CPM/PN modulation and ranging for bandwidth-limited multiple access links

Abstract

A prior paper introduced the waveform denoted GMSK/PN as a candidate for power-limited, single-access communications links on which simultaneous high data rate transmission and precision ranging are desired. The waveform combines traditional GMSK (Gaussian Minimum-Shift Keying) modulation and a PN (pseudonoise) spread-spectrum sequence used for ranging. A ranging code is placed on a subcarrier whose frequency is selected to minimize mutual interference between the data and ranging components. The waveform is entirely phase modulated (constant envelope) and has no residual carrier.In an obvious generalization, one notes that other forms of bandwidth-efficient modulation, such as continuous phase modulation (CPM), may replace GMSK. The generalized waveform, designated CPM/PN, is studied here. New considerations arise in studying frequency-division multiple access (FDMA) applications of CPM/PN. For example, the wide spectral spread of the ranging sidelobes in subcarrier ranging leaves the designer with a choice of: (1) maintaining a relatively large spacing between adjacent carriers (capacity limitation), and (2) tolerating the mutual interference encountered in closer carrier spacing (performance limitation). Under close carrier spacing, the single-user interference suppression advantage of spectral separation between data and range signals achieved via use of a subcarrier may be blunted in MA cases because of adjacent user interference. One alternative facilitating MA use of CPM/PN is to trade some interference immunity in order to narrow the per signal occupied bandwidth. A waveform that accomplishes this, CPM/PN without subcarrier, adds a ranging code at baseband to the CPM phase form prior to phase modulation. A significant interference-related consequence of deleting the subcarrier is the direct overlap of each user's data and ranging components. Since data typically is by far the stronger signal, it can have considerable impact on ranging performance. Though higher spectrum efficiency can be achieved without the subcarrier, the resulting mutual interference is significant and must be addressed. This paper describes the without-subcarrier CPM/PN waveform and its spectral characteristics. We show the signal processing required to both generate and receive it, in particular the decision-directed demodulator that removes the own-data phase modulation prior to recovery of the ranging signal. The spectrum models lead naturally to characterization of the mutual interference environment created when CPM/PN is used in a frequency-division multiple access (FDMA) format. The data and ranging signal-to-noise ratio degradation due to mutual interference, as well as the amount of transmit margin required to overcome that degradation, is predicted analytically and calculated for several scenarios defined by a National Aeronautics and Space Administration (NASA) task group studying future earth-lunar communication architectures. Both analytic and simulation models are used to determine the achievable CPM/PN ranging performance, and results from both sources are presented for two forms of CPM: GMSK and filtered offset QPSK (OQPSK). Certain national and/or international spectral emission regulations agreements (e.g., National Telecommunications and Information Administration, NTIA, and Space Frequency Coordination Group, SFCG) must be observed in the CPM/PN applications under study by NASA. Standard square-wave chip PN waveforms prove incompatible with the applicable spectral masks, whose sidelobe decay rates 9 are faster than the \l f decay of the square-wave. But design employing bandwidth-efficient chip waveforms such as half-sinusoidal and Butterworth filter responses can satisfy applicable spectral emission masks without sacrificing PN chip rate.