 length L is investigated. From Figures 1.9 and 1.10, it can be observed that for the multipath Rayleigh fading channels, mirror-mapping-based schemes have better CIR performance compared with the plain OFDM. The CIR performance of the plain OFDM and the conjugate-based schemes, namely MCSR and MCJT, is not affected by the channel length L. For conversion-based schemes, namely MSR and MCVT, the CIR performance is degraded with increase in channel length L. For OFDM-based communications, to maintain the spectral efficiency, N is usually much larger than the channel length L. Thus, conversion-based schemes can effectively reduce ICI for the long OFDM symbol design. In summary, based on the CIR results of flat fading channels and multipath Rayleigh fad- ing channels, we conclude that mirror-mapping-based schemes can effectively mitigate ICI. Experiment on Sea Fast time varying, phase variation, and Doppler shift constitute the features of the underwa- ter acoustic (UWA) communication channel. As a consequence, the effect of ICI becomes significantly critical for OFDM-based UWA communication systems. The applicability of mirror-mapping-based schemes in UWA communications is verified in this section by sea experiments. 1 ICI Cancellation Techniques Based on Data Repetition for OFDM Systems Normal OFDM MSR, L= 1 MSR, L = 10 MSR, L = 50 MSR, L = 100 Figure 1.9 CIR comparison among the plain OFDM, the MSR scheme, and the MCSR scheme for different values of E over multipath Rayleigh fading channels. Normal OFDM MCVT, L=1 MCVT, L = 10 MCVT, L = 50 MCVT, L = 100 Figure 1.10 CIR comparison among the plain OFDM, the MCVT scheme, and the MCJT scheme for different values of E over multipath Rayleigh fading channels. 1.5.1 Experiment Settings The experiment was conducted in a sea area about 3 km east of Gushan, Taiwan, on 21-22 May 2013, which is illustrated in Figure 1.11. Three nodes were deployed, i.e. node 4, node 5, and node 9, each of which consisted of one transducer and four hydrophones. 1.5 Experiment on Sea Node 4 Node