JP2010272901A - Offset compensating circuit and offset compensating method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an offset compensating circuit and an offset compensating method capable of quickly and highly accurately compensating without increasing a circuit scale. <P>SOLUTION: The offset compensating circuit comprises: a DC offset estimating/compensating means that estimates the value of a DC offset included in a signal obtained by orthogonally demodulating an OFDM signal to compensate the DC offset; a frequency offset estimating/compensating means that estimates the value of a frequency offset included in the compensated signal to compensate the frequency offset; an excessively compensated component calculating means that uses both the DC offset value estimated by the DC offset estimating/compensating means and the estimated frequency offset value to calculate an excessively compensated component of a subcarrier; and an excessively compensated component removing means that removes, from the DC offset value, the excessively compensated component calculated by the excessively compensated component calculating means to use the resultant in the DC offset compensation by the DC offset estimating/compensating means. Thus, the frequncy offset estimating/compensating means compensates the frequency offset of the signal with the DC offset compensated by the DC offset estimating/compensating means after the removal of the excessively compensated component and outputs the resultant. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to an offset compensation circuit and an offset compensation method for compensating an offset of an Orthogonal Frequency Division Multiplexing (OFDM) signal.

  The OFDM scheme has a high transmission rate within a limited band and is resistant to multipath, and has recently been used for various wireless communications. For example, in IEEE 802.11a, g, which is a wireless LAN standard, an OFDM wireless signal is used.

  FIG. 10 shows an OFDM baseband signal spectrum without DC offset and frequency offset. If the OFDM signal has no DC offset and no frequency offset, the subcarriers do not interfere with each other. Further, the center frequency of the DC subcarrier is 0 Hz.

  FIG. 11 shows an OFDM baseband signal spectrum including a DC offset and a frequency offset. Since the presence of the DC offset and the frequency offset affects the demodulation process, it is necessary to compensate for the DC offset and the frequency offset.

  Here, the DC offset and frequency offset of the OFDM signal will be described. The direct conversion OFDM receiver directly demodulates the RF (high frequency) signal received by the antenna to extract the I (in-phase) and Q (quadrature) components of the OFDM baseband signal, and then performs OFDM demodulation. Do.

  However, in this direct conversion method, the frequency of the RF signal and the local oscillation signal are the same, but the level of the RF signal is not sufficient and the S / N is not good compared to the local oscillation signal. A DC offset occurs due to the wraparound of the component to the RF signal. However, in the OFDM system, the frequency intervals of all subcarriers are set so as to satisfy orthogonality, and the DC offset does not affect the demodulation process if it is originally. However, the oscillation frequency of the local oscillator does not completely coincide with the frequency of the RF signal and is slightly shifted, that is, it has a frequency offset. This frequency shift itself is a part of the frequency correction processing in the subsequent stage. However, if there is the above-described DC offset, the frequency error estimate is estimated larger or less than the actual one.

  As a conventional offset compensation method for an OFDM signal, a first method (see, for example, Patent Document 1) in which a DC offset is first compensated and then a frequency offset is compensated, and a frequency offset is compensated first and then a DC offset is compensated. Second method for compensation (see, for example, Patent Document 2), frequency offset candidate values are obtained by singular value decomposition of a system matrix, optimal frequency offset candidate values are searched, and optimal frequency offset candidates There is a third method for estimating the DC offset from the value (see, for example, Patent Document 3).

  FIG. 12 shows a flowchart of an example of a conventional offset compensation method (first method) for OFDM signals. In FIG. 12, in step S1, a DC offset is estimated from the preamble of the OFDM signal, and the estimated DC offset is compensated. Thereafter, in step S2, the frequency offset is estimated, and the estimated frequency offset is compensated.

  FIG. 13 shows a flowchart of an example of a conventional offset compensation method (second method) for OFDM signals. In FIG. 13, in step S3, the frequency offset is estimated from the preamble of the OFDM signal, and the preamble frequency offset is compensated. Thereafter, in step S4, a DC offset is estimated from the preamble compensated for the frequency offset, and in step S5, the DC offset is compensated for the OFDM signal data and the frequency offset is compensated.

