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Claims  |
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What is claimed is:
1. A transmitter in a CDMA transmission system comprising:
long code generating means for generating a long code which is a spreading
code with a period longer than information symbol;
spreading means for generating a wideband spread signal by spreading
transmission information using said long code;
a priori code generating means for generating a priori code representing a
phase of said long code, a cross-correlation between said a priori code
and said long code being negligibly small;
a priori code insertion means for inserting said a priori code into said
spread signal at a predetermined fixed interval to form frames; and
transmission means for transmitting said frames.
2. The transmitter in the CDMA transmission system as claimed in claim 1,
wherein said a priori code comprises smoothly changing autocorrelation
values, and a zero-cross point.
3. The transmitter in the CDMA transmission system as claimed in claim 1,
wherein said autocorrelation values of said a priori code have a
triangular profile.
4. The transmitter in the CDMA transmission system as claimed in claim 1,
wherein said a priori code has a stepwise profile.
5. The transmitter in the CDMA transmission system as claimed in claim 1,
wherein said a priori code has a period shorter than said long code.
6. The transmitter in the CDMA transmission system as claimed in claim 1,
wherein said a priori code insertion means inserts said a priori code into
said spread signal within a predetermined range at a beginning of
communications.
7. The transmitter in the CDMA transmission system as claimed in claim 1,
wherein said a priori code generating means comprises means for generating
a predetermined code for a convolution, and calculation means for carrying
out said convolution between said code for a convolution and said long
code to output said a priori code, and said a priori code insertion means
inserts an output of said calculation means into said spread signal.
8. An acquisition apparatus of a spreading code in a CDMA transmission
system which demodulates desired information by receiving a received
signal including a spread signal and an a priori code, said spread signal
being spread by a long code which is a spreading code with a period longer
than information symbol, said a priori code having a negligibly small
cross-correlation with said long code, and representing a phase of said
long code, said acquisition apparatus comprising:
first correlation means for generating a replica of said long code, and for
calculating a correlation between said received signal and said replica of
said long code;
second correlation means for generating a replica of said a priori code,
and for calculating a correlation between said received signal and said
replica of said a priori code;
phase estimation means for estimating received phase of said long code
based on an output of said second correlation means; and
means for controlling a generation phase of said replica of said long code
based on an output of said phase estimation means.
9. The acquisition apparatus of the spreading code in the CDMA transmission
system as claimed in claim 8, wherein said a priori code is inserted into
said spread signal at a fixed period, and wherein
said second correlation means comprises paired correlators consisting of a
correlator that generates replicas of paired a priori codes consisting of
a replica of a first a priori code and a replica of a second a priori code
with their correlation detection start timings shifted by an amount of
.DELTA., and that calculates correlations between said replica of said
first a priori code and
said received signal, and a correlator that calculates correlations between
said replica of said second a priori code and said received signal; and
said phase estimation means estimates a received phase of said long code
based on outputs of said paired correlators.
10. The acquisition apparatus of the spreading code in the CDMA
transmission system as claimed in claim 9, wherein
each of said paired correlators integrates a product of said received
signal and said replica of said a priori code for one period of said a
priori code; and
said phase estimation means estimates a received phase of said long code
from two integrated values.
11. The acquisition apparatus of the spreading code in the CDMA
transmission system as claimed in claim 9, wherein said second correlation
means comprises m pairs of correlators, where m is an integer greater than
one, and wherein start timings of correlation detection and integration of
adjacent pairs of said correlators are shifted by T.sub.A /m in time,
where T.sub.A is a length of said a priori code.
12. The acquisition apparatus of the spreading code in the CDMA
transmission system as claimed in claim 9, wherein said phase estimation
means comprises means for selecting one pair of said correlators which
produces maximum outputs, and estimates a received phase of said long code
based on said maximum outputs and their detection timings.
13. The acquisition apparatus of the spreading code in the CDMA
transmission system as claimed in claim 12, wherein said phase estimation
means obtains a zero-cross point, at which a correlation value between
said received signal and said replica of said a priori code becomes zero,
from said maximum outputs and their detection timings, and estimates a
received phase of said a priori code from said zero-cross point, and said
control means controls a generation phase of said replica of said long
code in accordance with the estimated received phase of said a priori
code.
