EP1825625A1

EP1825625A1 - Method for estimating frequency/time offset and apparatus using the same in ofdm communication system

Info

Publication number: EP1825625A1
Application number: EP05780670A
Authority: EP
Inventors: Hee-Wook Kim; Kun-Seok Kang; Do-Seob Ahn; Ho-Jin Lee
Original assignee: Electronics and Telecommunications Research Institute ETRI
Current assignee: Electronics and Telecommunications Research Institute ETRI
Priority date: 2004-12-14
Filing date: 2005-07-21
Publication date: 2007-08-29
Also published as: WO2006065016A1; EP1825625A4; KR20060067034A; KR100626644B1

Abstract

Provided is a frequency and time offset estimation method and apparatus using the same for an orthogonal frequency division multiplexing (OFDM) communication system. The invention can establish frequency and time synchronization while improving data transmission rate in the OFDM communication system by estimating a time offset and a decimal of frequency offset using a first OFDM symbol and a Cyclic Prefix (CP) of an OFDM frame and compensating a received signal based thereon in a time domain, and estimating an integer of the frequency offset using the compensated signal and again compensating the compensated received signal based on the estimated integer of the frequency offset in a frequency domain.

Description

METHOD FOR ESTIMATING FREQUENCY/TIME OFFSET AND APPARATUS USING THE SAME IN OFDM COMMUNICATION SYSTEM

Description Technical Field

The present invention relates to a frequency and time offset estimation method and apparatus using the method for an Orthogonal Frequency Division Multiplexing (OFDM) communication system. More particularly, the invention is directed to a frequency and time offset estimation method and apparatus using the same for an OFDM communication system for estimating a time offset and a decimal of frequency offset using a first OFDM symbol and a Cyclic Prefix (CP) of an OFDM frame and compensating a received signal based thereon in a time domain, and estimating an integer of the frequency offset using the compensated signal and again compensating the compensated received signal based on the estimated integer of the frequency offset in a frequency domain.

Background Art

An OFDM system used herein includes Orthogonal Frequency Division Multiple Access (OFDMA) system, Orthogonal Frequency and Code Division Multiplexing (OFCDM) system, and so on.

Recently, such OFDM, which is known as efficient digital transmission scheme in use of bands, has been applied to digital transmission systems such as European digital audio or video broadcasting, Asymmetric Digital Subscriber Line (ADSL), Wireless Local Area Network (WLAN), and the like.

The OFDM scheme is strong in Inter-Symbol Interference (ISI) that is the important issue in high speed communications and has a merit that allows frequency selective fading to be shown as frequency nonselective fading.

However, the OFDM scheme is sensitive to carrier frequency error caused by Doppler frequency variance due to oscillator mismatching or movement of transmitter and receiver, compared to a single carrier system.

This carrier frequency error brings about Inter- Carrier Interference (ICI) since it destroys the orthogonality between subcarriers in the OFDM system. Accordingly, the frequency synchronization problem that yields considerable degradation of performance by little carrier frequency error has been noted as one of most important issues in implementing the OFDM system. In addition, since there is not exited "eye opening" capable of finding optimal sampling time in the OFDM system, symbol synchronization therein is different from that in the single carrier system. In the OFDM system, time synchronization implies finding a start estimation value of one OFDM symbol. Generally, the OFDM system uses CP and thus is less sensitive to symbol synchronization error; but has to estimate it not to escape from the error within CP.

Meanwhile, the conventional OFDM synchronization methods are largely classified into a pilot-aided method and a blind method.

The pilot-aided method employs two pilot symbols to estimate all of time synchronization and an integer and a decimal of frequency offset; but decreases data rate owing to the use of the two pilot symbols and causes the ambiguousness of time synchronization estimation corresponding to the length of CP in time synchronization.

The blind method estimates time synchronization and decimal of frequency offset by using CP, wherein the correlation between OFDM symbol and CP portions is employed. Therefore, the blind method has a drawback that an integer portion of frequency offset cannot be estimated.

Moreover, a shortcoming is that the blind method cannot learn the start point of frame although the symbol synchronization is established.

Disclosure Technical Problem

It is, therefore, an object of the present invention to provide a frequency and time offset estimation method and apparatus using the same in an OFDM communication system which is capable of establishing frequency and time synchronization while improving data transmission rate by estimating a time offset and a decimal of frequency offset using a first OFDM symbol and a CP of an OFDM frame and compensating a received signal based thereon in a time domain, and estimating an integer of the frequency offset using the compensated signal and again compensating the compensated received signal based on the estimated integer of the frequency offset in a frequency domain.

The other objectives and advantages of the invention will be understood by the following description and will also be appreciated by the embodiments of the invention more clearly. Further, the objectives and advantages of the invention will readily be seen that they can be realized by the means and its combination specified in the claims.

