WO2016039668A2

WO2016039668A2 - Interference mitigation

Info

Publication number: WO2016039668A2
Application number: PCT/SE2014/051027
Authority: WO
Inventors: Miguel Lopez
Original assignee: Telefonaktiebolaget L M Ericsson (Publ)
Priority date: 2014-09-08
Filing date: 2014-09-08
Publication date: 2016-03-17
Also published as: WO2016039668A3

Abstract

There is provided mitigating of interference from a narrowband signal during transmission of a multicarrier modulated signal. Modulation symbols of a multicarrier modulated signal are acquired. The multicarrier modulated signal comprises a plurality of sub-carriers. Frequency components of a narrowband signal overlapping with the sub-carriers are acquired. A signal representing at least one of the modulation symbols and the narrowband signal is generated. In the generated signal interference to a sub-carrier caused by a frequency component of the narrowband signal at a different frequency than the sub-carrier is suppressed by transmission at the sub-carrier frequency being modified. The generated signal is transmitted.

Description

INTERFERENCE MITIGATION

TECHNICAL FIELD

Embodiments presented herein relate to interference mitigation, and particularly to a method, a radio access network node, a computer program, and a computer program product for mitigating interference from a narrowband signal during transmission of a multicarrier modulated signal.

BACKGROUND

In communications networks, there may be a challenge to obtain good performance and capacity for a given communications protocol, its parameters and the physical environment in which the communications network is deployed.

For example, in some communications networks a first radio access technology (RAT) and a second RAT co-exists within the same frequency band. For example, the first RAT may represent a Global System for Mobile Communications (GSM) using narrowband transmission signals and the second RAT may represent a Long Term Evolution (LTE) using wideband transmission signals. Thus, in particular, in such a dual RAT deployment GSM and LTE may occupy the same frequencies, or frequency band, simultaneously. However, simultaneously transmitting signals using the first RAT and the second RAT may cause mutual interference.

It is desirable to ensure that both portable wireless devices receiving signals of the first RAT (such as legacy GSM mobile phones) and portable wireless devices receiving signals of the second RAT (such as LTE user equipment) have good reception performance.

Hence, there is a need for mitigation of interference from a narrowband signal when transmitting a wideband signal SUMMARY

An object of embodiments herein is to provide efficient mitigation of interference from a narrowband signal when transmitting a wideband signal.

According to a first aspect there is presented a method for mitigating interference from a narrowband signal during transmission of a multicarrier modulated signal. The method is performed by a radio access network node. The method comprises acquiring modulation symbols of a multicarrier modulated signal, the multicarrier modulated signal comprising a plurality of sub-carriers. The method comprises acquiring frequency components of a narrowband signal overlapping with the sub-carriers. The method comprises generating a signal representing at least one of the modulation symbols and the narrowband signal, wherein interference to a sub-carrier caused by a frequency component of the narrowband signal at a different frequency than the sub-carrier is suppressed by transmission at the sub-carrier frequency in the generated signal being modified. The method comprises transmitting the generated signal.

Advantageously this provides efficient mitigation of interference from a narrowband signal when transmitting a wideband signal in the form of a multicarrier modulated signal. Advantageously this enables a dual employment of a radio access technology utilizing a narrowband signal, such as used in GSM, and a radio access technology utilizing a multicarrier modulated signal, such as used in LTE, operating in overlapping frequency bands.

Advantageously this improves downlink performance, especially for high order modulations that require low error vector magnitude (EVM).

According to a second aspect there is presented a radio access network node for mitigating interference from a narrowband signal during transmission of a multicarrier modulated signal. The radio access network node comprises a processing unit. The processing unit is configured to acquire modulation symbols of a multicarrier modulated signal, the multicarrier modulated signal comprising a plurality of sub-carriers. The processing unit is

configured to acquire frequency components of a narrowband signal overlapping with the sub-carriers. The processing unit is configured to generate a signal representing at least one of the modulation symbols and the narrowband signal, wherein interference to a sub-carrier caused by a frequency component of the narrowband signal at a different frequency than the sub-carrier is suppressed by transmission at the sub-carrier frequency in the generated signal being modified. The processing unit is configured to transmit said generated signal. According to a third aspect there is presented a computer program for mitigating interference from a narrowband signal during transmission of a multicarrier modulated signal, the computer program comprising computer program code which, when run on a processing unit of a radio access network node, causes the radio access network node to perform a method according to the first aspect.

According to a fourth aspect there is presented a computer program product comprising a computer program according to the third aspect and a computer readable means on which the computer program is stored.

According to a fifth aspect there is presented a signal for mitigating

interference from a narrowband signal during transmission of a multicarrier modulated signal. The signal represents at least one of modulation symbols of a multicarrier modulated signal, the multicarrier modulated signal

comprising a plurality of sub-carriers, and a narrowband signal. Interference to a sub-carrier caused by a frequency component of the narrowband signal at a different frequency than the sub-carrier is suppressed by transmission at the sub-carrier frequency in the generated signal being modified.

It is to be noted that any feature of the first, second, third, fourth and fifth aspects may be applied to any other aspect, wherever appropriate. Likewise, any advantage of the first aspect may equally apply to the second, third, fourth, and/ or fifth aspect, respectively, and vice versa. 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, step, etc." are to be interpreted openly as referring to at least one instance of the element, apparatus, component, means, 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. l is a schematic diagram illustrating a communication network according to embodiments;

Fig. 2 is a schematic diagram of a transmitter architecture according to prior art;

Figs. 3a, 3b, and 3c are schematic diagrams of transmitter architectures according to embodiments; Fig. 4 is a schematic diagram showing functional units of a radio access network node according to an embodiment;

Fig. 5 is a schematic diagram showing functional modules of a radio access network node according to an embodiment;

Fig. 6 shows one example of a computer program product comprising computer readable means according to an embodiment;

Figs. 7, 8, and 9 are flowcharts of methods according to embodiments; Fig. 10 schematically illustrates a frequency spectrum at the transmitter of a radio access network node;

Fig. 11 schematically illustrates a frequency spectrum at the receiver of a portable wireless device; Fig. 12 schematically illustrates a waveform of a narrowband signal and symbols of a multicarrier modulated signal;

Fig. 13 schematically illustrates the spectrum of a 20 MHz LTE carrier;

Fig. 14 schematically illustrates an altered spectrum of the signal of Fig. 13;

Fig. 15 schematically illustrates the signal of Fig. 14 after propagation through a fading channel;

Fig. 16 schematically illustrates a 64 QAM constellation;

Fig. 17 schematically illustrates a pre-distorted 64 QAM constellation; and

Fig. 18 schematically illustrates the noise floor at the receiver for the received signal shown in Fig. 17. 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. Fig. 1 is a schematic diagram illustrating a communications network 10 where embodiments presented herein can be applied. The communications network 10 comprises a radio access network node (NN) 11 implementing functionality of at least one of a radio base station, a base transceiver station, a node B, and an evolved node B. The radio access network node 11 is operatively connected to a core network 13 which in turn is opera tively connected, via a communications link 15, to a wide area service and data providing Internet protocol network 14. A portable wireless device (WD) 12 such as a mobile station, a mobile phone, a handset, a wireless local loop phone, a user equipment (UE), a smartphone, a laptop a computer, or a tablet computer is thereby enabled to access services and data as provided by the network 14. As the skilled person understands, the communications network 10 may comprise a plurality of radio access network nodes 11.

