Synchronization for OFDM-Based Systems

Recently, orthogonal frequency division multiplexing (OFDM) techniques have received great interest in wireless communications for their high speed data transmission. OFDM improves robustness against narrowband interference or severely frequency-selective channel fades caused by long multipath delay spreads and impulsive noise. A single fade or interferer can cause the whole link to fail in a single carrier system. However, only a small portion of the subcarriers are damaged in a multicarrier system. In a classical frequency division multiplexing and parallel data systems, the signal frequency band is split into N nonoverlapping frequency subchannels that are each modulated with a corresponding individual symbol to eliminate interchannel interference. Nevertheless, available bandwidth utilization is too low to waste precious resources on conventional frequency division multiplexing systems. The OFDM technique with overlapping and orthogonal subchannels was proposed to increase spectrum efficiency. A high-rate serial signal stream is divided into many low-rate parallel streams; each parallel stream modulates a mutually orthogonal subchannel individually. Therefore, OFDM technologies have recently been chosen as candidates for fourth-generation (4G) mobile communications in a variety of standards, such as 802.16m and LTE/LTE-A.