  FIG. 14 shows a flowchart of an example of a conventional offset compensation method (third method) for OFDM signals. In FIG. 14, a frequency offset candidate value is obtained by singular value decomposition of the system matrix, and the optimum frequency offset candidate value is searched by multiplying the received signal by the frequency offset candidate value in step S6. A DC offset is estimated from a candidate value of a large frequency offset. Next, the DC offset estimated in step S7 is compensated, and the estimated frequency offset is compensated.

JP 2003-32216 A JP 2004-304507 A JP 2008-135888 A

  In the OFDM baseband signal spectrum of FIG. 11, the value observed as the DC offset value at the center frequency of 0 Hz includes the adjacent subcarrier component D ′. FIG. 15 shows an extracted subcarrier component D ′ adjacent thereto.

  In the first method shown in FIG. 12, the component D ′ of the adjacent subcarrier is compensated in step S1, so that interference with the adjacent subcarrier is caused as shown in FIG. There was a problem of deterioration.

  In the second method shown in FIG. 13, if the frequency offset is completely compensated in step S3, the DC offset can be accurately estimated in step S4. However, when the OFDM signal includes a DC offset and a frequency offset, if the frequency offset is estimated first, an error occurs in the estimated value of the frequency offset due to the influence of the DC offset. There has been a problem that the reception performance deteriorates due to the error of the estimated value.

  In the third method shown in FIG. 14, since the system matrix is subjected to singular value decomposition to obtain frequency offset candidate values, the circuit scale becomes large and the calculation takes time. Furthermore, it takes a long time to calculate the optimum frequency offset candidate value by multiplying the received signal by the frequency offset candidate value in step S6, and there is a problem that the real-time offset compensation operation cannot be performed.

  The present invention has been made in view of the above points, and an object thereof is to provide an offset compensation circuit and an offset compensation method for estimating and compensating a DC offset and a frequency offset with high speed and accuracy without increasing the circuit scale. .

An offset compensation circuit according to an embodiment of the present invention is an offset compensation circuit that estimates and compensates for a DC offset and a frequency offset of an OFDM signal,
DC offset estimation compensation means (32 to 34) for estimating and compensating for a DC offset value included in a signal obtained by orthogonal demodulation of the OFDM signal;
Frequency offset estimation compensation means (31, 37, 38) for estimating and compensating a frequency offset value included in the signal compensated by the DC offset estimation compensation means;
After the compensation by the frequency offset estimation compensation unit, the DC offset value estimated by the DC offset estimation compensation unit and the frequency offset value estimated by the frequency offset estimation compensation unit are used, and Overcompensation component calculation means (36) for calculating the carrier overcompensation component;
Overcompensation component removal means (35) used for DC offset compensation in the DC offset estimation compensation means by removing the overcompensation component calculated by the overcompensation component calculation means from the DC offset value estimated by the DC offset estimation compensation means. When,
Have
After removing the overcompensation component, the frequency offset estimation compensation means compensates the frequency offset of the signal compensated for the DC offset by the DC offset estimation compensation means and outputs the signal.

An offset compensation method according to an embodiment of the present invention is an offset compensation method for estimating and compensating for a DC offset and a frequency offset of an OFDM signal,
A first step (S11, S12) for estimating and compensating for a DC offset value included in a signal obtained by orthogonal demodulation of the OFDM signal;
A second step (S13, S14) for estimating and compensating for a frequency offset value included in the signal compensated in the first step;
A third step (S15) for estimating a DC offset value from the signal compensated for the frequency offset in the second step;
Using the DC offset value estimated in the third step and the frequency offset value estimated in the second step, a subcarrier overcompensation component in the first step is calculated, and is estimated in the first step. A fourth step (S16) for removing the overcompensation component from the obtained DC offset value;
A fifth step (S17) for compensating a signal obtained by orthogonally demodulating the OFDM signal with a DC offset value obtained by removing the overcompensation component in the fourth step;
And a sixth step (S18) for compensating and outputting the frequency offset value included in the signal compensated in the fifth step.

  Note that the reference numerals in the parentheses are given for ease of understanding, are merely examples, and are not limited to the illustrated modes.

