Galileo AltBOC Receiver

Alternative BOC, AltBOC(15,10) modulation on E5, is one of the most advanced and promising signals the Galileo satellites will transmit. Galileo receivers capable of tracking this signal will benefit from unequalled performance in terms of measurement accuracy and multipath suppression. However, the signal processing techniques required to process the AltBOC modulation are much more challenging than those for the traditional BPSK or even for the usual BOC modulation. This stems from the extremely large bandwidth and from the complex interaction of 4 components of the spreading code. Despite a high number of publications on the tracking of other Galileo signals, little has been published yet on the tracking of AltBOC. The purpose of this paper is to present the principles of tracking and receiver-side processing for the AltBOC(15,10) signal . First, the signal structure is discussed in detail. Then the principle of demodulation and the architecture of the tracking channel are discussed alongside with related algorithms. The hardware implementation of the prototype receiver is presented as well. Finally, the impact of various error sources on the tracking performance is evaluated. INTRODUCTION Galileo is to transmit four different signals in the E5 band. Two of them will carry navigation messages whilst two are data-free pilot channels. Signal component Modulation Data Center frequency E5aI BPSK(10) Yes 1176.45MHz E5aQ BPSK(10) No E5bI BPSK(10) Yes 1207.14MHz E5bQ BPSK(10) No Table 1 Main characteristics of the four components of the E5 signal band. In this paper, it is assumed that the four signal components in the E5 band are modulated as a single wideband signal generated following the AltBOC(15,10) 8-PSK modulation as described and analysed in [1, 2, 3]. This wideband signal is centered at the E5 frequency of 1191.795MHz, and has a bandwidth of at least 70 MHz. The AltBOC modulation offers the advantage that the E5a (I&Q) and E5b (I&Q) bands can be processed independently, as traditional BPSK(10) signals, or together, leading to tremendous performances in terms of tracking noise and multipath. Unlike most previous publications, this paper focuses on the AltBOC signal from a receiver perspective. After a brief description of the AltBOC modulation and demodulation principle, the noise and multipath errors are evaluated on both the code and carrier measurements, and the influence of the receiver frontend bandwidth is assessed. Also, the signal distortion due to the ionosphere dispersion within the E5 band is analysed. AltBOC SIGNAL STRUCTURE For the derivation of the demodulation principle of the AltBOC modulation, it is sufficient to approximate the baseband AltBOC signal by its AltLOC counterpart:         ) 2 / . .( 4 ) 2 / . .( 3 . . 2 2 . . 1 1 . ) ( . ) ( ) (      