Two and a half years ago in March 2015 I had my first post on 5G on this site about 3GPP kicking off its 5G activities. A lot has happened since then and I had the occasional post but now things are getting really exciting for me: Various 3GPP groups have started churning out the first drafts of Technical Specifications (TS) rather than refining the many different implementation options discussed in many different 3GPP Technical Reports (TR). In other words the technical specifications are now written based on the agreed-on options from the TRs. So now is the time to have a first look what is going to be put into practice.
First Versions Of The Specs Are Out
The 5G ‘New Radio’ (NR) Technical Specification documents for the RAN (radio) part of the network can be found in the 38.xxx folder on the 3GPP website and the first document to look at is 38.401, the ‘NG-RAN; Architecture description’. From there one can fan out and take a look at documents describing the NR air interface, frame structure, radio network component interface, the interface to the 5G core network, etc. etc. 3GPP TS 23.501 is the place to start looking for how the 5G core network will look like. In the next posts on this topic I will go through a number of different topics that I think are most relevant to understand where the industry will go with 5G and which features described in the specification will first see the light of day. But before that I have a more general topic:
When looking at what Verizon, Qualcomm and others are saying in public it is likely that the first 5G-like deployment we will see will be complementary to 4G LTE. It will be quite different to what we saw when 4G was launched, however. Back in the 2009/2010 timeframe, LTE was independently deployed alongside 3G UMTS. A mobile device was either connected to a 3G cell or a 4G LTE cell but not to both at the same time. While for a short period of time there were 4G-only data sticks, multi-mode data sticks and smartphones soon followed with GSM, UMTS and LTE capabilities. Still, however, a device was only connected to one of the three radio technologies at a time.
I’ve been stressing this independence because things will be quite different for first 5G-NR deployments. The first main use case is likely to increase system capacity and individual device throughput. As LTE is doing a good job at this already and carrier aggregation will ensure that more and more spectrum can be used with the technology, it was decided that 5G-NR will we used to complement LTE in higher frequency bands. There are two variants of this: Low-band and high-band deployments:
One flavor is to use 5G-NR in one of the still available and so far unused sub-5 GHz bands, e.g. in the 3.5 GHz range and use it in dense-urban scenarios as an extra frequency layer. It won’t have the range of LTE cells that are operated in much more propagation friendly bands from 700 to 2.6 GHz but due to its limited range and extra available bandwidth it promises a significant capacity enhancement. Due to its short range, however, it will only be available in the center of LTE macro cells. This is different from the time we moved from 3G to 4G as both technologies were used in similar frequency bands and hence a handover from one to the other technology was only required at the end of the LTE coverage area, i.e. not very often. This will be very different in early 5G deployments that will use e.g. the 3.5 GHz band. In a 4G/5G independent approach, frequent inter-system handovers would be required.
As a consequence, 3GPP decided to go for something different for initial deployments: 5G eUTRAN-NR dual connectivity (EN-DC). In the initial EN-DC approach, a 5G cell, the gNB, is directly connected to a 4G eNB LTE cell. Not only that but there’s not even a 5G core network yet and the 5G cells are directly controlled by the 4G LTE eNB. The advantage of this is that all UE control signaling will be done over the 4G cell operating in a lower frequency band and is thus much better reachable than the 5G cell. If the UE detects the presence of a 5G signal in an upper frequency band and reports it to the LTE eNB, the eNB can then decide to activate an additional transport channel for the UE via the 5G gNB and transfer data on 4G and 5G simultaneously. Hence the ‘dual-connectivity’. In other words, EN-DC is a bit like LTE carrier aggregation already done today on steroids!
While it would also be possible to use LTE for capacity extension in sub-5 GHz bands and hence some people regard this kind of 5G introduction as a bit of a strange thing, things are very different in much higher frequency bands. Here, propagation conditions are so different and hence the LTE air interface which was optimized for the sub-5 GHz bands is not suitable. Massive-MIMO, beam sweeping and beam forming are just a few must haves at such high frequencies to beam a signal to a device that is located more than just a few meters away from the base station. Time for a new air interface which I will discuss in future posts on this topic. One frequency band for such a deployment is in the 25-26 GHz range. Again, two options present itself: The first is a 5G stand-alone option, perhaps for stationary home connectivity connected to a 4G or 5G core network, with connectivity to a 4G or 5G core network. Another option is EN-DC with a dual-connectivity 4G-5G device that sets up a connection e.g. on a 700 MHz (0.7 GHz) LTE carrier to which a 26 GHz cell is added when reported by the device.
So much for today. In the next follow up posts, I’ll have a look at how the EN-DC dual-connectivity option will work and the 5G NR air interface and frame structure.