Sinem Coleri

Channel estimation techniques based on pilot arrangement in OFDM systems

Channel estimation techniques for OFDM systems based on a pilot arrangement are investigated. Channel estimation based on a comb type pilot arrangement is studied through different algorithms for both estimating the channel at pilot frequencies and interpolating the channel. Channel estimation at pilot frequencies is based on LS and LMS methods while channel interpolation is done using linear interpolation, second order interpolation, low-pass interpolation, spline cubic interpolation, and time domain interpolation. Time-domain interpolation is obtained by passing to the time domain by means of IDFT (inverse discrete Fourier transform), zero padding and going back to the frequency domain by DFT (discrete Fourier transform). In addition, channel estimation based on a block type pilot arrangement is performed by sending pilots in every sub-channel and using this estimation for a specific number of following symbols. We have also implemented a decision feedback equalizer for all sub-channels followed by periodic block-type pilots. We have compared the performances of all schemes by measuring bit error rates with 16QAM, QPSK, DQPSK and BPSK as modulation schemes, and multipath Rayleigh fading and AR based fading channels as channel models.

QoS aware adaptive resource allocation techniques for fair scheduling in OFDMA based broadband wireless access systems

A system based on orthogonal frequency division multiple access (OFDMA) has been developed to deliver mobile broadband data service at data rates comparable to those of wired services, such as DSL and cable modems. We consider the resource allocation problem of assigning a set of subcarriers and determining the number of bits to be transmitted for each subcarrier in OFDMA systems. We compare simplicity, fairness and efficiency of our algorithm with the optimal and proposed suboptimal algorithms for varying values of delay spread, number of users and total power constraint. The results show that the performance of our approach is appealing and can be close to optimal. We also consider another resource allocation scheme in which there is no fixed QoS requirements per symbol but capacity is maximized.

Traffic Measurement and Vehicle Classification with Single Magnetic Sensor

Wireless magnetic sensor networks offer a very attractive, low-cost alternative to inductive loops for traffic measurement in freeways and at intersections. In addition to vehicle count, occupancy and speed, the sensors yield traffic information (such as vehicle classification) that cannot be obtained from loop data. Because such networks can be deployed in a very short time, they can also be used (and reused) for temporary traffic measurement. This paper reports the detection capabilities of magnetic sensors, based on two field experiments. The first experiment collected a two-hour trace of measurements on Hearst Avenue in Berkeley. The vehicle detection rate is better than 99 percent (100 percent for vehicles other than motorcycles); and estimates of vehicle length and speed appear to be better than 90 percent. Moreover, the measurements also give inter-vehicle spacing or headways, which reveal such interesting phenomena as platoon formation downstream of a traffic signal. Results of the second experiment are preliminary. Sensor data from 37 passing vehicles at the same site are processed and classified into 6 types. Sixty percent of the vehicles are classified correctly, when length is not used as a feature. The classification algorithm can be implemented in real time by the sensor node itself, in contrast to other methods based on high scan-rate inductive loop signals, which require extensive offline computation. We believe that when length is used as a feature, 80-90 percent of vehicles will be correctly classified.

Lifetime analysis of a sensor network with hybrid automata modelling

In this paper, we focus on TinyOS, an event-based operating system for networked sensor motes. We show how to model TinyOS as a hybrid automata with HyTech and verify the correct operation of the system by using safety verification feature of HyTech. Since lifetime is an important metric for sensor nodes that are planned to be deployed once and unattended for long periods of time without maintenance, we perform power analysis of a sensor node by using trace generation feature of HyTech. Furthermore, we simulate a tree sensor network of TinyOS motes by using the programming language SHIFT to determine the lifetime of the network as a function of the distance from the central data collector.

Fault tolerant and energy efficient routing for sensor networks

The paper presents a fault tolerant routing algorithm that maximizes the lifetime of a sensor network by adjusting the number of packets traversing each node over multiple routes. An LP formulation gives the optimal single route. A distributed, iterative algorithm based on least cost path routing approximates the LP solution. Multiple path routing extends the iterative solution to increase resilience to link failures. Simulations show significant increase in network lifetime, and the tradeoff between the number of successful packet transfers and network lifetime for different multipath routing mechanisms.

Application of GPS to mobile IP and routing in wireless networks

We discuss how the location information provided by GPS can be used at different layers of the protocol stack of a mobile host. Our discussion focuses an application of the two GPS based networking protocols we have designed and implemented. The network topology we are considering is composed of fixed Internet with base station at the edges connected with an ad hoc networks of mobile nodes that are connecting information from sensors spread through a region. In the network, base stations and mobile nodes are GPS equipped. The idea is to leverage the information of the mobile and base station positions, obtained via the GPS, to improve performance of the ad hoc routing, to adaptively determine the appropriate base station capacity to be reserved strictly for handoffs, to collect data from sensors scattered in the medium and to inform mobiles about the prospective future location.

Fault tolerance and energy efficiency of data aggregation schemes for sensor networks

A sensor network is modelled as a platform to gather correlated data from sensor nodes and transfer the data to a sink node over a multi-hop network. Two coding strategies are analyzed: Slepian-Wolf coding and explicit communication. We show that while Slepian-Wolf coding is more energy efficient than explicit communication, it is less tolerant to link failures since decoding the encoded data from one node depends on data from other nodes. For the case of explicit communication, we propose multipath routing schemes based on finding k spanning trees such that all nodes remain connected to the sink after the failure of fewer than k links. We show that clustering schemes with large and small cluster size perform well for high and low correlation respectively even for the multipath case.

POSITION LEVERAGE SMOOTH HANDOVER ALGORITHM FOR MOBILE IP

Mobile IP is designed to support uninterrupted connectivity of mobile computers as they roam from place to place. We propose a fast intra-domain and inter-domain handoff scheme using the location of routers to meet the delay and packet loss requirements of real-time services. The scheme achieves an intelligent and dynamic neighborhood discovery and avoids the use of multicast addresses in intra-domain handoff. In addition, it decreases the registration time and distributes home agent data base dynamically in inter-domain handoff depending on how far from the home agent mobile host is. Simulation and implementation results for an interactive voice communication are presented.