U.S. patent application number 12/132817 was filed with the patent office on 2008-10-02 for pilot signal transmission technique and digital communication system using same.
This patent application is currently assigned to MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.. Invention is credited to Akihiko Matsuoka, Yutaka Murakami, Masayuki Orihashi, Morikazu Sagawa.
Application Number | 20080240289 12/132817 |
Document ID | / |
Family ID | 14422176 |
Filed Date | 2008-10-02 |
United States Patent
Application |
20080240289 |
Kind Code |
A1 |
Murakami; Yutaka ; et
al. |
October 2, 2008 |
PILOT SIGNAL TRANSMISSION TECHNIQUE AND DIGITAL COMMUNICATION
SYSTEM USING SAME
Abstract
In a digital communication system such as a mobile radio system,
fading distortion is compensated with a raised precision. In a
transmitter, pilot signals are regularly inserted in the
information signals. The amplitude of the pilot signals are set
larger than the maximum possible amplitude of the information
signals. The modulation scheme of the pilot signals may be
different from that of the information signals. In a receiver, the
fading distortion of each of the pilot signals is determined. The
fading distortions of the information signals are estimated by
interpolation using the determined fading distortion of the pilot
signal, and then compensated. The frequency band of each
information signal is preferably limited with a roll-off filter
with a roll-off coefficient ranging from 0.1 to 0.4.
Inventors: |
Murakami; Yutaka;
(Tsurumi-ku, JP) ; Orihashi; Masayuki;
(Ichikawa-shi, JP) ; Matsuoka; Akihiko;
(Midori-ku, JP) ; Sagawa; Morikazu; (Inagi-shi,
JP) |
Correspondence
Address: |
CONNOLLY BOVE LODGE & HUTZ LLP
1875 EYE STREET, N.W., SUITE 1100
WASHINGTON
DC
20036
US
|
Assignee: |
MATSUSHITA ELECTRIC INDUSTRIAL CO.,
LTD.
KADOMA-SHI
JP
|
Family ID: |
14422176 |
Appl. No.: |
12/132817 |
Filed: |
June 4, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10601591 |
Jun 24, 2003 |
|
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|
12132817 |
|
|
|
|
09292398 |
Apr 15, 1999 |
6608843 |
|
|
10601591 |
|
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Current U.S.
Class: |
375/298 |
Current CPC
Class: |
H04L 27/3455
20130101 |
Class at
Publication: |
375/298 |
International
Class: |
H04L 27/36 20060101
H04L027/36 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 16, 1998 |
JP |
10-105990 |
Claims
1. A method comprising the steps of: modulating digital data, by
using a QPSK (quadrature phase shift keying) scheme or 2.sup.m-QAM
(quadrature amplitude modulation) scheme (m is an integer equal to
or higher than 3) in which both absolute values of in-phase (I) and
quadrature-phase (Q) components of an information symbol with a
maximum amplitude take the same value in an I-Q plane, to generate
an orthogonal base band signal including one or more information
symbols, each information symbol consisting of I-Q components;
regularly inserting a pilot symbol into the orthogonal base band
signal, the pilot symbol being disposed in the I-Q plane so as to
equalize a distance in the I-Q plane between the pilot symbol and
every one of two neighboring information symbols with the maximum
amplitude, the amplitude of the pilot symbol being larger than the
maximum amplitude of the information symbol, the absolute value of
the amplitude of the pilot symbol taking a fixed value; and
transmitting the base band signal including the pilot symbol and
the information symbols by wireless.
2. The method according to claim 1, wherein a phase difference
between the pilot symbol and the information symbol with the
maximum amplitude is .pi./4.
3. The method according to claim 1, wherein the pilot symbol is
disposed on either the in-phase axis or the quadrature-phase
axis.
4. The method according to claim 1, wherein an amplitude of the
pilot symbol is not 1.6 times larger than the maximum amplitude of
the information symbol.
