Turbo receivers for multiple-input multiple-output underwater acoustic communications
Systems and methods for underwater communication using a MIMO acoustic channel. An acoustic receiver may receive a signal comprising information encoded in at least one transmitted symbol. Using a two-layer iterative process, the at least one transmitted symbol is estimated. The first layer of the two-layer process uses iterative exchanges of soft-decisions between an adaptive turbo equalizer and a MAP decoder. The second layer of the two-layer process uses a data-reuse procedure that adapts an equalizer vector of both a feedforward filter and a serial interference cancellation filter of the adaptive turbo equalizer using a posteriori soft decisions of the at least one transmitted symbol. After a plurality of iterations, a hard decision of the bits encoded on the at least one transmitted symbol is output from the MAP decoder.
1. A MIMO UWA modem comprising:
an acoustic receiver comprising a plurality of acoustic sensors configured to receive MIMO UWA transmissions;
a memory;
a signal processing unit in communication with the acoustic receiver, the signal processing unit configured to decode the MIMO UWA transmissions and output a hard decision of what bit of information is encoded on one or more symbols included in the MIMO UWA transmissions,
wherein the signal processing unit includes a plurality of streams for decoding the MIMO UWA transmissions, each stream of the plurality of streams includes a maximum a posteriori probability decoder and an adaptive turbo equalizer having a feedforward filter and a serial interference cancellation filter,
wherein the signal processing unit decodes the MIMO UWA transmissions by iteratively exchanging soft decisions of the one or more symbols between the adaptive turbo equalizer and the maximum a posteriori probability decoder and by iteratively adapting the feedforward filter and the serial interference cancellation filter based upon a posteriori soft decisions of the one or more symbols.
2. The MIMO UWA modem of claim 1 , wherein the adaptive turbo equalizer of each stream further comprises an a posteriori statistical calculation unit for determining the a posteriori soft decisions of the one or more symbols.
3. The MIMO UWA modem of claim 2 , wherein the a posteriori statistical calculation unit includes a time averaging statistical estimator that determines a mean and a variance of the soft symbols for use in determining the a posteriori soft decisions of the one or more symbols.
4. The MIMO UWA modem of claim 3 , wherein the time averaging statistical estimator determines the mean using the equation
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5. The MIMO UWA modem of claim 2 , wherein the a posteriori statistical calculation unit determines the a posteriori soft decisions of each symbol of the one or more symbols using the equation {hacek over (x)} n,k t =Σ α i ∈s α i P(x n,k =α i |{hacek over (x)} n,k t ).
6. The MIMO UWA modem of claim 1 , wherein each stream of the plurality of streams includes a respective soft demapper for determining a bit a priori log likelihood ratio from each estimated symbol.
7. The MIMO UWA modem of claim 1 , wherein the MIMO UWA transmissions received by the acoustic receiver comprise one of a QPSK modulated MIMO transmission, an 8PSK modulated MIMO transmission, a 16QAM modulated MIMO transmission, or a higher order QAM modulated MIMO transmission.