WO2004028105A1

WO2004028105A1 - An orthogonal frequency division multiplexing system

Info

Publication number: WO2004028105A1
Application number: PCT/KR2002/002398
Authority: WO
Inventors: Chang-Bok Joo; Ki-Yeul Han
Original assignee: Chang-Bok Joo; Ki-Yeul Han
Priority date: 2002-09-18
Filing date: 2002-12-20
Publication date: 2004-04-01
Also published as: KR100471538B1; KR20040024987A; AU2002359017A1

Abstract

An apparatus and a method for estimating a channel and determining a symbol timing for FFT windowing in an OFDM system which appends channel estimation PN codes as a preamble to a periodic symbol structure of a frame unit and allows a fast channel estimation by simplifying a hardware structure to estimate multi-paths of channel coefficients by using a correlation of the PN codes as well as determines an accurate symbol synchronization timing from the estimated channel coefficients and improves an equalization performance by utilizing values, obtained by fast Fourier transforming the estimated channel coefficients, as weight coefficient of one-tap equalization.

Description

AN ORTHOGONAL FREQUENCY DIVISION MULTIPLEXING SYSTEM

Field ofthe Invention

The present invention relates to an orthogonal frequency division multiplexing (OFDM) system; and, more particularly, to an apparatus and a method for estimating a channel and determining a symbol timing for FFT windowing control in an OFDM system which appends channel estimation PN codes as preamble to a signal having a periodic symbol structure of a frame structure such as an OFDM signal, allows real time channel estimation by simplifying a hardware structure for estimating multi-path channel coefficients by using an auto-correlation of the PN codes, determines an accurate symbol timing from the estimated channel coefficients and improve an equalization performance by utilizing values obtained by fast Fourier transforming the estimated channel coefficients as weight coefficient of one- tap equalization.

Background ofthe Invention

Generally, an orthogonal frequency division multiplexing (OFDM) is a communication technique that realizes a fast digital signal transmission against to multi-path by achieving a multi-carrier of data blocks using a fast

Fourier transform (FFT) and an inverse fast Fourier transform (IFFT) and inserting a guard interval (GI) for reducing the effects of inter-symbol

^• interference due to a multi-path delay.

However, it is impossible that channel equalization is realized by only a linear equalization method since inter-symbol interferences are occurred in a channel at which a delay profile can exceed the GI due to an affection of reflection by such as impedance mismatch or loading effect of electric apparatus connected to terminals of transmission line such as a power line communication.

Accordingly, a channel estimation method, capable of accurately knowing a channel coefficient as a delay profile of the channel, must be acquired for channel equalization inducing interferences between symbols.

A channel estimation and equalization method in a conventional OFDM system utilizes one-tap equalization method which multiplies symbol sample values of FFT output for the data symbol of the receiving end with a weighted coefficient value which is the symbol sample value of the FFT output in the symbol interval of a preamble is stored at a weighted coefficient storing block by considering an inverse of a value divided by a symbol sample value of the preamble or a sample value of a pilot signal as the weighted coefficient value, on the assumption that inter-symbol interference does not occur since a delay profile of channel is shorter than a guard interval (GI).

And also, although channel estimation studies have been implemented by using a signal process algorithm according to various reference documents, it still remains as a difficult problem to perform channel estimation with a high accuracy at a high speed in real time.

That is, in the method according to the conventional signal processing algorithm, generally it is difficult to apply it as a real time, high speed estimation method of channel coefficients since a number of multiplication and summation or division and inverse matrix operation must be iteratively operated from several times to dozens of times or hundreds of times until the algorithm converges.

SUMMARY OF THE INVENTION It is an object of the present invention to provide an apparatus and a method for estimating a channel and determining symbol timing for FFT windowing control in an orthogonal frequency division multiplexing (OFDM) system which enables precision channel equalization by means of a method for estimating channel coefficients at a high speed in a time domain even for a channel having a long delay profile which exceeds a guard interval (GI) in a multi sub-carrier system such as an OFDM and at the same time which simplifies a hardware structure for channel estimation and a symbol timing determination from estimated channel coefficients.