14. The acquisition apparatus of the spreading code of the CDMA
transmission system as claimed in claim 8, wherein said control means sets
the generation phase of said replica of said long code such that the
generation phase is centered at the received phase of said a priori code
estimated by said phase estimation means, and carries out non-uniform
search in which the generation phase of said replica of said long code is
gradually shifted in a wider range when a phase of said replica of said
long code is not synchronized with a phase of said long code in said
received signal.
15. The acquisition apparatus of the spreading code in the CDMA
transmission system as claimed in claim 8, wherein said a priori code is
inserted solely at an initial portion of said spreading code.
16. The acquisition apparatus of the spreading code in the CDMA
transmission system as claimed in claim 8, wherein said a priori code is
inserted solely at an initial portion of said spreading code of a control
channel.
17. The acquisition apparatus of the spreading code in the CDMA
transmission system as claimed in claim 8, wherein said first correlation
means is a sliding correlator.
18. The acquisition apparatus of the spreading code in the CDMA
transmission system as claimed in claim 8, wherein said second correlation
means comprises an I-correlator and a Q-correlator, said I correlator
detecting correlation between an I-component (inphase component) of a
carrier of said received signal after quadrature detection and an
I-component of said replica of said a priori code, said Q-correlator
detecting correlation between a Q component (quadrature component) of the
carrier of said received signal after quadrature detection and a
Q-component of said replica of said a priori code, and wherein said phase
estimation means estimates a received phase of said long code using both
said I-component and said Q-component.
19. The acquisition apparatus of the spreading code in the CDMA
transmission system as claimed in claim 18, wherein said phase estimation
means obtains a complex amplitude of an output of said I correlator and an
output of said Q-correlator, and estimates said received phase of said
long code using the complex amplitude.
20. The acquisition apparatus of the spreading code in the CDMA
transmission system as claimed in claim 18, wherein said phase estimation
means compares an output of said I-correlator with an output of said
Q-correlator, and estimates said received phase of said long code using
the output with a greater absolute value.
21. The acquisition apparatus of the spreading code in the CDMA
transmission system as claimed in claim 18, wherein said phase estimation
means obtains an estimated received phase of said long code from an output
of said I-correlator, and an estimated received phase of said long code
from an output of said Q-correlator, and estimates said received phase of
said long code by averaging these two estimated received phases.
22. A receiver in a CDMA system which demodulates desired information by
receiving a received signal including a spread signal and an a priori
code, said spread signal being spread by a long code which is a spreading
code with a period longer than information symbol, said a priori code
having a negligibly small cross-correlation with said long code, and
representing a phase of said long code, said receiver comprising:
first correlation means for generating a replica of said long code, and for
calculating a correlation between said received signal and said replica of
said long code;
second correlation means for generating a replica of said a priori code,
and for calculating a correlation between said received signal and said
replica of said a priori code;
phase estimation means for estimating received phase of said long code
based on an output of said second correlation means; and
means for controlling a generation phase of said replica of said long code
based on an output of said phase estimation means.
23. A CDMA transmission system comprising:
long code generating means for generating a long code which is a spreading
code with a period longer than information symbol;
spreading means for generating a wideband spread signal by spreading
transmission information using said long code;
a priori code generating means for generating a priori code representing a
phase of said long code, a cross-correlation between said a priori code
and said long code being negligibly small;
a priori code insertion means for inserting said a priori code into said
spread signal at a predetermined fixed interval to form frames;
transmission means for transmitting said frames;
receiving means for receiving said frames;
first correlation means for generating a replica of said long code, and for
calculating a correlation between a received signal and said replica of
said long code;
second correlation means for generating a replica of said a priori code,
and for calculating a correlation between said received signal and said
replica of said a priori code;
phase estimation means for estimating received phase of said long code
based on an output of said second correlation means; and
means for controlling a generation phase of said replica of said long code
based on an output of said phase estimation means.
24. A transmission method in a CDMA transmission system comprising the
steps of:
generating a long code which is a spreading code with a period longer than
information symbol;
generating a wideband spread signal by spreading transmission information
using said long code;
generating a priori code representing a phase of said long code, a
cross-correlation between said a priori code and said long code being
negligibly small;
inserting said a priori code into said spread signal at a predetermined
fixed interval to form frames; and
transmitting said frames.
