WO2004028055A1

WO2004028055A1 - An apparatus for detecting symbol timing in an ofdm system

Info

Publication number: WO2004028055A1
Application number: PCT/KR2003/000190
Authority: WO
Inventors: Chang-Bok Joo; Ki-Yeul Han
Original assignee: Chang-Bok Joo; Ki-Yeul Han
Priority date: 2002-09-18
Filing date: 2003-01-28
Publication date: 2004-04-01
Also published as: AU2003206197A1; KR20040024986A; KR100492359B1

Abstract

The present invention relates to an apparatus for detecting symbol timing in an OFDM system with the method that set up the optimal threshold value regardless of channel characteristics using an adaptive threshold establishment method that determines the threshold level according to channel noise power and with the specially designed training symbols that can make the timing detection performance of the present apparatus be less sensitive to channel characteristic variation of a multipath channel. In addition, the present apparatus includes the moving subtraction method that is the new technique to detect timing information. The moving subtraction method produces the effects of improving the symbol synchronization performance in compared with the conventional edge detection methods.

Description

AN APPARATUS FOR DETECTING SYMBOL TIMING IN AN OFDM SYSTEM

Field of the Invention

The present invention relates to an apparatus for detecting symbol timing in an orthogonal frequency division multiplexing (OFDM) system. Particularly, the present

invention provides the method that first sets up an optimal threshold value adaptively according to channel noise power by means of calculating its mean value, and then the present invention includes the specially designed training symbols that can make the timing detection performance of the present apparatus be less sensitive to the characteristics variation of a multipath channel. Thereby, the symbol timing detection performance of the present apparatus is less affected although channel characteristic varies in condition that the established threshold value is kept. Besides, the present apparatus includes the moving subtraction method that is the new technique to detect timing information. The moving subtraction method can make the present apparatus have better synchronization performance by reducing the generation possibility of negative timing

error in case that a frequency offset exists or unexpected noise is added compared as the conventional edge detection methods. In conclusion, the present invention can provide more stable and better symbol synchronization performance on any practical time-variant channel environments that we do not know in advance. From hardware point of view, the present apparatus has a small amount of computation because it detects symbol timing via the addition and subtraction operation on the whole and its moving average window length is very small (about 4). The present invention, thereby, achieves the simplification of hardware configuration and signal processing.

Background of the Invention

Generally, an orthogonal frequency division multiplexing (OFDM) is a block modulation scheme where data symbols are transmitted in parallel by employing a (large) number of orthogonal sub-carriers. The generation of such an OFDM signal is achieved by an inverse fast Fourier transform (IFFT) after a block of N serial data symbols is converted into a block of N parallel data symbols. The OFDM signal is transmitted in unit of packet that contains training symbols and a large number of data OFDM symbols that individually include guard interval that reduces ISI between those made by an IFFT process.

At the receiver, after the transmitted signal is first converted into the baseband signal and then the baseband OFDM symbols in which guard intervals are removed by symbol timing information are demodulated by fast Fourier transform (FFT). As mentioned, in recovering a useful data at the receiver, it is very important to synchronize the start timing of each data OFDM symbol. Such a synchronization of the start timing of an effective OFDM data symbol is called symbol synchronization. To achieve this symbol synchronization in the received signal, an apparatus for detecting symbol timing is positively necessary at an OFDM system receiver.

An OFDM symbol is composed of the guard intervals (GI) of length N_G samples and the data OFDM symbol duration of length N samples, i.e., the OFDM symbol interval of N_G + N samples. The guard interval for preventing an inter-symbol interference is put in the copy of length N_G samples of rear portions of a data OFDM

symbol. Generally the guard interval is 4-6 times greater than the mean delay spread time of the channel and the OFDM data symbol duration is 4-5 times greater than the guard interval.

