 OFDM-QH, the ICI from direct neighbors is removed. Figure 1.12 1 ICI Cancellation Techniques Based on Data Repetition for OFDM Systems Table 1.1 System parameters. Sampling rate at the transmitter 48 kHz Sampling rate at the receiver 48 kHz Signal bandwidth 5.36 kHz Carrier frequency 17 kHz Number of total subcarriers Number of data subcarriers Number of pilot subcarriers Number of null subcarriers Subcarrier spacing 3.35 Hz OFDM symbol duration 299 ms Guard interval 50 ms Number of hydrophones OFDM-B OFDM-QH E = 0.2 E = 0.1 Figure 1.12 BER performance comparison between OFDM-QH and OFDM-B for CFOs E = 0, ,0.1,0 0.2. The total subcarrier number is N = 1024, the bandwidth is 6 kHz, and the channel spread length is 9 ms. For OFDM-QH, Gray code is adopted for QPSK modulation. compares the bit error rate (BER) performance of OFDM-B and OFDM-QH under dif- ferent CFOs. It is found that OFDM-B has lower BER than OFDM-QH in low-medium signal-to-noise-ratio (SNR) range (SNR < 20, BER > 10-3). This is because of the higher symbol detection error for QPSK modulation. Considering the high signal attenuation of UWA channels and relatively low SNR at the receive hydrophones, OFDM-B, the one with better BER, is chosen as the benchmark in the sea experiment. 1.5 Experiment on Sea Plain OFDM (BPSK) ICI Cancellation (QPSK) Preamble Postamble 5 symbols 10 symbols Figure 1.13 Frame structure. The frame structure for transmission is given in Figure 1.13. The transmitted frame consists of a preamble, transmitted OFDM data symbols, and a postamble. We allocate 5 OFDM symbols with BPSK modulation for the plain OFDM and 10 OFDM symbols with QPSK modulation for either the ICI self-cancellation schemes or the two-path cancellation schemes. Therefore, 17 OFDM symbols are involved in each frame in the experiment. In addition, Gray code is adopted for QPSK modulation. Depending on the ICI cancellation structure at the last 10 OFDM symbols, there are four different frames, implementing the MSR scheme, the MCSR scheme, the MCVT scheme, and the