1.1 Introduction
Orthogonal Frequency Division Multiplexing (OFDM) is a multicarrier transmission technique, which divides the available spectrum into many carriers, each one being modulated by a low rate data stream. OFDM is similar to FDMA in that the multiple user access is achieved by subdividing the available bandwidth into multiple channels, which are then allocated to users. However, OFDM uses the spectrum much more efficiently by spacing the channels much closer together. This is achieved by making all the carriers orthogonal to one another, preventing interference between the closely spaced carriers. Coded Orthogonal Frequency Division Multiplexing (COFDM) is the same as OFDM except that forward error correction is applied to the signal before transmission. This is to overcome errors in the transmission due to lost carriers from frequency selective fading, channel noise and other propagation effects. For this discussion the terms OFDM and COFDM are used interchangeably, as the main focus of this work is on OFDM, but it is assumed that any practical system will use forward error correction, thus would be COFDM.
OFDM can be viewed as a collection of transmission techniques. When this technique is applied in wireless environment, it is referred to as OFDM. In the wired environment, such as asymmetric digital subscriber lines (ADSL), it is referred to as discrete multi tone (DMT). In OFDM, each carrier is orthogonal to all other carriers. However, this condition is not always maintained in DMT (Dusan, 1998). OFDM is an optimal version of multi carrier transmission schemes. OFDM started in the mid 60’s, Chang proposed a method to synthesize band limited signals for multi channel transmission (Chang, 1966). The idea is to transmit signals simultaneously through a linear band limited channel without inter channel (ICI) and inter symbol interference (ISI). After that, Saltzberg performed an analysis based on Chang’s work and he concluded that the focus to design a multi channel transmission must concentrate on reducing crosstalk between adjacent channels rather than on perfecting the individual signals (Saltzberg, 1967).
In FDMA each user is typically allocated a single channel, which is used to transmit all the user information. The bandwidth of each channel is typically 10kHz-30kHz for voice communications. However, the minimum required bandwidth for speech is only 3kHz. The allocated bandwidth is made wider than the minimum amount required to prevent channels from interfering with one another. This extra bandwidth is to allow for signals from neighbouring channels to be filtered out, and to allow for any drift in the centre frequency of the transmitter or receiver. In a typical system up to 50% of the total spectrum is wasted due to the extra spacing between channels. This problem becomes worse as the channel bandwidth becomes narrower, and the frequency band increases.
As a prelude to other parts of this study, this chapter will discuss the background upon which this study was initiated, the statement of problems that led to this study, the Aim and Objectives of the study. Others are Significance of the study, Scope of work, Limitations of the Study and Definition of technical terms.
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