106年 - 106 國立臺北大學_碩士班一般入學考試試題_通訊工程學系：通訊原理#147266

(i) (2%) Analog communication is preferred over digital in long distance communication since we can decode and retransmit analog signals when the signal-to-noise ratio (SNR) is large.

(ii) (2%) As long as the sampling frequency is larger than 2 times the signal bandwidth, one can easily reconstruct the original signal from samples.

(iii) (2%) Split phase line coding can provide self-synchronization.

(iv) (2%) For sending a same signal, linear modulation typically requires a larger bandwidth than angle modulation.

(i) (5%) Pass the received signal through an ideal bandpass filter centered at \( f_{c} \) with bandwidth \( B \geq 2W \). The noise then becomes \( n_{c}(t) \cos 2\pi f_{c}t - n_{s}(t) \sin 2\pi f_{c}t \) narrowband noise. Draw \( S_{n_c}(f) \) the PSD of \( n_{c}(t) \).

(ii) (8%) The output signal now is \( y(t) = x(t) + n_c(t) \cos 2\pi f_c t - n_s(t) \sin 2\pi f_c t \). Use a coherent demodulator followed by a low-pass filter with bandwidth \( W \) to demodulate it. Derive the demodulation signal and the SNR at the demodulator output.

(iii) (8%) Suppose the signal is further corrupted by an interference \( k(t) = \text{sinc}(wt) \) and the received signal after the bandpass filter becomes \( y_2(t) = (t) + k(t) \cos 2\pi(f_c + 2W)t + n_c(t) \cos 2\pi f_c t - n_s(t) \sin 2\pi f_c t \). Design a low-pass filter for the demodulator in part ii) such that the signal-to-interference-plus-noise ratio (signal power divided by the sum of interference and noise power) is maximized.

(i) (5%) Derive the condition on \( h(t) \), in time domain, such that there will be no inter-symbol interference?

(ii) (8%) Now, derive from part i) the condition on \( H(f) \), in frequency domain, which guarantees no inter-symbol interference.

(iii) (8%) Provide an example of \( H(f) \) satisfying the condition in part ii). Show in time domain that it indeed will not result in any inter-symbol interference.

4. (10%) In wireless communication systems, Additive White Gaussian Noise (AWGN) channel is often assumed. What do "Additive", "White", and "Gaussian" mean?

5. (10%) Plot the 16QAM and 8PSK constellations with normalized power 1. How many bits can one 16QAM symbol and one 8PSK symbol carry, respectively?

(i) Is the vector \( v_{1} = [1\ 0\ 0\ 1\ 0\ 1] \) a valid codeword vector? Why or why not?

(ii) How many valid codewords can you find?

(i) Show that \( \varphi_1(t) \) and \( \varphi_2(t) \) are mutually orthogonal over the interval (-2, 2)

(ii) Are \( \varphi_1(t) \) and \( \varphi_3(t) \) mutually orthogonal over the interval (-1, 1)? Why or why not?