Exercise 3.6Z:Optimum Nyquist Equalizer for Exponential Pulse

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Two-sided exponential input pulse

As in  Exercise 3.6  we consider again the optimal Nyquist equalizer.

• The input pulse  $g_x(t)$  is a two-sided exponential function:

$$g_x(t) = {\rm e }^{ - |t|/T}\hspace{0.05cm}.$$

• Through a transversal filter of  $N$–th order with the impulse response

$$h_{\rm TF}(t) = \sum_{\lambda = -N}^{+N} k_\lambda \cdot \delta(t - \lambda \cdot T)$$

it is possible that the output pulse  $g_y(t)$  has zero crossings at  $t/T = ±1, \ \text{...} \ , \ t/T = ±N$, 
while  $g_y(t = 0) = 1$. 

• However,  in the general case,  the precursors and trailers with  $| \nu | > N$  lead to intersymbol interference.

Note:  The exercise belongs to the chapter  "Linear Nyquist Equalization".

Questions

1

Give the signal values  $g_x(\nu) = g_x(t = \nu T)$  at multiples of  $T$. 

$g_x(0)\ = \ $

$g_x(1)\ = \ $

$g_x(2)\ = \ $

2

Calculate the optimal filter coefficients for  $N = 1$.

$k_0 \ = \ $

$k_1 \ = \ $

3

Calculate the output values  $g_2 = g_{y}(t = 2T)$  and  $g_3 = g_{y}(t = 3T)$.

$g_2 \ = \ $

$g_3\ = \ $

4

Which of the following statements is true?

	For the given input pulse  $g_x(t)$,  no improvement is possible with a second-order transversal filter.
	The first statement is independent of the input pulse  $g_x(t)$.
	For the given input pulse,  a further improvement is obtained with an  "infinite"  transversal filter.

Solution

Solution

(1)  The five first samples of the input pulse at distance  $T$  are:

$$g_x(0)\hspace{0.25cm}\underline{ = 1},\hspace{0.2cm}g_x(1) \hspace{0.25cm}\underline{= 0.368},\hspace{0.25cm}g_x(2) \hspace{0.25cm}\underline{= 0.135},\hspace{0.2cm}g_x(3) = 0.050,\hspace{0.2cm}g_x(4) {= 0.018} \hspace{0.05cm}.$$

(2)  According to  "solution to Exercise 3.6",  we arrive at the following system of equations:

$$2t = T \hspace{-0.1cm}:\hspace{0.2cm}g_1 \hspace{-0.2cm} \ = \ \hspace{-0.2cm} k_0 \cdot g_x(1) +k_1 \cdot [g_x(0)+g_x(2)]= 0\hspace{0.3cm}\Rightarrow \hspace{0.3cm} \frac{k_1}{k_0} = - \frac{g_x(1)}{g_x(0)+g_x(2)} \hspace{0.05cm},$$

$$t = 0 \hspace{-0.1cm}:\hspace{0.2cm}g_0 \hspace{-0.2cm} \ = \ \hspace{-0.2cm} k_0 \cdot g_x(0) + k_1 \cdot 2 \cdot g_x(1) = 1\hspace{0.3cm}\Rightarrow \hspace{0.3cm} k_1 = \frac{1-k_0}{0.736} \hspace{0.05cm}.$$

• This leads to the result:

$$k_0 - 0.324 \cdot 0.736 \cdot k_0 = 1 \hspace{0.3cm}\Rightarrow \hspace{0.3cm} k_0 \hspace{0.15cm}\underline {= 1.313}, \hspace{0.2cm}k_1\hspace{0.15cm}\underline { = -0.425} \hspace{0.05cm}.$$

Input pulse  (top),  output pulse for  $N = 1$  (bottom)

(3)  For time  $t = 2T$  holds:

$$g_2 \ = \ k_0 \cdot g_x(2) +k_1 \cdot [g_x(1)+g_x(3)]$$

$$\Rightarrow\hspace{0.3cm} g_2 \ = \ 1.313 \cdot 0.050 -0.425 \cdot [0.135+0.018]\hspace{0.15cm}\underline {\approx 0} \hspace{0.05cm}.$$

• Similarly,  the output pulse at time  $t = 3T$  is also zero:

$$g_3 \ = \ k_0 \cdot g_x(3) +k_1 \cdot [g_x(2)+g_x(4)$$

$$\Rightarrow\hspace{0.3cm}g_3 \ = \ 1.313 \cdot 0.135 -0.425 \cdot [0.368+0.050]\hspace{0.15cm}\underline {\approx 0} \hspace{0.05cm}.$$

• The figure shows that  for this exponentially decaying pulse, the first-order transversal filter provides complete equalization.

• Outside the interval  $-T < t < T$,   $g_y(t)$  is identically zero.

• Inside it results in a triangular shape.

(4)  Only the  first statement is correct:

• Since already with a first-order delay filter all precursors and trailers are compensated,  also with a second-order filter and also for $N → ∞$ no further improvements result.

• However,  this result applies exclusively to the (bilaterally) exponentially decaying input pulse.

• For almost any other pulse shape,  the larger  $N$  is,  the better the result.