Coded data generation or conversion – Analog to or from digital conversion
Reexamination Certificate
2003-10-07
2004-08-10
Tokar, Michael (Department: 2819)
Coded data generation or conversion
Analog to or from digital conversion
C370S206000, C370S509000
Reexamination Certificate
active
06774829
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an apparatus and a method for receiving signals modulated by a multi-carrier modulation scheme for transmitting the information code using a plurality of carriers, or in particular to a receiving apparatus for receiving the signals containing data carriers and regularly-inserted pilot carriers having a fixed amplitude and a fixed phase.
This invention is suitably applicable to a receiving apparatus for receiving the signals modulated by the OFDM (Orthogonal Frequency Division Multiplex) modulation scheme with pilot carriers inserted therein, and an OFDM receiving apparatus having the functions of clock synchronization or generation of a delay profile utilizing the correlation of guard intervals.
In recent years, the OFDM modulation scheme, which is highly resistant to multi-path fading and ghost, has been closely watched as a modulation scheme suitable for application to the digital audio broadcasting for mobile units and the terrestrial digital TV broadcasting. The OFDM modulation scheme is a type of multi-carrier modulation scheme in which n (n: several tens to several hundreds) carriers orthogonal to each other are digitally modulated.
The I-axis component and the Q-axis component of each of these carriers is assigned a discrete code as a modulated signal, which code is updated every symbol period (several tens of &mgr;s). As shown in
FIG. 2
, a multiplicity of digitally modulated waves &Dgr;f,
2
&Dgr;f,
3
&Dgr;f, . . . , (n−1)&Dgr;f are added to each other, and an OFDM signal obtained by orthogonal modulation of the I and Q axes is transmitted. The multi-carrier digital modulation schemes often used are multi-valued modulation schemes such as the 16 quadrature amplitude modulation (16QAM) and the 32 quadrature amplitude modulation (32QAM) as well as the differential quadrature phase shift keying (DQPSK).
The OFDM signal is obtained by subjecting a plurality of the carriers of the I and Q components to the inverse fast Fourier transform (IFFT) and converting the carriers into a temporal waveform. The OFDM signal is configured, as shown in
FIG. 3
, of an effective symbol providing a temporal waveform after the IFFT processing and a guard interval copied from a part of the valid symbol and added before the valid symbol. According to the OFDM modulation scheme, the addition of the guard interval can prevent the deterioration due to the inter-symbol interference of the delayed wave in the delay time during the guard interval period. Therefore, the OFDM modulation scheme has a high resistance against the multi-path fading. In this way, the signal identical with the end portion of the symbol is added to the head of the symbol as a guard interval, and therefore, in the OFDM transmission apparatus, the cross-correlation between the head and the end of the symbol is utilized for demodulation and synchronization.
Next, the processing of the received signal utilizing the correlation of the guard intervals is explained.
In the OFDM modulation scheme, the frequency intervals between subcarriers are so small that the interference between subcarriers is likely to occur due to the carrier frequency error between the transmitter and the receiver and the sampling clock frequency error of the demodulation system. The reproduction of these frequencies, therefore, requires a high accuracy. Specifically, in order to continue to receive the OFDM signal correctly, the sampling clock reproduction process is required in which the sampling clock frequency at the receiving end is kept coincident with the sampling clock frequency of the transmission signal at highly accuracy.
In view of this, the technique for reproducing the sampling clock utilizing the correlation of the guard intervals is disclosed in U.S. Pat. No. 5,602,835. An OFDM receiving apparatus utilizing the correlation of the guard intervals is briefly explained with reference to FIG.
16
.
The received signal input to the receiving antenna
1
is converted to a baseband signal through a high-frequency unit
2
and an intermediate-frequency unit
3
. The baseband signal is converted from an analog form to a digital form by an A/D converter
4
. The received sampling series (R) converted to digital form are returned by a FFT (fast Fourier transform) operation unit
8
to a frequency waveform signal from a temporal waveform signal subjected to the IFFT operation at the transmitting end, and demodulated to the original signal by a demodulation unit
9
.
The receiving sampling series (R) and the signal delayed by a time length equal to the valid symbol period in a valid symbol delay unit
11
are subjected to the cross-correlating process to each other by a correlator
13
. From the result of this cross-correlating operation, only the result of the correlating operation for the signal range corresponding to the guard interval period is extracted. Then, the range of extraction of the correlating operation result is shifted by one sample period and the corresponding correlating operation result is extracted. Over the entire signal section for the cross-correlating operation, the same extraction process is repeated while shifting the extraction range of the correlating operation result by one sample period each time. The values of the repeated correlating operation thus extracted are added to obtain a waveform having a peak indicating the position of a guard interval as shown in FIG.
4
. Outside the guard interval period, the two signals have no correlation and, therefore, the correlation result is approximately zero in value. With the arrival at the guard interval, however, the correlation begins to present itself and the result of addition value increases linearly. The correlation value reaches a peak at the time point when the addition period coincides with the guard interval. After this time point, the addition period begins to be displaced from the guard interval, and therefore the correlation value begins to decrease. The correlation waveform, therefore, assumes a triangle as shown in FIG.
4
. Actually, however, this triangular wave is usually contained noises due to the random characteristic of the OFDM signal as described later.
The peak of the triangular wave representing the correlation value indicates the ending time point of the guard interval. In the symbol timing detector
5
for detecting the peak position, therefore, the sampling clock frequency of a VCO
7
is controlled through a VCO controller
6
in such a manner as to maintain the peak at a predetermined position. In this way, the clock reproduction processing can be realized.
The OFDM transmission, for the feature of the modulation scheme thereof, is often used for the mobile communication. In outdoor transmission, a multiple communication paths are formed to transmit the main wave arriving directly from the transmitter and waves reflected on buildings and the like and arriving with a delay time in accordance with the topographic conditions. Further, in mobile communication, a fading environment may be generated with the levels of the main wave and the reflected waves changing each moment. The multi-path communication undesirably causes the synchronization of the FFT demodulation windows at the receiving end with the reflected waves or the inter-symbol interferences in the presence of reflected waves exceeding the range of the guard intervals.
This inter-symbol interference causes a deterioration of the carrier-to-noise ratio (C/N), and causes the deterioration of the code error rate. In order to improve the reliability of the transmission, therefore, it is necessary to select a propagation path environment free of reflected waves exceeding the guard intervals. Very effective means for improving the reliability of communication, therefore, is to observe the propagation path characteristics and to employ a transmission scheme suited to the particular propagation path characteristics.
The most widely used method for observing the propagation path characteristics is the measurement of a delay profile indi
Nakada Tatsuhiro
Takesue Hiroyuki
Antonelli Terry Stout & Kraus LLP
Hitachi Kokusai Electric Inc.
Tokar Michael
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