What is difference between OFDM - orthogonal frequency division multiplexing and GMSK - gaussian minimum shift keying?

Answer 1

Orthogonal FDM is a method of passing digital data that uses multiple carriers. The basic idea is, you take a high-rate data stream, convert it into a number of low-rate data streams, put each of these streams on a slightly different frequency and dissemble all of it back into the high-rate stream at the distant end. A low-rate stream doesn't require as much bandwidth as a high-rate one does, so you can afford to use multiple frequencies without knocking everyone else off the air. Error correction schema are used to prevent losing data, and monitoring the atmosphere allows the radio to move channels around as necessary to ensure reliability. The reason you want to go through all this trouble is to be able to communicate when atmospherics turn bad. A similar technique is used in tropospheric-scatter radios, which bounce radio waves off part of the upper atmosphere; because the troposphere changes all the time, the only way to reliably communicate via it is to transmit the same data over several different frequencies at once. But in troposcatter, it's called diversity operation.

Minimum-shift keying is a form of frequency-shift keying, in which one frequency stands for a 1 - or 'mark' in teleprinter terms - and a different frequency for a 0, or space. This is a technique that's over a century old, and a century ago the way you did it was to wire a small capacitor in series with the contacts on an electromechanical relay, wire that assembly in parallel with the frequency-determining capacitor in your transmitter, and wire the relay's coil to your teleprinter. When the system needed to send a mark, the teleprinter closed the relay which changed the total capacitance of the LC tank which caused the radio's transmitting frequency to change. And they did it this way for at least seventy years! The problem with it (besides needing all those mechanical parts to get the thing to work, and the slow transmitting speeds it required; you certainly wouldn't be able to watch YouTube videos on your phone by keying an oscillator with a relay) is the technique created huge, bandwidth-eating sidebands that didn't matter when the only people sending FSK data were police departments and newspapers, but these days the only people NOT sending FSK data are babies too young to use phones, your grandma who refuses to buy a cell phone, and people in prison who aren't allowed to own cell phones. Today you couldn't get away with eating this much bandwidth, so "minimum-shift keying" - an FSK technique that creates very small sidebands - is the way to go, and feeding the signal through a gaussian filter to create gaussian MSK gives you a very reliable way to transmit data.

A good answer to put on your paper might be "OFDM uses multiple carriers and GMSK uses a single carrier."