GB2447972A

GB2447972A - Synchronising OFDM Symbols by auto-correlating cyclic prefix then cross correlating scattered pilots in the time domain

Info

Publication number: GB2447972A
Application number: GB0706263A
Authority: GB
Inventors: Zhaohui Liu; Mike Page
Original assignee: Matsushita Electric Industrial Co Ltd
Current assignee: Panasonic Holdings Corp
Priority date: 2007-03-30
Filing date: 2007-03-30
Publication date: 2008-10-01
Also published as: GB0706263D0

Abstract

The invention synchronises the Symbol Timing in OFDM Systems using scattered pilots. Coarse symbol timing is achieved by autocorrelating 2 with a cyclic prefix and detecting a correlation peak 3. At the same time, a frequency offset can be estimated 4. Received samples are frequency offset compensated 5, and a symbol index is determined 6 which allows a pattern of transmitted pilots to be identified. These frequency domain pilot sub-carrier symbols are then transformed to the time domain via an IFFT (Inverse Fast Fourier Transform) 7. Given a search window around the coarse timing point, the frequency compensated samples are cross correlated with the time domain pilot sub-carriers waveform 8 to generate an estimate of the channel impulse response (CIR). Then the absolute value of the correlation is accumulated within a sliding window 9 and the position of the maximum 10 is regarded as the optimum timing point.

Description

Method and Appatus for Symbol Timing in OFDM Systems The present

invention relates to symbol timing and particularly symbol timing in orthogonal frequency division multiplexing (OFDM) systems with scattered pilots.

It is known in OFDM systems that in order to avoid inter-symbol interference (ISI) in a multipath channel, a guard interval (cyclic prefix) is included in the transmitted OFDM signal. The symbol timing is selected to minimize the inter-symbol interference.

Examples of OFDM communication systems based on scattered pilots are the Digital Video Broadcasting (DVB) system and the Digital Radio Mondiale (DRM) system. In general, there is no training symbol (such as a NULL symbol) in such an OFDM system. Instead, scattered pilots are provided for the purpose of channel estimation, so that a coherent receiver can be realised.

There are several approaches for symbol timing of an OFDM receiver: 1) Symbol timing based on correlation with the cyclic prefix.

2) Symbol timing based on correlation with training symbols.

3) Symbol timing based on maximizing the energy of the channel impulse response * 20 (CIR) estimate within a sliding window.

: It should be noted that there is usually no training symbol for a scattered pilot OFDM *:. system, so approach 2 may not be used.

Approach 1 can only give an approximate timing position for a multi-path channel.

Approach 2 cannot give the optimum timing position when the first path is not the strongest.

For approach 3, the CIR is usually calculated via interpolation and inverse fast Fourier transformation (IFFT) which is a frequency domain method and so may suffer from path "aliasing" where the incorrect signal is picked up for use in the lEFT. EP-A-1276291 relates to a frequency domain method which may suffer from such aliasing.

An object of the present invention is to achieve optimum symbol timing in a multi-path environment for a scattered pilots based OFDM system, even when the first path is not the strongest path and avoiding the problem of path aliasing.

The present invention provides a method of determining the symbol timing in an OFDM system, the method having a coarse timing and fine timing stage which are carried out with the aid of scattered pilots present in the symbol.

In particular, firstly, coarse symbol timing can be achieved by carrying out a correlation with a cyclic prefix. At the same time, a frequency offset can be estimated. Secondly, from the frequency offset compensated samples, the symbol index can be estimated and thus the known pilot sub-carriers can be transformed to the time domain via an IFFT. Thirdly, given a search window around the coarse timing point, the frequency compensated input samples can be correlated with the IFFT of the pilot sub-carriers. Finally, the absolute value of the correlation is accumulated within a sliding window and the position of the maximum is regarded as the optimum timing point. This approach has no path "alias" and is always able to synchronise to the first arriving path even if it is not the strongest path.

The present invention also provides an apparatus comprising means for performing the method.

In order that the present invention be more readily understood, an embodiment will be described with reference to the accompanying figures in which: Fig. 1 shows a schematic diagram according to a preferred embodiment of the present invention; Fig. 2 shows an OFDM symbol with cyclic prefix; Fig. 3 shows an example of the arrangement of scattered pilots in OFDM symbols; Fig. 4 shows a graph where windows are used to search for the optimum timing point; A preferred embodiment of symbol timing approach is shown Fig. 1. The figure shows the steps after analogue to digital conversion of the incoming signal in a receiver of digital broadcasting system which utilises OFDM with scattered pilots. Such a system is DVB and DRM but the invention is not limited to use in such systems.

After A/D conversion, the input samples are provided to memory 1 and auto-correlation 2 is carried out within a sliding window. The coarse symbol timing and frequency offset estimation 4 can be found from the position of maximum correlation 3. After frequency offset compensation 5, the symbol index 6 can be estimated from the location of the scattered pilots. Then the waveform of the pilot sub-carriers in the time domain can be constructed by carrying out IFFT 7. Applying cross-correlation 8 between the input samples from the frequency offset compensation 5 and the constructed pilot waveform, the amplitude of channel impulse response is estimated. Accumulation 9 within a sliding window will ensure synchronisation to the first arnval path even if it is not the strongest path. This is the overall method carried out in accordance with the preferred embodiment. The steps will be * considered in detail below. *. ** * * Se..

