Page 96 - DCAP207_NETWORKS_DCAP406_COMPUTER_NETWORKS

Unit 6: Multiplexing

Differential Phase Shift Keying (DPSK) Notes

DPSK changes the phase of the carrier wave instead of frequency. This is used for digital
transmission in which the phase of the carrier is discretely varied in relation to the phase of the
immediately preceding signal element and in accordance with the data being transmitted. The
shift in phase takes place from the present phase rather than from an absolute standard therefore
this technique is called DPSK. A disadvantage of DPSK is higher BER vs. SNR than BPSK (by
about 1 dB).
Figure 6.12 shows DPSK using two phase changes i.e. 0 is a 00 phase shift and 1 is a 1800 phase
change.
Figure 6.12: DPSK with Two Phase Angles

QAM (Quadrature Amplitude Modulation)

This technique is based on the amplitude modulation and phase modulation to improve the
performance of the amplitude modulation. Theoretically, any number of changes in amplitude
can be associated with any number of changes in phase. For example, two carrier signals are
transmitted simultaneously at the same frequency with a 90 degrees phase shift. The QAM
intends to combine the benefits of the amplitude and phase shift keying modulation. It involves
fewer number of amplitude shifts than phase shift because the amplitude modulation is susceptible
to the noise. The minimum bandwidth required for QAM is equivalent to the minimum bandwidth
required for ASK and PSK.

Task How does AM differ from ASK?

6.5 Modulation of Analog Signal

The modulation is the act of translating some low-frequency (base band signal) such as voice,
data, etc. to a higher frequency. The modulation/demodulation is a nonlinear process in which
two different sinusoids are multiplied. In the modulation process, some characteristic of a high-
frequency sinusoidal carrier fc as shown in Figure is changed in direct proportion to the
instantaneous amplitude of the base band signal as fm in Figure 6.13. Let us assume the two
sinusoids as shown in Figure 6.13 as fm and fc as base band signal and carrier respectively and
are represented as:
f = A Sinω t + φ ...(1)
m m 1
f = B Sinω t + φ ...(2)
c c 2
In equation 2, either amplitude B or angular frequency wc may be varied in accordance with
equation 1 and thus producing either amplitude modulation or frequency modulation or phase

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