Maximising Small-Cell Backhaul Performance

Small-cell deployment is being considered by mobile operators around the world as a means to satisfy the ever-increasing mobile broadband traffic demand in urban areas.

Microwave systems have dominated the backhauling of macro base station sites in most areas of the world for many years. A new generation of systems is now coming out of the development labs of wireless equipment manufacturers. These are being specifically designed to meet the particular requirements and challenges of space and range-limited street-level deployments.

The main operator requirements driving these developments are:

  • Small size and attractive equipment form factors - units may be visible at eye-level in public places
  • Flexibility - units need to be mounted on walls and lamp posts with basic powering facilities
  • Rapid speed of deployment
  • Ease of maintenance
  • Large reductions in CapEx and OpEx compared to macro-sites

Unlike traditional macro base stations, small-cell base stations are mainly deployed below building rooftop heights. Typically, installation locations include city structures such as street lamp posts, sign posts and building sidewalls. Moreover, they are expected to be deployed at much higher densities than macro cells.

As a result, wireless backhaul links are likely to be subject to obstructions arising from city clutter such as trees, posts and buildings. Such deployment characteristics, combined with the smaller coverage area per cell, create specific mechanical, functional and economical requirements for the small cells and their means of backhauling.

The grid-like nature of the streets and the obstructions imposed by city clutter at street level, necessitate the deployment of a combination of “chain”, “tree”, “star” and “ring” configurations. Such topologies can ideally be accommodated by radio operation in Point-to-Point (PtP), relay and add/drop, and Point to Multi-Point (PtMP) modes, capabilities. In addition, the use of area-licensed MW spectrum and unlicensed 60 GHz spectrum is suited ideally to small-cell backhaul characteristics and requirements.

Software-defined Point-to-Point (PtP) or Point to MultiPoint (PtMP) microwave radios can be used to optimise the performance of small-cell backhaul networks by combining them with PtP 60GHz radios. In a backhaul network design that uses a limited amount of area-licensed spectrum, selectively replacing microwave links with high-capacity 60GHz radio links can facilitate radio planning and improve network capacity.

Currently, mobile operators are closely examining wireless systems that operate in all frequencies for the purpose of Small-Cell backhauling. Of particular interest is the use of PtP/PtMP systems operating in the 26/28/32/42 GHz arealicensed bands, as well as in the 60GHz unlicensed band.

Both PtP and PtMP technologies share the advantage of using either relatively low-cost area-licensed or unlicensed spectrum, which expedites dense link planning while minimising licensing-related delays and costs. Large channel sizes allow for high capacity while high-frequency operation helps to reduce equipment sizes.

Deploying small cell backhaul on area-licensed frequencies takes advantage of both the operator’s investment and the extra assurance that licensed spectrum provides against interference. Area-licensed spectrum, as opposed to perlink licensed spectrum, allows the use of PtMP systems and doesn’t require a separate licensing procedure for planning and installing each link – which is a significant benefit if multiple links need to be deployed within a short time.

PtMP technology improves equipment CapEx and implementation time by reducing the number of deployed units. Spectrum for the bandwidth-rich, area-licensed 26/28/32/42 GHz bands usually comes at a reasonable price - especially considering the dense link deployment intended for small-cell backhaul.

Interference may occur depending on the amount of PtMP spectrum owned and the density of the installed links. This can lower the SINR and force the links to operate at lower capacities than would otherwise be possible. Wireless systems operating at 60GHz can be used to alleviate this difficulty. V-Band (60GHz) technology offers licence-free operation combined with high capacity across many global markets. For backhauling 4G small cells – a cost-sensitive but demanding application – these are considerable advantages, so this alternative solution is gaining significant interest from the mobile operator community.

Unlicensed operation at 60GHz has far smaller potential for interference from closely located links than unlicensed 5GHz systems, regardless of whether the links are owned by the same operator or not. This is due to the characteristics of radiation propagation at 60GHz - attenuation, narrow antenna beamwidths, small reflections – coupled with streetlevel interference. Fortunately, the multiple channels available in the 60GHz band can be used judiciously to minimise interference even in dense link deployment scenarios.

Microwave and 60GHz spectrum technologies can be combined to optimise the capacity of a small-cell backhaul network while maintaining its robustness.

Combining area-licensed microwave spectrum and 60GHz spectrum can minimise interference and allow for higher capacity/density than would be possible using a single spectrum band. Using licensed microwave spectrum offers increased assurance against interference, while also offering higher tolerance to potential partial obstructions, for example a row of lamp posts, to the link LOS.

StreetNode™, a software-defined radio platform, which was designed and developed by Intracom Telecom is an example of the latest innovative PtP / PtMP systems. StreetNode specifically addresses the challenges of smallcell backhaul, enabling operators to deploy backhaul networks of any complex configuration in the 26 / 28 / 32 / 42 GHz and 60 GHz bands and realizing the benefits of increased network capacity and robustness under a single management system.

Konstantinos Dimopoulos is the Senior Product Manager for mmWave at Intracom Telecom.

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