25. An acquisition method of a spreading code in a CDMA transmission system
which demodulates desired information by receiving a received signal
including a spread signal and an a priori code, said spread signal being
spread by a long code which is a spreading code with a period longer than
information symbol, said a priori code having a negligibly small
cross-correlation with said long code, and representing a phase of said
long code, said acquisition method comprising the steps of:
generating a replica of said long code;
calculating a correlation between said received signal and said replica of
said long code;
generating a replica of said a priori code;
calculating a correlation between said received signal and said replica of
said a priori code;
estimating received phase of said long code based on said correlation
between said received signal and said replica of said a priori code; and
controlling a generation phase of said replica of said long code based on
an estimated received phase of said long code.
26. A CDMA transmission method comprising the steps of:
generating a long code which is a spreading code with a period longer than
information symbol;
generating a wideband spread signal by spreading transmission information
using said long code;
generating a priori code representing a phase of said long code, a
cross-correlation between said a priori code and said long code being
negligibly small;
inserting said a priori code into said spread signal at a predetermined
fixed interval to form frames;
transmitting said frames;
receiving said frames;
generating a replica of said long code;
calculating a correlation between said received signal and said replica of
said long code;
generating a replica of said a priori code;
calculating a correlation between said received signal and said replica of
said a priori code;
estimating received phase of said long code based on said correlation
between said received signal and said replica of said a priori code; and
controlling a generation phase of said replica of said long code based on
an estimated received phase of said long code. |
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Claims |
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Description |
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TECHNICAL FIELD
The present invention relates to an apparatus and method for establishing
acquisition of a spreading code in a CDMA transmission system which
carries out multiple access by using spread spectrum in a mobile
communications system.
BACKGROUND ART
In a direct sequence (DS) CDMA transmission, information data is
transmitted after it undergoes a primary modulation, followed by a
secondary modulation which spreads the primary modulated signal with a
high rate spreading signal. At a receiving side, the wideband received
signal (spread signal) is first converted into the original narrow-band
signal through a process called despreading, and then undergoes a usual
demodulation. The despreading at the receiving side is carried out by
detecting correlation between the received signal and a replica of the
spreading code synchronized in phase with the spreading code in the
received signal. Accordingly, it is necessary for recovering the
information data to synchronize the spreading code replica with the
spreading code in the received signal.
The synchronization roughly falls into two processes: acquisition and
tracking. The acquisition captures the phase difference between the
received spreading code and the spreading code replica within a range
sufficiently narrower than .+-. one chip because the autocorrelation of a
quadrature code used as the spreading code can usually be obtained only
within .+-. one chip. Then, the tracking keeps the phase difference
between the two codes within this range. Since the present invention
relates only to the acquisition, a conventional acquisition method will be
described below.
The acquisition of the spreading code is generally carried out as follows:
First, the correlation between the received signal and the spreading code
replica is taken by multiplying the two codes, and by integrating the
product for one spreading code period. Next, the establishment of the
acquisition is decided by square-law detecting the correlation output
followed by deciding whether or not the detection output exceeds a
threshold. Since the phase of the carrier is not synchronized between the
transmission side and the reception side, it is difficult to know the
phase of the carrier before despreading. Thus, the non coherent method is
used in which the effect of the carrier phase is removed by square-law
detecting the correlated and integrated signal before performing the
threshold decision.
The correlation of the acquisition can usually be detected using a sliding
correlator carrying out time integral, or a matched filter performing
space integral. The matched filter is an FIR filter (transversal filer)
having the tap number corresponding to the processing gain and using the
spreading code as tap coefficients. Since the matched filter achieves the
correlation detection between the spreading code in the received signal
and the spreading code replica at once by using the space integral, the
acquisition is achieved in a fraction of time. However, the matched filter
has taps whose number corresponds to the processing gain. In addition, the
correlation detection must be performed at a rate at least twice the chip
rate because it is necessary for the matched filter to adjust its sampling
timing at a peak position with the band-limited received signal. Thus, the
matched filter requires the tap number twice the processing gain, which
presents a shortage that its circuit scale becomes larger than that of the
sliding correlator.
