Jianliang XU, Dik L. Lee, and Bo Li

Dept. of Computer ScienceHong Kong Univ. of Science & Technology

April 2002

On Bandwidth Allocation for Data Dissemination in Mobile Cellular Networks

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Outline

• Introduction

• Problem formulation

• Bandwidth allocation techniques

• Performance evaluation

• Wrap up

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• On-Demand Access– A client sends data requests uplink to

the server, and the server returns the

results to the client individually

– Fast response for a light-load system

Wireless Data Dissemination

• Data Broadcast– The server periodically broadcasts

info to the entire client population, and the

clients monitor the broadcast channel

to retrieve the data of their interest

– Scale to an unlimited client population

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Bandwidth Allocation – Previous Work

• Data dissemination– Band allocation among data items [Acharya et al., SIGMOD’95;

Hameed and Vaidya, ACM WINET]

– Band allocation between on-demand access and data broadcast [Acharya et al., SIGMOD’97; Lee et al., ACM MONET]

– Confined to a single-cell environment

• Voice communications– Pretty many studies [Oh & Tcha, IEEE ToC; Zhang & Yum, IEEE

ToVT; Li et al., ACM WINET; etc.]

– Minimize call blocking/dropping prob. or improve carried traffic while ensuring QoS

– Different objective with data dissemination (access delay)

• Our study: data dissemination & multi-cell

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A Motivating Example

• Cellular: 2 cells; 6 Kbps shared bandwidth• Database: 4 items, each of 1K bits• Data access rate: Cell A: 1 Cell B: 4• Flat broadcast

Band Alloc(Kbps) Exp. Latency(s) Overall Exp.

Latency(s)Cell A Cell B Cell A Cell B

Uniform 3.0 3.0 1.0 1.0 1.0

Proportional 1.2 4.8 2.5 0.625 1.0

Best 2.0 4.0 1.5 0.75 0.9

1 2 3 4

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Bandwidth Allocation Problem

Input parameters

Traffic pattern for each cell (long-term steady state) Total amount of bandwidth Frequency reuse pattern

Problem: how much bandwidth is assigned to each cell Objective: to minimize the overall access latency for a

multi-cell wireless data dissemination system

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Definitions

Min reuse distance: the min. distance at which frequencies can be reused at acceptable interference

Interference cluster: a maximal subset of cells which are within the distance of mutual interference

1

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3R

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Cost Model• Latency for on-demand access

• Latency for data broadcast

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Notationsbi amount of band to cell i data access rate for cell ili average item size for cell i

li,j size of item j in cell isi, j space distance between instances of item j in cell ili, j size of item j in cell i

pi, j access prob for item j in cell i

Mi num of items in cell i’s db

N num of cells in the systemNb num of cells using broadcast

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Problem Formulation

• Optimal bandwidth allocation problem

Find out (b1, b2, …bN)

Minimize

Subject to for any interference cluster Q

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• An interference cluster: Nc cells

• Problem formulation

Find out (b1, b2, …bc)

Minimize

Subject to BbcN

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• A constrained-minimum problem

Optimal Allocation for Interference Cluster

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Optimal Allocation for Interference Cluster (cont’d)

• Solved the optimization problem using the Lagrange

multiplier theorem

Theorem 1 The min overall expected latency is achieved when the bandwidth allocated to cell i, is given by

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Allocation Techniques for Cellular Networks

• Challenges– Frequency reuse – non-uniform traffic loads

• Heuristic allocation techniques– Compact allocation

– Cluster-step allocation

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Compact Allocation

• Equivalence class: the cells at min reuse distance• Assign the same bandwidth to all the cells in an

equivalence class• Reduced problem: bandwidth allocation for a cluster

consisting of the cells with one from each equivalence class– Take the average parameter values

– Apply the optimal allocation technique

– Favor homogeneous loads for the cells in an equivalence class

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Cluster-Step Allocation

• For all possible interference clusters, assign bandwidth one cluster after one depending on their importance– Importance is determined by the aggregate deserved

allocation factor ( for broadcast and for on-demand access)

– For each cluster only unassigned cells are considered: the total bandwidth is a min of B1 and B2

• B1: remaining available bandwidth for the cluster

• B2 : deserved allocated bandwidth for the unassigned

cells

– Apply the optimal allocation technique for each cluster

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Simulation Setup

• Database size: 1,000 data items• Total available bandwidth: 672 Kbps • Mobile cellular network: 7 × 7 cells • Min reuse distance: • Access rates

• RAND: uniformly distributed between 1 and 50

• HOMO: for the cells in equivalence class i, uniformly distributed between 1 and 2i+1

• HETERO: uniformly distributed between 1 and 10 except some have 100

R21

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0

5

10

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Exp

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d A

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RAND HOMO HETERO

Uniform Alloc

Compact Alloc

Cluster-step Alloc

Numerical Result – Data Broadcast

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1.E-01

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1.E+04

Exp

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RAND HOMO HETERO

Uniform Alloc

Compact Alloc

Cluster-step Alloc

Numerical Result – On-Demand Access

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1.E-01

1.E+00

1.E+01

1.E+02

1.E+03

1.E+04

Exp

ecte

d A

cces

s La

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y (s

)

RAND HOMO HETERO

Uniform Alloc

Compact Alloc

Cluster-step Alloc

Numerical Result – Hybrid Access

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• Limitations

- On-demand access with an M/M/1 model

- Bandwidth allocation granularity

• Formulated the band allocation problem for a wireless data dissemination system

• Analyzed the optimal bandwidth allocation technique for an interference cluster

• Proposed two heuristics for bandwidth allocation in a mobile cellular network

• Numerical results showed the superiority of the proposed solutions

Wrap Up …