Digital Signal Processing Reference
In-Depth Information
carrier frequency. The bandwidth requirement is 2/T
B
. The most significant differ-
ence between ASK and BPSK is that, ASK is linear modulation whereas PSK is
non-linear modulation technique. Therefore, PSK has a superior performance [
3
]
over ASK in a noisy environment for a given peak power (Fig.
6.19
).
PSD
(−f
c
−1/T
B
) −f
c
(−f
c
+1/T
B
)
(f
c
−1/T
B
) f
c
(f
c
+1/T
B
)
Fig. 6.19
PSD of binary PSK waveform
6.4.3 BPSK Demodulator
The
coherent detection
of BPSK is not very different from ASK detection (as shown
in Fig.
6.8
). The detector consists of a product modulator synchronized with a
locally sinusoid carrier, an integrator that's operates on the multiplier output for
successive bit interval and a decision device that compares the integrated (LPF)
output with a predefined threshold. Though this coherent detector suffers from the
problem of synchronization both in pulse and timing, PSK cannot e detected using
non-coherent detector. This is because of the fact that, the envelopes extracted from
the waveform representing symbol 1 and symbol 0 are not indifferent. To avoid
the necessity of synchronization, a modified PSK can be employed. In
Differential
Phase Shift Keying
there is no need of synchronous carrier to demodulate the PSK.
6.5 Differential Phase Shift Keying (DPSK)
Differential Phase Shift Keying necessarily deals with the signal transition from 0
to 1 and from 1 to 0. In this particular scheme of modulation, phase synchronization
is not at all required, as BPSK. For example, the symbol 0 may be used to represent
transition in the baseband binary sequence (with respect to the previous encoded
bit) and symbol 1 to represent no transition.
6.5.1 DPSK Modulator
The block diagram of DPSKmodulator is shown in Fig.
6.20
. The binary data stream
d(t) is applied to the input of the DPSK encoder. The output is then applied to a
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