contentskom.aau.dk/group/04gr896/report/report_may31.pdf · the radio resource allocation in...

Contents

1 Subject Presentation and Problem Delimitation 31.1 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . .31.2 Problem shaping . . . . . . . . . . . . . . . . . . . . . . . . . .51.3 Solution steps and Time Schedule . . . . . . . . . . . . . . . . .6

2 Introduction of GSM and GPRS 92.1 Cellular Introduction . . . . . . . . . . . . . . . . . . . . . . . . 9

2.1.1 Cellular Concept . . . . . . . . . . . . . . . . . . . . . .92.1.2 Effects of Propagation Channel . . . . . . . . . . . . . .102.1.3 Access Method . . . . . . . . . . . . . . . . . . . . . . .12

2.2 GSM Network . . . . . . . . . . . . . . . . . . . . . . . . . . . .132.2.1 Architecture . . . . . . . . . . . . . . . . . . . . . . . . .132.2.2 Protocols Stack . . . . . . . . . . . . . . . . . . . . . . .162.2.3 Channel Coding . . . . . . . . . . . . . . . . . . . . . .172.2.4 Logical Channels . . . . . . . . . . . . . . . . . . . . . .18

2.3 Circuit switch Versus Packet switch . . . . . . . . . . . . . . . .192.4 GPRS Network . . . . . . . . . . . . . . . . . . . . . . . . . . .20

2.4.1 Architecture . . . . . . . . . . . . . . . . . . . . . . . . .212.4.2 Protocols Stack . . . . . . . . . . . . . . . . . . . . . . .232.4.3 Coding Schemes and Data Unit . . . . . . . . . . . . . .252.4.4 Logical Channels . . . . . . . . . . . . . . . . . . . . . .28

2.5 Comparison Between GSM and GPRS . . . . . . . . . . . . . . .29

3 Radio Resource Management 303.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . .30

3.1.1 Dedicated channels . . . . . . . . . . . . . . . . . . . . .303.1.2 GSM transmission establishment . . . . . . . . . . . . . .303.1.3 GPRS transmission establishment . . . . . . . . . . . . .313.1.4 GSM call maintaining . . . . . . . . . . . . . . . . . . .313.1.5 GPRS call maintaining . . . . . . . . . . . . . . . . . . .32

3.2 ”Best effort” strategy . . . . . . . . . . . . . . . . . . . . . . . .32

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CONTENTS

3.2.1 Resource allocation . . . . . . . . . . . . . . . . . . . . .323.2.2 Resource reassignment . . . . . . . . . . . . . . . . . . .343.2.3 Scheduling . . . . . . . . . . . . . . . . . . . . . . . . .34

3.3 Proposed optimized RRM strategy . . . . . . . . . . . . . . . . .353.3.1 Resource Allocation . . . . . . . . . . . . . . . . . . . .363.3.2 Scheduling . . . . . . . . . . . . . . . . . . . . . . . . .37

3.4 RRM assumptions and parameters used in Simulation . . . . . . .38

4 Simulation Model of RRM 394.1 Cellular Network Assumptions . . . . . . . . . . . . . . . . . . .394.2 Radio Channel Model . . . . . . . . . . . . . . . . . . . . . . . .41

4.2.1 C/I Generation Model . . . . . . . . . . . . . . . . . . .424.3 Traffic Model . . . . . . . . . . . . . . . . . . . . . . . . . . . .48

4.3.1 GSM Voice Call . . . . . . . . . . . . . . . . . . . . . .494.3.2 GPRS Data Session . . . . . . . . . . . . . . . . . . . . .494.3.3 Mapping of the traffic load . . . . . . . . . . . . . . . . .51

4.4 RLC/MAC Functionalities . . . . . . . . . . . . . . . . . . . . .534.4.1 TBF establishment . . . . . . . . . . . . . . . . . . . . .534.4.2 ARQ . . . . . . . . . . . . . . . . . . . . . . . . . . . .534.4.3 MAC layer . . . . . . . . . . . . . . . . . . . . . . . . .54

4.5 Simulator structure and basic description . . . . . . . . . . . . . .544.5.1 Description of the different objects . . . . . . . . . . . . .544.5.2 Simulation process . . . . . . . . . . . . . . . . . . . . .55

5 Performance Evaluation 585.1 Performance Metrics and Simulation Parameters . . . . . . . . . .58

5.1.1 Performance Metrics . . . . . . . . . . . . . . . . . . . .585.1.2 Simulation Parameters . . . . . . . . . . . . . . . . . . .59

5.2 Performance Evaluation . . . . . . . . . . . . . . . . . . . . . . .605.3 Summary of output results . . . . . . . . . . . . . . . . . . . . .66

6 Conclusion and prospect 67

A Simulator validation 68A.1 Propagation model and air interface . . . . . . . . . . . . . . . .68A.2 Traffic model . . . . . . . . . . . . . . . . . . . . . . . . . . . .68A.3 RRM strategies . . . . . . . . . . . . . . . . . . . . . . . . . . .68

B Confidence Intervals 69B.1 About confidence intervals . . . . . . . . . . . . . . . . . . . . .69B.2 Output curves with computed confidence interval . . . . . . . . .69

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Chapter 1

Subject Presentation and ProblemDelimitation

The radio resource allocation in GSM/GPRS network is the focus in this project.Beginning with the study of GSM/GPRS network, the scope of RRM strategies isinvestigated. The delimitation of the project is described in the problem shaping.In addition, the time plan is carried out in order to organize and arrange the projectwork during the semester.

1.1 Background

The wireless communication has been possible due to the electromagnetic wavepropagation through the air interface and its fast development achieves a globalcommunication available from one person to another at any place and any time.

The impressive growth of the cellular mobile telephony as well as the numberof the Internet users poses an exciting potential for market that combines bothinnovations: cellular wireless data services. It is predicted that there will be higherdemand for wireless data services and in particular high-performance wirelessInternet access.

The overview of mobile communication system starts with several mobile radionetworks with low capacity, quality and small mobility range. These limitationswere not solved until the appearance of cellular concept used in mobile commu-nication systems.

The main idea of cellular concept is to divide a large area into small cells inorder to reuse the frequency in the distant cell without interference. Basic on

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CHAPTER 1. SUBJECT PRESENTATION AND PROBLEM DELIMITATION

this purpose, the first analog cellular communication system came into being.Although the analog cellular communication system invokes the revolution ofreusing frequency for the aim of saving limited spectrum resources, a series ofproblems still exist, such as only basic voice telephony, limited coverage and non-compatibility among different networks. Instead of analog, the second generationof digital cellular communication system has been applied. Global System forMobile Communications(GSM) is a typical representative among this stage andhas been very stable and widely accepted standard for mobile communication.GSM uses circuit switched technology to transmit both voice and data.

It inherently supports other technologies at its branches. In addition to voicecommunication, GSM provides mobile services based on digital data interchangeat up to 9.6kbps. Each GSM carrier band is 200 kHz wide and divided into eighttime slots. To provide a single voice channel, one time slot is used. It is alsoused to provide a single 9.6kbps data circuit. However, all eight slots are usedto provide one 64kbps full rate-circuit voice user using time division multipleaccess (TDMA) bearer slots. In addition, some other applications, such as SmallMessage Service(SMS), have been added.

The cellular data services do not fulfill the needs of users and providers. Fromthe user point of view, these data rates are too slow and the connection setup takestoo long. Moreover, the service is so expensive for most users. While from theprovider point of view, the radio resource is not utilized optimally, neither theoffer of data services.

However, the fast growth of Internet requires a wireless data access which GSMis inefficient to support because of the fixed data service. The General PacketRadio Service(GPRS), which is the extension of GSM, is implemented to providepacket data service over the GSM infrastructure. The channel capacity is highlyincreased and the amount of users is enlarged. The major new third generation,called the Universal Mobile Telecommunications System(UMTS), is definitelydesigned to achieve universal speech services and local multimedia systems andis in process of development worldwide.

When considering the wireless communication system, the channel effects shouldnot be ignored. The performance of a wireless system is strongly affected by itsenvironment. In the characterization of fading channels, different components aredistinguished, such as a large-scale path loss,a medium-scale slow-varying withlognormal distribution, and a small-scale fast varying component modelled witha Rician or Rayleigh distribution according to the presence or absence of Line Of

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1.2. PROBLEM SHAPING

Sight(LOS), respectively. In this way phenomenons like multipath fading, shad-owing, near-far effects are taken into account.

1.2 Problem shaping

The importance of GPRS lies in the increasing traffic, mainly data over voice,providing a packet switched extension for the access of IP orientated services.

Even though GPRS can supply many new data services as well as higher effi-ciency in utilizing the capability of transmission network, it has to share the sameradio resources with GSM (voice) services. That means the air interface becomesa bottleneck and the GSM/GPRS operators have to balance the quality of existingvoice service and that of newly introduced data services.

As far as radio resource allocation is concerned, European TelecommunicationsStandards Institute (ETSI) proposes the fixed and on-demand channel allocationmechanisms.

• Fixed Resource Allocation (FRA)

• Dynamic Resource Allocation (DRA)

• Fixed Resource Allocation with Queue capability (FRAQ)

• Dynamic Resource Allocation with Queue capability (DRAQ)

The purpose of this project is to develop a method/algorithm to optimally as-sign radio resources to GPRS services and avoid decreasing the quality of voiceservices.

Some key issues in this project include:

1. Set criterion for evaluating the service quality of GPRS, i.e, blocking rateor throughput. GSM voice service quality is also in consideration thoughits criterion has already been set in industry standard (blocking rate).

2. A certain GPRS data traffic model should be built.

3. An optimal algorithm will be designed to assign radio resources for packet-switched data to achieve the criterion in item 1 under the traffic model initem 2.

