Broadcast distribution of updates (e.g., security patches, machine learning models) from a server to end devices (EDs) is a critical requirement in the Internet of Things (IoT). In this article, we consider the problem of reliable over-the-air broadcast of updates in Long Range Wide Area Networks (LoRaWANs). Existing broadcast techniques for LoRaWANs suffer from long delivery delays due to low data rates and duty-cycle constraints. We address this problem by proposing a device-level cooperative mechanism, in which updated EDs broadcast a few update fragments to accelerate delivery to their neighbors. We demonstrate large reductions in the delivery time compared with conventional methods. For instance, in a 400-node network spanning 1 km radius and operating at 1% duty-cycle, the proposed scheme reduces the time required to deliver a 10 kB update to an ED at the network’s edge from 42 h to 45 min. The proposed solution thus provides a pathway toward improved security and efficient realization of edge intelligence in LoRaWAN IoT.

Over-the-air firmware updates are crucial for mitigating security threats and maintaining up-to-date device functionality in long range wide area networks (LoRaWANs). LoRaWAN end devices are usually energy-constrained, and LoRaWAN transmissions are subject to duty-cycle restrictions. Consequently, controlling the energy expenditure and update-delivery latency of over-the-air firmware updates are key challenges. We propose a flexible scheme that achieves a tunable tradeoff between the energy consumption and delivery delay. The scheme employs the LoRa spreading factors sequentially to transmit update-carrying frames, sending a fixed number of frames with a given spreading factor before moving to the next. By adjusting the smallest spreading factor to be used and the number of transmissions per spreading factor, a suitable energy-delay tradeoff can be achieved. Thus, time-sensitive updates, such as security patches, may be sent with a low-delay-high-energy setting, whereas a more energy-efficient but higher delay setting may be used for noncritical updates.

In [1], there are errors in two equations. In (5), the variable “A” was mistakenly included in the final expression. In (13) and (14), a multiplicative factor was missing in both equations.

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.

Message replication on the links from end devices (sensors) to gateways has been used to improve the reliability of data transfer in many wireless IoT applications. Existing replication schemes employ one of the following two approaches: 1) each replica is transmitted as a separate frame and 2) replicas are included in the payload of frames that carry new data (known aspiggybacking). These approaches provide either low latency or low energy expenditure, but not both. We propose a hybrid approach that reduces the energy consumption while operating within the delay constraints of the application. Aimed at sensor networking applications with small message sizes and reasonable delay tolerances, the proposed method is shown to outperform existing mechanisms in terms of energy expenditure and reliability.

Convenient analytical techniques are developed for use in preliminary design of systems and protocols for the delivery of fountain-coded files from a source node to a destination node with the assistance of a relay node. Each node has a half-duplex packet radio, and the time-varying disturbance on each link is represented by a lumpable finite-state Markov chain. The analytical techniques include the use of first passage times and recurrence times in the derivation of an exact expression for the completion time of file transfer over a radio link and the application of results on Markov-correlated Bernoulli trials to obtain exact expressions for completion times of relay-aided file transfer in wireless networks. In the development of analytical techniques, we employ capacity-achieving channel codes and ideal fountain codes; however, we present simulation results for packet radios that employ practical channel codes and for which the fountain code is a standard systematic raptor code.

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.

In this article, we propose application layer coding schemes to recover lost data in delay-sensitive uplink (sensor-to-gateway) communications in the Internet of Things. Built on an approach that combines retransmissions and forward erasure correction, the salient features of the proposed schemes include low computational complexity and the ability to exploit sporadic receiver feedback for efficient data recovery. Reduced complexity is achieved by keeping the number of coded transmissions as low as possible and by devising a mechanism to compute the optimal degree of a coded packet in$\mathcal {O} (1)$. Our major contributions are as follows: 1) an enhancement to an existing scheme calledwindowed coding, whose complexity is greatly reduced and data recovery performance is improved by our proposed approach; 2) a technique that combines elements of windowed coding with a new feedback structure to further reduce the coding complexity and improve data recovery; and 3) a coded forwarding scheme, in which a relay node provides further resilience against packet loss by overhearing source-to-destination communications and making forwarding decisions based on overheard information.

The chirp spread spectrum (CSS) modulation scheme is employed by the physical layer of the Long Range (LoRa) communication technology. In this paper, we examine the performance of CSS over time-varying channels whose gain may change during the reception of a LoRa frame. This is in contrast to the usually employed model in the literature, which assumes the channel gain to be constant throughout a frame. Specifically, we investigate the effects of exponentially correlated Rayleigh fading on the frame-error rate of a CSS receiver in which the channel gain is estimated at the beginning of each frame. Our primary observation is that over rapidly-varying channels, the robustness benefits of the larger spreading factors tend to disappear as the payload size grows. This observation, which is contrary to the common perception that higher spreading factors necessarily provide greater immunity against noise, highlights the need to consider channel characteristics and payload sizes in allocating the spreading factor for reliable and energy-efficient LoRa communications.

To enhance the reliability of LoRa sensor networks, we propose two opportunistic forwarding protocols in which relays overhear sensor-to-gateway messages and periodically forward them to a gateway. To reduce the number of transmissions, the relays forward XOR sums of the overheard messages. One protocol employs a single relay node, while two relays cooperate in the other. With a single relay, our proposed protocol requires fewer relay transmissions to achieve similar loss rates as conventional uncoded forwarding in which every received message is immediately forwarded. We demonstrate up to 55% reduction in relay transmission time relative to uncoded forwarding. The cooperative protocol provides better loss and delay performance than the single-relay protocol, and we demonstrate up to 16% reduction in the message-loss rate along with 33% reduction in relay transmission times compared to conventional forwarding.

We propose two application-layer coding schemes for delay-constrained point-to-point packet communications with restrictions on the transmitter's maximum duty-cycle. The schemes operate over GF(2) and utilize intermittently available receiver feedback for erasure correction. Applications that will benefit from the proposed schemes include wireless sensor networks in which energy constrained sensors must deliver readings to a gateway within a deadline. Simulation results for independent Bernoulli erasure channels, Gilbert-Elliott channels, and Long Range (LoRa) communications demonstrate orders-of-magnitude reductions in the delivery failure rate as compared to feedback-assisted repetition redundancy and a blind coding scheme that does not utilize feedback.

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.

We propose a communication scheme with relays to improve the reliability of a Long Range (LoRa) sensor network with duty-cycle limitations. The relays overhear the sensors' transmissions and forward them to a gateway. Simulations show that relaying is very beneficial, even though the nodes are not coordinated and duty cycling limits the number of sensor measurements that can be forwarded. In our setup, a single relay can halve the measurement loss rate and eight relays provide a gain of up to two orders of magnitude. Further improvements are achieved by including a few past measurements in each frame.

We devise and evaluate five methods for fountain-coded broadcast distribution of a file from a source to multiple destinations in an ad hoc wireless network that consists of half-duplex packet radios. The methods differ in their use of intermediate nodes, their use of forwarding, and their reliance on a network spanning tree. All five methods employ continued fountain coding to prevent nodes from receiving duplicate fountain-coded packets. We derive an analytical approximation for the throughput of fountain-coded broadcast file distribution in a four-node network with time-varying radio links modeled by independent two-state Markov chains, and we show that our approach to fountain-coded file distribution gives throughput that is very close to the approximation. We employ simulations to examine larger networks in which each radio link has correlated Rayleigh fading and the radios use adaptive modulation and channel coding.