EE 359: Wireless Communications  Fall 2003
SECOND EXAM
This exam is open book and notes, and calculators and your reader are needed. Graded exams and solutions
will be available within the next week. Good luck!
Problem 1 (20 points): Short Problems.
(a)
Equalizer Tracking:
Why does an equalizer that tracks the channel during data transmission
still need to train periodically? Name 2 beneﬁts of tracking.
(b)
Narrowband Interference Eﬀects:
Describe qualitatively what eﬀect a narrowband interfer
ence signal has on an OFDM signal, a direct sequence spread spectrum signal, and a frequency
hopped spread spectrum signal.
(c)
OFDM Design:
Consider a channel with maximum delay spread
T
M
=40
μ
sec. Is this likely
to be an indoor or outdoor channel and why? Suppose you build an OFDM system for this
channel with a total system bandwidth of 640 KHz.
How many subchannels are needed to
obtain approximately ﬂat fading on each subchannel? How many samples long does the cyclic
preﬁx need to be to insure no ISI between FFT blocks, assuming the subchannel symbol time
equals the inverse of the subchannel bandwidth?
Problem 2 (30 points): Capacity vs. Diversity in MIMO with Adaptive Modulation.
This problem illustrates how multiple antennas can be used for either diversity gain or capacity gain,
and how adaptive modulation can be used with either technique.
Consider a 2x2 MIMO system that
decomposes into two parallel AWGN channels:
y
1
=
α
1
x
1
+
n
1
.
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 Spring '08
 Goldsmith
 ADAPTIVE MODULATION, outage probability, total noise power, multipath components, maximum constellation size

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