Receiver designs for multiuser underwater acoustic communications

This dissertation focuses on multiuser communications through shallow, underwater acoustic channels. These channels are characterized by channel impulse responses with long delay spreads undergoing rapid fluctuations with respect to the digital signaling time. When multiple users (e.g. AUVs, gliders, or sensor nodes) need to transmit information to a common receiver, they must share the channel in some fashion. The designs presented in this dissertation utilize a sharing scheme known as Space Division Multiple Access (SDMA), where the inherent disparity in the impulse responses sampled at different spatial locations are leveraged by the system to provide users with interference-free uplinks to the common receiver. Compared to other channel sharing methods, SDMA benefits from high data throughput and a low reliance on feedback from the receiver, two desirable qualities in a bandwidth limited, rapidly evolving environment. The receivers discussed throughout this dissertation will employ successive decoding techniques to retrieve each user's information independently but will use knowledge from previous decoding cycles to model and remove multiple access interference along the way. With multiple iterations of estimation and interference cancellation, these receivers will progress towards the goal of providing each and all of the users with interference-free uplinks to the receiver. Three receivers will be discussed in this dissertation with each successive design more generally applicable than the previous: one will be applicable in time-invariant environments between geographically fixed users and a fixed, multiple-element receiver, the next will be applicable in a time-varying environment between fixed users and a fixed receiver array, and the final design will be applicable in situations with users in motion. All of the receivers discussed will require direct knowledge of the impulse response and will employ sparse channel estimation techniques to acquire this information and track any changes while decoding. The capabilities of all of the receivers will be analyzed with data collected during at-sea experiment