U.S. patent number RE40,997 [Application Number 10/717,424] was granted by the patent office on 2009-11-24 for spread spectrum communication transmitter and receiver, and cdma mobile communication system and method.
This patent grant is currently assigned to Mitsubishi Denki Kabushiki Kaisha. Invention is credited to Terumi Sunaga.
United States Patent |
RE40,997 |
Sunaga |
November 24, 2009 |
Spread spectrum communication transmitter and receiver, and CDMA
mobile communication system and method
Abstract
A transmitter used in a CDMA mobile communication system
includes a pilot channel transmit unit which intermittently
transmits a pilot signal in a spread spectrum formation, and
traffic channel transmit units which respectively transmit data
signals in respective traffic channels.
Inventors: |
Sunaga; Terumi (Yokohama,
JP) |
Assignee: |
Mitsubishi Denki Kabushiki
Kaisha (Tokyo, JP)
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Family
ID: |
14068859 |
Appl.
No.: |
10/717,424 |
Filed: |
November 19, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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Reissue of: |
08820608 |
Mar 19, 1997 |
06381233 |
Apr 30, 2002 |
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Foreign Application Priority Data
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Mar 25, 1996 [JP] |
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8-092954 |
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Current U.S.
Class: |
370/335; 375/141;
370/342 |
Current CPC
Class: |
H04B
7/2637 (20130101); H04B 1/707 (20130101); H04B
2201/70701 (20130101) |
Current International
Class: |
H04J
13/00 (20060101); H04B 7/216 (20060101) |
Field of
Search: |
;370/335,342,320,441,491,500 ;375/141,200,205,267 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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8 186558 |
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Jul 1996 |
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JP |
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08335899 |
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Dec 1996 |
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JP |
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Primary Examiner: Hsu; Alpus H.
Attorney, Agent or Firm: Ladas & Parry LLP
Claims
What is claimed is:
1. A transmitter used in a CDMA mobile communication system
comprising: a pilot transmit unit further comprising: a pilot data
generator which generates pilot data; a first modulator which
modulates the pilot data; a second modulator which spreads a
spectrum of modulated pilot data from the first modulator to
thereby generate a pilot signal; and a timing generator which
generates a timing signal applied to at least one of the pilot data
generators and the first and second modulators so that the pilot
signal is intermittently transmitted; and traffic channel transmit
units which respectively transmit data signals in respect of
traffic channels.
2. A transmitter used in a CDMA mobile communication system as
claimed in claim 1, wherein said pilot signal has a period shorter
than an interval in which the pilot signal is intermittently
transmitted.
3. A receiver used in a CDMA mobile communication system
comprising: a pilot channel receive unit which demodulates pilot
signals respectively transmitted intermittently in a spread
spectrum formation by transmitters, and detects from the pilot
signals, a timing for a traffic channel demodulation; and a traffic
channel receive unit which demodulates data at the timing detected
by said pilot channel receive unit; and the timing detected by
comparing peaks of the pilot signals intermittently transmitted,
the timing for the traffic channel demodulation corresponding to a
greatest one of the peaks.
.Iadd.4. A transmitter used in a CDMA mobile communication system,
comprising: a pilot channel transmit unit which is configured to
transmit a pilot signal in a spread spectrum formation; and traffic
channel transmit units which are configured respectively to
transmit data signals in respective traffic channels, wherein the
pilot channel transmit unit is configured to transmit the pilot
signal intermittently; a start timing of the pilot signal is offset
from a start timing of a pilot signal transmitted by another
transmitter in said CDMA mobile communication system; and said
pilot signal whose start timing is offset has a period shorter than
an interval at which said pilot signal whose start timing is offset
is transmitted, and wherein the data signals are transmitted
continuously even when the pilot signal is transmitted
intermittently..Iaddend.
.Iadd.5. A transmitter used in a CDMA mobile communication system,
comprising: a pilot channel transmit unit which is configured to
transmit a first pilot signal intermittently in a spread spectrum
formation; and traffic channel transmit units that are configured
respectively to transmit data signals in respective traffic
channels, wherein the pilot channel transmit unit is configured to
start to transmit the first pilot signal at a different timing from
a timing at which another pilot channel transmitter in said CDMA
mobile communication system starts to transmit a second pilot
signal, and said first pilot signal has a period shorter than an
interval at which said first pilot signal is transmitted, and
wherein the data signals are transmitted continuously even when the
pilot signal is transmitted intermittently..Iaddend.
.Iadd.6. A receiver for use in a CDMA mobile communication system,
comprising: a pilot channel receive unit that receives pilot
signals transmitted by transmitters in said CDMA mobile
communication system, wherein a start timing of the pilot signals
transmitted by different transmitters are offset from each other,
and each pilot signal has a period shorter than an interval at
which each pilot signal is transmitted, wherein data signals are
transmitted continuously even when the pilot signal is transmitted
intermittently..Iaddend.
.Iadd.7. A receiver for use in a CDMA mobile communication system,
comprising: a pilot channel receive unit which receives pilot
signals transmitted by transmitters in said CDMA mobile
communication system, wherein the pilot signals start to be
transmitted at different timing from each other, and each pilot
signal has a period shorter than an interval at which each pilot
signal is transmitted, wherein data signals are transmitted
continuously even when the pilot signal is transmitted
intermittently..Iaddend.