Each such radio access network node 11 may concurrently serve a plurality of portable wireless devices 12. Particularly, the radio access network node 11 may be configured to use a first radio access technology (RAT) and a second RAT. The first RAT may involve the radio access network node 11 to transmit sub-carriers of a multicarrier modulated signal. The second RAT may involve the radio access network node 11 to transmit a narrowband signal.

For example, the narrowband transmission signals of the first RAT may cause interference to the wideband transmission signals of the second RAT.

Particularly, a GSM carrier may cause interference to sub-carriers of an LTE signal. Further, as noted above, it may be desirable to ensure that both portable wireless devices receiving signals of the first RAT (such as legacy GSM mobile phones) and portable wireless devices receiving signals of the second RAT (such as LTE user equipment) have good reception performance. In general terms, for GSM this can only be achieved by having a power spectral density (PSD) high enough above the PSD of the LTE signal, so that the channel to interference ratio (C/I) experienced by the legacy GSM mobile phone is within its operating range. This however, will create interference for the LTE user equipment. Moreover, a GSM signal is never orthogonal to an LTE orthogonal frequency-division multiplexing (OFDM) signal. Due to the lack of orthogonality, the OFDM fast Fourier transform (FFT) unit at the UE receiver will spread some GSM interference over the whole LTE frequency band, with significant amounts of GSM interference being leaked to many sub-carriers of the LTE signal.

The part of the communications link 15 directed from the radio access network node 11 to the portable wireless device 12 is called a downlink, and the part of the communications link 15 directed from the portable wireless device 12 to the radio access network node 11 is called an uplink.

Fig. 2 schematically illustrates a transmitter architecture of a radio access network node 11. The radio access network node 11 comprises two radio chains; one for the first RAT and one for the second RAT. These two radio chains are merged by means of adding the signals of the two radio chains. In more detail, the radio access network node 11 comprises an OFDM module 11a for modulating frequency domain modulation symbols of a multicarrier modulated signal 17. The radio access network node 11 comprises a

modulation module 11a for modulating a narrowband signal 18. The OFDM modulated signal and the modulated narrowband signal are added and sent to an antenna arrangement 16 for transmission towards wireless portable devices 12. As the skilled person understands, this is a simplified illustration of the transmitter architecture and further various elements, components, and modules, such as amplifiers, etc., maybe inserted along the radio chains. As noted above, transmission using the first RAT may interfere with transmission using the second RAT and vice versa if the first RAT and the second RAT have overlapping frequency components, for example if frequency components of the narrowband signal overlap with sub-carriers of the multicarrier modulated signal. However, as also noted above, interference to e.g. OFDM subcarriers may occur even if the frequency components do not overlap.

The herein disclosed embodiments alleviate interference from the

narrowband signal as experienced by a portable wireless devices 12 configured to receive the multicarrier modulated signal. The herein disclosed embodiments are based on keeping the narrowband signal intact at least over its own narrow frequency band; some embodiments involve to pre-distort the multicarrier modulated signal whilst some embodiments involve to add a (typically wideband) pre-distortion signal to the narrowband signal, so as to achieve essentially the same compound signal (representing both the narrowband signal and the multicarrier modulated signal) for the

embodiments.

For example, the multicarrier modulated signal may be distorted in order to cancel the interference caused to the sub-carriers not overlapping the signal spectrum of the narrowband signal. At the portable wireless device side, a portable wireless device receiving the narrowband signal does not experience any degradation since the narrowband signal has not been altered within its own narrow bandwidth, whilst a portable wireless device receiving the multicarrier modulated signal will experience improved performance since the interference outside of a narrow frequency band of the narrowband signal has been cancelled at the transmitter side.

Fig. 10 schematically illustrates the frequency spectrum at the transmitter of the radio access network node 11. Hence, the frequency spectrum of Fig. 10 may be regarded as describing the situation in presence of an ideal, signal non-degrading, communications channel (i.e., without any noise, fading, etc.). From the situation in Fig. 10 it maybe conjectured that the narrowband frequency components 41 will not cause any interference to the frequency components 42 of the multicarrier modulated signal outside of the frequency band FBi. It may thus further be conjectured that a portable wireless device receiving the multicarrier modulated signal in a frequency band FBi would experience interference from the narrowband signal whilst a portable wireless device receiving the multicarrier modulated signal in a frequency band FB2 would not experience interference from the narrowband signal. For example, the floor of the narrowband frequency components 41 maybe in the range of 60 dB lower than the frequency components 42 of the multicarrier modulated signal. Feeding the narrowband signal through a bandpass filter centred around the carrier frequency of the narrowband signal would thus just marginally further suppress the narrowband signal outside the frequency band FB in comparison to the frequency components 42 of the multicarrier modulated signal.

However, due to the frequency content of the narrowband signal, some frequency components of the narrowband signal may leak to adjacent frequency bands after application of frequency transformation (e.g., by means of a Fast Fourier Transform, FFT) at the receiver of the portable wireless device receiving the multicarrier signal. As a non-limiting illustrative example, consider a narrowband signal being a GSM frequency correction burst and that the multicarrier signal is an LTE signal. A GSM frequency correction burst is a pure tone with a frequency offset of 67.7 kHz from the center of frequency of the GSM carrier. Because of the channel raster used in LTE and GSM, this pure tone will not have the same frequency as any of the tones composing the multicarrier LTE signal, which have a 15 kHz inter- carrier spacing. Hence, the application of the FFT at the receiver of the portable wireless device receiving the multicarrier signal will spread the energy of the pure tone over all the subcarriers of the LTE signal since only non-zero coefficients are obtained when determining the Fourier

decomposition of a 67.7 kHz tone in terms of tones located at the subcarriers of the LTE signal. Fig. 11 schematically illustrates a typical frequency spectrum at the receiver of the portable wireless device 12, after application of the FFT. From the schematic illustration of Fig. 11 it can be seen that some frequency components of the frequency representation 51 of the narrowband signal has leaked outside the frequency band CB and may thus cause interference to a portable wireless device receiving the multicarrier modulated signal, in Fig. 11 represented by the sub-carriers 52. This may be true regardless if the portable wireless device receives a multicarrier modulated signal in the frequency band FBi or in the frequency band FB2.

Before disclosing how to efficiently mitigate interference from the

narrowband signal during transmission of the multicarrier modulated signal some notation that will be used henceforth will be introduced before the embodiments are presented in detail. Column vectors will be denoted using boldface letters. For notational convenience the indices in a vector are allowed to start at any integer (even negative integers). For example u = [i_½]£L_M denotes the column vector

U_M

(1) u = ^;¹ .

