JPH07264093A - Saw spread spectrum demodulation and its constitution method - Google Patents

Abstract

PURPOSE:To eliminate the interference of pseudo carriers by shifting an encoded electrode interval within a range for not remarkably degradaing correlation characteristics. CONSTITUTION:The interval di of encoded electrodes 3-1, 3-2, 3-3,...3-n constituting an encoded output electrode is set at Mi/f0.vs {(b) or (c)}. Mi is a positive semi integer expressed by N+ki/2. f0 is a carrier frequency, vs is the propagation speed of SAW, N is a carrier cycle number (the positive semi integer) in one chip of a code and ki is an integer. Compared with a conventional technique (a) for which the Mi is set at the N, correlation output for the pseudo carriers for which the cycle of N-1 and N+1, etc., is generated in one chip of the code is suppressed.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to surface acoustic waves (SAW).
A SAW spread spectrum demodulator for spread spectrum demodulating a spread spectrum signal using a matched filter,
Furthermore, it relates to a method of configuring the same.

[0002]

2. Description of the Related Art A spread spectrum system is a radio communication system which has high confidentiality of signals and is resistant to interference from other signal sources. Several methods are known as the spread spectrum method. For example, in the direct modulation method, the transmission signal is phase-modulated by the code modulator installed in the transmitter,
The received signal is demodulated by the code demodulator mounted on the receiver. The code used in this modulation / demodulation (spread spectrum modulation / demodulation) is called a pseudo noise (PN) code, which is a code that repeats a series of chips that are time-aligned according to a predetermined rule. Spreads the spectral distribution of. Therefore, even if a transmission signal subjected to spread spectrum modulation (spread spectrum signal) is received by another person, it cannot be demodulated unless the PN code used at the time of modulation is known (concealment of the signal). Further, even if a radio signal in a close band is emitted from another signal source, the spectrum is spread, so that it is difficult to receive the interference.

Among the characteristics of the spread spectrum system, signal confidentiality is emphasized particularly in military-related communication equipment in which confidentiality of information is important. Furthermore, in addition to this confidentiality, the property of being resistant to interference is emphasized in various consumer devices. First, in recent years, the frequency of wireless systems has been increasing, and the deterioration of received signal quality due to multipath interference has become a serious problem. This problem is mitigated or reduced by a spread spectrum method that is strong against interference. Can be resolved. Secondly, a spread spectrum method for spreading the spectrum of a transmission signal is effective when it is desired to share a radio frequency with another radio system in order to effectively use frequency resources.

When the spread spectrum system is implemented, a code modulator (spread spectrum modulator) is installed in the transmitter and a code demodulator (spread spectrum demodulator) is installed in the receiver. In the spread spectrum demodulator, it is necessary to acquire synchronization with the code used in the transmitter. As a method of acquiring synchronization, for example, there is a method of repeating demodulation until synchronization is achieved, or a method of executing synchronization acquisition and spread spectrum demodulation in parallel. However, in these methods, there is a problem that it takes time to acquire synchronization,
There is a problem that the configuration of the receiver becomes complicated.

As a spread spectrum demodulator which does not cause this kind of problem, there is a configuration using a SAW convolver. That is, the same code as that used in the spread spectrum modulator is time-inverted and input to the SAW convolver, the correlation with the received signal is obtained, and the data is demodulated when the correlation value reaches a peak. Spread spectrum demodulation can be performed without causing the problems described above. However, when such a configuration is used, a code obtained by time-reversing the code used in the spread spectrum modulator is required. Moreover, the efficiency of the SAW convolver is not so good and the insertion loss is large.

As another configuration of the spread spectrum demodulator, there is a configuration using a SAW matched filter. SA
The W filter generally has a structure in which an input electrode and an output electrode are adhered and formed on a piezoelectric substrate. When an electric signal is input to the input electrode of the SAW filter, the input electrode excites the surface of the piezoelectric substrate to generate SAW, and the generated SAW propagates on the surface of the piezoelectric substrate. The output electrode receives this and converts it into an electric signal. Therefore, the transfer function of the SAW filter is represented by the product of the transfer function of the input electrode (electroacoustic conversion), the transfer function of the surface of the piezoelectric substrate (SAW propagation delay) and the transfer function of the output electrode (acoustoelectric conversion). S
The AW matched filter is a matched filter realized by utilizing SAW propagation delay.

