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BACKGROUND OF THE INVENTION
This invention relates to an earth station for carrying out communication
in a satellite communication system through a single satellite or a
plurality of satellites.
In addition to a satellite or satellites, a satellite communication system
comprises a base station and a fixed station. The earth station may be
used as a selected one of the base station and the fixed station.
Alternatively, a satellite communication system comprises a base station
and a movable station. The earth station may be used as one of the base
station and the movable station. The movable station is carried by an
airplane or an automobile and has a variable location. As a further
alternative, the satellite communication system comprises a base station,
a fixed station, and a movable station. The earth station may be used as
one of the base station, the fixed station, and the movable station.
When used as the movable station which has a small antenna of wide
directivity, the earth station is herein called a small earth station. The
small antenna has a wide directivity in order to cope with variation of
the variable location of the earth station. The satellite communication
system generally comprises a plurality of small earth stations. In
addition to the satellite communication system, another satellite
communication system may use the satellite or satellites and comprise
another plurality of small earth stations. In this event, undesirable
interference takes place between these satellite communication systems.
In order to avoid such interference, proposal is made about using a spread
spectrum technique in U.S. Pat. No. 4,455,651 issued to Paul Barran et al
and assigned to Equatorial Communications Company.
The spread spectrum technique is useful for a movable station in locating
the variable location at which the station is present. This field of
application of the spread spectrum technique is disclosed in U.S. Pat. No.
4,359,733 issued to K. O'Neill.
However, a wide frequency band is occupied when carrying out communication
by the use of the spread spectrum technique. This makes it difficult to
transmit other data signal through the frequency band and results in a
reduction of efficiency of transmission.
In order to improve the efficiency of transmission, the frequency band is
divided into a lower frequency band and a higher frequency band. The lower
frequency band is employed for frequency division multiplexed signals. The
higher band is employed for spread spectrum signals. Inasmuch as only the
higher frequency band is used, the spread spectrum signals are unavoidably
received with a reduced gain.
SUMMARY OF THE INVENTION
It is an object of this invention to provide an earth station for use in a
satellite communication system that comprises at least one satellite
assigned with a frequency band and is capable of effectively using a
frequency band of a satellite.
It is another object of this invention to provide an earth station of the
type described, that is capable of preventing interference between two
satellite communication systems.
An earth station to which this invention is applicable is for carrying out
communication in a satellite communication system through a satelllite by
the use of an up-link frequency band and a down-link frequency band. The
earth station comprises a transmission section responsive to first and
second input signals for transmitting first and second transmission
signals through the up-link frequency band towards the satellite.
According to this invention, each of the up-link and the down-link
frequency bands has a plurality of frequency subbands spaced apart from
one another with frequency gap bands interposed between the frequency
subbands. The transmission section comprises modulating means for
modulating a selected one of the frequency subbands of the up-link
frequency band by the first input signal into a transmission subband
signal; first transmitting means coupled to the modulating means for
transmitting the transmission subband signal as the first transmission
signal through the selected one of the frequency subbands; spread spectrum
processing means for processing the second input signal into a spread
spectrum transmission signal in the up-link frequency band; and second
transmitting means coupled to the spread spectrum processing means for
transmitting the spread spectrum transmission signal as the second
transmission signal through the up-link frequency band.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 schematically shows a conventional satellite communication system
having a plurality of earth stations;
FIG. 2 shows a process carried out on a reception signal in one of the
earth stations;
FIG. 3 diagrammatically shows division of a frequency band for use in
another conventional satellite communication system;
FIG. 4 schematically shows a satellite communication system which comprises
a plurality of earth stations according to a first embodiment of this
invention;
FIG. 5 shows a process carried out on a reception signal in one of the
earth stations illustrated in FIG. 4;
FIG. 6 shows a block diagram of a movable station used as one of the earth
stations depicted in FIG. 4;
FIG. 7 shows a view of a response of a matched filter used in one of the
earth stations illustrated in FIG. 4;
FIG. 8 shows an example of processing CDM spread spectrum signals in one of
the earth stations illustrated in FIG. 4;
FIG. 9 shows another example of processing CDM spread spectrum signals in
one of the earth stations illustrated in FIG. 4;
FIG. 10 shows a block diagram of a demodulation unit used in one of earth
stations illustrated in FIG. 4;
FIG. 11 shows a view for use in describing operation of the demodulation
unit illustrated in FIG. 10; and
FIG. 12 schematically shows a satellite communication system according to
another embodiment of this invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring to FIG. 1, a conventional satellite communication system will be
described at first in order to facilitate an understanding of the present
invention. The satellite communication system comprises a plurality of
satellites and a plurality of earth stations. In the example being
illustrated, only two satellites are exemplified as first and second
satellites 20 and 21. A plurality of movable stations 23-1 through 23-N
and a single base station 24 are illustrated as the earth stations.
