For randomly distributed nodes sending packets to a central gateway using slotted Aloha with a constrained number of retransmissions over a wireless fading channel, we take an iterative approach to compute the total traffic density of packet transmissions and collision-induced retransmissions as a function of the node-gateway distance. From this, we compute the outage probability, throughput, and energy consumption. This method can be used to analyze and dimension Low-Power Wide-Area Networks (LPWANs). Applying it in LoRa illustrates design tradeoffs concerning outage probability, spreading factors, energy consumption, and cell size. It notably shows that permitting more retransmissions helps the nodes near the gateway but hurts distant ones.

The concept of pulse-coupled oscillators for self-organized synchronization has been applied to wireless systems. Putting theory into practice, however, faces certain obstacles, particularly in radio technologies that cannot implement pulses but use common messages for interactions between nodes. This raises the question of how to deal with interference between messages. We show that interference can disturb the synchronization process and propose low-complex, randomization-based techniques to address this issue. First, we demonstrate that randomly switching between two transmit power levels (without increasing the average power) can expedite synchronization. The high-power transmissions temporarily boost network connectivity with negligible impact on the average interference. Second, we reduce interference by blindly distributing the messages over the entire oscillator cycle. Instead of using a fixed oscillator phase at which the pulses are sent, each node chooses its own, randomly selected phase to send a synchronization message. This node-specific “fire phase” is contained in the message to permit others to compute the timing. Third, we suggest that such interference management can also be beneficial for other synchronization techniques and validate this claim using Glossy as an example. Our insights may contribute to feasible solutions for self-organized wireless synchronization. Further work is needed to gain a comprehensive understanding of the effects of randomization and to develop algorithms for the adaptability of local parameters.

We propose a scheme to reduce and balance the energy consumption of nodes in dense IoT use cases implemented with LoRa. We study a multiple gateways network with nodes uniformly distributed around each gateway, and restricted to use only short spreading factors. Relays using longer spreading factors, are added to the network infrastructure to forward the nodes transmissions that cannot reach a gateway directly. We find that the impact of the interference added by the relays on the probability of successful transmissions becomes marginal for high density networks. Furthermore, in such dense networks the consumption at the nodes is lowered one order of magnitude with marginal increase in the overall energy consumption of the network. Also, the consumption spread between nodes close and far from the gateway is effectively reduced.

We derive the probability distribution of the link outage duration at a typical receiver in a wireless network with Poisson distributed interferers sending messages with slotted random access over a Rayleigh fading channel. This result is used to analyze the performance of random linear network coding, showing that there is an optimum code rate and that interference correlation affects the decoding probability and throughput.

This article takes an analytical approach to investigate the temporal dynamics of interference in wireless networks. We propose a framework to calculate the autocorrelation of interference in Poisson networks and derive closed-form expressions for the case of Nakagami fading. The framework takes three correlation sources into account: the location of interferers, the wireless channel, and the data traffic. We introduce the interference coherence time-in analogy to the well-established channel coherence time-and show how its basic qualitative behavior depends on the source of correlation. The insights gained can be useful in the design of medium access control and retransmission protocols.

A thorough understanding of the temporal dynamics of interference in wireless networks is crucial for the design of communication protocols, scheduling, and interference management. This article applies stochastic geometry to investigate the interference dynamics in a network of nodes that use carrier sense multiple access (CSMA). This type of network is approximated by a Matérn hard-core point process of type II with Nakagami fading. We derive and analyze expressions for the variance, covariance, and correlation of the interference power at an arbitrary location. Results show that even though the commonly used Poisson approximation to CSMA may have the same average interference as the Matérn model, the interference dynamics behaves significantly different. In this way, this study reveals a pitfall in the modeling of wireless systems and contributes to the theory of interference calculus.

Our research investigates the concept of interference prediction as an unprecedented approach for interference management and medium access in wireless networks. This paper is a first step in this direction: it proposes and evaluates a simple interference prediction technique that is based on low-complexity learning. Nodes predict the interference situation they expect to experience in the near future and select the most favorable time slot to start the transmission of a multislot message. The performance gain is evaluated in a small-scale fading environment in terms of link outage and delay against random slot selection. Simulation results show that interference prediction is a promising building block for wireless systems. Additional studies are needed to explore advanced techniques and assess their feasibility.