Generally, as shown in Fig. 1, the receiver of the OFDM system includes an analog-to-digital converter (ADC) 1 for converting a received analog baseband signal into a digital baseband signal, a reference symbol timing detector 2 for detecting a reference symbol timing, an FFT window controller 3, a guard interval removing block 4, a FFT 5, and a data demodulation block 6.

In the prior art, correlation-based methods are used to generate a timing pulse for controlling the FFT window position. The correlation-based methods are again divided into the auto-correlation based methods and the cross-correlation based methods. The autocorrelation based methods detect symbol timing by comparing between a threshold and the result correlated between the training symbols received via channel and the version delayed by the period of the training symbols, and the cross-correlation based methods estimate symbol timing by detecting peak position from the result correlated between the training symbols stored at the receiver and those received via channel. However, the correlation-based methods have the disadvantage that is to be sensitive to channel characteristic in the timing detection performance. That is, in the cross-correlation methods, the correlation peak position and value vary as channel delay profile. Moreover, the timing detection performance of the auto-correlation methods is influenced by not only channel characteristic but also a threshold value. Actually, because the optimal threshold value of such methods is different every channel and SNR, it is difficult to set up the value that is suitable for any channel environments. In conclusion, these conventional methods cannot guarantee stable and good symbol synchronization performance on the practical time- variant channel environments that we do not know in advance. From a viewpoint of hardware, correlation-based methods usually require a large amount of computation. Especially, cross-correlation based methods need high-complexity hardware. Moreover, although a method for finding a minimum position of the magnitude of a difference between receiving signals is existed, it is difficult to detect the timing for a symbol starting point in case that a frequency offset is occurred and requires a long moving average window register, which results in high-complexity hardware.

SUMMARY OF THE INVENTION The present invention was designed to solve the above-mentioned problems of the conventional symbol timing detection methods. That is, it is an object of the present invention to provide an apparatus for detecting symbol timing in an orthogonal frequency division multiplexing (OFDM) system which achieves more stable and better symbol synchronization on the practical time-variant channel environments by setting up the optimal threshold value always regardless of channel characteristics using an adaptive threshold establishment method that determines the threshold level according to channel noise power and by using the specially designed training symbols that make the symbol timing detection performance be less sensitive to power delay profile variation in a multipath channel. It is another object of the present invention to provide an apparatus for detecting symbol timing in an OFDM system which is capable of improving the symbol synchronization performance by effectively reducing the generation possibility of negative symbol timing by a moving subtraction method that is a new timing information detection method even in case that frequency offset and undesired noise exist, and which is capable of generating accurate symbol timing more reliably by real time processing method through a reference symbol timing decision block for distinguishing and determining proper reference symbol timing from timing candidates.

It is another object of the present invention to provide an apparatus for detecting symbol timing in an OFDM system which simplifies signal processing and attains low- complexity hardware configuration by performing a symbol timing detection on the basis of a subtraction operation and by make to have the moving average window of very small length (about 4) in compared with the methods of finding a minimum value for symbol timing detection, and which make to obtain symbol synchronization in the OFDM system by a real time processing method.

In accordance with the present invention, an apparatus for detecting symbol timing in an OFDM system generates a difference signal between a receiving signal and the signal of delaying it by reference-training symbol period. At this time, the intervals of only existing additive noise are generated. And then, the apparatus calculates an instantaneous power for the generated difference signal. The mean noise power in the generated noise interval is obtained either by an effective average noise power calculation and control block or by a mean noise power calculation block of calculating average channel noise power from packet detection part. Thereafter, an optimum threshold which is not influenced by channel characteristics such as a power delay profile and a delay spread is established by the calculated mean noise power, and symbol timing is detected in real time through the moving subtractor and the other timing decision blocks.

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 depicts a block diagram of a receiver of a conventional OFDM system;

Fig! 2 is an exemplary diagram showing the training symbols constitution; Fig. 3 shows a block diagram of a symbol timing detection apparatus of an OFDM system in accordance with the present invention;

Fig. 4 illustrates a block diagram of a moving subtractor shown in Fig. 3;

Fig. 5 depicts an operation principle of the moving subtractor shown in Fig. 3; and

Fig. 6 represents graphs of output waveforms of each component block of the symbol timing detection apparatus of the OFDM system in accordance of the present invention.