In the manner known in the art, the first and second satellites 20 and 21
are on the geostationary orbit. The first satellite 20 is displaced from
the second satellite 21 on the geostationary orbit. The base station 24
can communicate with the movable stations 23-1 through 23-N through the
first and the second satellites 20 and 21.
In the base station 24, a signal combination circuit 25 is supplied with a
frame synchronization signal of a predetermined bit pattern and a sequence
of message signals. The frame synchronization signal and the message
signals are supplied from an external device (not shown). The signal
combination circuit 25 positions the frame synchronization signal at the
head of each frame and the message signal following the frame
synchronization signal to produce a combination signal.
The combination signal is subjected to phase shift keying (PSK) by a
modulator 26 to be produced as a PSK signal. The PSK signal is sent to a
spread spectrum processing modulator 27. The spread spectrum processing
modulator 27 carries out forward spread spectrum processing of the PSK
signal by a predetermined pseudo noise code signal (PN signal) to produce
a code division multiplexed (CDM) spread spectrum signal. The CDM spread
spectrum signal will be called a CDM signal hereinafter. When such CDM
signals should be directed to some or all of the movable stations 23-1
through 23-N, different PN signals are used. The CDM signals are different
from one another depending on the PN signals.
The CDM signal is delivered to a transmitter-receiver 28 as a transmitted
CDM signal and is transmitted from the transmitter-receiver 28 through a
sharp directivity antenna 29 to the first satellite 20 by the use of an
up-link frequency band.
The transmitted CDM signal is repeated by the first satellite 20 as a
repeated CDM signal. Each of the movable stations 23-1 through 23-N
receives the repeated CDM signal through a down-link frequency band as a
received CDM signal.
When the CDM signal is transmitted at a transmission rate within a
bandwidth of .DELTA.f (Hz), the frame has a frame period 1/.DELTA.f. Each
of the up-link frequency band and the down-link frequency band must have a
bandwidth of N.DELTA.f (Hz), where N represents the spectrum spread
parameter as called in the art.
The received CDM signal is received at each of the movable stations 23-1
through 23-N by a low or wide directivity antenna. One of the movable
stations 23-1 through 23-N is assigned with a particular PN signal. When
the received CDM signal is identified by the particular PN signal, that
movable station carries out inverse spread spectrum processing of the
received CDM signal to reproduce the PN signal and the PSK signal. The PSK
signal is demodulated into a reproduced combination signal. The movable
station under consideration derives or extracts the message signal from
the combination signal.
It will now be assumed that it is desired by the movable station 23-1 to
carry out a position or location to locate its position or location. The
position or location is determined after the PN signal is reproduced as a
reproduced PN signal. In this event, the movable station 23-1 carries out
forward spread spectrum processing of a positioning message signal into a
locating CDM signal by the particular PN signal which is synchronized with
the reproduced PN signal.
Through the up-link frequency bands, the locating CDM signal is transmitted
to the first and the second satellites 20 and 21 as first and second
transmission signals, respectively. The first transmission signal is
repeated by the first satellite 20 and is received by the
transmitter-receiver 28 as a first reception signal. Likewise, the second
transmission signal is received as a second reception signal by a receiver
31 through a reception antenna 30. First and second matched filters 32 and
33 are for carrying out inverse spread spectrum processing of the first
and the second reception signals, respectively.
