Effects of mult-path channel upon constellation of OFDM systems

Based on time shift properties of discrete Fourier transform, the constellation point distribution of orthogonal frequency division multiplexing (OFDM) system in multi-path channel is analyzed. Although inter-symbol interference (ISI) of OFDM can be mitigated by using cyclic prefix whose length is longer than the maximum multi-path delay of wireless channel, the constellation of OFDM symbol sharply deteriorates, which will have an important effect on performance of OFDM systems. For different sub-channels with different carrier frequencies, the effects on constellation are quite distinct, that is, there exists frequency selectivity. In this paper, the frequency selective properties of constellation caused by multi-path channel are discussed and the corresponding simulation results are provided, which can be used as a theoretical reference for channel estimation of OFDM systems.