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TLT-5206 COMMUNICATION THEORY 2" Mid-term exam 08.12.2011

 

(Suomenkieliset kysymykset toisella puolella) Basic calculator allowed. Prepared by J. Talvitie.

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Explain shortly the following concepts: a) Instantaneous freguency 6) Raised-cosine
pulse c) Symbol constellation 4) Information e) Entropy f) Channel capacity

a) Explain briefly the basic idea of freguency modulation (FM). Sketch also the essential
waveform shape and spectrum of the modulated signal (at principal levels) when the
modulating signal is a single sine-wave (freguency fi). What advantages does FM
generally have compared to linear modulation methods? (4p)

b) Depict (draw) the conceptual post-detection noise power density spectrum of FM
signal. Based on this, explain the principle of pre-emphasis/de-emphasis filtering. Is it
possible to affect channel interference based issues with pre-emphasis/de-emphasis
filtering in case of FM, why?

Present the 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 both in time and freguency
domains. What is the fundamental difference between a discrete time signal and a digital
signal?

In the figure below a block diagram of a baseband digital transmission system based on
Nyguist pulse shaping is presented. Describe briefly the fundamental purpose/meaning of
each block in the system (1-3 sentences per block). To which blocks the terms symbol
alphabet and pulse shape are related and how they affect the system performance (bit rate,
noise sensitivity, bandwidth, etc.)?

 

 

  

  
   

SAMPLER

 

 

 

 

ESTIMATED — SLICEROR
SYMBOLS — DECISION DEVICE

 

Let's consider a carrier-modulated digital PAM/PSK/OAM system where the target bit
rate is 25 Mbits/s and the available transmission bandwidth around the center-freguency
is 15 MHz. Design the system at waveform level, i.e., determine reasonable values for the
key parameters (symbol rate, alphabet size, excess bandwidth, etc.). Explain the thinking
behind your design using written text and/or figures (So, just presenting the calculations
is not enough)

Maximum points: 5 x 6 = 30p.