Turning to FIG. 2, each of the first and the second reception signals has
frames D.sub.n-1, D.sub.n, and D.sub.n+1 in the manner depicted along an
upper line labelled (a). Each frame has the frame period 1/.DELTA.f. As
shown along a lower line indicated at (b), each of the matched filters 32
and 33 produces a sequence of pulse signals located at the heads of the
respective frames. Each pulse signal has a time duration 1/(N.DELTA.f),
respectively.
Turning back to FIG. 1, the pulse signals are delivered to a calculating
unit 34. Based on the pulse signal sequence supplied from the first
matched filter 32, the calculating unit 34 detects a first time instant of
arrival of the first reception signal from the first satellite 20.
Similarly, the calculating unit 34 detects a second time instant of
arrival of the second reception signal from the second satellite 21 by
using the pulse signal sequence produced by the second matched filter 33.
Based on the first and the second time instants, the calculating unit 34
calculates the position of the movable station 23-1 by the use of
triangulation in the manner known in the art. This position data is
transmitted to the movable station 23-1 through the first satellite 20 as
described above.
The locating CDM signal is transmitted by using a wide frequency bandwidth
N.DELTA.f of each of the first satellites 20 and 21. Therefore, it is
possible to determine the position with a high degree of accuracy.
However, the wide frequency band is occupied by the locating CDM signal.
This makes it difficult to transmit other CDM signals through the
frequency bandwidth and results in a reduction of efficiency of
transmission.
Referring to FIG. 3, the frequency band is divided into a lower frequency
band and a higher frequency band. The lower frequency band is used in
transmitting frequency division multiplexed signals. Only the higher
frequency band is used in transmitting the CDM signals.
The lower frequency band has a plurality of frequency subbands adjacent to
one another. The frequency division multiplexed signal consists of data
signals transmitted through the respective frequency subbands. The
frequency subbands are called first frequency channels of frequency slots.
Each spread spectrum signal is a CDM signal and carries the message
signals subjected to spread spectrum processing.
Inasmuch as only the higher frequency band is used, the spread spectrum
signals are unavoidably received with a reduced gain.
Referring to FIG. 4, a satellite communication system comprises a single
base station 35, first through m-th fixed stations 36-1 to 36-m, and first
through k-th movable or mobile stations 37-1 to 37-k. Each of the movable
stations 37-1 to 37-k may be carried by a vehicle, such as an automobile
or an airplane. Each of the base station 35 and the movable stations 37
(suffixes omitted) is according to a first embodiment of this invention as
will become clear as the description proceeds. The satellite communication
system may comprise a plurality of base stations which cooperate with the
fixed stations. At any rate, the illustrated base station 35, the fixed
stations 36-1 to 36-m, and the movable stations 37-1 to 37-k are
communicable with one another through the first and second satellites 20
and 21 which are assumed to be geostationarily located at different
positions of a geostationary orbit. However, it is to be noted that the
first and the second satellites 20 and 21 may not always be geostationary
satellites but orbiting satellites which run along different orbits.
In the example being illustrated, the base station 35 bidirectionally
communicates with the fixed stations 36-1 to 36-m and the movable stations
37-1 to 37-k not only through the first satellite 20 but also through the
second satellite 21. For this purpose, up-link and down-link frequency
bands are determined between the base station 35 and the movable stations
37-1 to 37-k and between the base station 35 and the fixed stations 36-1
to 36-m and may be common to the first and the second satellites 20 and
21.
Turning to FIG. 5 for a short while, each of the up-link and the down-link
frequency bands comprises a plurality of frequency subbands or channels
spaced apart from one another along a frequency axis with a frequency gap
or slots interposed between the frequency subbands, as shown along a top
line labelled (a). Each frequency subband is called a first frequency
channel. Likewise, each frequency gap is called a second frequency
channel.
