular, in a third embodiment, the phase-compensated frequency domain version

is composed of subcarriers to be transmitted over the channel 130, where the phase-compensated frequency domain version

for the subcarrier with index k is dependent on the frequency domain phase rotation sequence P (k) as sampled and an equally sampled channel H (n) for a non-phase-compensated frequency domain version X (n) of the transmission signal at sample index n as scaled with the channel H (k) for the subcarrier with index k. The knowledge of the channel 130 is then the equally sampled channel H (n) and the channel H (k) for the subcarrier with index k. For example, the phase-compensated frequency domain version

might be determined as:

where L is an odd integer, and where F (l) is the sampled frequency domain phase rotation sequence P (k) . Further aspects of F (l) will be disclosed below.

The third embodiment is similar to the first embodiment, except for the sampling, yielding reduced calculation complexity by extracting only part of the frequency domain signal, the phase rotation sequence, and the channel to derive the phase-compensated frequency domain signal

In some aspects, the second embodiment and the third embodiment are combined. That is, in some aspects, the impact of channel information is removed from the compensation because of the nature of flat frequency channel response for the above listed deployment scenarios. In particular, in a fourth embodiment, the phase-compensated frequency domain version

is composed of subcarriers to be transmitted over a channel 130 that is assumed to be frequency flat or quasi-flat, where the phase-compensated frequency domain version

for the subcarrier with index k is dependent on the frequency domain phase rotation sequence P (k) as sampled. The knowledge of the channel 130 is then the assumption that the channel 130 is frequency flat or quasi-flat. For example, the phase-compensated frequency domain version

might be determined as:

Where, again, L is an odd integer, and F (l) is the sampled frequency domain phase rotation sequence P (k) .

The fourth embodiment is similar to the second embodiment, except for the sampling, yielding reduced calculation complexity by extracting only part of the frequency domain signal and the phase rotation sequence to derive the phase-compensated frequency domain signal

In some examples, F (l) is expressed as:

0≤l≤L-1.

The value of L can be adaptively determined according to a normalized version of the frequency offset value f _o. For example, define a threshold θ, determine the number of samples from p (n) , the absolute value of which is greater than θ. Then L equals to the number of samples.

In some aspects, the phase-compensated frequency domain signal

is converted to the time domain and then transmitted (possibly after time or frequency domain inter-symbol frequency offset compensation) . Hence, in some embodiments, the network equipment 200 is configured to perform (optional) steps S108, S110.

S108: The network equipment 200 converts the phase-compensated frequency domain version

of the transmission signal into a phase-compensated time domain version

of the transmission signal; and

S110: The network equipment 200 transmits the phase-compensated time domain version

of the transmission signal over the air from the transmitter 110.

The phase compensation might be applied on the transmission signal within each OFDM symbol. Optionally, inter-symbol frequency offset compensation is applied after the symbol-level phase compensation, either in the frequency domain, or in the time domain. In the frequency domain, inter-symbol frequency offset compensation can be applied with coefficients in the form

where m is the symbol index within a slot starting from index 0.

Reference is here made to Fig. 6. In Fig. 6 (a) is illustrated a transmitter 110 where inter-symbol frequency offset compensation is applied in the frequency domain after the symbol-level frequency domain phase compensation. An IDFT is then applied to convert the frequency domain signal to a time domain signal that is subjected to further time domain processing before being transmitted. In Fig. 6 (b) is illustrated a transmitter 110 where inter-symbol frequency offset compensation is applied in the time domain after the symbol-level frequency domain phase compensation. An IDFT is thus applied after the symbol-level frequency domain phase compensation but before the time domain inter-symbol frequency offset compensation to convert the frequency domain signal to a time domain signal. The time domain signal is, after the time domain inter-symbol frequency offset compensation subjected to further time domain processing before being transmitted.

As above, denote by f _o the frequency offset normalized by the subcarrier spacing.

Then:

where ‘FFT’ denotes the fast Fourier transform.

If f _o=m, the frequency offset f _o is an integer multiple of the subcarrier spacing. Then

There is thus only one peak in P (k) whilst all other values are equal to zero.

If f _o=m+d, where m is an integer value and d is a decimal value,

According this equations above, the number of frequency domain samples of the phase rotation sequence with sufficient power is limited. As will be disclosed next, this property can be utilized to reduce the complexity of the frequency offset compensation at the transmitter side 110.

