JPH09162784A - Roll-off filter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce the number of multipliers and to shorten calculation time by performing exact calculation concerning a main lobe and performing approximate calculation of response waveforms of other sections by the use of sine waves. SOLUTION: Filter outputs are respectively calculated by a main lobe calculator 11 and a side lobe calculator 12 and added by an adder 13. A delay element 111 delays an input signal so that the response of main lobe can be linked behind the response section of side lobe. A main lobe response calculator 112 uses a transversal filter composed of a delay element, multiplier, tap line and adder. On the other hand, the side lobe adder 12 is composed of a waveform generator 121 for generating the waveform of side lobe while using the approximate calculation of coefficient due to the sine wave and side lobe response calculators(SLC) 122 and 123 for respectively calculating one side of side lobe. Then, the SLC 122 and 123 are added by an adder 125 and outputted to the adder 13.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a roll-off filter provided to limit a band in transmission of digital signals, and more particularly to a configuration of a digital circuit of a roll-off filter having a small roll-off rate.

[0002]

2. Description of the Related Art In the transmission of digital signals, an LPF is required to remove high frequency components and limit the band.
A commonly used LPF is a DTL (D
It is a roll-off filter that uses an elapsed Tap Line). By using DTL, it is possible to create a filter with a smaller roll-off rate α. The filter includes, as its constituent elements, a multiplier 22 and a delay tap 23 in addition to the plurality of delay elements 21 and the adder 24.
The number N is required. The number N of delay taps is proportional to the total number of samples required to calculate the filter output, that is, the number of samplings per symbol and the number of symbols used for filter calculation. The larger N is, the higher the frequency component of the filter output is removed. It will be easier.

[0003]

By the way, when designing a filter having a small roll-off rate α, it is difficult for the tap coefficient │c _k │ (k is a natural number of 1 to N) of the filter to converge. Need to be bigger. That is, when the roll-off rate α is reduced, the number of multipliers 22 increases. In general, a multiplier has a larger circuit scale and consumes more power than an adder or the like. Therefore, an increase in the number of multipliers has a large effect on the entire circuit, and in the conventional method, the performance improvement of the filter requires an increase in the scale of the entire device.

Therefore, an object of the present invention is to provide a roll-off filter which reduces the amount of calculation and is effective in reducing the circuit scale.

[0005]

A roll-off filter according to the present invention is a main lobe calculator for calculating a main lobe of an impulse response using a transversal filter, and an approximate calculation by a sine wave for side lobes other than the main lobe. And a side lobe calculator that performs calculation using the above, and an adder that adds the output of the main lobe calculator and the output of the side lobe calculator.

The main lobe calculator is composed of a delay element for delaying an input signal and a main lobe response calculator for receiving the signal of the delay element. The main lobe response calculator is (n -1) (n is a value obtained by adding 1 to the number of samples per symbol) delay elements, and tap coefficients a _i connected to the input side or the output side of these delay elements (i is a natural number of 1 to n) Of n multipliers and an adder that adds the outputs of these multipliers.

The sidelobe calculator is a waveform generator for generating a sidelobe waveform by using an approximate calculation of a coefficient by a sine wave, and a first side connected to the waveform generator for calculating one side of the sidelobe. One sidelobe response calculator, a delay element connected to the one sidelobe response calculator, a second sidelobe response calculator connected to the delay element to calculate the other side of the sidelobe; 2 and an adder that adds the outputs of the side lobe response calculators.

[0008]

In the present invention, the main lobe, which greatly affects the characteristics of the response waveform of the filter, is accurately calculated as before, but the other side lobes are calculated by approximation with a sine wave. Therefore, the amount of calculation, that is, the number of multipliers is reduced. That is, considering a roll-off filter that outputs a response waveform as shown in FIG. 3, the calculation is performed separately for the main lobe 31 portion and the other side lobe 32 portion in FIG.
Then, regarding the side lobe 32, c _k in FIG.
The calculation amount is reduced by using the coefficient approximated by the sine wave instead of obtaining the coefficient from the equation.

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION Consider a roll-off filter having a response waveform as shown in FIG. Here, the number of samples per symbol is 4, the period of the impulse response is 8 symbols, and in the circuit described below, the clock frequency is equal to the sampling frequency. The filter output is
As shown in FIG. 1, the main lobe calculator 11 and the side lobe calculator 12 respectively calculate and adder 13
Is added.

The main lobe calculator 11 includes a delay element 111.
And a main lobe response calculator 112. The delay element 111 delays the input signal in order to connect the response of the main lobe after the response portion of the side lobe. Here, since the number of samples per symbol is 4, the delay time t ₁ of the delay element 111 is t ₁ =
3 × 4 = 12. Mainlobe response calculator 112
Uses a transversal filter including n-1 delay elements 41, n multipliers 42, n tap lines 43, and an adder 44 as shown in FIG. When the transversal filter of FIG. 4 is used as the main lobe response calculator, n is 2 which is the number of samples around the symbol.
One times the sum, that is, n = 2 × 4 + 1 = 9. As the coefficient a _i (i = 1, ..., N) of the multiplier, a value set in advance to be optimal is used.