In FIG. 4, the base station 35 comprises a frequency division multiplexing
(FDM) unit 38 for carrying out frequency division multiplexing of a
plurality of data signals such as sound signals to produce an FDM signal.
A demultiplexing (DEMUX) unit 39 is for demultiplexing a reception FDM
signal to reproduce a plurality of data signals. A spread spectrum
processing (SSP) unit 40 carries out forward spread spectrum processing of
a message signal to produce a CDM signal. First and second inverse spread
spectrum processing (first and second ISSP) units 41 and 42 are for
carrying out inverse spread spectrum processing on first and second
reception CDM signals to reproduce message signals, respectively.
The FDM unit 38, the demultiplexing unit 39, and the first inverse spread
spectrum processing unit 41 are connected to a first transmitter-receiver
43 connected to a first sharp directivity antenna 44 directed to the
second satellite 21. The second inverse spread spectrum processing unit 42
and the spread spectrum processing unit 40 are connected to a second
transmitter-receiver 45 accompanied by a second sharp directivity antenna
46 directed to the first satellite 20.
Referring to FIG. 6, the first movable station 37-1 comprises a data
modulator 47, a demodulation (DEM) unit 48, a spread spectrum modulation
(SS MOD) unit 49, and a spread spectrum demodulation (SS DEM) unit 50. A
transmitter-receiver 51 is coupled to a low or wide directivity antenna 52
and is connected to the data modulator 47, the demodulation unit 48, the
spread spectrum modulation unit 49, and the spread spectrum demodulation
unit 50.
The data modulator 47 modulates a predetermined subcarrier assigned to the
movable station 37-1 by a transmitting data signal to produce a modulated
signal. The predetermined subcarrier is one of the first frequency
channels that is assigned to the first movable station 37-1. In this
manner, the data modulator 47 modulates the predetermined subcarrier by
the transmitting data signal and serves as a modulating arrangement for
modulating a selected one of the frequency subbands or the first frequency
channels into a transmission subband signal by the transmitting data
signal which serves as a first input signal.
A message signal has a predetermined code sequence assigned to the first
movable station 37-1 as a second input signal and is delivered to the
spread spectrum modulation unit 49. In the manner which will later be
described in detail, the spread spectrum modulation unit 49 carries out
spread spectrum processing of the message signal to provide a CDM (code
division multiplexed) signal as a spread spectrum transmission signal
having the up-link frequency band. Production of a CDM signal is possible
by the use of a spread spectrum technique which is described in a book
"Spread Spectrum Systems" written by R. C. Dixon and published 1976 by
John-Wiley and Sons, Inc. The spread spectrum technique will therefore not
be described in detail.
The transmission subband signal and the CDM signal are delivered to the
transmitter-receiver 51. The transmitter-receiver 51 transmits the
transmission subband signal through the predetermined subband as a first
transmission signal. The transmitter-receiver 51 serves as a first
transmitting arrangement for transmitting the transmission subband signal.
The transmitter-receiver 51 transmits the CDM signal through the up-link
frequency band as a second transmission signal. The transmitter-receiver
51 serves as a second transmitting arrangement for transmitting the CDM
signal. The first and the second transmission signals are collectively
called a transmission signal hereinabove.
In a like manner, the base station 35 (FIG. 4) transmits a transmission
subband signal and a spread spectrum transmission signal. A combination of
the transmission subband signal and the spread spectrum transmission
signal is termed a transmitted signal when transmitted from the base
station 35.
Through the second satellite 21, the first movable station 37-1 receives,
as a first reception signal, the transmission subband signal transmitted
from the base station 35 through one of the frequency subbands that is
assigned in the down-link frequency band to the movable station 37-1 as a
predetermined one of the frequency subbands. Furthermore, the movable
station 37-1 receives, as a second reception signal, the spread spectrum
transmission signal which is transmitted from the base station 35 through
the up-link frequency band and is repeated by the first satellite 20
through the down-link frequency band. Responsive to the first and the
second reception signals, the transmitter-receiver 51 produces first and
second reception band signals.
