So here we are in the fall of 2019. 5G networks have started around the globe and pretty much all of them use high-band spectrum at 3.5 GHz (band n78). US operators are experimenting with 2.5 GHz and mmWave spectrum in combination with an LTE anchor cell. The anchor cell is usually either at 1.8 GHz (band 3) or 2.6 GHz (band 7) and perhaps in some cases also on the 800 MHz low-band (band 20). That is all nice and well but 5G on 3.5 GHz won’t cut it when it comes to nationwide coverage. For that, 5G also needs to use the mid- and low-bands. But how do we get there?
The problem with 5G in the mid- and low-bands between 700 MHz and 2.1 GHz (I would count 2.6 GHz as high-band) is that all of this spectrum is already used today. One could of course do some spectrum refarming and replace existing 3G or 4G carriers with 5G. This is of course the simplest way forward but some network operators might not have that luxury due to a high load on their 4G network today and limited spectrum resources.
Now one network operator has started talking about an alternative. Verizon in the US recently announced that they are thinking about Dynamic Spectrum Sharing (DSS), a method to use 4G LTE and 5G NR on the same channel simultaneously. Ericsson has a blog post on the topic but remains pretty vague how using 4G and 5G simultaneously on the same channel might work. A recent article in the IEEE Wireless Communications magazine by Huawei engineers, however, sheds some light on this. Fast forward to the second final paragraph of the article for the details.
Dynamic Spectrum Sharing (DSS) in A Nutshell
In essence, 4G LTE and 5G NR use a very similar air interface on the physical layer. 4G LTE uses Orthogonal Frequency Division Multiple Access (OFDMA) with 15 kHz subcarriers for carrier bandwidths up to 20 MHz. 5G NR is more flexible in this regard and can also use other subcarrier bandwidths as well, especially on higher frequencies, e.g. 30 kHz in the 3.5 GHz band. To make 4G LTE and 5G NR work in the same channel, 5G NR has to use 15 kHz subcarriers as well. In addition to the 4G synchronization and management channels, their 5G counterparts are broadcast as well, just at different times. The combined 4G/5G scheduler in the base station then reserves a part of the channel for 4G transmissions and another part for 5G transmissions.
For 4G devices on the market today, this is completely transparent and no software changes are necessary. One problem on the 5G side, however, is that the way the Synchronization Service Blocks (SSBs) are transmitted in a 15 kHz subcarrier channel pattern would make them trample over the LTE Cell Specific Reference Signals (CRS) that are periodically inserted in the channel grid. To overcome this, even 3GPP Release 15 already contains an option to use a different subcarrier bandwidth for the SSBs than for the rest of the 5G channels. 30 kHz subcarriers for the SSBs and 15 kHz subcarriers for other 5G channels will do the trick. A bit of a kludge but 3GPP must have seen Dynamic Spectrum Sharing with LTE coming.
Drawbacks
One problem with Dynamic Spectrum Sharing is a waste of resources, as in addition to the 4G signalling channels, 5G signalling channels need to be broadcast as well. The signalling overhead in a 10 MHz channel is around 15% today, leaving 85% of the channel for user data transmission. 5G has about the same overhead which reduces the available bandwidth for user data to around 70% of the channel.
Benefits:
But in return, a network operator gets two things: First, if the base station hardware allows it, one can pretty quickly get to nationwide 5G coverage without deploying any additional spectrum. But 5G is not a magic bullet that miraculously increases speed with a given amount of spectrum.
Another reason Dynamic Spectrum Sharing in low- and mid-bands might make sense is that due to the nationwide 5G coverage it makes sense to roll-out a 5G core network. Legacy LTE devices can’t communicate with the 5G core and thus use the LTE part of the shared channel. New 5G core capable 5G devices will use the other part of the channel. How much of the channel is used for 4G and 5G can then be adjusted based on the traffic.
Will we see this in practice a lot? Time will tell.