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|Title:||Multicarrier Schemes for Efficient Underwater Acoustic Communications|
|Abstract:||In recent times orthogonal frequency division multiplexing (OFDM) has been preferred over single carrier communication for underwater acoustic (UWA) communication. Underwater channel is frequency selective, and there exist regions in the spectrum which completely attenuate the OFDM carriers and this results in loss of data/symbols carried by them. This degrades the bit-error-rate (BER) performance and results in loss of bandwidth due to the necessity of retransmission of erroneous bits. Retransmission is not acceptable for long delay channels like the UWA channel. An attractive approach to this problem is to spread the group of information symbols over the entire available bandwidth. The resulting variant of OFDM system is termed as spread OFDM (SOFDM). The receiver of such a system separates the spread symbols by performing simple linear matrix operations. The spreading operation not only reduces peak-to-average power ratio (PAPR) but also improves the BER performance over the UWA channels. Walsh-Hadamard (WH), discrete Fourier transform (DFT), discrete cosine transform (DCT) and carrier interferometry (CI) codes, used as spreading matrix, have been extensively evaluated over the UWA channels in this thesis. A new multiple access concept which combines gain diversity of WH matrix and phase properties of orthogonal CI codes, has been applied over the UWA channel. The robustness of CI code in frequency selective channel coupled with the spreading diversity of WH code significantly improves the BER performance. Future underwater applications will demand a design which can support a wide range of data transmission rates and a very large number of users/nodes while maintaining the desired quality-of-service (QoS) for each one. The major contribution of the thesis is the design of multicarrier multiple access schemes based on orthogonal codes for singlerate and multi-rate communication over the UWA channel. New spreading codes have been proposed to include additional users as well as higher data rates. CI-direct sequence code-division multiple-access (CI/DS-CDMA) and CI-multicarrier code-division multiple-access (CI/MC-CDMA) have been used to support variable data rates for UWA applications. Transmit diversity techniques have been employed to SOFDM systems to further enhance the system performance. Space-frequency block-coding (SFBC) has been used to provide transmit diversity to the SOFDM systems and the BER performance has been evaluated over the UWA channel in this thesis. The combination of OFDM and the non-coherent detection scheme maintains the receiver design simple, reliable and spectrally efficient. The gain in simplicity of the receiver has been utilized to improve the system performance by applying simple combining diversity techniques.|
|Appears in Collections:||03. EE|
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