Avoiding Effects of OFDM Adjacent

Channel Interference by Using

Combinations of Modulation Schemes

Keio university

Graduate school of Science and Technology -

School of Science for Open and Environmental

Systems

Nakagawa - Haruyama Laboratory

LE TIEN ANH

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Contents

Abstract

1

1 Introduction

2

2 OFDM adjacent channel interference

7

3 Conventional Methods

14

3.1

Windowing .

14

3.2

Filtering .

20

3.3

Guard-band and virtual sub-carrier .

22

3.4

Forward Error Correction coding .

23

3.5

Adaptive modulation .

25

4 Combinations of modulation schemes

29

4.1

Performance of OFDM modulation schemes

33

4.2

Proposal .

39

4.3

Simulation model and results .

41

4.3.1

Simulations of the proposed method .

41

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Contents

­ii­

4.3.2

Optimization .

61

5 Conclusion

68

Acknowledgments

69

References

70

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List of Figures

2.1

Channelization in lower and middle UNII band.

8

2.2

Adjacent channel interference .

10

2.3

Near-far problem in wireless communication

systems.

.

12

3.1

Magnitude transfer function of a raised-cosine

window.

.

17

3.2

IIR power gain with practical filtering orders.

21

4.1

BER performance of an OFDM system using

modulation schemes:BPSK, QPSK, 16QAM,

64QAM .

34

4.2

PER performance of an OFDM system using

modulation schemes:BPSK, QPSK, 16QAM,

64QAM .

35

4.3

Throughput performance of an OFDM sys-

tem using modulation schemes:BPSK, QPSK,

16QAM, 64QAM .

37

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List of Figures

­ii­

4.4

Proposal of modulation combination to avoid

adjacent channel interference in OFDM.

. .

40

4.5

Simulation model .

43

4.6

BER performance of the simulated 16QAM

OFDM system under adjacent channel inter-

ference with conventional and proposed mod-

ulation methods .

47

4.7

PER performance of the simulated 16QAM

OFDM system under adjacent channel inter-

ference with conventional and proposed mod-

ulation methods .

49

4.8

Throughput performance of the simulated 16QAM

OFDM system under adjacent channel inter-

ference with conventional (16QAM) and pro-

posed modulation methods .

51

4.9

Throughput performance of the simulated 16QAM

OFDM system under adjacent channel inter-

ference with conventional (16QAM, QPSK,

BPSK) and proposed modulation methods. .

53

4.10 BER performance of the simulated 64QAM

OFDM system under adjacent channel inter-

ference with conventional and proposed mod-

ulation methods .

54

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List of Figures

­iii­

4.11 PER performance of the simulated 64QAM

OFDM system under adjacent channel inter-

ference with conventional and proposed mod-

ulation methods .

56

4.12 Throughput performance of the simulated 64QAM

OFDM system under adjacent channel inter-

ference with conventional (64QAM) and pro-

posed modulation methods .

58

4.13 Throughput performance of the simulated 64QAM

OFDM system under adjacent channel inter-

ference with conventional (64QAM) and pro-

posed modulation methods .

60

4.14 Optimization of nos when applying the pro-

posal into a 64 sub-carrier 16QAM OFDM

system .

63

4.15 Optimization of nos when applying the pro-

posal into a 64 sub-carrier 64QAM OFDM

system .

65

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List of Tables

2.1

International 2.4 GHz ISM bands [1] . . . .

7

4.1

Main parameters of a simulation to show the

performance of modulation schemes in OFDM. 33

4.2

Main parameters of the simulated system.

.

42

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Abstract

Non-orthogonality among adjacent OFDM channels creates

OFDM adjacent channel interference and it heavily affects

the entire system′s performance. Conventional methods to

avoid OFDM adjacent channel interference are not only in-

sufficient but also are wasting a lot of frequency resources.

In this research, a method using combinations of modula-

tion schemes is proposed to avoid effects of OFDM adja-

cent channel interference. It can be obtained by modulat-

ing the sub-carriers at the outer sides of an OFDM chan-

nel with lower order modulation schemes (such as BPSK

or QPSK), while modulating the sub-carriers at the inner

side of the OFDM channel with higher order modulation

schemes (such as 16QAM or 64QAM). Intensive simulations

have been carried out to evaluate the performance of the

proposed method. The simulation results have shown an

increase in the OFDM system′s resistance against adjacent

channel interference while still maintain the bandwidth effi-

ciency.

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Chapter 1.

Introduction

The growth of demand on wireless mobile multimedia ser-

vices has made OFDM technology a very popular modula-

tion scheme for high-speed communication systems. OFDM

has been applied in almost all kinds of communication media

such as wireless, copper wires, power-line or fiber optic [7]...

It can be defined as either a modulation or a multiplex-

ing technique. It has been used in many applications such

as Digital Terrestrial Television Broadcasting, Digital Audio

Broadcasting, wireless networking and broadband internet

access. IEEE 802.11 standard extension targets a range of

data rate from 6 up to 54 Mbps using OFDM in the 5 GHz

band making OFDM effectively a world-wide standard for

this band [1].

One of the main reasons to use OFDM is because it can han-

dle and have the potential to handle efficiently the multipath

fading and interference problems in a wireless communica-

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