JP4157530B2 - Diversity receiver - Google Patents

Description

  The present invention relates to a diversity receiver.

  When a mobile phone is used in a vehicle that moves at a high speed, the reception level fluctuates due to a change in the propagation path of the received radio wave, and so-called fading phenomenon occurs. In order to alleviate the fading phenomenon, conventional diversity technology has been adopted in which vehicles or mobile phones are equipped with multiple receiving antennas, and the antenna with the highest reception level is selected or the received signals from each antenna are synthesized and output. Has been done. At this time, if the number of reception branches (the number of antennas) is increased, a large diversity gain can be obtained. However, the increase in diversity gain is smaller than the increase in the number of reception branches. For this reason, two-branch diversity has been put into practical use due to a trade-off between hardware scale and performance and cost constraints.

  As a diversity algorithm, the former algorithm, that is, a method of selecting an antenna having the highest reception level is generally adopted because of its ease of implementation. Actually, the received electric field strength constantly changes while the vehicle is traveling, and accordingly, for example, the reception state of each antenna also changes. In this case, in the conventional diversity algorithm, if even a slight level difference occurs between the antennas, the antenna is switched to an antenna having higher reception sensitivity, so that an antenna switching operation frequently occurs. As a result, in continuous reception systems other than systems that intermittently receive burst signals with a short frame length, such as TDMA and wireless LAN, deterioration of communication quality is noticeable due to frequent antenna switching even in a relatively stable reception state. Become.

  In Patent Document 1, an upper limit threshold and a lower limit threshold for antenna switching are provided, and antenna switching is performed so that a reception level is between these two thresholds and an antenna having a higher reception level is selected. A diversity reception method for avoiding demodulation failure due to frequent antenna switching when the reception level is too low or too high is disclosed.

On the other hand, Patent Document 2 shows a diversity control device that reduces the frequency of antenna switching by switching antennas when the absolute reception level difference between antennas exceeds a predetermined hysteresis threshold.
JP 2002-290296 A JP 2003-134014 A

  In general, a receiver employs automatic gain control (AGC) in which a signal amplitude is adjusted by a receiving unit (also referred to as a radio unit) in order to ensure a wide reception dynamic range. A received signal from the receiving unit is input to a digital signal processing unit, converted into a digital signal by an A / D converter, and then demodulated. The AGC in the receiving unit is such that even when an A / D converter with a minimum number of bits is used in the digital signal processing unit, the received signal amplitude is within the input dynamic range of the A / D converter. Adjust the amplitude.

  In the techniques of Patent Documents 1 and 2, antenna switching is performed depending on the reception level. Since AGC at the receiving unit cannot be handled immediately after antenna switching, when the reception level difference before and after antenna switching is large, the amplitude of the signal input to the A / D converter deviates from the input dynamic range of the A / D converter. Sometimes. In such a case, the output value of the A / D converter is saturated, and a correct digital value corresponding to the received signal is not output, so that correct demodulation cannot be performed.

  Accordingly, an object of the present invention is to provide a diversity receiver that can perform optimum antenna selection while avoiding saturation of an output value of an A / D converter that converts a received signal into a digital signal.

  According to a first aspect of the present invention, a plurality of antennas, a selection switch for selecting one of the antennas as a reception antenna, and a reception signal from the antenna selected by the selection switch are converted into digital signals. An A / D converter for conversion; a demodulator for demodulating the digital signal; and a first reception level of the first antenna currently selected by the selection switch and a second reception level of the second antenna of the next selection candidate Are both smaller than the first threshold, the first reception level is smaller than the second reception level, and the difference between the second reception level and the first reception level is smaller than the second threshold, the first antenna is replaced. A diversity receiver comprising switch control means for controlling the selection switch so that the second antenna is selected is provided.

  According to the second aspect of the present invention, N (N is a plurality of 3 or more) antennas and K (K is N> K and 2 or more) of the N antennas are selected. A selection switch, an A / D converter that converts received signals from the K antennas selected by the selection switch into digital signals, a demodulator that demodulates the digital signals, and an output signal from the demodulator. A combiner that performs diversity combining for each of the K first antennas having a relatively large first reception level that is currently selected by the selection switch; The second reception levels of the selection candidate second antennas are both smaller than the first threshold, the first reception level is smaller than the second reception level, and the difference between the second reception level and the first reception level is smaller than the second threshold. If the diversity receiver and a switch control means for the second antenna instead of the first antenna to control said selection switch to be selected is provided.

  In addition, for example, the switch control means, when both the first reception level and the second reception level are smaller than the first threshold and the difference between the second reception level and the first reception level is equal to or greater than the second threshold, The selection switch may be controlled so that the second antenna is selected instead of the first antenna. In this case, automatic gain control (AGC) gain setting means is provided.

  In one aspect, the gain setting means, for example, when the first reception level and the second reception level are both smaller than the first threshold and the difference between the second reception level and the first reception level is equal to or larger than the second threshold. The automatic gain control (AGC) gain is once set to an average value of the first AGC gain for the first antenna and the second AGC gain for the second antenna, and then changed to the second AGC gain.

  In another aspect, the gain setting means is automatic when the first reception level and the second reception level are both smaller than the first threshold and the difference between the second reception level and the first reception level is equal to or greater than the second threshold. A gain control (AGC) gain is changed stepwise from a first AGC gain for the first antenna to a second AGC gain for the second antenna.

