FTN 전송기법을 위한 효율적인 복호 알고리즘 연구

Abstract

FTN (Faster-than-Nyquist) signalling is a technique of transmitting information at a rate higher than the allowed Nyquist limit. Systems employing this technique have shown to achieve higher information rates at the cost of increased processing in the transmitter and the receiver. There have been some efforts to apply FTN theory to commercial applications, e.g. DVB-S2 (Digital Video Broadcasting - Satellite - Second Generation) for the satellite broadcasting system. There is increasingly growing demand to send high data rates over satellite channels. This thesis here discuss utilizing time packing or FTN signalling to satisfy this demand in combination with using tight frequency spacing is , where is the symbol rate and is roll-off factor.

In FTN signalling, information symbols are transmitted at a rate higher than that suggested by the Nyquist criterion, i.e., . Therefore, ISI (Inter-Symbol Interference) is unavoidable. Signaling above the Nyquist rate comes at the expense of higher receiver complexity and higher transmitted power, since more information symbols are sent per second. Since FTN method was first studied, the usefulness of FTN has not yet been determined. Several receivers have been suggested, some implying the feasibility of FTN, and others not considering it worthwhile. The presented model utilized interference as branch information and iteratively exchange probabilistic information between BCJR and LDPC (Low Density Parity Check code) decoder. In BCJR (Bahl Cocke Jelinek Raviv) decoder, the performance depends on Euclidean distance of branch metrics between possible transitions at each node.

This thesis proposed FTN re-mapper by reordering the branch matrices on trellis diagram in order to maximize Euclidean distances. This thesis confirmed that performance was improved compared to conventional methods as increasing throughput of FTN signal.

Furthermore, FTN method is applied to underwater communication. The performance of underwater communication system is sensitive to the ISI due to multipath signal propagation. And due to limited frequency using acoustic wave, underwater communication is a low transmission rate. In aspect to performance improved, turbo equalizations are employed. Iterative turbo equalization consists of inner codes and outer codes, decision feedback equalizer was employed as an outer codes and half rate convolutional codes was employed as inner codes. Equalizer and BCJR decoder are connected through the interleaving and de-interleaving that update each other's information repeatedly. The receiver is depend on powerful turbo equalization algorithms that iteratively exchange probabilistic information between inner decoder and outer decoder, thereby the error rates can be reduced significantly. In aspect to throughput efficiency, punctured coding method and FTN method are considered in underwater channel. Punctured coding methods are well known method to improve throughput. As a results of applying FTN method to underwater channel, this thesis confirmed FTN method also effective in the range of and .