Some mobile stakeholders were frustrated that the first wave of 5G standards, 3GPP Release 15, stuck with OFDM in the air interface, rather than adopting a more radical modulation technology. However, additional standards are likely to creep into Release 16 standards next year, provide renewed opportunities for new approaches from giants like Huawei, to start-ups like Cohere Technologies, with its OTFS (Orthogonal Time Frequency Space) modulation scheme. This sits on top of OFDM and claims significantly lower susceptibility to fading, which boosts data rates and reliability.
For now, companies like Cohere are keeping their inventions alive with friendly operator trials and by targeting use cases, like fixed wireless and backhaul, where standards-based interoperability are less important. The company, which came out of stealth mode in 2015, has announced successful tests with Telefónica and 5TONIC Labs in a fixed wireless environment.
The tests used Cohere’s 5G implementation, turboConnect FWA, which was unveiled at
Mobile World Congress last month. While the company waits and hopes for acceptance in mobile and IoT standards, it aims to take advantage of the recent burst of interest in FWA, especially in 5G mmWave spectrum, to push OTFS into the mainstream.
The tests were conducted over three days at 5TONIC Labs and Telefónica headquarters in Madrid. Cohere says the results showed spectral efficiency that was almost six times better than other solutions, boosting coverage and capacity at a fraction of the cost of fiber or alternative technologies. The system achieved aggregated throughput of 300Mbps in 10 MHz of spectrum, with a spectral efficiency of 57bps/Hz across 14 layers in a 90-degree sector, under real world conditions.
5G turboConnect uses a Massive Multiuser MIMO antenna design and software-defined baseband unit. It is designed for dense urban environments with a range of a few hundred meters to up to two kilometers.
“The objectives of the tests were to show the spectral efficiency based on measured throughput, and evaluate coverage, the role of interference, peak data rates, linear scaling with MIMO order, real time processing and the ability to scale. These are key performance indicators required for a successful deployment of FWA service,” said Arturo Azcorra, vice chairman of 5TONIC, in a statement. “We look forward to continue cooperating with Cohere in the testing of its OTFS solutions, both for FWA applications as well as for other use cases, such as ultra-reliable communications and connections with high speed vehicles.”
Telefónica has been actively exploring the potential of a 5G FWA network to supplement its fiber infrastructure in Spain, and even enable it to offer fixed-line services in countries where it has mainly or only mobile networks. It has not made any announcements about commercial deployments as yet.
Other operators have shown an interest in OTFS. Altice and Telstra are among the start-up’s investors, and it has conducted commercial trials with both those companies, as well as Charter Communications, C-Spire and Deutsche Telekom, all in fixed wireless. Cohere says several more operators are scheduled for FWA trials in the early part of this year.
OTFS sits on top of OFDM and claims to be significantly more reliable. It creates a two-dimensional view of the delay and signal fading of a wireless channel. spreading information across time and frequency. This allows signals to benefit from diversity in the channel while also penetrating through concrete and glass, and to achieve high levels of immunity to fading, multipath and other signal impairments.
All that adds up to higher data rates, greater spectral efficiency and reduced interference, claims the company, which says that, in its first trials in 2015-16, it tested its technology among tall buildings and mountains and in moving vehicles, and the OTFS radios never faded, even over distances above four kilometers, as OFDM ones would have done. A minimum of 4bps/Hz was achieved in 10 MHz of spectrum in the 3.5 GHz band during those initial trials, driving speeds of 120Mbps to 320Mbps. Those rates will be much increased as Cohere is incorporating Massive MIMO radios with 64×64 arrays into its plans, as well as implementation in millimeter wave bands.
Cohere stresses that its technology can coexist with OFDM to support smooth migration from current networks, or coexistence between different 5G flavors. Indeed, OTFS is not an entirely new modulation scheme as it sits on top of OFDM, but is a “scheme for diversity that adds to MIMO with an equalization technique”, as Phil Marshall of Tolaga Research explained.
That integration could be critical to win acceptance for a technology from a small player. Many observers believe OFDM will be the most efficient option for sub-6 GHz spectrum and mobile broadband use cases, but that alternatives will be needed to optimize performance in millimeter wave bands, or for emerging IoT and ultra-low latency applications.
“We solved some very fundamental problems around imperfect channel information and timing references that the wireless community has been struggling with forever,” Rakib told EETimes when the company emerged from stealth mode. “The good news is this is a thin layer above OFDM-based cellular silicon today, requiring small transforms in the modulation process that should require less than 10% in addition to today’s silicon.”
The firm has also hinted at changes to its model should it hit the 3GPP big time. “Today we are a systems company, but if we become part of the 5G standard, we will have to add on licensing models, and eventually we might have to build silicon.”
Larger companies are also hoping to get their favored air interface technologies into the standards in Release 16. For the mmWave bands, China Mobile has validated Huawei’s Filtered OFDM and ZTE’s FB-OFDM in its high frequency tests, but has provided no details of the results as yet. In 2016, 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.
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.
Some of the items included in the study list for Release 16 are the use of unlicensed spectrum for 5G NR; 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.
The departure from OFDM will come, then, in the second wave of 5G standards, and will be mainly targeted at low power IoT applications which require 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’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.
Cohere is not alone in thinking that some of this radical thinking should have been done for Release 15. 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.
Cohere’s VP, Anton Monk, 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.
But 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.
“Moreover, CP-OFDM is well-localized in time domain, which is important for latency critical applications and TDD deployments,” the researchers wrote. “It is also more robust to oscillator phase noise and Doppler than other multicarrier waveforms. Robustness to phase noise is crucial for operation at high carrier frequencies (e.g. mmWave band).”