Friday, August 21, 2009

Radio Technologies -- OFDM

OFDM stands for Orthogonal Frequency Division Multiplexing It is a digital modulation method that works well in challenging environments such as noisy or highly attenuated communication mediums. It translates multiple signals with each signal occupying its own frequency. A tutorial can be found here.

The technology has been around for over 40 years and patented in 1970 but found popularity in the early 2000s when bandwidth requirements for wireless demanded higher throughputs at a lower cost. In this article the author outlines a short history of OFDM and talks about why it was adopted. OFDM uses narrowband carriers with each signal transmitted in parallel but at a different frequency. By placing these signals called sub-carrieres closely but not overlapping. The term orthogonal refers to the mathematical relationship of the signals, and not the spatial. OFDM uses a guard interval to protect against multipath signal interference.

There are challenges with OFDM including a high average peak-to-average ratio. In this article the author indicates OFDM requires an outsized power amplifier and that power efficiency will be compromised and that heat dissipation will be an issue for the RF designer. OFDM also requires high linearity to avoid inter-modulation distortion and third order intermodulation products can pile up on the carrier. Non-linearity also shows up in adjacent channel interference. Also phase noise is an issue that must be controlled given the closely spaced carrier signals. These challenges impact the data rates one can achieve.

Best regards,

Hall T.

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