The latest cellular generation, the 5G, is the ultimate low latency, high data rate network solution but can still suffer from poor coverage unless you use two 5G modules. In this blog, we explore why some decide to use two 5G sub-6/FR1 modems instead of only one and what that means for product development, especially for antenna and RF.

5G sub-6 FR1

5G is split into two distinct frequency ranges, FR1 and FR2. The first covers frequencies below 6GHz and the latter refers to frequencies above 24GHz. Here we are going to focus only on FR1 which also covers the frequency bands of previous-generation LTE networks. There are many differences between LTE and 5G. However, the major differences from a hardware implementation point of view are added frequencies and the number of antennas.

With LTE typical solution only has 2 antennas. One Tx/Rx antenna, the one that transmits and receives a signal, and the Rx antenna that only receives a signal. The highest LTE categories do support more antennas but those 4 antenna solutions are mainly used in high data rate applications like smartphones. In the most complex designs, different frequency bands might benefit from their own antennas. This is why you might encounter antennas for low, mid, or high bands and therefore more LTE antennas.

Why use two 5G modules?

To understand why to use two 5G modems let’s first explore the limitations of cellular networks. Network carrier-driven networks are reliable because there is a business reason to keep them operational but the business-driven nature has its downside, the coverage. There are areas where you don’t have coverage for your selected carrier because there are insufficient users. This can be a geographical area or special conditions near the outskirts of a network cell like a basement. Another factor is the bandwidth available. Network carriers are driven to optimize the available bandwidth to serve the users but excess capacity is not profitable. Therefore, capacity might not be enough to support all in all situations when the amount of users varies.

When your use case relies on uninterrupted connection, poor coverage or lack of capacity can cause extreme havoc. One such example is drones controlled over cellular networks. You benefit from the low latency of 5G and you can control the drone where ever you are. However, what happens when you lose the connection? Nothing good. With two modems you can connect to two different networks minimizing the probability of losing a connection. You can also benefit from two data connections for double throughput.

HW Integration of dual modem solution

Hardware development becomes more complex with two modems but why exactly? We are going to skip the obvious module-related layout complexity and interface size topics. Instead, we focus on the RF and antenna parts. The two modems need to be active at the same time in order to get the benefits. This means that criteria for isolation between the antennas become more demanding. It is the isolation of Tx antennas from all other antennas that matters the most, not Rx to Rx. A good rule of thumb is that if 12dB isolations were found acceptable between antennas of a single modem you should aim for at least 15dB isolation. This is important when you have coupling between antennas of two different modems.

The number of cellular antennas has now also doubled from 4 to 8 as you introduced the second module. It means more space is needed for the antennas if you aim for the same antenna performance as with a single modem. However, your device will likely be a bit bigger due to the added modems (which are not very small) and with intelligent antenna design you can squeeze the size when you can. So it is not that big of a deal. Where those antennas should be placed is a big deal.

An example of antenna placement for dual 5G modem device

When you have a rectangular device, typically lowest frequency antennas should be placed at the corners of the device. This is to benefit from the best resonance modes of the PCB. However, using the same resonance modes will also increase the coupling between the antennas. With two antennas, that is very easy to manage, and severe coupling between antennas can be avoided with smart antenna placement. That is where the problem lies. You now have at least 4 antennas for low bands with two 5G modules. If you place one for each corner, you no longer have flexibility in antenna placement to avoid coupling. This is when you need to find all the tricks from the sleeves of antenna designers to avoid coupling between the antennas. The remaining 4 antennas can be placed more freely because higher-frequency antennas won’t need as much volume for the same performance. The compact placement of several antennas can also lead to a high correlation between antennas(ECC). This can become an issue between the antenna of a single module.

Antenna placement example for device with two 5G modules

The challenges in antenna design are not anything new for us. We at Radientum solve those every day for our customers. When you are about to embark on a product development journey with two 5G modules, we are here to help you to take the right steps and ensure that the ride is as smooth as possible.

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Disclaimer: The views and opinions expressed in this article are those of the author. It is intended only as a sharing of antenna design knowledge for educational purposes.

Mikko ParkkilaCMO & Co-Founder

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