JP3325731B2 - Adaptive equalization method in mobile communication - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an adaptive equalization system for compensating for frequency selective fading which occurs in land mobile communication and the like and hinders high speed transmission.

[0002]

2. Description of the Related Art Viterbi equalizers and Decision Feedback Equalizers (DFEs) are known as adaptive equalization methods for preventing such frequency selective fading.
Was effective. However, the Viterbi equalizer increases the number of states to be considered in Viterbi decoding when the range of delay time to be compensated increases or when applied to a multilevel modulation scheme such as 16QAM. Since the amount of calculation increases exponentially, implementation becomes difficult. On the other hand, DFE can be realized on a relatively small scale, and can be equalized even when the delay time is large. FIG. 7 shows a configuration diagram of the DFE. The feedforward filter (hereinafter referred to as FF filter) 1 in the first stage in the figure is
A matching process for obtaining a diversity effect incorporating a direct wave and a delayed wave is executed. A feedback filter (hereinafter, referred to as an FB filter) 2 at a subsequent stage removes a residual delay wave by feeding back a determination output from the detector 4. An equalization function is realized by such a diversity combining and canceling operation of the multiple wave components. In land mobile communication, D / U ( the ratio of the power of the direct wave to the power of the delayed wave: Desire
to Undesire Power Ratio) changes frequently. Here, when D> U, it is called a minimum phase system, and when D <U, it is called a non-minimum phase system. As described above, the residual delay wave is canceled in the adder 3 by the output of the FB filter 2. In the case of a non-minimum phase system, the level of the delay wave is large, so it is difficult to cancel and the equalization characteristics deteriorate. Therefore, the number of taps of the FF filter 1 is increased so that the residual delay wave does not remain. However, as in the case of the Viterbi equalizer, the amount of calculation increases rapidly in proportion to the square of the number of taps.

[0003] In order to improve such a drawback of the DFE, a bidirectional equalization system has been proposed. FIG. 8 shows the configuration of the bidirectional equalization system. The operation will be described according to the configuration. First, as shown in FIG. 9, the frame structure in this system is such that a training signal is arranged at both ends of a frame with data interposed therebetween, and a preamble signal (UW
1), called a postamble signal (UW2). After the received signal is A / D converted by the A / D converter 5, the frame synchronization signal is captured by the frame synchronization circuit 6 and stored in the reception buffer 7. Next, equalization is performed from both directions from the front of the reception buffer 7 and the rear of the reception buffer 7 by using two of the DFE 8 for forward equalization and the DFE 9 for backward equalization, and the primary accumulation is performed in the output buffers 10 and 11. I do. Next, the respective equalization errors are primarily stored in error buffers 12 and 13, which are compared by a comparator 14 to select a better equalized output by controlling a switch 16 with a direction controller 15. It is a method.

[0004]

In this two-way equalization method, since the backward equalization is performed by inverting the time, the delayed wave is processed as a direct wave. That is, D <
Since the condition of U can be processed assuming that D > U, the FF file
The equalization characteristics in the non-minimum phase system can be greatly improved without increasing the number of taps of the filter 1. However, in this method, since the equalization is performed from both directions, there is a disadvantage that the amount of calculation is inevitably doubled.

An object of the present invention is to greatly improve the equalization characteristics with a small increase in the amount of calculation for the conventional DFE, and to degrade the equalization characteristics for the bidirectional equalization method. It is an object of the present invention to provide an adaptive equalization method in mobile communication capable of facilitating real-time processing and reducing a hardware scale by reducing a huge amount of computation, which is a conventional disadvantage.

[0006]

In order to achieve this object, the adaptive equalization system in mobile communication according to the present invention uses a non-minimum phase system (D / U) in the case of a conventional bidirectional equalization system. In the case of <U), paying attention to the fact that equalization from the reverse direction of time is effective, if D / U can be detected in advance, it is not necessary to perform equalization in both directions. For a phase system (D> U), equalization only in the time direction is performed in the same manner as the conventional DFE, and for a non-minimum phase system (D <U), equalization only in the reverse time direction is performed. The configuration is such that the amount of calculation can be significantly reduced by selecting the conversion direction.

