The 3GPP decision to fast-track a subset of the 5G New Radio (NR) specifications – to give operators like AT&T the chance to deploy 5G six months earlier than previously expected – has caused a ripple effect on other projects within Releases 15 and 16, which will define broad 5G RAN, core and system architecture standards.
Critics of the splitting of the Release 15 radio specs have said it will delay essential aspects which should have been finalized in tandem with the radio. In particular, Nokia and others raised concerns that the work on the packet core and the RAN was becoming too separate.
The initial, accelerated NR standards define a non-standalone version (which requires an LTE anchor network, and so is not implemented with the 5G Core). Enrique Blanco, CTO of Telefonica, argued when the split was announced in March that it was a backwards step, preventing 5G NR technology from evolving to support emerging use cases. The initial standard will be only focused on a limited set of use cases, such as fixed wireless, and these will be of interest to only a subset of operators, while the really transformative uses of 5G, such as network slicing, will be driven from the 5G Core.
Now it seems that some Release 16 work has also been delayed because of the resources needed to get 5G NR Non-standalone completed in time. At a plenary 3GPP meeting in Japan last month, several study items were put on hold until December, though Balazs Bertenyi of Nokia, chairman of 3GPP RAN, insisted they would be back on the agenda in the first half of 2018.
“Given the challenges we have to finish Release 15 on time, we are going to put the study items on hold,” he said. “We’re very much going to be dealing with these in the first half of 2018.”
The delayed items include 5G-Unlicensed; non-terrestrial network (channel modeling); an enhanced V2X evaluation methodology; integrated access and backhaul; and non-orthogonal multiple access.
Signals Research Group, which attended the plenary, commented sceptically on the delays and wondered why there had been no discussion of extending the December deadline for completing 5G NR Non-standalone. “We’re sure this idea would be a non-starter given operators’ [overly aggressive] statements regarding their near term 5G ambitions, but we think it would have been the wiser decision given the industry is stuck with 5G NR and its evolutionary path for at least the next decade,” the analysts wrote.
It added, in a report on the meeting: “In theory, the 5G NR air interface is designed to be forwards compatible and capable of supporting new features, capabilities, and use cases that do not exist today. Only time will tell.”
One hot topic of debate before the business of defining the 5G specs started in earnest was whether the waveform would change radically. There was a level of consensus that while OFDM remained well-suited to mobile broadband use cases in sub-6 GHz spectrum, other technologies would be better optimized for some IoT applications, especially with ultra-low latency or massive numbers of end points, and for high frequency spectrum.
But discussion of non-OFDM waveforms was pushed into Release 16 and has now been further delayed because of the pressure to hit the aggressive timeline for the first release of 5G NR.
Stéphane Téral, executive director of research in mobile infrastructure at IHS Markit, wrote in a recent blog: “Even more surprising is that most engineers in the wireless standards community assume that we can’t do much better than OFDM in approaching capacity. This is simply not the case.”
He particularly highlighted the alternative technologies devised by Cohere Technologies, which gained considerable profile a couple of years back for its OTFS (Orthogonal Time Frequency and Space) approach. Its submission to 3GPP RAN1 was supported by AT&T, China Mobile, Deutsche Telekom, Telefonica and Telstra.
Cohere VP Anton Monk was critical of the fast tracking decision, saying in an interview earlier this year: “Some big vendors wanted to get something done fast and slap a 5G name on it — we’ll see how widely it gets deployed.” He claims that Release 15 adds little to what is also being developed for LTE-Advanced Pro. They are “LTE with Massive MIMO and beamforming — nothing really new except for including Huawei’s polar codes in the control channel and using LDPC (low density parity check) everywhere else,” he added.
Technologies like Cohere’s, as well as work on other waveforms by Qualcomm, Ericsson, Huawei and others, will have another chance in 5G Phase 2, which is seen as the first time that 5G standards will genuinely address brand new use cases such as high speed rail, enhanced V2X and others requiring extreme reliability.
