length of the rectangular function is a length of windowing the pulse response h u2032 m assuming that a preset windowing length of the receiving apparatus is l and the value of l is an actual length of a channel assumed by the receiving apparatus by windowing the pulse response h u2032 m the receiving apparatus reserves multi path information of the channel and filters noises outside the channel at the same time a specific operation of obtaining the windowing length l by the receiving apparatus may be that when the receiving apparatus does not know a type of the channel or the channel changes constantly the receiving apparatus estimates a multi path delay and obtains the length l according to an estimation result of the multi path delay it should also be noted that the preset windowing information in the receiving apparatus can also include two windows with the length being k fig5 illustrates the channel pulse response before and after the windowing the reason for adding the two windows with the length being k is that the n 2m pilots sequence is not the actual channel frequency response the actual channel frequency response should be the frequency response of all sub carriers of the ofdm symbol and the n 2m pilots sequence is a product of 3 times down sampling points of the actual channel frequency response and a rectangular function with the length being n 2m therefore the channel pulse response h u2032 m obtained after the ifft performed on the n 2m pilot points is not the actual channel pulse response.