Channel estimation and symbol boundary detection method

A channel estimation method for use with a received signal by a receiver is disclosed. The received signal comprises multiple data bursts which are transmitted to the receiver via multiple path channels, with each of the data bursts having a plurality of preamble symbols which are decoded. The channel estimation method includes the following steps: firstly, at least one correlation pattern is generated according to the decoded preamble symbols. Then, a cross correlation of the correlation pattern with the received signal is performed to yield at least one correlation result of channel impulse response (CIR). Wherein, the symbol boundary of the received signal is decided according to the correlation result.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to a symbol boundary detection method, and more particularly to a channel estimation and symbol boundary detection method in a digital video broadcasting-terrestrial 2 (DVB-T2) system.

2. Description of Related Art

An orthogonal frequency division multiplexing (OFDM) system comprises a high-efficiency multi-path modulation/demodulation technology which utilizes a multi-carrier to transmit OFDM signals, so as to improve data transmission rate. Recently, OFDM technology has been used in various wireless communication systems such as the digital video broadcasting-terrestrial 2 (DVB-T2) system.

A DVB-T2 signal is constructed by super frames, which consist of several T2-frames, to be transmitted in the DVB-T2 system. With reference toFIG. 1, a structure diagram of a T2-frame is shown in which the T2-frame1is composed of OFDM symbols, including one first preamble symbol (P1symbol)11, several second preamble symbols (P2symbol)13and data symbols15. To receive DVB-T2 signals, P1symbol11should first be detected and decoded for key parameters such as the transmission type, the P2symbols13then can be successive decoded to obtain the content of the data symbols15.

The DVB-T2 signals are encapsulated into several packets as the T2-frame1structure which are transmitted to the receiver via plural path channels. The channel impulse response (CIR) is usually under perfect channel, to avoid the problem of inter-symbol interference (ISI), a serial of a cyclic prefix (CP) information, as guard interval (GI), is additionally added between symbol packets generally. Furthermore, in order to avoid inter-symbol interference effectively, especially in the multi-path scheme, the symbol boundary should be positioned so that the least-possible ISI is incurred when receiving DVB-T2 signals.

FIG. 2shows a conventional symbol boundary detection scheme. As illustrated inFIG. 2, the packets23as the T2-frame1structure are transmitted via plural path channels (Pa1,Pa2), and the cyclic prefix information is added prior to each of the packets23. Under the channel, a typical symbol boundary detection scheme will position the OFDM symbol window25according to the first propagation path (Pa1), as shown by the dash-line frame inFIG. 2. Once the packet error is occurred when decoding the received signals by the present positioned OFDM symbol window25, it needs to shift the position of the OFDM symbol window25and then detect whether the packet error is occurred. Repeat the above steps until the least-ISI-achieving OFDM symbol window25is detected. However, one P1symbol11and several P2symbols13must be decoded in order as long as the position of the OFDM symbol window25is adjusted. Therefore, it consumes a lot of search time.

In view of the foregoing, a need has arisen to propose a novel channel estimation and symbol boundary detection method to estimate a channel profile efficiently to further detect an optimal symbol boundary position.

SUMMARY OF THE INVENTION

In view of the foregoing, it is an object of the embodiment of the present invention to provide a symbol boundary detection method for the digital video broadcasting-terrestrial 2 (DVB-T2) system to estimate a channel profile efficiently and detect an optimal symbol boundary position according to the estimated channel profile.

According to one embodiment, a symbol boundary detection method for detecting the symbol boundary of a received signal is disclosed. The received signal comprises a plurality of data bursts which are transmitted via a plurality of path channels, wherein each of the data bursts comprises a plurality of preamble symbols which are decoded. The symbol boundary detection method includes the following steps: firstly, at least one correlation pattern is generated according to the decoded preamble symbols. Then, a cross correlation of the correlation pattern with the received signal is performed to yield at least one correlation result of channel impulse response (CIR). Finally, a symbol window position is adjusted according to the correlation result and the total inter-symbol interference (ISI) power contributed by the path channels is calculated under different symbol window positions. An optimal symbol window is positioned as the symbol window corresponding to the achieved minimum ISI.

According to another embodiment, a channel estimation method for use with a received signal by a receiver is disclosed. The received signal comprises a plurality of data bursts which are transmitted to the receiver via a plurality of path channels, wherein each of the data bursts comprises a plurality of preamble symbols which are decoded. The channel estimation method includes the following steps: firstly, at least one correlation pattern is generated according to the decoded preamble symbols. Then, a cross correlation of the correlation pattern with the received signal is performed to yield at least one correlation result of channel impulse response (CIR). Wherein, the symbol boundary of the received signal is decided according to the correlation result.

