UNIVERSITI PUTRA MALAYSIA

CDMA SLOTTED ALOHA MAC PROTOCOL FOR ABR TRAFFIC OVER SATELLITE LINKS

SALEM KRIEM EMHEMED

FK 2000 24

CDMA S LO TIED A LOHA MAC PROTOCO L FOR ABR TRAFFIC OVER SATE LLITE LINKS

B y

SALEM KRIEM EMBEMED

Thesis Submitted in Ful filment of the Requirements for the Degree of Master of S cience in the Fa cult y of Engineering

U niveniti Putra Mal aysia

Jul y 2000

2

To my family ...

Abstract of thesis presented to the Senate ofUniversiti Putra Malaysia in fulfilment of the requirements for the degree of Master of Science

CDMA SLOTTED A LOHA MAC P ROTOCO L FO R AB R T RAFFIC O VER SATE LLITE LINKS

By

SA LEM KRIEM E MHEMD

Jul y 2000

Chairman: Associated Pro fessor Borhanuddin Mohd .Ali, Ph .D .

Faculty: Engin eerin g

3

The design of a packet radio network with a large number of terminals and

a single hub station involves the use of two basically different types of

communication channels and channel architectures. One type of channel is a

broadcast channel used to transmit data from the hub station to the terminals.

Transmitting data from a single hub station to a large number of terminals

(one to many) is a relatively simple problem. This channel architecture is almost

always configured in a simple time division multiplexed (TOM) mode. The other

type is transmitting data from terminals to a single hub (many to one) which is

much more challenging problem.

In general the choice of multiple access protocol for a particular

application should depend on two primary factors :

( t) The traffic characteristic of the data network of interest

4

(2) The state of technology development at the time the network is deployed

In this thesis we investigate the combination of two multip le access

schemes, slotted Aloha and DS- CDMA spread spectrum protocol (DS-CDMA

slotted Aloha) over LEO satellite link in the uplink from terminals to the satellite.

In this protocol the channel is divided into time slots. Each user is assigned

a time slot equal to the packet transmission. Prior to transmission each user

randomly choose unique code sequence different from other users. After

transmission the unsuccessful packets should be re-transmitted after a random

time delay.

The simulation is done by an OPNET package in the presence of LEO

satellite system with non-real time traffic type (ABR traffic). The investigation

includes the throughput performance of the DS-CDMA slotted Aloha, Packet loss

ratio, and the bit errors in the packet transmission in the presence of MAl, A WGN

and the error correction mechanism.

The simulation shows high throughput performance is obtained against

other conventional narrowband protocols such as pure Aloha and slotted Aloha

protocols due to the capability of the spread spectrum technique to increase the

channel capacity. The simulation also shows an improvement in the throughput

perfonnance by implementmg the error correction mechanism.

Abstrakt tesis yang dikemukakan kebada Senat Universiti Putra Malaysia sebagi memenuhi kepeduan untuk Ijazah Master Sains

CDMA A LOHA DISLOT PROTO KOL C APAlAN PE LBAGA I UNTUK KADAR BIT ADA KE ATAS P UATAN SATELLITE

Oleh

SA LEM KRIEM EMHEMD

Julai 2000

Pengerusi: Profesor Madya Borhanuddin M ohd Ali, Ph .D.

Fa kulti: Kejuruteraan

5

Merekabentuk radio rangkaian paket dengan bilangan terminal yang

banyak dan satu stesen hab, melibatkan dua jenis saluran komunikasi dan senibina

saluran yang berbeza. Salah satu jenis salman ialah salman penyiaran yang

digunakan untuk menghantar data dari stesen bah ke terminal.

Penghantaran data daripada satu stesen bah ke bilangan terminal yang

banyak (daripada satu ke banyak:) adalah satu masalah yang mudah manakala

salUTan senibena selalunya dikonfigurasikan dalam mod pemultipleksan

pembahagi masa (roM). Penghantaran data daripada banyak terminal kepada satu

hab (banyak ke satu) adalah lebih mencabar.

Pada amnya pemilihan protokol capman pelbagai (multiple access

protocol) untuk kegunaan tertentu adalah bergantung kepada dua faktor utama:

( 1) Ciri trafik bagt rangkaian data

6

(2) Tahap pembangunan teknologt pada masa rangkalan ltu dtperkenalkan

Dalam ka.J1an 1m, kanu mengkaJl dua Jenis skim capaIan peibagat Aloha

dislot dan protokol spektrum sebar (DS-CDMA slotted Aloha) ke atas pautan

sateht OrbIt Bunu Rendah (LEO) dan termmal ke satelit.

