# Reception and Poster Session

Monday, June 27, 2005 - 5:00pm - 6:30pm

Lind 400

**Reduction of Complexity for Systems Involving A Discrete-time Markov Chain: Time-scale Separation**

George Yin (Wayne State University)Qing Zhang (University of Georgia)

Many problems in wireless communications involve a

discrete-time Markov chain. Often, the state space of the Markov

chain is inevitably large. To reduce the computation complexity

becomes a practical concern. This poster presents a brief summary on

recent developments for such systems. To achieve the goal of complexity

reduction, time-scale separation and singular perturbation techniques are

used in the modeling and analysis. Asymptotic expansions of probability

vectors and structural properties of the Markov chain are provided

together with near optimality in related control problem and Markov

decision processes.**Temporal Properties of Congestion Events in Networks with**

Fractional Brownian Traffic

Yasong Jin (University of Kansas)

In packet networks congestion events tend to persist, producing large

delays and long bursts of consecutive packet loss resulting in perceived

performance degradations. The length and rate of these events have a

significant effect on network quality of service (QoS). The packet delay

resulting from these congestion events also influences QoS. A technique

for predicting these properties of congestion events, such as, the

inter-congestion events time and the congestion duration, is developed in

the presence of fractional Brownian Motion (fBM) traffic.**Particle Filtering in Wireless Communications**

Alain Tchagang (University of Minnesota, Twin Cities)

With the increase use of wireless communication systems, the demand for

fast and reliable filtering algorithm capable of coping with difficult

transmission conditions is becoming more and more prevalent. Physical

limitations and impairments of wireless channels, including multi-access

and co-channel interference, time-variation and frequency selectivity,

present a major technical challenge to the reliable transmission over a

wireless link. The task can be greatly facilitated by the use of an

efficient signal processing technique. In this poster, we present an

overview and the application of particle filtering to the problems

associated with transmission in wireless communications.**High-Throughput Random Access Via PHY-Layer Opportunism and Interference**

Cancellation

Yingqun Yu

We consider two cross-layer design examples of high-throughput random

access schemes via PHY-layer opportunism and interference cancellation.

The first example is the so-called channel-aware slotted Aloha, which

can exploit fading by effecting decentralized multi-user diversity. For

continuous channels with analog amplitude, we prove the optimality of

binary scheduling in terms of sum-throughput for homogeneous systems and

in terms of sum-log-throughput for heterogeneous systems. For finite

state Markov chain (FSMC) channels, we provide a convex formulation of

the corresponding throughput optimization problem, and derive a simple

binary-like access strategy.

The second example is SICTA, a contention tree algorithm with

successive interference cancellation (SIC). The conventional random

access algorithms, including tree algorithms (TAs), have relatively low

throughput, since collided packets are typically discarded when packet

collisions happen. We develop a novel protocol exploiting successive

interference cancellation in a tree algorithm, where collided packets

are reserved for reuse. We show that SICTA with binary splitting and

gated access can achieve a throughput of 0.693, which represents about

40% increase in throughput over the renowned 0.487

first-come-first-serve (FCFS) tree algorithm.**Effective capacity approach to providing statistical**

quality-of-service guarantees in wireless networks

Dapeng Wu (University of Florida)

The next-generation wireless networks are targeted at supporting

various applications such as voice, data, and multimedia with diverse

quality of service (QoS) requirements. To provide explicit QoS

guarantees such as a data rate, delay bound, and delay-bound violation

probability triplet, it is necessary to analyze a QoS provisioning

system in terms of these QoS measures. This task requires

characterization of the service (channel modeling), and queueing

analysis of the system. However, the existing channel models such as

finite-state Markov chain models, do not explicitly characterize a

wireless channel in terms of these QoS measures. This leads to high

complexity in characterizing the relation between the control

parameters of QoS provisioning and the calculated QoS measures.

Recognizing that the above difficulty is the lack of a channel model

that can easily relate the control parameters of a QoS provisioning

system to the QoS measures, we propose and develop a link-layer

channel model termed the effective capacity (EC) model. The EC model

captures the effect of channel fading on the queueing behavior of the

link, using a computationally simple yet accurate model, and thus, is

a critical tool for designing efficient QoS provisioning mechanisms.

Such an approach to channel modeling and QoS provisioning, is called

effective capacity approach. In this poster, I will present our

effective capacity approach to channel modeling.**A Charged Van der Waals Equation of State and Order-Decreasing Lattice-Path Mappings**

Dov Chelst (DeVry University)

One can successfully modify a standard derivation of a van der Waals

equation of state (see Lebowitz and Penrose, Journal of Math. Phys.

