Clement Kam

Age of information under random updates

We consider the system where a source randomly generates status update messages and transmits them via a network cloud to the intended destination. These update message can take different times to traverse the network, which we model as exponential service times, and may result in packets reaching the destination out of order, rendering some of the earlier transmissions obsolete. We analyze the status update age for such a system, and show that it tracks well with simulation results.

Effect of Message Transmission Path Diversity on Status Age

This paper focuses on status age, which is a metric for measuring the freshness of a continually updated piece of information (i.e., status) as observed at a remote monitor. In paper, we study a system in which a sensor sends random status updates over a dynamic network to a monitor. For this system, we consider the impact of having messages take different routes through the network on the status age. First, we consider a network with plentiful resources (i.e., many nodes that can provide numerous alternate paths), so that packets need not wait in queues at each node in a multihop path. This system is modeled as a single queue with an infinite number of servers, specifically as an M/M/∞ queue. Packets routed over a dynamic network may arrive at the monitor out of order, which we account for in our analysis for the M/M/∞ model. We then consider a network with somewhat limited resources, so that packets can arrive out of order but also must wait in a queue. This is modeled as a single queue with two servers, specifically an M/M/2 queue. We present the exact approach to computing the analytical status age, and we provide an approximation that is shown to be close to the simulated age. We also compare both models with M/M/1, which corresponds to severely limited network resources, and we demonstrate the tradeoff between the status age and the unnecessary network resource consumption.

Effect of message transmission diversity on status age

We investigate the performance of a status monitoring system, in which a sensor sends random status updates over a network to a remote monitor. Specifically, we analyze the status age metric, which characterizes how old the information at the monitor is from the last received status update. The system on which we focus is a single queue with 2 servers (specifically, an M/M/2). In a dynamic network, different status packets may take different routes to the monitor, which allows for the possibility of packets arriving out of order. In the case of the status monitoring system, only the latest status is useful. Studying a system with 2 servers allows for the possibility of packets to arrive out-of-order while still having to queue. We present the exact approach to computing the analytical status age, and we provide an approximation that matches very closely with the simulated age. We also compare with the M/M/∞ and M/M/1, and we demonstrate the tradeoff between status age and network resource consumption.

Cooperation in cognitive underlay networks: stable throughput tradeoffs

This paper addresses fundamental issues in a shared channel where the users have different priority levels. In particular, we study a two-user cognitive shared channel consisting of a primary (higher-priority) and a secondary user, operating in the cognitive underlay fashion, but in a novel way where interference suffered by the primary user is compensated by requiring the secondary user to cooperatively relay some of the primarys packets. We start by analyzing the case of no node cooperation, where nodes transmit their own packets to their respective destinations. We then extend the analysis to a system in which the secondary node acts as a relay for the primary user, in addition to serving its own packets. Specifically, in the cognitive cooperation case, the secondary node forwards those packets to the primary destination that it receives successfully from the primary source. In such cognitive shared channels, a tradeoff arises in terms of activating the secondary along with the primary so that both transmissions may be successful, but with a lower probability, compared to the case of the secondary node staying idle when the primary user transmits. Results show the benefits of relaying for both the primary as well as the secondary nodes in terms of the stable-throughput region.

Age of information with a packet deadline

We study the age of information, which is a recently introduced metric for measuring the freshness of a continually updated piece of information as observed at a remote monitor. The age of information metric has been studied for a variety of different queuing systems. In this work, we introduce a packet deadline as a control mechanism and study its impact on the average age of information for an M/M/1/2 queuing system. We analyze the system for a fixed deadline and derive a mathematical expression for the average age. We numerically evaluate the expression and show the relationship of the age performance to that of the M/M/1/1 and M/M/1/2 systems. We show that the system with a deadline constraint can outperform both the M/M/1/1 and M/M/1/2 without such a deadline.

