In this paper, we consider multiuser multihop relay communication systems, where the users, relays, and the destination node may have multiple antennas. We address the issue of source and relay precoding matrices design to maximize the system mutual information (MI). By exploiting the link between the maximal MI and the weighted minimal meansquared error (WMMSE) objective functions, we show that the intractable maximal MI-based source and relay optimization problem can be solved via the WMMSE-based source and relay design through an iterative approach which is guaranteed to converge to at least a stationary point. For the WMMSE problem, we derive the optimal structure of the relay precoding matrices and show that the WMMSE matrix at the destination node can be decomposed into the sum of WMMSE matrices at all hops. Under a (moderately) high signal-to-noise ratio (SNR) condition, this WMMSE matrix decomposition significantly simplifies the solution to the WMMSE problem. Numerical simulations are performed to demonstrate the effectiveness of the proposed algorithm.

Abstract-The underwater acoustic (UA) channel is dispersive in both time and frequency with severe frequency-dependent signal attenuation. Efficient channel estimation and tracking are crucial to coherent high-rate UA communication. In this paper, we propose a new compressed sensing (CS) based channel estimation method with block-by-block channel tracking for UA communication. Compared with conventional channel estimation algorithms, the proposed method efficiently exploits the sparsity of the UA channel, and improves the channel tracking capability of UA communication system. The proposed algorithm was tested during our UA communication experiment conducted in December 2012 in the Indian Ocean off Rottnest Island, Western Australia. At a data rate of 8 kbps (QPSK constellations), average uncoded bit-error-rates (BERs) of 3% and 14% have been achieved over 1 km and 6 km ranges, respectively, using MMSE equalization based on the proposed channel estimation and tracking method.

Abstract-This paper investigates a single-carrier iterative frequency-domain equalization (SC-IFDE) scheme for high-rate underwater acoustic (UA) communication systems. This scheme is based on the minimum mean-squared error (MMSE) criterion, and soft decision feedback is applied to improve the reliability of the equalizer decision. The proposed algorithm is applied to the data received during the UA communication experiment conducted in December 2012 in the Indian Ocean off Rottnest Island, Western Australia. It is demonstrated that using one transmitting transducer and one receiving hydrophone, the proposed SC-IFDE algorithm achieves an average of 3% uncoded bit-error-rate (BER) with quaternary phase shift keying (QPSK) modulated signals over a range of 1 km.