Dalam protokol 1m saluran dibahagtkan kebada slot-slot masa. Settap

pengguna mengambli slot masa yang sarna untuk penghantaran paket. Sebelum

penghantaran, setiap pengguna memihh secara rawak jujukan kod yang berbeza

dengan pengguna laIn. Selepas penghantaran paket yang tidak befjaya pedu

dihantar semula selepas melepasl kelewatan masa rawak.

Proses simulasi dijalankan menggunakan OPNET dengan kehadiran

sistem sateht LEO bukan masa-nyata Jems Kadar Bit Ada (ABR). Penyehdikan

1n1 termasuldah mengkaJi prestasl DS-CDMA Aloha dislot, msbah kehilangan

paket, ralat dalam paket penghantaran dengan kehadiran multiple access

mterference( MAI), additIve whIte guasslan noise ( A WGN) dan mekanisme

pebetulan ralat.

Hasd danpada S1II1ulasl yang diJalankan satu pre Stasi yang tInggt

dtperolebJ. dintara protokol Jalursemplt b1asa sepem Aloha ash dan Aloha dlslot

dlsebabkan kemampuan tekmk spektrum sebar yang boleh menmgkatkan kapasltI

sa luran . Slmulasl In1 Juga dapat memperbalkI ralat prestasl pengeluaran dengan

memperkenalkan mekanlsme pembetulan ralat

7

ACKNOWLEDGMENTS

In the name of Allah the Most beneficent and the Most Merciful. First and

for most, I wish to give all the praise to Almighty Allah for with His mercy He

has given me the life and sustenance and giving me the strength and time to

complete this research.

Secondly, I would like to extend my heartiest thanks to my supervisor

Professor Madya Dr Borhanuddin Mohd Ali for his valuable guidance and

encouragement. Also I would like to thank Mr. Ashraf Gasim and Dr.Sabira

Khatun for their guidance, advice and comments. I thank Mr. Mohd Hadi Habaebi

for his help. Next, I am grateful to the Faculty of Engineering University Putra

Malaysia for all the facilities provided to finish my research nicely.

I am thankful to all academic and non-academic members of department

of Computer and Communication System Engg.,UPM for their kind cooperation

and help. My gratefulness also goes to the administrations and staff of

PostGraduate Center at UPM for all their cooperation.

Special mention must be made to my wife and my two lovely kids for their

patience and inspiration they have given to me. Last but not least, my sincere

thanks to Libyan General Committee of Education for granting me financial

support for my entire study period in Malaysia, and thanks to all members of

Libyan People's Bureau in Malaysia for their support.

8

I certify that an Examination Committee met on 12 July 2000 to conduct that the final examination of Salem Kriem Emhemd on his Master of Science thesis entitled "CDMA Slotted Aloha MAC Protocol for ABR Traffic Over Satellite Links" in accordance with Universiti Pertanian Malaysia (Higher Degree) Act 1980 and Regulations 198 1 . The Committee recommends that the candidate be awarded the relevant degree. Members of the Examination Committee are as follows:

Dr.Y.Prakash, Ph.D Fakulty of Engineering Universiti Putra Malaysia (Chainnan)

Associated Professor Borhanuddin Mohd Ali, Ph.D Fakulty of Engineering Universiti Putra Malaysia (Member)

Dr.Sabira Kbatun, Ph.D F acuity of Engineering Universiti Putta Malaysia (Member)

Eng.Ashraf Gasim Faculty of Engineering Universiti Putta Malaysia (Member)

- '

HAZALI MOHA YDDIN, Ph.D, ProfessorlDeputy Dean of Graduate School, Universiti Putra Malaysia

Date: 0 2 AUG 2000

9

This thesis submitted to the Senate ofUniversiti Putra Malaysia and was accepted as fulfilment of the requirements for the degree of Master of Science.

KAMIS A WANG, Ph.D, Associate Professor Dean of Graduate School Universiti Putra Malaysia

Date: r11 NOV 2000

DECLARATION

I hereby declare that the thesis is based on my original work except for quotations and citations, which have been duly acknowledged. I also declare that it has not been previously or concurrently submitted for any other degree at UPM or other institutions.

