Theodore Antonakopoulos

CSMA/CA performance under high traffic conditions: throughput and delay analysis

The IEEE802.11 standard for wireless local area networks is based on the CSMA/CA protocol for supporting asynchronous data transfers. CSMA/CA uses an acknowledgment mechanism for verifying successful transmissions and optionally, a handshaking mechanism for decreasing collisions overhead. In both cases, an exponential backoff mechanism is used. This work investigates the theoretical performance of both mechanisms in terms of throughput and delay under traffic conditions that correspond to the maximum load that the network can support in stable conditions. We present extensive numerical results in order to highlight the effect of the backoff mechanism parameters on network performance for both mechanisms.

Millipede: a MEMS-based scanning-probe data-storage system

Ultrahigh storage densities of up to 1 Tbit/in./sup 2/ or more can be achieved by local-probe techniques to write, read back, and erase data in very thin polymer films. The thermomechanical scanning-probe-based data-storage concept called Millipede combines ultrahigh density, small form factor, and high data rate. After illustrating the principles of operation of the Millipede, we introduce system aspects related to the read-back process, multiplexing, and position-error-signal generation for tracking.

Multipath characterization of indoor power-line networks

The time- and frequency-varying behavior of an indoor power-line network is the result of variable impedance loads connected to its termination points. In fact, any signal transmitted through such a communications network is subject to time-varying multipath fading. In this paper, an analytical calculation method is presented, which can be used to determine the multipath components of any point-to-point channel in the indoor power-line environment. The method calculates all transmission characteristics of the network and, therefore, it can be exploited in the process of designing proper transmission algorithms for optimizing system performance. The proposed method is applied to an example network to demonstrate its usefulness in explaining the networks time-dependent behavior and in estimating channel parameters, such as subchannel bandwidth, multipath delay spread, fading conditions, etc.

Probe-based ultrahigh-density storage technology

Ultrahigh storage densities can be achieved by using a thermomechanical scanning-probe-based data-storage approach to write, read back, and erase data in very thin polymer films. High data rates are achieved by parallel operation of large two-dimensional arrays of cantilevers that can be batch fabricated by silicon-surface micromachining techniques. The very high precision required to navigate the storage medium relative to the array of probes is achieved by microelectromechanical system (MEMS)- based x and y actuators. The ultrahigh storage densities offered by probe-storage devices pose a significant challenge in terms of both control design for nanoscale positioning and read-channel design for reliable signal detection. Moreover, the high parallelism necessitates new dataflow architectures to ensure high performance and reliability of the system. In this paper, we present a small-scale prototype system of a storage device that we built based on scanning-probe technology. Experimental results of multiple sectors, recorded using multiple levers at 840 Gb/in2 and read back without errors, demonstrate the functionality of the prototype system. This is the first time a scanning-probe recording technology has reached this level of technical maturity, demonstrating the joint operation of all building blocks of a storage device.

Bit and power allocation in constrained multicarrier systems: the single-user case

Multicarrier modulation is a powerful transmission technique that provides improved performance in various communication fields. A fundamental topic of multicarrier communication systems is the bit and power loading, which is addressed in this article as a constrained multivariable nonlinear optimization problem. In particular, we present the main classes of loading problems, namely, rate maximization and margin maximization, and we discuss their optimal solutions for the single-user case. Initially, the classical water-filling solution subject to a total power constraint is presented using the Lagrange multipliers optimization approach. Next, the peak-power constraint is included and the concept of cup-limited waterfilling is introduced. The loading problem is also addressed subject to the integer-bit restriction and the optimal discrete solution is examined using combinatorial optimization methods. Furthermore, we investigate the duality conditions of the rate maximization and margin maximization problems and we highlight various ideas for low-complexity loading algorithms. This article surveys and reviews existing results on resource allocation in constrained multicarrier systems and presents new trends in this area.

A new computationally efficient discrete bit-loading algorithm for DMT applications

This letter presents a new bit-loading algorithm for discrete multitone systems that converges faster to the same bit allocation as the optimal discrete bit-filling and bit-removal methods. The algorithm exploits the differences between the subchannel gain-to-noise ratios in order to determine an initial bit allocation and then performs a multiple-bits loading procedure for achieving the requested target rate. Numerical results using asymmetric digital subscriber test loops demonstrate the computational efficiency of the proposed algorithm.

The IEEE 802.11 Distributed Coordination Function in Small-Scale Ad-Hoc Wireless LANs

The IEEE 802.11 standards for wireless local area networks define how the stations of an ad-hoc wireless network coordinate in order to share the medium efficiently. This work investigates the performance of such a network by considering the two different access mechanisms proposed in these standards. The IEEE 802.11 access mechanisms are based on the carrier sense multiple access with collision avoidance (CSMA/CA) protocol using a binary slotted exponential backoff mechanism. The basic CSMA/CA mechanism uses an acknowledgment message at the end of each transmitted packet, whereas the request to send/clear to send (RTS/CTS) CSMA/CA mechanism also uses a RTS/CTS message exchange before transmitting a packet. In this work, we analyze these two access mechanisms in terms of throughput and delay. Extensive numerical results are presented to highlight the characteristics of each access mechanism and to define the dependence of each mechanism on the backoff procedure parameters.

Resource Allocation Management for Indoor Power-Line Communications Systems

Power-line communications (PLC) is recognized as an alternative technology for delivering broadband services in home and small-office environments. Multicarrier modulation is one of the most effective solutions for combating the imperfections of the power-line channel. One of the technical challenges in increasing the potentiality of PLC technology is the utilization of the available spectrum when different users compete for the available resources. Due to the frequency and time-varying characteristics of the power-line network, the various users experience different channel conditions, thus the application of efficient resource allocation will increase the overall network performance. This paper addresses the problem of subchannel and power allocation for indoor PLC networks with multiple links employing power spectral density limitations, and presents an efficient multiuser loading algorithm. The proposed solution encounters both uplink and downlink traffic, guarantees a set of minimum data rates to all users, and allocates the available resources with fairness according to the channel quality of each link. We demonstrate the performance of the proposed algorithm using an illustrative example of a typical indoor network and we examine its effectiveness under various channel conditions

Direction finding in IEEE802.11 wireless networks

A novel direction-finding method for stations of IEEE802.11 wireless local area networks is presented in this paper. The method uses a switched beam array for determining the direction of arrival of the incident electromagnetic field in a time efficient way and associates certain medium access control (MAC) layer functions with different radiation patterns of the switched antenna array, in order to determine the proper orientation of directional beams on both entities of a communication link. The application of the proposed method to an IEEE802.11 wireless network is presented and it is depicted how the method improves the network performance without requiring any modifications to the existing MAC protocol.

A new efficient access protocol for integrating multimedia services in the home environment

This paper presents a new dynamically adaptable medium access method that can be applied to home networks in order to create a distributed environment for multimedia and data processing services. The method uses a mechanism that detects silent periods in voice transmissions and dynamically adapts the bandwidth allocation between isochronous and asynchronous traffic.