EFFECTS OF SPREADING IN OFDM-BASED SYSTEMS

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EFFECTS OF SPREADING IN OFDM-BASED

SYSTEMS

Thesis submitted in partial fulfillment

of the requirements for the degree of

Bachelor of Technology

in

Electronics and Electrical Communication Engineering

by

Varun Bedi(07EC1026)

Under the guidance of

Dr. Suvra Sekhar Das

Indian Institute of Technology, Kharagpur

May, 2011

Indian Institute of TechnologyKharagpur

Certificate

This is to certify that the thesis entitled ”Effects of Spreading in OFDM-based Sys-

tems” is a bona fide record of work carried out byVarun Bedi, under my supervision

and guidance, for the partial fulfillment of the requirements for the award of the degree

of Bachelor of Technology (Honors)in Electronics and Electrical Communication Engi-

neering at the Indian Institute of Technology, Kharagpur. The thesis has fulfilled all the

requirements as per the regulations of the institute and in my opinion reached the standard

for submission

Dr. Suvra Sekhar Das

Assistant Professor

G. S. Sanyal School of Telecommunications,

Indian Institute of Technology, Kharagpur.

Place: IIT Kharagpur

Date: May 2011

Acknowledgement

I am greatly indebted to my project supervisor Prof. Suvra Sekhar Das for his invaluable

guidance and encouragement throughout the course of this project. I would like to takethis opportunity to express my sincere and profound gratitude to him.

I would also like to gratefully acknowledge the suggestionsand discussions received

from Priyangshu, Arun, Narendra, Swathi, Prabhu, Myna, Sawan, Praveen, Jayant, Rohit,Raghvendra and Shanshank whenever I had any doubt.

My sincere thanks also go to all the faculty of E & ECE for theirkind co-operation in

all spheres during my project work. I would also like to thankall others whose direct orindirect help has benefited me during my stay at IIT Kharagpur.

Furthermore, I would like to thank all other friends and fellow students of IIT Kharagpur

and particularly my E&ECE mates for sparing great time throughout my B.Tech and verygood support I received from them during my stay.

Last but not the least, I thank my parents and all my family members for their support

and motivation they have provided throughout my project.

Varun Bedi

ii

Abstract

OFDM-based systems has shown good performance recently in terms of high spectral effi-

ciency and also robustness against multipath fading. However, they pose a serious problem

of high Peak to Average Power Ratio (PAPR). The operation of Spreading, using orthog-

onal Spreading codes, is used to obtain similar throughput advantage and robustness in

CDMA systems. To overcome the shortcoming of OFDM-based systems, various attempts

have been made to manage the high PAPR, without losing the benefits of OFDM. One of

them is introducing Spreading in OFDM-based Systems. Spread-spectrum OFDMA is one

such technique where we combine some benefits of both OFDMA and CDMA systems.

In this work, we first extend the concept of Overloading, already present in the downlink

of CDMA2000, to Spread Spectrum OFDMA, and compare the effects of doing so. Next,

we extend the Overloading concept to Underloading, where itis intended to investigate the

case when less number of users than the capaciy limit are present in the system. Hence, Un-

derloaded Spread-spectrum OFDMA performance has been evaluated next. This scheme

is particularly relevant to the femtocell scenario, where the number of users is less but the

throughput requirement may be more.

In the later part of the work, we have tried changing the interleaving depth, and to

investigate its effect on various flavors of Spread SpectrumOFDMA. In the last part, we

takenMCSdata from a VoIP simulation, and modelled our underloaded SS-OFDMA to the

partial PRB usage required here. We analyze the performanceof our scheme, and compare

it with other schemes applied to the same data.

iii

Contents

Acknowledgement ii

Abstract iii

1 Introduction 1

1.1 Background. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

1.2 Motivation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

1.3 Access Techniques. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

1.4 Problem Definition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

1.5 Work Done . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

2 Radio Access Techniques 5

2.1 Fundamentals of RATs. . . . . . . . . . . . . . . . . . . . . . . . . . . 5

2.2 Performance Requirements for IMT-A systems. . . . . . . . . . . . . . 5

2.3 Multiple Access Techniques - An Overview. . . . . . . . . . . . . . . . 7

2.4 OFDMA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

2.5 Spread Spectrum OFDMA. . . . . . . . . . . . . . . . . . . . . . . . . 11

2.5.1 MC-SS-MA. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

2.5.2 SC-FDMA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

3 Overloaded Spread Spectrum OFDMA 13

3.1 Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

3.2 System Description. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

3.2.1 Transceiver Description. . . . . . . . . . . . . . . . . . . . . . 14

3.2.2 System Block-Diagram for UL-Transmission. . . . . . . . . . . 16

3.2.3 System Block-Diagram for DL-Transmission. . . . . . . . . . . 16

3.3 Multi-Stage Detector. . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

3.4 Analytical Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

3.5 Code Selection. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

iv

CONTENTS CONTENTS

4 Underloaded Spread Spectrum OFDMA 25

4.1 Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

4.2 Transceiver Description. . . . . . . . . . . . . . . . . . . . . . . . . . . 25

5 The Interleaving operation 27

5.1 Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

5.2 Overloading. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

5.3 Underloading. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

6 SDIC in Overloaded SS-OFDMA 33

6.1 Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33

6.2 Measures of Confidence. . . . . . . . . . . . . . . . . . . . . . . . . . 33

6.2.1 Exact LLR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

6.2.2 Approximate LLR . . . . . . . . . . . . . . . . . . . . . . . . . 34

6.3 SDIC Algorithm. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

7 Results - Overloading and Underloading 36

7.1 Overloaded SS-OFDMA with HDIC receiver. . . . . . . . . . . . . . . 36

7.1.1 Low Interleaving. . . . . . . . . . . . . . . . . . . . . . . . . . 36

7.1.2 More interleaving. . . . . . . . . . . . . . . . . . . . . . . . . . 39

7.1.3 Comparison of BPSK 100% overload with QPSK 0% overload. . 39

7.2 Overloaded SS-OFDMA with SDIC receiver. . . . . . . . . . . . . . . . 44


7.2.2 More interleaving. . . . . . . . . . . . . . . . . . . . . . . . . . 44

7.2.3 Comparison of BPSK 100% overload with QPSK 0% overload. . 49

7.3 Underloaded SS-OFDMA. . . . . . . . . . . . . . . . . . . . . . . . . 51


7.3.2 More Interleaving. . . . . . . . . . . . . . . . . . . . . . . . . . 51

8 Underloaded SS-OFDMA for Dynamic VoIP data 57

8.1 OFDMA - Concept of PRB. . . . . . . . . . . . . . . . . . . . . . . . . 57

8.2 Applying Underloaded SS-OFDMA. . . . . . . . . . . . . . . . . . . . 58

8.3 BER Results. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59

8.3.1 AWGN Channel - Without Spreading. . . . . . . . . . . . . . . 59

8.3.2 Rayleigh Channel - Without Spreading. . . . . . . . . . . . . . 59

8.3.3 Rayleigh Channel - With Spreading. . . . . . . . . . . . . . . . 63

8.3.4 Rayleigh Channel - With Variable Spreading. . . . . . . . . . . 63

v

CONTENTS CONTENTS

9 Conclusions and Future work 66

9.1 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66

9.2 Future Work. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67

Appendices 68

.1 PAPR performance results. . . . . . . . . . . . . . . . . . . . . . . . . 69

.1.1 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69

List of Abbreviations 72

Bibliography 74

vi

List of Figures

2.1 Transmitter-Receiver Block Diagram of OFDMA. . . . . . . . . . . . . 10

2.2 Transmitter-Receiver Block Diagram of MC-SS-MA. . . . . . . . . . . 12

3.1 Transmitter Block Diagram for Overloaded OFDM. . . . . . . . . . . . 15

3.2 Receiver Block Diagram for Overloaded OFDM. . . . . . . . . . . . . . 15

3.3 UL-Transmission system Block Diagram for Overloaded SS-OFDMA . . 17

3.4 UL-Reception system Block Diagram for Overloaded SS-OFDMA . . . . 17

3.5 DL-Transmission system Block Diagram for Overloaded SS-OFDMA . . 18

3.6 DL-Reception system Block Diagram for Overloaded SS-OFDMA . . . . 19

3.7 Iterative Multi-Stage Detector Algorithm. . . . . . . . . . . . . . . . . . 20

4.1 Downlink Tx Block diagram from Base-station in Low interleaving case. 26

4.2 Downlink Rx Block diagram at each-user in Low interleaving case. . . . 26

5.1 Downlink Tx Block diagram from Base-station in Low interleaving case. 28

5.2 Downlink Rx Block diagram at each-user in Low interleaving case for

Overloading. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

5.3 Downlink Tx Block diagram from Base-station in High interleaving case. 30

5.4 Downlink Rx Block diagram at each-user in High interleaving case. . . . 30

5.5 SS-OFDMA Small Interleaver Transmitter Block Diagram. . . . . . . . 31

5.6 SS-OFDMA Small Interleaver Receiver Block Diagram. . . . . . . . . . 31

5.7 SS-OFDMA Small Interleaver Transmitter Block Diagram. . . . . . . . 32

5.8 SS-OFDMA Small Interleaver Receiver Block Diagram. . . . . . . . . . 32

6.1 The SDIC Algorithm for 2 bits/symbols case. . . . . . . . . . . . . . . . 35

7.1 BER performance of Overloaded SS-OFDMA for different modulation in

Low interleaving Indoor, with HDIC receiver. . . . . . . . . . . . . . . 37


Low interleaving Outdoor, with HDIC receiver. . . . . . . . . . . . . . 38

vii

LIST OF FIGURES LIST OF FIGURES


More interleaving Indoor, with HDIC receiver. . . . . . . . . . . . . . . 40


More interleaving Outdoor, with HDIC receiver. . . . . . . . . . . . . . 41

7.5 BER performance of Overloaded SS-OFDMA for BPSK 100 percent Over-

load with QPSK 0 percent overload Low interleaving case, with HDIC

receiver . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42


load with QPSK 0 percent overload More interleaving case, with HDIC

receiver . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43


Low interleaving Indoor, with SDIC receiver. . . . . . . . . . . . . . . . 45


Low interleaving Outdoor, with SDIC receiver. . . . . . . . . . . . . . . 46


More interleaving Indoor, with SDIC receiver. . . . . . . . . . . . . . . 47


More interleaving Outdoor, with SDIC receiver. . . . . . . . . . . . . . 48


load with QPSK 0 percent overload Low interleaving case, with SDIC re-

ceiver . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49


load with QPSK 0 percent overload More interleaving case, with SDIC

receiver . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50

7.13 Comparison of BER performance of Underloaded SS-OFDMAfor differ-

ent modulation in Low interleaving indoor case. . . . . . . . . . . . . . 52


ent modulation in Low interleaving Outdoor case. . . . . . . . . . . . . 53


ent modulation in More interleaving indoor case. . . . . . . . . . . . . . 54


ent modulation in More interleaving Outdoor case. . . . . . . . . . . . . 55

8.1 Available Downlink Bandwidth is divided into PRBs. . . . . . . . . . . 57

8.2 PDF of number of PRBs using a modulation scheme. . . . . . . . . . . 58

8.3 BER performance of OFDMA for Dynamic VoIP data under AWGNChannel 60

viii

LIST OF FIGURES LIST OF FIGURES

8.4 BER performance of OFDMA for Dynamic VoIP data under Rayleigh

Channel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61

8.5 BER performance of OFDMA - finding performance equivalent of Full-

loaded case. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62

8.6 BER performance of SS-OFDMA for Dynamic VoIP data under Rayleigh

Channel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64

8.7 BER performance of SS-OFDMA for Dynamic VoIP data under Rayleigh

Channel, with Variable Spreading. . . . . . . . . . . . . . . . . . . . . 65

1 Comparison of PAPR performance of SC-FDMA with OFDMA for differ-

ent modulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69

2 Comparison of PAPR performance of partial PRB cases for SG 12 and

variable SG with OFDMA . . . . . . . . . . . . . . . . . . . . . . . . . 70

ix

List of Tables

2.1 Cell Spectral Efficiency. . . . . . . . . . . . . . . . . . . . . . . . . . . 6

2.2 Cell Edge Spectral Efficiency. . . . . . . . . . . . . . . . . . . . . . . . 6

2.3 Peak Data Rate for IMT-A. . . . . . . . . . . . . . . . . . . . . . . . . 7

2.4 Supported environment and mobility types. . . . . . . . . . . . . . . . . 7

2.5 Summary of Multiple Access Techniques. . . . . . . . . . . . . . . . . 9

3.1 Values of Set Cross Correlation. . . . . . . . . . . . . . . . . . . . . . . 24

x

1Introduction

1.1 Background

Since the 1990’s, the cellular industry has seen a massive growth in the number of mobile

phone users and their requirements. As a result, there have been constant attempts to im-

prove the performance of cellular systems (data rate, spectral efficiency, and throughput),

while minimizing the costs incurred.

