Abstract: Advantageous underwater acoustic (UWA) apparatus, systems and methods are provided according to the present disclosure. The apparatus, systems and methods employ nonbinary low density parity check (LDPC) codes that achieve excellent performance and match well with the underlying modulation. The nonbinary LDPC codes of the proposed apparatus, systems and methods are formed, at least in part, from a generator matrix that has a high density to reduce the peak-to-average-power ratio (PAPR) with minimal overhead. The disclosed apparatus, systems and methods employ nonbinary regular LDPC cycle codes if the constellation is large and nonbinary irregular LDPC codes if the constellation is small or moderate. The nonbinary irregular and regular LDPC codes enable: i) parallel processing in linear-time encoding; ii) parallel processing in sequential belief propagation decoding; and iii) considerable resource reduction on the code storage for encoding and decoding.

Abstract: Advantageous online and/or real-time OFDM-based underwater acoustic (UWA) apparatus, systems and methods are provided according to the present disclosure. The apparatus, systems and methods employ a receiver with a bank of parallel branches, with each branch having a self-correlator matched to a different waveform scaling factor. A detection is declared when any of the branches leads to a correlation metric larger than a pre-defined threshold. The branch with the largest metric yields a Doppler scale estimate and a coarse synchronization point. The proposed apparatus, systems and methods use one OFDM preamble, thereby avoiding the need to buffer the whole data packet before data demodulation and enabling online and/or real-time operation. Thus, the disclosed apparatus, systems and methods are advantageously applicable to UWA communications.

Abstract: Advantageous underwater acoustic (UWA) apparatus, systems and methods are provided according to the present disclosure. The apparatus, systems and methods employ nonbinary low density parity check (LDPC) codes that achieve excellent performance and match well with the underlying modulation. The nonbinary LDPC codes of the proposed apparatus, systems and methods are formed, at least in part, from a generator matrix that has a high density to reduce the peak-to-average-power ratio (PAPR) with minimal overhead. The disclosed apparatus, systems and methods employ nonbinary regular LDPC cycle codes if the constellation is large and nonbinary irregular LDPC codes if the constellation is small or moderate. The nonbinary irregular and regular LDPC codes enable: i) parallel processing in linear-time encoding; ii) parallel processing in sequential belief propagation decoding; and iii) considerable resource reduction on the code storage for encoding and decoding.

Abstract: Advantageous online and/or real-time OFDM-based underwater acoustic (UWA) apparatus, systems and methods are provided according to the present disclosure. The apparatus, systems and methods employ a receiver with a bank of parallel branches, with each branch having a self-correlator matched to a different waveform scaling factor. A detection is declared when any of the branches leads to a correlation metric larger than a pre-defined threshold. The branch with the largest metric yields a Doppler scale estimate and a coarse synchronization point. The proposed apparatus, systems and methods use one OFDM preamble, thereby avoiding the need to buffer the whole data packet before data demodulation and enabling online and/or real-time operation. Thus, the disclosed apparatus, systems and methods are advantageously applicable to UWA communications.

Abstract: Advantageous OFDM-based underwater acoustic (UWA) apparatus, systems and methods are provided according to the present disclosure. In general, OFDM transmissions over UWA channels encounter frequency-dependent Doppler drifts that destroy the orthogonality among OFDM subcarriers. The disclosed apparatus, systems, and methods use a two-step approach to mitigate frequency-dependent Doppler drifts for zero-padded OFDM transmissions over fast-varying channels: (1) non-uniform Doppler compensation via resampling to convert a “wideband” problem into a “narrowband” problem; and (2) high-resolution uniform compensation on the residual Doppler. The disclosed apparatus, systems and methods are based on block-by-block processing and do not rely on channel dependence across OFDM blocks. Thus, the disclosed apparatus, systems and methods are advantageously applicable for fast-varying UWA channels.

Abstract: Trellis-based methods for reducing feedback while maintaining performance in a Multiple Input, Multiple Output Orthogonal Frequency Division Multiplexing (MIMO-OFDM) system. The methods involve: receiving a plurality of symbols from a plurality of sub-carriers at a receiver; selecting a plurality of indices of codewords corresponding to a codebook of pre-coding weighting matrices for the sub-carriers based on vector quantization compression of the codewords, including selecting a plurality of indices based on trellis-based feedback encoding; and transmitting the selected indices over a wireless channel to a transmitter. Optimal precoding matrices are selected at the same time for all subcarriers by searching for the optimum choice of matrices along a trellis. Selecting a plurality of indices based on trellis-based feedback encoding includes constructing a trellis by specifying a state transition table.