  According to the present invention, optimal antenna selection can be performed while avoiding saturation of the output value of the A / D converter.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
(First embodiment)
First, the diversity receiver according to the first embodiment of the present invention will be described with reference to FIG. The diversity receiver according to the present embodiment uses a selection diversity system that selects one antenna as a receiving antenna from a plurality of antennas. Here, the case where the number of reception branches, that is, the number of antennas and reception units connected thereto is two will be described, but the number of reception branches may be any number of two or more.

  Radio frequency band signals (RF signals) received by the antennas 1 and 2 are input to the receiving units (radio units) 3 and 4, respectively, and reception processing with analog signals is performed. Each of the reception units 3 and 4 includes an RF band filter 21, a low noise amplifier (LNA) 22, a frequency converter 23, a channel selection filter 24, and a variable gain amplifier (VGA) 25 that are input as shown in FIG. Have. The RF signal input to the receiving unit is amplified by the low noise amplifier 22 through the filter 21 and then converted into a baseband signal by the frequency converter 23. Here, the RF signal is directly converted into a baseband signal. However, a two-stage frequency converter is used, and after the RF signal is converted into an intermediate frequency signal by the first-stage frequency converter, the second-stage frequency is converted. You may convert into a baseband signal with a converter.

  The output signal from the frequency converter 23 is input to the channel selection filter 24, and a desired channel component is selected. The signal of the selected channel is amplified by the AGC variable gain amplifier 25 and output from the receiving units 3 and 4. The gain of the variable gain amplifier 25 (hereinafter referred to as AGC gain) is controlled so that the output signal level of the receiving units 3 and 4 is substantially constant, or is controlled by a baseband signal processing unit as will be described later. .

  Output signals from the receiving units 3 and 4 are input to the selection switch 5. The selection switch 5 selects an antenna to be used as a reception antenna under the control of the controller 9. In the present embodiment, antenna selection by the selection switch 5 is performed on the output side of the receiving units 3 and 4. That is, the selection switch 5 selects one of the output signals of the receiving antennas 1 and 2. The signal selected by the selection switch 5 is converted into a digital signal by the A / D converter 6 and then demodulated by the demodulator 7. The demodulated signal is further decoded by a decoder (not shown) to reproduce the original data.

  The reception level measuring device 8 measures the reception levels of the antennas 1 and 2, for example, received signal strength indicator (RSSI), at a constant interval using the reception signals of the reception units 3 and 4. As an input of the reception level measuring device 8, for example, the output signal of the low noise amplifier 22 in FIG. 2 can be used. In the following description, the reception level measured by the reception level measuring device 8 is also referred to as an RSSI value for convenience. The controller 9 determines which of the antennas 1 and 2 is selected from the RSSI value obtained by the reception level measuring device 8, and controls the switch 5 based on the determination result.

  For example, as shown in FIG. 3, the controller 9 sequentially stores the RSSI value measured by the reception level measuring device 8 and the RSSI value of the next selection candidate antenna from the RSSI value stored in the memory 31. A predictor 32 for prediction, a determiner 33 for determining a threshold value of the RSSI value predicted by the predictor 32, and a switch controller 34 for controlling the selection switch 5 according to the determination result of the determiner 33 are provided.

  The reference for selecting the reception antenna by the selection switch 5 under the control of the controller 9 is, for example, the reception level (RSSI value). Of the antennas 1 and 2, the higher reception level is selected as the reception antenna. As shown in FIG. 4, when the reception level is sufficiently high, the S / N of the baseband signal is saturated regardless of the magnitude of the reception level due to the influence of distortion and phase noise inside the receiver. End up. When the reception level of an antenna at which the S / N of the baseband signal is saturated is R [dBm], the reception quality does not change even if any antenna whose reception level exceeds R is selected. R depends on the circuit design of the receiver. As is also a problem in Patent Document 1, if the antenna is frequently switched, the amplitude and phase of the received signal may jump abruptly, leading to a demodulation error and loss of synchronization. It is desirable to control so as not to switch the antenna when the number is higher.

  In a receiver that performs AGC of a received signal for each antenna, the signal level after being converted into a digital signal by the A / D converter 6 is basically set to an appropriate amplitude at each antenna. On the other hand, when the number of baseband signal processing units that perform AGC control is small relative to the number of reception units, optimal AGC cannot be performed for all reception units. In such a case, the AGC is performed only for the receiving unit currently selected by the switch 5, or the AGC gains for all the receiving units are controlled in common.

  Here, when the temporal fluctuation of the reception level is fast, when the reception antenna is switched from an antenna having a high reception level at time t1 to an antenna having a high reception level at time t2, the signal input to the A / D converter 6 is changed. Rapid fluctuation occurs. As a result, the signal level deviates from the input dynamic range of the A / D converter 6 and the output value of the A / D converter 6 is saturated, so that a demodulation error or loss of synchronization of the demodulator 7 may occur. In particular, when the AGC follow-up speed is low, saturation continues for a long time after antenna switching, so that problems such as decoding errors and loss of synchronization are significant.

  In such a case, in the present embodiment, a constraint condition that “the antenna switching is permitted when the difference between the RSSI values before and after the antenna switching becomes smaller than a certain threshold D (second threshold)” is set. As a result, it is possible to avoid decoding errors and loss of synchronization due to saturation of the output value of the A / D converter 6 at the time of antenna switching, and perform high-quality and stable communication.

  Further, when the reception level fluctuates very quickly, the antenna switching timing (time t2) is obtained by performing antenna switching based on the RSSI value at the latest measurement timing (time t1) in the reception level measuring device 8. There is a concern that the optimum antenna is no longer selected. In such a case, by predicting the RSSI value at the time t2 by extrapolation using the RSSI values at the times t0 and t1 (t0 <t1), it is possible to select an optimum antenna at the timing of switching the antenna, and the reception quality Can be prevented.