Wireless communication started with the analog1st generation serviceAMPS. The2nd

Generation (2G) GSM and CDMA were digital in nature and used the multiple access

schemes Time Division Multiple Access (TDMA) and Code Division Multiple Access

(CDMA), along with Frequency Division Multiple Access (FDMA). Data services were

later introduced inGPRSandEDGE. 3rd Generation (3G) such asUMTS andW-CDMA

were the first to see data rates upto 2 Mbps and beyond. Finally, in the Beyond 3rd

Generation (B3G) services,LTE has adopted Orthogonal Frequency Division Multiplexing

(OFDM), the access technology dominating the latest evolutions of all mobile radio stan-

dards [1].

1

Motivation Introduction

1.2 Motivation

4th Generation (4G) is the fourth generation of cellular wireless standards. In 2008, the

International Telecommunication Union (ITU) released a set of requirements for any4G

technique by the name of International Mobile Telecommunications-Advanced (IMT-A ).

The key Radio Access Technologies that are a part of different upcoming mobile radio stan-

dards for IMT-A are Orthogonal Frequency Division MultipleAccess (OFDMA), Code

Division Multiple Access (CDMA) and Space Division Multiple Access (SDMA). Also,

Single Carrier/Multi-Carrier operation as well as combination of these technologies can

be considered to be an effective Radio Access Technique (RAT) which can satisfy the

requirements ofIMT-A [2].

The 3GPPhas also created a formal Study Item with the specific aim of evolving the

3GPPradio access technology into Long Term Evolution - Advanced(LTE-A), a proposed

4G mobile radio standard, to ensure competitiveness. Some of the main objectives were in-

creased data rates, greater flexibility of spectrum usage and reasonable power consumption

for the mobile terminal.

1.3 Access Techniques

The 3G schemes started off based onCDMA technology and its different forms. Later

standards like Long Term Evolution (LTE) were based onOFDMA technology, which

is considered a better technique for mitigating Multipath Fading effects and Inter Carrier

Interference (ICI), effects that are more adverse in high-mobility scenariosof the IMT-A

requirements.

The technique Orthogonal Frequency Division Multiplexing(OFDM) is a type of Fre-

quency Division Multiplexing (FDM), where all the carriers allocated to users are orthog-

onal to each other. Because of the orthogonality, OFDM mitigatesICI and hence improves

the system performance. This orthogonality can be effectively achieved by using Inverse

Fast Fourier Transform (IFFT) and Fast Fourier Transform (FFT) blocks, instead of a se-

ries of upconverters and downconverters, leading to drastic reduction in the Trasceiver

complexity. OFDM also uses a Guard Period at the end of each OFDM symbol which is

used to check Inter Symbol Interference (ISI) due to Multipath effect. OFDMA enabled

LTE to enhance data rate to a maximum of 100 Mbps in downlink and 50Mbps in uplink.

Howeverm, one of the main disadvantage ofOFDM is its high Peak to Average Power

2

Problem Definition Introduction

Ratio (PAPR), which introduces non-linearities in the power amplifier at the transmitter

and hence causes more distortions in received signal at userterminal.

For the Uplink,LTE uses single Single Carrier-Frequency Division Multiple Access

(SC-FDMA). SC-FDMA uses Discrete Fourier Transform (DFT)-Spreading before map-

ping data symbols on the carrier. This reduces the PAPR of thetransmitted signal, as one

symbol itself is spread over multiple carriers. This reduces power requirement at the user

terminal and leads to saving of mobile battery life. Also, due to the nature of SC-FDMA,

there is some scope on increasing the Spectral Efficiency andhence the average Through-

put of the system in Uplink.

1.4 Problem Definition

Increasing Capacity or Spectral Efficiency of OFDM-based access techniques is a big chal-

lenge to Telecommunication engineers at present. With thisrespect, the main objective of

this project has been to develop a new Radio Access Technique(RAT) that offers better

Capacity or Spectral Efficiency performance, or has a lower PAPR of the transmitted sig-

nal, and meets the ever-increasing demand for data rates of users to a greater extent than

existing techniques.

OFDMA has been widely accepted as a RAT for the future, due to its strong benefits.

In this work, we concentrated on ways to increase capacity ofOFDMA. One way to do

this is to introduce Spreading in the scheme, which reduces the PAPR and gives a Soft

Capacity limit to the scheme. This work investigates such techniques, and evaluates the

performance gain obtained in each case.

1.5 Work Done

Initially a literature survey was carried out to find out the benefits and shortfalls of different

Radio Access Techniques (RATs). This is presented in Chapter 2.

Next, we talk about the extending the concept of Overloaded CDMA into OFDMA

based systems, in what is called Overloaded Spread Spectrum-Orthogonal Frequency Divi-

sion Multiple Access (SS-OFDMA). Chapter 3 presents the theoretical idea of this scheme.

In Chapter 4, we present the theory for extending concepts ofUnderloading in CDMA

to SS-OFDMA.

3

Work Done Introduction

Chapter 5 presents the theory behind the Interleaver block of the transceiver. We have

investigated the effects of changing the Interleaver depth, in each of the above cases.

Chapter 6 presents a Soft Decision Interference Cancellation (SDIC) scheme for the

Receiver, to improve the performance of Overloaded SS-OFDMA.

In Chapter 7, we have analyzed Underloaded SS-OFDMA for a simulated picture of

Modulation levels in OFDMA PRBs in VoIP transmission.

Finally, in Chapter 8, we present the results for each of the cases. Evaluation of the pro-

posed schemes has been carried out with respect to its Bit Error Rate (BER), Throughput

and PAPR performances.

4

2Radio Access Techniques

2.1 Fundamentals of RATs

Radio Access Techniques (RATs) are the backbone of any Wireless Communication sys-

tem, enabling multiple users to share limited network resources, such as bandwidth, effi-

ciently. Design of newRATs must take into account different kinds of wireless channel

conditions, along with the mobility and the traffic conditions. Any new RAT being de-

signed for 4G cellular systems must meet with the following IMT-A requirements issued

by ITU-R.

2.2 Performance Requirements for IMT-A systems

Requirements presented below give a brief overview of the entire document [3].

1. Cell Spectral Efficiency

It’s the aggregate throughput of all the users in an Service Data Unit (SDU) divided

by Channel Band Width (BW) and divided by number of cells.

n =

∑N

i=1 |xi|

(T ∗ w ∗ M)(2.1)

5

Performance Requirements for IMT-A systems Radio Access Techniques

where, T: Time over which the Bits are to be transmitted

w: Channel BW

M: no. of Cells

Table 2.1shows the Cell Spectral Efficiency requirements for different environ-

mental conditions.

Table 2.1: Cell Spectral Efficiency

Test Environment Downlink(bits/sec/Hz/Cell)

Uplink(bits/sec/Hz/Cell)

Indoor 3 2.25Microcellular 2.6 1.8Base Coverage Urban 1.5 1.4High Speed 1.1 0.7

2. Bandwidth

The RAT should be able to scale BW up to and including 40 MHz.

It also suggests that operation can be extended up to wider BWof 100 MHz, and the

RAT should be able to handle this.

3. Cell edge user spectral efficiency

It is the 5% point of the Cumulative Distribution Function (CDF) of Normalized

User Throughput. Table2.2 shows the cell edge spectral efficiency for different

environments.

Table 2.2: Cell Edge Spectral Efficiency

Test Environment Downlink(bits/sec/Hz/Cell)

Uplink(bits/sec/Hz/Cell)

Indoor 0.1 0.07Microcellular 0.075 0.05Base Coverage Urban 0.06 0.03High Speed 0.04 0.015

4. Latency

C-Plane (Transition time from Idle state to Active state): 100msec

U-Plane (Transport delay from IP layer of Base station to IP layer of terminal): 10m-

sec

6

Multiple Access Techniques - An Overview Radio Access Techniques

5. Peak Data Rates

Table2.3shows peak Data Rate requirements of IMT-Advanced.

Table 2.3: Peak Data Rate for IMT-A

- Bandwidth DatarateDL-Peak 40MHz 600MbpsDL-Peak 100MHz 1.5GbpsUL-Peak 40MHz 270MbpsUL-Peak 100MHz 675Mbps

6. Mobility class and Test environment supported

Table2.4shows test environment supporting types of mobility (notation: Y).

Table 2.4: Supported environment and mobility types

Mobility \(Test Envi-ronment)

Stationary(0 Km/Hr)

Pedestrian(0-10 Km/Hr)

Vehicular(10-120Km/Hr)

Stationary(120-350Km/Hr)

Indoor Y Y - -Microcellular Y Y Y(up to 30 Km/Hr) -Base coverage Urban Y Y Y -High Speed - - Y Y

7. Handover

Handover interruption time is defined as time over which userterminal can’t ex-

change user plane packet with any of the serving base station. For different condi-

tions, Hand over interruption time is given below.

Intra frequency : 27.5 msec

Inter frequency:

(a) Within same Spectrum Band: 40 msec

(b) Between different Spectrum Band: 60 Msec

2.3 Multiple Access Techniques - An Overview

In the early stages of modern communication, scarce BW was shared between multiple

users with separate frequency channels without significantinterference among the users.

This is the simplest of way of having multiple access scheme among users and is referred

to as Frequency Division Multiple Access (FDMA).

7


With the invent of digital technology, many users were assigned time slots on the same

frequency channel instead of a separate frequency channel for each user. This kind of

technique is called Time Division Multiple Access (TDMA). Users can send bursts of data

in its alloted time slot and this process is repeated periodically in each frame. This required

additional transceiver circuitry to have Time Synchronization in the system.

Now, Frequency Division Multiple Access (FDMA) and Time Division Multiple Access

(TDMA) techniques have their limits of capacity that they can support(i.e. it supports only

a fixed number of users as number of time slots or frequency channels available are fixed).

A combination ofTDMA andFDMA is used to enhance the capacity of the system which

gave rise to Global System for Mobile communication (GSM), the global standard in 2G

and 2.5G.

Another technique calledCDMA was efficient compared to the earlier two with respect

to Soft Hand over, high BW efficiency and soft capacity limit.It involves allocation of

orthogonal code sequences to each user who are using the common frequency resource at

the same time, to faciliate multiple access. FurtherCDMA is also being used in its different

forms such asMC-CDMA [4], MC-DS-CDMA [4], OFDM-CDMA [5]. However, there

are many problems faced by the currently available CDMA Technology [6].

Table2.5 gives a brief evaluation, description, pros and cons of existing of multiple

access schemes [7].

8


Table 2.5: Summary of Multiple Access Techniques

Approach SDMA TDMA FDMA CDMAIdea Segment space

into cells orsectors

segment sendingtime into dis-joint time slots

segment sendingfrequency bandsinto disjoint fre-quency channels

Data Spreadover availableSpectrum

Terminal Only One termi-nal can be acti-vated

All terminals areactive but onlyfor a short pe-riod

every termi-nal has itsown frequencyuninterrupted

all the terminalscan be active atthe same placeat the same mo-ment

Signal Sepa-ration

cell structure di-rected antennas

synchronizationin the timedomain

filtering inthe frequencydomain

code along withspecial receiverarchitecture

Advantage very simple, in-creases capacityper sqr.km

easy HO, fullydigital, lowbattery con-sumption

simple, contin-uous transmis-sion and hencelow overhead,robust

flexible, lessfrequencyplanning, softhandover,soft capacity,privacy

Disadvantage Inflexible, an-tennas typicallyfixed

guard spaceneeded, highsynchronizationoverhead, robust

inflexible andmore carrierslead to ICI, highBS cost

complex re-ceiver, needsmore compli-cated powercontrol forsenders, softcapacity limit

Comment only with com-bination withTDMA, FDMA,CDMA useful

can be usedin low delayspread env. withlow mobility

gives frequencyselectivity de-pending onchannel sepa-ration betweenthe users andmaximum delayspread

for large delayspread, receivercomplexityincreases.