(SALEM KRIEM'EMHEMD)

Date: 2 108 / 2000

11

TABLE OF CONTENTS

Page DEDICATION . . . . . .. . . . . . . . . . . . . . . . . , ... '" ... .... , . ..... , ............ . , .................... 2

ABSTRACT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 3

ABSTRAK . . . . . . . . . . . . . . . ... . . . . . . .. . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 AKNOWLEDGMENTS . . . . . . . . . . . , . ........ ..... , ... '" ... ... ... ..... , ... . . . . . . . . . . , . . .. 7 APROV AL SHEETS . . . .. . . . . . .. . . . . . . . . .. . .. . . . . . . . . .. . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . .. 8

DECLARATION FORM . . . . . . . . . . , . ...... ... ...... ......... ... ... .. , ...... ......... ... 10

LIST OF TABLES . . . . . . . . . . . , '" ...... ...... ... ......... ... ..... , ...... ...... . , . ..... , ... 13

LIST OF FIGURES . . . . . . . . . . , . . . . . . . . . . ... '" . . . . , . . . . . . , . . . '" . . . . . . . . . . . . . . , . . . . . . . . . . 14 LIST OF ABBREVIATIONS ... . . , ... ... ......... ...... ......... .... , .... ............. 16

CHAPTER

I INTRODUCTION . . . . . . . . . . . . . . . . . . . .. . .. . , ...... , ... ...... '" ... '" ..... .17 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ... .. , ............... ' " .. , . . , ....... 17 Multiple access schemes in wireless channel.. ........ , ...... , ...... .... 19

Fixed assignment schemes . . . .. . . . . . . , . .. . . . ..... . . , . ..... , . . . 20 Demand assignment scheme . . . .. . . . . . . . . . . .. . . . . . . , . . . . ..... 20 Random assignment schemes . . . . . . . , . ... '" ......... ......... 21

Code division multiple access scheme . . . '" '" ... '" ....... 22

Statement of the problem . . . . . , ... .......... , . ... .. , ............ ... ..... , ... 23

Research objectives . . . . . . . .. . . . . . . . . . . . . . . . . , . ......... ...... .. . ....... . . .... 25 Thesis organization . . . . ,. '" ... '" .. , '" ..... , . . . . .. ... '" ., . ..... , ......... 25

II LITERA TlJRE REVIEW . . . '" . ..... ... .. . . ... . , ... . .. . ... . . ... ... .. , ... 26 Multiple Access Scheme in Satellite System . . . . . . . . . . . . . . . . . . . , . ...... 26

Frequency Division Multiple Access ... .. , . . . . . . . .. . . . . . . . . 27 Time Division Multiple Access . . . . . , ...... ... ... . , . ... .. , .. 28

Demand Assignment Multiple Access ... . , . ......... ... ... 30

Pure Aloha Multiple Access . . . . . . . . . . . , ...... ... ............ 31

Slotted Aloha Multiple Access . . . . . . . . . . . . . .. . . . .. , ... ...... 33

Code Division Multiple Access . . . . , . ...... ... ... ... ... ...... .. , ... ... ... 36

Basic Theory of CDMA system . . . . . . . . . . . . . . . .. . . .. . . . . . . . . . . . . . . . . . , .. .38

Satellite Communication . . . . . , ............ ...... ... ... ........ . . , . ..... , . .. 51 Orbits on Satellite system . . . . . . . . . '" ...... ............ ... ... 52

Multiple Access Schemes . . . . . . . .. . . , ........... . . , ...... , ..... . .. . .... , .. 54 Conclusion ......... ... . " ...... ... ... ... ... ... ..... , ... ... ...... ... ... ... ..... 58

ill METHODOLOGY . . .. . ... . .. . . . . . . . .. . .. . . . . . . . , ....... ... . ... , ......... 61 Proposed Protocol.. .... " . .......... , . ... .. , ... ... ... '" ... ...... ... ..... , 61

Simulation Model.. .. . , ................. .......... ... ..... , .. ............. 64 General Assumption and Characteristic of the Simulation Model . . . . . . ... . . . . , ...... , ................ 64

Aloha Simulation Model ..................... '" ........ , ...... .......... 66

12

Slotted Aloha simulation Model . . , ...... '" ., .... .. . '" ... ... ... ....... 72 CDMA Slotted Aloha Simulation Model. . . . . . . .. . . . . . . ... . .. . . . . . . . . . . 73