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4. All above will be implemented in a simplified simulation model to evaluatethe optimal RRM algorithm.

Since this project is designed to deal with a problem in real engineering world,and considering the time, human resource and facility limitations the design grouphas, the working scope of this project will be delimited as follows:

• Possible algorithm input and analysis model will be built on the basis ofsome current existing GPRS architectures and radio resource managementstrategies. That is, the developing methodology is not allowed to change thenetwork structure.

• Input data and traffic model will be based on the network of current GSM/GPRSoperator.

• The radio resources in this project refer to all the GSM time slots and fre-quency bands inside one cell cluster.

1.3 Solution steps and Time Schedule

First, a background of the GSM/GPRS system is analyzed, and concretely theGPRS air interface of this infrastructure. Next to this introduction, the qualityof the voice and data in GSM/ GPRS operators is matter of study. In this waythe assignment of radio resources to packet switched services is evaluated fromdifferent operator strategies, defining scenarios, input parameters and algorithms,and finding out optimal criterions by means of simulation methods. In order toget efficient radio resources utilization, some strategies on the dynamic channelallocation for data packets as the reservation of some time-slots in each TDMAto GPRS traffic will be study. The analytical results will be compared with thoseobtained from the simulations. Finally, in order to apply it to a real situation, aparticular operator strategy is included as an example.

In order to achieve the goal of the project, the group work has been organizedin the following phases( 1.1)

1. Problem Delimitation:Feb 9 - Mar 4, Week 7 - Week 10

Problem shaping. Understand the basic concepts of GSM and GPRS tech-nique and analyze the work scope of the project.

Milestone:Introduction chapter.

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1.3. SOLUTION STEPS AND TIME SCHEDULE

2. GPRS Technical issues:Mar 5 - Mar 17, Week 10 - Week 12

Understand and describe the GPRS air interface and the existing radio re-source management (RRM) strategies, especially those currently operatingon the network of a local operator, Sonofon.

Milestone:Technique background chapter.

3. Scenario Definition and Parameter Configuration:Mar 11th - 24th, Week 11- Week 13

(3.1) Model Division

The following objects are investigated during this time of period be-cause they are considered the key issues in the model building andscenario definition.

3.1.1 Propagation Model

3.1.2 Traffic Model/Generator

3.1.3 Radio Resource Management Model(including ARQ)

(3.2) Flowchart and Parameters Definition

Define scenarios, input parameters and evaluation criterions, for whichan optimal algorithm for radio resource allocation in GPRS can be de-veloped. These definitions will be modified a couple of times accord-ing to the updating work.

Milestone: Modelling foundation and parameter definition

4. Solution Development:Mar 25th - May 19th, Week 13 - Week 21

(4.1) Algorithm and RRM Strategy Design

Develop an optimal RRM solution and evaluate it in simplified simu-lation models or with the help of analytic models.

(4.2) Simulation

4.2.1 Best Effort Strategy Validation

4.2.2 Proposed RRM Model Simulation

(4.3) Report Writing In Parallel.

Milestone: Design and simulation chapter

5. Conclusion and Finalizing Report:May 20th - Jun 3rd, Week 21 - Week 23

(5.1) Finish the simulation part and Performance Evaluation.

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(5.2) Finalize the last version of report.

Milestone: Performance Evaluation and Conclusion

Figure 1.1: Time Plan

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Chapter 2

Introduction of GSM and GPRS

To introduce GSM and GPRS, first there is a need to talk about the cellularconcept on which those networks are based and then GSM and GPRS networkswill be presented to get an overview of the networks on which RRM is carriedout.

2.1 Cellular Introduction

Because of the very limited frequency bands, a mobile radio network has onlya small number of speech channels available. In order to serve millions of sub-scribers, frequency must be spatially and temporally reused. That’s why the cel-lular concept has been developed, as well as a multiple access method.

2.1.1 Cellular Concept

The cellular concept has first been proposed by Bell Labs in 1971. It is sup-posed to increase the system capacity.

The main idea of cellular concept is the division of the area to be covered intocells that receive a subset of the frequencies from the total set assigned to thenetwork. Two neighboring cells will not use the same set of frequencies to preventinterference. The same set of frequencies can be reused at a sufficiently largedistance. When moving from one cell to another during an ongoing conversation,an automatic channel/frequency change occurs (handover).

The coverage area of a cell is modelled by an hexagon in order to simplifyanalysis but the propagation model tends to represent a cell by a circle.

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CHAPTER 2. INTRODUCTION OF GSM AND GPRS

A group of adjacent cells which uses all of the frequencies is called a cluster.As in Figure 2.1, 7 cells make up a cluster. All the cells labelled ”1” use the samefrequencies, but no frequency can be reused within a cluster.

Figure 2.1: Clusters of 7 cells

Generally the signal strength is sufficient in nowadays cellular systems, hencethe interferences from neighbor cells with the same frequency(Co-channel Inter-ferences)are above the noise floor. This means we can consider the cellular systemas interference limited rather than noise limited.

Moreover, the sectoring is applied in the cellular network. Instead of havingomnidirectional antennas either in the center or the boundary of the cell, direc-tional antennas are used in order to decrease the interference, so that the cell canbe divided in 3 (for example for a 120 degree sectoring) equally sized sectors thatshare the frequencies of the cell. This method allows to reduce the number of cellscausing interference but increases the complexity of the network.

2.1.2 Effects of Propagation Channel

The signal being transmitted via electromagnetic waves through the air inter-face, many problems could arise either due to noise, interferences, attenuation ofthe wave with distance and obstacles (path loss and slow fading), or due to the nu-merous paths the wave can take from the transmitter to the receiver (fast fading).

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2.1. CELLULAR INTRODUCTION

Two types of interference can cause trouble to an air born communication. Ad-jacent interferences are caused by some other user transmitting on neighboringchannels, and guard-bands are used to fix this problem. Co-channel interferencesare caused by the use of the same channel by another customer (in a different clus-ter). The cell shaping and power control are very important tools to help preventthese problems. Noise can have numerous causes, but the most important ones arethe transceiver’s electronics, the background radiation, or man-made. Transmit-ting sufficient power is necessary to have a good SNR on the receiver’s side.

Path loss means the strength of the electromagnetic wave gets weaker as it getsfurther away from the transmitter. This effect not only limits the coverage of a cellbut also reduces the interferences from other cells far away. The decrease of signalstrength is proportional to the power of the distance. This power factor is usuallybetween 2 and 5, from free space to strong obstacle attenuation, and typically 3.5in an urban environment.

Shadowing is affected by prominent terrain contours (hills, forests, billboards,buildings, etc.) between the transmitter and receiver; it is likely that the line ofsight path in the transmission link is ’shadowed’ and the radio waves are diffractedand reflected around the obstacles. In cellular mobile communication, shadowingcauses a relative slow fluctuation in the local mean of the received signal power,where the local mean represents an average over a few tenth of wavelength.

Another phenomenon is the multipath propagation caused by reflection, diffrac-tion and scattering as well as the relative movement between the transmitter andthe receiver. Thus all the reflected radio wave components of one signal with dif-ferent amplitudes and phases will arrive at the receiver and the combination ofthese components can cause a very fast fluctuation according to the moving speedof a mobile user.

These different types of fading will also cause temporal effects on the transmit-ted signal, which can usually be fixed using hardware implementations or proto-cols.

Temporal Effects

The relative delay is due to multiple echoes corresponding to all the paths thesignal can take (multipath) which all arrive with their own delays. To correctthis delay problems that could cause inter symbol interference, GSM receiversuse equalizers that are updated with a training sequence used to learn about the

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channel and add up all the echoes. The use of this method makes an assumptionthat the movement is slow compared to the equalizer update.

The absolute delay is a delay caused by the distance separating both ends ofthe transmission. Electromagnetic waves are transmitted at the speed of light, sothe distance can have an influence if the received signals has a delay of bit times.Two solutions are used to prevent this problem which are the use of guard bandsbetween each block of transmitted data, and the use of a timing advance protocolto calculate the delay and send data at the right time.

In order to optimally share resources on the limited frequency band allocated toGSM through this interface, a multiple access method is used.

2.1.3 Access Method

Since radio spectrum is a limited resource shared by all users, a method mustbe devised to divide up the bandwidth among as many users as possible. Themethod chosen by GSM is a combination of Time and Frequency Division Mul-tiple Access (TDMA/FDMA) as shown in Figure 2.2. The FDMA part involvesthe division by frequency of the (maximum) 25MHz bandwidth into 124 carrierfrequencies spaced 200kHz apart. One or more carrier frequencies are assignedto each cell.

Each of these carrier frequencies is then divided in time, using a TDMA scheme.The fundamental unit of time in this TDMA scheme is called a Time Slot(TS) orburst and it lasts 15/26 ms(or approximately 0.577ms). Eight TS are grouped intoa TDMA frame 120/26 (ms) (or approximately 4.615ms), which forms the basicunit for the definition of logical channels. One physical channel is one TS perTDMA frame.

Channels are defined by the number and position of their corresponding burstperiods. All these definitions are cyclical. Channels can be divided into dedicatedchannels, which are allocated to a mobile station, and common channels, whichare used by mobile stations in idle mode.

TDMA realizes several users can share the same frequency in different timeduration.

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2.2. GSM NETWORK

Figure 2.2: Access Method

2.2 GSM Network

The digital communication system described was originally used by GSM asa circuit switched system. CS networks are based on regular cabled telephonynetworks (ISDN) where a physical connection is assigned by switches betweeneach end of the Point to Point communication. In GSM, circuit switched is phys-ically made by assigning a fixed time slot in every frame which is used for theentire connection. GSM was mainly developed for voice usage, but it providesslow rate circuit switched data services (9.6Kbps) and a pager-like service calledShort Message Service (SMS). The following sections will briefly describe thefunctional entities, the protocol stack, the radio interface signalling protocol, andthe logical channel structure based on GSM.

