WO2007004363A1 - Ofdm receiver apparatus - Google Patents

Abstract

An OFDM receiver apparatus for performing a diversity reception, wherein the power consumption and the cost can be reduced. An OFDM signal transmitted from a transmitter apparatus is received by two antennas (11A,11B). An antenna switching part (12) selectively outputs the signal, which was received by one of the two antennas, to a tuner (13). A signal outputted from the tuner (13) is supplied through an A/D converter (14), a quadrature detecting circuit (15) and through an FFT calculating circuit (16) to an equalizing circuit (17). A deviation determining circuit (21) calculates the positional deviations, on the constellation, of I and Q component signals outputted from the equalizing circuit (17). When the positional deviations exceed a predetermined threshold value (when the positional deviations of the signals are large), a deciding circuit (22) sends, to the antenna switching part (12), a control signal that instructs it to switch to the other antenna.

Description

 Specification

 OFDM receiver

 Technical field

 The present invention relates to a technique for diversity reception of OFDM signals.

 Background art

 [0002] In Japanese terrestrial digital television broadcasting, an OFDM (Orthogonal Frequency Division Multiplexing) system is adopted as a transmission system. The OFDM system is one of the multicarrier transmission systems that divides a transmission signal into multiple carriers and transmits it. The spectrum of each subchannel, which is strong against frequency selective fading of the multipath transmission path, can be arranged densely, There are advantages such as high utilization efficiency.

 [0003] In the future, terrestrial digital television broadcasting (one-segment broadcasting) for portable devices will be started. When designing mobile phones that can receive digital TV, card-type digital television receivers, etc., you must always consider miniaturization and low power consumption. In addition, mobile devices often receive broadcast signals while moving. There are two problems in receiving broadcast signals with mobile devices.

 [0004] (1) An antenna attached to a portable device is simple. Moreover, when a portable device antenna is used, the reception position is generally low. For this reason, reception may not be possible even if the reception position with a small tolerance for the change in reception sensitivity slightly fluctuates. (2) Since mobile devices are used while moving, it is likely to become a lay leaf receiving reception environment.

 [0005] For such a problem specific to signal reception using a portable device, there is a measure of performing diversity reception. In this method, OFDM signals are received using multiple antennas, and the signals of each branch are combined at the front end or at the subcarrier level.

 [0006] Note that Patent Document 1 exists as a prior art related to a diversity receiver.

[0007] Patent Document 1: Japanese Unexamined Patent Publication No. 2000-174726

Disclosure of the invention Problems to be solved by the invention

 [0008] The conventional technique for performing diversity reception has a problem that the circuit scale for synthesizing signals of a plurality of systems is increased, and the area and cost of the receiving LSI chip are increased. This problem is a problem that should be particularly improved in portable devices that are becoming smaller.

 [0009] In addition, the conventional technique for performing diversity reception requires a plurality of tuners. This is because signals received by a plurality of antennas need to be processed by a tuner provided for each antenna. The front-end combining diversity receiver has a smaller demodulator circuit size than the subcarrier-level combining diversity receiver, but it still requires two tuners. In general, since the proportion of power consumed by the tuner exceeds 80% of the power consumed by the receiver, these conventional diversity reception methods are intended to improve the quality of the demodulated signal. However, there is a problem of increasing power consumption. In particular, there is a strong demand for low power consumption, and this is an issue that should be improved for portable devices.

 [0010] In view of the above problems, the present invention demodulates a high-quality signal even in a mobile device or portable device having a poor reception environment, while reducing the power consumption and cost of the receiver. The purpose is to provide technology.

 Means for solving the problem

 [0011] In order to solve the above-mentioned problem, the invention according to claim 1 is characterized in that two antennas that receive OFDM signals and the two antennas are switched selectively, and the selected antenna is Switching means for outputting the received signal to the tuner, means for performing FFT operation on the signal output from the tuner, means for performing equalization processing of the signal after FFT operation, and constellation for the signal after equalization processing A calculating means for calculating a positional deviation of the first antenna, and when the positional deviation exceeds a predetermined threshold value, a control signal is given to the switching means to control to switch the selected antenna to the other antenna, And a means for controlling to use the selected antenna continuously when the positional deviation is less than the predetermined threshold value.

[0012] The invention according to claim 2 is the OFDM receiver according to claim 1, wherein the calculation means calculates the positional deviation of the signal on the constellation by averaging over all carriers. A stage.

 [0013] The invention according to claim 3 is the OFDM receiver according to claim 1 or claim 2, wherein the calculating means calculates a positional deviation of the signal on the constellation by averaging the symbols. Means.

 [0014] The invention described in claim 4 is the OFDM receiver according to claim 3, wherein the number of the plurality of symbols is less than 10 symbols.

 [0015] The invention according to claim 5 is the OFDM receiver according to claim 3, wherein the switching means selects the two antennas in the means for calculating the positional deviation by averaging the plurality of symbols. The average of the plurality of symbols is calculated while avoiding a predetermined period after switching automatically.

 [0016] The invention according to claim 6 is the OFDM receiver according to claim 5, characterized in that the predetermined period is at least 10 symbols or more.

 [0017] In the invention according to claim 7, in the OFDM receiver according to claim 2, the means for calculating the position deviation included in the calculating means by averaging with respect to all carriers is provided for each symbol. The means for calculating the positional deviation on average for all carriers and controlling the switching or continuous use of the antenna compares the positional deviation with the predetermined threshold value for each symbol, and calculates the predetermined number of symbols. When the positional deviation continuously exceeds the predetermined threshold value, a control signal is given to the switching means to control the selected antenna to be switched to the other antenna, and less than the predetermined number of symbols. When the positional deviation exceeds the predetermined threshold continuously, control is performed so that the selected antenna is continuously used.

 [0018] The invention according to claim 8 is the OFDM receiver according to claim 7, wherein the predetermined number of symbols is 2 !, 4 !, or any number of symbols.

