Pulse or digital communications – Receivers – Particular pulse demodulator or detector
Reexamination Certificate
1998-08-25
2002-06-25
Corrielus, Jean (Department: 2631)
Pulse or digital communications
Receivers
Particular pulse demodulator or detector
C375S259000
Reexamination Certificate
active
06411664
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a correlation detecting apparatus and a method for detecting a specific code contained in a received signal, and particularly relates to a code detecting apparatus and a method which are preferably applied to a case where a timing connects a plurality of terminals in a wireless fashion.
2. Description of the Prior Art
As the function of a computer becomes higher, it is now popularized to construct a LAN (Local Area Network) by connecting a plurality of computers in order to share files or data, or transfer electronic mails or data. The conventional LAN employs an optical fiber or a coaxial cable, otherwise a twist pair cable to connect each computer by a wire.
However, in such a wired LAN, connecting works are required, thus making it difficult to readily construct the LAN and besides, in the wired LAN the cables become complicated. Therefore, a wireless LAN is now attracting attention as a system which releases a user from wiring of the conventional wired LAN.
As the wireless LAN, such a version that data communication is performed on a CDMA (Code Division Multiple Access) system using a spread spectrum has heretofore been proposed. In the CDMA system, transmitting data is multiplied by a PN (Psuedo Noise Code) code, thereby causing the spectrum of transmitting data to be spread. Transmitted data with such a spread spectrum is demodulated by multiplying the similar PN code similar to the transmitting side. This CDMA system is characterized by high privacy property and excellent interfere-resistance.
Recently, information is increasingly turned into multimedia, so that a large amount of data such as video data and audio data is now often handled. For this reason, it has also been demanded for the wireless LAN to make a speed of transfer rate higher so that a large amount of data such as video data and audio data can be transmitted. Unfortunately, in the spectrum spread modulation, if data transfer is performed at a high rate, e.g. about 30 Mbps, a band width of 300 MHz or more will then be required. Such a wide band width cannot be ensured with the present frequency allocation and so it is difficult to secure such a wide band width for making communication.
Moreover, with the spectrum spread method, in order to demodulate, a synchronism capturing time is required for matching the phase of transmitted data code with the phase of a code generated to demodulate in a receiver. For this reason, in the spectrum spread method, a synchronizing bit sequence is inserted in each packet in order to capture the synchronism at a high speed. Due to such a synchronizing bit sequence, a problem in which bits other than effective data increase will arise.
Therefore, the applicant of this application has proposed that data is transmitted on an OFDM (Orthogonal Frequency Division Multiplexing) system, data communication being performed on a TDMA (Time Division Multiple Access) system making one frame as a unit, an M sequence being transmitted at the head of one frame, a transmitting/receiving time being determined with reference to this M sequence and the transmitting/receiving time of each wireless communication terminal being indicated by control information from a wireless communication control terminal. According to the OFDM system, the transfer rate can be raised and the correct demodulation can be achieved even if any jitter arises. Furthermore, because the transmitting/receiving timing is set up with reference to the M sequence at the head of one frame, data can be reproduced during reception by demodulating only a required symbol within a frame using that time information.
Where the M sequence is received and the timing is set in this manner, it is necessary to detect the M sequence from a received signal. For a circuit to detect such the M sequence, it is conceived to employ a correlation detector circuit using a matched filter.
FIG. 1
shows an example of the correlation detector circuit using the matched filter as described above. Referring to
FIG. 1
, a received signal from an input terminal
151
is supplied to a matched filter
152
. The matched filter
152
is a sort of an FIR filter and, as is shown in
FIG. 2
, is comprised of delay circuits
161
-
1
,
161
-
2
,
161
-
3
, . . . , multiplier circuits
162
-
1
,
162
-
2
,
162
-
3
, . . . and an adder circuit
163
. Coefficients of the multiplier circuits
162
-
1
,
162
-
2
,
162
-
3
, . . . are set to 1 or (−1) depending on a code to be detected. If a strong correlation exists between codes which are set as the coefficients of the multiplier circuits
162
-
1
,
162
-
2
,
162
-
3
, . . . and the received code, an output level of the adder circuit
163
will rise.
Thus, the matched filter
152
detects the correlation between the received code and the code set for the filter. The output of the matched filter
152
is supplied to a comparator circuit
153
. The comparator circuit
153
compares the output of the matched filter
152
with a predetermined threshold value TH.
If the code of M sequence is received, the output of matched filter
152
rises and the output of the matched filter
152
exceeds the threshold value TH. If the output of matched filter
152
exceeds the threshold value TH, a detecting output occurs at an output terminal
154
.
In such a system, however, signals from a plurality of wireless communication terminals are time division multiplexed within the frame for transmission and so different levels of signals are received from the respective wireless communication terminals and a wireless communication control terminal. Therefore, even if the matched filter
152
receives the M sequence and yields the received output of the M sequence, this signal output may be hidden by other signal components. This will raise a problem that the M sequence signal cannot be detected.
Specifically, the received signal is subjected to an AGC (Automatic Gain Control) and controlled so that the received signal level may be constant. Thus, when any signal with an excessive amplitude is input immediately before the M sequence signal, then the AGC will act to reduce its gain. If the M sequence signal is received in a condition that the gain is reduced, the detected level of correlation signal of M sequence will be lowered and buried in a strong received signal, thus making it impossible to be detected. Moreover, even though only a portion near an area where the M sequence is received may be extracted, the detected level of correlation signal of M sequence will turn below the threshold value, thereby making it impossible to detect the M sequence signal.
In this way, where the plurality of signals with different signal levels are transmitted on the time division fashion, it is difficult to ensure the detection of M sequence signal with such a construction as compares the output of matched filter with a predetermined threshold value.
SUMMARY OF THE INVENTION
Therefore, an object of the present invention is to provide a correlation detecting apparatus which is capable of ensuring the detection of a code sequence even if a plurality of signals with different amplitudes are time division multiplexed.
The present invention provides a correlation detecting apparatus comprising a matched filter means for detecting a predetermined code from a received signal, an average amplitude detecting means for detecting an average amplitude of the received signal, a normalizer means for normalizing an output of the matched filter means by an output of the average amplitude detecting means and a comparator means for comparing the normalized output of the matched filter means by the normalizer means with a threshold value to output a correlation detecting signal and its method.
Further, the present invention provides a communication apparatus comprising an input means inputted with a received signal, a synchronizing signal detecting means for detecting a synchronizing signal contained in the received signal from the received signal, and a demodulating means
Hiraiwa Hisaki
Sugita Takehiro
Usui Takashi
Corrielus Jean
Maioli Jay H.
LandOfFree
Correlation detecting apparatus, its detecting method and... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Correlation detecting apparatus, its detecting method and..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Correlation detecting apparatus, its detecting method and... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2900298