Adaptive OFDM modulation for underwater acoustic communications: Design considerations and experimental results
Date
2014Source Title
IEEE Journal of Oceanic Engineering
Print ISSN
0364-9059
Publisher
Institute of Electrical and Electronics Engineers Inc.
Volume
39
Issue
2
Pages
357 - 370
Language
English
Type
ArticleItem Usage Stats
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Abstract
In this paper, we explore design aspects of adaptive modulation based on orthogonal frequency-division multiplexing (OFDM) for underwater acoustic (UWA) communications, and study its performance using real-time at-sea experiments. Our design criterion is to maximize the system throughput under a target average bit error rate (BER). We consider two different schemes based on the level of adaptivity: in the first scheme, only the modulation levels are adjusted while the power is allocated uniformly across the subcarriers, whereas in the second scheme, both the modulation levels and the power are adjusted adaptively. For both schemes we linearly predict the channel one travel time ahead so as to improve the performance in the presence of a long propagation delay. The system design assumes a feedback link from the receiver that is exploited in two forms: one that conveys the modulation alphabet and quantized power levels to be used for each subcarrier, and the other that conveys a quantized estimate of the sparse channel impulse response. The second approach is shown to be advantageous, as it requires significantly fewer feedback bits for the same system throughput. The effectiveness of the proposed adaptive schemes is demonstrated using computer simulations, real channel measurements recorded in shallow water off the western coast of Kauai, HI, USA, in June 2008, and real-time at-sea experiments conducted at the same location in July 2011. We note that this is the first paper that presents adaptive modulation results for UWA links with real-time at-sea experiments.
Keywords
Adaptive modulationFeedback
Orthogonal frequency-division multiplexing (OFDM)
Underwater acoustic (UWA) communication
Computer simulation
Experiments
Feedback
Orthogonal frequency division multiplexing
Underwater acoustics
At-sea experiment
Average bit-error rates
Channel measurements
Design considerations
Modulation levels
Propagation delays
Sparse channel impulse response
Underwater acoustic communications
Adaptive modulation