[0019] The invention described in claim 9 is to switch between two antennas that receive OFDM signals and the two antennas, and output the signals received by the selected antennas to the tuner. A switching means, a means for measuring the signal quality of the signal output from the tuner, and if the signal quality is below a predetermined threshold value, a control signal is given to the switching means so that the selected antenna is Control to switch to the antenna, the signal And a means for controlling to continuously use the selected antenna when the quality exceeds the predetermined threshold value.

 [0020] The invention of claim 10 selectively switches between two antennas for receiving OFDM signals and the two antennas, and outputs the signals received by the selected antennas to the tuner. Switching means, means for performing FFT operation on the signal output from the tuner, means for performing equalization processing on the signal after FFT operation, means for performing demapping processing on the signal after equalization processing, and demapping processing A means for calculating a bit error rate of a later signal, and when the bit error rate exceeds a predetermined threshold, a control signal is given to the switching means to control to switch the selected antenna to the other antenna. And a means for controlling to continuously use the selected antenna when the bit error rate is lower than the predetermined threshold value.

 [0021] The invention described in claim 11 is to switch between two antennas for receiving OFDM signals and the two antennas, and output the signals received by the selected antennas to the tuner. A switching means, a means for calculating the CZN ratio of the output signal of the tuner force, and if the CZN ratio falls below a predetermined threshold, a control signal is given to the switching means to switch the selected antenna to the other Means for controlling to switch to an antenna, and when the CZN ratio exceeds the predetermined threshold, means for controlling to use the selected antenna continuously.

 [0022] The invention according to claim 12 is the OFDM receiver according to any one of claims 1, 2, 9, 9, 10 and 11, wherein the switching means includes a predetermined number of symbols. The antenna switching control is performed at the intervals described above.

 The invention's effect

 The present invention is an OFDM receiver having two antennas, comprising means for measuring signal quality, and alternatively selecting an antenna with good signal quality. Only the signal output from the selected antenna is received and processed by the tuner. The signal output from the tuner is demodulated.

[0024] Thus, the present invention can selectively receive a high-quality signal using two antennas, and further, the receiving apparatus includes only one tuner, so that low power consumption is achieved. Electric Power and low cost can be achieved.

 [0025] Further, as a means for measuring the signal quality, the positional deviation of the signal on the signal constellation is calculated. As a result, when the received signal is disturbed, the quality of the signal can be measured, and an antenna with good signal quality can be selected.

 [0026] As a means for measuring signal quality, a bit error rate of a signal is calculated. As a result, the antenna can be selected by accurately reflecting the quality of the received signal.

 [0027] Further, the present invention includes means for calculating a CZN ratio of a received signal, and alternatively selects an antenna having a good CZN ratio. Only the signal with the selected antenna power output is received and processed by the tuner.

 [0028] Thereby, of the two antennas, an antenna having a good reception state can be selectively used. In addition, since the receiving device has only one tuner, it is possible to reduce power consumption and cost.

 [0029] Further, according to the present invention, the means for calculating the positional deviation by averaging a plurality of symbols calculates the average positional deviation of the signal on the constellation by averaging less than 10 symbols. Thereby, the circuit configuration of the means can be reduced. In addition, it is possible to improve the responsiveness of the antenna switching process that is performed when a disturbance occurs during transmission and the signal is distorted. That is, the antenna switching process can be performed immediately when distortion occurs in the signal.

 [0030] Further, the average symbol value is calculated by averaging with less than 10 symbols. If the frequency is less than 10 symbols, for example, the Doppler shift frequency is low, the positional deviation on the constellation becomes large. It is about the same as the period.

 [0031] Therefore, a period in which the distortion of the signal occurs during the calculation of the symbol average value includes the symbol average value calculated in this case, and the distortion of the signal occurs during the calculation of the symbol average value. Thus, when the average period value is compared with the calculated symbol average value, the difference between the two symbol average values can be increased. As a result, the predetermined threshold value can be easily set, and the CZN ratio of the received signal can be improved. It was confirmed by experiments that the threshold value was easy to set and that the CZN ratio of the received signal was improved.

[0032] Further, the present invention provides a stationary period consisting of a predetermined period after the antenna is switched alternatively. Do not calculate the displacement of the signal on the constellation. As a result, even if the positional deviation on the constellation increases due to the antenna switching process, it is possible to reduce the overlap between the period in which the positional deviation has increased and the symbol average value calculation period. As a result, even if the positional deviation becomes large due to the antenna switching process, it is possible to suppress the influence of the positional deviation becoming large in the calculation of the symbol average value.

 [0033] Further, the period during which the positional deviation on the constellation due to the antenna switching process is large is less than 10 symbols. Therefore, by setting the predetermined period (stationary period) to at least 10 symbols in the present invention, it is possible to completely eliminate the influence of the antenna switching process when calculating the symbol average value.

 [0034] Further, the present invention compares the positional deviation with a predetermined threshold value for each symbol, and when the positional deviation exceeds a predetermined threshold value for a predetermined number of symbols, the antenna is switched, If the positional deviation exceeds a predetermined threshold continuously with less than a certain number of symbols, the selected antenna is continuously used.

 [0035] As a result, there is a period in which the positional deviation on the constellation increases due to disturbance in the transmission and distortion in the signal, and the positional deviation on the constellation is large due to the antenna switching process. It is possible to overlap the running period. Therefore, even if the signal is distorted and the antenna switching process is performed, the period during which a normal signal can be received can be made longer than when both the above periods are not overlapped.

 [0036] Further, the predetermined number of symbols is 2 !, 4! From this point of view, if a disturbance occurs in the middle of transmission and the distortion is detected accurately, the constellation position shift becomes large over the period of 2 or 4 symbols. It is desirable to implement the antenna switching process. Therefore, it is possible to maximize the period during which a normal signal can be received while taking the above matters into consideration.

 [0037] The objects, features, aspects, and advantages of the present invention will become apparent from the following detailed description and the accompanying drawings.

