Doubling Down on 5G: Integration of Two 5G Modules in One Device

March 26, 2024

Blog

We have now been living in a 5G world for a few years. The latest cellular generation has lowered latencies and increased data rates, but some challenges remain, like the possibility of poor coverage.

Let’s explore a solution to this coverage problem, which might initially seem straightforward: use two 5G sub-6/FR1 modems instead of one! Despite sounding like a simple solution, seamlessly incorporating these dual 5G modems into product development demands a careful approach. It's not as straightforward as merely adding another module alongside the first one.

In essence, 5G – the fifth generation of cellular networks – operates in two frequency ranges, FR1 (below 6GHz) and FR2 (above 24GHz). FR1 also covers the frequency bands of the previous-generation LTE networks. While LTE and 5G have many differences, in terms of hardware implementation we only need to consider the added frequencies and the number of antennas.

A typical LTE solution has two antennas: a Tx/Rx antenna, and an Rx antenna. The first one is for both signal transmitting and receiving, while the Rx only receives signals. Some high data rate applications, such as smartphones, might use 4 antenna solutions for different frequency bands.

The Reason for Two 5G Modules

The inherent limitation of cellular networks resides in bandwidth and coverage. When network carriers are optimizing the available bandwidth, they aim to minimize excess capacity. This is fine as long as the number of users in an area stays constant. However, if the user count increases, the capacity could reach its limit. Conversely, in a region with an insufficient user base, a network carrier might opt not to provide coverage in the first place.

If a device relies on an uninterrupted connection, this limitation may cripple several features, if not the whole device. Imagine a drone, which is controlled over cellular networks: what happens when your drone starts losing connection? The answer to this problem is two modems. The strategic decision to include another modem allows simultaneous connections to two different networks. In addition to the improved reliability of the connection, you also double the data throughput with two data connections.

An Example of a Dual Modem Hardware Integration

To keep things manageable, we are going to skip most of the hardware development issues of this topic and focus only on the RF and antenna parts.

With multiple active antennas, the challenge comes from isolating the two modules with sufficient dB. The Rx antennas are more forgiving, but the Tx antennas need to be properly isolated: with two modems that have coupling between them, an acceptable minimum would be 15dB of isolation. With the introduction of the second module, the number of cellular antennas doubles. While this increase demands more space, intelligent antenna design can mitigate the impact on device size. However, trying to place 8 antennas can still prove to be an intricate challenge.

Required extra space is not the only problem. To benefit from the best PCB resonance modes, low-frequency antennas are typically placed at the corners of a rectangular device. Unfortunately, using the same resonance modes also increases coupling between the antennas. Combine these two facts, and you have 2 modules that combined have at least 4 different low-frequency antennas. You can still place one for each corner, but doing so will eliminate flexibility in antenna placement, which helps to avoid coupling issues.

Luckily higher-frequency antennas do not require as much volume for the same performance, so placing them will be easier. However, this compact antenna placement may become an issue due to a higher correlation between antennas within the same module. This is also known as the envelope correlation coefficient (ECC).

Refined Solutions from Experts

As this example has probably shown you, a simple solution might not be so simple to implement. Professional antenna designers like those at Radientum are aware of these problems (and many more) and can work around them, creating customized antenna solutions for each device.