The following notation will be used extensively. If M≤m≤n≤N then

Given two vectors g, h, the notation toeplitz(g, h) will denote the Toeplitz matrix whose first column is g and first row is h.The notation circulant(h) denotes a circulant matrix whose first column is h. Each column in such matrix is obtained by circular shift of the preceding column. The notation || ||₂ indicates the second norm of the vector . In addition, i_M and o_MxW denote the identity matrix of dimension M x M and the matrix of zeros of dimension M x N.

In the derivations below it is assumed that the portable wireless device has only one receiving antenna. These derivations are used only to motivate the inventive concepts disclosed herein. In general terms, all the processing of the herein disclosed embodiments for mitigating interference from a narrowband signal 18 during transmission of a multicarrier modulated signal 17 occurs at the transmitter side in the radio access network node and it is transparent to the portable wireless device. Therefore there is no loss of generality assuming that the portable wireless device has only one receiving antenna.

Finally, it may be assumed that the portable wireless device receiving the receiving the multicarrier modulated signal is not aware that a narrowband carrier is being simultaneously transmitted in a radio frequency channel overlapping the multicarrier modulated signal transmission. This is typically the case for all legacy portable wireless devices receiving the multicarrier modulated signal. Without loss of generality, assume that a multicarrier modulated signal, such as an LTE signal, is transmitted. The multicarrier modulated signal uses a frequency band, such as a LTE frequency band. Consider transmission of a multicarrier modulated signal with a total bandwidth of JV_c subcarriers.

Without loss of generality, assume that a carrier of a narrowband signal, such as a GSM signal, is transmitted, overlapping the frequency band of the multicarrier modulated signal. Such a situation is schematically illustrated in Fig. 10. It is assumed that most of the energy of the narrowband signal is concentrated between sub-carriers k₀ and / , as shown in the figure.

Assuming a sub-carrier spacing of 15 kHz, a typical value of / - k₀ could be 24, corresponding to two LTE physical resource blocks, or 360 kHz.

It is assumed that the multicarrier modulated signal uses time division multiplexing, where time may be divided into frames, frames may be divided into sub-frames (such as 10 LTE sub-frames) and each sub-frame may be divided into time slots (such as 2 LTE time slots). Each time slot may comprise a number of orthogonal frequency-division multiplexing (OFDM) symbols. An example of how to modulate each OFDM symbol will follow next. However, as the skilled person will understand, at least some of the embodiments disclosed herein are independent from whether OFDM is used or not. For simplicity, and without loss of generality, it is assumed that the origin of the time axis coincides with the end of the cyclic prefix (CP) of the OFDM symbol under consideration, as schematically illustrated in Fig. 12

Using the timing convention from Fig. 12, the baseband signal corresponding to one modulated OFDM symbol has the form (3) s(t) =∑^N _ki₀ ¹ a_ke^-^ 0≤t≤T_u, where Δ/ is the sub-carrier spacing, T_u≡ l/Af and

a = [a^_kio^{1 are} the frequency domain modulation symbols of the multicarrier modulated signal.

The narrowband signal may be modulated using Gaussian Minimum-Shift Keying (GMSK) or Eight Phase Shift Keying (8PSK). However, as the skilled person will understand the herein disclosed embodiments are applicable to any narrowband modulation, not only GMSK or 8PSK; but for the sake of concreteness a GMSK modulated GSM signal is assumed. A GMSK signal can be expressed in the form (4) v(t) = A^■ e p ( ), where φ(ρ) is a continuous phase and A is an amplitude. A discrete baseband model can be constructed from (3) and (4) by sampling at the instants t_n = nT_s, with T_s = i/(N_cAf). Sampling yields the discrete signals

(5) s(n)≡ s(nT_s) = Σ^^"1 a_ke^^ and

(6) v(n)≡ v(nT_s) = A^■ exp(/<p(n7 )).

Modelling of the signal received at the LTE portable wireless device will be presented next. Assume that a discrete equivalent channel is used to model the radio propagation, as well as transmission and reception filtering.

Assume that L is the length of the cyclic prefix, and that the taps of the discrete equivalent channel are [h_k]^L _k=0. Some of the taps might be zero. The following notation is used:

(7) h = [h_L - h₀ 0 - o]^T, of length (N_C + L),

(8) h?^{u >} = [h₀■■■ h_L 0 · · · 0] ^T, of length JV_C,

(9) g = [h_Lo ··· of, of length JV_C,

(10) H_TOE = toepiitzig, h) of dimension JV_c x (JV_C + L).

The cyclic prefix of the transmitter chain of the multicarrier modulated signal can be described in matrix form as follows. Define the matrices

(11) p = [0_{LX (NC}-_L) i_L] of dimension L X N_C (12) c = of dimension (JV_C + L) X N_C The matrix c performs the cyclic prefix insertion. With this notation, the signal received at the LTE portable wireless device, after cyclic prefix removal, may be expressed in the following form

(13) y = H_Toe CF^→a + H_Toev__L:Nc→ + w. Equation (13) comprises the following parameters: The parameter a

represents frequency domain modulation symbols (i.e., desired symbols for LTE portable wireless devices), of length JV_c. The parameter y represents the time domain received digital signal (at an LTE portable wireless device), of length JV_c. The parameter H_Toe represents the discrete equivalent channel of dimension JV_c x (JV_C + L). The channel is assumed to be the same for both the LTE signal and the GSM signal, due to the transmitter architecture. The parameter F = [F_{m n}] represents the Discrete Fourier Transform Matrix of size

-j nmn

JV_c x JV_c, with F_{m n} =— -^—; n, m = o,— , N_C - 1. The parameter F^_1 is the Inverse

Discrete Fourier Transform. Both the Discrete Fourier Transform and the Inverse Discrete Fourier Transform are typically implemented by means of the Fast Fourier Transform (FFT) or IFFT, respectively. The parameter v__L._Nc→ represents a vector containing samples of a baseband GSM signal, of length (JV_C + L). See (6 for the definition of the components of v. The parameter w represents the noise vector, of length JV_C. An LTE portable wireless device would then apply the Fast Fourier

Transform (FFT) to the received signal of equation (13):

(14) Y≡ Fy = FH_ToeCF^~1a + FH_Toev__L._Nc_₁ + W, where w = Fw. It is easily verified that the matrix H_clrc≡ H_Toe c= circuiant(h^rUp). Hence, equation (14) maybe rewritten as follows: (15) Y = FH_clrcF- a + FH_Toev__{L Nc}_₁ + W.

It is well known that the Fourier matrix diagonalizes any circulant matrix. In other words, the matrix Λ≡ FH_clrcF^→ is diagonal. Thus, (15) becomes (16) Y = Aa + FH_Toev__L._Nc_₁ + W.

A detailed analysis of (i6) will now follow. To this end the narrowband is split into two components:

It is readily verified that

Plugging (19) into (16) yields

(20) Y = Aa + FH_Toe {Cv^c + v^?) + W

= Aa + FH_Toe Cv^c + FH_Toev^p + W

= Aa + FH_clrcF^~1Fv^c + FH_Toev^p + W.