* Reference numbers of main units in the drawings *

11: reference-training symbol period delay 12: subtractor 13: magnitude square calculator 14: moving average device

15: effective average noise power calculation and control block

16: level discriminator 17: moving subtractor

18: candidate timing extracter

19: reference symbol timing decision block 20: symbol timing generation block

21 : mean noise power calculation block (belong to a packet detection part)

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT A preferred embodiment of the present invention will now be described with reference to the accompanying drawings.

Referring to Fig. 3, there is shown a block diagram of a symbol timing detection apparatus of an OFDM system in accordance with the present invention. The symbol timing detection apparatus includes a reference-training symbol period delay 11 for delaying a complex baseband receiving signal of OFDM signals having a periodic structure by packet configuration by a reference-training symbol period T, a subtractor 12 for generating a difference signal between a complex baseband receiving signal and the signal delayed by the reference-training symbol period delay 11, a magnitude square calculator 13 for calculating a square of the magnitude of the difference signal from the subtractor 12, a moving average device 14 for moving-averaging the squares of the magnitude values calculated in the square calculator 13, an effective average noise power calculation and control block 15 for calculating mean noise power during a noise interval created by the offsetting of the signals during the difference signaling process and for setting an optimal threshold level using the calculated effective average noise power, a level discriminator 16 for outputting a signal as "1" if a signal level of the moving average device 14 is larger than the optimal threshold level established in either the effective average noise power calculation and control block 15 or the mean noise power calculation block (belong to a packet detection part) 21 and for outputting a signal as "0" if a signal level of the moving average device 14 is smaller than the optimal threshold level established in either the effective average noise power calculation and control block 15 or the mean noise power calculation block (belong to a packet detection part) 21, a moving subtractor 17 for calculating a difference of the output samples of the level discriminator 16 included in the moving subtraction window by sample intervals of sample period T_s , a candidate timing

extractor 18 for extracting a value being larger than 0 outputted from the moving subtractor 17, a reference symbol timing decision block 19 for distinguishing and determining the proper reference symbol timing from the timing candidates extracted from the candidate timing extractor 18, a symbol timing generation block 20 for outputting the symbol timing of indicating the start position of each OFDM symbol from the reference symbol timings determined by the reference symbol timing determination block 19, and a mean noise power calculation block (belong to a packet detection part) 21 for providing another method for establishing the optimal threshold level by calculating mean channel noise power in a packet detection part of an OFDM system.

In Fig. 4, there is shown the detailed signal flow diagram of the moving subtractor 17 and it represents a moving subtraction window having a length N. The moving subtractor 17 includes a plurality of delay devices 17-1 for delaying output signal samples from the level discriminator 16, an adder 17-2 for adding the delayed signals from the plurality of delay devices 17-1, and a subtractor 17-3 for calculating a difference between the output signal from the level discriminator 16 and the output signal from the adder 17-2. Referring to Figs. 5 and 6, the operations of the above-described apparatus for detecting the symbol timing in the OFDM system is described in detail in accordance with the present invention. Fig. 5 is a detailed diagram showing an operation principle of the moving subtractor shown in Fig. 3 and Fig. 6 is graphs showing output waveforms of each component block of the symbol timing detection apparatus of the OFDM system in accordance of the present invention. Prior to a detailed description of the present invention, a simple explanation of the preamble for configuring the packet of the OFDM symbols will be shown below.

