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FIELD OF THE INVENTION
The present invention relates to an improvement of a spread spectrum
communication device, which transmits and receives various sorts of
information by using spread spectrum.
BACKGROUND OF THE INVENTION
Heretofore various sorts of systems have been researched and developed as
communication systems. Among them the spread spectrum communication system
is known as a system having a high reliability.
By this spread spectrum communication system, on the transmitter side, a
primary modulated signal of data in a narrow frequency band of base band
information, sound, etc. is transmitted with spread spectrum obtained by
hopping to a plurality of frequence in a wide frequency band with a high
speed (FH method, frequency hopping), by spreading spectrum in a wide
frequency band by using a high spread pseudo noise code (PN code) (DS
method, Direct Sequence), or by combining them (FH/DS method), and on the
receiver side, an information signal is reproduced by inversely spreading
the wide band signal to the original narrow band primary modulated signal
by means of a correlator. Recently attention is paid to this spread
spectrum communication system as a communication system having a very high
reliability from several points of view that it is resistant to external
interference and noise, that it has a high secrecy, etc.
Now the greatest point in this spread spectrum communication system is the
construction of the correlator on the receiver side. At present, in the
wireless spread spectrum communication system, the correlator, which is
thought to be the simplest and to have a high reliability, is a device
using surface acoustic wave (SAW).
In SAW convolvers there are, in general, those of correlator type (tapped
delay line type) and those of convolver type. Here, although those of
correlator type have a simple construction and generally a high
efficiency, they are influenced seriously by the temperature coefficient
of the substrate. On the contrary, those of convolver type are hardly
influenced by temperature variations, but in general they have a low
efficiency. However, the PN code described above of those of correlator
type is fixed and they cannot deal with variations therein, while the kind
of the PN code can be freely varied for those of convolver type.
Consequently, if the efficiency is at a level usable in practice, the
convolver type correlator can be much easily used.
Further, in the spread spectrum communication system, although the DS
method can be realized in a very simple manner, because the frequency band
of a high speed PN code is widened by mixing it with the base band
information by means of one mixer, it has a weak point in the separation
from other channels or in the remotelocal problem.
Therefore it is said that the FH/DS method is advantageous, by which the DS
method is combined with the FH method, in order to overcome the weak point
in the separation or the remote-local problem.
This method is a direct spreading modulation method, by which the central
frequency hops periodically. FIG. 6 shows a spectrum of this modulation
system. The spread spectrum signal indicated in the figure is composed of
a number of spread signals. A direct spread signal covering a part of the
whole band appears at each instance and on the other hand the signal as a
whole has a frequency hopping pattern. The frequency hopping/direct
spreading signal is used for the reasons enumerated as follows. That is,
it is used for increasing the capacity of spreading the spectrum, for
multiple connection and discrete address, and for multiplexing signals.
The output of the FH/DS modulator is nothing but that obtained by
superposing a direct spreading modulation on a carrier hopping in
frequency, as indicated in FIG. 7. In FIG. 7, reference numeral 40 is a
frequency synthesizer; 41 is a code generator; 42 is a multiplier; 43 is a
balanced modulator; and 44 is a transmitting antenna. The difference
between this FH/DS modulation and the simple direct spreading modulation
consists in that the carrier frequency is constant in the latter, while it
varies in the former. Further, it is known that it is possible to
construct a system, where code data coming from one code sequence
generator 41 are supplied not only for determining the hopping pattern by
means of the frequency synthesizer 40 but also for the balanced modulation
for the direct spreading.
In the receiver, by this FH/DS method, it is a direct spreading correlator
and a frequency hopping correlator superposed on each other that are used
at demodulating the spread spectrum modulation before the demodulation of
the base band. That is, a local reference wave is the frequency
hopping/direct spread signal, which is multiplied by the input signal.
FIG. 8 shows the construction of a typical FH/DS type receiver, in which
reference numerals 50 and 54 are mixers; 51 is an IF filter; 52 is a base
band demodulator 53 is a balanced modulator; 55 is a frequency
synthesizer; 56 is a code generator; and 57 is a synchronizing circuit.
The local reference oscillator is substantially the same as the modulator
on the transmitter side except for the following two points, i.e. (a) that
the central frequency is off-set by an amount corresponding IF and (b)
that the direct spread code is not modulated by the base band input.
