Hybrid Modulation 11

Introduction

Quadrature amplitude modulation (QAM) is both an analog and a digital modulation scheme. It conveys two analog message signals, or two digital bit streams, by changing (modulating) the amplitudes of two carrier waves, using the amplitude-shift keying (ASK) digital modulation scheme or amplitude modulation (AM) analog modulation scheme. These two waves, usually sinusoids, are phase with each other by 90° and are thus called quadrature carriers or quadrature components hence the name of the scheme. The modulated waves are summed, and the resulting waveform is a combination of both phase-shift keying (PSK) and amplitude-shift keying (ASK), or in the analog case of phase modulation (PM) and amplitude modulation. In the digital QAM case, a finite number of at least two phases and at least two amplitudes are used. PSK modulators are often designed using the QAM principle, but are not considered as QAM since the amplitude of the modulated carrier signal is constant. QAM is used extensively as a modulation scheme for digital telecommunication systems.

 

We'll use a signal that is transmitting at 3600 bps, or 3 bits per baud. This means that we can represent 8 binary combinations.

We'll use 2 measures of amplitude, 1 and 2, just we did before. We'll also use 4 possible phase shifts, like we did before. Combining the two, we have 8 possible waves that we can send. How convenient.

First step is to generate a table to show us which waves correspond to which binary combination. This can basically be done at random, although modem manufacturers have agreed on standards.

Bit value	Amplitude	Phase shift
000	1	None
001	2	None
010	1	1/4
011	2	1/4
100	1	1/2
101	2	1/2
110	1	3/4
111	2	3/4

 

Let's encode a big bit stream:

001010100011101000011110

First, we break it up into 3-bit triads:

001-010-100-011-101-000-011-110

Now all we have to do is figure out what the resulting signal should look like. Remember that we shift each wave relative to the wave before it!

Divider  :-

                       A direct 16 quadrature amplitude modulation (QAM) modulator based on the six-port technique is proposed. It uses a six-port passive microwave network to implement the modulation scheme with suitable terminations. A microwave prototype was built to validate the 16 QAM modulation up to 200 Mbit/s data rate at 4.2 GHz carrier frequency