Also, this present invention provides an apparatus and a method for estimating a channel coefficients and determining symbol timing in an OFDM system with a hardware of a simple structure by allowing an operation of a correlator to be composed of only summation operation. In accordance with one aspect of the present invention, the apparatus and method for estimating a channel and determining symbol timing in an OFDM system are characterized in that: a transmitting end generates N_D -1 numbers of PN code symbol samples, i.e., channel estimation PN codes with a period of N_D -1 at front end of each frame of the OFDM signal at data sample intervals for 3 to 5 periods and the receiving end estimates M number of channel coefficients by a correlation operation between received signal of the preamble period and M number of PN codes circulated by one bit in order which is stored in the memory block ofthe receiving end. At this time, the apparatus and method for estimating a channel and determining symbol timing in an OFDM system is characterized in that: a correlator operates only (N_D^Λ-1)*M times of summation operations since a correlation of PN codes is applied in estimating channel coefficients.

And, the present invention can estimate channel coefficients with a high accuracy in real time for a power line communication with a long delay profile which exceeds a GI of a symbol in the OFDM system. Also, it can determine an accurate symbol timing by determining the first or peak value position of estimated channel coefficients for symbol timing determination. Moreover, it can allow data to be transmitted at a high speed by improving a channel equalization performance using the estimated channel coefficients as a weighted coefficient value of one-tap equalization by fast Fourier transforming the estimated channel coefficients.

In addition, the present invention is capable of one-tap equalization in frequency domain by allowing a channel characteristics to be exactly known in real time and at a high speed even in the channel environment at which a delay profile exceeds a GI.

BRIEF DESCRIPTION OF THE DRAWINGS The above objects, features and advantages of the present invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings, in which:

Fig. 1 is a block diagram showing an OFDM signal frame in accordance with the present invention; Fig. 2 depicts the construction of transmitting and receiving ends of the OFDM system utilizing correlation characteristics of PN codes in accordance with the present invention;

Fig. 3 shows a block diagram of a channel coefficient estimation block and a timing generation block for symbol synchronization shown in Fig. 2; Fig. 4 is a view explaining the correlation operation of a correlator shown in Fig. 3; and

Fig. 5 represents a channel profile for an estimation example and estimation error.

* Reference numbers of main units in the drawings *

20: channel coefficient estimation block

21 : circulated PN code memory block

22: correlator 23 : threshold setting block

24: threshold comparison and channel coefficient output block 30: symbol synchronization timing generation block

31 : peak value position extraction block

32: symbol timing determination block

33: FFT window control block DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT A preferred embodiment of the present invention will now be described with reference to the accompanying drawings.

Fig. 1 is a block diagram showing an OFDM signal frame in accordance with the present invention; Fig. 2 depicts the construction of transmitting and receiving ends of the OFDM system utilizing correlation characteristics of PN codes in accordance with the present invention; and Fig. 3 shows a block diagram of a channel coefficient estimation block and a symbol synchronization timing generation block shown in Fig. 2. The OFDM system employs a method that processes a data block in parallel and transmits the data block in parallel in a number of sub-carriers orthogonal to each other. In this method, the generation of OFDM signals is obtained by converting a series of data into a parallel data through a serial-to- parallel (S/P) converter 1 at the transmitting end shown in Fig. 2 and processing them at an inverse fast Fourier transform (IFFT) block 3.

At a receiving end, a base band signal from the transmitting end is converted into a digital signal by an analog-to-digital converter (ADC) 11. Thereafter, the data from the transmitting end is recovered by demodulating of a signal demapping (demodulation) block 15 and data decoding of a data decoder 17 after a symbol is recovered by a process of a fast Fourier transformer (FFT) 14.

As shown in Fig. 1, a frame of an OFDM signal in the present invention includes PN code symbols for channel estimation and OFDM data symbols. One symbol of the OFDM signal is composed of N_G^Λ number of samples of guard intervals and N_D' number of samples of data sample intervals, that is, N_G + N_D^Λ number of samples. The GI for preventing an inter-symbol interference is created by prefix N_G number of rear end portion samples of the data sample to front end portions of the data samples, and generally the GI is 4-6 times longer than the average delay spread time of the channel and the data sample interval is 4-5 times longer than the GI.

