The unique characteristics of the underwater environment, such as limited infrastructure, challenging acoustic communication channels, and constrained battery power of underwater sensor nodes, significantly impact the overall network lifetime of underwater wireless sensor networks (UWSNs). In a multi-hop UWSN, the death of a special node — cut-vertex (CV) — divides the network into the main network and an isolated sub-network (ISN). The UWSN may struggle to operate continuously and efficiently owing to the death of underwater sensor nodes, resulting in a shorter network lifetime and reduced data reliability. Consequently, the data generated by the ISN is lost. To address this issue, this paper presents an autonomous underwater vehicle (AUV)-aided ISN prevention protocol for UWSNs. The proposed protocol employs an AUV to explore and identify a CV by utilizing the information collected from the sensor nodes. Subsequently, the AUV predicts the future residual energy of the CV, ensuring its arrival near the CV prior to the energy depletion of the CV and the formation of an ISN. Then, instead of the CV, the AUV directly collects data from the CV-associated sensor nodes while the CV harvests energy. The CV replenishes its energy by harnessing ambient underwater sources and subsequently reintegrates into the network after attaining sufficient energy recharge. In this study, we evaluate the performance of the proposed protocol by comparing it with the Q-learning-based topology-aware routing protocol, a hybrid data-collection scheme, stratification-based data-collection scheme, and Q-learning-based energy-efficient and lifetime-aware routing protocol in terms of the lifetime of the network, lifetime of the CVs, energy consumption, end-to-end delay, and packet delivery ratio.

This article proposes a nondata-aided (NDA) signal-to-noise ratio (SNR) estimation method for molecular diffusive communication channels. In contrast to previous data-aided (DA) approaches, which require a known data sequence to be shared between transmitter and receiver nanomachines, the proposed NDA method eliminates the need for prior knowledge of data symbols for the estimation process. The absence of a training sequence significantly reduces resource costs by minimizing the number of molecules required for information transmission. The expectation–maximization (EM) algorithm was used to iteratively find the maximum-likelihood (ML) estimates. The proposed method is particularly advantageous in scenarios in which accessing known data sequences is not possible, thereby ensuring robust communication in the Internet of Bio-Nano Things. By employing the ML estimates obtained through the EM algorithm, the probabilities associated with the transmission of data symbols were derived, and the bit error probabilities were obtained to assess the system performance. The numerical results validated the effectiveness of the proposed NDA SNR estimator in accurately estimating SNR over a diverse range of conditions. The proposed NDA estimator demonstrated resilience by maintaining a low bit error probability.

Concentration shift keying (CSK) is a widely studied modulation technique for molecular communication-based nanonetworks, which is a key enabler for the Internet of Bio-NanoThings (IoBNT). Existing CSK methods, while offering optimal error performance, suffer from increased operational complexity that scales poorly as the number of transmitters, K, grows. In this study, a novel M-ary CSK method is proposed: CSK with common detection thresholds (CSK-CT). CSK-CT uses common thresholds, set sufficiently low to guarantee the reliable detection of symbols from all transmitters, regardless of distance. Closed-form expressions are derived to obtain the common thresholds and release concentrations. To further enhance error performance, the release concentration is optimized using a scaling exponent that also optimizes the common thresholds. The performance of CSK-CT is evaluated against the benchmark CSK across various K and M values. CSK-CT has an error probability between 10-7 and 10-4, which is a substantial improvement from that of the benchmark CSK (from 10-4 to 10-3). In terms of complexity, CSK-CT is O(n) and does not scale with K but M ≪ (MK), whereas the benchmark is O(n2). Furthermore, CSK-CT can mitigate inter-symbol interference (ISI), although this facet merits further investigation. Owing to its low error rates, improved scalability, reduced complexity, and potential ISI mitigation features, CSK-CT is particularly advantageous for IoBNT applications focused on data gathering. Its effectiveness is especially notable in scenarios where a computationally limited receiver is tasked with collecting vital health data from multiple transmitters.

In this paper, an energy-efficient Q-learning-based routing protocol, called the Q-learning-based topology-aware routing (QTAR) protocol, is proposed for underwater wireless sensor networks. Unlike existing protocols, QTAR considers the network topology to determine the next-forwarder (NF) candidates along the routing path and adopts Q-learning to aid in the optimal global decision-making of an NF from the NF candidates. In addition, QTAR utilizes implicit cut-vertex recognition to optimize NF selection, alleviating the energy wastage that arises from forwarding data packets away from the sink. In our study, we evaluated the performance of QTAR by comparing it with the Q-learning-based energy-efficient and lifetime-aware routing protocol (QELAR), energy-efficient depth-based routing protocol (EEDBR), Q-learning-based delay-aware routing (QDAR), and reinforcement learning-based opportunistic routing protocol (RLOR) in terms of the energy consumption, latency, and network lifetime. Our results revealed that QTAR demonstrated the advantages of a lower energy consumption, shorter latency, and longer network lifetime in the percentage ranges of 26.08 to 70.12, 22.2 to 50, and 37.8 to 75, respectively, than QELAR, EEDBR, QDAR, and RLOR.