9

OFDMA Radio Access Techniques

2.4 OFDMA

Along with techniques mentioned above, one of the techniquethat has got immense impor-

tance is OFDMA. The main advantages ofOFDMA over TDMA/CDMA is, its scalability,

uplink orthogonality and the ability to take advantage of the channel frequency selectivity.

Hence, recent wireless standards such as 3GPP-LTE, IEEE-802.16e, and W-LAN (IEEE-

802.11) has considered OFDM as access technique.

OFDM is multiplexing technique that subdivides available bandwidth into several par-

allel sub-streams of reduced datarate and hence each sub-stream is transmitted on orthog-

onal carrier. Fig.2.1 [7] shows transmitter-receiver block diagram of OFDMA system.

Its shows that, serial bit-stream is modulated to symbols which are transmitted in parallel

on each orthogonal carrier. Use of the IFFT ensures orthogonality between the carriers.

Also introduction of Cyclic Prefix (CP) completely eliminates effect ofISI as long as CP

duration is more than the maximum channel delay spread. CP isonly the repetition of few

of the last samples and hence makes the system resistant to fading effect.

At the receiver side, CP number of samples are removed from the receivedOFDMA

symbol. Further a reverse of the process mentioned above starts with IFFT replaced by

Figure 2.1: Transmitter-Receiver Block Diagram of OFDMA

10

Spread Spectrum OFDMA Radio Access Techniques

FFT operation and so on. After FFT at the receiver, Frequencydomain equalization is

performed and it is followed by de-mapper. One of the disadvantage of OFDMtechnique

is a high Peak to Average Power Ratio (PAPR).

2.5 Spread Spectrum OFDMA

In this SS-OFDMA, a symbol to be mapped on a carrier is instead spread over spreading

gain amount of carriers. This gives the advantage of frequency diversity and hence even

channel condition is not good for certain carriers, symbol mapped can be decoded easily.

Hence this kind of technique gives benefit of OFDMA and CDMA. Depending on correla-

tion properties, system performance of Spread Spectrum OFDMA varies. Hence, receiver

has to use Multiuser Detection techniques to mitigate interference. On the basis of codes

used for spreading, it can be categorized under following techniques.

2.5.1 MC-SS-MA

This is nothing but Multi Carrier Spread Spectrum Multiple Access (MC-SS-MA). Fig.2.2[7]

shows the block diagram of MC-SS-MA access technique. Symbols before giving to the

IFFT are passed through spreader. Number of symbols at the input of the spreader depends

on the spreading gain of spreading sequences used. This means, for 100% loading of sym-

bols, number of symbols are exactly equal to the spreading gain but for lower loading

factor, number of symbols to be spread are less.

One of the advantage as mentioned before, of this technique is we can get diversity gain

because of the spreading of one symbol over multiple carriers. But, at the receiver, this

technique increases receiver complexities.

2.5.2 SC-FDMA

SC-FDMAis one of the type ofMC-SS-MAtechnique wherein, DFT Spreading used at the

Spreading block in Fig.2.2 [7]. Use of this DFT spreading which are having orthogonal

sequences gains all the advantages of MC-SS-MA along with that reduces PAPR of the

system. SC-FDMA is used as UL access technique for 3GPP-LTE standard. [8] Reason

behind selecting SC-FDMA instead of OFDMA for UL transmission in LTE standard is,

OFDMA is having issue of PAPR because at the output of IFFT, all the carriers get added

constructively resulting large envelope fluctuations. Signal with this high PAPR requires

highly linear power amplifiers to avoid excessive intermodulation distortion. To achieve

11

Spread Spectrum OFDMA Radio Access Techniques

Figure 2.2: Transmitter-Receiver Block Diagram of MC-SS-MA

this linearity, amplifiers has to operate with large back offfrom this peak power means it

has to provide large amount of DC power, resulting in less efficiency. It gives significant

burden on portable wireless terminal. Second reason behindselecting SC-FDMA instead

of OFDMA for LTE-UL is Another problem with OFDMA in cellularuplink transmissions

derives from the inevitable offset in frequency referencesamong the different terminals that

transmit simultaneously. Frequency offset destroys the orthogonality of the transmissions,

thus introducing multiple access interference.

There are two techniques of subcarrier mapping for the design of the SC-FDMA scheme.

Localize subcarrier mapping gives good throughput performance compared to Interleaved

subcarrier mapping. But, Interleaved Subcarrier mapping gives good PAPR performance

by 3-7dB. Comparison of SC-FDMA and OFDMA performance resuls is given in [9] and

[10].

12

3Overloaded Spread Spectrum OFDMA

In this chapter, we investigate a OFDM-based access technique modified and combined

with CDMA to utilize certain advantages of it. The objective is to investigate and compare

its performance with existing access techniques suggestedin recent standards.

One of the merits of CDMA based techniques is Soft Capacity i.e. it can be overloaded

to get higher capacity, with certain degradation in probability of bit error. Overloading is

a term used in Cellular CDMA technique. Generally, CDMA allocates every sequence of

lengthN to maximumN number of users, i.e.,N = TTc

is the processing gain orSpreading

Gain of CDMA.

where,T : Symbol Duration andTc: Chip Duration

If less thanN number of users are allocated using chip sequence of lengthN , then

system is called anUnderloaded System. In this case, Orthogonality of the codes is not

violated and hence their performance is not affected.

When the number of usersM exceedN , system is said to be anOverloaded System.

So, in this case, it is necessary to assignM number of sequences (M > N). Hence,

the sequences are no more orthogonal and in effect, it increases the Multiple Access

Interference (MAI ). This kind of scheme can be used to increase the System Capacity,

13

Introduction Overloaded Spread Spectrum OFDMA

but in effect it makes the system performance worse

In literature, different approaches are described to mitigate this effect of overloading

and hence enable more number of users to share BW simultaneously. Several approaches

includes the use of Multi User Detection (MUD) at Base Station. [11] has shown CDMA

Overloading performance using Iterative Interference Cancelation Receiver. Several ap-

proaches also suggest use of Orthogonal Codes such as Quasi Orthogonal Sequences

(QOS) and Orthogonal Gold (OG) Codes can enhance system performance [12].

3.1 Introduction

Recently, OFDMA based access techniques have been dominantand are suitable for multi-

path environment. For next generation wireless sytems, we can still think about combina-

tion of CDMA and OFDMA. One of this kind of technique which is used in 3GPP-LTE

standard isSC-FDMA [13]. It consists of DFT spreading which enables the Code Divi-

sion Multiplexing of the symbols. This concept of Overloading in CDMA can be extended

further to OFDM based systems to increase the capacity.

This can be possible using different kinds of spreading codes, such asOG Codes or

QOS. These codes must have very little or no performance degradation as compared to

underloaded systems and should possess good correlation properties. Further, it is also

necessary to use a Multi Stage Detector [14] for the additional interference cancelation.

Keeping all this in mind, we need to evaluate this new scheme,and compare it with existing

techniques, for different channel conditions and amounts of Overloading.

3.2 System Description

3.2.1 Transceiver Description

One of the advantages of Overloaded OFDMA is that it increases the capacity of OFDM

system in proportion with the increase in the amount of Overload. Also, Symbols are

spread over carriers equivalent to the amount of spreading gain. So it also brings in ad-

vantage of Diversity Gain. This is because, under deep fade over certain carriers it avoids

the loss of those many number of symbols and hence distributes the fade over all the sym-

bols equally which can be further easily recovered at receiver side. But, this novel Access

technique has to deal with the highPAPRissue as it mainly depends on kind of spreading

sequences we are using [15].

14

System Description Overloaded Spread Spectrum OFDMA

Figure 3.1: Transmitter Block Diagram for Overloaded OFDM

Figure 3.2: Receiver Block Diagram for Overloaded OFDM

15


Fig. 3.1[7] shows that the incoming bitstream is mapped to symbols and then further

converted from serial to parallel.M > N number of symbols spread over theN Spreaded

Symbols. This implies that, Spreading Gain of theM chip sequences isN . After the

mapping, theN Spreaded Symbols are passed through as input to an IFFT operation,

which indicates an OFDM system. The output of IFFT is followed by a parallel to serial

converter.Then before the Upconversion, a Cyclic Prefix (CP) is appended at the end of the

OFDM symbol.

Fig. 3.2[7] shows the Receiver Architecture, wherein the reverse chronology of events

occurs as that of the Transmitter side, except a Iterative Multi-Stage Detector. The Iterative

Multi-Stage Detector is used to mitigate theMAI due to theM > N symbols being spread

to N spreaded symbols. Details regarding Detection algorithm is given later.

3.2.2 System Block-Diagram for UL-Transmission

So as to understand, where this access technique fits into thesystem, consider a scenario

for Uplink transmission. Base Station hasM number of useful sub-carriers with it and a

maximum ofN sub-carriers can be allocated to one user. Hence, in total,U = MN

is the

total number of users supported by the system. In this way, a particular user can transmit

the data over the allocated sub-carriers only.

Fig. 3.3[7] showsU number of users transmitting over whole system BW. Hence, each

user is mappingM number of symbols overN number of allocated carriers(M > N),

which makes the system overloaded.

Fig. 3.4[7] shows the receiver block diagram i.e. at the Base Station (BS). Each user’s

data is decoded by using Interference Cancelation along with Despreading for respective

sub-carriers set.

3.2.3 System Block-Diagram for DL-Transmission

This system is also fit for DL-Transmission. Consider the scenario given above, in which

the system supports totalU number of users each are allocated totalN sub-carriers. Hence

at the Base-Station (transmitter) if overloaded SS-OFDMA is used, BS will spreadM data

symbols of each user over allocated N carriers. To decode thesame data at particular User,

they already have information regarding the carriers allocated by BS to them. Hence, every

User decodes the information from allocated carriers only.In this way, capacity of existing

system is increased with proportion of amount of overload.

16


Figure 3.3: UL-Transmission system Block Diagram for Overloaded SS-OFDMA

Figure 3.4: UL-Reception system Block Diagram for Overloaded SS-OFDMA

17


Figure 3.5: DL-Transmission system Block Diagram for Overloaded SS-OFDMA

Following block diagram Fig.3.5[7] shows the transmission of data to individual user

from the BS. And Fig.3.6[7] shows the reception of the data from Base Station to in-

dividual User. Hence, in this case BS Spreads the data of individual user over allocated

sub-carriers.

18

Multi-Stage Detector Overloaded Spread Spectrum OFDMA

Figure 3.6: DL-Reception system Block Diagram for Overloaded SS-OFDMA

3.3 Multi-Stage Detector

This section gives brief overview of the Multi-stage Detectors used for interference can-

celation. One of the Iterative Multi-Stage Detector given in literature [16], is used for in-

terference cancelation. Following figure shows the Iterative Multi-Stage Detector Fig.3.7,

in which two sets of symbols are detected in parallel afterI number of iterations of Multi

Stage Detector algorithm. Basic principle of this detectoris to iteratively remove the inter-

ference from the other set of received symbols and to achieveperformance equivalent to

completely loaded system.

In the first iteration, interference from other set of symbols is considered to be zero.

Hence, detected symbols are given by equations (3.6) and (3.7). While in the following

iterations, interference from other set of symbols in removed. Hence System increases in

the succeeding iterations.

δ1,i andδ2,i used in the analysis are the Partial Cancelation Factors which decides the

amount of estimated interference for the set-2 and set-1 symbols, respectively. Generally,

as the iterations increases value of Partial Cancelation Factor (PCF) approaches unity and

it is selected to minimize theBER. δ1,i is set to unity as set-1 data estimates are reliable as

19

Analytical Model Overloaded Spread Spectrum OFDMA

Figure 3.7: Iterative Multi-Stage Detector Algorithm

compared to set-2 estimates. The following section gives analysis related to this Iterative

MUD.

3.4 Analytical Model

Consider that|∆u| number of carriers are allocated to a User Equipment (UE). A UE can

mapM number of modulated symbols over the available carriers. Incase of Overloading

M = N + K

where,N is equivalent to number of carriers allocated i.e.|∆u| andK decides Over-

load Factor.

Cv[n] is chip sequence matrix of dimensionM × |∆u|, used to spread theM number of

symbols over allocated carriers. In case of Overloading this chip sequence can be divided

into two setsS1 andS2 containingN andK number of codes respectively, each of length

|∆u| i.e.N . Let, chip sequence matricesCu,1 andCu,2 indicates 1st and 2nd set of codes,

respectively. Then, the transmitted Signal after Spreading operation is given as :-

ξu[n] =M

∑

v=1

du[sM + v]Cv[n] ∀n : 1, 2, · · · , |∆u| (3.1)

20

Analytical Model Overloaded Spread Spectrum OFDMA

where,du[sM + v] is vth mapped symbol on sth OFDM Symbol and

ξu[n] is spreaded signal i.e. input to IFFT.

