Do more antennas increase a cellular device’s performance? Yes — but with a few important considerations.
With 2G and early 3G, phones and other cellular devices used a single antenna for transmitting and receiving. As 3G technology matured, devices began adding a second antenna strictly for receiving. This “diversity” antenna increased throughput and reliability because reception no longer depended on a single antenna.
Here’s where the first caveat comes in. The device must be designed so the diversity antenna is oriented in a way that’s substantially different from the primary transmit/receive antenna’s position. In a handset, for example, the 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 separation and spatial diversity.
This orthogonal relationship provides two major benefits:
- It ensures that when the primary antenna is transmitting, its radiation pattern doesn’t cause its signal to interfere with the diversity antenna’s reception.
- The received signals are coming from different directions. So, if one signal is weak, the other one might be strong enough to provide a good user experience, such as a call that doesn’t drop or a download that doesn’t slow to a crawl.
Isolation Minimizes Interference
With 4G LTE, antenna diversity became standard. Smartphones, tablets, and Internet of Things (IoT) devices began using multiple input single output (MISO) architectures, where both antennas can transmit and receive. This design improves data speeds and link reliability. Eventually LTE devices began migrating to multiple input multiple output (MIMO) antennas, which also are the norm for 5G.
MIMO put a spotlight on the concept of isolation, which measures how much signal is being transferred from one antenna to another. This sometimes is referred to as coupling.
The envelope correlation coefficient (ECC) is the measure of the interference level between the MIMO antennas due to the similarity of their radiation patterns. It’s calculated in a lab environment using a vector network analyzer (VNA) in an anechoic chamber. (Most device OEMs don’t have this specialized equipment and expertise, so they outsource these types of tests to Taoglas.)
During these tests, isolation and ECC are measured at multiple frequencies, each of which has an ideal limit. By optimizing these two parameters, the device’s RF system throughput will be as high as possible because the two antennas are not interfering with each other and because they are working together to maximize reception from multiple directions.
Isolation, ECC, and other aspects of spatial diversity highlight the importance of considering antennas early on in the device design process. Minimizing the antennas’ interaction with each other through spatial separation and spatial diversity maximizes throughput and reliability by enabling the device to take full advantage of MIMO. 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.
That almost inevitably leads to problems such as performance not meeting customers’ performance requirements and/or failing carrier certification. When that happens, the device might have to be redesigned, or a custom antenna developed, both of which can delay the product’s time to market and time to revenue.