Although a combination of the frequency mixer 50 and the IF filter 51 is
used as the correlator indicated in FIG. 8 stated above, it has a drawback
that the synchronizing circuit 57 is complicated and the synchronization
catch time is long.
As an FH/DS type correlator an SAW convolver has a number of practical
advantages that the manner of the frequency hopping and the sort of the PN
code can be freely changed. However, when the hopping frequency is
selected simply arbitrarily, complexicity is produced in the timing
synchronization of the hopping and improvement in the channel separation
or local-distant problem cannot be obtained satisfactorily. Further, in
the case where the frequency band width, which can be used, is restricted
by a legal restriction, etc. the hopping method using a frequency interval
of about 1/2 of the spread band width by the DS method, which has been
studied heretofore, can take a small number of frequence and thus the
effect of improving characteristics owing to the adoption of the FH/DS
method is small.
OBJECT OF THE INVENTION
The object of the present invention is to realize the simplification of the
modulation-demodulation method by utilizing characteristics peculiar to
the convolver in the spectrum spreading process using the frequency
hopping and the PN code, in the case where a convolver is used as a
correlator.
SUMMARY OF THE INVENTION
In order to achieve the above object, an SSC device according to the
present invention is characterized in that it comprises a transmitter
including primary modulating means for obtaining base band primary
modulated information from data to be transmitted, secondary modulating
means for spreading the spectrum by hopping the frequency of the base band
primary modulated information described above and tertiary modulating
means for spreading the spectrum of the output of the secondary modulating
means by using a PN code; and a receiver including demodulating means
consisting of a convolver for correlating a mixture of a reference signal
PN code, which is inverted in time with respect to the PN code in the
transmitter described above and a frequency hopping signal, which is in
synchronism with the timing of the frequency hopping on the transmitter
side, with the received spread spectrum signal.
In the spread spectrum communication device according to the present
invention, since the FH/DS method is adopted, the frequency may be hopped
asynchronously with the base band information on the transmitter side and
in addition the PN code can be generated asynchronously therewith.
Therefore no complicated synchronization is necessary. In particular, when
an SAW convolver is used as the convolver, since the output disappears, if
the frequency on the reception side differs from that on the reference
side by a value greater than the inverse (f.sub.g) of a period of time
necessary for the SAW to propagate under the output gate electrode, only
the timing of the FH may be synchronized between the transmitter and the
receiver and therefore it is possible to construct them in a very simple
manner by increasing satisfactorily the speed of hopping the frequency of
the FH and the speed of the PN code of the DS with respect to the base
band information by setting the points, where the FH modulation described
above hops, approximately with an interval of this f.sub.g.
Consequently it is possible to use a number of hopping frequence in the
restricted frequency band and also to intend considerable improvement in
the channel separation, the distant-local problem, etc.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram for explaining how to use the SAW convolver,
which is the base of the present invention;
FIGS. 2 and 3 show correlation outputs of the SAW convolver indicated in
FIG. 1;
FIG. 4 is a block diagram illustrating the construction of the spread
spectrum transmitter and receiver, which is an embodiment of the present
invention;
FIG. 5 is a block diagram showing another embodiment of the present
invention;
FIG. 6 indicates the spectrum of the FH/DS system;
FIG. 7 is a block diagram indicating a prior art FH/DS transmitter; and
FIG. 8 is a block diagram indicating a prior art FH/DS receiver.
DETAILED DESCRIPTION
Hereinbelow several embodiments of the present invention will be explained,
referring to the drawings.
FIGS. 1 to 3 show how to use new basic characteristics of the SAW convolver
according to the present invention in a spread spectrum receiver using the
SAW convolver.