The matched filter is a transfer function H of a complex conjugate with respect to the frequency characteristic H (ω) of the input signal.
A filter having ^* (ω). When realizing a SAW matched filter, the distance between the input and output electrodes is set so as to realize the negative time included in the transfer function H ^* (ω), and the SAW propagation delay is given. Is defined by the pattern (electrode structure and arrangement) of the input and output electrodes. In determining the structure and arrangement of the input electrode and the output electrode, first, the transfer function to be realized by the cooperation of both the input electrode and the output electrode is expressed in the form of the product of two parts, and each part is represented by the input electrode. And the output electrode. The structure and arrangement of the input and output electrodes are such that the allocated transfer function part is realized.

When a spread spectrum signal is input to the SAW matched filter configured as described above, the spread spectrum code relating to the input signal and the pattern of the input electrode and the output electrode (that is, the code expressed thereby) An electrical signal indicative of the correlation is obtained from the output electrode. Therefore, the received signal can be spread spectrum demodulated by detecting the timing (phase) at which this correlation reaches a peak. When using such a configuration,
The time-reversed code as in the SAW convolver is not necessary, and the insertion loss is relatively good.

By the way, in the SAW matched filter, since the allocation method when allocating the required transfer function to both the input electrode and the output electrode is arbitrary, the input electrode and the output electrode which can obtain the target transfer function are obtained. There is not one combination of structure and arrangement, and generally there are many combinations even if they are relatively easy to realize. Among them, a widely used combination is an input electrode whose electrode length corresponds to one chip length, and an output electrode having a plurality of coded electrodes (generally electrode finger pairs) whose intervals correspond to one chip length. , Combination (hereinafter referred to as the first configuration)
Or, conversely, a combination of an input electrode having a plurality of encoded electrodes whose intervals are equivalent to one chip length and an output electrode whose electrode length is equivalent to one chip length (hereinafter referred to as the second configuration).
There is. The output electrode in the first configuration and the input electrode in the second configuration have a large number of coded electrodes corresponding to the number of chips in the spread spectrum code. In addition,
Since the second configuration has the same characteristics as the first configuration, the first configuration will be taken as an example in the following description for simplification of description.

In this structure, the distance between the coding electrodes is set to a distance corresponding to one chip length of the spread spectrum code, and the outputs of the plurality of coding electrodes (that is, the received SAWs are coded). Signal) is added in phase at the level of the carrier carrying the spread spectrum signal. With such a configuration, the carrier frequency at which the correlation peak can be satisfactorily detected is the frequency f ₀ such that its N cycle (N: positive half integer, that is, a number that becomes a positive integer when multiplied by 2) is equal to one chip of the code. Is. In the SAW matched filter suitable for such a carrier frequency f ₀ , a problem occurs when a pseudo carrier having the following frequency f ₁ or f ₂ is input.

[0011]

## EQU00004 ## f ₁ = (N-1) / Nf ₀ f ₂ = (N + 1) / Nf _{0 In} this way, the signals spectrum-spread by the same code have their N-1 cycle or N + 1. Also when a carrier is carried by a pseudo carrier whose period is equal to one chip of the code and input to the input electrode, a strong output due to the synchronous addition appears at the output electrode, and the correlation peak is detected.

However, in the above-mentioned first structure, the electrode length of the input electrode corresponds to one chip length, that is, the frequency f _0.
The carrier is set to N cycles. Under such a setting, the transfer function of the input electrode is a sinc function, and the zero point closest to the center frequency exists at a frequency separated from the center frequency by 1 / N of the center frequency. When the center frequency of the input electrode transfer function is selected as the carrier frequency f ₀ , the frequencies f _N1 and f _{N2 at the} zero point are as follows.

