A demonstration at IMS 2018 shows how a beam can stay focused on a target even as the antenna rotates or the target moves.
5G will ultimately become a combination of many technologies. The New Radio, commonly known as 5GNR, will incorporate 64QAM and 256QAM modulations. Another significant aspect is beam steering, which is making progress with new ICs and antennas.
At the 2018 International Microwave Symposium in Philadelphia, Keysight Technologies teamed with Anokiwave and Ball Aerospace to show a real beam-steering system. Anokiwave and Ball Aerospace have developed a series of phased-array antennas driven by Anokiwave ICs. In the video below, you can see the antenna rotate ±30° from boresight, the point at which the antenna array is pointed directly at the receiver. Because the beam steering is synchronized with the transmit antenna angle, the received power remains constant.
The array antenna in Figure 1 consists of 256 antennas arranged in a 16×16 grid. The antenna array transmitted a 64QAM data signal at 28 GHz.
Using analog beam steering, the array consists of 64 Anokiwave AWMF-0108 quad-core ICs, where each IC drives four antenna elements. The antenna can form a single steerable beam using all 256 elements or four independently steerable beams where each beam uses 64 elements.
In analog (also called RF) beam steering (Figure 2), a single ADC provides the signal for eight antennas. While analog beam steering minimizes the number of ADCs, problems such as phase shift, RMS phase error, and RMS amplitude error as a function of frequency must be carefully managed. Plus, all of the signal processing—phase shifting and signal attenuation/amplification—must take place at the antenna, making it practical only with highly-integrated silicon IC technology. Analog beam steering provides no flexibility in the number of beams that can be formed.
A full-digital signal chain uses a dedicated ADC/DAC pair per antenna. While that provides the most beam forming flexibility, it’s also impractical. The high cost of the ADCs and excessive heat they generate also make digital beam steering impractical for high frequency antennas where the spacing between antenna elements is very small. Thus, a hybrid approach is taking hold. Figure 3 shows the same eight antennas from Figure 2 driven by two ADCs handling four antennas each. This approach is popular because it allows good beam forming flexibility without the challenges of digital beam forming.
Figure 4 shows a block diagram of the AWMF-0108. Anokiwave’s Logan Minard explained the signal chains in a phone call following the Microwave Symposium. “Common” refers not to a common signal return, but to the ADC and DAC circuits included in the transmit and receive signal chains, respectively. Analog signals from the ADCs go to Wilkinson power dividers followed by amplitude and phase control and a power amplifier (PA). Another switch connects the signal to the antenna.
On the receive side, Minard explained, the received signal first comes through a low-noise amplifier (LNA), then through a Wilkinson Combiner and temperature compensator, which adjusts signal gain based on temperature.
In the demonstration shown at the Microwave Symposium, the beam steers ±30° from boresight, though the array is capable of steering up to ±60° from boresight. “Once you get beyond 60 degrees from boresight,” said Minard, “excessive scan loss occurs making the antenna gain drop below acceptable levels.” The screen image in Figure 5 shows all of the side lobes of the transmitted signal for the antenna array.
You can learn more about 5G beam steering antennas from this video by Keysight Technologies. The video shows you how to model transmission channels.
—Martin Rowe covers test and measurement for EDN and EE Times. Contact him at martin.rowe@AspenCore.com