In the past, cell towers were built as high as possible in order to blanket wide swaths of ground with radio frequency (RF) signals and cover as many subscribers as possible. Today, industry focus has shifted from simply coverage to “capacity coverage” in order to meet subscriber demand for bandwidth-intensive services. In other words, mobile operators aren’t just concerned with how many subscribers they can cover with a single tower. They’re concerned with how many subscribers they can cover while those subscribers are streaming up to 100 Mbps of video simultaneously.
The proliferation of smartphones and tablet computers will continue to drive the need for increased network capacity coverage over the next several years. Mobile device manufacturers are churning out products that can squeeze more and more bits per hertz from the spectrum, and consumers are buying them by the millions, often resulting in overloaded networks and sluggish or interrupted service.
Certainly, the wireless industry has responded with new technologies to improve capacity coverage. LTE is expected to increase data capacity of networks by 20 times over comparable GSM networks. Emerging small cell technologies can augment towers to boost network capacity in “hot spot” areas, but these technologies are still in their infancy. The widespread buildout of LTE may take at least another two years, and not all providers have committed to it yet.
In the meantime, strategies exist now for operators to optimize their existing networks with minimal capital outlay. Specifically, reconfiguring existing towers now in innovative ways can help mobile operators stay ahead of the capacity crunch, keep customers happy and maximize outside plant investments.
Boost backhaul: When LTE arrives, it will require prodigious backhaul on the order of 300-500 Mbps per cell site. Typical copper T1 lines feeding a 2G or 3G cell tower can deliver 1.5 Mbps per circuit, so a backhaul of 200-400 T1 lines would be required to implement LTE to a copper-fed site. That’s a highly unlikely scenario due to both cost and copper infrastructure availability. Much more likely is that, in the near future, every cell tower will be fed by fiber or microwave backhaul. While both have their advantages, fiber has proved so far to be the most rugged and reliable. Microwave can be susceptible to interference, weather and other natural phenomena, such as sunspots, while fiber is not. Even if a mobile operator is not quite ready to implement LTE at a particular cell site, now is the time to prepare. The first step to LTE is upgrading the backhaul, and operators headed in that direction should be talking to their backhaul provider now about replacing copper lines with higher bandwidth technologies.
Sub-sectoring: Cell towers today are typically tri-sectored. In fiber-to-the-antenna (FTTA) installations, each sector is serviced by a pair of directional antennas and a radio. As mobile data and video increases traffic, the capacity of a single-sector tower can become overloaded. Building more towers to bear the additional load offers one solution, but new tower builds have become increasingly difficult due to cost and regulation. Subdividing existing sectors can provide a simpler solution.
For example, a traditional tower may be equipped with three sets of antennas aimed in three different directions, each covering a 120-degree sector. The entire capacity of each 120-degree sector is supplied by the single radio covering that sector. If the sectors are sub-divided into six 60-degree slices with a corresponding radio per sector, the capacity of each radio is applied only to the smaller coverage radius. This permits concentration of capacity in a smaller area resulting in improved quality of experience for all of the subscribers serviced by the sub-sectored tower.
Sub-sectoring requires retrofitting towers with additional infrastructure, such as radio heads, antennas, fiber cable and tower cabling terminals. Many operators are already preparing for sub-sectoring by installing fiber cable to support 12 and even 18 radio heads per tier. Special tower-mounted fiber terminals can be used to terminate the spare fibers and hold them safely in reserve until new radios and antennas are installed to activate the additional sectors.
Shrinking cells & spectrum reuse: As everyone in the wireless industry is painfully aware, spectrum is a limited and precious resource. In order to boost capacity, mobile operators are maximizing their allocated spectrum by reusing it in smaller and smaller chunks. Cells have shrunk in diameter from seven miles to 70 feet in some instances. Small cells allow a particular frequency to be reused in multiple locations without interference. Each cell serves a smaller number of subscribers and therefore has more capacity to deliver the multi-media services they demand.
Small cell technologies that do not require towers are enabling spectrum reuse, but the tower can support spectrum reuse as well. While 2G installations are expected to remain at the top of the tower, LTE antennas and radios can be set on a lower tier. By placing them closer to the ground and appropriately steering their coverage, a smaller cell footprint is achieved, enabling more spectrum reuse in a given geographic area.
It’s clear to all that smartphones and computer tablets will continue to fly off store shelves at breakneck speed. These sophisticated devices are taking cell networks to the limit of their current capabilities. The industry has responded with new network technologies. LTE promises to improve mobile data speeds 20-fold, but the network buildout will take time. Smart mobile operators aren’t waiting around. They are taking steps to boost current capacity and prepare their networks for the future. It takes some investment, but it’s worth it to keep your customers happily streaming high-quality video while on the fly.
Stephen C. King is a wireless applications engineer with the 3M Communication Markets Division.