Multiplex communications – Communication techniques for information carried in plural... – Combining or distributing information via frequency channels
Reexamination Certificate
1998-08-12
2002-03-26
Vu, Huy D. (Department: 2731)
Multiplex communications
Communication techniques for information carried in plural...
Combining or distributing information via frequency channels
C375S326000, C375S354000
Reexamination Certificate
active
06363084
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to frequency synchronization for an Orthogonal Frequency Division Multiplexing (OFDM) receiver, and more particularly to a method for estimating a coarse frequency offset within ±½ range of intercarrier spacing and an apparatus employing the same.
2. Description of the Prior Art
In an OFDM method, serially-inputted symbol streams are divided into unit blocks. The symbol streams of each unit block are converted into N number of parallel symbols. After the conversion, these symbols are multiplexed and added by using a plurality of subcarriers having different frequencies, respectively, according to Inverse Fast Fourier Transform (IFFT) algorithm and transmitted via the channel. That is, the N number of parallel symbols are defined as one unit block, and each subcarrier of the unit block has an orthogonal characteristic, which does not have an influence on subchannels. Consequently, in the OFDM method, the Inter-Symbol Interference (ISI) caused by multi-path fading can be reduced by increasing symbol period in proportion to the number of subchannels (N) while maintaining the same symbol transmission rate as that of a single carrier transmission method. Especially, a guard interval is inserted between the transmitted symbols to enhance the capability of the ISI reduction. As a result, a channel equalizer of simplified structure can be implemented.
In the OFDM method, a symbol is detected according to each subchannel. In presence of frequency offset, that is, the difference of carrier frequencies between the transmitter and the receiver, an orthogonal characteristic between each subcarrier frequency is not maintained, causing an interference between adjacent subchannels. Particularly, each subcarrier is closely distributed within a band as the number of OFDM subchannels increase, such that the interference between adjacent subchannels is generated even at a small frequency offset value.
However, the classical frequency synchronization method used in single carrier systems can not be applied to OFDM systems because of the non-linearity of the Fast Fourier Transform (FFT) used in the receiver. For this reason, some methods have been proposed to apply some successive offsets on a rotator, to scan a predefined range and detect the final value to compensate the effect of frequency offset when the reference carriers are correctly positioned.
The method using the rotator above has some disadvantages that the total processing time is increased due to the time necessary for obtaining the frequency offset value within a given range on the time domain prior to the FFT processing. That is, a differential decoding method is used between the current and the previous OFDM symbols to obtain the frequency offset value. Since the differential decoding is performed, 2S+1 number of continuous offset values within the range of {−S;S} (here, S refers to a predetermined maximum frequency offset value) applied to the rotator are maintained during at least two OFDM symbols. Accordingly, in the rotator, the time necessary for scanning subcarriers within the range {−S;S} is the time necessary for calculating 2S+1 number of estimation values, {&egr;
−s
, &egr;
−s+1
, . . . , &egr;
s−1
, &egr;
s
}, namely the time necessary for processing of 2×(2S+1) number of OFDM symbols.
Typically, in terrestrial television broadcasting, transmission signal is composed of frames and each frame includes 68 OFDM symbols. Each symbol is composed of K=6817 carriers (K refers to the number of transmission carrier) in a 8K mode, or K=1705 carriers in a 2K mode. At this time, the delay for obtaining the frequency offset is about 70 OFDM symbols in the 2K mode, or 274 OFDM symbols in the 8K mode. As described above, since it takes more than one OFDM frame (68 OFDM symbols) to obtain the frequency offset, that causes a considerable processing delay in synchronization process.
Further, since the residual offset can be generated within ±½ range of subcarrier when performing the scanning in each subcarrier area, a fine frequency synchronization process posterior to the coarse frequency synchronization is performed. In processing of obtaining the coarse frequency offset, when the frequency offset value is close to ½ or −½ range of intercarrier spacing from the reference signal, the following fine frequency synchronization may be deviated. Accordingly, in order to obtain the accurate frequency synchronization, the second scanning shall be performed in smaller step sizes around the coarse frequency offset value, and as a result, the processing delay is again increased.
SUMMARY OF THE INVENTION
In view of the foregoing, it is an object of the present invention to provide a method for estimating a frequency offset value, which is not accessed to ±½ of intercarrier spacing after direct obtaining of offset values within a traceable range of the frequency domain posterior to a FFT processing by using the reference carrier signal inserted within an OFDM frame in an OFDM receiver.
It is another object of the present invention to provide an apparatus for estimating the coarse frequency offset in an OFDM receiver.
In order to achieve the first object, the present invention provides a method for estimating a coarse frequency offset from an OFDM signal in which a reference signal is inserted at L number of carrier positions within each symbol and information data are inserted at the remaining carrier positions in an OFDM receiver, comprising the steps of: a) performing differential decoding process for two adjacent OFDM symbols obtained by performing FFT on the OFDM signal; b) obtaining 2S+1 number of candidate offset values {&egr;
−s
; &egr;
s
}, which are present on a predetermined range {−S; S} by using L number of differential decoded values at the same position from the reference signal among differential decoded values obtained from the step a); and c) estimating a coarse frequency offset value according to positions on which a first and a second maximum values among the 2S+1 number of candidate offset values obtained from the step b) are present.
In order to achieve the second object, the present invention provides an apparatus for estimating a coarse frequency offset from an OFDM signal in which a reference signal is inserted at L number of carrier positions within each symbol and information data are inserted at the remaining carrier positions in an OFDM receiver, comprising: an input buffer for receiving continuous OFDM symbols in unit of sample clock on the frequency domain obtained by performing FFT on the OFDM signal, delaying them as much as symbol length, and outputting delayed symbols in unit of sample clock; a complex multiplier for performing differential decoding process for a previous OFDM sample outputted from the input buffer and a current OFDM sample; a storing unit for receiving serially and storing differential decoded values outputted from the complex multiplier and outputting them at random; an address controller for generating address and control signals to control the storing unit; a frequency offset acquisition unit for summing up L number of differential decoded values at the same position from the reference signal outputted from the storing unit by each sample clock, calculating an absolute value of the summed value, and generating 2S+1 number of candidate offset values &egr;
i
, where &egr;
i
refers to an integer of −S≦i≦S, and S refers to a predetermined maximum frequency offset; and a frequency offset tracking unit for checking the 2S+1 number of candidate offset values obtained from the frequency offset acquisition unit and determining the coarse frequency offset according to positions of the first and the second maximum values.
REFERENCES:
patent: 5802117 (1998-09-01), Ghosh
patent: 6035003 (2000-03-01),
Daewoo Electronics Co. Ltd.
Pillsbury & Winthrop LLP
Vu Huy D.
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