WO2010000297A1

WO2010000297A1 - Iq-imbalance compensation in presence of carrier offset

Info

Publication number: WO2010000297A1
Application number: PCT/EP2008/058383
Authority: WO
Inventors: Niklas Andgart; Michal Stala
Original assignee: Telefonaktiebolaget Lm Ericsson (Publ)
Priority date: 2008-06-30
Filing date: 2008-06-30
Publication date: 2010-01-07
Also published as: US8594212B2; ATE536689T1; EP2304914A1; EP2304914B1; CN102077538A; US20110135036A1

Abstract

An arrangement for processing a received wireless communication signal is disclosed. The arrangement comprises a frequency offset compensation unit (405) adapted to apply a frequency offset compensation to the received wireless communication signal, determination circuitry (405) adapted to determine one or more adjustment values associated with the frequency offset compensation, and an FFT-unit (410) adapted to transform the frequency offset compensated received signal to a frequency domain. The arrangement also comprises an IQ-imbalance estimator (440) adapted to estimate an IQ-imbalance, and an IQ-imbalance compensation unit (420) adapted to perform IQ-imbalance compensation of the received wireless communication signal based on the estimated IQ-imbalance. At least one of the IQ-imbalance estimator and the IQ-imbalance compensation unit is adapted to base its operation on the one or more adjustment values. Corresponding method and computer program product are also disclosed, application both to OFDM and non OFDM signals

Description

IQ-IMBALANCE COMPENSATION IN PRESENCE OF CARRIER OFFSET

Technical Field

The present invention relates generally to the field of compensating for in- phase/quadrature imbalance (IQ-imbalance). More particularly, it relates to IQ- imbalance compensation of a received wireless communication signal.

Background

Many Orthogonal Frequency Division Multiplexing (OFDM)-based systems, such as the Universal Mobile Telecommunications Standard Long Term Evolution (UMTS LTE), require that receivers of the system are able to process a received OFDM-signal to acquire a relatively high signal-to-noise ratio (SNR). The receiver should be able to process the signal without introducing impairments or noise (or at least without introducing impairments or noise that are of a severe nature). Further, the receiver should be able to adjust for impairments and/or noise introduced by the channel over which the received OFDM-signal was transmitted.

Furthermore, many OFDM-based systems employ complex transmission schemes, such as Multiple-Input Multiple-Output (MIMO) and/or large signal constellations, which may further increase the demands on the receiver. To be able to meet such demands of high SNR in receivers operating in

OFDM-systems, an expensive radio design may be required. Alternatively (or additionally) methods may be employed that are able to handle radio imperfections resulting from a non-optimal radio design.

One radio imperfection is IQ-imbalance. IQ-imbalance is one of the more Hm- iting radio imperfections, and is thus important to dispose of or at least suppress.

IQ-imbalance may be generated by anything that affects the in-phase (I) and quadrature (Q) components of the received OFDM-signal differently. One example source of IQ-imbalance is a local oscillator of a receiver or a transmitter (or both). Another example source of IQ-imbalance is mismatch between one or more blocks in the respective I- and Q-paths of the receiver chain. Examples of blocks that may experience such mismatch are amplifiers and channel filters. IQ-imbalance can be modeled, in the receiver, as a difference in phase and amplitude between the in-phase and quadrature oscillator components (i.e. the carriers). After down-converting the received signal to a baseband signal in down-conversion mixers, this difference in phase and amplitude results in a leakage between the in-phase and quadrature components of the baseband signal. Thus, the real part of the symbols will affect the imaginary part of the symbols, and vice versa;

where * denotes conjugate, p represents the IQ-imbalance and is a factor that depends on the phase and amplitude mismatch (for example in the local oscillator or of blocks in the respective I- and Q-paths of the receiver chain), y[n] is what the received signal would have been if there was no IQ-imbalance, and

is the actually received signal.

In OFDM, data is transmitted in parallel on a number of sub-carriers (or sub- carrier frequencies), which may be efficiently implemented by using an inverse fast Fourier transform (IFFT) in the transmitter, and a fast Fourier transform (FFT) in the receiver. If the size of the FFT is N, then N samples at the output of the FFT are referred to as an OFDM-symbol (i.e. a frequency domain OFDM-symbol).

Each OFDM-symbol thus comprises data on N sub-carriers. Each such piece of data will be referred to as a symbol (in contrast to an OFDM-symbol), and may com- prise a pilot symbol or an information symbol. In UMTS LTE, a symbol as described above may be denoted a resource element, and a pilot symbol may be denoted a reference signal.

In an OFDM-based system, the baseband signal is thus transformed, in the receiver, to a frequency domain signal and this is commonly achieved by applying an FFT to the baseband signal. When transformed to the frequency domain, the IQ-imbalance affects the frequency domain signal in frequency pairs. Thus, the symbols on sub-carrier N-k leak into sub-carrier k and vice versa. This may be expressed by the following frequency domain expression:

Y,_Q {k) = Y{k) + p

- k\ (2) where 7(A:) is what the received signal would have been if there was no IQ-imbalance, and Yio{k) is the actually received signal It may be noted that the notation of sub-carrier N-k is equivalent to sub-carrier -k This is due to the N-peπodicity of the FFT Throughout this application, sub-carrier N-k will be denoted the mirror sub-carrier of sub-carrier k, and sub-earners k and N-k will be denoted a frequency pair

The leakage from a sub-caπier to another sub-caπier is a form of inter-carrier interference (ICI), and will degrade the SNR in the receiver Thus, m order to achieve a high SNR while allowing for a less expensive radio design, it may be desirable to measure (or estimate) the IQ-imbalance and perform compensation on the received signal for the estimated IQ-imbalance For example, the value p can be estimated The estimated value p can then be used to perform compensation on the received signal The estimated value p may, for example, be determined based on known pilot values and known channel values (e g channel estimates) If the value p is accurately estimated, the compensation will cancel the leakage from the mirror sub-carrier completely Another imperfection experienced in wπeless communication systems is frequency offset A frequency offset experienced at a communication receivei may, for example, be due to mismatches between transmittei and receiver oscillators (which may in turn be caused by e g component mismatch, temperature variations, etc ) or Doppler phenomena Frequency offset compensation may be achieved in different ways For example, the frequency of the local oscillatoi may be adjusted based on a detected or estimated frequency offset Another possibility is to perform digital frequency offset compensation Digital offset compensation may be achieved via a multiplication of the received baseband signal with a phase ramp, e g exp(j 2πon I N) , where ύ is the normal- ized estimated fiequency offset A frequency offset compensatoi unit using this approach is commonly referred to as a digital iotator

When digital frequency offset compensation is applied to the baseband signal, equations (1) and (2) will no longer be valid Thus, if conventional IQ-imbalance estimation and compensation appioaches weie applied to such a signal, the result would not be optimal The value p would not be accuiately estimated Furthei, even if the value /3 were accurately known, the compensation itself would not be optimal if conventional compensation techniques were used

Similar problems may arise in wireless communication systems (and in receivers for such systems) not based on OFDM, but, for example, on SC-FDMA (Single Car- rier Frequency Division Multiple Access)

Thus, there is a need for improved methods of and arrangements for IQ- imbalance compensation of a received wireless communication signal when frequency offset compensation is applied Further, theie is a need for impioved methods of and arrangements for estimating IQ-imbalance in such cases

Summary

It should be emphasized that the term "comprises/comprising" when used in this specification is taken to specify the presence of stated features, integers, steps, or components, but does not preclude the presence or addition of one oi more other fea- tures, integers, steps, components, or groups thereof

It is an object of the invention to obviate at least some of the above disadvantages and to provide improved methods of and arrangements for IQ-imbalance estimation and compensation of a leceived wireless communication signal

According to a first aspect of the invention, this is achieved by a method of piocessing a received wireless communication signal, suitable for use m a communication receiver The method comprises applying frequency offset compensation to produce a frequency offset compensated received signal, determining one or more adjustment values associated with the frequency offset compensation, and tiansforming the frequency offset compensated received signal to a fiequency domain, wherein the fre- quency domain comprises a pluiahty of sub-carrier fiequencies The method also com- pπses estimating an m-phase/quadrature imbalance (IQ-imbalance), and pei forming IQ- imbalance compensation of the received wneless communication signal based on the estimated IQ-imbalance According to the method, at least one of the step of estimating the IQ-imbalance and the step of performing IQ-imbalance compensation is pei formed based on the one or more adjustment values The IQ-imbalance may be a property associated with a receiver of the wireless communication signal.

