JP5626688B2 - Wireless receiver - Google Patents

Description

  The present invention relates to a radio receiving apparatus that receives a radio signal transmitted by an orthogonal frequency division multiplexing (hereinafter referred to as OFDM (Orthogonal Frequency Division Multiplexing)), and in particular, the radio signal level is reduced due to multipath fading. The present invention relates to a radio receiving apparatus and a channel equalization method thereof that can compensate for error rate characteristic degradation caused by.

  In recent years, the OFDM scheme has been widely used in mobile communications because of its high frequency utilization efficiency, resistance to frequency selective fading due to multipath, and resistance to intersymbol interference due to delayed waves. However, it cannot escape from the deterioration caused by the decrease in received signal strength due to fading. Although maximum ratio combining diversity is effective in reducing performance degradation due to multipath, in order to obtain the maximum effect of spatial diversity, an RF receiver circuit, an A / D converter, a discrete Fourier transform (DFT (Discrete Fourier Transform) Transform)) Circuits are required for each antenna, and the increase in circuit scale is inevitable.

Japanese Patent Laid-Open No. 2004-096140. JP 2009-141740 A JP 2007-228248 A.

K. Gyoda et al. "Design of electronically steerable passive array radiator (ESPAR) antennas", Proceedings of Antennas and Propagation Society International Symposium, Salt Lake City, Utah, U.S.A., Vol.2, pp.922-925, July 2000. C. Plapous et al., "Reactance domain MUSIC algorithm for electronically steerable parasitic array radiator", IEEE Transactions on Antennas and Propagation, Vol.52, Issue 12, pp.3257-3264, December 2004. E. Taillefer et al., "Reactance-domain ESPRIT algorithm for a hexagonally shaped seven-element ESPAR antenna", IEEE Transactions on Antennas and Propagation, Vol.53, Issues 11, pp.3486-3495, November 2005. J. G. Proakis et al., "Digital Communications", Fourth Edition, McGraw-Hill, Singapore, 2001.

  FIG. 17 is a block diagram showing a configuration of an ESPAR antenna (Electronically Steerable Passive Array Radiator) antenna (hereinafter referred to as ESPAR antenna, for example, see Non-Patent Document 1) 60 according to the prior art. In FIG. 17, the ESPAR antenna 60 includes one feeding element 61, for example, six parasitic elements 62 to 67, and a ground conductor 68, and an electric length is provided at one end of each of the parasitic elements 62 to 67, respectively. Are connected to variable capacitance diodes 72 to 77 for changing the above. The length of the feeding element 61 is a quarter wavelength, and by changing the electrical length of each of the parasitic elements 62 to 67, the parasitic elements 72 to 77 located before and after the feeding element 61 with respect to the arrival direction of the radio wave. Can be made to have directivity like the Yagi-Uta antenna. The antenna directivity can be electrically changed freely by changing the DC bias voltage applied to the variable capacitance diodes 72 to 77 by the controller 70. In the ESPAR antenna illustrated in FIG. 17, directivity can be controlled in an arbitrary direction of 360 degrees in the horizontal plane. The ESPAR antenna according to the related art obtains a space diversity effect by appropriately controlling the directivity of the antenna by changing the bias voltage applied to the variable capacitance diodes 72 to 77 in accordance with the characteristics of the propagation path.

  In contrast to the above problems, the selection diversity using the ESPAR antenna is advantageous in terms of circuit scale, but it takes time to converge the calculation for directivity control. Directional control cannot follow. In addition, even in the case of frequency selective fading due to a multipath environment, there are many situations in which any subcarrier always falls even if the directivity is switched, and the effect is low.

  In the wireless receiver using the ESPAR antenna, a bias voltage necessary for realizing appropriate directivity for obtaining the spatial diversity effect can be calculated by an algorithm such as MUSIC or ESPRIT (for example, see Non-Patent Documents 2 and 3). .) However, since the information obtained by one reception is less than the degree of freedom of the antenna element, the convergence time tends to be long. If the propagation path changes rapidly due to high-speed movement, the tracking performance becomes a problem. Even in the ESPAR antenna in which only one parasitic element 62 is attached to the same feeding element 61 as used in the proposed method with a low degree of freedom of antenna elements, the optimum direction with a strong received signal strength is always selected. The state cannot be maintained. This is because a single RF input basically results in a selective switching diversity operation. In this operation, unless the directivity is switched in principle, it is not known whether the result will be better or worse than before. .

  FIG. 18 is a timing chart showing the operation by the diversity method according to the prior art. In FIG. 18, the broken line is a threshold value indicating the received signal strength for switching the antenna element. If control is performed so that the antenna element is switched below the threshold value, the antenna is switched at a timing indicated by a triangular arrow. For example, as shown in the part surrounded by the broken line, it is understood that the antenna with the lower reception strength is selected even though the reception strength of the other antenna is stronger, and the antenna that is not optimal may be selected continuously. . In addition, when applied to an OFDM system widely used in mobile radio communications, this method is not effective for frequency selective fading. This is because the optimum direction for controlling the directivity, that is, the weighting factor for directivity control differs depending on the frequency, so that the optimum weighting factor for all subcarriers of OFDM cannot be uniquely determined.

  FIG. 19 and FIG. 20 are spectrum diagrams of OFDM signals when they are received with two different directivities by the OFDM radio receiving apparatus according to the prior art. As shown in FIGS. 19 and 20, it can be seen that there are subcarriers that fall in either direction, and it is difficult to select an optimum direction for all subcarriers.

