March 26, 2024
blog
We have been living in a 5G world for several years now. Although the latest cellular generations have reduced latency and increased data speeds, some challenges remain, including the potential for poor coverage.
Let’s explore a solution to this coverage problem. This may seem simple, but it involves using two 5G sub-6/FR1 modems instead of one. While it sounds like a simple solution, seamlessly incorporating these dual 5G modems into product development requires a careful approach. This is not as simple as just adding another module next to the first one.
Essentially, 5G (fifth generation cellular networks) operates in two frequency ranges: FR1 (below 6GHz) and FR2 (above 24GHz). FR1 also covers the frequency bands of previous generation LTE networks. There are many differences between LTE and 5G, but when it comes to hardware implementation, only the added frequencies and number of antennas need to be considered.
A typical LTE solution has two antennas: a Tx/Rx antenna and an Rx antenna. The first one is used to both send and receive signals, while Rx only receives signals. Some high data rate applications, such as smartphones, may use four antenna solutions for different frequency bands.
Why two 5G modules?
The inherent limitations of cellular networks are bandwidth and coverage. When network operators optimize available bandwidth, they aim to minimize excess capacity. This is fine as long as the number of users in the area remains constant. However, as the number of users increases, you may reach capacity limits. Conversely, in areas with an insufficient user base, network carriers may choose not to provide coverage in the first place.
If your device relies on uninterrupted connectivity, this limitation may cause some, if not the entire device, to stop working. Imagine a drone controlled via a cellular network. What happens when the drone starts losing connection? The answer to this problem is two modems. The strategic decision to include a separate modem allows for simultaneous connections to two different networks. In addition to improving connection reliability, two data connections also double your data throughput.
Dual modem hardware integration example
To keep things manageable, we’ll omit most of the hardware development issues in this topic and focus only on the RF and antenna parts.
When using multiple active antennas, the challenge is to separate the two modules by enough dB. Rx antennas are more forgiving, but Tx antennas must be properly isolated. For two modems with coupling between them, the minimum acceptable separation is 15dB. The introduction of the second module doubles the number of cellular antennas. This increase requires more space, but intelligent antenna design can reduce the impact on device size. However, trying to place eight antennas can still prove to be a complex challenge.
The additional space required isn’t the only issue. To take advantage of the best PCB resonance modes, low frequency antennas are typically placed at the corners of rectangular devices. Unfortunately, using the same resonant mode also increases the coupling between the antennas. Combining these two facts results in two modules that combine at least four different low-frequency antennas. You can also place one in each corner, but that gives you less flexibility in antenna placement and avoids coupling issues.
Fortunately, high-frequency antennas do not require as much volume to achieve the same performance, making them easier to install. However, this compact antenna arrangement can be problematic due to the higher correlation between antennas within the same module. This is also known as the envelope correlation coefficient (ECC).
Sophisticated solutions from experts
As you can see from this example, a simple solution may not be so easy to implement. Professional antenna designers like Radientum are aware of these issues (and many others) and can create customized antenna solutions for each device to avoid them.
Mikko Parkkila is CMO and co-founder of Radientum. Parkkila started his career as an antenna engineer for Nokia and his Microsoft, designing antennas and systems for smartphones, including multiband LTE MIMO antennas. In 2015, Parkkila co-founded Radientum to provide simulation-driven custom design services for wireless products.
Other works by Mikko
