Jonathan R. Agre

Proportional fair scheduling algorithm in OFDMA-based wireless systems with QoS constraints

In this work we consider the problem of downlink resource allocation for proportional fairness of long term received rates of data users and quality of service for real time sessions in an OFDMA-based wireless system. The base station allocates available power and subchannels to individual users based on long term average received rates, quality of service (QoS) based rate constraints and channel conditions. We formulate and solve a joint bandwidth and power optimization problem, solving which provides a performance improvement with respect to existing resource allocation algorithms. We propose schemes for flat as well as frequency selective fading cases. Numerical evaluation results show that the proposed method provides better QoS to voice and video sessions while providing more and fair rates to data users in comparison with existing schemes.

Location enhancement to IEEE 802.11 DCF

In this paper, we propose an enhancement to the existing IEEE 802.11 distributed coordination function (DCF) MAC to improve channel spatial reuse efficiency, and thus improve overall network data throughput. Our modification, named the location enhanced DCF (LED) for IEEE 802.11, incorporates location information in DCF frame exchange sequences so that stations sharing the communication channel are able to make better interference predictions and blocking assessments. Utilizing an underlying physical layer design that supports frame capture, the LED enhanced interference estimation can increase overall network data throughput by permitting more concurrent transmissions. In this paper we also analytically study the potential performance enhancement of the LED over the original IEEE 802.11 DCF. The results are verified using the ns-2 simulator, which shows that up to 35% of DCF blocking decisions are unnecessary and our LED method can achieve up to 22% more throughput than the original DCF.

A layered architecture for location-based services in wireless ad hoc networks

The design of a location services module (LSM) that provides a foundation for location-aware services is described. The architecture enhances application software development by providing a simple interface that hides details of the particular underlying location determination technologies. Functions for multiple technology switching, error estimation, location tracking, multiple technology fusion and cooperative location determination are defined. Several self-organizing protocols for cooperative location determination algorithms appropriate for wireless ad hoc networks are described. An example implementation of location-based services for a world wide web browser using an LSM is provided.

A queueing network approach to the module allocation problem in distributed systems

Given a collection of distributed programs and the modules they use, the module allocation problem is to determine an assignment of modules to processors that minimizes the total execution cost of the programs. Standard approaches to this problem are based on solving either a network flow problem or a constrained 0-1 integer programming problem. In this paper we discuss an alternative approach to the module allocation problem where a closed, multiclass queueing network is solved to determine the cost of a particular module allocation. The advantage of this approach is that the execution cost can be expressed in terms of performance measures of the system such as response time. An interchange heuristic is proposed as a method of searching for a good module allocation using this model and empirical evidence for the success of the heuristic is given. The heuristic normally finds module allocations with costs within 10 percent of the optimal module allocation. Fast, approximate queueing network solution techniques based on mean-value-analysis allow each heuristic search to be completed in a few seconds of CPU time. The computational complexity of each search is O (M K (K + N) C) where M is the number of modules, K is the number of sites in the network, N is the number of communications processors, and C is the number of distributed program types. It appears that substantial problems of this type could be solved using the methods we describe.

Task computing for ubiquitous multimedia services

The Task Computing framework is designed to operate in dynamic ubiquitous environments in which a mobile computing user dynamically discovers the current set of available semantically defined services. Task Computing allows the end-user to easily and seamlessly integrate and manipulate services found on their own computer, the nearby devices and relevant remote web services. The user can create, execute and monitor complex tasks resulting from compositions of these services. In this paper, how Task Computing addresses some issues in mobile and ubiquitous multimedia applications and some of the multimedia related semantic services currently implemented in this framework are described.