  FIG. 21 is a block diagram showing a configuration of an OFDM radio receiving apparatus according to the prior art disclosed in Patent Document 1. In FIG. In FIG. 21, the ESPAR antenna 10 includes a feed element 11 and a parasitic element 12 loaded with a variable capacitance diode 13, and is applied to the variable capacitance diode 13 from a controller 55 via a high frequency blocking inductor 14. The directivity of the ESPAR antenna 10 is changed by changing the DC bias voltage (hereinafter referred to as control voltage) Vc. Here, the radio signal received by the feed element 11 is input to the RF front end circuit 51, and then subjected to processing such as low noise amplification, frequency conversion to a baseband signal, A / D conversion, and the like, and then an OFDM base signal. Input to the band demodulator 50. The OFDM baseband demodulator 50 Fourier-transforms an input digital signal into a plurality of subcarriers using an FFT 52, and then performs error correction using an error correction decoder 53 to obtain a digital demodulated signal. In Patent Document 1, in particular, the received signal quality detection circuit 54 detects received signal quality such as RSSI based on the signal after Fourier transform, and the controller 55 changes the control voltage Vc so that, for example, RSSI increases. Thus, it is possible to compensate for the deterioration of the error rate characteristics caused by the decrease in the received signal level due to multipath fading.

  However, in the OFDM radio receiving apparatus according to the prior art of FIG. 21, the directivity is feedback controlled based on the measured reception quality. However, in a fading communication path where the control delay is large and the propagation path characteristics fluctuate at high speed, In packet transmission such as a wireless LAN system that has a problem in followability, data transmission is completed with a very short packet, and therefore, delay due to control is not allowed. Also, in a propagation path with a large multipath delay wave and frequency selectivity, even if the time variation is low and control delay does not become a problem, the diversity effect is sufficient if not combined with an error correction code. There was a problem that it could not be obtained.

  The object of the present invention is to solve the above-described problems, and in an OFDM radio receiving apparatus, antenna directivity control can follow a received signal in a fast fading environment, and radio that can reliably equalize intersubcarrier interference. To provide a receiving apparatus and a channel equalization method thereof.

According to a first aspect of the present invention, there is provided a radio receiving apparatus that receives radio signals of an orthogonal frequency division multiplexing system including a plurality of subcarrier signals juxtaposed at predetermined subcarrier frequency intervals.
An array antenna for receiving the radio signal;
Directivity changing means for changing the directivity of the array antenna at a control frequency obtained by multiplying the subcarrier frequency interval by a value obtained by dividing the first natural number by the second natural number;
After Fourier transforming the radio signal received by the array antenna with the above directivity to a plurality of subcarrier signals, channel equalization is performed for each subcarrier signal so that intersubcarrier interference is substantially eliminated. And demodulating means for demodulating the received radio signal.

  In the radio receiving apparatus, the directivity changing unit obtains the subcarrier frequency interval based on a radio signal received by the array antenna, and sets a first natural number to a second natural number with respect to the subcarrier frequency interval. The directivity of the array antenna is changed so as to be frequency-synchronized with a control frequency obtained by multiplying by a value obtained by dividing by.

In the wireless receiver, the array antenna includes a feeding element, at least one parasitic element provided so as to be electromagnetically coupled to the feeding element, and a variable reactance loaded on the parasitic element. With elements,
The directivity changing means changes the directivity of the array antenna by changing a reactance value of the variable reactance element.

Furthermore, in the above wireless receiver, the array antenna includes a plurality of antenna elements provided to be separated from each other by a predetermined distance,
The directivity changing means changes the directivity of the array antenna by shifting the phase of each radio signal received by the plurality of antenna elements relative to each other.

A channel equalization method for a radio reception apparatus according to a second aspect of the invention includes a channel for a radio reception apparatus that receives radio signals of an orthogonal frequency division multiplexing system including a plurality of subcarrier signals juxtaposed at predetermined subcarrier frequency intervals. In the process
The wireless receiving device includes an array antenna that receives the wireless signal,
In the channel equalization method, the directivity of the array antenna is changed while the directivity of the array antenna is changed at a control frequency obtained by multiplying the subcarrier frequency interval by a value obtained by dividing the first natural number by the second natural number. After the radio signal received by the array antenna with varying characteristics is Fourier transformed into a plurality of subcarrier signals, the above reception is performed by channel equalization for each subcarrier signal so that intersubcarrier interference is substantially eliminated. And an equalizing step for demodulating the received radio signal.

  In the channel equalization method of the radio reception apparatus, the equalization step obtains the subcarrier frequency interval based on a radio signal received by the array antenna, and calculates a first natural number with respect to the subcarrier frequency interval. The directivity of the array antenna is changed so as to be frequency-synchronized with a control frequency obtained by multiplying by a value obtained by dividing by a second natural number.

In the channel equalization method of the wireless receiver, the array antenna includes a feeding element, at least one parasitic element provided to be electromagnetically coupled to the feeding element, and the parasitic element. A variable reactance element loaded,
In the equalizing step, the directivity of the array antenna is changed by changing a reactance value of the variable reactance element.

Furthermore, in the channel equalization method of the wireless reception device, the array antenna includes a plurality of antenna elements provided to be separated from each other by a predetermined distance,
The equalizing step is characterized in that the directivity of the array antenna is changed by shifting the phase of each radio signal received by the plurality of antenna elements relative to each other.