2.2.1 Architecture

The network architecture of GSM consists of three main entities called the Mo-bile Station (MS), the Base Station Subsystem (BSS) and the Network and Switch-ing Subsystem (NSS), each containing one or several components as shown inFigure 2.3.

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Figure 2.3: GSM Network

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2.2. GSM NETWORK

The MS contains the physical equipment used by a subscriber such as a portablehandset or a vehicle mounted unit, which is identified by International MobileEquipment Identity(IMEI). In addition to that, a Subscriber Identity Module (SIM)is attached to the mobile equipment and provides the subscriber information calledInternational Mobile Subscriber Identity (IMSI) used to distinguish the differentusers.

The BSS, handling the radio link with the MS, has two components which arethe Base Transceiver Station (BTS) and the Base Station Controller (BSC). TheBTS comprises the radio transmission and reception devices, and also managesthe signal processing related to the air interface. The BSC manages the radiointerface, mainly through the allocation, release and handover of radio channels.

The Network Switching Systems(NSS) handles the switching between bothends of the connection, and can connect to other networks such as the regularcabled telephony network. The main component is the Mobile Switching Cen-ter(MSC) which is basically an ISDN-switch, coordinating and setting up callsto and from MSs. It is connected to four databases which are the Visitor Loca-tion Register (VLR), the Home Location Register (HLR), the Equipment IdentityRegister (EIR), and the Authentication Centre (AuC). The VLR contains all thesubscriber data, both permanent and temporary, which is necessary to control aMS in the MSCs coverage area. The VLR is commonly realized as an integralpart of the MSC, rather than a separate entity. The HLR database is used to storepermanent and semi-permanent subscriber data such as the location area (assum-ing the MS is in a coverage area), and this data is used to locate an MS in theevent of a MS terminating call set-up. The AuC database contains the subscriberauthentication keys and the algorithm required to calculate the authentication pa-rameters to be transferred to the HLR. The Equipment Identity Register (EIR)database contains information on the MS and its capabilities. The IMSI (Interna-tional Mobile Subscriber Identity) is used to interrogate the EIR.

These network components are interconnected through interfaces named by let-ters from A to I with an exception for the air interface called Um. The three mostimportant interfaces are the Um interface, the interface between the BSC and BTS(Abis) and the one between the BSS and the MSC (A). They use different signal-ing protocols to convey information between the MS and the MSC.

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2.2.2 Protocols Stack

The GSM protocols stack corresponds to the OSI reference model. Layers 1and 2 of the GSM model correspond to the physical and link layers as defined bythe OSI model. Figure 2.4 shows the different protocol layers in the Um, A andAbis interfaces.

Figure 2.4: GSM Protocols

The stacks in Figure 2.4 are implemented in hardware or software, dependingon the nature of the entity which resides on them. In order for different proto-col developers to write inter-operable code, the European TelecommunicationsStandards Institute (ETSI) has produced a set of specifications to follow whenimplementing GSM protocols.

The physical layer (GSM-RF) specifies how data is transmitted from one entityto another across the physical transport medium.

The link layer (LL) provides a link between the networking layers above it, andthe physical layer below it. It provides error detection and correction of packetsreceived from the physical layer. A modified LAPD (Link Access Protocol for theISDN D-channel) protocol, called LAPDm, is used over the Um interface. TheMTP (Message Transport Part) level 2 protocol of the SS7 protocol suite is usedover the A-interface.

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2.2. GSM NETWORK

The similarity between the GSM protocol stack and the OSI model ends at thelink layer. Layer 3 of the GSM protocol stack does not correspond to the net-work layer of the OSI model. GSM layer 3 is composed of 3 parts, which isknown as the message or signaling layer. It is used to set up and maintain voicecircuits between users of a mobile cellular network. It does this by managing ra-dio resources, information about user whereabouts, and voice circuit information.These operations are all specific to cellular radio networks because other networksdo not have to keep track of user whereabouts or movement of users from one cellto another. The message layer in GSM is composed of three sub-layers. The RadioResource (RR) Management layer is in charge of establishing and maintaining astable uninterrupted communications path between the MSC and MS over whichsignalling and user data can be conveyed. Handovers are part of the RR layersresponsibility. Most of the functions are controlled by the BSC, BTS, and MS,though some are performed by the MSC (in particular for inter-MSC handovers.).The RR’ layer is the part of the radio resource functionality which is managedby the BTS. The Mobility Management (MM) layer is in charge of maintainingthe location data, in addition to the authentication and ciphering procedures. TheCommunication Management (CM) layer consists of setting up calls at the users’request. Its functions are divided in three: Call control, which manages the cir-cuit oriented services, Supplementary services management, which allows mod-ifications and checking of the supplementary services configuration, and ShortMessage Services, which provides Point to Point short message services.

In order for these signaling protocols to be implemented over the air interfacein GSM, we will introduce the logical channels which represent the informationcarried by the physical channels.

2.2.3 Channel Coding

To protect the signal from interferences we must add a number of bits for errorcontrol. These bits are called redundancy bits. The GSM system uses convolu-tional encoding to achieve this protection. The exact algorithms used differ forspeech and data services. The method used for speech blocks will be describedbelow.

Bit Composition of the Speech Signal Recall that the RPE-LPC Encoder pro-duces a block of 260 bits every 20 ms. It was found (though testing) that some ofthe 260 bits were more important when compared to others. Below is the compo-sition of these 260 bits.

• Class Ia:50 bits (most sensitive to bit errors)

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• Class Ib:132 bits (moderately sensitive to bit errors)

• Class II:78 bits (least sensitive to error)

Channel Coding As a result of some bits being more important than others,GSM adds redundancy bits to each of the three Classes differently. The 50 ClassIA bits are encoded in a cyclic encoder (that adds 3 parity bits). The 132 ClassIb bits (together with the 53 encoded Class IA bits and 4 trailing zero bits) areencoded using convolutional encoding with rate 1/2 (that produce a new sequenceof 378 bits). Finally, the Class II bits are merely added to the result of the convolu-tional encoder. The channel encoded bit sequence is now 456 bits long. Therefore,each 20 ms burst produces 456 bits at a bit rate of 22.8 kbps. To further protectagainst bit errors, the 456 bit sequence is then diagonally interleaved.

Interleaving Interleaving is the processes of rearranging the bits. Interleavingallows the error correction algorithms to correct more of the errors that could haveoccurred during transmission. By interleaving the code, there is less possibilitythat a whole chuck of code can be lost.

The 456 bits output by the convolutional encoder are divided into 8 blocks of57 bits.These eight blocks are shuffled to form eight new blocks. One shuffledblock of the current speech packet and another from the previous speech packetare written into a normal burst.

2.2.4 Logical Channels

GSM distinguishes between physical channels (the time slot) and logical chan-nels (the information carried by the physical channels). Several recurring timeslots on a carrier constitute a physical channel, which are used by different logicalchannels to transfer information - both user data and signalling.

Traffic Channels

The GSM traffic channels are used to transfer information such as voice or dataat rates depending on the channel coding, and the type of traffic channel used.The Full rate traffic channel (TCH/F) in GSM is used to convey voice or datainformation in a circuit switched manner. A TCH/F is mapped on a time slotevery frame, thus allowing to transfer 114 bits of coded information every timeslot. The voice rate is about 13Kbps, as only the most important data is highlycoded, but circuit switched data information has a stronger channel coding whichallows a data rate of only 9.6Kbps. The Half rate traffic channel(TCH/H) can be

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2.3. CIRCUIT SWITCH VERSUS PACKET SWITCH

used to transfer data information. It is mapped on a time slot every other frame,and has half the data rate of TCH/F(4.8Kbps).

Control Channels

Control channels(Table 2.1) deal with network management messages and chan-nel maintenance task. Any non-traffic communication between the BS and the MSuses these channels. Three types of control channels exist which are either one ortwo-way communication channels.

Table 2.1: GSM Control ChannelsType Control Channel

Frequency correctionBroadcast FCCH

SynchronisationSCH

System informationBCCH

PagingCommon PCH

Random AccessRACH

Ressource AssignementAGCH

Signalling procedureDedicated SDCCH

MeasurementsSACCH

Time Critical msgsFACCH

This table is just to give a general picture of GSM control channel but the detailsinside is none of our interests in this RRM project thus it will not be discussedfurther more.

2.3 Circuit switch Versus Packet switch

GSM is based on circuit switched system, where an end to end connection isbuilt by occupying a fixed channel during the whole call session, even during idle

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periods. Circuit switch is suitable for constant bandwidth data flows, while it haslow efficiency in conveying data services with a bursting nature. Besides GSMcan only support a low data rate service like SMS.

In packet switched system, channels are allocated dynamically on demand, usu-ally not end to end. Several users therefore can share one channel in time, and thechannel efficiency can be high when users send data in a burst behavior. The pur-pose of GPRS is to provide compatibility to packet switched network like Internetand supply advanced data services on existing GSM infrastructure.

In order to establish a comparison between GSM and GPRS, we will now focuson the GPRS network, which uses the same physical layer and has many similar-ities with GSM.

2.4 GPRS Network

With the rapid growth of wireless users, new data services and higher transmissionspeed are on demand, which can not be satisfied by GSM network. Thus, GPRS,which is based on GSM, came into being.

General Packet Radio Service(GPRS) is an enhancement of GSM system andestablished on the platform of GSM. It is also known as the 2.5 Generation anda step toward the 3rd Generation (3G). Furthermore interworking specificationshave been developed between ANSI/ISA-136 and GSM platforms to get a logicalextension of the overall scheme.