:* Fig. 2 shows the transmitted OFDM signal with its cyclic prefix. By calculating the auto- *:. 20 correlation of thc input samples, the coarse symbol timing and frequency offset estimate can be found. The details of such steps can be found in many references, e.g., reference [1]. a.. a

Figure 3 shows the pilot position within each OFDM symbol. In general, the power of each pilot is boosted and their phases and positions are changed depending on the symbol index.

After obtaining the coarse symbol timing, the scattered pilots can be used to obtain the optimal timing. To use this information, the symbol index must first be detected, since the positions and phases of the scattered pilots are different for each symbol. This can be achieved by the following steps: 1) The input samples are frequency offset compensated, generating a sample vector x[n] 2) An FFT is applied to x[n], generating the frequency domain output Y, where s is the symbol index and k is the sub-carrier index 3) The absolute value of YSk is accumulated at each presumed position, the maximum corresponding to the pilot pattern (i.e. the pilot sub-carrier positions in the symbol).

This is because the pilot sub-carriers are power boosted.

4) Once the pilot pattern has been detected, the symbol index can be searched within the set that comprises of all symbols with the detected pilot pattern. (Note that as shown in figure 3, several symbols may share the same pattern of pilot positions, this pattern repeating after a pilot pattern cycle). Denote the set as F and the symbol index. can be found from the following: for each s E P . exp(-j( - ) end = arg max(z) S. Where c is the pilot pattern cycle, is the phase of the pilot at sub-carrier k and 0SS* * . *5** symbol s. S...

The estimated symbol index corresponds to the maximum of z,.

Once the symbol index is known, then the waveform of pilots in the time domain may be * generated using an IFFT. Given a search window around the coarse symbol timing point, the : recreated pilot waveform may be cross-correlated with the frequency offset compensated input samples, and the optimum timing can be found by maximizing the energy of the CW within a sliding window. As shown in Figure 4, assuming the coarse symbol timing point is, n0, the length of the accumulation window is L, the search window is W and let p[n] be the IFFT of the pilot sub-carriers, then the optimal timing n can be found as follows: (1) Calculate the cross-correlation for n=OtoW-1 y[n] = p [k]x[k + n0 + n -w/2] end (2) Calculate the energy of the CIR within a sliding window for n=OtoW-L z[nJ = y[k + n]j end (3) flop1 =argmaxz[nJ As the CuR is estimated in time domain (rather than the frequency domain), any path "alias" is avoided.

In general, the length of accumulation window L should be equal to the channel size, which is not known. However, as the transmission mode is related to the channel characteristics, L should be no more than the length of the cyclic prefix. On the other hand, because the :. scattered pilots are regularly spaced, its time domain waveform is cyclic. So if the

. accumulation window is too large, peaks introduced by periodicity will also be involved. S....DTD: This can be solved by including irregularly spaced pilot sub-carriers to break up the periodic S...

sequence. In general, more pilot sub-carriers are available for the first symbol for the * 15 purpose of frame synchronization. For example, in DRM, gain pilots are regularly spaced * : and extra, irregularly spaced time reference pilots are available for the first symbol. *5S

Further approaches can be taken to mitigate the effects of noise. For instance, the output of the cross-correlation can be compared with a threshold; if the output is less than the threshold, it can be regarded as mainly caused by noise and set to zero.

References 1) M. Speth and etc., "Optimum receiver design for wireless broad-band systems using OFDM -part II," IEEE Trans. Comm., vol. 49, No. 4, pp. 571 -578, 2001 2) Wei Chee Lim and etc., "Joint channel estimation and OFDM synchronization in multipath fading, "in Proc. ICC'04, June, 2004, pp. 983 -987 * * * *** S... * . S...

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Claims

1. A method of determining symbol timing in a scattered pilot based OFDM system comprising: receiving an OFDM symbol having a cyclic prefix; calculating auto-correlation of the received symbol in order to determine coarse symbol timing and a frequency offset estimation; performing frequency offset compensation of the received OFDM symbol on the basis of the estimated frequency offset; detecting the symbol index from the location of scattered pilots in the symbol; generating a waveform of the pilots in the time domain using an inverse Fast Fourier Transform (IFFT); estimating an amplitude of a channel impulse response (CIR) by cross correlating the generated waveform and frequency offset compensated OFDM symbol accumulating an absolute value of the cross-correlation within a sliding window, and determining the maximum value wherein the maximum value is the optimum symbol timing point.

2. The method of claim 1 wherein the length of the sliding window is related to the length of the cyclic prefix.

3. The method of claim 1 or 2 wherein the OFDM symbol includes irregularly spaced scattered pilots.

4. The method of any preceding claim wherein the output of the cross-correlation is compared with a threshold, and if the output is less than the threshold, it is set to zero.

5. An apparatus for determining symbol timing in a scattered pilot based OFDM system, comprising: means for receiving an OFDM symbol having a cyclic prefix; means for auto-correlating the received symbol in order to determine coarse symbol timing and a frequency offset estimation; means for performing frequency offset compensation of the received OFDM symbol on the basis of the estimated frequency offset; means for detecting the symbol index from the location of scattered pilots in the symbol; means for generating a waveform of the pilots in the time domain using an inverse Fast Fourier Transform (1FF!'); means for estimating an amplitude of a channel impulse response (CIR) by cross correlating the generated waveform and frequency offset compensated OFDM symbol; means for accumulating an absolute value of the cross-correlation within a sliding window and determining the maximum value, wherein the maximum value is the optimum symbol timing point.