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13 December 2022

Orange and Nokia take E-band microwave backhaul to 20Gbps over 3.6km

It is clear microwave backhaul is here to stay through the 5G era and even expand its role in reaching more remote areas, as well as some dense urban settings where fiber cannot readily be deployed. Extending microwave backhaul into the E-band (71-86 GHz) has become essential for many operators as the only band currently available commercially with sufficient bandwidth to meet anticipated backhaul demand for 5G Standalone services that may themselves be operating over rather lower millimeter wave frequencies.

Such backhaul might connect terrestrial stations, or in remote areas also be used for point-to-point links to non-terrestrial systems such as satellite or high altitude platforms (HAPS).

With such objectives in mind, France’s Orange is one operator that has been pushing the boundaries of microwave backhaul in the E-band in conjunction with Nokia, whose R&D on this front has been performed by Bell Labs as a semi-autonomous subsidiary.

In a recent joint trial with Nokia, Orange demonstrated a throughput of 20Gbps over 3.6 kilometers, a feat that will have been noted by other operator customers of the Finnish vendor.

Carrier aggregation was required to achieve the combined throughout and range, in this case combining two legacy microwave radios operating in the 18 GHz band with two E-Band radios at 80 GHz, using a single dual-band (18+80 GHz) antenna.

Such a combination of E-band and traditional microwave signals through carrier aggregation in microwave is not new, nor unique to Nokia. CommScope published a white paper in 2020 arguing that carrier aggregation would extend considerably the previously limited distance that could be reliably traversed by E-band signals, as has now been reaffirmed in that Nokia/Orange trial.

CommScope contended that the cost and capacity advantages of E-band dovetailed well with the increased distance and reliability of traditional microwave. Previously, efforts to combine E-band and microwave signals had focused chiefly on radio network design, rather than the actual antennas, which had to be able to support the configuration.

CommScope found, as Nokia has also, that dual-band antennas and link aggregation maximized capacity and distance. A key point demonstrated by CommScope over two years ago was that a newly designed dual-band antenna capable of transmitting and receiving both spectrum bands reduced both cost and complexity, aggregating data from the two separate links into one.

CommScope’s tests then found that E-band signal range was more than doubled as a result from 1.5 to 3.8 miles (2.14 to 6.18km), while throughput rose by over 1,600% from 186Mbps to 3.124Gbps. Nokia and Orange have taken that further, admittedly over a shorter distance. Another key point though was that service availability could be maintained at the magical ‘five nines’, that is 99.999%, equating to maximum five minutes downtime a year, by incorporating the more robust lower frequency 23 GHz signal. This effectively gave a reliable carrier-grade backup for the E-band link.

Although signal degradation does in general increase with frequency, the E-band is considered a sweet spot in the millimeter wave area because it lies in a relatively well-behaved spectral zone, with less atmospheric attenuation in that 70 to 80 GHz range than at either adjacent lower or higher frequencies. A key point is that oxygen molecules resonate with RF signals at 60 GHz and so exhibit an absorption peak, which then plummets in the E-band such that signal degradation is barely worse than for legacy microwave radios.

Above 100 GHz, atmospheric attenuation generally increases again with numerous molecular absorption bands caused by both oxygen and water molecules. That is why the E-band has come to be seen as a sweet spot for microwave backhaul, rather as some midband frequencies have for the RAN.

There are still other fronts for improvement, including higher modulation schemes and more advanced MIMO techniques.  But it is clear that despite the success of E-band trials further spectrum at even higher frequencies will need to be liberated above 100 GHz.

There is good news there in the fact that despite the existence of those absorption peaks, the rate of increase in signal attenuation is less at higher with frequencies, so it is not a linear relationship. Also antennas can be smaller at the shorter wavelengths so that more can accommodated in a given size of RF equipment.  There is hope still then of achieving range of several Kms even well above 100 GHz.

So, as E-band microwave backhaul starts to roll out the industry has been conducting R&D on opening up those higher bands, which will also require significant work on spectrum regulation.

It took about 15 years for work on the E-band to reach the current status demonstrated by Orange and Nokia. Microwave backhaul at those even higher frequencies is not likely to be available until towards 2030, the time when 6G is mooted to be coming along. The extra capacity will certainly be needed. With those absorption peaks to negotiate, multiple carrier aggregation will become even more essential.