Unlike GSM that was designed for voice services and requires a circuit switch-ing transmission mode, GPRS provides a packet switching transmission mode.This feature allows an easy adoption to the bursty traffic generated by Internetapplications like e-mail, WWW and FTP. In comparison with 9.6kbps data trans-mission rate of GSM, GPRS offers a maximum theoretical data transmission rateof 172.4kbps. Another important goal of the technology is to make it possiblefor GSM license holders to share physical resources on a dynamic, flexible basisbetween packet data services and other GSM services.

As a consequence, GPRS shares GSM frequency bands with telephone andcircuit-switched data traffic, and makes use of many properties of the physicallayer of the original GSM system most importantly the TDMA frame structure,modulation technique(GMSK) and structure of GSM time slots. GPRS provides

20

2.4. GPRS NETWORK

synchronous and asynchronous interworking with X.25 networks, IP networksand other GPRS networks. The different types of bearer services described withinGPRS are Point-To-Point (PTP) and Point-To-Multipoint (PTM). An example ofPTP is the access to the Internet, whereas, PTM caters Traffic Information.

The increase of speed provided by Packet Switched (PS) networks as GPRSenlarged the amount of data services on mobile phones. The dynamic use ofmultiple time slots provided a way of not wasting resources and the packet archi-tecture allowed users to connect to IP services such as the web (e-mail, ftp andhttp protocols). The possibility of having a PTM connection also allowed moreservices such as multiuser video-conferences or chatting.

2.4.1 Architecture

GPRS attempts to reuse the existing GSM platform as much as possible, butin order to build a packet based mobile network, some network elements, inter-faces and protocols that handle packet data are required. Figure 2.5 describes theGSM/GPRS network which will more detailed in the following.

21


Figure 2.5: GPRS Network

22

2.4. GPRS NETWORK

GPRS introduces a minimum impact on the BSS infrastructure and no newphysical radio interface. The following elements are implemented on the net-work: on the BSS side, Packet Control Unit Support Nodes (PCUSN), on the corenetwork side, Serving GPRS Support Nodes (SGSN), Gateway GPRS SupportNodes (GGSN) and SS7/IP Gateways (SIG).

PCUSN The PCUSN is a stand-alone node in the BSS whose main purpose isto complement BSCs with the specific packet processing of GPRS. It is respon-sible for the capacity on demand feature. It decides which radio resources aredynamically allocated to packet switched and circuit switched use. The BSC thenmanages the radio resources allocated for circuit switched use, while the PCUSNmanages radio resources for the GPRS traffic itself. Its primary function is toprovide the interworking function between the radio interface (synchronous con-nection) and the packet network Gb interface (asynchronous and connectionless).

SGSN/SIG The main functions of the SGSN are to detect GPRS MSs in itsservice area, to perform mobility management, to implement authentication pro-cedures and to send/receive data packets to/from the MS. It requests location in-formation from the HLR through the Gr interface. These messages are routedthrough the SIG, which provides the interworking between GPRS nodes in an IPnetwork and GSM nodes in a signalling system 7 (SS7) network.

GGSN The GGSN provides the point of interconnection with external Pub-lic Data Networks (PDN) through the Gi interface. It stores routing informa-tion for attached GPRS users and requests location information for mobile ter-minated data packet from the HLR (this is accomplished transparently throughthe SGSN). Its primary functions are Packet Routing and Transfer (Routing, Tun-nelling,Encapsulation, Compression, etc.).

In order to allow those elements to communicate, some specific protocols areimplemented.

2.4.2 Protocols Stack

In the GPRS Protocol Stack, the focus will be in the air interface and lowerlayers, which can be observed on Figure 2.6.

23


Figure 2.6: GPRS Protocols Stack

LLC (Logical Link Control) The LLC layer is responsible for handling thevirtual connection between the SGSN and the GPRS MS and exists even whenno physical resources are available between the two. It supports peer-to-peer datatransfer between the SGSN and the MS.

When the LLC packet arrives at the BSS (PCUSN), it is forwarded to the MS.There the Radio Link Control (RLC) is responsible for efficient use of the physi-cal link on the air interface and the MAC(Medium Access Control) for handlingaccess to the physical link.

RLC (Radio Link Control) The RLC is responsible for segmentation and re-assembly of the LLC packets. The segmentation results in RLC blocks. Con-trol information is added to each RLC block to allow Backward Error Correction(BEC). The size of these segments is such that when applying the coding schemes,they precisely fit on four normal bursts (= radio block). With BEC, both acknowl-edge and unacknowledged mode are possible.

As several subscribers can be multiplexed on one physical channel, each con-nection has to be (temporarily) uniquely identified. These connections are referredto as Temporary Block Flows (TBF). A TBF is a physical connection between themobile station and the PCUSN. The TBF is maintained only for the duration ofthe data transfer. The TBF is identified by a Temporary Flow Identifier (TFI). TheTFI is added to the RLC block.

24

2.4. GPRS NETWORK

MAC (Medium Access Control) The MAC layer handles procedures relatedto common transmission resource management. The layer allows point-to-pointtransfer of signaling and user data within a cell. The medium access can be real-ized by fixed and dynamic allocation. In MAC header, Uplink State Flag(USF) isused as identifier.

GSM-RF This layer is based on the GSM specification, which describes thephysical characteristics of the air interface. While the RLC/MAC layer is imple-mented in the PCUSN, the GSM-RF layer is located in the BTS.

BSSGP (BSS GPRS Protocol) This layer is responsible for the transport ofrouting and QoS information between PCUSN and SGSN. It provides a connec-tionless link with unconfirmed data transfer between BSS and SGSN. It acts as aninterface between LLC frames and the RLC/MAC blocks in the BSS, and as aninterface between the RLC/MAC derived information and the LLC frames in theSGSN.

Frame Relay/Network Service This layer provides a packet type commutationbetween SGSN and BSS.

2.4.3 Coding Schemes and Data Unit

An important feature of GPRS is the presence of four coding schemes (CS-x)with different levels of resistance to transmission problems. So the throughputwill be determined by the choice among the four coding schemes according to thechanges in the channel conditions.The most important variables that affect thischoice are the channel quality as measured by carrier to interference ratio(C/I)and the packet size.

Among the 4 coding schemes, CS-1 has the lowest coding rate, it is also themost robust coding scheme. Therefore, CS-1 is used for all control messages.CS-2 and CS-3 have higher code rate and more information bits. CS-4 is the mostdata efficient coding scheme and is the most vulnerable to channel impairment.

The characteristics of each type are in the following Table 2.2:

25


Table 2.2: GPRS Coding SchemesCHANNEL CODINGSCHEMES

CS-1 CS-2 CS-3 CS-4

Code Rate 1/2 2/3 3/4 1Data Rate (kb/s) 9.05 13.4 15.6 21.4Maximum Data Speedwith 8 TS

72.4(kb/s) 107.2(kb/s) 124.8(kb/s) 171.2(kb/s)

Now we can depicted the whole GPRS data unit mapping process from PDU totime slot in Fig2.7.

26

2.4. GPRS NETWORK

Figure 2.7: GPRS Data Unit

27


2.4.4 Logical Channels

Traffic Channels

Packet Data Traffic Channel (PDTCH) This channel is allocated for user datatransfer. It is temporarily dedicated to one mobile station. In the multislot oper-ation, a MS may use multiple PDTCHs in parallel for individual packet transfer.All PDTCHs are uni-directional: PDTCH/U for UpLink Transfers and PDTCH/Dfor DownLink. One PDTCH is mapped onto one physical channel. Up to eightPDTCHs with different time slots but with the same frequency parameters may beallocated to one MS at the same time.

Control Channels

Packet Common Control Channel(PCCCH) This channel is used for a com-mon control signalling required to initiate packet transfer. PCCCH is mapped onone or several physical channels according to a 51 or 52 multi frame.

Four different channels are defined: PRACH (random access used by the MS toaccess the network), PPCH (paging used to page a MS belonging to a given paginggroup), PAGCH (Access Grant used to assign resources to a MS during the packettransfer establishment phase) and PNCH (used to send a PTM-M notification to agroup of MSs).

Packet Broadcast Control Channel (PBCCH) This channel is used to broad-cast System Information. Alternatively, the BCCH of GSM can be used. ThePBCCH is mapped on one or several physical channels.

Packet Associated Control Channel (PACCH) This channel conveys signal-ing information related to a given MS. It is used to send signaling associated to apacket transfer and resource assignment. PACCH shares resources with PDTCHsthat are currently assigned to a MS. Moreover a MS that is currently involved inpacket transfer can be paged for circuit switched services on this channel. PACCHis a bi-directional nature, it is dynamically allocated (on the block basis of thesame physical channel as carrying PDTCHs or both on the UpLink and the Down-Link regardless on whether the corresponding PDTCH assignment is for UpLinkor DownLink).

Packet Timing advance Control Channel (PTCCH) PTCCH/U (UpLink) isused to transmit access burst to allow estimation of the timing advance for a MS.

28

2.5. COMPARISON BETWEEN GSM AND GPRS

PTCCH/D (DownLink) is used to transmit timing advance updates for severalMSs.

2.5 Comparison Between GSM and GPRS

The differences between GSM and GPRS in the fields of architecture, proto-col stack, coding scheme and logical channel have been analyzed in the previoussection. The general overview of comparison is shown in Table 2.3.

Table 2.3: Comparison of GSM and GPRSGSM GPRS

Transmission Circuit Switch Packet SwitchMaximum Trans-mission Rate

9.6kbps 172.4kbps(data)

Coding Scheme unique CS-x(x=1,2,3,4)Access Time Long ShortCharging duration-based volume-

based(always”On” mode)

Application Sup-port

Limited(Largevolumes)

Robust(Smallvolumes)

• GSM- Circuit switched transmission.- Transmission rate per user: 13.6kbps or 9.6kbps.- Duration-based charging.

