Pulse or digital communications – Synchronizers – Frequency or phase control using synchronizing signal
Reexamination Certificate
1998-12-02
2001-06-12
Le, Amanda T. (Department: 2634)
Pulse or digital communications
Synchronizers
Frequency or phase control using synchronizing signal
C375S368000, C370S514000, C370S520000
Reexamination Certificate
active
06246735
ABSTRACT:
CROSS-REFERENCES TO RELATED APPLICATIONS
U.S. patent application Ser. No. 09/099,390 filed on Jun. 18, 1998 in the names of Atsushi Miyashita et al., entitled “OFDM MODULATOR AND OFDM MODULATION METHOD FOR DIGITAL MODULATED WAVE HAVING GUARD INTERVAL” and claiming priority based on Japanese Patent Application No. 9-162579 filed on Jun. 19, 1997 and assigned to the same assignee of the present invention is related to the present invention and the disclosure thereof is hereby incorporated by reference.
U.S. patent application Ser. No. 09/198,346 filed on Jun. 17, 1998 in the name of Toshiyuki Akiyama et al., entitled “TRANSMITTING AND RECEIVING METHOD OF ORTHOGONAL FREQUENCY DIVISION MULTIPLEXED MODULATION SIGNAL AND COMMUNICATION SYSTEM” and claiming priority based on Japanese Patent Application No. 9-161486 and assigned to the same assignee of the present invention is also related to the present invention and the disclosure thereof is hereby incorporated by reference.
U.S. patent application Ser. No. 09/096,454 filed on Jun. 11, 1998 in the name of Seiichi Sano et al., entitled “DATA TRANSMISSION APPARATUS AND RECEIVING APPARATUS USING ORTHOGONAL FREQUENCY DIVISION MULTIPLEX MODULATION SYSTEM” and claiming priority based on Japanese Patent Application No. 9-155591 filed on Jun. 12, 1997 and assigned to the same assignee of the present invention is also related to the present invention and the disclosure thereof is hereby incorporated by reference.
BACKGROUND OF THE INVENTION
The present invention relates to a synchronization detection method of a data transmission apparatus using a digital modulation system and a data transmission apparatus to which the synchronization detection method is applied and more particularly to the technique for improving the accuracy of detection of synchronization of a demodulator in a receiver of a transmission apparatus for transmitting information by means of an orthogonal frequency division multiplex (OFDM) system.
In recent years, as a multiplex system for digital radio communication of mobiles or terrestrial systems, an orthogonal frequency division multiplex (OFDM) system has been getting attention which is characterized to be robust to multi-path fading and ghost.
In the OFDM system, signals obtained by digitally modulating a multiplicity of carrier waves of several tens to several hundreds kinds arranged at intervals of a frequency fs with a symbol frequency fsy (=1/Tsy), that is, OFDM signals (orthogonal frequency division multiplex modulated signals) are used to transmit information codes.
When a transmission signal modulated and transmitted by the OFDM system is received and demodulated on a receiving side, it is first necessary to reproduce or recover synchronization from the received OFDM signal.
For this purpose, the following system has been proposed “A Study of Field Pickup Unit using OFDM Modulation Scheme” by S. Moriyama et al., Institute of Television Engineers of Japan, Technical Report Vol. 19, No. 38, pp. 7-12, August 1995. On a transmission side of this system, there are previously inserted into a starting portion of a frame which is a unit of data transmission processing a null section constituted by a no-signal period and a group of synchronization symbols constituted by a sweep signal or the like having a signal component varying from a maximum frequency to a minimum frequency of a transmission band within a predetermined time. On a receiving side, the null section and the synchronization symbol group are detected to recover synchronization. Further, an example of a specific method of detecting the null section is described in copending U.S. patent application Ser. No. 09/096,454.
SUMMARY OF THE INVENTION
The example of U.S. Ser. No. 09/096,454 is now described in brief with reference to FIG.
1
.
The OFDM signal received on the receiving side shown in
FIG. 1
is converted into a baseband signal in an RF/IF demodulator
61
to obtain a baseband OFDM signal. A typical waveform of the baseband OFDM signal is shown in FIG.
2
(
a
).
The baseband OFDM signal is converted into a digital signal by an A/D converter
62
and supplied to an electric power calculator
15
in which an electric power value of the received signal is obtained. The electric power value of the received signal is supplied to an input terminal S
11
of a level-of-received signal decision unit
14
′. A typical waveform of the electric power value of the received signal inputted to the input terminal S
11
is shown in FIG.
2
(
b
).
The electric power value is compared with a predetermined decision level previously set in an amplitude-decision-level-of-received signal setting unit
13
by an amplitude-of-received signal decision unit
12
(which can be realized by, for example, a general-purpose logic IC74LS85, although its configuration is not shown).
When the electric power value of the received signal supplied to the input terminal S
11
is smaller than the decision level set in the decision level setting unit
13
, the amplitude decision unit
12
produces an “L” level signal and when the electric power value is larger than the decision level, the amplitude decision unit
12
produces an “H” level signal.
The output signal of the amplitude decision unit
12
is produced to an output terminal S
12
. A typical waveform at the output terminal S
12
is shown in FIG.
2
(
c
). Since an electric power value in null sections N
1
and N
2
of the baseband OFDM signal shown in FIG.
2
(
a
) is 0, the decision result of the null sections becomes continuous “L” level as shown in FIG.
2
(
c
).
That is, when the “L” level continues during a predetermined length, this system confirms the existence of the null section and recovers synchronization.
When such a data transmission apparatus is moved during operation, noise is sometimes mixed into signals received in the receiving side, so that a C/N ratio is reduced or level variation occurs due to fading.
The level variation of the received signal is normally reduced by the provision of an AGC (Automatic Ga in Control) circuit in the RF/IF demodulator
61
. However, when the period of the level variation of the received signal is long, the AGC circuit cannot fully follow the level variation varying from a short period to a long period to be assumed. Therefore, the level variation remains in the signal after the AGC processing. For example, when it is assumed that the level variation of the received signal which has been subjected to the AGC processing is a standard signal level ±3 dB, the level of the received signal is varied in the range from two times the standard signal level to a half thereof.
A relation of an electric power value of the received signal in the null section and the predetermined decision level (threshold) set in the decision level setting unit
13
is now described with reference to
FIGS. 3A and 3B
.
FIG. 3A
shows an example of an electric power value of the received signal in case where any level variation does not occur and numeral
115
represents a null section.
In order to decide and detect the null section, the threshold
45
is set between an electric power value of a supposed fixed received signal in a data transmission section and an electric power value of the received signal in the null section
115
, so that the time when the electric power value of the received signal is smaller than or equal to the threshold continues during a predetermined length in the case of FIG.
3
A and accordingly the null section can be detected.
However, the electric power value of the received signal is varied as described above. For example, as shown in
FIG. 3B
, when the received signal level is increased and noise in the null section is also increased due to variation of a gain of the AGC circuit not shown, the electric power value of the received signal in the null section
115
exceeds the threshold
45
.
Therefore, the electric power value of the received signal in the null section is not smaller than the threshold level continuously during the predetermined period. Consequently
Akiyama Toshiyuki
Miyashita Atsushi
Sano Seiichi
Tsukamoto Nobuo
Antonelli Terry Stout & Kraus LLP
Hitachi Denshi Kabushiki Kaisha
Le Amanda T.
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