Application of channel estimation to underwater,acoustic communication.
详细信息
文摘
The underwater channel poses numerous challenges for acoustic communication. Acoustic waves suffer long propagation delay, multipath and fading, limited bandwidth, and potentially high spatial and temporal variability. In addition, there is no typical underwater acoustic channel； even body of water exhibits quantifiably different properties. Consequently, current modems---implemented in hardware with a fixed, conservative set of transmission parameters---are often ill-suited for a particular channel, resulting in performance that is far from optimum. Very little work has been done in the area of channel characterization, especially for waters only several meters deep. As a result, network simulations often make conservative and/or inaccurate assumptions about shallow underwater channels. In this thesis the Hudson River estuary is characterized as an acoustic communication channel. The analysis reveals that the Hudson is a multipath fading channel Rician fading over 200 m and Gamma fading over 505 m) with an extremely short coherence time of approximately 50 ms. A subset of the estimation techniques is then employed to develop a network simulation and an adaptive, real-time software modem. The simulator converts a transmitted packet into a modulated signal and digitally mixes it with the channel estimates to produce a signal that approximates what would have been received after transmission through the physical channel. When simulating a time-invariant channel, the achieved bit error rates are, on average, within 3.34% of those obtained by transmission through the actual channel. The simulator is modular and can easily accommodate new channel estimates, modulation schemes, receiver techniques, and alternate implementations of higher layers in the network stack. In the software modem each packet is preceded by an acoustic signal that is used for impulse response estimation. The modem then processes the signal in real time and uses the inverse impulse response to equalize the channel, allowing for the transmission of packets at higher data rates with symbols whose duration is less than the multipath spread of the channel. In a time-invariant shallow water test channel, the modem correctly decoded packets at up to 6 kbps. In an AWGN channel, the modems BER approached the theoretical limit for the given SNR.