For some operators, neither fixed wireless access nor old-style mobile broadband will be the services that justify investing heavily in 5G. They are looking for entirely new revenue streams, often from industrial, IoT and mission critical communications. For most of these applications, high frequency spectrum is unnecessary, and companies are far more interested in questions of the most efficient air interface, channelization methods and coding techniques to support IoT services optimally.
For some, these have been neglected in favor of the interests of a small group of early adopters which are heavily focused on fixed wireless and high capacity mobile data. But the majority are more focused on squeezing as much out of LTE as they can for the coming years, and meanwhile evolving new profit models. For that majority, for whom time is less of the essence, a thorough, high quality standards process may be more important than a short deadline.
These views were aired loudly at an IEEE conference in San Francisco last week. Of course, IEEE is the home of the other main family of wireless standards, including 802.11/WiFi, so it was unsurprisingly that a major theme of the event was the challenges LTE – and implicitly 5G NR – will face to harness unlicensed spectrum.
Other delegates were frustrated that the first wave of 5G NR specs had not drawn more widely on technologies outside the 3GPP inner circle. 5G was once expected to include more inputs from the WiFi community itself, but successive attempts to combine efforts have yielded few concrete results so far. Others hoped 5G would take a clean slate approach and think outside of the 3GPP/IEEE box.
One start-up which was proposing a radical new air interface was Cohere Technologies, whose VP, Anton Monk, was at the Dubrovnik meeting and the San Francisco event.
He believes that, in the haste to get first standards out and pre-empt operators which might deploy pre-standard kit, 3GPP has failed to consider all its options fully. “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 said.
He claims that the first set of specs – which will be frozen at the end of this year, six months ahead of the original schedule – add 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.
These error correction codes are in fact an important development within 5G NR, and like any technology debate, their choice has been highly political – and the end result something of a compromise to satisfy various vested interests and 3GPP’s consensual process. There were three main candidates – turbocoding, which is used in 3G and 4G; LDPC, which is widely used in WiFi; and polar codes, not in significant use today but heavily backed by some stakeholders, especially Huawei.
Orange, Ericsson and LGE led the support for turbo, and claimed recent work on high throughput/low latency turbo would get round its limitations in those areas and cope with 5G requirements. Majority support was for LDPC, led by Qualcomm, Nokia, Intel and Samsung.
Polar codes are the clean slate option, with the benefits and risks of that, and was mainly backed by Huawei, with some support from Vodafone and the Chinese operators, This is a new coding scheme which claims to deliver increased gain and better flexibility than the others, because polar techniques can support channel coding of any code rate with an appropriate code construction to fit any future service requirements. “Polar code provides an efficient channel coding technology for 5G allowing significantly higher spectrum efficiency than today’s cellular accesses,” Huawei claims, adding that this technique can optimize channel activity to the point it is running at close to its maximum transfer rate, or Shannon limit, on the encoding side. It also allows close-to-optimal performance on the decoding side, with less complex implementation, it said.
While selective use of polar codes implies that the initial 5G NR release is not all rehashing of old ideas, many other more radical approaches will not join the standard until Release 16, which will be kicked off in mid-2018 and could reach commercial equipment in 2021.
Among items which were added to the study list in Dubrovnik, and so could potentially find their way into the program for R16, were:
the use of unlicensed spectrum for 5G NR (see item below);
integrated access/backhaul in the same millimeter wave channels, for small cells;
non-terrestrial networks (using 5G NR for satellite communications, especially in underserved areas);
enhanced vehicle communications;
and use of non-orthogonal multiple access.
Matt Branda, Qualcomm’s director of technical marketing, told Wireless Week that non-orthogonal multiple access would be important for future low power Internet of Things (IoT) applications, as it can support non-scheduled network access for uplinks. That would allow devices to wake up, transmit and immediately sleep again, without having to wait to be scheduled, which would save energy and signalling overhead, and could also be important for urgent mission critical messages.
Qualcomm is just one of the companies which believes an OFDM-based air waveform will still be suitable for most applications in 5G, but not for ultra-reliable or ultra-low power IoT services. Alan Carlson, European head of InterDigital, a major contributor to 3GPP standards, wrote in a blog post: “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.
Huawei has also been working on a ‘unified’ air interface which supports multiple approaches at once, depending on the application. Its concept is based on polar code (see below) plus Filtered OFDM (fOFDM) and Sparse Code Multiple Access (SCMA). fOFDM allows sub-bands to be ‘filtered’ so that they can be configured differently from one another to support different use cases with different demands on the network; and the approach can double the system throughput compared to LTE’s OFDM interface, says Huawei. Meanwhile, SCMA supports huge numbers of connections and boosts system throughput by optimizing power allocation across multiple SCMA layers.
Huawei has trialled these three concepts in early-stage tests on outdoor macrocells in China, along with multiuser MIMO and full duplex radio. And last autumn, it partnered with Vodafone to demonstrate that f-OFDM, SCMA and polar code work in a stable manner with Massive MU-MIMO, to achieve three times the spectral efficiency of LTE, with air interface latency of 0.5ms in the user plane.