A simple technique for the detection of multiple access interference in the parallel acquisition of weak gps signals

Current trends towards the development of High Sensitivity (HS-GNSS) receivers require that the receiver operate in relatively restrictive environments. Some signals are received with severe attenuation, while others may be received with the nominal, open-sky, signal strength. As the disparity in the relative strengths of the signals being processed increases, the cross-correlation margin becomes insufficient to distinguish cross-correlation peaks from the auto-correlation peak, a problem referred to as Multiple Access Interference (MAI). Thus the HS-GNSS receiver requires some mechanism to counteract MAI. In this paper we propose a simple and effective technique for the detection of MAI. Our approach relies on the noncoherent combining technique, and the parallel consideration of many code-phase bins during the acquisition process, both of which are very commonly employed in weak signal acquisition. Our approach takes the form of a binary hypothesis test: a single MAI decision statistic is created from observations of the received signal, if this decision statistic exceeds a threshold then MAI is deemed present in the observed signal, otherwise MAI is deemed absent. The formation of this decision statistic requires very little extra computation, consisting simply of the sum of all the noncoherent accumulator outputs. The proposed detector requires only N - 1 extra additions, where N is the number of code-phase bins considered in parallel during the acquisition process. The proposed detector, therefore, offers the combined benefits of low computational burden with no a priori limit on the minimum detectable signal strength. Of course, the minimum detectable signal is still a function of the strongest signal impinging on the receiver antenna.