Reception signal processing device and method for processing reception signal

A reception signal demodulation device which converts a modulated reception signal into a digital signal and performs signal reconstruction includes a demodulator which demodulates the received signal, a matched filter which improves a signal-to-noise power ratio of the signal from the demodulator, a threshold value discriminator which determines a compression rate based on the signal from the matched filter, and a reconstruction circuit which compresses the signal from the matched filter, converts it into the digital signal, and performs signal reconstruction from the signal converted into the digital signal based on the compression rate.

This application is a National Stage Entry of PCT/JP2013/000755 filed on Feb. 13, 2013, which claims priority from Japanese Patent Application 2012-033999 filed on Feb. 20, 2012, the contents of all of which are incorporated herein by reference, in their entirety.

TECHNICAL FIELD

The present invention relates to a reception signal processing device and a method for processing a reception signal.

BACKGROUND ART

According to the sampling theorem, a bandwidth required when an original signal is reconstructed from a received signal is more than two times larger than a signal bandwidth. However, when a compressive sensing technology is used, it is not always necessary to satisfy the sampling theorem. Therefore, the original signal can be reconstructed.

For example, in non-patent document 1, a technology by which the signals reflected from a plurality of objects can be reconstructed under an undersampling condition in a radar device is disclosed.FIG. 5is a block diagram of the radar device which reconstructs the signals under the undersampling condition by using the compressive sensing technology.

The radar device shown inFIG. 5includes a reception antenna101, a demodulator102, an integrator103, an A/D converter104, and a signal reconstruction unit105. A modulated signal is received from a space by the reception antenna101and outputted to the demodulator102. The demodulator102demodulates the received signal and outputs this to the integrator103as a demodulated signal. The integrator103compresses the demodulated signal by performing integration with a time of a sampling interval required for the reconstruction intrinsically. Further, the signal to be compressed is requested to be a chirp signal or a PN signal which has a good RIP characteristic. Hereinafter, a degree of integration when the demodulated signal is compressed by performing the integration is defined as a compression rate. Accordingly, the integrator103performs a compression process at the compression rate set in advance. Further, the signal which has been compressed is described as a compressed signal.

Here, the RIP characteristic is an index indicating whether or not the signal can be reconstructed that is disclosed in non-patent document 1 and is determined by a configuration for compressing the signal (in this case, the integrator103) and the signal to be compressed (in this case, the demodulated signal).

The compressed signal from the integrator103is inputted to the A/D converter104. When the integrator103outputs the compressed signal obtained by performing the integration with for example, four sampling intervals to the A/D converter104, the sampling rate that is one-fourth of the intrinsically required sampling rate can be used in the A/D converter104. Accordingly, the A/D converter104converts the compressed signal into the digital signal at this sampling rate and outputs the digital signal to the signal reconstruction unit105.

In the signal reconstruction unit105, by solving the “L1 norm minimization problem” described in non-patent document 1, the desired signal is reconstructed from the output value of the A/D converter104. At this time, when a condition in which an amount of information of the desired signal is sufficiently small compared to the signal bandwidth and information is not lost when the signal is compressed is met, a correct reconstruction result can be obtained. For example, in the radar device, when the number of objects is sufficiently small in an observation range, such conditions can be met.

PRIOR ART DOCUMENT

DISCLOSURE OF THE INVENTION

Problems to be Solved by the Invention

However, in the radar device using the compressive sensing technology shown inFIG. 5, as described later, when a signal-to-noise power ratio (SNR) at the demodulator102is small, a problem in which there is a case in which the signal cannot be correctly reconstructed and the process in the signal reconstruction unit105becomes redundant occurs.

Namely, in a state in which the SNR of the demodulated signal is small (it is equal to or less than 0 dB), there is a case in which a noise component is regarded as the object. In this case, because the condition at which the number of objects is sufficiently small compared to the inside of the observation range is not met, the signal cannot be reconstructed correctly.

