CN106911607A - The unbalanced method of estimation of inphase/orthogonal and its module in a kind of ofdm system - Google Patents

Abstract

The invention discloses the unbalanced method of estimation of inphase/orthogonal and its module in a kind of ofdm system.The method of estimation is comprised the following steps：S1：Time-domain signal to receiving carries out frequency deviation compensation；S2：After removal Cyclic Prefix, two times upper neighbouring different frequency pilot signs are transformed into frequency domain；S3：The subcarrier that frequency pilot sign takes is extracted according to system bandwidth；S4：For two frequency pilot signs, the reception signal of two pilot signals on each subcarrier is obtained；S5：The coupling of removal IQ imbalance parameters and channel, asks for IQ imbalance estimates.Not only wide application of the invention, the timing requirements for receiver treatment are low；And meeting most of practical application scene demand, algorithm is simple, and amount of calculation is small.

Description

A method and module for estimating in-phase/quadrature imbalance in OFDM system

technical field

The invention relates to a method and module for estimating in-phase/quadrature-phase (In-phase/Quadrature-phase, referred to as IQ) imbalance in an OFDM (orthogonal frequency division multiplexing) system, which belongs to wireless communication technology field.

Background technique

In common OFDM communication systems (such as 802.11a/g/n/ac series and LTE, etc.), in order to improve spectrum utilization efficiency, high-order modulation or MIMO technology is generally used. These complex transmission schemes require the receiver to be able to achieve a relatively high signal-to-noise ratio (SNR). However, the transceiver is limited by the production cost and may need to use relatively low-priced devices. Its non-ideal IQ imbalance characteristics will lead to a decrease in the signal-to-noise ratio at the receiving end and seriously affect the receiving performance. In order to improve the receiving performance, it is necessary to accurately estimate the IQ imbalance so as to compensate the receiving end.

For broadband communication systems based on OFDM technology, the entire frequency band itself has been divided into multiple sub-carriers, so common IQ imbalance estimation methods often use calculations performed in the frequency domain. For example, for the estimation of IQ imbalance in OFDM systems, Broadcom Corporation of the United States proposed the use of paired pilots that are symmetric about DC based on the symmetrical pilot structure in the US patent application "IQ imbalance estimation using broadcast signals" (publication number US2015103960A1). Subcarriers are used to obtain statistics, and then the statistics are provided to the channel estimation module to obtain channel estimation, and finally the result of channel estimation is sent to the IQ imbalance estimation module to eliminate the coupling of IQ imbalance and channel information, and obtain IQ imbalance Balance parameter estimates. It is required that the design of the pilot signal be symmetrical about DC, which places great restrictions on the design of the system pilot signal. For example, in some communication standards of UMTS LTE, this method is not applicable because the pilots are not placed on symmetrical frequencies. Moreover, this patent application needs to complete the pilot channel estimation first, and then estimate the IQ imbalance, so there are requirements for the specific implementation timing. Furthermore, the patent application assumes that adjacent subcarrier channels are flat in channel estimation. This assumption is more reasonable for smaller subcarrier spacing, such as LTE systems (subcarrier spacing is 15kHz); and for larger subcarrier spacing For OFDM systems, such as 802.11a/g/n/ac systems (subcarrier spacing is 312.5kHz), this assumption may not be true.

In order to remove the dependence of the IQ imbalance estimation algorithm on the symmetrical pilot structure, Ericsson proposed to use data symbols to replace part of the pilot symbols in the Chinese invention patent "Method and Arrangement for Estimating IQ Imbalance" with the announcement number CN102057644B , that is to form a pilot/data subcarrier pair that is symmetrical about DC to estimate IQ imbalance. The main idea is similar to the above-mentioned patent application of Broadcom Company. Treated as known symbols (equivalent to pilots). This method has the following shortcomings: the demodulation of the data symbols needs to be completed first, and then the IQ imbalance can be estimated, so it has requirements for the timing of the specific implementation, and the complexity is high; the channel estimation on the data symbols is generally performed by the channel on the pilot The channel estimation interpolation generated by the channel itself contains IQ imbalance and interpolation errors, resulting in the accuracy of IQ imbalance estimation is not as good as using symmetric pilot signals directly; if the data symbol demodulation error occurs, it will directly lead to errors in IQ imbalance estimation.

