29-10-2012, 10:50 AM
A NON-PROPRIETARY, CONSTANT ENVELOPE, VARIANT OF SHAPED OFFSET QPSK (SOQPSK) FOR IMPROVED SPECTRAL CONTAINMENT AND DETECTION EFFICIENCY
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ABSTRACT
Shaped BPSK (SBPSK) and Shaped Offset QPSK
(SOQPSK), as dejined in various MIL standards, are
widely employed on SATCOM links because of their
combination of good spectral efjiciency, constant envelope
characteristics, and interoperability with legacy
equipments. Because of the limited link margin available
on most SATCOM links, spectral occupancy at levels of
-30 dBc and lower has not received much attention. More
recently, however, numerous terrestrial applications of
OQPSK and similar waveforms (GMSK, Feher-patented
FQPSK) have been proposed. In these applications, the
power spectral density is relevant to levels of -70 dBc or
lower, and these new waveforms are superior to the MILSTD
SOQPSK waveform in this regard. The present paper
describes a simple non-proprietary modification of the
MIL-STD SOQPSK waveform which offers spectral
containment and detection eficiency comparable to or
better than FQPSK-B (Revision AI) and GMSK, while
preserving a constant envelope characteristic and
backward compatibility with existing equipment.
INTRODUCTION
Shaped BPSK (SBPSK) was introduced in the early 1980’s
(MILCOM ’84, SBPSK: A Robust Bandwidth-Efficient
Modulation for Hard-Limited Channels) as a means of
bandlimiting a BPSK signal, while keeping the signal
envelope constant. The advantage to maintaining a
constant envelope is that the spectral sidelobes do not
regenerate when the signal in passed through a limiting or
otherwise nonlinear amplifier. The initial application of
SBPSK, in the early 1980’s, was in the ANIPSC-3,
operating in the 225 - 400 MHz SATCOM band. The
waveform’s success in that role led eventually to its
adoption as a standard for the UHF SATCOM terminals as
described in MIL-STD-188-18 1, and 188-1 82. Further
development of the SBPSK concept led to an Offset QPSK
variant called, naturally enough, SOQPSK, which is also
defined in the same MIL-STDs.
LABORATORY MEASURED RESULTS
The results presented above have been produced by
computer simulation. To corroborate the simulation
predictions with results from measured hardware, we
present data from the Advanced Range Telemetry
(ARTM) Project at Edwards AFB. The ARTM Project has
extensive experience with Feher-patented FQPSK-B (Rev.
Al), which offers performance comparable to SOQPSK-A
and -B; these three waveforms could be considered as
alternatives to each other for many applications
CONCLUSIONS
The results presented here answer several questions
concerning constant envelope (or near constant envelope)
variants of offset QPSK. First, we have provided a
parametric description of a family of bandlimited, constant
envelope OQPSK waveforms. Two members of this
family have been analyzed in detail, and shown to provide
BER and spectral characteristics which compare favorably
with Feher-patented FQPSK-B (Rev. Al). We have
shown by simulation that the BER performance of the -A
and -B versions are within a small fraction of a dB of the
MIL-STD version, using an ordinary offset QPSK
demodulator.
In addition, we have shown via measured results from the
ARTM lab that the members of the SOQPSK family
interoperate with FQPSK-B (Rev. Al) in the sense that
SOQPSK-A and SOQPSK-B have been correctly
demodulated with a custom FQPSK-B (Rev. Al)
demodulator. When operated through a non linear
amplifier, the BER performance of the SOQPSK-B is
about 0.7 dB better than FQPSK-B (Rev. Al), which in
turn, is about 0.5 dB better than SOQPSK-A.