  According to the radio receiving apparatus and the channel equalization method thereof according to the present invention, the array antenna at a control frequency obtained by multiplying the subcarrier frequency interval by a value obtained by dividing the first natural number by the second natural number. For each subcarrier signal, the inter-subcarrier interference is substantially eliminated after Fourier transform of the radio signal received by the array antenna with the above-described directivity changing into a plurality of subcarrier signals while changing the directivity of the subcarrier. The received radio signal is demodulated by channel equalization. Therefore, since feedback control is not performed, the problem of control delay in the above-described directivity diversity does not occur. Moreover, the diversity effect in a frequency selective propagation path can be obtained effectively. Therefore, in the OFDM radio receiving apparatus, it is possible to realize the same effect as the state in which the antenna directivity control can sufficiently follow the change in the received signal under the high-speed fading environment, and to reliably equalize the inter-subcarrier interference. Can do.

It is a block diagram which shows the structure of the OFDM radio | wireless receiver which concerns on the 1st Embodiment of this invention. It is a block diagram which shows the structure of the OFDM radio | wireless receiver which concerns on the modification of the 1st Embodiment of this invention. It is a block diagram which shows the structure of the OFDM radio | wireless receiver which concerns on the 2nd Embodiment of this invention. It is a block diagram which shows the structure of the OFDM radio | wireless receiver which concerns on the modification of the 2nd Embodiment of this invention. It is a figure which shows the spectrum of the radio reception signal received with the OFDM radio reception apparatus of FIG. 2 is a timing chart showing a relationship between an OFDM symbol and a control signal in the OFDM wireless reception device of FIG. 1. When a cyclic prefix (CP) signal or a guard interval (GI) is inserted in the OFDM wireless receiver of FIG. 1 (when CP or GI has a time length of 1/4 of one symbol). FIG. 5 is a timing chart showing the relationship between an OFDM symbol of control signal frequency fcont = subcarrier frequency interval fsp and four types of equalization weights (E1 to E4) and a control signal. When a cyclic prefix (CP) signal or a guard interval (GI) is inserted in the OFDM wireless receiver of FIG. 1 (when CP or GI has a time length of 1/4 of one symbol). The control signal frequency fcont = subcarrier frequency interval fsp × 4 when one type of equalization weight is used) is a timing chart showing the relationship between the OFDM symbol and the control signal. When a cyclic prefix (CP) signal or a guard interval (GI) is inserted in the OFDM wireless receiver of FIG. 1 (when CP or GI has a time length of 1/8 of one symbol). The control signal frequency fcont = subcarrier frequency interval fsp × 4 when two types of equalization weights are used) is a timing chart showing the relationship between the OFDM symbol and the control signal. When a cyclic prefix (CP) signal or a guard interval (GI) is inserted in the OFDM wireless receiver of FIG. 1 (when CP or GI has the same time length as one symbol, It is a timing chart which shows the relationship between the OFDM symbol and control signal of control signal frequency fcont = subcarrier frequency interval fsp × 1/2 when one type of equalization weight is used). FIG. 2 is a spectrum diagram of a first example for explaining equalization of inter-carrier interference in the OFDM radio reception apparatus of FIG. 1. FIG. 6 is a spectrum diagram of a second example for explaining equalization of inter-carrier interference in the OFDM wireless reception device of FIG. 1. FIG. 6 is a spectrum diagram of a third example for explaining equalization of inter-carrier interference in the OFDM wireless reception device of FIG. 1. FIG. 10 is a spectrum diagram of a fourth example for explaining equalization of inter-carrier interference in the OFDM wireless reception device of FIG. 1. FIG. 10 is a spectrum diagram of a fifth example for explaining equalization of inter-carrier interference in the OFDM wireless reception device of FIG. 1. FIG. 10 is a spectrum diagram of a sixth example for explaining equalization of inter-carrier interference in the OFDM radio receiving apparatus of FIG. 1. 2 is a table showing specifications at the time of simulation execution of the OFDM wireless reception device of FIG. FIG. 3 is a graph showing simulation results of the OFDM wireless reception apparatus of FIG. 1, showing bit error rate BER with respect to ratio E _b / E ₀ of transmission power and noise power per symbol in additive white noise (AWGN) and Rayleigh fading; is there. It is a block diagram which shows the structure of the ESPAR antenna which concerns on a prior art. It is a timing chart which shows the operation | movement by the diversity system based on a prior art. It is a spectrum figure of the OFDM signal when it receives with the 1st directivity by the OFDM wireless receiver which concerns on a prior art. It is a spectrum figure of an OFDM signal when it receives with the 2nd directivity by the OFDM wireless receiver concerning a prior art. It is a block diagram which shows the structure of the OFDM radio | wireless receiver which concerns on a prior art.

  Hereinafter, embodiments according to the present invention will be described with reference to the drawings. In addition, in each following embodiment, the same code | symbol is attached | subjected about the same component.

  FIG. 1 is a block diagram showing a configuration of an OFDM radio receiving apparatus according to the first embodiment of the present invention. The OFDM radio receiving apparatus according to the present embodiment uses the maximum ratio combining diversity method using the ESPAR antenna 10 with a single RF input, which solves the above-described problems, to achieve directivity in frequency synchronization with OFDM signal symbols. By performing channel equalization by the channel equalizer 24 while changing, the channel estimation and inter-subcarrier interference are reduced and equalized.

  In FIG. 1, a received signal is input to a frequency converter 17 via a low noise amplifier (LNA) 16 received by a feed element 11 of an ESPAR antenna 10 and converted into a baseband signal. The baseband signal is converted into a digital signal by the A / D converter 18 and then input to the automatic frequency controller (AFC) 21 of the OFDM baseband demodulator 20.

