OFDM - signal processing The principles of OFDM...

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30 www.rfdesign.com January 2001 T he principles of orthogonal frequency division multiplexing (OFDM) modulation have been in existence for several decades. However, in recent years these techniques have quickly moved out of textbooks and research laboratories and into prac- tice in modern communications systems. The tech- niques are employed in data delivery systems over the phone line, digital radio and television, and wireless networking systems. What is OFDM? And why has it recently become so popular? This article will review the fundamentals behind OFDM tech- niques, and also discuss common impairments and how, in some cases, OFDM mitigates their effect. Where applicable, the impairment effects and tech- niques will be compared to those in a single carrier system. A brief overview of some modern applica- tions will conclude the article. The single-carrier modulation system A typical single-carrier modulation spectrum is shown in Figure 1. A single carrier system modu- lates information onto one carrier using frequency, phase, or amplitude adjustment of the carrier. For digital signals, the information is in the form of bits, or collections of bits called symbols, that are modu- lated onto the carrier. As higher bandwidths (data rates) are used, the duration of one bit or symbol of information becomes smaller. The system becomes more susceptible to loss of information from impulse noise, signal reflections and other impairments. These impairments can impede the ability to recover the information sent. In addition, as the bandwidth used by a single carrier system increases, the sus- ceptibility to interference from other continuous sig- nal sources becomes greater. This type of interfer- ence is commonly labeled as carrier wave (CW) or frequency interference. Frequency division multiplexing modulation system Frequency division multiplexing (FDM) extends the concept of single carrier modulation by using multiple subcarriers within the same single channel. The total data rate to be sent in the channel is divided between the various subcarriers. The data do not have to be divided evenly nor do they have to originate from the same information source. Advantages include using separate modula- tion/demodulation customized to a particular type of data, or sending out banks of dissimilar data that can be best sent using multiple, and possibly dif- ferent, modulation schemes. Current national television systems committee (NTSC) television and FM stereo multiplex are good examples of FDM. FDM offers an advantage over single-carrier modulation in terms of narrowband frequency interference since this interference will only affect one of the frequency subbands. The other subcarriers will not be affected by the interference. Since each subcarrier has a lower information rate, the data symbol periods in a digital system will be longer, adding some additional immunity to impulse noise and reflections.
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This note was uploaded on 01/25/2011 for the course SCE 5403 taught by Professor Hafez during the Spring '10 term at Carleton CA.

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OFDM - signal processing The principles of OFDM...

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