Multiplex communications – Generalized orthogonal or special mathematical techniques – Particular set of orthogonal functions
Reexamination Certificate
1999-12-16
2004-03-30
Olms, Douglas (Department: 2661)
Multiplex communications
Generalized orthogonal or special mathematical techniques
Particular set of orthogonal functions
C370S210000, C375S260000
Reexamination Certificate
active
06714511
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a transmission/reception apparatus used for digital mobile communications using an orthogonal frequency division multiplexing (hereinafter referred to as “OFDM) system.
2. Description of the Related Art
A conventional OFDM transmission/reception apparatus is explained using FIG.
1
and FIG.
2
.
FIG. 1
is a block diagram showing an outlined configuration of a transmission system of a conventional OFDM transmission/reception apparatus.
FIG. 2
is a block diagram showing an outlined configuration of a reception system of the conventional OFDM transmission/reception apparatus.
In
FIG. 1
, serial-parallel converter (hereinafter referred to as “S/P converter”)
11
converts a serial input signal to a plurality of parallel signals. IDFT circuit
12
performs inverse discrete Fourier transform (hereinafter referred to as “IDFT”) on the input signals. Guard interval inserter
13
inserts a guard interval for every valid symbol. D/A converter
14
performs D/A conversion on the signal with guard interval inserted.
In
FIG. 2
, A/D converter
15
performs A/D conversion on a reception signal. Delayer
16
delays the input signal by a valid symbol length. Correlator
17
despreads the input signal. Timing generator
18
detects the timing of the reception signal at which the correlation result becomes largest. Guard interval eliminator
19
eliminates a guard interval inserted for every symbol. DFT circuit
20
performs discrete Fourier transform (hereinafter referred to as “DFT”) on the input signal. Coherent detectors
21
to
24
perform coherent detection on the input signal. Deciders
25
to
28
judge the input signal. Parallel-serial converter (hereinafter referred to as “P/S converter”)
29
converts a plurality of parallel signals to a serial signal.
Then, the operation of the conventional OFDM transmission/reception apparatus is explained. Here, suppose the number of carriers is 4, for example.
First, the operation of the transmission system is explained using
FIG. 1. A
modulated signal input to the transmission system is S/P-converted by S/P converter
11
. This results in four modulated signals, which are transmitted by a first, second, third and fourth carriers, respectively.
Then, the 4 modulated signals are IDFT-processed by IDFT circuit
12
.
A general OFDM transmission/reception apparatus has a frame format as shown in the frame format schematic diagram in FIG.
3
. That is, in a frame format used for a general OFDM transmission/reception apparatus, a signal with the same waveform as that of the last part of a valid symbol is added at the start of the valid symbol as a guard interval. The OFDM transmission/reception apparatus can eliminate a delayed signal with a shorter delay time than this guard interval through DFT processing of the reception system.
Guard interval inserter
13
inserts a guard interval into the IDFT-processed signal. The signal with the guard interval inserted is converted to an analog signal by D/A converter
14
. In this way, a transmission signal is obtained.
Then, the operation of the reception system is explained using FIG.
2
. The reception signal input to the reception system is converted to a digital signal by A/D converter
15
.
Generally, the OFDM transmission/reception apparatus finds a correlation between a pre-DFT signal and the pre-DFT signal delayed by a valid symbol length. Then, the OFDM transmission/reception apparatus detects a DFT integration interval by detecting the timing at which the correlation result becomes largest. To be more specific, delayer
16
delays the reception signal by a valid symbol length, then correlator
17
finds a correlation and timing generator
18
detects the timing at which the correlation result becomes largest. Guard interval eliminator
19
eliminates the guard interval from the reception signal according to this detection result.
The reception signal stripped of the guard interval is DFT-processed by DFT circuit
20
. This results in 4 baseband signals, which are carried by 4 carriers. The 4 baseband signals are each subjected to coherent detection by coherent detectors
21
to
24
. In this way, coherent detected signals are obtained.
Here, coherent detectors
21
to
24
are explained using FIG.
4
.
FIG. 4
is a block diagram showing an outlined configuration of the coherent detector of the OFDM transmission/reception apparatus. Digital multipliers
41
and
42
multiply the DFT-processed signals by pilot symbols. Conjugate complex number generator
43
generates a conjugate complex number for the input signal.
In a general frame format, a pilot symbol, a known reference signal, is added before a message interval. In a general coherent detection method, a fading variation is detected using a pilot symbol.
In(nT) which is a DFT-processed input signal in a pilot symbol interval is expressed as In(nT)=P(nT)·A(nT)·exp(j&THgr;(nT)), where P(nT) is a pilot symbol, A(nT) is an amplitude variation due to fading and exp(j&THgr;(nT)) is a phase variation due to fading.
F(nT), which represents a variation due to fading, is expressed as follows:
F
(
nT
)
=
In
(
nT
)
·
P
(
nT
)
=
{
P
(
nT
)
·
A
(
nT
)
·
exp
(
j
Θ
(
nT
)
)
}
·
P
(
nT
)
=
P
(
nT
)
2
·
A
(
nT
)
·
exp
(
j
Θ
(
nT
)
)
1
◯
Here, in a modulation system such as a QPSK modulation system in which the amplitude is constant and only the phase contains information, P(nT)
2
=1. Therefore, expression {circle around (1)} is expressed as follows:
F
(
nT
)=
A
(
nT
)·exp(j&THgr;(
nT
))
Then, digital multiplier
41
obtains signal F(nT) that represents a variation due to fading by multiplying DFT-processed input signal (baseband signal) In(nT) by pilot symbol P(nT) in a pilot symbol interval.
Then, conjugate complex number generator
43
generates a conjugate complex number about F(nT), a signal representing a variation due to fading. In this way, conjugate complex number F(nT)* of F(nT) signal expressing a variation due to fading is obtained. Conjugate complex number generator
43
inverts the polarity of the Q component of the input signal and generates a conjugate complex number. Therefore, conjugate complex number F(nT)* is expressed in the following expression:
F
(
nT
)*=
A
(
nT
)·exp(−j&THgr;(
nT
))
Then, digital multiplier
42
multiplies the DFT-processed input signal (baseband signal) by the conjugate complex number of the signal representing a variation due to fading. In this way, a coherent detected signal is obtained.
Here, suppose the fading variation is sufficiently slow compared to the interval of pilot symbols and the fading variation is constant between pilot symbols. Based on this supposition, coherent detected signal D
out
(nT) is expressed in the following expression:
D
out
(
nT
)
=
D
in
(
nT
)
·
A
(
nT
)
·
exp
(
j
Θ
(
nT
)
)
·
A
(
nT
)
·
exp
(
-
j
Θ
(
nT
)
)
=
D
in
(
nT
)
·
A
(
nT
)
2
◯
2
In expression {circle around (2)}, A(nT)
2
is the component with a constant phase and variable amplitude. Therefore, the phase variation of coherent detected signal D
out
(nT) is only dependent on D
in
(nT). Therefore, the phase of the reception signal is demodulated by digital multiplier
42
multiplying the DFT-processed input signal (baseband signal) by a conjugate complex number of the signal indicating a variation due to fading. The QPSK modulation system is a modulation system with a constant amplitude and variable phase. Therefore, the OFDM transmission/reception apparatus performs coherent detection by demodulating the phase of the reception sig
Sudo Hiroaki
Uesugi Mitsuru
Greenblum & Bernstein P.L.C.
Matsushita Electric - Industrial Co., Ltd.
Olms Douglas
Phunkulh Bob A.
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