CDMA — How Everyone Can Speak at Once Without Mixing

Putting a voice onto a wave

Start with what a telephone does. A voice is a trembling of air. Turning that trembling into a trembling of electricity and sending it down a wire was the wired telephone.

To send it without wire you load it onto a radio wave — invisible, but the same family as light, only with far longer ripples.

Three ways to share one sky

The trouble is that there is only one sky. If many are to use it at once, it has to be shared somehow. There are three ways.

1. Share by frequency — give each person a different channel, as radio stations have.
2. Share by time — slice one channel into tiny turns, too fast for anyone to notice the gaps.
3. Share by manner of speaking — do not divide at all. Everyone speaks at once, each in a different way.

The first two divide the room, either into compartments or into a timetable. The third is CDMA.

What the manner of speaking is

The manner here is a code. The signal is multiplied by an agreed pattern before it goes out, which spreads it thin and wide — thin enough that to anyone else it sounds like noise.

At the far end the same pattern is applied again. Your signal gathers back into focus; everyone else's stays scattered. Only the matching pattern comes back to life; the rest remains murmur.

Two things come free with this. Without the code a listener hears only noise, so eavesdropping is hard. And one more caller needs no new channel carved out, so far more people fit under the same sky.

A little further in

If it is so good, why had no one done it?

The theory had been known for a long time. Building it was the problem — and the nastiest part was loudness.

Since everyone speaks at once, a caller standing right beneath the tower arrives loud and buries the caller far away. It is the party guest shouting into your ear while you try to follow a conversation across the room.

A small American company brought the method forward and the American industry looked away. They had already sunk large money into sharing by time, and nothing like this had ever been run commercially.

Inventing it and being first to do it

Korea was placed differently. It had asked the leading countries for the time-sharing technology and been refused each time, on the grounds that it had no technology of its own.

So the Electronics and Telecommunications Research Institute in Daedeok took the road nobody was on and went into commercialisation with that American company. Unable to wait for drawings, they took the outline and worked out the detail themselves — with experience from building telephone exchanges as their capital.

A prototype carried its first call in the spring of 1994, and in 1996 a service ran on a method that had never run anywhere. It was the first in the world.

What remained was not merely a way of making calls. An industry in handsets and equipment appeared, and the same pattern repeated later in mobile internet and in fifth-generation networks.

What you would study to do this

Telecommunications does not run on one discipline.

1. Someone who puts patterns onto signals and takes them off — signal processing, in electronic and communication engineering.
2. Someone who measures how waves bend and fade around buildings and terrain — radio engineering.
3. Someone who fits all that arithmetic onto a chip the size of a fingernail — semiconductors and circuit design.
4. Someone who writes the agreements machines keep with each other — computer engineering.
5. Someone who turns those agreements into a world standard — a fight conducted in meetings and documents.

That last line may surprise you. But in communications, getting everyone to agree to use a technology matters as much as building it. However good it is, a method only you use will not connect a call.

This happened at ETRI in Yuseong-gu, Daejeon — in the middle of Daedeok Science Town when you open Daejeon on the map.

The question that remainsIf there was a way to stop things mixing without dividing the space at all — what else do we divide up out of habit, believing we have to?