Microcells: What's Needed at the Pole?

It has become readily apparent that dramatic changes must occur in mobile networks to deliver the anticipated capacity per user that LTE networks have committed to carry. Simply put, LTE only delivers two times the spectral efficiency of 3G networks, yet it promises more than ten times the user bandwidth. The promised capacity can only be delivered by a fivefold increase in radio access network (RAN) spectrum, or an 80 percent reduction in the number of users per cell site. Coupled with that is an ongoing shift from voice centric services to video centric services that will increase bandwidth per user even more, further complicating capacity issues.

Looking forward to LTE Advanced, spectral efficiency isn’t expected to increase significantly but still higher bandwidth per user is promised. All this can only be delivered by increasing the allocation of RAN spectrum, or by further reducing the number of users per cell site.

These realities have led operators to explore a wide variety of technology options, such as network offloading using femto cells or WiFi hotspots and cell size reduction using pole mounted microcells. Commercial options, such as tiered pricing for data plans to curb customer demand, as well as political options, such as lobbying for more RAN spectrum to be auctioned off by the governments are also in play. While it’s likely that all of these will be required to bridge the gap between network capability and customer expectation, one of the most promising options is to reduce the number of users per cell site by utilizing microcells. Doing so allows operators to deliver more capacity per user with the same RAN spectral efficiency, and without spending billions of dollars buying additional RAN spectrum at government auctions. What’s more, it places the network deployment timeline under the operators control rather than them being subject to political processes.

A closer look at microcell requirements illuminates that what’s needed for deployment at the pole is very much dependent upon the type of network being built. Consider backhaul technology – the use of fiber is assumed in many of the microcell or distributed antenna product designs, yet the reality is very little fiber is available at the light pole or lamp post. Hence, a wireless backhaul option is required in the vast majority of situations. As a result, what’s needed is hardware to convert the Ethernet signal from the base station to the wireless frequency, as well as some sort of antenna structure. These need to be packaged in something that is visually pleasing and that, preferably, looks nothing like a microwave radio.

Another consideration is network topology. For star type configurations, where each pole mounted microsite is connected directly back to a macrosite or fiber point of presence, the equipment required at the pole is quite simple: a base station, either a wireless or fiber backhaul link, a battery backup (if required by the operator) and an aesthetically pleasing environmental enclosure that meets municipal zoning requirements. The problem with this network topology is that the hub site can become a point of congestion, and visibility (also known as line of site) between the hub site and the micro site limits the deployed network’s size. A much more flexible network topology would include some level of interconnectivity between microsites at the street level to enable aggregation of traffic prior to connecting back to hub sites. Examples of these topologies are ring with spurs or add/drop chains.

In addition to significantly reducing the number of hub sites required, these types of networks offer higher reliability by providing alternate paths for traffic in the case of equipment failure or power outage. The implication of these network topologies is that the number of backhaul links per site must be increased and the capacity per link must also be increased in order to carry the traffic from multiple microsites. Additionally, the microsite now needs to include packet switching capability in order to route traffic between the local site and the various backhaul links. The number of backhaul links required per site is naturally limited by street level topologies, too. With most cities laid out on some sort of grid system, on any given street corner there are only four possible paths available. Since the pole or light standard usually blocks one of the directions, the majority of microsites will require two or three backhaul links – all of which need to be housed within the same volume and weight constraints imposed by the municipal requirements, and be designed for simple and quick installation to keep the cost per site in control.

In order to deliver the capacity to support the aggregated traffic from multiple microsites, high frequency, high capacity radios will be required. These higher frequency radios also have the advantage of reducing self-interference due to higher propagation losses, and narrower beam widths when compared to lower frequency radios. However, due to street level clutter (trees, signage, etc.) microsites can also take advantage of lower frequency wireless technologies that can cope with reduced line-of-site propagation that will be required in some instances, meaning a mixture of radio technologies will be required in most networks.

The insatiable demand for capacity is driving us to reinvent mobile networks – both on the RAN side, as well as on the backhaul side. Microcells offer one of the most promising methods to increase total network capacity by effectively reducing the number of users per cell and increasing the total number of cell sites. While useful in some instances, a simple product offering for base stations or a single backhaul link does not provide what’s needed for a comprehensive microcell network deployment. In point of fact, what’s needed is a platform-based approach that provides the flexibility to adapt to the multitude of different network deployment scenarios envisioned today and for the future.