Pulse or digital communications – Systems using alternating or pulsating current – Plural channels for transmission of a single pulse train
Reexamination Certificate
1998-06-18
2001-10-16
Chin, Stephen (Department: 2734)
Pulse or digital communications
Systems using alternating or pulsating current
Plural channels for transmission of a single pulse train
C370S210000, C332S125000
Reexamination Certificate
active
06304611
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an OFDM (Orthogonal Frequency Division Multiplex) modulation method and an OFDM modulator used in digital transmission apparatuses of OFDM modulation system.
In recent years, digitization of television broadcasting have been studied. As a modulation system for television broadcasting, the adoption of the OFDM modulation system is regarded as promising.
The OFDM modulation system is one kind of multi-carrier modulation system. In the OFDM modulation system, a large number of digital modulated waves are added together. As the modulation system of each carrier at this time, the QPSK (quadrature phase shift keying) system or the like is used. According to this system, a composite wave (OFDM signal) as shown in
FIG. 1
can be obtained.
FIG. 1
illustrates the case where the number of carriers is 24. In
FIG. 1
, T
S
denotes an effective symbol duration (duration of all effective symbols) of one symbol data.
Expression of this OFDM signal using a numerical expression will now be described.
Representing the QPSK signal of each carrier by &agr;
k
(t), it can be expressed by equation (1).
&agr;
k
(
t
)=
a
k
(
t
)×cos (2
&pgr;kft
)+
b
k
(
t
)×sin (2
&pgr;kft
) (1)
Here, k represents a carrier number, and a
k
(t) and b
k
(t) are data of a kth carrier, and assume the value of [−1] or [1].
Assuming the number of carriers is N, the OFDM signal is a combination of N carriers. Denoting this by &bgr;
k
(t), it can be represented by equation (2).
β
k
(
t
)
=
∑
k
=
1
N
α
k
(
t
)
(
2
)
Assuming now that each component has coefficient values a
k
=0 and b
k
=1 in the equation (1) and N=24 in the equation (2), waveforms of the coefficient values &agr;
k
(t) in the equation (2) are exemplified in FIG.
1
. Representing the equation (2) in the form of a frequency spectrum, a spectrum shown in
FIG. 2
is obtained.
In the OFDM system, a guard interval is typically added to each effective symbol duration of the composite carrier signal in order to mitigate the influence of the multi-path. In other words, a guard interval &Dgr;T is added to an effective symbol duration V
S
as shown in
FIGS. 3 and 4
.
FIG. 3
illustrates an example of the case where a guard interval has been added to a carrier wave of k=1 for simplifying the description.
FIG. 4
illustrates an example of the case where a guard interval has been added to a composite carrier signal obtained by combining N=448 carriers.
As shown in
FIG. 3
, a waveform (a) of an interval &Dgr;T/2 located in a start portion of the effective symbol duration V
S
is added after an end edge of the effective symbol duration as a rear guard interval waveform (a′) of an interval &Dgr;T/2. In the same way, a waveform (b) of an interval &Dgr;T/2 located in an edge portion of the effective symbol duration V
S
is added before a start edge of the effective symbol duration as a front guard interval waveform (b′) of an interval &Dgr;T/2. By the sum total of these front and rear guard interval waveforms (b′) and (a′), a guard interval of an interval &Dgr;T is added to one effective symbol duration V
S
. The entire symbol duration containing the effective symbol duration V
S
and the guard interval &Dgr;T, i.e., the entire symbol duration (symbol duration with guard) becomes T
S
.
By the way, (c) is a rear guard interval added to an immediately preceding effective symbol duration, and (d) is a front guard interval added to an immediately succeeding effective symbol duration. At connection points (changeover points) t
1
and t
2
of the entire symbol duration, the waveform becomes discontinuous. Especially at the time t
1
forming the connection point between the entire symbol duration of X=0 and the entire symbol duration of X=1, the waveform step (level change) becomes large and side lobes are generated as described later.