Optimal radio resource management in multihop relay networks

Multihop relay networks (MR) use relay stations (RS) to extend or enhance the coverage of a base station (BS) in a cellular network. The base station is attached to a wired backhaul. Relay stations use wireless transmission to connect to the base station and to the mobile stations (MS), while direct BS-MS connection is also possible. Low cost relay technologies are recently proposed for OFDM based broadband wireless technologies. Especially for low-cost relay stations that have single radio interface how to share the channels and allocate proper amount of bandwidth/power to the base station (BS), relay stations (RSs) and the mobile stations (MSs) is an important issue. This work develops methods for radio resource management in MR networks supporting heterogeneous traffic. We propose a scheduling method, where the transmission of base station to each relay station and transmissions of each relay station to its respective mobile stations (i.e. composite links) are scheduled in a TDMA fashion and the resource allocation in those microcells are performed in an OFDMA manner. We numerically evaluate the performance of our proposed scheme with respect to a number of parameters.

Modeling reentrant and nonreentrant software

A description of software module models for computer systems is presented. The software module models are based on a two level description, the software level and the hardware level, of the computer system. In the software module level it is possible to model performance effects of software traits such as reentrant and nonreentrant type software modules. The resulting queueing network models are, in general, not of the product form class and approximation schemes are employed as solution techniques. An example of a software module model of a hypothetical computer system is presented. The model is solved with a simulation program and three approximation schemes. The approximation results were compared with the simulation results and some schemes are found to produce good estimates of the effects of changing from reentrant to non-reentrant software modules.

Enhancing 802.11 Wireless Networks with Directional Antenna and Multiple Receivers

When directional antennas are used in 802.11 based wireless LANs, higher network capacity is often accompanied with more collisions. This is due to the enhanced hidden node problem and the deafness problem that arise in the directional transmission/reception scenario. These problems are caused by an inconsistent view of the medium status by neighboring nodes. We have developed a new variation to the 802.11 protocol called sectorized MAC (S-MAC), which employs a novel architecture consisting of multiple directional antennas and multiple receivers. With a new self-interference cancellation scheme, an S-MAC node can continuously monitor the channel status in all directions in order to avoid the aforementioned problems. It is fully compatible with the standard 802.11 protocol and can inter-operate with omni-antenna-based 802.11 nodes. It is applicable to both infrastructure mode and ad hoc mode. Simulation studies show that it achieves significant capacity gains, even if only used in part of the network.

Crucial Differences between Commercial and Military Communications Technology Needs: Why the Military Still Needs Its Own Research

Military establishments constitute a relatively much smaller part of the wireless communications market than they once did, and the commercial sector is now a primary driver of communications technology development. However, there are crucial differences between military and commercial communications needs, especially at the tactical edge. First, commercial wireless services depend on a well-maintained and reliable infrastructure, whereas military operations may require communications between mobile platforms and command posts in areas without such an infrastructure. Second, military communications tend to involve multihop networks, and the theoretical foundations of such networks are not as well established as those involving single pairwise links. Third, military communications can involve multiple heterogeneous networks operating in the same area. While commercial networks can also be heterogeneous, commercial systems do not need to deal with the same extent of temporal and spatial variability. Connecting across heterogeneous networks in a dynamic environment remains a significant challenge. Fourth, military systems must also deal with highly complex and contested electromagnetic environments. They must confront adversarial action such as jamming and electronic warfare, as well as the benign complexity arising from the proliferation of computers, sensors, and radios. They must deal with low and intermittent bandwidth. They must also contend with the continuing loss of spectrum previously dedicated to military use. If we combine all these factors with the more commonly observed ones such as the need for ruggedness, security, and stealth, we see that much research and development, unlikely to be undertaken by the commercial sector, is still needed in the area of military communications. While military establishments can and do make use of commercial technology, they will continue to require purpose-driven research and development.

Practical Resource Allocation Algorithms for QoS in OFDMA-Based Wireless Systems

In this work we propose an efficient resource allocation algorithm for OFDMA based wireless systems supporting heterogeneous traffic. The proposed algorithm provides proportionally fairness to data users and short term rate guarantees to real-time users. Based on the QoS requirements, buffer occupancy and channel conditions, we propose a scheme for rate requirement determination for delay constrained sessions. Then we formulate and solve the proportional fair rate allocation problem subject to those rate requirements and power/bandwidth constraints. Simulations results show that the proposed algorithm provides significant improvement with respect to the benchmark algorithm.