Multi-hop networks are vital for establishing emergency communications, in case the terrestrial communication infrastructures are compromised. Organizing nodes into specific structures can enhance system resilience and efficiency. To this end, we consider using directional antennas to enhance transmission, and the nodes form a Z-chain structure based on the location information provided by the satellite. The core mechanism for performance gain lies in the spatial separation that shifts the dominant interference from the high-gain antenna main lobes to the attenuated side lobes. An exhaustive search demonstrates the optimality of the Z-chain configuration. To combat performance degradation from antenna pointing errors and node drift, we introduce an alternating Newton method (ANM) that jointly optimizes network structure and beam configurations by minimizing single-hop outage probability. Simulations show that the proposed Z-chain structure surpasses existing linear relay designs by converting most intra-chain interference from main-lobe to side-lobe directions, albeit with additional relay nodes. The Z-chain structure offers a promising paradigm for high-performance wireless networks with potential applications in terrestrial, aerial, and space communications.

Directional antennas can synergize with suitable network structure to complement each other in multi-hop wireless networks. This work addresses the combination of directional transmission and network structure to improve the capacity. Firstly, Z-chain structure considering the antenna radiation pattern is proposed, which is consistent with the best chain structure obtained by exhaustive search. Secondly, theoretical analysis indicates that spectral efficiency$\eta_{S}$behaves as a sigmoid function of node density$\rho$, with its asymptotic value increasing as$\Theta\left[\log \left(G_{r} \sin ^{2} \theta\right)\right]\left(\theta \leq \theta^{*}\right)$for large values of$G_{r}$, where$G_{r}$denotes the relative antenna gain,$\theta^{*}$is a function of the antenna beam width and$2 \theta$is the$\mathbf{Z}$-chain link angle. Thirdly, energy efficiency and consumption is assessed from a traffic load perspective. The findings reveal that the Z-chain network can accommodate higher data traffic compared to conventional frequency reuse method and straight structure. Experiments conducted using OMNeT++ validate above conclusions.

Relay nodes (RNs) enhance the connectivity and coverage of Wireless Sensor Networks (WSNs), but their deployment remains an NP-hard problem. This work incorporates directional antennas to improve network performance while addressing the analytical and computational challenges they introduce. We adopt the Rician fading model to capture realistic wireless channel conditions, including path loss, fading, and interference, enabling accurate signal-to-interference-plus-noise ratio (SINR) estimation. Outage probability is embedded into edge weights for relay placement optimization. To solve this problem, we formulate the constrained relay node placement problem, prove its NP-hardness, and develop a polynomial-time approximation algorithm that balances minimizing outage probability and limiting relay node deployment. Comparative analysis with existing algorithms shows that our method significantly reduces outage probability and improves network performance while keeping relay deployment costs within a reasonable range.

Polling and interrupt has long been a trade-off in RDMA systems. Polling has lower latency but each CPU core can only run one thread. Interrupt enables time sharing among multiple threads but has higher latency. Many applications such as databases have hundreds of threads, which is much larger than the number of cores. So, they have to use interrupt mode to share cores among threads, and the resulting RDMA latency is much higher than the hardware limits. In this paper, we analyze the root cause of high costs in RDMA interrupt delivery, and present FastWake, a practical redesign of interrupt-mode RDMA host network stack using commodity RDMA hardware, Linux OS, and unmodified applications. Our first approach to fast thread wake-up completely removes interrupts. We design a per-core dispatcher thread to poll all the completion queues of the application threads on the same core, and utilize a kernel fast path to context switch to the thread with an incoming completion event. The approach above would keep CPUs running at 100% utilization, so we design an interrupt-based approach for scenarios with power constraints. Observing that waking up a thread on the same core as the interrupt is much faster than threads on other cores, we dynamically adjust RDMA event queue mappings to improve interrupt core affinity. In addition, we revisit the kernel path of thread wake-up, and remove the overheads in virtual file system (VFS), locking, and process scheduling. Experiments show that FastWake can reduce RDMA latency by 80% on x86 and 77% on ARM at the cost of < 30% higher power utilization than traditional interrupts, and the latency is only 0.3 ∼ 0.4 μ s higher than the limits of underlying hardware. When power saving is desired, our interrupt-based approach can still reduce interrupt-mode RDMA latency by 59% on x86 and 52% on ARM.

The Internet-of-Things (IoT) paradigm holds the promise to revolutionize the way we live and work through connecting various machine-type communication terminals. In this work, we investigate a symbiotic network from a cross-layer perspective, where passive IoT devices coexist in symbiosis with an ambient network that uses carrier sense multiple access with collision notifications (CSMA/CNs) MAC protocol. In the ambient network, each full-duplex mobile user (MU) aims to transmit its own packets to the common access point (AP) while receiving the signal backscattered from its associated backscatter device (BD). Considering the imperfectness of carrier sensing of the MU, two key parameters, i.e., the probability of detection and the probability of false alarm, are quantified. Then we derive the PHY-layer outage probabilities and analyze the corresponding diversity orders for both the CSMA/CN and the BD system. By incorporating the outage probabilities and the carrier sensing metrics into the MAC-layer analysis, the cross-layer outage capacities of the CSMA/CN and the BD system are derived. Simulation results demonstrate that the system performance can be improved by appropriately setting the PHY-layer parameters, such as the BD reflection coefficient α and the number of samples for carrier sensing K, as well as the MAC-layer parameters, such as the sensing duration and the initial contention window. With the BD reflection coefficient being 0.05, the outage capacity of the overall system has improved by 41.85% compared with carrier sense multiple access with collision avoidance protocol.

In this paper, we study the cross-layer performance of a symbiotic system comprising passive internet-of-things (IoT) devices and the ambient system using carrier sense multiple access (CSMA) with collision notification (CSMA/CN) MAC protocol. Different from the CSMA with collision avoidance (CSMA/CA), the CSMA/CN protocol can enhance the throughput of the ambient system notably by using physical layer techniques to detect collisions. We first evaluate the physical-layer outage probabilities for the ambient and the backscatter device (BD) system respectively, and then introduce the outage probability of the ambient system into the MAC-layer performance analysis. Thereafter, the crosslayer outage capacities of the ambient system and the BD system are derived by considering the available transmission time for the BD system. We find that the overall system performance can be improved by appropriately setting the reflection coefficient of the BD and the parameters of the MAC protocol. The simulation results are provided to verify our theoretical analysis and demonstrate the system performance.