.Iadd.8. A CDMA mobile communication method in a CDMA mobile
communication system, comprising: a transmitter, comprising: a
pilot channel transmit unit which is configured to transmit a pilot
signal in a spread spectrum formation; and traffic channel transmit
units which are configured respectively to transmit data signals in
respective traffic channels, wherein the pilot channel transmit
unit is configured to transmit the pilot signal intermittently; a
start timing of the pilot signal is offset from a start timing of a
pilot signal transmitted by another transmitter in said CDMA mobile
communication system; and said pilot signal whose start timing is
offset has a period shorter than an interval at which said pilot
signal whose start timing is offset is transmitted, wherein the
data signals are transmitted continuously even when the pilot
signal is transmitted intermittently; and a receiver, comprising: a
pilot channel receive unit that receives pilot signals transmitted
by transmitters in said CDMA mobile communication system, wherein a
start timing of the pilot signals transmitted by different
transmitters are offset from each other, and each pilot signal has
a period shorter than an interval at which each pilot signal is
transmitted..Iaddend.
.Iadd.9. A CDMA mobile communication method in a CDMA mobile
communication system, comprising the steps of: a) transmitting, on
transmit sides, pilot signals in a spread spectrum formation
intermittently; and b) receiving, on a receive side, pilot signals
transmitted by transmitters in said CDMA mobile communication
system, wherein a start timing of a pilot signal is offset from a
start timing of another pilot signal transmitted by another
transmitter in said CDMA mobile communication system, and said
pilot signal whose start timing is offset has a period shorter than
an interval at which said pilot signal whose start timing is offset
is transmitted, and wherein data signals are transmitted
continuously even when the pilot signal is transmitted
intermittently..Iaddend.
.Iadd.10. A CDMA mobile communication method in a CDMA mobile
communication system, comprising the steps of: a) transmitting, on
transmit sides, pilot signals in a spread spectrum formation
intermittently; and b) receiving, on a receive side, pilot signals
transmitted by transmitters in said CDMA mobile communication
system, wherein the pilot signals start to be transmitted at
different timing from each other, and each pilot signal has a
period shorter than an interval at which each pilot signal is
transmitted, and wherein data signals are transmitted continuously
even when the pilot signal is transmitted
intermittently..Iaddend.
.Iadd.11. A CDMA mobile communication method in a CDMA mobile
communication system, comprising: a transmitter, comprising: a
pilot channel transmit unit which is configured to transmit a pilot
signal in a spread spectrum formation; and traffic channel transmit
units which are configured respectively to transmit data signals in
respective traffic channels, wherein the pilot channel transmit
unit is configured to transmit the pilot signal intermittently; a
start timing of the pilot signal is offset from a start timing of a
pilot signal transmitted by another transmitter in said CDMA mobile
communication system; and said pilot signal whose start timing is
offset has a period shorter than an interval at which said pilot
signal whose start timing is offset is transmitted, wherein the
data signals are transmitted continuously even when the pilot
signal is transmitted intermittently; and a receiver, comprising: a
pilot channel receive unit which receives pilot signals transmitted
by transmitters in said CDMA mobile communication system, wherein
the pilot signals start to be transmitted at different timing from
each other, and each pilot signal has a period shorter than an
interval at which each pilot signal is transmitted..Iaddend.
.Iadd.12. A CDMA mobile communication method in a CDMA mobile
communication system, comprising: a transmitter, comprising: a
pilot channel transmit unit which is configured to transmit a first
pilot signal intermittently in a spread spectrum formation; and
traffic channel transmit units that are configured respectively to
transmit data signals in respective traffic channels, wherein the
pilot channel transmit unit is configured to start to transmit the
first pilot signal at a different timing from a timing at which
another pilot channel transmitter in said CDMA mobile communication
system starts to transmit a second pilot signal, and said first
pilot signal has a period shorter than an interval at which said
first pilot signal is transmitted, wherein the data signals are
transmitted continuously even when the pilot signal is transmitted
intermittently; and a receiver, comprising: a pilot channel receive
unit that receives pilot signals transmitted by transmitters in
said CDMA mobile communication system, wherein a start timing of
the pilot signals transmitted by different transmitters are offset
from each other, and each pilot signal has a period shorter than an
interval at which each pilot signal is transmitted..Iaddend.
.Iadd.13. A CDMA mobile communication method in a CDMA mobile
communication system, comprising: a transmitter, comprising: a
pilot channel transmit unit which is configured to transmit a first
pilot signal intermittently in a spread spectrum formation; and
traffic channel transmit units that are configured respectively to
transmit data signals in respective traffic channels, wherein the
pilot channel transmit unit is configured to start to transmit the
first pilot signal at a different timing from a timing at which
another pilot channel transmitter in said CDMA mobile communication
system starts to transmit a second pilot signal, and said first
pilot signal has a period shorter than an interval at which said
first pilot signal is transmitted, wherein the data signals are
transmitted continuously even when the pilot signal is transmitted
intermittently; a receiver, comprising: a pilot channel receive
unit which receives pilot signals transmitted by transmitters in
said CDMA mobile communication system, wherein the pilot signals
start to be transmitted at different timing from each other, and
each pilot signal has a period shorter than an interval at which
each pilot signal is transmitted..Iaddend.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a CDMA (Code Division Multiple
Access) mobile communication system and method using a spread
spectrum communication system. Further, the present invention is
concerned with a spread spectrum communication transmitter and
receiver used for such a CDMA mobile communication system.
