Digital audio broadcasting receiver

Details Digital audio broadcasting receiver Digital audio broadcasting receiver

1998-01-29

2002-01-22

Nguyen, Chau (Department: 2663)

Multiplex communications

Generalized orthogonal or special mathematical techniques

Fourier transform

C375S340000, C375S344000

Reexamination Certificate

active

06341123

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to a digital audio broadcasting receiver in which each carrier is subjected to differential phase modulation and orthogonal frequency division multiplexing (OFDM).
As a system which permits transmission of digital data to a mobile object which is strongly affected by the problems of radio wave propagation, such as the multipath and fading, the orthogonal frequency division multiplexing (OFDM) transmission system is known, and the use of this system in broadcasting is under way. Its typical example is seen in digital audio broadcasting (DAB) which is set forth in ITU-R Recommendation BS.774.
FIG. 16
is a block diagram of a digital audio broadcasting receiver.
In the drawing, reference numeral
1
denotes an antenna;
2
, an RF amplifier;
3
, a frequency converter (MIX);
4
, a local oscillator (LO),
5
, an intermediate frequency amplifier (IF AMP);
6
, an orthogonal demodulator (DEMOD);
7
, an A/D converter;
8
, a synchronizing signal detector (synchronous detection);
9
, a synchronization control means;
10
, a complex discrete Fourier transform processing (hereafter referred to as “DFT”) means;
11
, a differential demodulator;
12
, a phase error detector;
13
, a frequency tuning control means;
14
, a Viterbi decoder;
15
, an MPEG audio decoder;
16
, a D/A converter;
17
, an audio amplifier; and
18
, a speaker.
In the receiver configured as described above, the broadcast wave received by the antenna
1
is amplified by the RF amplifier
2
, is subjected to frequency conversion by the frequency converter
3
, is subjected to removal of unwanted components such as adjacent channel waves and amplification by the intermediate frequency amplifier
5
, is subjected to detection by the orthogonal demodulator
6
, and is imparted to the A/D converter
7
as a baseband signal.
The signal sampled by the A/D converter
7
is subjected to DFT by the DFT means
10
, and the phase of each transmission carrier subjected to quadrature phase shift keying (QPSK) is detected. In the ensuing differential demodulator
11
, modulated phases of the same carrier of two transmitted symbols which are timewise adjacent to each other are compared, and processing (differential demodulation) for outputting a phase shift in the mean time is effected. The data subjected to differential demodulation is then outputted to the Viterbi decoder
14
in accordance with a rule on the order of carriers used in modulation on the transmitting side.
In the Viterbi decoder
14
, interleaving is canceled during the time spanning over the range of a plurality of symbols transmitted by the transmitting side, the data transmitted through convolutional coding is decoded, and correction of errors of data occurring on the transmission path is effected at that time.
In accordance with the provisions of the layer-
2
of ISO/MPEG
1
, the MPEG audio decoder
15
expands the compressed DAB broadcast audio data outputted from the Viterbi decoder
14
, and sends the same to the D/A converter
16
. The audio signal subjected to analog conversion by the D/A converter
16
is reproduced by the speaker
18
via the amplifier
17
.
Here, the synchronizing signal detector
8
detects the null symbol (the period during which no signal is present) by envelope detection, in the frame alignment signal included in the transmitted signal of DAB. This output serves as a timing signal by which DFT effected by the DFT means
10
through the synchronization control means
9
is executed correctly in synchronism with the transmission frame and each symbol of the signal.
The phase error detector
12
detects an error between an original phase point and the phase data of each carrier outputted from the differential demodulator
11
. That is, in DAB, if the frequency of the signal imparted to the orthogonal demodulator
6
is correct, the phase of the differentially demodulated data outputted from the differential demodulator
11
in correspondence with each carrier becomes substantially one of &pgr;/4, 3·&pgr;/4, 5·&pgr;/4, and 7·&pgr;/4.
Accordingly, if the data corresponding to each carrier is multiplied by 4 and the remainder is obtained with respect to 2&pgr;, this value becomes &pgr; if there is no error in the original data, and becomes a multiple of 4 of that value if there is a phase error in the original data, so that phase error detection is carried out. In practice, in the phase error detector
12
, the aforementioned operation is performed with respect to the data of the multiplicity of carriers, and the accuracy of detection is improved by averaging the results.
Since the phase error &egr; thus determined is an output from the differential demodulator
11
, the relationship of the following Formula (1) holds between an error &zgr; of the signal frequency at this time and the phase error &egr;:
&zgr;=&egr;/T   (1)
Here, T is a symbol period including a guard interval.
The frequency tuning control means
13
operates in such a manner as to cause the frequency error &zgr; of the baseband signal imparted from the orthogonal demodulator
6
to approach 0 by controlling the frequency of the intermediate frequency signal outputted from the frequency converter
3
by controlling the frequency of the local oscillator
4
in such a manner that this phase error &egr; becomes small.
As already described, the DAB signal is comprised of a multiplicity of carriers. To separate the carriers, DFT has an output characteristic shown in
FIG. 17
, and when the frequency is pulled in correctly, components from other carriers do not leak.
However, when the frequency is not pulled in correctly, components from other carriers leak, as shown in FIG.
18
.
Here, if there is no leakage from other carriers even if there is a frequency deviation, adjacent carriers s1and s2can be expressed by the following Formula (2):
s1=exp{j(2&pgr;(f0+&Dgr;f−n·fcc)t}
s2=exp{j(2&pgr;(f0+&Dgr;f−n·fcc)
(t+tsym)+&thgr;c+&thgr;n)}  (2)
where, f0: transmission frequency
&Dgr;f: frequency deviation
n: carrier number
fcc: interval between carrier frequencies
tsym: the period of one symbol
&thgr;n: (2N+1)p/4, N is an arbitrary integer
&thgr;c: 2&pgr;(f0−n·fcc)·tsym
Accordingly, the phase error from (2·.N+1)&pgr;/4 of the same carrier of adjacent symbols can be expressed by the following Formula (3):
&thgr;=&Dgr;f·tsym  (3)
Hence, it can be seen that the phase error is proportional to the frequency deviation.
In practice, however, when the frequency has deviated, if there is leakage from other carriers, e.g., a frequency of −80 Hz, large variations appear in the differential modulated data, as shown in FIG.
19
. Here, the differentially demodulated data is divided into four quadrants of 0−&pgr;/2, &pgr;/2−&pgr;, &pgr;−3&pgr;/2, and 3&pgr;/2−2&pgr;, but there occurs data which enters adjacent quadrants as shown in
FIG. 19
, and the sign of the data which shifted to adjacent quadrants becomes opposite and such data constitutes a large phase error. Since erroneous data in which the sign of phase error is opposite is also used in averaging processing by the phase error detector, the detected phase error assumes a value smaller than a real value.
In addition, the greater the deviation of the frequency, the greater the leakage of components from other carriers, so that the variation becomes larger, and the data is located closer to the adjacent quadrants, with the result that the aforementioned error is liable to occur. For this reason, as for the frequency deviation and the average value of phase errors, the phase error becomes small starting from the frequency deviation of 70 Hz or thereabouts, where the frequency deviation and the average phase error cease to be proportional. For this reason, if the frequency deviation is large, there has been a problem in that it takes time in the pulling in of the frequency.
SUMMARY OF THE INVENTION
The present invention has bee

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