Article
DQPSK
Differential Quadrature Phase Shift Keying Modulation
Mr. Akshay Dattatray Shelke
B.E. Electronics and telecommunication
Email: (akky487@gmail.com)
This technical brief explains what differential
QPSK is and why it can be advantageous compared to ordinary QPSK.
Supporting Information:
If you
are not familiar with QPSK modulation, you should start with this article.
In
theory, QPSK is an excellent RF communication scheme. It is conceptually
straightforward, it transfers two bits per symbol instead of one, and it can be
conveniently implemented using I/Q modulation techniques.
As
usual, though, real life is not quite as neat and tidy as the theoretical
version. The particular problem we’re concerned with here is an additional and
unpredictable phase shift introduced by a lack of phase or frequency
synchronization between the transmitter hardware and the receiver hardware.
The
QPSK transmitter has a local oscillator that generates the sinusoid used as the
carrier wave. The receiver has a local oscillator that generates a sinusoid
used in demodulating the incoming signal. Ideally, these two oscillators have
exactly the same phase and frequency.
In
reality, of course, there will be discrepancies. The frequencies can be matched
quite well thanks to high-precision oscillation devices, but synchronizing the
phase is not so easy. A phase or frequency offset between the received signal
and the receiver’s local oscillator will introduce error into the phase of the
received signals, and this error could cause the receiver to assign an
incorrect two-bit code to a particular symbol.
It is
possible to design a receiver that can extract the phase and frequency of the
incoming carrier. This process is known as carrier recovery, and it can be used
to achieve coherent (i.e., phase-and-frequency-synchronized) demodulation. The
trouble is, coherent receivers are more complicated and more expensive. Many
systems would benefit from a modulation scheme that avoids the error associated
with phase or frequency offset yet does not require the additional cost and
complexity of carrier recovery.
This is
where differential Quadrature phase shift keying (DQPSK) comes into play.
In
QPSK, information is conveyed by the absolute phase of each symbol. DQPSK, in
contrast, conveys information by establishing a certain phase of one symbol relative to the previous symbol. The following
diagram illustrates this distinction.
The
relative phase is simply the phase of the current symbol minus the phase of the
previous symbol. If we use the standard four QPSK phase values—45°, 135°, 225°,
and 315°—the DQPSK phase options become 0°, 90°, –90°, and 180° (or,
equivalently, –180°).
By
using relative phase instead of absolute phase, DQPSK is not affected by a
fixed phase offset introduced by lack of phase synchronization between
transmitter and receiver; the fixed offset affects both symbols equally and is
eliminated in the subtraction process. DQPSK is also robust against
transmitter–receiver frequency discrepancies.
Even
though a frequency offset introduces a time-varying phase error, as long as
this error changes slowly relative to the symbol rate, the differential phase
from one symbol to the next will remain accurate enough for reliable data
transfer.
Compared
to carrier recovery, this differential phase detection process does not
constitute a major increase in the complexity of the receiver; this is
especially true if the conversion from analog baseband to digital data is
performed in software.
One
disadvantage to keep in mind, though, is the effect of noise: theoretically, a
coherent QPSK system would have a lower bit error rate because the received
symbol is compared to an ideal reference signal, whereas in DQPSK a noisy
symbol is compared to another noisy symbol.
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