A case for variable-range transmission power control in wireless multihop networks

Abstract
In this paper, we study the impact of individual variable-range transmission power control on the physical and network connectivity, network capacity and power savings of wireless multihop networks such as ad hoc and sensor networks. First, using previous work by Steele (16) and Gupta (7) we derive an asymptotic expression for the average traffic carrying capacity of nodes in a multihop network where nodes can individually control the transmission range they use. For the case of a path attenuation factor α =2 we show that this capacity remains constant even when more nodes are added to the network. Sec- ond, we show that the ratio between the minimum transmission range levels obtained using common-range and variable-range based routing protocols is approximately 2. This is an important result because it suggests that traditional routing protocols based on common-range transmission can only achieve about half the traffic carrying capacity of variable-range power control approaches. In addition, common-range approaches consume ∼ (1 − 2 2α) % more transmission power. Second, we derive a model that approximates the signaling overhead of a routing protocol as a function of the transmission range and node mobility for both route discovery and route maintenance. We show how routing protocols based on common-range transmission power limit the capacity available to mobile nodes. The results presented in the paper highlight the need to design future wireless network protocols (e.g., routing protocols) for wireless ad hoc and sensor networks based, not on common-range which is prevalent today, but on variable-range power control. I. INTRODUCTION Effective transmission power control is a critical issue in the design and performance of wireless ad hoc networks. Today, the design of packet radios and protocols for wireless ad hoc networks are primarily based on common-range transmission control. We take an alternative approach and make a case for variable-range transmission control. We argue that variable- range transmission control should underpin the design of future wireless ad hoc networks, and not, common-range transmission control. In this paper, we investigate the tradeoffs and limits of using a common-range transmission approach and show how variable-range transmission control can improve the overall network performance. We analyze the impact of power control on the connectivity at both the physical and network layers. We compare how routing protocols based on common-range and variable-range transmission control techniques impact a number of system performance metrics such as the connectivity, traffic carrying capacity, and power conserving properties of wireless ad hoc networks. Power control affects the performance of the physical layer in two ways. First, power control impacts the traffic carrying capacity of the network. On the one hand, choosing too high a transmission power reduces the number of forwarding nodes needed to reach the intended destination, but as mentioned above this creates excessive interference in a medium that is commonly shared. In contrast, choosing a lower transmission power reduces the interference seen by potential transmitters but packets require more forwarding nodes to reach their intended destination. Second, power control affects how con- nected the resulting network is. A high transmission power increases the connectivity of the network by increasing the number of direct links seen by each node but this is at the expense of reducing network capacity. In this paper, we consider the use of variable-range transmission control to allow nodes to construct a minimum spanning tree (MST). We show that the use of a minimum spanning tree can lead toward lower total weight than a tree based on common-range transmission links that minimally avoid network partitions. The type of power control used can also impact the con- nectivity and performance of the network layer. Choosing a higher transmission power increases the connectivity of the network. In addition, power control impacts the signaling overhead of routing protocols used in mobile wireless ad hoc networks. Higher transmission power decreases the number of forwarding hops between source-destination pairs, therefore reducing the signaling load necessary to maintain routes when nodes are mobile. The signaling overhead of routing protocols can consume a significant percentage of the available resources at the network layer, reducing the end user's bandwidth and power availability. Existing routing protocols discussed in the mobile ad hoc networks (MANET) working group of the IETF (9) are designed to discover routes using flooding techniques at common-range maximum transmission power. These protocols are optimized to minimize the number of hops between source- destination pairs. Such a design philosophy favors connectivity to the detriment of potential power-savings and available capacity. Modifying existing MANET routing protocols to promote lower transmission power levels in order to increase network capacity and potentially higher throughput seen by

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