  The ESPAR antenna 10 includes a feed element 11 and a parasitic element 12 that are separated by, for example, a half wavelength so as to be electromagnetically coupled to each other, and a variable capacitance diode that changes an electrical length at one end of the parasitic element 12. 13 is connected, and the control voltage Vc from the control voltage generator 15 is applied to the variable capacitance diode 13 via the high frequency blocking inductor 14. As the control voltage Vc changes, the capacitance of the variable capacitance diode 13 changes and the electrical length of the parasitic element 12 changes. Thereby, the parasitic element 12 operates as a director or a reflector and changes the directivity of the ESPAR antenna 10. Here, the number of parasitic elements 12 may be plural, and a controller that generates a bias voltage to change directivity may be provided. In the ESPAR antenna 10 illustrated in FIG. 1, directivity in directions of 0 degrees and 180 degrees in a horizontal plane and non-directivity can be controlled.

  The OFDM baseband demodulator 20 includes an automatic frequency controller (AFC) 21, a guard interval (hereinafter referred to as GI) remover 22, a discrete Fourier transformer 23, a channel equalizer 24, and a phase tracking circuit 25. And a subcarrier demodulator 26, a deinterleaver 27, and an error correction decoder 28. The automatic frequency controller 21 detects the data symbol length of the received signal from the input digital baseband signal, determines the subcarrier frequency interval fsp from this, and uses this to determine the antenna directivity change frequency (control signal frequency fcont). ) To synchronize the frequency. Here, preferably, fcont = fsp. The relationship between the control signal frequency fcont and the subcarrier frequency interval fsp will be described later in detail.

  By adding, as a GI, a cyclic prefix (CP) signal in which the signal component in the effective symbol is partially duplicated on the OFDM terminal symbol as a GI, a delay wave generated in the multipath transmission path and The intersymbol interference caused by the interference is reduced. Therefore, the data symbol length is the length of the data portion that becomes the input of the discrete Fourier transformer 23 obtained by removing the cyclic prefix (CP) or GI from the symbol of the OFDM signal. The synchronization method considering CP or GI will be described later in detail. In this embodiment, the OFDM signal symbol and the control signal are frequency-synchronized, but may be phase-synchronized (completely synchronized) using a PLL circuit. This will be described in detail later.

  The GI remover 22 removes GI from the digital baseband signal after automatic frequency control, and the discrete Fourier transformer 23 performs DFT processing on the digital baseband signal after GI removal to a signal for each subcarrier of OFDM. Next, the channel equalizer 24 performs propagation path estimation based on the preamble signal and interference equalization processing between subcarriers for each subcarrier based on each subcarrier signal after DFT processing, as will be described in detail later. . After phase tracking circuit 25 performs phase tracking processing on each channel-equalized subcarrier signal, subcarrier demodulator 26 performs demodulation processing for each subcarrier signal, and deinterleaver 27 performs each subcarrier signal. A predetermined deinterleaver process is performed based on the above. Further, the error correction decoder 28 performs an error correction decoding process on the deinterleaved signal using, for example, a Viterbi decoding method, and then outputs a demodulated signal.

  In the OFDM radio receiving apparatus configured as described above, the directivity of the ESPAR antenna 10 is changed with the period of the symbol timing. FIG. 6 is a timing chart showing the relationship between the OFDM symbol and the control signal in the OFDM radio receiving apparatus of FIG. In the example of FIG. 6, the control signal is a sine wave, but the control signal may be a rectangular wave or a triangular wave. In the case of a rectangular wave or a triangular wave, as will be understood from the spectrum, interference occurs not only in the subcarriers on both sides but also in other subcarriers, but equalization can be performed. Further, since the capacitance change of the variable capacitance diode 13 is not actually a straight line with respect to the bias voltage, it is not a simple sine wave, and a sine wave curve is obtained by changing the capacitance in consideration of the characteristics of the variable capacitance diode 13. It is desirable to apply such a bias voltage. This oscillation frequency is 1 MHz or less in most standards used in current mobile radio communication systems.

  In the OFDM baseband demodulator 20, the channel equalizer 24 is configured by adding an inter-subcarrier interference countermeasure function to a conventional equalizer that equalizes a phase change in a propagation path and a frequency offset between transmitters and receivers. Is done. This equalization processing seems to change the propagation path at a high speed by changing the directivity of the ESPAR antenna 10 and also causes interference between subcarriers, so that the conventional method is deteriorated. . This equalization processing will be described later in detail. The wireless receiver according to the present embodiment is characterized in that the directivity of the ESPAR antenna 10 is changed in accordance with the OFDM symbol period. That is, under the condition that the received signal strength varies depending on the antenna direction, the received radio signal seems to have undergone AM modulation by periodically changing the directivity.

  FIG. 5 is a diagram showing a spectrum of a radio reception signal received by the OFDM radio reception apparatus of FIG. In FIG. 5, by changing the directivity in the symbol period, the received signal overlaps with an adjacent subcarrier to cause intersubcarrier interference. However, in this embodiment, OFDM orthogonality is maintained when the modulation frequency (control frequency of the control signal) is equal to a natural number multiple of the subcarrier frequency even after being modulated by directivity change. Since only the adjacent subcarriers on both sides are included as interference, this effect can be easily equalized by digital signal processing. In general, when control is performed with a control frequency obtained by multiplying the subcarrier frequency interval by a value obtained by dividing the first natural number n by the second natural number m (control frequency fcont = (n / m) × fsp (subcarrier frequency interval), which is 1 times the above when m = n.) Since the control signal waveform is already known, the modulation effect can be equalized if an increase in calculation amount is allowed. It is. On the other hand, in the present embodiment, the received signal strength is averaged within one symbol period by directivity switching in spite of a single RF. Therefore, unlike selection switching diversity, there is no waste. Any subcarrier can achieve a diversity effect as in the case of ratio combining.

