Maritime and shipping are unambiguously the cornerstones of the global economy and transportation. To improve efficiency, maritime sector activities are focused on the realization of Smart Shipping (SMS), leveraging 6G Communications, Energy Efficiency (EE) and Machine Learning (ML). However, conventional Centralized Machine Learning (CML) cannot be easily applied in the maritime, mainly due to the drawbacks: (i) prohibitive data communication overhead and bandwidth limitations, since CML requires centralization of massive data through transmissions from heterogeneous sources, (ii) excessive energy consumption associated with massive data transfers, (iii) remarkable transmission errors due to harsh propagation conditions, and (iv) data privacy violation, since the data carries sensitive and commercial information. This article proposes a two-fold Federated Learning (FL) scheme (FedShip) to improve the privacy, EE and communication-efficiency of future 6G maritime networks. FedShip uses the Over-the-Air computation (AirComp) principles to exploit the signal superposition property and ensure that local models are accurately and efficiently combined. Using real data regarding the fuel consumption of multiple cargo ships, we compared the FL performance, building multiple timeseries forecasting models, with collaborative ML baselines. AirComp performance was also assessed using simulation data about channel measurements. After optimizing the hyperparameters of the local models, extensive results revealed that: (i) FL shows enhanced fuel prediction accuracy (95.5% relative to the CML), while ensuring data privacy and (ii) AirComp can be adopted to combine the local models with low computation error, offering significant EE and spectrum efficiency improvements, especially when dense 6G scenarios are considered.

Abstract Non-orthogonal multiple access (NOMA) is a promising solution to enhance the spectral efficiency of sixth generation (6G) networks. NOMA enables multiple users to be concurrently served, relying on superposition coding at the transmitter and successive interference cancellation (SIC) at the receivers. However, in practice, hardware impairments and channel estimation errors will lead to incorrect signal ordering. This study analyses inverse successive interference cancellation (ISIC) in downlink power-domain NOMA networks. In the worst case, ISIC will inversely order the N received signals, compared to conventional SIC. First, ISIC will decode the signal with the minimum allocated power, while treating the other $$N-1$$ N - 1 signals as interference. This process is repeated $$N-1$$ N - 1 times and in the last step, the signal with the maximum allocated power will be decoded without interference. Various key performance metrics for ISIC are analytically derived, including the outage probability, throughput, error probability, ergodic rate, achievable sum-rate, energy and spectral efficiency. In order to provide further insights on ISIC, an asymptotic analysis is also provided. Comparisons with SIC reveal that for the signal with the second-lowest power allocation level, ISIC achieves better performance, while the performance of the information signal with the minimum allocated power is degraded. Overall, our findings shed light on the performance of NOMA in practical settings where SIC can not be correctly performed and focuses on the worst-case where the decoding order at the receiver is completely inversed.

Full-duplex relaying is an enabling technique of sixth generation (6G) mobile networks, promising tremendous rate and spectral efficiency gains. In order to improve the performance of full-duplex communications, power control is a viable way of avoiding excessive loop interference at the relay. Unfortunately, power control requires channel state information of both source-relay and relay-destination channels, as well as of the loop interference channel, thus resulting in increased overheads. Aiming to offer a low-complexity alternative for power control in full-duplex relay networks, we adopt reward-based learning in the sense of multi-armed bandits. More specifically, we provide bandit-based power control algorithms, relying on acknowledgements/negative-acknowledgements observations by the relay. The proposed algorithms avoid the need for channel state information acquisition and exchange, and can be employed in a distributed manner. Performance evaluation results in terms of outage probability, average throughput and accumulated regret over time highlight an interesting performance-complexity trade-off compared to optimal power control with full channel knowledge and significant performance gains over the cases without power control and random power level selection.

In this work, we combine the two notions of timely delivery of information to study their interplay; namely, deadline-constrained packet delivery due to latency constraints and freshness of information. More specifically, we consider a two-user multiple access setup with random-access, in which user 1 is a wireless device with a queue and has external bursty traffic which is deadline-constrained, while user 2 monitors a sensor and transmits status updates to the destination. We provide analytical expressions for the throughput and drop probability of user 1, and an analytical expression for the average Age of Information (AoI) of user 2 monitoring the sensor. The relations reveal that there is a trade-off between the average AoI of user 2 and the drop rate of user 1: the lower the average AoI, the higher the drop rate, and vice versa. Simulations corroborate the validity of our theoretical results.