Next, define

(21) V≡Fv^c, and, again, split the frequency domain vector v into two orthogonal components. Recall that it is assumed that in the frequency domain most of the energy of the narrowband signal is concentrated between sub-carriers k₀ and / .

It is clear that v^r orthogonal to V^s, and (24) V = V^r + V^s.

Using (21), (24), we equation (20) can be rewritten into the following form:

(25) Y = Aa + FH_clrcF^→V + FH_Toev^p + W = Aa + A(V^r + V^s) + FH_Toev^p + W. Finally, Y may thus be expressed as:

(26) Y = Aa + AV^r + AV^S + FH_Toev^p + W.

It is noted that (26) and (i6) are completely equivalent. The advantage of (26) is that it allows for an interpretation to each of the terms in (26). The terms in (26) may thus be expressed as follows. The parameter a represents the frequency domain modulation symbols (i.e., desired symbols for LTE portable wireless devices). The parameter v^r represents the narrowband signal in the frequency domain, centered around the center of frequency of the carrier frequency of the narrowband signal. The parameter V^s represents the narrowband signal interference in the frequency domain that has leaked to the multicarrier subcarriers outside the narrowband carrier frequency. The parameter FH_Toev^p represents the residual narrowband signal interference due to the lack of cyclic prefix in the narrowband signal.

An estimate of the signal-to-interference-plus-noise ratio (SINR) of the signal received by the LTE portable wireless device will now be determined. In general terms, the residual interference FH_Toev^p contains much less energy than v^ror V^s. Ignoring this term yields an approximate estimate of the SINR as follows:

ΙΐΛαΙΙΙ

(27) SINR≡

ΙΙΛΚΠΙΙ + ^ΙΙ!

As noted above, the embodiments disclosed herein relate to mitigating interference from a narrowband signal 18 during transmission of a

multicarrier modulated signal 17. This would increase the SINR in (28). In order to obtain such interference mitigation there is provided a radio access l6 network node 11, a method performed by the radio access network node 11, a computer program comprising code, for example in the form of a computer program product, that when run on a processing unit of the radio access network node 11, causes the radio access network node 11 to perform the method.

Fig. 4 schematically illustrates, in terms of a number of functional units, the components of a radio access network node 11 according to an embodiment. A processing unit 21 is provided using any combination of one or more of a suitable central processing unit (CPU), multiprocessor, microcontroller, digital signal processor (DSP), application specific integrated circuit (ASIC), field programmable gate arrays (FPGA) etc., capable of executing software instructions stored in a computer program product 31 (as in Fig. 6, see below), e.g. in the form of a storage medium 23. Thus the processing unit 21 is thereby arranged to execute methods as herein disclosed. The storage medium 23 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 radio access network node 11 may further comprise a communications interface 22 for

communications with at least one portable wireless device 12. As such the communications interface 22 may comprise one or more transmitters and receivers, comprising analogue and digital components and a suitable number of antennae for radio communications. The processing unit 21 controls the general operation of the radio access network node 11 e.g. by sending data and control signals to the communications interface 22 and the storage medium 23, by receiving data and reports from the communications interface 22, and by retrieving data and instructions from the storage medium 23. Other components, as well as the related functionality, of the radio access network node 11 are omitted in order not to obscure the concepts presented herein. Fig. 5 schematically illustrates, in terms of a number of functional modules, the components of a radio access network node 11 according to an

embodiment. The radio access network node 11 of Fig. 4 comprises a number of functional modules; an acquire module 21a, a generate module 21b, and a transmit/receive module. The radio access network node 11 of Fig. 4 may further comprises a number of optional functional modules, such as any of a determine module 2 id, a feed module 2ie, a pre-distort module, and an add module 2ig. The functionality of each functional module 2ia-g will be further disclosed below in the context of which the functional modules 2ia-g may be used. In general terms, each functional module 2ia-g maybe implemented in hardware or in software. Preferably, one or more or all functional modules 2ia-g may be implemented by the processing unit 21, possibly in cooperation with functional units 22 and/ or 23. The processing unit 21 may thus be arranged to from the storage medium 23 fetch instructions as provided by a functional module 2ia-g and to execute these instructions, thereby performing any steps as will be disclosed hereinafter.

Fig. 6 shows one example of a computer program product 31 comprising computer readable means 33. On this computer readable means 33, a computer program 32 can be stored, which computer program 32 can cause the processing unit 21 and thereto operatively coupled entities and devices, such as the communications interface 22 and the storage medium 23, to execute methods according to embodiments described herein. The computer program 32 and/ or computer program product 31 may thus provide means for performing any steps as herein disclosed.

In the example of Fig. 6, the computer program product 31 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 31 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 32 is here schematically shown as a track on the depicted optical disk, the l8 computer program 32 can be stored in any way which is suitable for the computer program product 31.

Figs. 7, 8 and 9 are flow charts illustrating embodiments of methods for mitigating interference from a narrowband signal 18 during transmission of a multicarrier modulated signal 17. The methods are performed by the radio access network node 11. The methods are advantageously provided as computer programs 32.

Reference is now made to Fig. 7 illustrating a method for mitigating interference from a narrowband signal 18 during transmission of a

multicarrier modulated signal 17 as performed by the radio access network node 11 according to an embodiment.

As noted above, the radio access network node 11 may be configured to transmit a multicarrier modulated signal 17 and a narrowband signal 18. The radio access network node 11 is therefore configured to, in a step S102, acquire modulation symbols of the multicarrier modulated signal. The multicarrier modulated signal comprises a plurality of sub-carriers 52.

The radio access network node 11 is further configured to, in a step S104, acquire frequency components of a narrowband signal 18 overlapping with the sub-carriers 52. In other words, the narrowband signal 18 may at least overlap with the transmission frequency band of the subcarriers. Different ways for how the radio access network node 11 may acquire these frequency components of the narrowband signal 18 will be disclosed below.

A signal is generated in order to mitigate interference from the narrowband signal 18 during transmission of the multicarrier modulated signal 17.

Particularly, the radio access network node 11 is configured to, in a step S106, generate a signal representing at least one of the modulation symbols and the narrowband signal. Interference to a sub-carrier caused by a frequency component of the narrowband signal at a different frequency than the sub- carrier is suppressed by transmission at the sub-carrier frequency being modified in the generated signal 19. Interference of at least one of the frequency components of the narrowband signal interfering with sub-carriers of the multicarrier modulated signal is thereby suppressed in the generated signal 19.

Different sub-carriers may thereby be compensated with a factor

corresponding to the interference that, through OFDM modulation (at the transmitter) and demodulation (at the receiver) and non-orthogonality between the sub-carriers and the narrowband signal, is caused by the narrowband signal.

,The interference from signal components of the narrowband signal being present within a band-pass filter at the OFDM demodulator (at the intended receiver) is thereby removed.

Further features and/ or properties of the generated signal 19 will be disclosed below.

The generated signal 19 may then be transmitted. Thus the radio access network node 11 is configured to, in a step S108, transmit the generated signal 19. The generated signal 19 maybe transmitted to the portable wireless device 12.

