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Technology Stocks : LAST MILE TECHNOLOGIES - Let's Discuss Them Here

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To: Doughboy who wrote (3484)	4/28/1999 12:17:00 PM
From: Bernard Levy	of 12823

Hi Doughboy: DMT - discrete multi-tone. DMT modulation is a multicarrier modulation whereby the transmission spectrum is divided into frequency bins, and a different carrier is used for each bin. The advantage of this approach is that the modulation order (say QAM-16, QAM-64 ...) is adjusted to take into account the signal to noise ratio (SNR) in each bin (some bins can be more affected than others by interferers). The equalization is also extremely easy to accomplish. By opposition QAM/CAP modulation uses a single carrier, but a very long and complex equalizer. DMT is also known in the wireless context as OFDM= orthogonal frequency division multiplexing. The rationale for using different carriers in this context is that different frequencies undergo different levels of fading, leading to different SNRs for different frequency bins. DMT/OFDM modulation has a few drawbacks: higher circuit complexity and power consumption, and higher latency due to the Fourier transform which implements the signal division into different bins. Best regards, Bernard Levy

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