Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02361997 2002-07-24
DS-CDMA
TRANSMISSION METHOD
The present invention relates to a direct secN.ence
code division multiple access (DS-CDMA) transmission
method carrying out multiple access using spread spectrum
in mobile communications, and particularly to a DS-CDMA
transmission method that carries out code multiplexing of
multiple code channels.
Recently, intensive research and development of the
DS-CDMA system have been made as one of the next
generation mobile communication systems. The DS-CDMA
transmission system carries out communications between
multiple users using the same frequency band, and
individual users are identified by a spreading code
properly assigned to each user.
The DS-CDMA system has advantages over the frequency
division multiple access or time division multiple access
in that it can increase the capacity in terms of the
number of simultaneous subscribers within the same
available frequency band, and is suitable for high speed
signal transmission because it transmits information
signals after spreading them into wideband signals.
In the mobile communication environment, straight
paths between a base station and mobile stations are
seldom unobstructed, thereby constituting multipath
propagation. As a result, the received signal undergoes
Rayleigh fading. In Rayleigh fading, the amplitude of a
received signal has Rayleigh distribution, and its phase
has a uniform distribution. It is necessary for a
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receiver to estimate the randomly varying phase of the
received signal to carry out coherent detection which is
more efficient than differential detection. One of the
methods for estimating the received phase is implemented
by inserting pilot symbols of a known pattern into
information symbols at fixed intervals, and by estimating
the received phase of each information symbol on the basis
of the received phases estimated using the pilot symbols.
In this case, the pilot symbols must be inserted at every
time interval during which the phase fluctuation due to
fading is nearly negligible.
There are mainly two methods for implementing high
bit rate signal transmission in the DS-CDMA system: (1) a
method which varies a spreading factor (processing gain)
in accordance with the transmission information rate; and
(2) a code multiplexing method which multiplexes multiple
channels each having a fundamental information rate.
Here, we will consider the second method.
Fig. 16 shows a conventional channel structure when
carrying out the absolute coherent detection which makes
the channel (amplitude and phase) estimation using the
pilot symbols as mentioned above. In this figure, N
denotes the number of code channels (the number of code
multiplexing). Each code channel is spread using a short
code (SC-1, ..., SC-N) with a period equal to that of
information symbol, and is further spread using a
spreading code referred to as a long code (LC-Y) with a
period much longer than that of the common information
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symbol. The short codes serve to identify the individual
code channels, and the long code serves to distinguish a
user from the other simultaneous users in the same cell in
reverse link channels, and from the other simultaneous
users in the other cells in forward link channels. Fig.
17 shows a frame structure of a single code channel
transmission.
The conventional system has the following problems:
(1) The pilot symbols, which are inserted into each
code channel as
shown in Fig. 16, are spread by the same spreading code
assigned to each code channel for spreading data symbols.
This results in some cross-correlation between the
multiplexed code channels, which degrades the accuracy of
the channel estimation by the pilot symbols. In other
words, the conventional DS-CDMA code multiplexing method
has a problem in that the accuracy of the channel
estimation using the pilot symbols degrades owing to the
cross-correlation between other code channels because the
pilot symbols of respective code channels are spread using
different spreading codes. The degradation is remarkable
when the received signal power per path decreases under
the multipath environment.
(2) The pilot symbols, which are inserted at the
same positions in frames in the respective code channels
as shown in Fig. 18A, are used to estimate the received
phases for each code channel so as to obtain the transfer
function of the channel on information data sequence by
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means of interpolation at the insertion intervals of the
pilot symbols as shown in Fig. 18B. This results in a
problem in that the accuracy of the channel estimation
degrades with an increase in the fading fluctuation speed.
In addition, since the transmission power control is also
carried out at the insertion intervals of the pilot
symbols by measuring the received signal power at the
positions of the pilot symbols which are inserted in the
same positions in the frames of respective code channels
as shown in Fig. 19, there is~another problem in that the
accuracy of the transmission power control also degrades
with an increase in the fading fluctuation speed.