  Further, as described above, according to the present embodiment, the directivity is not controlled in accordance with the propagation path, so that the tracking performance does not become a problem even if the propagation path changes rapidly. In other words, the tracking performance with respect to the propagation path change can be expected to be equivalent to that of a single receiver that does not perform diversity. In addition, since directivity is not controlled in the optimum direction by calculation, it has a unique effect of operating without problems even in a frequency selective fading environment.

  Next, it will be described below that the channel equalizer 24 of FIG. 1 can equalize the propagation path estimation and the inter-subcarrier interference. Assuming a preamble signal having signals in all subcarriers of the OFDM signal, assuming that the vector of this signal is P, the following expression is obtained.

[Equation 1]
P = [p ₀ , p ₁ ,..., P _N−1 ] ^T (1)

Here, p _k represents the preamble of the k-th subcarrier. Here, if F is a Fourier transform matrix for performing FFT, the transmission signal p is expressed by the following equation.

[Equation 2]
p = F ⁻¹ P (2)

  The received signal qi at the i-th element affected by multipath fading due to the propagation path is expressed by the following equation.

[Equation 3]
q _i = C _i p (3)

Here, C _i is a cyclic impulse response of the i-th antenna element, and is an impulse response matrix in the frequency domain. An input signal r to the demodulation circuit, which is a signal converted into baseband after being received by the OFDM radio receiving apparatus according to the present embodiment, is expressed by the following equation.

[Equation 4]
r
= Q ₀ + Dq _i + n
= (C ₀ + DC ₁ ) p + n
= (C ₀ + DC ₁ ) F ⁻¹ P + n (4)

Here, n is noise, D represents a received signal change due to antenna directivity control, and is represented by a matrix in which the coefficients d _k of the sub antennas at time k are arranged diagonally, that is, by the following equation.

[Equation 5]
D = diag (d _k ) (5)
[Equation 6]
u = Fr = (H ₀ + GH ₁ ) P + z (6)

Where z is thermal noise in the frequency domain. Here, _Hi is represented by the following equation.

[Equation 7]
H _i = FC _i F ⁻¹ (7)

H _i is a matrix having the frequency impulse response of the i-th antenna element as a diagonal element. G is expressed by the following equation.

[Equation 8]
G = FDF ⁻¹ (8)

  G is a matrix representing intersubcarrier interference. Here, a diagonal matrix Pd and Hi having P and hi as diagonal elements is introduced and expressed by the following equation.

[Equation 9]
P _d = diag (P) (9)
[Equation 10]
H _i = diag (h _i ) (10)

  Here, if the expression 6 is modified while ignoring some deterioration, the following expression can be obtained.

[Equation 11]
u = P _d h ₀ + GP _d h ₁ + z (11)

From this, the following covariance matrix R and cross-correlation matrix B _i are obtained:

[Equation 12]
^{_{R = E [uu H] =}} P d R h P d H + GP d R h P d H G H + σ 2 z I (12)
[Equation 13]
B _i = E [uh _i ^H ] (13)

  Rh is represented by the following formula.

[Formula 14]
R _h = E [h _i h _i ^H ] (14)

  Rh is a channel response covariance matrix. Here, E [x] represents an ensemble average of x. Further, the cross-correlation matrix is expressed by the following equation.

[Equation 15]
B ₀ = P _d R _h (15)
[Equation 16]
B ₁ = GP _d R _h (16)

Thus, the channel response can be expressed by the following equation using the vector W _i.

[Equation 17]
h _i = W _i ^H u (17)

Accordingly, the equalization weight vector W _i is expressed by the following equation.

[Equation 18]
W _i = R ⁻¹ B _i (18)

  This is a weight that realizes the least square error, and it can be seen that the same effect as the maximum ratio combining diversity can be obtained.

Next, the channel is equalized using this result. Here, representing the received signal u _d of the data portion by the following equation.

[Equation 19]
u _d = Hc _d + z (19)

However, the following formula is assumed.
[Equation 20]
Hc = Hc ₀ + GHc ₁ (20)
[Equation 21]
Hc _i = diag (h _i ) (21)

Here, when b is a coefficient representing the received signal power, the covariance matrix R _d at the time of channel equalization of the following equation can be expressed by using the cross-correlation matrix B _d at the time of channel equalization of the following equation. it can.

[Equation 22]
B _d = E [u _d d ^H ]
= BHc (22)

[Equation 23]
R _d = E [u _d u _d ^H ]
= HcE [dd ^H ] Hc ^H + σ ² _z I
= BHcHc ^H + σ ² _z I (23)

  Therefore, the estimated value d of the transmitted signal is expressed by the following equation.

[Equation 24]
^{_{d = R d -1 B d (}} 24)
[Equation 25]
dc = b (bHcHc ^H + σ ² _z I) ⁻¹ · Hc (25)

  As a result, reception by maximum ratio combining is possible. Further, in the OFDM radio receiving apparatus according to the present embodiment, since interference is an interference signal from peripheral subcarriers of its own subcarrier, the received signals are arranged in the order of subcarrier frequencies with the number of subcarriers regarded as the frame length. It is also possible to equalize by Viterbi decoding.

  FIG. 2 is a block diagram showing a configuration of an OFDM radio receiving apparatus according to a modification of the first embodiment of the present invention. In the OFDM radio receiving apparatus of FIG. 1, the control signal frequency fcont is controlled by the automatic frequency controller 21 so as to coincide with the subcarrier frequency interval fsp. However, the present invention is not limited to this, as shown in FIG. Automatic frequency control may not be performed. In this case, the control signal frequency fcont of the control signal generator 15 and the subcarrier frequency interval fsp of the transmitted OFDM signal are set to substantially coincide with each other and the oscillation frequency accuracy of each oscillator is extremely high. It is preferable to use it. Thereby, interference between OFDM subcarriers can be reduced and eliminated.