DETAILED DESCRIPTION OF THE INVENTION

Turning to the drawings,FIG. 3is a block diagram illustrating the typical wireless communication system according to one embodiment of the present invention. As shown inFIG. 3, the wireless communication system3comprises a transmitter31and a receiver33, wherein the transmitter31and the receiver33has antennas35,37respectively. The transmitter31emits signals via the antennas35, and the receiver33receives signals via the antennas37. The received signals are processed such as demodulating and decoding to be useful information. The signals are transmitted to the receiver33via a plurality of path channels39. Specifically, the wireless communication system3comprises the digital video broadcasting-terrestrial 2 (DVB-T2) system, which utilizes the T2-frames to transmit digital broadcasting signals such as the signals of various TV channels.

Under multi-path scheme, the receiver33receives a received signal, i.e., the digital broadcasting signal. The received signal comprises a plurality of data bursts which are transmitted via a plurality of path channels39respectively. The data bursts are encapsulated into several packets as the T2-frame1structure and are transmitted to the receiver33.FIG. 4shows a structure diagram of a T2-frame4according to one embodiment of the present invention. The T2-frame4is composed of OFDM symbols, including one first preamble symbol (P1symbol)41, several second preamble symbols (P2symbol)43and data symbols45, as shown inFIG. 4. P1symbol41carries information to indicate key transmission parameters such as the fast Fourier transform (FFT) size and transmission type. P2symbols43carry remaining parameters such as the guard interval (GI), code rate, etc. To receive DVB-T2 signals, P1symbol41should first be detected and decoded for key parameters. Once P1symbol41has been identified, the symbol boundary detection could follow.

After the received P1symbol41is detected and decoded, the receiver33regenerates the P1symbol41according to the decoded parameters. As shown inFIG. 5, the regenerated P1symbol41is composed of 2048-symbol which is divided into four parts (part1-part4)411-414. Based on the structure of P1symbol41, at least one tailor-made correlation pattern is devised to take advantage of the special formatting of P1symbol41.

The diagram ofFIG. 6illustrates the correlation patterns devised by P1symbol41and the corresponding correlation results according to one embodiment of the present invention. As shown inFIG. 6, the first correlation pattern61is constructed by the part2412and the part3413of P1symbol41, the second correlation pattern62is constructed by the part1411,5420-pedding-bit and the part3413of P1symbol41, and the third correlation pattern63is constructed by the part2412,4820-pedding-bit and the part4414of P1symbol41. Under perfect channel, the cross-correlation of the correlation patterns61-63with the received signal yields the correlation results of channel impulse response (CIR) shown next to the correlation patterns61-63inFIG. 6.

Taking the first correlation pattern61for example, the correlation result has three major pulses. However, under perfect channel, there should only be one propagation path, hence one pulse. Therefore, two extra undesired pulses with smaller power, due to the artifact of the cross-correlation, should be eliminated for correct CIR estimation.

In order to facilitate better working of cancellation, two correlation patterns, the second correlation pattern62and the third correlation pattern63, for cross-correlation are proposed. Taking the second correlation pattern62and its corresponding correlation result for example, the main path is at 0 and the undesired artifact paths (at482and542) are positioned in the right of the main path. A Left-to-Right (LtoR) cancellation is employed to eliminate the two undesired artifact paths at482and542. The implementation of the LtoR cancellation is mathematically described in formula (1).
t=0˜N
s(t+482)=s(t+482)−s(t)×α1
s(t+542)=s(t+542)−s(t)×α2(1)

Where s(t) is the received signal, t is the received sample index, and N is the range for successive cancellation. α1is the gain of pulse(482) relative to gain of pulse(0), that is, α1=c(482)/c(0); α2is the gain of pulse(542) relative to gain of pulse(0), that is, α2=c(542)/c(0).

Similar, taking the third correlation pattern63and its corresponding correlation result for example, the main path is at 542 and the undesired artifact paths (at 0 and 60) are positioned in the left of the main path. A Right-to-Left (RtoL) cancellation is employed to eliminate the two undesired artifact paths at 0 and 60. The implementation of the RtoL cancellation is mathematically described in formula (2).
t=N˜0
s(t−482)=s(t−482)−s(t)×α3
s(t−542)=s(t−542)−s(t)×α4.  (2)

Where s(t) is the received signal, t is the received sample index, and N is the range for successive cancellation. α3is the gain of pulse(60) relative to gain of pulse(542), that is, α3=c(60)/c(542); α4is the gain of pulse(0) relative to gain of pulse(542), that is, α4=c(0)/c(542).

