final03 - EE 359: Wireless Communications - Fall 2003...

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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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This note was uploaded on 08/05/2008 for the course EE 359 taught by Professor Goldsmith during the Spring '08 term at Stanford.

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final03 - EE 359: Wireless Communications - Fall 2003...

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