In some embodiments, the IQ-imbalance compensation is performed using a feed forward approach. In other embodiments, the IQ-imbalance compensation is per- formed using a feedback approach.

In some embodiments, the step of estimating the IQ-imbalance and the step of performing IQ-imbalance compensation may be performed after the step of transforming the frequency offset compensated received signal and may be based on the one or more adjustment values. In some embodiments, the IQ-imbalance compensation is per- formed using a feed forward approach.

In some embodiments, the step of estimating the IQ-imbalance may be performed after the step of transforming the frequency offset compensated received signal and may be based on the one or more adjustment values. The step of performing IQ- imbalance compensation may be performed prior to the step of transforming the fre- quency offset compensated received signal and after the step of applying frequency offset compensation and may be based on the one or more adjustment values.

In some embodiments, the step of performing IQ-imbalance compensation may comprise adjusting the estimated IQ-imbalance based on the one or more adjustment values and performing the IQ-imbalance compensation based on the adjusted estimated IQ-imbalance.

In some embodiments, the step of estimating the IQ-imbalance may be performed after the step of transforming the frequency offset compensated received signal and may be based on the one or more adjustment values. The step of performing IQ- imbalance compensation may be performed prior to the step of applying frequency off- set compensation and may not be based on the one or more adjustment values.

In some embodiments, the step of estimating the IQ-imbalance may comprise defining a residual IQ-imbalance by a numerator and a denominator and estimating the numerator and denominator.

In some embodiments, the step of estimating the IQ-imbalance may comprise estimating the IQ-imbalance based on at least the one or more adjustment values, information related to a first symbol of a first sub-carrier frequency and information re- lated to a second symbol of a second sub-carrier frequency, wheiein the second sub- carrier frequency is a mirror sub-earner frequency to the first sub-carrier frequency

In some embodiments, the step of estimating the IQ-imbalance may comprise determining estimated channel values for at least the first sub-earner frequency, the first sub-earner frequency having a pilot symbol, and estimating the IQ-imbalance based on at least the pilot symbol, the at least one determined estimated channel value, information related to the second symbol, and at least one of the one or more adjustment values In some embodiments, the step of estimating the IQ-imbalance may comprise estimating the IQ-imbalance based on information related to a number of symbols of sub-earner frequencies being neighbonng sub-cam er frequencies to the mirror sub- earner frequency

In some embodiments, the step of estimating the IQ-imbalance may comprise estimating the IQ-imbalance based on a plurality of pilot symbols and information related to symbols of corresponding mirroi sub-earner frequencies In some embodiments, the step of estimating the IQ-imbalance may be performed prior to the step of transforming the frequency offset compensated leceived signal and after the step of applying frequency offset compensation and may be based on the one or moie adjustment values

In some embodiments, the step of estimating the IQ-imbalance may be per- formed prior to the step of applying frequency offset compensation and may not be based on the one oi more adjustment values

In some embodiments, the wireless signal may be an OFDM-signal and each of the one or moie adjustment values may be associated with a respective OFDM-symbol of the received OFDM-signal and may comprise a complex numbei associated with the respective OFDM-symbol Performing at least one of the step of estimating the IQ- imbalance and the step of pel forming IQ-imbalance compensation based on the one oi moie adjustment values may comprise a multiplication with a first function of the complex number The step of applying frequency offset compensation may compose digital iotation The complex number may be a second function of a phase of the digital rota tion conespondmg to a particular sample of the respective OFDM-symbol The second function may comprise a doubling of the phase of the digital rotation corresponding to the particular sample.

The step of determining the one or more adjustment values may comprise using a digital rotator to generate the complex number during a cyclic prefix of the respective OFDM-symbol or of an adjacent OFDM-symbol.

A second aspect of the invention is a computer program product comprising a computer readable medium, having thereon a computer program comprising program instructions, the computer program being loadable into a data-processing unit and adapted to cause the data-processing unit to execute method steps according to the first aspect of the invention when the computer program is run by the data-processing unit.

A third aspect of the invention is an arrangement for processing a received wireless communication signal. The arrangement comprises a frequency offset compensation unit adapted to apply a frequency offset compensation to the received wireless communication signal to produce a frequency offset compensated received signal, de- termination circuitry adapted to determine one or more adjustment values associated with the frequency offset compensation, an FFT-umt adapted to transform the frequency offset compensated received signal to a frequency domain, wherein the frequency do- mam comprises a plurality of sub-carrier frequencies, an IQ-imbalance estimator adapted to estimate an IQ-imbalance, and an IQ-imbalance compensation unit adapted to perform IQ-imbalance compensation of the received wireless communication signal based on the estimated IQ-imbalance. At least one of the IQ-imbalance estimator and the IQ-imbalance compensation unit is adapted to base its operation on the one or more adjustment values.

In some embodiments, the IQ-imbalance estimator may be adapted to estimate the IQ-imbalance based on at least the one or more adjustment values, information related to a first symbol of a first sub-carrier frequency and information related to a second symbol of a second sub-carrier frequency, wherein the second sub-carrier frequency is a mirror sub-carrier frequency to the first sub-carrier frequency The arrangement may further compπse a channel estimator adapted to determine estimated channel val- ues for at least the first sub-carrier frequency, the first sub-carrier frequency having a pilot symbol, and the IQ-imbalance estimator may be adapted to estimate the IQ- imbalance based on at least the pilot symbol, the at least one determined estimated channel value, information related to the second symbol, and at least one of the one or more adjustment values.

In some embodiments, the wireless signal may be an OFDM-signal and the de- termination circuitry may be adapted to determine each of the one or more adjustment values to be associated with a respective OFDM-symbol of the received OFDM-signal and to comprise a complex number associated with the respective OFDM-symbol. The operation of at least one of the IQ-imbalance estimator and the IQ-imbalance compensation unit may comprise a multiplication with a first function of the complex number. The frequency offset compensation unit may comprise a digital rotator. The determination circuitry may be further adapted to determine the complex number as a second function of a phase of the digital rotator corresponding to a particular sample of the respective OFDM-symbol. The second function may comprise a doubling of the phase of the digital rotator corresponding to the particular sample. The digital rotator may com- prise the determination circuitry.

In some embodiments, the third aspect of the invention may additionally have features identical with or corresponding to any of the various features as explained above for the first aspect of the invention.

A fourth aspect of the invention is an electronic communication device com- prising a wireless communication receiver, which comprises the arrangement according to the third aspect of the invention.

One of the advantages of embodiments of the invention is that accurate estimation of IQ-imbalance is possible in receivers where frequency offset compensation is applied. Another advantage with embodiments of the invention is that accurate IQ- imbalance compensation is possible in receivers where frequency offset compensation is applied.

Another advantage with embodiments of the invention is that IQ-imbalance estimation and compensation schemes are provided that are robust against frequency off- set compensation. Another advantage with embodiments of the invention is that a cost-effective implementation is provided. For example, a digital rotator for frequency offset compensation may be utilized to calculate the one or more adjustment values. Alternatively or additionally, a residual IQ-imbalance may be defined by a numerator and a denominator to simplify the estimation process.

Brief Description of the Drawings

Further objects, features and advantages of the invention will appear from the following detailed description of embodiments of the invention, with reference being made to the accompanying drawings, in which:

Fig. l is a schematic front view of a mobile terminal connected, through a radio link, to a base station site, wherein the mobile terminal may comprise arrangements according to some embodiments of the invention; Fig. 2 is a diagram illustrating an example of pilot symbol placements in

OFDM-signals;

Fig. 3 is a flow chart illustrating example method steps according to some embodiments of the invention;

Fig. 4A is a block diagram illustrating an example arrangement according to some embodiments of the invention;

Fig. 4B is a block diagram illustrating an example arrangement according to some embodiments of the invention;

Fig. 4C is a block diagram illustrating an example arrangement according to some embodiments of the invention; Fig. 4D is a block diagram illustrating an example arrangement according to some embodiments of the invention;

Fig. 4E is a block diagram illustrating an example arrangement according to some embodiments of the invention,

Fig. 4F is a block diagram illustrating an example arrangement according to some embodiments of the invention; Fig. 5A is a block diagram illustrating an example arrangement according to some embodiments of the invention;

Fig. 5B is a signaling diagram illustrating example input and control signals to the example arrangement of Figure 5 A according to some embodiments of the inven- tion; and

Fig. 6 is a plot illustrating advantages of applying some embodiments of the invention.