IV RESULTS AND DISCUSSION .. . . .. . . . .. . . . . . . . . . . . . . . . . . . . . . . . . , ... 78 Pure Aloha Protocol . . . . . . . .. .. , '" ... .......... , . .... , . .... , . ............. 79 Slotted Aloha Protocol . . . .. . . .. . .. . . , ............... ... '" ......... ....... 80 Pure Aloha and Slotted Aloha Protocols .. . ... . . .. , . .... , . ... . , .... '" .. 81 DS-CDMA Slotted Aloha with Different Modulation Schemes . . . . . 82 Throughput of the DS-CDMA Slotted Aloha Protocol in the AWGN ChanneL . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84 Throughput of DS-CDMA Slotted Aloha with Different Number of Packets . . . .. . . . . . . . . , . ... ......... ... ...... '" ...... ... ..... , ... 86 Throughput of DS-CDMA Slotted Aloha Protocol in the Presence of Noise Figure . . . . . , ... .. , .... ,. '" ... ... ... ... ... ......... ... 87 Packet Loss Ratio of the DS-CDMA Slotted Aloha in the Presence of the Noise Figure . . . . . . . . . .... . . . . . . . . . . . . . . . . . .... .. . . . . . .. .. 88 The Bit Errors Per Packet of the DS-CDMA Slotted Aloha in the presence of the Noise Figure . . . . . , .. , '" ... '" ." '" .. , '" ." ....... 89 The Packet Loss Ratio of the DS-CDMA Slotted Aloha in the A WGN Channel.. . . . , '" ........ , ... . " ...... ............ ...... ...... 90 The Bit Errors Per Packet of the DS-CDMA Slotted Aloha in the AWGN Channel. . . . . , ... ...... ... ...... ............ .................. 91 The Packet Loss Ratio of the DS-CDMA Slotted Aloha with Different Number of Packets .. . . . . . . . . . . . . . . . . . . . . . . . . . . . , ..... , .. 92 Throughput Improvement of the DS-CDMA slotted Aloha With Error Correction Mechanism ... .. . . . . . . , ..... , ..... , ... .......... 93 Packet Loss Ratio of the DS-CDMA slotted Aloha in the Presence of the Error Correction Mechanism . . . . . . . .. . . . .. . . . , ....... 95

V CONCLUS ION AND FUTURE WORK . . . . . . . .. ..... . .. . . . , ... .. , .. 97 REFERENCES . . . '" '" ... ............ '" ...... ...... ...... ...... ...... 100 A P PEND ICES . ... , . ..... , ... .. , ...... '" ... '" ......... ...... .......... 102

A OPNET Simulation Package .. . . . . . . . . . . .. . . .. . . . . . . . .103 B Codes Source for Aloha Protocol . .. . .. . , . ........... 112

B IODATA OF A UmORS . . . . . . . . . . . . ... . . . . , ....... ... .............. 1 15

13

LIST OF TABLES

TABLE . PAGE

1 Frequency spectrum allocation for some common band . . . . . . . . . . . . 51 in satellite system

FIGURE 1. 2.

3.

4.

5. 6.

7.

8.

9.

10.

II. 12.

13.

14.

15.

16.

17.

18.

19. 20.

21.

22. 23. 24. 25 .

26 .

27.

28.

29.

30.

31.

32.

33.

34.

35.

36.

37.

38.

39.

14

UST OF FIGURES

PAGE Frequency division multiple access ......... . , . .. , ......... .. , ... '" ., .. 28

Time division multiple access . . . . . . . . . .. . ... .. , ...... ... ... ......... ...... 29

Danger time for the pure Aloha protocol . . . .. . . .... . . .. .. , ........... , .. 32

Pure Aloha multiple access protocoL .. . . . '" ..... , ....... , . .. , .......... 32

Danger time for slotted Aloha protocol . . . ... . . . . . . ... . .. .. . . .. . .. . . . . . . 33

Slotted Aloha multiple access protocol ... . . . . .. .. . . . . . . . . . . . . , ...... ... .34

Single user transmission in CDMA channel. ....... . . . . . " ......... ... .39

The basic concept of the spread spectrum .. . . . . . . . . . . . . . . .. ... .. . . . . ... . 41

Sample waveform of spectrum spreading . .. . .. . . . ..... . .. . .... . . .. . . .. .42