It is therefore an object of the present invention to
provide a DS-CDMA transmission method which can improve
the accuracy of the channel estimation without much
increasing the circuit scale of a transmitter and receiver
in the DS-CDMA code multiplexing.
Another object of the present invention is to provide
a DS-CDMA transmission method which can improve the
tracking ability to fading of the channel estimation and
the transmission power control using the pilot symbols
without much increasing the circuit scale of the
transmitter and receiver in the DS-CDMA code multiplexing.
According to a first aspect of the present invention,
a~DS-CDMA transmission method using a code multiplexing
method which transmits a signal by generating a high bit
rate transmission channel by code multiplexing a plurality
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of code channels, the DS-CDMA transmission method
comprises the steps of:
assembling frames for respective code channels by
inserting pilot symbols into information symbols at fixed
intervals, the pilot symbols being used for channel
estimation for coherent detection;
spreading the information symbols in each of the code
channels using a spreading code properly assigned to the
each of the code channels, the spreading code being
selected from a group of orthogonal spreading codes that
are orthogonal to each other and have a period equal to an
information symbol period; and
spreading the pilot symbols in the code channels
using one of the spreading codes being selected from the
group of the orthogonal spreading codes, or any of the
spreading codes other than the spreading codes assigned to
the information symbols in the code channels from the
group of the orthogonal spreading codes.
According to a second aspect of the present
invention, a DS-CDMA transmission method using a code
multiplexing method which transmits a signal by generating
a high bit rate transmission channel by code multiplexing
a plurality of code channels, the DS-CDMA transmission
method comprises the steps of:
assembling frames for one of the code channels by
inserting pilot symbols into information symbols at fixed
intervals, the pilot symbols being used for channel
estimation for coherent detection;
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spreading the information symbols in each of the code
channels using a spreading code properly assigned to
each of the code channels, the spreading code being ',
selected from a group of orthogonal spreading codes that
are orthogonal to each other and have a period equal to an
information symbol period; and
spreading the pilot symbols, which are generated only
in the one of the code channels, using one of the
spreading codes being selected from the group of the
orthogonal spreading codes, or any of the spreading codes
other than the spreading codes assigned to the information
symbols in the code channels from the group of the
orthogonal spreading codes.
According to a third aspect of the present invention,
a DS-CDMA transmission method using a code multiplexing
method which transmits a signal by generating a high bit
rate transmission channel by code multiplexing a plurality
of code channels, the DS-CDMA transmission method
comprises the steps of:
convolutionally encoding transmitted data
collectively;
writing a convolutionally encoded information data
sequence collectively at every N x X2 interval, where N is
a number of code channels;
reading, after having written entire information data
in Z frames in a direction, where Z is a natural number,
the information data in a direction perpendicular to the
direction in which the entire information data in Z frames
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are written at every Y2 interval, where X2 and Y2 are
natural numbers that satisfy the relations N X X2 - Y2.
and N x X2 x Y2 - the total number of information data in
the Z frames; and
carrying out interleaving that distributes the read
information data to N code channels after interleaving.
According to a fourth aspect of the present
invention, a DS-CDMA transmission method using a code
multiplexing method which transmits a signal by generating
a high bit rate transmission channel by code multiplexing
a plurality of code channels, the DS-CDMA transmission
method comprises, when assembling frames for respective
code channels by inserting into information symbols pilot
symbols used for channel estimation for coherent detection
at fixed intervals, the steps of:
dividing N code channels to K blocks each consisting
of H code channels, where N is a number of multiplexed
code channels, K is a number of blocks, H is a number of
code channels in the block, and N = H x K;
. inserting pilot symbols into same positions of frames
in the H code channels in a same block; and
shifting,positions of inserting the pilot symbols
from block to block such that intervals between closest
pilot symbols become uniform with regard to entire pilot
symbols in the K blocks.