The demodulation unit 48 produces a reception subband signal from the first
reception band signal and demodulates the reception subband signal into a
data signal. The spread spectrum demodulation unit 50 produces a frequency
gap signal from the second reception band signal and carries out inverse
spread spectrum processing on the frequency gap signal to produce a
message signal.
More particularly, the demodulation unit 48 comprises a channel selection
filter 48a for selecting the reception subband signal from the first
reception band signal. A data demodulator 48b demodulates the reception
subband signal into the data signal. The channel selection filter 48a
serves as a first receiving arrangement. The data demodulator 48b serves
as a first producing arrangement.
The spread spectrum demodulation unit 50 comprises a comb filter bank 50a
for selecting the frequency gap signal from the second reception band
signal. A matched filter 50b carries out inverse spread spectrum
processing of the frequency gap signal to produce a frequency matches
signal. A message demodulator 50c demodulates the frequency matched signal
into the data signal. The comb filter bank 50a serves as a second
receiving arrangement. The matched filter 50b and the message demodulator
50c serves as a second producing arrangement.
The other movable stations 37-2 to 37-k are similar in structure and
operation to the movable station 37-1 and will not be described any
longer.
Referring to FIGS. 4 and 6 together with FIG. 5, each of the first
frequency channels or the frequency subbands and the second frequency
channels or the frequency gaps has a prescribed bandwidth .DELTA.f. The
first frequency channels are assigned to the fixed stations 36-1 and 36-m
and the movable stations 37-1 to 37-k, respectively.
In case where the base station 35 communicates with the fixed stations 36-1
to 36-m and the movable stations 37-1 to 37-k by sending data signals, the
base station 35 communicates with the fixed stations 36-1 to 36-m and the
movable stations 37-1 to 37-k by using the first frequency channels
corresponding to the fixed stations 36-1 to 36-m and the movable stations
37-1 to 37-k, respectively.
The FDM unit 38 comprises the first through (k+m)-th data modulators 38-1
to 38-(k+m) corresponding to the fixed stations 36-1 to 36-m and the
movable stations 37-1 to 37-k, respectively. The data modulators 38-1 to
38-(k+m) modulate subcarriers different from each other by the data
signals to produce a plurality of modulated signals, respectively.
A multiplexer 53 carries out FDM of the modulated signals to produce an FDM
signal. The FDM signal is transmitted to the second satellite 21 through
the first frequency channels of the up-link frequency band by the first
transmitter-receiver 43.
On the other hand, te second transmitter-receiver 45 transmits a sequence
of standard bursts as a CDM signal by using the up-link frequency band to
the first satellite 20. The standard bursts are in a predetermined period.
A signal combination unit 54 combines a frame synchronization signal with
the standard burst sequence into a sequence of message signals to produce
a sequence of combined signals. A PSK modulator 55 modulates the combined
signals according to PSK to produce a modulation signal. A spread spectrum
modulator 56 carries out forward spread spectrum processing of the
modulation signal to produce a CDM signal. The CDM signal is transmitted
to the first satellite 20 through the up-link frequency band by the second
transmitter-receiver 45.
Each of the data modulators 38-1 to 38-(k+m) serves as the modulating
arrangement for modulating a selected one of the frequency subbands into a
transmission subband signal by the data signal which serves as a first
input signal. The spread spectrum processing unit 40 carries out spread
spectrum processing of the message signal sequence to produce a CDM signal
as a spread spectrum signal having the up-link frequency band in the
manner described above. The first transmitter-receiver 43 serves as the
first transmitting arrangement described above, and the second
transmitter-receiver 45, as a second transmission signal.
Although not shown, each of the fixed stations 36-1 to 36-m comprises a
data modulator, a demodulation unit, and a transmitter receiver similar in
structure and operation to the modulator 47, the demodulation unit 48, and
the transmitter-receiver 51 described in conjunction with FIG. 6. The data
modulator and the demodulation unit are connected directly to the
transmitter-receiver which is coupled, in turn, to an antenna directed to
the second satellite 21 alone like the first sharp directivity antenna 44.
