Theoretical and Practical Evaluation of an Overlay Multi-band Front-end

An overlay front-end architecture exploits the properties of direct sequence spread spectrum GNSS signals by intentionally overlaying two or more signal reception paths in the intermediate frequency (IF) or complex baseband domain. While the radio frequency (RF) reception paths are still kept separately, a combiner stage is introduced, including variable gain amplifiers (VGAs) that control the amount of overlay signal power before its combination. Thanks to the overlay carried out in this combiner, only one common baseband path is required. This allows savings in terms of cost, size, power consumption, and digital bandwidth. Moreover, continuous tracking of both received signal bands is possible. However, the overlay introduces inevitable signal degradation. This paper analyses the impact of this overlay signal degradation using the spectral separation coefficient (SSC) theory and presents practical results obtained using a data recorder front-end with digital signal processing. The introduced overlay theory is validated by the measurements evaluated by a GNSS receiver. Finally, it is shown that this gain controlled by a VGA provides another degree of freedom that can be used to optimize the reception conditions in such an overlay front-end architecture. An optimization of the overlay VGA setting for an ionosphere free dual-frequency linear combination is analytically derived and carried out for the practical measurements.