  According to the present invention, it is possible to compensate for a DC offset and a frequency offset with high speed and accuracy without increasing the circuit scale.

It is a block block diagram of one Embodiment of an OFDM receiver. It is a block block diagram of one Embodiment of an offset compensation circuit. It is a block diagram of an example of an OFDM radio frame. It is a wave form diagram of a symbol. It is a figure which shows an OFDM baseband signal spectrum. It is a waveform diagram of a symbol for explaining the present invention. 6 is a flowchart of an embodiment of an offset compensation circuit operation. It is a flowchart in case offset compensation is performed in multiple times. It is a figure which shows a simulation result. It is a figure which shows the OFDM baseband signal spectrum without DC offset and frequency offset. It is a figure which shows the OFDM baseband signal spectrum containing DC offset and frequency offset. 5 is a flowchart of a conventional OFDM signal offset compensation method. 5 is a flowchart of a conventional OFDM signal offset compensation method. 5 is a flowchart of a conventional OFDM signal offset compensation method. It is a figure which shows the OFDM baseband signal spectrum for demonstrating the interference with respect to an adjacent subcarrier.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

<OFDM receiver>
FIG. 1 shows a block diagram of an embodiment of an OFDM receiver to which an offset compensation circuit of the present invention is applied. In FIG. 1, an RF signal received by an antenna 10 is amplified by a low noise amplifier (LNA) 11 and then supplied to mixers 12 and 13, respectively.

  The local oscillation signal output from the local oscillator 14 is directly supplied to the mixer 12, and phase-shifted by π / 2 by the phase shifter 15 and supplied to the mixer 13. The mixers 12 and 13, the local oscillator 14, and the phase shifter 15 constitute a direct conversion type quadrature demodulator. The mixer 12 multiplies the RF signal and the phase-shifted local oscillation signal to output an I (in-phase) signal of the OFDM baseband signal. The mixer 13 multiplies the RF signal and the local oscillation signal and outputs a Q (orthogonal) signal of the OFDM baseband signal.

  The I signal output from the mixer 12 is amplified by an AGC (automatic gain control) amplifier 17, then unnecessary high frequency components are removed by a low-pass filter 18, digitized by an A / D converter 19, and sent to a digital compensation / demodulation unit 20. Supplied. The Q signal output from the mixer 13 is amplified by an AGC (automatic gain control) amplifier 21, then unnecessary high frequency components are removed by a low-pass filter 22, and digitized by an A / D converter 23 to be digitally compensated / demodulated. 20 is supplied.

  The digital compensation / demodulation unit 20 estimates and compensates DC offset and frequency offset from the I and Q signals, and then performs demodulation. That is, offset compensation is executed in the digital compensation / demodulation unit 20.

<Offset compensation circuit>
FIG. 2 shows a block diagram of an embodiment of the offset compensation circuit of the present invention. In FIG. 2, the I and Q signals output from the A / D converters 19 and 23 are supplied to the multiplier 31 after passing through various blocks. When a frequency offset value Δf is supplied from a frequency offset estimation unit 38, which will be described later, the multiplier 31 performs carrier recovery from the frequency offset Δf, performs frequency offset compensation by performing complex multiplication on each of the I and Q signals, and outputs the result. To do. The I and Q signals output from the multiplier 31 are supplied to a subtracter 32 and a DC offset estimation unit 33.

Here, FIG. 3 shows a configuration diagram of an embodiment of an OFDM radio frame in the standard IEEE 802.11a. The OFDM radio frame has a PLCP preamble, a SIGNAL that is an OFDM header, and data in the data area. The PLCP preamble has a short training symbol that is a fixed pattern using short symbols and a long training symbol that is a fixed pattern using long symbols. Short training symbol has a symbol _S 1 _{~S 10.} The short training symbol and the long training symbol are used for synchronization, AGC, and offset estimation.

  In the present embodiment, the period of one symbol is sampled n (n is a numerical value such as 16) times, and the I and Q signals are supplied from the A / D converters 19 and 23, respectively.