5. A wireless transmitting system, comprising: a base band signal
generator that modulates digital data, by using a QPSK (quadrature
phase shift keying) scheme or 2.sup.m-QAM (quadrature amplitude
modulation) scheme (m is an integer equal to or higher than 3) in
which both absolute values of in-phase (I) and quadrature-phase (Q)
components of an information symbol with a maximum amplitude take
the same value in an I-Q plane, to generate an orthogonal base band
signal including one or more information symbols, each information
symbol consisting of I-Q components; a pilot signal inserter that
regularly inserts a pilot symbol into the orthogonal base band
signal, the pilot symbol being disposed in the I-Q plane so as to
equalize a distance in the I-Q plane between the pilot symbol and
every one of two neighboring information symbols with the maximum
amplitude, the amplitude of the pilot symbol being larger than the
maximum amplitude of the information symbol, the absolute value of
the amplitude of the pilot symbol taking a fixed value; and a
wireless transmitter that transmits the base band signal including
the pilot symbol and the information symbols by wireless.
6. The system according to claim 5, wherein a phase difference
between the pilot symbol and the information symbol with the
maximum amplitude is .pi./4.
7. The system according to claim 5, wherein the pilot system is
disposed on either the in-phase axis or the quadrature-phase
axis.
8. The system according to claim 5, wherein an amplitude of the
pilot symbol is not 1.6 times larger than the maximum amplitude of
the information symbol.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is a Continuation of application Ser. No.
10/601,591 filed on Jun. 24, 2003, which is a Divisional of
application Ser. No. 09/292,398 filed on Apr. 15, 1999, now U.S.
Pat. No. 6,608,843, and for which priority is claimed under 35
U.S.C. .sctn. 120; and this application claims priority of
application Ser. No. 10-105990 filed in Japan on Apr. 16, 1998
under 35 U.S.C. .sctn. 119; the entire contents of all are hereby
incorporated by reference.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The invention relates to a digital communications system and
more specifically to modulation methods facilitating fading
distortion compensation that performs a quasi-synchronous detection
using a pilot signal.
[0004] 2. Description of the Prior Art
[0005] In a digital communications system, especially in a digital
mobile radio system, the envelope of information signals or symbols
is distorted (i.e., the phases and the amplitudes of information
signals involves phase errors and amplitude errors) due to fading.
The phase error of the received signal causes an error in the
frequency of a local oscillator in the receiver. The error of the
local oscillation frequency with respect to the carrier frequency
is hereinafter referred to as a "frequency offset". The phase error
(or frequency offset) and the amplitude error of the received
signal have to be estimated and compensated for in the
receiver.
[0006] A fading distortion compensation scheme using a pilot signal
is described by S. Sampei, "Rayleigh Fading Compensation Method for
16-QAM MODEM in Digital Land Mobile Radio Systems," Trans. IEICE
(The Institute of Electronics, Information and Communication
Engineers) Japan, Vol. J72-B-II, No. 1, January 1989, pp. 7-15
(which is hereby incorporated by reference). FIG. 1 is a diagram
showing a signal constellation used in this 16-QAM system. In FIG.
1, small black-filled circles indicate 16 signal points in an
in-phase (I) and quadrature-phase (Q) plane. One of the signal
points with the maximum amplitude, that is, any of the signal
points A, B, C and D is assigned to a pilot signal. (Since one of
the 16 signal points in the signal constellation is used for a
pilot signal, the remaining 15 points are available for the
information signals.) A pilot signal is inserted in every frame or
every N-1 information symbols (assuming that N symbols constitute
one frame) in a transmitter. The estimation and compensation of
distortions (due to fading) of information signals or symbols are
achieved by interpolation using the pilot signals.
[0007] In such quasi-synchronous detection as just described,
larger-amplitude pilot symbols yields a higher precision in
estimation of the frequency offset and the amplitude error of the
information signals, which results in an improvement of the bit
error rate, which is a function of the ratio of the carrier signal
power to the noise power density per one symbol. However, enlarging
the amplitude of the pilot signals without taking any measure
lowers the power efficiency of the power amplifier of the
transmitter system due to an increase in the ratio of the peak to
the average transmission power.
[0008] It is therefore an object of the invention to provide a
method of and a system for compensating for fading distortion of
the received signal with a raised precision and thereby to provide
a digital communication system that permits a reception of a
reduced bit error rate.