  In the above embodiment and its modification, the control signal frequency fcont is substantially matched with the OFDM subcarrier frequency interval fsp (fc≈fs). However, the present invention is not limited to this, and fcont = ( n / m) × fsp (where m and n are integers of 1 or more, that is, natural numbers).

  The frequency fsp in the transmission signal and the control signal frequency fcont are the same or fsp × (n because there is always a change in the reference frequency in the wireless transmission device and a frequency shift caused by the Doppler effect accompanying movement. / M) = If frequency synchronization is not performed even if it is set to be fcont, the frequency fsp and fcont of the received signal are completely the same in the wireless reception device, or completely fsp × (n / m) = fcont may not be satisfied. Equalization is performed because the orthogonality between subcarriers is lost as the relation deviates from the equality relation in the relation of fsp = fcont or fsp × (n / m) = fcont and becomes non-equal. As a result, it becomes a cause of deterioration of error characteristics, and the gain due to the channel equalization processing according to the present embodiment is offset. However, in a range in which this deterioration does not exceed the gain by the channel equalization processing according to the present embodiment, even if the frequencies fsp and fcont do not match or fsp × (n / m) = fcont is not satisfied. There is an effect of the channel equalization processing according to the present embodiment. That is, even if the inter-subcarrier interference cannot be completely equalized, it can be substantially equalized, and the interference can be greatly reduced.

  For equalization, the combined effect of propagation path and directivity change is obtained by calculation using a preamble signal, and this value changes as the phase changes. Therefore, the phase changes with the passage of time due to the frequency shift, which causes a need to frequently insert and recalculate the preamble signal. In other words, if phase synchronization is not realized as a result, equalization weights adapted to the phase state at that time must be prepared and equalized. However, this is not considered a requirement of the present invention.

When CP or GI exists, the length is usually an integer ratio such as 1/4 or 1/8 of the data portion. Therefore, the phase relationship between the start portion of the data excluding this and the control signal is fsp = fcont. There are multiple states, such as 4 states or 8 states. Therefore, it can be realized by preparing necessary types such as 4 types of values or 8 types of values corresponding to this, and switching and setting them for use.
Further, if the frequency of the control signal is multiplied by n so that the type of the phase relationship between the data start portion and the control signal is reduced, the necessary types can be reduced.

  7 shows a case where CP or GI is inserted in the OFDM wireless receiver of FIG. 1 (when CP or GI has a time length of 1/4 of one symbol, and control signal frequency fcont = subcarrier frequency interval fsp). It is a timing chart which shows the relationship between the OFDM symbol of 4 types of equalization weights (when using E1-E4) and a control signal. 8A shows a case where CP or GI is inserted in the OFDM radio receiving apparatus of FIG. 1 (when CP or GI has a time length of 1/4 of one symbol, control signal frequency fcont = subcarrier frequency interval). 6 is a timing chart showing a relationship between an OFDM symbol and a control signal (when fsp × 4 uses one type of equalization weight). In the case of FIG. 7, interference between subcarriers can be easily equalized in both cases of FIG. 8A, but in the case of FIG. 8A, interference from a frequency separated by a frequency fsp × 4, that is, from an adjacent channel. Prone to interference.

  8B shows a case where CP or GI is inserted in the OFDM radio receiving apparatus of FIG. 1 (when CP or GI has a time length of 1/8 of one symbol, and control signal frequency fcont = subcarrier frequency interval fsp × 4 is a timing chart showing a relationship between an OFDM symbol and a control signal (when two equalization weights are used in 4). In the case of FIG. 8B, it becomes easy to receive interference from a frequency separated by the frequency fsp × 4, that is, interference from an adjacent channel.

  8C shows a case where CP or GI is inserted in the OFDM radio receiving apparatus of FIG. 1 (when CP or GI has a time length of one symbol, one type of control signal frequency fcont = subcarrier frequency interval fsp / 2). 6 is a timing chart showing a relationship between an OFDM symbol and a control signal. In the case of FIG. 8C, not only is it easy to receive interference from adjacent channels, but the amount of calculation required for equalization increases due to the occurrence of intersubchannel interference over a higher order, but the effect of reducing intersubcarrier interference can be obtained. .

  When equalizing, if the change in amplitude caused by the change in directivity is a sine wave, the interference is only one at each of the upper and lower frequencies with respect to the fundamental wave. Desirable in terms of improvement. However, in an actual device, the change in directivity does not change linearly with respect to the control voltage, and the change in directivity and the change in amplitude are not necessarily linearly related, so there is no need to add an accurate sine wave to the control signal. thin. The basic effect can be expected in the same way if the directivity change has a periodicity such as a rectangular wave or a triangular wave, but in this embodiment, an amplitude change closer to a sine wave can be expected, and the example of a sine wave is simple. showed that.

  Further, the equalization of the natural numbers m and n of the setting ratio of the control signal frequency and the inter-carrier interference in the OFDM radio receiving apparatus of FIG.

