Wing Ho Yuen

Exploiting data diversity and multiuser diversity in noncooperative mobile infostation networks

In wireless networks, it is often assumed that all nodes cooperate to relay packets for each other. Although this is a plausible model for military or mission based networks, it is unrealistic for commercial networks and future pervasive computing environments. We address the issue of noncooperation between nodes in the context of content distribution in mobile infostation networks. We assume all nodes have common interest in all files cached in the fixed infostations. In addition to downloading files from the fixed infostations, nodes act as mobile infostations and exchange files when they are in proximity. We stipulate a social contract such that an exchange occurs only when each node can obtain something it wants from the exchange. Our social contract enables much higher system efficiency compared to downloading from fixed infostations only while not requiring true cooperation among nodes. We show by analysis and simulations that network performance depends on the node density, mobility and the number of files that are being disseminated. Our results point to the existence of data diversity for mobile infostation networks. The achievable throughput increases as the number of files of interest to all users increases. We have also extended the common interest model to the case where nodes have dissimilar interests. Our simulation results show that as mobile nodes change from having identical interests to mutually exclusive interests, the network performance degrades dramatically. We propose an alternative user strategy when nodes have partially overlapping interests and show that the network capacity can be significantly improved by exploiting multiuser diversity inherent in mobile infostation networks. We conclude that data diversity and multiuser diversity exist in noncooperative mobile infostation networks and can be exploited.

On energy efficiency and network connectivity of mobile ad hoc networks

In mobile ad hoc networks, it is often more important to optimize for energy efficiency than throughput. In this paper we investigate the effect of transmit range on energy efficiency of packet transmissions. We determine a common range for all nodes such that the average energy expenditure per received packet is minimized. In the first part of this paper, we consider stationary networks. We show that energy efficiency depends on various system parameters that includes path loss exponent of the channel, energy dissipation model and network offered load. In particular, when the path loss exponent is large, energy efficiency decreases when the transmit range increases. Hence, the network should be operated at the critical range that just maintains network connectivity. However, when the path loss exponent is small, operating at the critical range yields inferior throughput and energy efficiency. Our results show that energy efficiency is intimately connected to network connectivity. Three network connectivity regimes are identified as the transmit range of all nodes increases. In the second part, we examine the effect of node mobility on energy efficiency. We show that at normal offered load, an optimal transmit range exists such that energy efficiency is maximized The optimal range turns out to be insensitive to node mobility, and is much larger than the critical range. We show that the energy expenditure can be reduced by 15% to 73% in different mobility scenarios, if the network is operated at the optimal range.

Noncooperative content distribution in mobile infostation networks

In wireless networks, it is often assumed that all nodes cooperate to relay packets for each other. Although this is a plausible model for military or mission-based networks, it is unrealistic for commercial networks and future pervasive computing environments. We address the issue of noncooperation between nodes in the context of content distribution in mobile infostation networks. All nodes have common interest in all files cached in the fixed infostations. In addition to downloading files from the fixed infostations, nodes act as mobile infostations and exchange files when they are in proximity. We stipulate a social contract such that an exchange occurs only when each node can obtain something it wants from the exchange. We show by analysis and simulations that network performance depends on node density, mobility and the number of files that are being disseminated. Our results point to the existence of data diversity for mobile infostation networks. As the number of files of interest to all users increases, the achievable throughput increases. Moreover, each user has a fairer share of the total network throughput. In particular, the transmission of each channel is only limited by contention, indicating the noncooperation strategy achieves near optimum resource utilization.

Effect of node mobility on highway mobile infostation networks

In a mobile infostation network, any two nodes communicate when they are in proximity. Under this transmission constraint, any pair of nodes is intermittently connected as mobility shuffles the node locations. In this paper, we evaluate the effect of node mobility on highway mobile infostation networks. Each node enters a highway segment at a Poisson rate with a random speed drawn from a known but arbitrary distribution. Moreover, each node changes speed at each highway segment. Since nodes have different speed, a node may overtake other nodes or be overtaken as time evolves. Using arguments from renewal reward theory, the long run fraction of time an observer node is connected, and the long run average data rate can be derived. In this paper, however, we consider the special case of no speed change in each highway segment. In this case, the performance metrics are functions of the observer node speed. We consider both forward traffic scenarios, in which two nodes moving in the same direction have a transient connection when they are within range from each other, and reverse traffic scenarios in which two nodes travelling in opposite directions are connected transiently when they are in range. For node speed that is uniformly distributed, we reveal that the expected fraction of connection time, or expected number of connections in queuing terminology, is independent of the observer node speed in reverse traffic. In forward traffic, on the other hand, the fraction of connection time increases with observer speed. That is, the network performance improves with node mobility, which is unique to the mobile infostation networking paradigm.

Inter-relationships of performance metrics and system parameters in mobile ad hoc networks

In this paper, the network behavior of a routing algorithm for mobile ad hoc networks is investigated. Extensive simulations are performed using ns-2 in a variety of mobility scenarios and offered load regimes. In the literature, performance metrics (goodput, delay and path length) are often obtained through ensemble averaging of many flows. Here we advocate an alternate graphical interpretation of simulation results similar to that used by Holland and Vaidya (see Proceedings of the fifth annual ACM/IEEE international conference on Mobile computing and networking, August 1999). Performance metrics of individual monitored flows are plotted instead. By identifying the correlations between performance metrics and system parameters, inter-relationships between them are revealed. For example, we have shown that path length is dependent on system parameters such as mobility, offered load and even the node distribution. These observations often give us insights to the mechanisms that underline the network behavior. In particular we have resolved a conjecture that goodput improvement under high mobility is due to the load balancing effect. We show that at high mobility, goodput improvement for heavy offered load regimes is a consequence of the reduction of path length in the flows. Furthermore, we have introduced the concept of fraction of congested flows as a new performance metric. This and some other metrics such as fairness can be visualized from our graphs and are important in characterizing network performance.

Performance evaluation of highway mobile infostation networks

A mobile infostation network stipulates all transmissions to occur when nodes are in proximity. We evaluate the effect of mobility on highway mobile infostation networks. Each node enters a highway segment at a Poisson rate with a constant speed drawn from a known but arbitrary distribution. Both forward and reverse traffic are considered. For node speed that is uniformly distributed, the expected fraction of connection time, or expected number of connections in queueing terminology, is independent of observer node speed for reverse traffic, while it increases with observer node speed for forward traffic. We also extend our mobility model such that each node changes speed at each highway segment. The long run fraction of connection time of an observer node is dependent on the ratio of transmit range and connection time limit. Forward traffic connection yields better performance when the ratio is small and vice versa. We also compute the optimal transmit range and the corresponding data rate for both traffic types. We conclude that forward traffic connections yield much higher data rate in most scenarios.

Optimum transmit range and capacity of mobile infostation networks

A mobile infostation network stipulates all transmissions to occur when nodes are in proximity. In this paper, the effect of transmit range on the capacity of four transmission strategies is studied. We show that a stipulated transmit range improves the capacity relative to the reference strategy with an unconstrained transmit range. This indicates an optimal trade-off exists between spatial transmission concurrency and spectral efficiency on individual links. The optimal number of neighbors is invariant to node density, and is between 0.6 to 1.2 for our transmission strategies. This result should be contrasted to a magic number of 6 to 8 neighbors for multihop networks, where the expected forward progress per hop is maximized. This reflects the different optimization criteria of mobile infostation and multihop ad hoc networks. In addition, the capacity per unit area increases linearly with node density. This is counter-intuitive but can be explained using a rescaling argument drawn from percolation theory.

Throughput analysis of one-dimensional vehicular ad hoc networks

We consider content distribution in a one-dimensional vehicular ad hoc network. We assume that a file is encoded using fountain code, and the encoded message is cached at infostations. Vehicles are allowed to download data packets from infostations, which are placed along a highway. In addition, two vehicles can exchange packets with each other when they are in proximity. As long as a vehicle has received enough packets from infostations or from other vehicles, the original file can be recovered. In this work, we derive closed-form expressions for the average per-node throughput, under both discrete and continuous velocity distributions for the vehicles. Our result shows that the average per-node throughput can be expressed as a linear function of the densities of different classes of nodes in the highway. This result implies that the average per-node throughput scales linearly with vehicle arrival rate, and the average system throughput scales quadratically with vehicle arrival rate. Besides, system throughput reduces when overall mobility increases.