Amplitude Modulation

Amplitude modulation (AM) is a linear baseband modulation technique in which the message modulates the amplitude of a constant frequency signal.

Communications Toolbox™ software includes these modulation and demodulation functions, System objects, and blocks to model pulse amplitude modulation (PAM) and quadrature amplitude modulation (QAM).

Functions	System objects	Blocks
genqammod, genqamdemod pammod, pamdemod qammod, qamdemod	comm.GeneralQAMModulator, comm.GeneralQAMDemodulator	General QAM Modulator Baseband, General QAM Demodulator Baseband M-PAM Modulator Baseband, M-PAM Demodulator Baseband Rectangular QAM Modulator Baseband, Rectangular QAM Demodulator Baseband

PAM

In digital PAM, quantized pulses modulate a carrier signal. A pulse train is the carrier signal for digital data. As described in Proakis ([2]), the signal waveforms may be expressed as

s_m(t) = A_m p(t), 1 ≤ m ≤ M

where:

• p(t) is a pulse of duration T.

• {A_m, 1 ≤ m ≤ M} denotes the set of M possible amplitudes corresponding to M = 2^b possible b-bit blocks of symbols.

QAM

In digital QAM, two carrier signals of the same frequency are out of phase with each other by 90°, a condition known as orthogonality or quadrature. The transmitted signal is created by adding the two carrier signals together. At the receiver, the two signals can be coherently separated (demodulated) because of their orthogonality property.

The QAM signal waveforms may be viewed as combined amplitude (r_m) and phase (ϕ_m) modulation. As described in Proakis ([2]), the signal waveforms may be expressed as

s_m(t) = Re[r_me^jϕ_me^j2πf_ct] = r_mcos(2πf_ct+ϕ_m)

• $$r_m=\sqrt{A_{mi}^2+A_{mq}^2}$$

• ϕ_m = tan^-1(A_mq/A_mi)