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ELT-41308 COMMUNICATION THEORY 12.12.2019
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No materials, no calculator. Prepared by and return to: Mikko Valkama

NB 1: If you wish to take the Full Exam, answer to all the guestions 1-6. If you wish to take only the 2nd
Midterm Exam, answer only to the guestions 4-6. Maximum number of response sheets is two for the full
exam and one for the midterm. I

 

NB 2: Please 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 in a clear manner. Thank you.

 

1. Explain shortly the following concepts: a) amplitude spectrum, b) freguency response,
c) wide-sense stationary random signal, d) intermodulation distortion, e) white noise. No need to
dwell on details, rough explanations which show your understanding are enough.

2. Explain and characterize (in both time and freguency domains) the concept of distortionless
(distortion-free) transmission? What kind of different distortions are commonly encountered in
communication systems ? As an example, consider a system where the transmitted signal x(t) is
received as y(t) = x(t) — ax(t — 7). Determine the impulse and freguency responses of this
system and illustrate them graphically. What kind of distortions does this system cause (if any) ?
How do these distortions depend on the values of a and 7 and on the input signal bandwidth ?

3. Explain the general concept of I/O modulation. Illustrate the principle by drawing a block-diagram of
an I/O modulator, and some example spectral contents of the relevant signals in different stages. How
does 1/0 modulation utilize the structure of a general bandpass signal, sketched below. Explain also
how can the receiver recover the I and O components. Finally, explain the concept of lowpass or
baseband eguivalent and how is it related to I/O modulation.

Typ(t) = A(t) cos(wot + Y(t)) = x,(t) cos(wyt) — 7, (2) sin(w,t)

4. Present the basic time domain mathematical model for ideal signal 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.

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 2.6 Gbit/s, and that you have 400 MHz bandwidth available
around a center-freguency of 28 GHz. 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
pulse. Draw also an elementary block-diagram of the transmitter, starting from the transmit bit
seguence towards the I/O modulated waveform.

6. (a) Explain shortly what is meant by (i) information, (ii) entropy and (iii) mutua! information, in the
context of communications systems. Explain also what is meant in this context by channel capacity.

(b) Explain briefly the basic idea of multicarrier modulation and OFDM.

Maximum points, Full Exam: 6 x 5 = 30p

Maximum points, 2"! Midterm Exam: 3 x 5=15p