Detailed Description In the following, embodiments of the invention will be described with reference to OFDM-systems and receivers for such systems. It should be understood, however, that embodiments of the invention are equally applicable to other wireless communication systems and receivers, such as, for example, SC-FDMA-based systems and receivers. Both frequency offset compensation and IQ-imbalance compensation may be non-linear operations. Therefore, the order in which they are performed may be crucial and may affect which way is optimal for performing the frequency offset compensation and/or the IQ-imbalance compensation. If both the IQ-imbalance estimation and compensation are performed in the time domain, and if there are no constraints relating, for example, to already implemented blocks of the receiver (e.g. a radio with a built-in IQ- imbalance compensator and a built in frequency offset compensator but with no IQ- imbalance estimator), then the IQ-imbalance estimation and compensation may be placed freely so that the IQ-imbalance estimation and compensation are performed before the frequency offset compensation and the conventional expressions of equations (1) and (2) are valid.

However, there may be constraints to a receiver implementation, such as the one mentioned above. Furthermore it may be attractive to perform at least the IQ- imbalance estimation in the frequency domain because there exist a symmetry between sub-carrier frequency pairs as will be explained in the following. The IQ-imbalance compensation may also benefit from this symmetry. In the time domain, each sample comprises data related to pilot and information symbols of the entire OFDM-symbol (in addition to noise and other interference). In the frequency domain, each sub-carrier frequency comprises data related to the pilot or information symbol of that particular sub- carrier and data of the corresponding mirror sub-carrier (in addition to noise and other interference). This simplifies the IQ-imbalance estimation and compensation as will be explained in the following.

However, if the IQ-imbalance estimation and/or compensation is performed after the frequency offset compensation, care must be taken so that the performance of the IQ-imbalance estimation and compensation is not adversely affected by the frequency offset compensation. In the following, example embodiments of the invention will be described in which IQ-imbalance of a received OFDM-signal is compensated for. Embodiments of the invention take into account one or more adjustment values associated with a frequency offset compensation applied to the received OFDM-signal. Some embodiments use the one or more adjustments values in estimation of the IQ-imbalance, some em- bodiments use the one or more adjustments values when compensating for the IQ- imbalance based on an IQ-imbalance estimate, and some embodiments use the one or more adjustments values both in estimation of and in compensation for the IQ- imbalance.

When digital frequency offset compensation is applied to the baseband signal, equations (1) and (2) will no longer be valid since the frequency offset compensation introduced by, for example, a digital rotator may change the manner in which the IQ- imbalance affects the received signal. Equation (1) may, for example, be transformed into y'^Q [«] = y[n]+ P exp{βπ{2ύ)(n + m_s )/ N) y [n] , (3) where m_s is the index of the first sample of OFDM-symbol s, and equation (2) may, for example, transform into

Y₁₀ (k) = Y(k) + p 0_(or (N - k + 2(5), (4) where ΘIΛ = exp{j2π(2ύ)m_s / N) corresponds to twice the phase value of the phase ramp used by the rotator at the first sample of OFDM-symbol s. Thus, when the frequency is adjusted with, for example, a digital rotator, the IQ-imbalance parts of equations (3) and (4) may, for each symbol, rotate around origin with an amount depending on the estimated frequency offset used in the frequency offset compensation. Even if the frequency offset compensation only comprises very small rotations, the approaches to IQ-imbalance compensation where no account is taken of the frequency offset compensation will cease to function properly.

Furthermore, when frequency offset compensation is present, the influence of the IQ-imbalance is no longer restricted to cause interference between the two mirrored sub-frequencies of a frequency pair. In these cases, the IQ-imbalance part has an argu- ment 2(3, which causes the symbols of each sub-carrier to slide slightly off the OFDM frequency grid. This in turn causes IQ-imbalance leakage between all sub-carriers; expO^"2τr(- / + A: - 2u))- l

¹ IO ^v '*-" N(exp[j2π{-l + k - 2υ)/ N)- I)

For a small frequency offset ( ύ « 1 ), the sum in equation (5) is dominated by the term for / = N-A: and the scaling factor at the end of each teπn of the sum in equation (5) is approximately equal to 1. Thus, an approximate expression for equation (5) is

Y_IQ {k) * Y{k) + p θ_(s)r {N - k). (6)

Thus, for relatively small frequency offset compensations, equation (6) may be used as a basis for determining how IQ-imbalance estimation and compensation should be perfoπned. For large frequency offset compensations, however, equation (5) should be reverted to for determining how IQ-imbalance estimation and compensation should be perfoπned, as will be demonstrated in connection to Figure 6. In equation (5), the number of terms used in the summation may be a variable depending of the frequency offset compensation. Preferably, the terms being neighbors to the / = N-A: term (the mirror sub-frequency) are used. Embodiments of the invention utilize the estimated frequency offset used for frequency offset compensation to de-rotate the components of the IQ-imbalance estimate, and also to properly rotate the entire IQ-imbalance estimate when it is used in IQ- imbalance compensation. IQ-imbalance of a received OFDM-signal may be estimated based on pilot symbols, channel values and, m some embodiments, estimates of transmitted symbols

Estimating the value p accurately requires knowledge of the channel as well as of the transmitted symbols for each frequency pair used in the estimation Channel in- formation could, for example, be obtained from a channel estimator Knowledge of transmitted symbols is commonly obtained by the use of pilot symbols Thus, to be able to perform an accurate estimation of the value p, pilots need to be distributed on both sub-carriers of each frequency pair used m the estimation

However, pilot symbol distribution is in general defined in the standard to be applied Thus, access to pilot information on frequency pairs depends on how the pilot distribution is defined m the standard In UMTS LTE, foi example, the pilots are not placed on mirror frequency pairs

When no frequency offset compensation is present, the IQ-imbalance, represented by the value p, may be estimated as

where P(k) and P(N k) denote known pilot values of a fiequency pan, and H(k) and H(N-k) denotes known channel values

In geneial, the channel values are not known, and must be replaced by estimates H(k) and H(N - k) In some embodiments, the IQ-imbalance is estimated based on one or moie fiequency pans, where pilot symbols aie only available on one sub-caniei of a frequency pair In such embodiments, information relating to the othei (mirroi) sub-camer of the fiequency pan may be used m the estimation In some embodiments, the information relating to the mirror sub-earn ei may be an estimate of the symbol tiansmitted on the mirror sub-carrier of the frequency pan In some embodiments, the information relating to the mirror sub-carriei may be a leceived symbol of the mirror sub-camer of the fiequency pan

The estimate of the transmitted symbols may be achieved in vanous ways Foi example, the estimate may not be explicitly calculated, but is implicit in the IQ- imbalance estimation. In some embodiments, the estimate is the received symbol divided by a channel estimate and a soft symbol estimate results The estimate may be calculated according to

An old estimation of p may, for example, be used as p . If p is set to zero the special case S(N - K) - Y₁₀ (N - K) H(N - k) results In some embodiments, a hard decision is taken regarding the tiansmitted symbol and used as an estimate That is, the estimate of the transmitted information symbol may be generated through a decision directed approach In these embodiments, a decision S(N - K) is taken with regard to the transmit- ted symbol In some of these embodiments, the decision is made based on

Y₁₀ (N - k)j H(N - k) In yet some embodiments, the received signal is decoded and the estimate of the transmitted information symbol S(N - k) may be geneiated through re-generatmg the corresponding supposedly sent signal from the decoded signal and used to provide an estimate of the transmitted symbol If, for example, no pilot is available on sub-carrier N-k, then P(N-k) may be replaced by an estimate of the transmitted symbol S(N - k) , and the IQ-imbalance estimation would be represented as

It is possible to do the opposite substitution, i e replacing P(k) by an estimate of the transmitted symbol S(K) ,

Ηowevei, m such an embodiment, using an estimate of the transmitted symbol achieved by dividing the leceived symbol by a channel estimate may be an mfenoi solution (Since, if substituting S (k) by Y₁₀ (K) H(K) m equation (10), the expression be- comes zero.) Using any other estimate of the transmitted symbol may be used though. Alternatively, S(H) may be set to zero in (10) and the IQ-imbalance estimation would result from

thus relying on the zero-mean of the data signal S(k) . This IQ-imbalance estimate will, however, be less accurate, since the data signal S(k) is not modeled and thus influences the IQ-imbalance estimate as noise. It is also noted that this IQ-imbalance estimate improves with the measuring time due to the reliance on the zero-mean of the data signal. Whether to use equation (9), (10) or (1 1) may also depend on the presence or not of information data symbols on the mirror frequencies. If no information data is sent on some frequency blocks, these frequencies are referred to as quiet frequencies. However, pilots may still be present even though no information data is transmitted (depending on the communication standard in use). If, for example, N-k belongs to a range of quiet frequencies, then equation (10) or (11) may be preferable, while equation (9) may be preferable if k belongs to a range of quiet frequencies.