Frequency Selection and Relay Placement for Energy Efficiency in Underwater Acoustic Networks

When planning the deployment of an underwater network, the frequency-dependent nature of the underwater acoustic channel has a significant impact on network performance. The quality of acoustic communication in the underwater medium is heavily dependent on the choice of frequency and link length. In this paper, we consider the problems of choosing the proper frequency and node locations for minimizing energy consumption in the network. First, we look at the single frequency that minimizes the transmission power for a network with links of varying lengths. We propose approximate models for this frequency for cases when the link lengths are known exactly and when only probabilistic knowledge of the link lengths is available. Second, to better understand the importance of the actual locations of nodes, we study the problem of relay placement in a multihop line network. We demonstrate that utilizing different frequencies has an impact on the optimal relay placement, and we provide conditions for which the optimal placement is equidistant on the line. Finally, we solve the problem of jointly optimizing the relay placement and the single-frequency choice for a line network. To validate our results, we test our models using simulations of network deployment, and we test our optimal relay placement strategy for a variety of underwater channel settings.

Experimental evaluation of the age of information via emulation

This work focuses on evaluating a new metric for the timeliness of information in a status monitoring system, referred to as the age of information or the status age. Investigation into the age of information metric is fairly recent and has primarily been focused on theoretical analysis, and there has been no evaluation of the metric in a realistic system of networked nodes. We evaluate the age metric in a realistic wireless system using the open source network emulation tools CORE and EMANE. Our goal is to validate theoretical results on age for a single link across a wide range of system parameters. In addition to verifying existing theoretical results, we go further by adding more theoretical results that better model the emulated system than currently offered in the theory. Our results indicate that the theoretical results are indicative of real world behavior, but we also identify the limitations of the theoretical models and the impact on the emulated age.

Controlling the age of information: Buffer size, deadline, and packet replacement

We study the age of information, which is a recently introduced metric for measuring the freshness of a continually updated piece of information as observed at a remote monitor. The age of information metric has been studied for a variety of different queuing systems, and in this work, we consider the impact of buffer sizes, packet deadlines, and packet replacement on the average age of information for queuing systems. We conduct a simulation-based study in which we modeled a wide variety of queuing systems and control mechanisms in simulation and computed the average age of information. We first study the buffer size alone to see how it affects the average age, and then we look at adding a packet deadline for such a system. We consider packet deadline control in the buffer only and in both the buffer and server, and we also compare the performance with a random deadline. We observe how the buffer size and deadline are optimized for the age, and we identify general trends for how to choose values of control mechanisms under different conditions of the packet generation rate. Lastly, we study the ability to replace packets in the buffer with newly arriving packets, and we are particularly interested in whether we can achieve the performance of such a system by controlling buffer size and deadline alone, for systems in which we do not have the ability to do packet replacement.

Impact of relay placement on energy efficiency in Underwater Acoustic Networks

In spite of the body of work in modeling the underwater channel, there is still considerable lack of fundamental understanding that is required to enable the widespread deployment of Underwater Acoustic Networks (UANs). In this paper we study one such aspect, namely the impact of relay placement on the overall transmit power required to maintain a certain quality of communication between two nodes, in a multi-hop underwater line network. We start by analyzing a simple case involving one relay and a common signal frequency over all hops. We then extend the analysis to include different signal frequencies over different hops. Specifically, in each case, we find through analysis and numerical evaluation the optimal relay placement that minimizes total transmit power, thereby improving energy efficiency. We observe that the optimal relay locations are not always equidistant, especially when each hop can choose from a set of signal frequencies for transmission.

Cognitive Cooperative Random Access for Multicast: Stability and Throughput Analysis

In this paper, we investigate the queue stability and throughput of a two-user cognitive radio system with multicast traffic. We study the impact of network-level cooperation, in which one of the nodes can relay the packets of the other user that are not received at the destinations. Under this approach, if a packet transmitted by the primary user is not successfully received by the destination set but is captured by the secondary source, then the secondary user assumes responsibility for completing the transmission of the packet; therefore, the primary releases it from its queue, enabling it to process the next packet. We demonstrate that the stability and throughput regions of this cooperative approach is larger than that of the noncooperative approach, which translates into a benefit for both users of this multicast system. Our system model allows for the possibility of multipacket reception (MPR), and the optimal transmission strategies for different levels of MPR capability are observed in our numerical results. In addition to achieving a larger stability region, our results show that cooperation can result in reduced average delay for both primary and secondary users.