• GPRS- Packet switched transmission.- Transmission rate per user: up to 172.4kbps(Multiple time-slot can be integrated)- Different coding schemes with GSM.- Volume-based charging(always ”On” mode).

Although the way of transmission in GSM(CS) and GPRS(PS) are different,they use the same air interface to transmit information, which brings about theproblem of how to assign the limited radio resource between GSM and GPRSwithout decreasing the quality of voice. In the next chapter, the Radio ResourceManagement(RRM) will be analyzed in detail.

29

Chapter 3

Radio Resource Management

In this chapter we present the play scenarios of existing RRM strategies first,and then describe in more detail the Best Effort policy and our proposed RRMstrategy, to investigate how we can improve the GSM/GPRS system performancecomparing to Best Effort.

3.1 Introduction

Radio Resource Management is the function responsible for the establishment,the maintaining, and the release of voice calls (CS) or data transmission (PS). Theestablishment of connections is about allocating resources or blocking the incom-ing CS or PS calls if necessary. Maintaining a connection requires dealing withhandovers and power control, but also reassignment of resources and transmissionscheduling for GPRS.

3.1.1 Dedicated channels

It is possible to dedicate resources either for GSM or GPRS. Dedicated PDCHfor example will only be used for GPRS transfers, thus allowing a smaller GPRSblocking rate in case the voice has the highest priority.

3.1.2 GSM transmission establishment

Concerning GSM voice calls, resource allocation is quite simple as a CS con-nection only requires one TS in both uplink and downlink. In the case of unavail-able resources, the GSM call is simply blocked, meaning that the call will notsucceed. In a network that support GPRS, and in the case no free time slots are

30

3.1. INTRODUCTION

available, resources can be taken from existing GPRS transmissions. This processis called preemption.

3.1.3 GPRS transmission establishment

For GPRS connections, the procedure of resource allocation is more compli-cated as multiple time slots can be allocated to the same call, and a time slotcan be used by more than one GPRS user. This allocation process is called TBF(Temporary Block Flow) assignment.

A TBF, associated with one GPRS transmission, corresponds to a certain amountof time slots that this transmission will be able to use. The TBF for uplink anddownlink are different and independent, and different identifiers are used to com-municate to the mobile in which frames it is allowed to upload or download itsblocks.

Downlink: In downlink, a TBF is assigned a Temporary Flow Identity number(TFI), and at the establishment of a connection, the TBF is communicated to themobile with its TFI. During the connection, the mobile will check for its TFI in theassigned TS to be able to download the corresponding blocks. The TFI is a 5 bitnumber, thus 32 different TFI numbers can be assigned. If more than one sessionhave the same TFI, in the case where there are more than 32 active sessions, theycannot share the same TS.

Uplink: Two different strategies can be used to assign resources in uplink. TheFixed Resource Allocation (FRA) strategy will send the TBF to the mobile at theestablishment of a connection with directions on when it is able to send blockson the given time slots. The Dynamic Resource Allocation (DRA) strategy willsend the TBF information to the mobile with an identifier called Uplink State Flag(USF). The mobile will then listen to its allocated downlink TS and checks therefor its USF to know when it is allowed to send its uplink blocks.

3.1.4 GSM call maintaining

In addition to handovers which is the RRM part handled by the MSC, GSMcalls can be dropped during a communication, given certain conditions such as abad propagation leading to loss of frames and unacceptable voice communication.It is also possible to reassign GSM resources if we want to free a particular timeslot (intra-cell handover). The policy for dropping and intra-cell handover is partof the RRM strategy.

31

CHAPTER 3. RADIO RESOURCE MANAGEMENT

3.1.5 GPRS call maintaining

For GPRS, the equivalent of handovers are routing area update, when mov-ing from an SGSN region to another or from a BSS to another. The part of callmaintaining that interest us and happens in the BSS is the scheduling procedure.Scheduling is the part of RRM which chooses how to share multiplexed TS andwhen to transmit the blocks in the TBF. TBF reassignment can also take place, (ifwe want to upgrade or downgrade a GPRS session) as well as dropping accordingto criteria that are also defined in the RRM strategy.

3.2 ”Best effort” strategy

The RRM strategy called ”best effort” for GPRS is the one currently used bymobile operators and does not take QoS or propagation conditions into accountfor either choosing the resources to allocate or schedule the transmission of RLCblocks. It then considers all GPRS users equally and try to share the availablebandwith fairly between all sessions. Voice GSM users have the absolute priority,and when no resources are available, GPRS users can be downgraded to free atime slot used by the voice user (preemption). As we only consider downlink inthe simulation, we will describe the downlink strategy assumed to be ”best effort”.

3.2.1 Resource allocation

GSM users are allocated the first available time slot starting from the first TRX.If no time slot is available at the time of an incoming voice call, this call will beallowed to preempt a GPRS time slot, thus downgrading all GPRS users multi-plexed in this TS. The TS selected for preemption will be the first GPRS time slotstarting from the first TRX. In the extreme case where all TS are already taken byvoice calls, the incoming GSM call will be blocked.

For GPRS users requiring K channels (depending only on the capacity of themobile as QoS is not considered), a TRX will be chosen which will be the lastleast-loaded one in terms of number of TBF and GSM calls assigned. In thisTRX, the K least loaded time slots will be selected and if less than K TS areavailable, the maximum amount possible will be allocated. The selection of TimeSlots to be allocated for the TBF is illustrated in Figure 3.1.

32

3.2. ”BEST EFFORT” STRATEGY

Figure 3.1: Time Slot selection in best effort

If no TS is available (taken by voice or filled by TBFs), the GPRS request willbe queued in the access queue, which is a First In First Out (FIFO). If the queueis filled, the incoming GPRS request will be blocked. The resource allocationstrategies described above for ”best effort” are shown in figure 3.2.

33


Figure 3.2: Resource request

3.2.2 Resource reassignment

During transmission, if a TBF possesses less than K channels due to voicepreemption or lack of resources, a TBF assignment can be done to reassign someTS and upgrade the throughput.

When resources become free after a congestion period, the priority of TBFassignment goes first to the TBFs having the least resources to improve the userthroughput and release resources more rapidly and then to the access queue todecrease access delay and GPRS blocking rate.

3.2.3 Scheduling

Once a GPRS call has been allocated resources, a scheduling technique is usedto choose blocks that has to be sent in TS holding multiple TBF (multiplexed timeslots). In the ”best effort” strategy, in order to send blocks fairly in multiplexedtime slots, the queue used is a Round Robin queue as illustrated in Figure 3.3,meaning that blocks will be sent from a different TBF every block period.

34

3.3. PROPOSED OPTIMIZED RRM STRATEGY

Figure 3.3: Round-Robin scheduling

3.3 Proposed optimized RRM strategy

The proposed RRM strategy is based on a grading system which differentiateGPRS transmissions based on parameters that are chosen according to the de-sired performance value to be optimized, such as throughput or access delay. Itthen does not consider GPRS sessions equally as in best effort, but give accesspriority and share the available resources according to the assigned grade. GSMvoice calls still have the priority over GPRS transmissions, and preemption is thusconsidered on all shared TS.

In the simulated scenario, the grading is based on the Carrier to Interference ra-tio (C/I). The main purpose to choose C/I is to try to improve the system through-put. Since the link adaptation is employed, the throughput is always consideredoptimized for a chosen CS-x, which is elected according to C/I. In the case wegive priority to high C/I values, the throughput per cell should increase and re-sources should be released more rapidly, thus decreasing the access delay and theblocking rate.

The grading could be based on other parameters such as QoS. The QoS prece-dence and required access delay could be used for grading a request in the accessqueue, thus giving access priority corresponding to the requirements. Moreover,

35


the QoS mean and peak throughput classes (in addition of precedence and C/I)could be used to compute grades for resource allocation and scheduling.

3.3.1 Resource Allocation

The access of resources is the same as Best Effort for GSM voice, whereas thedifference lies in GPRS strategy. The GPRS data is assigned a grade that equalsto C/I in our simulation (which is obtained according to the propagation channel)but that could be based on any parameters as described above.

Access Queue If no free TS is available for GPRS data, the incoming requestwill be put into the access queue. The principle for the resource allocation isFIFO with priority, where the priority is based on the grading(G) of GPRS datamentioned before. If the grades of the requests are different in the access queue,the highest one will get the available TS first. When several requests have thesame grade, FIFO strategy will be applied. Moreover, when the access queue isfilled, the request with smallest grade is blocked instead of blocking the incomingrequest.

Allocation As the scheduling is based on allocating a certain percentage of TSaccording to the grade, optimizing the allocation means choosing the TRX and TSgiving the best possible throughput.

Below a significant example is explained. The next request waiting in the queuecan be allocated in one of the two TRXs that are occupied as in the figure:

36

3.3. PROPOSED OPTIMIZED RRM STRATEGY

Figure 3.4: RRM Allocation Strategy

Each TS has a grade(TSG) calculated as the sum of all the TBF grades assignedto this TS. The possible % of a TS utilization for an incoming TBF is calculatedby the following expression:

TSavailable =GincomingTBF∑

TSG + GincomingTBF

(3.1)

In this example, the grade of the first request waiting in the access queue is12. For the first TRX, there are 6 TSs allocated for GSM, and only one is free.The TS utilization of this TRX by this TBF would be 1TS. In the second TRX,the TS utilization would be 0.93TS. The calculation of theTSG is shown in thepicture. Although the second TRX has more TSs available to be used, the actualTS utilization would only be 0.93TS, which is less than for the first TRX. In thiscase, the first TRX is selected.

3.3.2 Scheduling

In the case of multiple TBFs occupying one TS, as mentioned before, thescheduling of the optimized RRM strategy is also based on the grading which,in our simulation, depends on the C/I in the propagation channel. The probabilityfor each TBF in the same TS to use the assigned resource is calculated from its

37


grade(G) divided by the sum of all TBFs grades in this TS(TSG).

