We abstract the core logical functions from applications that require ultra-low-latency wireless communications to provide a novel definition for reliability. Real-time applications - such as intelligent transportation, remote surgery, and industrial automation - involve a significant element of control and decision making. Such systems involve three logical components: observers (e.g. sensors) measuring the state of an environment or dynamical system, a centralized executive (e.g. controller) deciding on the state, and agents (e.g. actuators) that implement the executive's decisions. The executive harvests the observers' measurements and decides on the short-term trajectory of the system by instructing its agents to take appropriate actions. All observation packets (typically uplink) and action packets (typically downlink) must be delivered by hard deadlines to ensure the proper functioning of the controlled system. In-full on-time delivery cannot be guaranteed in wireless systems due to inherent uncertainties in the channel such as fading and unpredictable interference; accordingly, the executive will have to drop some packets. We develop a novel framework to formulate the Observer Selection Problem (OSP) through which the executive schedules a sequence of observations that maximize its knowledge about the current state of the system. To solve this problem efficiently yet optimally, we devise a branch-and-bound algorithm that systematically prunes the search space. Our work is different from existing work on real-time communications in that communication reliability is not conveyed by packet loss or error rate, but rather by the extent of the executive's knowledge about the state of the system it controls.

Many control applications demand stringent latency and reliability requirements that cannot be met by existing wireless technologies. Prior work has modeled a pilot-assisted, variable-rate communication procedure that describes the transmission of commands from a centralized controller to a number of agents (e.g. actuators) through a number of access points (APs). This procedure comprises two phases with pre-allocated time budgets per transmission cycle. There is a training phase where channel states between AP-agent pairs are estimated, and a downlink phase where the commands are sequentially transmitted at adapted rates before the start of a new cycle - a hard deadline. System reliability is compromised, though, when the agents fail to receive the controller's commands before the start of a subsequent cycle. In this paper, we calculate a closed-form expression for the probability of hard-deadline violation when there are two agents. We find upper and lower bounds on this probability when there are more agents. We observe that these bounds approximate the probability of hard-deadline violation which reflects the reliability of the system.

We consider a downlink periodic wireless communications system, where multiple access points cooperatively transmit packets to a number of devices, e.g., actuators in an industrial control system. Each period consists of two phases: an uplink training phase and a downlink data transmission phase. Each actuator must successfully receive its unique packet within a single transmission phase; else, an outage is declared. Such an outage can be caused by two events: a transmission error due to transmission at a rate that the channel cannot actually support or time overflow, where the downlink data phase is too short, given the channel conditions to successfully communicate all the packets. We determine the closed-form expressions for the time overflow probability when there are just two field devices, as well as the transmission error probability for an arbitrary number of devices. In addition, we provide upper and lower bounds on the time overflow probability for an arbitrary number of devices. We propose a novel variable-rate transmission method that eliminates time overflow. Detailed system-level simulations are used to identify system design guidelines, such as the optimal amount of training time, as well as for benchmarking the proposed system design versus non-cooperative cellular, cooperative fixed-rate, and cooperative relaying.

Competing cellular operators aggressively share infrastructure in many major US markets. If operators also share spectrum licenses, intra- cellular interference will become correlated with inter-cellular interference. We propose a mathematical framework to model a two-operator millimeter-wave (mmWave) cellular network with co- located base-stations (BSs). We then characterize the SINR distribution for an arbitrary network to understand the impact of varying the spatial correlation between the operators' networks. An interesting observation is that sharing spectrum and infrastructure yields a higher rate coverage probability for higher rate thresholds, but has a lower coverage for lower thresholds. This suggests that networks catering for low-rate, limited-QoS devices, are at a disadvantage when spectrum and infrastructure are shared.