GUARD INTERVAL OCCURRENCE IN DMT MODULATION TECHNIQUE

PROBLEM TO BE SOLVED: To specially reduce a transmission delay by permitting a last sample forming a symbol before a shift to be placed at the beginning of the symbol, after the shift to directly form a prefix. SOLUTION: A complex coefficient Ai.ej i is added to an IFFT circuit 12 through a complex multiplier 22 in the case that i belongs to [1, N]. The N coefficients eiKiτ are respectively added to the second input of the multiplier 22 in correspondence. The IFFT circuit outputs a first sample S1' of the symbol Dt' with a time t1 and a memory 24 stores the sample which is controlled in a writing mode. A multiplexer 16 is changed-over and selects the output of the circuit 12. The state of a circuit 10 is kept in a state without changes, until a time tτ. Thus, the samples from S1' to Sj' can be stored in the memory 24 and cyclic prefixes formed by S1' to Sj' are given to the output of the multiplexer 16.