The DC offset estimation unit 33 obtains a time average value of n samples of one symbol period for, for example, the symbol S ₇ of the short training symbols of the I and Q signals, and uses this time average value as the DC offset estimation value of each of the I and Q signals. And

  Specifically, assuming that a received signal (training symbol) without a DC offset is s (t) and a DC offset is D, a received signal r (t) having a DC offset is expressed as follows.

r (t) = s (t) + D
Here, since s (t), which is a training symbol, is a periodic signal having no DC component, the average value in one symbol section is zero. Therefore, when r (t) is averaged over one symbol interval (N samples), the DC offset can be obtained by the following equation.

なお、ＤＣオフセット及び周波数オフセットがない場合には、各シンボルのＩ，Ｑ信号は図４（Ａ）に示すように１シンボル周期の時間平均値が０となる。しかし、ＤＣオフセット及び周波数オフセットがある場合には各シンボルのＩ，Ｑ信号は１シンボル周期の時間平均値は０以外の値となる。ＤＣオフセット推定部３３はＩ，Ｑ信号それぞれで推定したＤＣオフセット値Ｄ_０を保持し、セレクタ３４及び減算器３５に供給する
ところで、制御部４０はＯＦＤＭ無線フレームの受信開始からＡＧＣアンプ１７，２１やその他回路の動作が安定した時点（例えばショートトレーニングシンボルのシンボルＳ_７のタイミング、なお、例えばＳ_５やＳ_６等の場合もある）でＤＣオフセット推定部３３にＤＣオフセット推定動作の開始を指示している。また、制御部４０はＤＣオフセット推定部３３の出力するＤＣオフセット値Ｄ_０をセレクタ３４に選択させて減算器３２に供給する。

When there is no DC offset and no frequency offset, the I and Q signals of each symbol have a time average value of 0 for one symbol period as shown in FIG. However, when there is a DC offset and a frequency offset, the I and Q signals of each symbol have values other than 0 for the time average value of one symbol period. The DC offset estimator 33 holds the DC offset value D ₀ estimated for each of the I and Q signals and supplies it to the selector 34 and the subtractor 35. The controller 40 starts the reception of the OFDM radio frame and starts the AGC amplifiers 17 and 21. and (timing of the symbol S ₇ for example short training symbols, noted, for example, in some cases, such as S ₅ and S ₆₎ operation stable point other circuits instructing start of the DC offset estimation operation in DC offset estimator 33 is doing. Further, the control unit 40 causes the selector 34 to select the DC offset value D ₀ output from the DC offset estimation unit 33 and supplies it to the subtracter 32.

The subtracter 32 subtracts the DC offset value D ₀ for each of the I and Q signals output from the multiplier 31 and supplies the result to the multiplier 37 and the frequency offset estimation unit 38.

The frequency offset estimator 38 estimates the frequency offset using the 2n-sample I and Q signals that are the next two symbols (for example, the symbols S ₈ and S ₉ ) after the symbol (for example, the symbol S ₇ ) used for the DC offset estimation. Do. The frequency offset estimator 38 performs a complex correlation operation with the I and Q signals of n samples (one symbol period) before, and calculates a phase change amount with the I and Q signals before n samples, thereby calculating the frequency. Estimate the offset.

  Specifically, assuming that a received signal (training symbol) without a frequency offset is s (t), the training symbol is a periodic signal, and the same data is repeated for each symbol period Δt, and therefore is expressed by the following equation.

s (t) = s (t−Δt)
A signal r (t) having a frequency offset Δf is expressed by the following equation.

r (t) = s (t) exp (j2πΔft)
Since r (t) is a periodic signal, the following equation can be obtained by taking the autocorrelation of one symbol section. Note that s ^* represents a complex number conjugate with s.

r (t) s ^* (t−Δt) = | s (t) | ² exp (j2πΔfΔt)
The phase Δω in this equation is expressed by the following equation.

Δω = arg | s (t) | ² exp (j2πΔfΔt)
Therefore, the frequency offset Δf can be expressed as follows.

