Mobile operators have finally come to accept that small cells will be an essential component of their strategy to increase network capacity--and more precisely to increase capacity density in those areas where they already see congestion and adding macro cells is too expensive, unfeasible, or simply not effective.
As they start planning for small cell deployments, mobile operators have to figure out the business case. Installing a small cell is easier and cheaper than a macro cell, but operators have to install many more small cells than macro cells, and that changes both the cost dynamics and the operational processes. In this context, backhaul has become one of the top key issues - and one that is tricky to address.
More of the same will not do
Wireless backhaul is well established in mobile networks with multiple solutions and vendors. What's special about small cells? Can't the same equipment be used for small cells?
The wireless backhaul market for macro cells is dominated by point-to-point (PTP) microwave and increasingly E-band links that operate over licensed spectrum that requires line of sight (LOS). This solution does not work for small cells installed close to the ground and often lacking the required LOS to the other end of the PTP link. PTP equipment also requires good alignment between the two ends of a link, something that cannot be taken for granted in a world where small cells re often mounted on lampposts or utility that frequently move (hit by a truck, or swaying in the wind).
Furthermore, in a PTP environment each cell requires a two-end dedicated link. Even as the hardware for each link becomes more compact, cheaper, energy efficient, and easier to install, the number of terminals to install is inevitably higher than for a point-to-multipoint (PMP) solution where multiple small cells can connect to a single hub location where traffic is aggregated. Where space is difficult and expensive to secure, as it is in most small-cell deployments in dense urban areas, reducing the amount of hardware required has a huge impact not only on equipment and installation costs, but also on operating costs (i.e., site rental), and rollout time.
Figure 1 shows the cost per mbps of capacity for LTE macro cells and small cells, for different technologies (the full TCO analysis is available here). While in both scenarios--macro and small cells--PMP solutions are less expensive because of the more efficient use of equipment and spectrum, the cost on a per-mbps basis is much higher for small cells. This is because the combined capex and opex for each cell site is comparable for small and macro cells, but the capacity requirements for small cells are lower.
Figure 1. Comparison of backhaul costs over a five year periods for a macro-cell (3 sectors) and a small-cell (1 sector) LTE deployment.
A separate backhaul market for small cells is emerging
A new approach to small-cell backhaul is required to bring down the per-mbps costs to the level of macro cells. Small cells are forcing vendors to rethink wireless backhaul for an environment where most cell locations are not in LOS with each other or with aggregation points, and where minimizing the size and units of equipment to be installed is a high priority both for costs and time-to-market.
At the same time, small cells have lower throughput requirements as they typically have only one sector. Ideally, a backhaul module to be installed within the small cell enclosure can substantially lower equipment costs and complexity of installation, especially if there are no LOS requirements.
As a result, we expect to see the emergence of a separate market for small cell backhaul with specific requirements that the macro-cell equipment is not well suited to meet - and vendors are already moving in this direction. This is a market where wireless technologies are likely to enjoy a dominant position, as direct fiber connectivity to small cells is often too expensive or impractical. Fiber is typically available in urban high density areas, but may not be easily accessible from a lamppost--or at least not cost effectively.
No single solution
Unlike the macro-cell backhaul market, it is unlikely that a dominant wireless backhaul technology will prevail, at least in the short to medium term. Multiple solutions are available, and many more are being developed, but none is able to address the needs of the entire small-cell market. Even vendors are ready to acknowledge this. Increasingly they talk about a portfolio of solutions, among which operators opportunistically choose the right backhaul module that most appropriate for each location. In this context operators need to combine different backhaul solutions, possibly using the same form factor and consistent network management tools.
Difficult tradeoffs: how to choose among available options
Microwave PTP is a mature, well established technology that can be used where LOS is available and the link locations can be secured.
PMP solutions require less hardware and this leads to large cost savings, and also allow more flexibility in the location of terminals, as they do not require a fixed, narrow link. Microwave PMP requires LOS, so it can be effectively used only in a subset of small-cell locations within LOS to the aggregation point. If the aggregation point is located at street level, a higher percentage of small cells is within LOS.
Unlicensed backhaul can also be used for small cells in areas where interference is low or can be to some extent controlled (e.g., within a corporate campus). The equipment is widely available and cheap, but interference is major limitation, as small cells will be deployed mostly in high-density areas where unlicensed bands are also heavily used. In practice, this is a solution that most mobile operators may adopt as a short-term solution as they explore licensed-spectrum options.
There has been recently a large increase in vendor and operator interest in non-line-of-sight (NLOS) solutions using the sub 6-GHz bands. This would be a perfect solution if only licensed spectrum in these bands was available in large channels and at a low price. In nearly all markets this is not the case. The high spectrum prices and limited spectrum availability mean that the spectrum is typically used for the radio access network (RAN) infrastructure as this provides a better ROI. Furthermore, most spectrum allocations in the sub-6 GHz bands are too narrow to provide the capacity required by LTE small cells. In markets where mobile operators have access to this spectrum, we expect sub-6 GHz solutions to be widely deployed, even if they provide more limited capacity, because they combine both the advantages of both PMP and NLOS solutions.
The best approach to small-cell wireless backhaul maybe to not try to resolve the tradeoffs once and for all, or to find the best solution. Instead, operators may be better served by developing new strategies for the coexistence of multiple backhaul solutions within their small-cell footprint, and effective processes for the selection of the best suited solution at each small-cell location.