The present invention relates to the acquisition of a long code with a very
long period as compared with an information symbol, and hence it is
considered very difficult to realize by using the matched filter. Thus,
the acquisition with the sliding correlator will be considered in
connection with the present invention.
The correlation is calculated between the spreading code replica and the
received signal including noise, and the correlation output undergoes an
envelope detection through a square-law detector. The detected output is
integrated for a dwell time .tau. in an integral & dump circuit. The
integrated output undergoes threshold processing. Although the sliding
correlation method is simple in circuit configuration, it has a shortage
that it takes a long time for the acquisition.
In the CDMA applied to a cellular, control information is exchanged between
a base station and a mobile station through a control channel before a
traffic channel is established for transmitting information data
represented by voice. Generally speaking, an increasing period of
spreading code will provide a greater number of the spreading codes,
thereby increasing the number of multiplexing, although a time taken for
the acquisition grows longer because of an increase in phase uncertainty.
Taking account of this characteristic, a method is proposed in which a
short code is used for a control channel and a long code is applied to a
traffic channel by superimposing it over the short code, the short code
having a period equal to one symbol interval of the information data, and
the long code having a much longer period than the information data
symbol. In this method, the acquisition of the control channel is achieve
using the short code, and that of the traffic channel is carried out by
inserting phase information on the starting position of the long code in
the control channel. Thus, the acquisition of the traffic channel is
started from a state in which the chip phase is nearly synchronized
between the base station and the mobile station. As a result, the
acquisition of both channels can be established in a short time.
This method, however, presents a problem in that it employs a short code in
the control channel. Spreading the control channel with the short code
will substantially restrict the number of the control channel because the
number of the quadrature codes is determined by the code length. When
assigning the limited number of the control channels, spreading code
management is required in connection with the control channel. To avoid
such management, an increasing number of control channel is required, in
which case, it is necessary to increase the code length of the control
channels to some extent. Thus, it is also required for the control channel
to use spreading codes with a period longer than one symbol period of the
information data. In this case, speed up of the acquisition becomes
important.
DISCLOSURE OF THE INVENTION
An object of the present invention is to provide an apparatus and method
for establishing acquisition of a spreading code in a CDMA transmission
system which can establish the acquisition in a short time in the direct
sequence CDMA transmission system using a long code.
In a first aspect of the present invention, there is provided a transmitter
in a CDMA transmission system comprising:
long code generating means for generating a long code which is a spreading
code with a period longer than information symbol;
spreading means for generating a wideband spread signal by spreading
transmission information using the long code;
a priori code generating means for generating a priori code representing a
phase of the long code, a cross-correlation between the a priori code and
the long code being negligibly small;
a priori code insertion means for inserting the a priori code into the
spread signal at a predetermined fixed interval to form frames; and
transmission means for transmitting the frames.
Here, the a priori code may comprise smoothly changing autocorrelation
values, and a zero-cross point.
The autocorrelation values of the a priori code may have a triangular
profile.
The a priori code may have a stepwise profile.
The a priori code may have a period shorter than the long code.
The a priori code insertion mean may insert the a priori code into the
spread signal within a predetermined range at a beginning of
communications.
The a priori code generating mean may comprise means for generating a
predetermined code for a convolution, and calculation means for carrying
out the convolution between the code for a convolution and the long code
to output the a priori code, and the a priori code insertion means may
insert an output of the calculation means into the spread signal.
In a second aspect of the present invention, there is provided an
acquisition apparatus in a CDMA system which demodulates desired
information by receiving a received signal including a spread signal and
an a priori code, the spread signal being spread by a long code which is a
spreading code with a period longer than information symbol, the a priori
code having a negligibly small cross-correlation with the long code, and
representing a phase of the long code, the acquisition apparatus
comprising:
first correlation means for generating a replica of the long code, and for
calculating a correlation between the received signal and the replica of
the long code;
second correlation means for generating a replica of the a priori code, and
for calculating a correlation between the received signal and the replica
of the a priori code;
phase estimation means for estimating received phase of the long code based
on an output of the second correlation means; and
means for controlling a generation phase of the replica of the long code
based on an output of the phase estimation means.
