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modnorm

Scaling factor for normalizing modulation output

Syntax

scale = modnorm(const, 'avpow', avpow)
scale = modnorm(const, 'peakpow', peakpow)

Description

scale = modnorm(const, 'avpow', avpow) returns a scale factor for normalizing a PAM or QAM modulator output such that its average power is avpow (watts). const is a vector specifying the reference constellation used to generate the scale factor. The function assumes that the signal to be normalized has a minimum distance of 2.

scale = modnorm(const, 'peakpow', peakpow) returns a scale factor for normalizing a PAM or QAM modulator output such that its peak power is peakpow (watts).

Examples

The code below illustrates how to use modnorm to transmit a quadrature amplitude modulated signal having a peak power of one watt.

M = 16; % Alphabet size
% QAM Modulation
hMod = comm.RectangularQAMModulator(M);
hDemod = comm.RectangularQAMDemodulator(M);
% AWGNChannel System object
hChan = comm.AWGNChannel('NoiseMethod', 'Signal to noise ratio (SNR)', ...
    'SNR', 10);
const = step(hMod,(0:M-1)'); % Generate the constellation.
x = randi([0 M-1], 100,1);
scale = modnorm(const,'peakpow',1); % Compute scale factor.
y = scale * step(hMod,x); % Modulate and scale.
hChan.SignalPower = (y' * y)/ length(y); % Calculate Signal Power
ynoisy = step(hChan,y); % Transmit along noisy channel.

ynoisy_unscaled = ynoisy/scale; % Unscale at receiver end.
z = step(hDemod,ynoisy_unscaled); % Demodulate.

% See how scaling affects constellation.
h = scatterplot(const,1,0,'ro'); % Unscaled constellation
hold on; % Next plot will be in same figure window.
scatterplot(const*scale,1,0,'bx',h); % Scaled constellation
hold off;

In the plot below, the plotting symbol o marks points on the original QAM signal constellation, and the plotting symbol x marks points on the signal constellation as scaled by the output of the modnorm function. The channel in this example carries points from the scaled constellation.

Additional examples using modnorm are in Examples of Signal Constellation Plots.

See Also

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