Multiplex communications – Generalized orthogonal or special mathematical techniques – Particular set of orthogonal functions
Reexamination Certificate
1999-12-20
2002-09-10
Olms, Douglas (Department: 2732)
Multiplex communications
Generalized orthogonal or special mathematical techniques
Particular set of orthogonal functions
C375S229000
Reexamination Certificate
active
06449245
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a signal receiving apparatus and method and a providing medium.
As a digital broadcast transmitting system using a ground wave, recently, attention is being paid to an OFDM (Orthogonal Frequency Division Multiplex) modulating system. As a service using the OFDM modulating system, in Europe, a radio service using the Eureka 147 DAB (Digital Audio Broadcasting) system has already been started. With respect to a television broadcasting as well, in Europe, the DVB (Digital Video Broadcasting)-T system has already been developed and standardization of the television broadcasting is recommended by the ITU-R (International Telecommunication Union-Recommendation).
In the Eureka 147 DAB system of which service has already been started, since a main service is intended for a mobile unit audio signal, the &pgr;/4 offset differential QPSK (Quadrature Phase Shift Keying) is used for each of carrier waves of the OFDM. Since the system is intended for a mobile unit, resistance to fading is a necessary condition. The system is employed since there is no information in the amplitude direction and it is unnecessary to reproduce an absolute phase.
On the other hand, in the television broadcasting, different from broadcasting service of which target is sound, it is not so much necessary to correspond to be adapted to a mobile unit. Instead, since it is necessary to transmit mainly video information of a large information amount, a high transmission speed is required. Specifically, in the audio service for the mobile unit, very reliable transmission even in hostile environments is demanded. On the contrary, in the television broadcasting service, high speed transmission is demanded. From such a background, in the DVB-T system intended for the television broadcasting service, it is proposed to use a modulating system such as QPSK, 64 QAM, or 16 QAM for modulation of each of carrier waves of the OFDM.
In the ground wave transmission, generally, a multipath exists and the frequency characteristics of a reception signal are distorted by the multipath. It is therefore an important subject to reduce the influence of the multipath. In the OFDM system, therefore, a signal of a copy of a part of a signal to be inherently transmitted is added as a guard interval. By adding the guard interval, with respect to a multipath shorter than the guard interval, a proper signal process is performed on the reception side, thereby enabling the influence of the multipath to be eliminated.
In the OFDM modulating system such as the DVB-T system using modulation of a QAM system as a system of modulating each of carrier waves, when a distortion caused in the multipath occurs, the amplitude and phase of each carrier wave become different from those on the transmission side. It is therefore necessary to equalize (correct) a signal distorted by the multipath so that the amplitude and phase are unchanged. In the OFDM system, the OFDM modulation is carried out by performing the FFT (Fast Fourier Transform) on the reception side. By dispersing pilot signals in transmission signals and by monitoring the amplitude and the phase of the pilot signal on the reception side, the characteristics of a transmission path are estimated and a reception signal is equalized according to the estimated characteristics of the transmission path.
In the DVB-T system, it is proposed that the pilot signals are inserted in a pattern as shown in FIG.
9
. In the same figure, the abscissa axis shows a frequency f and the ordinate axis represents a time t. As illustrated in the same figure, in the example, a carrier wave signal for pilot (shown by a black circle) is inserted per
12
carrier waves of one OFDM symbol (its frequency is shown by f
0
) and the insertion position of the carrier wave signal for pilot is shifted by three carrier waves at every OFDM symbol. A blank circle shows a carrier wave signal for information. Also, a tg denotes a guard interval.
The pilot signals arranged discretely in both the time and frequency directions shown in
FIG. 9
were subjected to two-dimensional Fourier transform, the structure of sampling lattice points was examined, and the transmission band width was checked. The result is as shown in FIG.
10
. From the same figure, it is understood that the transmission band width when there is no fluctuation in the time direction in the transmission path is within a time corresponding to an interval of three carrier waves. In other words, since there is the transmission band width of ⅓ of effective time of the OFDM symbol (duration of the OFDM symbol except for the guard interval), the pilot signal pattern in the DVB-T system has equalizing capability for the time within ⅓ of the OFDM effective symbol length.
FIG. 11
shows an example of the construction of a conventional signal receiving apparatus for estimating the transmission path characteristics from such a pilot signal and equalizing (correcting) a reception signal. A tuner
2
converts a signal received by an antenna
1
into an intermediate frequency (IF signal) which is outputted to multipliers
3
and
4
. Carrier waves whose phases are different from each other by 90 degrees generated by a carrier wave generating circuit
7
are supplied to the multipliers
3
and
4
. Each of the multipliers
3
and
4
multiplies the input intermediate frequency signal by the carrier wave, converts the signal into an OFDM signal in a base band, and outputs a resultant signal to an FFT circuit
5
. The FFT circuit
5
performs an FFT process on the input signal, thereby OFDM demodulating the OFDM signal in the base band.
An FFT window circuit
6
generates a window as a reference of start of the FFT operation of the FFT circuit
5
by using the correlation of the guard intervals of the OFDM signals from the OFDM signals in the base band outputted from the multipliers
3
and
4
and outputs the window to the FFT circuit
5
. The carrier wave generating circuit
7
generates carrier waves whose phases are different from each other by 90 degrees from an output of the FTT window circuit
6
and outputs them to the multipliers
3
and
4
.
Each of the carrier waves of the OFDM signal outputted from the FFT circuit
5
is supplied to a dividing circuit
10
and a pilot signal extracting circuit
8
which construct an equalizing circuit
13
. The pilot signal extracting circuit
8
extracts the pilot signal from the input signal and outputs it to an interpolating filter
9
. The interpolating filter
9
performs an interpolating process on the input pilot signal, thereby estimating the transmission path characteristics of each of the carrier waves of the OFDM signal, and outputs the estimation result to the dividing circuit
10
. The dividing circuit
10
divides each of the carrier waves of the OFDM signal input from the FFT circuit
5
by the transmission path characteristics input from the interpolating filter
9
, removes the distortion occurred in the transmission path, and outputs a resultant to a demapping circuit
11
. The demapping circuit
11
restores the transmission information from a signal point of the signal input from the dividing circuit
10
. When an error correcting circuit using a convolutional code or the like exists at the post stage of the demapping circuit
11
, a metric to be supplied to a Viterbi decoder is generated by the demapping circuit
11
.
A TPS detecting circuit
12
extracts a transfer control signal called a TPS (Transfer Parameter Signal) from an output of the FFT circuit
5
. The transfer control signal includes a coding ratio of the convolutional code, a system of modulating the OFDM carrier wave, guard interval information and the like in the next super frame (one super frame consists of eight frames). The TPS detecting circuit
12
controls each of the circuits on the basis of the extracted transfer control signal. For example, the demapping circuit
11
is controlled on the basis of the modulating system of the OFDM carrier wave included in the transfer control si
Ikeda Yasunari
Okada Takahiro
Lerner David Littenberg Krumholz & Mentlik LLP
Olms Douglas
Vanderpuye Ken
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