A NASA Small Business Innovation Research (SBIR) project has led to the development of high-rate (155.52-Mb/s) modems for use in high-quality wireless data communications. The research was directed toward conceiving innovative signal-processing techniques to reduce costs below those of conventional modems, implementing these techniques in a low-power complementary metal oxide semiconductor (CMOS) application-specific integrated-circuit (ASIC) modulator and a corresponding demodulator, and developing the necessary circuit boards (see figure) and control software. An additional goal of the research was to determine the maximum rate at which data can be transmitted through a standard 72-MHz-wide satellite-transponder channel.

This Low-Cost Modem enables wireless digital communication at a rate up to 155.52 Mb/s. An even higher data rate can be achieved by operating multiple such modems in parallel.

The research revealed that the primary barriers to reducing the costs of high-rate modems lay in (1) expensive high-power amplifiers (HPAs) needed to support high rates and higher-order, more bandwidth-efficient modulations; (2) expensive frequency converters that satisfy stringent phase-noise requirements; and (3) expensive high-resolution, high-rate analog-to-digital converters (ADCs). The research then focused on the development of modulation/demodulation and coding/decoding techniques to overcome each of these cost barriers.

One of the innovative features of the modem design is the use of pragmatic trellis coded modulation (PTCM) to reduce (1) the required power and other required performance parameters and thus (2) the cost of the power amplifier. The modem is capable of a fully rotationally invariant octonary-phase-shift-keying (8PSK) PTCM; this means that it is possible to flywheel through a cycle slip with no loss of data  a crucial benefit for Earth/satellite wireless data communications.

The design incorporates analog-matched-filtering and single-sample-per-baud concepts that, in combination, greatly reduce cost and complexity, relative to modems developed previously for the same purpose. The design supports low-complexity modulators by providing for square-root-Nyquist pulse shaping with programmable predistortion and programmable values of a. This programmability enables a communication-link designer to trade a and HPA backoff for a fixed baud rate to achieve the maximum possible data rate through a transponder channel.

Another innovative feature of this modem stands in contrast to the conventional means of converting a stream of 4 complex samples per baud into quadrature analog baseband waveforms. In the conventional approach, the streams are multiplexed and passed through a very fast digital-to-analog converter (DAC); not only is a fast DAC expensive, but the board and assembly costs are high because of the need to use emitter-coupled-logic components (latches, clock drivers, and so forth). In this modem, the conventional high-speed DAC is replaced with four baud-rate DACs, thereby effecting a significant reduction in cost. Moreover, unique data encoding and pulse shaping reduce (in comparison with that of a typical previously developed modulator) the maximum stop-band energy level by nearly 12 dB, further enhancing modulation quality.

In a comparative test against a state-of-the-art high-rate European modem, this modem demonstrated superior performance in the transfer of data through a satellite transponder, the ability to function properly with frequency translators that cost only 1/40 as much as do the frequency translators in the European modem, and an overall cost of one-fourth of that of the European modem. In other tests,

  • Transfer of data at the design rate of 155.52 Mb/s was demonstrated in a laboratory environment; and
  • Transfer of data at a rate of 75 Mb/s was demonstrated through a 36-MHz-wide transponder channel, using a rate-5/6 PTCM and 8PSK.

The modem is also expected to support a 16-state quadrature amplitude modulation (16QAM), 155.52-Mb/s link, using a Reed-Solomon (187, 204) outer code and a rate-3/4 pragmatic trellis code. The link occupies a nominal frequency band 73.5 MHz wide, and it should be possible to accommodate the link in a standard 72-MHz-wide transponder channel; however, at the time of reporting the information for this article, a field test of the modem with such a link had not yet been performed.

This work was done by Ron McCallister, Bruce Cochran, and Jim Crawford of SiCOM, Inc. for Glenn Research Center.

Inquiries concerning rights for the commercial use of this invention should be addressed to

NASA Glenn Research Center
Commercial Technology Office
Attn: Steve Fedor
Mail Stop 4 -8
21000 Brookpark Road
Ohio 44135

Refer to LEW-16668.

Electronics Tech Briefs Magazine

This article first appeared in the April, 2000 issue of Electronics Tech Briefs Magazine.

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