Abstract: Techniques are described for multicarrier multiple access wireless transmission, e.g. orthogonal frequency-division multiple access (OFDMA) transmissions, over frequency selective fading channels. The techniques are designed to maintain constant modulus transmissions for uplink while effectively mitigating intersymbol interference. Specifically, the techniques utilize non-redundant unitary precoding across OFDMA subcarriers to maintain constant modulus transmissions for uplink communications. For example, the techniques involve precoding a block of information symbols and assigning a different subcarrier for each symbol of the block. The subcarriers are selected to be equi-spaced and may be selected, for example, from a phase-shift keying constellation. The number of symbols per block is equal to the number of subcarriers assigned per user. Importantly, even with multiple subcarriers per user, the techniques enable constant modulus transmissions for uplink.

Abstract: Techniques are described for space-time block coding for single-carrier block transmissions over frequency selective multipath fading channels. Techniques are described that achieve a maximum diversity of order NtNr (L+1) in rich scattering environments, where Nt (Nr) is the number of transmit (receive) antennas, and L is the order of the finite impulse response (FIR) channels. The techniques may include parsing a stream of information-bearing symbols to form blocks of K symbols, precoding the symbols to form blocks having J symbols, and collecting consecutive Ns blocks. The techniques may further include applying a permutation matrix to the Ns blocks, generating a space-time block coded matrix having Nt rows that are communicated through a wireless communication medium. The receiver complexity is comparable to single antenna transmissions, and the exact Viterbi's algorithm can be applied for maximum-likelihood (ML) optimal decoding.

Abstract: Advantageous OFDM-based underwater acoustic (UWA) apparatus, systems and methods are provided according to the present disclosure. In general, OFDM transmissions over UWA channels encounter frequency-dependent Doppler drifts that destroy the orthogonality among OFDM subcarriers. The disclosed apparatus, systems, and methods use a two-step approach to mitigate frequency-dependent Doppler drifts for zero-padded OFDM transmissions over fast-varying channels: (1) non-uniform Doppler compensation via resampling to convert a “wideband” problem into a “narrowband” problem; and (2) high-resolution uniform compensation on the residual Doppler. The disclosed apparatus, systems and methods are based on block-by-block processing and do not rely on channel dependence across OFDM blocks. Thus, the disclosed apparatus, systems and methods are advantageously applicable for fast-varying UWA channels.

Abstract: The invention is directed to techniques for space-time coding in a wireless communication system in which the transmitter makes use of multiple transmit antennas. The transmitter uses channel information estimated by a receiving device and returned to the transmitter, e.g., as feedback. In one exemplary embodiment, the transmitter receives a mean feedback information that defines a mean channel value associated with the different channels of the different antennas. In another exemplary embodiment, the transmitter receives covariance feedback, e.g., statistical values associated with each of the different channels.

Abstract: Techniques are described for performing block equalization on a received wireless communication signal formed according to interleaved chips generated from sub-blocks of symbols. For example, a one-step block equalization process is described which produces estimates of the information-bearing symbols from a wireless communication signal received from two or more transmitters in a soft handoff environment. The techniques provide improved performance in high load, soft handoff environments with low complexity, highly flexible equalization. The wireless communication signal may be a CIBS-CDMA signal in which a symbol block is divided into sub-blocks and spread by a user-specific block-spreading matrix. The CIBS signal is received through M subchannels and a de-spreading matrix is applied to produce a multi-user interference (MUI) free sub-block output for the mth channel.

Abstract: Techniques are described for space-time block coding for single-carrier block transmissions over frequency selective multipath fading channels. Techniques are described that achieve a maximum diversity of order NtNr (L+1) in rich scattering environments, where Nt (Nr) is the number of transmit (receive) antennas, and L is the order of the finite impulse response (FIR) channels. The techniques may include parsing a stream of information-bearing symbols to form blocks of K symbols, precoding the symbols to form blocks having J symbols, and collecting consecutive Ns blocks. The techniques may further include applying a permutation matrix to the Ns blocks, generating a space-time block coded matrix having Nt rows that are communicated through a wireless communication medium. The receiver complexity is comparable to single antenna transmissions, and the exact Viterbi's algorithm can be applied for maximum-likelihood (ML) optimal decoding.

Abstract: Techniques are provided for reducing feedback while maintaining performance in a MIMO-OFDM system. The disclosed techniques employ finite-rate feedback methods that uses vector quantization compression. The disclosed methods/techniques generally involve: receiving a plurality of symbols from a plurality of sub-carriers at a receiver; selecting a plurality of indices of codewords corresponding to a codebook of pre-coding weighting matrices for the sub-carriers based on vector quantization compression of the codewords; and transmitting the selected indices over a wireless channel to a transmitter. Finite state vector quantization feedback makes use of a finite state vector quantizer (FSVQ), which is a recursive vector quantizer (VQ) with a finite number of states. In finite state vector quantization feedback, optimal precoding matrices (beamforming vectors) are selected sequentially across subcarriers.