In another Chinese invention patent "IQ Imbalance Estimation for Asymmetric Pilot Symbols" with the announcement number CN102668483B, Ericsson proposes that based on the assumption that IQ imbalance is a static parameter, in the early stage of receiving OFDM signals, downmixing An additional frequency offset is added to the frequency, so that a symmetrical pilot subcarrier is obtained in the baseband signal (at this time, the frequency spectrum is offset and is no longer centered on DC). Then adopt the IQ imbalance estimation algorithm based on the symmetric pilot, and finally need to compensate the additional frequency offset. After the estimated value of IQ imbalance is obtained, normal down-mixing is used for reception, and IQ imbalance estimation is no longer performed. This method has the following shortcomings: different down-mixing methods need to be used in the initial stage of receiving OFDM signals, and the additional frequency offset added in the down-mixing depends on the specific pilot design, and the implementation complexity is high; assuming IQ imbalance It is a static value, and it is impossible to realize real-time estimation of IQ imbalance; in the actual environment, IQ imbalance is also affected by time-varying factors such as temperature and voltage, so the practicability of this method is questionable.

Contents of the invention

The primary technical problem to be solved by the present invention is to provide a method for estimating in-phase/quadrature imbalance in an OFDM system.

Another technical problem to be solved by the present invention is to provide an estimation module of in-phase/quadrature imbalance in an OFDM system.

For realizing the above-mentioned purpose of the invention, the present invention adopts following technical scheme:

A method for estimating in-phase/quadrature imbalance in an OFDM system, comprising the following steps:

S1: Perform frequency offset compensation on the received time domain signal;

S2: After removing the cyclic prefix, transform two different pilot symbols adjacent in time to the frequency domain;

S3: Extracting subcarriers occupied by the pilot symbols according to the system bandwidth;

S4: For the two pilot symbols, obtain received signals of the two pilot signals on each subcarrier;

S5: Remove the coupling between the IQ imbalance parameter and the channel, and obtain an estimated value of the IQ imbalance.

Preferably, in step S5, removing the coupling between the IQ imbalance parameter and the channel is to obtain the ratio of the IQ imbalance parameter on one of the subcarriers

Wherein preferably, on at least two subcarriers in the subcarriers, obtain with the at least two subcarriers to find an estimate of IQ imbalance.

Wherein preferably, each subcarrier in the subcarriers is obtained separately on each subcarrier to find an estimate of IQ imbalance.

Preferably, the following step is further included: the two different pilot symbols adjacent in time refer to two preamble symbols STF and LTF.

Preferably, the two different pilot symbols adjacent in time refer to two preamble symbols STF and LTF on the same carrier.

An estimation module of in-phase/quadrature imbalance in an OFDM system, comprising:

Frequency offset compensation unit: used to perform frequency offset compensation on the received time domain signal;

Remove cyclic prefix unit: used to remove cyclic prefix;

Frequency domain transformation unit: used to transform two different pilot symbols adjacent in time to the frequency domain;

A subcarrier extraction unit: used to extract the subcarriers occupied by the pilot symbols according to the system bandwidth; and

ν/μ calculation unit: used to obtain the received signals of the two pilot signals on each subcarrier for the two pilot symbols, and then remove the coupling between the IQ imbalance parameter and the channel to obtain the IQ imbalance estimated value.

Preferably, the estimation module further includes a ν/μ averaging unit, which is used to average ν/μ on subcarriers.

Compared with the prior art, the method for estimating the in-phase/quadrature imbalance and its modules provided by the present invention have the following advantages:

1. Avoid the dependence on the symmetrical pilot structure in the conventional method, but use two different pilot symbols that are adjacent in time, and have wider applications.

2. There is no need to rely on channel estimation and data demodulation, and the timing requirements for receiver processing are low.

3. It is suitable for scenarios with frequency selective channels and meets the requirements of most practical application scenarios.

4. The algorithm is simple, the amount of calculation is small, and it is easy to implement.

Description of drawings

Fig. 1 is the block flow diagram of IQ imbalance estimation method in the OFDM system provided by the present invention;

Fig. 2 is the simulation result of IQ imbalance parameter α estimation;

Fig. 3 is the simulation result of IQ imbalance parameter θ estimation;

Fig. 4 is a structural block diagram of the IQ imbalance estimation module in the OFDM system provided by the present invention.

detailed description

The technical content of the present invention will be described in detail below in conjunction with the accompanying drawings and specific embodiments.

The present invention assumes that there are two different pilots adjacent in time in the OFDM system, and the channel of the two pilot signals on the same subcarrier is flat. This assumption is generally established in common OFDM communication systems. For example, (1) STF/LTF in the 802.11a/g/n/ac system is separated by 8us in time, which is used in the LAN environment and has a high time correlation; (2) two columns of DMRS signals on the PUSCH in the LTE system, Contains different cyclic shifts, with a time interval of 0.5ms. When used in common scenarios with a speed of less than 120km/h, the time correlation is relatively high.

