Synchronization of OFDM with null subcarriers in channels with significant ICI

In this paper we propose and analyze a new approach for synchronization of OFDM based underwater communication systems with significant Doppler spread. In order to keep cyclic prefix overhead low - long OFDM symbols are needed which in turn leads to significant inter-carrier interference (ICI). For signal acquisition and synchronization we propose the use of a special OFDM symbol which has regions of null subcarriers i.e. subcarriers where no power is transmitted but also still has subcarriers for channel estimation and payload data. By estimating the signal to noise ratio on candidate demodulation positions we can calculate a “would-be” signal to noise ratio by adding up the power on the active subcarriers and dividing it by the power in the null subcarriers. By only using the inner subcarriers in a region of null subcarreries resistance against Doppler spread is achieved. We optimize and compare the proposed solution against a base-line solution which uses a pre-amble with a hyperbolic frequency modulated signal. We find our solution to compare well and present bit error rate and frame error rate results on a complete system which uses our proposed acquisition and synchronization scheme.