Spectrum de-spreading .............. , . . . . .. . . . . . ... . . . . . . . . . . . . . .. . . . . . . . . . 43 Waveform of the signal in de-spreading process . . .. . . .. . .. . . .. . . . . ... . 44

Transmission process . .. . .. .. , ...... ... ...... ........ , ......... ............ 45

Frequency hopping waveform . . . . . . . .. . .. . .. . . , ............ ............. .46 Frequency hopping transmitter . . . . . . '" '" ... ... ........ , .. , ... ...... ... .47

Frequency hopping receiver .. . .. . ... . . ..... . . . . , ... ......... ......... .... 47

Different orbits on satellite system ... ...... ......... ...... ......... ...... 52

Hierarchical model of the OPNET simulation package . . . ... . .. ..... 103

Pipe line procedure for the transmitter channeL . . . .. . . . '" ........... 107

Pipe line procedure for the receiver channel .. . . . .. . . . . . . . , ... ... . , .... 108

Block diagram of the DS-CDMA slotted Aloha protocol. . . . . . . .. .. , . 63 DS-CDMA slotted Aloha model . .. . .. . . . .. , ... .... , ... , " . '" . , . ... , '" .. 65 Error correction code in the satellite receiver . . . . . . . . . . . . . . . . . . . . . . . . . , .66

Centralized Aloha network ... ........ . . . . . .. . ... . . . . . .... .. .. . . . . . . ... . .. . 67

Aloha transmitter node . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68

Aloha_tx transmitter process module (FSM) . . . . . . . . . . .. . , . .. , ..... .... 68

Aloha receiver node . . . . .. . . . '" ... ........... , ... ............ ... '" .. , ..... 70

Aloha rx receiver process module (FSM) . . . . . , ........... , ....... , ..... 71

Slotted Aloha transmitter node . .. . . . . . . ..... . ... . . . . . . . . . . . . . . . . . . . . . . . ... 73

DS-CDMA slotted Aloha simulation modeL .. . . . . . . .. . .... . ....... . . . .. 74

Transmitter node ofDS-CDMA slotted Aloha . . . '" ..... , ......... ..... 75

Receiver node of DS-CDMA slotted Aloha . . .... . , . .. , ....... , . ........ 76

Wireless ATM packet format . .. ... . , . .............. , ... .............. ..... 77 Comparison of pure Aloha, simulation and analytical . . .. . . .. . .. . . .. .. 80

Comparison of slotted Aloha, simulation and analytical . . . ...... . .. . . 81

Comparison of pure Aloha and slotted Aloha . .. . . . . .. ... '" ... ......... 82

DS-CDMA slotted Aloha throughput with different modulation schemes ..... . . ..... .. . .. .... .... .. . . . ... " . .. , ... " ............ 83

Channel throughput vs traffic load of DS-CDMA slotted Aloha in the presence of AWGN .. . .... " .. , ......... ... ........... , .... 85

The throughput of DS-CDMA slotted Aloha in different number of packets .. . ... . .. . " ...... ... ...... .................... 87

The throughput vs traffic load of the DS-CDMA slotted Aloha in the presence of the noise figure ...... . .. .. . . .. ... .... . .... '" .. 88

40. Mean packet loss ratio vs simulation time ofDS-CDMA

l5

slotted Aloha in the presence of the noise figure . . . . . . '" . , . ........... 89

41. Mean bit errors per packet vs simulation time ofDS-CDMA slotted Aloha in the presence of noise figure . . . . . . . . , ........ , '" ......... 90

42. Mean packet loss ratio vs simulation time ofDS-CDMA Slotted Aloha in the AWGN channel.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ... 91

43. Mean bit errors per packet vs simulation time in the AWGN channel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92

44. Mean packet loss ratio vs simulation time with different Number of Packets . . . . . . . . . . . . . . . . . . . . , ... ............ .. , ... ..... , ............ 93

45. Channel throughput vs traffic load ofDS-CDMA slotted Aloha in the presence of the error correction threshold . . . . . , '" ...... , .. 95

46. Mean packet loss ratio vs simulation time of the DS-CDMA Slotted Aloha in the presence of the error correction threshold . . . . . . , .. 96