2. Description of the Related Art
FIG. 1 is a block diagram of a base station transmitter used in a
CDMA mobile communication system using a conventional spread
spectrum communication system, which is typically described in the
IS/95 that is a standard system in the U.S. Telecommunications
Industry Association/Electronic Industries Association (TIA/EIA).
FIG. 2 is a block diagram of a mobile station receiver in the CDMA
mobile communication system.
The transmitter shown in FIG. 1 can simultaneously communicate with
n mobile stations where n is an integer. More particularly, the
transmitter includes traffic channel transmit units 31.sub.1,
31.sub.2, . . . , and 31.sub.n, which respectively communicate with
the first, second, . . . , and nth mobile stations. Each of the
traffic channel transmit units 31.sub.1 through 31.sub.n includes
an information modulator 2 and a spread spectrum modulator 5. The
information modulator 2 of each traffic channel modulates transmit
data (information) 4 by a BPSK, QPSK or another modulation method.
The modulated transmit data is applied to the spread spectrum
modulator 5. The spread spectrum modulators 5 of the traffic
channel transmit units 31.sub.1 through 31.sub.n generate
respective spreading codes (PN codes). The spread spectrum
modulator 5 of each traffic channel spread the spectrum of the
modulated transmit data from the information modulator 2.
The transmitter shown in FIG. 1 has a pilot channel transmit unit
30. The mobile receivers discriminate the base stations from each
other by referring to the pilot channel. The pilot channel transmit
unit 30 includes a pilot data generator 1, an information modulator
2 and a spread spectrum modulator 3. The information modulator 2
modulates pilot data generated by the pilot data generator 1 by the
BPSK, QPSK or another modulation method. The spread spectrum
modulator 3 spreads the spectrum of the modulated pilot data by
using a spreading code specifically used for the pilot channel and
different from the spreading codes used for the traffic channels.
The pilot signal thus generated can be arbitrary data which can be
known in the base stations and the mobile receivers. For example,
data consisting of only binary ones or binary zeros can be used as
the pilot data.
The output signals of the traffic channel transmit units 31.sub.1
through 31.sub.n and the pilot channel transmit unit 30 are
combined so that the pilot channel and the traffic channels are
simultaneously transmitted in a given frequency band. Then, the
combined radio signal is transmitted via an antenna.
FIG. 3 shows a relation between the pilot and traffic channels with
respect to time. As shown in FIG. 3, the pilot signal is always
transmitted without any interval. In this regard, the pilot signal
is a continuous signal.
Referring to FIG. 2, the mobile receiver used in the conventional
CDMA mobile communication system includes a pilot channel receive
unit 34, and a traffic channel receive unit 35. The pilot channel
receive unit 34 includes a despreader 8, a path detector 11 and a
hand-over controller 19. The traffic channel receive unit 35
includes despreaders 9 and 10, a RAKE combiner 12, an information
demodulator 13, and a level measuring unit 14 for controlling a
transmit power.
The despreader 8 performs a despreading process on the received
signal by using the spreading code for the pilot channel. The
despreaders 9 and 10 perform a despreading process on the received
signal by using the spreading code allocated to the receiver shown
in FIG. 2 at the transmitter. The path detector 11 detects multiple
paths from the pilot signal. The hand-over controller 19 performs a
hand-over control by using the results of the multipath detection
obtained by the path detector 11. The output signal of the path
detector 11 is also used as a timing signal used for the
despreading process carried out by the despreaders 9 and 10. The
RAKE combiner 12 performs a RAKE process on the despread signals
from the despreaders 9 and 10. The information demodulator 13
demodulates the output signal of the RAKE combiner 12 to thereby
generate the original information. The level measuring unit 14
performs a level measuring operation for controlling the transmit
power.
FIG. 4 shows a cell structure of the CDMA mobile communication
system having the above transmitter and receiver. There are
illustrated first, second, third and fourth base stations 21, 22,
23 and 24, which cover service areas (cells) 26, 27, 28 and 29,
respectively. All the base stations 21 through 24 have transmitters
as shown in FIG. 1. A reference number 25 indicates a mobile
receiver (station) having the structure shown in FIG. 2. The mobile
station 25 is located within the cell 26 covered by the base
station 21, and can communicate with the base station 21.
FIG. 5 is a timing chart of timings at which the base stations 21
through 24 respectively transmit the pilot signal. In the
conventional CDMA mobile communication system, all the base
stations 21 through 24 employ the same spreading code for spreading
the pilot data. The period of the spreading code used to spread the
pilot data is sufficiently longer than one symbol time of
information (data). As shown in FIG. 5, the base stations 21
through 24 transmit the same spreading code for the pilot channel
with respective inherent offset times equal to a time t'. That is,
the starting points of the spreading codes used in the base
stations 21 through 25 are offset by the time t'.