The following takes the IQ imbalance estimation process in the 802.11a/g/n/ac system as an example to illustrate the method for estimating the in-phase/quadrature imbalance in the OFDM system provided by the present invention. In the 802.11a/g/n/ac system, 2 STFs (short training field, short training sequence) and 2 LTFs (1ong training field, long training sequence) in the preamble are used to estimate the IQ imbalance. The processing block diagram is as follows Figure 1 shows.

In the IQ imbalance evaluation method provided by the present invention, it is assumed that α is the gain imbalance of the IQ branch, and θ is the phase imbalance of the IQ branch. Define the variables μ, ν as follows:

It is easy to get, if there is no IQ imbalance, then μ=1, ν=0.

As shown in Figure 1, the method includes the following steps;

S1: Perform frequency offset compensation on the received time domain signal.

For frequency offset, it needs to be estimated and compensated in advance. This step can be realized by adopting the existing technology, which will not be repeated here.

S2: After removing the cyclic prefix, transform two different pilot symbols adjacent in time (the time indexes are n and m respectively) into the frequency domain.

Specifically, FFT is performed to transform the two pilot symbols of STF/LTF into the frequency domain.

For the received signal with IQ imbalance and frequency selection channel in OFDM system, due to the influence of IQ imbalance, the received signal in time domain is s _d =μs+νs ^* . Where s is the original time domain signal without IQ imbalance, s ^* represents the conjugate of s, and s _d is the distorted time domain signal with IQ imbalance.

Transformed into the frequency domain by FFT, the received signal S _d =μS+νF·conj(F ^H S )=μS+νZ of the pilot symbol in the time domain is obtained. To simplify the expression, Z=F·conj(F ^H S) is defined in the formula. Since S is a known pilot frequency, Z can be calculated in advance to reduce the amount of real-time calculation.

S3: Extract subcarriers occupied by the pilot symbols according to the system bandwidth.

Here it is assumed that the number of subcarriers specifically occupied by the received signal is Nsubc. This step can be realized by adopting the existing technology, which will not be repeated here. It should be noted that the number Nsubc of subcarriers occupied by the received signal is different from the number N of subcarriers. Generally speaking, Nsubc<N, Nsubc is the occupied subcarriers, which will be extracted to calculate the IQ imbalance. Usually, Nsubc=48 (subcarrier bandwidth 80MHz); Nsubc=24 (subcarrier bandwidth 40MHz); Nsubc=12 (subcarrier bandwidth 20MHz). N is the FFT size, which generally includes guard sidebands.

S4: For two pilot symbols, obtain received signals of two pilot signals on each subcarrier.

Using the obtained S and Z, for each pilot symbol, a pair of binary linear equations is formed on each subcarrier.

Considering the influence of the frequency selection channel, the received signal can be expressed as S _d2 =H(μS+νZ). H is the channel experienced by each subcarrier. Among them, S and Z can be pre-stored in advance and obtained by reading to construct a binary linear equation system; S and Z can also be obtained by real-time calculation.

Two different pilot symbols adjacent in time (the time index is n, m respectively), that is, the two preamble symbols STF/LTF, the channel can be regarded as flat on the same subcarrier k, that is, H _k,n = H _k,m . Therefore, the received signals of two pilot signals (time indexes are n, m respectively) on the kth subcarrier are:

In the case of known S _d2,k,n ,S _d2,k,m ,S _k,n ,S _k,m ,Z _k,n ,Z _k,m , it is easy to know μH _k,n ,νH _{k, n} can be obtained by solving a binary linear equation. It can be seen from the equations that IQ imbalance is coupled with the channel and cannot be separated.

S5: Remove the coupling between the IQ imbalance parameter and the channel, and obtain an estimated value of the IQ imbalance.

In order to remove the coupling between the IQ imbalance parameter and the channel, a simplified algorithm can be used to obtain Therefore, the present invention selects two different pilot symbols STF/LTF adjacent in time in the OFDM system, and calculates the ratio of the IQ imbalance parameter on the same subcarrier k The influence of the channel H _k,n is offset, and the decoupling of the IQ imbalance parameter and the channel is realized.

Although as mentioned above, μH _k,n , νH _k,n can be obtained by solving the binary linear equation, but in order to obtain the IQ imbalance parameters α and θ, it is not necessary to solve the above binary linear equation, the present invention adopts Simplified method as follows:

Under the assumption of |α|<<1, |θ|≤5degree (greater than the typical value range of IQ imbalance, refer to "Widely linear block-diagonalization type precoding in massive MIMO system with IQ imbalance", ICC 2015, p3392- 3397, Jun 2015), the following approximation can be obtained:

Therefore it is only necessary to calculate That is to say, it is convenient to obtain the IQ imbalance parameters α, θ.