  As an example of extrapolation, the RSSI value RSSI (t2) at time t2 is obtained by linear interpolation as follows using the RSSI values RSSI (t0) and RSSI (t1) at the past two times t0 and t1. Conceivable.

Next, a specific antenna selection procedure in the present embodiment will be described with reference to the flowchart of FIG.
First, assume that antenna 1 is initially selected. The RSSI value when the antenna 1 is used is periodically measured by the reception level measuring device 8 (step S1), and sequentially stored in the memory 31 (step S2). Next, based on the RSSI value stored in the memory 31, the predictor 32 predicts RSSI value = RSSI1 (t2) at time t2 when the next antenna switching is determined (step S3). Next, the determination unit 33 determines RSSI (t2) as a threshold, that is, compares RSSI1 (t2) with the threshold R (step S4). Here, if RSSI1 (t2) is larger than the threshold value R, it is considered that there is sufficient S / N, so the antenna is not switched (step S5).

  On the other hand, when RSSI1 (t2) is smaller than the threshold value R, the prediction is made in step S8 based on the RSSI value of the antenna 2 measured in step S6 and stored in the memory 31 in step S7 as in steps S1 to S3. The RSSI value = RSSI2 (t2) is compared with the threshold value R (step S9). If RSSI2 (t2) is smaller than R in step S9, the determiner 33 compares the RSSI values of the antenna 1 and the antenna 2 = RSSI1 (t2), RSSI2 (t2) (step S10).

  If RSSI1 (t2) is larger than RSSI2 (t2) in step S10, antenna switching is not performed (step S5), and antenna 1 continues to be used as a receiving antenna. If RSSI2 (t2) is larger than RSSI1 (t2) in step S10, the determination unit 33 sets RSSI value before selecting the next antenna (before antenna switching) = Ra (in this case, Ra = RSSI1 (t1)). ) And the next selection candidate antenna (in this case, antenna 2) RSSI value = Rb (in this case, Rb = RSSI2 (t2))) to obtain the absolute difference | Rb−Ra | Rb−Ra | is compared with a threshold value D (step S11). If | Rb−Ra | is smaller than the threshold value D, the receiving antenna is switched from antenna 1 to antenna 2 (step S12). Similarly, if RSSI2 (t2) is greater than or equal to the threshold value R in step S9, the RSSI difference before and after antenna switching is compared with the threshold value D (step S11).

  When | Rb−Ra | is very large, the input level of the A / D converter 6 is suddenly changed and the output value of the A / D converter 6 is saturated. In order to avoid such a problem, the antenna is not switched when | Rb−Ra | is equal to or greater than the threshold D as described above. As the threshold value D, for example, a backoff amount of the target gain of AGC from the maximum input level of the A / D converter 6 can be adopted, and is set to a value of, for example, 12 dB.

(Second Embodiment)
Next, a second embodiment of the present invention will be described. In the second embodiment, as shown in FIG. 6, the controller 10 is also connected to the receiving units 3 and 4, and in addition to the control of the selection switch 5 similar to the controller 9 in FIG. The gain of the variable gain amplifier 33 is set. Therefore, in addition to the memory 31, the predictor 32, the determiner 33, and the switch controller 34 similar to the controller 9 shown in FIG. 3, the controller 10 is based on the determination result of the determiner 33, as shown in FIG. A gain setting unit 35 for setting the gain of the variable gain amplifier 33 is added.

  It is assumed that the antenna 1 having a relatively large reception level is selected when both the reception levels of the signals of the antenna 1 and the antenna 2 are low. In the first embodiment, after that, only the reception level of the antenna 2 suddenly increases, and when the reception level difference between the antenna 1 and the antenna 2 (absolute difference in RSSI value) is greater than or equal to the threshold value D, the A / D converter In order to avoid collapse of the output of the A / D converter 6 due to the saturation of the output value of 6 and the influence of quantization noise, the antenna 1 continues to be used as it is despite the reception level of the antenna 2 being sufficiently high. In the second embodiment, in such a case, the reception antenna is switched from the antenna 1 to the antenna 2 and the AGC gain is changed at the same time, thereby avoiding the saturation of the output value of the A / D converter 106, and the S Select an antenna with a high / N.

  In the antenna selection procedure in this embodiment, steps S13 to S15 are added in addition to FIG. 5 showing the antenna selection procedure of the first embodiment as shown in FIG. At the time of antenna switching, the AGC gain of the receiving unit connected to the antenna of the next selection candidate is set to G. In step S11, the difference between the RSSI value before antenna switching = Ra (for example, Ra = RSSI1 (t1)) and the RSSI value of the next candidate antenna = Rb (for example, Rb = RSSI2 (t2))) | Rb−Ra If | is equal to or greater than the threshold value D, the AGC gain of the receiving unit 4 connected to the antenna 2 is set to G (step S13), and the receiving antenna is switched from the antenna 1 to the antenna 2 (step S14).

  When the AGC gain (G1) of the receiving unit 3 connected to the antenna 1 before switching and the AGC gain (G2) of the receiving unit 4 connected to the antenna 2 that is the next selection candidate are known, the antenna The AGC gain G (t2) at the switching time t2 is set to an average value of G1 and G2, for example, as follows.