If the estimate S(N ~ k) = Y₁₀ (N - k)j ' H(N - k) of the transmitted information symbol is used, the IQ-imbalance estimation of equation (9) may be expressed as ∑, Y₁₀ (N - k){Y_IQ (k) - H(H)P[H))

P = (12)

Thus, the actual estimate of the transmitted information symbol need not be calculated, but is implicit in the IQ-imbalance estimation of equation (12).

Estimating the IQ-imbalance may, in some embodiments, comprise using the Synchronization CHannel (SCH) of UMTS LTE. It is noteworthy that the SCH has symmetrically placed pilots, i.e. pilots are placed on mirror sub-carriers. This gives the advantage that the IQ-imbalance may be estimated based on pilot symbols and channel values alone and no estimates of transmitted symbols have to be used. When no frequency offset compensation is present, the estimated value p can be used for compensation, either of the time domain signal or of the frequency domain signal. If performed on the frequency domain signal, the compensation may comprise

Y_Comp(k) = Y_lo {k) - pY;_Q (N - k) . (13) If p is estimated accurately, this operation will cancel the leakage from the mirror sub-carrier N-k.

Now, when frequency offset compensation is present, embodiments of the invention suggest that equations (7), (9), (10), (11) and (12) are adjusted using the adjustment value Θ_(S). For example, equation (7) may be adjusted to

and similarly for equations (9), (10), (11) and (12). The multiplication with the adjustment value aims at removing the complex rotation of the numerator that is due to the applied frequency offset compensation.

The summations in equation (14) may be over several sub-carrier frequencies of the same OFDM-symbol and/or over several OFDM-symbols. The adjustment value may be constant over the different sub-carrier frequencies of the same OFDM-symbol and may change between different OFDM-symbols. The more information that is collected in time and frequency by the summations, the more robust the IQ-imbalance estimation gets against noise and fading dips. On the other hand, if the summations cover a large time span, an estimation delay is introduced which may be undesirable. Several estimations may also be calculated according to equation (14), where each estimation covers a subset of the sub-carrier frequencies. In this way, frequency dependent IQ- imbalance may be estimated (and thus compensated for).

The more frequency pairs that are used in the summations in equation (14), the more complex the IQ-imbalance estimation becomes. To reduce the complexity, a smaller number of samples can be used in accordance with some embodiments of the invention. Thus, of the available pilot symbols in an OFDM-symbol (or in several OFDM-symbols) only a subset may be used for the IQ-imbalance estimation. In these embodiments, the pilot symbols (and hence the frequency pairs) to be used in the IQ- imbalance estimation may, for example, be chosen according to a pseudo-random pattern over the available time and frequency span.

Embodiments of the invention also suggest that, when frequency offset compensation is present; equation (13) should be adjusted using the adjustment value %. For example, equation (13) may be adjusted to

Y,o,,_Ψ (*) = Yio (*) - Pθ(,)Yj'o (N - k) , (15) when the IQ-imbalance compensation is performed in the frequency domain. If the IQ- imbalance compensation is performed after the AFC in the time domain, the compensation may be expressed as

If the IQ-imbalance compensation is performed before the AFC in the time domain, the compensation may be expressed as

As before, the adjustment value may be different for different OFDM-symbols. Thus, the IQ-imbalance compensation may be performed either pre-FFT or post-FFT. Further it may be perfoπned in a feedback or feed forward manner.

If the compensation is to be perfoπned in a feedback loop, a previous IQ- imbalance estimate p₀ may be combined with the new (residual) IQ-imbalance estimate Ap to improve the result. This is due to that the IQ-imbalance is estimated based on an already IQ-imbalance compensated signal (compensated with a previous IQ- imbalance estimate), and the new IQ-imbalance estimate represents an estimate of the residual IQ-imbalance. In some embodiments, the combination of new and previous IQ- imbalance estimates may be performed as p = (p₀ + Ap ^■ a) /(I + Ap ^■ p_oa) , and used in, for example, any of equations (13), (15), (16) or (17). The factor a ( 0 < a ≤ 1 ) specifies a first-order filter to smoothen the imbalance estimates.

In order to simplify the implementation, the residual IQ-imbalance may be expressed as Ap = L_x I L₂ , and the combination of new and previous IQ-imbalance estimates may be calculated as p = (L₂p₀ + L_xCt)/ (L₂ + L_tp_o ^*a) . This may reduce the com- putational complexity since the division of equation (14) does not need to actually be performed.

Figure 1 illustrates an example mobile terminal 110 connected, through a radio link 120, to a base station site 130. The base station site 130 comprises one or more an- tennas 135 and at least one base station 150. The mobile terminal 110 may comprise an arrangement for IQ-imbalance compensation according to embodiments of the invention.

The mobile terminal 110 is illustrated as a mobile telephone in a schematic front view. This example mobile terminal 1 10 comprises an antenna 111 mounted on the housing of the apparatus. Alternatively, the mobile terminal 110 may have an intei- nal antenna mounted within the housing of the apparatus. The mobile terminal 110 may even comprise multiple antennas. The mobile terminal 110 may further comprise a display, a keypad, a loudspeaker, and a microphone, which together provides a man- machme interface for operating the mobile terminal 110. The example mobile terminal 1 10 is adapted to connect to a mobile telecommunication network via the wireless link 120 to the radio base station 150. Hence, a user of the mobile terminal 110 may use conventional circuit- switched telecommunication services such as voice calls, data calls, video calls, and fax transmissions, and/or packet-based services such as electronic messaging, VoIP, Internet browsing, electronic commerce, etc. To this end, the mobile terminal 110 and the base station 150 may be compliant with at least one mobile telecommunication standard employing OFDM, for instance UMTS LTE, IEEE 802.1 Ia, IEEE 802.1 Ig, and/or WiMAX (IEEE 802.16). Alternatively or additionally, the mobile terminal 110 may be adapted to connect to a digital broadcast system and to receive signals on accordance with at least one standard for digital broadcast employing OFDM, such as DVB-T, DVB-H, T-DMB and/or DAB.