[0013]

F _N1 = f ₀ −f ₀ / N = (N−1) / N · f ₀ = f ₁ f _{N 2} = f ₀ + f ₀ / N = (N + 1) / N · f ₀ = f ₂ Therefore Even if the pseudo carriers having the above-mentioned frequencies f ₁ and f ₂ are input, they are blocked at the input electrode, so that it is possible to preferably prevent the spread spectrum signal carried by the pseudo carriers from being demodulated.

Such a configuration is applicable when performing spread spectrum communication using a plurality of carriers having different frequencies. That is, by setting the carrier frequencies of the adjacent channels to the zero points f _N1 and f _N2 of the input electrodes, it is possible in principle to suppress interference from the adjacent channels. This also applies to the second configuration.

[0015]

However, it is actually difficult to sufficiently suppress the interference from the carriers of the adjacent channels by such a configuration. That is, the frequency that acts as a pseudo carrier for the spread spectrum demodulator of one channel is the carrier frequency of another channel, which itself has a high level. Therefore, it is difficult to sufficiently cancel the carrier of the other channel at the zero point of the input electrode transfer function.

The present invention has been made to solve the above problems, and firstly, by shifting the coding electrode interval within a range that does not significantly deteriorate the correlation characteristic, it is possible to prevent the occurrence of pseudo carriers. Reduce the correlation output,
This aims to realize a high-performance spread spectrum demodulator suitable for spread spectrum communication using a plurality of carriers. Secondly, it is an object to provide a method for constructing such a spread spectrum demodulator.

[0017]

In order to achieve such an object, the SAW spread spectrum demodulator of the present invention comprises a first electrode formed on a piezoelectric substrate and a surface acoustic wave along a propagation direction. A second electrode having n (n: an integer of 2 or more) coded electrodes which are arranged on the piezoelectric substrate so as to be spaced apart from each other and commonly connected to the output terminal, and are modulated by a spread spectrum code. Signal has a predetermined frequency f ₀
Surface acoustic waves excited on the piezoelectric substrate by the first electrode when being carried by the carrier and received by the first electrode, as a result, the surface acoustic waves are obtained from the respective coded electrodes. In the SAW spread spectrum demodulator which synthesizes the signal output at the second electrode and outputs it as a correlation signal from the output terminal, when the number i is given in order to the interval between the adjacent coding electrodes, the above n codes are represented. The distance between the electrodes is

D _i = M _i / f ₀ · v _s where M _i = N + k _i / 2: positive half integer, N: positive number closest to the number of carrier periods included in one chip length of the spread spectrum code Half-integer, k _i : an integer, d _i , and for an interval in which k _i is an odd number, all the coding electrodes on one side of the adjacent coding electrodes are configured to have inverted phases. It is characterized by

In addition, the SAW spread spectrum demodulator of the present invention has the first electrode formed on the piezoelectric substrate and the output terminal arranged and arranged on the piezoelectric substrate so as to be spaced from each other along the propagation direction of the surface acoustic wave. A second electrode having n (n: an integer equal to or greater than 2) coded electrodes commonly connected to, and a signal modulated by a spread spectrum code is carried by a carrier having a predetermined frequency f ₀ . When input to the electrode of, the surface acoustic wave of velocity v _s excited on the piezoelectric substrate by each encoding electrode is received by the first electrode, and as a result, the signal output obtained from the first electrode is output. In the SAW spread spectrum demodulator which outputs the correlation signal from the output terminal, the numbers i are sequentially arranged in the intervals of the adjacent coding electrodes.
Is given, the interval between the n coded electrodes is calculated by the following equation.