As shown in Fig. 2, in the present invention, the transmitting end includes a PN code generation block 5 for estimating the channel coefficient in time domain and generating channel estimation PN codes at a symbol sample rate(interval) at the front ends of each frame of the OFDM signals. And, the receiving end includes a channel coefficient estimation block 20 for calculating estimated channel coefficients by a correlation between M number of circulated PN codes at a memory block in the unit of PN code length by receiving the PN codes of the transmitting end, a timing generation block 30 for use in a symbol synchronization for determining a symbol synchronization timing as a position of a periodic peak channel coefficient value appearing by the periodicity of correlation between the PN codes among the estimated channel coefficients calculated from the channel coefficient estimation block 20, a weighted coefficient storing block 18 and one-tap equalizer 19 for utilizing the channel coefficients calculated from the channel coefficient estimation block 20 as weighted coefficient values for one-tap equalization by the FFT.

That is, at the transmitting end, channel estimation PN codes having a period of N_D -1 number are generated at a data sample interval by 3 to 5 cycles at front ends of each frame of OFDM signals through the PN code generation block 5 for channel coefficient estimation.

At the receiving end, M number of PN codes (transmitted PN code^'s and M-1 number of PN codes circulated by a bit in turn by corresponding these PN codes to the delay profile and M being the number of channel coefficients to be estimated: M ="N_D) stored at the memory block of the receiving end takes a correlation with respect to PN codes received through the channel coefficient estimation block 20, wherein the obtained M number of correlation values become estimated channel coefficients.

The received preamble symbol with PN code is composed of an overlap of transmitted PN codes delayed by M-1 sample time by M number of paths, by a delay profile of the channel. There are received signals being received by a first path, a second path delayed by one sample interval and a third path delayed by two sample intervals. And also, the transmitted PN codes received at each path are received with a magnitude and a phase corresponding to the delay profile.

Therefore, in the channel coefficient estimation block 20, since a correlation value between the overlapped PN codes of the receiving end and the non-circulated PN codes ofthe memory block has an auto-correlation with respect to the first path receiving PN code and a cross-correlation with respect to the other delay receiving PN codes, the cross-correlation value becomes 0 by the correlation of the PN code. Therefore, the auto-correlation value corresponding to the first path becomes a first channel coefficient value corresponding to a direct path at the receiving delay profile.

Similarly, since a correlation between the overlapped PN codes of the receiving end and the 1-bit circulated PN code of the memory block has an auto-correlation with respect to the second path receiving PN code and a cross-correlation with respect to the other delay receiving PN codes, the correlation becomes a second channel coefficient value corresponding to the second path at the delay profile by the correlation ofthe PN codes.

By the same method, since a correlation between the overlapped PN codes of the receiving end and the 2-bit circulated PN code has an auto- correlation with respect to the PN code received through the third path and a cross-correlation with respect to the other PN code received after a delay, the correlation becomes a third channel coefficient value corresponding to the third path at the delay profile by the correlation ofthe PN codes.

By using the above-described method, M numbers of channel coefficients are estimated by a correlation between the overlapped PN codes ofthe receiving end and M number of sequentially circulated PN codes ofthe memory block.

A correlation between a noise sample additionally received through the channel during the above correlation processes and the circulated PN codes of the memory block becomes a cross-correlation and the cross-correlation value influences on an estimation accuracy ofthe channel coefficient value.

As described above, the present invention allows channel equalization with respect to the transmission channel exceeding the GI of the symbol causing inter-symbol interferences in an OFDM system. For this purpose, as shown in Fig. 1, the transmitting end includes a PN code generation block 5 for channel coefficient estimation to generate channel estimation PN codes with a period of NJD'-l numbers at front end of each frame of the OFDM signals at a data sample interval for 3 to 5 cycles. Whereas, as shown in Fig. 2, the receiving end includes a channel coefficient estimation block 20 containing M number of circulated PN codes obtained by circulating the transmitted PN code by one bit in order and generates the estimated channel coefficients to input terminals of the timing generation block 30 for use in a symbol synchronization and the FFT block 14. At this time, the output of the FFT 14 is stored at the weighted coefficient-storing block 18 for use in one-tap equalizer 19 for the channel equalization for the OFDM data symbols.

