Direct-sequence spread spectrum (DSSS) communication, as a key technology in code-division multiple access networks, demonstrates excellent performance in multiple-access capability and interference suppression. However, its effectiveness significantly degrades in mobile underwater acoustic (UWA) networks, where the platform mobility induces severe Doppler effect and exacerbates the instability of multipath propagation. These impairments are further exacerbated by the inherently low data rates of UWA DSSS systems, where the prolonged signal duration leads to time-varying Doppler effect and fast-fading multipath structure within a single frame, collectively degrading the UWA DSSS receiver performance. To address these limitations, this paper proposes an improved DSSS receiver to achieve time-varying UWA multipath tracking, which employs Kalman filtering for channel parameter estimation. A multipath management technique is implemented to determine the appearance and disappearance of paths, followed by a coherent combination of all valid paths. Comprehensive simulations demonstrate the superior performance of the proposed receiver under various time-varying channel scenarios. The effectiveness of the proposed receiver is further validated by experimental data collected from Danjiangkou Lake and Zhang River experiments.

Due to the time‐varyingproperty of the underwater acoustic (UWA) channel, the significant Doppler spread will severely degrade the performance of direct‐sequence spread‐spectrum (DSSS) communications. The relative velocity variation between the transmitter and the receiver will cause both the phase rotation and the magnitude loss of correlation peak, during the long transmission of the DSSS packet. To solve this problem, the authors propose a novel transceiver design for the UWA DSSS communications. At the transmitter, the triple differential phase shift keying (D PSK) modulation is adopted to overcome the phase rotation, whereas the phase noise will be amplified resulting in the signal‐to‐noise ratio (SNR) loss. At the receiver, the improved bit‐interleaved coded modulation with iterative decoding algorithm for D PSK is used to recover the SNR loss, in which the D PSK demodulator is treated as the convolutional decoder, and the linear prediction is adopted to track the channel variation. Furthermore, an adaptive selection of local reference signal is also applied to recover the correlation loss. Theoretical simulation shows that the proposed transceiver can effectively mitigate the performance loss caused by the motion acceleration, and the performance gain is significant over the conventional.

Frequency‐hopped M‐ary frequency‐shift‐keyed (FH‐MFSK) has the ability to handle with long multipath, large Doppler spreads, high ambient noise and rapid phase fluctuation, and is proved to be a robust modulation scheme in mobile shallow underwater acoustic channels. Aiming at this topic, a convolutionally encoded FH‐MFSK modulation scheme with a novel iterative multi‐channel reception is proposed. To obtain the novel reception suitable for non‐coherent M‐ary frequency‐shift‐keyed, the ideas of iterative multi‐channel demodulation and decoding are combined and extended. A modified multi‐channel soft‐input–soft‐output demodulator based on maximum‐a‐posterior criterion is derived, and substantial gains of several decibels in power efficiency are achieved. Extra gain can be obtained by utilising time diversity with long multipath delay. Simulation shows that a non‐coherent receiver equipped with five arrays obtains around 5.5 dB gain over the single non‐iterative receiver. A shallow water field testing at Songhua Lake with 300 m distance confirms that the proposed receiver verifies its robustness and usability.

In this study, the authors propose an l 0 ‐norm penalised shrinkage linear least mean squares ( l 0 ‐SH‐LMS) algorithm and an l 0 ‐norm penalised shrinkage widely linear least mean squares ( l 0 ‐SH‐WL‐LMS) algorithm for sparse system identification. The proposed algorithms exploit the priori and the posteriori errors to calculate the varying step‐size, thus they can adapt to the time‐varying channel. Meanwhile, in the cost function they introduce a penalty term that favours sparsity to enable the applicability for sparse condition. Moreover, the l 0 ‐SH‐WL‐LMS algorithm also makes full use of the non‐circular properties of the signals of interest to improve the tracking capability and estimation performance. Quantitative analysis of the convergence behaviour for the l 0 ‐SH‐WL‐LMS algorithm verifies the capabilities of the proposed algorithms. Simulation results show that compared with the existing least mean squares‐type algorithms, the proposed algorithms perform better in the sparse channels with a faster convergence rate and a lower steady‐state error. When channel changes suddenly, a filter with the proposed algorithms can adapt to the variation of the channel quickly.

In face recognition for criminal identification, the training data are always clean while the probe data are occluded by sunglasses, scarf or other facial accessories. Occlusions in the probe data severely degrade the recognition performance. We find that introducing artificial occlusions into the training data is helpful in this situation. The incremental training data is decomposed into a class-specific dictionary, a non-class-specific dictionary and a sparse noise or corruption by the sparse and dense hybrid representation framework (SDR). The artificially introduced occlusions play an important role in building the discriminative faces for classification during SDR. Experimental results demonstrate that the proposed method can provide higher recognition accuracy under benchmark face database.

In this study, the authors propose a novel decision feedback equaliser (DFE)‐based receiver, which combines channel shortening methods and dichotomous coordinate descent (DCD) recursive least squares (RLS) adaptive algorithm with variable forgetting factor (VFF). The proposed receiver can obtain its performance gain via two aspects: (i) reducing the computing complexity by channel shortening method, such as passive time reversal (pTR) filter or minimised mean‐square error (MMSE) filter and (ii) improving the performance of the receiver, such as a lower bit error rate and mean‐square error. Underwater acoustic (UWA) channel has the features of long multi‐path spread and time‐varying property. When the multi‐path spread is very long, pTR or MMSE filter is used as a channel shortening pre‐processing method to reduce the computing complexity in the DFE receiver. When the channel is time varying, VFF is incorporated into DCD‐RLS adaptive algorithm to improve the tracking capability of the DFE receiver. The proposed receiver is appealing for practical implementations. Numerical examples and lake experimental results show that, the proposed DFE receiver can achieve a better performance in the UWA communication system.