Further, the smaller the number of objects is, the higher the value of the compression rate can be set. The higher the value of the compression rate is, the more the signal processing load on the signal reconstruction unit105can be reduced. However, to the contrary, when it is too high, there is a case in which the signal cannot be appropriately reconstructed.

In the radar device shown inFIG. 5, the signal reconstruction unit105does not know the number of objects in advance. For this reason, in order to perform the reconstruction for all the expected number of objects, the signal reconstruction unit105always performs the reconstruction process at the compression rate based on the maximum number of objects. However, because the number of objects is different for each observation actually, the signal processing load can be reduced by using the higher compression rate when the number of objects is sufficiently small compared to the maximum number of objects. In such case, the radar device shown inFIG. 5performs a redundant process.

Accordingly, a main purpose of the present invention is to provide a reception signal processing device which can perform a signal reconstruction with high reliability even when the SNR of the demodulated signal is small and reduce the load on the signal reconstruction process at the time of the signal reconstruction and a method for processing the reception signal.

Means for Solving the Problems

In order to solve the above-mentioned problem, a reception signal demodulation device which converts a modulated reception signal into a digital signal and performs signal reconstruction includes a demodulator which demodulates the received signal, a matched filter which improves a signal-to-noise power ratio of the signal from the demodulator, a threshold value discriminator which determines a compression rate based on the signal from the matched filter, and a reconstruction circuit which compresses the signal from the matched filter, converts it into the digital signal, and performs signal reconstruction from the signal converted into the digital signal based on the compression rate.

A method for demodulating a reception signal by which a modulated reception signal is converted into a digital signal and signal reconstruction is performed includes a demodulation procedure for demodulating the received signal, a noise suppression procedure for improving a signal-to-noise power ratio of the signal from the demodulation procedure, a compression rate determination procedure for determining a compression rate based on the signal from the noise suppression procedure, and a reconstruction procedure for compressing the signal from the noise suppression procedure, converting it into the digital signal, and performing signal reconstruction from the signal converted into the digital signal based on the compression rate.

Effect of the Invention

By using the present invention, because the compression rate is determined from the signal whose signal-to-noise power ratio (SNR) is improved and the compression according to the compression rate, the A/D conversion, and the signal reconstruction are performed, the signal reconstruction can be performed with high reliability even when the SNR of the demodulated signal is small and the load on the signal reconstruction process can be reduced at the time of the signal reconstruction.

MOST PREFERRED MODE FOR CARRYING OUT THE INVENTION

Next, a first exemplary embodiment of the present invention will be described.FIG. 1is a block diagram of a reception signal processing device2A according to the first exemplary embodiment of the present invention. Such reception signal processing device can be utilized for an object detection in a radar device, a sonar device, or the like and also, utilized for reconstruction of the received signal in a communication system.

Such reception signal processing device2A includes a reception antenna3, a demodulator4, a matched filter5, a threshold value discriminator6, and a reconstruction circuit10A. Further, the reconstruction circuit10A includes a signal compression unit7, an A/D converter8, and a signal reconstruction unit9.

The reception antenna3is an antenna which receives a signal. The demodulator4demodulates a received signal G1and outputs it as a demodulated signal G2. The matched filter5improves a signal-to-noise power ratio (SNR) of the demodulated signal and outputs it as a small noise demodulated signal G3.

The threshold value discriminator6counts the number of times at which the amplitude value of the small noise demodulated signal G3exceeds a threshold value set in advance and estimates the number of objects to be detected by the radar device based on this count value. Hereinafter, the estimated number of objects is described as “estimated object number”. The threshold value discriminator6calculates a compression rate based on an estimated object value and outputs a compression rate signal G4.

The signal compression unit7performs weighting to the small noise demodulated signal G3from the matched filter5with a sampling interval that is intrinsically required for the reconstruction and outputs a cumulative sum as a compressed signal G5. The A/D converter8converts the compressed signal G5into a digital signal G6. The signal reconstruction unit9performs a signal reconstruction from the digital signal G6.