As shown in Fig. 3, the channel coefficient estimation block 20 includes a circulated PN codes memory block 21 for storing M number of circulated PN codes obtained by circulating the PN code of the transmitting end by 1 bit in order, a correlator 22 for calculating M number of correlation values by performing a correlation between the PN codes of frame of the transmitted OFDM signal converted at the ADC 11 of the receiving end and the M number of circulated PN codes obtained by circulating the transmitted PN code by 1 bit in order which stored at the circulated PN code memory- block 21, a threshold setting block 23 for setting a threshold level to determine the channel coefficient and a threshold level comparison and channel coefficient output block 24 for finally outputting the estimated channel coefficient values by comparing the M number of estimation channel coefficient calculated at the correlator 22 with the threshold set at the threshold setting block 23.

The timing generation block 30 for use in a symbol synchronization includes a peak value position extraction block 31 for extracting positions of the periodic peak channel coefficient values appeared by the periodicity of correlation of PN code among the estimated channel coefficients calculated from the channel coefficient estimation block 20, a symbol timing determination block 32 for determining the positions of the peak channel coefficient values extracted from the peak value position extraction block 31 as a symbol synchronization timing and an FFT window control block 33 for outputting to a GI removing block 12 a GI removing control signal to remove the GI in response to the symbol timing determined from the symbol timing determination block 32.

The correlation operation of PN codes according to the present invention is described with reference to Fig. 4 as follows.

The OFDM signals are received through the transmission channel of multi-paths, and the channel estimation PN code symbols placed at the front end of each frame ofthe OFDM signal are received in the overlapped form of M number of different signals attenuated and time-delayed by the delay profile corresponding to the channel characteristics.

Referring to Fig. 4, the received PN code of the receiving end are inputted in order to N_D'-' 1 number of shift registers 41 in the correlator 22 through the ADC block 11 in serial. Regarding the correlations between the inputted PN code and M number of PN codes circulated by one bit in order which are stored in M number of shift registers placed in parallel, i.e., NJLT- l number of shift registers 42 storing non-circulated PN code, N_D^,; 1 number of shift registers 43 storing PN code circulated by 1 bit... N_D^Λ- 1 number of shift registers 44 storing PN code circulated by M-1 bit, the M number of correlation values are obtained at the same time by summing the binary EX-OR operation at the same bit positions in the shift register, and therefore, these correlation values become M number of estimated channel coefficients.

These estimated values are outputted as the determined channel coefficient values by comparison with the tlireshold of the threshold-setting block 23 in the threshold comparison and channel coefficient output block 24.

Hereinafter, the above operation is further illustrated in detail, that is, the received preamble PN code ofthe received end is composed of an overlap of transmitted preamble PN code signal through M number of paths delayed by one to M-1 sample intervals which are received through the first path, the second path delayed by one sample time and the third path delayed by two sample time, etc., by the delay profile ofthe channel. And also, the PN codes ofthe transmitting end received from each path are received with a magnitude and a phase corresponding to the delay profile. Therefore, in the channel coefficient estimation block 20, since a correlation value between the overlapped PN code signal ofthe receiving end ofthe shift register 41 and the non-circulated PN code ofthe memory block of the shift register 42 has an auto-correlation with respect to the first path receiving PN code and a cross-correlation with respect to the other delay receiving PN code, the cross-correlation value becomes 0 by the correlation of the PN code. Therefore, the auto-correlation value corresponding to the first path becomes a first channel coefficient value corresponding to a direct path at the receiving delay profile. Similarly, since a correlation between the overlapped PN code signal ofthe receiving end and the 1-bit circulated PN code ofthe memory block 21 ofthe shift register 43 has an auto-correlation with respect to the second path receiving PN code and a cross-correlation with respect to the other delay receiving PN codes, the correlation becomes a second channel coefficient value corresponding to the second path at the delay profile.

By the same method, since a correlation between the overlapped PN code signal of the receiving end and the M-1 bit circulated PN code of the shift register 44 has an auto-correlation with respect to the PN code received through the M^ path and a cross-correlation with respect to the other delayed PN codes, the correlation becomes the M^ channel coefficient value corresponding to the M^ path at the delay profile by the correlation ofthe PN codes.

