A factor-graph based ZP-OFDM receiver for deep water acoustic channels

In this paper, we present a factor-graph based receiver for zero-padded orthogonal frequency division multiplexing (ZP-OFDM) transmissions over deep water channels with extremely long delay spreads. Due to the geometric structure of deep oceans, the channel delay spread between the direct paths and surface/bottom reflections can be on the order of hundreds of milliseconds, which is usually several times larger than the ZP-OFDM block duration, leading to severe inter-block-interference (IBI). Channel time variation also introduces inter-carrier-interference (ICI). The proposed receiver addresses both IBI and ICI in a unified factor-graph representation. Data detection is performed according to the Gaussian message passing (GMP) principle which operates on the factor graph in an iterative manner. The proposed structure is further extended to the progressive/iterative framework, while the channel estimation is updated in each iteration loop. Compared with a multiuser receiver, which views overlapped blocks as coming from different users, the proposed receiver enjoys better block-error-rate performance and appealing expansion capability when multiple receiving-elements are used. Both simulation and experimental results are provided to validate the performance of the proposed receiver.