FIG. 2is a block diagram of the signal compression unit7. The signal compression unit7includes weighting units70(70A to70D) which include phase shifters71(71ato71d) and integrators72(72ato72d), respectively and a selector73. Further, in the exemplary embodiment, although the signal compression unit7includes four weighting units, this is shown as an example. Therefore, the number of the weighting units can be arbitrary determined.

The phase shifter71shifts the phase of the inputted demodulated signal by “0” or “π” radians at random with a sampling interval that is intrinsically required for the reconstruction to perform weighting. The integrator72outputs the cumulative sum of the signals that are weighted by the phase shifter71. Further, this integrator72has a buffer function to store the value of the cumulative sum until the value is read from the selector73. The selector73selects one of the outputs from the weighting units70in synchronization with the sampling timing of the A/D converter8.

Next, the operation of the reception signal processing device having such configuration will be described.FIG. 3is a flowchart showing a method for processing a reception signal.

Steps S1and S2: The reception antenna3receives the signal and outputs this as the received signal G1. The received signal G1includes not only a reflected signal from the object but also a signal reflected by another body or the like existing in a periphery as a noise. The received signal G1including such noise is inputted to the demodulator4and demodulated.

Step S3: The demodulated signal G2from the demodulator4is inputted to the matched filter5and the autocorrelation between the demodulated signal and the transmission waveform is obtained. Whereby, the SNR of the demodulated signal is improved. The demodulated signal whose SNR is improved is outputted to the threshold value discriminator6and the signal compression unit7as the small noise demodulated signal G3.

Step S3: The threshold value discriminator6counts the number of the times at which the amplitude value of the small noise demodulated signal G3is greater than the threshold value set in advance. The small noise demodulated signal G3is a signal obtained by improving the SNR of the demodulated signal G2. For this reason, because the noise included in the small noise demodulated signal G3is small, the main component of the signal is the signal reflected by the object. Because it is expected that the signal component reflected by the object has a large amplitude, the number of objects can be estimated by setting the threshold value and counting the number of times at which the amplitude exceeds this threshold value. Of course, because the noise cannot be removed completely, in a strict sense, the counted number does not correspond to the number of objects. For example, the counted number counted by the threshold value discriminator6can be used as the estimated object number.

Steps S4and S5: The threshold value discriminator6converts the estimated object number into the compression rate, and outputs it to the signal compression unit7, the A/D converter8and the signal reconstruction unit9as the compression rate signal G4. Further, when the estimated object number is converted into the compression rate, the function transformation can be used. However, for example, a conversion table between the estimated object number and the compression rate or the like is provided in advance and the compression rate may be obtained based on the conversion table.

Steps S6and S7: In the signal compression unit7, the phase shifter71performs a weighting process in which a weighting factor of “1” or “−1” is used for the small noise demodulated signal G3from the matched filter5at random and the integrator72performs an integration process by which the weighted signals are integrated to generate the compressed signal G5. The signal compression unit7performs the integration according to the compression rate signal G4and outputs it to the A/D converter8as the compressed signal G5.

In the weighting process, the weighting is performed by shifting the phase by “0” or “π” radians at random with a sampling interval that is intrinsically required for a phase shift amount. In this case, the phase shift by “0” radian corresponds to the weighting using the weighting factor of “1” and the phase shift by “π” radians corresponds to the weighting using the weighting factor of “−1”.

The integration process performs an integration of a weighted signal G7with a sampling interval that is intrinsically required. The phase shifter71of each weighting unit70performs the weighting process.

For example, it is assumed that “T” is the sampling interval that is intrinsically required to correctly reconstruct a signal before compression and the time width of the small noise demodulated signal G3is “8T”. Further, it is assumed that the compression rate signal G4from the threshold value discriminator6is “2”. In this case, because the compression rate is “2”, the sampling interval of the A/D converter8is “2T” and the sampling rate is “½T”. Therefore, the sampling rate is a half of the sampling rate that is intrinsically required. Four signals that correspond to the first to fourth sampling values are outputted from the integrators72, respectively.

