User's Manual Part 2

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Tech-X Flex

(NG2) Tech-X Flex User Guide - Firmware v06.50

8-19

Intro

Overview

Wi-Fi

Ethernet

System

IP/Video

MoCA

Specs

With this method of modulation, the strength and consistency of the carrier frequency are critical for

reliable data transmission. For any given scheme, a weaker signal forces the data points closer together,

which decreases the tolerance for error if the waveform peaks do not fall exactly at the expected level.

Conversely, an increase in amplitude “space” with stronger signals allows the use of schemes with a

greater number of data points, increasing the amount of binary data that can be modulated on each wave

cycle.

QAM uses this technique of modulation with one variation: the addition of a second waveform in order to

double the amount of data that can be sent with each cycle. The two waves, known as the “I” and “Q”

signals, travel at a fixed 90 degrees out-of-phase. With each cycle, both waves represent amplitude-

modulated binary numbers which collectively represent a larger set of bits. Continuing with the previous

example of four data points per cycle, the following QAM cycle might represent a binary “1011”:

Figure 8-12 16-QAM cycle showing a 1011-modulated symbol

This particular scheme is known as “16-QAM,” as each modulated cycle (also known as a symbol)

transports a total of four bits, allowing one of 16 different binary numbers to be modulated (0000 - 1111).

Other QAM schemes such as 64-QAM and 256-QAM operate in a similar fashion, except that they

contain more amplitude data points. For example, 64-QAM defines eight data points which allows each

wave to individually carry a binary number between 000 - 111 (three bits) per cycle. Collectively, then, the

I and Q signals transport six bits per symbol, a binary 000000 - 111111 (0 - 63 in decimal notation).

A constellation diagram is a common means of graphically representing a QAM scheme. These

diagrams use a grid to indicate each potential binary number possible per symbol, using the x- and y-

axes to represent the amplitude data points of the I and Q signals, respectively. For example, the

following constellation diagram represents 16-QAM:

I-signal

(10)

Q-signal

(11)

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