GPS World, March 2015

DESIGN TESTING Signal Simulators With four RF outputs the received signal of a four element antenna can be configured very easily FIGURE 3 shows the dialog to configure a four element antenna with the geometry shown in FIGURE 4 Note that the antenna elements are configured in the body fixed system with the x axis to front and the y axis to the right inline with a north east down NED system when facing to north while the geometry shown in Figure 4 follows an eastnorth up ENU convention The following sections give an overview of multiantenna systems and discuss results from a measurement campaign of the German Aerospace Center DLR utilizing the simulator and the DLR GALileo ANTenna array GALANT four element multi antenna receiver Multi Antenna Receivers Multi antenna receivers utilize an antenna array with a number of antenna elements The signals of each antenna element are mixed down and converted from analog to digital for baseband processing In the baseband the signals received by the different antenna elements are multiplied with complex weighting factors and summed The weighting factors are chosen in such a way that the received signals from each antenna element cancel out into the direction of the interferers nulling and additionally for advanced digital beamforming such that the gain is increased into the direction of the satellites by forming of individual beams to each satellite Because all these methods work with carrier phases it is important that in the simulation setup the signals contain the correct carrier phases at the RF outputs of the simulator corresponding to the user satellite and user interferer geometry and the position and attitude of the simulated array antenna FIGURE 5 presents the geometry of a rectangular antenna array with 2x2 elements and a signal s t impinging from direction ϕ θ The spacings of the elements dx 30 40 50 60 70 80 90 100 179 1792 1794 1796 1798 180 30 40 50 60 70 80 90 100 Suppression dB Phase delay degree FIGURE 2 Carrier suppresion as a function of phase delay FIGURE 3 Configuration of individual antennas per receiver dy are typically half a wavelength but can also be less The range difference for antenna element i relative to the reference element in the center of the coordinate system depends on the incident direction ϕ θ and the position m 01 n 01 of the element within the array 4 The corresponding carrier phase shift is 5 For CRPA and adaptive beam forming applications the differential code delays may be neglected if they are small compared to the code chip length However it is essential that the carrier phase differences are precisely simulated because they contain the information about the incident direction of the signal and are the basis for the array processing in the receiver For instance the receiver can estimate the directions of arrival of the incident signals from these carrier phase differences Now we consider a 2x2 array antenna It can be simulated with the simulator with four RF outputs where each output corresponds to one antenna element In the simulator control software a user with four antennas is set up where the position of each antenna element is defined as an antenna position offset relative to the user position In this approach both differential code and carrier delays due to the simulated array geometry are taken into account because the code and carrier pseudoranges are computed by the simulator for the position of each antenna element However the RF hardware channels of the receiver front end may have differential delays against each other which may even vary with time If the direction of the satellites and interferers shall be estimated correctly by the receiver algorithms a calibration signal is required to measure and compensate these differential hardware delays For the real antenna system a binary phase shift keying BPSK signal with zero delay for each antenna channel is generated by the array receiver and fed into the antenna calibration port For the simulation this calibration signal must also be generated by the constellation simulator In a simple way a satellite in the zenith of the user antenna can be simulated which has the same distance and delay to all antenna elements Unfortunately this simple solution includes some limitations to the simulated position and attitude of the user because the user position must be at the Equator if a real satellite is simulated in form of a GPS World March 2015 www gpsworld com 28