Generalized unitary rotation based precoder for super-imposing of synchronization sequences in OFDM

This paper proposes a novel algorithm for obtaining an orthogonal frequency division multiplexing (OFDM) signal with a super-imposed synchronization sequence, which is orthogonal to the rest of the signal. It allows to completely eliminate an interference between the super-imposed sequence and the useful signal. The super-imposed sequence does not consume dedicated time and frequency resources and provide additional means for timing and carrier frequency offset estimation and can be used for the channel estimation. The proposed precoder is based on a new class of fast unitary transformations and it is compatible with classic OFDM systems with pilot signal assisted modulation (PSAM) and frequency domain equalization. Moreover, it can be used in conjunction with existing OFDM timing offset and carrier frequency synchronization methods. The impact of frequency domain equalization on the orthogonality between the synchronization sequence and data subcarriers is analyzed. Additionally, the compatibility of the proposed precoder with popular data-aided and decision-directed timing offset estimation algorithms have been verified using Monte Carlo computer simulations.