Received signal after FFT operation at Receiver is given by

~Rs = ~Hs × ~Xs + ~η (3.2)

where,~Hs is channel matrix of length as that of size of FFT andXs is output of FFT.

If ~Zs indicates equalizer coefficients, then output of equalizeris given by

~Rseq = ~Zs × ~Hs × ~Xs + ~Zs × ~η (3.3)

where,~η is Noise Vector, size as that of the FFT.

Following section gives Iterative Multi User Detector Algorithm that is used to cancel

interference from other set of symbols. Algorithm uses parallel interference cancelation

technique.

Step 1. In the first iteration, output of matched filter for 1st set of symbols is given by

y11,l =

[

du,1[l] +N

∑

v=1;v 6=l

du,1[v] ∗ ρu,1,l,v +M

∑

v′=1

du,2[v′] ∗ ρu,1,l,v′

]

N∑

k=1

Zs[k]Hs[k] + ηl,1(3.4)

l; 1, 2, · · · , N

where, ηl,1 =∑N

k=1 Zs[k]η[k]C∗u,1,l[k]

Step 2. Similarly, in the same iteration, output of matched filter for second set of sym-

bols is given as

y12,l′ =

[

du,2[l′] +

M∑

v′=1;v′ 6=l′

du,2[v′] ∗ ρu,1,l′,v′ +

N∑

v=1

du,1[v] ∗ ρu,1,l′,v

]

N∑

k=1

Zs[k]Hs[k] +(3.5)

ηl′,1

l′ : 1, 2, · · · , K

where, ηl′,1 =∑N

k=1 Zs[k]η[k]C∗u,1,l′[k]

Step 3. using Hard Decision Interference Cancelation (HDIC), Symbol detection in 1st

iteration is given as

d̂1u,1,l = φ̄(y1

1,l) l : 1, 2, · · · , N (3.6)

d̂1u,2,l′ = φ̄(y1

2,l′) l′ : 1, 2, · · · , K (3.7)

21

Code Selection Overloaded Spread Spectrum OFDMA

Step 4. for iterations i = 2 to I we have

matched filter output of first set of symbols is given by,

yi1,l = y1

1,l − δ2,i

K∑

v′=1

d̂i−1u,2 [v′]ρu,1,l,v′ (3.8)

d̂iu,1,l = ¯φ(yi

1,l) l : 1, 2, · · ·N

In similar way, we can write for 2nd set of Symbols

yi2,l′ = y1

2,l′ − δ1,i

N∑

v=1

ˆdi−1u,1 [v]ρu,2,l′,v (3.9)

d̂iu,2,l′ = ¯φ(yi

2,l′) l′ : 1, 2, · · ·K

This is an Iterative Multi-user Detector Algorithm, where from, we can get two set of

detected symbols in I number of iteration i.e. we get,d̂Iu,1,l for 1st set and̂dI

u,2,l′ for 2nd

set.

cross correlation parameters given in algorithm, can be enumerated as follow

1. ρu,1,l,v: cross correlation oflth set-1 symbol withvth set-1 symbol.

2. ρu,1,l,v′: cross correlation oflth set-1 symbol withvth set-2 symbol.

3. ρu,1,l′,v: cross correlation ofl′th set-2 symbol withvth set-1 symbol.

4. ρu,1,l′,v′ : cross correlation ofl′th set-2 symbol withvth set-2 symbol.

3.5 Code Selection

Selection of codes in CDMA system depends on its correlationproperties i.e. both cross

correlation and Auto correlation. Cross correlation of codes ensures the mitigation of

interference while auto correlation properties deal with the synchronization problems. This

means, good auto correlation properties inhibits less interference even system has some

synchronization error.

Same concept can be applied here, but in case of overloading we are dealing with two

sets of codes for two sets of symbols to be mapped [12]. This means that, orthogonality

between two sets and codes within and between the sets has to be ensured. In literature,

we find studies related to selection criterion such as, selection of codes in over saturated

22


CDMA channels and Optimal Signature Sets for Over saturatedQuasi-Scalable Direct-

Sequence Spread-Spectrum Systems by [17]. Also, [18] has given performance of Over-

loaded CDMA technique using Orthogonal Gold Codes and also usingQOS.

This simulation uses, Orthogonal Gold Codes as we can form multiple sets of orthogonal

codes using these sequences and also, these codes are orthogonal within sets, but cross

correlation is nonzero between the two sets. Generation of OG Gold codes of lengthN is

presented below [19].

Step 1. Consider two Maximum Length (ML) sequences A and B such that

A = [a0, a1, a2, · · · , aN2]

B = [b0, b1, b2, · · · , bN−2]

Step 2. following procedure is used for the Gold Code formation

Ci,j =

aj ⊕ T ibj for 0 ≤ i < N − 1

aj for i = N − 1

0 otherwise

(3.10)

Where,j = 0, 1, · · · , N − 2

Here, T ibj represents cyclic shift ofbj by T i amount of chips forith Gold

Code. Hence, with this procedure we can get N-1 number of spreading Gold

Codes of chip length N-2.

Step 3. To construct Orthogonal Gold codes from this sequence set wehave to follow

following procedure.

☛ Append a zero at the end of every sequence

☛ replace zeros in sequence set with 1 and 1 with -1

This produces N Gold sequences of length N which are Orthogonal to each other. Using

The same procedure given above and using the sameML sequences but having different

time shifts, we can produce entirely different set of orthogonal sequences. These ML

sequences are given bellow

ai,2 = (a1, a2, · · · , aN−2, a0) = T 1ai (3.11)

bi,2 = (b1, a2, · · · , bN−2, b0) = T 1bi

23


For each time shift we can produce new set of Orthogonal sequence. Hence for N-1 length

two ML sequence, we can produce N-1 number of Orthogonal GoldCode sets.

Even These sets of Orthogonal Gold Codes are orthogonal within a set but between two

sets, their correlation is non zero.

Table 3.1: Values of Set Cross Correlation

Sequence Length No. of Sets Correlation Coeffi-cient

8 7 0.516 15 0.532 31 0.2564 63 0.25128 127 0.162256 255 0.125512 511 0.06251024 1023 0.0625

24

4Underloaded Spread Spectrum OFDMA

4.1 Introduction

In case ofUnderloading, less than or equal toN number of users are allocated Chip

Sequences of Spreading GainN . Since, these sequences have good cross-correlation

characteristics, the orthogonality between them is maintained under the Underloading

operation. As compared to the Overloaded case, there is no additional Multiple Access

Interference (MAI ) seen in the Underloaded case.

4.2 Transceiver Description

Fig4.1 shows the block diagram of downlink transmission from base station where each

users data is interleaved among themselves and less number of symbols are spreaded over

more number of subcarriers so that there is no interference at the transmitter side.

Fig4.2 shows the Block diagram of downlink reception at each user where each user

receives their own data.There is no need of Iterative Multi-stage Detector for parallel in-

terference Cancellation at the receiver

25

Transceiver Description Underloaded Spread Spectrum OFDMA

Figure 4.1: Downlink Tx Block diagram from Base-station in Low interleaving case

Figure 4.2: Downlink Rx Block diagram at each-user in Low interleaving case

26

5The Interleaving operation

5.1 Introduction

Interleaving is an integral component of any digital communication system, especially

when the channel exhibits phenomena such as Fading, which cause Burst Errors. Interleav-

ing can help control such burst errors, and hence, improve the Bit Error Rate performance

of the system.

At the transmitter, the Interleaver reorders the input source symbols. This reordering

may be random or fixed, both trying to ensure that order of the symbols is made random.

The reordering is denoted byπ(i) operation on the indices, which gives an indexj ∈

1, 2, . . . , n. It is a one-to-one mapping, and hence, is reversible.

Input Symbols: X1, X2, X3, . . . , Xn

After Interleaving: Xπ(1), Xπ(2), Xπ(3), . . . , Xπ(n)

At the receiver, a corresponding deinterleaver is present,which reverses the work of the

interleaver, and puts the received symbols back into order.This operation may be denoted

27

Overloading The Interleaving operation

by π−1(j) which gives an indexi, such thatπ(i) = j.

Received Symbols: X̂π(1), X̂π(2), X̂π(3), . . . , X̂π(n)

After Deinterleaving: X̂1, X̂2, X̂3, . . . , X̂n

As a result of interleaving, the correlated noise introduced in the channel appears to be

statistically independent at the receiver. This allows a better Bit Error Rate performance of

the Error Correction Coding scheme used, and hence a better system performance.

In this work, we have analyzed the effect of replacing a normal interleaver over spread-

ing gain number of source symbols, with a larger interleaverover all the source symbols in

an OFDM Symbol. The corresponding configuration of the transmitter and receiver block

diagrams is shown in the following figures, for both the cases, for both Overloading and

Underloading.

5.2 Overloading

Fig 5.1shows the block diagram of downlink transmission from base station where each

users data is interleaved among themselves and more number of symbols are spreaded over

less number of subcarriers so that interference is introduced at the transmitter side.

Figure 5.1: Downlink Tx Block diagram from Base-station in Low interleaving case

28


Figure 5.2: Downlink Rx Block diagram at each-user in Low interleaving case for Overloading

Fig 5.2 shows the Block diagram of downlink reception at each user where each user

receives their own data.To reduce the interference effect which is introduced at the trans-

mitter side Iterative Multi-stage detector for Parallel Interference Cancellation is used at

the receiver.

Fig 5.3shows the block diagram of downlink transmission from base station where all

the users data is interleaved combindly and more number of symbols are spreaded over

less number of subcarriers so that interference is introduced at the transmitter side.

Fig 5.4 shows the Block diagram of downlink reception at each user where each user

receives all users data.To reduce the interference effect which is introduced at the trans-

mitter side Parallel Interference Cancellation is used at the receiver.

29


Figure 5.3: Downlink Tx Block diagram from Base-station in High interleaving case

Figure 5.4: Downlink Rx Block diagram at each-user in High interleavingcase

30

Underloading The Interleaving operation

5.3 Underloading

In Fig 5.5, we use asmaller interleaver block (normal case). At the transmitter, the in-

terleaving is done over each set ofM bits individually, that are sent by each of theU

users.

Figure 5.5: SS-OFDMA Small Interleaver Transmitter Block Diagram

In Fig 5.6, at the receiver, the deinterleaver block works on the sameM bits, and puts them

back into order.

Figure 5.6: SS-OFDMA Small Interleaver Receiver Block Diagram

In Fig 5.7, we use alarger interleaver block instead of the normal one. At the transmitter,

the interleaving is done overU ∗ M bits together, whereM bits each are sent by each of

theU users.

In Fig 5.8, at the receiver, each user has to receive all theU ∗ M bits that were broad-

casted by the transmitter, and deinterleave them. Out of theU ∗ M bits, each user now

31

Underloading The Interleaving operation

Figure 5.7: SS-OFDMA Small Interleaver Transmitter Block Diagram

takes only itsM bits, and drops the other bits.

Figure 5.8: SS-OFDMA Small Interleaver Receiver Block Diagram

32

6SDIC in Overloaded SS-OFDMA

6.1 Introduction

To improve the interference cancellation at the receiver incase of Oveerloaded SS-OFDMA,

we replace the Hard Decision Interference Cancelation (HDIC) by Soft Decision Interfer-

ence Cancellation (SDIC).

SDIC has been known to improve performance of the system by reducing the number

of errors made during the intermediate hard decisions, at the expense of added receiver

complexity. Hence, in this case also, we expect an improvement in the performance of

Overloaded SS-OFDMA.

6.2 Measures of Confidence

In this case, we use the Log-Likelihood Ratio (LLR) to measure the confidence we have on

any intermediate decision. This confidence can then be incorporated into further decisions,

resulting in what is call Soft Decisions. LLR also comes in 2 flavours, given below.

33

SDIC Algorithm SDIC in Overloaded SS-OFDMA

6.2.1 Exact LLR

The expression for the Exact LLR of any received symbol r is given as :

ExactLLR = log(Prob(bi = 0|r = (x, y))

Prob(bi = 1|r = (x, y))) (6.1)

6.2.2 Approximate LLR

The expression for the Approximate LLR of any received symbol r is given as :

ApproxLLR =−1

σ2[mins∈S0

{(x − sx)2 + (y − sy)

2} − mins∈S1{(x − sx)

2 + (y − sy)2}]

(6.2)

6.3 SDIC Algorithm

In our SDIC algorithm, the input is taken as the received and equalized symbols. We then

despread these symbols and then, calculate the LLR values per bit by using one of the

discussed LLR metrics.