Δf = Δω / 2πΔt
When there is a frequency offset, the I and Q signals of each symbol have an apparent one symbol period changed as shown in FIG. 4B, and the correlation between the current sample and the sample one symbol period before is correlated. Lower. The frequency offset estimation unit 38 estimates the frequency offset value Δf and supplies it to the overcompensation component calculation unit 36 and the switch 39.

The control unit 40 switches the switch 39 so as to supply the multiplier 31 with the carrier regenerated from the frequency offset value Δf at the next symbol (for example, the symbol S ₁₀ ) for which the frequency offset has been estimated. As a result, the multiplier 31 outputs I and Q signals compensated for the frequency offset.

Here, as described above, in the OFDM signal including the DC offset and the frequency offset, as shown in FIGS. 11 and 15, the value D ₀ observed as the DC offset value at the center frequency 0 Hz is adjacent to the value D _0. A subcarrier component D ′ is also included. If the true value of the DC offset is D, the relationship of equation (1) is established.

D ₀ = D + D ′ (1)
Therefore, as shown in FIG. 5, the OFDM baseband signal spectrum compensated for the DC offset and the frequency offset is DC offset compensated up to the adjacent subcarrier component D ′, and the adjacent subcarrier is interfered (overcompensated). It is in the state.

The DC offset estimator 33 obtains a time average value of n samples of one symbol period in the next symbol (for example, symbol S ₁₀ ) subjected to frequency offset estimation, and uses this time average value as the DC offset of each of the I and Q signals. The estimated value is d ′. As shown in FIG. 5, the DC offset estimated value d ′ in this case is a component caused by a component D ′ that is regarded as an offset of an adjacent subcarrier and is overcompensated. The control unit 40 controls the overcompensation component calculation unit 36 to supply the DC offset estimation values d ′ of the I and Q signals output from the DC offset estimation unit 33.

The overcompensation component calculator 36 calculates an overcompensation component D ′, which is a component of the adjacent subcarrier, using the equation (2) from the DC offset estimated values d ′ of the I and Q signals. Here, f ₀ is a subcarrier interval.

ここで、図６（Ａ）に示すように、時刻０から時刻Ｔまで一定の複素数Ａを持つ矩形波を（３）式で表す。

Here, as shown in FIG. 6A, a rectangular wave having a constant complex number A from time 0 to time T is expressed by equation (3).

上記の矩形波をフーリエ変換して得られる周波数スペクトラムは（４）式で表すことができる。ここで、Ｔ＝１／ｆ_０とすると、周波数スペクトラムは図６（Ｂ）に示すようになる。

The frequency spectrum obtained by Fourier transforming the above rectangular wave can be expressed by equation (4). Here, when T = 1 / f ₀ , the frequency spectrum is as shown in FIG.

一方、ＯＦＤＭ信号に加わった複素数のＤＣオフセットについても、（３）式の矩形波と同様に考えることができる。したがって、ＤＣオフセットの周波数スペクトラムは図６（Ｃ）に示すようになる。

On the other hand, the complex DC offset added to the OFDM signal can also be considered in the same manner as the rectangular wave of equation (3). Therefore, the frequency spectrum of the DC offset is as shown in FIG.

Next, the equation (4) is replaced with a DC offset calculation equation. Therefore, when T = 1 / f ₀ , AT = D ′, and f = Δf are substituted into the equation (4), the equation (5) is obtained.

（５）式をＤ’について解くと（２）式が得られる。なお、この（２）式を使用してＤ’を求めることができる条件としては、Ｄ’をｄ’との比を用いて計算するため、ｄ’が０以外の場合である。しかし、ｄ’が０となる場合は、周波数オフセットΔｆが、Δｆ＝±ｆ_０，±２ｆ_０，±３ｆ_０，…となる場合であり、このようなΔｆ＝±ｆ_０，±２ｆ_０，±３ｆ_０，…となる場合は規格ＩＥＥＥ８０２．１１ａ，ｇにおいて補償が必要とされる周波数オフセットの範囲外である。

Solving equation (5) for D ′ yields equation (2). In addition, as a condition for obtaining D ′ by using this equation (2), since D ′ is calculated using a ratio with d ′, d ′ is other than 0. However, when d ′ is 0, the frequency offset Δf is Δf = ± f ₀ , ± 2f ₀ , ± 3f ₀ ,..., And such Δf = ± f ₀ , ± 2f ₀ , When ± 3f ₀ ,... Is outside the range of the frequency offset that requires compensation in the standard IEEE 802.11a, g.