The mobile station 25 shown in FIG. 4 receives the pilot signals
from the base stations 21, 22, 23 and 24. Usually, the pilot signal
from the base stations 21 closet to the mobile station 25 has the
strongest level. The despreader 8 of the pilot channel receive unit
34 shown in FIG. 2 performs the despreading process on the received
signal by using the same spreading code as used in the
transmitter.
FIG. 6A shows a correlation between the spreading code for the
pilot channel and the pilot signal transmitted by the base station
21 and received by the mobile station 25. Similarly, FIGS. 6B, 6C
and 6D show correlations with the pilot signals transmitted by the
base stations 22, 23 and 24 and received by the mobile station 25.
Peaks 201 through 204 respectively shown in FIGS. 6A through 6D
indicate timing synchronization points in the pilot channels of the
base stations 21 through 24. Variations in the waveforms other than
the peaks 201 through 204 shown in FIGS. 6A through 6D result from
a self-correlation of the spreading code for the pilot channel.
These variations in the waveforms are noise components for the
mobile station 25 (receiver).
The mobile station 25 shown in FIG. 4 receives the signals of the
pilot channels transmitted by the base stations 21 through 24 in
such a state that the signals are superimposed. Hence, the output
signal of the despreader 8 of the pilot channel receive unit 34 has
a formation in which the four waveforms shown in FIGS. 6A through
6D are superimposed. It should be noted that the correlations shown
in FIGS. 6A through 6D are not affected by multipath fading or
Rayleigh fading.
The path detector 11 shown in FIG. 2 detects the greatest peak in
the output signal of the despreader 8 (the greatest peak in the
superimposed correlation waveform). In the case of FIG. 4, the
mobile station 25 is located within the cell 26 of the base station
21. Hence, the propagation distance between the base station 21 and
the mobile station 25 is shorter than the propagation distances
from the base stations 22, 23 and 24. Hence, the path between the
base station 21 and the mobile station 25 has the smallest
propagation loss. Hence, the greatest peak in the despread received
signal output by the despreader 8 corresponds to the correlation
peak 201 of the pilot channel of the base station 21 having the
cell 26 in which the mobile station 25 is located.
Since the pilot signals transmitted by the base stations 21 through
24 have respective inherent time offsets. Hence, by detecting the
greatest peak of the superimposed correlation waveform, it is
possible for the mobile station 25 to discriminate the base station
21 from the other base stations 22 through 24 and detect the timing
of spectrum-spreading. The path detector 11 informs the despreaders
9 and 10 of the traffic channel receive unit 35 of the timing of
the greatest peak 201.
The despreaders 9 and 10 perform the despreading processes on the
received signal of the allocated traffic channel at the timing
informed by the path detector 11. The RAKE combiner 12 performs a
RAKE combination process (path diversity combination) on the output
signals of the despreaders 9 and 10 by using information concerning
the pilot channel supplied from the path detector 11. The above
information includes information concerning the timing, amplitude
(receive power level) and phase of the pilot signal. The
information demodulator 13 demodulates the output signal of the
RAKE combiner to thereby produce the original information
(data).
The level measuring unit 14 measures the received signal of the
traffic channel by using the output signal of the RAKE combiner 12,
and controls the transmission power of the mobile station 25. It
will be noted that a transmit part of the mobile station shown in
FIG. 2 is omitted for sake of convenience.
The hand-over controller 19 performs a control by using the output
signal of the path detector 11 so that the mobile station 25 is
handed over to the area of the base station which transmits the
pilot signal received as the greatest peak at the mobile station
25.
However, the conventional CDMA mobile communication system thus
configured has a disadvantage in that a good S/N ratio cannot be
obtained at the time of receiving the pilot signals from the base
stations due to the fact that all the base stations continue to
transmit the pilot signals. The mobile station 25 shown receives
the pilot signal from the base station 21 to which the mobile
station 25 belongs so that the signals of the pilot channels
transmitted by the other base stations 22, 23 and 24 are
superimposed, as noise components, on the pilot channel data signal
from the base station 21. Hence, the pilot channel receive unit 34
does not have a goon S/N ratio.
The signals of the pilot channels transmitted by the base stations
22 through 24 serve as interference signals with respect to the
signal of the traffic channel processed by the traffic channel
receive unit 35 of the mobile station 25. That is, the mobile
station 25 always receives the signals of the pilot channels
transmitted by the base stations 22 through 24 to which the mobile
station 25 does not belong, and thus always receives interference
by the base stations 22 through 24. Hence, the given frequency
range can accommodate only a reduced number of stations
(corresponding to a channel capacitance).
SUMMARY OF THE INVENTION
It is a general object of the present invention to eliminate the
above disadvantages.
A specific object of the present invention is to provide a CDMA
transmitter and a CDMA receiver which can realize a CDMA mobile
communication system in which an interference by signals
transmitted via pilot channels by base stations is eliminated and
an increased channel capacity and an improved SIN ratio can be
obtained.
Another object of the present invention is to provide such a CDMA
mobile communication system and a CDMA mobile communication method
employed in the system.
The above objects of the present invention are achieved by a
transmitter used in a CDMA mobile communication system comprising:
a pilot channel transmit unit which intermittently transmits a
pilot signal in a spread spectrum formation; and traffic channel
transmit units which respectively transmit data signals in
respective traffic channels.