Figure 2 illustrates an example distribution of pilot symbols 210a-h within a time-frequency grid 200 of transmitted signals in an OFDM-system. Embodiments of the invention may be employed to estimate IQ-imbalance of a signal such as the one demonstrated in this figure. In Figure 2, example OFDM-signals are illustrated before IFFT-processmg m the transmitter. The corresponding time-frequency grid will be found in the receiver aftei FFT-piocessing. In Figure 2, it may be noted that pilot symbols 210a and 210b are transmitted m OFDM-symbol 201, that pilot symbols 210c and 21Od are transmitted m OFDM- symbol 205, and that intermediate OFDM-symbols do not comprise any pilot symbols Furthermore, it may be noted that pilot symbols 210a and 210b are transmitted on dif- ferent sub-carriers compared to pilot symbols 210c and 21Od It may also be noted that in this example time-frequency grid 200 there are no pilot symbols on the mirror sub- carriers For example, there is no pilot symbol on sub-carrier 220b, which is the mirror sub-carrier of pilot symbol 210b

When performing channel estimation on a signal such as the one demonstrated in Figure 2, the channel at the positions where there are pilot symbols available may first be estimated, using any known method for channel estimation Thereafter, the channel may also be estimated at the other positions m the time-frequency grid One way of performing this estimation is by means of interpolation in frequency and/or m time Figuie 3 illustrates an example method 300 according to some embodiments of the invention and suitable for use in an OFDM-ieceivei In step 310, a received baseband OFDM-signal undergoes frequency offset compensation For example, the received baseband OFDM-signal may pass a digital lotator, which rotates the received baseband OFDM-signal appropiiately to produce a frequency offset compensated ie- ceived signal

In step 320, one or more adjustment values aie determined The one or more adjustment values are associated with the frequency offset compensation The one or more adjustment values may differ between different OFDM-symbols They may, foi example, comprise twice the phase value used by a digital rotator in the fiequency offset compensation foi a particular sample of an OFDM-symbol

In step 330, the frequency offset compensated received signal is tiansformed to the frequency domain by application of an FFT

In step 340, the IQ-imbalance is estimated The IQ-imbalance is estimated based at least on the pilot symbol of one oi more fiequency pans, on the corresponding estimated channel value and on mfoimation related to the mirroi fiequency of the pilot symbol The mirroi fiequencies may oi may not comprise a pilot symbol One or sev- eral frequency pairs may be used in the estimation The frequency pans may be picked from a single OFDM-symbol or from several OFDM-symbols Further, the frequency pairs may compose all available frequency pairs (corresponding to all available pilot symbols) in a time- frequency interval, or they may comprise only some of the available frequency pairs in the time-frequency interval

The IQ-imbalance estimation in step 340 may utilize the one or more adjustment values deteimined m step 320 For example, the IQ-imbalance may be estimated as descπbed in connection to equation (14) above

Finally, in step 350, the IQ-imbalance estimate is used to compensate the re- ceived signal for the IQ-imbalance, either in the time domain or m the fiequency domain The IQ-imbalance compensation in step 350 may utilize the one or more adjustment values determined m step 320 For example, the IQ-imbalance may be compensated for as described in connection to any of the equations (13), (15), (16) or (17) above If (15) or (16) are used, the IQ-imbalance estimate of step 340 may, for example, first be adjusted using the one or moie adjustment values as illustrated in sub-step 351 Then, the adjusted IQ-imbalance estimate may be used (in sub-step 352) for IQ- imbalance compensation

It is noted that the steps of the example method 300 may be performed in another order than what is illustrated in Figuie 3 For example, the steps 340 and 350 may change place so that the IQ- compensation is done m accoi dance with a feedback appioach as will be illustrated further in connection to Figure 4A Using a feedback appioach, the step 350 may also be placed just before step 330 oi just before step 310 In such cases, the IQ-imbalance compensation is pei formed in the time domain If step 350 is placed befoie step 310, then step 350 does not use the adjustment values from step 320 as will be illustrated by Figure 4C This is because the IQ-imbalance compensation is not distuibed by the frequency offset compensation in step 310

In yet some embodiments, step 340 may be placed just befoie step 330 or just before step 310 so that the IQ-imbalance estimation is peifoπned m the time domain If step 340 is placed before step 310, then step 340 does not use the adjustment values from step 320 because the IQ-imbalance estimation is not disturbed by the frequency offset compensation in step 310. In some situations, the IQ-imbalance may be constant or rather slowly varying. In such situations a possibility is to measure the IQ-imbalance once (e.g. in the factory, during or in connection to production or at each power on), and then there need not be any update of this estimate. That is, the received OFDM-signal is not used to estimate the IQ-imbalance and there is no estimator running continuously. Instead, the IQ-imbalance may be measured using a signal generated for this (and possibly other) purpose in the terminal. Such embodiments are also meant to be embraced by the above description.

In embodiments where step 340 is placed just before step 330, step 350 may, for example, be placed just before or just after step 340, after step 330 or just before step 310. In embodiments where step 340 is placed just before step 310, step 350 may, for example, be placed after step 330 or just before step 330.

Figure 4A illustrates parts of an example arrangement 400 according to some embodiments of the invention. The arrangement may, for example perform method steps of the example method 300 as described in Figure 3. The arrangement 400 may perform IQ-imbalance estimation and compensation according to embodiments of the invention, and may be comprised in an OFDM-receiver of an electronic communication device.

The arrangement 400 comprises a frequency synchronization and adjustment value calculation unit 405. The unit 405 may, for example comprise a frequency offset compensation unit, such as an automatic frequency control (AFC) unit, adapted to apply a frequency offset compensation to the received OFDM-signal to produce a frequency offset compensated received signal and determination circuitry adapted to determine one or more adjustment values associated with the frequency offset compensation. It is understood that the unit 405 may alternatively be implemented as two or more separate units performing different tasks of the unit 405.

To describe the operations of the unit 405 logically, it generates a constant length (e.g. a unit length) complex vector with a phase angle corresponding to the required frequency offset compensation, and multiplies a received OFDM-signal with the vector and thereby achieves frequency offset compensation. These operations may, for example, be performed by a digital rotator. Before being fed to the frequency synchronization and adjustment value calculation unit 405, the received OFDM-signal may typically have been received by one or more antennas, and been filtered, down-converted, etc , in a receiver front end circuit

The frequency synchronization and adjustment value calculation unit 405 also generates the one or more adjustment values These values may also comprise a constant length complex vector with a phase angle related to the frequency offset compensation Such vectors may be generated at suitable moments m time and fed to an IQ- compensation unit (IQ-comp ) 420 and an IQ-imbalance estimation unit (IQ-est ) 440 The suitable moments in time may, for example, be once every OFDM-symbol The arrangement 400 further comprises an FFT unit 410 The FFT unit 410 applies an FFT to the frequency offset compensated received OFDM-signal to transform it to the frequency domain

The arrangement 400 also comprises the IQ-compensation unit (IQ-comp ) 420 In this embodiment, the IQ-compensation unit 420 performs compensation on the transformed signal (i e after the FFT 410) based on an IQ-imbalance estimate and the one oi more adjustment values In othei embodiments, the IQ-compensation unit 420 may be located pπor to the FFT 410 The IQ-imbalance estimate is leceived via a delay element 450 from the IQ-imbalance estimation unit (IQ-est ) 440 The IQ-imbalance estimate may be supplied as a value ( p ) The one or more adjustment values are pro- vided by unit 405

The arrangement may further comprise a channel estimator (Ch est ) 430 and an equalizer (Eq ) 460 After equalization, the leceived OFDM signal may be forwarded to a decision unit (DU) 470 and/or a decoder (Dec ) 480

The IQ-estimation unit 440 may be adapted to estimate the IQ-imbalance based on pilot symbols, estimated channel values and possibly on (implicitly or explicitly) estimated transmitted symbols as described before The IQ-estimation unit 440 also uses the one or more adjustment values to estimate the IQ-imbalance The one or more adjustment values are provided by unit 405 The estimated channel values may be received from the channel estimator 430 or they may be calculated in a channel value calculation unit 445 comprised in the IQ-imbalance estimation unit 440 If the transmitted symbols are not implicit in the IQ-imbalance estimation, the transmitted symbols may be esti- mated by circuitry 443 within the IQ-imbalance estimation unit 440. Alternatively, the transmitted symbols may be estimated by the decision unit 470 and fed to the IQ- imbalance estimation unit 440. Yet alternatively, the transmitted symbols may be estimated by re-generating, in re-generating circuitry (Re-gen.) 490, the supposedly trans- mitted signal based on the output from the decoder 480 and fed the result to the IQ- imbalance estimation unit 440.

Figure 4A demonstrates a situation where the compensation is performed post- FFT in a feedback loop. Thus, the IQ-imbalance estimate is an estimate Ap of the residual IQ-imbalance, which may be supplied as two values ( Z₁ and L₂ ) as explained before and could be combined with a previous IQ-imbalance estimate p₀ to improve the result.

In an alternative embodiment, the compensation is performed pre-FFT in a feedback loop, and block 420 is placed before block 410 in the receiver chain.

