Multiple input multiple output (MIMO) antenna systems boost application performance and reliability in 4G, 5G, and Wi-Fi. That means device OEMs can leverage MIMO to differentiate on performance rather than price and help meet demanding customer requirements, such as mission-critical enterprise applications and first responder communications.
But to take full advantage of MIMO, device OEMs need to understand how spatial diversity affects antenna integration. They also need to understand how mobile operator requirements determine which type of MIMO system to use and how that affects their device’s ability to pass carrier certification.
First, some history: With 2G and early 3G, cellular devices used a single antenna for transmitting and receiving, an architecture known as single input single output (SISO). As 3G technology matured, devices added a second antenna devoted to receiving. This second antenna increased throughput and reliability because reception no longer depended on a single antenna. Instead, the two antennas received signals from different directions. If the signal coming from one direction was weak, there was a chance the signal from the other direction was solid or at least usable.
LTE smartphones, tablets, and Internet of Things (IoT) devices initially used multiple input single output (MISO) architectures, where both antennas can transmit and receive. Over time, 4G devices migrated to MIMO, which also is standard for 5G devices.
MIMO systems often are described in terms of their “order.” The more antennas there are, the higher the order. For example, in a 2×2 order MIMO system, there are two antennas transmitting and two receiving, while a 4×4 order has four antennas transmitting and four receiving.
Modules, Bands, and Operator Requirements
When choosing a MIMO antenna, one factor is the transceiver module. It doesn’t make sense to choose a MIMO antenna system with a higher order than the module can support.
Two other factors are the bands that the device will use and the mobile operators whose networks it will use. For example, AT&T requires 5G devices to support 4×4 MIMO by default and won’t allow single-antenna implementations for NR FR1.
MIMO affects device performance, which the carrier certification process measures in terms of total radiated power (TRP), total isotropic sensitivity (TIS), and other metrics. This is another reason why it’s important to consider operator requirements when choosing a MIMO system. (For more information about TRP, TIS, and the carrier certification process, see “Why Cellular Pre-Certification is Critical.”)
Tips for Successful MIMO Antenna Integration
Isolation measures how much signal is being transferred from one antenna to another in a MIMO system. Also known as coupling, isolation can be optimized so the antennas don’t interfere with one another and to ensure that they are working together to maximize reception from multiple directions.
Spatial diversity helps achieve isolation. The diversity antenna should oriented in a way that’s substantially different from the primary transmit/receive antenna’s position. For example, a handset’s diversity antenna typically is at the top and the primary antenna is at the bottom, with a 90-degree difference in orientation. This achieves both spatial diversity and spatial separation. (For a deeper dive, see “Understanding Antenna Spatial Diversity and How It Affects Device Performance.”)
Spatial diversity and separation also are examples of why it’s critical to start considering antennas early on in the device design process. Waiting until the device’s form factor and PCB layout have been finalized runs the risk of having to put the MIMO antennas in less-than-ideal locations. It also can result in an extensive, expensive redesign, or a custom antenna, if the initial design doesn’t meet performance and operator requirements.
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