In a non-limiting example, only L=7 samples are reserved:

F (k) = [P (N-3) P (N-2) P (N-1) P (0) P (1) P (2) P (3) ]

Assume that the channel variation among 7 continuous subcarriers is insignificant, for examples due to the subcarriers being coherent. Then the phase-compensated frequency domain signal

can be expressed as follows (excluding all but the first three and last three subcarriers) :

This implies that the phase-compensated frequency domain signal

does not depend on the channel information at the transmitter side 110.

In another non-limiting example, if the channel information of all the subcarriers could be obtained, then:

Figs. 7 to 10 show the amplitude of P (k) for different frequency offset values. In Fig. 7 the frequency offset (as normalized by subcarrier spacing) is 0.03, in Fig. 8 it is 0.17, in Fig. 9 it is 1, and in Fig. 10 it is 1.03. It can be seen that only a few of the coefficients have an amplitude that warrants the coefficient to be considered in the convolution operation. Therefore, the computational complexity for performing the convolution could be hugely decreased by considering only the coefficients P (k) with significant amplitude values.

Fig. 11 shows an example which expresses the relation between the normalized frequency offset and filter order. The filter order corresponds to the number of samples in P (k) which have large enough amplitude. Only those samples are essentially involved in the convolution operation.

A comparison of the throughput performance will now be made with reference to Fig. 12 and Fig. 13. Results for the herein disclosed embodiments are denoted “w. FD-FOC” whereas results where only inter-symbol frequency offset in frequency domain is performed are denoted “w.o. FD-FOC” . Results in Fig. 12 are given for the HST channel and results for the TDL-D channel are given in in Fig. 13. As can be seen, the herein disclosed embodiments provide significant gains.

A further comparison of the throughput performance will now be made with reference to Fig. 14 and Fig. 15 for different frequency offset in channel affected by additive white Gaussian noise (AWGN) . Fig. 14 shows results where only inter-symbol frequency offset in frequency domain is performed. Fig. 15 shows results for the herein disclosed embodiments. The notation “f0=0, 400, …Hz” in Figs. 14 and 15 means that the frequency offset is 0, 400, …Hz. Further, the subcarrier spacing was set to 30 kHz.

Fig. 16 schematically illustrates, in terms of a number of functional units, the components of a network equipment 200 according to an embodiment. Processing circuitry 210 is provided using any combination of one or more of a suitable central processing unit (CPU) , multiprocessor, microcontroller, digital signal processor (DSP) , etc., capable of executing software instructions stored in a computer program product 1810 (as in Fig. 18) , e.g. in the form of a storage medium 230. The processing circuitry 210 may further be provided as at least one application specific integrated circuit (ASIC) , or field programmable gate array (FPGA) .

Particularly, the processing circuitry 210 is configured to cause the network equipment 200 to perform a set of operations, or steps, as disclosed above. For example, the storage medium 230 may store the set of operations, and the processing circuitry 210 may be configured to retrieve the set of operations from the storage medium 230 to cause the network equipment 200 to perform the set of operations. The set of operations may be provided as a set of executable instructions.

Thus the processing circuitry 210 is thereby arranged to execute methods as herein disclosed. The storage medium 230 may also comprise persistent storage, which, for example, can be any single one or combination of magnetic memory, optical memory, solid state memory or even remotely mounted memory. The network equipment 200 may further comprise a communications interface 220 at least configured for communications with other entities, functions, nodes, and devices. As such the communications interface 220 may comprise one or more transmitters and receivers, comprising analogue and digital components. The processing circuitry 210 controls the general operation of the network equipment 200 e.g. by sending data and control signals to the communications interface 220 and the storage medium 230, by receiving data and reports from the communications interface 220, and by retrieving data and instructions from the storage medium 230. Other components, as well as the related functionality, of the network equipment 200 are omitted in order not to obscure the concepts presented herein.

Fig. 17 schematically illustrates, in terms of a number of functional modules, the components of a network equipment 200 according to an embodiment. The network equipment 200 of Fig. 17 comprises a calculate module 210e configured to perform step S106. The network equipment 200 of Fig. 17 may further comprise a number of optional functional modules, such as any of a receive module 210a configured to perform step S102a, a receive module 210b configured to perform step S102b, an estimate module 210c configured to perform step S104a, an estimate module 210d configured to perform step S104b, a convert module 210f configured to perform step S108, and a transmit module 210g configured to perform step S110.

In general terms, each functional module 210a: 210g may in one embodiment be implemented only in hardware and in another embodiment with the help of software, i.e., the latter embodiment having computer program instructions stored on the storage medium 230 which when run on the processing circuitry makes the network equipment 200 perform the corresponding steps mentioned above in conjunction with Fig 17. It should also be mentioned that even though the modules correspond to parts of a computer program, they do not need to be separate modules therein, but the way in which they are implemented in software is dependent on the programming language used. Preferably, one or more or all functional modules 210a: 210g may be implemented by the processing circuitry 210, possibly in cooperation with the communications interface 220 and/or the storage medium 230. The processing circuitry 210 may thus be configured to from the storage medium 230 fetch instructions as provided by a functional module 210a: 210g and to execute these instructions, thereby performing any steps as disclosed herein.

