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Two-Stage Dynamic Uplink Channel and Slot Assignment for GPRS

Author: Ying-Dar Lin, Yu-Ching Hsu, Mei-Yan ChiangReporter: Chen-Nien Tsai

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Outline

GPRS Background Introduction Two-Stage Dynamic Channel and

Slot Assignment Results Summary

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GPRS Background

GPRS Introduction GPRS Network Architecture GPRS Air Interface GPRS Logical Channels Mapping Logical Channels to

Physical Channels Packet Data Transfer Operations

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GPRS Introduction

GPRS: Stands for General (or generic) Packet

Radio Services developed by European

Telecommunication Standard Institute (ETSI)

is one of the standards of Global System for Mobile communications (GSM) Phase 2+

is designed as a packet switching system

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GPRS Network Architecture

It fits in with the existing GSM PLMN Two new network elements

Serving GPRS Support Node (SGSN) Gateway GPRS Support Node (GGSN)

Many new interfaces Gb, Gi, Gn, etc.

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GPRS Network Architecture

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GPRS Air Interface (1/3) GPRS uses the existing GSM resources. GPRS uses a two-dimensional access schem

e (FDMA and TDMA). Total 25 MHz bandwidth 125 carrier frequencies of 200 kHz bandwidth

GPRS users will share the same TDMA frame with GSM voice users.

GPRS air interface will dynamically allocate resources (timeslots).

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GPRS Air Interface (2/3) TDMA Frame

8 timeslots Period = 4.615 ms

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GPRS Air Interface (3/3)52 Multiframes

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GPRS Logical Channels

PDCH is the generic name for the physical channel allocated to carry packet logical channels

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Mapping Logical Channels to Physical Channels (1/3) Logical channels are carried on

physical channels. Multiple logical channels can be

mapped onto the same physical channels.

Three possible combinations: PBCCH+PCCCH+PDTCH+PACCH+PTCCH PCCCH+PDTCH+PACCH+PTCCH PDTCH+PACCH+PTCCH

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Mapping Logical Channels to Physical Channels (2/3)

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Mapping Logical Channels to Physical Channels (3/3)

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Packet Data Transfer Operations

Before data can transfer GPRS Attachment PDP (Packet Data Protocol) context

activation. After these two steps, the mobile

can access the network, request resources, and send data.

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Uplink Packet Data Transfer Request resources

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Uplink Packet Data Transfer Fixed radio block allocation

According to bit map

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Uplink Packet Data Transfer

Dynamic radio block allocation USF (Uplink State Flag) is

attributed to each MS.

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Outline

GPRS Background Introduction Two-Stage Dynamic Channel and

Slot Assignment Results Summary

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Introduction

Two-stage assignment Stage-1: BS assigns several PDCHs to an

MS. Stage-2: BS selects one of the multiplexe

d MSs in a PDCH to use the radio resource. Objective of this paper

Load balance in stage-1 Good prediction in stage-2

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Outline

GPRS Background Introduction Two-Stage Dynamic Channel and

Slot Assignment Results Summary

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Two-Stage Dynamic Channel and Slot Assignment

Stage 1 Multiple PDCHs with the corresponding U

SFs are assigned to an MS. Stage 2

To utilize the radio resource, the BS has to predict who has data to send and the assign the following time slot to that MS.

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Stage-1 Channel Assignment After receiving the Packet Channel Request,

BS must decide the number of as well as which specific PDCHs to be assigned to the MS.

Deciding which PDCHs to assign is more critical. (load balance)

Two load measurement methods Number of Assigned Flow Effective transmission over last cycle

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Number of Assigned Flow (NoAF) The number of multiplexed MSs withi

n a PDCH is chosen as the load measurement metric.

This scheme can be considered a frequency-wise and PDCH-wise balance.

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Effective Transmission over Last Cycle (EToLC) The load metric employed by EToLC is

defined as the number of transmissions occurred during the previous PRR (Pure Round-Robin) cycle.

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Stage-2 Slot Assignment The BS has to predict who has data to

send. If the selected MS has no data

impending, the slot is wasted. Three schemes are considered:

Pure Round-Robin Round-Robin with Linearly-Accumulated

Adjustment Optimal

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Pure Round-Robin (PRR)

Each multiplexed MS in a PDCH is round-robined to use the uplink channel.

All MSs are assumed having impending data to send.

A PRR cycle equals the number of MSs multiplexed in this PDCH.

The highest mis-selection rate.

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Round-Robin with Linearly-Accumulated Adjustment (RRLAA)

Basis principle to reduce the transmission chance for th

e MSs that failed to utilize the last assigned slot, and increase the chance for those who had.

For RRLAA, a Penalty cycle and a Reward cycle are defined and appear alternately.

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Penalty Cycle (1/2) A Penalty cycle is derived from PRR cycle by

skipping MSs who waste their last assigned timeslots in Penalty cycles.

An MS will be skipped in n successive penalty cycles when it wastes n successive assigned timeslots in Penalty cycles.

When the MS begins to send packets, the penalty accumulation is reset.

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Penalty Cycle (2/2)M: number of multiplexed MS in a PDCH

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Reward Cycle (1/2) An MS is authorized to transmit during the

following Reward cycle if it transmits data in the previous Penalty cycle.

An MS will be rewarded n timeslots in a Reward cycle when it successively employs the assigned timeslots in n penalty cycles.

An MS will be selected to send data at most once in a Penalty cycle but possibly multiple times in a Reward cycle.

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Reward Cycle (2/2)Infinite loop?

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Optimal (OPT) Assume that whether an MS has data to

send or not is known in advance. For compare performance.

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Outline

GPRS Background Introduction Two-Stage Dynamic Channel and

Slot Assignment Results Summary

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Results

Comparison between Load Balancing Schemes for Stage-1 Channel Assignment

Comparison between Selection Schemes for Stage-2 Slot Assignment

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Comparison for Stage-1 Channel Assignment (1/3)

FNoP-NoAF is designed to be compared with other three models and is considered as the most load-balanced case.

RND: Random FNoP: Fixed Number of PDCH (?)

NoAF: Number of Assigned Flow

EToLC: Effective Transmission over Last Cycle

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Comparison for Stage-1 Channel Assignment (2/3) Standard deviation of PDCH utilization

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Comparison for Stage-1 Channel Assignment (3/3) System throughput

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Comparison for Stage-2 Slot Assignment (1/3) Mis-selection rate

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Comparison for Stage-2 Slot Assignment (2/3) System throughput

throughput ≈ offered load * ( 1 – mis-selection rate)

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Comparison for Stage-2 Slot Assignment (3/3) Average Packet Queuing Delay

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Outline

GPRS Background Introduction Two-Stage Dynamic Channel and

Slot Assignment Results Summary

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Summary (1/2)

Two load-balancing schemes for stage-1 channel assignment. Number of Assigned Flow (NoAF) Effective Transmission over Last Cycle (E

ToLC) EToLC outperforms NoAF.

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Summary (2/2)

One selection scheme for stage-2 slot assignment. Round Robin with Linearly-Accumulated

Adjustment (RRLAA). Reward and penalty cycles.

RRLAA has the lower mis-selection rate, better system throughput, and lower packet queuing delay.