Multiplex communications – Generalized orthogonal or special mathematical techniques – Particular set of orthogonal functions
Reexamination Certificate
2000-01-31
2004-03-23
Ngo, Ricky (Department: 2664)
Multiplex communications
Generalized orthogonal or special mathematical techniques
Particular set of orthogonal functions
C370S206000, C370S210000, C370S480000
Reexamination Certificate
active
06711123
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a receiving apparatus for an OFDM communication system used in mobile communication or the like. More particular, this invention relates to a receiving apparatus for performing timing synchronization on OFDM transmission by using specific known patterns included in a reception signal.
BACKGROUND OF THE INVENTION
A conventional receiving apparatus for an OFDM communication system will be explained below. For example, in wide-band mobile communication, arrival timing of a signal dynamically changes due to changes in the distances or the multi-path. In particular, since multi-carrier communication represented by OFDM (Orthogonal Frequency-Division Multiplexing) is sensitive to an error of reception timing, it is necessary to do some amends.
As literature related to the receiving apparatus for the OFDM communication system which performs some kind of compensations, for example, “A High Performance Frequency and Timing Synchronization Technique for OFDM, THE INSTITUTE OF ELECTRONICS, INFORMATION AND COMMUNICATION ENGINEERS, TECHNICAL REPORT OF IEICE., SSE98-21, RCS 98-21 (1998-04)” is known. The conventional art will be explained with reference to this literature.
FIG. 7
is a block diagram showing the configuration of a receiving apparatus in a conventional mobile radio communication system.
FIG. 8
shows a burst format of a conventional OFDM signal.
A reception signal (OFDM signal) S
11
received by a receiver is input into a correlation calculation unit
14
. In the reception signal S
11
, as shown in
FIG. 8
, subsequent to preamble written in a start portion of the burst, specific patterns each having a length B indicated by “pattern A” are repeatedly written (
FIG. 8
shows a case in which the patterns are continuously written for two times). In the correlation calculation unit
14
into which the reception signal is input, the correlation value of the section of the continuous lengths B is calculated while shifting the calculation position. More specifically, while the calculation position is shifted at every predetermined time, correlation between the earlier “pattern A” and the later “pattern A” is taken a plurality of times, and correlation value distribution information S
15
is output.
In a maximum correlation value searching unit
15
, a position indicating the maximum correlation value is selected from the obtained correlation value distribution information S
15
, and maximum correlation value position information S
16
is output. A timing adjustment unit
11
receives the reception signal S
11
and the maximum correlation value position information S
16
. This timing adjustment unit
11
determines a start position of a data portion in the reception signal S
11
based on the reception signal S
11
and the maximum correlation value position information S
16
, and outputs a data signal S
12
constituted by only the data portion to a time/frequency conversion unit
12
.
Since the OFDM signal, i.e., the data signal S
12
is multiplexed such that information is dispersed to a plurality of sub-carriers, the time/frequency conversion unit
12
converts the received data signal S
12
from a signal on a time axis into a signal on a frequency axis, and information S
13
on each sub-carrier is extracted. This extraction is performed using the FFT (Fast Fourier Transform).
Finally, the information S
13
converted into data on each sub-carrier is transmitted to a detection unit
13
. The detection unit
13
detects the information S
13
and demodulates the information S
13
to output reception information S
14
.
As described above, in the conventional receiving apparatus for an OFDM communication system, correlation between continuous repetitive patterns is used to detect a synchronization position, and a position where a reception signal having the maximum power is input in an ideal state being free from noise, i.e., a position indicating the maximum correlation value is estimated as the synchronization position.
However, in the conventional receiving apparatus for an OFDM communication system, since correlation between continuous repetitive patterns is used to detect a synchronization position, for example, as shown in
FIG. 9
, portions C in
FIG. 9
are the same as each other, and portions D and E are different from each other. Thus, a difference between correlation values sometimes becomes very small. Therefore, a method of estimating a synchronization position based on correlation value distribution information obtained by the calculation method is very poor at noise in signal reception, and preferable synchronization characteristics cannot be obtained in an actual communication environment in which multi-path fading or the like occurs.
In the conventional receiving apparatus for an OFDM communication system, as described above, a position where a reception signal having the maximum power is input is estimated as a synchronization position in an ideal state being free from noise. However, in an actual communication environment, it is unsure that the power of a preceding wave is always stronger than that of a delay wave. In general, in the OFDM communication system, as shown in
FIG. 10
, a redundant portion called a guard interval is formed ahead of the information portion. In the guard interval, the influence of interference caused by the delay wave is suppressed.
Therefore, although the influence of the delay wave in the guard interval is not very large, a received wave outside the guard interval serves as an interference component to adversely affect the operation of the OFDM communication system. Therefore, as shown in
FIG. 10
, in a channel having the characteristic that the power of a delay wave is higher than that of a preceding wave, a preceding wave (interference wave component) is arranged ahead of the guard interval, and the preceding wave degrades synchronization characteristics.
SUMMARY OF THE INVENTION
The present invention has been made in light of the problems described above. It is an object of the present invention to obtain a receiving apparatus which can obtain preferable synchronization characteristics even in a communication environment having high noise power.
According to one aspect of the present invention, unlike the conventional art where a correlation between continuous repetitive patterns is used to detect a synchronization position, the correlation between the continuous repetitive patterns is not used but signal patterns identical to specific known patterns are calculated. A calculation result is prepared in the apparatus in advance, and correlation calculation between a reception signal and the signal patterns is performed while shifting the position of the correlation calculation for every specific time. Further, a correlation value calculation unit for forming signal patterns identical to the known patterns in such a manner that an OFDM signal is converted from a frequency axis signal to a time axis signal is provided.
According to another aspect of the present invention, unlike the conventional art, a position where a reception signal having a maximum power (maximum correlation value) is input is not estimated as a synchronization position. For example, a intensity distribution of reception signals is calculated based on the plurality of obtained correlation values, a synchronization position where the power value of the reception signal arranged out of a guard interval is minimum is determined, and the synchronization position is set as a reference position of timing synchronization. In this manner, the maximum signal/interference wave power ratio can be realized for the reception wave, and preferable synchronization characteristics can be always obtained.
According to still another aspect of the present invention, unlike the conventional art where correlation between continuous repetitive patterns is used to detect a synchronization position, the correlation between the continuous repetitive patterns is not used but signal patterns identical to known specific multi-carrier time
Ha Yvonne Q.
Mitsubishi Denki & Kabushiki Kaisha
Ngo Ricky
Rothwell Figg Ernst & Manbeck
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