  As a result, the saturation of the output value of the A / D converter 6 can be more effectively reduced as compared with the case where the antenna is switched without changing the gain. Then, after an appropriate time has elapsed (for example, at time t3), the gain G is updated again so that the gain G ′ becomes an appropriate value for the demodulator 7 so that the input amplitude to the A / D converter 6 becomes an appropriate value ( Step S15), and thereafter the original AGC operation is performed. Alternatively, when the AGC of the receiving unit connected to the antenna of the selection candidate at the next antenna selection timing is not performed, the AGC gain G ′ ( t2) may be determined.

  Further, by taking a fine resolution when changing the AGC gain stepwise, it is possible to avoid demodulation errors caused by sudden amplitude fluctuations. For example, when the change from the AGC gain G1 to G2 is divided into n steps, the gain change step amount is Δ = (G2−G1) / n, and G1 + Δ, G1 + 2Δ,..., G1 + (n−1) * It is changed in steps like Δ and G2. For the period T in which the AGC gain is changed, the number n of gain change steps depends on the AGC control speed f (period T = 1 / f). In this way, especially when the absolute difference between the RSSI values of the antennas 1 and 2 is large, there is no large stepwise amplitude fluctuation due to the gain adjustment range being too large when the AGC gain is made closer to G1 from G1. The occurrence of demodulation errors due to amplitude fluctuations can be avoided.

(Third embodiment)
Next, a third embodiment according to the present invention will be described with reference to FIG. In this embodiment, the number of reception branches is N (N is a plurality of 3 or more), and a combined diversity method is used. RF signals received by the N antennas 41-1 to 41-N are respectively input to reception units (radio units) 42-1 to 42-N as shown in FIG. 2 and subjected to reception processing.

  Output signals from the receiving units 42-1 to 42-N are input to the N input / K output selection switch 43. The selection switch 43 selects K antennas (K is N> K and 2 or more) among the N antennas 41-1 to 41-N under the control of the controller 48, that is, the receiving units 42-1 to 42-. The output signal from N is selected. In FIG. 9, N = 4 and K = 2 are set as an example.

  The signal selected by the selection switch 43 is converted into a digital signal by the A / D converters 44-1 to 44-K, and then demodulated by the demodulators 45-1 to 45-K. The output signals from the demodulators 45-1 to 45-K are subjected to synthesis diversity by digital signal processing by the diversity synthesizer 46, and become one output signal. The output signal from the diversity synthesizer 46 is decoded by a decoder (not shown) to reproduce the original data. Demodulators 45-1 to 45-K appropriately perform the AGC gains of the receiving units connected to the currently selected K antennas, and the other AGC gains are controlled in the other receiving units. Is held.

  The reception level measuring device 47 measures the reception levels of the antennas 41-1 to 41-N, for example, RSSI at a constant interval using the reception signals of the reception units 42-1 to 42-N, and the RSSI value is determined by the controller 48. Notify The controller 48 determines K antennas in order from the highest reception level (RSSI value) among the antennas 41-1 to 41-N from the notified RSSI value, and controls the switch 43 based on the determination result. The controller 48 is basically configured as shown in FIG. 3, for example, as described in the first embodiment.

  Next, the selection procedure of K antennas in the selection switch 43 will be described with reference to FIG. Of the antennas 41-1 to 41-N, it is fundamental to select K antennas in descending order of reception level (RSSI value).

  First, it is assumed that two antennas A and B (hereinafter also referred to as AntA (t1) and AntB (t1)) are selected at the current time t1. The RSSI values of all the antennas 41-1 to 41-N are periodically measured (step S101) and sequentially stored in the memory (step S102). Based on the accumulated RSSI value, the RSSI value at time t2 when the antenna switching is determined next is predicted (step S103).

  The RSSI values of the currently selected antennas AntA (t1) and AntB (t1) are Pa (t1) and Pb (t1), and the antenna having the maximum RSSI value at time t2 is Ant1 (t2). The second, third, and fourth antennas in the descending order are Ant2 (t2), Ant3 (t2), and Ant4 (t2), respectively. Further, the RSSI values of the antennas Ant1 (t2) to Ant4 (t2) at time t2 are P1 (t2), P2 (t2), P3 (t2), and P4 (t2). The antenna number and RSSI value of the currently selected antenna, and further the set of the antenna number and RSSI value of the antenna selected at time t2 are stored in the memory (step S104).

  Next, the RSSI value Pa (t1) of one antenna AntA (t1) out of the currently selected antennas AntA (t1) and AntB (t1) is compared with the threshold R (step S105), and Pa (t1) If it is larger than R, it is considered that the S / N of the output of the antenna AntA (t1) is sufficiently large, so the selection of the antenna AntA (t1) is maintained (step S106).

  Subsequently, the threshold R is compared with RSSIPb (t1) of the other antenna AntB (t1) of the currently selected antennas AntA (t1) and AntB (t1) (step S107), and Pb (t1) is greater than the threshold R. If it is larger, the S / N of the output of the antenna AntB (t1) is considered to be sufficiently large, so the selection of the antenna AntB (t1) is maintained (step S108).

  If Pb (t1) is equal to or smaller than the threshold value R in step S107, the process proceeds to processing for selecting one of the three antennas other than antenna A (FIG. 11) (step S109).

