# Exercise 5.7Z: Matched Filter - All Gaussian

At the input of a reception filter there is

• a Gaussian pulse   $g(t)$  with amplitude  $g_0$  and equivalent duration $\Delta t_g = 1\hspace{0.08cm} \rm ms$,
$$g(t) = g_0 \cdot {\rm{e}}^{ - {\rm{\pi }}\left( {t/\Delta t_g } \right)^2 } ,$$
• superimposed by white Gaussian noise with power density $N_0 = 10^{-4}\hspace{0.08cm} \rm V^2 \hspace{-0.1cm}/Hz$.

The pulse energy is  $E_g = 0.01\hspace{0.08cm} \rm V^2 s$.  Let the reception filter be an acausal Gaussian low-pass filter with frequency response

$$H_{\rm E} (f) = {\rm{e}}^{ - {\rm{\pi }}\left( {f/\Delta f_{\rm E} } \right)^2 } .$$

The associated impulse response is thus:

$$h_{\rm E} (t) = \Delta f_{\rm E} \cdot {\rm{e}}^{ - {\rm{\pi }}\left( {\Delta f_{\rm E} \hspace{0.03cm}\cdot \hspace{0.03cm}t} \right)^2 } .$$

The system-theoretical filter bandwidth  $\Delta f_{\rm E}$  should be chosen so that the Gaussian low-pass filter is optimally matched to the input impulse  $g(t)$.  This is then referred to as a:  "matched filter".

Notes:

• The exercise belongs to the chapter  Matched Filter.
• Use the following definite integral to solve:
$$\int_0^\infty {{\rm{e}}^{ - a^2 x^2 } {\rm{d}}x = \frac{{\sqrt {\rm{\pi }} }}{2a}} .$$

### Questions

1

Calculate the pulse amplitude.

 $g_0 \ = \$ $\ \rm V$

2

What is the maximum S/N ratio at the filter output in  $\rm dB$?

 $10 \cdot \lg\ \rho_d(T_\text{D, opt}\hspace{-0.05cm}) \ = \$ $\ \rm dB$

3

At what filter bandwidth is this S/N ratio achieved?

 $\Delta f_\text{E, opt}\ = \$ $\ \rm kHz$

4

Which of the following statements are true if the filter bandwidth  $\Delta f_{\rm E}$  is smaller than calculated in subtask  (3)

 The signal component  $d_{\rm S}(T_\text{D, opt}\hspace{-0.05cm})$  is smaller than with matching. The noise power  $\sigma_d^2$  is larger than with matching. The S/N ratio is smaller than calculated in subtask  (2).

### Solution

#### Solution

(1)  For the energy of an pulse  $g(t)$  applies in general,  or for this example:

$$E_g = \int_{ - \infty }^{ + \infty } {g^2(t) \hspace{0.1cm}{\rm{d}}t} = g^2_0 \cdot \int_{ - \infty }^{ + \infty } {{\rm{e}}^{ - 2{\rm{\pi }}\left( {t/\Delta t_g } \right)^2 } \hspace{0.1cm}{\rm{d}}t} .$$
• This equation can be transformed as follows:
$$E_g = 2 \cdot g_0 ^2 \cdot \int_0^\infty {{\rm{e}}^{ - \left( {\sqrt {2 \rm{\pi }} /\Delta t_g } \right)^2 \cdot \hspace{0.05cm} t^2 }\hspace{0.1cm} {\rm{d}}t} .$$
• With   $a = (2\pi)^{1/2}\cdot 1/\Delta t_g$   and the formula given,  the following relationship holds:
$$E_g = 2 \cdot g_0 ^2 \cdot \frac{{\sqrt {\rm{\pi }} }}{2a} = \sqrt 2 \cdot g_0 ^2 \cdot \Delta t_g .$$
• Solving this equation for  $g_0$,  the final result is:
$$g_0 = \sqrt {\frac{E_g }{\Delta t_g \cdot \sqrt 2 }} = \sqrt {\frac{{0.01\;{\rm{V}}^{\rm{2}} {\rm{s}}}}{{0.001\;{\rm{s}} \cdot 1.414}}} \hspace{0.15cm}\underline { = 2.659\;{\rm{V}}}.$$

(2)  Assuming a matched filter,  the S/N ratio at the output is:

$$\rho _{d} ( {T_{{\rm{D,}}\,{\rm{opt}}} } ) = \frac{2 \cdot E_g }{N_0 } = \frac{{2 \cdot 10^{ - 2} \;{\rm{V}}^{\rm{2}} {\rm{s}}}}{{10^{ - 4} \;{\rm{V}}^{\rm{2}} {\rm{/Hz}}}} = 200.$$
• In logarithmic representation,  the following result is obtained:
$$10 \cdot \lg \rho _{d} ( {T_{{\rm{D,}}\,{\rm{opt}}} } ) = 10 \cdot \lg \left( {200} \right) \hspace{0.15cm}\underline {\approx 23\;{\rm{dB}}}.$$

(3)  A comparison between the input pulse and the filter frequency response shows that when  $\Delta f_{\rm E} = 1/\Delta t_g$  is fitted,  it must hold:

$$\Delta f_{{\rm{E,}}\,{\rm{opt}}} \hspace{0.15cm}\underline { = 1\;{\rm{kHz}}}.$$

(4)  Solutions 1 and 3  are correct:

• A smaller filter bandwidth is favorable with respect to interference,
• but unfavorable with respect to the useful signal.
• The negative influence  (smaller useful signal)  outweighs the positive influence (less noise).