Antenna

The demonstrator antenna is an 8x8 slot array organized as 4-slot subarrays in an eight column, two row configuration (see Figure 7). As the subarray is periodic, the antenna can be extended in both horizontal and vertical directions. The λ/2 spacing and vertical grouping enable scanning without grating lobes beyond 45 degrees in azimuth and up to 10 degrees in elevation. The demonstrator antenna has a single polarization. Both vertical and horizontal versions of the antenna have been developed and tested, although this article only presents results from the single polarization antenna.

The subarray is a series, end-fed resonant slotted waveguide. The antenna covers the band from approximately 26.5 to 31 GHz, greater than 15 percent relative bandwidth. The measured and simulated array patterns show very good agreement (see Figure 8), with no notches in the elevation patterns. The antenna has a total gain of 24 dBi, 12 dBi per subarray per channel. Embedded matching is better than 20 dB, isolation better than 16 dB and active matching better than 10 dB over all scan angles.

 

Figure 8 Gapwaves array antenna patterns: azimuth simulated (a) and measured (b), elevation simulated (c) and measured (d).

Thermal Design

Thermal handling is challenging at mmWave frequencies, where the many densely packed components create substantial heat in a limited area. High thermal handling is critical to ensure semiconductor and other components operate within their specified operating temperature ranges, assuring reliability and optimum performance. A SiGe IC solution for a mmWave, dual polarized array, for example, dissipates several W/cm². The thermal situation is complicated by the desire for passive cooling, which is challenging for a system dissipating more than a few hundred Watts. For even moderate heat dissipation, PCB designs rely on using thermal vias.

A Gapwaves waveguide system has excellent thermal capabilities. The all-metal antenna assembly doubles as an integrated heatsink, extracting heat from both the top and bottom sides of the components. Back-of-the-envelope calculations show that the antenna assembly can extract up to 2 W/°C from top-sided cooling of a typical 5 mm x 5 mm component area. The subarray architecture also provides benefits: for a fixed EIRP, it reduces the required power and decreases component density.

Measurements of the demonstration platform confirmed the effectiveness of the design using top-side cooling of the SiGe ICs. The steady state temperature measured by the on-chip sensors at maximum output power was only 60°C, when maximum junction temperatures are typically around 150°C.

ARRAY PERFORMANCE

Figure 9 Analog beamformer board.

The active part of the demonstration platform consists of an analog beamforming board using commercially available components (see Figure 9). The 16 antenna ports are fed by four SiGe ICs, each with four Tx/Rx channels and connected to a single input/output RF connector. A fifth IC on the board serves as an optional buffer and preamplifier. The SiGe Tx/Rx chains provide 17 dBm saturated output power per channel, backed off to 8 dBm per channel to ensure good linearity. The amplitude and phase of each port can be independently controlled digitally, enabling full control of the beam. The total power consumption is about 13 W. The ICs operate from 26.5 to 29.5 GHz, less than the bandwidth of the antenna.

Active measurements validate the system performance in both Tx and Rx. Only the Tx results are presented here, with the measured antenna beams shown in Figure 10. The beams are well behaved: scanning beyond the design requirements of ±45 degrees in azimuth and ±10 degrees in elevation and stable over frequency (see Figure 11). The measured EIRP is approximately 52 dBm at saturation, backed off to 44 dBm with an error vector magnitude (EVM) of approximately 3 percent (see Figure 12).

 

Figure 10 Gapwaves array azimuth (a) and elevation (b) beam steering using analog beamforming.

INCREASING EIRP

Figure 11 Azimuth antenna pattern vs. frequency, 26.5 to 29.5 GHz, no calibration.

Figure 12 Measured antenna EIRP.

To increase the EIRP of mmWave arrays, a logical step is to adopt GaN on SiC in the RF front-end. Originally developed for defense applications, GaN technology has matured and the cost is becoming competitive for commercial applications.2 Its high breakdown, electron mobility, power density and excellent thermal properties make the compound semiconductor attractive for mmWave front-ends. For Tx, GaN achieves a saturated power of 2 W with a power-added efficiency of approximately 10 percent. Backed off to an EVM of 3 percent, GaN can deliver an average output power of 24 dBm. GaN also achieves a noise figure about 1.5 dB better than SiGe, which significantly improves the uplink margin.

However, the high-power density of GaN requires considerable cooling capability in the antenna array, even though GaN’s maximum rated junction temperature is about 75 degrees higher than SiGe. The Gapwaves waveguide approach offers the thermal handling needed for GaN power amplifiers. Although Gapwaves is agnostic to the choice of semiconductor technology, combining the low loss, high gain waveguide array with the high-power and high efficiency of GaN is very attractive. The combination can reduce the number of components and power consumption for a given EIRP, reducing the cost of the array. A gap waveguide antenna using a GaN front-end is being developed for the 28 GHz band (see Figure 13). The subarray design contains eight slots in a single row, using analog beamforming with a GaN front-end module. Eight subarrays can be combined to form a 64-slot antenna with 24 dBi gain and 56 dBm EIRP at 9 dB back-off. The array will scan ±45 degrees in azimuth, and no elevation scan is planned. A total power consumption of about 40 W is expected.

SUMMARY

Figure 13 Analog beamformer using GaN front-end modules.

Gapwaves’ demonstration platform shows the performance capabilities of the Gapwaves waveguide technology for mmWave antenna arrays. The low loss and thermal advantages of waveguide; the ability to integrate antennas, filters, radio and baseband components; and a cost-effective, producible platform position this technology as a strong contender for 5G and other mmWave systems. While the platform is agnostic to RF semiconductor technology, its thermal performance is particularly beneficial for the high-power density of GaN.

References

1. International Telecommunications Union, ITU-R Radiocommunications Sector of ITU, “IMT Vision - Framework and Overall Objectives of the Future Development of IMT for 2020 and Beyond,” August 2015.
2. B. Peterson and D. Schnaufer, “5G Fixed Wireless Access Array and RF Front-End Trade-Offs,” Microwave Journal, Vol. 61, No. 2, February 2018, pp. 22–43.
3. Ericsson, “AIR 5121 Pre-NR Base Radio in FCC ID TA8AKRD901059-1,” fccid.io/TA8AKRD901059-1.
4. P. S. Kildal, E. Alfonso, A. Valero-Nogueira and E. Rajo-Iglesias, ”Local Metamaterial-Based Waveguides in Gaps Between Parallel Metal Plates,” IEEE Antennas and Wireless Propagation Letters, Vol. 8, 2009.
5. 3GPP Technical Specification Group Radio Access Network, “Study on New Radio Access Technology: RF and Co-Existence Aspects,” TR 38.803 V14.2.0, https://portal.3gpp.org/desktopmodules/Specifications/SpecificationDetails.aspx?specificationId=3069.