Performances of data transmission by multichannel modulations

OFDM je sustav prijenosa informacije kod kojeg se serijski slijed simbola dijeli na više sporijih slijedova koji se koriste za modulaciju ortogonalnih podnosioca. OFDM se koristi u mnogim praktičnim sustavima jer je pogodan za prijenos informacije u bežičnim i ožičenim medijima. Kod frekvencijski selektivnih kanala SNR znatno varira s frekvencijom. DMT je vrsta OFDM-a koja dobro iskorištava frekvencijski selektivan kanal a koristi se u ožičenim sustavima kao što je DSL. DMT koristi WFA za optimalni raspored snage po tonovima te odgovarajuću alokaciju bitova. Simulacijama je pokazano kako je poboljšanje uz WFA obrnuto proporcionalno kvaliteti kanala. Orthogonal frequency division multiplexing (OFDM) is a transmission technique which divides the serial data stream into a number of slower streams which are used to modulate parallel orthogonal subcarriers whose bandwidth is much less than that of the total data rate. The OFDM technique is widely used in many practical system as it has many advantages in wireline and wireles communications. If the channel is frequency selective, the signal to nosie ratio of each subchannel can vary with frequency. One of the transmission schemes to overcome the frequency selectivity is discrete multi tone (DMT) which is used in wireline systems such as DSL. In a DMT system, power level of each subchannel is optimized with the water filling algorithm (WFA) so the proper bit - loading could be executed. It is shown trough a simulation that usage of WFA makes performance improvement that is inversely proportional with channel quality.