In a general digital broadcasting system, because data are transmitted through a limited frequency bandwidth, a time dispersion effect, which means that pulse energy of a predetermined symbol is dispersed to neighboring symbols, generates interference to the neighboring symbols. Furthermore, the transmitted data is affected from various channel distortions.
As channel distortion, there are such as a multipath, a frequency offset and a phase jitter. The channel distortion occurs an Inter-symbol Interference (ISI), which means interference or a distortion between a predetermined symbol and its neighboring symbols and generates a problem to receive desired data in the digital transmission system.
To prevent the channel distortion and decrease a symbol error due to an ISI, the general receiver, e.g., a digital broadcasting receiver, uses a channel equalizer.
Most of the communication channels need an adaptive equalizer for updating a tap coefficient according to time because the distortions in the general digital broadcasting system are time variant.
Configuration of a conventional channel equalizer is described in FIG. 1.
As shown in FIG. 1, the conventional channel equalizer includes a digital filter 11, a symbol detector 12, a tap coefficient updating unit 13, a training sequence storage 14, a statistical data calculator 15, a switch 16 and an equalizer input signal storage 17.
The digital filter 11 eliminates an inter symbol-interference (ISI) causing a distortion of a baseband signal received to a digital broadcasting receiver. The symbol detector 12 receives the output signal of the digital filter 11 and calculates a decided data by comparing the output signal to a predetermined threshold to thereby output the decided data.
The tap coefficient updating unit 13 receives output signals of the equalizer input signal storage 17 and the digital filter 11 and an error data selected by the switch 16 to thereby update a tap coefficient of the digital filter 11.
Also, the training sequence storage 14 stores the training data sequence already known in a transmitter of a digital broadcasting system. The training data sequence is read in a training mode and is outputted to the tap coefficient update unit 13.
The statistical data calculator 15 calculates a statistical error in order to output the statistical error to the tap coefficient update unit 13 in a blind mode.
The switch 16 selects one of the training storage 14, the statistical data calculator 15 and the symbol detector 12 according to an operating mode and outputs error data outputted from the selected one to the tap coefficient update unit 13. Then, the tap coefficient update unit 13 calculates an error signal corresponding to the error data and updates the tap coefficient by using data corresponding to the tap coefficient of the digital filter 11 in the equalizer input signal storage 17 to output the tap coefficient to the digital filter 11.
As the channel equalizer, the decision feedback equalizer (DFE) is used broadly in the digital broadcasting receiver. In general, the an eye diagram of an output signal in the decision feedback equalizer is opened, wherein the eye diagram is a kind of factor for determining performance of the equalizer, i.e., a function of making an output signal decision rightly and easily. Also, if an output signal of the symbol detector is determined correctly, the feedback filtering block deletes an Inter Symbol Interference (ISI) according to the predetermined symbol and there is not a problem such as a noise amplifying phenomenon in channel equalization of the a linear equalizer. Therefore, the decision feedback equalizer is widely used in the digital broadcasting receiver.
Therefore, for an appropriate use, it is important that the output signal of a symbol detector should not have a decision error and, for the most of all, the eye diagram is opened.
For the eye diagram is opened in Advanced Television System Committee (ATSC) digital broadcasting system which is American standard for a terrestrial digital television, it is used a method for opening the eye diagram by inserting one training sequence segment, wherein one segment is 208 bytes, in every 312 data segment although a data efficiency is decreased.
However, since the inserted training sequence segment is short under a multipath environment having a long ghost, it is often failed to open the eye pattern. In particular, though there is the training sequence, it is often failed if a tap coefficient is not enough long to converge or if the environment is under an inferior multipath environment having a time-varied channel, a long ghost or a large level signal ghost. If the eye diagram is not opened, possibility of the decision error in the symbol detector is very high. As a result, it can be generated an error propagation problem that the decision error is accumulated through a feedback loop of the decision feedback equalizer.
Therefore, it is required a method for decreasing the decision error during data period having no training sequence segment; and, in particular, total tap energy should be decreased in order to decrease the decision error by opening the eye diagram of the decision feedback equalizer.