In this paper, we propose a joint relay and transmit/ receive (Tx/Rx) antenna mode selection scheme (RAMS) in the general full-duplex (FD) relay networks consisting of one source, one destination, and N FD amplify-and-forward (AF) relays. Each FD relay is equipped with two antennas, one for receiving and the other for transmitting. In the proposed scheme, each antenna of the FD relay is able to transmit/receive the signal. Each relay adaptively selects its Tx antenna and Rx antenna based on the instantaneous channel conditions, and the optimal single relay with the optimal Tx/Rx antenna configuration is selected to maximize the end-to-end signal to interference and noise ratio (SINR) of the FD relay system. The performance of the proposed scheme is analyzed. The closed-form expressions of the outage probability, average symbol error rate, and the ergodic capacity are derived. The analytical results are verified by the simulations. To reduce the error floor and capacity ceiling caused by the self-loop interference in FD relay, we propose a RAMS scheme with adaptive power allocation (RAMS-PA). We provide an upper bound and a lower bound of the end-to-end SINR for RAMS-PA scheme, and prove that the error floor can be removed in the RAMS-PA scheme. Results show that the proposed scheme achieves an extra spatial diversity in the medium SNR region due to the FD antenna selection at the relay nodes and considerably improve the system performance compared to the conventional FD relay selection scheme with fixed relay Tx and Rx antennas.

In this paper, we propose a joint relay and antenna selection scheme in general full-duplex (FD) relay networks with one source, one destination and N FD amplify-and-forward (AF) relays. Each FD relay is equipped with two antennas, one for receiving and one for transmitting. We consider a joint antenna and relay selection scheme to optimize the end-to-end error performance. In the proposed scheme, each relay adaptively selects the transmit antenna and receive antenna based on the instantaneous channel conditions, and the optimal single relay with the optimal Tx/Rx antenna configuration is selected to optimize the end-to-end performance of the system transmission. This is in contrast to the conventional pure FD relay selection, where the Tx and Rx FD antenna of each relay are fixed. The proposed scheme achieves an extra space diversity due to the antenna selection at the relay nodes, and considerably improves the system performance compared to the conventional FD relay selection. Furthermore, closed-form expressions for the outage probability and average symbol error rate (SER) are derived. The analytical results are verified by the computer simulations. Results show that the proposed scheme outperforms the conventional full-duplex relay selection scheme with fixed relay Tx and Rx antennas.

Most researches on relay selection (RS) in bidirectional relay network typically assume perfect channel state information (CSI). However, outdated CSI, which is caused by the the time-variation of channel, cannot be ignored in the practical system, and the performance of the conventional bidirectional RS scheme degrades greatly with outdated CSI. In this paper, to improve the performance of RS with outdated CSI, we propose a weighted bidirectional RS scheme, in which a deterministic weight factor decided by the correlation coefficient of outdated CSI, is introduced in the selection process. The outage probability bound of the weighted bidirectional RS is derived and verified, along with the asymptotic expression in high signal-to-noise ratio (SNR). Based on the analytical expressions, the optimal weight factor and the optimal power allocation scheme in minimizing the outage probability are obtained. Simulation results reveal that when the CSI is outdated, the diversity order reduces from full diversity to one. Furthermore, the weighted bidirectional RS scheme with the optimal weight factor yields a significant performance gain over the conventional bidirectional RS scheme, especially in high SNR.

The full duplex (FD) technique, which allows the communication node to transmit and receive signals over the same frequency band simultaneously, has the potential to double the spectral efficiency in comparison with the traditional half duplex (HD) technique. However, self-interference, leaking from the FD node's transmission to its own reception, has the detrimental impact on the performance of FD communication. In this paper, we analyze and optimize the two-way FD relay system using amplify-and-forward protocol, when the multi-relay scenario is considered. The optimal relay selection scheme in maximizing the effective signal-to-interference and noise ratio is proposed, which significantly improves the system performance than a single relay network. Furthermore, to facilitate the comparisons with the traditional two-way HD relay, the analytical expressions of the two-way FD relay are derived in a closed form, including bit error rate (BER), ergodic capacity, and outage probability. Based on the analytical expressions, the optimal power allocation and the optimal choice of duplex mode, i.e., FD and HD, are obtained by minimizing the outage probability. Monte-Carlo simulations are fulfilled to verify the analytical expressions. The results reveal that the residual self-interference after interference suppression limits the performance of two-way FD relay: when the residual interference is small, the FD mode has lower BER/outage probability and higher ergodic capacity since it utilizes the resources effectively; otherwise, the HD mode achieves lower BER/outage probability and higher ergodic capacity since it can completely cancel the self-interference at the cost of lower resource utilization.

In this paper, we investigate the performance of multi-pair two-way relaying, in which multiple pairs of users exchange information within pair, with the help of a shared relay. Each user has a single antenna, and the relay is equipped with very large number of antennas. The relay adopts the amplify-and-forward protocol, and the beamforming matrixes of maximum-ratio combining/maximum ratio transmission and zero-forcing reception/zero-forcing transmission are both considered. Due to array gain of antenna array, the power of each user or the relay (or both) can be made inversely proportional to the number of relay antennas, without compromising the performance. Thus, three power-scaling schemes are studied. Furthermore, the asymptotic spectral and energy efficiencies of the system are obtained analytically, when the number of relay antennas approaches to infinity. The asymptotic results are beneficial to provide more insightful understandings for the fundamental limits of the very large antenna system, and verified by the Monte-Carlo simulations. The analytical and simulation results reveal that very large antenna arrays in such system can average the small-scale fading, eliminate the inter-pair interference, and reduce the total power consumption.