In addition, in order to improve the estimation accuracy, this estimation method further includes: Do subcarrier averaging to improve the estimation accuracy of α, θ. This is because the IQ imbalance parameter is slowly changing and can be obtained by finding And averaged to improve the accuracy of α, θ estimation.

Specifically, on each subcarrier, based on the STF/LTF received signal, the Then on each subcarrier Summing, arithmetic mean. get The average value, and then use the formulas (4) and (5) to calculate the estimates of α, θ, and then obtain the estimated value of IQ imbalance.

It is easy to know that this method is not sensitive to time offset because the time offset has the same impact on subcarrier k for two different pilots. Frequency offset needs to be estimated and compensated in advance.

The invention can be applied in the OFDM system to realize the estimation of the IQ imbalance parameter so that the receiving end can compensate the IQ imbalance.

The estimation method provided by the present invention completely avoids the dependence on the symmetrical pilot structure in the conventional method, and obtains the IQ imbalance parameters α, θ by using reasonable simplified approximation. There is no need for channel estimation and data demodulation during the processing, so the implementation has low requirements on the timing of receiver processing. This estimation method is robust in performance to scenarios with frequency-selective channels and time offsets. The calculation amount of the algorithm is small, which is very beneficial to the realization.

After frequency offset compensation, the processing branch of the estimation method provided by the present invention is independent of processing modules such as channel estimation, channel equalization, and data demodulation. This feature optimizes receive timing, whether applied on a receiver or test instrument.

Through fixed-point simulation, the estimation accuracy of the estimation method provided by the present invention within the range of typical values of α and θ is verified. The formula for converting the linear value α into dB is α _dB =10*log10(α+1). In the simulation, according to the 802.11a/g/n/ac specification for the test definition of the modulation characteristic, the time average of 20 frames is used to obtain accurate measurement results.

Simulation 1 is mainly to study the accuracy and applicability of the algorithm under different values of α and θ (the range is larger than the typical value range of IQ imbalance).

The simulation parameters are set as follows:

The mean square error of the simulation results is as follows:

MSE _α ＝0.0011dB

MSE _θ ＝0.0207degree

The simulation result diagram of the IQ imbalance parameter α, θ estimation is given below.

As shown in Figure 2, given θ (value [-5:5], corresponding to 11 curves), α is estimated accurately in the range of [-0.3:0.02:0.3] dB. The asterisks in the figure indicate the reference value for comparison, that is, the set α value.

As shown in Figure 3, given α (with a value of [-0.3:0.02:0.3] dB, corresponding to 31 curves), θ is estimated accurately within the range of [-5:5] degrees. The asterisks in the figure indicate the reference value for comparison, that is, the set θ value.

Simulation 2 is mainly to study the accuracy of the algorithm under typical values of α and θ.

The simulation parameters are set as follows:

The mean square error of the simulation results is shown in the table below:

According to the above simulation results, it can be seen that the method for estimating the in-phase/quadrature imbalance in the OFDM system provided by the present invention has high accuracy and meets the actual needs.

Taking the 802.11a/g/n/ac system as an example, the principle and implementation of the present invention are described. However, the specific examples should be regarded as illustrative rather than restrictive, and the invention should not be considered limited to the specific examples described above. In a specific OFDM system, the specific implementation of the present invention can be changed accordingly. For example, in the LTE system, two columns of DMRS signals on the PUSCH can be used as two different pilot symbols adjacent in time in the present invention to perform frequency domain conversion, and then establish a receiving signal equation group. The ratio of the IQ imbalance parameter obtained by calculating on the subcarrier Find the IQ imbalance estimates α, θ. Similarly, you can also pass the Averaging over subcarriers improves accuracy.

The method for estimating IQ imbalance suitable for OFDM systems provided by the present invention uses two different pilot signals on the same subcarrier, and utilizes the characteristic that channels on adjacent OFDM symbols change less with time to offset the influence of channels. Through reasonable simplification and approximation, the amount of calculation is greatly reduced. Finally, the simulation results prove that this method can accurately estimate IQ imbalance.

The main advantages of the above estimation method are: no DC symmetric pilot structure is required, no dependence on channel estimation and data demodulation, robustness to frequency-selective channels, and the estimation algorithm has a small amount of calculation and is easy to implement.

It can be understood that the present invention also provides an IQ imbalance estimation module, which is used to implement each step in the aforementioned OFDM system IQ imbalance estimation method, so as to estimate the IQ imbalance estimation value.