The successive cancellation could be implemented by either LtoR cancellation or RtoL cancellation, or combined. In the combined scheme, the results of LtoR and RtoL cancellation are added together to increase the power ratio of the main path to noise (SNR), as shown inFIG. 7.

With reference toFIG. 8, an example of the cross-correlation using pattern2(62) and pattern3(63), and estimated CIR after LtoR and RtoL successive cancellation respectively is shown according to one embodiment of the present invention. The channel is a single frequency network (SFN) channel with some particular delay profile. As shown inFIG. 8, the undesired artifact pulses with smaller power of the correlation results can be eliminated by the LtoR and RtoL successive cancellations for correct CIR estimation. In one embodiment, in order to simplify the computation, a pulse-threshold is pre-determined. If the power of the remaining pulse is smaller than the pulse-threshold, the path with smaller power could be set to zero.

Under multi-path scheme, the receiver33must decide the symbol boundary position of the received signal. To avoid ISI, the symbol boundary should be positioned so that the least-possible ISI is incurred. The incurred ISI can be estimated by the estimated CIR information.

Attention is directed next toFIG. 9, which shows a structure diagram illustrating a more complicated multi-path propagation channel according to one embodiment of the present invention, in which the delay spread (the length of the channels) is larger than the guard interval. As shown inFIG. 9, the data bursts of the received signal are encapsulated into several packets as the T2-frame4structure which are transmitted to the receiver33via multiple path channels. The cyclic prefix (CP) information is added prior to each of the packets23, as guard interval (GI). Under the symbol window91is positioned according to the first propagation path, as shown by the solid-line frame inFIG. 9. However this is not the least-ISI-achieving OFDM symbol window91under the estimated channel profile (obtained from the correlation result). For an OFDM signal transmitted through this multi-path channel, the incurred ISI can be roughly estimated by the estimated CIR (obtained from the correlation result). For example, the incurred ISI of the OFDM symbol window91could be estimated as formula (3).

In formula (3), ISI(n) is the total ISI contributed to the (n+GI)thsymbol point in the symbol window91. The total incurred ISI at all the points in the symbol window91could be calculated. Repeat the above steps each time the symbol window91is shifted. Then, the calculated ISI at all the points are summed up by formula (4). For example, EISI(0) is the total ISI power contributed by path channels CGI-CCH—Len-1, incurred at all the points in the symbol window91. EISI(0) is estimated by calculating and summing up the ISI incurred at (GI)thsample (=ISI(n)), (GI+1)thsample (=ISI(1)), . . . , and (CH_Len−1)thsample (=ISI(CH_Len−1)). Wherein, CH_Len is the length of the path channels. According to formula (4), the minimum ISI is achieved when shifting d symbols, corresponding to the dash-line OFDM symbol window93as the optimal symbol window position inFIG. 9.

Finally, reference is made toFIG. 10, which shows a flow diagram illustrating the symbol boundary detection method according to one embodiment of the present invention. The method comprises the following steps.

The receiver33receives the data bursts and P1symbol of a received signal in step S101, and decodes the received P1symbol in step. S103. Then, a correlation pattern is generated according to the decoded P1symbol and its special format in step S105. Taking the second correlation pattern62for example as below.

Sequentially, a cross-correlation of the generated correlation pattern (second correlation pattern62for example) with the received signal yields a correlation result of channel impulse response in step S107. The successive cancellation (LtoR or RtoL cancellation) is employed to eliminate the pulses with smaller power of the correlation result for correct CIR estimation in step S109. Then, it determines that the power of the remaining pulse is smaller than the pre-defined pulse-threshold in step S111. If yes, the path with smaller power could be set to zero to simplify calculation in step S113.

After the CIR information is estimated, the symbol boundary position is adjusted in order to calculate the ISI at all the points of all path channels by formula (3), (4) in step S115. Finally, an optimal symbol window93is positioned as the symbol window corresponding to the achieved minimum ISI in step S117. In most cases, the least-ISI OFDM symbol window will have timing offset, a symbol timing shift procedure is performed to avoid the aliasing caused by frequency interpolation in step S119.

As mentioned above, the traditional symbol boundary detection consumes a lot of search time. According to the present invention, the correlation pattern is generated according to the decoded P1symbol, and the correlation result of CIR can be estimated by cross-correlation. The successive cancellation procedure eliminates the smaller pulses for correct CIR estimation. After the CIR is estimated, the symbol boundary position can be optimized with least-ISI according to the formulas in the present invention. Therefore, the present invention can estimate a channel profile efficiently and detect an optimal symbol boundary position quickly.