Ofdm Tx

Introduction

In older multi-channel systems using FDM, the total available bandwidth is divided into N non-overlapping frequency sub-channels. Each sub-channel is modulated with a separate symbol stream and the N sub-channels are frequency multiplexed. OFDM is a multi-channel modulation system employing Frequency Division Multiplexing (FDM) of orthogonal sub-carriers, each modulating a low bit-rate digital stream. In FDM, the prevention of spectral overlapping of sub-carriers reduces Interchannel Interference, but leads to an inefficient use of spectrum. The guard bands on either side of each sub-channel are a waste of precious bandwidth. To overcome this problem, OFDM uses N overlapping (but orthogonal) sub carriers, each carrying a baud rate of 1/T and spaced 1/T apart. Because of the frequency spacing selected, the sub-carriers are all mathematically orthogonal to each other. This permits the proper demodulation of the symbol streams without the requirement of non overlapping spectra.

Another way of specifying the sub-carrier orthogonality condition is to require that each sub-carrier have exactly integer number of cycles in the interval T. It can be shown that the modulation of these orthogonal sub-carriers can be represented as an Inverse Fourier Transform. Alternatively, one may use a DFT operation followed by low-pass filtering to generate the OFDM signal. It must be noted that OFDM can be used either as a modulation or a multiplexing technique.

OFDM using Inverse DFT

The use of Discrete Fourier Transform (DFT) in the parallel transmission of data using

Frequency Division Multiplexing was investigated in 1971 by Weinstein and Ebert [1].Consider a data sequence d0, d2, …, dN-1, where each dn is a complex symbol.(the data sequence could be the output of a complex digital modulator, such as QAM, PSK etc). Suppose we perform an IDFT on the sequence 2dn (the factor 2 is used purely for scaling...