Multiplex communications – Communication techniques for information carried in plural... – Combining or distributing information via frequency channels
Utility Patent
1998-03-09
2001-01-02
Ton, Dang (Department: 2732)
Multiplex communications
Communication techniques for information carried in plural...
Combining or distributing information via frequency channels
C370S503000
Utility Patent
active
06169751
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an OFDM (Orthogonal Frequency Division Multiplex) receiving apparatus, and particularly to a frame synchronizing circuit and a bit synchronizing circuit.
2. Description of the Background Art
In recent years, communication using OFDM (Orthogonal Frequency Division Multiplex) signal is attracting attention in the fields of digital sound broadcasting for the movable terminals and the terrestrial digital television broadcasting, etc. This is because the OFDM signal is capable of high-speed transmission of a large amount of data because of its superior frequency efficiency, is less prone to characteristic deterioration due to a reflected wave even in the absence of a waveform equalizer, and is also less prone to interfere with other services because of its signal waveform is close to random noise.
FIG. 7
is a diagram showing a configuration of an OFDM signal. An arrow GA indicates symbols of the OFDM signal S along the time base, and an arrow GB indicates the part a of the symbols indicated by the arrow GA in an enlarged manner. The OFDM signal S is formed of symbols Sm (m=1, 2, . . . ) arranged along the time base. Each symbol Sm is formed by digital-modulating (e.g., QPSK, 16 QAM, etc.) a plurality (several tens to several thousands, e.g., 512) of carriers differing in frequency (orthogonal to each other in the symbol time ts) with data to be transmitted and multiplexing the modulated carriers on the frequency base by an inverse FFT (Fast Fourier Transform) calculation. The symbols Sm therefore all exhibit random amplitude distribution as shown by the arrow GB. On the transmission path, this OFDM signal S takes the form of a complex signal with overlapping real and imaginary parts for each symbol Sm.
Such OFDM signal is transmitted from a transmitting party to a receiving party through wired or radio transmission path. On wired transmission path, the occupied frequency band is regulated by transmission characteristics of the transmission path. On radio transmission path, the occupied frequency band is regulated by law. Hence the transmitting party converts the OFDM signal from the intermediate frequency band to the occupied frequency band of the transmission path. When demodulating the data, the receiving party converts the received OFDM signal from the occupied frequency band of the transmission path to the intermediate frequency band for the demodulating process.
The received OFDM signal can be OFDM-demodulated with the aid of an FFT calculation. Accordingly, it is necessary for the OFDM receiving apparatus to regenerate the symbol timing of the OFDM symbols for the process of FFT calculation. For this purpose, generally, reference symbols for timing synchronization are periodically transmitted in OFDM transmission.
FIG. 8
shows a frame configuration of the OFDM signal S. A duration with no signal, called “a null symbol NS,” is provided at the head of a frame DFr, by detecting which frame synchronization is obtained. The null symbol NS is also used to detect rough synchronization of symbol timing. The null symbol NS is followed by a reference symbol RS for timing synchronization (hereinafter referred to as a synchronizing symbol), with which precise symbol timing is detected. The synchronizing symbol RS is followed by information symbols DS.
Conventional OFDM receiving apparatuses that detect synchronization by using such a frame configuration include one disclosed in a laid open Japanese Patent Publication No.8-265291. According to this example, two synchronization detecting symbols are required the null symbol NS and the synchronizing symbol RS. This reduces the amount of information symbols DS that can be transmitted, resulting in lowering of the transmission efficiency.
Furthermore, since the synchronizing symbol cannot be detected without establishing frame synchronization by detecting the null symbol NS, a doubled time is required to establish the synchronization.
In detecting the synchronizing symbol, it is necessary that the sample timing signal for analog-to-digital conversion has its frequency aligned with the clock frequency of data so that the data can be sampled at identify points and its phase precisely synchronized with that of the data. In the absence of the bit synchronization, the phase of the sample timing varies with respect to the received signal moment by moment. Then the original data may be converted into wrong data. If the digital data obtained by converting the received signal has errors, the synchronizing symbol may not be detected, or may be wrongly detected. Then the symbol timing cannot be obtained correctly.
Establishing the bit synchronization requires extracting the clock component of the received signal to obtain frequency alignment with the data clock by using a PLL (Phase-Locked Loop) circuit, etc., and obtaining phase alignment in the data sample timing.
However, it is difficult to extract the clock component from the received OFDM signal since its signal waveform is close to that of random noise. Accordingly, a clock must be regenerated by using only the synchronizing symbol to establish the bit synchronization.
SUMMARY OF THE INVENTION
The present invention has been made to solve the above-described conventional problems. A first aspect of the present invention is an OFDM receiving apparatus receiving an OFDM signal. The receiving apparatus comprises plural data symbols and plural reference symbols inserted in each of the data symbols at a predetermined interval based on a clock signal. The said apparatus comprises:
a timing generator for generating a first timing signal on the basis of a control signal;
a first synchronizing symbol detector for detecting the reference symbol in the OFDM signal based on the first timing signal to produce a first synchronizing symbol detection signal;
a second synchronizing symbol detector for detecting the reference symbol in the OFDM signal based on a second timing signal being delayed from the first timing signal by a predetermined time to produce a second synchronizing symbol detection signal; and
a decider for selecting which of the first and second synchronizing symbol detection signals are synchronized with the OFDM signal.
As apparent from the above, according to a first aspect of the present invention, it is possible to establish the frame synchronizing and the bit synchronization between the transmitted OFDM signal and received OFDM signal based on the reference symbols having predetermined patterned waveform but on the data symbols having random patterned waveforms.
According to a second aspect, in the first aspect of the present invention, an OFDM receiving apparatus, further comprises:
a synchronization detector for establishing frame synchronization based on the selected synchronizing symbol detection signals;
a phase comparator for comparing phases of the clock signal and the first timing signal to produce a phase error signal;
a hold unit for holding the phase error signal based on the selected synchronizing symbol detection signals to produce a hold signal; and
a loop filter for smoothing the output signal from the hold unit to output the control signal.
According to a third aspect, in the first aspect of the present invention,
the first synchronizing symbol detecting unit comprises:
a first sampling unit for sampling the OFDM signal based on the first timing signal to produce a first digitalized signal; and
a first synchronizing symbol detecting unit for detecting the reference symbol to produce the first synchronizing symbol detection signal based on the first digitalized signal; and
the second synchronizing symbol detector comprises a delayer for delaying the first timing signal by a predetermined time to produce a second timing signal;
a second sampler for sampling the OFDM signal based on the second timing signal to produce a digitalized signal; and
a second synchronizing symbol detector for detecting the reference symbol to produce the second synchronizing symbol detection signal based on
Harada Yasuo
Hayashino Hiroshi
Shirakata Naganori
Matsushita Electric - Industrial Co., Ltd.
Ton Dang
Wenderoth , Lind & Ponack, L.L.P.
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