Returning to FIG. 2, the overcompensation component calculator 36 supplies the overcompensation component D ′ of each of the I and Q signals to the subtractor 35. The subtracter 35 subtracts the overcompensation component D ′ from the DC offset value D ₀ held and output by the DC offset estimator 33 and supplies it to the selector 34. Selector 34 under the control of the controller 40, for example, supplied to the subtracter 32 selects the output of the subtracter 35 is at the reception timing of the symbol S ₁₀ and later. The switch 39 supplies the carrier 37 reproduced from the frequency offset value Δf to the multiplier 37 under the control of the control unit 40.

  As a result, the subtractor 32 subtracts the true value D of the DC offset from each of the I and Q signals that have passed through the multiplier 31, performs DC offset compensation, and supplies the result to the multiplier 37 and the frequency offset estimation unit 38. This DC offset compensation is not affected by the overcompensation component D '. The multiplier 37 performs frequency offset compensation by performing complex multiplication of the carrier reproduced from the frequency offset Δf on each of the I and Q signals, and outputs the result.

<Flow chart of offset compensation circuit operation>
FIG. 7 shows a flowchart of one embodiment of the offset compensation circuit operation. In FIG. 7, the DC offset estimation unit 33 estimates the DC offset value D ₀ in step S11. In step S12, the subtractor 32 performs DC offset compensation of the I and Q signals. Next, in step S13, the frequency offset estimation unit 38 estimates the frequency offset value Δf. In step S14, the multiplier 31 performs frequency offset compensation of the I and Q signals.

Thereafter, in step S15, the DC offset estimation unit 33 estimates the DC offset estimated value d ′. Next, in step S <b> 16, the overcompensation component calculation unit 36 calculates the overcompensation component D ′ using the DC offset estimation value d ′ supplied from the DC offset estimation unit 33. The subtractor 35 subtracts the overcompensation component D ′ from the DC offset value D ₀ to obtain the DC offset D and supplies it to the subtractor 32.

  Thereafter, in step S17, the subtractor 32 performs DC offset compensation of the I and Q signals using the DC offset D. Next, in step S18, the multiplier 31 performs frequency offset compensation of the I and Q signals and outputs them. After executing this step S17, demodulation of the OFDM received signal is started.

  By the way, after executing step S12 described above, steps S13 to S17 may be executed a plurality of times, then step S18 may be executed, and then demodulation of the OFDM received signal may be started. FIG. 8 shows a flowchart in this case. In FIG. 8, steps S23 to S27 corresponding to steps S13 to S17 are repeated m (m is an integer) times. That is, in step S23, the frequency offset estimation unit 38 estimates the frequency offset value Δf. In step S24, the multiplier 31 performs frequency offset compensation of the I and Q signals.

Thereafter, in step S25, the DC offset estimation unit 33 estimates the DC offset estimated value d ′. Next, in step S26, the overcompensation component calculation unit 36 calculates the overcompensation component D ′ using the DC offset estimation value d ′ supplied from the DC offset estimation unit 33. The subtractor 35 subtracts the overcompensation component D ′ from the DC offset value D ₀ to obtain the DC offset D and supplies it to the subtractor 32. Thereafter, in step S27, the subtractor 32 performs DC offset compensation of the I and Q signals using the DC offset D. Finally, in step S28 corresponding to step S18, the multiplier 31 performs frequency offset compensation of the I and Q signals and outputs them. After executing this step S28, demodulation of the OFDM received signal is started.

  As described above, by executing a part of the processing of FIG. 7 a plurality of times as shown in FIG. 8, the offset estimation accuracy can be further increased.

<Simulation results>
FIG. 9 shows an EVM (Error Vector Magnitude) with respect to the frequency offset value when the DC offset and the frequency offset are compensated. This is a simulation result based on the standard IEEE 802.11a. Here, the solid line indicates the result of compensation according to the prior art, and the one-dot chain line with a black circle indicates the result of compensation according to the present embodiment.