Where d _i = M _i / f ₀ · v _s, where M _i = N + k _i / 2: positive half integer, N: positive number closest to the number of carrier periods included in one chip length of the spread spectrum code Half-integer, k _i : an integer, d _i , and for an interval in which k _i is an odd number, all the coding electrodes on one side of the adjacent coding electrodes are configured to have inverted phases. It is characterized by

Then, the SAW spread spectrum demodulator of the present invention has a method of forming a first electrode on a piezoelectric substrate,
The second electrodes, which have n (n: an integer of 2 or more) coded electrodes and are commonly connected to the output terminal, are arranged on the piezoelectric substrate so that the coded electrodes are spaced from each other in the propagation direction of the surface acoustic wave. In the method of constructing the SAW spread spectrum demodulator to be formed, when the number i is given to the intervals of the adjacent coding electrodes in order, the intervals of the n coding electrodes are expressed by the following formula.

D _i = M _i / f ₀ · v _s where M _i = N + k _i / 2: positive half-integer, N: positive number closest to the number of carrier cycles included in one chip length of the spread spectrum code Half-integer, k _i : integer, is set to d _i , and for the interval in which k _i is an odd number, the configuration is such that all the coding electrodes on one side of the adjacent coding electrodes are inverted in phase. It is characterized by doing.

[0020]

In the SAW spread spectrum demodulator of the present invention, the coefficient M that determines the interval (i-th) between the coding electrodes.
_i is set to a value shifted by k _i / 2 with respect to the coefficient N related to the 1-chip length N / f ₀ of the spread spectrum code. That is, the interval of the coding electrode, with respect to _{N /} f 0 · _{v s,}
It is set to a value shifted by (k _i / 2) / f ₀ · v _s . Therefore, from the signal carried by the carrier of frequency f ₀ and modulated by the spread spectrum code, as in the case where the spacing of the coding electrodes is set to N / f ₀ · v _s ,
A correlation signal related to addition synthesis is obtained. On the other hand, signals carried by carriers (pseudo carriers) of different frequencies do not generate correlation signals related to addition combining even if they are modulated by the same spread spectrum code. Therefore, by setting k _i to a value such as 0, ± 1, ± 2, etc. so that k _i / 2 becomes sufficiently small with respect to N, the interference of the pseudo carrier is not substantially impaired. A high-performance SAW spread spectrum demodulator that does not suffer from noise is obtained.

[0021]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT A preferred embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 shows a SAW according to an embodiment of the present invention.
The configuration of a spread spectrum demodulator is shown. In this embodiment, the input electrode 2 and the coded output electrode 3 are adhered and formed on the surface of the piezoelectric substrate 1 on which SAW is generated and propagates by excitation. A spread spectrum signal is input to the input electrode 2 via the input terminal 4, whereby the input electrode 2 excites SAW on the surface of the piezoelectric substrate 1. This SA
W constitutes the encoded output electrode 3 and the encoded output electrode 3
From the predetermined number of encoded electrodes (electrode finger pairs in the figure) 3-1, 3-2, 3-3, ... 3-n respectively, received and encoded, and added by the encoded output electrode 3. To be done. Electric signal obtained by this (addition output)
Is output to the subsequent circuit via the output terminal 5.

The feature of this embodiment is that the structure of the coded output electrode 3, particularly the coded electrodes 3-1, 3-2, 3-3, ... 3-
It is the interval d _i (i: 1, 2, 3, ... N-1) between n.

It is now assumed that the carrier frequency is f _0, and the one-chip length of the spread spectrum code is N periods (N: positive half integer) of this carrier = N / f ₀ . For the input electrode 2, the electrode length is set to a distance d _p equivalent to one chip length of the spread spectrum code, as in the conventional case. The distance d _p can be expressed as follows. However, v _s is the SAW propagation velocity on the surface of the piezoelectric substrate 1.

[0025]

D _p = N / f ₀ · v _{s In} the prior art shown above, as shown in FIG. 2A, the coding electrodes 3-1, 3-2, 3-3, ... 3-n
The mutual intervals are all set to a distance d _p equivalent to one chip length of the spread spectrum code. Therefore, the input terminal 4
When the signal applied to the input electrode 2 via the signal is carried by the pseudo carriers having the frequencies f ₁ and f ₂ shown above, it is reduced by the electrode length setting of the input electrode 2, but ignored. An uncorrelated output is obtained from the coded output electrode 3.

