Every now and then I get the question why virtually all network operators that want to offer 5G mobile service go for the non-standalone setup first that requires LTE as the anchor instead of going directly to a standalone setup. There are a number of reasons for this but I guess that spectrum availability is the main reason.
In practice, most network operators today use most of the low- and mid-band spectrum they have acquired in the past for LTE services. In this context, low-bands occupy the 700-900 MHz range and the 1700-1800 MHz spectrum would fall in the mid-band category. Everything above isn’t really suited to provide coverage but rather provides capacity at the center to the mid-range area of the cell. In other words, there is little spectrum left today to deploy 5G in most parts of the world to provide a coverage layer that would provide significantly higher speeds compared to today. Instead, the 3.5 GHz range has been opened for mobile services and this is where many operators choose to deploy 5G with a bandwidth of up to 100 MHz (compare to the maximum LTE carrier bandwidth of 20 MHz). In addition, some operators have chosen to launch 5G services in the mmWave bands, for example in the 39 GHz range that is even more challenging.
Narrow Channels Don’t Make A Lot Of Sense
One could of course carve out a bit of spectrum in the low-bands and give it to 5G but that would have two main disadvantages. The first one is that one can’t take a lot of spectrum from LTE as this would reduce the available capacity for the majority of the users. But even if this was done it also wouldn’t serve 5G users well, as their speed would be more limited than the speeds for LTE users when they move away from the center of the cell. One could of course add LTE cells to the connection but then you are in a similar situation as today, i.e. it’s not a pure 5G setup anymore.
A 5G Anchor Requires a 5G Core
Actually, a configuration with 5G NR as the anchor that aggregates LTE cells for capacity is more complicated compared to having LTE as the anchor and 5G as extra capacity because a 5G anchor cell requires a 5G core network. And the 5G core specification was done later and is not quite ready yet for prime time.
Why Was It Different When LTE Launched?
Let’s compare that to the LTE rollout from back then a decade ago. At that time, the situation was entirely different. GSM was used in around 10 MHz of spectrum by most network operators in the 900 and 1800 MHz spectrum and UMTS was deployed in another 10 MHz in the 2100 MHz band. In this situation it was easy to start with a standalone LTE network, because there was ample capacity available in the newly opened 800 MHz band, ample capacity in the 1800 MHz band despite a part of it being used by GSM and even more capacity available in the 2600 MHz band. No comparison to the spectrum starved situation we have today.
So even if the 4G/5G Option 3 EN-DC setup sounds and looks a bit weird, it makes a lot of sense.