GPS World, January 2017

streams of data along with the size and weight of the complex electronics required to process the antennas received signals are significant issues for portable or consumer level applications Unlike conventional or traditional beamforming technology the new correlator beamforming approach combines RF signals received by any number of individual antenna elements into a single switched RF signal This time multiplexed signal is then downconverted and digitized by a single RF frontend The correlator beamforming design should offer manufacturing cost savings because the resulting data stream is processed using a single correlator channel per beam This reduces the complexity when compared to the traditional beamforming methodology The architectural differences between a standard singleantenna setup a traditional beamforming CRPA system and correlator beamforming are shown in Figure 1 and FIGURE 2 CORRELATOR BEAMFORMING The correlator beamforming technique performs antenna array signal processing to form beams as part of a receivers correlation process The complete explanation of this technology can quickly get complex even for the seasoned RF engineer To describe this process more simply we will assume noiseless signals and no multipath except as noted as well as equal noise figures for all front end processing chains To further simplify our explanation modulation on the carrier and switching losses will be ignored FIGURE 3 illustrates traditional beamforming processing as applied to a four element CRPA The four sinusoids shown depict the baseband sampled signal carriers received by each element from a satellite at a particular azimuth and elevation angle with respect to the center element Note that the phases of the signals for Elements 1 through 3 prior to the phase shifters are different from the reference Element 0 The reasons for these phase differences are twofold 1 slightly different signal propagation distances from the satellite to each elements phase center as a function of array geometry and orientation and 2 differences in the electrical path lengths from each elements phase center to the frontend analog to digital converter ADC The latter effects are a combination of angle of arrival AoA dependent and independent inter channel biases and comprise what is normally referred to as the antenna manifold Note the unequal amplitudes of the received signals This is intended to represent differences in the gain patterns of each individual antenna element as well as minor gain differences in the signal processing chains amplifiers filters mixers transmission lines and ADCs In general for beamforming applications as opposed to null forming it is not necessary to compensate for these Amplitude compensation at the sample level significantly increases the signal processing burden Furthermore in the context of this article one or two bits of sample amplitude quantization is adequate for multipath rejection as long as no significant interference is expected As shown in Figure 3 phase shifts are applied such that all signals are phase aligned to the reference element The coherent sample streams can then be summed to maximize received signal power In the spatial domain this corresponds to steering a beam in the direction of the desired signal This visual interpretation arises from the fact that the specific set of phase shifts that aligns the signals coherently only applies to signals arriving from this desired signals direction Under the conditions described above if a multipath signal arrives from a different direction than that which is intended the phase of the multipath signals in the four elements will not be coherent so the multipath signal will not experience the same N2 power gain as the direct signal This is the fundamental reason that such a system rejects multipath signals by steering the beam the effective gain of the direct signal is higher than the effective gain of the multipath signals Even though not shown in Figure 3 it should be clear that the coherently combined sample stream undergoes typical GNSS receiver baseband processing that is correlation with a locally generated replica carrier code Element 0 Reference Element 1 Element 2 Element 3 Phase shifters Integration interval Summation of phase shifted sample streams with blanking 1 Blanking waveform 0 FIGURE 4 Illustration of traditional beamforming with 25 percent dutycycling JANUARY 2017 WWW GPSWORLD COM GPS WORLD 57