SUMMARY OF THE INVENTION
[0009] In accordance with the principles of the invention, a point
that differs in phase from any of the signal points for possible
information symbols and that is larger in amplitude than any of the
signal points is selected for a pilot signal point in a signal
constellation (or a signal point map plotted on a in-phase and
quadrature-phase plane). A pilot signal is inserted in every frame
or every predetermined number of information signals.
[0010] In a receiver, the fading distortion of each of the pilot
signals regularly inserted in the received signals is determined.
The fading distortions of the information signals are estimated by
interpolation using the determined fading distortion of the pilot
signal, and then compensated for.
[0011] In a preferred embodiment, the amplitude of the pilot signal
is set not larger than 1.6 times a maximum possible amplitude of
the information signals.
[0012] The frequency band of each information signal is preferably
limited with a roll-off filter with a roll-off coefficient ranging
from 0.1 to 0.4.
BRIEF DESCRIPTION OF THE DRAWING
[0013] The features and advantages of the present invention will be
apparent from the following description of an exemplary embodiment
of the invention and the accompanying drawing, in which:
[0014] FIG. 1 is a diagram showing a signal constellation used in
this 16-QAM system;
[0015] FIG. 2 is a schematic block diagram showing a part of an
illustrative embodiment of a mobile telephone terminal that
incorporates a fading distortion compensation system in accordance
with the principles of the invention;
[0016] FIG. 3 is a diagram of an exemplary signal constellation for
16-APSK (amplitude phase shift keying) used in a first example;
[0017] FIG. 4 is a diagram showing a symbol stream transmitted in a
digital communication system that serves the mobile telephone
terminal 1 of FIG. 1; and
[0018] FIGS. 5 through 8 are signal constellations for .sup.m-QAM,
16-QAM, 8-PSK and QPSK according to the principles of the
invention.
[0019] Throughout the drawing, the same elements when shown in more
than one figure are designated by the same reference numerals.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0020] FIG. 2 is a schematic block diagram showing a part of an
illustrative embodiment of a mobile telephone terminal 1 that
incorporates a fading distortion compensation system in accordance
with the principles of the invention. A transmission system (shown
in upper part of FIG. 2) of the mobile telephone terminal 1
includes a serial-to-parallel (S/P) converter 10; a base band
signal generator 20 having its input connected with a converter 10
output; a pilot (or frame) signal inserter 110 having its in-phase
(I) input and quadrature-phase (Q) input connected with respective
outputs of the base band signal generator 20; low pass filters
(LPF) 30 having their inputs connected with pilot signal inserter
110 outputs; a radio transmitter portion 40 having its I and Q
inputs connected with I and Q LPF 30 outputs, respectively; an
antenna duplexer 50 having transmission input connected with the
transmitter portion 40 output; and an antenna 60 used for both
transmission and reception. A reception system (shown in lower part
of FIG. 2) of the mobile telephone terminal 1 includes a radio
receiver portion 70 having its input connected with a duplexer 50
reception output; LPFs 80 having their input connected with radio
receiver portion 70 I and Q outputs; a fading distortion
compensator 100 having its I and Q inputs connected with respective
LPF 80 outputs; and a decision maker 90, having its input connected
with a compensator 100 output, for providing received data. The
fading distortion compensator 100 includes a phase error estimator
120, an amplitude error estimator 130 and a phase and amplitude
compensator 140. The mobile telephone terminal 1 further includes a
controller 170 for controlling overall operation of the terminal
1.
[0021] In transmission operation, binary data is supplied in the
form of a bitstream to the S/P converter 10. The S/P converter 10
converts the serial binary data into a parallel data of a
predetermined number of bits. The base band signal generator 20
generates an in-phase (I) and quadrature-phase (Q) components or
signals for a symbol associated with the parallel data.
[0022] FIG. 3 is a diagram of an exemplary signal constellation for
16-APSK (amplitude phase shift keying) used in a first example.
