HW 10 Problem 1: How is the effect of frequency-selective fading on basic OFDM s

HW 10 Problem 1: How is the effect of frequency-selective fading on basic OFDM signaling? Ans: The bandwidth of each sub-carrier is usually made so narrow that each subcarrier experiences only flat fading. Moreover if cyclic prefix is used each sub-carrier is essentially behaves like a single frequency signal for any symbol period. The channel response is the transfer function at that frequency. However different sub-carriers may experience different channel responses (fadings). Problem 2: What techniques can one use in OFDM to overcome fading? Ans: a.
HW 10 Problem 1: How is the effect of frequency-selective fading on basic OFDM signaling? Ans: The bandwidth of each sub-carrier is usually made so narrow that each subcarrier experiences only flat fading. Moreover if cyclic prefix is used each sub-carrier is essentially behaves like a single frequency signal for any symbol period. The channel response is the transfer function at that frequency. However different sub-carriers may experience different channel responses (fadings). Problem 2: What techniques can one use in OFDM to overcome fading? Ans: a.) Pilot-symbol aided channel estimation and equalization b.) Frequency diversity (Frequency-domain coding or Multicarrier CDMA) Problem 3: a.) Let the symbol duration T be 0.1msec guard time duration (for cyclic prefix) is 0.05msec and the number of subcarrier is 8. The data sequence of user a is: -1 1 -1 1 The data sequence of user b is: 1 -1 -1 1 The data sequence of user c is: -1 -1 1 1 The data sequence of user d is: 1 -1 -1 -1 The spreading code for user a is -1 -1 -1 -1 The spreading code for user b is -1 1 -1 1 The spreading code for user c is -1 -1 1 1 The spreading code for user d is -1 1 1 -1 These users use sub-carrirs k=0 2 4 6. If chip energy Ec is 1 and bit energy Eb is 4.
What is the baseband MC-CDMA signal? b.) Let the signal in (a) go though a channel with the following 2-path model: h(t)=d(t)-d(t-0.04msec). What is the received baseband MC-CDMA signal? c.) Perform the demodulation process (FFT) (For this step assume no noise) d.) Add constant noise to each demodulated carrier output. Assume ?=4 (No=16) perform dispreading equalization combining and detection to decode the transmitted data and find out the BER. {using the following three different approaches} 1. (maximum ratio combining) 2. (orthogonal restoring combining) 3. (MMSE) e.) Assume ?=2 No=4; Repeat d.) f.) Assume ?=1 No=1; Repeat d.) g.) Assume ?=0.5 No=0.25; Repeat d.) h.) Assume ?=0.1 No=0.01; Repeat d.)
Attachments: