In this chapter, we focus on channel estimation within each received OFDM block. The underwater acoustic channel is well-known to consist of sparsely distributed propagation paths. Compared to the canonical least squares based method which estimates all the discretized channel samples within the channel delay spread, channel sparsity can be exploited to reduce the number of unknown parameters to estimate. This chapter treats sparse channel estimation for OFDM in both time-invariant and time-varying environments.
Sparse channel estimation has been underpinned by the striking development of compressive sensing theory in the last decade. To facilitate the development of sparse channel estimation in this chapter, the basics of compressive sensing are presented in Appendix A. The reader is encouraged to read this chapter and Appendix A in parallel for an in-depth understanding.
This chapter is organized as follows.