The OFDM utilizes ten short training symbols and 2 or 3 long training symbols at a front end of the packet for AGC (Automatic Gain Controller), synchronization, channel estimation, and so on as shown in Fig. 2. This exemplary preamble constitution is constructed in such a way that the 10 short training symbols corresponding to 2 OFDM symbol durations are used for AGC and estimating the average noise power, and 3 long training symbols corresponding to 3 OFDM symbol durations are used for detecting the symbol timing and channel estimation. The training symbols for detecting the symbol timing constitute one OFDM symbol including its guard interval. That is, the OFDM symbol consists of the guard interval (GI) of length N_G and two identical training symbols (TI, T2) of each length N_ms (=N/2 ). This training symbol is the reference-

training symbol with the period T. Moreover, the training symbols for detecting the symbol timing are generated by the elements of sequence with indices that are a multiple of 2 have non-zero amplitude; +1 or -1, and the elements must be arranged so that the difference power of the corresponding samples that are within the shadowy region is high to guarantee the superior performance regardless of channel characteristic variation. The size of the shadowy region can be only one sample provided the power of the first difference signal within the region is sufficiently large compared with the difference power of noise at low SΝR (e.g., 5dB or OdB).

Fig. 6(a) is a waveform representing the magnitude of a complex baseband signal received via a mutipath channel at an OFDM system with frequency offset. Fig. 6(b) is a waveform of the complex baseband receiving signal delayed by reference-training symbol period T = T_SN_P and Fig. 6(c) is the magnitude of a complex difference signal obtained

from the subtractor 12 by calculating a difference between a complex baseband receiving signal and the signal delayed by the reference-training symbol period delay 11, wherein portions at which a level is slightly represented are noise intervals created by the offsetting of the signals during the difference signal process. Fig. 6(d) is a signal consisting of an instantaneous power of a difference signal at each sample time f_s ^{s an} output from the

magnitude square calculator 13 for calculating the power of the difference signal from the output of the subtractor 12, and Fig. 6(e) is an output signal of the moving average device 14 for moving averaging the power signals from the magnitude square calculator 13. The effective average noise power calculation and control block 15 for calculating mean noise power during a noise interval created by the offsetting of the signals during the difference signaling process and for setting optimal threshold first calculates average noise power intervals representing a level slightly before training symbols for symbol timing in Fig. 6(e), and then establishes the optimal threshold level (p_th) using the calculated mean noise power; ,_Λ = ,_/lc x (effective mean noise power) where p_thc is the constant

optimal-threshold-coefficient determined by simulation (about 3.2). Subsequently, the output signal from the moving average device 14 is compared with the established threshold level in the level discriminator 16 and results in outputting a signal level being larger than the optimal threshold level as "1" and a signal level being smaller than the optimal threshold level as "0".

Next, an output signal of the level discriminator 16 which outputs the waveform shown in Fig. 6(f) is inputted to the moving subtractor 17 for calculating a difference of the output samples of the level discriminator 16 included in the moving subtraction window by sample intervals of sample period _s according to the operation principle shown in Fig. 5

and is finally converted into 3 types of signals having +, - and 0 values such as a waveform of Fig. 6(g). Fig. 5 illustrates the operation principle of the moving subtractor 17 having a

length 3, wherein (a) is an original signal, (b) and (c) are signals delayed by 1 sample and 2 samples from the original signal, respectively and (d) is a signal obtained by subtracting (b) and (c) signals from the original signal.

And then, the candidate timing extractor 18 extracts a signal having a value being larger than 0 in the output of the moving subtractor 17 and sends the extracted candidate signal to the reference symbol timing determination block 19. Then, in the reference symbol timing decision block 19, the first candidate timing coming in the specific range is outputted as a deterministic reference symbol timing as shown in Fig. 6(i). That is, the reference symbol timing decision block 19 recognizes the timing candidate extracted from the candidate timing extractor 18 as the counted sample ordinal number in a packet and detects the first candidate timing that is entering between D_TLB and D_TUB ^as the proper reference symbol timing in real-time processing where D_TLB and D_TUB is the Timing Decision Lower Bound and the Timing Decision Upper Bound

related to a packet detection error limit respectively.