Figure 4B illustrates parts of an example arrangement 400' according to some embodiments of the invention. The arrangement may, for example perform method steps of the example method 300 as described in Figure 3. The arrangement 400' may perform IQ-imbalance estimation and compensation according to embodiments of the invention, and may be comprised in an OFDM-receiver of an electronic communication device. The arrangement 400' comprises a frequency synchronization and adjustment value calculation unit 405', which operates similarly to the unit 405 described in connection to Figure 4A.

Before being fed to the frequency synchronization and adjustment value calculation unit 405', the received OFDM-signal may typically have been received by one or more antennas, and been filtered, down-converted, etc., in a receiver front end circuit.

The arrangement 400' comprises an FFT unit 410'. The FFT unit 410' applies an FFT to the frequency offset compensated received OFDM-signal to transform it to the frequency domain.

The arrangement 400' further comprises an IQ-compensation unit (IQ-comp.) 420'. In this embodiment, the IQ-compensation unit 420' performs compensation on the transformed received OFDM-signal based on an IQ-imbalance estimate and the one or more adjustment values. The IQ-imbalance estimate is received from an IQ-imbalance estimation unit (IQ-est.) 440'. The one or more adjustment values are provided by unit 405'. The arrangement may further comprise a channel estimator (Ch. est.) 430' and an equalizer (Eq.) 460'. After equalization, the received OFDM-signal may be forwarded to a decision unit (DU) 470' and/or a decoder (Dec.) 480'.

The IQ-estimation unit 440' may be adapted to estimate the IQ-imbalance based on pilot symbols, estimated channel values and possibly on (implicitly or explic- itly) estimated transmitted symbols as described before. The IQ-estimation unit 440' also used the one or more adjustment values to estimate the IQ-imbalance. The one or more adjustment values are provided by unit 405'. The estimated channel values may be received from the channel estimator 430' or they may be calculated in a channel value calculation unit 445' comprised in the IQ-imbalance estimation unit 440'. If the trans- mitted symbols are not implicit in the IQ-imbalance estimation, the transmitted symbols may be estimated by circuitry 443' within the IQ-imbalance estimation unit 440'. Alternatively, the transmitted symbols may be estimated by the decision unit 470' and fed to the IQ-imbalance estimation unit 440'. Yet alternatively, the transmitted symbols may be estimated by re- generating, in re- generating circuitry (Re-gen.) 490', the supposedly transmitted signal based on the output from the decoder 480' and fed the result to the IQ-imbalance estimation unit 440'.

Figure 4B demonstrates a situation where the compensation is performed post- FFT in a feed forward loop.

Figure 4C illustrates parts of an example arrangement 400" according to some embodiments of the invention. The arrangement may, for example perform method steps of the example method 300 as described in Figure 3. The arrangement 400" may perform IQ-imbalance estimation and compensation according to embodiments of the invention, and may be comprised in an OFDM-receiver of an electronic communication device. The arrangement 400" comprises a frequency synchronization and adjustment value calculation unit 405", an FFT unit 410", a channel estimator (Ch. est.) 430", an IQ-estimation umt 440", a delay element 450", an equalizer (Eq ) 460", a decoder (Dec ) 480", and may comprise circuitry 443", a channel value calculation unit 445", a decision unit (DU) 470" and/or re-generatmg circuitry (Re-gen ) 490", which may all operate similarly to the corresponding units described in connection to Figure 4A The arrangement 400" also comprises the IQ-compensation umt (IQ-comp )

420" In this embodiment, the IQ-compensation unit 420" performs compensation before the frequency offset compensation, and based on an IQ-imbalance estimate Since the IQ-imbalance compensation is performed befoie the frequency offset compensation, the IQ-compensation unit 420" does not use the one oi more adjustment values The IQ-imbalance estimate is leceived via delay element 450" from the IQ-imbalance estimation umt (IQ-est ) 440"

Figure 4C demonstrates a situation where the compensation is performed pre- FFT and pre-AFC in a feedback loop Thus, the IQ-imbalance estimate is an estimate

Ap of the residual IQ-imbalance, which may be supplied as two values (L₁ and L₂ ) as explained before and could be combined with a pievious IQ-imbalance estimate p₀ to improve the iesult

In the aiiangement illustrated in Figure 4C, the IQ-imbalance compensation unit 420' ' may be adapted to perform the compensation m the analog or in the digital domain In some embodiments, the IQ-imbalance compensation umt 420" and possibly unit 405 ' 'may be comprised m a radio en cult

Further vaπations to the placement of the IQ-imbalance estimation unit and the IQ-imbalance compensation umt may be envisioned based on the variations of the oi- deπng of method steps that weie desciibed in connection to Figuie 3

The one oi more adjustment values, e g the complex value θ(_s), which may be used m the IQ-imbalance estimation and/or compensation, may be generated in different ways The complex value θβ) may coπespond to a unit vector with twice the phase of the fust sample of each OFDM-symbol If the adjustment value is calculated in hard- waie it may be desiiable to avoid implementing an additional digital iotator for the adjustment value generation Instead it may be desiiable to utilize the digital rotator used for fiequency offset compensation Typically, the digital rotator iotates each received sample before the FFT- apphcation Thus, the digital rotator may be used once every sample According to some embodiments of the invention, a sample from the cyclic prefix (CP) of a current OFDM-symbol or of a subsequent OFDM-symbol may be excluded from the frequency offset compensation During that sample, the digital rotator may be used to calculate the adjustment value (or a value related to the adjustment value) instead, e g by rotating a "1" (or any other real or close to real constant) a desired angle m the complex plane The desired rotation for generating θ_(Sj may be twice the rotation phase of a particular sample, such as the first sample of an OFDM-symbol This rotation can be achieved either by rotating the constant ("1") by the double argument compared to the iotation phase of the first sample of an OFDM-symbol or by rotating the constant ("1") by the rotation phase of the first sample of an OFDM-symbol and squaring the result

Figures 4D-F describe examples of different alternative implementations 405a, 405b, 405c of frequency synchiomzation and adjustment value calculation units 405, 405' and 405" of Figures 4A-C respectively Any of the alternatives of Figures 4D-F may be used in any of the units 405, 405' and 405"

In Figure 4D, the leceived OFDM-signal is input to a digital rotator 406a performing frequency offset compensation The digital rotator is also fed a phase ramp signal from a phase iamp generator 407a The phase ramp is also fed to an adjustment value calculation unit 408a, which calculates a complex number using one or more values of the phase ramp signal For example, an adjustment value may be calculated as a constant (e g unit) length vector having a phase corresponding to a phase ramp value multiplied by two

In Figure 4E, the received OFDM-signal is input to a digital rotatoi 406b per- forming frequency offset compensation The digital rotator is also fed a phase ramp signal from a phase ramp generator 407b At suitable moments in time, the digital iotatoi is adapted to pioduce a complex numbei i elated to the one or more adjustment values by using one or more values of the phase i amp signal as will be explained in connection to Figure 5A B For example, the complex number may be calculated as a constant (e g unit) length vectoi having a phase coπespondmg to a phase ramp value At these moments in time, the output from the digital iotator 406b is feed to a phase doubling unit 409b by changing the setting of the switch 403b The phase doubling unit thus produces an adjustment value, which is a complex number having the same length but the double phase compared to the complex number produced by the digital rotator 406b This may, for example, be achieved by a squaring operation In Figure 4F, the received OFDM-signal is input to a digital rotator 406c performing frequency offset compensation The digital rotator is also fed a phase ramp signal from a phase ramp generator 407c At suitable moments m time, the digital rotator is fed a double phase angle from phase doubling unit 409c instead of the phase ramp, by changing the setting of the switch 403c At these moments in time, the digital rotator operates on a real or close to real constant (such as a "1") instead of on the input signal (similarly to the operations that will be described in connection to the constant 504 of Figure 5A) The digital rotator thus produces an adjustment value, which is a complex number having constant length and the double phase compared to the phase lamp value Figuie 5 A illustrates an example implementation of an adjustment value gen- eiation unit 500 Figure 5 B illustiates example corresponding input signals to the adjustment value generation unit 500 The adjustment value generation unit 500 may, foi example, be implemented in block 406b of Figure 4E The adjustment value geneiation unit 500 comprises a digital rotator 501 In normal operation, the digital iotatoi appropriately rotates a received signal 502, 502' using a phase ramp 507, 507' to achieve fre- quency offset compensation The switches 503 and 506 are set to pass the received signal 502, 502' and the phase ramp 507, 507' respectively to the digital rotator 501 At some suitable moment in time, a contiol signal 505, 505' tnggers the switches 503 and 506 Thus, the switch 503 is momentarily set to pass a real or close to real constant 504 (such as a "1") to the digital iotatoi 501 The switch 506 is set to pass a delayed version of the phase ramp to the digital rotator 501 Thus, the digital rotator iotates the constant (e g "1") by an angle coπespondmg to a value of the phase ramp chosen by the delay value The resulting iotated constant is output from the digital iota- tor 501 and may be used to produce the one oi moie adjustment values, foi example by a squaiing operation as explained m connection to Figure 4E The suitable moment m time may, foi example, be duiing a sample of a cyclic prefix (CP) of the current or following OFDM-symbol The contiol signal 505, 505' may ieset the switches 503 and 506 to pass the received signal 502, 502' and the phase ramp 507, 507' respectively for the following sample

