1. An apparatus for achieving robust channel estimation in a communication system, comprising: (A) a training sequence generator to generate a training sequence, wherein the training sequence generator further comprises (I) a training symbol generator to generate a plurality of training symbols satisfying a predetermined constraint such that the training symbols are insensitive to synchronization error, wherein the training symbol generator further comprises a cyclic left shifter to perform cyclic left shift on a current training symbol in time domain to generate a next training symbol with the amount of the shifting determined by the number of segments in a cyclic prefix to be added to each training symbol; and (II) a training sequence forming unit configured to form the training sequence from the training symbols generated by the training symbol generator; (B) a formatter to insert the training sequence into a frame; and (C) a transmitting module to transmit the frame.