Buffer-aided relays can improve the diversity of multi-hop networks, however, they increase packet delays. Thus, various delay-aware protocols have been developed, but without considering the transmission diversity. Moreover, most works adopt ideal assumptions, such as symmetric links, perfect channel state information (CSI), and error-free feedback channels. So, we propose a low-complexity (LoCo) link selection algorithm, herein called LoCo-Link. The proposed algorithm may experience delays during CSI updates, and hence, by using outdated CSI its performance may deteriorate. To alleviate this issue, we next propose a distributed version of LoCo-Link (d-LoCo-Link) dealing with outdated CSI. In both algorithms, the source performs broadcasting toward multiple relays; when the packets are transmitted by a relay to the destination, they are discarded from all other relays. This coordination relies on feedback channels. For non error-free feedback channels, we propose a scheme in which the relays listen to the transmission of the best relay and drop duplicate packets. Results show that LoCo-Link surpasses other algorithms, by decreasing the delay in asymmetric networks. Moreover, d-LoCo-Link avoids diversity losses due to outdated CSI, while the effect of non error-free feedback channels is mitigated by taking advantage of the inter-relay channels.

Nowadays, various techniques and systems have been proposed for the implementation of wireless indoor localization systems. Recently, significant focus has been given to Multiple Input-Multiple Output (MIMO) implementations of Ultra-Wideband (UWB) systems, in order to enhance the localization precision while introducing high communication performance. The main disadvantages of such approaches are the high deployment cost and computational complexity, due to the increased data processing. These facts are compensated by adopting the Time Reversal (TR) transmission scheme, providing scaling capabilities and Spatial Focusing (SF) to the system, at the same time. In this paper, a 2 × 2 Multi-Antenna Time-Reversal Indoor Positioning (MATRIP) system model is proposed to track the mobile Blind Node (BN) in a harsh indoor environment, measuring its time-range from one fixed-position Anchor Node (AN). This kind of implementation overcomes the restrictions imposed by the legacy localization systems, regarding the minimum number of ANs of the system. The efficiency of the MATRIP system is demonstrated through simulation results evaluating the average localization error versus the Signal to Noise Ratio (SNR), the Focusing Gain (FG), and the channel capacity.

While buffer-aided relaying improves the diversity of a multi-hop network, its deployment introduces time-delays, thus rendering buffering unreliable for delay-intolerant applications. To alleviate excessive delays, various studies propose delayaware protocols, but at the expense of reduced diversity, and consequently, increased outage probability. Attempts to maintain the diversity of the system while trying to reduce delays, however, may lead to even higher delays, especially in asymmetric topologies. In this work, we propose a Low-Complexity (LoCo) link selection algorithm, herein called LoCo - Link, that aims at reducing packet delays and enhancing the performance of practical asymmetric two-hop networks. The complexity of LoCo - Link is derived and compared with other state-of-the-art relay selection policies. The performance of the proposed algorithm is evaluated in terms of outage probability, average throughput and average delay, focusing on scenarios with asymmetric links.

In this paper, we propose novel relay selection policies that aim at reducing the average delay by incorporating the buffer size of the relay nodes into the relay selection process. More specifically, we propose two delay-aware protocols that are based on the max - link relay selection protocol. First, a delay-aware only approach while it reduces the delays considerably it starves the buffers and increases the outage probability of the system. Towards this end, we propose a delay- and diversity-aware buffer-aided relay selection policy that aims at reducing the average delay considerably and at the same time maintaining good diversity. The protocols are analyzed by means of Markov Chains and expressions for the outage, throughput and delay are derived. The performance and use of our proposed algorithms is demonstrated via extensive simulations and comparisons.

The knowledge of direction information of the transmitted signals requires angle-based position estimation techniques, as well as the orientation of the individual nodes of the system. When using omnidirectional antennas, this information is difficult to be calculated in order to find the Blind Node's (BLN or BN) position. Also, the combination of Ultra Wide-Band (UWB) systems with angle-based techniques is not the best option when tracking down an unknown target, especially indoors, due to the large bandwidth of the UWB signals and the huge number of paths. This paper proposes a geometrical angle-based positioning approach for UWB omnidirectional systems which also combines Fingerprint (FP) and Time-of-Flight (ToF), aiming to calculate the BN's position with the minimum positioning error. The efficiency of the proposed positioning scheme is demonstrated through simulation results in terms of error probability and comparisons with other state-of-the-art positioning schemes.

