Multi-Modulator for Bandwidth-Efficient Communication

NASA’s Jet Propulsion Laboratory

A modulator circuit board has recently been developed to be used in conjunction with a vector modulator to generate any of a large number of modulations for bandwidth-efficient radio transmission of digital data signals at rates than can exceed 100 Mb/s. The modulations include quadrature phase-shift keying (QPSK), offset quadrature phase-shift keying (OQPSK), Gaussian minimum-shift keying (GMSK), and octonary phase-shift keying (8PSK) with square-root raised-cosine pulse shaping. The figure is a greatly simplified block diagram showing the relationship between the modulator board and the rest of the transmitter. The role of the modulator board is to encode the incoming data stream and to shape the resulting pulses, which are fed as inputs to the vector modulator. The combination of encoding and pulse shaping in a given application is chosen to maximize the bandwidth efficiency.

The Modulator Board is part of a radio transmitter, wherein it processes an incoming data stream in such a way as to generate modulator inputs for bandwidth- efficient modulation.

The modulator board includes gallium arsenide serial-to-parallel converters at its input end. A complementary metal oxide/semiconductor (CMOS) field-programmable gate array (FPGA) performs the coding and modulation computations and utilizes parallel processing in doing so. The results of the parallel computation are combined and converted to pulse waveforms by use of gallium arsenide parallel-to-serial converters integrated with digital-to-analog converters. Without changing the hardware, one can configure the modulator to produce any of the designed combinations of coding and modulation by loading the appropriate bit configuration file into the FPGA.

At the time of reporting the information for this article, a prototype of the modulator board had been tested in laboratories, and tests in a two-way ground-to-spacecraft communication link were planned. Although the modulator board was conceived for original use in spacecraft-to-spacecraft and spacecraft-to-ground communications, there are potential terrestrial uses in microwave tower-to-tower links and aircraft remote sensing systems.

By making it possible to implement many different high-rate modulators in the same piece of hardware, the underlying design concepts of this modulator can be expected to afford economies of scale: It would cost less to manufacture many identical modulator hardware units to satisfy market demands for many types of modulators than to manufacture smaller numbers of specialized modulator units having different designs.

This work was done by Andrew Gray, Dennis Lee, Norman Lay, and Craig Cheetham of Caltech; Wai Fong, Pen-Shu Yeh, Robin King, and Parminder Ghuman of Goddard Space Flight Center; and Scott Hoy and Dave Fisher of Lockheed-Martin for NASA’s Jet Propulsion Laboratory.

In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to:

Innovative Technology Assets Management

JPL

Mail Stop 202-233

4800 Oak Grove Drive

Pasadena, CA 91109-8099

(818) 354-2240

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Refer to NPO-40807, volume and number of this NASA Tech Briefs issue, and the page number.