The network equipment 200 may be provided as a standalone device or as a part of at least one further device. For example, the network equipment 200 may be provided in a network node 102 (such as in a node of a radio access network or in a node of a core network) , or in a user equipment 104. Alternatively, functionality of the network equipment 200 may be distributed between at least two devices, or nodes. These at least two nodes, or devices, may either be part of the same network part or may be spread between at least two such network parts. Thus, a first portion of the instructions performed by the network equipment 200 may be executed in a first device, and a second portion of the of the instructions performed by the network equipment 200 may be executed in a second device; the herein disclosed embodiments are not limited to any particular number of devices on which the instructions performed by the network equipment 200 may be executed. Hence, the methods according to the herein disclosed embodiments are suitable to be performed by a network equipment 200 residing in a cloud computational environment. Therefore, although a single processing circuitry 210 is illustrated in Fig. 16 the processing circuitry 210 may be distributed among a plurality of devices, or nodes. The same applies to the functional modules 210a: 210g of Fig. 17 and the computer program 1820 of Fig. 18.

Fig. 18 shows one example of a computer program product 1810 comprising computer readable storage medium 1830. On this computer readable storage medium 1830, a computer program 1820 can be stored, which computer program 1820 can cause the processing circuitry 210 and thereto operatively coupled entities and devices, such as the communications interface 220 and the storage medium 230, to execute methods according to embodiments described herein. The computer program 1820 and/or computer program product 1810 may thus provide means for performing any steps as herein disclosed.

In the example of Fig. 18, the computer program product 1810 is illustrated as an optical disc, such as a CD (compact disc) or a DVD (digital versatile disc) or a Blu-Ray disc. The computer program product 1810 could also be embodied as a memory, such as a random access memory (RAM) , a read-only memory (ROM) , an erasable programmable read-only memory (EPROM) , or an electrically erasable programmable read-only memory (EEPROM) and more particularly as a non-volatile storage medium of a device in an external memory such as a USB (Universal Serial Bus) memory or a Flash memory, such as a compact Flash memory. Thus, while the computer program 1820 is here schematically shown as a track on the depicted optical disk, the computer program 1820 can be stored in any way which is suitable for the computer program product 1810.

The inventive concept has mainly been described above with reference to a few embodiments. However, as is readily appreciated by a person skilled in the art, other embodiments than the ones disclosed above are equally possible within the scope of the inventive concept, as defined by the appended patent claims.

Claims

A method for frequency offset compensation of a transmission signal to be transmitted by a transmitter (110) over a channel (130) , wherein the method is performed by a network equipment (200) , and wherein the method comprises:

calculating (S106) a phase-compensated frequency domain version
of the transmission signal by using circular convolution between a frequency domain version X (k) of the transmission signal and a frequency domain phase rotation sequence P (k) , and by using knowledge of the channel (130) , wherein the frequency domain phase rotation sequence P (k) is determined as a function of an estimated frequency offset value f _o for the transmitter (110) .
The method according to claim 1, wherein the method further comprises:

receiving (S102a) a reference signal; and

estimating (S104a) the frequency offset value f _o from measurements made on the reference signal.
The method according to claim 1 or 2, wherein the method further comprises:

receiving (S102b) a feedback signal; and

estimating (S104b) the frequency offset value f _o from information obtained from the feedback signal.
The method according to any preceding claim, wherein the frequency domain phase rotation sequence P (k) is determined by determining a time domain phase rotation sequence p (n) as a function of the frequency offset value f _o, and then converting the time domain phase rotation sequence p (n) into frequency domain.
The method according to claim 4, wherein the time domain phase rotation sequence is determined as:

where T _s denotes sampling time interval, and n≥0 denotes sample index within an OFDM symbol.
The method according to claim 5, wherein the frequency domain phase rotation sequence P (k) is determined from a DFT of size N from p (n) as:
The method according to any preceding claim, wherein the phase-compensated frequency domain version
is composed of subcarriers to be transmitted over the channel (130) , wherein the phase-compensated frequency domain version
for the subcarrier with index k is dependent on the channel H (n) for a non-phase-compensated frequency domain version X (n) of the transmission signal at sample index n as scaled with the channel H (k) for the subcarrier with index k, and wherein the knowledge of the channel (130) is the channel H (n) and the channel H (k) for the subcarrier with index k.
The method according to claim 7, wherein, for a frequency domain channel estimate H (k) , the phase-compensated frequency domain version
for the subcarrier with index k is determined as:

where ‘mod’ denotes the modulus operation.
The method according to any of claims 1 to 6, wherein the phase-compensated frequency domain version
is composed of subcarriers to be transmitted over the channel (130) , and wherein the knowledge of the channel (130) is an assumption that the channel (130) is frequency flat or quasi-flat.
The method according to claim 9, wherein the phase-compensated frequency domain version
for the subcarrier with index k is determined as:

where ‘mod’ denotes the modulus operation.
The method according to any of claims 1 to 6, wherein the phase-compensated frequency domain version
is composed of subcarriers to be transmitted over the channel (130) , wherein the phase-compensated frequency domain version
for the subcarrier with index k is dependent on the frequency domain phase rotation sequence P (k) as sampled and an equally sampled channel H (n) for a non-phase-compensated frequency domain version X (n) of the transmission signal at sample index n as scaled with the channel H (k) for the subcarrier with index k, and wherein the knowledge of the channel (130) is the equally sampled channel H (n) and the channel H (k) for the subcarrier with index k.
The method according to claim 11, wherein the phase-compensated frequency domain version
is determined as:

where L is an odd integer, and where F (l) is the sampled frequency domain phase rotation sequence P (k) .
The method according to any of claims 1 to 6, wherein the phase-compensated frequency domain version
is composed of subcarriers to be transmitted over the channel (130) , wherein the knowledge of the channel (130) is an assumption that the channel (130) is frequency flat or quasi-flat, and wherein the phase-compensated frequency domain version
for the subcarrier with index k is dependent on the frequency domain phase rotation sequence P (k) as sampled.
The method according to claim 13, wherein the phase-compensated frequency domain version
is determined as:

where L is an odd integer, and where F (l) is the sampled frequency domain phase rotation sequence P (k) .
The method according to claim 12 or 14, wherein
The method according to claim 12, or 14, or 15, wherein the value of L is adaptively determined according to a normalized version of the frequency offset value f _o.
The method according to any preceding claim, wherein the method further comprises:

converting (S108) the phase-compensated frequency domain version
of the transmission signal into a phase-compensated time domain version
of the transmission signal; and

transmitting (S110) the phase-compensated time domain version
of the transmission signal over the air from the transmitter (110) .
The method according to any preceding claim, wherein the transmission signal is to be transmitted in an OFDM-FDMA operation based system.
The method according to any preceding claim, wherein the network equipment (220) is part of either a network node (102) or a user equipment (104) .
A network equipment (200) for frequency offset compensation of a transmission signal to be transmitted by a transmitter (110) over a channel (130) , the network equipment (200) comprising processing circuitry (210) , the processing circuitry being configured to cause the network equipment (200) to:

calculate a phase-compensated frequency domain version
of the transmission signal by using circular convolution between a frequency domain version X (k) of the transmission signal and a frequency domain phase rotation sequence P (k) , and by using knowledge of the channel (130) , wherein the frequency domain phase rotation sequence P (k) is determined as a function of an estimated frequency offset value f _o for the transmitter (110) .
A network equipment (200) for frequency offset compensation of a transmission signal to be transmitted by a transmitter (110) over a channel (130) , the network equipment (200) comprising:

a calculate module (210e) configured to calculate a phase-compensated frequency domain version
of the transmission signal by using circular convolution between a frequency domain version X (k) of the transmission signal and a frequency domain phase rotation sequence P (k) , and by using knowledge of the channel (130) , wherein the frequency domain phase rotation sequence P (k) is determined as a function of an estimated frequency offset value f _o for the transmitter (110) .
The network equipment (200) according to claim 20 or 21, further being configured to perform the method according to any of claims 2 to 19.
A computer program (1820) for frequency offset compensation of a transmission signal to be transmitted by a transmitter (110) over a channel (130) , the computer program comprising computer code which, when run on processing circuitry (210) of a network equipment (200) , causes the network equipment (200) to:

calculate (S106) a phase-compensated frequency domain version
of the transmission signal by using circular convolution between a frequency domain version X (k) of the transmission signal and a frequency domain phase rotation sequence P (k) , and by using knowledge of the channel (130) , wherein the frequency domain phase rotation sequence P (k) is determined as a function of an estimated frequency offset value f _o for the transmitter (110) .
A computer program product (1810) comprising a computer program (1820) according to claim 23, and a computer readable storage medium (1830) on which the computer program is stored.