Massive MIMO is one of the technologies being developed to enable 5G to reach the advertised data rates of 10 Gbps, particularly at frequencies below mmWave. Developing massive MIMO systems requires simulation and test of the electromagnetic signal environment, over-the-air (OTA) beamforming algorithms, antenna arrays and the entire 5G system. Once 5G is ready to be deployed, the massive MIMO arrays will have to be tested as they are manufactured. To support development and testing of these systems, Mitron has developed a highly accurate, multichannel, phase/amplitude control matrix.
All the phase/amplitude control matrices and modules now on the market use digitally-controlled MMIC phase shifters and attenuators. The smallest steps available for such phase shifters and attenuators are 1.4 degrees and 0.25 dB, respectively, up to 6 GHz. Theoretically, these step sizes provide ±2.8 degree phase and ±0.5 dB attenuation resolution; in the real world, actual products have worse performance because of inaccuracies. At the mmWave frequency bands presently proposed for 5G, primarily in the range from 25 to 40 GHz, few digitally-controlled MMIC phase shifters and attenuators are found. Reported products have only 5.6 degree and 0.5 dB step resolution, and their accuracies will be worse than the ±11.2 degree and ±1 dB theoretical values. These existing products cannot meet the accuracy requirements of future 5G systems. Further, most of these products are narrowband, typically with only 200 MHz bandwidth. So testing a massive MIMO system across the various 5G bands will require many narrowband phase and amplitude control channels, adding significant cost to the test system.
Mitron adopted a completely different approach to achieving phase and amplitude control: designing analog control circuits that are controlled digitally—which improves the resolution—and calibrating the frequency response of the phase and amplitude circuits using software algorithms on a fast automatic test system. Compared to the MMIC-based products discussed, Mitron achieves better phase and amplitude resolution and accuracy, as well as wider bandwidth. Mitron’s analog phase and amplitude control circuits cover 1.7 to 6, 6 to 18 and 25 to 40 GHz, such that a single unit handles multiple 5G frequency bands, reducing the investment in test equipment for 5G R&D and production testing.
Using these analog control circuits, Mitron has developed 1x16 phase/amplitude control matrix systems covering 1.7 to 6 GHz and 24 to 40 GHz (see Figure 1). Each system fits in a 2U 19 in rack and is controlled via a graphical user-interface (GUI) on a PC (see Figure 2). The products handle very wide instantaneous bandwidth, and just two systems support the primary microwave and mmWave bands proposed for 5G.
Each 1×16 matrix has maximum control ranges of 360 degrees and 50 dB, with steps of 1 degree and 0.1 dB, respectively. At any phase and amplitude value over the whole frequency band, the phase and amplitude accuracies are 2 degrees and 0.2 dB maximum from 0 to 30 dB attenuation and 2.5 degrees and 0.4 dB maximum from 30 to 50 dB attenuation. Figure 3 shows the phase and amplitude accuracy at 1.7 GHz for attenuation values from 0 to 50 dB. Figure 4 shows similar performance data for the mmWave unit measured at 40 GHz.
Using the analog control circuits as “LEGO® building blocks,” additional matrix configurations with the same accuracy can easily be developed: 1×32, 1×64, 2×32, 4×8, 4×32 and M×N.