The transmitter may be configured so that the pilot channel
transmit unit comprises: a pilot data generator which generates
pilot data; a first modulator which modulates the pilot data; a
second modulator which spreads a spectrum of modulated pilot data
from the first modulator to thereby generate the pilot signal; and
a timing generator which generates a timing signal applied to at
least one of the pilot data generator and the first and second
modulators so that the pilot signal can be intermittently
transmitted.
The transmitter may be configured so that the pilot signal has a
period shorter than an interval at which the pilot signal is
intermittently transmitted.
The above objects of the present invention are also achieved by a
receiver used in a CDMA mobile communication system comprising: a
pilot channel receive unit which demodulates pilot signals
respectively transmitted intermittently in a spread spectrum
formation by transmitters and detects, from the pilot signals, a
timing for a traffic channel demodulation; and a traffic channel
receive unit which demodulates data at the timing detected by the
pilot channel receive unit.
The receiver may be configured so that the pilot channel receive
unit detects the timing for the traffic channel demodulation by
comparing peaks of the pilot signals intermittently transmitted,
the timing for the traffic channel demodulation corresponding to a
greatest one of the peaks.
The receiver may be configured so that it further comprises an
estimating unit which estimates states of paths from the pilot
signals intermittently transmitted.
The receiver may be configured so that the estimating unit supplies
the traffic channel receive unit with information necessary to the
traffic channel demodulation and based on an estimated state of the
path to be demodulated.
The above objects of the present invention are also achieved by a
CDMA mobile communication system comprising transmitters and
receivers; each of the transmitters comprising: a pilot channel
transmit unit which intermittently transmits a pilot signal in a
spread spectrum formation; and traffic channel transmit units which
respectively transmit data signals in respective traffic channels.
Each the receivers comprises: a pilot channel receive unit which
demodulates pilot signals respectively transmitted intermittently
in the spread spectrum formation by the transmitters and detects,
from the pilot signals, a timing for a traffic channel
demodulation; and a traffic channel receive unit which demodulates
data at the timing detected by the pilot channel receive unit.
The CDMA mobile communication system may be configured so that the
transmitters intermittently transmit the pilot signals with time
offsets.
The CDMA mobile communication system may be configured so that the
transmitters intermittently transmit the pilot signals with the
time offsets so that the pilot signals are serially transmitted one
by one.
The CDMA mobile communication system may be configured so that the
transmitters intermittently transmit the pilot signals with the
time offsets so that only one of the pilot signals is transmitted
at any time.
The CDMA mobile communication system may be configured so that the
transmitters intermittently transmit the pilot signals with the
time offsets so that a time period is provided during which none of
the pilot signals are transmitted.
The above objects of the present invention are also achieved by a
CDMA mobile communication method comprising the steps of: a)
transmitting, on transmit sides, pilot signals in a spread spectrum
formation; b) demodulating, on a receive side, the pilot signals
respectively transmitted intermittently; and c) detecting, on the
receive side, from the pilot signals, a timing for a traffic
channel demodulation.
The CDMA mobile communication method may be configured so that the
step a) comprises the step of intermittently transmitting the pilot
signals with time offsets.
The CDMA mobile communication method may be configured so that the
step a) intermittently transmits the pilot signals with the time
offsets so that the pilot signals are serially transmitted one by
one.
The CDMA mobile communication method may be configured so that the
step a) intermittently transmit the pilot signals with the time
offsets so that only one of the pilot signals is transmitted at any
time.
The CDMA mobile communication method may be configured so that the
step a) intermittently transmits the pilot signals with the time
offsets so that a time period is provided during which none of the
pilot signals are transmitted.
BRIEF DESCRIPTION OF THE DRAWINGS
Other objects, features and advantages of the present invention
will become apparent from the following detailed description when
read in conjunction with the accompanying drawings, in which:
FIG. 1 is a block diagram of a spread spectrum communication
transmitter used in a conventional CDMA mobile communication
system;
FIG. 2 is a block diagram of a spread spectrum communication
receiver used in the conventional CDMA mobile communication
system;
FIG. 3 is a diagram showing transmissions in a pilot channel and
traffic channels in the conventional system;
FIG. 4 is a diagram of a cell arrangement;
FIG. 5 is a diagram showing transmissions of pilot signals in the
cells in the conventional system;
FIGS. 6A, 6B, 6C and 6D are waveform diagrams showing correlations
obtained after a despreading process in the conventional
system;
FIG. 7 is a block diagram of a spread spectrum communication
transmitter used in a CDMA mobile communication system according to
a first embodiment of the present invention;
FIG. 8 is a diagram showing transmissions in a pilot channel and
traffic channels in the system according to the first embodiment of
the present invention;
FIG. 9 is a diagram showing transmissions of pilot signals in cells
in the system according to the first embodiment of the present
invention;
FIG. 10 is a block diagram of a spread spectrum communication
receiver used in the system according to the first embodiment of
the present invention;
FIGS. 11A, 11B, 11C, 11D and 11E are waveform diagrams showing
correlations obtained after a despreading process in the system
according to the first embodiment of the present invention;
FIG. 12 is a flowchart of an operation of the spread spectrum
communication receiver shown in FIG. 10;
FIG. 13 is a block diagram of a spread spectrum communication
transmitter according to a second embodiment of the present
invention;
FIG. 14 is a block diagram of a propagation path state estimating
unit shown in FIG. 13;
FIGS. 15A, 15B and 15C are diagrams of despread output signals;
and
FIG. 16 is a diagram showing an operation of the propagation path
state estimating unit shown in FIG. 14.
