ZTE is bringing a number of mobile network infrastructure innovations to market under the banner of Pre5G this year. As the name implies, the technologies involved are seen as critical to the development of the next mobile standard – 5G. They also enable ZTE to set its stall out as the big OEMs battle to get as many of their proposals as possible adopted into the 5G standard.
Speaking to Wireless at Mobile World Congress in February, Dr Xiang Jiying, chief scientist, ZTE, observed: ‘From the perspective of ZTE and the industry in general, 5G is still four to five years away. First we need to finalise the standard (expected in 2019) and after that we need to develop the chipsets for the handsets and build them.
‘The transition from 4G to 5G we believe will be a step-by-step migration,’ he continues. ‘Our Pre5G solutions represent two steps towards 5G. We are using 4G eNodeB technology for the infrastructure, but we are not touching the handset yet – that comes later.
‘The benefit of this’, he explains, ‘is that we can experience some aspects of 5G now, because we do not have to modify the air interface or handsets yet. However, we have developed a complex pre-5G base station.
‘It provides three to six times the capacity over normal 4G,’ says Ziying. ‘We can get four times higher data rates with the same handset, so the customer can benefit because of this. We developed the first prototype of the base station in 2014 and last year we promoted field deployments of it with a couple of customers.
‘We believe a lot of mobile operator CTOs around the world are interested in what our Pre5G solutions can offer,’ says Ziying. ‘This is because it is very easy for them to go to the next step, because the eNodeB is already there from 4G deployments.
‘But with a small software modification they can migrate to a 5G base station and then later introduce the handsets when the new 5G air interface is decided; that way they break the migration to 5G into two steps.’
The second phase of 5G will introduce the use of high frequency microwave and millimetre wave spectrum and this requires a modification of the radio frequency. Ziying says ZTE’s baseband unit is ready for microwave frequencies now. ‘This illustrates the basic nature of our Pre5G work, as it shows we have done a lot of the necessary innovation for 5G already,’ he points out.
Unified Air Interface
Perhaps the key innovation on the road to 5G developed by ZTE is its unified air interface (UAI), which is made up of three layers. The unified bottom layer contains a transparent physical layer designed to meet a diverse range of service and spectral requirements, which the huge range of 5G use case being proposed will demand. Its unified structure and OFDM (orthogonal frequency division multiplexing) waveform dynamically adapts to any service and any spectrum.
The intermediate layer contains flexible network slices, which can be customised for specific services and applications, as each layer of the network slice is designed to meet a different set of performance characteristics according to the service type and traffic requirements. The smart top layer features service awareness technology to enable the efficient sharing of resources to achieve intelligent service convergence.
The UAI is made possible by four key technologies: the FB-OFDM (filter bank-OFDM) waveform; the unified interface itself; multiple access technology in the shape of ZTE’s multi-user shared access (MUSA) solution; and massive MIMO (multiple input, multiple output).
The FB-OFDM waveform is designed to dynamically adapt to the service requirements of a wide variety of applications and services including; enhanced mobile broadband (eMBB), the Internet of Things (IoT) and vehicle-to-infrastructure (V2X) technology.
One of its characteristics is its ability to support an increasing range of IoT services due to its longer transmission time interval (TTI) and narrow frequency range. This enables the network to support a larger number of connected devices at a low bitrate. The short TTI and wide spectrum efficiency also benefits new types of services such as ultra-low latency V2X applications.
The major benefit on the 5G unified interface is its ability to scale from low spectrum bands to the extremely high frequency millimetre wave bands being proposed for the second phase of 5G, as well as providing effective subcarrier distribution to intelligently allocate spectrum frequencies.
ZTE is proposing its FB-OFDM solution as a possible air interface standard for the second phase of 5G when higher frequency bands are expected to be used and a new air interface will be required.
Ziying notes that a wide variety of use cases have been proposed for 5G, some of which have very different, if not polar opposite, characteristics. Some IoT use cases only require low power and to send very small amounts of data without any great urgency. Other use cases require large amounts of data to be sent at very low latencies.
5G use case extremes
‘It is difficult for an air interface to handle these use cases extremes,’ says Ziying. ‘However, we have defined a new layer below Layer 1 – Layer 1 minus – whose function is to aggregate all the different applications. It means we can keep the same frame structure for 5G to handle everything; we call it the unified air interface (UAI). In fact, we proposed this three years ago and now someone else has also proposed something similar.’
ZTE believes that its FB-OFDM solution has advantages over some of the rival 5G air interface proposals, arguing that it is very good for implementation by mobile operators and for chipset development. ‘We understand the implementation limitations for the second phase of 5G and if you look at ZTE’s Pre5G proposals and our FB-OFDM solution, it is more close to the implementation and the product than some others,’ says Ziying.
The FB-OFDM waveform is just one of the potential 5G waveform technologies being developed by ZTE. But Jiying explains that by using MUSA and FB-OFDM it is able to reduce inter-carrier interference through subcarrier-level filtering and it helps users flexibly configure air interface resources in accordance with service requirements.
Multi-user shared access (MUSA)
ZTE’s MUSA (multi-user shared access) technology uses non-orthogonal complex spreading sequences to allow multiple users to transmit at the same time and frequency. This hugely increases the number of IoT devices that can share the same air interface resources, thereby improving the number of IoT terminal connections by three to six times, according to the company.
Ziying points out that IoT terminals have to synchronise with the network now and again and that uses up power – in fact the synchronisation process is the main source of power consumption in IoT. ‘But by using MUSA, the IoT terminals do not need to synchronise, so that saves a lot of power and therefore makes it a good technology for IoT.’
‘MUSA also allows us to overcome the near-far effect where weaker signals are drowned out by stronger ones,’ he continues. ‘In 2G, 3G and 4G networks, if the handset is close to the base station then the capacity is high, but at the edge of the cell it goes down hugely - by up to 100 times – this is not good. But we have been able to even out the peaks and troughs between signals, so the signals are much smoother and even across the cell to the edge.’
Finally, there is ZTE’s massive MIMO solution, which it first proposed in mid-2015. Essentially, this involves the deployment of very large numbers of antennas, which focus the transmission and reception of signal energy into smaller regions of space. This enables major improvements in throughput and energy efficiency.
‘Massive MIMO uses hundreds of antenna elements,’ says Ziying. ‘At the moment we only use 2, 4 or 8 antennas. But customers do not want big antennas, so we had to do a lot of innovation to get 128 antennas into the size of 8 antennas.’
Another ZTE innovation centres on the chipset. Mobile operator customers are concerned about power consumption and the cost of that, so ZTE has developed its own chipset for its new base station.
‘We have two chipsets that can handle 128 antennas. Without this innovation and by using existing chipsets it would have been a huge board. We have the first prototype of this ready,’ says Ziying.
A third innovation is beamforming, which often goes hand in hand with massive MIMO. Massive MIMO implements beamforming with smart antennas to send out multiple signals, which have the capability of determining the best path the signal should take to reach a device.
‘Traditional antennas do well in open spaces, but not so well in dense areas,’ observes Ziying. ‘So, massive MIMO will be used in dense hotspots, but it is a big challenge and requires a lot of effort as in urban areas you cannot identify the beams because they are coming from all directions.
‘You need to do very complex algorithms, but we have solved this problem, so we can now support the use of massive MIMO with beamforming in dense urban environments. The beam is not a physical beam; instead it should be a logical one. A lot of effort has gone in to enabling the solution to automatically generate a logical beamform to handle the moving subscriber,’ says Ziying.
See also: ZTE unveils Pre5G and 5G-ready solutions at MWC 2016