 Brief Description of Drawings

FIG. 1 is a block diagram of an OFDM receiving apparatus according to a first embodiment.

FIG. 2 is a diagram showing the constellation of the original signal. FIG. 3 is a diagram showing a constellation when there is a 5% frequency shift.

 FIG. 4 is a diagram showing a constellation under an environment where the Doppler frequency is 80 Hz.

 FIG. 5 is a diagram showing a constellation under a CZN = 15 dB environment.

 FIG. 6 is a block diagram of an OFDM receiving apparatus according to a second embodiment.

 FIG. 7 is a block diagram of an OFDM receiving apparatus according to a third embodiment.

 FIG. 8 is a diagram illustrating a state in which measurement of the force symbol average value immediately after antenna switching is started.

 FIG. 9 is a diagram showing a state in which the measurement of the symbol average value is started after the stationary period has elapsed after antenna switching.

 FIG. 10 is a diagram showing a state where the original distortion and the distortion caused by the switching process overlap before and after the antenna switching process!

 FIG. 11 is a diagram showing how the original distortion overlaps with the distortion caused by the switching process before and after the antenna switching process.

 FIG. 12 is a diagram showing a case where the calculated positional deviation continuously exceeds a predetermined threshold value with less than a predetermined number of symbols.

 BEST MODE FOR CARRYING OUT THE INVENTION

 Hereinafter, embodiments of the present invention will be described with reference to the drawings. As a transmission system for terrestrial digital broadcasting, an OFDM system in which hundreds to thousands of multiple carriers (subcarriers) are multiplexed and transmitted within one channel band is adopted in Japan, Europe and the United States. This OFDM method is a multicarrier modulation method in which transmission data is divided into a plurality of subcarriers for transmission, and is therefore resistant to frequency selective fading interference that occurs during mobile reception where the frequency utilization efficiency is very high. Also, under the same conditions, the total bit rate transmitted over a 6 MHz bandwidth, the symbol period of each carrier is the number of carriers (hundreds to thousands of minutes) compared to that of the normal single carrier modulation system. In addition, by providing a guard period called guard interval between each effective symbol, the effect of multipath (ghost) can be reduced, which has the advantage that image quality degradation can be suppressed.

 [0040] {First embodiment)

FIG. 1 is a functional block diagram of an OFDM receiver according to the first embodiment of the present invention. The OFDM receiver of this embodiment is a mobile phone, a portable digital TV, etc. It is a receiving device used in a mobile device, and is a one-segment receiving device that receives a layer in which only one segment is powerful among OFDM signals composed of a plurality of layers. As shown in FIG. 1, the OFDM receiver of this embodiment is a receiver that includes two antennas 11A and 11B having directivity and performs diversity reception.

 [0041] An RF (Radio Frequency) signal transmitted from an OFDM transmitter (not shown) is received by two receiving antennas 11A and 11B through a transmission path. The received RF signal is output to the tuner 13 via the antenna switching unit 12, and the tuner 13 converts the frequency into an IF (Intermediate Frequency) signal. The IF signal is output to the AZD conversion 14 via BPF (band pass filter), mixer, LP F (low pass filter) and the like. The signal input to the AZD conversion 14 is converted into a digital signal at a predetermined sampling frequency.

 The signal output from the AZD conversion 14 is output to multipliers 151 and 1 52 included in the quadrature detection circuit 15. Multiplier 151 multiplies the signal by the COS wave, and multiplier 152 multiplies the signal by the SIN wave. Multipliers 151 and 152 output signals of in-phase component (1) and quadrature component (Q), respectively. Further, the high frequency component is removed from the I component and Q component signals by the LPFs 153 and 154, and the signal is output to the FFT operation circuit 16.

[0043] The FFT operation circuit 16 Fourier-transforms an input time domain signal (I component and Q component signals) into a frequency domain signal (I component and Q component signals). The signal converted into the frequency domain is output to an equalization circuit 17 that performs equalization processing on the received signal. The equalization circuit 17 calculates an estimated transmission path response of the received signal using a pilot signal having a known amplitude and phase, and interpolates the estimated transmission path response in the symbol direction and the carrier direction. Then, the received signal equalization process is executed using the interpolated estimated transmission line response. The received signal after the equalization processing is output to the demapping circuit 18. In the demapping circuit 18, a complex signal on the constellation such as QPSK and 16QAM is demapped to an integer signal. The signal after the demapping processing is output to the FEC circuit 19, where Viterbi decoding and Reed-Solomon decoding are performed, and error correction is performed. The signal outputted from the FEC circuit 19 is decoded by a source decoder (not shown) such as MPEG (Moving Picture Experts Group) 2 method and used as a decoded signal. Next, features of the present invention will be described. The OFD M receiver according to the present embodiment as described above includes two antennas 11 A and 11 B and an antenna switching unit 12. The antenna switching unit 12 has a function of selectively switching the two antennas 11A and 11B in response to a control signal from the antenna switching control unit 20. That is, the antenna switching unit 12 outputs to the tuner 13 a signal received by one selected antenna among the signals received by the two antennas 11A and 11B.

 [0045] The antenna switching control unit 20 includes a shift detection circuit 21 and a determination circuit 22, and performs antenna switching determination using the I component and Q component signals output from the equalization circuit 17. It has a function.

 The deviation detection circuit 21 is a circuit that receives the I component and Q component signals output from the equalization circuit 17 and detects a positional deviation on the constellation.

 [0047] The constellation represents the signal arrangement with the in-phase component (I component signal) as the horizontal axis and the quadrature component (Q component signal) as the vertical axis. In the present embodiment, the OFDM signal is a QPSK-modulated signal, and FIGS. 2 to 5 are diagrams showing the constellation of the QPSK-modulated signal. Figure 2 shows the constellation of the original signal transmitted from the transmitter. Thus, the original signal is either (I, Q) = (l, 1), (1, —1), (— 1, 1), (—1, —1) It is. If disturbance is applied during the process from the transmission of the OFDM signal from the transmitter to the reception by the receiver, the constellation of the received signal will deviate by the above four points. For example, Figure 3 shows the constellation when a 5% frequency shift occurs, Figure 4 shows the constellation when received in an 80 Hz Doppler frequency environment, and Figure 5 shows the white Gaussian noise. Constellation under the environment of C ZN = 15dB with (AWGN) added.