Embodiments relating to further details of mitigating interference from a narrowband signal 18 during transmission of a multicarrier modulated signal 17 will now be disclosed.

Some properties of the narrowband signal and the multicarrier modulated signal will now be disclosed. For example, the narrowband signal 18 may have a carrier frequency. The subcarrier frequencies to which interference is suppressed in the generated signal 19 may be outside a frequency band (FB) surrounding the carrier frequency of the narrowband signal 18. The frequency band may overlap with at least one sub-carrier 52 of the

multicarrier modulated signal 17. The narrowband signal may be a Global System for Mobile Communications (GSM) signal, or an Enhanced Data Rates for GSM Evolution (EDGE) signal. The modulation symbols of the multicarrier modulated signal 17 maybe frequency domain modulation symbols. Additionally or alternatively, the multicarrier modulated signal may be a Long Term Evolution (LTE) or a WiFi signal. Reference is now made to Fig. 8 illustrating methods for mitigating

interference from a narrowband signal 18 during transmission of a

multicarrier modulated signal 17 as performed by the radio access network node 11 according to further embodiments.

Different ways for how the radio access network node 11 may acquire the frequency components of the narrowband signal 18 will now be disclosed.

As noted above, a parameter V^s may be determined during the derivation of the SINR for the multicarrier modulated signal 17, see equation (29). The parameter V^s represents frequency components of the narrowband signal 18 as seen after OFDM demodulation. Particularly, the frequency components of the narrowband signal 18 may be frequency components of the narrowband signal 18 that would otherwise leak to the adjoining sub-carriers of the multicarrier modulated signal 17 (at a portable wireless device 12 receiving the modulation symbols of the multicarrier modulated signal) after

application of the FFT. Such frequency components of the narrowband signal 18 are given by the parameter V ^s .

As further noted above, a parameter parameter v comprises the parameter V^s, see equation (30) and is the frequency representation of a time segment of the narrowband signal. Thus, the acquired frequency components of the narrowband signal may be all frequency components of a time segment of the narrowband signal. The radio access network node 11 may then be configured to, in an optional step Sio6a, determine frequency components of the narrowband signal that would otherwise leak to the adjoining sub-carriers of the multicarrier modulated signal 17 (at a portable wireless device 12 receiving the modulation symbols of the multicarrier modulated signal) after application of the FFT. The radio access network node 11 may then be configured to, in an optional step Sio6b, generate the signal based on the determined frequency components.

As further noted above, a parameter v^c may be determined during the derivation of the SINR for the multicarrier modulated signal 17, see equation (31). The parameter v^c represents a time segment of the narrowband signal 18. Further, the parameter v is a frequency domain representation of v^c, see equation (32). Thus, the frequency components of the narrowband signal may be provided by means of a time segment of the narrowband signal. The radio access network node 11 may then be configured to acquire the frequency components of the narrowband signal by, in an optional step Si04b, determine first frequency components of the time segment. The first frequency components of the time segment correspond to v. The radio access network node 11 may then be configured to generate the signal by, in an optional step Sio6a', determine second frequency components as those first frequency components that leak to the adjoining sub-carriers of the multicarrier modulated signal after application of the FFT at the receiver; and, in an optional step Sio6b', generate the signal based on the determined frequency components. The second frequency components correspond to V^s.

The time segment may comprises the Nc-2L center-most samples of the narrowband signal taken at the multicarrier signal sampling rate, see equation (33). These Nc-2L center-most samples maybe in a segment of the narrowband signal time-wise overlapping an orthogonal frequency-division multiplexing (OFDM) symbol of said modulation symbols of the multicarrier modulated signal. The OFDM symbol has a total length Nc+L modulation symbols, including a cyclic prefix of length L modulation symbols. Fig. 12 schematically illustrates a waveform of a narrowband signal and OFDM symbols of a multicarrier modulated signal comprising a cyclic prefix along a time line.

As noted above, some of the herein disclosed embodiments involve to pre- distort modulation frequency domain symbols in each multicarrier modulated symbol. Some of the herein disclosed embodiments involve to pre-distort the narrowband signal. The pre-distortion depends on the narrowband signal. The pre-distortion is such that it removes most of the narrowband signal interference outside a narrow band around the center of frequency of the carrier frequency of the narrowband signal. The carrier frequency of the narrowband signal itself is left intact within its own narrow band.

There may be different ways to perform the pre-distortion. Different embodiments relating thereto will now be described in turn.

The generation of the signal may result in a pre-distortion signal. For example, the radio access network node n may be configured to generate the signal by, in an optional step Sio6c, pre-distort the modulation symbols with a pre-distortion signal. The pre-distortion signal is based on the frequency components of the narrowband signal determined at the sub-carrier frequencies. Two cases will now be considered. First, embodiments relating to the case where there is only one narrowband signal (i.e., one frequency carrier of one narrowband signal). Then, embodiments relating to the case where there are several narrowband signals (i.e., several frequency carriers, each one of which corresponding to one narrowband signal) will be addressed. Case i: One frequency carrier

Particularly, define

(34) b≡ -V^s, and set

(35) a_pre = a + b. in (36) the parameter a_pre represents the pre-distorted constellation of the modulation frequency domain symbols. A pre-distorter may thus be added to the transmitter architectures of Fig. 2 as shown in Figs. 3a, 3b, and 3c. Using these latter transmitter architectures, the signal received by an LTE portable wireless device, after cyclic prefix removal, may be expressed as follows.

(37) y = H_ToeCF ¹a_pre + H_Toev__L:Nc→ + w.

It is noted that (37) is analogous to (13), except that the pre-distorted frequency domain symbols used.

Using (35) it can be seen that

(38) y = H_ToeCF^→a + H_ToeCF^→b + H_Toev__L:Nc→ + w.

Next, the portable wireless device applies the FFT. The result is similar to (26), but with one extra term due to the pre-distortion: (39) Y≡Fy = Aa + AV^r + AV^S + FH_ToeCF^→b + FH_Toev^p + W

= Aa + AV^r + AV^S + Ab + FH_Toev^p + W

= Aa + A(V^S + b) + AV^r + FH_Toev^p + W.

It is noted that (34) implies

(40) A(V^s + b) = A(V^S - V^S) = 0. Hence (39) becomes

(41) Y = Aa + AV^r + FH_Toev^p + W.

Equation (41) should be compared to (26). Next, the SINR experienced by an LTE portable wireless device is presented, ignoring the contribution of the smallest interference term FH_Toev^p.

By comparing (27) and (42), it can be seen that

(43) SINR_pre__dlstort > SINR. The inequality (43) shows that applying the pre-distortion (35) to the frequency domain symbols increases the SINR experienced by LTE portable wireless device, while keeping the narrowband signal intact within its own narrow bandwidth surrounding its carrier frequency. Case 2: Several frequency carriers

The extension to several signal narrowband signals is straightforward. It is assumed that several narrowband signal frequency carriers are overlaid on the frequency band of the multicarrier modulated signal.