By using the above-described method, M numbers of channel coefficients are estimated at the same time by a parallel processing of correlation between the overlapped PN code signal of the receiving end and M number of sequentially circulated PN codes ofthe memory block 21.

A cross-correlation value between a noise sample additionally received through the channel during the above correlation processes and the circulated PN codes of the memory block 21 becomes smaller by spreading operation with the circulated PN codes for thereby increasing the estimation accuracy of the channel coefficient value.

The above described operations can be represented by the following equations: The p^th PN code symbol vector of the transmitted sample length

L=N_D^,-1 is represented by the following Equation (1);

^••.*,.:> Equation (1)

The multi-channel coefficient vector is represented by the following Equation (2); m^ ^^ -^O Equation (2)

The receiving signal vector appending a noise to the multi-path is represented by the following Equation (3);

Equation (3) wherein a_0 and a_f represent a coefficient of a direct path without a symbol delay and a coefficient of i-sample time delayed path, respectively. And, r (p is n (p)^,=("n_l, n _2, n^Λ_3..., n _L) as a noise sample and s (p) is a signal delayed by i-sample time along the multi-path, i.e., represented as the following Equations as an i-bit circulated PN code signal;

^S2 <JP'~)=⁼ (^XZ.-l Z. l," -»^X .-2 ^

^sM-τ. p>=<y. Z.-ΛΪ--2, '-^-C, 1, ='- --_.-Λ-'-3 - Equation (4) In the channel coefficient estimation, by taking a correlation between the received signal r (p)' and the s_f (ρ) code circulated by.i bit in turn, the channel coefficient a_f can be obtained in the range of an error such as

∑ , p)*,,.(p)/ =_Ω;+ Σ∑ »(p) *,_ι.(_P)/L . .

That is, m the correlation operation,

the cross-correlation ofthe PN codes becomes 0 and also the cross correlation between the noise sample and the PN codes becomes a very small value. Therefore, this error range can be considered as the accuracy of estimation.

Referring to Fig. 5, there are shown an example of channel estimation in case that a signal to noise power ratio at the input ofthe receiving end is 13 dB and an estimation error thereof. It is known by the Fig. 5 that the estimation of the channel delay profile is well performed. Since the degree of error is appeared within ±0.005 (normalizing the largest channel coefficient value to 1 which corresponds to a attenuation below -46 dB in power). Therefore, even if the threshold is set to 0, it is possible that a channel coefficient with a high accuracy of estimation is determined.

Referring back to Fig. 3, the present invention extracts the position of the peak value of the estimated channel coefficients outputted from the threshold comparison and channel coefficient output block 24 through the peak value position extraction block 31 in the timing generation block 30 for the purpose of symbol synchronization. And, the present invention outputs to the FFT window controller 33 the decided symbol timing through the symbol timing decision block 32 by determining the count number of position of peak estimated channel coefficient extracted from the peak value position extraction block 31 as a symbol timing which is a start point of the OFDM data symbol following the OFDM preamble PN code in a frame by using the periodicity ofthe output ofthe correlator.

And also, in accordance with the present invention, the weighted coefficient storing block 18 stores the one-tap equalization weight coefficient values after Fast Fourier Transform (TTF) the estimated channel coefficients outputted from the threshold comparison and channel coefficient output block 24 and then one-tap equalizer 19 performs channel equalization by utilizing the channel coefficients stored in block 18. As described above, the present invention is capable of estimating a transmission channel coefficients accurately in real time in a time domain even for a multi-path transmission channels having a long delay profile which exceeds a guard interval (GI) in an OFDM system and determining symbol timing at the same time, and particularly, simplifying a hardware structure of a symbol synchronization by using a correlation between PN code which accurately extracting channel coefficients at a high speed by configuring an operation of the correlator with only summation operation and determining positions of peak channel coefficient values for a symbol synchronization timing. And also, the present invention is capable of implementing channel equalization to eliminate an inter-symbol interference for a multi-path having a long delay profile which exceeds a GI since weighted tap coefficients of one-tap equalizer is set by the value obtained by FFT the estimated channel coefficients. Since the present invention described above allows channel characteristics to be exactly known in real time even in a power line communication with a long delay spread, it achieves a very excellent and reliable channel equalization and determines a symbol synchronization timing at the same time.

