Richard S. Orr

Combined GMSK Modulation and PN Ranging for Communications and Navigation

Renewed interest in human exploration of space suggests a future need for high throughput earth-space links. Anticipating crowding within spectrum used for deep- space communications, particularly at X-band, NASA is investigating the use of bandwidth-efficient modulation (BEM). Requirements for trajectory/orbit determination and vehicle guidance will persist, however, implying that signals for radiometric tracking must coexist with communications within the available spectrum. Because navigation signals in current use may be insufficiently compatible with BEM, the question arises as to how to combine ranging and BEM communications. This paper presents one waveform approach potentially applicable to the space-to-earth return link problem. GMSK modulation is combined with a pseudonoise (PN) ranging signal on a subcarrier. Waveform parameters are adjusted to achieve: (1) a constant-envelope waveform; (2) adequate separation between the communications and ranging components; (3) high data rates (~ 6 Msps); and (4) conformance to international standards such as Space Frequency Coordination Group (SFCG). We define the waveform, present its power spectrum, exhibit receiver structures for simultaneous GMSK data demodulation and PN tracking for accurate time-of-arrival estimation, and show relationships among the waveform parameters that must hold to satisfy the demands of applicable standards.

Evaluation of Multiple Access Techniques for Simultaneous Space Communications and Tracking

The National Aeronautics and Space Administration has conducted a comprehensive study to identify the most appropriate and efficient modulation, coding, multiple access and link protocol options for future space communication links supported by NASAs Ground Network, space network, deep space network, and earth-based ground terminals and in-situ relay satellites envisioned for Constellation Program mission support at the Moon and Mars. This paper briefly describes the study process and summarizes the multiple access recommendations for future NASA space communications.

CDMA is unfair: Transmit margin in an inhomogeneous user community

In a CDMA system where multiple users share bandwidth in common, multiple access interference (MAI) acts as noise that adds to the receiver thermal noise. To overcome MAI, margin must be added to each users transmit power. The amount of margin required to achieve this for the homogeneous case in which all users have the same parameters has been known for some time. The general solution for required transmit margin when the user parameters are arbitrary—the inhomogeneous case—is developed in this paper. The solution exhibits an “inverse Robin Hood” characteristic in which less demanding users “subsidize” the more demanding ones by providing a greater share of MAI margin. This behavior is an inherent attribute of CDMA that cannot be overcome except by the elimination of MAI through strictly orthogonal signals.

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

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.1 2

Code phase and delay settings that minimize CDMA interference

When code-division multiple access (CDMA) signals are linearly multiplexed (phase-coherently summed) at a transmit terminal, a receiver demodulating information from one component signal experiences multiple access interference (MAI) from all others to the extent that the waveforms are not strictly orthogonal. Under a model for which: (1) the users are homogeneous, i.e., identical in those parameters that govern the amount of MAI they produce at a receiver, and (2) the various codes are random binary sequences, one may develop an expression for the amount of induced MAI as a function of the spreadspectrum chip delays and phases. It is not immediately obvious from this expression, however, what selection of delays and phases produces the most benign distribution of MAI among users. This paper presents an analytic characterization of MAI for linear multiplexing of a set of CDMA codes, culminating in a deterministic design of delays and phases that achieves MAI noise levels substantially smaller than those predicted by worst-case or random assignment results. It is believed that the design achieves the minimum MAI level, although it is not proved.

Multiple access interference properties of constant-envelope CDMA

CDMA systems transmitting multiple signals from a single source normally employ linear superposition of codes. The resulting signal has a high peak-to-average power ratio (PAPR), which is undesirable from a high power amplifier (HPA) viewpoint. Several techniques have been proposed to reduce PAPR and ease the HPA requirements. Among these, constant-envelope CDMA (CE-CDMA) is an alternative that eliminates envelope variation, achieving PAPR = 0 dB. It can use a saturating (Class C) amplifier that runs at lower peak power and may be more robust and inexpensive than the Class A HPA needed for linear superposition. The latter requires significant linear dynamic range and must operate at several dB output power backoff. Constant-envelope multiplex techniques are nonlinear and thus exhibit both a multiplexing efficiency loss—part of the price one pays for a substantial reduction in peak power—and multiple access interference (MAI) among the user signals. To date there has been no systematic, quantitative exploration of multiplexing efficiency and MAI in CE-CDMA. The principal results reported in this paper are the derivation of formulas for multiplexing loss and MAI in CE-CDMA and their use to discover, for the case of homogeneous users, multiplexing methods optimum with respect to minimizing the combined losses. A comparison of MAI for linearly multiplexed and CE-CDMA concludes the paper.