The delay value used for producing the delayed version of the phase ramp is determined by a time difference between the control signal 505, 505' and a register con- trol signal 509, 509' The register control signal 509, 509' may, at some suitable moment in time, trigger a register 508 to hold a sample of the phase ramp 507, 507' The suitable moment in time may, for example, be at the beginning of the payload part of an OFDM-symbol The register content is then passed, as the delayed version of the phase ramp, by the switch 506 to the digital rotator 501 as explained above Figure 6 is a plot 600 showing the performance of different IQ-imbalance estimation and compensation approaches to illustrate advantages of embodiments of the invention The resulting bit error rate (BER) is shown as a function of the fiequency offset The simulated channel is an additive white Gaussian noise (AWGN) channel with 20 dB signal-to-noise ratio (SNR) In the simulations, it is assumed that the fre- quency offset is accurately estimated, that the IQ-imbalance has a 0 35 dB gam imbalance and a 1-degiee phase imbalance

In the plot 600, the ideal BER, i e when either the IQ-imbalance is perfectly estimated and compensated for or no IQ-imbalance is present and no IQ-imbalance compensation is peiformed, is shown by the curve 601 The result if no IQ-imbalance compensation is applied is shown by the curve 602 The curve 603 shows the result if IQ-imbalance compensation is applied but neither the IQ-imbalance estimation noi the IQ-imbalance compensation are adjusted for the frequency offset compensation The curve 604 shows the result if IQ-imbalance compensation is applied and both the IQ- imbalance estimation and the IQ-imbalance compensation are adjusted for the fie- quency offset compensation according to embodiments of the invention

Fi om Figuie 6 it can be seen that when no compensating for the frequency offset compensation is done to the IQ-imbalance algoiithms (cmve 603), the BER starts do increase already at low fiequency offsets It can also be seen that at some point (approximately at 10 Hz frequency offset m this example) the gam fiom applying IQ- imbalance compensation is lost The BER may even get worse than without IQ- imbalance compensation (curve 602) At which frequency offset the cuives 602 and 603 will cross each other may depend on the length of the time interval used for IQ- imbalance estimation

The curve 604, showing BER when the IQ-imbalance estimation and compensation are adjusted for the frequency offset rotation according to embodiments of the invention, reveals a situation corresponding to an IQ-imbalance free situation (curve 601) up to a frequency offset of approximately 1 kHz The BER starts to increase for larger frequency offsets due to mter-carrier mterfeience (ICI), since the approximation in equation (6) is not valid for such high frequency offsets To make the algorithm robust for higher frequency offsets, more frequencies from the sum m equation (5) may be used m the estimation and/or compensation

In some embodiments, the arrangements and methods described above may be applied in MIMO leceivers In such embodiments, the desciibed IQ-imbalance estimation and/or IQ-imbalance compensation techniques may be applied to each MIMO stream separately The desciibed embodiments of the invention and their equivalents may be performed in hardware or software or m a combination of both haidware and software

Embodiments of the invention may be performed by geneial-purpose circuits associated with or integral to a receiver, such as digital signal processors (DSP), central processing units (CPU), co-processor units, or by specialized circuits such as for exam- pie application- specific integrated circuits (ASIC) The digital rotator may be implemented using a CORDIC (Coordinate Rotation Digital Computer) algorithm All such forms aie contemplated to be within the scope of the invention

Embodiments of the invention may be implemented within an electionic apparatus comprising a receiver chain having aπangements or performing methods accord - ing to any of the embodiments of the invention The electionic appaiatus may, for example, be a portable or handheld mobile radio communication equipment, a mobile radio terminal, a mobile telephone, a pager, a communicator, an electronic organizer, a smartphone, a computer, an embedded drive, a mobile gaming device, or a (wπst) watch The electronic appaiatus may alternatively be a base station in a communication system According to some embodiments of the invention, a computer program product comprises a computer readable medium such as, for example, a diskette or a CD-ROM. The computer readable medium may have stored thereon a computer program comprising program instructions. The computer program may be loadable into a data-processing unit, which may, for example, be comprised in the mobile terminal 1 10. When loaded into the data-processing unit, the computer program may be stored in a memory associated with or integral to the data-processing unit. According to some embodiments, the computer program may, when loaded into and run by the data-processing unit, cause the data-processing unit to execute method steps according to, for example, the method shown in Figure 3.

The invention has been described herein with reference to various embodiments. However, a person skilled m the art would recognize numerous variations to the described embodiments that would still fall within the scope of the invention. For example, the method embodiments described herein describes example methods through method steps being performed in a certain order. However, it is recognized that these sequences of events may take place in another order without departing from the scope of the invention. Furthermore, some method steps may be performed in parallel even though they have been described as being performed in sequence.

In the same manner, it should be noted that in the description of embodiments of the invention, the partition of functional blocks into particular units is by no means limiting to the invention. Contraπly, these partitions are merely examples. Functional blocks described herein as one unit may be split into two or more units. In the same manner, functional blocks that are described herein as being implemented as two or more units may be implemented as a single unit without departing from the scope of the invention

Hence, it should be understood that the limitations of the described embodiments are merely for illustrative purpose and by no means limiting. Instead, the scope of the invention is defined by the appended claims lather than by the description, and all variations that fall within the range of the claims are intended to be embraced therein

Claims

1. A method of processing a received wireless communication signal, suitable for use in a communication receiver, comprising: applying (310) frequency offset compensation to produce a frequency offset compensated received signal; determining (320) one or more adjustment values associated with the frequency offset compensation; transforming (330) the frequency offset compensated received signal to a fre- quency domain, wherein the frequency domain comprises a plurality of sub-carrier frequencies; estimating (340) an in-phase/quadrature imbalance - IQ-imbalance; and performing (350) IQ-imbalance compensation of the received wireless communication signal based on the estimated IQ-imbalance; wherein at least one of the step of estimating the IQ-imbalance and the step of performing IQ-imbalance compensation is performed based on the one or more adjustment values.

2. The method of claim 1, wherein the step of estimating the IQ-imbalance and the step of performing IQ-imbalance compensation are performed after the step of transforming the frequency offset compensated received signal and are based on the one or more adjustment values.

3. The method of claim 2, wherein the IQ-imbalance compensation is per- formed using a feed forward approach.

4. The method of claim 1, wherein the step of estimating the IQ-imbalance is performed after the step of transforming the frequency offset compensated received signal and is based on the one or more adjustment values, and wherein the step of per- forming IQ-imbalance compensation is performed prior to the step of transforming the frequency offset compensated received signal and after the step of applying frequency offset compensation and is based on the one or more adjustment values

5 The method of any of claims 2 through 4, wherein the step of performing IQ-imbalance compensation comprises adjusting (351) the estimated IQ-imbalance based on the one or more adjustment values, and performing (352) the IQ-imbalance compensation based on the adjusted estimated IQ-imbalance

6 The method of claim 1, wherein the step of estimating the IQ-imbalance is performed after the step of transforming the frequency offset compensated received signal and is based on the one or more adjustment values, and wherein the step of performing IQ-imbalance compensation is performed prior to the step of applying fre- quency offset compensation and is not based on the one or more adjustment values

7 The method of any of claims 2, 4 or 6, wherein the IQ-imbalance compensation is performed using a feedback approach

8 The method of claim 7, wherein the step of estimating the IQ-imbalance compπses defining a residual IQ-imbalance by a numerator and a denommatoi and estimating the numeratoi and denominator

9 The method of any of claims 2 through 8, wherein the step of estimating the IQ-imbalance comprises estimating the IQ-imbalance based on at least the one or more adjustment values, information related to a first symbol of a fust sub-can iei fiequency and information related to a second symbol of a second sub-carrier frequency, wherein the second sub-carrier fiequency is a mirror sub-carrier frequency to the first sub-earner frequency

10. The method of claim 9, wherein the step of estimating the IQ-imbalance comprises: determining estimated channel values for at least the first sub-carrier frequency, the first sub-carrier frequency having a pilot symbol; and estimating the IQ-imbalance based on at least the pilot symbol, the at least one determined estimated channel value, information related to the second symbol, and at least one of the one or more adjustment values.