In the block diagram indicated in FIG. 1 reference numeral 25 is an SAW
convolver; 26 and 29 are mixers; 27 and 30 are oscillators; and 28 and 31
are PN code generators. A mixed signal obtained by mixing a signal having
a frequency f.sub.1 coming from the oscillator 27 and a PN code coming
from the PN code generator 28 in the mixer 26 is inputted in the SAW
convolver 25 as the received signal. Further a mixed signal obtained by
mixing a signal having a frequency f.sub.2 coming from the oscillator 30
and a PN code coming from the PN code generator 31, which is inverted in
time with respect to the PN code generator 28, is inputted in the SAW
convolver 25 as the reference signal. At this time, the output waveform of
the convolver output 32 has a convolution peak, as indicated in FIG. 2, in
the case where
.DELTA.f=.vertline.f.sub.1 -f.sub.2 "<f.sub.g (1)
(f.sub.g : inverse of the period of time necessary for the SAW to propagate
under the convolution gate electrode of the SAW convolver) and a
convolution peak considerably reduced, as indicated in FIG. 3, so that
only a small almost noise-like output is obtained, in the case where
f=.vertline.f.sub.1 -f.sub.2 .vertline.>f.sub.g (2)
Consequently, in view of the fact described above, the hopped frequency of
the FH in the spread spectrum communication device, which is an embodiment
of the present invention, stated later is selected so as to be
approximately an integer times as great as f.sub.g, a characteristic
peculiar to the SAW convolver described above, or frequency hopping points
are selected so that there exists a difference in frequency greater than
about f.sub.g between the different channels. By selecting such frequency
hopping points in the FH spreading, improvement in S/N at the reception
has been considerably increased with respect to that obtained in the case
where they are selected arbitrarily.
As an example, when f.sub.a =215 MHz and when the frequency on the f.sub.2
side was away from 215 MHz by more than about 110 kHz, the convolver
output 32 was reduced considerably. Since the central frequency of the SAW
convolver used at this time was about 215 MHz and the effective band width
was about 30 MHz, the frequency hopping points was able to be set with an
interval of about 110 kHz within this frequency band of about 30 MHz.
In this way, it was found that it is possible to set a number of frequency
hopping points in a frequency band approximately identical to that of the
SAW convolver. For this reason, it was possible to obtain a correlation
output by frequency-hopping the reference signal of the SAW convolver.
FIG. 4 shows an embodiment of the spread spectrum communication device of
the FH/DS system using an SAW convolver according to the present
invention.
In the figure, T represents the transmitting section and R the receiving
section. The transmitting section T consists of mixers 1, 3, an oscillator
2, a PN code generator 4, a band pass filter 5, an amplifier 6, a
transmitting antenna, an oscillator controller 23, and a clock generator
24. On the other hand, the receiving section R consists of a receiving
antenna 7', band pass filters 8, 10, 14, amplifiers 9, 15, an oscillator
11, mixers 12, 21, an SAW convolver 13, an envelope detector 16, a
synchronizing circuit 17, a PN code controller 18, a PN code generator 19,
a carrier synchronizing circuit 20, a demodulating circuit 22 and an
oscillator controller 23'. In the transmitting section T, the base band
primary modulated information to be transmitted (e.g. a primary modulated
signal generated by a modulator 39 according to digital data) is given to
the mixer 1 and a signal for hopping the frequency with an interval of the
frequency hopping as described above is produced by means of the
oscillator controller 23 by the clock coming from the clock generator 24
synchronized with the base band information. A carrier for the FH signal
is produced by the oscillator 2 by using this signal and the FH spread
modulation of the information is effected by the mixer 1. Thereafter a
fast PN code is produced by the PN code generator 4 in synchronism with
the clock coming from the clock generator 24, which fast PN code is mixed
with the FH spread modulated signal described above in the mixer 3 to
effect the DS modulation. Thereafter the signal passes through the band
pass filter 5 making the necessary frequency band pass through and further
through the amplifier 6 in order to obtain necessary power. After that,
the spread spectrum of the FH/DS system is transmitted through the antenna
7.
In the receiving section R, the spread spectrum signal received by the
antenna 7' is supplied to the SAW convolver 13 after having been amplified
in the high frequency region by the amplifier 9 as well as the band pass
filters 8 and 10.
Further the FH/DS signal is supplied to the SAW convolver 13 as the
reference signal 11, which FH/DS signal is obtained by modulating the
carrier coming from the oscillator 11 generating the FH carrier frequency
supplied to the SAW convolver 13 and hopping in the same way as the
received signal, by means of the PN code generator 19 generating the PN
code inverted in time with respect to the PN code generated by the PN code
generator 4 on the transmitter side. The PN code used for the reference
signal generation described above is kept to be in synchronism with the PN
code, which is sent by the transmitter side and received, by the envelope
detecting circuit 16, the synchronizing circuit 17 and the PN code control
circuit 18. At this time, the output of the convolver 13 (whose frequency
is 2fc, i.e. twice as high as the input carrier frequency fc) is mixed
with a signal synchronized with the carrier of the received signal by the
synchronizing circuit 20 (frequency 2fc) in the mixer 21 through the band
pass filter 14 and the amplifier 15 and led to the envelope detecting
circuit 16 for the synchronization of the PN code described above. Further
the signal (frequency fc) coming from the carrier synchronizing circuit 20
is led also to the oscillator controller 23' and synchronized also with
the oscillator 11 oscillating with the FH carrier frequency described
above. The base band primary modulated information is obtained from the
modulating circuit 22, when the carrier, the FH signal and the PN code are
in synchronism with each other.
