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OFDM shoulders heavy RF traffic

Posted: 16 May 2001 ?? ?Print Version ?Bookmark and Share

Keywords:ofdm? orthogonal fdm? rf? idtf? vofdm?

Orthogonal frequency division multiplexing (OFDM) has grown to be the most popular communications system in high-speed communications in the last decade. In fact, many industry leaders have said that OFDM technology is the future of wireless communications as we know it.

While the term 'modulation' is commonly used to describe OFDM, it may be more appropriate to call it a communications system like direct sequence or frequency hopping. It can also be referred to as a transport technology or scheme. Comparisons are often made between OFDM and narrowband systems. However, since OFDM can be either narrowband or wideband, OFDM should only be compared to single-carrier systems.

Basically, OFDM is the concept of multicarrier communications, where the different carriers are orthogonal to each other. Orthogonal in this respect means that the signals are totally independent; it is achieved by ensuring that the carriers are placed exactly at the nulls in the modulation spectra of each other. In OFDM the data on each carrier overlaps the data in adjacent carriers. That overlap is an extra source of spectral efficiency in OFDM.

Another source of OFDM spectral efficiency is the fact that the drop-off of the signal at the band is primarily due to a single carrier that is carrying a low data rate, allowing for sharp edges that correspond closer to the desired rectangular shape of the spectral power density of the signal.

OFDM provides several advantages. It leads to less intersymbol interference than if the overall throughput was attempted on a single-carrier system.

OFDM is generally implemented through the application of a Fourier transform to frames of modulated, coded data just before it would have been ready for upconversion in a conventional wireless communications system.

The presence of an inverse discrete Fourier transform (IDTF) suggests that if the data entering the IDTF is correlated in certain ways, the output of the IDTF could be a pulse of magnitude n, the size of the IDTF, or a sinusoid of peak 1. That has been demonstrated to increase the peak-to-average ratio of OFDM transmission. The peak- to-average power problem has been mitigated in various ways. For example, the use of highly linear amplifiers, although expensive, can be used.

The IEEE802.11a standard relies on retransmission with scrambling of the data to ensure that a data packet will eventually make it across even if the peak is clipped during the first transmission.

It is not typical to use uncoded OFDM. In fact, almost all current OFDM implementations use some form of forward error correction (FEC). Early use of FEC with OFDM became known as coded-OFDM or COFDM. It is, however, inappropriate to distinguish the standard through the use of prefix "C" since all OFDM in use today is coded.

Early OFDM systems featured a narrow frequency band as part of their design criterion. That was to ensure that all carriers suffered the same attenuation due to the channel and was over come by introducing a channel estimator in the receiver. That estimation could be achieved by using training OFDM frames with no data on them. In the case of these training OFDM frames, the signal Y(k) would be an estimate of the channel's frequency response. Data frames transmitted in the same packet of OFDM frames can then be divided by or multiplied by the reciprocal of this frequency response to remove the effects of the channel.

Often the estimate of the channel response contains nulls, resulting from multiple rays' destructively interfering with each other. Those nulls could be so deep that dividing by the channel response leads either to an increase in the noise level or to a division by zero. That is often circumvented through truncating the response to a certain value if it drops lower than a particular value. A more appropriate solution is either to use an FEC that allows erasures of suspect symbols like Reed Solomon coding or to feed the entire channel response into the FEC in the receiver and use it for some form of weighting.

It is believed that the mobile multimedia network, Magic, which is part of the NTT Docomo vision 2010, will provide 10Mbps to the user through OFDM technology. The IEEE 802.16 has invited interested parties to submit proposals for a high-speed fixed wireless-access standard to operate below 11GHz. That submission includes possible applications in the 2.5GHz multipoint, multichannel distribution systems band. As of the end of January, there are six proposals under discussion: Three are OFDM, two are single-carrier and one is a turbo-coding proposal. A PHY was not specified.

? Hatim Zaghloul

Chairman and CEO

Wi-LAN Inc.

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