Avoiding Effects of OFDM Adjacent Channel Interference by Using Combinations of Modulation Schemes

Contents

1  Introduction  NA

2  OFDM adjacent channel  NA

3  Conventional  NA

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  NA

4.1  Performance of OFDM modulation  NA

4.2  Proposal  39

4.3  Simulation model and results  41

4.3.1  Simulations of the proposed method  41

Contents

4.3.2  Optimization  61

5  Conclusion  NA

70

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

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

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

List of Tables

2.1  International GHz ISM bands  NA

4.1  Main parameters of simulation to show  NA

performance  of modulation schemes in OFDM  33

4.2  Main parameters of the simulated system  42

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.

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-

Introduction

3 tion

channel [2]. In a single carrier system, if the transmitted signal has a greater bandwidth than the bandwidth of the multipath fading channel and the interference, the received signal will be distorted and interfered [3]. As a result, a single fade or interference can cause the entire link to fail. However, if the transmitter has a narrower bandwidth as compared to the multipath channel and interference, the received signal will not be distorted in time domain. Since OFDM is a multi carrier transmission technique, its transmitting bandwidth is divided into subcarriers each has a much smaller bandwidth. Only a small percentage of the sub carriers will be affected by the multipath fading and interference [17]. Error detection and correction coding can then be used to make necessary corrections to the few erroneous sub carriers. OFDM is also an efficient way to deal with multipath propagation; for a given delay spread, the implementation complexity is signif- icantly lower than that of a single carrier system with an equalizer [18]. In a classical multi carrier system, the total single frequency band is divided into N nonoverlapping frequency subchan- nels. All sub channels are then modulated and frequency multiplexed. Guard bands must be used to avoid spectral overlap of the frequency sub channels in order to eliminate

Introduction

4 inter-channel

interference. However, it leads to an inefficient usage of the available spectrum [16]. OFDM is a special case of multi carrier transmission, where a single data stream is transmitted over a number of lower rate sub carriers. The main difference between OFDM and a classical multi carrier system is that the sub carriers in OFDM are overlapped. To realize the overlapping multi car- rier technique, however we need to reduce crosstalk among sub carriers, which means that we want orthogonality among the different sub carriers [7]. Orthogonality means that there is a precise mathematical relationship between the signals of the sub carriers in an OFDM system. In a normal frequency-division multiplexing system, many carriers are spaced apart in such a way that the signals can be received using conventional filters and de- modulators. In such receivers, guard-bands are introduced between the different carriers and in the frequency domain which results in a lowering of spectrum efficiency. In an OFDM system, the receiver acts as a bank of demodulators, translating sub carriers down to DC, with the resulting sig- nal integrated over a symbol period to recover the raw data. If the other sub carriers all beat down the frequencies that, in the time domain, have a whole number of cycles in the

Introduction

5 symbol

period T, then the integration process results in ze- ros contribution from all other sub carriers. Thus, the sub carriers are orthogonal if the sub-carrier spacing is a multiple of 1/T [1]. To eliminate the banks of sub carriers oscillators and co- herent demodulators required by frequency-division multi- plex, completely digital implementations can be built around the fast Fourier transform (FFT). Using this method, both transmitter and receiver are implemented using an efficient FFT technique [8]. The orthogonality of sub carriers in OFDM can be main- tained, and individual sub carriers can be completely sep- arated by using an FFT circuit at the receiver when there are no inter-symbol interference (ISI) and inter-carrier in- terference (ICI) introduced by transmission channel distor- tion. In practice, however, these conditions can not be ob- tained [19]. To reduce the distortion, a simple solution is to increase the symbol duration or the number of sub car- riers. However, this method may be difficult to implement in terms of carrier stability against Doppler frequency and FFT size [1]. Each sub-carrier can be modulated with a different modulation scheme. In WLAN environment, dif- ferential encoding and detection-based modulation schemes,

