Digital Communication Techniques

2Some Developments in Modulation Techniques

2.1. Orthogonal frequency division multiplexing

2.1.1. Introduction

Adaptation, coding, the information to be transmitted and the propagation channel are vital questions regarding telecommunications. Modulations are often used for several frequency-selective channels; blocks are modulated using Fourier transforms. This technique, called orthogonal frequency division multiplexing (OFDM), is a market leader; it converts a broad multipath channel into simple, single-path subchannels, which are easy to equalize. In addition, proper use of cyclical redundancies at transmission makes it possible to reduce the complexity of terminals via fast Fourier transforms (FFTs); there are also advantages (simplicity of equalization, use of FFT/Direct Cosine Transform (DCT) algorithms, which can be seen as a Fourier transform) and drawbacks (a lack of diversity). There is also a precursor system called Kineplex, for military radio links in a HF band: 1.8– 30 MHz. However, it is expensive to create orthogonal analog filters; this system has had scant success. At the start of the 1980s, digital modulators based on FFTs began to be introduced into multicarrier modulations. Hence it was easy to achieve speedy take-up of this technology. The algorithm for calculating the FFT was “invented” by Cooley and Tukey (among others), researchers at IBM at the start of the 1960s, and had a significant impact on the development of digital signal processing applications. ...