In this work, relay selection policies for a network where a terrestrial source communicates with a terrestrial destination through multiple relay nodes which reside in the stratosphere are presented. More specifically considering realistic three-dimensional (3-D) non-isotropic scattering channel modelling combined with opportunistic relay selection for various fading conditions. Moreover, the stratospheric nodes are battery-dependent and relay selection aims at power reduction. As Channel State Information (CSI) overhead is introduced by the selection process, proactive and reactive versions are presented based on instantaneous CSI knowledge. In this setting, the selection policies in terms of outage probability, throughput and power reduction are evaluated and comparisons with statistical channels and with a relaying scheme that does not perform power adaptation are provided.

In this paper, we present a relay selection scheme which combines the spectral efficiency of successive opportunistic relaying with the robustness of single-link relay selection. More specifically, we propose a scheme that minimizes the total energy expenditure per time slot under an inter-relay interference cancellation scheme. The new relay selection policy is analyzed in terms of outage probability and diversity by modeling the evolution of relay buffers as a Markov Chain. We construct the state transition matrix of the Markov Chain and obtain its stationary distribution, which in turn, yields the outage probability. The proposed scheme outperforms relevant state-of-the-art relay selection schemes in terms of throughput, diversity, energy efficiency and average delay, as demonstrated via representative numerical examples.

In this work, we study a cooperative network with multiple full-duplex buffer-aided relays. A hybrid cooperative relaying policy is proposed that employs power adaptation and consists of two alternative schemes: (i) full-duplex transmission through the relay which requires the least total power expenditure and loop interference is mitigated through power adaptation; (ii) buffer-aided max - link selection with power adaptation, when full-duplexity is not feasible. Aiming to reduce the overhead of channel state information (CSI) acquisition and processing, we propose a suboptimal distributed method for relay selection, for which the network performance is not degraded significantly. We show that power adaptation offers reduced overhead of CSI acquisition. Numerical results and comparisons with other state-of-the-art relaying schemes are provided and performance evaluation in terms of throughput, power minimization and switching rate, show the benefits of the proposed hybrid scheme.

In this article, we study relay selection policies in applications which are of interest in the fifth generation (5G) of wireless networks. More specifically, we classify relays based on their distinct capabilities such as processing, multiple antennas, channel estimation, storage and density. In addition, we discuss the level of efficiency exhibited by each relay class, regarding their impact in high mobility, delay critical and green communications applications. Then, we formulate relay selection policies taking into consideration the goals set in the context of each application. Numerical results depicting the average throughput, average delay and power reduction are indicative of the applications the selection policies are chosen for.

In this paper we consider a simple cooperative network consisting of a source, a destination and a cluster of decode-and-forward relays characterized by the half-duplex constraint. At each time-slot the source and (possibly) one of the relays transmit a packet to another relay and the destination, respectively. When the source and a relay transmit simultaneously, inter-relay interference is introduced at the receiving relay. In this work, with the aid of buffers at the relays, we mitigate the detrimental effect of inter-relay interference through either interference cancellation or mitigation. More specifically, we propose the min-power opportunistic relaying protocol that minimizes the total energy expenditure per time slot under an inter-relay interference cancellation scheme. The min-power relay-pair selection scheme, apart from minimizing the energy expenditure, also provides better throughput and lower outage probability than existing works in the literature. The performance of the proposed scheme is demonstrated via illustrative examples and simulations in terms of outage probability and average throughput.

Cooperative relaying with Distributed Space-Time Coding (DSTC) and cooperative Opportunistic Relaying (OR) are the two main relaying techniques that are typically employed in current wireless systems. However, OR is less complex and proven to be outage optimal compared to DSTC. Thus, in this paper we propose a Multi-Mode Relaying (MMR) technique that is based on OR and combined with the Successive Relaying (SR) technique in order to deal with the half-duplex constraint of the relay nodes. MMR, by employing Multi-Mode Decode-and-Forward (MMDF) relay nodes and incorporating two interference mitigation algorithms, not only combines the merits of OR and SR but also is able to alleviate the increased inter-relay interference caused by SR. The performance of MMR is evaluated through simulation and the results show that it is able to increase the average end-to-end capacity of the wireless path as well as the percentage of high quality packets received at the terminal of the user.

In this work, we present a successive opportunistic relaying protocol with an inter-relay interference (IRI) mitigation scheme, in a quasi-static environment with Rayleigh fading. In order to recover the half duplex loss of the transmission, we adopt a successive relaying (SR) strategy, coupled with reactive relay selection considering the Signal to Noise Ratio (SNR) and the Signal to Interference plus Noise Ratio (SINR) of each hop, after every source transmission. The main degrading factor in this scheme is the IRI introduced by the best relay, which is selected to forward the previous frame, while the rest of the relays are receiving the current source frame. Trying to cancel the effect of IRI, we employ interference cancelation (IC) to subtract the interference signal depending on the capacity of the interference channel.