To be specific, assume that there are M narrowband signal frequency carriers. The discrete signals are given by

(44) v_m (n)≡ v_m (nT_s) = exp(/<jP_m(n7 )) , m = 1, - , M.

It is assumed that each narrowband signal frequency carrier contains most of its energy (in the frequency domain) between sub-carriers k₀ (m) and k^m), m = Ι, ·· · , Μ. It is further assumed that the different narrowband signal frequency carriers are well spaced in the frequency domain and do not overlap: k^m < k₀ (m + l), m = I,— , M - 1.

Define

(46) V≡Fv^c,

(48) V^s = V - V^r, and

(49) b≡ -V^s. The signal represented by the parameter b may then be added to the multicarrier modulated signal comprising the modulation symbols. The radio access network node 11 may thus be configured to generate the signal by, in an optional step Sio6d, add the pre-distortion signal to the modulation symbols. The pre-distorted constellation of the modulation frequency domain symbols may thus be given by

(50) a_pre = a + b.

The pre-distortion signal may be added to the modulation symbols before feeding the modulation symbols to an OFDM modulator. Figs. 3a, 3b, and 3c schematically illustrate three different transmitter architectures of the radio access network node 11. In Figs. 3a, 3b, and 3c module 11a is an OFDM modulator for the multicarrier modulated signal; module 11b is a modulator for the narrowband signal; module 11c is a pre-distorter; and module nd is a separate OFDM modulator for the pre-distortion signal. In all the

embodiments of Figs. 3a, 3b, and 3c all resulting signals are added and fed to an antenna arrangement 16

In the embodiment of Fig. 3a frequency components of a narrowband signal 18 are from a narrowband signal modulator 11b fed to a pre-distorter 11c which in turn feeds a pre-distorted version of the multicarrier modulated signal 17 to the OFDM modulator 11a.

In the embodiment of Fig. 3b frequency components of a narrowband signal 18 are from a narrowband signal modulator 11b fed to a pre-distorter 11c which in turn feeds a pre-distorted version of the narrowband signal to an OFDM module lid. The OFDM module lid is separate from the OFDM module 11a of the multicarrier modulated signal 17.

In the embodiment of Fig. 3c frequency components of a narrowband signal 18 are from a narrowband signal modulator 11b added to the multicarrier modulated signal 17. This composite signal is then fed to an OFDM

modulator 11a. Particularly, the radio access network node 11 may be configured to generate the signal by, in an optional step Sio6e, feed the modulation symbols to an OFDM modulator. The signal fed to the OFDM modulator may be pre- distorted or not pre-distorted. For example, there may be separate OFDM modulators for the pre-distortion signal and the multicarrier modulated signal. Hence, the radio access network node 11 may be configured to generate the signal by, in an optional step Sio6f, feed the modulation symbols to a first OFDM modulator; in an optional step Sio6g, feed the pre- distortion signal to a second OFDM modulator; and, in an optional step Sio6h, add the OFDM modulated pre-distortion signal to the OFDM modulated modulation symbols. Further, the generated signal 19 may be generated by determining an adjustment factor based on components in the narrowband signal that are not orthogonal to said sub-carriers. Not orthogonal may here be defined as the narrowband signal provides a non- zero signal contribution after having been OFDM demodulated.

As noted above, there maybe more than one narrowband signal 18.

Particularly, the radio access network node 11 maybe configured to, in an optional step Si04a, acquire further frequency components of at least one further narrowband signal. All narrowband signals have their own carrier frequency. The radio access network node 11 may then be configured to, in an optional step Sio6j, generate the signal based on the further frequency components.

As noted above, the generated signal 19 represents at least one of the modulation symbols and the narrowband signal. The radio access network node 11 may therefore be configured to, in an optional step Sio8a, and in a case where the generated signal 19 only represents the modulation symbols, to also transmit modulation symbols of the narrowband signal. Likewise, the radio access network node 11 maybe configured to, in an optional step Sio8b, and in a case where the generated signal 19 only represents the narrowband signal, to also transmit the modulation symbols of the multicarrier

modulated signal. Particularly, the radio access network node 11 may be configured to, in an optional step Sio8c, transmit the generated signal 19 and the modulation symbols of the narrowband signal concurrently. The modulation symbols of the multicarrier modulated signal and the modulation symbols of the narrowband signal may be transmitted using a single antenna arrangement. A signal for mitigating interference from a narrowband signal 18 during transmission of a multicarrier modulated signal 17 may thus represent at least one of modulation symbols of a multicarrier modulated signal, the multicarrier modulated signal comprising a plurality of sub-carriers 52, and a narrowband signal. In the generated signal 19 interference to a sub-carrier caused by a frequency component of the narrowband signal at a different frequency than the sub-carrier may thus be suppressed by transmission at the sub-carrier frequency in the generated signal 19 being modified.

Thus there may be transmitted a signal which is within a frequency band intended for OFDM transmission (for example an LTE frequency band) and which comprises OFDM subcarriers and further comprises a narrowband signal (for example a GSM signal). In the signal at least one OFDM subcarrier is pre-distorted to cancel, or at least mitigate, interference that would be caused to a demodulation result for the subcarrier by the application of FFT to the narrowband signal in the intended receiver of the OFDM transmission. According to an embodiment the OFDM subcarriers may be pre-distorted so as to cancel, or at least mitigate, the contribution to the demodulation result of the subcarrier resulting from the application of FFT to a time segment of the narrowband signal, which time segment is within, but shorter than, the time segment during which an OFDM symbol is processed by the intended receiver of the OFDM transmission. The time segment maybe a segment starting one cyclic prefix later that the start of transmission of the symbol and ending one cyclic prefix earlier than the end of the transmission of the symbol (as disclosed above with reference to Fig. 12).

Such a signal may be generated by a step of obtaining a time segment of a narrowband signal (or alternatively information from which the time segment of the signal can be constructed, such as one or more modulation symbols), by a step of determining a (interference) contribution that the time segment would produce to a subcarrier frequency when subjected to FFT (typically FFT in the same way as would be done in a receiver), and a step of pre distorting the transmission on the subcarrier frequency to counteract the (interference) contribution. In particular, pre-distortion may be carried out by pre-distorting a modulation symbol for the subcarrier prior to OFDM modulation.

For example, consider a wideband OFDM signal being transmitted within a certain frequency band intended for such transmission, and within this frequency band a narrowband signal for another RAT is also transmitted. As the narrowband signal is present within the frequency band intended for OFDM transmission, the narrowband signal is thus not filtered out by the band-pass filter in a receiver of the OFDM signal, but will be passed on for demodulation in the same way as the OFDM signal itself. The narrowband signal may thereby cause interference not only to OFDM subcarriers overlapped by the spectrum of the narrowband signal, but potentially to the demodulation result for all subcarriers of the OFDM signal, as the part of the narrowband signal which is outside the discrete subcarrier frequencies of the OFDM signal is not orthogonal to the subcarriers in the OFDM sense.

