Antennas, antennas, and more antennas are one of the keys to successfully developing 5G new radio (NR) devices employing millimeter wave (mmWave) bandwidths. 5G NR handsets are highly complex devices. They have to handle the new 5G mmWave bands, including 4×4 multiple input and multiple output (MIMO) antennas, 3GPP specified Evolved Universal Terrestrial Radio Access (EUTRA) dual connectivity, and emerging standards such as UWB. And they still must support all the existing demands of 4G connectivity as well as Bluetooth, WiFi, and GNSS. As discussed in the previous FAQ on “What is the 5G RF/mmWave signal chain?” the circuit board space available for the RF signal chain in a 5G NR handset is about 35 percent less than in an LTE Advanced handset, mostly as a result of the need for larger batteries and added features in the 5G designs.
Base stations face the same dilemma of a growing need for more and more antennas, while the space available is not keeping pace. Due to the high propagation loss of the mmWaves employed in 5G NR base stations and handsets, plus the high bandwidth demands of users, beamforming, beam switching, multiplexing, and beam steering techniques in addition to MIMO are critical for increasing spectral efficiencies and providing cost-effective, reliable coverage.
Active antenna systems and massive MIMO
Unlike conventional antennas that transmit and receive based only on fixed radiation patterns, active antenna systems, and beamforming antennas dynamically shape their main and null beam directions according to the location of their connected users. Beamforming antenna systems can reduce interference, improve the signal-to-interference-and-noise ratio (SINR), and deliver a significantly better system performance.
Beamforming is applying multiple radiating elements transmitting the same signal at an identical wavelength and phase, which combine to create a single antenna with a longer, more targeted stream formed by reinforcing the waves in a specific direction. The more radiating elements that make up the antenna, the narrower the beam. An artifact of beamforming is side lobes. These are essentially unwanted radiation of the signal that forms the main lobe in different directions. The more radiating elements that make up the antenna, the more focused the main beam is and the weaker the side lobes are. Beamforming is typically implemented using both amplitude and phase components to improve sidelobe suppression.
Beamforming can be implemented with digital or analog techniques. With digital beamforming (also called baseband beamforming or precoding), the signal is precoded with the amplitude and phase modifications in baseband processing before RF transmission. Multiple beams (one per each user) can be formed simultaneously from the same set of antenna elements.
With analog beamforming, the signal phases of individual antenna signals are adjusted in the RF domain. Analog beamforming impacts the radiation pattern and gain of the antenna array, improving coverage. Unlike in digital beamforming, only one beam per set of antenna elements can be formed.
In general, digital beamforming can deliver higher performance. But that higher performance comes at the cost of more complex and costly hardware and higher energy consumption. Digital beamforming is more suited to base stations since performance is more important than mobility or efficiency. Analog beamforming is easier to integrate into mobile devices and delivers higher efficiency.
Beam steering is achieved by changing the phase of the input signal on all radiating elements. Phase-shifting allows the signal to be targeted at a specific receiver. An antenna array can employ radiating elements with a common frequency to steer a single beam in a specific direction. Different frequency beams can also be steered in different directions to serve different users. The direction a signal is sent in is calculated dynamically by the base station as the endpoint moves, effectively tracking the user. If a beam cannot track a user, the endpoint may switch to a different beam. This granular degree of tracking is made possible because 5G base stations are significantly closer to users than previous generations of mobile infrastructures.
Massive MIMO and MU-MINO
Massive MIMO uses many base station antennas to communicate with multiple users, using beamforming, beam steering, and beam switching techniques in phased adaptive array technology. Massive MIMO improves capacity without the increase in design complexity of intercell coordination. Using massive MIMO, it’s possible to form beams such that there is almost always only a single user in each beam. Thus, giving each user their interference-free, high-capacity link to the base station.
Massive MIMO technology uses large antenna arrays (typically comprising 16, 32, or 64 array elements) to exploit spatial multiplexing. Spatial multiplexing delivers multiple parallel streams of data within the same resource block. Expanding the total number of virtual channels increases capacity and data rates without additional towers and spectrum.
In high-density urban settings, MIMO technology can also allow multiple users to share the same network resources simultaneously. Multi-User MIMO (MU-MIMO) allows messages for different users to travel securely along the same data pipelines, then be sorted to individual users when the data arrives at their mobile devices. Serving multiple users with the same transmission increases capacity and allows for better utilization of resources. That results in the ability to download or stream with high bandwidth, even in crowded areas.
Selected mmWave antenna development activities
There are many efforts around the world to improve mmWave antennas’ performance for 5G. The following are two examples:
Issues like path loss, rain absorption, conduction losses in metals, substrate losses, and changes in substrate properties are well-known contributors to antenna losses. There are challenges in choosing an appropriate substrate and challenges in its fabrication, but overcoming these challenges can be key to successfully reducing losses.
One variable to consider when selecting a substrate is the substrate’s dissipation loss. Though conductor losses are dominant at low frequencies, at frequencies above 10GHz, dielectric loss becomes the dominating factor contributing to the board’s overall loss. A substrate with a low loss tangent becomes very desirable at mmWave frequencies.
Recently, liquid crystal polymer (LCP) (Dk=3, Df=0.0016) has gained attention as a mmWave substrate solution. Characteristics like low coefficient of thermal expansion, flexibility, low water absorption, and low dissipation loss make LCP a promising candidate for a mmWave antenna substrate.
In addition to looking for better substrates and other materials to use in antenna fabrication, there are efforts to develop completely new fabrication and integration technologies. Looking at the antennas and other components in mmWave devices, by using metasurfaces, an innovative hardware integration technology, so-called Multi-layer waveguide (MLW), may be able to provide the desired features for an optimum hardware technology: high performance, simple integrability, cost-effectiveness, and mass production capability. An improvement over current solutions is needed for two primary reasons:
- First, mmWave front-end modules cost more than the ones operating at lower frequency bands. One factor is that critical passive components, such as filters and antennas, shrink in size and require high precision manufacturing and assembly, which is expensive and slows down the development cycle of new products.
- Secondly, the hardware integration of mmWave front-end subsystems requires low-loss and cost-efficient interconnect and packaging solutions to minimize the loss of the signal power.
MLW technology is a novel, cost-effective air-filled waveguide for mmWave applications, which is made by stacking thin but unconnected metal layers. Since there is an air gap between the layers, the electromagnetic field may leak and cause unwanted losses. By applying an Electromagnetic Bandgap (EBG) structure (a type of metasurface) created by means of periodic through holes allocated in a glide-symmetric configuration, the expected field leakage among the unconnected layers is suppressed.
Various groups are investigating MLW with alternative emerging waveguide (WG) technologies: Gap waveguide, 3D printed waveguide, micromachined waveguide, and substrate integrated waveguide (SIW). At this time, it appears that the MLW technology may provide a balanced tradeoff among the three key parameters.
That concludes this three-part FAQ on mmWaves. Part one was a high-level view of the “Basics of mmWave and its applications.” The second FAQ was a deeper dive into one specific emerging application for mmWaves and considered, “What is the 5G RF/mmWave signal chain?” And this concluding FAQ reviewed various aspects of “mmWave antennas and antenna management for 5G.”
References:
Making waves: A new method for mmWave antennas and components, Ericsson
Millimeter-Wave Beamforming: Antenna Array Design Choices & Characterization, Rohde & Schwarz
The next big thing – advances in 5G millimeter wave antennas, Benchmark
Through the 5G Antenna Design Maze with Antenna-plexers, Qorvo
What is 5G beamforming, beam steering and beam switching with massive MIMO, Metaswitch Networks