DETAILED DESCRIPTION
FIG. 7 is a block diagram of a spread spectrum communication
transmitter used in a CDMA mobile communication system according to
a first embodiment of the present invention. In FIG. 7, parts that
are the same as those shown in the previously described figures are
given the same reference numbers.
The transmitter shown in FIG. 7 includes a pilot channel transmit
unit 40, and n traffic channel transmit units 41.sub.1, 41.sub.2, .
. . , and 41.sub.n, which communicate with the first, second, . . .
, and nth mobile stations. The pilot channel transmit unit 40
includes a pilot transmission timing generator 50 in addition to
the aforementioned pilot data generator 1, the information
modulator 2 and the spread spectrum modulator 3. The pilot
transmission timing generator 50 generates a pilot transmission
timing signal, which is applied to the pilot data generator 1, the
information modulator 2 and the spread spectrum modulator 3. In
this regard, the pilot channel transmit unit 40 shown in FIG. 7
differs from that shown in FIG. 1.
The pilot transmission timing signal controls the pilot data
generator 1, the information modulator 2 and the spread spectrum
modulator 3 so that the pilot signal is intermittently transmitted.
This will be described later with reference to FIG. 9.
The traffic channel transmit units 41.sub.1 through 41.sub.n have
an identical structure. Each of the traffic channel transmit units
41.sub.1 through 41.sub.n has an error correction encoder 51 and an
interleave unit 52 in addition to the information modulator 2 and
the spread spectrum modulator 5. The transmit data 4 is subjected
to an error correction encoding process by the error correction
encoder 51, and is then subjected to an interleaving process by the
interleave unit 52. The output signal of the interleave signal is
modulated by the information modulator 2. The output signal of the
information modulator 2 is subjected to the spectrum spreading
process by the spread spectrum modulator 5. The modulated signals
generated by the pilot channel transmit unit 40 and the traffic
channel transmit units 41.sub.1 through 41.sub.n are combined by a
combiner 53. The error correction encoder 51 and the interleave
unit 52 are also employed in FIG. 1, but are not shown for the sake
of simplicity.
A combined signal thus produced passes through a band limiter 54, a
frequency converter 55, and a power amplifier 56, and is
transmitted via an antenna 57.
FIG. 8 is a timing chart of an operation of the transmitter shown
in FIG. 7. The pilot channel transmit unit 46 intermittently
transmits the pilot signal at an interval .tau.. One cycle of the
spreading code for the pilot channel is completed in the period
during which the pilot signal is transmitted. The cycle of the
spreading code for the pilot channel is shorter than the
transmission interval .tau. of the pilot signal. The above
intermittent transmission of the pilot signal is controlled by the
pilot transmission timing signal generated by the generator 50.
During the interval between two consecutive pilot signals, only the
traffic channel signals are transmitted.
When the base stations 21 through 24 shown in FIG. 4 have
transmitters configured as shown in FIG. 7, the transmitters of the
base stations 21 through 24 transmit respective pilot signals, as
shown in FIG. 9. The base stations 21 through 24 intermittently
transmit the pilot signals at the intervals .tau., and start to
transmit them at different timings corresponding to respective
inherent time offsets so that a plurality of base stations .Iadd.do
not .Iaddend.simultaneously transmit the respective pilot
signals.
In the case shown in FIG. 9, sections TS are provided in which none
of the base stations transmit the respective pilot signals. When
the sections TS are set longer than the delay time of the
multipath, it is possible to prevent a delay wave of the pilot
signal transmitted by a base station and propagated through the
multipath from overlapping with the pilot signal next transmitted
by another base station. If the distances between the base stations
are short and there are short delay times, as in the case of a
radio LAN system, it will be not necessary to provide the time
sections TS.
FIG. 10 is a block diagram of a spread spectrum communication
receiver used in the CDMA mobile communication system according the
first embodiment of the present invention. In FIG. 10, parts that
are the same as those shown in the previously described figures are
given the same reference numbers. The receiver shown in FIG. 10
includes an antenna 61, an amplifier 62, a frequency converter 63,
a band limiter 64, a pilot channel receive unit 44 and a traffic
channel receive unit 45.
The pilot channel receive unit 44 includes a receive level
measuring unit 18, a hand-over controller 19 and a timing
regenerator 65. The despreader 8 performs the despreading process
on the received signal by using the spreading code for the pilot
channel. The path detector 11 detects the paths of the received
signal having respective delay times. The timing regenerator 65
regenerates a timing signal indicative of the beginning of the
pilot signal transmission interval .tau. by using the output signal
of the path detector 11. The hand-over controller 19 performs the
hand-over process by using the output signal of the path detector
11 and the timing signal regenerated by the timing regenerator 65.
The receive level measuring unit 18 measures the receive power
level of the detected path at the timing indicated by the timing
signal.
A further description of the pilot channel receive unit 44 will be
given with reference to FIGS. 11A through 11E.