ABR ACK AWGN BER BPSK CDMA CLSP CRC CSN DAMA DPSK DSSS Eb FDMA FHSS FSM GEO m LEO MAC MAl MCLSP MEO MSK PCN PDF PLR PN PT QPSK SNR TDMA UTIA VCI WATM

LIST OF ABBRIV ATIONS

Available bit rate Acknowledgement Additive White Guassian Noise Bit error rate Binary phase shift keying Code division multiple access Channel load sensing protocol Cyclic reduncy check Cell sequence number Demand assignment multiple access Differential phase shift keying Direct sequence spread spectrum Bit energy Frequency Division Multiple Access Frequency hopping spread spectrum

Finite state machine Geostationary earth orbit Header identifier Low earth orbit Medium access control Multiple access interference Modified channel load sensing protocol Medium earth orbit

Minimum shift keying Personal communication network Probability density function Packet loss ratio Pseudo noise Payload type Quadrature phase shift keying Signal to noise ratio Time Division Multiple Access Us telecommunication industry association Virtual circuit identifier Wireless asynchronous transfer mode

16

17

CHAPTER I

INTRODUCTION

Background

Future global personal communication networks (PCNs) promise to be

far more ambitious in terms of the number and category of user terminals. It is

expected that there will be an increase in the order of millions of subscribers,

as mobile and personal services are provided by satellites (Rizwan, 1998).

The services to subscribers of the future satellite networks based on

personal (PCNs) will not be limited to metropolitan areas and will be spread

out in all of the world including developing countries and even unpopulated

areas. Satellite of various sizes and capabilities has been la\Dlched to serve

almost any country in the world

The mam advantages inherent to satellite communication are the

broadcasting capability, the full connectivity of stations, the flexibility of

station organization, the capacity to support mobile users, and high

transmission quality.

The most crucial problem in the satellite system is the long propagation

delay of the user satellite links, noise, and limited bandwidth, which require

18

special consideration at the data link control layer (DLC) of the OSl mode]

(Rizwan.,1998).

The low earth orbit satellite (LEO) system is the most promising

candidate, since these systems can provide global coverage to small hand held,

low power terminals without essential need of any existing terrestrial facilities.

In addition the LEO satellite provides additional advantages to the

global communication network, e.g. low propagation delay (2SOms), low

propagation loss, and high elevation angle at high latitudes resulting from non­

equatorial orbits constellation (Jamalipour and Akira.,1997).

A satellite link is normally used by many earth stations. Since not all

stations can transmit data on the link at the same time (at least not using the

same frequency) there has to be some mechanism to determine which station is

currently allowed to use the link to avoid the collision between stations during

the transmission process. The protocols that are used to provide this

mechanism are called Medium Access Control (MAC).

The MAC protocols are responsible for deciding which station gets to

use the link when there is a competition for it The MAC protocols are only

needed on the satellite up link (many to one). The down link has only one

sender (broadcast), the satellite, and thus has no need for the mechanisms

19

provided by a MAC protocol because there is only one sender (one to many)

and any access schemes such as time division multiple access TDMA can be

applied.

Multiple access schemes are used to allow many users to share

simultaneously a fInite amount of radio spectrum. The sharing of spectrum is

required to achieve high capacity by simultaneously allocating the available

bandwidth (or the available amount of channels) to multiple users. For high

quality communication, this must be done without severe degradation in the

performance of the system (Jamalpour and AkiTa 1997).

In addition, medium access can be defined as the method to control the

access to the channel to minimize the probability of collision, which may occur

between multiple users when sending their packets simultaneously through a

common receiving point.

Multiple Access Sehemes in Wireless Channel

Multiple access schemes in wireless channel can be classified into:

• Fixed assignment schemes

• Demand assignment schemes

• Random assignment schemes

20

Fixed Assignment protocols

In fixed assignment protocols, the transmission or the users are

scheduled in either time (TDMA) or frequency (FDMA). By assigning a

specific duration of time or frequency band to each user, it is possible to avoid

the collision between their signals. In fixed assignment protocols the traffic

from each user in the network: is steady and fixed (Prasad, 1996).

Demand Assignment Protocols

When the traffic varies with time on day or seasons using fixed

assignment schemes leads to inefficient use 01 the channel capacity .In this

case assigning the capacity on demand in response to user requests is more

practical. In Demand Assignment Multiple Access (DAMA), prior to the

transmission, a user asks for a channel and after the permission it can transmit

its signal.