Thereafter, the symbol timing generation block 20 generates symbol timings in a packet on the basis of the determined reference symbol timing. Finally, the mean noise power calculation block 21 (belong to packet detection part) for providing another method for calculating average channel noise power and establishing the optimal threshold level first calculates the channel noise power σ²< in a

packet detection part of an OFDM system when an effectual signal is not received and then establishes the optimal threshold level {P_th) using the calculated mean channel noise power (σ² _w ),' P& = P_th (²a² σ² ) where p_thc is the constant optimal-threshold- coefficient determined by simulation (about 3.2) and is the fixed VGA (Variable Gain Amplifier) gain after AGC (Automatic Gain Controller) stabilizes within a previous training symbols duration (a short training symbols duration).

In the present invention, the moving subtraction method can produce the effect of reducing the generation possibility of negative timing error in the operation process. Therefore, the moving subtraction method can make the subcarrier symbols of an OFDM symbol be less affected by ISI (inter-symbol interference) when symbol synchronization is achieved, can thereby improve the performance of the timing synchronization consequently.

As described above, the present invention relates to an apparatus for detecting a symbol timing of an OFDM system. The present apparatus provides the method that always sets up the optimal threshold value regardless of channel characteristics using an adaptive threshold establishment method that determines the threshold level according to channel noise power and then includes the specially designed training symbols that can make the timing detection performance of the present apparatus be less sensitive to power delay profile variation of a multipath channel. Thereby, the symbol timing detection performance of the present apparatus is less affected by channel characteristic variation. In addition, the present apparatus includes the moving subtraction method that is the new technique to detect timing information. The moving subtraction method can make the present apparatus have better synchronization performance by reducing the generation possibility of negative timing error in a case that a frequency offset exists or unexpected noise is added compared as the conventional edge detection methods. In conclusion, the present apparatus can provide more stable and better symbol timing detection performance on the practical time-variant channel environments in compared with conventional correlation-based methods and conventional subtraction-based methods for finding a minimum position.

From hardware point of view, the present apparatus has a small amount of computation because it detects symbol timing via the addition and subtraction operation on the whole and its moving average window length is very small (about 4). The present invention, thereby, achieves the simplification of hardware configuration and signal processing.

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. An apparatus for detecting symbol timing of an orthogonal frequency division multiplexing (OFDM) system, comprising: a reference training symbol period delay for delaying a receiving signal of

OFDM signals having a periodic structure by packet configuration by a reference- training symbol period T; a subtractor for generating a difference signal between the receiving signal and the signal delayed by the reference-training symbol period delay; a magnitude square calculator for calculating a square of the magnitude of the difference signal from the subtractor and obtaining an instantaneous power; a moving average device for moving-averaging the squares of the magnitude values calculated in the square calculator. an effective average noise power calculation and control block for calculating effective mean-noise-power during a noise interval created by the offsetting of the signals during the difference signaling process and for setting an optimal threshold level using the calculated effective average-noise-power; a level discriminator for outputting a signal as "1" if a signal level of the moving average device is larger than the optimal threshold level established in either the effective average noise power calculation and control block or the mean noise power calculation block (belong to a packet detection part) and for outputting a signal as "0" if a signal level of the moving average device is smaller than the optimal threshold level established in either the effective average noise power calculation and control block or the mean noise power calculation block (belong to a packet detection part); a moving subtractor for calculating a difference of the output samples of the level discriminator included in the moving subtraction window by sample intervals of sample period T_s ⁼ TI N_P ;

a candidate timing extractor for extracting a value being larger than 0 outputted from the moving subtractor; a reference symbol timing decision block for distinguish and determining the proper reference symbol timing from the timing candidates extracted from the candidate timing extractor; a symbol timing generation block for outputting the symbol timing of indicating the start position of each OFDM symbol from the reference symbol timings determined by the reference symbol timing decision block; and a mean noise power calculation block (belong to a packet detection part) for providing another method for establishing the optimal threshold level by calculating mean channel noise power in a packet detection part of an OFDM system.