U.S. patent application number 10/601591 was filed with the patent office on 2004-05-20 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 Matsuoka, Akihiko, Murakami, Yutaka, Orihashi, Masayuki, Sagawa, Morikazu.
Application Number | 20040095899 10/601591 |
Document ID | / |
Family ID | 14422176 |
Filed Date | 2004-05-20 |
United States Patent
Application |
20040095899 |
Kind Code |
A1 |
Murakami, Yutaka ; et
al. |
May 20, 2004 |
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; (Yokohama,
JP) ; Orihashi, Masayuki; (Ichikawa-shi, JP) ;
Matsuoka, Akihiko; (Yokohama, JP) ; Sagawa,
Morikazu; (Tokyo, JP) |
Correspondence
Address: |
CONNOLLY BOVE LODGE & HUTZ LLP
SUITE 800
1990 M STREET NW
WASHINGTON
DC
20036-3425
US
|
Assignee: |
Matsushita Electric Industrial Co.,
Ltd.
|
Family ID: |
14422176 |
Appl. No.: |
10/601591 |
Filed: |
June 24, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10601591 |
Jun 24, 2003 |
|
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|
09292398 |
Apr 15, 1999 |
|
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6608843 |
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Current U.S.
Class: |
370/310 |
Current CPC
Class: |
H04L 27/3455
20130101 |
Class at
Publication: |
370/310 |
International
Class: |
H04B 007/00 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 16, 1998 |
JP |
10-105990 |
Claims
What is claimed is:
1. A method of transmitting digital data by wireless so as to
permit a reception in a reduced bit error rate in a digital
wireless communications system, the method including the steps of:
converting said digital data into a first stream of information
signals through a more-than-7-signal-point modulation scheme;
inserting a pilot signal regularly in said first stream of said
information signals into a second stream of said information
signals and said pilot signal, an amplitude of said pilot signal
being larger than that of any of said information signals; and
transmitting said second stream by wireless.
2. A method as defined in claim 1, wherein said step of inserting a
pilot signal includes the step of setting said amplitude of said
pilot signal not larger than 1.6 times a maximum possible amplitude
of said information signals.
3. A method as defined in claim 1, further including the step of
the transmitter limiting a frequency band of each information
signal with a roll-off filter with a roll-off coefficient ranging
from 0.1 to 0.4.
4. A method as defined in claim 1, wherein said
more-than-7-signal-point modulation scheme is a 16-amplitude phase
shift keying.
5. A method as defined in claim 4, wherein said step of inserting a
pilot signal includes the step of generating said pilot signal so
as to make an angle of .pi./8 with adjacent information signals in
an in-phase and quadrature-phase signal space.
6. A method as defined in claim 1, wherein said
more-than-7-signal-point modulation scheme is a
2.sup.m-signal-point quadrature amplitude modulation, where m is a
positive integer not smaller than 3.
7. A method as defined in claim 6, wherein said step of inserting a
pilot signal includes the step of generating said pilot signal so
as to be disposed on one of an in-phase axis and a quadrature-phase
axis in a signal space of said 2.sup.m-signal point quadrature
amplitude modulation.
8. A method as defined in claim 1, wherein said
more-than-7-signal-point modulation scheme is an 8 phase shift
keying.
9. A method as defined in claim 8, wherein said step of inserting a
pilot signal includes the step of generating said pilot signal so
as to make an angle of .pi./8 with adjacent information signals in
an in-phase and quadrature-phase signal space of said 8 phase shift
keying.
10. A method of transmitting digital data by wireless so as to
permit a reception in a reduced bit error rate, the method
including the steps of: converting said digital data into a first
stream of information signals through a quadrature phase shift
keying modulation; inserting a pilot signal regularly in said first
stream of said information signals into a second stream of said
information signals and said pilot signal, an amplitude of said
pilot signal being larger than that of any of said information
signals; and transmitting said second stream by wireless.
11. A method as defined in claim 10, wherein said pilot signal is
so disposed as to make an angle of .pi./4 with adjacent information
signals in an in-phase and quadrature-phase signal space of said
quadrature phase shift keying modulation.
12. A method as defined in claim 10, wherein said step of inserting
a pilot signal includes the step of setting said amplitude of said
pilot signal not larger than 1.6 times a maximum possible amplitude
of said information signals.
13. A method as defined in claim 10, further including the step of
limiting a frequency band of each information signal with a
roll-off filter with a roll-off coefficient ranging from 0.1 to
0.4.
14. A system capable of transmitting digital data by wireless so as
to permit a reception in a reduced bit error rate, the system
including: means for converting said digital data into a first
stream of information signals through a more-than-7-signal-point
modulation scheme; means for inserting a pilot signal regularly in
said first stream of said information signals into a second stream
of said information signals and said pilot signal, an amplitude of
said pilot signal being larger than that of any of said information
signals; and means for transmitting said second stream by
wireless.
15. A system as defined in claim 14, wherein said inserting means
includes means setting said amplitude of said pilot signal not
larger than 1.6 times a maximum possible amplitude of said
information signals.
16. A system as defined in claim 14, further including means for
limiting a frequency band of each information signal with a
roll-off filter with a roll-off coefficient ranging from 0.1 to
0.4.
17. A system as defined in claim 14, wherein said
more-than-7-signal-point modulation scheme is a 16-amplitude phase
shift keying.
18. A system as defined in claim 17, wherein said pilot signal is
so disposed as to make an angle of .pi./8 with adjacent information
signals in an in-phase and quadrature-phase signal space.