There is a strong demand in the aviation industry to replace cables in airplanes by wireless connectivity to gain flexibility and reduce weight. Such in-plane wireless communications must be reliable and robust against interference. As part of our activities in this domain, we present a proof-of-concept for an ultra-wideband (UWB) sensor network deployed in a mockup of a small passenger cabin of a commercial aircraft with a few passengers and report experimental results on the packet loss rate with off-the-shelf IEEE~802.15.4-2011 compliant UWB transceivers. It is shown that a combination of spatial and temporal diversity can significantly lower the packet loss rate of different link types without degrading throughput.

A time synchronization technique for networked devices with low-precision oscillators and low computational power is proposed and evaluated. We apply a dynamic stochastic model inserted into a Kalman filter formulation to track the clock evolution of oscillators and achieve synchrony to a central time reference. Indoor and outdoor experiments performed with commercially available wireless sensor platforms over several days serve as a proof of concept and show the synchronization accuracy under stable and varying temperature conditions.

This work explores the benefits in terms of energy efficiency that can be provided by device-to- device (D2D) communications in cellular systems. Analyzing a large cellular network using stochastic geometry, considering Nakagami fading and realistic system parameters, we explore the reduction that can be achieved in network power consumption, and the impact this reduction has on the performance of cellular users. When the network is not fully loaded, switching some cellular users into D2D mode results in a significant reduction of the network power consumption while the overall network throughput is maintained. Furthermore, it is observed that the impact of the reduction in the inter-cell interference arising from switching cellular users to D2D mode surpasses that from the new non- orthogonal transmissions. This translates into a slight improvement in the users' rates. Both analytical and simulation results are provided.

Time synchronization provides the basis for several applications in wireless sensor networks but the limited memory and computational power, and the use of low precision oscillators make the task of time synchronization non-trivial. In this demonstration, we present a novel time synchronization scheme that is based on time series analysis. To provide a general model for the practical behavior of low precision oscillators, autoregressive integrated moving average models are explored. Based on the analysis of experimental data, an autoregressive integrated model (ARI (1,1)) is derived. Unlike the resource hungry Kalman filter based formulations, the proposed scheme is resource efficient as it results in simple linear regression processing. Experiments are performed on real sensor devices including Zolertia and TelosB, where an accuracy below 1 clock tick 1 is achieved.

Time synchronization is challenging in wireless sensor networks due to the use of low-precision oscillators and the limited computational capacity of resources limited sensor nodes. While several schemes exist, the performance analysis of a majority of them is based on simulations and fail to capture key features of real world deployments. This paper explores the use of autoregressive integrated moving average models to provide a general clock model for sensor nodes with low precision oscillators and limited computational power. Based on measurements with off-the-shelf sensor devices Z1, an autoregressive integrated model for time synchronization is proposed. We derive a synchronization scheme (ARI-Sync) based on this model and compare it against the well known Flooding Time Synchronization Protocol (FTSP) observing significantly improved accuracy, roughly doubling the resynchronization period of Z1 nodes for a typical wireless sensor network application.

Device-to-device (D2D) communications is a promising candidate for increasing the throughput of cellular systems towards 5G. Its current consideration for inclusion in LTE-A is attracting a lot of research interest, including work on mode selection, bandwidth allocation, and power control. Some consider the uplink for D2D operation while others focus on operation over the downlink. It is still unclear, however, which link is preferable for D2D operation. This paper addresses this issue focusing on the outage performance of D2D receivers. We use stochastic geometry tools for our analysis, considering realistic system parameters and Nakagami fading. We find that the interference from the cellular system on the downlink, which is correlated over consecutive time slots, favors underlay D2D operation, resulting in a lower outage probability for the D2D transmissions.

Active interference cancellation (AIC) is an effective technique to shape the OFDM spectrum, providing deep notches over protected bands without affecting receiver design. However, AIC typically introduces spectrum overshoot, of concern if compliance with a spectral emission mask is required. Most AIC designs neglect these spectral spurs, focusing on minimization of out-of-band radiation. We present a novel design that explicitly takes spectral spurs into account. The resulting optimization problem is convex and can be efficiently solved, although for systems with a large number of subcarriers the computational effort can be significant. A suboptimal solution based on a previous AIC design with a maximum power constraint is also proposed. Its performance is close to optimal while obtained at a much lower computational cost, comparing favorably to previous schemes.