Here, the DS-CDMA transmission method may further
comprise, when carrying out the channel estimation for the
coherent detection using the pilot symbols, the steps of:
CA 02361997 2001-11-09
obtaining channel estimates at the positions of the
pilot symbols in the code channels using the pilot symbols
inserted into the information symbols according to a pilot
symbol insertion pattern in the block to which the code
channel belongs;
obtaining channel estimates at the positions of the
pilot symbols in .each of the blocks by averaging the
channel estimates of H code channels in that block; and
obtaining transfer functions of the code channels on
an information data sequence at pilot symbol insertion
intervals of the entire pilot symbols inserted to the
entire code channels, the transfer functions being
obtained by interpolation using, in common to the entire
code channels, the channel estimates at the positions of
the pilot symbols in the blocks.
Here, the DS-CDMA transmission method may further
comprise, when measuring received signal power for
carrying out transmission power control, the steps of:
measuring the received signal power of the code
channels using the pilot symbols inserted into the
information symbols according to a pilot symbol insertion
pattern in the block to which the code channel belongs;
obtaining measured values of the received signal
power at the positions of the pilot symbols in each of the
blocks by averaging the measured received signal power of
the H code channels in that block;. and
carrying out the transmission power control at pilot
symbol insertion intervals of the entire pilot symbols
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distributed to the entire code channels using, in common
to the entire code channels, the measured values of the
received signal power at the positions of the pilot
symbols in the blocks.
The above and other objects, effects, features and
advantages of the present invention will become more
apparent from the following description of the embodiments
thereof taken in conjunction with the accompanying
drawings.
Fig. 1 is a diagram illustrating an example of a
channel structure in the DS-CDMA transmission system in
accordance with the present invention;
Fig. 2 is a diagram illustrating another example of a
channel structure in the DS-CDMA transmission system in
accordance with the present invention;
Fig. 3 is a block diagram showing a configuration of
an embodiment of an error correcting encoder in the DS-
CDMA transmission system in accordance with the present
invention;
Fig. 4 is a block diagram showing another
configuration of an embodiment of an error correcting
encoder in the DS-CDMA transmission system in accordance
with the present invention;
Fig. 5 is a block diagram showing a configuration of
an embodiment of a transmitter in the DS-CDMA transmission
system in accordance with the present invention;
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Fig. 6 is a block diagram showing a configuration of
an embodiment of a receiver in the DS-CDMA transmission
system in accordance with the present invention;
Fig. 7 is a block diagram showing a configuration of
an embodiment of an error correcting decoder in the DS-
CDMA transmission system in accordance with the present
invention;
Fig. 8 is a block diagram showing another
configuration of an embodiment of an error correcting
decoder in the DS-CDMA transmission system in accordance
with the present invention;
Fig. 9A is a diagram illustrating the operation of an
interleaves in the DS-CDMA transmission system in
accordance with the present invention;
Fig. 9B is a diagram illustrating the operation of a
deinterleaver in the DS-CDMA transmission system in
accordance with the present invention;
Fig. 10A is a diagram illustrating the operation of
an interleaves in the DS-CDMA transmission system in
accordance with the present invention;
Fig. lOB is a diagram illustrating the operation of a
deinterleaver in the DS-CDMA transmission system in
accordance with the present invention;
Fig. 11 is a diagram illustrating another example of
a channel structure in the DS-CDMA transmission system in
accordance with the present invention;
Fig. 12 illustrates the relationship between Fig. 12A
and 12B.
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Fig. 12A is a diagram illustrating a pilot symbol
insertion pattern (when a block number K = 2) in the DS-
CDMA transmission system in accordance with the present
invention;
Fig. 12B is a diagram illustrating a channel
estimation method (when a block number K = 2) in the DS-
CDMA transmission system in accordance with the present
invention;
Fig. 13 is a diagram illustrating transmission power
control timings (when a block number K = 2) in the DS-CDMA
transmission system in accordance with the present
invention;
Fig. 14 illustrates the relationship between Fig. 14A
and 14B.
Fig. 14A is a block diagram showing another
configuration of the embodiment of the transmitter in the
DS-CDMA transmission system in accordance with the present
invention;
Fig. 14B is a block diagram showing another
configuration of the embodiment of the transmitter in the
DS-CDMA transmission system in accordance with the present
invention;
Fig. 15 illustrates the relationship between Fig. 15A
and 15B.