Specifically, with reference to Fig. 1 and Fig. 4, the IQ imbalance estimation module includes the following units:

Frequency offset compensation unit: used to perform frequency offset compensation on the received time domain signal;

Remove cyclic prefix unit: used to remove cyclic prefix;

Frequency domain transformation unit: used to transform two different pilot symbols adjacent in time to the frequency domain;

A subcarrier extraction unit: used to extract the subcarriers occupied by the pilot symbols according to the system bandwidth;

ν/μ calculation unit: used to obtain the received signals of the two pilot signals on each subcarrier for the two pilot symbols, and then remove the coupling between the IQ imbalance parameter and the channel to obtain the IQ imbalance estimated value.

In order to improve the accuracy, a ν/μ averaging unit may also be added for averaging ν/μ on the subcarriers.

The method for estimating the in-phase/quadrature imbalance in the OFDM system provided by the present invention and its modules are described above in detail. For those skilled in the art, any obvious changes made to it without departing from the essence of the present invention will constitute an infringement of the patent right of the present invention and will bear corresponding legal responsibilities.

Claims (10)

1. the unbalanced method of estimation of inphase/orthogonal in a kind of ofdm system, it is characterised in that including Following steps：

S1：Time-domain signal to receiving carries out frequency deviation compensation；

S2：After removal Cyclic Prefix, two times upper neighbouring different frequency pilot signs are transformed into frequency Domain；

S3：The subcarrier that the frequency pilot sign takes is extracted according to system bandwidth；

S4：For described two frequency pilot signs, the described two pilot tones letter on each subcarrier is obtained Number reception signal；

S5：The coupling of removal IQ imbalance parameters and channel, asks for IQ imbalance estimates.

2. the unbalanced method of estimation of inphase/orthogonal in ofdm system as claimed in claim 1, It is characterized in that：

The coupling of IQ imbalance parameters and channel is removed in step S5, is in a subcarrier On ask for IQ imbalance parameter ratio

3. the unbalanced method of estimation of inphase/orthogonal in ofdm system as claimed in claim 2, Characterized by further comprising following steps：

Tried to achieve respectively at least two subcarriers in the subcarrier

With at least two subcarrierAverage value come ask for IQ imbalance estimate.

4. the unbalanced method of estimation of inphase/orthogonal in ofdm system as claimed in claim 2, Characterized by further comprising following steps：

Tried to achieve respectively on each subcarrier in the subcarrier

With on described each subcarrierAverage value come ask for IQ imbalance estimate.

5. the unbalanced estimation side of inphase/orthogonal in ofdm system as claimed in claim 2 or claim 3 Method, it is characterised in that：

Upper neighbouring different frequency pilot signs of described two times refer to two leading code sign STF and LTF。

6. the unbalanced method of estimation of inphase/orthogonal in ofdm system as claimed in claim 5, It is characterized in that：

Upper neighbouring different frequency pilot signs of described two times refer to two lead codes on same carrier wave Symbol STF and LTF.

7. the unbalanced estimation side of inphase/orthogonal in ofdm system as claimed in claim 2 or claim 3 Method, it is characterised in that：

Upper neighbouring different frequency pilot signs of described two times refer to two on PUSCH in LTE system Row DMRS signals.

8. inphase/orthogonal is uneven in the ofdm system as described in any one in claims 1 to 3 The method of estimation of weighing apparatus, it is characterised in that：

In the IQ imbalances estimate,

$\mathrm{\&alpha;}=real\left(\frac{\mathrm{\&nu;}}{\mathrm{\&mu;}}\right)$

$\mathrm{\&theta;}=-2imag\left(\frac{\mathrm{\&nu;}}{\mathrm{\&mu;}}\right).$

9. the unbalanced estimation module of inphase/orthogonal in a kind of ofdm system, it is characterised in that including With lower unit：

Frequency offset compensation element：For carrying out frequency deviation compensation to the time-domain signal for receiving；

Removal cyclic prefix unit：For removing Cyclic Prefix；

Frequency-domain transform unit：For two times upper neighbouring different frequency pilot signs to be transformed into frequency domain；

Subcarrier extraction unit：For extracting the son load that the frequency pilot sign takes according to system bandwidth Ripple；

ν/μ computing units：For to described two frequency pilot signs, obtaining the institute on each subcarrier Two reception signals of pilot signal are stated, the coupling of IQ imbalance parameters and channel is then removed, asked Take IQ imbalance estimates.

10. estimation module as claimed in claim 9, it is characterised in that also include：

ν/μ averaging units, for being averaged on sub-carriers to ν/μ.