KR20070075542A - Equalizer - Google Patents

Abstract

An equalizer is provided to adaptively compensate a channel having a long signal delay and a time-varying channel. An equalizer includes a pilot extractor(20) for extracting pilot symbols from a received signal, a time interpolator(30) for time-interpolating the pilot symbols output from the pilot extractor, and a channel characteristic determination unit(65) for determining whether a channel which transmits the time-interpolated signal is a time-varying channel or a channel having a long signal delay by using a channel impulse response of the time-interpolated signal. The equalizer further includes a pilot selector(35) for receiving channel characteristic determined by the channel characteristic determination unit and determining whether the time-interpolated pilot symbols are selected from the symbols output from the time interpolator, and a frequency interpolator(40) for interpolating symbols output from the pilot selector in a frequency domain to calculate a channel.

Description

Equalizer

1 is a diagram illustrating a pattern of a transmission signal of DVB-T or DVB-H including a distributed pilot.

2 is a structural diagram illustrating an example of an equalizer capable of compensating for a channel of a transmission signal

3 is a structural diagram showing an embodiment of a preferred equalizer according to the present invention

<Description of Symbols of Major Parts of Drawings>

10: temporary storage unit 20: pilot extractor

30: time interpolator 35: pilot selector

40: frequency interpolator 50: compensator

60: Fourier transform section 65: channel characteristic determination section

70: symbol offset detection unit

The present invention relates to an equalizer, and more particularly, to an equalizer capable of adaptively responding to channel changes.

As one of the modulation and demodulation methods of a broadcast signal, orthogonal frequency division multiplexing (OFDM) can extend the symbol period by the number of subcarriers, and is robust to a frequency-selective fading channel by multipath and has a narrowband interference signal. Since only a part of carriers is affected, it is widely used for high speed data transmission method of wired and wireless. Currently, OFDM-based broadcasting standards include digital audio broadcasting (DAB), digital video broadcating terrestrial (DVB-T), and digital video broadcating handheld (DVB-H). In addition, the ADSL and VDSL using the existing line is using the DMT method, which can be called a wired OFDM method.

DVB-T (Digital Video Broadcasting Terrestrial) supports two transmission modes, 2K and 8K. In 2K mode, the mobile reception performance is excellent. In 8K mode, a long channel delay occurs in a single frequency network (SFN) channel. It can have robust characteristics.

However, in case of portable reception, both time-varying channel and long delay channel should be considered. In the case of digital video broadcasting terrestrial (DVB-T), each channel characteristic has mutual weakness, so DVB-H supports 4K transmission mode.

The receivers of the transmission signals of the DVB-T and DVB-H can perform channel estimation using this distributed pilot. The distributed pilot includes pseudo random binary sequences (PRBS) that the transmitting end and the receiving end know each other, and may be transmitted at 4/3 times greater power than the data signal.

The DVB-T and DVB-H transmission methods use time slicing to reduce power consumption of the mobile terminal, which divides the transmission capacity into a certain time slot and then sends a packet to each time slot. It is a method of transmitting a normalized broadcast signal.

In the DVB-T and DVB-H transmission schemes, the receiver of the signal interpolates received symbols in the time domain and estimates a channel by interpolating in the frequency domain. When the received symbols are time interpolated, channel estimation performance for a wide SFN channel can be improved.

However, the interpolation performed in the time domain has a problem that the channel estimation error may be increased when the channel changes rapidly with time. The center frequency range used by DVB-T and DVB-H uses the bands from VHF to UHF, which can suffer from poor Doppler transitions, especially in the UHF bands of 700 to 800 MHz. There is a problem that can increase the error of channel estimation.

In addition, if the order of interpolation is increased for more accurate time domain interpolation, the number of OFDM symbols to be stored in the storage device increases exponentially and the complexity of channel estimation increases.

SUMMARY OF THE INVENTION The present invention has been made to solve the above problems, and an object of the present invention is to provide an equalizer capable of adaptively channel compensation for a channel having a long signal delay occurring in SFN and a time-varying channel that may occur in portable reception. It is.

Another object of the present invention is to provide an equalizer capable of channel compensation to have robust characteristics in a time-varying channel without increasing the order of the interpolation filter in the time domain.

In order to achieve the above object, the present invention includes a pilot extractor for extracting and outputting a pilot symbol from a received signal; A time interpolation unit outputting time-interpolated pilot symbols output by the pilot extractor; A channel characteristic determiner determining whether a channel to which the signal is transmitted is a time-varying channel or a long delay spread channel using a channel impulse response of the time interpolated signal; A pilot selector which receives the channel characteristic determined by the channel characteristic determiner and determines whether to select a time-interpolated pilot symbol among symbols output by the time interpolator according to the channel characteristic; And a frequency interpolator configured to calculate a channel by interpolating symbols output by the pilot selector in a frequency domain.