  On the other hand, if Pa (t1) is less than or equal to the threshold R in step S105, the RSSI value Pb (t1) of the currently selected antenna AntB (t1) is compared with the threshold R (step S110), and Pb (t1) is If it is larger than the threshold value R, the S / N of the output of the antenna AntB (t1) is considered to be sufficiently large. Therefore, the selection of the antenna AntB (t1) is maintained (step S111), and one of the three antennas other than the antenna B is selected. The process proceeds to the process of selecting (FIG. 12) (step S112). Furthermore, if Pb (t1) is greater than or equal to the threshold value R in step S110, the process proceeds to the process of selecting two from the four antennas (FIG. 13) (step S113).

  Next, the procedure of step S109 in FIG. 10 for selecting one of the three antennas other than antenna A will be described with reference to FIG. In step S109, first, the absolute difference between the RSSI value Pb (t1) of the currently selected antenna AntB (t1) and the RSSI value P1 (t2) of the antenna Ant1 (t2) having the maximum RSSI value at time t2 | Pb (t1) -P1 (t2) | is obtained and compared with the threshold value D (step S201). If | Pb (t1) -P1 (t2) | is smaller than D, Ant1 (t2) is selected ( Step S205).

  If | Pb (t1) −P1 (t2) | is larger than D in step S201, the RSSI value Pb (t1) of antenna B and the RSSI value of antenna Ant2 (t2) having the second largest RSSI value at time t2 The absolute difference | Pb (t1) −P2 (t2) | of P2 (t2) is obtained and compared with the threshold value D (step S202). When | Pb (t1) −P2 (t2) | is smaller than D, Ant2 (t2) is selected (step S206).

  If | Pb (t1) −P2 (t2) | is greater than D in step S202, the RSSI value Pb (t1) of antenna B and the RSSI value of antenna Ant3 (t2) having the third largest RSSI value at time t2 An absolute difference of P3 (t2) is obtained and compared with a threshold value D (step S203). If | Pb (t1) −P3 (t2) | is smaller than D, Ant3 (t2) is selected (step S207). Otherwise, the selection of the currently selected antenna AntB (t1) is maintained (step S204).

  Next, the procedure of step S112 in FIG. 10 for selecting one of the three antennas other than the antenna B will be described with reference to FIG. In step S112, first, the absolute difference between the RSSI value Pa (t1) of the currently selected antenna AntA (t1) and the RSSI value P1 (t2) of the antenna Ant1 (t2) having the maximum RSSI value at time t2 | Pa (t1) -P1 (t2) | is obtained and compared with the threshold value D (step S210). If | Pa (t1) -P1 (t2) | is smaller than D, Ant1 (t2) is selected ( Step S214).

  If | Pa (t1) −P1 (t2) | is greater than D in step S210, the RSSI value Pa (t1) of the antenna AntA (t1) and the antenna Ant2 (t2) having the second largest RSSI value at the time t2. Absolute difference | Pa (t1) −P2 (t2) | is obtained and compared with a threshold value D (step S211), and | Pa (t1) −P2 (t2) | is smaller than D In this case, Ant2 (t2) is selected (step S215).

  If | Pa (t1) −P2 (t2) | is greater than D in step S211, the RSSI value Pa (t1) of the antenna AntA (t1) and the antenna Ant3 (t2) having the third largest RSSI value at the time t2. Absolute difference | Pa (t1) −P3 (t2) | is obtained and compared with the threshold D (step S212), and | Pa (t1) −P3 (t2) | is smaller than D In the case, Ant3 (t2) is selected (step S216). Otherwise, the selection of the currently selected antenna AntA (t1) is maintained (step S213).

  Next, the procedure of step S113 in FIG. 10 for selecting two from the four antennas will be described with reference to FIG. In step S113, an absolute difference | Pa (t1) −P1 (t2) | between Pa (t1) and P1 (t2) is obtained and compared with a threshold value D (step S301), and | Pa (t1) −P1 (t2). ) | Is smaller than D, Ant1 (t2) is selected and the process proceeds to step S310.

  If | Pa (t1) −P1 (t2) | is larger than D in step S301, the absolute difference | Pa (t1) −P2 (t2) | Compared with D (step S302), if | Pa (t1) −P2 (t2) | is smaller than D, Ant2 (t2) is selected and the process proceeds to step S410.

  If | Pa (t1) −P2 (t2) | is larger than D in step S302, the absolute difference | Pa (t1) −P3 (t2) | Compared with D (step S303), if | Pa (t1) −P3 (t2) | is smaller than D, Ant3 (t2) is selected and the process proceeds to step S510.

  If | Pa (t1) −P3 (t2) | is larger than D in step S303, antenna switching is prohibited and the currently selected antenna is used as it is (step S304).

  If Ant1 (t2) is selected in step S310, the procedure proceeds to a procedure for selecting another antenna. The absolute difference | Pb (t1) −P2 (t2) | is obtained from Pb (t1) and P2 (t2) and compared with the threshold value D (step S311), and | Pb (t1) −P2 (t2) | If it is smaller, Ant2 (t2) is selected (step S312).

  If | Pb (t1) −P2 (t2) | is larger than D in step S311, the absolute difference | Pb (t1) −P3 (t2) | Compared with D (step S313), if | Pb (t1) −P3 (t2) | is smaller than D, Ant3 (t2) is selected (step S314).

  If | Pb (t1) −P3 (t2) | is greater than D in step S313, the absolute difference | Pb (t1) −P4 (t2) | between Pb (t1) and P4 (t2) | Compared with D (step S315), if | Pb (t1) −P4 (t2) | is smaller than D, Ant4 (t2) is selected (step S316).

  If | Pb (t1) −P4 (t2) | is greater than D in step S315, the currently selected antenna AntB (t1) is used as it is (step S317). However, if AntB is the same antenna as Ant1, Ant2 is selected.

