Signal Multiplex Techniques for GNSS

Fundamentals
Title	Signal Multiplex Techniques for GNSS
Author(s)	J.A Ávila Rodríguez, University FAF Munich, Germany.
Level	Advanced
Year of Publication	2011

Introduction

All the existing and planned Global Satellite Navigation Systems will be transmitting more than two services in each transmission band.

• In E1/L1

GPS plans to transmit at least four signals: The old C/A Code and P(Y) Code, the modernized military M-Code and the new L1 Civil signal (L1C).

Similarly, Galileo will provide on E1 the Public Regulated Service (PRS) and the E1 OS data and pilot signals.

GLONASS transmits the C/A Code and P-Code, and plans new CDMA signals

and Compass plans to provide two additional signals with services that are still to be specified.

• In L2

GPS will be transmitting the L2 Civil Signal (L2C) together with the P(Y) Code and M-Code,

and GLONASS transmits the C/A Code and P-Code.

• In E5/L5

GPS L5 will provide data and pilot signals, while

Galileo plans to transmit four signals using the Alt-BOC modulation,

GLONASS plans to transmit new open CDMA signals in the L5 band,

and Compass plans to provide two additional signals with services that are still to be specified.

• In E6

Galileo plans to transmit the Public Regulated Service (PRS) and a Commercial Service (CS) with improved characteristics with respect to the free Open Service (OS),

and Compass plans to provide two additional signals with services that are still to be specified.

• Finally in L3

GLONASS will also transmit one civil signal (C/A Code) and the military P-Code.

The availability of spectrum allocations is a limited resource as we have seen in previous chapters and thus the existing bands have to be reused. As new services are demanded, new signals are also required to fulfill needs that did not exist previously. Moreover it is highly desirable and of greatest importance that the new navigation signals that are introduced do not cause significant distortion on other signals sharing the frequency. In other words, they have to be compatible with the already existing ones. Additional constraints are that we can transmit the new signals through the same High Power Amplifier (HPA), to be able to accommodate new data messages and new Pseudo Random Noise (PRN) code families, to have spectral isolation with the rest of signals in the band and to be capable of providing flexibility to control the distribution of power between the signals in and outside of the allocated band.

If we take a look at all these constraints, we can clearly recognize the importance of developing good signal multiplex techniques based on a high efficiency constant-envelope. Multiplexing aims at providing a multiplicity of signals that must coexist on the same carrier without mutual interference [A.R. Pratt and J.I.R. Owen, 2005]. Moreover, we cannot ignore that there are clear constraints to fulfil at payload level and that the multiplex technique must therefore be carefully selected to avoid undesirable sources of degradation. In this chapter we will study the characteristics of current and future multiplexing techniques and the recently proposed modifications for GPS and Galileo in order to be capable of accommodating the optimized MBOC signal modulation.

Finally, it is interesting to note that, as shown in [A.R. Pratt and J.I.R. Owen, 2005] , alternative multiplexing methods to those discussed in this chapter could be used. In fact, separate antenna and amplifier chains (that is separate aperture) which allow for signal combination in the far field of the satellite antenna system could be employed. In addition, different signals could be multiplexed on the carrier frequency on several different antenna beams as suggested in [G.L. Cangiani, 2005]. Nevertheless, the extra complexity that the spacecraft payload would have to deal with would be of consideration and the antenna design would suffer from poor efficiency and important cost and weight drawbacks. Moreover, the more challenging problem of transmitting these signals would be the development of a general modulation approach with a single modulator, up-converter, power amplifier chain and antenna aperture [P.A. Dafesh et al., 2006].

Multiplexing Schemes

The first multiplexing technique used in navigation (GPS) was employed to send the C/A Code and the P(Y) Code providing two bi-phase signals on the same carrier frequency in phase quadrature (QPSK). Demultiplexing was relatively simple. However, the need to have more navigation signals has made this multiplexing scheme obsolete for future modernized implementations. In fact, the possible solution of adding another signal slightly offset in frequency would give rise to a non-constant envelope with the consequent distortion after passing through the High Power Amplifier (HPA). The following multiplexing techniques are further described:

• Linear Modulation (Spatial Combining)

Tri-code Hexaphase Modulation

• Majority Signal Voting
• Hard Limiting
• Quadrature Product Sub-carrier Modulation (QPSM)

Interplex and CASM

Modified Interplex Modulation

• Intervoting

In spite of the important advances realized in the past years, the research field on signal multiplex is still subject to active studies as shown in [T. Fan et al, 2005]. Out of all the multiplex techniques presented above, the Linear Modulation and the Tri-code Hexaphase Modulation have maximum efficiencies limited to roughly seventy-one percent [P.A. Dafesh et al., 2006] , what is an important disadvantage. Moreover, they are limited to multiplexing only binary signals. The rest of multiplex techniques offer a superior performance as we will see in next chapters. The efficiency is defined as the sum of the effective transmitted power [math]\displaystyle{ P_{useful} }[/math] plus any band limiting losses, divided by the total transmitted power [math]\displaystyle{ P_{total} }[/math].

[math]\displaystyle{ \eta = \frac{P_{useful}}{P_{total}} }[/math]