The equalizer includes a temporary storage unit for temporarily storing and outputting the received symbols; And a compensation unit for receiving symbols output from the temporary storage unit and the frequency interpolator, respectively, and compensating for channels of symbols output from the temporary storage unit.

The time interpolator may time interpolate successively input pilot symbols at the same subcarrier position.

When the square of the channel impulse response of the input symbol is equal to or greater than a predetermined value, the channel characteristic determiner determines that the channel characteristic of the symbol is a long delay spread channel and outputs the information accordingly.

The channel characteristic determiner may calculate a channel characteristic by summing a square of the channel impulse response in a signal section having a predetermined length.

The channel characteristic determiner may output a flag value according to whether a channel characteristic of an input signal is a time-varying channel or a long delay spread channel.

The pilot selecting unit outputs only the pilot symbols extracted by the pilot extracting unit among the input pilot symbols when the channel characteristic determining unit outputs information indicating that the channel characteristics of the input signal are time-varying channels, and the channel characteristic determining unit outputs the pilot symbols. When outputting information indicating that the channel characteristic of L is a long delay spread channel, it is preferable that the time interpolator outputs interpolated pilot symbols.

Hereinafter, with reference to the accompanying drawings, preferred embodiments of the present invention that can specifically realize the above object will be described.

1 is a diagram illustrating a pattern of a transmission signal of a DVB-T or DVB-H including a distributed pilot. The transmission signal pattern of the DVB-T or DVB-H will be described with reference to FIG. 1.

1 is a direction in which the frequency of subcarriers transmitted at the same time is increased (frequency axis), and a vertical direction is a direction in which time proceeds (time axis). Circles arranged in a horizontal line indicate the positions of subcarriers forming an OFDM symbol.

The distributed pilot is repeated at every 12 subcarrier positions in the OFDM symbol transmitted at the same time. In the OFDM symbol transmitted at the next time, the distributed pilot may be located at a position where the subcarrier frequency interval is separated by 3 from the subcarrier position of the distributed pilot in the OFDM symbol transmitted at the previous time. Thus, the same distributed pilot pattern can be repeated every four OFDM symbols consecutive in time.

In FIG. 1, Tu represents the number of usable subcarriers, Dt represents a distance between distributed pilots on the time axis, and Df represents a distance between distributed pilots on the frequency axis, respectively.

The distance Df between the distributed pilots in the frequency domain determines the delay range of the ghost that can be estimated in the channel.

Since the same pilot pattern appears every four input symbols, time interpolation can be performed at the same pilot position. That is, the first input symbol (t = 1) represents the same symbol pattern as the symbol input at t = 5, and the time interpolation for the symbol input at t = 2, 3, 4 at the distributed pilot of the symbols is Can be performed.

The symbol input at t = 6 has the same distributed pilot pattern as the symbol input at t = 2, and t = 3,4,5 on the distributed pilot position of the symbol input at t = 6 and the symbol input at t = 2. Time interpolation may be performed on the symbols of.

Therefore, if a symbol is input at t = 7 and then time interpolated in the above manner, the symbols input at t = 4 are distributed in the frequency domain of the input symbol at t = 4 since distributed pilots are located at four subcarrier positions. The spacing between pilots is originally reduced to one quarter of the spacing between distributed pilots, and a symbol input at t = 4 has a pattern in which distributed pilots are located every four subcarrier positions.

2 shows an example of an equalizer capable of compensating for a channel of a transmission signal of DVB-T or DVB-H. An example of an equalizer capable of compensating for a channel will be described with reference to FIG. 2.

The temporary storage unit 10 temporarily stores and outputs an input symbol of an equalizer. The pilot extractor 20 extracts a pilot symbol included in the received signal to calculate a channel. An example of calculating the channel by the pilot extractor 20 may use a zero forcing method. The time interpolator 30 interpolates the extracted pilot symbols in the time domain. The frequency interpolator 40 performs interpolation of the frequency domain on the symbols input at the same time among the time interpolated symbols, and the compensator 50 uses the channel characteristics output from the frequency interpolator 50. By performing the channel compensation of the received signal stored in the temporary storage unit 10.

The inverse Fourier transform unit 60 may convert input symbols in the frequency domain into signals in the time domain, and the symbol offset detector 70 may detect offsets of the symbols in the time domain.

The equalizer according to the present invention may determine whether to perform interpolation, in particular, whether to interpolate a symbol in the time domain according to the channel characteristics of the input signal, and thereby compensate the channel according to each channel characteristic. Hereinafter, an embodiment of a preferred equalizer according to the present invention will be described.

3 is a structural diagram showing an embodiment of a preferred equalizer according to the present invention. The same components as the above description are given the same names and the same reference numerals for convenience of description, and detailed description thereof will be omitted.