  If Ant2 (t2) is selected in step S410, the procedure proceeds to a procedure for selecting another antenna. An absolute difference | Pb (t1) −P1 (t2) | between Pb (t1) and P1 (t2) is obtained and compared with a threshold value D (step S411), and | Pb (t1) −P1 (t2) | If it is smaller, Ant1 (t2) is selected (step S412).

  If | Pb (t1) −P1 (t2) | is greater than D in step S411, the absolute difference | Pb (t1) −P3 (t2) | Compared with D (step S413), if | Pb (t1) −P3 (t2) | is smaller than D, Ant3 (t2) is selected (step S414).

  If | Pb (t1) −P3 (t2) | is larger than D in step S413, the absolute difference | Pb (t1) −P4 (t2) | between Pb (t1) and P4 (t2) | Compared with D (step S415), if | Pb (t1) −P4 (t2) | is smaller than D, Ant4 (t2) is selected (step S416).

  If | Pb (t1) −P4 (t2) | is larger than D in step S415, the currently selected antenna AntB (t1) is used as it is (step S417). However, if AntB is the same antenna as Ant2, Ant1 is selected.

  If Ant3 (t2) is selected in step S510, the procedure proceeds to a procedure for selecting another antenna. The absolute difference | Pb (t1) −P1 (t2) | is obtained from Pb (t1) and P1 (t2) and compared with the threshold value D (step S511), and | Pb (t1) −P1 (t2) | If it is smaller, Ant1 (t2) is selected (step S512).

  If | Pb (t1) −P1 (t2) | is greater than D in step S511, the absolute difference | Pb (t1) −P2 (t2) | Compared with D (step S513), if | Pb (t1) −P2 (t2) | is smaller than D, Ant2 (t2) is selected (step S514).

  If | Pb (t1) −P2 (t2) | is larger than D in step S513, the absolute difference | Pb (t1) −P4 (t2) | between Pb (t1) and P4 (t2) | Compared with D (step S515), if | Pb (t1) −P4 (t2) | is smaller than D, Ant4 (t2) is selected (step S516).

  If | Pb (t1) −P4 (t2) | is greater than D in step S515, the currently selected antenna AntB (t1) is used as it is (step S517). However, if AntB is the same antenna as Ant3, Ant1 is selected.

  FIG. 14 shows a specific example of antenna selection according to the above-described procedure. Numerical values representing the RSSI values of the antennas # 1, # 2, # 3, and # 4 at time t1 in dBm units are RSSI (t1), and are −50 dBm, −60 dBm, −62 dBm, and −58 dBm, respectively. In this case, it is assumed that antennas # 1 and # 4 are selected. Assume that RSSI (t2) at time t2 for determining the next antenna switching is predicted to be −45 dBm, −47 dBm, −52 dBm, and −56 dBm as shown in FIG.

  At this time, when two antennas are selected in descending order of the RSSI value, antennas # 1 and # 2 are selected. However, by switching from antenna # 4 to # 1, the absolute difference between the RSSI values becomes 11 dB, Since it exceeds the predetermined back-off value D = 10 dB, the output values of the A / D converters 46-1 to 46-K are saturated, and a demodulation error occurs. In order to avoid such a phenomenon, the antenna # 3 having the third RSSI value that satisfies D <10 dB is selected in the present embodiment. As a result, at time t2, antenna # 1 is left as it is, and an operation of switching from antenna # 4 to # 3 is performed.

  In this case, when the absolute difference between the RSSI values before and after the switching of the second antenna exceeds D, it is also effective to use the antenna used at time t1 as it is at time t2 without performing antenna switching. is there.

  Demodulators 45-1 to 45-K in FIG. 9 simultaneously perform time and frequency synchronization processing and the like. Here, any one of the demodulators 45-1 to 45K is responsible for most of these synchronization processes as a master branch, and the other demodulators are synchronized as a result of processing in the master branch as slave branches. Such a form is also conceivable. In this case, whether or not the synchronization is lost depends on the synchronization processing of the master branch. Therefore, when selecting an antenna, a constraint condition is set such that the antenna connected to the master branch and the slave branch is not switched to another antenna at the same time. For example, the antenna switching is not permitted unless the reception level of the master branch is lowered to such an extent that the synchronization is lost. Thereby, in the antenna branch connected to the demodulator of the master branch, the influence of phase discontinuity due to antenna switching can be reduced as much as possible, and errors can be made difficult to occur.

  The diversity receiver according to the third embodiment described above is particularly useful as a receiver of an orthogonal frequency division multiplexing (OFDM) system. For example, in an OFDM receiver, a mode in which switching diversity processing is performed in the time domain before Fast Fourier Transform (FFT) processing and a mode in which the switching diversity processing is performed in the frequency domain after FFT processing are conceivable. In the former case, an optimum diversity gain cannot be obtained particularly when there are many delayed waves and the frequency selective fading is affected. For this reason, the form which performs diversity for every subcarrier in the frequency domain after the latter FFT processing is examined. In particular, S / N (signal to noise power ratio) can be maximized by employing independent maximum ratio combining diversity for each subcarrier.

  In this case as well, the subcarrier maximum ratio combining diversity of two elements is appropriate as in the above consideration. At this time, since the diversity effect after the FFT processing is weakened when the levels of the received signals of the two antennas are low, it is desirable that the antenna has as high a received power as possible. Therefore, as described in the third embodiment, N (N is a plurality of three or more) antennas are provided, and K = 2 antennas are selected from the highest reception level, and diversity combining processing is performed. By doing so, we can expect great improvements. In this case, an FFT unit is included in the demodulators 45-1 to 45K, and diversity combining processing by the diversity combiner 46 is performed on the signal after FFT processing.

