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This paper presents a multi-carrier digital modulation technique-orthogonal frequency division multiplexing. A review of the technique and its development and implementation are provided. The advantages and disadvantages in comparison to other modulation techniques are also discussed. The ideas of using a guard interval to accommodate long delay ghosts and of creating spectrum notches to combat co-channel interferences, as well as of assigning different order of modulations or power levels to different sub-carriers for layered services are discussed. The effects of phase noise and the peak-to-average power ratio of the OFDM signal are also analysed.< >