The original small cells, femtocells for residential and enterprise customers, used standard broadband internet for their backhaul connections into the mobile network core. Metrocells, deployed by operators for widespread public use in streets and densely populated urban areas, typically don't have this option. This has led to a range of innovative new wireless products both from established and upcoming vendors.
The Industry has already made its views known
In June 2012 the Next Generation Mobile Network Alliance (NGNM), an organisation of leading mobile communications companies, published a white paper outlining the requirements for small cell backhaul. The paper, written jointly by contributors from EE, Alcatel-Lucent, Huawei and others, provides an excellent overview of the topic.
"The raison d'etre of backhaul is to provide connectivity between the small cells and core nodes with the desired QoS level".
Using existing macrocell sites to consolidate backhaul for small cells
Most operators have pre-existing backhaul solutions that serve their macro cellular sites, even if in some cases that backhaul capacity is no longer adequate for the huge quantities of data required by smartphone users. These operators need to factor in the needs of their new small cells with the capabilities of their existing macro cells to ensure simple and cost-effective backhaul solutions.
In Dec 2012 EE revealed that they were in the process of upgrading every aggregation hub in their new LTE network from 1Gigabit/s to 10Gigabit/s as standard to cope with expected capacity demands.
There are also likely to be a few new "Small Cell Greenfield Operators" who will look to roll-out networks and provide backhaul with no existing legacy network, which presents a whole new set of challenges. These operators will start with a clean sheet and be able to use the most cost-effective solutions, but will suffer from the lack of a macrocellular base sites where backhaul can be aggregated.
Differing requirements for coverage "not-spots" and capacity "hot-spots"
Effectively there are two types of small cell known as "hot spots" and "not spots":
• Hot spots sit within the macro site coverage footprint and provide additional capacity, effectively providing the ability to offload data from the macro site. Backhaul to these hot spots can be less resilient (typically 99.0-99.9% availability) because in the event of a problem the user will failover to the macro cell. Backhaul failure will reduce capacity only.
• Not-spots sit outside of the macro cell coverage footprint and provides both coverage and capacity to the end user. The not-spot needs to have a much greater level of backhaul resilience than the hot spot. If the backhaul fails to the not spot then coverage is lost resulting in unhappy customers. Typically not spot backhaul resilience should be in the 99.9-99.99% range.
Metrocell backhaul will be predominantly wireless
While network planners would prefer to use wireline backhaul with it's higher throughput and availability, this typically isn't practical due to high costs (especially OPEX), installation time and/or planning constraints (it may not be feasible to dig up the street to lay cables). So we can expect the vast majority of the last few hundred metres of Metrocell backhaul to be wireless.
Wireless backhaul technologies fall into the following categories:
• Line-of-Sight (LOS)
• near-Line-of-Sight (nLOS)
• Non-Line-of-Sight (NLOS).
Wireless includes free-space optics (FSO) which is a non-radio LOS technology using lasers and is particularly effective in urban environments over short distances. One of the benefits of wireless technologies is the manner in which they can be deployed. Multiple hops can be deployed to go over terrain features or around corners of buildings. They can be deployed in chains and the chains linked up to produce resilient rings. Wireless is particularly effective in urban environments to connect street furniture assuming that you can get adequate power...and of course planning permission.
Other factors include:
| Spectrum | The cost and availability of spectrum drives the commercial viability of each solution making lightly-licenced (e.g. 60GHz) or licence-free (e.g. free space optical) technologies more attractive. |
| Range | The limited range of some technologies makes them more suitable for Metrocell links of a few hundred metres, avoiding interference with those further away. |
| Antenna alignment | Where narrow beams are used, specialist skills may be needed to align during installation. A rigid structure for the Metrocell is important to avoid outages due to sway or twisting |
| Environmental Protection | Backhaul units must be hermetically sealed for protection against the elements, including rain, direct sunshine, extreme heat/cold, rapid temperature changes |
| Power consumption | Connected through a single Ethernet cable, the use of Power over Ethernet constrains maximum power consumption. |
| Tamper-proof | Being deployed in unsupervised locations demands that backhaul units must be secure against all but the most determined vandals |
| Installation and commissioning I&C | Metrocell backhaul units must be lightweight, simple to install and require minimal site preparation. Commissioning must be simple, highly-automated and not require high skill levels |
Physical equipment security more important
One of the notable differences between the traditional macro-cellular networks and small cell networks is how much closer they are to the general public. Typically small cells are deployed between 3-6m above head height making them both more noticeable and potentially more accessible. Security must be considered for small cell base stations and their associated backhaul, and in particular the choice of form factor is important. Deploying a small cell base station with a separate (wireless) backhaul component may provide most flexibility to the operator but may present an opportunity for mis-use, vandalism or hacking. For this reason enclosed and integrated solutions are likely to be popular. In addition operators need to consider health and safety considerations of the small cell solution. They must be safe-to-touch, environmentally sealed and not easily prone to tampering. No one can prevent people doing stupid things, but if safety and security aren't taken into consideration there will undoubtedly be lawsuits from injured vandals!
A toolkit approach essential
The advent of small cells will significantly change the approach to backhaul in urban areas. Backhaul will move from wired to wireless to reduce costs and support rapid roll-out. New and innovative backhaul solutions will need to be implemented to meet capacity demands and increased capability of the LTE air-interface. The good news is that there are lots of options, but this means there is no standard "one-size fits all" solution to reduce logistics and inventory management issues.