Next, we have used a linear transformation to map the LLR value of each bit to a soft bit

value. For this we have considered the following function, using a delta parameter. This is

the way in which the confidence value has been incorporated into the model.

SoftBiti =

{

−LLRi

2∗θ+ 1

2|LLRi| < θ

1 − sgn(LLRi) |LLRi| > θ(6.3)

where,θ = maxallbits(LLR)/10 is a design parameter.

Next, after obtaining an estimate of the soft bit values thatwere sent, we simulate the

channel and spreading effects and cancel the interference due to these estimated soft bits to

the other symbols. This is achieved by using the Iterative Multi-Stage Detector discussed

in Ch3. Fig 6.1summarizes the SDIC algorithm.

34

SDIC Algorithm SDIC in Overloaded SS-OFDMA

Figure 6.1: The SDIC Algorithm for 2 bits/symbols case

35

7Results - Overloading and Underloading

7.1 Overloaded SS-OFDMA with HDIC receiver

7.1.1 Low Interleaving

Fig. 7.1 and Fig.7.2 show the BER performance of Overloaded SS-OFDMA for BPSK,

QPSK and 16QAM, with complex scrambling OCDMA/TDMA codes, with a low inter-

leaving depth (over each user’s bits). The simulation has been done using the Rayleigh

channel for both Indoor and Outdoor scenarios. We have considered the HDIC receiver

here, varying the spreading gains between 32, 128 and 256 with 1/2 rate Error Correction

Coding (ECC) and an FFT size of 2048 subcarriers.

Observations Results show that there is a degradation in the SNR requirement, for a

required BER, with Overloading. This degradation is higherfor higher modulation. This

is because, as we increase the number of symbols in set-2, interference increases with that

respect. Hence make performance worse. As we increase the SG, performance improves

because of decrease in Cross-Correlation between different codes.

36

Overloaded SS-OFDMA with HDIC receiver Results - Overloading and Underloading

0 5 10 15 20 25 30 35 40 4510

−3

10−2

10−1

100

BER performance of Overloaded SS−OFDMA for BPSK modulation with Complex ScramblingOCDMA/TDMA codes in RAYLEIGH channel for HDIC receiver with ECC and interleaving

SNR in dB

pro

b. o

f e

rro

r

%Overload = 0;SG = 32;LILA−Indoor%Overload = 50;SG = 32;LILA−Indoor%Overload = 0;SG = 128;LILA−Indoor%Overload = 50;SG = 128;LILA−Indoor%Overload = 0;SG = 256;LILA−Indoor%Overload = 50;SG = 256;LILA−Indoor

(a) BPSK

0 5 10 15 20 25 30 35 40 4510

−3

10−2

10−1

100

BER performance of Overloaded SS−OFDMA for QPSK modulation with Complex ScramblingOCDMA/TDMA codes in RAYLEIGH channel for HDIC receiver with ECC and interleaving

SNR in dB

pro

b. o

f e

rro

r


(b) QPSK

0 5 10 15 20 25 30 35 40 4510

−3

10−2

10−1

100

BER performance of Overloaded SS−OFDMA for 16QAM modulation with Complex ScramblingOCDMA/TDMA codes in RAYLEIGH channel for HDIC receiver with ECC and interleaving

SNR in dB

pro

b. o

f e

rro

r


(c) 16QAM

Figure 7.1: BER performance of Overloaded SS-OFDMA for different modulation in Low inter-leaving Indoor, with HDIC receiver

37


0 5 10 15 20 25 30 35 40 4510

−3

10−2

10−1

100


SNR in dB

pro

b. o

f e

rro

r

%Overload = 0;SG = 32;LILA−Outdoor%Overload = 50;SG = 32;LILA−Outdoor%Overload = 0;SG = 128;LILA−Outdoor%Overload = 50;SG = 128;LILA−Outdoor%Overload = 0;SG = 256;LILA−Outdoor%Overload = 50;SG = 256;LILA−Outdoor

(a) BPSK

0 5 10 15 20 25 30 35 40 4510

−3

10−2

10−1

100


SNR in dB

pro

b. o

f e

rro

r


(b) QPSK

0 5 10 15 20 25 30 35 40 4510

−3

10−2

10−1

100


SNR in dB

pro

b. o

f e

rro

r


(c) 16QAM

Figure 7.2: BER performance of Overloaded SS-OFDMA for different modulation in Low inter-leaving Outdoor, with HDIC receiver

38


7.1.2 More interleaving

Fig. 7.3and Fig.7.4shows the BER performance of Overloaded SS-OFDMA for BPSK,

QPSK and 16QAM, with complex scrambling OCDMA/TDMA codes, with a larger In-

terleaver depth (over all the bits in an OFDM symbol). The simulations have been done in

Rayleigh channel for both Indoor and Outdoor scenarios. We have considered the HDIC

receiver here, varying the spreading gains between 32, 128 and 256 with 1/2 rateECCand

an FFT size of 2048 subcarriers.

Observations Results shows that there is a degradation in SNR requirementwith over-

loading similar to the Low Interleaving case. But the BER performance improves as com-

pared to the Low Interleaving case due to More Interleaving gain among the users data.

The SNR degradation is more for higher modulation. This is because, as we increase

the number of symbols in set-2, Multiple Access Interference (MAI ) increases with that

respect and hence making the performance worse. Also, the gain of SNR for More inter-

leaving over Low interleaving is seen decreasing with increasing Spreading gain. So, we

should choose to use lower SGs for More Interleaving Gain.

7.1.3 Comparison of BPSK 100% overload with QPSK 0% overload

The Fig.7.5and Fig.7.6show the comparison of BER performance of BPSK with 100%

overload with QPSK 0% overload with complex scrambling OCDMA/TDMA codes, for

LILA and MILA cases respectively. The simulations have beendone in Rayleigh channel

for both Indoor and Outdoor scenarios. We have considered the HDIC receiver here, vary-

ing the spreading gains between 32, 128 and 256 with 1/2 rateECC and an FFT size of

2048 subcarriers.

Observations It shows that there is some gain of using overload at high SNRsin both

indoor and outdoor cases, for LILA case. Howevere, there is no gain of using overload

at high SNRs in both indoor and outdoor cases, for MILA case. Hence, it is clear that

Overloading is giving advantage only for Low Interleaving.So we should choose Low

interleaving instead of More Interleaving for this case.

39


0 5 10 15 20 25 30 35 40 4510

−4

10−3

10−2

10−1

100


SNR in dB

pro

b.

of

err

or

%Overload = 0;SG = 32;MILA−Indoor%Overload = 50;SG = 32;MILA−Indoor%Overload = 0;SG = 128;MILA−Indoor%Overload = 50;SG = 128;MILA−Indoor%Overload = 0;SG = 256;MILA−Indoor%Overload = 50;SG = 256;MILA−Indoor

(a) BPSK

0 5 10 15 20 25 30 35 40 4510

−4

10−3

10−2

10−1

100


SNR in dB

pro

b.

of

err

or


(b) QPSK

0 5 10 15 20 25 30 35 40 4510

−4

10−3

10−2

10−1

100


SNR in dB

pro

b.

of

err

or


(c) 16QAM

Figure 7.3: BER performance of Overloaded SS-OFDMA for different modulation in More inter-leaving Indoor, with HDIC receiver

40


0 5 10 15 20 25 30 35 40 4510

−4

10−3

10−2

10−1

100


SNR in dB

pro

b.

of

err

or

%Overload = 0;SG = 32;MILA−Outdoor%Overload = 50;SG = 32;MILA−Outdoor%Overload = 0;SG = 128;MILA−Outdoor%Overload = 50;SG = 128;MILA−Outdoor%Overload = 0;SG = 256;MILA−Outdoor%Overload = 50;SG = 256;MILA−Outdoor

(a) BPSK

0 5 10 15 20 25 30 35 40 4510

−4

10−3

10−2

10−1

100


SNR in dB

pro

b.

of

err

or


(b) QPSK

0 5 10 15 20 25 30 35 40 4510

−4

10−3

10−2

10−1

100


SNR in dB

pro

b.

of

err

or


(c) 16QAM

Figure 7.4: BER performance of Overloaded SS-OFDMA for different modulation in More inter-leaving Outdoor, with HDIC receiver

41


0 5 10 15 20 25 30 35 40 4510

−3

10−2

10−1

100

BER performance of Overloaded SS−OFDMA BPSK100% with QPSK 0% with Complex Scrambling OCDMA/TDMA codes in RAYLEIGH channel for HDIC receiver with ECC and interleaving

SNR in dB

pro

b. o

f e

rro

r

%Overload = 100;SG = 32;LILA−Indoor(bpsk)%Overload = 0;SG = 32;LILA−Indoor(qpsk)%Overload = 100;SG = 128;LILA−Indoor(bpsk)%Overload = 0;SG = 128;LILA−Indoor(qpsk)%Overload = 100;SG = 256;LILA−Indoor(bpsk)%Overload = 0;SG = 256;LILA−Indoor(qpsk)

(a) Low interleaving indoor

0 5 10 15 20 25 30 35 40 4510

−3

10−2

10−1

100

BER performance of Overloaded SS−OFDMA for BPSK100% with QPSK 0% with Complex ScramblingOCDMA/TDMA codes in RAYLEIGH channel for HDIC receiver with ECC and interleaving

SNR in dB

pro

b. o

f e

rro

r

%Overload = 100;SG = 32;LILA−Outdoor(BPSK)%Overload = 0;SG = 32;LILA−Outdoor(QPSK)%Overload = 100;SG = 128;LILA−Outdoor(BPSK)%Overload = 0;SG = 128;LILA−Outdoor(QPSK)%Overload = 100;SG = 256;LILA−Outdoor(BPSK)%Overload = 0;SG = 256;LILA−Outdoor(QPSK)

(b) Low interleaving outdoor

Figure 7.5: BER performance of Overloaded SS-OFDMA for BPSK 100 percentOverload withQPSK 0 percent overload Low interleaving case, with HDIC receiver

42


0 5 10 15 20 25 30 35 40 4510

−4

10−3

10−2

10−1

100

BER performance of Overloaded SS−OFDMA for BPSK 100% with QPSK 0% with Complex ScramblingOCDMA/TDMA codes in RAYLEIGH channel for HDIC receiver with ECC and interleaving

SNR in dB

pro

b. o

f e

rro

r

%Overload = 100;SG = 32;MILA−Indoor(BPSK)%Overload = 100;SG = 32;MILA−Indoor(BPSK)%Overload = 100;SG = 32;MILA−Indoor(BPSK)%Overload = 0;SG = 32;MILA−Indoor(QPSK)%Overload = 0;SG = 128;MILA−Indoor(QPSK)%Overload = 0;SG = 256;MILA−Indoor(QPSK)

(a) More interleaving indoor

0 5 10 15 20 25 30 35 40 4510

−4

10−3

10−2

10−1

100

BER performance of Overloaded SS−OFDMA for BPSK 100% with QPSK 0% with Complex ScramblingOCDMA/TDMA codes in RAYLEIGH channel for HDIC receiver with ECC and interleaving

SNR in dB

pro

b. o

f e

rro

r

%Overload = 100;SG = 32;MILA−Outdoor(BPSK)%Overload = 100;SG = 32;MILA−Outdoor(BPSK)%Overload = 100;SG = 32;MILA−Outdoor(BPSK)%Overload = 0;SG = 32;MILA−Outdoor(QPSK)%Overload = 0;SG = 128;MILA−Outdoor(QPSK)%Overload = 0;SG = 256;MILA−Outdoor(QPSK)

(b) More interleaving outdoor

Figure 7.6: BER performance of Overloaded SS-OFDMA for BPSK 100 percentOverload withQPSK 0 percent overload More interleaving case, with HDIC receiver

43

Overloaded SS-OFDMA with SDIC receiver Results - Overloading and Underloading

7.2 Overloaded SS-OFDMA with SDIC receiver


Fig. 7.7and Fig.7.8 show the BER performance of Overloaded SS-OFDMA for BPSK,

QPSK and 16QAM, with complex scrambling OCDMA/TDMA codes, with a low inter-

leaving depth (over each user’s bits). The simulation has been done using the Rayleigh

channel for both Indoor and Outdoor scenarios. We have considered the SDIC receiver

here, varying the spreading gains between 32, 128 and 256 with 1/2 rateECCand an FFT

size of 2048 subcarriers.