The path detector 11 detects peaks of the pilot signals in sections
A, B, C and D shown in FIG. 11E, which corresponds to the offset
times between the base stations 21, 22, 23 and 24 shown in FIG. 4.
The path detector 11 detects peaks 201, 202, 203 and 204 in the
sections A, B, C and D, respectively, and compares them with each
other in order to select the greatest peak from among them. In the
case shown in FIG. 4, the mobile station 25 is closet to the base
station 21, and the peak 201 is greater than the peaks 202, 203 and
204. The traffic channel receive unit 45 shown in FIG. 10 operates
based on the greatest peak 201. The timing regenerator 65
regenerates the timing signal from the timing of the greatest peak
201. The pilot signal transmission interval .tau. of each base
station is known. Hence, it is possible to estimate the next pilot
signal transmission time from the timing of the peak 201
transmitted by the base station 21. In this manner, the timing
signal can be reproduced.
The hand-over controller 19 performs the hand-over control when the
path detector 11 detects the greatest peak from another base
station. In response to the timing signal based on the timing of
the greatest peak of another base station, the hand-over control is
carried out. The receive level measuring unit 18 measures the
receive power level of the greatest peak and thus determines a
transmit power level of the mobile station 25.
Turning to FIG. 10 again, the traffic channel receive unit 45
includes the despreaders 9, and 10, the RAKE combiner 12, the
information demodulator 13, a deinterleave unit 66 and an error
correction decoder 67. The deinterleave unit 66 performs a
deinterleaving operation on the demodulated signal from the
information demodulator 13. The error correction decoder 67
performs an error correction and decoding process on the output
signal of the deinterleave unit 66.
FIG. 12 is a flowchart of an operation of the spread spectrum
communication receiver shown in FIG. 10 according to the first
embodiment of the present invention.
At step S11, the despreader 8 despreads the received signal by
using the spreading code for the pilot channel. At step S12, the
path detector 11 detects the greatest peak (the peak having the
greatest amplitude) as has been described previously. At this time,
peaks propagated through some delayed paths following the greatest
peak are also detected for the RAKE combine process, and timing
information concerning these peaks is applied to the traffic
channel receive unit 45, as indicated by a broken arrow in FIG. 12.
At step S13, the timing regenerator 65 regenerates the timing
signal, as has been described previously. At step S14, the receive
level measuring unit 18 measures the receive power levels of the
peaks detected by the path detector 11.
At step S15, the path detector 11 detects that the greatest peak is
transmitted by a base station other than the base station currently
identified. Thus, the hand-over control is started at step S16, and
the timing regenerator 65 starts to regenerate the timing signal
based on the peak detected by step S15, at step S17. At this step,
the timing information concerning the peak detected at step S15 is
supplied to the traffic channel receive unit 45.
The despreaders 9 and 10 of the traffic channel receive unit 45
despread the received signal by the spreading codes with an offset
time at step S21. For example, the despreader 9 despreads the
received signal at the timing when the greatest peak is detected by
the path detector 11, and the despreader 10 despreads the received
signal with an offset time corresponding to a delay time of the
second greatest peak detected by the path detector 11. At step S22,
the RAKE combiner 12 combines the despread received signals by the
RAKE combine process. At step S23, the information demodulator 13
demodulates the RAKE-combined signal. Then, the deinterleaving
process and the error-correction coding process are successively
carried out.
According to the first embodiment of the present invention, the
following advantages can be obtained. The output signal of the
despreader 8 has the signals shown in FIGS. 11A through 11D
superimposed. At the pilot signal transmission timing of the base
station 21 to which the mobile station 25 belongs, the other base
stations 22, 23 and 24 do not transmit the pilot signals. Hence, at
the pilot signal transmission timing of the base station, the pilot
signals of the base stations 22, 23 and 24 to which the mobile
station 25 does not belong are not superimposed and no noise is
added to the pilot signal transmitted by the base station 21.
Hence, a high S/N ratio can be obtained.
All the base stations 21 through 24 intermittently transmit the
pilot signals at the different timings. Hence, the traffic channel
receive unit 45 of the mobile station 25 receives interference
signals for a short time, as compared to the prior art in which all
the base stations continue to transmit the pilot signals. As a
result, an increased number of stations in the same frequency band
can be accommodated. In other words, the channel capacity can be
increased.
The spreading code which has one period in the pilot signal
transmission interval .tau. is used in the spread spectrum
modulator 3 shown in FIG. 7. Alternatively, it is possible to use a
spreading code that has a plurality of periods in the pilot signal
transmission interval .tau.. Even in this case, the same effects as
those obtained when the spreading code having one period in the
interval .tau. can be obtained. It is also possible to use a
spreading code having a period longer than the pilot signal
transmission interval .tau.. In this case, a part of the spreading
code is transmitted in the pilot signal transmission interval
.tau.. Even in this case, the same effects as those obtained when
the spreading code having one period in the interval .tau. can be
obtained.
In the above description of the first embodiment of the present
invention, the base stations transmit the pilot signals, and the
mobile stations receive them. However, the concept of the first
embodiment of the present invention can be applied to a structure
in which the mobile stations transmit signals such as pilot signals
and the base stations receive these signals.
The above description of the first embodiment of the present
invention is directed to use of four cells. However, the same
effects as those obtained in the case of four cells can be obtained
even when a different number of cells are used.