There are many technical challenges to realize the myriad of applications envisaged for underwater acoustic sensor networks (UASNs), in particular, determining the location of each node or localization. While various schemes have been proposed recently, the impact of MAC protocols for localization has not been studied. A MAC protocol that can enable many sensor nodes in large-scale networks to share the limited channel resource is an indispensable component to maximize localization coverage and speed, while minimizing communication costs. This can be achieved with MAC schemes that require little or no node coordination. In this paper, we evaluate the performance of a multi-stage localization scheme for a large-scale two-dimensional UASN under CSMA (requiring no node coordination) and T-Lohi (requiring light coordination). Our simulation results show that CSMA achieves better localization performance than T-Lohi at the expense of higher communication costs.

In this paper, a preemptive priority queueing model is developed to describe the traffic behavior of secondary users in cognitive radio systems. As the performance measures, the system dwelling time for non-real-time traffic and the blocking and forced termination probabilities for real-time traffic are derived.

In this paper, a resource allocation matrix (RAM) for an orthogonal frequency division multiple access (OFDMA) system is derived to maximize overall data throughput under a strict condition for user fairness. The RAM identifies the most preferable allocation state of OFDMA sub-bands. It is subject to a user fairness condition such that one sub-band is allocated per user during an OFDMA time-slot, allowing each user an equal opportunity to use the sub-bands. The proposed resource allocation scheme using the RAM is evaluated in terms of throughput and user fairness by comparison with the Proportional Fairness (PF) scheme and the Max C/I scheme. Numerical results show that the proposed scheme has much higher throughput under the strict user fairness condition. Even though the PF and Max C/I are given an advantage of ignoring the user fairness condition, the throughput performance of the proposed scheme is quite comparable to that of the PF and Max C/I.

In this paper, a packet scheduling scheme that supports real-time traffic with multi-level delay constraints in OFDMA systems is proposed. Under the delay constraint, a throughput enhancement is jointly studied. A mathematical model describes the differentiated behaviors of packet scheduling according to the delay constraint level, and a decision parameter is eventually derived and used in the determination of whether a given packet should be transmitted at current OFMDA transmit time interval (TTI) or would be tolerable for delay to the next TTI. Assuming that the packets violating the delay constraint are lost, the proposed scheme is evaluated in terms of the packet loss probability and throughput. It is then compared with existing schemes for real-time traffic support such as the exponential scheduling (EXP) scheme, the modified largest weighted delay first (M-LWDF) scheme, and the round robin scheme. The numerical results show that the proposed scheduling scheme performs much better than the aforementioned scheduling schemes in terms of the packet loss probability, with slightly larger throughput compared to the EXP and M-LWDF schemes.

In this paper, a novel channel allocation and scheduling algorithm in OFDMA system is proposed. A new parameter named alternative factor is defined to easily select the best channel of each user for overall downlink throughput maximization. And the user fairness is jointly cared and is evaluated by a modified Jain index which considers the relative amount of transmitted data to that of required one rather than transmitted amount itself. The numerical results show that the proposed scheme is superior to the proportional fairness (PF) and the round robin algorithms in terms of both throughput and user fairness.

In mobile IP, mobile node (MN) registers its location whenever it changes its foreign agent (FA) and this results in unnecessary registrations if the traffic activity of the MN is low. IP paging has been proposed in order to avoid the unnecessary registration signaling load and power consumption of mobile IP. In IP paging, the concept of paging area (PA), which consists of a number of FA areas (i.e., cells), is proposed and MN updates its location only if it changes PA rather than cell when the traffic activity of the MN is low. The IP paging is, however, basically based on IP layer protocol and it alone cannot provide sufficient power saving. Thus, it has been proposed to incorporate layer 2 power saving mode (PSM) in IEEE 802.11 wireless local area network (WLAN) with IP paging protocol in order to improve the power saving capability. In this paper, the performance of IP-based mobile networks with IP paging is analyzed first. Then, three representative PSM schemes are introduced and the performance of these PSM schemes is analyzed in terms of power consumption cost, and wake-up and delay cost. These results can be used in selecting system parameters appropriately.

This paper describes a comparison of system performance using two different chip waveforms of spreading sequences in multiple-chip-rate (MCR) direct-sequence (DS)/code-division multiple-access (CDMA) systems. The chip pulses used in this study are closely related to the characteristics of output filter employed at transmitter. In general, the chip waveform is an important factor to determine the link performance. The raised cosine chip pulse with a roll-off factor of /spl alpha/ will be adopted for IMT-2000 systems in order to reduce both the intersymbol effect and the spectral width of the modulated signal. However, due to the complexity of obtaining quantitative results on the performance of MCR-DS/CDMA systems, rectangular chip pulses are mainly utilized in performance analysis. Therefore, it is necessary to investigate the effect of the chip pulses used, i.e., a rectangular and a raised cosine chip pulses on system performance in order to evaluate MCR-DS/CDMA systems accurately. Thus, the effect of the chip pulses used on the performance in MCR-DS/CDMA systems is investigated in terms of the system capacity and blocking probability. It is shown that the system using a raised cosine chip pulse (i.e., RC system) supports at least 80% more capacity and 57% more traffic than that using a rectangular chip pulse (i.e., R system).