TABLE OF CONTENTS When a frequency offset is introduced in the channel, or when there is a mismatch in the carrier frequency between transmitter and receiver local oscillators, the BER degrades drastically, since severe intersubcarrier interference occurs because of the overlapping power spectra between neighboring subcarriers [see Figure 3.8(c)]. This sensitivity to frequency offset is often pointed out as a major OFDM disadvantage.
Assume that a frequency offset f 0ff and a phase offset ? off are introduced in the channel. Then the received signal in equivalent baseband expression is written as
| (4.64) |  |
Substituting (4.1), (4.30) to (4.32), and (4.64) into (4.49), the nth DFT output is written as
| (4.65) |  |
Similarly, as in Section 4.5.2, the components of p are written as [28]:
| (4.66) |  |
| (4.67) |  |
| (4.68) |  |
Substituting (4.35) and (4.66) to (4.68) into (4.28), we can theoretically evaluate the BER of a DPSK-based OFDM system in Rayleigh fading channels with frequency offset.
Figure 4.18 shows the BER of a QDPSK-based OFDM system against the frequency offset normalized by subcarrier separation. Table 4.3 shows the transmission parameters to demonstrate the performance, where the number of subcarriers and the length of guard interval in the OFDM system are optimized for the transmission and channel parameters in Table 4.1. The theoretical analysis agrees well with the computer simulation result. As compared with the case when ignoring...
