ITR: Cross-Layer Optimization For 4G Wireless Networks: Heavy-Tailed Traffic, Multiuser Channels, and Pseudocells

University Of California-Santa Barbara, Santa Barbara CA

Investigators

Abstract

This project addresses some fundamental aspects of the theory and practice of wireless networking. An integrated approach, combiningphysical layer innovations with new protocols for medium access control and scheduling, while accounting for application requirements and transport protocol dynamics, is employed for solving the research problems that are identified. Two major research thrusts are considered. In the first research thrust, the concept of ``pseudocellular'' wireless networks, which combine the best features of cellular and ad hoc networks, is considered as a paradigm for plug-and-play fourth generation (4G) wireless networks. Such a flexible architecture is clearly critical for quick set-up of wireless networks in emergency situations, in which stationary, or perhaps even mobile, base stations are deployed at convenient (but not optimized) sites to serve both slow-moving and fast-moving users. However, it is also a key ingredient of our vision of achieving a quantum jump in wireless link speeds, by going beyond the current cellular frequency bands of 1-2 GHz to the large bandwidths available in frequency bands in the 10s of GHz. The path loss in such bands is high, forcing the use of a dense network of base stations on the one hand, and enabling more aggressive frequency reuse on the other. The focus of the research is to support a mix of user mobilities, and a mix of real-time and non real-time applications, over a packetized pseudocellular infrastructure. This setting differs from conventional cellular networks, in that the cell sizes are small, and cells may have substantial overlap. It differs from wireless Local Area Networks (WLANs), in that it allows for rapidly mobile users despite the small cell sizes. Instead of a conventional hierarchical structure (i.e., large cells for fast-moving users, overlaid on small cells for slow-moving users) to deal with a range of mobility, a mobile-centric approach, which combines handoffs and reservation-based medium access control, is considered to allow for flexible deployment. A novel idea to be investigated is the support of priorities on the reservation channel, so as to allow, for example, highly mobile users with real-time calls in progress to rapidly reserve resources when entering a new pseudocell, thus implicitly achieving a handoff. Another important issue is transceiver optimization of the reservation channel, which requires solution of new problems in multiuser communications. The second research thrust is motivated by the well-known observation that Internet traffic has a heavy-tailed distribution, which typically calls for more conservative resource provisioning than for traditional Markovian traffic models. Since overprovisioning is unattractive in resource-constrained wireless environment, the approach considered is toemploy a new Quality of Service (QoS) framework that allows foraggressive resource utilization, by serving the bulk of the transactions (which are short) rapidly, and penalizing the small fraction of long transactions that contribute to the heavy tails. Scheduling disciplines that achieve this goal are very different from popular round robin or fair queueing schedulers, and were considered in the queueing theory literature more than three decades ago. The implication of these results for heavy-tailed Internet traffic is explored for the first time (to the best of our knowledge) in this project. The scheduling strategy is extended to a shared wireless channel, where fairness is traded off against system efficiency, with the latter dictating that users seeing the best channels are the ones that should get link access. The tradeoff is expected to be biased towards efficiency in order to support heavy-tailed traffic effectively. The interaction between scheduling and the dynamics of TCP connections (TCP is the Internet data transport protocol on top of which most transactions run) is explored, keeping in mind that a TCP connection that is starved of network resources can get locked out of the network due to repeated timeouts and rate cutbacks. Finally, the dependence of scheduling on mobility is explored, with the concept of assigning priority to highly mobile users (who have a smaller chance of getting access to the link during their sojourn in a given pseudocell), while keeping overall QoS and fairness in mind. The scheduling methods we develop place a high importance on overall system efficiency, and are therefore well-suited to flat rate pricing, which is arguably an effective mechanism of promoting usage growth in wireless data networks.