At first, most of conventional methods for decreasing the decision error use a viterbi decoder having a decoding delay. There is a method for making a predetermined delay in an equalizer tap coefficient adjustor be identical to the decoding delay of the viterbi decoder. Herein, this method is proposed by G. Long, entitled “The LMS Algorithm with Delayed Coefficient Adaptation”, IEEE Trans. Acoust., Speeach, Signal Processing, vol. ASSP-37, October 1989.
In addition, there is a method for solving a decoding delay of the viterbi decoder by periodically adding an interleaver and deinterleaver, proposed by M. V. Eyuboglu, entitled “Detection of Coded Modulation Signals on Linear, Severely Distorted Channels Using Decision-Feedback Noise Prediction with Interleaving”, IEEE Trans. Commun., vol. COM-36, pp. 401-409, April 1988 and in U.S. Pat. No. 4,833,693 issued to Eyuboglu, May 23, 1989.
Because above described methods for decreasing the decision error are used in the viterbi decoder having the decoding delay as TDB-1 behind the equalizer served as the symbol detector of the decision feedback equalizer, it is required that additional devices for deleting the decoding delay. Also, for the viterbi decoder behind the equalizer has enough performance, the TBD should be 5 times more than the number of memories in the trellis encoder used for encoding process. However, the decoding delay should be as small as possible in order to use the output of the viterbi decoder as a feedback input of the decision feedback equalizer.
Particularly, the decoding delay is not TBD-1 but 12×(TBD-1) in the digital broadcasting system which uses 12 TCM encoders by the trellis code interleaver as shown in FIG. 5; and, approximately, the decoding delay becomes 168 because there are 2 memories in the TCM encoder. It is very inefficient that the viterbi decoder having 168 decoding delay is actually used in the digital broadcasting system.
Therefore, for using the viterbi decoder as the symbol detector of the decision feedback equalizer in the digital broadcasting receiver, the decoding delay is as small as possible and, further, it is the best that there is no decoding delay. And a complexity for decoder implementation should be as low as possible.
Meanwhile, in order to decrease the tap energy of the feedback filter, there are methods for increasing the tap number which removes post ghost and for changing channel property of a receiving signal by using a beam-forming or a channel-matched filter.
The method for increasing the tap number of the feed forward filter is inefficient and the improvement of the performance is small in comparison to the increased tap number. The method for changing the channel property by using the channel-matched filter, which is proposed in a document by Richard Citta, entitled “A VSB Receiver Designed for Indoor and Distributed Transmission Environments”, IEEE 52nd Annual Broadcast Symposium, Oct. 9-11, 2002, is more effective than the method for increasing the tap number.
Because the channel equalization method proposed by Richard Citta generates the channel-matched filter based on over-sampling data and uses a fractionally-spaced equalizer, the complexity is very high. Also, since a simple slicer is used as the symbol detector, the error propagation problem due to the decision error can occur.
Therefore, development for a channel-matched filter having low complexity on a symbol basis and a symbol detector having rare decision error are highly required.
Disclosure
Technical Problem
It is, therefore, an object of the present invention to provide an apparatus and a method of decision feedback equalization that make a channel property of an inferior receiving signal to mild by using a channel-matched filter and decreases decision errors of symbol detector output signals by using a trellis decoder with decreased complexity, whose trace back depth is 1 (TBD=1).
The other object of the present invention provide a symbol detection method of the trellis decoder with decreased complexity, of which TDB is 1, which can use an output signal of trellis decoder as an input signal of the feedback filter of the decision feedback equalizer by decreasing a decoding delay.