FIG. 4 is a diagram showing an operation of the pilot signal
inserter 110 of FIG. 3. In FIGS. 3 and 4 and the following figures
of signal communications, small black-filled circles indicate
information symbols and small white-filled circles indicate pilot
(or frame) symbols.
[0023] In this specific example, the base band signal generator 20
generates I and Q components of one (ISi) of the 16 possible
information symbols IS.sub.1, IS.sub.2, . . . , IS.sub.P which is
associated with each of the parallel data supplied to the generator
20, where i=1, 2, . . . P, and P is the number of possible
information symbols (16 in this specific example) in the modulation
scheme. The information symbol stream supplied from the base band
signal generator 20 is shown in the upper part of FIG. 4.
[0024] The pilot signal inserter 110 inserts a pilot signal PS in
every predetermined number of information symbols, say, N-1 symbols
S.sub.1, . . . S.sub.N-1 to make a frame of N symbols S.sub.0,
S.sub.1, . . . S.sub.N-1 as shown in FIG. 4. In each frame of the
symbol stream output from the pilot signal inserter 110,
Sj = { PS for j = 0 IS i for j = 1 , 2 , , N - 1. ( 1 )
##EQU00001##
[0025] It is seen from FIG. 3 that the pilot signal PS (=S.sub.0)
is preferably disposed so as to make an angle of .pi./8 with
adjacent information signal points. Thus, the pilot signal
PS=(PS.sub.I, PS.sub.Q) is preferably set to any of the following
points:
PS I = R * cos ( .pi. 4 + .pi. 8 ) ##EQU00002## PS Q = R * sin (
.pi. 4 + .pi. 8 ) , ##EQU00002.2##
[0026] where i=1, 2, . . . , 16 and R is the amplitude of the pilot
signal PS.
[0027] According to the invention, the amplitude R of the pilot
signal PS is set larger than that of any information signal Sj
(=ISi), Ri, as shown in FIG. 3. Specifically, it is preferable to
set the pilot signal amplitude R for a range larger than the
maximum amplitude Rmax of the information symbols and not larger
than 1.6 times the maximum amplitude Rmax, that is,
Rmax<R.ltoreq.1.6*Rmax. (2)
[0028] It is noted that each signal Sj is processed in the form of
corresponding I and Q components Sj.sub.I and Sj.sub.Q. The signals
Sj from the pilot signal inserter 110 is limited in frequency band
by the LPFs 30. The LPFs 30 are preferably roll-off filters (or
Nyquist filters) having the following characteristic:
H ( .omega. ) = { 1 .omega. .ltoreq. .omega. 0 ( 1 - .alpha. ) 1 2
[ 1 - sin ( .pi. 2 .alpha..omega. 0 ( .omega. - .omega. 0 ) ) ]
.omega. 0 ( 1 - .alpha. ) .ltoreq. .omega. .ltoreq. .omega. 0 ( 1 +
.alpha. ) 0 .omega. .gtoreq. .omega. 0 ( 1 + .alpha. ) ( 3 )
##EQU00003##
[0029] where H(.omega.) is a amplitude characteristic of the
roll-off filters 30, .omega. is an angular frequency, .omega..sub.0
is a Nyquist angular frequency and .alpha. is a roll-off
coefficient. It is preferable to set the roll-off coefficient
.alpha. for a range from 0.1 to 0.4.
[0030] The filtered signals are modulated and amplified by the
radio transmitter portion 40, and eventually transmitted via the
duplexer 50 and the antenna 60.
[0031] In reception operation, the I and Q components of the
received signals received by the antenna 50, the duplexer 60 and
the radio receiver portion 70 is filtered by the LPFs 80 and
supplied to the phase error estimator 120, the amplitude error
estimator 130 and the phase and amplitude compensator 140. The
phase error estimator 120 provides an estimated phase error signal
to the compensator 140. The amplitude error estimator 130 provides,
to the compensator 140, an estimated amplitude error signal for
each information signal Sj (j=1, 2, . . . N-1) through
interpolation using a pilot signal S.sub.0. The phase and amplitude
compensator 140 responsively compensates each information signal by
using the estimated phase and amplitude error signals to provide
compensated I and Q components. The decision maker 90 provides data
associated with the compensated I and Q components.
