JPS6362134B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a feedback type equalizer using a transversal filter having discrete tap coefficients.

Equalizers with various configurations are already known, and feedback type equalizers using transversal filters are also known. In such an equalizer, the transversal filter has discrete tap coefficients, and the quantization steps of the tap coefficients are all the same. If this quantization step is made smaller, the equalization accuracy will improve, but the configuration will become more complicated. Therefore, in the past, an appropriate quantization step was selected. However, the tap coefficient corresponding to the tail portion of the impulse response of the transmission line has a relatively poor approximation, resulting in a low equalization accuracy.

The present invention improves the above-mentioned drawbacks, and
By weighting the quantization step of the tap coefficient,
The purpose is to improve the approximation to the tail portion of the impulse response and improve the equalization accuracy. Examples will be described in detail below.

FIG. 1 is a block diagram of the main parts of an embodiment of the present invention, where IN is an input terminal, OUT is an output terminal, SUB is a subtracter, T is a delay circuit, and K is a tap coefficient a ₀ to a _o set. It is a coefficient multiplier. The output signal is sent to the delay circuit T.
Each delayed signal is output from the coefficient unit K at a level corresponding to the set tap coefficients a ₀ to a _o and is added to the subtractor SUB, where it is subtracted from the input signal. A feedback type equalizer using a versatile filter is constructed.

Sample value hl(n) of impulse response of transmission line
(where n = 0, 1, 2, 3, ...) and the sample value ht (n) of the impulse response of the transversal filter of the equalizer (where n = 0, 1, 2, 3, ...), When hl(n) = ht(n), there is no waveform distortion, and if the impulse response contains an echo component as shown in Figure 2a, the tap coefficient a ₀ ~ _{a o}
is set corresponding to the sample value of the impulse response, as shown in FIG. 2b.

Since the tap coefficient of the transversal filter is a discrete value, if the quantization step is Δ, the quantization error ε(n) is |ε(n)|=|hl(n)−ht(n)|Δ/ 2…(1
) becomes. This quantization error ε(n) appears as waveform distortion, and this waveform distortion is evaluated by the peak distortion σ. That is, if the maximum value of the sample values hl(n) is expressed as D, and the sum of the sample values excluding the maximum value D is expressed as Σ', then σ ₁ = 1/DΣ' | ε(n) | ...(2) The maximum value σ _1(nax) is σ _1(nax) = 1/D(M+N)Δ/2 (3).

However, M and N indicate the number of samples, and of the total number of samples (M+N), M indicates the first number and N indicates the remaining number.

The first M sample values of the impulse response are
If the amplitude is less than or equal to the amplitude S and the amplitude is greater than or equal to kS (k<1), and the remaining N sample values are less than or equal to the amplitude kS, the first M sample values are quantized with the quantization step Δ, and the remaining N samples are The value is quantized by kΔ (k<1). That is, as shown in FIG. 2c, the quantization steps are set to Δ and kΔ depending on the amplitude distribution of the impulse response. With such a quantization step, the maximum value σ _2(nax) of the peak distortion σ ₂ becomes σ _2(nax) =1/D(MΔ/2+N·kΔ/2) (4). Therefore, the ratio of the maximum value of peak distortion when the quantization step is weighted and when it is not weighted is σ _2(nax) /σ _1(nax) = 1/D(MΔ/2+N・kΔ/2
)/1/D(M+N)Δ/2=1+kN/M/1+N/
M...(5) becomes.

Figure 3 shows (5) when k is 0.25, 0.5, and 0.75.
This shows the calculation result of the formula, for example, if N/M is 2,
When k is set to 0.25, the peak distortion can be reduced to 1/2 by weighting the quantization step.

As mentioned above, the tap coefficients a ₀ to _{a o} shown in FIG. 2b
By setting the quantization step to Δ and kΔ and giving the weight k as shown in Figure 2c,
The degree of approximation of the tail portion of the impulse response can be improved, and as a result, the accuracy of equalization can be improved.

As explained above, the present invention provides a feedback equalizer using a transversal filter having discrete tap coefficients, in which some tap coefficients are adjusted according to the amplitude distribution of the impulse response of the transmission path. For example, the quantization steps are set to Δ and kΔ (k<1) so that the quantization step of tap coefficients is different from the quantization steps of other tap coefficients. There is an advantage that the quantization error in the tail portion of the response can also be reduced and the equalization accuracy can be improved.

[Brief explanation of drawings]

FIG. 1 is a block diagram of essential parts of an embodiment of the present invention.
FIG. 2 is an explanatory diagram of the impulse response, tap coefficient, and quantization step, and FIG. 3 is a curve diagram showing the improvement in peak distortion. IN is an input terminal, OUT is an output terminal, SUB is a subtracter, K is a coefficient unit, and T is a delay circuit.

Claims (1)

[Claims] 1. In a feedback equalizer using a transversal filter with discrete tap coefficients, the quantization step of some tap coefficients is changed to that of other tap coefficients depending on the amplitude distribution of the impulse response of the transmission path. An equalizer characterized in that the quantization step is weighted so as to be different from the quantization step of the coefficients.