OFDM Wireless LANs: A Theoretical and Practical Guide

• Authors: Juha Heiskala;John Terry, Ph.D.
• Venue: OFDM Wireless LANs: A Theoretical and Practical Guide
• Year: 2001

From the Book:Preface This book will address the subject of broadband communications using orthogonal frequency division multiplexing (OFDM). OFDM is a special case of multicarrier modulation (MCM), which is the principle of transmitting data by dividing the stream into several parallel bit streams and modulating each of these data stream onto individual carriers or subcarriers. Although the origin of MCM dates back to the 1950s and early 1960s with military high frequency (HF) radio links, R.W. Chang in the mid 60s first published a paper demonstrating the concept we today call OFDM. Chang demonstrated the principle of transmission of multiple messages simultaneously through a linear band-limited channel without interchannel interference (ICI) and intersymbol interference (ISI). The multichannel or OFDM system developed by Chang differed from tradi-tional MCM in that the spectra of the subcarriers were allowed to overlap under the restriction they were all mutually orthogonal. This characteristic of OFDM systems required the abandon-ment of steep bandpass filters used in older MCM systems to separate the spectra of the individ-ual subcarriers. Weinsten and Ebert were the first to suggest using the discrete Fourier Transform (DFT) and inverse discrete Fourier Transform (IDFT) to perform baseband modulation and demodulation in 1971. Currently, OFDM systems utilize the Fast Fourier Transform (FFT) and Inverse FFT to perform modulating and demodulating of the information data. Saltzberg performed a perfor-mance analysis of OFDM, shortly after Chang published his paper, and concluded that the domi-nate impairment in OFDM is ICI. To combat ICI and ISI, Peled and Ruiz introduced the concept of a cyclic prefix (CP). Rather than using an empty guard space, a cyclic extension of the OFDM symbol is used instead. This effectively simulates a channel performing circular convolution as long as the CP is longer than the impulse response of the channel. The penalty of using a CP is loss of signal energy proportional to the length of the CP, yet the benefits of using a CP generally outweighs any loss of signal energy. Presently, OFDM appears in several standards relating to wireless communications at high data rates such as terrestrial digital audio broadcasting (DAB) and digital video broadcasting (DVB-T) in Europe. Presumably, one of the reasons OFDM was chosen as the DAB standard is that it is possible to deploy single frequency subnetworks within its main networks. Hence, main and relay broadcast transmitters may use the same set of subcarriers. In areas with reception from multiple transmitters, receive diversity gains are experienced. Based on coded OFDM, DVB-T is the youngest and most sophisticated of the three core DVB systems. Combining channel coding with OFDM permits reliable transmission over dispersing channels. Furthermore, the inherent structure of OFDM allows for flexible transmission rates. Finally, WLAN, the main subject of this book, is another application for OFDM technology. For instance, next generation wireless LAN standards such as IEEE 802.11a, High Performance Local Area Network type 2 (HiperLAN/2), and Mobile Multimedia Access Communication (MMAC) system have accepted OFDM as their physical layer specifications. These WLAN sys-tems also incorporate coding with OFDM to combat dispersing channels. It has been shown that coded OFDM modulation over modest dispersing channels can improve, rather than deteriorate, the reliability of the transmission. This interesting counterintuitive phenomenon can be attributed to the inherent frequency diversity provided by OFDM. Arguably, this characteristic is the most attractive feature of OFDM. Interactive Learning Clearly with the growing in interest in OFDM for high data rate wireless communications, in par-ticular WLANs, there is a need in the technical community for a book that reviews the subject of OFDM WLANs. Typically, this is accomplished in a classroom setting. Unfortunately, many engineers and scientists today cannot afford the time required to attend classes at a university. What is needed is a tool to allow each reader to learn each of the concepts presented in the chap-ters at his/her own pace. We have provided that by means of an interactive simulation environ-ment. Please visit our Web site at http://www.samspublishing.com and search for the OFDM book. More specifically, the site contains a complete OFDM WLAN physical layer simulation developed in MATLAB. We developed the simulation tool to illustrate the concepts discussed in Chapters 2聳5. To aid in the learning process, exercises are provided in each of these chapters. The exercises require the use of the OFDM system simulation tool and the simple programs you develop. Most of the examples given in this book are reproducible with the simulation program. The OFDM system simulation is executed through a graphical user interface (GUI) to facilitate system recon-figurability. The GUI is called from the MATLAB command window, which allows users to test quickly and easily many of the concepts in this book with a few clicks of the mouse. The novice and expert alike will thoroughly enjoy the endless combinations of test conditions available to them. With this learning tool, readers can further improve their understanding of the concepts presented in this book. In addition, readers interested in testing their algorithms over a WLAN environment will save months of software development time by using