In the above described example, the front and rear guard intervals having the same interval &Dgr;T/2 are added to the start edge and the end edge of the effective symbol duration, respectively. Alternatively, a front guard interval and a rear guard interval differing in duration and having total duration equivalent to &Dgr;T may be added to the start edge and the end edge of the effective symbol duration, respectively.
Alternatively, a guard interval having duration of &Dgr;T may be added to the start edge or the end edge of the effective symbol duration.
FIG. 5
is a waveform diagram of the case where a guard interval having duration of &Dgr;T has added to the end edge of the effective symbol duration for the carrier wave of k=1.
FIGS. 6A and 6B
are basic configuration block diagrams of an OFDM modulation/demodulation apparatus using a conventional technique shown in, for example, JP-A-7-321762. Hereafter, the modulation/demodulation operation of the OFDM signal will be described by referring to
FIGS. 6A and 6B
. As illustrated, an IFFT (Inverse Fast Fourier Transform) unit
81
is used for the OFDM modulation, and a FFT (Fast Fourier Transform) unit
95
is used for demodulation.
In a system of adding guard intervals to a temporal waveform generated by modulating an input data train into a large number of (sub)carriers, a modulation unit (sending side)(OFDM modulator) for conducting orthogonal frequency division multiplex (OFDM) includes an IFFT unit (
81
) for conducting inverse fast Fourier transform (IFFT) processing to modulate the input data train into a large number of (sub)carriers, a guard interval adder for adding a guard interval to each effective symbol duration of a combined carrier signal supplied from the IFFT unit (
81
) and outputting a resultant signal, and an quadrature processor (
84
) for applying quadrature modulation to the signal supplied from the guard interval adder and outputting an OFDM signal.
In other words, in the sending side (modulation unit) T shown in
FIG. 6A
, inverse Fourier transform is conducted in the IFFT unit
81
by regarding an in phase component I of each carrier as real part data R
f
and a quadrature component Q as imaginary part data I
f
. Thus, a real part signal R and an imaginary part signal I in the time domain are obtained.
To this signal, a signal corresponding to the guard interval is added in the guard interval adder
82
. Resultant signals R
g
and I
g
are subject to D/A conversion in a digital-to-analog (D/A) converter
83
(
83
a
and
83
b
). By using a carrier signal having a frequency f
C
supplied from an oscillator
85
for an analog signal of the real part signal R
g
and using a carrier signal shifted in phase by 90° by a phase shift circuit
86
for an analog signal of the imaginary part signal I
g
, quadrature modulation is conducted in the quadrature modulator
84
. Resultant signals are combined in a synthesizer (adder)
87
. An OFDM signal is thus obtained. In
FIG. 6A
,
84
a
and
84
b
denote multipliers.
In the receiving side (demodulation unit) R shown in
FIG. 6B
, operation opposite to that of the sending side is conducted. An output obtained by demodulating the received OFDM signal in an quadrature demodulator
91
with a carrier signal of a frequency f
C
supplied from an oscillator
93
is taken out as a real component. An output obtained by demodulating the received OFDM signal in the quadrature demodulator
91
with the carrier signal shifted in phase by 90° in a phase shifting circuit
92
is taken out as an imaginary component. These demodulated signals are converted to digital signals by an analog-to-digital (A/D) converter
94
(
94
a
,
94
b
). The digital signals are subject to Fourier transform in a FFT unit
95
. OFDM demodulated signals are thus obtained. In
FIG. 6B
,
91
a
and
91
b
denote multipliers.
When conducting fast Fourier transform, a timing signal regenerator
96
is used to attain timing as shown in the receiving side R of FIG.
6
B.
For attaining the timing of this fast Fourier transform,
Akiyama Toshiyuki
Miyashita Atsushi
Nakada Tatuhiro
Sano Seiichi
Tsukamoto Nobuo
Antonelli Terry Stout & Kraus LLP
Chin Stephen
Hitachi Denshi Kabushiki Kaisha
Jiang Lenny
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