FIG. 5 shows another embodiment of the FH/DS type spread spectrum
transmitter and receiver according to the present invention. In the
figure, reference numerals identical to those in FIG. 4 represent circuits
identical or analogous thereto. The transmitting section T is almost
identical to the transmitting section T indicated in FIG. 4, except that
the base band primary modulated information is applied to the clock
generator 24. On the other hand, in the receiving section R, reference
numeral 33 is a mixer; 34 is an AGC circuit; 35 is a local oscillator; 36
is a clock generator; and 37 is an FH timing synchronizing circuit. In the
present embodiment, owing to the fact that the FH signal, i.e. the speed,
with which the oscillator 2 hops by the oscillator controller 23, is
faster than the base band information and that the speed of the PN code
generated by the PN code generator 4 is higher than it, it is not
necessary to synchronizing the oscillator controller 23 and the PN code
generator 4 with the base band information.
Further, if the speed of the PN code generated by the PN code generator 4
is sufficiently higher than the speed of the oscillator controller 23,
when the receiver uses the SAW convolver 13, it is not necessary to
synchronize the PN code generator 4 with the oscillator controller 23 on
the transmitter side.
Consequently the construction of the receiving section T indicated in FIG.
5 is simple, in which the synchronization is required only for the FH
signal.
At first, the spread spectrum signal received by the antenna 7' in the
receiving section is converted into an intermediate frequency band by the
local oscillator 35 and the mixer 33 after having been amplified by the
band pass filter 8 and the amplifier 9.
At this time, the AGC circuit 34 can work easily, if the amplitude of the
modulated signal transmitted by the transmitting side is constant, and
thus it is possible to eliminate instabilities in the intensity of the
received electromagnetic wave such as fading. It is inputted in the SAW
convolver 13 as the transmitted signal after having passed through the
band pass filter 10.
On the other hand, as the reference signal to the SAW convolver 13, the
same signal as the FH/DS spread in the spectrum on the transmitting side
(however the PN code for the DS is inverted in time with respect to that
used on the transmitting side) is added to form the correlation. The base
band primary modulated information is restored by removing noise,
taking-out the transmitted signal and making it pass through the
demodulating circuit 22 after having made it pass through the band pass
filter 14 and the amplifier 15. At this time, the FH timing synchronizing
circuit 37 is driven so that the output peak of the SAW convolver 13 is
maximum and the timing, with which the FH signal is generated, is
regulated by the oscillator controller 23' and the oscillator 11. At this
time, the frequency hopping points of the FH signal are set so as to be
approximately an integer times as great as f.sub.g, or they are set so
that there exists a difference in frequency greater than about f.sub.g
between the different channels, just as on the transmitting side. However,
in order to synchronize the receiver side with the transmitting side
concerning the timing and the order of the frequency hopping, the FH
timing circuit 37 is driven.
It is not necessary to synchronize the clock generator 36 for the DS signal
and the PN code generator 19 with the transmitting side, but the PN code
inverted in time with respect to that used on the transmitting side is
produced, which is mixed with the FH signal by the mixer, and the FH/DS
signal is applied to the SAW convolver 13 as the reference signal.
In this way, in the FH/DS type spread spectrum communication device, if the
frequency hopping points of the FH are so set that there exists always a
difference greater than f.sub.g between the frequency points, at which the
different channels hop, from the characteristics of the SAW convolver,
improvement in S/N is easily considerably increased.
As explained above, according to the present invention, in a spread
spectrum communication device, since it is possible to construct easily
the FH/DS system, information communication can be effected with a high
reliability and a small number of errors by means of a very simple
transmitter and receiver. In particular, in the case where communication
is effected by using an SAW convolver and a weak electromagnetic wave,
whose frequency band is restricted, this effect is remarkable and the
effect in practice is great.
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