G =G∑TSG

(3.2)

For example, there are 3 TBFs in one TS. The grades are 3,1 and 6 respectively.Therefore, the possibility for the three TBFs to obtain the TS is 30%, 10% and60%, which guarantees the highest graded TBF will have most time to get TS totransmit data.

3.4 RRM assumptions and parameters used in Sim-ulation

Several assumptions were taken in the simulation regarding to the RRM strate-gies. In order to simplify the TBF assignment process, it is considered that eachTS can be occupied by 32 different TBFs regardless of their TFI. The access queuesize has been chosen to be of 7 sessions, which is a compromise in high loadingconditions between the access delay and the blocking probability. We considereda GPRS dropping criteria on the number of consecutive retransmission of an RLCblock. When this number reaches a certain threshold of 5, the GPRS session willbe dropped (which would correspond to a packet drop in real networks). Further-more, as we simplified the propagation model (only one Carrier to Interferenceratio per session), and do not consider GSM errors, no GSM dropping criteria hasbeen used for the simulation.

In the next chapter the simulation platform used to test and compare the twoexplained RRM strategies is described. The output results will be analyzed andcompared to validate the proposed optimized algorithm.

38

Chapter 4

Simulation Model of RRM

In this section, the network simulator and simulation assumptions are presented.The simulator is a time-driven network level simulator based on events,where thebasic simulation time step is 20ms (1 RLC block time of GPRS), but if eventscome between two time steps the simulation will jump at that time before goingto the next block (event-based simulation). Only the downlink traffic of 1 cell isinvestigated and simulated. Hence the simulation scenario was limited to Base-Station and Mobile-Stations and up till RLC layer.

Since the purpose of this RRM algorithm is to improve GPRS performance andonly take GSM voice call parameters as its limitation, the role of GSM voice callwas limited to occupy some radio resources. In this case, blocking rate was theonly service quality identifier of GSM voice and call dropping and handover wasnot in our consideration. Therefore the transmission time step was fixed to 20ms(1 RLC block time of GPRS, made up of 4 TDMA frames).

4.1 Cellular Network Assumptions

The simulations were made in a typical hexagonal micro cell environment. Thecell was assigned 4 TRXs with 32 TSs, four of which are for signaling (one in eachTRX) and the rest 28 traffic channels are shared between voice and data traffic.Since control channels are not implemented and the signaling delay is consideredinsignificant compared to the delay caused by access queuing, the access delay, asa GPRS performance indicator, is considered the same as access queuing delay.

Figure4.1 shows the channel with the radio resources available in a mediumcapacity cell without applying any partition for dedicated channels for voice ordata.

39

CHAPTER 4. SIMULATION MODEL OF RRM

Figure 4.1: Radio Resources

According to the mobility, a Typical Urban scenario with mobile user speedof 3Km/h (TU3)[?] was assumed, since it is a typical case for pedestrian user.The simulated mobiles are uniformly distributed in the cell and their movement isreduced to a small area around their initial position when setting up a voice callor data transmission session, so no mobility tracking was simulated.

All the mobiles are capable of using at most 4 TSs in one frame. Also all themobiles are in the active state to receive a GPRS data transmission.

The parameters from general cellular assumptions are presented in Table 4.1.

40

4.2. RADIO CHANNEL MODEL

Table 4.1: General Cellular AssumptionsParameter Name VALUESimulation Duration 4 hoursSimulation Time Step 20msRADIO CELLPower Control NoMobility Tracking NoHand-Over NoFrequency Hopping Ideal Random HoppingReuse Pattern 1/3TRXs 4 (32 TSs)Number of signaling channels 4Number of traffic channels 28Number of Mobile Nodes 30PARAMETERS FOR GPRS TERMINALMulti-slot capacity of Mobile 4TSs

4.2 Radio Channel Model

In a GPRS system, block error rate(BLER) in its air interface is a key param-eter to evaluate transmission quality and directly effects on RRM strategies(e.g.retransmission in upper layers). Given a certain GPRS coding scheme, BLERmainly depends on the C/I ratio(The ratio between a desired signal and the in-terference signal is described as C/I), which is decided by several aspects in thepropagation channel. These aspects are path loss, shadowing effect, multi-pathfading (fast fading), and co-channel interference, as described in Chapter 2.

Due to the complexity of a cellular system, in most case link level and net-work level simulators make up of a entire simulator but are studied and operatedseparately. A link level simulator generate a point-to-point MS-BSS transmissionprocess and the result (BER, BLER, etc) will be included in the network levelsimulator. Since the mapping curves from C/I to BLER in given coding schemesand propagation models have already been validated in the link level simulation in[?], we propose a C/I generation model that includes the mapping process in ournetwork level simulator. An overall picture of our radio channel model is depictedin Figure 4.2.

41


Figure 4.2: Radio Channel Model

4.2.1 C/I Generation Model

We propose a TU3 (Typical Urban, 3 km/h) model as the propagation environ-ment to generate C/I.

Path Loss

In free space, the path loss is proportional to the distance square [?]. In an urbanarea, it is typically proportional tor−3.5. The detailed locations and movementsof each mobile were not modelled, instead each user was assigned a random ini-tial distance from BTS according to their spatial distribution function. AssumeMS were evenly distributed in the hexagonal cell, we got the distance ProbabilityDensity Function(PDF):

f(r) = kr, 15m < r < 500m (4.1)

where r is the distance between MS and BTS and k is a coefficient after normal-ization.

Shadowing

Shadowing or slow fading can be seen as the gradual changes of the local meanreceived power for a MS, where the local mean power represent the average overa few tens of wavelengths [?]. The shadowing effect can be described by a log-

42


normal distribution(Equation 4.2) [?].

f(a) =1√

2πδ2aexp[−(ln a− µ)2

2δ2] (4.2)

a: fading coefficient, withE(ln a) = µ andV ar(ln a) = δ

In the situation of TU3, where the speed of the mobile is slow (3km/h), theeffect of shadowing causes the signal attenuation nearly flat during approximate1000 RLC blocks, where the wavelength equals to 30cm and 1 RLC block lasts20ms. In this case the fading level of shadowing was fixed to one value duringone session in the proposed simulator.

Multi-path fading

In corresponding to the slow fading, the multi-path fading is defined as fastfading. It can be described as a Rician or Rayleigh distribution [?]. In the formercase, there is a significant Line of Sight(LOS) between BS and MN; while if noLOS exists, it is referred to the latter case. The Probability Density Function(PDF)of Rayleigh distribution is expressed as:

f(r) =r

σ2exp(− r2

2σ2) (4.3)

The fast fading characterizes the fast variation of the signal envelop over a shortdistance of a few wavelengths or over short time durations of seconds [?]. InTU3 situation, the variation of the signal changes relatively slow, nearly 50 RLCblocks unchanged . Detailed analysis on the time correlation and frequency cor-relation of multipath fading is presented in Appendix A. It was proposed thatgiven a frequency hopping distance greater than 500kHz every TDMA frame [RefMarco](separations of 600to 800 KHz are reported to be adequate [?]), the mul-tipath fading levels for consecutive RLC blocks will be fully uncorrelated, thatmeans only random block error need to be simulated.

In reference [?], fast fading effect has been included in the proposed CIR-BLERmapping tables and mean BLERs during one session were given. Hereby onlypathloss and shadowing need to be implemented in proposed simulator.

Co-channel Interference

We consider Co-channel interference as the predominant aspect of C/I, whichincludes the 6 interferes from the first tier(6 adjacent cells with the same fre-quency), and neglects all other interferers. This model is depicted in Fig 4.3.

43


Figure 4.3: Radio Channel Model

Assume the same transmitted power for both desired and interference signal,based on pathloss, the CIR is

CIR =1

2(Ru− 1)−3.5 + (Ru− 0.5)−3.5 + (Ru + 0.5)−3.5 + (Ru + 1)−3.5 + Ru−3.5

(4.4)[?] Where Ru=D/r, D: reuse distance, r: MS-BTS distance.

Conclusion

The situation of propagation simulation is under TU3 model, where the fastfading plays a dominant role in the effect of propagation channel, given the slowspeed of the mobile. Since the values of BLER are taken from the mapping ta-ble in [?], where the multi-path fading has already been applied, and shadowinglevel during one session will keep almost constant, the path loss effect will play apredominant role in deciding the C/I value for each session.

When considering different users with the same distance from BTS, with thesame path-loss effects, log-normal fading can be considered as a spatial variationamong different users with the same distance from the BTS. Assumed the receiveddesired signal and the interferers (which have experienced path-loss) face to in-dependent log-normal distributed stochastic variations, which can be presented indB as Equation 4.5.

C/I = DesiredSignal −∑

Interferes (4.5)

44


and both of the components on right side follow normal distribution, C/I followsa normal distributionN(µ, δ)(since the sum of independent normal distribution isstill a normal distribution), so the C/I is also a log-normal distribution. The meanvalue was calculated from path-loss and the standard deviation was set to 7dB.

Finally, the whole propagation model can be presented as the diagram in Figure4.4, in which the average BLER of a user can be calculated.

Figure 4.4: Propagation Model Diagram

When we link the user distribution and the corresponding CIR mean value frompath-loss(before imposing log-normal fading), the PDF of mean CIR in a cell canbe shown in Fig 4.5.

45


Figure 4.5: Distribution of CIR in a cell

In actual implementation, we accumulated this PDF on 1 dB step, and the prob-abilities above 30 dB were merged since form 30 dB all the coding schemes havealready achieved their maximum throughput and the increase of C/I will not bringfurther beneficial, as shown in Fig??.

All the assumed propagation settings are listed in Table 4.2.

