OFDM and Its Block-Level Implementation: A Case Study

摘要

The design of any wireless application aims at acquiring high data rates with minimal interference issues. The inside story of the world of wireless communication is that due to the channel being random and time-variant, multipath propagation takes place. It generates multiple replicas of the transmitted signal, which are all delayed in time differently. This time delay is governed by the length of the propagation path. As a consequence, fading and inter-symbol interference (ISI) occur. If the data rate is low and the symbol duration is large as compared to the maximum delay of the channel, then ISI can be combated without any equalization. As the distance range or the data rate increases, ISI becomes more severe and channel equalization has to be provided-otherwise it imposes a constraint on the maximum achievable data rate. Thus, the time-varying nature of the wireless channel and its inherent randomness makes the modulation technique very critical. It is required to create a situation where a high data rate is to be transmitted over a channel with a relatively large maximum delay. In a nutshell, an ISI-immune and bandwidth-efficient communication system is needed to mitigate the effects of multipath propagation so that wireless applications requiring a high data rate and minimal interference can be supported.rnOrthogonal frequency division multiplexing (OFDM) is a wideband multi-carrier modulation (MCM) technique in which a high-rate data stream is divided into a number of low-rate data streams and transmitted simultaneously over a number of orthogonal sub-carriers. Thus, in each subchannel, the symbol duration is low compared to the maximum channel delay and ISI is mitigated. Moreover, the orthogonality of sub-carriers ensures bandwidth-efficiency of system. It can be seen as either a modulation or a multiplexing technique. This technology combats ISI by avoidance as well as elimination. It relies on the use of inverse fast Fourier transform (IFFT)/fast Fourier transform (FFT)-like signal-processing tools for OFDM signal generation. This paper begins with an overview of the wireless channel and addresses the inherence of ISI in brief. The case study covers the rationale behind OFDM technology, the mathematical intricacies of signal generation, and its implementation details. Further, a brief outline of the challenges faced andrnan insight into its multitudes of applications is given. An OFDM transmitter is simulated for two cases: 64-point IFFT and 128-point IFFT using Matrix Laboratory 7 (MATLAB7) as a simulation tool. It comprises, for example, a serial-to-parallel converter, a symbol mapper, and cyclic prefixing. Lastly, an analysis of the OFDM signal spectrum and its time-domain plots is done. In a nutshell, issues concerning OFDM signal generation and its block-level implementation is taken up as a case study in this paper.