Fig. 15A is a block diagram showing another
configuration of the embodiment of the receiver in the DS-
CDMA transmission system in accordance with the present
invention;
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Fig. 15B is a block diagram showing another
configuration of the embodiment of the receiver in the DS-
CDMA transmission system in accordance with the present
invention;
Fig. 16 is a diagram illustrating a conventional code
multiplexing method;
Fig. 17 is a diagram illustrating a frame structure
of a single code channel transmission;
Fig. 18A is a diagram illustrating a conventional
pilot symbol insertion pattern;
Fig. 18B is a diagram illustrating a conventional
channel estimation method;
Fig. 19 is a diagram illustrating conventional
transmission power control timings; and
Fig. 20 is a flowchart showing a transmission and
reception method in accordance with the present invention.
The invention will now be described with reference to
the accompanying drawings.
EMBODIMENT 1
Fig. 1 shows an example of a channel structure of a
DS-CDMA transmission system in accordance with the present
invention. As shown in Fig. 1, each frame of a code
channel with a fundamental transmission rate fb consists
of pilot symbols and information data whose information
rate is expanded by spreading factor (processing gain)
into a wideband signal. The N code multiplexing of such
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fundamental channels enables the information to be
transmitted at the transmission rate of N x fb bps, if all
the channels have the same quality as the fundamental
channel. In this case, if a common spreading code is used
to spread the pilot symbols of the N code channels, the
cross-correlation between the individual code channels can
be eliminated. Since the code channels undergo the same
fading in the multicode multiplex transmission, the same
pilot symbols can be used in common. Fig. 2 illustrates a
channel structure different from that of Fig. 1, in which
only one code channel transmits the pilot symbols.
Fig. 3 is a block diagram showing an error correcting
encoder in the multicode multiplex transmission. Input
information data is coded by an outer code encoder 1 using
an outer code of a concatenated error correcting code,
undergoes interleaving by an interleaver 2, and is
distributed to N code channels through a serial-to-
parallel converter 3. Subsequently, convolutional coding
by a convolutional encoder 4 and interleaving by an
interleaver 5 are carried out successively for each code
channel. Fig. 9A illustrates an interleaving method of
the DS-CDMA transmission system. The entire information
data in Z-frames are written in the X1 data direction for
each code channel, and are read in the Y1 data direction
perpendicular to the write direction. Here, X1 and Y1 are
natural numbers satisfying the relations N x X1 x Y1 - the
total number of information data in the Z frames, and X1 =
Y1.
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Fig. 4 shows a configuration of an error correcting
encoder of the transmitter of the DS-CDMA transmission
system. As in Fig. 3, the input information data is
encoded by an outer code encoder 6 using an outer code of
a concatenated error correcting code, and undergoes
interleaving by an interleaver 7 to be output. The output
data collectively undergo a convolutional coding by a
convolutional encoder 8, and the convolutionally encoded
information sequence is collectively interleaved by an
interleaver 9. Fig. 10A illustrates an interleaving
method of the present DS-CDMA transmission system. The
convolutionally encoded information data sequence is
written at every N x X2 period, and after thus writing the
entire information data in the Z frames, the data are read
at every Y2 information data period in the direction
perpendicular to the write direction. Here, X2 and Y2 are
natural numbers satisfying the relations N x X2 x Y2 - the
total number of the information data in the Z frames, and
N x X2 = Y2 .
After that, the interleaved information data are
distributed into N code channels by a serial-to-parallel
converter 10.
Fig. 5 is a block diagram showing the transmitter of
the DS-CDMA transmission system. Each frame assembler 11
inserts pilot symbols, which are used for channel
estimation for the coherent detection, into the coded
information data of each code channel shown in Figs. 3 and
4-at fixed intervals (the pilot symbols may be inserted
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into only one code channel if it is desired to do so).
Subsequently, the data are modulated by each modulator 12.
The modulated data symbols of each code channel output
from each modulator 12 are spread by spreading modulator 13
using a spreading code (SC-X~LC-Y) for the pilot symbols, and
using spreading codes(SC-P~LC-Y, where P represents 1-N) for the
information symbols of respective code channels. The
spread signals of respective code channels are summed up
by an adder 14 to be transmitted.