On the other hand, in this embodiment, as shown in FIG.
As shown in (b) or (c), the coded electrode 3
The interval between -1, 3-2, 3-3, ... 3-n is set to a value d _i represented by the following equation. Further, when k _i is an odd number, the arrangement phase of all the coding electrodes on the right side thereof (the order of the electrode fingers forming each coding electrode: that is, the electrode finger on the upper side in the drawing is on the input electrode 2 side). 2 (a) or vice versa) is inverted as compared with FIG. Of course, the left side may be phase-inverted.

[0027]

D _i = M _i / f ₀ · v _s where M _i = N + k _i / 2 k _i : integer When set in this way, from the spread spectrum signal carried by the carrier of frequency f ₀ and input The correlation can be preferably detected, and the correlation output can be remarkably suppressed for the spread spectrum signal that is carried and input by the pseudo carriers of the frequencies f ₁ and f ₂ described above.

First, consider points other than the sign change point. In this case, if k _i is an even number, all encoded electrodes 3-1 and 3
The carriers are in-phase with respect to the encoded outputs of −2, 3-3, ... 3-n, and the addition output output from the encoded output electrode 3 via the output terminal 5 is the in-phase addition output. If k _i is an odd number, coding electrode 3-i of the odd k _i
If the arrangement phase of one of the two is reversed, the carriers are in phase with respect to the encoded outputs of all the encoding electrodes 3-1, 3-2, 3-3, ... 3-n, and the addition output thereof is the in-phase addition output. Becomes

Considering the sign change point, the phases related to the addition do not match, and strictly speaking, in-phase addition is not performed. However, in this embodiment, the shift of M _i with respect to N is 1 /
Since 2 is given as a unit, the degree of disagreement remains at most 1/4 period of the carrier. In the SAW matched filter, in general, the carrier frequency f ₀ is set sufficiently high with respect to the code rate of the spread spectrum code (the reciprocal of 1 chip length) so that transmission and reception can be performed easily.
The width of the correlation peak is about one chip long. Therefore, even if the uncertainty of detection of the correlation peak at the sign change point is 1/4 period of the carrier as described above, it does not cause much problem.

When pseudo carriers having frequencies f ₁ and f ₂ are input, the coding electrodes 3-1 and 3- are used.
For encoded outputs of 2, 3-3, ... 3-n, the carriers are generally not in-phase, and therefore the summing output is not the in-phase summing output.

As described above, according to this embodiment, the interval d _i between the coded electrodes 3-1, 3-2, 3-3, ... 3-n is set within a range that does not significantly deteriorate the correlation characteristic. Since the spread code is displaced from the distance d _p corresponding to one chip length, the response to the pseudo carrier can be reduced. Further, this makes it possible to realize a high-performance spread spectrum demodulator suitable for spread spectrum communication using a plurality of carriers.

In the example shown in FIG. 2B, M ₁ =
N, M ₂ = N + 1/2, ..., And the example shown in FIG. 2C is an example of M ₁ = N, M ₂ = N + 1 ,. However, the present invention is not limited to such a setting. That is, M _i can be appropriately set as long as the correlation characteristics are not significantly deteriorated. k _i may be a negative integer. Further, although the present invention has been described according to the above-mentioned first configuration, the present invention can be applied to the second configuration.

[0033]

As described above, according to the SAW spread spectrum demodulator of the present invention, the coefficient M _i that determines the interval between the coding electrodes is set to 1 chip length N / f of the spread spectrum code.
_{Since the} coefficient N related to ₀ is set to a value shifted by k _i / 2, from the signal carried by the carrier of frequency f ₀ and modulated by the spread spectrum code,
It is possible to obtain a correlation signal related to addition synthesis. On the other hand, the correlation output related to the signals carried by the carriers of different frequencies does not become the output related to the addition and combining even if the signals are modulated by the same spread spectrum code.
However, by setting k _i to a value such as 0, ± 1, ± 2, etc. so that k _i / 2 becomes sufficiently small, the correlation detection performance is not substantially impaired and the interference of pseudo carriers is not generated. A high performance SAW spread spectrum demodulator is obtained.