 [0048] In this way, the received signal to which the disturbance is added is plotted on the constellation at a position where the four-point position force of the original signal is also shifted. Then, the deviation detection circuit 21 calculates the position deviation on the constellation of the I component and Q component signals output from the equalization circuit 17 by the calculation method shown in Equation 1.

[0049] [Equation 1] R = -τ∑ [(| Μ-ι) '+ (! 0ι | -ΐ) Ί

^Ν i = 1

[0050] In Equation 1, N is the number of carriers in one symbol, Ii is the I component (in-phase component) of the i-th subcarrier, and Qi is the Q component (orthogonal component) of the i-th subcarrier. Show. In other words, for the signals of all carriers in one symbol, (I, Q) = (l, 1), (1, —1), (-1, 1), (1 1, —1) The average value is calculated by adding the square values of the distances. For example, when there is no disturbance, i I = I Qi I = 1 for all subcarriers and R = 0 is calculated. In other words, it is determined that there is no distortion in the signal without any signal shift. On the other hand, when there is a disturbance, R becomes a real number other than 0, and a signal displacement is detected. The average deviation R increases as the Ii component or Qi component departs from 1, and it can be determined that the reception situation is worse.

 [0051] When the deviation detection circuit 21 calculates the carrier average value R of the signal position deviation by the calculation shown in Equation 1, the deviation detection circuit 21 outputs this value to the determination circuit 22. The determination circuit 22 integrates the R values over a period of several tens of milliseconds or several hundreds of milliseconds, and further calculates the average value. That is, the deviation detection circuit 21 calculates the average value of the signal position deviation on the constellation for all subcarriers in one symbol, and the decision circuit 22 further determines the signal position deviation for a plurality of symbols. The average value of is obtained. Equation 2 is an arithmetic expression of the symbol average value Rave of the signal position deviation calculated in the determination circuit 22.

[0052] [Equation 2]

[0053] In equation (2), Rk represents the carrier average value of the signal position deviation in the k-th symbol, and M represents the number of symbols to be averaged. For example, Rave is calculated every 50 symbols.

[0054] After calculating the symbol average value Rave of the signal position deviation, the determination circuit 22 performs a comparison process between a predetermined threshold and Rave. When Rave exceeds a predetermined threshold (that is, when the signal misalignment is large and the signal quality is poor), the antenna switching unit 12 receives the reception amplifier. A control signal for switching the tenor is sent. On the other hand, when Rave is below the predetermined threshold (that is, when the signal position deviation is small and the signal quality is good), the antenna switching unit 12 is controlled to continue using the currently active antenna. Send a signal.

 The control signal sent from the determination circuit 22 is converted into an analog signal by the DZA conversion 23 and sent to the antenna switching unit 12 as an analog control signal. The antenna switching unit 12 performs antenna switching control based on the input control signal. As a result, when the symbol average value Rave of the signal deviation exceeds a predetermined threshold, the antenna 11A, 11B is switched to the antenna that is not currently active, and the symbol average value Rave is set to the predetermined value. If it falls below the threshold value, the antenna that is currently active is selected as it is among the antennas 11A and 11B.

 [0056] Thus, according to the present embodiment, signal distortion is determined based on the positional deviation of the received signal on the constellation, and an antenna with a small degree of distortion is selected. Since the antenna switching unit 12 outputs only the signal received by the selected antenna to the tuner 13, it is possible to select and demodulate a signal with good signal quality out of the two signals received with diversity. is there. Since the OFDM receiver of this embodiment requires only one tuner, low power consumption and low cost can be achieved.

 [0057] It should be noted that determination circuit 22 preferably performs antenna switching control with an interval of a predetermined number of symbols (for example, 50 symbols). This is to prevent frequent switching of antennas within a short period of time (for example, several symbols), resulting in large fluctuations in transmission path conditions.

 [0058] {Second embodiment}

 Next, a second embodiment of the present invention will be described. In the second embodiment, antenna switching control is performed based on the bit error rate (BER) of the received signal. FIG. 6 is a block diagram of an OFDM receiving apparatus according to the second embodiment. In the figure, the same reference numerals are used for the same components as those in the first embodiment. Hereinafter, differences from the first embodiment will be described.

As shown in FIG. 6, the OFDM receiver includes a determination circuit 31 that inputs a bit error rate signal from the FEC circuit 19. Specifically, the FEC circuit 19 performs communication by Viterbi decoding. The ability to correct the error of the signal The Viterbi decoding circuit performs error correction processing and counts the number of occurrences of bit errors. Based on this count value, the FEC circuit 19 can calculate the bit error rate. When the FEC circuit 19 outputs this bit error rate signal to the determination circuit 31, the determination circuit 31 performs a comparison process between the bit error rate and a predetermined threshold value. When the bit error rate exceeds a predetermined threshold (that is, when there are many bit errors and the signal quality is poor), the determination circuit 31 controls the antenna switching unit 12 to switch the antenna. Send a signal. In addition, when the bit error rate is lower than a predetermined threshold (that is, when the signal quality is good with few bit errors), the determination circuit 31 is currently active with respect to the antenna control unit 12. The control signal is transmitted so that the current antenna is continuously used.

 The control signal sent from the determination circuit 31 is converted into an analog signal in the DZA conversion and sent to the antenna switching unit 12 as an analog control signal. The antenna switching unit 12 performs antenna switching control based on the input control signal. As a result, when the bit error rate of the signal exceeds the predetermined threshold value, the antenna 11A, 11B is switched to the antenna that is not currently active, and the bit error rate falls below the predetermined threshold value. In this case, the antenna that is currently active is selected as it is among the antennas 11A and 11B.