Fig. 6 is a block diagram showing a receiver of the
DS-CDMA transmission system. The received spread signal
is input in common to matched filters 15-0, ..., 15-N
corresponding to the spreading codes. The pilot symbols
in the received signal are despread by the matched filter
15-0 using the spreading code (SC-X~LC-Y) as the
spreading code replica. Then, the received phase of the
pilot symbols is estimated by a pilot symbol channel
estimator 16 which averages several pilot symbols using
the output from a frame synchronizer 17. An information
symbol channel estimator 18 estimates the received phase
at each position of the information symbols by
interpolating the estimated information fed from the pilot
symbol channel estimator 16. Since the code channels in
the received signal undergo the same fluctuations due to
fading, the estimated phase fluctuations in the
information symbols can be used in common to the entire
code channels. On the other hand, the information symbols
on individual code channels are despread by the matched
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filters 15-1 - 15-N using different spreading codes (SC-
P~LC-Y, where P denotes 1-N) as spreading code replicas
for respective channels. Using the signal fed from the
information symbol channel estimator 18, each channel
compensator 19 compensates the despread information
symbols on the code channels for the received phase
fluctuations which are estimated using the pilot symbols.
In a multipath configuration, the phase estimator and
compensator (17, 18 and 19), which use pilot channels
corresponding to N code channels as shown in Fig. 6, are
used for each multipath to be combined. The information
symbols from respective paths which have been compensated
for the fading phase fluctuations by the channel
compensator l9 of each channel are RAKE combined by a RAKE
combiner 20 which sums up the multipath components using
estimated received complex envelopes of individual paths
as weights.
The RAKE combined signals are each input to an error
correcting decoder as shown in Figs. 7 and 8.
Fig. 7 shows a configuration of the error correcting
decoder of the DS-CDMA transmission system. The RAKE
combined signals are each deinterleaved by a deinterleaver
21 separately for each code channel. Fig. 9B illustrates
a.deinterleaving method of the DS-CDMA transmission
system, in which write and read are carried out in the
directions opposite to those in the interleaving method as
shown in Fig. 9A. The deinterleaved signals are each
decoded by a Viterbi decoder 22 separately for each
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channel. The decoded data of the respective code channels
undergo parallel-to-serial conversion by a parallel-to-
serial converter 23, followed by the deinterleave by a
deinterleaver 24 and the decoding by an outer code decoder
25, to be output.
Fig. 8 shows another configuration of the error
correcting decoder of the DS-CDMA transmission system.
The RAKE combined signals of the N code channels undergo
parallel-to-serial conversion by a parallel-to-serial
converter 26, and then are collectively deinterleaved by a
deinterleaver 27. Fig. 10B illustrates a deinterleaving
method of the DS-CDMA transmission system, in which write
and read are carried out in the directions opposite to
those in the interleaving method as shown in Fig. 10A.
The deinterleaved signals are collectively decoded by a
Viterbi decoder 28, followed by the deinterleave by a
deinterleaver 29 and decoding by an outer code decoder 30,
to be output.
2 t7 EMBODIMENT 2
Fig. 11 shows another example of a channel structure
of a DS-CDMA transmission system in accordance with the
present invention. As shown in Fig. 11, each frame of a
code channel with a fundamental transmission rate fb
consists of pilot symbols and information data whose
information rate is expanded by the spreading factor
(processing gain), thereby generating a wideband signal.
The N code multiplexing of such fundamental channels
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enables the information to be transmitted at the
transmission rate of N x fb bps, if all the channels have
the same quality as the fundamental channel. Here, the N
code channels are divided into K blocks each consisting of
H code channels, where N is the number of multiplexed code
channels, K is the number of blocks, H is the number of
code channels in each block, and N = H x K. The integer K
may be even or odd. The H code channels in the same block
have the pilot symbols inserted at the same positions in
the frames. With regard to the entire pilot symbols in
the different K blocks of the code channels, the inserted
positions of the pilot symbols in the K blocks are shifted
such that the intervals become uniform between the closest
pilot symbols.