Technical Solution

In accordance with one aspect of the present invention, there is provided a frequency and time offset estimation method for an orthogonal frequency division multiplexing (OFDM) communication system, the method comprising the steps of: calculating a time offset allowing a Log- Likelihood (LL) function to have a maximum value when an OFDM signal is received in a time domain; calculating a decimal of frequency offset making the LL function to have the maximum value using the time offset; compensating a received signal based on the calculated time offset and decimal of the frequency offset; converting the compensated received signal into a corresponding frequency domain signal; calculating an integer of the frequency offset allowing a metric function to have a minimum value; and compensating the compensated received signal again using the integer of the calculated frequency offset.

In accordance with another aspect of the present invention, there is provided a frequency and time offset estimation apparatus for an OFDM communication system, the apparatus comprising: a first offset calculation means for calculating a time offset and a decimal of frequency offset allowing an LL function to have a maximum value in a time domain; a first compensation means for compensating an OFDM signal based on the time offset and decimal of the frequency offset calculated by the first offset calculation means; a signal conversion means for converting the OFDM received signal compensated at the first compensation means into a corresponding frequency domain signal; a second offset calculation means for calculating an integer of the frequency offset making a metric function to have a minimum value using the frequency domain signal converted by the signal conversion means; and a second compensation means for compensating the compensated received signal again using the integer of the frequency offset calculated by the second offset calculation means.

Advantageous Effects

The present invention can improve the data transmission rate and synchronize the frequency and time by estimating a time offset and a decimal portion of frequency offset using a first OFDM symbol and CP of OFDM frame and compensating a received signal based thereon in a time domain, and estimating an integer portion of the frequency offset using the compensated signal and again compensating it based on the estimated integer portion of the frequency offset in a frequency domain.

Description of Drawings

The above and other objects and features of the present invention will become apparent from the following description of the preferred embodiments given in conjunction with the accompanying drawings, in which:

Fig. 1 is a diagram showing a structure of OFDM frame in a time domain and structure of OFDM symbol in a frequency domain in accordance with a preferred embodiment of the present invention; Fig. 2 is a diagram showing a procedure of estimating a decimal of frequency offset and a time offset in an OFDM communication system in accordance with a preferred embodiment of the invention;

Fig. 3 is a diagram showing a procedure of estimating an integer of frequency offset in an OFDM communication system in accordance with a preferred embodiment of the invention;

Fig. 4 is a flowchart describing a frequency and time offset estimation method for an OFDM communication system in accordance with a preferred embodiment of the invention; and

Fig. 5 shows a block diagram of a frequency and time offset estimation apparatus for an OFDM communication system in accordance with a preferred embodiment of the invention. Best Mode for the Invention

The above-mentioned objectives, features, and advantages will be more apparent by the following detailed description associated with the accompanying drawings; and based on this, the invention will be readily conceived by those skilled in the art to which the invention pertains. Further, in the following description, well-known arts will not be described in detail if it seems that they could obscure the invention in unnecessary detail. Hereinafter, a preferred embodiment of the present invention will be set forth in detail with reference to the accompanying drawings.

Fig. 1 is a diagram showing a structure of OFDM frame in a time domain and a structure of OFDM symbol in a frequency domain in accordance with a preferred embodiment of the present invention.

As shown in Fig. 1, the OFDM frame in a time domain in accordance with the invention includes a first OFDM symbol 11, a second OFDM symbol, a third OFDM symbol, etc.

In this structure, the other OFDM symbols excluding the first OFDM symbol 11 are transmitted in the same manner as the prior art. That is, OFDM symbols are arranged in such a way that data is not transmitted onto the remaining subcarriers as virtual carriers shown in dotted lines following the symbols but transmitted onto only ^" subcarriers among a total of N subcarriers so as to overcome the nonlinearity of filter.

In the meantime, the first OFDM symbol is transmitted to have a recursive structure. Namely, data is carried on only odd subcarriers of the first OFDM symbol 12 and then transmitted, but transmitted without its carrying on even subcarriers. By doing so, the recursive structure is implemented. In the foregoing, since no data is transmitted onto the half subcarriers of the first OFDM symbol, data with double power in the frequency domain is transmitted for the first OFDM symbol to maintain a constant signal power in the time domain.

Fig. 2 is a diagram showing a procedure of estimating a decimal of frequency offset and a time offset in an OFDM communication system in accordance with a preferred embodiment of the invention.

As illustrated therein, the first OFDM symbol has a recursive structure of A and B portions by the recursive structure and redundancy of CP.

First of all, the present invention employs an observation window capable of observing 2N+L sample size for time synchronization. Here, N implies a size of OFDM symbols and L indicates a length of CP. The correlation between samples observed via the observation window may be represented by the following equation:

wherein eans a kth observation sample, represents a signal power, indicates a noise power, and ^ε implies a frequency offset.

