One of the main challenges in small cell networks is backhaul. Where there is no available fibre it has been assumed that operators will need to bring in line of site wireless technologies (such as P2MP, P2P in V Band and E Band) because NLOS “must” operate below 6GHz and in doing so it cannot provide the capacities required because there isn’t enough spectrum there. The problem is, line of sight is not easy either, as small cells will probably be sited low down, with not much visibility to hub or macro sites, where aggregation points might be situated.
That takes us back to the view that NLOS would still be of great benefit to small cells, if it could provide the required capacities.
Companies like FastBack Networks, Cambridge Communication Systems and Tarana Wireless developing systems that combine NLOS and LOS operation in different ways and work at higher spectrum bands. But not many have proposed NLOS backhaul as an answer on its own to high capacity small cell backhaul solutions, because the general problem with NLOS wireless systems is that they must operate below 6GHz, where there is not enough spectrum available to support high throughputs.
To illustrate this view, take this quote from Esteban Monturus, Backhaul Market Analyst at Rethink-Maravedis, who told ThinkSmallCell’s David Chambers the following as he ran through the options for small cell backhaul:
“Non-Line-Of-Sight (NLoS). The main challenge here is availability of spectrum, which must be below 6GHz. Some vendors in the industry such as Fastback Networks are promising high capacity also in NLoS conditions, where throughput has traditionally decreased considerably compared to LoS. If capacity is insufficient, then operators can’t ensure the quality of experience. So I will be very interested to see what real-world performance these new vendors can achieve and learn how they have surmounted the capacity constraints in sub-6GHz spectrum. Advanced antenna techniques are certainly among the main tools to achieve this.””
The other option for NLOS is not to develop technologies that increase capacity below 6GHz, but to provide NLOS backhaul at higher frequencies. And that is exactly what Ericsson announced at the recent Mobile World Congress. Ericsson said that it has developed techniques that allow it to backhaul traffic from small cells without a line of sight connection, but that it can do so at dedicated bands above 20GHz.
Indeed, far from suffering performance degredation as a result of being deployed at these higher bands, throughput actually increases, Ericsson claims.
Ola Gustafsson, Head of Product Line Microwave and Mobile Backhaul, Ericsson, said in a press release “Conventional NLOS products work in the sub-6GHz band using OFDM (orthogonal frequency division multiplexing) technology, for multipath handling. The problem is that the available spectrum below 6GHz is very limited and would be insufficient for small-cell backhaul. Ericsson has now proved, with extensive research, that MINI-LINK products operating in the 23-60GHz bands offer higher throughput and increased stability compared with sub-6 systems also under NLOS conditions. This expands the possibilities to deploy MINI-LINK as the backhaul solution for small cells, thereby enabling increased network performance.”
Ericsson’s proposal is that over short hops, diffraction and reflections caused by the environment can actually be used to overcome obstructions between the macro layer nodes that are often positioned on top of high buildings, and the small cells positioned at street level. With this approach, throughput will be very high and stable, which is important for mobile backhaul.
In the vast amount of dedicated spectrum available above 20GHz, microwave backhaul is not only capable of providing fiber-like multi-gigabit capacity, but also supports high performance backhaul for small cells, even in locations where there is no direct line of sight.
If you want the full picture, then read this article written for Ericsson’s own journal Telecoms Review . If I may provide a quick precis for those who are time-pressed, Ericsson’s case is the following.
That at 28GHz (and other bands above 20GHz) it can achieve significant performance gains over comparable systems at 5.8GHz. Ericsson found that NLOS systems at bands above 20GHz perform better in diffraction and relection scenarios than a 5.8GHz system with comparable antenna size.
5.8Gz systems can be suitable for small cell backhaul, but “contrary to common belief, but in line with theory, MINI-LINK microwave backhaul in bands above 20GHz will outperform sub-6GHz systems under most NLOS conditions”. The key system parameter enabling the use of high-frequency bands is the much higher antenna gain for the same antenna size.
The tests compared two systems:
The first system used the unlicensed 5.8GHz band with a typical link configuration for applications in this band. The air interface used up to 64QAM modulation in a 40MHzwide TDD channel with a 2×2 MIMO (cross-polarized) configuration providing full duplex peak throughput of 100Mbps (200Mbps aggregate). The second system, a MINI-LINK PT2010 [an Ericsson product], used a typical configuration for the licensed 28GHz band, based on FDD, 56MHz channel spacing and single-carrier technology with up to 512QAM modulation, providing full duplex throughput of 400Mbps (800Mbps aggregate).
What Ericsson found was that with comparable antenna sizes, the link margin is about 20dB higher at a peak rate of 400Mbps for the 28GHz system compared with the 5.8GHz system at a peak rate of 100Mbps.
In a diffraction situation, Ericsson found that full throughput – 400Mbps – was achieved at 28GHz up to 6m below the line of sight, equivalent to a 30-degree diffraction angle, while the 5.8GHz system dropped to under 50Mbps at 3m below the line of sight.
Using reflection, the narrower beam at 28GHz also led to improved performance, Ericsson said. Ericsson’s engineers also found that NLOS systems at 28GHz could penetrate at least as well through sparse greenery as systems at 5.8GHz, in terms of comparative path loss.
There are a few tweaks necessary for the higher band NLOS. For example, on account of the wide antenna lobe of the 5.8GHz system, realignment was not needed for the hub antenna for measurement purposes. For the 28GHz system, realignment of the narrow antenna beam was needed at each measurement point – but Ericsson said this is “a fairly simple procedure even under NLOS conditions”.
“And so,” concludes the paper, “in the vast amount of dedicated spectrum available above 20GHz, microwave backhaul…supports high performance backhaul for small cells, even in locations where there is no direct line of sight.”