  In the case of an OFDM system, a guard interval (GI) obtained by copying a part of a symbol is usually inserted for each OFDM symbol. As is well known, the influence of intersymbol interference can be reduced by performing FFT processing after removing the GI portion when there is a delayed wave. In this case, it is preferable to perform diversity antenna switching at a certain timing within the GI. The start timing of the GI is obtained in the process of obtaining frame synchronization and symbol synchronization before FFT. As a result, antenna switching in the data symbol following GI does not occur, and a sudden change in phase does not occur, so that the probability of occurrence of bit errors can be reduced.

  On the other hand, it is also effective to perform antenna switching in a section other than the GI, that is, in the data part in the OFDM symbol. This is because GI is often used in synchronization processing before FFT processing, and there is a method that uses the phase continuity of a signal in GI. Therefore, when antenna switching is performed in GI, synchronization occurs due to a phase discontinuity state. There is a high probability of acquisition failure. In such a case, it is better to first perform antenna switching in the data section in order to give first priority to establishing synchronization. In this case, since there is a phase discontinuity in the data, the orthogonality between subcarriers in the signal after FFT processing is lost, and an error may occur in the symbol depending on the subcarrier. However, errors may be reduced by the effects of deinterleaving and error correction codes, and as a result, no errors may occur. In other words, it is a method that puts the highest priority on keeping out of sync. From the above, when the diversity receiver of this embodiment is applied to an OFDM receiver, antenna switching may be performed at an arbitrary timing.

  In the case of packet communication, the above-described problem does not occur by performing antenna switching in a portion not involved in data decoding, such as a preamble at the beginning of a packet. However, in the case of a receiver that receives a continuous signal such as a terrestrial digital broadcast signal, there is no preamble and the effect of antenna switching occurs.

  In systems where time interleaving is used to remove the effects of fading, such as digital terrestrial broadcasting, the probability of symbol errors due to antenna switching is reduced by making the minimum time interval for switching antennas longer than the time interleaving length. be able to. When there are two or more antenna switchings within the interleaving time, the error correction code correction capability is exceeded, and conversely, error bits frequently occur, which can be avoided.

(Fourth embodiment)
Next, a fourth embodiment according to the present invention will be described. In the fourth embodiment, the configuration of the receiver is the same as in FIG. 6 of the third embodiment, and the antenna selection method is different from that of the third embodiment as follows.

  First, it is assumed that two antennas A and B are selected at the current time t1. In the subsequent processing, the processing flow before step S109, step S112, and step S113 is as shown in FIG. 10 as in the third embodiment.

  The procedure of step S109 in FIG. 10 for selecting one of the three antennas other than antenna A will be described with reference to FIG. In step S109, an absolute difference | Pb (t1) −P1 (t2) | between Pb (t1) and P1 (t2) is obtained and compared with a threshold value D (step S601), and | Pb (t1) −P1 (t2) If | is smaller than D, Ant1 (t2) is selected (step S604).

  If | Pb (t1) −P1 (t2) | is larger than D in step S601, after selecting Ant1 (t2), the AGC gain of the receiving unit is set to G (step S602). When the AGC gain G1 of the receiving unit connected to the antenna before switching and the AGC gain G2 of the receiving unit connected to the antenna of the selection candidate at time t2 are known, G at time t2 is set to G1 and G2, for example. The average value of Then, after an appropriate time, for example, at time t3, G is updated again so as to asymptotically approach the gain G ′ so that the input amplitude to the A / D converter 6 becomes an appropriate value for the demodulator 7 (step S603). After that, the original AGC operation is performed.

  Next, the procedure of step S112 in FIG. 10 for selecting one of the three antennas other than the antenna B will be described with reference to FIG. In step S112, an absolute difference | Pa (t1) −P1 (t2) | between Pa (t1) and P1 (t2) is obtained and compared with a threshold value D (step S605), and | Pa (t1) −P1 (t2). ) | Is smaller than D, Ant1 (t2) is selected (step S608).

  If | Pa (t1) −P1 (t2) | is greater than D in step S605, the same processing as in FIG. 12 is performed thereafter (step S606 and step S607).

  Next, the procedure of step S113 in FIG. 10 for selecting two from the four antennas will be described with reference to FIG. In step S113, an absolute difference | Pa (t1) −P1 (t2) | between Pa (t1) and P1 (t2) is obtained and compared with a threshold value D (step S610), and | Pa (t1) −P1 (t2). ) | Is smaller than D, Ant1 (t2) is selected (step S616).

  If | Pa (t1) −P1 (t2) | is greater than D in step S610, after selecting Ant1 (t2), the AGC gain of the receiving unit is set to G (step S612). When the AGC gain G1 of the receiving unit connected to the antenna before switching and the AGC gain G2 of the receiving unit connected to the antenna of the selection candidate at time t2 are known, G at time t2 is set to G1 and G2, for example. The average value of Then, after an appropriate time, for example, at time t3, G is again updated so that the input amplitude to the A / D converter 6 becomes asymptotic to the gain G ′ so that the input amplitude to the demodulator 7 becomes an appropriate value (step After step S613), the original AGC operation is performed.