With reference to the above disclosed embodiments, this interference to the demodulation result of an OFDM subcarrier not overlapped by the spectrum of the narrowband signal may be mitigated by adding a compensating signal to that subcarrier. The interference of the narrowband signal may thus be mitigated by adding a signal at a different frequency outside of the spectrum of the narrowband signal. The interference may thereby be mitigated without affecting the narrowband signal. Even though the compensating signal itself may be at a discrete frequency, it may compensate the demodulation result for that subcarrier for interference not only from discrete frequencies but potentially from the entire spectrum of the narrowband signal, which may span a bandwidth corresponding to several subcarriers. Interference to a subcarrier overlapped by the spectrum of the narrowband signal may also be compensated for in the same way; however this may affect the narrowband signal.

Prior to the transmission of the compound OFDM/narrowband signal (as in step S106), the narrowband signal to be sent may be analyzed and an interference that it may cause to a subcarrier of a multicarrier modulated signal may be determined. From this interference, a compensating signal may be determined and added to the original signal representing the modulation symbols for that subcarrier. The resulting signal may then be transmitted together with the narrowband signal (as in step S108).

The generated signal 19 may thus be a resulting compound signal being a signal within a frequency band intended for OFDM transmission, comprising a plurality of OFDM subcarriers but also comprising a narrowband signal of another RAT within the frequency band. In the generated signal, one or more of the subcarriers (which may be outside the spectrum of the narrowband signal) are compensated at the respective subcarrier frequency to mitigate interference to the demodulation of the subcarrier by the narrowband signal, including interference to the demodulation caused by components of the narrowband signal having a different frequency than the subcarrier. As disclosed above, the interference is caused by the narrowband signal not being orthogonal (in the OFDM sense) to the subcarriers. OFDM

demodulation is made by performing an FFT on a time segment of the received signal. The determination of an interference that would be caused by the narrowband signal to the demodulation of a symbol or symbols transmitted on one or more subcarriers may thus be determined by performing an FFT (as will be done during OFDM demodulation) on a portion of the narrowband signal which is to be transmitted in a same time interval as the symbols, or otherwise calculating the result of such FFT using knowledge of the content of the portion of the narrowband signal. Compensation to a subcarrier may advantageously be made prior to OFDM modulation by pre-distorting the modulation symbols (see above with references to Figs. 3a, 3b, and 3c for different embodiments relating thereto). However, according to alternative embodiments, the transmission of OFDM subcarriers overlapped by the spectrum of the narrowband signal may be omitted, so that the transmitted OFDM subcarriers and the spectrum of the transmitted narrowband signal occupy different parts of spectrum. However, since the narrowband signal for such alternative embodiments is still assumed to be within a frequency band intended for OFDM transmission (and to which frequency band the band-pass filter of an OFDM receiver is thus adapted), the narrowband signal will still be part of the signal processed by the FFT in the OFDM receiver and (due to non-orthogonality) thus will cause interference to the subcarriers, which interference can be mitigated as described above. A particular embodiment based on at least some of the above disclosed embodiments for mitigating interference from a narrowband signal 18 during transmission of a multicarrier modulated signal 17 will now be disclosed with reference to the flow chart of Fig. 9

S202: The narrowband signals is received and in the time domain split into two components v^p and v^c as in equations (51) and (52). One way to

implement step S202 is to perform step S104.

S204: The Fourier transform of v^c is determined as in equation (53), i.e., v≡ Fv^c. One way to implement step S204 is to perform step Si04b.

S206: The frequency components of the Fourier transform of v^c are split into to components Ψ and V^s, as in equations (54) and (55). One way to implement step S206 is to perform any of step Sio6a' and

S208: The frequency domain multicarrier modulation symbols a are pre- distorted by means of V^s as in equations (56) and (57). One way to implement step S208 is to perform step Sio6b'. S210: The pre-distorted frequency domain multicarrier modulation symbols a_pre are fed to an OFDM modulator and then transmitted. One way to implement step S210 is to perform step S108.

Simulation results and further examples will now be presented. In these simulation results and further examples LTE will be used as an example of a RAT using multicarrier modulated signals and GSM will be used as an example of a RAT using a narrowband signal. However, as is understood by the skilled person, the results and further examples also extend to other RATs. Fig. 13 schematically illustrates the spectrum of a 20 MHz LTE carrier. Fig. 14 schematically illustrates how the spectrum is altered when one GSM carrier is overlaid. The energy of the GSM carrier is concentrated on fewer than 24 sub-carriers, each with a width of 15 kHz. Fig. 15 schematically illustrates the signal of Fig. 14 after propagation through a fading channel. Fig. 16 shows a 64 QAM constellation to be used as modulation symbols for a multicarrier modulated LTE signal without pre-distortion, and Fig. 17 shows a

corresponding 64 QAM constellation which has been pre-distorted according to the teachings of at least some embodiments as herein disclosed. By comparing Figs. 16 and 17 it is clear that the herein disclosed pre-distortion hardly changes the total energy of the multicarrier modulated LTE signal. Fig. 18 shows the noise floor at the receiver for the received signal shown in Fig. 17. It can be seen that pre-distortion removes a significant amount of interference and increases the SINRby about 5.7 dB. The increase of SINR is around 10 dB in the sub-carriers adjacent to the GSM carrier. In more detail, the LTE signal of Fig. 18 spans 100 PRBs corresponding to 1200 sub-carriers. Fig. 18 depicts the error floor in one OFDM symbol. The dashed line shows the error floor due to non-orthogonality between the GSM signal and the OFDM sub-carriers, when using a 64 QAM constellation and the fading channel as shown in Fig. 15. The solid line shows the error floor if the 64 QAM constellation is pre-distorted according to embodiments as herein disclosed. In this example, the application of the pre-distortion yields a 5.7 dB decrease in GSM interference power for the LTE portable wireless device. The improvement is larger in the sub-carriers adjacent to the GSM carrier. On the other hand, the GSM signal is not distorted within a bandwidth of 2 PRBs corresponding to 360 kHz, which ensures that the GSM performance is not degraded. The receiver filter bandwidth in a GSM portable wireless device is usually between 180 kHz and 270 kHz.

In summary, at least some of the herein disclosed embodiments involve to pre-distort the modulation (QAM) symbols in each OFDM symbol of a multicarrier modulated signal. The pre-distortion depends on a narrowband signal, such as a GSM signal, and is applied in such a way that it removes most of the interference from the narrowband signal outside a narrow band around the center of frequency of the carrier frequency of the narrowband signal. The carrier itself of the narrowband signal is left intact within its own narrow frequency band. Thus the reception at a portable wireless device of the narrowband signal is not degraded, whilst the reception at a portable wireless device of the multicarrier modulated signal is improved. The portable wireless device receiving the multicarrier modulated signal does not need to be aware of the interference caused by the narrowband signal.

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. For example, a radio access network maybe built wherein the functions of a radio access network node may be distributed over several network nodes, some of which maybe centralized, e.g. in a data center. The pre-distortion of subcarriers or pre-distortion of symbols to be modulated onto the subcarriers, and the determination of such pre-distortion may thus be performed elsewhere than in a radio access network node handling the actual transmission of radio signals. For example, it may be performed in a baseband processing facility which may be located elsewhere, and which facility further may be centralized and/ or pooled to serve many access nodes.