1966) to accomodate a system of charged particles. Along the way,

significant modifications to the original arguments must be made to

accomodate the long-range nature of the Coulomb potential. Particularly,

in a one-dimension two-component plasma, a purely combinatorial result

regarding lattice path mappings proves useful. I will define the term

order-decreasing mapping, to explain the nature of the combinatorial

result and its connection to the original statistical mechanical

problem. In addition, I will discuss attempts at generalizing the

mathematical result in order to accomodate a wider class of physical

systems.**Wireless Local Area Network Simulation**

Martin Eiger (Telcordia)

We describe a software system that simulates wireless local area networks supporting heterogeneous services and multiple

protocols. We present applications of this system in three areas: analysis of voice capacity, maximization of data throughput while

protecting voice quality of service, and the design and evaluation of scheduling algorithms in a polling-based system.**Bandwidth Guaranteed Provisioning in Network-Based Mobile VPNs**

Katherine Guo (Alcatel-Lucent Technologies Bell Laboratories)

Provision of VPN services to mobile customers is a revenue

generating opportunity for Network Service Providers (NSPs). To

provide network-based Mobile-VPN services, the NSPs terminate the

VPN sessions from remote mobile users within the NSP network and

aggregate VPN traffic destined to the same enterprise intranet

onto one or a few VPN sessions from within the network to the

enterprise VPN gateway. In between, value-added services are

provided for this customer traffic.

We propose the use of a hierarchical network

architecture to efficiently realize network-based Mobile VPNs. The

hierarchical architecture consists of three main components:

a) the Mobile Access Point (MAP), b) the IP Services

Gateway (IPSG), and c) the enterprise VPN gateway or the

Customer Premise Equipment (CPE). We address the problem of

provisioning customers on the IP Service Gateways (IPSGs) in a

bandwidth constrained network such that the profit realized by the

NSP by delivering Mobile-VPN services is maximized. The parameters

considered in the optimization problem include the cost of

provisioning customers on the IPSGs, the bandwidth capacity of

links on the network, the bandwidth cost and the revenue that a

NSP realizes from a customer.

We derive an integer programming formulation for the problem and

provide solutions for a few practical cases. The results provide

insights into the design of Mobile-VPN service architectures and

illustrates the different trade-offs that are involved.**Capacity-approaching LDPC Codes Based on Markov Chain Monte Carlo MIMO Detection**

Rong-Rong Chen (The University of Utah)

We study joint code design and MIMO (multiple-input multiple-output)

detection based on Markov Chain Monte Carlo (MCMC) approach. The MCMC

detector is highly effective for large antenna systems because it has a

much lower complexity than traditional MIMO detectors including the sphere

decoding detector. We apply the extrinsic information transfer (EXIT)

technique to find optimal irregular LDPC codes that are matched to the

characteristics of the MCMC detector. For a large system of 8 transmit and

8 receive antennas and 16 QAM modulation, the optimized LDPC code achieves

within 1.4 dB of the channel capacity. This result improves best published

results by 2.3 dB, where 1.3 dB is due to the MCMC detection and 1 dB is

due to code optimization.**Capacity-approaching LDPC Codes Based on Markov Chain Monte Carlo MIMO Detection**

Rong-Rong Chen (The University of Utah)

We study joint code design and MIMO (multiple-input multiple-output)

detection based on Markov Chain Monte Carlo (MCMC) approach. The MCMC

detector is highly effective for large antenna systems because it has a

much lower complexity than traditional MIMO detectors including the sphere

decoding detector. We apply the extrinsic information transfer (EXIT)

technique to find optimal irregular LDPC codes that are matched to the

characteristics of the MCMC detector. For a large system of 8 transmit and

8 receive antennas and 16 QAM modulation, the optimized LDPC code achieves

within 1.4 dB of the channel capacity. This result improves best published

results by 2.3 dB, where 1.3 dB is due to the MCMC detection and 1 dB is

due to code optimization.**On the optimum power allocation in interference-free cooperative systems**

Paul Anghel (University of Minnesota, Twin Cities)

We consider an interference-free fixed wireless cooperative system where

transmissions from different terminals are orthogonal. In the proposed

system the idled terminals help a source by relaying its information signal

to the destination. We consider both regenerative and non-regenerative idled

terminals (relays). For the inference-free system with non-regenerative

relays, we show that maximizing mutual information for each user under total

power constraint is a strictly quasi-concave maximization problem whose

global optimal solution can be found efficiently. In fact, we provide a

simple analytic formula for the optimum power allocation strategy, which is

reminiscent of the water-filling principle. We also illustrate how the setup

can be extended in order to allow for terminal mobility. For the

regenerative system we consider two cases - full collaboration between

relays and no-collaboration between relays - and show that in both cases the

maximum mutual information under total power constraint can be found in

polynomial-time.**Optimizing Performance Metrics for Cellular Networks**

John Hobby (Alcatel-Lucent Technologies Bell Laboratories)

We have developed an interactive tool for modeling and optimizing

various types of cellular networks including CDMA, GSM, UMTS, EV-DO.