  In this embodiment, the subcarrier frequency interval is fsp, and the control signal frequency for periodically changing the directivity is fcont. Here, n × fsp (n = 1, 2, 3,...) May not exactly coincide with the control signal frequency fcont, but preferably n × fsp = fcont. In this case, if the control signal is a sine wave, inter-subcarrier interference appears only on the nth subcarrier of the upper and lower frequencies. The operation is possible even in the case of fsp = m × fcont (m = 1, 2, 3,...). In this case, the sine wave is a full-wave rectified sine wave of m × fcont period (when m = 2). Since it is a special waveform like a rounded sawtooth wave (when m = 4) that is a 90 degree sine wave cut, as expected from the frequency spectrum of the repetitive waveform, it is the same as a rectangular wave or triangular wave Although the equalization is more complicated, it works functionally.

In a system using OFDM in practical use since it is set to a natural fraction of the data symbol length is generally the CP / GI, symbol period is a natural number ratio of the subcarrier frequency interval n _{0 /} m ₀ × (here , N ₀ and m ₀ are natural numbers (may be different or the same), which is related to the type of weight used for equalization and the ease of equalization. Not essential.

  Furthermore, how sub-carrier interference appears in the case of a triangular wave and a rectangular wave when the subcarrier frequency interval fsp = fcont will be described below. FIGS. 9 to 14 are spectrum diagrams of examples for explaining equalization of inter-carrier interference in the OFDM radio receiving apparatus of FIG.

  The original OFDM signal has a plurality of subcarriers arranged as shown in FIG. In this example, there are 9 subcarriers. When attention is paid to one of the subcarriers, it is as shown in FIG. When the directivity is changed with a sine wave, two sidebands are generated as shown in FIG. 11 by the same operation as that of amplitude modulation. Therefore, in the entire OFDM signal, sidebands are generated on both sides thereof as shown in FIG. 12, and two sidebands are combined in the seven subcarriers in the central portion to cause intersubcarrier interference. On the other hand, when a rectangular wave or a triangular wave is applied, sideband components over a high order are generated for one subcarrier according to the spectrum as shown in FIG. As shown in FIG. 14, interference from surrounding subcarriers is added to the original OFDM signal as a high-order sideband portion, as shown in FIG.

Second embodiment.
FIG. 3 is a block diagram showing a configuration of an OFDM radio receiving apparatus according to the second embodiment of the present invention. Compared to the first embodiment of FIG. 1, the second embodiment replaces the ESPAR antenna 10 with, for example, two antenna elements 31 and 32 that are separated from each other by a half wavelength, a phase shifter 33, and the like. An array antenna 30 configured to include an adder 34 is provided.

  In FIG. 3, the radio signal received by the antenna element 31 is input to the adder 34. On the other hand, the radio signal received by the antenna element 32 is input to the adder 34 via the phase shifter 33 whose amount of phase shift is changed by the control signal voltage Vc. The adder 34 adds the two input radio signals and outputs the addition result radio signal to the low noise amplifier 16.

  By configuring as described above, the directivity of the array antenna 30 is changed by the control signal voltage Vc, and the same effect as the ESPAR antenna 10 of FIG. 1 can be obtained. Other operations and processes are the same as those in the first embodiment.

  Although one phase shifter 33 is provided, the present invention is not limited to this, and the directivity is changed by changing the relative phase of the radio signals received by the two antenna elements 31 and 32. Therefore, two phase shifters inserted in the power feeding circuits of the antenna elements 31 and 32 may be provided. The array antenna 30 includes the two antenna elements 31 and 32. However, the present invention is not limited to this, and the array antenna 30 includes three or more N antenna elements, and relative radio signals received by the respective antenna elements. In order to change the directivity by changing the general phase, N or N-1 phase shifters inserted in the feeding circuit of each antenna element may be provided. Furthermore, although the variable capacitance diode 13 is provided, the present invention is not limited to this, and a variable reactance element may be provided.

  FIG. 4 is a block diagram showing a configuration of an OFDM radio receiving apparatus according to a modification of the second embodiment of the present invention. Similar to the modification of the first embodiment, this modification has a configuration in which the frequency control to the control signal generator 15 is eliminated, and has the same effects as the modification of the first embodiment.

The inventors performed a simulation using the OFDM radio receiving apparatus according to the first embodiment. FIG. 15 is a table showing specifications at the time of simulation of the OFDM wireless reception apparatus of FIG. FIG. 16 is a simulation result of the OFDM wireless receiver of FIG. 1, and shows a bit error rate with respect to a ratio E _b / E ₀ of transmission power and noise power per symbol in additive white noise (AWGN) and Rayleigh fading. It is a graph which shows BER.

  In the simulation, a simple model is used to confirm basic effects, and encoding is not performed. FIG. 7 shows the simulation result. The solid line is the theoretical value of the bit error rate characteristic under the Rayleigh fading environment, and the broken line is the characteristic according to the proposed method. It can be seen that gain is obtained by the diversity effect. From the computer simulation results, it was shown that the diversity gain can be obtained in the OFDM wireless reception apparatus according to the present embodiment.

  As described above in detail, according to the radio reception apparatus and the channel equalization method thereof according to the present invention, the subcarrier frequency interval is obtained by multiplying the value obtained by dividing the first natural number by the second natural number. The inter-subcarrier interference is substantially eliminated after Fourier transform of the radio signal received by the array antenna with the changing directivity into a plurality of subcarrier signals while changing the directivity of the array antenna at a controlled frequency. Thus, the received radio signal is demodulated by performing channel equalization for each subcarrier signal. Therefore, since feedback control is not performed, the problem of control delay in the above-described directivity diversity does not occur. Moreover, the diversity effect in a frequency selective propagation path can be obtained effectively. Therefore, in the OFDM radio receiving apparatus, the directivity control of the antenna can follow the received signal in a fast fading environment, and the inter-subcarrier interference can be reliably equalized.