The fixed stations 36-1 and 36-m and the movable stations 37-1 to 37-k are
supplied with the FDM reception signal and CDM reception signal as first
and second reception signals, respectively. In the manner depicted in FIG.
5 along the top line (a), the first reception signal has a first partial
spectrum of FDM reception signal assigned to the first frequency channels.
The second reception signal has a second partial spectrum of CDM reception
signal assigned to both of the first and the second frequency channels.
The first and the second partial spectra are collectively called a
reception spectrum.
In FIG. 6, the transmitter-receiver 51 receives the FDM reception signal
and the second reception signal as first and second reception signals,
respectively. It is assumed that the channel selection filter 48a has a
band-pass characteristic for allowing an i-th frequency subband to pass
therethrough, as shown in FIG. 5(b). As a result, the channel selection
filter 48a supplies the demodulator 48b with a modulated signal assigned
to the i-th frequency subband as shown in FIG. 5(c). The modulated signal
is demodulated by the demodulator 48b into a data signal, such as a sound
signal.
On the other hand, the comb filter bank 50a has a filter characteristic, so
as to allow the second frequency channels to pass therethrough as shown in
FIG. 5(d). Consequently, the comb filter bank 50a separates the CDM signal
from the FDM signal. The CDM signal is spread over a frequency bandwidth
N.DELTA.f and is divided into a plurality of partial spectrum signals each
of which has a frequency bandwidth of .DELTA.f and which is spaced apart
from one another as shown in FIG. 5(e). The CDM signal is supplied through
the matched filter 50b to the message demodulator 50c to be demodulated
into a demodulated PSK signal (or a message signal) and a demodulated PN
signal.
In case where the movable station 37-1 communicates with the base station
35 by sending a data signal, the modulated signal is transmitted from the
modulator 47 to the second satellite 21 through the first frequency
channel assigned to the movable station 37-1 of the up-link frequency band
as the first transmission signal. In the base station 35 (FIG. 4), the
reception signal from the second satellite 21 is received by the first
transmitter-receiver 43 through the first antenna 44 and is supplied to a
demultiplexer 57. In the example being illustrated, the demultiplexer 57
supplies the reception signal as the modulated signal to a selected one of
first through (k+m)-th data demodulators 39-1 to 39-(k+m), for example,
the data demodulator 39-1. The data demodulator 39-1 demodulates the
modulated signal to produce the data signal.
On locating the movable station 37-1, a position signal is sent as the
message signal to a PSK modulator 49a to be subjected to PSK and to be
produced as a PSK signal. The PSK signal is supplied to a spread spectrum
modulator 49b. The spread spectrum modulator 49b carries out forward
spread spectrum processing of the PSk signal by using the own PN signal in
synchronism with the demodulated PN signal to produce a CDM signal. The
CDM signal is transmitted to the first and second satellites 20 and 21 by
using the first and second frequency channels of up-link frequency bands.
In FIG. 4, the reception CDM signals from the first and second satellites
20 and 21 are received by the first and the second transmitter-receivers
43 and 46 through the first and the second antennae 44 and 46 to be
supplied to comb filter banks 41a and 42a, respectively. The comb filter
banks 41a and 42a pass only the second frequency channels, respectively.
The reception CDM signals are subjected to inverse spread spectrum
processing by matched filters 41b and 42b and are delivered to the message
demodulators 41C and 42c to be demodulated into first and second
positioning message signals.
The first and second positioning message signals are supplied to the
calculation unit 58. The calculation unit 58 comprises a detecting circuit
58a and a calculating circuit 58b. The detecting circuit 58a detects a
first arrival time instant and a second arrival time instant based on the
first and the second positioning message signals, respectively. The
calculating circuit 58b calculates the position of the movable station
37-1 in question by the use of triangulation with reference to the first
arrival time instant and the second arrival time instant. The position
signal is transmitted to the movable station 37-1 by using forward spread
spectrum processing.