Introduction

6 such

as D8PSK, are used. However, according to several standardization committees, the use of a broadband data terminal is possible not only in an indoor environment but also in an outdoor micro-cellular environment. The common modulation schemes used in such kinds of OFDM systems are BPSK, QPSK, 16QAM, and 64QAM [1]. The main motivation and contribution of this research are to find combinations of modulation schemes (which are com- monly used in OFDM systems) in order to increase the OFDM system’s resistance against Adjacent Channel Inter- ference while still maintain its bandwidth efficiency. The thesis is organized as followed: Chapter 2 will introduce the background knowledge of adjacent channel interference and its effects on OFDM system’s performance. In Chap- ter 3, main conventional methods will be introduced to see their advantages and disadvantages. The proposal will then be explained in detail in Chapter 4, followed by intensive simulations and simulation results. In Chapter 5, some con- clusions will be derived from the simulation results.

Chapter

2. OFDM adjacent channel interference OFDM

operators are assigned different channels to provide their services. For example, the 2.4GHz frequency band is available for license-exempt use in Europe, the United State and Japan. Table 2.1 lists available frequency bands and restrictions to devices which use this band for communica- tions. Table.

2.1 International 2.4 GHz ISM bands [1] Location Regulatory range Maximum output power North America 2.400-2.4835 GHz 1000 mW Europe 2.400-2.4835 GHz 100 mW (EIRP) 1

Japan 2.471-2.497 GHz 10 mW

In Japan, equipment manufacturers, service providers and 1 EIRP: Effective Isotropic Radiated Power.

OFDM

adjacent channel interference 8 the

Ministry of Post and Telecommunications are cooperat- ing in a Multimedia Mobile Access Communication (MMAC) project to define new wireless standards similar to those of IEEE 802.11. Additionally, MMAC is also looking into the possibility for ultra-high-speed wireless indoor LANs sup- porting large-volume of data transmission at speeds up to 156 Mbps using frequencies in the 30 to 300 GHz band [1]. Š‡w¤ŸÑ

‰‡w¤ŸÑ ‰‡‡w¤ŸÑ Œ…ˆŒ Œ…ˆ Œ…‰‡ Œ…‰‰ Œ…‰‹ Œ…‰ Œ…‰ Œ…Š‡ Œ…Š‰ Œ…ŠŒ Š‡w¤ŸÑ „‰‡ „‹‡ ‡»™ Fig.

2.1 Channelization in lower and middle UNII band. Figure 2.1 shows the channelization for the lower and mid- dle UNII (Unlicensed National Information Infrastructure) bands (in GHz). Eight channels are available with a channel spacing of 20 MHz and a guard spacing of 30 MHz at the band edges in order to meet FCC restricted band spectral density requirements. FCC also defined an upper UNII band from 5.725 to 5.825 GHz, which carries four other OFDM channels. For this upper band, the guard spacing (guard

OFDM

adjacent channel interference 9 band)

from the band edges is only 20 MHz, as the out-of- band spectral requirements for the upper band are less severe than those of the lower and middle UNII bands. In Europe, a total of 455 MHz is available in two bands, one from 5.15 to 5.35 GHz and another from 5.470 to 5.725 GHz. In Japan, a 100 MHz wide band is available from 5.15 to 5.25 GHz, carrying four OFDM channels. Obviously, the signals generated by commercially available OFDM wireless equipments are by no means perfect. In- deed, these signals generate some amount of energy outside of their approved spectrum bands. This is called side-band emissions [6]. This also is true in other wireless devices us- ing the same OFDM technology, such as Bluetooth, cordless telephones, Digital Terrestrial Television Broadcasting, Dig- ital Audio Broadcasting... Although band-pass filters are usually applied to minimize the RF interference and the side-band emission from adjacent channels, they still gen- erate side lobe energy that falls into the pass-band of other adjacent channels’ signals. This kind of interference is called Adjacent Channel Interference (ACI). If the total ACI from adjacent channels is much stronger than the wanted signal from the wanted channel, side band energy from the ACI can dominate the channel’s pass-band. This is shown in Figure