# 无线通信代写 | Spectrum Management Homework 2

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Problem 1 (14 points)

Why does the FCC authorize some AM radio stations for daytime operation only?

Problem 2 (16 points)

Companies like Teligent and Winstar lost a lot of money trying to build fixed broadband
wireless systems operating at frequencies of 24-30 GHz. What were some of the
challenges they faced using these frequencies? (No, you don’t have to look up these dead
companies.)

Problem 3 (16 points)

Why is it that when you are sitting out on the cut using your wireless LAN connection
that moving your laptop a foot in either direction can sometimes cause loss of signal?
Would the same thing be likely to happen if you were receiving a signal from a
transmitter on a tower in the middle of the prairie?

Problem 4 (16 points)

CMU has a campuswide wireless LAN system operating at 2.4 GHz. Assume that an
outdoor base station is transmitting at 10 milliwatts to a laptop that is 100m away,
with sufficiently few obstacles that it can be considered line—of—sight. Assume the
base station gain is a factor of 4 and the laptop antenna gain is a factor of 2.

a) What is the attenuation over the link, including the antenna impact? (in dB)

b) What is the received power? i. in dBm? ii. in milliwatts?

Problem 5 (18 points)

If offered the opportunity to swap frequency assignments, would either or both of PCS
cellular (at 1.9 GHz) and DBS broadcasters (at 12.2 GHz) be interested in such a swap?
Why or why not?

Problem 6 (20 points)

Imagine a company in rural Iowa using a point to point microwave system to link two
sites 20 Km apart. At each site they have a 75m tower with a 1 m diameter dish antenna
pointing towards the other site. There are no significant obstacles within the Fresnel
zone. They are currently operating at 4.5 GHz. Suppose that they were given the option
of changing to a frequency of 750 MHz, while keeping the same antennas, and using
transmitters operating at the same transmit power level. Would they be interested in
changing? Why or why not? (15 points)

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