[0032] In this way, the invention enables the bit error rate to be
reduced without influencing on the ratio of peak to average power
at the amplifier (AMP) 44 in the radio transmission portion 40
because the precision in estimation of frequency and amplitude
errors of the information signals is enhanced.
[0033] Modification
[0034] Though the embodiment has been described in conjunction with
the 16-APSK the invention is applicable to any
more-than-7-signal-point modulation scheme. Examples are presented
for 2.sup.m-QAM (quadrature amplitude modulation) (m.gtoreq.3),
16-QAM, 8-PSK (phase shift keying) and QPSK (quadrature phase shift
keying) in the following.
[0035] FIGS. 5 through 8 are signal constellations for 2.sup.m-QAM
(m.gtoreq.3), 16-QAM, 8-PSK and QPSK according to the principles of
the invention.
[0036] As for 2.sup.m-QAM as shown in FIG. 5, if the information
signal points ISi (i=1, 2, . . . , 2.sup.m) are written as
(ISi.sub.I, ISi.sub.Q) in I-Q coordinates, the points are expressed
as follows:
ISi.sub.I=s(2.sup.m-1a.sub.1+2.sup.m-2a.sub.2+ . . .
+2.sup.0a.sub.m)
ISi.sub.Q=s(2.sup.m-1b.sub.1+2.sup.m-2b.sub.2+ . . .
+2.sup.0b.sub.m) (4)
[0037] where s is a constant and each of a.sub.k and b.sub.k (k=1,
2, . . . m) represents 1 and -1,i.e., (a.sub.k, b.sub.k) represents
four points (1, 1), (1, -1), (-1, 1) and (-1, -1). In this case,
the pilot signal PS is disposed on either of the I and Q axes such
that the amplitude (R) of PS is larger than that (Ri) of any
possible symbol points. In the specific example of FIG. 5, the
pilot signal is disposed on the positive range of the I axis.
[0038] In case of 16-QAM, the possible symbol points are expressed
as follows:
ISi.sub.I=s(2.sup.1a.sub.1+2.sup.0a.sub.2)
ISi.sub.Q=s(2.sup.1b.sub.1+2.sup.0b.sub.2). (5)
[0039] In this case, the pilot signal PS is preferably disposed on
either of the I and Q axes such that the amplitude (R) of PS is
larger than that (Ri) of any possible symbol points ISi as shown in
FIG. 6
[0040] In case of 8-PSK as shown in FIG. 7, the possible signal
points are expressed as:
ISi.sub.I=r*cos(i.pi./4)
ISi.sub.Q=r*sin(i.pi./4). (6)
[0041] In this case, since the pilot signal PS is preferably
disposed so as to make an angle of .pi./8 with adjacent information
points, the pilot signal PS=(PS.sub.I, PS.sub.Q) is preferably set
to any of the following points:
PS I = R * cos ( .pi. 4 + .pi. 8 ) ##EQU00004## PS Q = R * sin (
.pi. 4 + .pi. 8 ) ##EQU00004.2##
[0042] where k=1, 2, . . . , 8 and R is the amplitude of the pilot
symbol that satisfy:
r<R.ltoreq.1.6*r.
[0043] In case of QPSK as shown in FIG. 8, the possible signal
points are expressed as:
ISi I = r * cos ( .pi. 4 + .pi. 2 ) ISi Q = r * sin ( .pi. 4 + .pi.
2 ) . ( 7 ) ##EQU00005##
[0044] In this case, since the pilot signal PS is preferably
disposed so as to make an angle of .pi./4 with adjacent information
points, the pilot signal PS=(PS.sub.I, PS.sub.Q) is preferably set
to any of the following points:
PS I = R * cos ( .pi. 2 ( 1 + i ) ) ##EQU00006## PS Q = R * sin (
.pi. 2 ( 1 + i ) ) . ##EQU00006.2##
[0045] Many widely different embodiments of the present invention
may be constructed without departing from the spirit and scope of
the present invention. It should be understood that the present
invention is not limited to the specific embodiments described in
the specification, except as defined in the appended claims.
* * * * *