What can mmWave do for 5G Backhaul?

Originally published on RF Globalnet.

MIKE GEEN – Chief Scientist – Filtronic

The challenges of implementing 5G radio access have been widely discussed, and one of 5G’s most pressing areas of
need is the technology connecting the network to the core. As the first 5G networks are rolling out, network architects
are seeking the highest capacities and data speeds possible for backhaul, in a format that can be rapidly and flexibly
deployed wherever it is needed.

The backhaul segment of the network is traditionally made up of the intermediate point-to-point links between the
core (or backbone) network and the small subnetworks at the network edge, where the base stations are. Since 5G is
aiming for multi-Gbps data rates for subscribers, all this data will need to be backhauled at much higher rates, too.

In addition to the evolution of the air interface, the radio access network (RAN) is itself evolving, with its functions
being ‘decomposed’ into a number of different locations. In 4G LTE, distributed RAN (D-RAN) and cloud (or centralised)
RAN (C-RAN), respectively, have allowed a remote or centralised baseband unit (BBU) to be used, connected to the
remote radio unit (RRU) via an eCPRI ‘fronthaul’ interface.

In 5G, the BBU will further be split into a distributed unit (DU) and one or more central units (CU), and these will be
connected using a ‘mid-haul’ link. Collectively, all these types of link are known as X-haul. More details may be found
in a new report published by ETSI’s millimeter Wave Transmission Industry Specification Group (mWT ISG), GR mWT
012.

Now more than ever, a wireless solution — rather than fibre — will be the obvious choice for X-haul. Already in Europe,
around 50 percent of backhaul between the edge and the core network is wireless. In some countries, including India,
the proportion is greater1. With the higher densities of small cells necessary to implement 5G, this share is set to
increase rapidly.

Wireless backhaul is quicker and simpler to deploy than fibre, and it is more cost-effective. Laying new fibre typically
costs between $35,000 and $100,000 per kilometer2,3, whereas the cost for a wireless link is an order of magnitude
lower2. Furthermore, in a complex urban environment, it can often be impossible to lay new fibre exactly where it is
needed.

The traditional bands for wireless backhaul between 6 GHz and 42 GHz served well for 3G and 4G. However, they
would struggle to meet the needs of 5G, where the dual requirements of higher data rates and increased capacity
mean much higher bandwidth requirements.

One way to achieve the additional bandwidth required is to move higher up the spectrum into mmWave bands —
particularly effective in urban environments, where link distances are relatively short. In the existing bands, there exist
a number of narrow channels; the total bandwidth available for mobile backhaul below 42 GHz is just 15 GHz. This
bandwidth is heavily used, and expensive licences are required to operate within it.

In contrast, the mmWave bands above 50 GHz will provide over 20 GHz of additional bandwidth in large chunks,
allowing very high data rates to be achieved. Some of the traditional wireless bands — notably, 26 GHz and 28 GHz —
have an uncertain future for backhaul, since they are now being targeted for 5G radio access.

ETSI’s mWT ISG already has expressed its concern regarding the need, when allocating mmWave bands for 5G, to
consider the ability of operators to continue operating backhaul for their 3G and 4G networks, as well.

The main mmWave bands for telecom transport are V-Band (57 – 71 GHz) and E-Band (71 – 86 GHz). There is even
some interest for the future in W-Band (92 – 114.25 GHz) and D-Band (130 – 174.8 GHz) but working at such high
frequencies introduces additional technical and manufacturing challenges.

In 2015, just 0.2 percent of fixed wireless links were at V-Band and less than 2 percent were at E-Band4. E-Band,
however, is increasing its share very rapidly — by 2018 E-Band accounted for around 7 percent of links and, as it is now
accepted as an essential element in 5G transport networks, this trend is forecast to continue with year on year growth
rates of around 36 percent, according to Dell’Oro Group5.

The licensing situation for V-Band has restricted its development. Although V-Band may seem attractive because it is
licence-free in many countries, operator concerns over interference and availability have hampered its application for
mobile backhaul.

V-Band is, however, finding a place in multipoint-to-multipoint meshed networks in dense urban scenarios, where each
network element, with beam-steerable antenna, could be reached by more than one direction by other equipment. This is helped by the extension of the licence-free band to 71 GHz, which provides more bandwidth and better propagation characteristics.

At higher frequencies, it also becomes necessary to take atmospheric attenuation into consideration. There are
well-characterised bands where absorption by water and oxygen molecules can be a problem, and rain attenuation
increases rapidly with frequency up to around 70 GHz, after which it begins to flatten out. Nevertheless, system
simulations carried out by the ETSI mWT ISG (GR mWT 008) suggest that link distances of several hundred metres are
practical at D-Band frequencies with antenna sizes comparable to those at E-Band.

Broader contiguous bandwidth allocations are not the whole story. Capacity and throughput can also be increased by
techniques such as network topology changes (densification, RAN sharing, increased fibre penetration from the core
to the edge); shorter link distances utilising star topologies from the fibre aggregation point, and; increasing channel
width in the traditional bands by the use of carrier aggregation.

Combining carrier bandwidth from different parts of the spectrum not only offers an increase in the total available
bandwidth, it can ensure availability where one band has superior propagation characteristics, allowing longer links.
Several aggregation scenarios are now either being deployed or being proposed for future deployment: two or more of the traditional microwave and sub-6 GHz bands, offering 1 – 5 Gbps; a microwave band (15 GHz, 18 GHz or 23 GHz) plus E-Band, providing up to 10 Gbps; and, in the future, E-Band plus D-Band can provide up to 100 Gbps.

Higher order modulation techniques can also increase data rates, but these demand higher signal-to-noise ratios and
very linear components in order to keep error rate to a minimum. Multi-channel systems like XPIC (cross-polarization
interference cancelling) and line-of-sight MIMO have also been demonstrated to provide enhanced data rates, but
require more expensive radio equipment.

In considering all these factors, we believe that wide-bandwidth mmWave radios provide the optimum solution to
meet the increasing capacity demands for backhaul and other types of transport in next-generation communications
system. mmWave links currently have the ability to give fibre-like capacity — up to 40 Gbps in multichannel
configurations — and, with carrier aggregation, can operate at high capacity up to 10km.

Further, new spectrum allocations above 95 GHz will provide a path to even higher capacities in the future.

References

1. Ericsson Microwave Outlook 2018: https://www.ericsson.com/en/microwave-outlook/reports/2018
2. “Backhauling with fibre?”, Fibre Systems, Winter 2015: https://www.corning.com/media/worldwide/coc/
   documents/Fiber/FSwin15pp3334.pdf
3. GSMA Mobile Backhaul: An Overview, June 19, 2019: https://www.gsma.com/futurenetworks/wiki/mobilebackhaul-an-overview/
4. SkyLight Research 2015
5. Dell’Oro Group Press Release, 23 July 2019: https://www.delloro.com/news/17-billion-of-microwave-transmissionequipment-needed-over-five-years/