Alan Carlson, European head of InterDigital, a major contributor to 3GPP standards, wrote in a blog post last year: “Two licensed band cellular definitions are expected below 6 GHz. The second will be especially tailored for IoT support. It is less likely in my opinion that this will be based on OFDM. OFDM is great for video, but in the IoT world of trillions of randomly occurring access events, its strict synchronization needs render a severe handicap.”
Qualcomm’s own pitch has been non-orthogonal RSMA (resource spread multiple access) technology, plus a new multiplexing technique which would allow traffic requiring very low latency to take priority automatically and to use RSMA. This technology uses time and frequency spreading and overlaps users in a way that aims to improve network efficiency and power consumption. It can support mobility and downlink meshing, as well as network-assisted mesh on the uplink.
Even in OFDM itself, there were passionate arguments for different variations, not least between Ericsson and Huawei to push their preferred implementations – and so enhance their own IPR position in 5G.
A year ago, the 3GPP NR Release 15 study item agreed to support CP-OFDM (Cyclic Prefix OFDM) for the downlink in enhanced mobile broadband applications, with the complementary technology, DFT-Spread OFDM, working alongside CP-OFDM on the eMBB uplink. This has some continuity with LTE, which uses OFDM in the downlink and DFT-S OFDM in the uplink, but increases the importance of CP-OFDM.
Qualcomm argues that DFT-S OFDM provides link budget benefits while CP-OFDM supports MIMO spatial multiplexing advantages. It believes there are advantages to using both technologies on the uplink, which can adaptively switch between them to get the best of both worlds.
Ericsson has also been a big supporter of the CP-OFDM waveform and has welcomed its inclusion in the first phase of 5G NR specs. Senior researcher Ali Zaidi and master researcher Robert Baldemair explained in a recent blog post why Ericsson backs CP-OFDM over the many waveform options presented to 3GPP.
“The trend has been to tweak OFDM in any possible way – sub-carrier wise filtering or pulse shaping, filtering of groups of sub-carriers, allowing successive symbols to overlap in time, dropping cyclic prefix, replacing cyclic prefix with nulls or with another sequence,” they said. “Various waveforms became strong contenders for 5G—numerous research publications showed CP-OFDM being outperformed. At one point, we felt that every multicarrier waveform was going to be part of 5G, except CP-OFDM.”
Ericsson Research said its tests showed CP-OFDM performing best on the indicators that matter most to operators – compatibility with multi-antenna technologies, high spectral efficiency and low implementation complexity.
The Chinese majors have been investing large sums in developing potential 5G technology. For the mmWave bands, China Mobile has validated Huawei’s Filtered OFDM and ZTE’s FB-OFDM in its high frequency tests.
Last year, Huawei described a “unified” air interface which draws on several approaches at once and allows different sub-bands, within the baseband, to be configured individually for different purposes.
Its design is based on three key concepts – Filtered OFDM (fOFDM), Sparse Code Multiple Access (SCMA) and polar code. Each has a contribution to make to a standard which can be adaptable without compromising on the performance requirements, says Huawei.
fOFDM enables a flexible OFDM interface in which sub-bands are ‘filtered’ so that they can be configured differently from one another to support different use cases with different demands on the network. It can also, according to Huawei, provide twice the system throughput of LTE’s OFDM interface.
SCMA supports huge numbers of connections and also boosts system throughput by optimizing power allocation across multiple SCMA layers. Huawei said that, in its test, SCMA delivered three times more uplink connections per link and increased downlink system throughput by 80%. Meanwhile, polar code is a new coding scheme which claims to deliver increased gain, compared to the turbocoding used in LTE. Polar techniques claim to support channel coding of any code rate with an appropriate code construction to fit any future service requirements.
Huawei says it has trialled these three concepts in early-stage tests on outdoor macrocells, which also used two of the most hotly tipped technologies to underpin 5G – advanced multiuser MIMO and full duplex radio (the latter allowing for simultaneous transmit and receive on the same frequency, doubling spectral efficiency). The MU-MIMO implementation supported up to 24 users, and up to 24 parallel layers of transmission on the same time-frequency resources. The tests took place under the auspices of China’s IMT-2020 Promotion Group, an umbrella initiative for 5G R&D.