While the present invention has been described with respect to the preferred embodiments, other modifications and variations may be made without departing from the spirit and scope of the present invention as set forth in the following claims.

Claims

What Is Claimed Is:

1. A method for estimating a channel coefficient and determining a symbol synchronization timing in an orthogonal frequency division multiplexing (OFDM) system, said OFDM system transmitting data blocks in parallel by processing the data blocks in parallel and carrying them in parallel in a multi sub-carrier orthogonal to each other, the method comprising the steps of: generating channel estimation PN codes at a symbol sample interval of an OFDM at a front end of each frame of the OFDM signal by providing a PN code generation block for a channel coefficient estimation at a transmitting end that transmits an OFDM signal of a base band by converting a serial data into a parallel data through a serial-to-parallel converter and then processing them by inverse fast Fourier transform (IFFT); and obtaining estimation channel coefficients by a correlation between PN codes transmitted from the transmitting end and M number of PN codes of a memory block which is the circulated PN code in order and determining position of peak channel coefficient values periodically appearing by a periodicity of correlation ofthe PN code for a symbol synchronization timing decision using estimated channel coefficients by providing a channel coefficient estimation block and a timing generation block for symbol synchronization at the receiving end that recovers data ofthe transmitting end by data demodulation and decoding after converting a base band signal of the transmitting end into a digital signal and recovering a symbol by fast Fourier transform.

2. The method of claim 1, wherein a PN code generation in the PN code generation block for the channel coefficient estimation is controlled by an OFDM symbol generation clock.

3. The method of claim 1, an estimation channel coefficient calculated from the channel coefficient estimation block of the receiving end is utilized as a weighted coefficient value for one-tap equalization by the FFT.

4. An apparatus for estimating a channel coefficient and determining a symbol synchronization timing in an orthogonal frequency division multiplexing (OFDM) system, said OFDM system transmitting data blocks in parallel by processing the data blocks in parallel and carrying them in parallel in a multi sub-carrier orthogonal to each other, the apparatus comprising: a PN code generation block for channel coefficient estimation generating channel estimation PN codes at a symbol sample interval of an OFDM at a front end of each frame ofthe OFDM signal at a transmitting end that transmits an OFDM signal of a base band by converting a serial data into a parallel data through a serial-to-parallel converter and then processing them by inverse fast Fourier transform (IFFT); a channel coefficient estimation block for obtaining estimation channel coefficients by a correlation between PN codes transmitted from the transmitting end and M number of PN codes of a memory block circulated a PN code in turn at the receiving end that recovers data ofthe transmitting end by data demodulation and decoding after converting a base band signal of the transmitting end into a digital signal and recovering a symbol by fast Fourier transform; and a timing generation block for determining positions of peak channel coefficient values periodically appearing by a periodicity of correlation of the PN codes for a decision of symbol synchronization timing among calculated estimation channel coefficients.

5. The apparatus of claim 4, wherein the channel coefficient estimation block includes: a circulated PN code memory block for storing M number of circulated

PN codes obtained by circulating the PN codes ofthe transmitting end by one bit in order; a correlator for calculating M number of correlation values by processing a correlation between M number of PN codes circulated by one bit in order and stored at the circulated PN code memory block and PN codes among frames of the OFDM signal of the receiving end converted from the analog-to-digital converter at the receiving end; a threshold setting block for setting and storing a threshold to determine the channel coefficients; and a threshold comparison and channel coefficient output block for finally outputting estimation channel coefficients by comparing the M number of channel coefficients calculated from the correlator with the threshold set by the threshold setting block.

6. The apparatus of claim 4, wherein the timing generation block for a symbol synchronization includes: a peak coefficient value position extracting block for extracting positions of periodic peak channel coefficient values appeared by the periodicity of correlation of PN codes among the estimated channel coefficients calculated from the channel coefficient estimation block; a symbol timing determination block for determining the symbol timing by determining the count number of positions of peak estimated channel coefficient values extracted from the peak value position extraction block as a symbol synchronization timing of the OFDM data symbol following OFDM preamble PN codes; and a FFT window control block for outputting a guard interval (GI) removing control signal in the GI removing block in response to the symbol timing determined by the symbol timing determination block.