Options for Wireless Backhaul in Heterogeneous Networks
Everybody agrees that when it comes to small cell backhaul, wireless technologies will play a leading role. But there are several types of wireless backhaul that are suitable for small cell backhaul. Which one is right for you?
There are three main categories of contending wireless backhaul solutions:
1- Line-of-sight (LOS) microwave systems typically operating in the 10 GHz – 42 GHz bands.
2- “E-band” LOS solutions that operate in the 60 GHz band (or in some cases at 80 GHz).
3- Non-line-of-sight (NLOS) utilizing sub 6 GHz licensed TDD spectrum.
There are pros and cons to each of the solutions above, so let’s get start with LOS microwave systems. LOS systems are point to point systems that operate on scalable channels of up to 2x56 MHz , require one license per link, and frequency coordination between licensees to manage interference. The antenna size is relatively large, some tens of centimeters to achieve high gain and enable connectivity over longer distances. When it comes to small cell backhaul, microwave systems would be slightly adapted by changing, the antenna footprint to a smaller one which reduces gain and directivity. This leads to shorter range, which is acceptable, but would also necessitate greater interference management. For these reasons, it makes more sense to use microwave systems in the higher bands such as in the range between 24 GHz and 42 GHz which is less congested and where antennas can be smaller. Nevertheless, every deployed node needs to be manually aligned with its peer, a costly exercise that does not allow scalability of deployment in a short timeframe.
E-band solutions have similar characteristics to microwave systems. They also require LOS alignment which is very critical because E-band has less margin of error than in LOS microwave systems due to the very narrow beam between the two nodes. A sturdy mounting structure is required for the E-band nodes so the wind cannot sway the pole and move E-band modules out of alignment. The nice thing about E-band solutions is they offer very high capacity as they utilize very large channels (500 MHz or 1 GHz) although they limit modulation rates to minimum to achieve reliability. The 60 GHz band has a unique feature: the oxygen in the air absorbs the electromagnetic energy and leads to greater losses for the transmitted signal. While this is viewed in standard communication systems as a disadvantage, in the case of small cell backhaul, it can be used as an advantage because there will be less interference between E-band links. This is more important given that the 60 GHz is unlicensed, so anyone can deploy systems within that band. Having the signals more confined means there will be less interference and therefore an easier way to handle larger deployments.
So, both LOS microwave and E-band solutions provide very high capacity but they are point-to-point systems that need alignment. Remember that small cells will be deployed in urban areas first at low height such as 3 m to 6 m. Therefore, these solutions run the risk of losing connectivity if a building is constructed, a tree is planted, a banner is placed or simply a large truck passes in between nodes!
NLOS systems address these problems and presents a solution that is easy to plan and deploy everywhere in an urban area thereby significantly reducing the cost and duration of small cell network roll out. This is the major advantage of NLOS systems over LOS microwave and E-band systems. A small cell can be deployed in under 30 minutes freeing up the installation team to deploy multiple cells in a day as opposed to one or two cells per day with the LOS solutions.
NLOS systems come in different flavors, too. For example, some operate in licensed frequency bands such as the 3.5 GHz while others operate in unlicensed bands such as 5.8 GHz. Since small cells are most needed in the urban core, unlicensed band NLOS systems run a large risk of interference with other communication systems used in enterprise, municipal, and other applications in addition to increasingly for the case of 5.8 GHz with other consumer electronics such as portable phones (other unlicensed bands such as the ISM 2.4 GHz are already considered too noisy for backhaul application while the lower U-NII 5 GHz bands have low power requirements or indoor use limitation). Interference results in lower reliability and service interruption.
To obtain higher reliability in NLOS deployment and obtain tight performance variability, licensed bands are used. Spectrum in the 3.5 GHz band is very low cost – often selling at below a cent per MHz per inhabitant. Recent spectrum auctions in Europe established a price on the order of 2 euro cents per MHz per inhabitant in the 2.6 TDD band. Furthermore, licensed spectrum in sub-6 GHz band is licensed on a block basis, so the marginal cost of deploying NLOS backhaul systems decreases leading to cost effective business case. The business case can be further enhanced by using a point-to-multipoint configuration which reduces the cost of connectivity to the core network.
All three solutions are complementary, and there are different mobile backhaul applications for each technology. E-band offers the highest capacity but shorter range. LOS microwave offers lower capacity than E-band but can go over very long distances. NLOS solutions offer similar spectral efficiency to LOS microwave and significantly higher capacity over that required by a small cell. The coverage ubiquity feature of NLOS is the one feature that no other wireless solution matches. This translates into an attractive business case which will be one of the main determining factors of heterogeneous network deployment.