JPH09331312A - Narrow band type orthogonal frequency division multiplex receiver - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve a reception rate by comparing levels of reception signals sampled before and after the switching of a diversity antenna for a guard interval period and selecting a diversity antenna with a higher level so as to attain diversity reception with respect to the effect of multi-path. SOLUTION: Two sample-and-hold circuits 19A, 19B being switching objects are connected to a switch 18 and the sample-and-hold circuits 19A, 19B sample and hold an output signal of a level detector 14. Then a switching timing generating section 16 applies switching control of the switch 18 corresponding to the changeover of the antenna changeover device 2. A level comparator 20 connected to the output of the sample-and-hold circuits 19A, 19B compares the holding levels and outputs a selection signal to select the antenna changeover device 2 to receive a higher level from the antenna 1A or 1B to the antenna changeover device 2.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a narrow band type orthogonal frequency division multiplexing (OFDM) receiver, and more particularly to diversity reception in consideration of narrow band type characteristics.

[0002]

2. Description of the Related Art There are a wide band type and a narrow band type as a broadcasting system, and there is a receiver corresponding thereto. The digital audio broadcasting (DAB) adopting the orthogonal frequency division multiplexing method as a modulation method is resistant to multipath interference and is suitable for mobile reception. In order to receive this digital audio broadcasting, there are a receiver compatible with wideband type broadcasting and a receiver compatible with narrowband type broadcasting as follows.

FIG. 5 is a diagram showing frequency bands of a wide band type receiver and a narrow band type receiver for receiving digital audio broadcasting. The wideband type receiver shown in FIG. 3A simultaneously receives broadcast programs of 5 to 6 channels at once with a bandwidth of about 1.5 MHz in which one symbol is superimposed on a large number of orthogonal carriers. The narrow band type receiver shown in FIG. 3B has a bandwidth of 300 kHz and receives one broadcast program of one channel. From the standpoint of the broadcasting side, narrow band type broadcasting is adopted when there are many broadcasting stations and there is a desire to broadcast individually. In addition, narrow band type broadcasting has a characteristic that the power consumption is 1/6 that of wide band type broadcasting, which makes it easy to allocate radio waves and should be installed at the same location as the existing FM transmitting station. Also has the feature of being easy.

Although the wideband type receiver is strong against multipath, interference between symbols, error correction, etc. are performed by guard intervals as a measure against multipath. However, diversity reception is typically not employed because it is difficult, as explained below.

[0005]

On the other hand, the narrow band type receiver has the error correction capability by frequency interleaving, but its capability is lower than that of the wide type receiver, and the reception rate when moving is slightly lowered. There is a problem.
FIG. 6 is a diagram for explaining multipath when receiving digital audio broadcasting. It is assumed that the wideband receiver is performing diversity reception, and it is assumed that one antenna is affected by the multipath as shown in FIG. In the other antenna, as shown in this figure (b), a dip occurs at a position different from that of this figure (a), so that it is often affected by different multipaths. This is because the frequency band width is large in the blow band type receiver.
Therefore, the multipath cannot be resolved by simply performing diversity reception. In order to eliminate this, it is necessary to divide the frequency into a plurality of bands and perform diversity reception for each frequency band. In this way, diversity reception becomes very difficult due to the configuration. Therefore, in the wideband receiver, as described above, the diversity reception is not usually adopted. On the other hand, in the diversity reception of the narrow band type receiver, since the frequency band is small, as shown in this figure (c) with one antenna,
Even if affected by multipath, the other antenna is not affected by multipath and normal reception is possible, as shown in FIG. This is because the narrow band receiver has a small frequency band. Therefore, it can be said that the narrowband receiver is suitable for diversity reception.

Accordingly, the present invention has been made in view of the above problems.
It is an object of the present invention to provide a narrow band type orthogonal frequency division multiplex receiver that enables diversity reception against the influence of multipath and can improve the reception rate.

[0007]

In order to solve the above problems, the present invention provides a narrow band type in which a received signal having a narrow frequency band is input by a plurality of orthogonal carriers on which symbols having a guard interval are superimposed. In an orthogonal frequency division multiplex receiver, a diversity antenna that inputs the received signal, an antenna switch that switches the diversity antenna, a timing of the guard interval is detected, and the diversity antenna is used during the guard interval period. To switch the antenna,
A switching timing generator that outputs a switching signal to the antenna switching device, a level detector that detects the received signal to detect the level of the received signal, and before and after switching the diversity antenna during the guard interval period. And a level comparator which compares the magnitudes of the levels of the received signals sampled in 1. and outputs a selection signal for switching to the diversity antenna having a higher level to the antenna switching device. By this means, diversity reception becomes possible and the reception rate can be improved.

The level detector outputs the level of the received signal to an automatic gain control circuit for automatically controlling the gain, and the automatic gain control circuit outputs the received signal during the guard interval. The gain is held constant during the sampling period of. By this means, it is possible to avoid fluctuations in the reception level during diversity reception. In-phase combined output obtained by combining not only the outputs of the plurality of antennas constituting the diversity antenna but also the same phase of the outputs of the plurality of antennas, and phase shift combination obtained by shifting the phases of the outputs of some antennas Diversity reception is performed from the output. By this means, not only the diversity reception but also the antenna can be effectively used.