 Thus, according to the present embodiment, switching control is performed so that an antenna with good signal quality is selected based on the bit error rate of the received signal. Since the antenna switching unit 12 outputs only the signal received by the selected antenna to the tuner 13, it is possible to select and demodulate a signal with good signal quality from the two signals received with diversity. It is. Since the OFDM receiver of this embodiment requires only one tuner,

Thus, low power consumption and low cost can be achieved.

[0062] (Third embodiment)

Next, a third embodiment of the present invention will be described. In the third embodiment, the antenna is switched and controlled based on the CZN ratio (Carrier to Noise ratio) of the received signal. FIG. 7 is a block diagram of an OFDM receiving apparatus according to the third embodiment. In the figure, the same reference numerals are used for the same configurations as those in the first embodiment. The following Differences from the first embodiment will be described.

 [0063] As shown in FIG. 7, the OFDM receiver includes a CZN calculation unit 41. The C / N calculation unit 41 calculates the CZN ratio of the received signal based on the calculation result of the FFT circuit 16. To do. The C / N ratio is the ratio of signal to noise, and the higher the value, the stronger the signal power. Note that the CZN calculation unit 41 may calculate the CZN ratio using the signal before the FFT calculation. Then, when the CZN calculation unit 41 outputs the calculated CZN ratio signal of the received signal to the determination circuit 42, the determination circuit 42 performs a comparison process between the CZN ratio and a predetermined threshold value. When the CZN ratio is below a predetermined threshold (that is, when there is a lot of noise and the signal power is weak), the determination circuit 42 instructs the antenna switching unit 12 to switch the antenna. Is sent out. In addition, when the CZN ratio exceeds a predetermined threshold value (that is, when the signal power is strong with little noise), the determination circuit 42 is connected to the antenna control unit 12 with the currently active antenna. A control signal is sent to continue using.

 [0064] The control signal sent from the determination circuit 42 is converted into an analog signal by the DZA conversion 23 and sent to the antenna switching unit 12 as an analog control signal. The antenna switching unit 12 performs antenna switching control based on the input control signal. As a result, when the CZN ratio of the signal falls below the predetermined threshold, the antenna 11A, 11B is switched to the antenna that is not currently active, and when the CZN ratio exceeds the predetermined threshold. Of the antennas 11A and 11B, the currently active antenna is selected as it is.

 Thus, according to the present embodiment, switching control is performed so as to select an antenna that can obtain a strong signal power based on the CZN ratio of the received signal. Since the antenna switching unit 12 outputs only the signal received by the selected antenna to the tuner 13, it can select and demodulate the signal with good CZN ratio out of the two signals received with diversity. It is. Since the OFDM receiver of this embodiment requires only one tuner, low power consumption and low cost can be achieved.

[0066] It is desirable that determination circuit 42 performs antenna switching control with an interval of a predetermined number of symbols (for example, 50 symbols). This is within a short period (eg several symbols) This is to prevent frequent switching of antennas and large fluctuations in transmission path conditions.

 [0067] {Fourth embodiment)

 When the Doppler shift frequency is low (this corresponds to the case where the OFDM receiver is used while being carried around), the position shift on the constellation continuously over a long period (Equation 1 R2) hardly increases. In other words, when the Doppler shift frequency is low, the constellation misalignment (R2 in Equation 1) becomes relatively large over a period of several symbols, and then in a period of several tens of symbols. Therefore, the displacement on the constellation (R2 in Equation 1) becomes relatively small.

 [0068] Therefore, for example, when the Doppler shift frequency is low, the symbol average value Rave over the period between several tens of symbols and several hundreds of symbols in the formula 2 described in the first embodiment is used. If measured, it is difficult to set the predetermined threshold value to an optimum value.

 [0069] This is because, during the calculation of the symbol average value Rave, even though the position deviation on the constellation is relatively large, the period is as small as several symbols. When the symbol average value Rave is measured over several tens to hundreds of symbols, the symbol average value Rave is a force that decreases. Therefore, when the symbol average value Rave is measured for several tens of symbols over several hundred symbols, the symbol average value Rave calculated including the period in which the signal is distorted over several symbols, The difference from the average symbol Rave calculated without including the period in which the signal is distorted is extremely small.

 [0070] In this situation, for example, if the predetermined threshold is set to a relatively large value, even if the positional deviation on the constellation becomes relatively large over a period of several symbols (that is, several Even if the signal is distorted over the symbol period), the signal cannot be detected.

[0071] On the other hand, when the predetermined threshold is set to a relatively small value, even in a period of several tens of symbols where the positional deviation on the constellation is relatively small (that is, signal distortion over several tens of symbols). But the power of normal signal reception is small enough) Tena switching is performed (that is, antenna switching processing is frequently performed).

 [0072] The difficulty in setting the predetermined threshold as described above results in a bad CZN ratio of the received signal. In the present embodiment, an OFDM receiver for solving the problem is provided.

 That is, the determination circuit 22 included in the OFDM receiving apparatus according to the present embodiment calculates the positional deviation of the signal on the constellation by averaging with less than 10 symbols. That is, in Equation 2, “M” is less than ten, and the determination circuit 22 calculates the symbol average average Rave for every less than 10 symbols.

 [0074] The OFDM receiver according to the present embodiment operates as follows.

 When the deviation detection circuit 21 calculates the carrier average value R of the signal position deviation by the calculation shown in Equation 1, the deviation detection circuit 21 outputs this value to the determination circuit 22. The determination circuit 22 integrates the values of R over a predetermined period and further calculates the average value.

 More specifically, the shift detection circuit 21 calculates the average value of the signal position shift on the constellation for all subcarriers in one symbol (Equation 1), and further determines the determination. In the constant circuit 22, the average value of the signal position deviation is obtained for less than 10 symbols (= M) (Equation 2).