When a small number of cells are provided, it is possible to
realize an arrangement in which, when one base station transmits
the pilot signal, the other base stations do not transmit the pilot
signals. If a large number of cells are provided, it may be
difficult to realize the above arrangement. In this case, a
plurality of base stations are allowed to simultaneously transmit
the pilot signals under a condition that these base stations are
sufficiently away from each other and the mobile station located
therebetween receives sufficiently attenuated pilot signals
therefrom due to propagation-based attenuation.
In the aforementioned description, the time sections TS are
provided as shown in FIG. 9, during which none of the base stations
transmit the pilot signals. However, the time sections TS are
completely or partially omitted.
In the aforementioned description, the pilot transmission timing
signal is applied to the units 1, 2 and 3, as shown in FIG. 7.
However, it is possible to modify the structure shown in FIG. 7 so
that the pilot transmission timing signal is applied to only one or
two of the units 1, 2 and 3 to thereby intermittently transmit the
pilot signal.
A description will now be given of a second embodiment of the
present invention.
FIG. 13 is a block diagram of a spread spectrum communication
receiver according to the second embodiment of the present
invention. In FIG. 13, parts that are the same as those shown in
the previously described figures are given the same reference
numbers. The receiver shown in FIG. 13 has a pilot channel receive
unit 44A in which a propagation path state estimating unit 17 is
provided. The propagation path state estimating unit 17 estimates
the state of the propagation path by using the pilot signals
intermittently transmitted.
FIG. 14 shows a structure of the propagation path state estimating
unit 17. As shown in FIG. 14, the unit 17 includes a fading
variation measuring part 250 and a fading variation estimating part
251 receiving an output signal of the fading variation measuring
part 250.
In the actual mobile communication systems, a radio wave propagated
through a transmission path is affected by multipath fading and
Rayleigh fading.
FIGS. 15A through 15C show examples of the despread output signals.
In these figures, the signals transmitted by the base station 21 to
which the mobile station 25 belongs are shown. Further, vectors are
used to indicate the peak points (locations) and magnitudes of the
correlation waveform necessary for the demodulating process. A
reference number 101 indicates an orthogonal axis and a reference
number 102 indicates an in-phase axis. Further, a reference number
103 denotes a time axis.
FIG. 15A shows the despread output signal which has not been
affected by fading variation. A vector 104 indicates the amplitude
and phase of each peak 201 shown in FIG. 11A. FIG. 15B shows the
despread output signal which has been affected by Rayleigh fading
so that the amplitude and phase of a vector 105 are varied with
time. The amplitude and phase of the vector 105 are varied due to
the state of the propagation path. FIG. 15C shows the despread
output signal which has been affected by two-wave multipath fading.
A reference number 106 indicates a leading wave, and a reference
number 107 indicates a delayed wave. The amplitudes and phases of
both of the waves 106 and 107 are varied.
The pilot signal transmitted by the base station 21 is known data.
Hence, the despread output waveform of the pilot signal transmitted
by the base station 21 without being affected by any fading (FIG.
15A) is also known for the mobile station 25. Hence, it is possible
to estimate, in the mobile station 25, variations (FIGS. 15B and
15C) in the amplitude and phase of the pilot signal affected by
fading during propagation as well as the difference between the
leading wave and the delayed wave by comparing the despread output
waveform without being affected by fading and the despread output
waveform affected by fading.
As has been described previously, the pilot signal is
intermittently transmitted by each of the base stations with time
offsets. Hence, the magnitudes of variations in the amplitude and
phase of the pilot signal caused by fading and measured by the
fading variation measuring part 250 shown in FIG. 14 correspond to
data obtained by sampling the pilot signal at intervals .tau..
Hence, the fading variation estimating part 251 interpolate the
sampled data output by the fading variation measuring part 250, and
thus estimates fading variations in each pilot signal transmission
interval with respect to the same base station.
FIG. 16 shows estimated results output by the fading estimating
part 251. The fading variation estimating part 251 outputs an
estimated fading variation 108 of the leading wave and an estimated
fading variation 109 of the delayed wave. These estimated
variations 108 and 109 are used to determine the timings at which
the despreaders 9 and 10 start to despread the received signal and
weight coefficients for the RAKE combine process carried out by the
RAKE combiner 12.
In the aforementioned first embodiment of the present invention,
the RAKE combine is carried out by using the information concerning
the phase, amplitude and timing of the pilot signal that is
intermittently transmitted. Hence, the RAKE combine carried out
during the time when the pilot signal is not received employs the
information obtained when the pilot signal is actually received. On
the other hand, according to the second embodiment, variations in
the despread output signal during the time when the pilot signal is
not received are estimated as described above. Thus, the RAKE
combine in the second embodiment of the present invention uses the
estimated results 108 and 109 and the received signal of the
traffic channel. Hence, the performance of the receiver according
to the second embodiment of the present invention can be further
improved.
The receive level measuring unit 18 shown in FIG. 13 can determine
the receive power level taking into account an influence of fading.
Hence, the transmission power can be determined more precisely. The
hand-over controller 19 also utilizes the variations due to fading,
and can perform the take-over process more precisely.
The present invention is not limited to the specifically disclosed
embodiments, and variations and modifications may be made without
departing from the scope of the present invention.
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