QAM

quadrature amplitude modulation. Two signals (in phase and quadrature) independently modulated.

Why QAM?
Just as the explorers in the 15th and 16th century were searching for more land (and gold), so are the communication explorers in the last half of the 20th century resolutely searching for more bandwidth (and gold), or ways to send more information in a given bandwidth. Quadrature amplitude modulation provides one of many techniques for incorporating more information per unit of bandwidth.

QAM is one of the more interesting forms of modulation. It is constructed using the principles of several forms of both analog and digital modulation techniques including amplitude modulation, phase-shift (digital keying) modulation, and the principles of clocking pulses. It also includes the ideas of vectors with their 0 in-phase (I) components, and their 90 or quadrature (Q) components. For more background, we will create a QAM signal, step by step. First, Figure 41 shows signals that incorporate more than one level and phase.

Figure 41 Changing the amplitude, phase, and position of signal waveforms. Sine wave a is a zero phase, baseband signal centered around the zero units axis. Sine wave b is this same signal "riding" on an offset, a bias, of a +1 unit. Sine wave c is similar to b except its amplitude is twice that of b. Waves a', b' and c' are similar to their aforementioned counterparts, except these waves each show a phase reversal of 180. d shows two waveshapes positioned at different (predetermined) levels above the zero-level axis. Thus, this figure shows modulations (changes) in amplitude, phase and position. These examples use sine waves for clarity. You could use other signals and their parameters would be changed in a similar way. Likewise, other signal phases will work.

In addition to the functions of changing the amplitude, phase, and position of signals, you can use the balanced modulator function to also include complex waves. QAM uses these combined functions to transmit the codes of these signals, rather than the actual signals. As an introduction, two examples of the coding are shown in Figure 42.

Figure 42 The fundamental idea used in coding complex numbers (amplitude and phase) for QAM. -A- shows the 1-bit codes ([0] and [1]) that are used for a sinusoidal wave with angles of +90 and -90. The +90 wave is inverted (a cosine wave) and the -90 wave is a zero-degree sine wave. -B- shows the 2-bit codes for the waves at +45, +135, +225 (-135) and +315 (-45). For simplicity, the waveshapes are not shown in -B-.

These waveshapes and positions are representative, but should not be considered as exactly those in QAM. For example, the sine waves on the pedestals in Figure 41 (d) are likewise only representative -- this is not exactly how it is done in QAM. In QAM the “dots” representing codes do not reside on a phase-plane circle as depicted in Figure 42. Rather they form an x-y square (really an I-Q) matrix. For example, a very simple 4-sample system -- denoted 4-QAM -- would be represented by a 2-by-2 dot matrix. A 16-QAM system would be shown by a 4-by-4 dot matrix, and a 256-QAM system would be represented by an 16-by-16 dot matrix. The meaning of each dot will be made clear in Figure 43.

Figure 43 The yellow square "dots" show one method of representing the I-Q phase plane positions (sometimes called the constellation) of the digital codes for 16-QAM. Thus, with 16-QAM there are 16 possible states, that is, 16 possible combinations of signal amplitude and phase. However, this is not the complete story.

The preceding paragraphs presented several ideas that must be combined to produce Quadrature Amplitude Modulation. These ideas can be used as follows.

1. QAM is a modulation process that incorporates the techniques of digital (coded) Phase Shift Keying (PSK) and digital (quantized) Amplitude Modulation. In addition, QAM can process and transmit the codes of two input signals phased 90 apart: the I and Q signals. This is the "quadrature" part of QAM.
2. The two quadrature signals are processed, then coded, and the resulting representative codes can be visualized as being placed on pedestals with different plus and minus amplitudes. This is the "amplitude modulation" part of QAM, as shown in Figure 44.

Figure 44 The QAM process can be visualized as first digitizing (coding) two quadrature (I and Q) analog signals. These coded signals are then sorted by their codes, and placed (amplitude modulated) at discrete levels above and below a zero amplitude reference (x) axis. The I and Q coded signals are then brought together, serialized, and amplitude modulated.

Information Contributed By: Bob Libbey, Retired RCA Engineer and Adjunct Professor, New Jersey Institute of Technology

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