 When the determination circuit 22 calculates the symbol average value Rave of the signal position deviation for less than 10 symbols, the determination circuit 22 compares the predetermined threshold with the calculated Rave. When the calculated Rave exceeds a predetermined threshold value, a control signal for switching the receiving antenna is sent to the antenna switching unit 12. On the other hand, when the calculated Rave falls below a predetermined threshold, a control signal is sent to the antenna switching unit 12 so that the currently active antenna is continuously used.

 The control signal sent from the determination circuit 22 is converted into an analog signal by the DZA conversion 23 and sent to the antenna switching unit 12 as an analog control signal. The antenna switching unit 12 performs antenna switching control based on the input control signal.

[0079] Thus, when the symbol average value Rave of signal deviation (symbol average value averaged with less than 10 symbols) exceeds a predetermined threshold, the current activity of the antennas 11A and 11B is determined. When switching to an antenna is performed and the symbol average value Rave (average symbol value averaged with less than 10 symbols) falls below a predetermined threshold value, antennas 11A and 11 B The currently active antenna is selected as is.

As described above, according to the present embodiment, the positional deviation of the received signal on the constellation is averaged with less than 10 symbols (that is, “M” in Equation 2 is less than 10). Is).

 Accordingly, the received signal distortion in which the positional deviation on the constellation becomes relatively large over a period of several symbols (this phenomenon occurs frequently in a case where, for example, the Doppler shift frequency is low as described above. It is possible to easily set the predetermined threshold value that can be detected.

 That is, in the present embodiment, for example, the average symbol value Rave is calculated over a period of time comparable to the period of reception signal distortion that frequently occurs when the Doppler shift frequency is low (that is, about several symbols). Has been implemented.

 [0083] Therefore, the distortion of the signal occurs during the calculation of the symbol average value Rave, and the period is included! /, The symbol average value Rave calculated when the symbol average value Rave and the symbol average value Rave are calculated. If the signal is distorted and includes a period of time, the difference between the two symbol average values Rave can be increased when compared with the symbol average value Rave calculated in this case. This facilitates the setting of the predetermined threshold.

 [0084] As described above, in the OFDM receiving apparatus according to the present embodiment, the predetermined threshold value can be easily set, so that the CZN ratio of the received signal can be improved.

 The inventors measured the CZN ratio of the received signal by changing the value of “M” in Equation (2). As a result, the C ZN ratio of the received signal was significantly improved when the symbol average value Rave was calculated with less than 10 symbols. Also, when the value of “M” in Equation 2 is changed from several tens to several hundreds of symbols, the case where the value of “M” is set to less than 10 symbols as in this embodiment. However, the predetermined threshold value could be set more easily.

[0086] Further, as described above, in this embodiment, the symbol average value Rave is calculated over the number of symbols less than 10 symbols. Therefore, this is more effective than calculating the symbol average value Rave over tens or hundreds of symbols and performing antenna switching control. It is possible to improve the responsiveness of the antenna switching process when the received signal is distorted, and to further reduce the circuit scale of the determination circuit 22.

[0087] (Fifth embodiment)

 When the antenna switching process is performed in the OFDM receiver according to each of the above embodiments, the antenna switching process is performed over several symbols (approximately the length that can be processed by the symbol direction interpolation filter and less than 10 symbols). Due to this, the displacement on the constellation increases.

 [0088] Therefore, in spite of the fact that the received signal is distorted inherently (that is, due to a disturbance in the middle of transmission, etc.) or the distortion is small! /, Immediately after the antenna switching process. When the calculated symbol average value Rave is compared with a predetermined threshold, antenna switching processing may be performed. That is, the antenna switching process may be performed again due to the antenna switching process.

 [0089] Since the received signal received by the antenna newly selected by the switching process may be more distorted than the received signal received by the antenna before switching, there is a case where such a signal is received. The situation (that is, the situation where the antenna is switched although the received signal is not originally distorted or the distortion is small) is not preferable.

 [0090] In the present embodiment, an OFDM receiver for solving the problem is provided. In the OFDM receiver according to the present embodiment, the average of a plurality of symbols (Equation 2) is calculated avoiding a predetermined period after the switching unit selectively switches the two antennas.

In other words, the shift detection circuit 21 or the determination circuit 22 included in the OFDM receiver according to the present embodiment has a predetermined period after the antenna switching unit 12 selectively switches between the two antennas 11A and 11B. During the quiescent period consisting of, the signal misalignment on the constellation is not calculated.

Furthermore, after the quiescent period has elapsed, the deviation detection circuit 21 obtains an average value (Equation 1) of the signal position deviation on the constellation for all subcarriers in one symbol, and further, the determination circuit In 22, the average value of the signal position deviation (Equation 2) is obtained for a plurality of symbols. [0093] The operation of the OFDM receiver according to the present embodiment is the same as that of the OFD according to the first embodiment.

This will be described in comparison with the operation of the M receiver.

[0094] FIG. 8 is a diagram illustrating an example of an antenna switching operation of the OFDM receiver according to the first embodiment.

 In the state where the antenna 11 A is selected, the determination circuit 22 calculates the symbol average value R based on the calculation result from the deviation detection circuit 21 according to the procedure described in the first embodiment. Calculate ave. As a result of the comparison between the symbol average value Rave and the predetermined threshold by the determination circuit 22, if the symbol average value Rave exceeds the predetermined threshold, the antenna switching process is performed and the antenna 11B is selected. The Then, immediately after the antenna switching process, the determination circuit 22 calculates the symbol average value Rave again based on the calculation result from the deviation detection circuit 21 according to the procedure described in the first embodiment.

 [0096] FIG. 9 is a diagram illustrating an example of the antenna switching operation of the OFDM receiver according to the present embodiment.