Observations In both Indoor and Outdoor environments, there is a degradation in the

SNR requirement with Overloading, for a required BER. Similar to the HDIC case, this

degradation is more for higher modulations due to increasedinterference. However, as ex-

pected, the degradation for each case with SDIC is quite lessthan the degradation observed

with HDIC.

7.2.2 More interleaving

Fig. 7.9and Fig.7.10shows the BER performance of Overloaded SS-OFDMA for BPSK,

QPSK and 16QAM, with complex scrambling OCDMA/TDMA codes, with a larger In-

terleaver depth (over all the bits in an OFDM symbol). The simulations have been done in

Rayleigh channel for both Indoor and Outdoor scenarios. We have considered the SDIC

receiver here, varying the spreading gains between 32, 128 and 256 with 1/2 rateECCand


Observations Results shows that there is a degradation in SNR requirementwith Over-

loading similar to the Low Interleaving case. But the BER performance improves as com-

pared to the Low Interleaving case due to More Interleaving gain among the users data,

especially at higher SNRs. The SNR degradation is more for higher modulations, as in the

previous cases. However, as expected, the degradation for each case with SDIC is quite

less than the degradation observed with HDIC.

44


−5 0 5 10 15 20 25 30 35 40 4510

−4

10−3

10−2

10−1

100

BER Overloaded SS−OFDMA − M = 2 moduatn− Rayleigh−Indoor SDIC ECC LILA

SNR in dB

prob

. of b

it er

ror

%Ov=0;SG=32;LILA−Indoor; M=2%Ov=50;SG=32;LILA−Indoor; M=2%Ov=0;SG=128;LILA−Indoor; M=2%Ov=50;SG=128;LILA−Indoor; M=2%Ov=0;SG=256;LILA−Indoor; M=2%Ov=50;SG=256;LILA−Indoor; M=2

(a) BPSK

−5 0 5 10 15 20 25 30 35 40 4510

−4

10−3

10−2

10−1

100


SNR in dB

prob

. of b

it er

ror


(b) QPSK

−5 0 5 10 15 20 25 30 35 40 4510

−4

10−3

10−2

10−1

100


SNR in dB

prob

. of b

it er

ror


(c) 16QAM

Figure 7.7: BER performance of Overloaded SS-OFDMA for different modulation in Low inter-leaving Indoor, with SDIC receiver

45


−5 0 5 10 15 20 25 30 35 40 4510

−4

10−3

10−2

10−1

100

BER Overloaded SS−OFDMA − M = 2 moduatn− Rayleigh−Outdoor SDIC ECC LILA

SNR in dB

prob

. of b

it er

ror

%Ov=0;SG=32;LILA−Outdoor; M=2%Ov=50;SG=32;LILA−Outdoor; M=2%Ov=0;SG=128;LILA−Outdoor; M=2%Ov=50;SG=128;LILA−Outdoor; M=2%Ov=0;SG=256;LILA−Outdoor; M=2%Ov=50;SG=256;LILA−Outdoor; M=2

(a) BPSK

−5 0 5 10 15 20 25 30 35 40 4510

−4

10−3

10−2

10−1

100


SNR in dB

prob

. of b

it er

ror


(b) QPSK

−5 0 5 10 15 20 25 30 35 40 4510

−4

10−3

10−2

10−1

100


SNR in dB

prob

. of b

it er

ror


(c) 16QAM

Figure 7.8: BER performance of Overloaded SS-OFDMA for different modulation in Low inter-leaving Outdoor, with SDIC receiver

46


−10 −5 0 5 10 15 20 25 30 35 40 45

10−4

10−3

10−2

10−1

100

BER Overloaded SS−OFDMA − M = 2 modulatn− Rayleigh−Indoor SDIC ECC LILA

SNR in dB

prob

. of b

it er

ror

%Ov=0;SG=32;MILA−Indoor; M=2%Ov=50;SG=32;MILA−Indoor; M=2%Ov=0;SG=128;MILA−Indoor; M=2%Ov=50;SG=128;MILA−Indoor; M=2%Ov=0;SG=256;MILA−Indoor; M=2%Ov=50;SG=256;MILA−Indoor; M=2

(a) BPSK

−5 0 5 10 15 20 25 30 35 40 4510

−4

10−3

10−2

10−1

100

BER Overloaded SS−OFDMA − M = 4 modulatn− Rayleigh−Indoor SDIC ECC MILA

SNR in dB

prob

. of b

it er

ror


(b) QPSK

−5 0 5 10 15 20 25 30 35 40 4510

−4

10−3

10−2

10−1

100


SNR in dB

prob

. of b

it er

ror


(c) 16QAM

Figure 7.9: BER performance of Overloaded SS-OFDMA for different modulation in More inter-leaving Indoor, with SDIC receiver

47


−5 0 5 10 15 20 25 30 35 40 4510

−4

10−3

10−2

10−1

100

BER Overloaded SS−OFDMA − M = 2 modulatn− Rayleigh−Outdoor SDIC ECC MILA

SNR in dB

prob

. of b

it er

ror

%Ov=0;SG=32;MILA−Outdoor; M=2%Ov=50;SG=32;MILA−Outdoor; M=2%Ov=0;SG=128;MILA−Outdoor; M=2%Ov=50;SG=128;MILA−Outdoor; M=2%Ov=0;SG=256;MILA−Outdoor; M=2%Ov=50;SG=256;MILA−Outdoor; M=2

(a) BPSK

−5 0 5 10 15 20 25 30 35 40 4510

−4

10−3

10−2

10−1

100


SNR in dB

prob

. of b

it er

ror


(b) QPSK

−5 0 5 10 15 20 25 30 35 40 4510

−4

10−3

10−2

10−1

100


SNR in dB

prob

. of b

it er

ror


(c) 16QAM

Figure 7.10: BER performance of Overloaded SS-OFDMA for different modulation in More in-terleaving Outdoor, with SDIC receiver

48


7.2.3 Comparison of BPSK 100% overload with QPSK 0% overload

The Fig. 7.11and Fig.7.12show the comparison of BER performance of BPSK with 100%

overload with QPSK 0% overload with complex scrambling OCDMA/TDMA codes, for

LILA and MILA cases respectively. The simulations have beendone in Rayleigh channel

for both Indoor and Outdoor scenarios. We have considered the SDIC receiver here, vary-

ing the spreading gains between 32, 128 and 256 with 1/2 rateECC and an FFT size of

2048 subcarriers.

−5 0 5 10 15 20 25 30 35 40 4510

−4

10−3

10−2

10−1

100


SNR in dB

prob

. of b

it er

ror


(a) Low interleaving indoor

−5 0 5 10 15 20 25 30 35 40 4510

−4

10−3

10−2

10−1

100


prob

. of b

it er

ror

SNR in dB


(b) Low interleaving outdoor

Figure 7.11: BER performance of Overloaded SS-OFDMA for BPSK 100 percentOverload withQPSK 0 percent overload Low interleaving case, with SDIC receiver

49


−5 0 5 10 15 20 25 30 35 40 4510

−4

10−3

10−2

10−1

100


SNR in dB

prob

. of b

it er

ror


(a) More interleaving indoor

−5 0 5 10 15 20 25 30 35 40 4510

−4

10−3

10−2

10−1

100


SNR in dB

prob

. of b

it er

ror


(b) More interleaving outdoor

Figure 7.12: BER performance of Overloaded SS-OFDMA for BPSK 100 percentOverload withQPSK 0 percent overload More interleaving case, with SDIC receiver

50

Underloaded SS-OFDMA Results - Overloading and Underloading

Observations Here, the BPSK 100% overload graph has come closer to the QPSK0%

overload graph as compared to previous section, due to the SDIC algorithm used. It shows

that there is some gain of using overload at high SNRs in both indoor and outdoor cases,

for LILA case. But similar to previous case, there is no gain of using overload at high

SNRs in both indoor and outdoor cases, for MILA case.

7.3 Underloaded SS-OFDMA


Fig. 7.13and Fig.7.14show the BER performance of Underloaded SS-OFDMA for BPSK,

QPSK, 16QAM and 64QAM with complex scramblingOG codes, with a low interleaving

depth (over each user’s bits). The simulation has been done using the Rayleigh channel for

both Indoor and Outdoor scenarios. We have considered spreading gains between 32, 128

and 256 with 1/2 rateECCand an FFT size of 2048 subcarriers.

Observations Results show that there is a big improvement in SNR requirement with

the increase in underloading. This improvement for higher modulation is somewhat lesser,

if not similar. This is because, as we increase the underloading, the number of symbols

spread over the number of subcarriers are less. Due to this, the interference introduced

over the channel is less where there are less number of Cross-correlated spreaded symbols.

7.3.2 More Interleaving

Fig. 7.15 and Fig.7.16 show the BER performance of Underloaded Spread spectrum

OFDMA for for BPSK, QPSK, 16QAM and 64QAM with complex scrambling OGcodes,

with a large interleaving depth (over all users’ bits). The simulation has been done using

the Rayleigh channel for both Indoor and Outdoor scenarios.We have considered spread-

ing gains between 32, 128 and 256 with 1/2 rateECCand an FFT size of 2048 subcarriers.

Observations Results show that there is a similar improvement in SNR requirement with

the increase in underloading similar to the Low interleaving case. To quantify, wew ob-

serve an improvement in SNR of about 3 dB for 50% underload,approximately 6 dB for

25% underload and around 9 dB for 12.5% underload compared tofull-load(underload

100%) atPe = 10−3. In absolute value, BER performance improves compared to Low

interleaving due to More Interleaving gain among the users data. The improvement for

higher modulations is also similar. However, this gain of SNR for More interleaving over

51


−10 0 10 20 30 4010

−3

10−2

10−1

100

BER performance of underloaded SS−OFDMA for different modulation in RAYLEIGH channel with Complex Scrambling OG codes with ECC and interleaving

SNR in dB

pro

b. o

f e

rro

r

%underload=12.5;SG = 32;LILA indoor(BPSK)%underload=25;SG = 32;LILA indoor(BPSK)%underload=50;SG = 32;LILA indoor(BPSK)%underload=100;SG = 32;LILA indoor(BPSK)%underload=12.5;SG = 32;LILA indoor(QPSK)%underload=25;SG = 32;LILA indoor(QPSK)%underload=50;SG = 32;LILA indoor(QPSK)%underload=100;SG = 32;LILA indoor(QPSK)%underload=12.5;SG = 32;LILA indoor(16QAM)%underload=25;SG = 32;LILA indoor(16QAM)%underload=50;SG = 32;LILA indoor(16QAM)%underload=100;SG = 32;LILA indoor(16QAM)%underload=12.5;SG = 32;LILA indoor(64QAM)%underload=25;SG = 32;LILA indoor(64QAM)%underload=50;SG = 32;LILA indoor(64QAM)%underload=100;SG = 32;LILA indoor(64QAM)

(a) Spreading gain 32

−10 0 10 20 30 4010

−3

10−2

10−1

100


SNR in dB

pro

b. o

f e

rro

r


(b) Spreading gain 128

−10 0 10 20 30 4010

−3

10−2

10−1

100


SNR in dB

pro

b. o

f e

rro

r


(c) Spreading gain 256

Figure 7.13: Comparison of BER performance of Underloaded SS-OFDMA for different modula-tion in Low interleaving indoor case

52


−10 0 10 20 30 4010

−3

10−2

10−1

100


SNR in dB

pro

b. o

f e

rro

r

%underload=12.5;SG = 32;LILA Outdoor(BPSK)%underload=25;SG = 32;LILA Outdoor(BPSK)%underload=50;SG = 32;LILA Outdoor(BPSK)%underload=100;SG = 32;LILA Outdoor(BPSK)%underload=12.5;SG = 32;LILA Outdoor(QPSK)%underload=25;SG = 32;LILA Outdoor(QPSK)%underload=50;SG = 32;LILA Outdoor(QPSK)%underload=100;SG = 32;LILA Outdoor(QPSK)%underload=12.5;SG = 32;LILA Outdoor(16QAM)%underload=25;SG = 32;LILA Outdoor(16QAM)%underload=50;SG = 32;LILA Outdoor(16QAM)%underload=100;SG = 32;LILA Outdoor(16QAM)%underload=12.5;SG = 32;LILA Outdoor(64QAM)%underload=25;SG = 32;LILA Outdoor(64QAM)%underload=50;SG = 32;LILA Outdoor(64QAM)%underload=100;SG = 32;LILA Outdoor(64QAM)


