NAME.............................................................................................................................................

Please try to answer all of the following questions in the spaces provided, but notice that on the first two questions you have a modicum of choice. If you need additional space, use the back of the sheets.

Partial credit is attainable, so include all of your calculations and clearly indicate what you are trying to do. The relative credit assigned to each question is indicated as a prudent time allocation. Nominally, this is a 125 minute exam.
 

1. (Prudent time allocation 49 minutes)

Please answer SEVEN (7) of the following NINE questions. Please keep your answers brief and to the point. Please note that you have an element of choice in this question!
 

1. Five Olympic athletes (called here for simplicity A, B, C, D, and E) are in contention for a gold metal. A and B are rated to have an equal chance of success in winning the gold. C is rated to be twice as likely to win as A (or B). Similarly, D is rated to be twice as likely as C and, finally, E is rated to be twice as likely as D. The message conveying the results of this contest contains   bits of information.

2. Two rock bands are playing and one band produces 200 times more sound intensity at 1 kHz than the other, because some of the amplifiers of the second band are not working. This diminution of sound corresponds to a intensity difference of dB and a relative loudness of, approximately, .

 
3. The time period of 125 microseconds ubiquitous in digital communication protocols? Explain.

 
4. The figure below represents a "snapshot" in time of the of the electric field lines associated with radiation from an oscillating electrical charge. 
   
   

If the charge is oscillating at a rate of 100 MHz, we know that the line segment from A to B is, approximately  centimeters long.

5. We have asserted that loudness and brightness are similar attributes. Explain.

 
6. In discussions of radio wave propagation there are three terms commonly used to wit: line-of-sight waves, surface waves and sky waves. What do these terms mean?

 
7. In one of the lectures, I asserted that, at this moment in history,
 
"There is essentially no physical limit on the rate at which we can convey information from one place to another -- an age old communication problem is solved."


Given what we know about the data rates of extant communication systems, this may seem to be an absurd declaration. What did I mean and how did I qualify the assertion? What is so special about "this moment in history?" Has the Sampling Theorem been repealed?
 

8. Localized spatial harmonics may be used to represent visual information. Explain what this assertion means and briefly discuss its implications for visual communication.

 
9. It is known that the cellular telephone system serving some particular metropolitan area is organized on the basis of seven-cell clusters. A segment of that system is depicted in the following figure:

Indicate on this figure a set of co-channel cells -- i.e. a set of cells that have the same set of allocated frequencies.

2. (Prudent time allocation 10 minutes)

Please answer ONE (1) of the following TWO questions. Please note that you have an element of choice in this question!

 
1. The SIMULINK setup illustrated above is a fragment taken from a configuration studied in one of your course assignments.




Based on your work on that assignment and the parameters given in the illustration, sketch, in the space below, the graphs that would be observed on the Scope and Power Spectral Density devices. Please be as specific as you can be -- i.e. note approximate time and frequency values in appropriate units.







2. Make a block diagram of the essential elements of a PCM communication link and explain how it works. In particular, indicate how, say, four voice channels might be carried by a single PCM link.


3. (Prudent time allocation 10 minutes)

The following figure was generated with the MATLAB program sndwave. It depicts a sound wave propagating from left to right. At the point in space noted "0" there is an interface between two different sound propagation media. It is known that the velocity of sound propagation is 2,000 meters/sec in the medium on the right side of the interface.



1. Estimate the frequency of the sound wave (include units)  

 
2. Estimate the temporal period of the sound wave (include units)  

 
3. Estimate the velocity of sound propagation in the medium on the left side of the interface (include units)  


4. (Prudent time allocation 12 minutes)

In discussing communication systems we have tried to demonstrate the crucial role played by signal filters. The filtering operation may symbolize by the following block diagram:



Let us suppose the following test signal is injected into the filter:
 


(remember )
 

1. Sketch below the approximate output signal if the filter is a low pass filter with a passband of 200 kHz (i.e. a filter which allows through all frequencies below 200 kHz).




2. Sketch the approximate output signal if the filter is now taken to be a band pass filter with a bandwidth of 100 kHz centered at 375 kHz (i.e. a filter which passes all frequencies between 325 and 425 kHz).

5. (Prudent time allocation 13 minutes)

In the late 1940's, Claude Shannon formulated a general model of communication. As part of his formulation, Shannon devised a diagrammatic representation which relates the various essential processes involved in communication.
 

1. Carefully sketch and label such a generalized communication (Shannon) diagram.

 
2. Choose a communication system or process about which you feel particularly well informed and, then, explain how each function in the Shannon diagram is accomplished in that system or process.

6. (Prudent time allocation 18 minutes)

In lecture we have often plotted the temporal (time) and spectral (frequency) representations of signals of interest. For the cases cited below, I would like you to sketch both the temporal and spectral representations of the cited signals. Please be careful and as specific as you can be -- viz. note approximate time and frequency values in appropriate units.
 

1. a. A 10 nanosecond (i.e. ) ASK light pulse which could be used to represent a "bit" in a digital fiber optic communication link:




2. A typical broadcast FM signal:




3. An unmodulated broadcast AM carrier signal:




4. The output of a Marconi era spark-gap wireless transmitter:




5. A NTSC video channel:




6. A segment of human speech communicated via a telephone link:




7. (Prudent time allocation 13 minutes)

In lecture we discussed the following fluid flow analogy and corresponding physical model of electrical signal propagation in a wire transmission line:



Fluid flow analogy of signal propagation.



Physical model of wire transmission line
 

1. Identify the elements in the analogous fluid flow system and describe the propagation of fluid signal in the system.

 
2. Identify the elements in the electrical system and describe the propagation of a pulse in the system.

 
3. What determines the data rate in the wire transmission system?