 In the state where the antenna 11 A is selected, the determination circuit 22 calculates the symbol average value R based on the calculation result from the deviation detection circuit 21 according to the procedure described in the first embodiment. Calculate ave. As a result of the comparison between the symbol average value Rave and the predetermined threshold by the determination circuit 22, if the symbol average value Rave exceeds the predetermined threshold, the antenna switching process is performed and the antenna 11B is selected. The

 [0098] After the antenna switching process, the deviation detection circuit 21 and the determination circuit 22 do not calculate the positional deviation on the constellation for a predetermined period. The predetermined period in which the positional deviation on the constellation is not calculated is the stationary period.

 After the stationary period has elapsed, according to the procedure described in the first embodiment, the determination circuit 22 calculates the symbol average value Rave again based on the calculation result from the deviation detection circuit 21.

 [0100] Thus, in the present embodiment, the stationary period is set. That is, in the present embodiment, the positional deviation on the constellation is not calculated for a predetermined period immediately after the antenna switching process.

[0101] Therefore, even if the positional shift on the constellation increases due to the antenna switching process, the period during which the positional shift has increased and the calculation period of the symbol average value Rave And duplication can be reduced. As a result, even if the above-mentioned position shift becomes large due to the antenna switching process, it is possible to suppress the influence of the position shift becoming large in the calculation of the symbol average value Rave.

[0102] As described above, the positional deviation due to the antenna switching process occurs over several symbol periods (more specifically, a period of less than 10 symbols). Therefore, by setting the stationary period to at least 10 symbols or more, it is possible to completely eliminate the influence of the antenna switching process when calculating the symbol average value Rave.

 [0103] Further, as described in the fourth embodiment, the determination circuit 22 according to the present embodiment performs the symbol average value Rav of the signal position deviation for less than 10 symbols after the stationary period. e may be calculated, and a comparison process between a predetermined threshold and the calculated symbol average value Rave may be performed.

 [Sixth Embodiment]

 Figure 10 shows an example of the antenna switching operation of the OFDM receiver according to the first embodiment.

 [0105] In FIG. 10, it is assumed that a large distortion occurs in the received signal due to a disturbance in the middle of transmission with the antenna 11A selected (hereinafter, the distortion due to the factor is referred to as the original distortion). . “X” shown in FIG. 10 indicates a symbol whose positional deviation on the constellation calculated by Equation 1 exceeds a predetermined threshold. Assume that the original distortion occurs over several symbols and then disappears (or is not a problem from the point of view of normal signal reception!).

 In the above situation, the symbol average value Rave is calculated over a predetermined period based on Equation 2, and the calculated symbol average value Rave is compared with a predetermined threshold value. As a result, the antenna switching process is performed. And antenna 11B is selected. Here, as described above, a positional shift of the received signal on the constellation occurs due to the antenna switching process (this is referred to as distortion caused by the switching process for convenience).

[0107] If the operation described with reference to Fig. 10 is performed while applying force, the following problems occur. In other words, the original distortion occurred and the force was also switched, resulting in the switching process. Between the time when distortion occurs and the distortion caused by the switching process can be ignored, the total of the period when the original distortion occurred and the period when distortion caused by the switching process occurred (in the case shown in Fig. 10, During the period of 16 symbols), the OFDM receiver cannot receive signals normally.

 In the OFDM receiver according to the present embodiment, as can be seen from the following description, when the antenna switching process is performed, the period during which signals cannot be normally received can be further shortened.

 [0109] The OFDM receiver according to the present embodiment continuously detects the positional deviation (Equation 1) for each symbol calculated by the deviation detection circuit 21 from a predetermined threshold value over a predetermined number of symbols. When it exceeds, antenna switching processing is performed.

 [0110] Hereinafter, the operation of the OFDM receiving apparatus according to the present embodiment will be described more specifically with reference to FIG. Here, FIG. 11 is a diagram illustrating an example of the antenna switching operation of the OFDM receiver according to the present embodiment.

 [0111] The shift detection circuit 21 calculates the positional shift of the signal constellation for each symbol by averaging for all carriers (Equation 1). Then, the deviation detection circuit 21 outputs the calculation result toward the determination circuit 22. The determination circuit 22 compares the positional deviation (Equation 1) calculated by the deviation detection circuit 21 with a predetermined threshold value for each symbol.

 [0112] As shown in FIG. 11, it is assumed that the original distortion occurs in a state where the antenna 11A is selected. As a result of the comparison processing in the determination circuit 22, if the calculated positional deviation exceeds a predetermined threshold value continuously for a predetermined number of symbols (two symbols in FIG. 11), the determination circuit 22 A control signal for switching the receiving antenna is sent to unit 12.

 [0113] The control signal sent from the determination circuit 22 is converted into an analog signal by the DZA conversion 23 and sent to the antenna switching unit 12 as an analog control signal. The antenna switching unit 12 performs antenna switching control based on the input control signal. As a result, the antenna in the active state is also switched to the antenna 11B.

[0114] On the other hand, unlike FIG. 11, as a result of the comparison process performed for each symbol in the determination circuit 22, the calculated positional deviation is continuously performed with less than a predetermined number of symbols. Is greater than the predetermined threshold value, the determination circuit 22 sends a control signal to the antenna switching unit 12 so as to continue using the currently active antenna. Therefore, the currently active antenna (antenna 11A) is selected as it is among the antennas 11A and 11B.

 [0115] For example, the predetermined number of symbols is "3", and as a result of the comparison process performed for each symbol, the force that the calculated positional deviation exceeds a predetermined threshold value is continuously 2 symbols. Assume that there were (see Figure 12). In this case, the determination circuit 22 sends a control signal to the antenna switching unit 12 so as to continue using the currently active antenna. Therefore, the currently active antenna (antenna 11A) is selected as it is among the antennas 11A and 11B.

 As described above, according to the present embodiment, the antenna switching process is performed when the calculated positional deviation exceeds a predetermined threshold value for a predetermined number of symbols. Therefore, the period in which the original distortion occurs and the distortion period resulting from the switching process can be overlapped (see FIG. 11).