−10 0 10 20 30 4010

−3

10−2

10−1

100


SNR in dB

pro

b. o

f e

rro

r



−10 0 10 20 30 4010

−3

10−2

10−1

100


SNR in dB

pro

b. o

f e

rro

r



Figure 7.14: Comparison of BER performance of Underloaded SS-OFDMA for different modula-tion in Low interleaving Outdoor case

53


−10 0 10 20 30 4010

−4

10−3

10−2

10−1

100


SNR in dB

pro

b. o

f e

rro

r

%underload=12.5;SG = 32;MILA indoor(BPSK)%underload=25;SG = 32;MILA indoor(BPSK)%underload=50;SG = 32;MILA indoor(BPSK)%underload=100;SG = 32;MILA indoor(BPSK)%underload=12.5;SG = 32;MILA indoor(QPSK)%underload=25;SG = 32;MILA indoor(QPSK)%underload=50;SG = 32;MILA indoor(QPSK)%underload=100;SG = 32;MILA indoor(QPSK)%underload=12.5;SG = 32;MILA indoor(16QAM)%underload=25;SG = 32;MILA indoor(16QAM)%underload=50;SG = 32;MILA indoor(16QAM)%underload=100;SG = 32;MILA indoor(16QAM)%underload=12.5;SG = 32;MILA indoor(64QAM)%underload=25;SG = 32;MILA indoor(64QAM)%underload=50;SG = 32;MILA indoor(64QAM)%underload=100;SG = 32;MILA indoor(64QAM)


−10 0 10 20 30 4010

−4

10−3

10−2

10−1

100


SNR in dB

pro

b. o

f e

rro

r



−10 0 10 20 30 4010

−4

10−3

10−2

10−1

100


SNR in dB

pro

b. o

f e

rro

r



Figure 7.15: Comparison of BER performance of Underloaded SS-OFDMA for different modula-tion in More interleaving indoor case

54


−10 0 10 20 30 4010

−4

10−3

10−2

10−1

100


SNR in dB

pro

b. o

f e

rro

r

%underload=12.5;SG = 32;MILA Outdoor(BPSK)%underload=25;SG = 32;MILA Outdoor(BPSK)%underload=50;SG = 32;MILA Outdoor(BPSK)%underload=100;SG = 32;MILA Outdoor(BPSK)%underload=12.5;SG = 32;MILA Outdoor(QPSK)%underload=25;SG = 32;MILA Outdoor(QPSK)%underload=50;SG = 32;MILA Outdoor(QPSK)%underload=100;SG = 32;MILA Outdoor(QPSK)%underload=12.5;SG = 32;MILA Outdoor(16QAM)%underload=25;SG = 32;MILA Outdoor(16QAM)%underload=50;SG = 32;MILA Outdoor(16QAM)%underload=100;SG = 32;MILA Outdoor(16QAM)%underload=12.5;SG = 32;MILA Outdoor(64QAM)%underload=25;SG = 32;MILA Outdoor(64QAM)%underload=50;SG = 32;MILA Outdoor(64QAM)%underload=100;SG = 32;MILA Outdoor(64QAM)


−10 0 10 20 30 4010

−4

10−3

10−2

10−1

100


SNR in dB

pro

b. o

f e

rro

r



−10 0 10 20 30 4010

−4

10−3

10−2

10−1

100


SNR in dB

pro

b. o

f e

rro

r



Figure 7.16: Comparison of BER performance of Underloaded SS-OFDMA for different modula-tion in More interleaving Outdoor case

55


Low interleaving is seen to decrease with increasing Spreading gain. Hence, we should

choose lower SGs for More Interleaving Gain.

56

8Underloaded SS-OFDMA for Dynamic

VoIP data

8.1 OFDMA - Concept of PRB

In OFDMA, users are allocated a specific number of subcarriers for a predetermined

amount of time. These are referred to as Physical Resource Blocks (PRBs) in theLTE

specifications. PRBs thus have both a time and frequency dimension.

Figure 8.1: Available Downlink Bandwidth is divided into PRBs

57

Applying Underloaded SS-OFDMA Underloaded SS-OFDMA for Dynamic VoIP data

The total number of available subcarriers depends on the overall transmission bandwidth

of the system. The LTE Specifications define parameters for the system bandwidths from

1.25 MHz to 20 MHz as shown in Fig 8.1[20]. A Physical Resource Block (PRB) is defined

as consising of 12 consecutive subcarriers for one slot (0.5msec) in duration. A PRB is

the smallest addressable element by the scheduler, in an OFDMA-based system.

8.2 Applying Underloaded SS-OFDMA

In this analysis, we have considered Dynamic VoIP data from another simulation. The

main purpose is to evaluate the Underloaded SS-OFDMA schemefor such a practical case,

where the Modulation schemes being used in different PRBs may be different. Fig8.2

shows the results of initial analysis of the data, revealingthe probability distribution func-

tions of number of PRBs being used with each modulation scheme at any time.

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25250

0.025

0.05

0.075

0.1

0.125

0.15

0.175

0.2

0.225

0.25

0.275

0.3PDF of number of PRBs (out of 25 in 1 TTI) using each of the modulation schemes

No. of PRBs

Prob

QPSK : Mean = 7.2105, Var = 37.025

16−QAM : Mean = 4.1422, Var = 6.0626

64−QAM : Mean = 3.9115, Var = 2.3390

Figure 8.2: PDF of number of PRBs using a modulation scheme

Then, we chose the number of PRBs for each modulation scheme,as the mean obtained

in Fig 8.2in one case, and a value away from the mean in another case. Using these values,

we simulated the case when only the chosen number of randomlyselected PRBs are being

used in the specific modulation scheme, and rest are unused, and the bit error rate has

been calculated for each case. Hence, the scheme is equivalent to a form of Underloaded

SS-OFDMA, as discussed in previous chapters. The Spreadinghere is achieved by using

58

BER Results Underloaded SS-OFDMA for Dynamic VoIP data

DFT-Spreading and DFT-Despreading operations. Since, SC-FDMA also uses the same

spreading codes, our scheme here can be thought to be SC-FDMAitself.

8.3 BER Results

8.3.1 AWGN Channel - Without Spreading

Fig. 8.3 shows the BER performance of OFDMA for QPSK, 16QAM, and 64QAM, for

both LILA (Interleaving over one PRB) and MILA (Interleaving over all PRBs) cases. The

simulation has been done using the AWGN channel. We have considered 1/2-rateECCand


Observations For the AWGN Channel, as expected, the BER performances is almost

equal on changing the number of PRBs under use. Also, there isno interleaving advantage

observed, as we have used an AWGN channel instead of a Rayleigh Channel.

8.3.2 Rayleigh Channel - Without Spreading

Fig. 8.4 shows the BER performance of OFDMA for QPSK, 16QAM, and 64QAM, for

both LILA (Interleaving over one PRB) and MILA (Interleaving over all PRBs) cases.

The simulation has been done using the Rayleigh channel for Outdoor scenario. We have

considered 1/2-rateECCand an FFT size of 512 subcarriers.

Observations Similar to the previous case, there is not much difference observed in the

BER performance with changes in number of PRBs under use. However, contrary to the

AWGN case, the More Interleaving result in each case has a better performance than the

Low Interleaving result. This is attributed to the larger interleaver depth in MILA, which

gives it a greater amount of Diversity gain. At higher modulations, a gap in the BER

performances begins to emerge at high SNRs, with change in number of PRBs used. Also

the interleaving benefit reduces for higher modulation cases.

For this case, we investigated the number of PRBs/modulations scheme, that would give

us the same performance as the Full-loaded case, as shown in Fig 8.5

Observations For QPSK, in MILA, the case with 13 PRBs under use has almost equal

BER performance as the Full-loaded case, and in LILA, the case with 7 PRBs under use

has almost equal BER performance as the Full-loaded case. For 16QAM, in both LILA

59


−10 −5 0 5 10 15 20 25 30 35

10−4

10−3

10−2

10−1

100

BER − Full−loaded OFDMA for QPSK Modulation in AWGN channel − HDIC − ECC − LILA − No Spreading

SNR in dB

Pro

b. o

f Bit

Err

or

QPSK7−LILAQPSK7−MILAQPSK13−LILAQPSK13−MILA%full−load=100;LILA%full−load=100;MILA

(a) QPSK

−10 −5 0 5 10 15 20 25 30 35

10−4

10−3

10−2

10−1

100

BER − Full−loaded OFDMA for 16QAM Modulation in AWGN channel − HDIC − ECC − LILA − No Spreading

SNR in dB

Pro

b. o

f Bit

Err

or

16QAM4−LILA16QAM4−MILA16QAM7−LILA16QAM7−MILA%full−load=100;LILA%full−load=100;MILA

(b) 16QAM

−10 −5 0 5 10 15 20 25 30 35

10−4

10−3

10−2

10−1

100

BER − Full−loaded OFDMA for 64QAM Modulation in AWGN channel − HDIC − ECC − LILA − No Spreading

SNR in dB

Pro

b. o

f Bit

Err

or


(c) 64QAM

Figure 8.3: BER performance of OFDMA for Dynamic VoIP data under AWGN Channel

60


−10 −5 0 5 10 15 20 25 30 35

10−4

10−3

10−2

10−1

100

Comparison of BER − Full−loaded OFDMA for QPSK Modulation in RAYLEIGH channel − HDIC − ECC − No Spreading

SNR in dB

Pro

b. o

f Bit

Err

or

QPSK7−LILAQPSK7−MILAQPSK13−LILAQPSK13−MILA%full−load=100;LILA%full−load=100;MILA

(a) QPSK

−10 −5 0 5 10 15 20 25 30 35

10−4

10−3

10−2

10−1

100

BER − Full−loaded OFDMA for mix of 16QAM in RAYLEIGH channel − HDIC − ECC− No Spreading

SNR in dB

Pro

b. o

f Bit

Err

or


(b) 16QAM

−10 −5 0 5 10 15 20 25 30 35

10−4

10−3

10−2

10−1

100

BER − Full−loaded OFDMA for mix of Modulations in RAYLEIGH channel − HDIC − ECC − No Spreading

SNR in dB

Pro

b. o

f Bit

Err

or


(c) 64QAM

Figure 8.4: BER performance of OFDMA for Dynamic VoIP data under Rayleigh Channel

61


−10 −5 0 5 10 15 20 25 30 35 40 45

10−4

10−3

10−2

10−1

100

Comparison of BER − Full−loaded OFDMA for QPSK Modulation in RAYLEIGH channel − HDIC − ECC − No Spreading

SNR in dB

Pro

b. o

f Bit

Err

or

QPSK2−LILAQPSK2−MILAQPSK4−LILAQPSK4−MILAQPSK6−LILAQPSK6−MILAQPSK7−LILAQPSK7−MILAQPSK13−LILAQPSK13−MILA%full−load=100;LILA%full−load=100;MILA

(a) QPSK

−10 −5 0 5 10 15 20 25 30 35 40 45

10−4

10−3

10−2

10−1

100

BER − Full−loaded OFDMA for mix of 16QAM in RAYLEIGH channel − HDIC − ECC− No Spreading

SNR in dB

Pro

b. o

f Bit

Err

or


(b) 16QAM

−10 −5 0 5 10 15 20 25 30 35 40 45

10−4

10−3

10−2

10−1

100

BER − Full−loaded OFDMA for mix of Modulations in RAYLEIGH channel − HDIC − ECC − No Spreading

SNR in dB

Pro

b. o

f Bit

Err

or

64QAM4−LILA64QAM4−MILA64QAM6−LILA64QAM6−MILA64QAM8−LILA64QAM8−MILA64QAM10−LILA64QAM10−MILA64QAM13 − MILA64QAM15 − MILA64QAM20 − MILA%full−load=100;LILA%full−load=100;MILA

(c) 64QAM

Figure 8.5: BER performance of OFDMA - finding performance equivalent ofFullloaded case

62


and MILA, the case with 7 PRBs under use has almost equal BER performance as the

Full-loaded case. For 64QAM, in MILA, the case with 20 PRBs under use has almost

equal BER performance as the Full-loaded case. Hence, as thenumber of PRBs under use

increases from 1 to 25, the BER performance comes closer and closer to the fullloaded

case, as expected.

8.3.3 Rayleigh Channel - With Spreading

Fig. 8.6shows the BER performance of SS-OFDMA for QPSK, 16QAM, and 64QAM ,

for both LILA (Interleaving over one PRB) and MILA (Interleaving over all PRBs) cases.


considered the spreading gain of 12 (Spreading over each PRB) with 1/2-rateECCand an

FFT size of 512 subcarriers.