  INDUSTRIAL APPLICABILITY The present invention can contribute to the improvement of transmission quality of a wireless reception device in a digital wireless communication system, and more specifically, specifically, a wireless LAN system compliant with IEEE802.11a / g, or wireless of terrestrial digital television broadcasting. It can be applied to a receiving device or the like.

10 ... ESPAR antenna,
11 ... feeding element,
12 ... parasitic element,
13: Variable capacitance diode,
14: Inductor for high frequency blocking,
15 ... Control voltage generator,
16: Low noise amplifier (LNA),
17 ... frequency converter,
18 ... A / D converter,
20 ... OFDM baseband demodulator,
21 ... Automatic frequency controller (AFC),
22 ... GI converter,
23 ... Discrete Fourier Transform,
24 ... Channel equalizer,
25. Phase tracking circuit,
26: Subcarrier demodulator,
27 ... Deinterleaver,
28 ... error correction decoder,
30 ... Array antenna,
31, 32 ... antenna elements,
33 ... Phase shifter,
34: Adder.

Claims (8)

A radio reception apparatus that receives an orthogonal frequency division multiplexing radio signal including a plurality of subcarrier signals juxtaposed at predetermined subcarrier frequency intervals using a single radio reception circuit,
An array antenna for receiving the radio signal;
The directivity of the array antenna is independent of the received radio signal and is asynchronous and has a control frequency substantially equal to the subcarrier frequency interval or a control frequency that is a natural number two or more times the subcarrier frequency interval. Directivity changing means to change gender,
After substituting the radio signal received by the array antenna having the directivity into a plurality of subcarrier signals, the subcarrier interference is substantially eliminated by changing the directivity. A radio receiving apparatus comprising: demodulation means for demodulating the received radio signal by channel equalization for each signal. The array antenna includes a feeding element, at least one parasitic element provided so as to be electromagnetically coupled to the feeding element, and a variable reactance element loaded on the parasitic element,
The radio receiving apparatus according to claim 1, wherein the directivity changing means changes the directivity of the array antenna by changing a reactance value of the variable reactance element. The array antenna includes a plurality of antenna elements provided to be separated from each other by a predetermined distance,
The directivity changing means changes the directivity of the array antenna by shifting the phase of each radio signal received by the plurality of antenna elements relative to each other. Wireless receiver. A channel equalization method for a radio reception apparatus for receiving radio signals of an orthogonal frequency division multiplexing system including a plurality of subcarrier signals juxtaposed at predetermined subcarrier frequency intervals using a single radio reception circuit,
The wireless receiving device includes an array antenna that receives the wireless signal,
The channel equalization method does not depend on the received radio signal and is asynchronous and controls a control frequency substantially equal to the subcarrier frequency interval or a natural number multiple of 2 or more of the subcarrier frequency interval. A subband generated by changing the directivity after the Fourier transform of a radio signal received by the array antenna whose directivity is changed into a plurality of subcarrier signals while changing the directivity of the array antenna at a frequency. A channel equalization method for a radio reception apparatus, comprising: an equalization step for demodulating the received radio signal by channel equalization for each subcarrier signal so that inter-carrier interference is substantially eliminated . The array antenna includes a feeding element, at least one parasitic element provided so as to be electromagnetically coupled to the feeding element, and a variable reactance element loaded on the parasitic element,
5. The channel equalization method for a radio reception apparatus according to claim 4 , wherein the equalizing step changes the directivity of the array antenna by changing a reactance value of the variable reactance element. The array antenna includes a plurality of antenna elements provided to be separated from each other by a predetermined distance,
5. The radio according to claim 4 , wherein the equalizing step changes the directivity of the array antenna by shifting the phases of the radio signals received by the plurality of antenna elements relative to each other. Channel equalization method for receiving apparatus . A radio reception apparatus that receives an orthogonal frequency division multiplexing radio signal including a plurality of subcarrier signals juxtaposed at predetermined subcarrier frequency intervals using a single radio reception circuit,
A first antenna element for receiving the radio signal;
A second antenna element provided to be electromagnetically coupled to the first antenna element;
A variable reactance element loaded on the second antenna element;
The variable reactance element has a control frequency that is asynchronous and does not depend on the received radio signal and substantially matches the subcarrier frequency interval, or a control frequency that is a natural number two or more times the subcarrier frequency interval. Directivity changing means for changing the directivity of the first antenna by changing a reactance value;
After Fourier transforming a radio signal received by the first antenna element whose directivity changes to a plurality of subcarrier signals, the intersubcarrier interference newly generated by the change in directivity is substantially eliminated. Thus, a radio receiving apparatus comprising demodulating means for demodulating the received radio signal by channel equalization for each subcarrier signal. A channel equalization method for a radio reception apparatus for receiving radio signals of an orthogonal frequency division multiplexing system including a plurality of subcarrier signals juxtaposed at predetermined subcarrier frequency intervals using a single radio reception circuit,
The wireless receiver is
A first antenna element for receiving the radio signal;
A second antenna element provided to be electromagnetically coupled to the first antenna element;
A variable reactance element loaded on the second antenna element,
The channel equalization method is
The variable reactance element has a control frequency that is asynchronous and does not depend on the received radio signal and substantially matches the subcarrier frequency interval, or a control frequency that is a natural number two or more times the subcarrier frequency interval. After changing the directivity of the first antenna by changing the reactance value, the radio signal received by the first antenna element whose directivity changes is Fourier transformed into a plurality of subcarrier signals, Including an equalization step of demodulating the received radio signal by channel equalization for each subcarrier signal so that the inter-subcarrier interference newly generated due to the change in directivity is substantially eliminated A channel equalization method for a wireless reception device.

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