Pulse or digital communications – Receivers – Particular pulse demodulator or detector
Reexamination Certificate
2000-11-16
2004-12-28
Chin, Stephen (Department: 2634)
Pulse or digital communications
Receivers
Particular pulse demodulator or detector
Reexamination Certificate
active
06836518
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a synchronization control method for a data transmission system utilizing an orthogonal frequency division multiplex and its transmission system.
In recent years, as a signal multiplex system for a digital radio communications for mobile terminals or terrestrial systems, attention has been paid to an orthogonal frequency division multiplex (hereinafter, referred to as an OFDM system) having a feature such that it is strong against a multipath fading or ghost.
According to such a system, an information code is transmitted by using a signal obtained by digitally modulating tens to hundreds of kinds of a number of carrier waves arranged with a same frequency interval fs by a symbol frequency fsy (=1/Tsy), namely, by using an OFDM signal (orthogonal frequency division multiplexed signal).
In the case where a transmission signal which was modulated by such a system and transmitted is received and demodulated on a reception side, first, it is necessary to obtain synchronization from the received OFDM signal.
In this case, the operation to obtain synchronization of the OFDM signal denotes that a head position of a data symbol is detected from the OFDM reception signal by a receiver apparatus and a demodulating process is started in accordance with a timing of the head position. To obtain the synchronization, therefore, it is necessary to obtain a reference signal showing the timing of the head position of the data symbol.
For this purpose, there has been proposed a method whereby, on a transmission side, a null interval serving as a non-signal period and a sync symbol group such as a sweep signal or the like having a signal component which changes from the highest frequency to the lowest frequency of a transmission band for a predetermined period are previously inserted to the head of a transmission frame serving as a unit of a data transmitting process and they are detected and synchronization is obtained on a reception side (“A Study or Field Pickup Unit using OFDM Modulation Scheme”, Technical Report of The Institute of Television Engineers of Japan, Vol. 19, No. 18, issued on August, 1995). As an example of a specific method for the detection of the null interval and the clock synchronization using the sweep signal, there is the invention disclosed in U.S. patent application Ser. No. 09/203,564 filed on Dec. 2, 1998, according to the invention of the same applicant as that of the present invention.
The method of obtaining synchronization of the null interval and the OFDM signal having no sweep signal has been disclosed in U.S. Pat. No. 5,602,835 registered on Feb. 11, 1997. In this method, as mentioned later, a transmitted OFDM signal includes a time-axis base data signal obtained by OFDM-modulation of one symbol signal and a guard interval added to a head of the time-axis base data signal, wherein the guard interval is produced by copying a predetermined tail part of the time-axis base data signal. According to the method, a calculation for obtaining a mutual correlation value, that is a degree (intensity) of correlation between two signals, of the OFDM reception signal and the signal obtained by delaying the reception signal by one effective symbol is implemented. The effective symbol means a single symbol without the guard interval. According to the method, since there is a delay of one effective symbol period between the received signal and the delayed signal, a point where a data signal for a predetermined period at the end of a data symbol of the reception signal and a guard interval added at the head of the data symbol of the delay signal coincide on the time axis, and a correlation value becomes the maximum value is obtained. The demodulating operation of the reception signal is performed by setting a position on a time base at the time of obtaining the maximum value to a reference.
A method of obtaining synchronization of an OFDM signal by using a sync symbol group will be briefly explained hereinbelow with reference to FIG.
3
.
FIG. 3
shows a synchronization detecting unit of a demodulating unit on a receiving unit side of a digital data transmission system in which a transmission signal in which null intervals have been inserted at regular periods is received, an electric power value of the reception signal is obtained, a magnitude of the obtained power value is discriminated by a comparator, the null interval is detected, and synchronization with the reception signal is obtained.
An RF transmission signal of the OFDM system which has been transmitted from a transmitter Tx and in which the null intervals have been inserted at regular periods is received by a receiver Rx, the RF signal is converted into a baseband signal S
21
by a down-converter
21
of the receiver Rx, and a digital reception signal obtained by digitally converting the signal S
21
by an A/D converter
22
is supplied to a terminal
1
. An instantaneous power value of the digital reception signal supplied to the terminal
1
is obtained by a power calculator
15
.
An average power of a power value S
11
outputted from the power calculator
15
is obtained by an average power calculator
6
. The average power is delayed by time corresponding to one symbol or more by a delay
7
. A multiplier
9
increases an output (average power) of the delay
7
by 1/N time (N is a positive real number), thereby obtaining a threshold value S
13
for comparing with the power value S
11
.
The level of the power value is discriminated by a comparator
12
of an adaptive type reception level discriminator
14
. When the power value S
11
is larger than the threshold value S
13
, an output S
12
of the level discriminator
14
is set to the “High (H)” level. If S
11
is smaller than the threshold value, the output S
12
is set to the “Low (L)” level. Since the output itself of the adaptive type reception level discriminator
14
is used merely for discriminating the magnitude of the reception signal as mentioned above, whether the “L” level continues for a predetermined length (time) or not is not discriminated.
In a null interval discriminator
19
, therefore, in the case where a state in which the output of the reception level discriminator
14
is at the “H” level continues for the predetermined length (time), it is determined that a null interval exists, and a null interval detection pulse S
19
is generated.
By the construction as mentioned above, the null interval during which the “H” level or the “L” level continues for the predetermined length (time) is detected from the reception signal, thereby enabling a synchronizing position of a frame start point to be coarsely matched.
However, in order to more correctly demodulate the reception signal by the receiver Rx, in the receiver Rx, a count start point (start point of sampling a data symbol in a demodulator
40
) of a frame counter
24
of the receiver Rx needs to be matched up to a precision of a 1-clock period based on the received reception signal.
As a method of accomplishing the above object, in the Transmitter Tx, besides a null symbol, a sync symbol for showing a specific time point on a time base is inserted into a transmission signal to be transmitted.
As a sync symbol to be inserted, there is a sweep signal, a PN code, or the like which changes from the predetermined highest frequency to the lowest frequency.
A case of using the baseband signal S
21
in which a sweep symbol (sync symbol) has been inserted subsequently to the null symbol will be described hereinbelow with reference to a signal diagram of FIG.
4
. A frequency component included in the sweep symbol of the baseband signal S
21
is shown at (q) in FIG.
4
.
A correlation between a reference signal (the same sweep signal as that of (q) in
FIG. 4
) which is equivalent to a frequency pattern of a sweep signal set in the receiver Rx in a sweep correlation arithmetic operating device
2
in FIG.
3
and the received baseband signal S
21
shown (p) in
FIG. 4
is arithmetically operated. The sweep correlation arithmetic operation range
Miyashita Atsushi
Sano Seiichi
Wagatsuma Yoshikatsu
Antonelli Terry Stout & Kraus LLP
Chin Stephen
Hitachi Kokusai Electric Inc.
Perilla Jason M.
LandOfFree
Synchronization control method for receiver apparatus of... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Synchronization control method for receiver apparatus of..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Synchronization control method for receiver apparatus of... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3297702