Modeling network coverage and capacity as smooth, differentiable

functions of parameters such as antenna tilts and azimuths

facilitates efficient optimization even for large networks.

The model involves explicit probability computations for effects

such as shadow fading, and it includes steady-state approximations

to dynamic processes such as the effect of the add/drop timer.

Reverse-link power control and power amplifier sharing lead to

highly-coupled systems that are nontrivial to solve and make the

analytical derivative functions especially challenging.**Reducing energy consumption for power-constrained mobiles**

Eric van den Berg (Telcordia)

We present a new method to increase the lifetime of energy constrained

mobile devices. Each mobile node powers off its radio interface during

time periods where it does nog expect to originate, receive or relay

traffic. Nodes make autonomous decisions on whether and when to power down

and wake up, without signaling messages to/from other nodes.

We present results to show that the proposed method can significantly

increase the lifetime of energy constrained nodes.**Cooperation vs. compression for sensor networks**

Chulhan Lee (The University of Texas at Austin)

In a sensor network, nodes share information about their

observations, and the amount of

shared information can often be substantial. This paper compares two

different strategies

that exploit this redundant information - compression and cooperation. It

finds compression

to be the winning strategy when energy savings is the most important

objective, but finds cooperation to be the throughput maximizing strategy. A

weighted optimization problem

is formulated to obtain intermediate schemes that allow a critical level of

cooperation in the system, thereafter compressing all other redundant

information.**High SNR analysis of MIMO broadcast channels**

Nihar Jindal (University of Minnesota, Twin Cities)

The behavior of the multiple antenna

broadcast channel at high SNR is investigated.

The sum rate capacity (achievable by dirty paper coding),

the sum rate achievable using transmitter beamforming,

and an upper bound to the sum rate capacity are studied.

Though these three terms are equivalent in the sense of

the multiplexing gain, i.e. in terms of first order growth, there

is an absolute difference between the corresponding rates.

These difference terms are calculated and

simple and intuitive geometrical interpretations are given.**Power Requirements in CDMA Networks with Multiple Classes**

of Traffic

Komandur Krishnan (Telcordia)Arnie Neidhardt (Telcordia)

We propose methods for calculating downlink power-requirements in

a CDMA network that supports multiple classes of traffic. In

providing for fluctuations of transmitter-power requirements in

response to traffic-rate fluctuations, we make use of the notion

of an effective power of a transmitter, analogous to the notion

of an effective bandwidth of a traffic source. The methods

consist of perturbation analysis by means of an expansion about

approximate average values of transmitter-powers.**Random multi-access protocols - A mean field analysis**

Alexandre Proutiere (France Telecom R&D, LLC)

Joint work with Charles Bordenave (ENS, Paris) and David McDonald (University of Ottawa, Canada).

Although random multi-access protocols, such as the Decentralized

Coordination Function (DCF) in IEEE 802.11-based networks, are widely

used, their performance remains largely unknown. This is due to the

fact that the inherent interactions between competing sources have

proven to be extremely complex to model and analyze. A very popular

approach to circumvent this difficulty consists in decoupling the

source behaviors; i.e. assuming that the evolutions of the back-off

processes of the different sources are mutually independent. This

assumption allows one to derive explicit estimates of

the performance. This approach was applied by Bianchi (IEEE trans. on

Wireless Comm. 2000) to analyze the DCF protocol and since then has

been widely used to accurately predict the performance of similar

protocols. In this work, using mean field techniques, we prove that,

for a wide range of random multi-access protocols, the decoupling

assumption is asymptotically exact as the number of competing sources

grows. In the specific case of the DCF, the mean field analysis

provides the transient and the stationary distributions of the backoff

processes.**Achieving Wireline Random Access Throughput in Wireless Networking via User Cooperation**

Georgios Giannakis (University of Minnesota, Twin Cities)

Well appreciated at the physical layer, user cooperation is introduced here

as a diversity enabler for wireless random access (RA) at the medium access

control sub-layer. This is accomplished through a two-phase protocol in

which active users start with a low power transmission attempting to reach

nearby users, and follow up with a high power transmission in cooperation

with the users recruited in the first phase. We show that such a

cooperative protocol yields a significant increase in throughput.

Specifically, we prove that for networks with a large number of users, the

throughput of a cooperative wireless RA network operating over Rayleigh

fading links approaches the throughput of a RA network operating over

additive white Gaussian noise (AWGN) links. As a result, user cooperation

migrates diversity benefits to the wireless RA regime, thus bridging the

gap to wireline RA networks, without incurring a bandwidth or energy

penalty.