29-10-2012, 11:01 AM
AN ENHANCED, CONSTANT ENVELOPE, INTEROPERABLE SHAPED OFFSET QPSK (SOQPSK) WAVEFORM FOR IMPROVED SPECTRAL EFFICIENCY
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ABSTRACT
Shaped BPSK (SBPSK) and Shaped Offset QPSK (SOQPSK), as defined in various MIL standards, are widely employed on SATCOM links because they offer an attractive combination of good spectral efficiency, constant envelope characteristics, and interoperability with legacy equipments. More recently, numerous terrestrial applications of OQPSK and similar waveforms (Feher-patented FQPSK) have been proposed. The present paper describes a simple non-proprietary modification of the MIL-STD SOQPSK waveform which offers spectral containment and detection efficiency comparable to or better than FQPSK-B (Revision A1), 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 first implementation of SBPSK was in the AN/PSC-1, operating in the 225 – 400 MHz SATCOM band. The waveform’s high degree of success in that role led eventually to its adoption as a standard for the UHF SATCOM terminals as described in MIL-STD-188-181, and 188-182. 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.
ARTM MEASURED RESULTS
The results presented above have been produced by computer simulation. To corroborate the simulation predictions with results from measured hardware, we used the facilities of the Advanced Range Telemetry (ARTM) Project at Edwards AFB. This facility was chosen because they have extensive experience with Feher-patented FQPSK-B (Rev. A1), and they have a custom-built FQPSK-B (Rev. A1)
demodulator, developed by RF Networks. This is significant because FQPSK-B (Rev. A1) offers performance comparable to SOQPSK-A and –B. Indeed, based on the results presented here, 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) versions 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. A1). 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.