[Brief description of drawings]

FIG. 1 is a perspective view showing a configuration of a SAW spread spectrum demodulator according to an embodiment of the present invention.

2A and 2B are diagrams showing a configuration of this embodiment in comparison with a conventional configuration, in which FIG. 2A is a diagram showing a conventional coding electrode interval, and FIGS. 2B and 2C are coding in the embodiment. It is a figure which shows an electrode space | interval.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Piezoelectric substrate 2 Input electrode 3 Coded output electrode 3-1, 3-2, 3-3, ... 3-n Coded electrode 4 Input terminal 5 Output terminal

Claims (3)

[Claims] 1. A first electrode formed on a piezoelectric substrate,
A second electrode having n (n: an integer of 2 or more) coded electrodes which are arrayed and formed on the piezoelectric substrate so as to be spaced from each other along the propagation direction of the surface acoustic wave and which are commonly connected to the output terminal; When the signal modulated by the spread spectrum code is carried by the carrier of the predetermined frequency f ₀ and input to the first electrode, the surface acoustic wave excited on the piezoelectric substrate by the first electrode is encoded. The signal output received from each of the coded electrodes is received by the electrodes
In the SAW spread spectrum demodulator which is synthesized by the electrodes of and output from the output terminal as a correlation signal, when the number i is given in order to the intervals of the adjacent coding electrodes, the intervals of the n coding electrodes are , D _i = M _i / f ₀ · v _s where M _i = N + k _i / 2: positive half integer, N: the number of carrier cycles included in one chip length of the spread spectrum code The closest positive half-integer, k _i : an integer, is d _i , and for an interval in which k _i is an odd number, all the coded electrodes on one side of the adjacent coded electrodes are inverted in phase. A SAW spread spectrum demodulator having the above structure. 2. A first electrode formed on a piezoelectric substrate,
A second electrode having n (n: an integer of 2 or more) coded electrodes which are arrayed and formed on the piezoelectric substrate so as to be spaced from each other along the propagation direction of the surface acoustic wave and which are commonly connected to the output terminal; When a signal modulated by a spread spectrum code is carried by a carrier having a predetermined frequency f ₀ and input to the second electrode, a surface acoustic wave of velocity v _s excited on the piezoelectric substrate by each coded electrode. In the SAW spread spectrum demodulator that receives the signal from the first electrode and outputs the resulting signal output from the first electrode as a correlation signal from the output terminal. In this case, the interval between the n coded electrodes is expressed by the following equation: d _i = M _i / f ₀ · v _s where M _i = N + k _i / 2: positive half integer , N: spread spectrum code Nearest positive half integer number of carriers period included in one chip length, k _i: integer, in a d _i represented, for interval k _i is an odd number, on one side of the adjacent coding electrode A SAW spread spectrum demodulator having a structure in which all the encoding electrodes are inverted in phase. 3. A second electrode formed by forming a first electrode on a piezoelectric substrate and having n (n: an integer of 2 or more) coded electrodes and commonly connected to an output terminal is a coded electrode. In the method of constructing the SAW spread spectrum demodulator in which the elements are arranged on the piezoelectric substrate so as to be spaced apart from each other in the propagation direction of the surface acoustic wave, when the numbers i are sequentially given to the intervals between the adjacent coding electrodes, The interval between n coded electrodes is expressed by the following equation: d _i = M _i / f ₀ · v _s where M _i = N + k _i / 2: positive half integer, N: 1 of spread spectrum code The positive half-integer closest to the number of carrier periods included in the chip length, k _i : an integer, is set to d _i , and the interval at which k _i is an odd number is on one side of the adjacent encoding electrode. S is characterized in that all the encoding electrodes have a structure in which the phases are inverted. Method of configuring AW spread spectrum demodulator.