Reference no: EM13275537

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.)