The other FM receiver, TV receiver also serves as the diversity antenna and the antenna switching device. By this means, the FM receiver, T
It becomes easier to coexist with a V receiver.

[0010]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a diagram showing a narrow band type orthogonal frequency division multiplexing receiver according to the present invention. As shown in the figure, in a narrow band type orthogonal frequency division multiplex receiver 100 for digital audio broadcasting (DAB), antennas 1A and 1B for diversity in a vehicle and antenna switching for selecting one of these antennas are performed. Unit 2 is installed to receive digital audio broadcasting,
It forms the input signal of the receiver. Here, an outline of modulation in digital audio broadcasting will be described. After the information such as the audio signal is converted into the symbol sequence, the inverse FFT is performed and superimposed on each carrier. The symbol string is, for example, QPSK-modulated data. The symbol sequence is provided with a guard interval for preventing interference between symbols due to multipath, an error correction code for correcting an error of a received signal, and the like. On the transmitting side, the symbol length is lengthened by the guard interval. On the receiving side, data of the guard interval where inter-symbol interference is expected due to multipath interference is ignored, and only symbols of effective length are demodulated. D / A conversion after inverse FFT processing,
A baseband time series is obtained and this time series is converted to the desired high frequency RF. Note that this demodulation is the reverse of this modulation, as described below. Returning to FIG. 1, a radio frequency amplifier (RF) 3 connected to the antenna switching device 2 amplifies an input signal from either the antenna 1A or 1B. The first mixer 4 connected to the high frequency amplifier 3 has a first
Connected to the local oscillator 5 for performing a first down conversion on the amplified input signal. A first bandpass filter (BPF) 7 connected to the first mixer 4 removes unnecessary components after the first downconversion. A second mixer 8 connected to the first bandpass filter 7 is connected to a second local oscillator 9 for a second downconversion. The second bandpass filter 10 connected to the second mixer 8 removes unnecessary components after the second down-conversion. The A / D converter 11 connected to the second bandpass filter 10 converts an analog input signal into digital data. A / D converter 11
The FFT decoding unit 12 connected to is composed of a DSP (digital signal processor) and performs fast Fourier transform to demodulate into symbols. The error correction unit 13 connected to the FFT decoder 12 corrects the received signal based on the error correction code, and outputs the result to an audio decoder (not shown). The level detector 14 connected to the output of the second bandpass filter 10 detects the output and detects the level of the input signal of the antenna 1A or 1B from the magnitude of the DC component extracted. AG connected to the level detector 14
The C (automatic gain control) circuit 15 adjusts the gain of the high frequency amplifier according to the magnitude of its input level. The switching timing generation unit 16 connected to the FFT decoding unit 12 detects a guard interval from the demodulated symbol to form the switching timing of the diversity antenna 1A or 1B, and switches the antenna switching unit 2 by this detection. Amplifier 17 for amplifying the output signal of level detector 14
The switch 18 is connected to. Two sample-holds 19A and 19B to be switched are connected to the switch 18, and the sample boards 19A and 19B sample and hold the output signal of the level detector 14.
Then, the switching timing generator 16 controls the switching of the switch 18 corresponding to the switching of the antenna switching device 2. The level comparator 20 connected to the outputs of the sample-holds 19A and 19B compares the magnitudes of the respective held levels and sets the antenna switching device 2 so that reception can be performed at the higher level of the antenna 1A or 1B. The selection signal to be switched is output to the antenna switch 2. The state of the antenna switching device 2 is maintained during the symbol period, that is, until the next guard interval. Diversity control is performed during the guard interval that is not related to demodulation, and tie diversity control is not performed during demodulation. That is, data loss due to antenna switching due to diversity control during demodulation is avoided. At the time of sampling, the switching timing generator 16 causes the AGC
The gain of the circuit 15 is kept constant. This is because it does not affect the comparison judgment of the level comparator 20.

FIG. 2 is a timing chart for explaining the timing of diversity. As shown in this figure, the symbol sequence has 250 μs between symbols of about 1 msec.
The ec guide interval is inserted. The switching timing generator 16 detects this guard interval. By this detection, one of the sample-holds 19A and 19B connected to the switch 18 is sampled and the result is held. After that, the operation is switched to the other, sampling is similarly performed, and the result is held.

Therefore, according to the present invention, in the narrow band type orthogonal frequency division multiplex receiver, it is possible to realize the initial purpose of diversity reception, so that the reception rate can be improved. FIG. 3 shows a modified example of the antenna section including the antennas 1A and 1B and the antenna switching device 2. As shown in the figure, the antenna selector 2 is composed of switches 2A, 2B and 2C. The antenna 1A is connected to the switch 2A, and one output of the antenna 1B is connected to the switch 2B. The other output of the antenna 1B is connected to the phase shifter 30 that shifts the output signal of the antenna 1B by a constant phase. The phase shifter 30 is connected to the switch 2C. Then, after the guard interval is detected by the switching timing generator, the switch 2A is ON, the switch 2B is ON, the switches 2A and 2B are ON (in-phase combination), and the switches 2A and 2C are ON (phase shift combination).
It is said. Accordingly, the sample hold 19 of FIG.
In addition to A and 19B, sample holds 19C and 19D are provided. Further, the switch 18 is also changed so that these four sample and hold 19A, 19B, 19C and 19D can be switched. Then, the level comparator 20 also compares the maximum levels among the sample-holds 19A, 19B, 19C, and 19D and outputs a selection signal for combining the antenna switching devices 2 based on the comparison result. here,
When only the switch 2A is turned on, the antenna 1
A is normally received, and antenna 1B is affected by multipath. When only the switch 2B is turned on, the antenna 1B is normally received and the antenna 1A is affected by multipath. When the switches 2A and 2B are turned on, both the antennas 1A and 1B are normally receiving. When the switches 2A and 2C are turned on, the antenna 1A is normally received, and the antenna 1B is affected by multipath, but the effect causes a peak. In this way, many antenna states are formed to enable diversity control and effective antenna reception.

In the above description, diversity reception having two antennas has been described, but the diversity reception is not limited to two, and a greater number of antennas may be used for diversity reception. FIG. 4 is a diagram showing an example in which the antenna unit is shared with another FM receiver or the like. As shown in the figure, a narrow band type orthogonal frequency division multiplexing receiver 100 and an FM (frequency modulation)
The receiver or TV (television) receiver 200 serves as both the antennas 1A and 1B and the antenna switching device 2. In this case, the switch 30 is provided after the antenna selector 2.
0 is provided to enable the switch 300 to selectively connect the narrow band type orthogonal frequency division multiplex receiver 100 or the FM receiver or the TV receiver 200 to the antenna selector 2. In this case, a controller 400 for controlling the switching of the antenna switching device 2 is provided, and the controller 40
0 performs switching control based on a selection signal of either the narrow band type orthogonal frequency division multiplexing receiver 100 and the FM receiver or the TV receiver 200. As described above, the narrow band type orthogonal frequency division multiplex receiver 100 is used in other FM of the vehicle.
By using the receiver and the like as the antenna and the like, it is possible to coexist with them.

[Brief description of drawings]

FIG. 1 is a diagram showing a narrow band type orthogonal frequency division multiplexing receiver according to the present invention.

FIG. 2 is a timing chart for explaining diversity timing.

FIG. 3 is a modified example of an antenna unit including antennas 1A and 1B and an antenna switching device 2.

FIG. 4 is a diagram showing an example in which an antenna unit is shared with another FM receiver or the like.

FIG. 5 is a diagram showing frequency bands of a wideband type receiver and a narrowband type receiver for receiving digital audio broadcasting.

[Fig. 6] Fig. 6 is a diagram for describing multipath at the time of receiving a digital audio broadcast.

[Explanation of symbols]

 1A, 2A ... Diversity antenna 2 ... Antenna switcher 14 ... Level detector 15 ... AGC circuit 16 ... Switching timing generator 19A, 19B ... Sampling hold 20 ... Level comparator 30 ... Phase shifter 100 ... Narrow band type orthogonal frequency division Multiple receiver 200 ... FM receiver or TV receiver

Claims (5)

[Claims] 1. A narrow band type orthogonal frequency division multiplex receiver for inputting a received signal having a narrow frequency band with a plurality of orthogonal carriers on which symbols having a guard interval are superposed, and a diversity antenna for inputting the received signal. An antenna switch for switching the diversity antenna; and a switch for outputting a switching signal to the antenna switch to detect the guard interval timing and switch the diversity antenna during the guard interval period. A timing generator; a level detector for detecting the level of the received signal by detecting the received signal; and a level of the received signal sampled before and after the switching of the diversity antenna during the guard interval period. The levels are large Square narrowband type orthogonal frequency division multiplexing receiver, characterized in that a selection signal for switching the antenna for diversity and a level comparator for outputting said antenna switching equipment of. 2. The level detector outputs the magnitude of the level of the received signal to an automatic gain control circuit for automatically controlling a gain, the automatic gain control circuit, during the guard interval, The narrow band type orthogonal frequency division multiplex receiver according to claim 1, wherein the gain is kept constant during the sampling of the received signal. 3. The in-phase combined output obtained by combining not only the outputs of the plurality of antennas constituting the diversity antenna but also the same phase of the outputs of the plurality of antennas, and shifting the phases of the outputs of some of the antennas. The narrow band type orthogonal frequency division multiplex receiver according to claim 1, wherein diversity reception is performed from the combined phase shift combined outputs. 4. The narrow band type orthogonal frequency division multiplex receiver according to claim 1, wherein the other FM receiver also serves as the diversity antenna and the antenna switch. 5. The narrow band type orthogonal frequency division multiplex receiver according to claim 1, wherein another TV receiver is used also as the diversity antenna and the antenna switching device.