> Submitted to QShine 2008 Conference 1 Exploiting Time and User Diversity in Distributed Medium Access Control for Mobile Ad-Hoc Networks Qinqing Zhang, Senior Member, IEEE, Lotfi Benmohamed, Senior Member, IEEE, I-Jeng Wang, Senior Member, IEEE Abstract—In this paper, we consider a distributed medium access control (MAC) for mobile ad-hoc networks (MANET). We propose an adaptive threshold-based medium contention scheme to exploit diversity gain in multiple dimensions, in particular, the time and user diversity. In our scheme, each node performs a carrier sensing multiple access with collision avoidance (CSMA/CA) MAC protocol and access the channel only when the channel quality is above a threshold. We develop a practical solution to address node mobility and time varying fading channels in a general ad-hoc network model. Our solution consists of channel estimation and prediction, transmission rate selection, and threshold adaptation. We show that an efficient threshold-based solution will need to employ a dynamic load- based threshold adaptation. Moreover, we show that a distributed MAC algorithm requires additional overhead for time and user diversity control and thus need appropriate tradeoff design to maximize the system capacity. We conduct network simulations to evaluate the performance of our proposed protocol and demonstrate promising throughput increase via time and user diversity. We conclude that the achievable diversity gain in MANET highly depends on the network topology and heterogeneity of link quality and thus remains as a challenging task. Index Terms—medium access control, mobile ad-hoc networks, time and user diversity, 802.11, distributed algorithms I. INTRODUCTION Mobile Ad-Hoc Network (MANET) has become an emerging communication technology for commercial and military networks. Different from cellular wireless networks, MANET has a number of unique characteristics including 1)rapid and unpredictable changes in topology and connectivity; 2) lack of a centralized controller, i.e., a decentralized network; 3) multi-hop transmission for a data packet between a source and destination. These main characteristics have brought many research challenges in the Q. Zhang, L. Benmohamed and I. Wang are with Milton S. Eisenhower Research Center at Johns Hopkins University, Applied Physics Laboratory, Laurel, Maryland, USA. E-mails: qinqing.zhang@jhuapl.edu , lotfi.benmohamed@jhuapl.edu , I-jeng.wang@jhuapl.edu . design of efficient and effective mobile ad-hoc networks to meet the diverse application and deployment requirements. Many of the resource management algorithms and medium access control (MAC) protocols are designed for a centralized system where an anchor node (such as a base station in cellular systems or access point in wireless LANs) manages and allocates resources to mobile nodes in order to make efficient use of the radio resources. However, in a MANET environment, the lack of a central control entity makes medium access control and scheduling a challenging task. There is a need to design and develop efficient MAC and resource management algorithms in a distributed manner to maximize the network throughput and meet the application- level QoS requirements. The time varying channel condition in MANET is a significant challenge for network design. Advances in wireless communications have created innovative techniques that take advantages of the time varying channel and mobility to improve the system capacity and performance [1]. One of the key techniques is to explore diversity gain in multiple dimensions, including frequency, spatial, time and user diversity. Exploiting these diversities requires developing appropriate methods and algorithms to gain knowledge of the channel by tracking and predicting the channel quality information. Improving the overall network capacity and performance requires developing appropriate methods and algorithms to gain knowledge of the user specific radio frequency (RF) condition and application traffic requirement. Innovative techniques in radio resource management have been designed for infrastructure based commercial wireless systems and networks [2][3]. In this paper, we propose to develop an intelligent MAC protocol with a design approach that focuses on the development of distributed algorithms to exploit time and user diversity to improve the network efficiency. We propose a novel adaptive threshold-based medium access control protocol that includes channel estimation and prediction, transmission rate selection, and threshold adaptation. Note that channel or frequency diversity, spatial diversity such as MIMO techniques, and multi-path diversity are additional dimensions that can be explored but are not within the scope of this paper. Over the recent years there has been some work on exploiting multi-user diversity in a distributed manner, in particular [4], [5], and [6]. In [4], the authors deal with