Here, the a priori code may be inserted into the spread signal at a fixed
period, wherein
the second correlation means may comprise paired correlators consisting of
a correlator that generates replicas of paired a priori codes consisting
of a replica of a first a priori code and a replica of a second a priori
code with their correlation detection start timings shifted by an amount
of .DELTA., and that calculates correlations between the replica of the
first a priori code and the received signal, and a correlator that
calculates correlations between the replica of the second a priori code
and the received signal; and
the phase estimation means may estimate a received phase of the long code
based on outputs of the paired correlators.
Each of the paired correlators may integrate a product of the received
signal and the replica of the a priori code for one period of the a priori
code; and
the phase estimation means may estimate a received phase of the long code
from two integrated values.
The second correlation means may comprise m pairs of correlators, where m
is an integer greater than one, wherein start timings of correlation
detection and integration of adjacent pairs of the correlators are shifted
by T.sub.A /m in time, where T.sub.A is a length of the a priori code.
The phase estimation means may comprise means for selecting one pair of the
correlators which produces maximum outputs, and may estimate a received
phase of the long code based on the maximum outputs and their detection
timings.
The phase estimation means may obtain a zero-cross point, at which a
correlation value between the received signal and the replica of the a
priori code becomes zero, from the maximum outputs and their detection
timings, and may estimate a received phase of the a priori code from the
zero-cross point, and the control means may control a generation phase of
the replica of the long code in accordance with the estimated received
phase of the a priori code.
The control means may set the generation phase of the replica of the long
code such that the generation phase is centered at the received phase of
the a priori code estimated by the phase estimation means, and may carry
out non-uniform search in which the generation phase of the replica of the
long code is gradually shifted in a wider range when a phase of the
replica of the long code is not synchronized with a phase of the long code
in the received signal.
The a priori code may be inserted solely at an initial portion of the
spreading code.
The a priori code may be inserted solely at an initial portion of the
spreading code of a control channel.
The first correlation means may be a sliding correlator.
The second correlation means may comprise an I-correlator and a
Q-correlator, the I correlator detecting correlation between an
I-component (inphase component) of a carrier of the received signal after
quadrature detection and an I-component of the replica of the a priori
code, the Q-correlator detecting correlation between a Q component
(quadrature component) of the carrier of the received signal after
quadrature detection and a Q-component of the replica of the a priori
code, wherein the phase estimation means may estimate a received phase of
the long code using both the I-component and the Q-component.
The phase estimation means may obtain a complex amplitude of an output of
the I correlator and an output of the Q-correlator, and may estimate the
received phase of the long code using the complex amplitude.
The phase estimation means may compare an output of the I-correlator with
an output of the Q-correlator, and may estimate the received phase of the
long code using the output with a greater absolute value.
The phase estimation means may obtain an estimated received phase of the
long code from an output of the I-correlator, and an estimated received
phase of the long code from an output of the Q-correlator, and may
estimate the received phase of the long code by averaging these two
estimated received phases.
In a third aspect of the present invention, there is provided a receiver in
a CDMA system which demodulates desired information by receiving a
received signal including a spread signal and an a priori code, the spread
signal being spread by a long code which is a spreading code with a period
longer than information symbol, the a priori code having a negligibly
small cross-correlation with the long code, and representing a phase of
the long code, the receiver comprising:
first correlation means for generating a replica of the long code, and for
calculating a correlation between the received signal and the replica of
the long code;
second correlation means for generating a replica of the a priori code, and
for calculating a correlation between the received signal and the replica
of the a priori code;
phase estimation means for estimating received phase of the long code based
on an output of the second correlation means; and
means for controlling a generation phase of the replica of the long code
based on an output of the phase estimation means.
In a fourth aspect of the present invention, there is provided a CDMA
transmission system comprising:
long code generating means for generating a long code which is a spreading
code with a period longer than information symbol;
spreading means for generating a wideband spread signal by spreading
transmission information using the long code;
a priori code generating means for generating a priori code representing a
phase of the long code, a cross-correlation between the a priori code and
the long code being negligibly small;
a priori code insertion means for inserting the a priori code into the
spread signal at a predetermined fixed interval to form frames;
transmission means for transmitting the frames;
receiving means for receiving the frames;
first correlation means for generating a replica of the long code, and for
calculating a correlation between a received signal and the replica of the
long code;
second correlation means for generating a replica of the a priori code, and
for calculating a correlation between the received signal and the replica
of the a priori code;
phase estimation means for estimating received phase of the long code based
on an output of the second correlation means; and
means for controlling a generation phase of the replica of the long code
based on an output of the phase estimation means.