19. A system as defined in claim 14, wherein said
more-than-7-signal-point modulation scheme is a
2.sup.m-signal-point quadrature amplitude modulation, where m is a
positive integer not smaller than 3.
20. A system as defined in claim 19, wherein said pilot signal is
disposed on one of an in-phase axis and a quadrature-phase axis in
a signal space of said 2.sup.m-signal point quadrature amplitude
modulation.
21. A system as defined in claim 14, wherein said
more-than-7-signal-point modulation scheme is an 8 phase shift
keying.
22. A system as defined in claim 21, wherein said pilot signal is
so disposed as to make an angle of .pi./8 with adjacent information
signals in an in-phase and quadrature-phase signal space of said 8
phase shift keying.
23. A system capable of transmitting digital data by wireless so as
to permit a reception in a reduced bit error rate, the system
including: means for converting said digital data into a first
stream of information signals through a quadrature phase shift
keying modulation; means for inserting a pilot signal regularly in
said first stream of said information signals into a second stream
of said information signals and said pilot signal, an amplitude of
said pilot signal being larger than that of any of said information
signals; and means for transmitting said second stream by
wireless.
24. A system as defined in claim 23, wherein said pilot signal is
so disposed as to make an angle of .pi./4 with adjacent information
signals in an in-phase and quadrature-phase signal space of said
quadrature phase shift keying modulation.
25. A system as defined in claim 23, wherein said inserting means
includes means setting said amplitude of said pilot signal not
larger than 1.6 times a maximum possible amplitude of said
information signals.
26. A system as defined in claim 23, further including means for
limiting a frequency band of each information signal with a
roll-off filter with a roll-off coefficient ranging from 0.1 to
0.4.
27. A mobile telephone that communicates digital data with a
reduced bit error rate, the mobile telephone comprising: a
transmission system; and a receiver system, said transmission
system including: means for converting said digital data into a
first stream of information signals through a
more-than-7-signal-point modulation scheme; means for inserting a
pilot signal regularly in said first stream of said information
signals into a second stream of said information signals and said
pilot signal, an amplitude of said pilot signal being larger than
that of any of said information signals; and means for transmitting
said second stream.
28. A mobile telephone as defined in claim 27, wherein said
inserting means includes means setting said amplitude of said pilot
signal not larger than 1.6 times a maximum possible amplitude of
said information signals.
29. A mobile telephone as defined in claim 27, further including
means for limiting a frequency band of each information signal with
a roll-off filter with a roll-off coefficient ranging from 0.1 to
0.4.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] 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.
[0003] 2. Description of the Prior Art
[0004] 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.
[0005] 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.
[0006] 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.
[0007] 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
[0008] 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.
[0009] 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.
[0010] 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.
[0011] 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
[0012] 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:
[0013] FIG. 1 is a diagram showing a signal constellation used in
this 16-QAM system;
[0014] 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;
[0015] FIG. 3 is a diagram of an exemplary signal constellation for
16-APSK (amplitude phase shift keying) used in a first example;
[0016] 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
[0017] FIGS. 5 through 8 are signal constellations for .sup.m-QAM,
16-QAM, 8-PSK and QPSK according to the principles of the
invention.
[0018] 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
[0019] 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.
[0020] 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.
[0021] 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.
[0022] 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.
[0023] 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, 1 Sj = {
PS for j = 0 IS i for j = 1 , 2 , , N - 1. ( 1 )
[0024] 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: 2 PS I = R * cos ( i 4 + 8 ) PS Q = R * sin ( i 4 + 8 )
,
[0025] where i=1, 2, . . . ,16 and R is the amplitude of the pilot
signal PS.
[0026] 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)
[0027] It is noted that each signal Sj is processed in the form of
corresponding I and Q components Sj.sub.r 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: 3 H ( ) = { 1
0 ( 1 - ) 1 2 [ 1 - sin ( 2 0 ( - 0 ) ) ] 0 ( 1 - ) 0 ( 1 + ) 0 0 (
1 + ) . ( 3 )
[0028] 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.
[0029] The filtered signals are modulated and amplified by the
radio transmitter portion 40, and eventually transmitted via the
duplexer 50 and the antenna 60.
[0030] 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.
[0031] 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.
[0032] Modification
[0033] 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.ltoreq.3),
16-QAM, 8-PSK (phase shift keying) and QPSK (quadrature phase shift
keying) in the following.
[0034] FIGS. 5 through 8 are signal constellations for 2.sup.m-QAM
(m.ltoreq.3), 16-QAM, 8-PSK and QPSK according to the principles of
the invention.
[0035] 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.2+2.sup.m-2b.sub.2+. . .
+2.sup.0b.sub.m) (4)
[0036] 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.
[0037] 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)
[0038] 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
[0039] 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)
[0040] 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: 4 PS I = R * cos ( i 4 + 8 ) PS Q =
R * sin ( i 4 + 8 )
[0041] where k=1, 2, . . . ,8 and R is the amplitude of the pilot
symbol that satisfy:
r<R.ltoreq.1.6*r.
[0042] In case of QPSK as shown in FIG. 8, the possible signal
points are expressed as: 5 ISi I = r * cos ( 4 + i 2 ) ISi Q = r *
sin ( 4 + i 2 ) . ( 7 )
[0043] 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: 6 PS I = R * cos ( 2 ( 1 + i ) ) PS
Q = R * sin ( 2 ( 1 + i ) ) .
[0044] 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.
* * * * *