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)=δ(t)-δ(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 η=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 η=2 , No=4; Repeat d.)
f.) Assume η=1 , No=1; Repeat d.)
g.) Assume η=0.5 , No=0.25; Repeat d.)
h.) Assume η=0.1 , No=0.01; Repeat d.)