In a fifth aspect of the present invention, there is provided a
transmission method in a CDMA transmission system comprising the steps of:
generating a long code which is a spreading code with a period longer than
information symbol;
generating a wideband spread signal by spreading transmission information
using the long code;
generating a priori code representing a phase of the long code, a
cross-correlation between the a priori code and the long code being
negligibly small;
inserting the a priori code into the spread signal at a predetermined fixed
interval to form frames; and
transmitting the frames.
In a sixth aspect of the present invention, there is provided an
acquisition method of a spreading code in a CDMA system which demodulates
desired information by receiving a received signal including a spread
signal and an a priori code, the spread signal being spread by a long code
which is a spreading code with a period longer than information symbol,
the a priori code having a negligibly small cross-correlation with the
long code, and representing a phase of the long code, the acquisition
method comprising the steps of:
generating a replica of the long code;
calculating a correlation between the received signal and the replica of
the long code;
generating a replica of the a priori code;
calculating a correlation between the received signal and the replica of
the a priori code;
estimating received phase of the long code based on the correlation between
the received signal and the replica of the a priori code; and
controlling a generation phase of the replica of the long code based on an
estimated received phase of the long code.
In a seventh aspect of the present invention, there is a CDMA transmission
method comprising the steps of:
generating a long code which is a spreading code with a period longer than
information symbol;
generating a wideband spread signal by spreading transmission information
using the long code;
generating a priori code representing a phase of the long code, a
cross-correlation between the a priori code and the long code being
negligibly small;
inserting the a priori code into the spread signal at a predetermined fixed
interval to form frames;
transmitting the frames;
receiving the frames;
generating a replica of the long code;
calculating a correlation between the received signal and the replica of
the long code;
generating a replica of the a priori code;
calculating a correlation between the received signal and the replica of
the a priori code;
estimating received phase of the long code based on the correlation between
the received signal and the replica of the a priori code; and
controlling a generation phase of the replica of the long code based on an
estimated received phase of the long code.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram showing a configuration of a transmitter of a
first embodiment of a CDMA system in accordance with the present
invention;
FIG. 2 is a schematic diagram showing a transmission frame consisting of a
spread signal and an a priori code in accordance with the present
invention;
FIGS. 3A and 3B are graphs each illustrating an example of the a priori
code, and its autocorrelation and correlation with a spreading code;
FIGS. 4A and 4B are graphs each illustrating another example of the a
priori code, and its autocorrelation and correlation with a spreading
code;
FIGS. 5A-5C are waveform diagrams illustrating a method for generating an a
priori code by convolution, wherein FIG. 5A illustrates the waveform of a
spreading code, FIG. 5B illustrates the waveform of a convolution code X,
and FIG. 5C illustrates a waveform of the a priori code;
FIG. 6 is a block diagram showing the configuration of a receiver of the
first embodiment of the CDMA system in accordance with the present
invention;
FIG. 7 is a block diagram showing the internal configuration of a
despreader in the receiver shown in FIG. 6;
FIG. 8 is a schematic diagram illustrating timing relationships between
paired correlators and a received signal;
FIG. 9 is a graph illustrating a method for estimating the start timing of
receiving the a priori code on the basis of maximum correlation outputs
R.sub.1 and R.sub.2 of the paired correlators and their detection timings
t.sub.1 and t.sub.2 ;
FIG. 10 is a schematic diagram illustrating the sequences for setting
estimation phases in a non-uniform search;
FIGS. 11A and 11B are flowcharts showing the operation of the first
embodiment;
FIGS. 12A and 12B are graphs illustrating inphase and quadrature components
of a correlation between a received signal and an a priori code; and
FIGS. 13-15 are block diagrams showing configurations of major portions of
receivers in a CDMA system in accordance with the present invention.
BEST MODE FOR CARRYING OUT THE INVENTION
The invention will now be described with reference to the accompanying
drawings.
EMBODIMENT 1
FIG. 1 is a block diagram showing the configuration of a transmitter of a
CDMA transmission system in accordance with the present invention.
The transmitter generates a frame as shown in FIG. 2, and transmits it.
In FIG. 2, a frame 10 is formed by inserting an a priori code 12 at a fixed
interval T.sub.B into long code spread sections 11 spread by a long code.
Here, the a priori code 12 is information used to estimate the received
phase of the long code, and the length of the a priori code is about 10
symbol periods. The length of the long code spread section 11, on the
other hand, is equal to the long code period of about 1,000 symbol
periods. These lengths can be set rather arbitrarily. In the description
below, the length of the a priori code 12 is termed a priori code length
T.sub.A, and the combination of the a priori code 12 and the long code
spread section 11 is called an a priori block, whose period T.sub.B is
referred to as an a priori period.
FIG. 3A illustrates an a priori code 12A with a step profile, and FIG. 3B
illustrates its autocorrelation and cross-correlation with the long code.
As shown in FIG. 3B, the autocorrelation value of the a priori code 12A
has a triangular profile 13, and its cross correlation value with the long
code is suppressed to nearly zero. Therefore, connecting the a priori code
12A and the long code into frames in a fixed phase relationship to be
transmitted, and detecting the autocorrelation value of the a priori code
12A at the receiving side makes it possible to detect the position of the
a priori code 12A.
FIG. 4A illustrates another a priori code 12B, and FIG. 4B illustrates its
autocorrelation and cross-correlation with the long code. As shown in FIG.
4B, the autocorrelation value of the a priori code 12B has a triangular
profile 14 as in FIG. 3B, and its correlation value with the long code is
suppressed nearly to zero.
FIGS. 5A-5C illustrate a method for generating the a priori code 12B. The a
priori code 12B as shown in FIG. 5C can be obtained by taking a
convolution between the code X with a period T.sub.A as shown in FIG. 5B
and a spreading code 15 with a period TA as shown in FIG. 5A. The
convolution is a well-known operation.
Returning to FIG. 1, transmission information fed to an input terminal 21
of the transmitter is supplied to a frame generating/mapping block 22. The
frame generating/mapping block 22 makes frames from the transmission
information, and carries out mapping in accordance with a modulation
method. The inphase component D.sub.I (t) and the quadrature component
D.sub.Q (t) of the transmission information output from the
generating/mapping block 22 are fed to multipliers 23.sub.I and 23.sub.Q
in a code spreader 23.
On the other hand, the inphase component C.sub.I (t) and the quadrature
component C.sub.Q (t) of the long code as a spreading code, are supplied
to the multipliers 23.sub.I and 23.sub.Q from a long code generator 24,
respectively. The multiplier 23.sub.I multiplies the transmission
information D.sub.I (t) by the long code C.sub.I (t), and the multiplier
23.sub.Q multiplies the transmission information D.sub.Q (t) by the long
code C.sub.Q (t). By this, the transmission information is spread by the
long code, and the long code spread section 11 in FIG. 2 is generated. The
generated long code spread section 11 is fed to a signal switch 25.
On the other hand, the inphase component P.sub.I (t) and quadrature
component P.sub.Q (t) of the a priori code generated by an a priori code
generator 26 is directly fed to the signal switch 25 from the a priori
generator 26. The signal switch 25 switches the a priori code 12 and the
long code spread section 11 in accordance with a switching signal from a
switch controller 27 to provide D/A converters 31.sub.I and 31.sub.Q with
the frame 10 of the format as shown in FIG. 2.
The transmission information (spread signal) supplied to the D/A converters
31.sub.I and 31.sub.Q is fed to a quadrature modulator 21 after converted
into analog signals. The quadrature modulator 21 carries out quadrature
modulation of the carrier signal, which is supplied from an oscillator 33,
by the spread signal. The quadrature modulated spread signal is
band-limited by a BPF (Band-Pass Filter) 34, and is fed to a frequency
converter 35. The frequency converter 35 performs frequency conversion of
the quadrature modulated spread signal by a signal from a local oscillator
36, and its output is band-limited by the BPF 37, and is transmitted from
an antenna 38.
FIG. 6 is a block diagram showing the entire configuration of a receiver of
the CDMA transmission system in accordance with the present invention. The
radio wave received by an antenna 51 is band-limited by a BPF 52 to such a
degree that a desired received signal is not distorted. The band-limited
received signal is mixed by a mixer 53 with a loca | | |
