Cross-correlated trellis-coded quadrature modulation (XTCQM) has been proposed as a generic scheme with specific embodiments that potentially offer superior alternatives to other highly power- and bandwidth-efficient phase-shift-keying (PSK) modulation schemes. Examples of such schemes include Gaussian minimum-shift keying (GMSK), staggered quadrature offset raised cosine (SQORC), Feher's patented quadrature PSK (FQPSK), and pulse-shaped offset quadrature PSK (OQPSK).
Previously developed trellis-coded-modulation (TCM) techniques combine (1) the bandwidth efficiency of such conventional multilevel-modulation techniques as multiple-phase-shift keying (MPSK) and quadrature amplitude modulation (QAM) with (2) the power efficiency of error-correction coding into (3) unified modulation schemes that, through suitable mappings, simultaneously exploit the desirable properties of (1) and (2). While the previously developed TCM techniques afford the bandwidth efficiency inherent in multilevel modulation, less attention was given, in the development of those techniques, to achieving high levels of spectral containment as quantified, for example, by keeping out-of-band power levels low to minimize adjacent-channel interference.
Considering only quadrature modulations, the innovative aspect of the development of XTCQM lies in a focus on the spectral occupancy of the transmitted signal, along with careful attention to a desirable constant-envelope property and to the power efficiency of the demodulation/decoding operation at the receiver. The more-generic nature of XTCQM (as compared with FQPSK and other schemes) affords considerably more flexibility for trading off between power and spectral efficiencies.
In XTCQM, a cross-correlation and a suitable waveform mapping are introduced into the in-phase (I) and quadrature (Q) baseband signals transmitted on quadrature carriers, in such a way as to provide a high level of spectral efficiency while also maintaining high power efficiency and a constant or pseudo-constant envelope. XTCQM would be implemented by use of such standard, currently available subsystems as an offset-QPSK modulator and a receiver containing such items as matched filters, and a Viterbi decoder. The transmitter portion of a conceptual XTCQM communication system has been shown, in a computational simulation, to perform as predicted by theory. At the time of reporting the information for this article, the software for computational simulation of the performance of the receiver portion of the system was in an early stage of development.
This work was done by Marvin K. Simon and Tsun-Yee Yan of Caltech for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line www.nasatech.com/tsp at under the Electronics & Computers category. NPO-20532
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Cross-Correlated Trellis-Coded Quadrature Modulation
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Overview
The document presents a technical overview of Cross-Correlated Trellis-Coded Quadrature Modulation (XTCQM), a novel modulation scheme developed by Marvin K. Simon and Tsun-Yee Yan. XTCQM is proposed as a versatile alternative to existing phase-shift-keying (PSK) modulation techniques, such as Gaussian minimum-shift keying (GMSK), staggered quadrature offset raised cosine (SQORC), Feher’s patented quadrature PSK (FQPSK), and pulse-shaped offset quadrature PSK (OQPSK).
The primary innovation of XTCQM lies in its ability to combine the bandwidth efficiency of multilevel modulation techniques, like multiple-phase-shift keying (MPSK) and quadrature amplitude modulation (QAM), with the power efficiency of error-correction coding. This integration results in a unified modulation scheme that effectively exploits the desirable properties of both bandwidth and power efficiency through suitable mappings.
A significant focus of the invention is on improving spectral occupancy while maintaining a pseudo-constant envelope, which is crucial for minimizing adjacent channel interference. The document emphasizes that while traditional trellis-coded modulation (TCM) techniques have addressed bandwidth efficiency, they often overlook the importance of spectral containment. XTCQM aims to rectify this by enhancing the spectral efficiency of transmitted signals without compromising power efficiency during demodulation and decoding at the receiver.
The document also discusses the flexibility of the XTCQM structure, which allows for a more adaptable trade-off between power and spectral efficiencies compared to more restrictive existing techniques. This flexibility is particularly beneficial for optimizing well-known bandwidth-efficient modulation techniques that can be further refined through the generic structure of XTCQM.
Additionally, the document includes a disclaimer regarding the endorsement of specific commercial products and clarifies that the work was conducted at the Jet Propulsion Laboratory under NASA's contract. It highlights the relevance of the technology to ongoing NASA programs and suggests that the findings may be disseminated through NASA Tech Briefs, pending patent determinations.
In summary, the document outlines the innovative aspects of XTCQM, its potential advantages over traditional modulation schemes, and its implications for future communication technologies, particularly in the context of NASA's ongoing research and development efforts.