Now, description will be made about inverse spread spectrum processing of
the reception CDM signal carried out in the base station 35 in detail.
When the reception CDM signal is assumed to be represented as S(t) in a
time base, namely, as a function of time t, the reception CDM signal may
be represented as S(.omega.) in a frequency base, namely, as a function of
frequency. A relationship between S(t) and S(.omega.) is given by:
S(.omega.)=.intg.S(t)e.sup.-j.omega.t dt.
Let each of the comb filter banks 41a and 42a have a frequency response
characteristic F(.omega.) represented by:
##EQU1##
where {a.sub.n } is representative of a Fourier coefficient.
From Equations (1) and (2), it is seen that each output signal S(t) from
comb filter banks 41a and 42a is represented by:
##EQU2##
In Equation (3), first and second terms on the righthand side represent a
primary response and a subsidiary or echo response. As illustrated in FIG.
7, the echo response accompanies forward and rearward the primary response
at every time instant of n/.DELTA.f, where n represents a natural number.
Each of the primary response and the echo response is restricted to a pulse
width of 1/(N.DELTA.f) in FIG. 7. This shows that the pulse width is in
inverse proportion to each frequency width of the up-link frequency band
and the down-link frequency band.
From this fact, it is readily understood that each output signal from the
comb filter banks 41a and 42a appears at a frequency interval of
1/.DELTA.f, due to the above-mentioned primary response and echo response.
If the frequency interval .DELTA.f is equal to the transmission rate R of
the reception CDM signal, the echo response adversely effects the primary
response among the codes of the reception CDM signal. As a result,
interference takes place among the codes of reception CDM signal.
In order to avoid the above-mentioned effect, Inequality (4) must hold.
##EQU3##
Inequality is rewritten into:
##EQU4##
When the spread spectrum parameter N is considerably large, the
transmission rate R may be selected so as to become larger than the
bandwidth .DELTA.f of the second frequency channel as understood by
Equation (5). The transmission rate R may be selected so as to become
larger than the bandwidth .DELTA.f of the second frequency channel as
understood by Equation (6). As a result, it is possible to avoid the
interference among the codes due to the echo pulse.
Referring to FIG. 8, the reception CDM signal is produced in the form of a
sequence of codes depicted at a.sub.1 to a.sub.5 through e.sub.1 to
e.sub.4 along first through fifth lines of FIG. 8, respectively, and may
be made to correspond to the output signal of the comb filter bank. The
comb filter bank produces, as the output signal, a primary pulse a.sub.1
resulting from the primary response. The remaining pulses a.sub.2 through
a.sub.5 are produced from the echo response. Likewise, primary pulses
b.sub.1 through e.sub.1 appear as a result of the primary response while
the remaining pulses b.sub.2 to b.sub.5 ; c.sub.2 to c.sub.5 ; d.sub.2 to
d.sub.5 ; and e.sub.2 to e.sub.4 appear as results of the echo response.
As illustrated along the first line of FIG. 8, the primary pulse a.sub.1
and the echo pulses a.sub.2 through a.sub.5 are arranged at the interval
of 1/.DELTA.f one another. Similarly, the primary pulses b.sub.1 to
e.sub.1 and the echo pulses b.sub.2 to b.sub.5 through e.sub.2 to e.sub.4
are arranged at the interval 1/.DELTA.f, respectively.
When the transmission rate R is smaller than the frequency bandwidth
.DELTA.f, the matched filter produces a code sequence arranged as shown
along a bottom line of FIG. 8(f).
Referring to FIG. 9, the reception CDM signal is produced in the form of a
sequence of codes a.sub.1 to a.sub.5 through f.sub.1 to f.sub.4 when the
transmission rate R is greater than the frequency bandwidth .DELTA.f. In
this event, the matched filter produces an output signal as shown in FIG.
9(g).
Thus, the primary pulses and the echo pulses can be separated from each
other and interference is therefore avoidable due to the primary and the
echo responses.
Now, description will be made about the message demodulator in the base
station.