The probability distribution function of sample vector composed of 2N+L samples derived by using Eq. (1) above may be defined as Eq. (2) below. The remaining portion excluding Log-Likelihood (LL) function portion in the probability distribution function has a constant value, regardless of a value to be estimated.

where r stands for a sample vector, " means a time offset, and ^εindicates the frequency offset as defined above.

On the other hand, the LL function that is the function of ^θ , ^s and r may be represented as Eq. (3) below by taking log in Eq. (2) above and using dependant portion of samples for Maximum Likelihood (ML) estimation. The ML estimation implies how to find parameter values when Eq. (3) below has a maximum value.

Eq. (3) above may be modified to derive ^θ , ^e having the maximum value as:

And an estimation value of decimal of frequency offset and an estimation value of time offset using Eq. (4) above may be calculated by the following equations:

Then, are defined as follows

and may be represented in a recursive way as:

Meanwhile, since finding allowing the LL function to have the maximum value in Eq. (6) above needs much time, it may be derived by setting a threshold * suitable for the system requirements as follows:

At a next process of the invention, a received signal is compensated based on the time offset and the decimal of the frequency offset as derived above and then converted into a corresponding frequency domain signal using, e.g.,

Fast Fourier Transform (FFT), in order to recovery data.

Fig. 3 is a diagram showing a procedure of estimating an integer of frequency offset in an OFDM communication system in accordance with a preferred embodiment of the invention.

As shown in Fig. 3, in the first OFDM symbol 31 before transmission, data is carried on only odd carriers ' and there exists virtual carrier to overcome the nonlinearity of filter.

In the first OFDM symbol which is formed by receiving it and compensating time synchronization and integer of frequency offset, the position of virtual carrier indicated by dotted lines is moved by the offset of the integer portion due to the integer of frequency offset.

Accordingly, the integer of frequency offset can be found by detecting the position of the virtual carrier. In other words, since no data is transmitted onto the virtual carrier, the present invention uses the fact that only noise is received thereby lowering the reception power.

When ^l indicates a sample value corresponding to an ith subcarrier, metric M(i) to detect the position of the virtual carrier may be defined as Eq. (10) below. Deriving i value to minimize the same may estimate the integer of the frequency offset. In Eq. (10) below, Q means the number of the virtual carriers.

wherein, if the threshold ' is set to shorten the detection time, the above equation may be represented as:

Fig. 4 is a flowchart illustrating a frequency and time offset estimation method for an OFDM communication system in accordance with a preferred embodiment of the invention.

First, when an OFDM signal is received in a time domain, the process of the invention calculates a time offset ^ allowing an LL function to have a maximum value (401). To be more specific, and are obtained to derive the LL function. Then, it is compared with the threshold to confirm whether the LL function has the maximum value at for ML estimation. In the comparison, if i is greater than the threshold, " at that time is a value corresponding to the time offset to derived; and if it is less than the threshold, the value is obtained by repeatedly performing the above step after settin until finding ^ greater than the threshold.

Next, the process calculates a decimal f the frequency offset allowing the LL function to have the maximum value using the time offset (402).

Thereafter, the process compensates the received signal based the time offset and the decimal of the frequency offset to estimate an integer of the frequency offset (403). Subsequently, the process converts the compensated received signal into a corresponding frequency domain signal using, e.g., FFT (404). Performing FFT can simply find the integer of the frequency offset since OFDM symbol in the frequency domain is shifted by the integer of the frequency offset that presents after FFT.

Next, the process derives the integer of the frequency offset allowing the metric function M(i) to have a minimum value (405). Namely, since the power of the virtual carriers is relatively lower than that of other carriers owing to the non-transmission of data onto the virtual carrier, position with minimum power sum value is estimated by moving an observation window by the number of virtual carriers . Specifically, the metric M(i) is first derived by using the frequency domain reception signal Then, it is compared with the threshold to conf whether the metric function has the minimum value at to estimate the integer of the frequency offset. In the comparison, if it is less than the threshold, i at that time is a value corresponding to the integer of the frequency offset to be derived; and if it is greater than the threshold, i is obtained by repeatedly performing the above step after setting until finding i less than the threshold.

And then, the process again compensates the received signal that is made by compensating the time offset and the decimal of the frequency offset by using the integer of the frequency offset as obtained above (406). Through the above processes, a received signal or data can be detected.