[0007]

BRIEF DESCRIPTION OF THE DRAWINGS FIG. FIG. 1 shows a block diagram of one embodiment of the present invention. A frame configuration similar to that of the bidirectional equalization system is used. After the received signal is converted into a digital signal by the A / D converter 20, a synchronization signal of a wave having a high average power is captured by the frame synchronization circuit 21 and stored in the reception buffer 22. At this time, in consideration of the case where the D / U is synchronized with the delay wave (U wave) when the D / U is in the non-minimum phase system, the maximum delay time expected on the propagation or the number of taps of the equalizer Is stored in the reception buffer 22 before the maximum equalizable delay time (τ) determined by the above.
Also, the length of the receiving buffer is ensured to be longer than the actual frame length by τ or more. Then the received signal to have you in the unique word correlator (following UW correlator) 23 and the known unique word (training signal: preamble or post-amble)
And a cross-correlation operation is performed. The cross-correlation is shown in equation (1).

[0008]

(Equation 1) The correlation value sequence y _j is detected in a range from the head of the reception buffer 22 to 2τ. FIG. 2 shows an example of the detected correlation value.

Next, the correlation peak detector 24 detects a point where the peak value of the correlation value is the maximum and a second point from the correlation value y. In the case of a two-wave model, these two points can be regarded as a direct wave (D wave) and a delayed wave (U wave). Next, it is determined by D / U determination whether the D / U of this processing frame is a minimum phase system or a non-minimum phase system. As shown in FIG. 2, when the point having the largest correlation value is before and the second point is after on the time axis, D> U, and it can be regarded as the minimum phase system. Conversely, as shown in FIG. 3, when the maximum point is on the time axis and the second point is on the time axis, D <
U, which can be regarded as a non-minimum phase system. As a result, in the case of the minimum phase system in the direction controller 25, the direction switcher 28 performs the equalization process in the forward direction from the point of the D wave.
To the forward direction, and conversely, for non-minimum phase systems, U
The direction switch 28 is switched in the opposite direction so that the equalizer 26 performs the equalization process with the time axis reversed after the frame length from the wave arrival point. The switch 29 on the output side of the output buffer 30 after the equalization is similarly switched.

The above example is for the two-wave model,
The radio waves arriving on the actual transmission path are not necessarily two waves, but may be only one wave or a case where a plurality of three or more delayed waves are combined. Therefore, a processing method considering these conditions will be described. First, as in the above-described example, the UW correlator 23 performs a cross-correlation operation between the received signal and the known unique word. Next, the correlation peak detector 2
4, the maximum peak point is detected. Next, a threshold based on the maximum peak point is set. This threshold value is set to a value between 0% and 100% of the amplitude of the maximum peak value. The reason for setting the threshold value is that when the level of the delayed wave is small enough to be regarded as noise in performing the equalization process, the delayed wave can be ignored.
Next, a plurality of peak points exceeding the threshold value _Vth are detected from the correlation value sequence y in descending order of the amplitude of the correlation value. These plural peak points are the portions shown in FIG. If the second peak exceeding the threshold is not detected, it is determined that only direct waves are present, and a forward equalization process is performed from the peak point. Also,
If the third and fourth peaks exceeding the threshold are detected, it is assumed that there are a plurality of delayed waves to be equalized,
The equalizing direction is selected from the positional relationship on the time axis. In this method, among three or more detected correlation peaks, the direction is determined by regarding the amplitude ratio between the earliest correlation peak and the latest correlation peak on the time axis as D / U. As shown in FIG. 5, when the first correlation peak on the time axis is larger than the last correlation peak, the minimum phase system (D>
U). Conversely, if the earliest correlation peak on the time axis is smaller than the last correlation peak as shown in FIG. 6, it can be regarded as a non-minimum phase system (D <U). As a result, as in the case of the two-wave model, in the case of the minimum phase system, the direction switch is switched in the forward direction so as to perform equalization processing in the forward direction from the point of the D wave. Switches the direction switch to the opposite direction in order to perform the equalization processing in the opposite direction on the time axis after the frame length from the arrival point of the U wave. By the above processing, even when a plurality of multipaths arrive, the direction of equalization can be accurately detected.

