OFDM for Underwater Acoustic Communications

Chapter 10OFDM Receiver with Clustered Channel Adaptation

Chapter 9 focuses on the block-by-block receiver processing, where channel paths are estimated solely based on the measurements from one individual block. This chapter will present an OFDM receiver leveraging the measurements from both the current and past blocks. This allows the system to reduce the number of pilots while maintaining robust operation over fast-varying channels.

This chapter is organized as follows.

• Section 10.1 illustrates channel dynamics, notably different variations across multipath clusters. Section 10.2 presents a cluster-based channel variation model to parameterize the channel variation from block to block.
• Section 10.3 describes the design of an adaptive OFDM receiver, developing key receiver modules incorporating channel adaptation.
• Sections 10.4 and 10.5 contain performance results of the block-to-block adaptive receiver using data sets collected in two field experiments.

10.1 Illustration of Channel Dynamics

In general, underwater acoustic channels have large temporal variations. From the receiver point of view, whether the channels vary fast or slow depending on the relationship between the channel coherence time and the transmission block length. Fig. 10.1 presents the channel estimate samples in two experiments. Two important observations are as follows

• Channel paths tend to be clustered; according to the ray-tracing model [141], there are many small paths centering around the eigen-paths ...