Observations At higher modulations, the difference in BER performance between the

Fullloaded case and the partial PRB case increases. In that also, the case using lower

number of PRBs is observed to have worse BER performance thanthe other case. We may

also observe that at higher modulations, the gain of MILA over LILA reduces.

8.3.4 Rayleigh Channel - With Variable Spreading

Fig. 8.7 shows the BER performance of SS-OFDMA for QPSK, 16QAM, and 64QAM,

for both LILA (Interleaving over one PRB) and MILA (Interleaving over all PRBs) cases.


considered 1/2-rateECC and an FFT size of 512 subcarriers. In this case, by Variable

Spreading, we imply that here the Spreading has been done over all used PRBs.

Observations By applying Variable Spreading, the BER performance of partial PRB

cases becomes better than the Fullloaded case. Also the BER performance is equal for

both partial PRB cases for all modulations. This can be explained by the lower interference

present in the partial PRB cases, due to many PRBs being unsed. Variable Spreading

also has a PAPR benefit to offer, making this scheme beneficialand capable of replacing

OFDMA.

63


−10 −5 0 5 10 15 20 25 30 35 40 4510

−4

10−3

10−2

10−1

100

Comparison of BER − PRB + Fullload cases − SSOFDMA − QPSK − RAYLEIGH channel − HDIC − ECC − Spreading

prob. of bit error

SNR in dB

QPSK7 − LILAQPSK7 − MILAQPSK13 − LILAQPSK13 − MILA%underload=100;SG = 32;LILA Outdoor%underload=100;SG = 32;MILA Outdoor

(a) QPSK

−10 −5 0 5 10 15 20 25 30 35 40 4510

−4

10−3

10−2

10−1

100

Comparison of BER − PRB + Fullload cases − SSOFDMA − 16QAM − RAYLEIGH channel − HDIC − ECC − Spreading

prob. of bit error

SNR in dB

16QAM4 − LILA16QAM4 − MILA16QAM7 − LILA16QAM7 − MILA%underload=100;SG = 32;LILA Outdoor%underload=100;SG = 32;MILA Outdoor

(b) 16QAM

−10 −5 0 5 10 15 20 25 30 35 40 4510

−4

10−3

10−2

10−1

100

SNR in dB

prob. of bit error

Comparison of BER − PRB + Fullload cases − SSOFDMA − 64QAM − RAYLEIGH channel − HDIC − ECC − Spreading


(c) 64QAM

Figure 8.6: BER performance of SS-OFDMA for Dynamic VoIP data under Rayleigh Channel

64


−10 0 10 20 30 4010

−4

10−3

10−2

10−1

100

BER − Partial PRB and Fullloaded SS−OFDMA QPSK RAYLEIGH Outdoor HDIC ECC Variable Spreading FFT512

SNR in dB

prob. of bit error

QPSK7 − LILAQPSK7 − MILAQPSK13 − LILAQPSK13 − MILA%underload=100;SG = 32;LILA Outdoor%underload=100;SG = 32;MILA Outdoor

(a) QPSK

−10 0 10 20 30 4010

−4

10−3

10−2

10−1

100

prob. of bit error

BER − Partial PRB and Fullloaded SS−OFDMA 16QAM RAYLEIGH Outdoor HDIC ECC Variable Spreading FFT512

SNR in dB

16QAM4 − LILA16QAM4 − MILA16QAM7 − LILA16QAM7− MILA%underload=100;SG = 32;LILA Outdoor%underload=100;SG = 32;MILA Outdoor

(b) 16QAM

−10 0 10 20 30 4010

−4

10−3

10−2

10−1

100

prob. of bit error

BER − Partial PRB and Fullloaded SS−OFDMA 64QAM RAYLEIGH Outdoor HDIC ECC Variable Spreading FFT512

SNR in dB


(c) 64QAM

Figure 8.7: BER performance of SS-OFDMA for Dynamic VoIP data under Rayleigh Channel,with Variable Spreading

65

9Conclusions and Future work

This chapter briefly summarizes the work done in this projectand some of the potential

areas where this work can be extended in future.

9.1 Conclusion

• In Overloaded SS-OFDMA with HDIC receiver, we observed that the degradation of

SNR is more for higher modulations, for the same overload factor.

• In Overloaded SS-OFDMA with SDIC receiver, the BER performance in all over-

loaded cases was found to be slightly better than with HDIC. However, the higher

modulations still have higher SNR degradation for same overload.

• Only BPSK shows some potential of using Overloading.

• SDIC is not able to resolve the SNR degradation issue of Overloaded SS-OFDMA.

• In Underloaded SS-OFDMA, we observe a big improvement in the BER perfor-

mance as the underload percentage increases.

• LargeInterleaving shows better BER performance as compared to Low interleaving.

66

Future Work Conclusions and Future work

• PAPR performance of Underloaded SS-OFDMA for the partial PRB case is better

compared to OFDMA.

9.2 Future Work

Future work in this regard will focus primarily on followingareas.

• Work could be pursued to curb the SNR degradation seen in Overloaded SS-OFDMA.

This may involve applying a technique such as Turbo Coding, or may involve search-

ing for more efficient pairs of spreading codes, with even lower Inter-Set cross-

correlation.

• A proper case could be prepared of the Underloaded SS-OFDMAscheme for Fem-

tocells. This may involve including multiple antenna concepts (MIMO) into the

system.

• Another big benefit that was observed was that of having a larger interleaver depth.

More work could be pursued in this direction in working out a feasible model for

implementation of this technique in communication systems.

• In the Partial PRB Analysis, Underloaded SS-OFDMA has shown a lower PAPR

than OFDMA. This may be more deeply analyzed, and may help in coming up with

a new standard for Future Wireless Communications.

67

Appendices

68

PAPR performance results

.1 PAPR performance results

Figure. 1shows a comparison between PAPR of partial PRB case with one PRB for

QPSK,16QAM and 64QAM (FFT size = 512).Results show that the 1PRB case is giv-

ing better PAPR performance compared to normal OFDMA for different modulations. We

also observe that the OFDMA PAPR performance is almost similar for different modula-

tions, but for the 1 PRB case PAPR is more for higher modulation.[21]

1 2 3 4 5 6 7 8 9 100

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1Comparison of PAPR performance of partial PRB case

PAPR in dB

CD

F

QPSK1PRB−with spreading(SG 12)QPSK1PRB−without spread(OFDM)16QAM1PRB−with spreading(SG 12)16QAM1PRB−without spread(OFDM)64QAM1PRB−with spreading(SG 12)64QAM1PRB−without spread(OFDM)

Figure 1: Comparison of PAPR performance of SC-FDMA with OFDMA for different modulation

Figure.2 Shows the PAPR performance for partial PRB schemes for QPSK,16QAM

and 64QAM with different PRBs. Results shows that as we increase the number of PRBs

for a particular modulation PAPR is more, because as we increase the number of PRBs

more number of carriers are present in the same OFDM symbol duration.This increases the

PAPR of the transmitted signal.With Variable SG, PAPR performance is better compare to

Spreading Gain (SG) 12 for fixed number of Physical Resource Blocks(PRBs).[21]

.1.1 Conclusion

• PAPR performance is better for partial PRB cases compared to OFDMA.

• For OFDM PAPR is similar for different modulation, but for partial PRB cases,

PAPR is more for higher modulation.

69


0 2 4 6 8 10 12

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1Comparison of PAPR performance of partial PRB case for QPSK modulation with different number of PRBs

PAPR in dB

CD

F

QPSK1−without spread(OFDM)QPSK2−without spread(OFDM)QPSK4−without spread(OFDM)QPSK8−without spread(OFDM)QPSK16−without spread(OFDM)QPSK25−without spread(OFDM)QPSK1−with variable spread(SG 12)QPSK2−with variable spread(SG 24)QPSK4−with variable spread(SG 48)QPSK8−with variable spread(SG 96)QPSK16−with variable spread(SG 192)QPSK25−with variable spread(SG 300)QPSK1−with spreading(SG 12)QPSK2−with spreading(SG 12)QPSK4−with spreading(SG 12)QPSK8−with spreading(SG 12)QPSK16−with spreading(SG 12)QPSK25−with spreading(SG 12)

(a) QPSK

0 2 4 6 8 10 12

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1Comparison of PAPR performance of partial PRB case for 16QAM modulation with different number of PRBs

PAPR in dB

CD

F

16QAM1−without spread(OFDM)16QAM2−without spread(OFDM)16QAM4−without spread(OFDM)16QAM8−without spread(OFDM)16QAM16−without spread(OFDM)16QAM25−without spread(OFDM)16QAM1−with variable spread(SG 12)16QAM2−with variable spread(SG 24)16QAM4−with variable spread(SG 48)16QAM8−with variable spread(SG 96)16QAM16−with variable spread(SG 192)16QAM25−with variable spread(SG 300)16QAM1−with spreading(SG 12)16QAM2−with spreading(SG 12)16QAM4−with spreading(SG 12)16QAM8−with spreading(SG 12)16QAM16−with spreading(SG 12)16QAM25−with spreading(SG 12)

(b) 16QAM

0 2 4 6 8 10 12

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1Comparison of PAPR performance of partial PRB case for 64QAM modulation with different number of PRBs

PAPR in dB

CD

F

64QAM1−without spread(OFDM)64QAM2−without spread(OFDM)64QAM4−without spread(OFDM)64QAM8−without spread(OFDM)64QAM16−without spread(OFDM)64QAM25−without spread(OFDM)64QAM1−with variable spread(SG 12)64QAM2−with variable spread(SG 24)64QAM4−with variable spread(SG 48)64QAM8−with variable spread(SG 96)64QAM16−with variable spread(SG 192)64QAM25−with variable spread(SG 300)64QAM1−with spreading(SG 12)64QAM2−with spreading(SG 12)64QAM4−with spreading(SG 12)64QAM8−with spreading(SG 12)64QAM16−with spreading(SG 12)64QAM25−with spreading(SG 12)

(c) 64QAM

Figure 2: Comparison of PAPR performance of partial PRB cases for SG 12and variable SG withOFDMA

70


• Variable Spreading is giving better performance comparedto Spreading gain 12 for

a fixed number of PRBs.[21]

71

List of Abbreviations

2G 2nd Generation

3G 3rd Generation

3GPP The Third Generation Partnership Project

4G 4th Generation

AMPS Analogue Mobile Phone System

B3G Beyond 3rd Generation

BER Bit Error Rate

BS Base Station

BW Band Width

CDF Cumulative Distribution Function

CDMA Code Division Multiple Access

CP Cyclic Prefix

DFT Discrete Fourier Transform

ECC Error Correction Coding

EDGE Enhanced Data rates for GSM Evolution

FDMA Frequency Division Multiple Access

FDM Frequency Division Multiplexing

FFT Fast Fourier Transform

72


GPRS General Packet Radio Service

GSM Global System for Mobile communication

HDIC Hard Decision Interference Cancelation

ICI Inter Carrier Interference

IFFT Inverse Fast Fourier Transform

IMT-A International Mobile Telecommunications-Advanced

ISI Inter Symbol Interference

ITU International Telecommunication Union

LTE Long Term Evolution

LTE-A Long Term Evolution - Advanced

MAI Multiple Access Interference

MC-CDMA Multi Carrier Code Division Multiple Access

MC-DS-CDMA Multi Carrier Direct Sequence Code Division Multiple Access

MC-SS-MA Multi Carrier Spread Spectrum Multiple Access

MCS Modulation and Coding Scheme

ML Maximum Length

MUD Multi User Detection

OFDM Orthogonal Frequency Division Multiplexing

OFDM-CDMA Orthogonal Frequency Division Multiplexing - Code Division Multiple

Access

OFDMA Orthogonal Frequency Division Multiple Access

OG Orthogonal Gold

PAPR Peak to Average Power Ratio

PCF Partial Cancelation Factor

73


QOS Quasi Orthogonal Sequences

RAT Radio Access Technique

SC-FDMA Single Carrier-Frequency Division Multiple Access

SDIC Soft Decision Interference Cancellation

SDMA Space Division Multiple Access

SDU Service Data Unit

SG Spreading Gain

SS-OFDMA Spread Spectrum-Orthogonal Frequency Division Multiple Access

TDMA Time Division Multiple Access

UE User Equipment

UMTS Universal Mobile Telecommunications Systems

W-CDMA Wideband Code Division Multiple Access

PRB Physical Resource Block

74

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76

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