In the present embodiment, the input information data
also undergo the error correcting encoding by the error
correcting encoder as shown in Fig. 3. In addition, the
data are interleaved by the same method as shown in Fig.
9A.
Fig. 14A and Fig. 14B are block diagrams showing the
transmitter of the DS-CDMA transmission system, and Fig.
20 is a flowchart of the transmission and reception (S200-
S250). Each frame assembler 31 inserts pilot symbols,
which are used for channel estimation for the coherent
detection, into the coded information data of each code
channel fed from the circuit as shown in Fig. 3, at fixed
intervals in accordance with a pilot symbol insertion
pattern of the block to which the code channel belongs
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(S200-S210). The modulated data symbols of the code
channels output from respective modulators 32 are
separately spread by spreading modulators 33 using
spreading codes (SC-P~LC-Y, where P represents 1-N)
assigned to respective code channels (S215). The spread
signals of the respective code channels are summed up by
an adder 34 to be transmitted (S220).
Fig. 15A and Fig. 15B are block diagrams showing a
receiver of the DS-CDMA transmission system. The received
spread signal is input in common to matched filters 35
corresponding to the respective spreading codes. The
pilot symbols and information symbols in the code channels
are despread separately for respective channels by the
matched filters 35 using the spreading codes (SC-P~LC-Y,
where P represents 1-N) as the spreading code replicas
(S225). A demultiplexer (DEMUR) 36 corresponding to each
code channel extracts from the information symbols the
pilot symbols inserted into different positions in the
respective blocks (S230). Then, the received phase of the
pilot symbols is estimated by a pilot symbol channel
estimator 37 which averages separately for each code
channel between several pilot symbols using the output
from a frame synchronizer 38 which carries out coherent
detection of the pilot symbols in response to the output
of the matched filter 35 (S235). The estimates of the
received phase at the pilot symbol positions in each block
are obtained by averaging the estimates of the received
phase of the code channels in that block (S240). Fig. 12A
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shows a pilot symbol insertion pattern in each block. An
information symbol channel estimator 39 can obtain
transfer functions of the channels on the information data
sequence by making interpolation at every insertion
interval of the entire pilot symbols throughout the entire
code channels by using, in common to all the code
channels, the estimates of the received phase at the pilot
symbol positions of the respective blocks as shown in Fig.
12B (S245). This can improve the tracking ability to
fading in the channel estimation because of the reduced
interval of the interpolation in the channel estimation.
Using the signal fed from the information symbol channel
estimator 39, each channel compensator 40 compensates the
information symbols on the code channels fed from the
demultiplexers 36 for the received phase fluctuations
which are estimated using the pilot symbols (5250).
In terms of transmission power control, a received
signal power measurer 41 measures the received signal
power at the pilot symbol positions of each code channel
on the basis of the signal fed from the pilot symbol
channel estimator 37. Then, it averages in each block the
measured values of the received signal power of the code
channels belonging to that block, thereby obtaining the
measured values of the received signal power (SIR) at the
pilot symbol positions of the block. A transmission power
control signal generator 42 generates a transmission power
control (TPC) signal based on the measured values. The
measured values of the received signal power at the pilot
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symbol positions in respective blocks are used in common
to the entire code channels as shown in Fig. 13. This
makes it possible to achieve the transmission power
control at the insertion intervals of the entire pilot
symbols inserted into all the code channels, and hence to
improve the fading tracking ability in the transmission
power control because of the virtually reduced
transmission power control period.
In a multipath configuration, the phase estimator and
compensator (37, 38, 39 and 40), which use pilot channels
corresponding to N code channels as shown in Fig. 15, are
used for each multipath to be combined. The information
symbols from respective paths which have been compensated
for the fading phase fluctuations by each channel
compensator 40 are RAKE combined by each RAKE combiner 43
which sums up the multipath components using estimated
received complex envelopes of individual paths as weights.
The RAKE combined signals are each input to an error
correcting decoder as shown in Fig. 7. Its operation and
the deinterleave method (Fig. 9B) are the same as those of
the embodiment 1.
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