Namely, the signal that is obtained by weighting the signal part from 0 to 2T of the small noise demodulated signal G3having the entire time width of 8T with the interval T is outputted from the integrator72aas the first sampling value. The signal that is obtained by weighting the signal part from 0 to 4T of the small noise demodulated signal G3with the interval T is outputted from the integrator72bas the second sampling value. The signal that is obtained by weighting the signal part from 0 to 6T of the small noise demodulated signal G3with the interval T is outputted from the integrator72cas the third sampling value. Further, the signal that is obtained by weighting the signal part from 0 to 8T of the small noise demodulated signal G3with the interval T is outputted from the integrator72das the fourth sampling value.

The selector73changes a connection destination for each sampling time of the A/D converter8so as to input the signal from each weighting unit70to the A/D converter8as the compressed signal G5. The sampling timing is set based on the compression rate signal.

Step S8: The A/D converter8converts the compressed signal G5into the digital signal at the sampling rate corresponding to the compression rate signal G4from the threshold value discriminator6and outputs it to the signal reconstruction unit9.

Step S9: The signal reconstruction unit9calculates the output signal from the matched filter5that is converted in a digital form from the output value of the A/D converter8by solving the “L1 norm minimization problem”.

As described above, even when the SNR of the received signal is small, the SNR is improved by the matched filter. Therefore, the highly accurate signal demodulation can be performed. Further, because the number of objects is estimated from the small noise demodulated signal whose SNR is improved and the compression rate is determined based on the estimated object number, the load on the signal reconstruction process can be reduced.

Next, a second exemplary embodiment of the present invention will be described. Further, the same reference numbers are used for the elements having the same function as the first exemplary embodiment and the description of the element will be omitted appropriately.

In the first exemplary embodiment, the compression rate determined by the threshold value discriminator6is inputted to the signal compression unit7, the A/D converter8, and the signal reconstruction unit9and these elements operate according to the compression rate. The value of the compression rate changes according to the number of objects. Accordingly, the processes of the signal compression unit7, the A/D converter8, and the signal reconstruction unit9change according to the number of objects. In this case, if a configuration in which the processes of the signal compression unit7and the A/D converter8are changed according to the number of objects is used, these processes have to be performed as an adaptive process and there is a possibility that the configuration becomes complex.

Accordingly, in this exemplary embodiment, the signal compression unit7and the A/D converter8perform the process by using a fixed compression rate corresponding to the maximum number of objects that is set in advance and the signal reconstruction unit9performs the reconstruction process by using the compression rate determined by the threshold value discriminator9.

FIG. 4is a block diagram of a reception signal processing device2B that has such configuration. The reception signal processing device2B includes the reception antenna3, the demodulator4, the matched filter5, the threshold value discriminator6, and a reconstruction circuit10B like the reception signal processing device2A shown inFIG. 1. Further, the reconstruction circuit10B includes the signal compression unit7, the A/D converter8, and the signal reconstruction unit9.

However, the compression rate signal G4from the threshold value discriminator6is inputted to only the signal reconstruction unit9. The compression rate corresponding to the maximum number of objects set in advance is set to the signal compression unit7and the A/D converter8as the fixed value (the fixed compression rate). When such setting is used, the processing timing of the signal compression unit7and the A/D converter8is different from that of the signal reconstruction unit9. As a result, the thinning of the sampled digital signal occurs in the signal reconstruction unit9and the load on the process in the signal reconstruction unit9is reduced.

Accordingly, even when the SNR of the received signal is small, the SNR is improved by the matched filter. Therefore, the highly accurate signal demodulation can be performed. Further, the number of objects is estimated from the small noise demodulated signal whose SNR is improved and the compression rate is determined based on the estimated object number. Whereby the load on the signal reconstruction process can be reduced by the thinning of the sampled digital signal.

This application claims priority based on Japanese Patent Application No. 2012-033999, filed on Feb. 20, 2012, the disclosure of which is hereby incorporated by reference in its entirety.

DESCRIPTION OF SYMBOL

2reception signal processing device

7signal compression unit

9signal reconstruction unit