  Further, after step S610, an absolute difference | Pb (t1) −P2 (t2) | between Pb (t1) and P2 (t2) is obtained and compared with a threshold value D (step S611), and | Pb (t1) −P2 If (t2) | is smaller than D, Ant2 (t2) is selected (step S617). If | Pb (t1) −P2 (t2) | is greater than D in step S611, after selecting Ant2 (t2), the AGC gain of the receiving unit is set to G (step S614). The AGC gain is updated so as to asymptotically approach G 'after an appropriate time (step S615).

  Further, as in the second embodiment, the update step width of the AGC gain may be reduced and the update in n stages may be performed.

  Note that the present invention is not limited to the above-described embodiment as it is, and can be embodied by modifying the constituent elements without departing from the scope of the invention in the implementation stage. In addition, various inventions can be formed by appropriately combining a plurality of constituent elements disclosed in the embodiment. For example, some components may be deleted from all the components shown in the embodiment. Furthermore, constituent elements over different embodiments may be appropriately combined.

The block diagram of the diversity receiver which concerns on the 1st Embodiment of this invention Block diagram of an example of the receiving unit in FIG. Block diagram of an example of the controller in FIG. The figure which shows the relationship between a reception level and S / N of a baseband signal The flowchart explaining the antenna selection procedure in the 1st Embodiment of this invention. The block diagram of the diversity receiver which concerns on the 2nd Embodiment of this invention Block diagram of an example of the controller in FIG. The flowchart explaining the antenna selection procedure in the 2nd Embodiment of this invention. The block diagram of the diversity receiver which concerns on the 3rd Embodiment of this invention The flowchart explaining the antenna selection procedure in the 3rd Embodiment of this invention. The flowchart explaining the antenna selection procedure in the 3rd Embodiment of this invention. The flowchart explaining the antenna selection procedure in the 3rd Embodiment of this invention. The flowchart explaining the antenna selection procedure in the 3rd Embodiment of this invention. The figure explaining the antenna selection procedure in the 3rd Embodiment of this invention. The flowchart explaining the antenna selection procedure in the 4th Embodiment of this invention. The flowchart explaining the antenna selection procedure in the 4th Embodiment of this invention. The flowchart explaining the antenna selection procedure in the 4th Embodiment of this invention.

Explanation of symbols

1, 2, ... antennas;
3, 4 ... receiving unit;
5 ... Antenna selection switch;
6 ... A / D converter;
7 ... demodulator;
8: Reception level measuring device;
9, 10 ... Controller;
31 ... Memory;
32 ... Predictor;
33 ... Judgment device;
34 ... switch controller;
35 ... Gain setting device;
41-1 to 41-N ... antenna;
42-1 to 42-N: reception unit level 43 ... antenna selection switch;
44-1 to 44-K ... A / D converter;
45-1 to 45-K ... demodulator;
46: Diversity synthesizer;
47. Reception level measuring device;
48 ... Controller.

Claims (5)

Multiple antennas,
A selection switch for selecting one of the antennas;
An A / D converter that converts a received signal from the antenna selected by the selection switch into a digital signal;
A demodulator for demodulating the digital signal;
The first reception level of the first antenna currently selected by the selection switch and the second reception level of the second antenna of the next selection candidate are both smaller than the first threshold, and the first reception level is smaller than the second reception level. And a switch control means for controlling the selection switch so that the second antenna is selected instead of the first antenna when a difference between the second reception level and the first reception level is smaller than a second threshold. Diversity receiver provided. N antennas (N is a plurality of 3 or more);
A selection switch for selecting K antennas (K is N> K and 2 or more) among the N antennas;
An A / D converter that converts received signals from the K antennas selected by the selection switch into digital signals;
A demodulator for demodulating the digital signal;
A combiner for performing diversity combining on the output signal from the demodulator;
For each of the K first antennas having a relatively large first reception level currently selected by the selection switch, the first reception level of the first antenna and the second reception of the second antenna of the next selection candidate. When both the levels are smaller than the first threshold, the first reception level is smaller than the second reception level, and the difference between the second reception level and the first reception level is smaller than the second threshold, the first antenna is replaced with the first antenna A diversity receiver comprising switch control means for controlling the selection switch so that the second antenna is selected. The switch control means is configured to switch the first antenna when the first reception level and the second reception level are both smaller than the first threshold and the difference between the second reception level and the first reception level is equal to or greater than a second threshold. Instead of being configured to control the selection switch so that the second antenna is selected,
When the first reception level and the second reception level are both smaller than the first threshold and the difference between the second reception level and the first reception level is greater than or equal to the second threshold, the automatic gain control (AGC) gain is temporarily set to 3. The gain setting unit according to claim 1, further comprising: a gain setting unit that sets the average value of the first AGC gain for the first antenna and the second AGC gain for the second antenna, and then changes to the second AGC gain. Diversity receiver. The switch control means is configured to switch the first antenna when the first reception level and the second reception level are both smaller than the first threshold and the difference between the second reception level and the first reception level is equal to or greater than a second threshold. Instead of being configured to control the selection switch so that the second antenna is selected,
When the first reception level and the second reception level are both smaller than the first threshold and the difference between the second reception level and the first reception level is equal to or greater than the second threshold, the automatic gain control (AGC) gain is set to the first gain. 3. The diversity receiver according to claim 1, further comprising gain setting means for stepwise changing from a first AGC gain for one antenna to a second AGC gain for the second antenna.   The apparatus further comprises a level measuring device for measuring reception levels of the plurality of antennas, and the control means stores the measured reception levels sequentially and a memory that stores two or more past reception levels stored in the memory. The diversity receiver according to claim 1, further comprising: a predictor that predicts two reception levels by extrapolation.

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