  In the standard IEEE802.11a, the maximum frequency used is 5.805 GHz ± 232.2 kHz. From this figure, it can be seen that, at a frequency offset of -232.2 kHz, the present embodiment shows an improvement of about 25 dB compared to the prior art, and this embodiment suppresses the degradation of the EVM with respect to the frequency offset.

  In this embodiment, the time offset is used for estimating the DC offset, the complex correlation operation is used for estimating the frequency offset, and the calculation of the equation (2) is added to compensate for the DC offset and the frequency offset with high accuracy. It can be carried out. In addition, since the multiplier 31 and the overcompensation component calculator 36 may be added as compared with the prior art (Patent Document 1), it is not necessary to add a particularly large block. For this reason, an increase in circuit scale can be suppressed. The time required for the estimation of the DC offset, the estimation of the frequency offset, and the calculation of equation (2) is four symbol periods of the short training symbol, and the DC offset and the frequency offset can be compensated at high speed.

  In this embodiment, IEEE802.11a, g of the wireless LAN standard is described as an example, but the present invention is not limited to this. The present invention can be applied to the OFDM system adopted in the entire wireless LAN standard IEEE 802.11. Further, the present invention can be applied not only to a wireless LAN but also to general wireless communication using OFDM.

DESCRIPTION OF SYMBOLS 10 Antenna 11 Low noise amplifier 12, 13 Mixer 14 Local oscillator 15 Phase shifter 17, 21 AGC amplifier 18, 22 Low-pass filter 19, 23 A / D converter 20 Digital compensation / demodulation part 31, 37 Multiplier 32, 35 Subtractor 33 DC offset estimation unit 34 selector 36 overcompensation component calculation unit 38 frequency offset estimation unit 40 control unit

Claims (4)

An offset compensation circuit that estimates and compensates for a DC offset and a frequency offset of an OFDM signal,
DC offset estimation compensation means for estimating and compensating for a DC offset value included in a signal obtained by orthogonal demodulation of the OFDM signal;
Frequency offset estimation compensation means for estimating and compensating for a frequency offset value included in the signal compensated by the DC offset estimation compensation means;
After the compensation by the frequency offset estimation compensation unit, the DC offset value estimated by the DC offset estimation compensation unit and the frequency offset value estimated by the frequency offset estimation compensation unit are used, and An overcompensation component calculating means for calculating an overcompensation component of the carrier;
An overcompensation component removing unit used for DC offset compensation in the DC offset estimation compensation unit by removing the overcompensation component calculated by the overcompensation component calculation unit from the DC offset value estimated by the DC offset estimation compensation unit;
Have
An offset compensation circuit, wherein after the overcompensation component is removed, the frequency offset is compensated by the frequency offset estimation compensation means for the signal compensated for the DC offset by the DC offset estimation compensation means. The offset compensation circuit according to claim 1.
The overcompensation component calculation means sets the frequency offset value as Δf, the DC offset estimation value as d ′, the overcompensation component as D ′, and the subcarrier interval as f ₀ .

An offset compensation circuit which calculates the overcompensation component from An offset compensation method for estimating and compensating for a DC offset and a frequency offset of an OFDM signal,
A first step of estimating and compensating for a DC offset value included in a signal obtained by orthogonal demodulation of the OFDM signal;
A second step of estimating and compensating for a frequency offset value included in the signal compensated in the first step;
A third step of estimating a DC offset value from the signal compensated for the frequency offset in the second step;
Using the DC offset value estimated in the third step and the frequency offset value estimated in the second step, a subcarrier overcompensation component in the first step is calculated, and is estimated in the first step. A fourth step of removing the overcompensation component from the obtained DC offset value;
A fifth step of compensating a signal obtained by orthogonally demodulating the OFDM signal with a DC offset value obtained by removing the overcompensation component in the fourth step;
A sixth step of compensating and outputting a frequency offset value included in the signal compensated in the fifth step;
An offset compensation method comprising: The offset compensation method according to claim 3, wherein
An offset compensation method, wherein the second to fifth steps are repeated a plurality of times.

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