the simulation program located at our Web site. Intended Audience The primary audiences for this book are engineers and scientists without prior knowledge of OFDM. In the development of the text, we consider our primary audience to fall within two broad categories of readers: novice and advanced. For the novice, we envision someone with a background in engineering, mathematics, and some knowledge of communication theory. For that audience, this book provides the basics of OFDM theory with many examples and illustra-tions demonstrating concepts. An example novice reader might be a researcher in digital image processing, who is in interested in understanding what effects does an OFDM WLAN network might have on the quality of the video. Another example of the novice reader could be a radio fre-quency (RF) engineer, who is interested in the additional requirements imposed by OFDM mod-ulation on the RF subsystems in the access point (AP) and mobile terminal (MT). An example of an advanced reader is an engineer or scientist familiar with basic OFDM con-cepts. For those individuals, this book is intended as a source for practical guidelines as well as introductory material of advanced research topics in OFDM. The secondary audiences for this book include individuals, such as network system engineers, product engineers, or managers, for whom some of the mathematical development presented in this text is slightly beyond their scope of understanding. For those individuals, explanatory text is provided throughout this book to give an intuitive feel of many of the concepts discussed. It is assumed that the all audiences have a background in calculus, physics, and random and sto-chastic processes. Thus, the majority of the text in this book is written at the undergraduate level, with the exception of the advanced research topics, which are written at the first-year graduate level. In addition, the reader will be provided in each chapter all the relevant mathematical foun-dations necessary to understand the OFDM principles discussed. As mentioned earlier, explana-tory text is also given to provide a better understanding of these OFDM principles from the mathematical expressions. A final point concerning the audience: to reap the fullest benefit of this book, it is advantageous to the reader to become proficient in the use of MATLAB. We expect this book to attract a broad range of readers, as it is written to do so. Certainly, no book can be all things to everyone. However, no matter your interest level in OFDM WLANs, this book has some insight to offer. Organization of this Book This book is organized as follows. Chapter 1, "Background and WLAN Overview," is dedicated to background material as well as an overview of OFDM WLANs. The background material cov-ers relevant concepts in digital signal and stochastic processing. It expected that readers will refer to this chapter as needed to understand the concepts in latter chapters. Chapters 2聳5 focus on the physical layer specifications of OFDM WLAN. Chapter 2, "Synchronization," provides a detailed discussion of many of the popular synchronization algorithms used in OFDM networks. Specifically, timing synchronization algorithms, which include packet detection, symbol timing recovery, and sample clock tracking, are covered. Also covered are frequency, channel estima-tion, and clear channel assessment (CCA) algorithms. Chapter 3, "Modulation and Coding," pro-vides a brief overview of modulation and coding techniques. In particular, the phase-shift keying (PSK) and quadrature amplitude modulations (QAM) found in OFDM WLAN standards are cov-ered. With respect of channel coding, discussions on block and convolutional codes are provided. Performance evaluation of several operational modes of the IEEE 802.11a physical specification are given. Chapter 3 can be thought of as the central theme or key technology area of current OFDM WLAN systems. Chapter 4, "Antenna Diversity," is dedicated to the central theme or key technology area of future OFDM WLAN, antenna diversity. Several popular transmit and receive diversity schemes are discussed in their context to OFDM systems. Examples show that drastic improvement in error rate performance is achievable when these techniques are deployed. Chapter 5, "RF Distortion Analysis for OFDM WLANs," focuses on the system impairments of the OFDM system resulting from RF nonlinearities. Particularly attention is given to the peak-to-average power (PAPR) prob-lem present in all OFDM systems. In this chapter, a survey of the more popular techniques to handle this problem is analyzed. In addition, other system impairments such as phase noise and in-phase and quadrature (IQ) imbalances are covered. In Chapters 6 and 7, an introduction of the medium access control (MAC) layer is given. Chapter 6, "Medium Access Control (MAC) for IEEE 802.11 Networks," summarizes the IEEE 802.11a MAC, while Chapter 7, "Medium Access Control (MAC) for HiperLAN/2 Networks," summa-rizes the HiperLAN/2 MAC. Both chapter details of the interaction between the MAC layer and the physical layer. Interestingly, a major criticism of OFDM has been the complexity issues associated with real-time implementation of the FFT and IFFT. However, steady improvements in semiconductor process technology has allowed for real-time prototyping of OFDM systems with Field Programmable Gate Array (FPGA) technology and cost effective solutions with Application Specific Integrated Circuit (ASIC) technology. Chapter 8, "Rapid Prototying of WLANs Using FPGA," is dedicated to the issues associated with real-time prototyping of an IEEE 802.11a radio using FPGA technology.