Figure 1 (a) Omnidirectional mmWave front-end module in plastic housing. (b) Omnidirectional mmWave front-end module hardware.
5G mmWave communication technology is supported in many commercial mobile phones today. Despite broad commercial adoption, the 5G mmWave and 6G frequency bands show a large path loss. In addition, the signal is strongly attenuated when propagating from an outdoor to an indoor environment. To compensate for this and to enable high network capacity requires a small cell size. These cells need to be omnidirectionally covered with sufficient signal strength. All these requirements suggest the use of phased antenna array concepts along with conformal antenna arrays. Based on these indications, IMST has developed a scalable omnidirectional active front-end module called the 5G CAN. The 5G CAN module uses printed circuit board (PCB) technology, includes RF and DC circuitry, incorporates a milled metal block as an antenna aperture and includes a heat sink for the MMICs. The antenna module is intended to be used as an evaluation kit to support the development of 5G/6G communication, along with investigating signal processing and propagation scenarios. As a second use case, the flexible and scalable concept can be used as a basis for customer-specific development. The concept delivers an excellent solution for a small 5G base station unit that might be used in applications like streetlamps or an indoor environment like an exhibition show floor.
THE 5G CAN FRONT-END MODULE CONCEPT
The omnidirectional RF front-end module covers frequencies from 24.25 to 29.5 GHz and it supports horizontally and vertically polarized RF signals. All the antenna beams can be controlled directly via a USB interface and the device operates from a 12 V, 120 W DC source. The antenna module has a cylindrical shape to provide full omnidirectional coverage. The front-end module integrated into a plastic housing is shown in Figure 1 (a) with the underlying module hardware shown in Figure 1 (b).
Even though many front-end modules are currently designed in a tile architecture1 and achieve a scanning capability of up to ± 60 degrees, this module is designed in a brick architecture. This brick design is based on stacking PCBs and milled metal plates as shown in Figure 2. The big advantages of this concept are the scalability and the possibility of integrating a very efficient cooling concept. Thermal management is a major aspect in highly integrated phased antenna arrays and this barrier needs to be considered in the very early stages of the design.
In the proposed antenna module, the brick design is based on four circularly shaped multilayer PCBs that include horizontal and vertical antenna apertures, active circuits and distribution networks. The antenna PCBs, which include beamforming ICs, are combined on an RF backplane PCB that is integrated into the middle of the circularly shaped antenna structure. The DC, control and RF interfaces are integrated at the top of the antenna module. For control and DC circuits, an additional PCB is attached to the top metal plate. DC voltages are converted from 12 V to about 2 V for the internal chip power supply and a microcontroller is used for SPI control. SPI and DC signals are directly connected to the RF PCBs.
Figure 2 mmWave 5G/6G front-end module brick architecture showing antenna aperture arrangement.
Figure 3 (a) Antenna array PCB bottom side. (b) Antenna array beam configuration.
The two RF input signals (H- and V-pol) are distributed on each antenna PCB to six Anokiwave beamforming ICs. To improve the isolation between different channels, horizontally and vertically polarized signals are routed on the top and bottom layers, respectively. Each beamforming IC generates four horizontally and four vertically polarized output signals, which can be independently controlled in amplitude and phase. These eight output signals feed the radiators that are realized on the edges of the PCBs.
Five milled metal plates are stacked between the PCBs and these operate as heat spreaders, shielding structures and antenna apertures. To reduce weight, these metal plates are designed with cavities in areas that are not used for heat dissipation. The heat sinking capability of the aluminum parts is enhanced by creating an airflow between the central hole in the metal disc and the outer surface of the disc where the antennas are located. The antenna array is designed in a sparse configuration to allow for a high integration density and excellent RF performance.
This concept scales easily by adding more boards and metal plates to the stack, along with modifying the control and DC boards accordingly. To increase the effective isotropic radiated power (EIRP), the antenna PCB’s diameter and the number of elements can be modified. Upscaling the frequency to 39 GHz (5G mmWave band n260) is also supported by this concept.
RF ANTENNA ARRAY CONFIGURATION
The array concept is presented in detail in this section. The single antenna PCB is shown in Figure 3 (a). It has a diameter of 93 mm and it contains six beamforming chips. Each of these beamforming chips feeds four dual-polarized antenna elements. This results in a total of 24 vertically polarized and 24 horizontally polarized antennas. The radiators are arranged at the edge of the PCB with an angle of 15 degrees. In the 5G CAN module, four PCBs are stacked up with an azimuthal rotation of 7.5 degrees. This creates the sparse array configuration shown in Figure 3 (b).
The full omnidirectional radiation characteristic is achieved in the horizontal plane, parallel to the stacked PCBs. This is done by switching from one beam to the next. The 3 dB beamwidth of this configuration is 8.5 degrees. Spatially adjacent beams overlap with a maximum signal reduction of about 2 dB between beams, which seems feasible for most applications. Depending on the application, it might also be feasible to increase or decrease the number of radiators used to generate one beam. Users can create flexible configurations in this concept by adapting the control software. A beam scanning in the vertical direction is realized by adapting the feeding phase of the elements with the beamforming ICs, similar to the techniques used in phased arrays. The configuration described here achieves an elevation scan range of ±30 degrees. Larger scan ranges are possible by reducing the height of the metal plates of the single elements in the vertical direction. The specific configuration of the antenna module allows for four beams to be individually configured and scanned in the vertical plane at the same time. The only limitation is that the beams cannot overlap.
Single Antenna Radiator Design
This section describes the design of the antenna module in detail. The different building blocks are shown in Figure 4. The vertically polarized antenna is realized as a substrate-integrated waveguide (SIW) antenna. The horizontally polarized antenna is a partially filled waveguide built into the milled metal plates placed on the top and bottom of the PCB. Figure 5 shows a single radiator in detail. Both polarizations are integrated into one aperture. Due to the 90-degree orthogonal arrangement, the coupling between both polarizations can be kept below -28 dB in the operating frequency range.
Figure 4 Building blocks of the 5G CAN front-end.
Figure 5 Single radiator concept.
The vertically polarized antenna and the horizontally polarized antenna are fed with a microstrip line and with a stripline, respectively. To minimize the coupling between the polarizations and in the feeding lines, these two feeding lines are routed in different layers. The antenna matching and decoupling performance, including the feeding network to the chip, are shown in Figure 6 (a) for the air mode and in Figure 6 (b) for the SIW mode.
All vertically polarized antenna elements show broadband matching (solid lines) of more than 10 dB for a frequency range from 23 to 29.5 GHz. Isolation to all other IC ports (dashed lines) is greater than 26 dB from 24 to 29.5 GHz. Similarly, all horizontally polarized antenna elements show broadband matching (solid lines) of more than 8 dB for a frequency range from 23 to 29.5 GHz and greater than 13 dB from 24 to 29 GHz. Isolation to all other IC ports (dashed lines) is greater than 23 dB from 23 to 29.5 GHz.