On the other hand, the side lobe calculator 12 adds the waveforms generated by the waveform generator 121 and the waveform generator 121 for generating the side lobe waveform by using the approximate calculation of the coefficient by the sine wave, Sidelobe response calculators 122 and 12 for calculating one side of the sidelobe, respectively
3, delay element 124, and adder 125.

The waveform generator 121 uses the transversal filter of FIG. 4 similarly to the main lobe response calculator 112. In this case, n = 4 because n is the number of samples per symbol, and the multiplier coefficient a _i (i =
1, ..., N), the section from 0 to π of the sine wave is divided by n, and each amplitude value is used as a _i .

Both sidelobe response calculators 122 and 123 have (n-1) .times.m delay elements 51, n multipliers 52, and n tap lines 53 as shown in FIG.
And a circuit consisting of an adder 54 is used. Here, n is the number of symbols to be calculated on one side minus the part of the main lobe, and n = (total number of symbols / 2) -1 = 3.
Becomes Also, since the delay time between each tap is one symbol time, m = 4. Multiplier coefficient b _i (i =
For 1, ..., N), the optimum value obtained in advance is used. At this time, as can be seen from the response waveform of FIG. 3, the response waveform of the side lobe is the object across the main lobe, so that the coefficient of the multiplier of the side lobe response calculators 122 and 123 can be the object. . That is, when the coefficient of the multiplier of the sidelobe response calculator 122 is b _2i and the coefficient of the multiplier of the sidelobe response calculator 123 is b _3i , b ₂₁ = b _3n , ..., B _2i = b _{3 (ni- 1)} , ..., b _2n
= A b _31. This means that the number of elements for storing coefficients is not the total number of multipliers used in the sidelobe response calculators 122 and 123, but half of them.

The delay element 124 between the sidelobe response calculators 122 and 123 receives the output of the main lobe, so the delay time t ₂ is 3 symbol times, in this case t ₂ =
3 × 4 = 12.

When the above circuit to which the present invention is applied is used, the total number of multipliers is 9 + 4 + 6 = 19. Conventionally, the number of multipliers required is equal to the total number of samples N, N = 4
Since x8 + 1 = 33, it can be seen that the number of multipliers is obviously reduced.

[0016]

In the conventional filter, basically, the outputs from the taps are multiplied by different coefficients over the entire response waveform for calculation, so that the same number of multipliers as the number of taps is required. In the present invention, the main lobe, which greatly affects the characteristics of the filter, is accurately calculated, and the coefficient calculation is reduced by approximating the response waveform of the other part with a sine wave, and as a result, the multiplier The number of components has been reduced, the circuit scale has been reduced, and the calculation time has been shortened.

[Brief description of the drawings]

FIG. 1 is a configuration diagram showing an embodiment of a roll-off filter in the present invention.

FIG. 2 is a circuit configuration diagram of a roll-off filter which has been conventionally used.

FIG. 3 is a diagram showing a response of an embodiment of a roll-off filter according to the present invention.

FIG. 4 is a detailed diagram of a DTL used in the main lobe response calculation and side lobe waveform generation in the present invention.

FIG. 5 is a detailed diagram of a circuit used for sidelobe response calculation in the present invention.

[Explanation of symbols]

 11 Main Lobe Calculator 12 Side Lobe Calculator 21, 41, 51 Delay Element 22, 42, 52 Multiplier 23, 43, 53 Tap Line 24, 44, 54 Adder

Claims (3)

[Claims] 1. A main lobe calculator for calculating a main lobe of an impulse response by using a transversal filter, and a side lobe calculator for calculating side lobes other than the main lobe by using an approximate calculation by a sine wave. A roll-off filter comprising an adder for adding the output of the main lobe calculator and the output of the side lobe calculator. 2. The roll-off filter according to claim 1, wherein the main lobe calculator includes a delay element for delaying an input signal and a main lobe response calculator for receiving a signal from the delay element. The main lobe response calculator has (n-1) (n is a value obtained by adding 1 to the number of samples per symbol) delay elements and a tap coefficient a connected to the input side or the output side of these delay elements. A roll-off filter comprising a transversal filter including n multipliers of _i (i is a natural number of 1 to n) and an adder that adds outputs of these multipliers. 3. The roll-off filter according to claim 1 or 2, wherein the side lobe calculator generates a side lobe waveform by using an approximate calculation of a coefficient by a sine wave, and the waveform generator. A first side lobe response calculator connected to the first side lobe response calculator, a delay element connected to the first side lobe response calculator and a second side lobe response calculator connected to the delay element to calculate the remaining one side of the side lobe. A roll-off filter comprising a sidelobe response calculator and an adder for adding outputs of the first and second sidelobe response calculators.