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COMM.SYS.300 Communication theory - 19.12.2022

Exam text content

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Original exam




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
Second Midterm Exam, answer only to the guestions 4-6.


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

NB 3: For Second Midterm, only one full response sheet is allowed. For Full Exam, two full response
sheets are allowed.

1. Explain shortly the following concepts in the context of electrical or electromagnetic
communications: a) spectrum, b) intermodulation distortion, c) correlation function, d) spectral
density, €) thermal noise. No need to dwell on finest details, rough explanations which show your
understanding are enough.


Explain the general concept of /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 the receiver can recover the I and O components. Finally, explain the concept of lowpass or
baseband eguivalent and how is it related to I/O modulation.

Xgp(1) = Alt) cos(act + p(t)) = x, (1) cos(at) — xo (t)sin(act)

3. Explain shortly the fundamental core elements or processing stages that every digital communication
system contains and what are their basic roles/purposes. Explain also shortly what are the most
fundamental parameters or features that definc the external operation of a digital communication

4. — Suppose you are to design an I/O modulated single-carrier M-OAM digital communication system
where the target physical-layer bit rate is 1 Gbit/s, and that you have 200 MHz bandwidth available
around a center-freguency of 30 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 high-freguency I/O modulated waveform.

5. Explain briefly the basic ideas of multicarrier modulation / OFDM and multi-antenna / MIMO
communications. Discuss also shortly the benefits of OFDM compared to single-carrier
PAM/OAM/PSK, and the corresponding benefits of multi-antenna/MIMO compared to single-
antenna systems.

6. — Explain shortly what is meant by (i) information, (ii) entropy and (iii) mutual information, in the
context of electrical or electromagnetic communications. Explain also what is meant in this context
by channel capacity. Assuming a bandlimited additive white Gaussian noise (AWGN) channel, what
factors are determining the channel capacity ? Give also a feasible numerical example.

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

Maximum points, Second Midterm Exam: 3 x 5=15p

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