A REMINDER: Pay Attention to Harry Nyquist and Claude Shannon

摘要

It's a very easy and normal thing to focus on the principal characteristics of the principal components of a communications system or link, and regard other items and issues as peripheral and not so important. That it has happened a surprising amount with satellite communications links, especially remote mobile ones, is the reason for this missive. Here's a hypothetical situation. You are a soldier. You're deployed in a mountainous area of the Middle East, carrying an older Intelsat satellite terminal for immediate access back to headquarters, and the headquarters has leased a 72-MHz transponder for your exclusive use. A nice, safe place to be is in a cave, so you string a communications cable to the satellite terminal at the cave's entrance and sit back to check in with your workstation that's connected to the terminal with a quadrature phase-shift keying (QPSK) modem. Checking in is fine, but the comm is lousy. The satellite company guarantees you 72 Mb/s (1 bit per Hz), but you're barely getting less than 35 Mb/s. Should you have gotten a newer terminal? Should you be using another system?