Table 4.2: Propagation Channel ParametersParameter Name VALUEReuse Pattern 1/3Co-channel Interferers 6, first tierCell Radius 500mPath-loss exponent 3.5Minimum MS-BTS distance 15mLog-normal fading STD 7dBFrequency Hopping Ideal Random Hopping

After C/I for each session was generated, the next step is to map it to BLERaccording to the mapping table[?] under a selected coding scheme. The process

46


to select the most appropriate coding scheme is referred to as link adaption (LA).We have assumed an ideal case where we can measure or estimate the C/I ofeach user before the real data transmission start, so we are able to select a codingscheme to achieve the maximum session throughput based on figure 4.6 [?] .

Figure 4.6: Throughput of Different Codes

Finally, the proposed LA policy is presented in Table 4.3.

Table 4.3: Proposed Coding SelectionCIR Selected Code≤ 5dB CS16∼ 9dB CS210∼ 16dB CS3≥ 17dB CS4

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4.3 Traffic Model

The GSM/GPRS network serving both circuit-switched voice and packet-switcheddata is the focus of attention.

The Traffic Models are defined to characterize the behavior of the users in acellular system. So the time variation of traffic is studied, both for voice andGPRS data. Although GPRS has a wide range of applications and its usage isincreasing, currently the presence of GSM voice service is much higher than data,so their representative probabilities are assumed over the global traffic load, e.g.the 70% voice traffic and 30% data load [?]. This values can be varied later on inthe simulation to evaluate the results with different traffic load probabilities.

The term of session is referred to both voice and data transmission. The sessionis defined from the moment when an event arrives to the BSS until the transmis-sion is finished, such as for voice the end time is reached or for GPRS all theRLC blocks have been sent to a specific Mobile Node or in the worse case whenthis event has been blocked or dropped. For GPRS data, a session consists of anumber of information bits,corresponding to 1 TBF, simplifying the hierarchicallayers for GPRS data.

The arrival and duration of sessions are statistically distributed depending onspecific parameters for each type of service. These events are represented in asimple way in the following figure (Fig.4.7)

Moreover each generation session is associated with one MN using uniform dis-tribution. One MN representing only a propagation condition at a specific distancefrom the BS, it is considered to be able to accept as many sessions as possible. Itcan correspond to the situation with some real mobiles being at the same distancebut with different shadowing factor, which is calculated at the beginning of thesession.

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4.3. TRAFFIC MODEL

Figure 4.7: Mixture of Voice and Data Traffic

4.3.1 GSM Voice Call

The voice service is delimited by means of the following parameters[?]:

• arrival time. The number of events arriving in one radio cell is describedas a Poisson distribution and it requires the definition of the mean arrivalrate,λGSM . It models the number of random occurrences of voice calls(N(t))in a specified unit of time. The probability that the variable N(t)reaches a value n is given by the formula:

f(N(t) = n) =(λGSM t)ne−λGSM t

n!(4.6)

It is assumed that the inter-arrival times are independent and identicallydistributed according to an exponential distribution[?].

• holding time, call average duration. It is defined as an exponential distribu-tion, and represented with the mean holding time, 1/µGSM .

The probability density function of the exponential distribution is (parame-terµv >0):

f(t) = µGSMe−µGSM t, t >= 0 (4.7)

4.3.2 GPRS Data Session

Unlike the GSM voice, the data traffic is more difficult to model due to theapplication and the bursty nature. According to the application, the traffic datacan be mainly WWW-browsing and e-mail, FTP, WAP, and other applications[?].

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However, this aproach to the higher level is not necessary since the sessionis assumed to be composed by a number of bits whose size can be calculatedaccording to a certain distribution function.

As it was seen for GSM calls, a traffic model consists of two parts: the arrivalprocess and the activity phase.

• The arrival process is described most frequently with the Poisson functionwhich counts the number of events coming within a certain time period.The mean arrival rate per session,λGPRS is specified and assigned a value.As it was defined for GSM calls, an exponential distribution for the inter-arrival time process is used to determine the exact time when an GPRS eventarrives within the simulation time.

• For the activity phase, the duration of the session is characterized withthe number of bits transmitted and calculated by means of the geometricprocess specified below. Then, according to the number of bits obtainedand the coding scheme specified, the number of RLC blocks is carried out(Fig.4.8). The testing of the following distribution function will be carriedout in Chapter 5.

The Geometric distribution is characterized as follows (parameter 0<p<1)[?]:The probability density function:

p(x) = p(1− p)x−1, x = 1, 2, .. (4.8)

The variable x is referred to the number of bits, and this distribution is usedfirst because it is simpler to model.

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4.3. TRAFFIC MODEL

Figure 4.8: GPRS Session

Under those traffic conditions, the simulator is implemented, and the valuesof these parameters will be specified.This traffic generation for voice and data isshown in Fig.4.9.

Figure 4.9: Traffic Generation

4.3.3 Mapping of the traffic load

In the analysis of the results, the view of the traffic load is useful to mea-sure the capacity of the network. For GSM voice call, the traffic load is definedas the measurement of channel time utilization, i.e. the average channel occu-pancy (Erlang=call-hour/hour).It is expressed by means of the average arrival rate

51


(λGSM ) and the average holding time (1/µGSM ), where Aoff−GSM is called theoffered traffic for voice.

Aoff−GSM = λGSM ∗ 1

µGSM

(4.9)

For GPRS, the offered load (Aoff−GPRS) is calculated with the arrival rate(λGPRS), and the mean holding time (1/µGPRS), which is estimated by meansof average values of the session size and the data rate in coding scheme . Thesession size (number of bits per session) is obtained with the previous distributionfunction and together with the coding scheme selected in the propagation model,the number of RLC blocks is determined. The mean holding time is calculated inthe following equation:

1/µGPRS =meansessionsize

meandatarate(4.10)

The overall offered load is simply carried out by adding the voice offered loadto the data offered load [?] :

Aoff = Aoff−GSM + Aoff−GPRS (4.11)

This mapping from the traffic load to the traffic parameters (λGSM , 1/µGSM ,λGPRS, session size) is shown as follows in Fig.4.10:

Figure 4.10: TrafficBlock

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4.4. RLC/MAC FUNCTIONALITIES

Table 4.4: Traffic Model ParametersTRAFFIC MODEL IDENTIFIERS VALUESVOICE TRAFFICarrival rate,λGSM λGSM=Aoff−GSM*µGSM )mean holding time, 1/µGSM 120 sPACKET TRAFFICarrival rate,λGPRS λGPRS=Aoff−GPRS*µGPRS)session sizeGeometric:p 0<p<1

Therefore, in the simulation the input parameters the user can introduce linkedto the traffic model are the traffic load for both voice and data, the mean holdingtime for call voice, and the probability for the distribution function in the sessionsize (See Table 4.4):

4.4 RLC/MAC Functionalities

4.4.1 TBF establishment

TBF establishment’s signaling process is not simulated in the model as norUplink nor Control Channels are simulated. As soon as the BSS has decided toassign a TBF, the first data block is being transmitted at the following time step.

4.4.2 ARQ

In order to ensure reliable, verified, error-free delivery of data in the presenceof different propagation conditions, the simulated RLC layer is working in theacknowledged mode so that automatic repeat request (ARQ) schemes can be usedto synchronize and acknowledge the transmission of data between the base stationand the mobile node.

As an ARQ protocol,Selective Repeatprotocol is used in the simulation. Weassume ACKs/NACKs are sent by the MS on the control channel PACCH (thuscorrectly received by the BTS) for each correct/erroneous block. The receptionof a NACK triggers the retransmission of the erroneously received block and thereception of an ACK updates the transmitter window. Because we assume thesignalling channels are error free, the choice of the window has no importance.

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This ARQ protocol insures a good use of the channel capacity as only erroneousblocks are retransmitted.

4.4.3 MAC layer

As uplink is not simulated in this model, the MAC functionalities against con-tention are not to be taken into account.

4.5 Simulator structure and basic description

Figure 4.11 is a basic picture of the object oriented simulator we developed inorder to compare the two different RRM strategies (”best effort” and optimizedRRM). It is an event based simulation, which means that the time step is not fixedas incoming sessions can be arriving anytime. Below are described the differentobjects and how they are used in the simulation process.

Figure 4.11: Overview of the simulator’s objects

4.5.1 Description of the different objects

The MS object is the description of a propagation condition associated witha certain location. The propagation values (CIR,BLER) are generated initiallyaccording to the earlier described propagation model. We consider that it canaccept more than one session, as it only represents a propagation condition (uplinkis not considered).

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4.5. SIMULATOR STRUCTURE AND BASIC DESCRIPTION

The traffic model object generates all traffic before the simulation start accord-ing to the earlier described model. It links a generated session with a mobile in auniformly distributed manner and communicates the events to the BSS object atthe right time.

The air interface object contains the actual transmission process and links thescheduler of the BSS with the different MS to compute how blocks are transmittedand return the different acknowledgment to the BSS (simplified uplink part).

The BSS object contains the RRM (channel allocation, scheduling, and channelrelease process), and the handling of acknowledgments. It’s divided into 3 parts,an access part, handling the arrival of a session from the traffic model object, a re-source update part dealing with updating active session queues and (re)assignmentof resources when necessary, and a scheduling part, linked with the air interfaceobject and preparing the next transmission.

The trace object is linked with most of the other objects and keeps track of thesessions to be able to output the results at the end of the simulation in a matlabcompatible file. Outputs are then plotted with some matlab functions using thedata traced from this object.

4.5.2 Simulation process

Figure 4.12 describes the simulation process linked with the above design, fromthe initialization part where traffic is generated and propagation values from theMS are computed, to the actual simulation part.

