Metrocells will need wireless backhaul with sufficient data throughput to deliver the high speeds which 3.5G and LTE are capable of. It's also important that the total capacity of that wireless backhaul within a geographic area matches or exceeds that of the metrocells themselves.

The key tradeoffs are the cost (and availability) of suitable spectrum, the cost of the equipment itself and the ease of deployment and maintenance. The relatively unused 42GHz sits between the licence free 60GHz and the more heavily utilized 28GHz bands, attracting some spectrum licence fees while offering lower costs by using a Point-to-Multipoint topology.

An overlooked and underused frequency band?

Looking across the available spectrum bands for relatively unfilled gaps, we can find approximately 3GHz of relatively unused bandwidth around the 42GHz range (specifically 40.5 GHz to 43.5 GHz). Use of these frequencies has been in its infancy and they have not yet been regulated or licensed in many countries, largely because there has been no equipment available which uses it. However normalization and harmonization of this band has already been applied by both ITU Worldwide and CEPT in Europe to make it available. Several vendors, both new entrants and mainstream, have developed products to exploit this emerging opportunity.

These frequencies require line of sight but are very appropriate for Point to MultiPoint (PtMP) backhaul. Spectrum has already become available in several countries, including the UK, Russia and Norway.

For example, Ofcom, the UK regulator, auctioned the 42GHz band back in 2007. Three companies own the rights to a 15 year nationwide licence. It's been little used up to now, but there is a clearly stated intent to use it for LTE cellular backhaul.

Active lobbying of regulators is ongoing to release this spectrum in other countries.

A slightly more complicated story in the US

In the US, the 41-42 GHz segment is allocated to both Fixed Service and for Fixed Satellite Service downlinks, plus broadcast satellite. The adjacent 42-42.5 GHz region has no Fixed Satellite Service allocation, but does have the same allocations for the Fixed Service, broadcast satellite, and others. To date there has been no actual licensing in either part of the band. However, the Fixed Wireless Communications Coalition (FWCC) has been working with the FCC since July 2012 to adopt new rules for fixed microwave service in the 41-42.5 GHz band.
Limited range

From an engineering perspective the millimeter wave bands (30GHz and above) are usually more sensitive to rain attenuation and atmospheric gaseous absortion, which can consequently limit the maximum hop length for these bands in comparison with lower ones.

This is an advantage for Metrocell deployments, which are typically only a few hundred metres within street canyons but benefit from high levels of spectrum reuse.

Typical hop lengths range from 1.5 km to 3 km in most climate zones (rain zone D) for carrier frequencies between 30GHz and 42GHz with a minimum four-nines (99.99 percent) availability. However, most of the microwave equipment is able to operate in very heavy rain conditions (30-50 mm/hr). On rain zones E-F in Western Europe, e.g. UK, the range is closer to 1.5km. This is all assuming a ninety degree sector but longer range could be achieved using narrower beam antennas.

What does the future hold?

Karolina Wikander, Ericsson's Product Line Head for Microwave comments on the current situation, "today, 5% of our volumes in GHz 38-42 band are related to 42 GHz which is promising considering that the band only has been open for a few years. When more countries adopt the band, the wide unused spectrum and product availability caters for that the band will be important piece of the puzzle both macro and small cell backhaul the coming years".

A recent Point-to-multipoint backhaul research report from ExelixisNet (March 2013) forecasts a dramatic 54% CAGR revenues growth of the 42GHz band in the next 5-years. That is a very positive upward trend from the current very small baseline, and won't outnumber the traditional 26 and 28GHz PtMP bands, which will still lead the PtMP revenue chart. Field testing and lobbying are expected to be the key drivers, along with the evolution of the small cells deployment between 2014 and 2015. Millimetre PtMP solutions in 42GHz, when ultimately offering "gigabits per square kilometre" for a limited range could position as a very good alternative to meet the challenges of the small cell backhaul.

Elias Aravantinos, Leading Analyst at ExelixisNet, highlights both Aviat Networks and Bluwan as key vendors who have already realized the essence of the 42GHz spectrum and have been lobbying key regulators to open this band. Several regions are under the microscope outside of Europe, such as Canada, the Middle East and Africa. Their attention has also turned towards the United States and whether the FCC will open this band for use by the fixed service in 2013.

Shayan Sanyal, Chief Commerical Officer for Bluwan adds on the prospects of this spectrum from a millimeter PtMP perspective: "We are at the beginning of what can be achieved from an RF performance perspective; already today, radios have been produced that can provide up to 2.3 Gbps on a single sector at 64 QAM and even more based on chipset improvements. Applications cover not just small cell backhaul, but multi-layered heterogeneous network backhaul, backhaul/RAN sharing initiatives and high capacity enterprise access".

Our thanks to Elias Aravantinos, Leading Analyst at ExelixisNet, for his contribution in writing the above article.