DAMA needs a separate channel to request the capacity. DAMA can be

allocated the channel based on fixed allocation or random allocation depending

on the traffic states 01 the channel. If the number of terminals in the channel is

limited the proper way is to employ the request channel based on the fixed

request such as in Single.-<hannel-per.-<arrier pulse-code-modulated multiple­

access demand-assignment equiprnent( SPADE) system (Cacciamani, t 970)

and the indonesian PALAPA network (Suryawan et aI., 1982). The SPADE

21

system is limited to a maximum of 50-request channel for sO terminals while

the PALAPA system is limited to 120 terminals.

When the number of potential data terminal sharing the request channel

is much larger than the number of terminals active at any given time, the fixed

channel request becomes impractical. In this case the channel request based on

random access allocation is the solution. An example of this method is in

International Maritime Satellite Organization ( INMARSAT satellite system).

The IN-MARSAT network employs a simple ALOHA random access request

channel for a large number of ship stations (Abramson.1977). The random

access in INMARSAT network is used only to request channel capacity for full

time voice and data.

Random Acc ess protocols

In random access protocols the users send their packets randomly

without any coordination among them. The users send their packets whenever

they have packets ready to send. Aloha protocol is the earliest random access

protocols invented by Abramason in 1970 in University of Hawaii

(Abramson., 1970). In Aloha protocol the users send their packets at any time

without any synchronization mechanism. If there is no ACK received it means

a collision has occurred. When the collisions occur, the collided packets should

be retransmitted after waiting a random time period.

22

The other random access protocol is Slotted Aloha which is a modified

version of Aloha protocol. In this scheme the users are allowed to transmit only

at the beginning of the time slot and the synchronization is required between all

the stations in the system to avoid the overlap of packets.

Code Division Multiple A ccess (CDMA)

In CDMA scheme multiple users can access the channel at any time

with the use of the whole bandwidth and for the complete duration by using

spread spectrum technique. Each user is allocated a unique Pseudorandom code

(PN code) that can be separated with other users. In the spread speCtn.ml the

transmitted signal is spread over a wider frequency band much wider than the

required signal to be sent. Such that the users are allowed to share the same

radio frequency spectrum simultaneously.

Spread spectrum based scheme is recommended because of its

advantages such as effective in mitigating multipath fading because of their

bandwidth introduces frequency diversity (Kohno et al.,1995). They are also

useful in mitigating interference, again because of their widely spread

bandwidth.

23

Statement of Problem

The choice of multiple access architecture for a data network should

begin with an understanding of the statistic of the traffic to be transmitted on

the multiple access channels. When the traffic is transmitted by a fixed set of

transmitters the best choice is to use the fixed assignment protocols such as

FDMA and TDMA. If the transmitters on the network changes rapidly enough,

DAMA architecture to assign capacity on demand will be the best choice.

Capacity within a DAMA can be allocated on the basis of frequency, on the

basis of time or on the basis of both frequency and time.

The design of the request channel for DAMA system presents a new

level of multiple access choice. When the set of transmitters is small, fixed

allocation architecture in the request channel is possible. However, if the

number of transmitters increase the best choice is to use the Aloha architecture

in the request channel as used in both INMARSAT DAMA system and the

Qualacornm CDMA system.

The DAMA system with the channel request based on Aloha will not

solve the problem of the access with the increase of the number of transmitters

with bursty traffic. In this case the solution is to design a data network using a

random access protocol in the primary channel rather than in the request

channel. The way to solve this problem is by using a hybrid multiple access

scheme including CDMA and slotted Aloha protocols.

24

This is a combination of two multiple access schemes, slotted Aloha

and CDMA (COMA slotted Aloha). Slotted Aloha is known as the simplest

and flexible random access method that can realize the share of communication

channel to a large number of users. CDMA provide high bandwidth and high

throughput spread spectrum.

In this protocol, the channel is divided into time slots equal to the

packet length and each user is assigned a time slot and allowed to transmit only

at the beginning of a time slot and all users are synchronized. When the packets

are ready to transmit each user randomly chooses a unique code (PN code) and

send their packets simultaneously in the next time slot.

The unsuccessful transmission are due to multiple access interference

(MAl) and Additive White Gaussian Noise (A WGN). Retransmission then is

required. CDMA slotted Aloha in the presence of large number of users with

bursty traffic can improve the performance of the system by reducing the

collision which may occur during transmission with high throughput

performance.