Abstract: In general, joint error-control coding and linear precoding techniques are proposed for fading-resilient transmissions over frequency- at and frequency-selective fading channels encountered with high-rate wireless OFDM transmissions. For example, a wireless communication system includes a transmitter that applies non-linear codes and a linear precoder to a data stream to produce a joint coded-precoded waveform. A receiver receives the joint coded-precoded waveform from the transmitter via a wireless communication channel, and demodulates the joint-coded precoded waveform to produce estimated data. The combination offers a multiplicative benefit to the diversity achievable by the wireless communication system.

Abstract: Techniques are described for space-time block coding for single-carrier block transmissions over frequency selective multipath fading channels. Techniques are described that achieve a maximum diversity of order NtNr(L+1) in rich scattering environments, where Nt(Nr) is the number of transmit (receive) antennas, and L is the order of the finite impulse response (FIR) channels. The techniques may include parsing a stream of information-bearing symbols to form blocks of K symbols, precoding the symbols to form blocks having J symbols, and collecting consecutive NS blocks. The techniques may further include applying a permutation matrix to the NS blocks, generating a space-time block coded matrix having Nt rows that are communicated through a wireless communication medium. The receiver complexity is comparable to single antenna transmissions, and the exact Viterbi's algorithm can be applied for maximum-likelihood (ML) optimal decoding.

Abstract: Techniques are described for efficiently estimating and compensating for the effects of a communication channel in a multi-carrier wireless communication system. The techniques exploit the fact that the transmitted symbols are drawn from a finite-alphabet to efficiently estimate the propagation channel for multi-carrier communication systems, such systems using OFDM modulation. A transmitter transmits data through a communication channel according to the modulation format. A receiver includes a demodulator to demodulate the data and an estimator to estimate the channel based on the demodulated data. The channel estimator applies a power-law operation to the demodulated data to identify the channel. The techniques can be used in both blind and semi-blind modes of channel estimation.

Abstract: Techniques are described for multicarrier multiple access wireless transmission, e.g. orthogonal frequency-division multiple access (OFDMA) transmissions, over frequency selective fading channels. The techniques are designed to maintain constant modulus transmissions for uplink while effectively mitigating intersymbol interference. Specifically, the techniques utilize non-redundant unitary precoding across OFDMA subcarriers to maintain constant modulus transmissions for uplink communications. For example, the techniques involve precoding a block of information symbols and assigning a different subcarrier for each symbol of the block. The subcarriers are selected to be equi-spaced and may be selected, for example, from a phase-shift keying constellation. The number of symbols per block is equal to the number of subcarriers assigned per user. Importantly, even with multiple subcarriers per user, the techniques enable constant modulus transmissions for uplink.

Abstract: Techniques are described for maintaining the orthogonality of user waveforms in multi-user wireless communication systems, such as systems using the code division multiple access (CDMA) modulation scheme in the presence of frequency-selective fading channels. Unlike conventional systems in which spreading is performed on individual information-bearing symbols, the “chip-interleaved block-spreading” (CIBS) techniques described herein spread blocks of symbols. A transmitter includes a block-spreading unit to form a set of chips for each symbol of a block of information-bearing symbols and to produce a stream of chips in which the chips from different sets are interleaved. A pulse shaping unit within the transmitter generates a transmission signal from the stream of interleaved chips and transmits the signal through a communication channel.

Abstract: Techniques are described for performing block equalization on a received wireless communication signal formed according to interleaved chips generated from sub-blocks of symbols. For example, a one-step block equalization process is described which produces estimates of the information-bearing symbols from a wireless communication signal received from two or more transmitters in a soft handoff environment. The techniques provide improved performance in high load, soft handoff environments with low complexity, highly flexible equalization. The wireless communication signal may be a CIBS-CDMA signal in which a symbol block is divided into sub-blocks and spread by a user-specific block-spreading matrix. The CIBS signal is received through M subchannels and a de-spreading matrix is applied to produce a multi-user interference (MUI) free sub-block output for the mth channel.

Abstract: Adaptive modulation techniques for multi-antenna transmissions with partial channel knowledge are described. Initially, a transmitter is described that includes a two-dimensional beamformer where coded data streams are power loaded and transmitted along two orthogonal basis beams. The transmitter optimally adjusts the basis beams, the power allocation between two beams, and the signal constellation. A partial CSI model for orthogonal frequency division multiplexed (OFDM) transmissions over multi-input multi-output (MIMO) frequency selective fading channels is then described. In particular, an adaptive MIMO-OFDM transmitter is described in which the adaptive two-dimensional coder-beamformer is applied on each OFDM subcarrier, along with an adaptive power and bit loading scheme across the OFDM subcarriers.