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*Subject*: Re: [amsat-bb] Geosynchronous Satellites*From*: Phil Karn <karn@xxxxxxxx>*Date*: Tue, 2 Jan 2001 19:35:04 -0800*In-reply-to*: <B66EC219.BA1F%na9d@mindspring.com> (message from Jon Ogden onTue, 26 Dec 2000 21:46:02 -0600)

>but it could possibly count as a constant envelope form of modulation), I >think BPSK can do be done in AM, PSK can (as we see on HF). I'll defer to >Phil for more definitive answers. I think there are multiple versions of Suppressed-carrier BPSK can be seen either as phase shift keying, with signal points on the unit circle at 0 and 180 degrees, or as suppressed-carrier double sideband AM. Both are equivalent. The design of a classic digital modulation scheme involves laying out a "constellation" of signal points on a two-dimensional plot that represents the instantaneous state of the RF carrier. The plot axes are generally labeled I (in phase) and Q (quadrature), because the usual way to generate RF from the baseband I & Q data is to run each channel through a balanced modulator and combine the results. The in-phase carrier is fed to the I-channel modulator while the carrier fed to the Q-channel modulator is shifted exactly 90 degrees ("quadrature"). The distance from the origin to the signal point is the instantaneous carrier amplitude for that point, and the angle at the origin between the +I axis and the signal point is the instantaneous carrier phase. It is not necessary for the each signal element to be a single point on this graph. E.g., in FSK the signal vector is in constant motion, tracing out either a clockwise or counterclockwise circle around the origin at a speed that depending on the current frequency "shift". In MSK (minimum shift keying) the vector always rotates exactly 90 degrees (clockwise or counterclockwise) during each bit interval. If the signal constellation points (or trajectories, as in MSK/FSK) all fall on the unit circle, then the signal envelope is constant and it can be amplified by a nonlinear amplifier without distortion. This is the case for unfiltered BPSK and QPSK as well as MSK and FSK. It's possible to lay out a constellation where the carrier amplitude varies as well as the carrier phase, i.e., the points are not all on the unit circle. QAM is the generic term here. Generally the points in QAM are laid out evenly in a square grid. The main reason to do this is to pack more data bits into each signal point, thus improving bandwidth efficiency. (Bandwidth depends on how fast you change the signal point, not on how many bits are represented by each signal point). But this comes at a cost. Because the points are packed more closely, noise is more likely to move one point onto another, causing an error. And any amplitude nonlinearities in the channel will compress the outer points together, or even squash them all onto the unit circle (if the amplifier is totally nonlinear). This obviously makes it not work very well. QAM is great when SNR is high, the channel is reasonably linear and bandwidth is limited. But it's a poor choice on a satellite channel when the SNR is low, the channel is quite possibly nonlinear, and bandwidth is more plentiful. That's why you see QAM used on telephone lines and in cable modems, but not on satellites, where BPSK and QPSK are usually the way to go. Phil ---- Via the amsat-bb mailing list at AMSAT.ORG courtesy of AMSAT-NA. To unsubscribe, send "unsubscribe amsat-bb" to Majordomo@amsat.org

**Follow-Ups**:**Re: Geosynchronous Satellites***From:*Louis A. Mamakos

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