الفهرس 
INTRODUCTIONOnly 14 pages are availabe for public view
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Abstract
Underwater acoustic channels (UWAC) are characterized by frequency selective fading. Due to its robustness to channel’s frequency selectivity, orthogonal frequency division multiplexing (OFDM) is considered one of the most suitable techniques for UWAC. As acoustic transmission is supported at low frequencies, the channels suer from low capacity. Using sparse Non-Orthogonal frequency division multiplexing (S-NOFDM), spectral eciency of OFDM is improved. The subcarriers of S-NOFDM are generated from a subset of OFDM orthogonal sub-channels, hence it requires fewer spectral resources. Then, time shift and multiplexing is used to generate a number of nonorthogonal subcarriers to substitute for the reduction of OFDM sub-channels used in transmission. First, channel-Matched S-NOFDM (CM-S-NOFDM) approach is proposed, where the set of selected sub-channels corresponds to maximum transmission coecients through the channel. The results indicate that the proposed scheme has better performance than S-NOFDM and it approaches the performance of OFDM while using less spectral resources. Second, security and condentiality of transmitted signals are considered. UWAC fading channels is modelled as Gaussian parallel broadcast channels with condential messages (BCCM). The sourcenode tends to transmit a condential message to each user over N independent sub-channels, where each sub-channel is degraded for one user rather than others. According to channel state information(CSI) sent by reliable feedback from receivers to source node, sub-channels are assigned to each user to achieve a higher secrecy rate under the constraint of average power input to the channel. Although OFDM is a common physical layer scheme used to cope with BCCM, OFDM-based BCCM consumes large bandwidth due to the sub-carrier orthogonality requirement. A novel S-NOFDM transmission scheme for the parallel Gaussian broadcast channel with condential messages (BCCM) is proposed to improve spectral eciency of OFDM in order to overcome capacity limitations.