I Can't Hear You

In-building wireless solutions proliferate as customers depend more on their wireless phones.

I'm in my office. Let me call you back on my landline; my coverage is lousy in here.

It's a line most heavy wireless users have uttered at some point, and carriers are becoming more concerned about it. With the increasing proliferation of cellular and PCS devices, more and more wireless customers would like to use their devices indoors - whether it is in their homes, in the mall, while they roam around their office complexes, or while stuck in rush-hour traffic a hundred feet below the Hudson River in the Lincoln Tunnel.

Ask any wireless analyst about trends in the industry, and it is a good bet the list will include the increasing number of people whose primary telephone is their wireless phone. Although this is good news for wireless operators looking to increase minutes of use, it creates a problem for customers who spend most of their day indoors - in RF-fortified buildings, tunnels or subways that most wireless carriers didn't plan on having to penetrate and are unsuitable for a standard base-station installation.

Most wireless carriers now are searching for solutions to this issue - hoping to keep the high-end business customers happy with their coverage on the top floor of their corporate headquarters, as well as satisfying the gabbing needs of the shopping-mall-bound teenagers. Fortunately, a variety of products and methods have been developed and put into practice to solve these problems. In the high-wireless-penetration markets of Asia and Europe, such solutions have been deployed in massive numbers, perhaps even making in-building wireless solutions as important a product as the base station itself.

Rev Up Base Station

Perhaps the easiest solution is to simply maximize the performance of existing base stations. Many deployed base stations across all technologies are often under used, leading to a smaller footprint and thus less building penetration. Very often, the limiting factor is not the downlink but rather the uplink. The downlink often can be improved via increased power or a change in antenna orientation, although there are some limitations on this such as co-channel interference in TDMA systems, or pilot pollution in CDMA systems. Thus, in most cases performance engineering can improve the forward link so that it is not the limiting factor. The problem lies in the uplink because most handsets only are capable of transmitting from 200mW to 600mW and no more. The only way to increase the performance of the uplink is to improve the reception, or sensitivity, at the base station. One of the best methods to improve a base station's sensitivity as well as its selectivity is to employ state-of-the-art superconductor-based filters and super-cooled, low-noise amplifiers. This technology gives a base station's receiver front end a lower noise figure, which allows for increased sensitivity. In addition, the superconductor-based filter has brick-wall filtering to get rid of unwanted interferers. The end result is a stronger footprint, with signals that are often strong enough to penetrate into and out of the building or enclosed location.

Repeater Pros & Cons

In situations where the base station has been maximized, but there still is not enough coverage in a particular location, repeaters often are a solution. One type of repeater is the over-the-air repeater. Such a repeater essentially consists of two low-noise amplifiers connected to an antenna, which amplify received mobile signals back to the base station and base-station signals to the mobile users. Some over-the-air repeaters do get a bit more complex by translating channels. The over-the-air repeater is marketed as an easy solution since it requires no hookups other than power and, of course, the antenna. Many of these repeaters are small in size, able to be hidden away in the rafters or above a dropped ceiling and are available at a low price.

The problem with this solution is it depends on the serving base station for its channels. In other words, the base station is expanding its footprint, but it is not increasing the number of channels available. Although this may not be a problem in certain low-capacity situations, it can cause big problems in many typical in-building situations. For instance, a typical application might be a suburban indoor shopping mall. A base station might serve the outside area of the mall, perhaps the parking lot as well as the surrounding business area. An over-the-air repeater, placed inside the mall to give coverage, would be served by the outside base station and would be using the capacity of the particular sector of the base station serving the mall area. Thus the serving base-station sector would have to carry all of its normal traffic from its own footprint as well as the traffic picked up by the repeater in the mall. It is conceivable that a mall would demand the capacity of perhaps an entire sector or more itself, and thus this solution would not be the most effective. An additional problem is that since the repeater depends on two RF links in both directions, there is often degradation in the quality of the channels.

A third in-building solution that is used extensively is the fiber-optic-fed repeater. This product works essentially the same way as the over-the-air repeater, except the link between the serving base station and the repeater is a fiber-optic line. In this way, an entire sector of a base station can be located remotely from the base station. Since a carrier can dedicate an entire sector or more to the distributed repeaters and their antennas, capacity is not an issue, and because the base-station-to-repeater link is over fiber, the quality is significantly better than with over-the-air repeaters. A common technique is to distribute the capacity of a serving sector via multiple fiber-fed repeaters. The drawback to this method is the need to run fiber from the serving site to the repeater locations. This often can be time consuming as well as having the added complexity associated with real-estate issues.

Picocell Base Stations

Another method for getting in-building coverage is to install dedicated base stations and distributed antennas. Although a standard macro base station would be overkill for most applications, infrastructure vendors now offer picocell base-station solutions. These pico base stations can be hidden in most locations, require lower power and can be replaced if they malfunction. Although such a solution does offer the best capacity and coverage benefit, installation does require a backhaul hookup that would complicate the installation, and the cost would generally be higher than the repeater alternative.

Getting adequate coverage in difficult locations is a challenge to which many wireless carriers are now devoting a lot of effort. As customers demand better-quality service in their homes, offices, subways and tunnels, infrastructure vendors will continue to develop better solutions, and performance engineers will find themselves indoors performing walk tests instead of their traditional drive tests.