MIMO Antenna Design and PCB Layout Tips
Multiple-input multiple-output (MIMO) has become a more popular term now that 5G is becoming more publicized, but this term and the technology have been around for awhile. MIMO can be traced all the way back to research papers from the 1970s, and significant development was required before the technology could be commercialized. Recently, the drastic increase in wireless services direct to consumers and in offices has been enabled by MIMO.
If you’re designing RF products to support telecom or networking infrastructure, then there’s a chance you’ll need to design your product to support MIMO. Part of this is a component selection task as you need to select a set of baseband transceiver/conversion ICs to support MIMO. The other part of this is a layout task in order to support multiple antennas required in MIMO.
What is MIMO?
Multiple-input multiple-output (MIMO) supports the use of multiple data streams being sent between a transmitting device and a receiving device. When two MIMO-capable devices are connected, multiple data streams can be transferred between them in parallel within the same channel. This effectively increases throughput without taking up additional frequency bands.
Most smartphones support 4x4 cellular MIMO (4 Tx and Rx antennas at each end of the link), meaning 4 channels can be used to transmit and receive data. New research from the past few months has looked at the use of MIMO in handsets with 8 antennas, which may have multiple resonant frequencies. Take a look at this MDPI article for an interesting wideband 8x8 MIMO design, and this IEEE article for 4 dual-band MIMO antenna design.
There are different flavors of MIMO, which basically refers to the number of users receiving data from a MIMO transmitter. Single-user MIMO (SU-MIMO) and multi-user MIMO (MU-MIMO) are as their names describe, where single or multiple users take advantage of available MIMO resources to receive data. 5G takes MIMO to a new level, with base stations employing massive MIMO to serve a huge number of subscribers and smart devices. Current towers simply can’t fit the required number of antennas to support massive MIMO, which is one reason tower counts for 5G networks are projected to reach ~10 million.
MIMO Antenna Design in Your PCB
Designing your PCB to support MIMO is all about incorporating multiple antennas into your board. Whether you’re designing a WiFi access point or a new smartphone, there are a few basic tasks to consider when designing multiple antennas into your PCB. These are:
The antennas used in the device need to be designed to accommodate the desired frequency and bandwidth. Opting for a slot or loop antenna can provide higher bandwidth, possibly allowing compatibility with a more diverse set of data streams. Omnidirectional antennas can be used, and beamforming can still be achieved if needed by operating the group as a phased array.
Many of the current experimental MIMO antenna design options for handsets/IoT products are printed directly on the PCB. Examples include loop antennas, patch antennas, center-fed patch antennas, inverted-F antennas, or any other design that provides high gain/efficiency. For larger systems, multiple feedlines will need to be sent into an external radome to connect to antennas.
Some method is needed to provide antenna diversity. These methods include beamforming, polarization coding, or spatial multiplexing. Beamforming follows the typical procedure with phased array layout and an RF antenna switch (either inside the RF transceiver or as its own IC). Polarization coding is a robust method that is easy to implement by simply rotating the antenna. Finally, spatial multiplexing is like beamforming on steroids; it’s more complex but it can maximize data throughput for a single user.
The antennas need to be properly placed, routed, impedance matched, and grounded. This is the primary domain of the PCB designer and depends on things like stackup, component selection/placement, grounding strategy, and routing. Antennas are normally placed at the edge of the board to separate them from digital components by as much as possible. Placement will also determine the grounding strategy, which should be designed such that digital return paths do not run near analog components.
Antennas in a MIMO-capable system need to be isolated from each other as well as other circuit blocks. The typical design goal is at least 20 dB isolation between antenna lines (defined as insertion loss between two antenna lines).
There are a number of ways to do this. The poor man’s method is to place shielding cans; some radio baseband ICs already reside under some shielding to suppress radiated noise from digital circuits or other analog components. A more sophisticated method beyond guard vias/traces is the use of electromagnetic bandgap structures, which are desirable for higher frequency systems. Coplanar waveguide or stripline routing are also preferred.
Crosstalk is the major factor here as you do not want to have the signal in one antenna line corrupted by a signal in a neighboring line, especially because these analog signals are not in-phase. This underscores the need for isolation between antenna lines in your MIMO-capable PCB.
No matter what you need to design, the integrated 3D field solver from Simberian in Altium Designer® can help you determine impedance and propagation delay in your PCB layout for MIMO antenna design. Simulation tools are also available to account for signal integrity effects in your board.
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