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Figure 4.12: Simulation Process

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4.5. SIMULATOR STRUCTURE AND BASIC DESCRIPTION

Initialization Before starting the simulation, all the MS objects are initialized(propagation condition), and traffic for all the simulation is generated accordingto the specified traffic load parameters.

Incoming event When an event is incoming, which corresponds to a GPRSsession or GSM call arrival, the simulator calls the access part of the BSS objectwith all the session parameters taken from the traffic model object.

Next block time step When a block time is reached, first the transmission ofall blocks is simulated using the air interface object. The resource update partof the BSS is then called with earlier acknowledgments to update the active ses-sion queues and release if a session or GSM call has ended, assign or reassignresources if necessary. Finally, the scheduler of the BSS is called to compute thenext transmission.

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Chapter 5

Performance Evaluation

This chapter mainly aims at analyzing the simulation result of the proposedRRM strategy. Validation of the simulator can be found in Annexe 1. First of all,the choice of the performance metrics used to evaluate the proposed strategy incomparison with the Best Effort are described, as well as the simulations parame-ters. Then the simulation results are analyzed for each considered metric in orderto validate the expected improvements of this strategy.

5.1 Performance Metrics and Simulation Parame-ters

In order to compare the two strategies, several performance metrics have beenconsidered. In addition simulation parameters have been defined to output signif-icant results.

5.1.1 Performance Metrics

The performance evaluation was based on the following indicators.

• Mean Access Delay

Since the incoming data which can not obtain a TS will be put into an ac-cess queue, an access delay referring to the waiting time in the queue isintroduced. The mean value of this parameter for an entire simulation mea-sures the average waiting time for GPRS sessions before they are allocatedresources.

• Throughput per Cell/User

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5.1. PERFORMANCE METRICS AND SIMULATION PARAMETERS

Throughput is the measurement of the data transmission speed. Two dif-ferent types of throughput have been considered. The throughput per cell(Equation (5.1) ) measures the overall system performance. It is computedusing the total number of bits transmitted and the simulation duration. Themean throughput per session (Equation (5.2) ) describes the mean transmis-sion speed offered to GPRS users. It is computed using the number of bitstransmitted in all non-blocked/non-dropped sessions with the duration ofthe corresponding sessions (not including access delay).

µthroughput/cell =

∑transmitted data

simulation duration(5.1)

µthroughput/session =

∑throughputs of successfull sessions

number of successfull sessions(5.2)

• Blocking Rate

When resources cannot be allocated to an incoming session, a session maybe blocked (the criteria used depends on the RRM strategy). Therefore, theaverage blocking rate for the simulation is described with this metric.

GSM blocking rate is not used. Indeed voice calls have the priority overGPRS sessions and no TS are dedicated to GPRS for both strategies, there-fore the resource available for GSM is the same for both strategies. Thusthe GSM blocking rate is not influenced by the choice of the strategy.

5.1.2 Simulation Parameters

The input parameters used for all outputs are described in Table 5.1. They arethe same for the two different BSS, in order to have a fair comparison. The movingparameter chosen to compare the efficiency of both RRM strategies is the trafficload. It shows the effect of the two policies in different traffic conditions.

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CHAPTER 5. PERFORMANCE EVALUATION

Table 5.1: Simulation ParameterParameter Name VALUESimulated Duration 4 hoursTraffic Load (TL) from 5 to 29 ”Erlangs”Number of simulations (per TL) 15TRXS 4 (32 TSs)Number of signaling channels 4Number of traffic channels 28Maximum number of multiplexed sessions perTS

32

Number of Mobile Nodes 30Multi-slot capacity of Mobile 4TSsMean GSM call duration 120sMean GPRS session size 100kBAccess queue size 7 sessionsMaximum number of block retransmission 25

5.2 Performance Evaluation

This section will present and analyze the results obtained using the parametersdefined in previous section according to the performance indicators.

Confidence Intervals In order to analyze the validity of our results, 95% confi-dence intervals of the shown output figures have been observed in Annexe 2.

Throughput Per Cell Figure 5.1 shows the throughput per cell of both strate-gies.

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5.2. PERFORMANCE EVALUATION

Figure 5.1: Throughput Per Cell Vs. Traffic Load

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It can be observed that up to a certain traffic load, the throughput per cell in-creases with the traffic load. Until higher traffic loads, the system throughput isequivalent for both strategies. That can be explained by the fact that for low trafficloads, there are not many GPRS users in the cell, whereas this number increaseswith the traffic load.

The enhancement provided by the proposed RRM appears as traffic load in-creases. It can be observed that the maximum throughput is higher for the pro-posed RRM than for the Best Effort. This performance enhancement is due tothe fact that our grading system prioritizes the GPRS users with higher CIR inthe scheduler and the access queue, thus users that are using Coding Scheme withhigher throughput.

After a certain point is reached, the throughput decreases as the number of GSMcalls increases and voice has the priority. We can notice that this point appears fora higher traffic load on the proposed strategy.

Mean throughput per user Figure 5.2 shows the mean throughput per user ofboth strategies.

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Figure 5.2: Throughput Per User Vs. Traffic Load

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The mean throughput per user decreases with the traffic load as fewer resourcesare available. For low traffic load, the result is equivalent for both strategies but itcan be seen that the trend is sharper for the Best Effort. The mean throughput peruser is thus better with the proposed strategy for high traffic loads. The reason forthat result is the same as for the throughput per cell.

Mean access delay Figure 5.3 shows the mean access delay of both strategies.

Figure 5.3: Best Effort: Access Delay Vs. Traffic Load

The access delay for both strategies is increasing with the traffic. As the moreusers there are, the more filled is the access queue. The performance result is muchbetter for the proposed algorithm because it prioritizes the better CIR sessions thusthe average data transmission time is less and resource are released faster allowingother sessions to get out of the access queue.

GPRS Blocking Rate Figure 5.4 shows the GPRS Blocking Rate of both strate-gies.

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Figure 5.4: Best Effort: GPRS Blocking Rate Vs. Traffic Load

The GPRS blocking rate is much higher for best effort for the same reasons asfor the delay. Indeed blocking a session happens as the access queue gets filled.

Throughput according to CIR A possible drawback of the CIR grading sys-tem could have been that GPRS users with high CIR would have had a betterthroughput than with the best effort whereas the users with lower CIR would havehad a worse one. This could be explained by the fact that users with high CIR areprioritized for the use of resource. From this point of view, the grading systemaccording to CIR does not seem fair as users under bad radio conditions are notadvantaged.

In order to check this hypothesis, the throughput according to CIR was outputin Figure 5.5. It is a mean for all simulated traffic loads.

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Figure 5.5: Best Effort: Throughput Vs. CIR

This figure shows that the hypothesis that users with bad CIR have a lessthroughput with our strategy than with the best effort is wrong. In average, everyGPRS user should be more satisfied with this algorithm.

5.3 Summary of output results

The chosen outputs show better results for the proposed strategy compared tothe best effort. By prioritizing high CIR values for resource access and use, ithas been shown that the user and system throughput increases and resources arereleased faster, thus affecting access delay and blocking rate. Many other metricsand parameters could have been investigated such as the mean number of retrans-missions, the mean throughput per TS, the use of dedicated PDTCH and TCH, butwe focused on the most relevant ones from our point of view and tried to make afair comparison between the two different RRM.

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Chapter 6

Conclusion and prospect

This project mainly aimed at investigating an optimized RRM strategy for GSM/GPRSlike networks in order to improve the system performance. Based on this purpose,the principle of GSM and GPRS network has been studied first. A simulationplatform has then been implemented with basic propagation and traffic model as-sumptions. The currently used RRM strategy (Best Effort) has been implementedas well as a newly designed optimized RRM strategy. Both strategies have thenbeen compared and analyzed to conclude that RRM schemes are a very importantfeature of wireless networks and could be enhanced in the future especially forUMTS like networks.

The simulation results of the proposed optimized RRM strategy is comparedwith the Best Effort method in order to verify the improvement of the system per-formance. From the simulation results, it can be concluded that the throughputincreased and the access delay decreased for the proposed optimized RRM strat-egy, which certifies the conformity between the practical simulation scenario andthe theoretical description.

Due to the time limitation, the RRM strategy developed is very simple andcould be improved in future work by considering a more optimized grading crite-ria that could take into account QoS for example. In addition to that, the effect ofdedicating GPRS or GSM channels would also have to be investigated.

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Appendix A

Simulator validation

A.1 Propagation model and air interface

In order to validate the behavior of the air interface model and transmission partof our simulator, a simple way was to output the throughput of one session usingan entire time slot (no multiplexing) according to its CIR. This curve has beencompared to the one used in the mapping model between CIR, CS, and BLER,as the best Coding Scheme is used for each transmission. The transmission partis also verified with the values of the throughput obtained. Figure?? shows thecomparison between the two curves.

A.2 Traffic model

The traffic model validation has been carried out by checking the generated ses-sionsparameters such as the mean number of GSM/GPRS sessions per hour, meancall duration/mean session size compared to the input parameters.

A.3 RRM strategies

In order to make sure the simulation of proposed RRM strategy was under thedesigned scenario, simple simulations has carried out, and debugging informationhas been checked to compare the simulator behavior with the expected one.

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Appendix B

Confidence Intervals

B.1 About confidence intervals

Confidence interval is used in order to estimate a range of values of an unknownoutput parameter for given samples. This technique is used because a mean valuecan vary from sample to sample. Instead of having an estimate of a mean value, alower and upper limit are obtained. It gives an idea on how much uncertainty thereis in the estimate of the ”true” mean. The wider the interval is, the less precise theestimate is.

The confidence level is the percentage of the intervals (computed for each in-dependent sample) that are likely to include the searched mean value. The confi-dence level considered in the following section is 95% (most used value).

B.2 Output curves with computed confidence inter-val

This section shows the different curves resulting from the simulations with thelower and upper limit (dotted curves).

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