Digital Signal Processing Reference
In-Depth Information
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Illustration 274:
OFMD-signal with zero phases (ASK-OFDM)
The Illustration shows why there should not be a pure ASK-OFDM because it would mean that all carrier
oscillations would have the same phase. This would have serious consequences at the output of the adder.
The sum of all carrier oscillations might have high local peaks (“needle pulses”). This in turn would mean
that the transmission appliances - e.g.amplifiers - would not be able to cope.
If the whole AFDM-signal is to resemble white noise in the frequency domain, pseudo-random phase key
shifting (as an image of the relevant bit pattern) of carrier oscillations of the same amplitude is the
suitable option.
But which sinusoidal oscillations are suitable? As we know, only those sinusoidal
oscillations which are integer multiples of the basic frequency can be considered in
connection with (periodic) signals as a digital signal of a certain block length is defined
as periodic (see Chapter 9 “Digitalization). A FOURIER-synthesis, however, is obtained
by moving from the frequency into the time domain, i.e. via an Inverted FOURIER-
Transformation IFT, or in this case via an IDFT (Inverted Discrete FOURIER-
Transformation).
Thus the bit code at the output of the demultiplexer can be interpreted as a frequency
spectrum and the enormous number of multipliers and oscillators can be replaced by an
IDFT block. Thanks to the orthogonality of the carrier frequencies the procedure using
IDFT is simplified.
Illustration 272 demonstrates in a simplified way that the symbol duration with OFDM
equals the period length of the basic oscillation. This means that an integer multiple of the
period length of all carrier oscillations fits into each OFDM-symbol.
