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COMM.SYS.300 COMMUNICATION THEORY 1* Midterm Exam 21.10.2021

 

No materials, no calculator, Preparcd by M. Valkama.

NB 1: Pay special attention to clear handwriting. If 1 cannot read your text with reasonable effort, your
paper cannot be unfortunately graded. So, please, try to write your responses and solutions in a clear
manner, Danke schön.

NB 2: You are allowed to return only | response sheet that is distributed once we start. You can certainly
write to all 4 individual sides of the response sheet. Also, extra sheets are available for possible
intermediate/persona! notes, but only one sheet can be returned.

Thank you. Enjoy. O

1. — Explain shortly the following concepts in the context of electrical/electromagnetic communications:
a) spectrum, b) nonlinear distortion, c) correlation, d) spectral density, €) thermal noise.

No need to dwell on all possible finest details, basic explanations which show your understanding are
enough. (5p)

2. — Explain the concepts of I/O modulation and I/O demodulation, in general. Illustrate the principles by
drawing appropriate block-diagrams and some example spectral contents of relevant signals in
different stages. Explain also how I/O modulation principle utilizes the general bandpass signal
characteristics. Finally, explain shortly the concept of lowpass eguivalent or baseband eguivalent

signal. (5p)

3. — Present the basic time domain mathematical model for ideal sampling process. Given the spectrum of
a continuous-time signal, sketch (i.e., draw) also the corresponding spectrum after ideal sampling.
Based on that, formulate the basic reguirement in order to avoid aliasing. Explain also the basic idea
of reconstruction in both time and freguency domains. Finally, tell the basic idea of sub-sampling
principle in bandpass signal context. (5p)

Maximum points: 5+5+5=15p.