On potential CBOC/TMBOC common receiver architectures

Under the 2004 Agreement on the Promotion, Provision, and Use of Galileo and GPS Satellite-Based Navigation Systems and Related Applications, the member states of the European Union and the United States agreed on working together, intensifying thus the cooperation on interoperability and compatibility issues between Galileo and GPS. Among other topics, one important focus was the E1/L1 frequency band, centred at 1575.42 MHz, where the Galileo E1 Open Service (OS) signal and the modernized GPS L1 civil (L1C) signal are going to be transmitted along with many other RNSS signals. Recent efforts made by US and European experts identified a common optimized Power Spectral Density (PSD) frame, known as Multiplexed BOC (MBOC), in which both the Galileo E1 OS and the GPS L1C signals would fit. This normalized MBOC PSD is actually formed by the sum of 10/11 of the normalized BOC(1,1) PSD and 1/11 of the normalized BOC(6,1) PSD. Because the MBOC is defined in the frequency domain, the time representation cannot be uniquely defined, and at least two different implementations that would still comply with the MBOC spectrum exist: CBOC and TMBOC. Indeed, the latest developments indicate that the main Galileo E1 OS and GPS L1C candidates will exhibit different features [3],[4]: - The current GPS L1C main candidate will have a pure BOC(1,1) data channel gathering 25% of the total signal power while the pilot channel will use a Time- Multiplexed BOC (TMBOC) modulation with 75% of the total civil signal power. - The current Galileo E1 OS main candidate will equally share its power between its data and pilot channels, with the important difference with respect to TMBOC that in both channels a Composite BOC (CBOC) modulation with BOC(6,1) will be used. It is well-understood that the definition of a common PSD for the GPS and Galileo civil signals on E1/L1 calls for an increased interoperability and compatibility of these signals at the user level. However, to really promote the use of GPS/Galileo E1/L1 combined receivers, it is of greatest importance to find GPS L1C and Galileo E1 OS tracking architectures that minimize the receiver complexity while maintaining high quality measurements. This is particularly true since the main candidates for implementation of MBOC for GPS L1C and Galileo E1 OS are already baseline of their respective systems [3]. The purpose of this article is to thoroughly investigate possible GPS/Galileo receiver architectures that could be adapted to CBOC, TMBOC or both waveforms and to assess their performance. The first step of the proposed analysis is to assess the interference that both GPS and Galileo signals will cause on each other. This step is necessary in order to evaluate the degradation that Galileo and GPS signals will cause on each other, and thus, to assess precisely the quality of the forthcoming tracking loops. In a second part, several CBOC/TMBOC tracking architectures meant to minimize the complexity of a combined GPS/Galileo receiver will be presented and their performances in terms of resistance to thermal noise and multipath, as well as their complexity, will be compared to optimal architectures dedicated to CBOConly or TMBOC-only receivers. This part aims at giving an insight on which key parameters can be modified to find a relevant trade-off between receiver complexity and performance for the general user. Finally, in the last section, different multipath mitigation techniques will be tested according to different receiver configurations - BOC(1,1) or MBOC receiver - and flexibility in the number of correlators available for each channel - pilot or data + pilot tracking. In particular, innovative multipath mitigation techniques based on a multi-correlator receiver are also investigated.