Chapter 1

CHAPTER 1

INTRODUCTION

Overview

Literature Survey

Objective of Thesis

Outline of Thesis

INTRODUCTION

1. Overview

• Digital Modulation

In most media for communication, only a fixed range of frequencies is available for transmission. One way to communicate a message signal whose frequency spectrum does not fall within that fixed frequency range, or one that is otherwise unsuitable for the channel, is to alter a transmittable signal according to the information in the message signal. This alteration is called modulation, and it is the modulated signal that we transmit. The receiver then recovers the original signal through a process called demodulation.

Fig. 1.1 Modulation Techniques [2]

The Fig. 1.1 shows the modulation techniques that are used in digital communication systems. Like analog modulation, digital modulation alters a transmittable signal according to the information in a message signal. However, in this case, the message signal is restricted to a finite set.

The design of any digital communication system begins with a description of the channel (AWGN and other impairments, such as fading), and a definition of the system requirements (error performance, data rate etc). Given the channel description, we need to determine design choices that best match the channel and meet the performance requirements.

In this thesis we start with examines the BER performance of two types of modulation techniques: spectral efficient modulation schemes and power efficient modulation schemes. Spectral efficient modulation schemes are linear modulation e.g. BPSK, QPSK, M-ary PSK, D-PSK, M-ary PAM, M-ary QAM. These are used for Bandwidth limited system. Power efficient modulation schemes are constant envelope modulation e.g. M-ary FSK, MSK, GMSK etc. These are used for Power-limited system.

We deal with real-time communication systems, in which the term coded (or uncoded) refers to the presence (or absence) of error-correction coding schemes involving the use of redundant bits and expanded bandwidth.

2. Literature Survey

Significant work has been carried out by Odenwalder in the field of BER performances in coded and uncoded AWGN and Rayleigh fading channel [3] in 1976. Cho, Yoon gives BER expression of amplitude modulations [4] in 2002. In 2006, Simon determine the BER of differentially encoded QPSK and OQPSK[5]. [6] explain simulation of communication.

A detailed description of block codes, including the decoding algorithms for RS codes is explained in [7]. Convolutional codes are detailed in [8]. TCM was first invented in the early 1980s by Ungerboeck in 1987[9] and excellent tutorial expositions can be found in [10].

OFDM was invented by Chang in 1966 [11], and the cyclic prefix was proposed by Weinstein and Ebert in 1971 [12]. The performance of a coded OFDM system in a multipath channel is analyzed by Kim in 1999 [13]. OFDM Mitigation technique is well explained by Schniter in 2004 [14]. Bingham [15] studied the performance and complexity of MC modulation and concluded that MC has higher potential in future.

OFDM is used for Digital Audio Broadcasting (DAB), Digital Video Broadcasting (DVB), and wireless Local Area Networks (WLANs) (IEEE 802.11a, IEEE 802.11g). It will also be used in fourth generation cellular systems, including Third Generation Partnership Project-Long-Term Evolution (3GPP-LTE) and WiMAX.

MIMO systems are systems with Multiple Element Antennas (MEAs) at both link ends. Originally suggested by Winters in 1987 [16]. They attracted great attention through theoretical investigations in 1990s by 1990s Foschini and Gans [17] and by Telatar 1999 [18]. Jankiraman [19] explain space-time codes and MIMO systems in 2004. Advances in practical implementation of MIMO systems have also greatly helped in their adoption by international standards organizations. MIMO was included in fourth-generation cellular systems as well as high-throughput wireless Local Area Networks (LANs).

A detailed description can also be found in [20], [21], [22], [24], [25], [26], [27] and [28].

3. Objective of Thesis

The objectives of this thesis involved in:

• Overview the BER performance of various Modulation Techniques, Channel Codes in AWGN Channel and Multipath Rayleigh Fading Channel.
• Simulation study and comparisons of the BER performance of OFDM, MIMO, TCM and combination of these three techniques in Rayleigh Fading Channel.
• Simulation study of the 4-level Quality of Service (QoS) system using MATLAB/SIMULINK.
• Simulation study of Offset 8-PSK and compare the BER performance with non-offset 8-PSK.
• Simulation study of the various (N, K) Reed-Solomon codes to determine Optimum Code Rate and corresponding (N, K) tuples.

4. Outline of Thesis

This section presents the outline of the thesis. The thesis has been divided into seven chapters; remaining six chapters are described here.

Chapter 2:  Chapter 2 overviews the effect of several radio impairment factors like AWGN, delay spread, multipath fading etc on the error performance through analytical analysis.

Chapter 3:  In this chapter, Simulation study is carried out for each modulation techniques present in chapter 2 and their performances are compared. Also in case of Rayleigh Fading Channel, RS code, convolution code and mitigating methods namely OFDM and MIMO are analyzed. It is found that the first two methods RS code and convolution code are not so efficient, because they do not address to the major cause of ISI due to delay dispersion.

Chapter 4: In this chapter a multilevel QoS model is proposed in terms of BER and SER using encoder and decoder only (without changing actual MODEM). Also a simulation study is carried out to compare BER and SER performances of different levels.

Chapter 5: An offset technique for 8-PSK to reduce amplitude fluctuation is proposed in this chapter.

Chapter 6: In this chapter, optimum code rate and corresponding perfect (N, K) tuples of RS codes for which BER is least is determined through extensive simulation study and results comparison.

Chapter 7: Chapter 7 summarizes and concludes the total thesis work in brief. Simulation study and results show that better BER performance in Rayleigh fading channel is achieved using OFDM and MIMO systems. Advantage, limitation of work and some direction for future work is also presented here.