A proposed method of FQPSK modulation and demodulation of a radio signal would incorporate any of a number of relatively simple (short-constraintlength) outer codes. By affording significant coding gains even when using a reduced-complexity (and thus suboptimal) FQPSK receiver, this method would offer the concomitant potential to enhance efficiency in power and spectral width of an FQPSK communication system.
The term “FQPSK” denotes Feherpatented quadrature-phase-shift keying, which
is a bandwidth-efficient phasemodulation scheme named after its inventor. Among the notable features of FQPSK is shaping of what would otherwise be square in-phase (I) and quadrature (Q) pulse waveforms, such that the signal envelope (in effect, the power of the transmitted signal) remains nearly constant. The shaping involves, among other things, a cross-correlation between the I and Q channels. The nature of the cross-correlation is such as to effectively incorporate a trellis coding scheme into FQPSK.
According to the proposed method, a short-constraint-length code (in effect, an outer code) would be introduced into a data stream via an interleaver prior to modulation of the carrier signal in an FQPSK transmitter. The combination of this outer code with the trellis or convolutional code inherent in FQPSK (in effect, an inner code) would form a concatenated coding arrangement which allows for iterative decoding. At the receiver, the iterative decoding would be part of the demodulation process.
The figure depicts one of a number of generic coding/decoding schemes, admitted by this method, that includes the use of a reduced-complexity receiver. Computational simulations for this scheme with various outer codes, interleaver block sizes, and numbers of decoding iterations demonstrated the potential to obtain coding gains (in terms of signal-to-noise ratio needed to keep the bit-error rate below a specified value) ranging from 3.75 to 7.7 dB.
This work was done by Marvin Simon and Dariush Divsalar of Caltech for NASA’s Jet Propulsion Laboratory.
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Refer to NPO-30135, volume and number of this NASA Tech Briefs issue, and the page number.