Adaptive Doppler-shift compensation for OFDM underwater acoustic communications system

  1. Lookup NU author(s)
  2. Ammar Abdelkareem
  3. Professor Bayan Sharif
  4. Dr Charalampos Tsimenidis
  5. Jeffrey Neasham
Author(s) Tsimenidis CC; Sharif BS; Neasham JA; Abdelkareem AE
Publication type Conference Proceedings (inc. Abstract)
Conference Name4th International Conference and Exhibition Underwater Acoustic Measurements: Technologies & Results
Conference LocationKos, Greece
Year of Conference2011
Source Publication Date20-24 June 2011
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An adaptive Doppler-shift compensation scheme for wideband orthogonalfrequency division multiplexing (OFDM) is proposed in this paper. This approachaccommodates a time-varying Doppler, where the time scaling factor for each OFDMsymbol is frequently estimated depending on the inherent periodicity of the cyclic-prefixand its preceding neighbours' values. The proposed technique relies on a single preambleof a packet consisting of multiple OFDM symbols to detect the start of the packet, theconcept of nearest neighbour rule to facilitate smoothing between symbols, and a dynamicsymbol synchronization point to update the time scaling factor. To accomplish this, anadaptation step is derived involving the weight of the nearest neighbour's time scaling inestimating the current re-sampling factor. In this paper, the synchronization point isupdated frequently and therefore an adaptive time scaling is obtained implicitly. Thevelocity perturbation is tackled by employing the concept of nearest neighbour rule inaddition to adopting a dynamic synchronization point. Instead of utilizing the whole guardinterval, the proposed system has exploited a finite length window of the cyclic prefix forcorrelation in each OFDM symbol to frequently estimate the Doppler shift. No iterativecomputation is required for the interpolation factor estimation. Furthermore, theproposed algorithm needs to only buffer one OFDM frame before data demodulationinstead of buffering the whole data packet. Thus, it necessitates a lesser degree ofcomplexity and memory requirements. An experiment has been conducted in the North Seaduring 2009 and the proposed algorithm has been compared with the block Dopplercompensation technique. Results show that there is variation in speed during the packettime, therefore the proposed system surpasses the block technique. It is confirmed that thetime scaling factor of the adaptive system is estimated for each OFDM symbol, whereasthe block approach fails in estimating these variations.