Claims

1. A method for mitigating interference from a narrowband signal (18) during transmission of a multicarrier modulated signal(i7), the method being performed by a radio access network node (n), comprising the steps of: acquiring (S102) modulation symbols of a multicarrier modulated signal, the multicarrier modulated signal comprising a plurality of sub- carriers (52);

acquiring (S104) frequency components of a narrowband signal overlapping with said sub-carriers;

generating (S106) a signal (19) representing at least one of said modulation symbols and said narrowband signal, wherein interference to a sub-carrier caused by a frequency component of the narrowband signal at a different frequency than the sub-carrier is suppressed by transmission at the sub-carrier frequency in said generated signal being modified; and

transmitting (S108) said generated signal.

2. The method according to claim 1, wherein the narrowband signal has a carrier frequency, and wherein said suppressed frequency components are outside a frequency band (FB) surrounding said carrier frequency of the narrowband signal.

3. The method according to claim 2, wherein said frequency band overlaps with at least one sub-carrier (52) of the multicarrier modulated signal.

4. The method according to any one of the preceding claims, wherein said frequency components are frequency components of the narrowband signal that would leak to said adjoining sub-carriers of the multicarrier modulated signal.

5. The method according to any one of the preceding claims, wherein said frequency components are all frequency components of a time segment of the narrowband signal, and wherein generating said signal further comprises: determining (Sio6a) frequency components of the narrowband signal that would leak to said adjoining sub-carriers of the multicarrier modulated signal; and

generating (Sio6b) said signal based on said determined frequency components.

6. The method according to any one of the preceding claims, wherein said frequency components are provided by means of a time segment of the narrowband signal,

wherein acquiring said frequency components further comprises:

determining (Si04b) first frequency components of said time segment; and wherein generating said signal further comprises:

determining (Sio6a') second frequency components as those first frequency components that leak to said adjoining sub-carriers of the multicarrier modulated signal; and

generating (Sio6b') said signal based on said determined frequency components.

7. The method according to claim 5 or 6, wherein said time segment comprises the Nc-2L center-most samples of the narrowband signal taken at the multicarrier signal sampling rate, in a segment of the narrowband signal time-wise overlapping an orthogonal frequency-division multiplexing,

OFDM, symbol of said modulation symbols of the multicarrier modulated signal, said OFDM symbol having a total length Nc+L modulation symbols including a cyclic prefix of length L modulation symbols.

8. The method according to any one of the preceding claims, wherein said modulation symbols of the multicarrier modulated signal are frequency domain modulation symbols.

9. The method according to any one of the preceding claims, wherein generating said signal further comprises:

feeding (Sio6e) said modulation symbols to an orthogonal frequency- division multiplexing, OFDM, modulator.

10. The method according to any one of the preceding claims, wherein generating said signal further comprises:

pre-distorting (Sio6c) said modulation symbols with a pre-distortion signal, said pre-distortion signal being based on said frequency components of the narrowband signal determined at the sub-carrier frequencies.

11. The method according to claim 9 and claim 10, wherein generating said signal further comprises:

adding (Sio6d) said pre-distortion signal to said modulation symbols before feeding said modulation symbols to said OFDM modulator.

12. The method according to claim 10, wherein generating said signal further comprises:

feeding (Sio6f) said modulation symbols to a first orthogonal frequency-division multiplexing, OFDM, modulator;

feeding (Sio6g) said pre-distortion signal to a second OFDM

modulator; and

adding (Sio6h) said OFDM modulated pre-distortion signal to said OFDM modulated modulation symbols.

13. The method according to any one of the preceding claims, wherein said generated signal only represents said modulation symbols, the method further comprising:

transmitting (Sio8a) modulation symbols of the narrowband signal.

14. The method according to any one of claims 1 to 12, wherein said generated signal only represents said narrowband signal, the method further comprising:

transmitting (Sio8b) said modulation symbols of said multicarrier modulated signal.

15. The method according to claim 13 or 14, further comprising:

transmitting (Sio8c) said generated signal and said modulation symbols of the narrowband signal concurrently.

16. The method according to any one of the preceding claims wherein said modulation symbols of said multicarrier modulated signal and said modulation symbols of the narrowband signal are transmitted using a single antenna arrangement.

17. The method according to any one of the preceding claims wherein said generated signal is generated by determining an adjustment factor based on components in the narrowband signal that are not orthogonal to said sub- carriers.

18. The method according to any one of the preceding claims, further comprising:

acquiring (Si04a) further frequency components of at least one further narrowband signal, wherein all narrowband signals have their own carrier frequency; and

generating (Sio6j) said signal based on said further frequency components.

19. The method according to any one of the preceding claims, wherein said multicarrier modulated signal is a Long Term Evolution, LTE, or a WiFi signal.

20. The method according to any one of the preceding claims, wherein said narrowband signal is a Global System for Mobile Communications, GSM, signal, or an Enhanced Data Rates for GSM Evolution, EDGE, signal.

21. A radio access network node (11) for mitigating interference from a narrowband signal during transmission of a multicarrier modulated signal (17), the radio access network node comprising a processing unit (21), the processing unit being configured to:

acquire modulation symbols of a multicarrier modulated signal, the multicarrier modulated signal comprising a plurality of sub-carriers (52); acquire frequency components of a narrowband signal overlapping with said sub-carriers;

generate a signal (19) representing at least one of said modulation symbols and said narrowband signal, wherein interference to a sub-carrier caused by a frequency component of the narrowband signal at a different frequency than the sub-carrier is suppressed by transmission at the sub- carrier frequency in said generated signal being modified; and

transmit said generated signal.

22. A computer program (32) for mitigating interference from a

narrowband signal during transmission of a multicarrier modulated signal (17), the computer program comprising computer program code which, when run on a processing unit (21) of a radio access network node (11) causes the processing unit to:

acquire (S102) modulation symbols of a multicarrier modulated signal, the multicarrier modulated signal comprising a plurality of sub-carriers (52); acquire (S104) frequency components of a narrowband signal overlapping with said sub-carriers;

generate (S106) a signal (19) representing at least one of said

modulation symbols and said narrowband signal, wherein interference to a sub-carrier caused by a frequency component of the narrowband signal at a different frequency than the sub-carrier is suppressed by transmission at the sub-carrier frequency d in said generated signal being modified; and

transmit (S108) said generated signal.

23. A computer program product (31) comprising a computer program (32) according claim 22, and a computer readable means (33) on which the computer program is stored.

24. A signal (19) for mitigating interference from a narrowband signal (18) during transmission of a multicarrier modulated signal (17),

wherein the signal represents at least one of modulation symbols of a multicarrier modulated signal, the multicarrier modulated signal comprising a plurality of sub-carriers (52), and a narrowband signal, and

wherein interference to a sub-carrier caused by a frequency component of the narrowband signal at a different frequency than the sub-carrier is suppressed by transmission at the sub-carrier frequency in said generated signal is modified.