5G hits reality as 3GPP postpones features
It’s not too surprising that some items would get cut from the wish list, especially given the compressed deadlines that envision a 2018 finalization of the standard, instead of 2020. While the timeframe for completion is clear, and the physical layer (PHY) is largely defined, “A lot of new concepts for 5G have to be put on the back burner,” said James Kimery, director of marketing at National Instruments, which is heavily involved in the 3GPP group developing what will eventually be the 5G standard.
The promise of 5G, which is being based on the 5G NR (new radio) proposal put forth by a group led by Qualcomm and others, is that it will have massive, multi-user multiple input, multiple output (MU-MIMO), beamforming, sub-millisecond latency, many bands of operation, carrier aggregation to make optimal use of those bands, massive IoT support, and high spectral efficiency through higher-order modulation. There’s more, of course, but the point is that it’s a wish list, not an option list.
The problem is one of resources. The RAN 1 group has defined the PHY layer such that it can theoretically perform to expectations in terms of throughput, bandwidths, and bands of operation. These bands stretch from sub-6 GHz to higher millimeter-wave (mmWave) bands at 28 GHz and above, with almost 60 bands in total.
The catch is that all the features and frequency bands need to be simulated accurately so that the appropriate test beds can be prepared. This is the task of the RAN 4 group, and that’s where things go a bit sideways.
The RAN 4 group doesn’t have sufficient resources to simulate all of the requested features by the December deadline. There are 53 proposed frequency bands (as of last Spring). Due to the resource constraints, according to Kimery, some of those bands will be dropped, for now. The RAN 4 group will decide which ones on a case-by-case basis (Figure 1).
Figure 1 Not all of the proposed bands of operation for 5G that were on the list in the Spring will make the cut for 5G due to RAN 4 resource constraints.
(Source: Approved TDoc # RP-170847, RP-170826, R4-1702504)
Kimery says, “mmWave should make it in, along with some <6-GHz bands and base functionality. However, there are lots of use cases and reality is setting in.”
Other reality checks for Phase 1 include the use of SISO (single input, single output) instead of MIMO and an initial baseline modulation scheme of 64 QAM per carrier instead of enabling 256 QAM from the start. Based on a 100-MHz carrier, Kimery expects to be able to start with a data throughput of 2.4 Gbits/s. “That’s 3× what’s currently doable on LTE, which is a big improvement,” he said. The data throughput will increase as higher-order modulation and MU-MIMO are added.
Good news on latency
The good news is that many other hoped-for features will be included in Phase 1. These include lower latency, defined in terms of the turnaround time from when a packet enters a basestation to when it’s sent out.
“LTE as it stands has to wait 4 slots (FDD LTE),” said Kimery. “Now the turnaround time has been shortened to 0.5 slots, so it’s 0.5 ms.”
The 3GPP will stick with the plan to start with non-standalone (NSA) where 5G NR access points are collocated with current LTE basestations so there’s no need to lay new fiber. “It’s not a forklift upgrade, which is good, as operators want 5G as soon as possible,” said Kimery. “SA [standalone] will take longer and has higher costs.”
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