We consider a wireless sensor network with n sensor nodes. The sensed data needs to be transferred in a multi-hop fashion to a common processing center. We consider the standard data sampling/sensing scheme where the sensor nodes have a sampling process independent of the transmission scheme. In this paper, we study the problem of optimizing the end-to-end delay in a multi-hop single-sink wireless sensor network. We prove that the delay-minimization objective function is strictly convex for the entire network. We then provide a distributed optimization framework to achieve the required objective. The approach is based on distributed convex optimization and deterministic distributed algorithm without feedback control. Only local knowledge is used to update the algorithmic steps. Specifically, we formulate the objective as a network level delay minimization function where the constraints are the reception-capacity and service-rate probabilities. Using the Lagrangian dual composition method, we derive a distributed primal-dual algorithm to minimize the delay in the network.We further develop a stochastic delay control primal-dual algorithm in the presence of noisy conditions. We also present its convergence and rate of convergence. The proposal is extensively evaluated by analysis and simulations.

Stability and delay constraints have significant impact on the design and operation of wireless sensor networks. In this paper, we propose a closed architecture for data sampling in wireless sensor networks. Examples show that the proposed scheme outperforms the traditional layered scheme, both in terms of stable operating region as well as the end-to-end delays. We then propose a distributed routing scheme for a broad class of wireless sensor networks which converges (in the Cesaro sense) to the set of Cesaro-Wardrop equilibria. The scheme is based on multiple time-scale stochastic approximation algorithms. Convergence is established using standard results from the related literature and validated by simulation results. Our algorithm can adapt to changes in the network traffic and delays.

Underwater acoustic sensor networks (UASNs) consist of sensors that are deployed to perform collaborative monitoring of tasks over a given volume of water. The quality of the underwater acoustic link is highly unpredictable, since it mainly depends on fading and multipath, which are not easily modeled phenomena. This is return severely degrades the performance at higher layers such as extremely long and variable propagation delays. In addition, this variation is generally larger in horizontal links than in vertical ones. In this paper, we analyze a modulation scheme and associated receiver algorithms. This receiver design take advantage of the time reversal (phase conjugation) and properties of spread spectrum sequences known as Gold sequences. This technique improves the signal-to-noise ratio (SNR) at the receiver and reduces the bit error rate (BER). We then applied the phase conjugation to the case of network communication. We show that this approach can give almost zero BER for a 2-hop communication compared to single hop. This link layer information can be used at the network layer to formalize routing decisions. We show these improvements by means of analytical analysis and simulations.

Wireless sensor networks are commonly used to monitor and control the physical world. To provide a meaningful service such as disaster and emergency surveillance, meeting real-time constraints and the stability of transmit queues are the basic requirements of communication protocols in such networks. In this paper, we propose a closed architecture with two transmit queues at each sensor i, i.e., one for its own generated data, and the other for forwarding traffic. Our first main result concerns the stability of the forwarding queues at the nodes. It states that whether or not the forwarding queues can be stabilized (by appropriate choice of weighted fair queueing weights) depends only on routing and channel access rates of the sensors. Further, the weights of the weighted fair queues play a role in determining the tradeoff between the power allocated for forwarding and the delay of the forwarding traffic. We finally propose a distributed routing scheme for a broad class of wireless sensor networks. Each link is assigned a weight and the objective is to route through minimum weight paths using iterative updating scheme. The proposal is validated by analytical analysis and simulations.

Distributed systems based on networked sensors and actuators with embedded computation capabilities are commonly used to monitor and control the physical world. To provide a meaningful service such as disaster and emergency surveillance, meeting real-time-and-energy constraints and the stability of transmit queues are the basic requirements of communication protocols in such networks. In settings with sparse distribution of actuator nodes, multi-hop routing is traditionally used to relay information to a remote sink. A problem with this approach is that the loss of connectivity of actuator nodes may lead to partitioning of the network. In this paper, we address the problem of minimizing power consumption at each actuator node and minimizing assignment overhead at each sensor node while ensuring network connectivity. We propose that each actuator is enabled with two wireless interfaces: one to communicate with its assigned sensor network, and the other to communicate with the network of neighboring actuators. At any instant of time, an actuator can adjust its transmit power level to ensure connectivity and pro-actively inform its attached sensor network in case of mobility. These strategies have an associated cost. We show in this paper that at any instant of time, the strategy chosen by an actuator to adjust power and control-overhead due to mobility are optimal subject to constraints. The proposal is validated by means of analysis and simulations.

Delay and energy constraints have a significant impact on the design and operation of wireless sensor-actuator networks (SANETs). Furthermore, preventing sensor nodes from being inactive is very critical. The problem of sensor inactivity arises from the path loss and fading that degrades the quality of the signals transmitted from actuators to sensors, especially in anisotropic deployment areas, e.g., rough and hilly terrains. Sensor data transmission in SANETs heavily relies on the scheduling information that each sensor node receives from its associated actuator. Therefore if the signal containing scheduling information is received at a very low power due to the impairments introduced by the wireless channel, the sensor node might be unable to decode it and consequently it will remain inactive. In this paper, it is proposed that each sensor node transmits its data to only one of the actuators. However all actuators cooperate and jointly transmit scheduling information to sensors with the use of beamforming. This results to an important reduction in the number of inactive sensors comparing to single actuator transmission for a given level of transmit power. The reduction is due to the resulting array gain and the exploitation of macro-diversity that is provided by the actuator cooperation. In order to maximize network lifetime and attain minimum end-to-end delays, it is essential to optimally match each sensor node to a particular actuator and find an optimal routing solution. A distributed solution for optimal actuator selection subject to energy-delay constraints is also provided.

Sensor-actuator networks are often limited in battery capacity and processing power. Therefore, it is exigent to develop solutions that are both energy and delay efficient. In this paper, we propose a low-energy and delay-sensitive TDMA based MAC for wireless sensor-actuator networks (SANETs). These networks are organized into clusters and each cluster is managed by a single actuator. To avoid inter- cluster downlink interference, we introduce an easy use of CDMA codes. The actuators schedule sensors to reduce the total network energy consumption per successful transmission. We identify the advantages of our proposal over existing TDMA-MAC schemes for cluster-based sensor networks. Our simulation results demonstrate that the proposed MAC greatly improves the SANET lifetime and achieves a good trade-off between the packet delay and sensor energy consumption.