11. The method of any of claims 9 through 10, wherein the step of estimating the IQ-imbalance comprises estimating the IQ-imbalance based on information related to a number of symbols of sub-carrier frequencies being neighboring sub-carrier frequencies to the mirror sub-carrier frequency.

12. The method of any of claims 9 through 11, wherein the step of estimating the IQ-imbalance comprises estimating the IQ-imbalance based on a plurality of pilot symbols and information related to symbols of corresponding mirror sub-carrier frequencies.

13. The method of claim 1, wherein the step of estimating the IQ-imbalance is performed prior to the step of transforming the frequency offset compensated received signal and after the step of applying frequency offset compensation and is based on the one or more adjustment values.

14. The method of claim 1, wherein the step of estimating the IQ-imbalance is performed prior to the step of applying frequency offset compensation and is not based on the one or more adjustment values.

15. The method of any of claims 1 through 14, wherein the wireless signal is an Orthogonal Frequency Division Multiplexing signal - OFDM-signal, wherein each of the one or more adjustment values is associated with an OFDM-symbol of the received OFDM-signal and comprises a complex number associated with the OFDM-symbol, and wherein performing at least one of the step of estimating the IQ-imbalance and the step of performing IQ-imbalance compensation based on the one or moie adjustment values comprises a multiplication with a first function of the complex number.

16 The method of claim 15, wherein the step of applying frequency offset compensation compπses digital rotation.

17 The method of claim 16, wherein the complex number is a second function of a phase of the digital rotation corresponding to a particular sample of the OFDM- symbol.

18 The method of claim 17, wherein the second function comprises a doubling of the phase of the digital rotation corresponding to the particular sample

19. The method of any of claims 15 thiough 18, wheiein the step of determining the one or moie adjustment values compiises using a digital rotator to generate the complex number during a cyclic prefix of the OFDM-symbol oi of an adjacent OFDM- symbol.

20 A computer progiam product comprising a computer readable medium, having thereon a computer progiam comprising program instructions, the computer progiam being loadable into a data-processing unit and adapted to cause the data- processmg unit to execute method steps according to any of claims 1 to 19 when the computer program is run by the data-processing unit

21 An arrangement for piocessmg a received wireless communication signal comprising a frequency offset compensation unit (405, 405', 405", 406a-c, 500) adapted to apply a frequency offset compensation to the received wireless communication signal to produce a frequency offset compensated leceived signal, determination circuitry (405, 405', 405", 408a, 406b-c, 409b-c, 500) adapted to determine one or more adjustment values associated with the fiequency offset compensation, a fast Fourier transform unit - FFT-umt - (410, 410', 410") adapted to trans- form the frequency offset compensated received signal to a frequency domain, wherein the frequency domain comprises a plurality of sub-carrier frequencies, an m-phase/quadiature imbalance - IQ-imbalance - estimator (440, 440', 440") adapted to estimate an IQ-imbalance, and an IQ-imbalance compensation unit (420, 420', 420") adapted to perform IQ- imbalance compensation of the received wireless communication signal based on the estimated IQ-imbalance, wherein at least one of the IQ-imbalance estimator and the IQ-imbalance compensation unit is adapted to base its operation on the one or moi e adjustment values

22 The arrangement of claim 21 , wherein the IQ-imbalance estimator and the

IQ-imbalance compensation unit are located after the FFT unit in a processing chain and are adapted to base their operation on the one or more adjustment values

23 The arrangement of claim 21, wherein the IQ-imbalance estimator is lo- cated after the FFT unit in a processing chain and is adapted to base its operation on the one oi more adjustment values, and wheiem the IQ-imbalance compensation unit is located pnor to the FFT unit and aftei the fiequency offset compensation unit in the piocessmg chain and is adapted to base its operation on the one oi moie adjustment values

24 The arrangement of any of claims 22 through 23, wherein the IQ-imbalance compensation unit is adapted to adjust the estimated IQ-imbalance based on the one or moie adjustment values, and perform the IQ-imbalance compensation based on the adjusted estimated IQ- imbalance

25 The arrangement of claim 21, wherein the IQ-imbalance estimator is located after the FFT unit m a processing chain and is adapted to base its operation on the one or more adjustment values, and wherein the IQ-imbalance compensation unit is located prior to the frequency offset compensation unit in the processing chain and is adapted to not base its operation on the one or more adjustment values.

26. The arrangement of any of claims 22 through 25, wherein the IQ-imbalance compensation unit is located prior to the IQ-imbalance estimator in accordance with a feedback approach

27. The arrangement of claim 26, wherein the IQ-imbalance estimator is adapted to estimate a numerator and a denominator defining a residual IQ-imbalance.

28. The arrangement of any of claims 22 through 27, wherein the IQ- imbalance estimator is adapted to estimate the IQ-imbalance based on at least the one or more adjustment values, information related to a first symbol of a first sub-carrier frequency and information related to a second symbol of a second sub-carrier frequency, wheiem the second sub-carrier frequency is a mirror sub-camer frequency to the fust sub-caiπer frequency

29. The arrangement of claim 28, further comprising a channel estimator (430, 445, 430', 445', 430", 445") adapted to determine estimated channel values foi at least the fust sub-carπei fie- quency, the first sub-carrier frequency having a pilot symbol, and wherein the IQ-imbalance estimator is adapted to estimate the IQ-imbalance based on at least the pilot symbol, the at least one determined estimated channel value, information i elated to the second symbol, and at least one of the one or more adjustment values

30 The arrangement of any of claims 28 through 29, wherein the IQ-imbalance estimator is further adapted to estimate the IQ-imbalance based on information related to a number of symbols of sub-carrier frequencies being neighboring sub-carrier frequencies to the mirror sub-carrier frequency

31 The arrangement of any of claims 28 through 30, wherein the IQ-imbalance estimator is adapted to estimate the IQ-imbalance based on a plurality of pilot symbols and information related to symbols of corresponding mirror sub-carrier frequencies

32 The arrangement of claim 21, wherein the IQ-imbalance estimator is located prior to the FFT unit and after the frequency offset compensation unit in a processing chain and is adapted to base its operation on the one or more adjustment values

33 The aπangement of claim 21, wherein the IQ-imbalance estimator is lo- cated pnor to the frequency offset compensation unit in a piocessmg chain and is adapted to not base its operation on the one oi more adjustment values

34 The arrangement of any of claims 21 through 33, wherein the wneless signal is an Orthogonal Fiequency Division Multiplexing signal - OFDM-signal, wherein the determination circuitry is adapted to determine each of the one or more adjustment values to be associated with an OFDM-symbol of the leceived OFDM-signal and to compiise a complex number associated with the OFDM-symbol, and wheiem the operation of at least one of the IQ-imbalance estimatoi and the IQ-imbalance compensation unit comprises a multiplication with a first function of the complex numbei

35 The arrangement of claim 34, wherein the frequency offset compensation unit comprises a digital iotatoi

36 The anangement of claim 35, wherein the determination circuitry is furthei adapted to determine the complex numbei as a second function of a phase of the digital rotatoi corresponding to a particulai sample of the OFDM-symbol

37. The arrangement of claim 36, wherein the second function comprises a doubling of the phase of the digital rotator corresponding to the particular sample.

38. The arrangement of any of claims 35 through 37, wherein the determination circuitry comprises the digital rotator.

39. An electronic communication device (110) comprising a wireless communication receiver, which comprises the arrangement of any of claims 21 through 38.