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ELT-41307 COMMUNICATION THEORY 21.3.2017

 

No materials, no calculator. Prepared by and return to: Mikko Valkama

NB: Pay special attention to clear handwriting. If I 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.
Thank you.

1. Explain shortly the following concepts: a) spectrum, b) cross-correlation,
c) intermodulation distortion, d) spectral density, e) white noise. No need to dwell on details, rough
explanation which shows your understanding is enough. (5p)

2. Assume x(t) is white noise. Suppose another random signal y(t) is created as
ylt) = x(t) + z(t — T) where T' isa known constant. First, calculate the autocorrelation function
of y(t). Then, calculate also the spectral density of y(t) and illustrate it graphically. (5p)

Sa Explain the general concepts of (i) amplitude modulation, (ii) freguency modulation, and (iii) V/O
modulation. (5p)

4. Explain shortly the basic principle of Nyguist pulse-shaping filtering and pulse amplitude modulation
(PAM), in the context of baseband digital communication. What does the concept of intersymbol
interference (ISI) mean ? What is the minimum bandwidth that still allows for avoiding ISI ? How is
the achievable physical layer bitrate determined in a baseband PAM system? —(5p)

5. Suppose you are to design an I/O modulated single-carrier M-OAM digital communication system
where the target physical layer bit rate is 90 Mbit/s, and that you have 20 MHz bandwidth available
around a center-freguency of 2600 MHz. Design the system in terms of the needed OAM symbol
alphabet size, symbol rate and feasible nonzero excess bandwidth (rolloff) factor for a raised-cosine
type of pulse. — (5p)

6. Explain shortly the concepts of entropy, mutual information and channel capacity. What is the
meaning of channel capacity for a communications engineer ? In a bandlimited additive Gaussian
noise channel, what factors are dictating or determining the channel capacity ? —(5p)

Maximum points, Full Exam: 5+5+5+5+5+5 = 30p.