[0011]

According to the present invention, it is not necessary to perform equalization from both directions by detecting the D / U in advance and determining the direction of the time axis for equalization, and the amount of calculation is reduced to about half that of the conventional method. And real-time processing is facilitated, and the hardware scale can be reduced.

[Brief description of the drawings]

FIG. 1 is a configuration diagram showing an embodiment of the present invention.

FIG. 2 is a time chart showing a correlation value sequence in a minimum phase system.

FIG. 3 is a time chart showing a correlation value sequence in a non-minimum phase system.

FIG. 4 is a time chart for explaining detection of a plurality of correlation peaks.

FIG. 5 is a time chart for explaining a correlation value sequence (D> U) when a multipath arrives.

FIG. 6 is a time chart for explaining a correlation value sequence (D <U) when a multipath arrives.

7 is a block diagram showing a basic configuration of the DF E (Decision Feedback Equalizer).

FIG. 8 is a diagram illustrating a configuration example of a conventional bidirectional equalization method.

FIG. 9 is a diagram illustrating an example of a frame configuration in a case of a bidirectional equalization method.

[Explanation of symbols]

Reference Signs List 1 FF filter 2 FB filter 3 Adder 4 Detector 5 A / D converter 6 Frame synchronization circuit 7 Receive buffer 8 DFE for forward direction 9 DFE for reverse direction 10, 11 Output buffer 12, 13 Error buffer 14 Comparator 15 Direction Controller 16 Switch 20 A / D Converter 21 Frame Synchronization Circuit 22 Receive Buffer 23 UW Correlator 24 Correlation Peak Detector 25 Direction Controller 26 Decision Feedback Equalizer (DFE) 28,29 Switching Device 30 Output Buffer

Claims (1)

(57) [Claims] 1. A signal wirelessly transmitted from a transmission side is received and stored in a reception buffer in a time-series manner.
From the power ratio D / U of the direct wave D and the delayed wave U, the stored
Whether the received signal sequence is the minimum phase system or non-minimum phase system
Is determined by the D / U determination means, and the equalization processing direction is selected in the time axis direction or the reverse direction in accordance with the determination result, and the stored feedback signal sequence is equalized by the determination feedback equalizer. a structure adaptive equalization scheme as the D / U determination means, a correlation value by performing correlation calculation between the received samples and the known unique word of the received signal sequence stored in the receiving buffer A unique word correlator that calculates and outputs a correlation value sequence, and detects from the correlation value sequence
0% to less than 100% of the amplitude of the maximum peak value
Is set as a threshold value, and
Correlation peak to detect multiple correlation peak points exceeding the threshold
A peak detector and a correlation peak before the plurality of correlation peak points.
The last correlation peak value is compared with the last correlation peak value.
The received signal is greater than the last correlation peak value.
The sequence is determined to be a minimum phase system (D> U) and the received signal
Equalization processing in the time axis direction from the point when the wave D arrives directly in the column
And conversely, the last correlation peak value is the previous correlation peak value
When the received signal sequence is larger than the non-minimum phase system (D <
U) and the delayed wave U arrives at the received signal sequence.
The time axis in the reverse direction after the processing frame length
Direction control to perform equalization processing to obtain equalized output
A plurality of delayed waves U, and a direct wave D can be efficiently transmitted even when a plurality of delayed waves U arrive.
Mobile communication characterized in that a conversion process is performed
Adaptive equalization scheme in.