Frequency Offset Compensation in OFDM Transmission

Orthogonal Frequency Division Multiplexing (OFDM) has been a signature technology in today’s wireless communications such as Long Term Evolution (LTE) and Wireless Local Area Network (WLAN). With its high spectral efficiency, robustness in multipath environment, flexible allocation in subcarriers and other merits, OFDM provides higher transmission speed comparing to previous physical layer technologies. However, performance of the orthogonal and overlapping subcarriers in OFDM signal is highly vulnerable to poor frame synchronization that introduce Inter-Symbol Interference (ISI) and carrier frequency offset that results in Inter-Carrier Interference (ICI). In other words, without feasible algorithms for frame synchronization and frequency offset correction, OFDM may perform worse than previous technologies.

This research grows from an air-to-ground communications project in upper C band (between 5925-6700 MHz) with airborne test articles having a speed of up to Mach 2, or twice the speed of sound. This will introduce a large Doppler offset into the system and corrupt constellations if not well compensated. As a part of OFDM transmission study, the thesis will cover the background of OFDM systems, but mainly focus on frequency offset corrections for this type of implementations, analyzing how the frequency offset is introduced, how it would affect performance, comparing some frequency estimator performance, and the algorithms for compensation to get good constellations.