Conventional OFDM transmission system uses a fixed-length Cyclic Prefix to counteract Inter-Symbol Interferences (1SI) caused by channel delay spreading under wireless mobile environment. This may cause considerable performance deterioration when the CP length is less than the channel RMS delay spread, or may decrease the system power and spectrum efficiency when it is much larger. A novel Orthogonal Frequency Division Multiplexing (OFDM) transmission scheme is proposed in this paper to adapt the CP length to the variation of channel delay spread. AOFDM-VCPL utilizes the preamble or pilot sub-carriers of each OFDM packet to estimate the channel RMS delay spread; and then uses a criterion to calculate the CP length, which finally affects the OFDM transmitter. As illustrated in the simulation section, by deploying this scheme in a typical wireless environment, the system can transmit at data rate 11.5Mb/s higher than conventional non-adaptive system while gaining a 0.65dB power saving at the same BER performance.
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