Referring to FIG. 10, the matched filter 41b supplies a delay circuit 61
and an adder 65 with an output signal as shown along the bottom line of
FIG. 8. The delay circuit 61 produces a first delay signal delayed by a
delay time 1/.DELTA.f relative to the output signal. The first delay
signal is successively delayed by delay circuits 62 to 64 and thereafter
sent to the adder 65. The delay circuits 62 to 64 produce second through
fourth delay signals delayed by two, three, and four times the delay time
1/.DELTA.f relative to the output signal, respectively.
The adder 65 adds the output signal to the first through the fourth delay
signals to produce a sum signal. In the sum signal, the echo pulses
a.sub.2 through a.sub.5 are added to the primary pulse a.sub.1. Likewise,
the echo pulses b.sub.2 through b.sub.5, c.sub.2 through c.sub.5, d.sub.2
through d.sub.5, and e.sub.2 through e.sub.5 are added to the primary
response pulses b.sub.1, c.sub.1, d.sub.1, and e.sub.1, respectively.
The sum signal is supplied to a sampling circuit 66 and a square-law
detector 56.
The square-law detector 67 calculates a square of the sum signal to produce
a square signal and supplies the square signal to first and second
samplers 68 and 69. The samplers 68 and 69 send first and second sampled
signals to a differential amplifier 70 to produce an amplified signal. The
amplified signal is supplied to a voltage-controlled oscillator (VCO) 72
through a low-pass filter 71. The VCO 72 delivers a controlled signal to a
counter 73 and a digital delay circuit 74.
The counter 73 outputs a clock signal based on the output signal from the
VCO 72. The clock signal is supplied to the digital delay circuit 74 and
the sampler 66. The digital delay circuit 74 delays the clock signal with
reference to the controlled signal given from the VCO 72 to produce a
delayed clock signal delayed to the clock signal.
Referring to FIG. 10 together with FIG. 11, the clock signal and the
delayed clock signal are supplied to the samplers 68 and 69, respectively,
so that the samplers 68 and 69 sample the detected signal at sampling
points Z.sub.1 and Z.sub.2 to produce a first sampled signal and a second
sampled signal, respectively. When the amplitude of the first sampled
signal is equal to the amplitude of the second sampled signal, the output
signal from the differential amplifier 70 becomes zero. As a result, the
counter 73 generates a third clock signal in the point corresponding to a
sampling point Z.sub.3. The third clock signal is supplied to the sampling
circuit 66 so that the sampling circuit 66 samples the sum signal at the
peak level of the sum signal to produce a third sampled signal. The third
sampled signal is supplied to a demodulation circuit 75. The demodulation
circuit 75 demodulates the third sampled signal to reproduce the message
signal. The message signal is supplied to the calculation unit 58.
The message demodulator 49c is similar in structure and operation to the
message demodulator 48c.
Each of the message demodulators in the movable stations is similar in
structure and operation to the message demodulator in the base station.
In addition, the message demodulator may have a plurality of delay
circuits. For example, the message demodulator has the delay circuits 61
and 62. The adder 65 adds the output signal from the matched filter, the
first delay signal, and the second delay signal to produce a sum signal.
In order to determine the position of the movable station, a satellite 100
may be used together with the satellites 20 and 21. In this case, the base
station 35 may further comprise a third inverse spread spectrum processing
(third ISSP) unit 101. The third inverse spread spectrum processing unit
101 comprises a comb filter bank 101a, a matched filter 101b, and a
message demodulator 101c. A receiver 102 is connected to the comb filter
bank 101a and has a third sharp directivity antenna 102a directed to the
satellite 100. The message demodulator 101c is connected to the
calculation unit 58 as shown in broken line in FIG. 4.
Under the circumstances, the movable station can transmit a position
message signal to the satellites 20, 21, and 100 without receiving the
time standard signal. The calculation unit 58 detects a third arrival time
instant of the reception signal from the satellite 100 in response to the
positioning message signal given from the message demodulator 101c. The
calculation unit 58 calculates the position of the movable station based
on the first, | | |
