You can thank the telecom community for making MIMO and beamforming new buzzwords in the technical lexicon. These two concepts are not so well understood and are intimately related. Beamforming is the key to increasing range in 5G and WiFi 6/6E, as well as providing multi-user access with a single antenna array.
There are different types of beamforming that are used in RF systems, which then relate back to the way in which antennas are controlled as the system operates. All types of beamforming in RF PCBs rely on the use of an antenna array to steer beams in the desired direction and provide half duplex communication with end users. There is also sophisticated signal processing algorithms implemented at the send and receive ends of a wireless link to ensure connections with users are not lost during communication.
If you want to add beamforming to your system, you’ll need success in three areas: DSP, antenna design and layout, and component selection to minimize system size. I’ll touch on these last two areas as DSP for beamforming and MIMO systems generally is extensive enough to warrant its own textbook.
Depending on who you ask, you’ll be told there are two types of beamforming. These are either analog or digital, or switched and adaptive beamforming. Anything classified into one of these areas then gets sub-classified into different categories depending on the type of antenna array used (linear, circular, rectangular, etc.). These different types of beamforming techniques find applications in scanning sonar, radar (see the large Duga-3 phased array above), WiFi, and soon MU-MIMO in 5G.
The poor-man’s way of doing beamforming is to simply switch the transmitted signal between different directional antennas with an RF switch. This is not done in modern systems, which now use adaptive beamforming with a phased array antenna. A phased array is simply a group of omnidirectional antennas spaced at a regular interval. The direction of the output beam can be tightly controlled within a limited range by controlling the phase difference between signals arriving at different antennas. The radiation from each antenna interferes to produce a lobe with high directionality. Similarly, by tracking the phase difference between signals received in the array, the controller can determine the direction of the transmitter. Systems with MU-MIMO may use sub-arrays to carefully track users or groups of users that are interacting with a given array.
Adaptive beamforming requires delaying the Tx signal sent to an antenna array. The required delay is a simple function of the emission angle from the antenna.
The difference between analog and digital beamforming simply refer to the design of the RF front end that interfaces with the antenna array. Modern systems involve some fusion of analog and digital components, which is referred to as hybrid beamforming. In other words, it really depends on where you put the ADC/DAC sections.
The principle factor that distinguishes digital and analog is the way in which the phase delay is applied to the signal sent to each feedline. For duplexed MIMO systems, digital beamforming is arguably the best choice to reduce component count and more precisely control the phase delay between feedlines. Analog beamforming involves use of some analog phase delay line, while in digital beamforming, the delay is applied by simply delaying the signal output from the transceiver.
Analog vs digital beamforming.
Although the beamforming community likes to try and distinguish between analog and digital methods, the two types of beamforming contain an analog section in the RF front end, which needs to be carefully designed to prevent signal degradation. The primary factors to consider are isolation between analog feedlines and the digital section in the system, which then relate to PCB stackup design and antenna placement. I’ve touched on PCB stackup design and its relation to signal integrity previously, so I’ll focus on the other points below.
The antenna feedlines in the phased array need to be isolated from each other to suppress analog crosstalk. The simplest way is to put down guard vias along surface feedlines or route in different layers, the latter of which becomes a bad strategy in the high GHz range. Ideally, routing everything on the same layer provides the most precise phase control between different beams, so you need to use elements on the surface layer to isolate antennas. Many designs use electronic bandgap structure, coplanar waveguide routing (car radar modules are a perfect example), or substrate integrated waveguide routing to provide the needed isolation (20 dB minimum is desirable).
Isolation between the analog and digital sections is also important. Shielding cans are too bulky for modern mobile devices, although they may be fine in something like a drone. In addition to gridding with ground pour (such as in PCBs for modern cell phones), antenna placement becomes critical here.
The antenna array needs to be placed away from the digital section, preferably on its own board with the transceiver/baseband chip (e.g., in radar modules), or at the edge of the board (typical in WiFi routers). This will also determine the right grounding strategy, which should be designed such that analog and digital sections do not interfere. If you’re very careful, you could take a pseudo-star ground approach with multiple ground plane layers to provide clear return paths.
Ultra-wideband 5G antenna design for MIMO in 5G. These 8 elements collectively provide beamforming as part of a larger array. Source: MDPI.
No matter which type of beamforming you want to use in your next system, getting your design right takes some expertise to maximize power output, ensure isolation, and minimize power consumption in your RF system. Working with the right design firm is the place to start when you want to design an advanced RF system to operate at high frequencies.
At NWES, we’re familiar with the types of beamforming used in modern RF systems, and we’re here to help innovators in private industry and government build the most advanced systems to solve the world’s toughest technical challenges. We’re here to help companies in the electronics industry design high speed, high frequency, and high density PCBs for a variety of applications, including quantum computing hardware. We've also partnered directly with EDA software companies and advanced PCB manufacturers, and we'll make sure your next layout is fully manufacturable at scale. Contact NWES today for a consultation.