LOCATION COVERAGE AND SENSITIVITY WITH A-GPS
Ari Kangas, Torbjörn Wigren, Ericsson Ab · 2004
Assisted GPS (A-GPS) is a standardized feature in most cellular standards of today, like GSM, WCDMA and IS95. The use of assistance data improves the sensitivity and time to first fix compared to a conventional GPS receiver. This paper compares the coverage and sensitivity of conventional GPS receivers and A-GPS receivers. It is shown that the best sensitivity is achieved by using accurate timing assistance to enable coherent integration over navigation data bit edges. k k tr tr k tr k k t n t j t d t c P t y + + − + + = φ ω τ τ . (1) Here P is the received SV signal power, c(t) is the C/A code of the SV, a known sequence of -1 and +1 values that switch with a rate of 1/Tc =1.023⋅10 6 Hz. The C/A code repeats itself every 1 ms. The quantity τtr is called the code phase. d(t ) is a sequence of -1 and +1 values containing navigation data. This sequence switches at a rate of 20 ms. ωtr is the unknown Doppler shift, and φ is an unknown phase offset. The noise n(t) is assumed to be white with spectral density NF kT0 , where NF is the receiver noise figure, k=1.38⋅10 -23 J/K, and T0 the receiver noise temperature. Interference from other satellites is not modelled here. At the SV signal levels of interest, the multiple access interference only adds about 0.1 dB to the noise power. The task of the GPS receiver is to find the correct code phase and doppler, detect the data bit edges, perform navigation data demodulation. The A-GPS receiver attempts to improve or eliminate some of the steps above. In order to do so, assistance data is collected from a network of GPS reference receivers. These reference receivers are located at sites with favorable signal conditions. The receivers continuously tracks visible SVs, demodulate their messages and transfers the information to the cellular network for further distribution to the GPS receiver in the cellular phone. In this way the demodulation may be avoided and the GPS receiver in the cellular phone gets access to compete navigation models and correction parameters. The search in the Doppler domain can also be reduced. It is also possible to provide the GPS receiver with GPS time information, accurate to a few microseconds, by exploiting e.g. GPS receiver in the base stations of the cellular network. With such information, only coherent code and doppler search would remain.