Fig. 5 illustrates a block diagram of a frequency and time offset estimation apparatus for an OFDM communication system in accordance with a preferred embodiment of the invention. As shown in Fig. 5, the inventive frequency and time offset estimation apparatus for the OFDM communication system comprises a first offset calculator 51 for calculating a time offset and a decimal of frequency offset allowing an LL function to have a maximum value in a time domain, a first compensator 52 for compensating an OFDM signal based on the time offset and the decimal of the frequency offset calculated at the first offset calculator 51, an FFT unit 53 for converting the compensated OFDM signal from the first compensator 52 into a frequency domain signal, a second offset calculator 54 for calculating an integer of the frequency offset allowing a metric function to have a minimum value using the frequency domain signal converted by the FFT unit 53, and a second compensator 55 for compensating the OFDM signal compensated at the first compensator 52 again using the integer of the frequency offset calculated by the second offset calculator 54. In this structure, the first offset calculator 51 includes a multiplier 511 by multiplying the OFDM signal by a given coefficient, a delay 512 for delaying by an index Z, a conjugate processor 513 for converting a sign of imaginary component of complex number, a counter 514 for adding N/2-L number of inputted values and subtracting a value therefrom when it is outputted and again adding a new value thereto when it is received, a detector 515 for detecting * having the maximum value, a calculator 516 for deriving an angle of **™' radian unit, and a square value calculator 517 for calculating a square value of envelop of inputted signal. In addition, the second offset calculator 54 includes a signal extractor 541 for extracting odd subcarrier signals since the first OFDM symbol is carried and transmitted onto the odd subcarriers, and a detector 542 for detecting i allowing the metric function M(i) to have the minimum value.

The method of the present invention as mentioned above may be implemented by a software program and stored in a computer-readable storage medium such as CD-ROM, RAM, ROM, floppy disk, hard disk, optical magnetic disk, etc. This process may be readily carried out by those skilled in the art; and therefore, details of thereof are omitted here. While the present invention has been described with respect to certain preferred embodiments, it will be apparent to those skilled in the art that various changes and modifications may be made without departing from the scope of the invention as defined in the following claims.

Claims

What is claimed is:

1. A frequency and time offset estimation method for an orthogonal frequency division multiplexing (OFDM) communication system, the method comprising the steps of: calculating a time offset allowing a Log-Likelihood (LL) function to have a maximum value when an OFDM signal is received in a time domain; calculating a decimal of frequency offset making the LL function to have the maximum value using the time offset; compensating a received signal based on the calculated time offset and decimal of the frequency offset; converting the compensated received signal into a corresponding frequency domain signal; calculating an integer of the frequency offset allowing a metric function to have a minimum value; and compensating the compensated received signal again using the integer of the calculated frequency offset.

2. The method as recited in claim 1, wherein the time offset calculating step includes the steps of: applying "0" to the LL function as the time offset; comparing the result of the LL function with a first threshold; if the LL function result is greater than the first threshold, calculating the applied value as the time offset; and if the LL function result is not greater than the first threshold, repeatedly performing said steps after increasing the applied value by "1" in sequence.

3. The method as recited in claim 1, wherein the frequency offset integer calculating step includes the steps of: applying "0" to the metric function as the integer of the frequency offset; comparing the result of the metric function with a second threshold; if the metric function result is not greater than the second threshold, calculating the applied value "0" as the integer of the frequency offset; and if the metric function result is greater than the second threshold, repeatedly performing said steps after increasing the applied value by "1" in sequence.

4. The method as recited in claim 1, wherein the OFDM signal contains OFDM symbols with recursive structure.

5. The method as recited in claim 1, whe the LL function uses Eq. (1) to calculate the decimal of the frequencv offset and Eq. (2) to calculate an estimation value f the time offset as follows:

A(&), B(θ), C(ff), D(θ), X(θ), Y(θ) where are defined as:

nts a sample vector, "means a time offset, implies a frequency offset, N stands f size of OFDM symbols, L indicates a length of CP, implies a kth observation sample, means a signal power, and indicates a noise power.

6. The method as recited in claim 1, wherein the metric function M(i) is defined as follows:

wherein ^'' indicates a frequency domain signal, Q implies the number of virtual carriers, and represents a power of signal.

7. A frequency and time offset estimation apparatus for an OFDM communication system, the apparatus comprising: a first offset calculation means for calculating a time offset and a decimal of frequency offset allowing an LL function to have a maximum value in a time domain; a first compensation means for compensating an OFDM signal based on the time offset and decimal of the frequency offset calculated by the first offset calculation means; a signal conversion means for converting the OFDM received signal compensated at the first compensation means into a corresponding frequency domain signal; a second offset calculation means for calculating an integer of the frequency offset making a metric function to have a minimum value using the frequency domain signal converted by the signal conversion means; and a second compensation means for compensating the compensated received signal again using the integer of the frequency offset calculated by the second offset calculation means.

8. The apparatus as recited in claim 7, wherein the apparatus performs the method of any one of claims 1 to 3.