Microcellular Networks Are Crucial to LTE Rollouts

By John Spindler, ADC

Since cellular networks switched from analog to 2G digital more than 10 years ago, carriers have sometimes had to split cells to improve capacity in dense markets and inside heavily populated venues like airports and stadiums. However, because 2G and 3G services don’t promise huge amounts of bandwidth per subscriber, this kind of cell-splitting has always been done as an afterthought – as a way to correct specific problems in specific venues. 700 MHz will be the first new cellular service that will require small-cell network architecture from Day One.

Today, major cellular providers in the U.S. are planning for 700 MHz LTE service deployments well in advance of actual rollouts by deploying small cells from the beginning, and they are using distributed antenna systems (DAS) to extend the range of these systems while reducing frequency and backhaul management issues. The reason is that capacity, not coverage, will be the primary challenge in delivering LTE, and that capacity will be needed in far more locations than before.

How LTE Impacts Network Capacity

Generally speaking, the lower the frequency of a radio wave, the better it penetrates obstacles such as building walls. LTE rollouts at 700 MHz will use a much shorter frequency than 3G frequencies such as 1900 and 2100 MHz, so carriers can expect better coverage inside buildings with LTE. But coverage won’t be the issue with LTE – capacity will. With LTE reaching inside buildings, there will be far denser user populations to be served. Because LTE represents a quantum leap in the amount of data that must be delivered to each subscriber and it must serve denser populations, cells will have to be smaller.

Published estimates by Verizon and other major carriers suggest that LTE service will range from 5-12 Mbps on the downlink and 2-5 Mbps on the uplink. This is far greater than the .8-1.2 Mbps downlink and .2 -.4 Mbps on the uplink promised for 3G services such as EV-DO, UMTS, and HSPA. As a result, each of the subscribers in a given area will need 5-10 times as much available data capacity.

While there are some efficiencies to be gained from the LTE protocol through the use of 2x2 multiple input/multiple output (MIMO) configurations, the plain fact is that in order to bring 5-10 times as much data to each subscriber, it will be necessary to divide that capacity among fewer users. As a result, cells must be far smaller than they are today. 

How Small Cells Improve Capacity

Cell-splitting, or breaking up a large cell into smaller ones, has been a common practice among carriers for several years in areas where capacity demands outstrip network capabilities. This situation typically occurs in dense urban areas or large public venues (airports, stadiums, etc.) where an unusually high concentration of cell phone users demands network services at the same time. This problem is frequently addressed by dividing these areas into “sectors,” and delivering discrete capacity to each of these sectors. However, in dense areas where heavy sectorization has not been used, the user experience can be dropped calls, slow call setup times, or slow network responses in areas where we have “four bars” of signal strength, and this is because while there is plenty of coverage, there is not enough capacity to service our needs.

With LTE, which is all about wireless data, the capacity crunch will extend to more areas than ever before. Along with stadiums and transit hubs, we will see capacity constraints in city centers, suburban shopping malls, office buildings, universities, hospitals and many other locations. But by deploying a higher number of smaller cells, carriers can provide adequate capacity for everyone under LTE. The key question becomes, “What’s the best way to deliver small-cell architecture?”

The Small Cell Challenge

Traditionally, mobile operators have simply deployed cell sites (on towers, rooftops, etc.) wherever they need more coverage or capacity. A new generation of pico and femtocells is smaller and less expensive and can be deployed fairly unobtrusively, and these will be added to the mix of options as carriers build out their LTE networks. However, if we can estimate that LTE infrastructure will require at least three times as many cell sites (and more likely 4-5 times as many) there will be many key challenges for operators no matter what types of cell sites they deploy:

• Each pico or femto BTS can carry only one frequency and it is likely that carriers will need to support multiple frequencies over time. This means that they will need a unique pico or femto BTS in each location for each frequency to be provided.  This will have a negative impact on the economics of the solution from both an equipment cost and on-going maintenance perspective.
• Each pico or femto will need its own backhaul connection, again negatively impact cost.
• The carrier must negotiate locations, real estate leases, power supplies, and other issues related to deployment of a new base station.
• Many cities are already reluctant to approve new large cell site locations due to aesthetics or zoning restrictions.
• Increasing the number of cell site locations by 300-500 percent causes proportional increases in management complexity and operational costs.
• Base stations deliver both coverage and capacity, so operators must deploy more coverage (even if it isn’t needed) to gain more capacity.

Figure 1 shows an urban core with a 3G infrastructure. Figure 2 shows an LTE infrastructure. Both examples rely on base stations only for coverage and capacity.

Using DAS to Improve Small Cell Architecture

Distributed antenna systems (DAS) extend the reach of cell base stations by delivering the base station signals through a series of small remote antennas. These antennas can be inside buildings, parking lots, in underground subways or other structures, or they can be located in any outdoor areas where more coverage is required.

Unlike base stations, which must be laboriously configured and deployed, DAS products simply extend a base station signal – they can carry whichever frequencies the base station puts out, and the more advanced active DAS products deliver the same high, uniform signal strength at every antenna, no matter how far it is away from the base station source. Once deployed, DAS are virtually transparent to the network and are largely trouble-free. Their small antennas fit easily on the sides of buildings, in office ceilings, or even in street furniture so they can be deployed in more places without objections from city planners.

Rather than adding cell sites to extend coverage and capacity, the carrier can extend coverage using cost-effective DAS antennas, as shown in Figure 3.

Figure 3: DAS extend the coverage of cellular base station hotels while delivering the required capacity.

This approach offers many significant benefits for mobile operators:

• Using DAS brings signals closer to users to deliver more uniform, high-quality coverage than base stations alone can provide
• Using DAS makes it possible to easily and economically add either capacity (by adding new base stations to existing sites or creating centralized “base station hotels”), or to add coverage by increasing the size of the DAS.
• DAS make it possible to use far fewer base stations, thereby reducing management and deployment costs as well as backhaul costs and overall network complexity
• Using small DAS antennas helps overcome zoning and aesthetics issues and speeds deployment
• Using DAS improves spectral efficiency because directional antennas can control the area of signal radiation to help eliminate interference among adjacent sectors and channels.
• Using DAS enables easier MIMO deployments, which can further improve coverage or capacity, as needed.

To date, DAS have been seen as an ad hoc measure taken to address specific coverage or capacity issues in specific venues. But with the coming of LTE and its enormous new requirements for capacity in the network, DAS will be the most cost-effective and flexible means of providing that capacity.

John Spindler is vice president of product management for the Network Solutions business unit at ADC.