 [0117] Therefore, the period in which the OFDM receiver cannot normally receive the signal until the original distortion occurs and the force antenna switching process is performed and the distortion due to the antenna switching process disappears (see Fig. 11). In some cases, the period of 10 symbols can be reduced compared to the case of FIG.

 [0118] As is clear from the above, in order to overlap the period in which the original distortion occurs and the distortion period resulting from the switching process, the antenna is in the middle of the occurrence of the original distortion. It is necessary to perform switching processing.

 [0119] Generally, the period during which the original distortion occurs is the number of symbols of 2 or more and less than 20 symbols as a result of observation (when the Doppler shift frequency is low (this is the case of carrying and using an OFDM receiver) Therefore, the predetermined number of symbols is 2 or more and less than 20 symbols (low Doppler shift frequency, 10 symbols in some cases)! / Desirable to have a number of symbols! /.

[0120] It should be noted that the viewpoint of maximizing the superposition period (that is, the viewpoint of minimizing the period during which the OFDM receiver cannot normally receive a signal) and the measurement results by the inventors. As a result, it is desirable that the predetermined number of symbols is 2 to 4 symbols (any number of symbols 2 to 4).

[0121] Further, in order to prevent the antenna switching process caused by the distortion caused by the switching process from being performed, the quiescent period described in the fifth embodiment is set in the OFDM receiver according to the present embodiment. You may do it.

[0122] Although the present invention has been described in detail, the above description is illustrative in all aspects, and the present invention is not limited thereto. Innumerable variations not illustrated The power of the scope of the present invention It is understood that the power can be assumed without departing.

Claims

The scope of the claims  [1] Two antennas (11A, 11B) that receive OFDM signals,  A switching means (12) for selectively switching the two antennas and outputting a signal received by the selected antenna to the tuner (13);  Means (16) for performing an FFT operation on the output signal of the tuner force;  Means to equalize the signal after FFT calculation (17);  A calculation means (21) for calculating a positional deviation on the constellation for the signal after equalization processing;  When the positional deviation exceeds a predetermined threshold value, a control signal is given to the switching means to control the selected antenna to be switched to the other antenna, and the positional deviation is below the predetermined threshold value. Includes means (22) for controlling to use the selected antenna continuously,  An OFDM receiver characterized by comprising: [2] In the OFDM receiver according to claim 1,  The calculating means includes  An OFDM receiver comprising: means for calculating an average of signal misalignment on a constellation for all carriers. [3] In the OFDM receiver according to claim 1 or claim 2,  The calculating means includes  An OFDM receiver comprising: means for calculating an average of a positional deviation of a signal on a constellation by a plurality of symbols. [4] In the OFDM receiver according to claim 3,  The number of the plurality of symbols is  Less than 10 symbols,  An OFDM receiver characterized by that. [5] In the OFDM receiver according to claim 3,  In the means for calculating the positional deviation by averaging a plurality of symbols, Avoid the predetermined period after the switching means alternatively switches the two antennas, An average of the plurality of symbols is calculated;  An OFDM receiver characterized by that. [6] In the OFDM receiver according to claim 5,  The predetermined period is  At least 10 symbols or more,  An OFDM receiver characterized by that. [7] The OFDM receiver according to claim 2,  The calculating means includes the means for calculating the positional deviation by averaging for all carriers, and for each symbol, calculating the average for the positional deviation for all carriers,  The means for controlling switching or continuous use of the antenna is:  For each symbol, the positional deviation is compared with the predetermined threshold, and when the predetermined number of symbols continuously exceeds the predetermined threshold, a control signal is given to the switching means to select The selected antenna is controlled to be switched to the other antenna, and when the positional deviation exceeds the predetermined threshold continuously with less than the predetermined number of symbols, the selected antenna is continuously used. An OFDM receiver characterized by controlling as follows. [8] In the OFDM receiver according to claim 7,  The predetermined number of symbols is  2 !, then 4 !, the number of symbols,  An OFDM receiver characterized by that. [9] Two antennas (11A, 11B) that receive OFDM signals,  A switching means (12) for selectively switching the two antennas and outputting a signal received by the selected antenna to the tuner (13); A means (21, 19, 41) for measuring the signal quality of the signal output from the tuner, and when the signal quality is lower than a predetermined threshold value, a control signal is given to the switching means and selected. The antenna is controlled to switch to the other antenna, and if the signal quality exceeds the predetermined threshold, the selected antenna is continuously used. Means to control (22, 31, 42),  An OFDM receiver characterized by comprising: [10] Two antennas (11A, 11B) that receive OFDM signals,  A switching means (12) for selectively switching the two antennas and outputting a signal received by the selected antenna to the tuner (13);  Means (16) for performing an FFT operation on the output signal of the tuner force;  Means to equalize the signal after FFT calculation (17);  Means (18) for demapping the equalized signal;  Means (19) for calculating a bit error rate of the signal after the demapping process;  When the bit error rate exceeds a predetermined threshold, a control signal is supplied to the switching means to control the selected antenna to be switched to the other antenna, and the bit error rate is lower than the predetermined threshold. In this case, means (31) for controlling to use the selected antenna continuously,  An OFDM receiver characterized by comprising: [11] Two antennas (11A, 11B) that receive OFDM signals,  A switching means (12) for selectively switching the two antennas and outputting a signal received by the selected antenna to the tuner (13);  Means (41) for calculating a CZN ratio of the tuner force output signal;  When the CZN ratio falls below a predetermined threshold, a control signal is given to the switching means to control the selected antenna to be switched to the other antenna, and when the CZN ratio exceeds the predetermined threshold. Means to control the continuous use of the selected antenna (42),  An OFDM receiver characterized by comprising: [12] In the OFDM receiver according to any one of claim 1, claim 2, claim 9, claim 10, and claim 11,  The OFDM receiving apparatus, wherein the switching means performs antenna switching control over an interval equal to or greater than a predetermined number of symbols.