Technical Solution
In accordance with one aspect of the present invention, there is provided a decision feedback equalizer in a terrestrial digital broadcasting receiver, including: a channel estimating unit for estimating a channel of a symbol-based receiving signal based on the receiving signal and a training sequence; a channel-matched filtering unit for changing a channel property of the receiving signal by maximizing a signal-to-noise ratio (SNR) of the estimated channel; an input signal storing unit for storing a receiving symbol of which channel property is changed by the channel-matched filtering unit; a channel equalizing unit for performing a decision feedback equalization by repeatedly filtering the receiving signal which passed through the channel-matched filtering unit; a trellis decoding unit for detecting a symbol, which is decision data, based on trellis decoding algorithm with decreased complexity, whose trace back depth is 1, from channel equalized receiving symbols and outputting the symbol in a decision directed mode; a statistical data calculating unit for calculating statistical error data used in a blind mode and outputting the statistical error data; a training sequence storing unit for storing the training sequence; a switching unit for selecting a mode among the training mode, the decision directed mode and the blind mode; an error signal calculating unit for calculating an error signal by comparing an output signal in the mode selected by the switching unit to an output signal of the channel equalizing unit; and a tap coefficient updating unit for updating a tap coefficient to be provided to the channel-matched equalizing unit based on the error signal, the output signal of the trellis decoding unit and the output signal of the input signal storing unit.
In accordance with one aspect of the present invention, there is provided a decision feedback equalizing method in a terrestrial digital broadcasting receiver, including the steps of: a) estimating a channel of a symbol-based receiving signal based on a receiving signal and a training sequence; b) changing a channel property of the receiving signal in order to maximize a signal-to-noise ratio (SNR) of the estimated channel by passing the receiving signal through a channel-matched filter; c) determining a parameter used for a decision feedback of the receiving symbol whose channel property is changed and initializing a channel equalization parameter; d) detecting a symbol from an output signal of an equalizer in a specific time index signal according to the determined parameter based on a trellis decoder with decreased complexity, whose trace back depth is 1; e) calculating statistical error data used in a blind mode; f) selecting one mode among a training mode, a decision mode and the blind mode; g) calculating an error signal by comparing an output signal of the mode selected in the step f) to output signal of equalizer in a decision directed mode, and updating a tap coefficient based on the error signal; and h) performing a decision feedback equalization based on the updated tap coefficient.
In accordance with one aspect of the present invention, there is provided a symbol detecting method for channel equalization in a terrestrial digital broadcasting receiver, the method including the steps of: a) calculating an absolute distance pairs between symbol pairs in an input signal of a symbol detector and a trellis diagram; b) selecting an absolute distance having a small value for each absolute distance pair among absolute distance pairs; c) calculating an accumulated absolute distance by adding a previous absolute distance to a current calculated absolute distance for each state in the trellis diagram in a time index; d) deleting the accumulated absolute distances except the smallest accumulated distance for each state in the trellis diagram in the time index; e) selecting a state in which the accumulated absolute distance is smallest among all states shown in the trellis diagram in the time index and obtaining an output signal of the symbol detector, which is the trellis decoding unit, from a branch shown in the trellis diagram transited to the selected state; and f) repeatedly performing the steps a) to e) for each symbol time index.
The present invention relates to an apparatus and a method for decision feedback equalization having a channel-matched filter and a trellis decoder in a terrestrial digital broadcasting receiver, wherein the channel estimator estimates the channel of the receiving signal based on the receiving signal and the training sequence, generates the channel-matched filter based on information of the estimated channel, equalizes the receiving signal passed though the channel-matched filter by using the decision feedback equalizer having the viterbi decoder with decreased complexity, whose TBD is 1, and thereby the channel equalization is performed effectively under an inferior environment such as in a room or in mobile
The present invention can change the channel property of the receiving signal under an inferior environment such as in a room or in mobile to be mild by placing the channel-matched filter generated from the symbol-based channel estimator in front of the conventional decision feedback equalizer, and decreases the decision error of the symbol detector by using the viterbi decoder with decreased complexity, whose TBD is 1, in stead of the simple slicer served as the symbol detector in the conventional decision feedback equalizer and thereby a convergence speed and stability of the digital filtering block are increased, and the residual MSE is decreased in a normal state after the convergence.
Advantageous Effect
As mentioned above, the present invention can change the channel property of the receiving signal under an inferior environment such as in a room or in mobile to be mild by placing the channel-matched filter generated from the symbol-based channel estimator in front of the conventional decision feedback equalizer, and decreases the decision error of the symbol detector the viterbi decoder with decreased complexity, whose TBD is 1, in stead of the simple slicer served as the symbol detector in the conventional decision feedback equalizer and thereby a convergence speed and stability of the digital filtering block are increased, and the residual MSE is decreased in a normal state after the convergence.