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Unformatted text preview: EE 359 - Wireless Communications - Winter 2008 Finals: Total 100 Points, Duration: 8:30 to 11:30 am This exam is open book and notes, and calculators are needed. Please state all assumptions used in your calculations. You may use any derivations or statements from the book as long as you cite where they come from. Good Luck! 1. [ 25 pts ] Short Answer (a) [ 5 pts ] Consider an OFDM system with a passband bandwidth of 100 MHz sent over a channel with a delay spread of 50 μ sec. How many subchannels are needed in this system to have approximately independent fading on each subchannel? If the sample time of the OFDM system equals the bit time T b = 5 μ sec and BPSK is transmitted on each subchannel, what is the data rate loss associated with the cyclic prefix of the system? (b) [ 5 pts ] Consider a 2x2 MIMO system whose singular value decomposition has one large singular value and one small one. If there is some channel estimation error at the receiver, will this impact the BER of both spatial streams equally or not and why? (c) [ 5 pts ] In spread spectrum systems, what is the problem in using long spreading codes for synchroniza- tion? What is the problem in using short spreading codes for data transmission? (d) [ 5 pts ] Give two possible reasons why most next-generation high speed wireless systems (Wimax, Wifi, 4th generation cellular) have chosen OFDM over spread spectrum for their design. What is one advantage of spread spectrum over OFDM? (e) [ 5 pts ] Will a 2x2 MIMO system with transmit selection combining and receiver MRC have better, worse, or the same performance as a 2x2 MIMO system with MRC transmit diversity and selection combining in the receiver, assuming perfect channel knowledge on both ends? Justify your answer.combining in the receiver, assuming perfect channel knowledge on both ends?...
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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.
- Spring '08