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Type of Interference and intensity level problems

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1. There are two speakers at the front of the room. Each is outputting a 250Hz pure tone. For each of the following situations, will you hear constructive interference (a loud noise) or destructive interference (soft or no noise). Lambda represents the wavelength of the 250Hz tone. Choices: Constructive, Destructive.
a. You are 2.5 lambda away from speaker A; 3 lambda away from speaker B and at the speakers the waves are exactly in phase.
b. You are 3 lambda away from speaker A; 2 lambda away from speaker B and at the speakers the waves are exactly in phase.
c. You are 3 lambda away from speaker A; 2.5 lambda away from speaker B and at the speakers the waves are exactly out of phase.
d. You are 3 lambda away from speaker A; 2 lambda away from speaker B and at the speakers the waves are exactly out of phase.

2. You are standing in the middle of a large room listening to acacaphony of sounds. Rank the intensity level of the sounds from each source (1=loudest, 2= next loudest, ...) Consider all objects to be right next to you. (Remember that if you have a tie, you skip some ranks.)
a. One hundred buzzers each are producing a sound with intensity level of 73dB.
b. An object emitting a sound with an intensity of 90dB.
c. Two buzzers each are producing a sound with intensity level of 88dB.
d. Ten buzzers each are producing a sound with intensity level of 78dB.

3. You are standing in the middle of a large room listening to a cacaphony of sounds. Rank the intensity of the sounds from each source (1=loudest, 2= next loudest, ...) Consider all objects to be right next to you.
a. An object emitting a sound with an intensity of 1.0*10^5 W/m2.
b. An object emitting a sound with an intensity level of 110 dB.
c. Two buzzers each are producing a sound with intensity level of 40 dB.
d. An object emitting a sound with an intensity level of 50 dB.

4. A stereo is playing music with an output of 55 dB. If the volume is turned up so that the output intensity is 100 times greater, what will be the new intensity level?

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1. First let's define what path difference is: length of the path of one wave minus length of path of other wave.
If the wave is in phase, then the path difference is whole number of wavelengths (e.g. 0 lambda, 1 lambda, 2 lambda ...) ---- ...

Solution Summary

For the interference problems, we use the path difference and its phase state (whether in or out of phase) to know the type of interference. For the intensity level problems, we use the main equation and derive some simpler equations so that we can eliminate some of the unknown variables.

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TDMA cellular system, Power efficiency and spectral efficiency for a modulation scheme, Propagation index, Multipath, Fast fading, Direct sequence spread spectrum, Outgoing traffic intensity

I need the answers to the questions so that my study group can have the correct information in preparation for a group assignment. This is just for reference and will not be submitted for a grade to any school.

Answer the following questions. Explain your answer and show your work in a logical manner. Use any needed formula -- show the formula, the substitutions made, and explain why.

1. A TDMA cellular system uses a 200 KHz bandwidth carrier, which carries 8 channels. What defines or determines each channel, and how much bandwidth does each channel occupy and why?

2. Define what is meant by power efficiency and spectral efficiency for a modulation scheme? What is the measure used for each?

3. Order the following modulation types in terms of spectral efficiency (best to worse) and explain why (be specific, show a number and how you got it): 64-QAM, QPSK, 8-PSK, and BPSK. Assume that the r=0, perfect filtering.

4. Propagation in a wireless channel is sometimes modeled using the propagation index n. Explain how the propagation index n varies and why. If there are two channels, where n=2 for the first one and n=3 for the second one (as for both the same power is transmitted and the same modulation/coding is used), in which channel will the signal be detectable further away and why?

5. Describe why multipath occurs in wireless. Describe fast fading, and some technique to counter it.

6. Describe how direct sequence spread spectrum (DSSS) suppresses interference from other transmissions in the same frequency band.

7. A telephone switching board can handle 120 phones. Assuming the following, determine the outgoing traffic intensity and the number of channels:
* On average 5 calls/hour per phone
* 60% of all calls made are external
* Average call duration time is 4 minutes
* QoS = 0.9%

Please refer to the attachment for multiple choice questions.

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