First, the time interpolator 30 performs time interpolation on the pilot symbols extracted by the pilot extractor 20. When the inverse Fourier transform unit 60 converts the interpolated symbols in the frequency domain into signals in the time domain, the channel characteristic determiner 65 determines the channel characteristics of the interpolated and converted signals in the time domain. The channel characteristic determiner 65 determines whether the channel over which the signal is transmitted is a long delay spread channel or a time varying channel. The determined information is then output to the pilot selector 35 and the frequency interpolator 40.

When the pilot selector 35 receives information from the channel characteristic determiner 65 that the channel to which the received signal is transmitted is a time-varying channel, the pilot selector 35 converts the extracted pilot symbol and the time-interpolated symbol of the symbol into the frequency interpolator 40. To pass). On the other hand, when receiving the information that the received signal is a channel having a long delay spread, the extracted pilot symbol does not select the time-interpolated symbol, and selects only the pilot symbols extracted by the pilot extractor 20 to frequency Output to the executive 40.

The frequency interpolator 40 interpolates the input symbols according to the channel information in the frequency domain by using the channel information received from the channel characteristic determiner 65. The compensator 50 performs channel compensation by using the output symbol of the frequency interpolator 40 and the output symbol of the temporary storage unit 10.

The channel characteristic determiner 65 of the equalizer according to the present invention determines the channel state of the received signal. When detecting the channel state of the signal, whether the delay spread or the time-varying channel of the maximum channel can be determined as the magnitude of the square of the channel impulse response. That is, if the size is larger than the user-definable threshold, it is determined that the channel has a property of long delay spread, otherwise it is determined that it is a time-varying channel. Equation 1 represents a case where a channel for each response is determined by the square of the magnitude of the channel impulse response.

However, in the case of a channel having poor reception performance due to poor signal to noise ratio (SNR) of the receiver, it is necessary to accumulate the value obtained by Equation 1 in an appropriate interval to improve reliability in determining the maximum delay spread. desirable.

The win size of Equation 2 represents the accumulation of the channel impulse response.

As shown in Equation 3, it is possible to determine whether the channel is a long delay spread channel or a time-varying channel according to the accumulated channel impulse response, and the time-interpolated channel may be selected only in the case of a long delay spread channel.

Here, Tu represents the number of subcarriers. Equation 3 shows an example in which the channel characteristic determiner 65 adaptively determines a channel and outputs a flag signal. That is, Tu / 24, the determiner of Equation 3, may use a different value or may output a value other than a flag.

Accordingly, the channel may be adaptively determined and compensated for the long delay spread channel and the time-varying channel.

Referring to the effect of the equalizer according to the present invention described above are as follows.

First, the equalizer according to the present invention can adaptively compensate for a channel having a long signal delay occurring in SFN and a time-varying channel that may occur in portable reception.

Second, according to the equalizer according to the present invention, the channel compensation can be performed so that the robustness of the time-varying channel can be achieved without increasing the order of the interpolation filter in the time domain.

Claims (7)

A pilot extracting unit extracting and outputting a pilot symbol from a received signal; A time interpolation unit outputting time-interpolated pilot symbols output by the pilot extractor; A channel characteristic determiner determining whether a channel to which the signal is transmitted is a time-varying channel or a long delay spread channel using a channel impulse response of the time interpolated signal; A pilot selector which receives the channel characteristic determined by the channel characteristic determiner and determines whether to select a time-interpolated pilot symbol among symbols output by the time interpolator according to the channel characteristic; And And a frequency interpolator configured to calculate a channel by interpolating symbols output by the pilot selector in a frequency domain. The method of claim 1, The equalizer includes a temporary storage unit for temporarily storing and outputting the received symbols; And An equalizer further comprising a compensator configured to receive symbols output from the temporary storage unit and the frequency interpolator to compensate channels of symbols output from the temporary storage unit; The method of claim 1, The time interpolator is a time equalizer, characterized in that for interpolating the successive pilot symbols in the same subcarrier position. The method of claim 1, And if the square of the channel impulse response of the input symbol is equal to or greater than a predetermined value, the channel characteristic determiner determines that the channel characteristic of the symbol is a long delay spread channel, and outputs determination information accordingly. The method of claim 4, wherein And the channel characteristic determiner calculates a channel characteristic by summing a square of the channel impulse response in a signal section having a predetermined length. The method of claim 1, And the channel characteristic determiner outputs a flag value according to whether the channel characteristic of the input signal is a time-varying channel or a long delay spread channel. The method of claim 1, The pilot selecting unit outputs only the pilot symbols extracted by the pilot extracting unit among the input pilot symbols when the channel characteristic determining unit outputs information indicating that the channel characteristic of the input signal is a time-varying channel, and the channel characteristic determining unit outputs the pilot symbols. And outputting the interpolated pilot symbols when the channel characteristics of the channel information are long delay spread channels.

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