Repeater's New Role

Mar 15, 2001 12:00 PM, Mark Davis

3G networks will mean new license costs and the cost of new infrastructure. New base stations and packet-handling equipment must be added, and leased-line costs will rise as facilities are expanded to accommodate the additional capacity.

The technical challenges of offering wide-area, high-speed data adds to the cost of new systems. Although the new technologies offer higher data rates, data throughput degrades as the user moves away from the cell. The implication is a requirement for two to five times as many cell sites in 3G networks as in 2G networks.

Carriers also must consider the uncertainty surrounding future subscriber voice- and data-service usage. A Forrester research study (www.forrester.com) predicts that European voice-revenue streams, for example, will decline 36% by 2005. As for data, a Herschel Shosteck Associates study (www.shosteck.com) concludes that if operators bear the costs of providing ubiquitous high-data-rate coverage, “the tariffs they would have to charge for transmitting high bandwidth would prove too onerous for end users to accept.”

The Shosteck study’s conclusion is that a compelling business case, more so than any killer app, holds the key to profitability for 3G operators. The use of repeaters can help achieve that profitability.

Early Success Limited

The role of repeaters in the early days of cellular was limited. Most commonly, higher-power-translating repeaters were used in remote areas due to the absence of leased facilities. These repeater locations often were the last site in an area, after which coverage ceased. Lower-power repeaters occasionally were used as outdoor RF hole fillers and indoor coverage enhancers. Fiber-linked repeaters were introduced quite successfully into such metro areas as Los Angeles, Chicago and Vancouver where carriers were able to supply their own fiber.

These first-generation repeaters achieved some modest successes but were limited by their own technology and by the technology of the networks in which they operated. Low-power repeaters probably never could approach the per-carrier output power of AMPS and TDMA base stations. High-powered enhancers had per-channel output power comparable to that of a base station but the requirement for translation made handoffs a significant challenge.

With some notable exceptions, carrier experiences with repeaters rarely were rewarding.

Moving Into 2G

The year 1996 saw the arrival of the first PCS carriers and the first widespread deployment of North American CDMA networks. Today, most cities have a minimum of four service providers, all of which must accept a smaller share of the market and lower per-minute prices than their predecessors.

The more competitive environment raised new financial and economic challenges, particularly for the smaller local or regional providers.

In an era of rapidly increasing subscriber penetration, 2G digital technologies, through their efficient spectrum use, have made urban build-outs possible. In fact, the high capacities of CDMA networks in particular have made it possible for some operators to offer “all you can eat” service plans.

The challenge of deploying such capable technology is achieving effective capacity usage. For every CDMA site that is experiencing blocking, there are often many more in the network that are underused. This situation is exacerbated in 1.9GHz networks, where many more sites are required for coverage. In PCS CDMA networks, a significant percentage of even urban cell sites are underused.

For the same reasons, operators serving rural and suburban areas — where coverage is the driving factor behind site placement — can be faced with an economic predicament. Not only do they have several competitors, the population density in their licensed coverage areas is low enough so that most base stations deployed will be underused, particularly with CDMA networks.

High-powered CDMA repeaters have provided these coverage-driven carriers with the means to cost-effectively deploy CDMA networks in their suburban and rural BTAs. The approach that many of these operators use is to design their networks from the outset with a combination of base stations or microcells and repeaters.

Such a network is known as a hybrid network. Other carriers have constructed hybrid networks after the initial build-out by replacing underused base stations with repeaters and moving those base stations to spots where capacity is required.

CDMA hybrid networks are possible because CDMA base stations require lower-power amplifiers than other technologies. A reasonably priced repeater can be manufactured with similar output power to a CDMA base-station sector.

CDMA networks operate using a 1:1 frequency-reuse scheme. Consequently, there is no requirement for frequency-reuse planning. Repeaters can be used anywhere in the network and can simplify planning by reusing existing pilots.

Finally, CDMA networks thrive on delay and multipath signals. These multipath signals are processed by the RAKE receiver that is present in both CDMA base stations and CDMA handsets. The CDMA RAKE receiver can discriminate at least three multipath signals and, by doing so, yields a system gain known as diversity gain. If engineered properly, repeaters can take advantage of this same diversity gain. Specifically, a CDMA repeater must have a diversity capability that allows the donor base station to receive two separate uncorrelated signals from the repeater. Without the diversity capability, a repeater will degrade the quality of the service, as measured by increased mobile-transmit power requirements and higher reverse-frame error rates.

Unlike base stations and microcells, repeaters do not have any downstream hardware or software costs, nor do they require backhaul, resulting in capital and operating savings.

Repeaters for 3G

Repeaters that are engineered for operation in cdma2000 and WCDMA networks are tools for operators seeking to maximize high-data-rate coverage and to minimize capital and operating expenses. Specifically, repeaters can help carriers meet these challenges:

• Cost effectively maintain data throughput over a given suburban area. As described above, a 1-for-1 3G overlay of a 2G network will not enable high data throughput everywhere, due to inadequate signal-to-noise ratios. More sites must be added at a time when subscriber requirements for data usage are uncertain and the inherent capacity of each base-station site is substantial.

By deploying new repeater sites rather than new base sites, signal-to-noise ratios can be augmented so as to maintain high data throughput. In this way, subscribers reap all the benefits offered by a base station, but the operator reduces capital and operating costs. If subscriber demand rises to the level where additional capacity is required, a base station then can be placed at the repeater location. This 3G-hybrid-network approach is the least expensive way for operators to enable higher-data-rate applications without committing to the expense of base stations.

• Reduce site costs in urban areas. Urban areas undoubtedly will be the place where operators first deploy 3G equipment. By co-deploying repeaters at the same sites with base stations, operators can use single-sector base stations and reap a corresponding backhaul savings, while still maintaining a 3-sector footprint. Like the hybrid network, this hybrid-site approach minimizes capital and operating expense, allowing the operator to add capacity as user demand and revenue increase.

• Provide cost-effective high-data-rate coverage to rural areas. Although many operators will not place an immediate priority on deploying high-data-rate infrastructure in rural areas, rural inhabitants eventually will need to be offered the same opportunities as the rest of the population.

Providing this service profitably, however, will be difficult due to the extremely low population density, roamers included. The use of repeaters offers one alternative for making high-data-rate rural coverage for operators more cost-effective.

A single base station can be used in combination with as many as two repeaters per sector. By strategically placing repeaters exactly where coverage is required, the available base-station capacity can be used the most efficiently. This is especially critical where small population centers are connected by long highway stretches. Along these stretches, capacity needs are very limited, and a cell site’s coverage potential inherently is underused.

As today’s operators and license holders begin to deploy 3G networks, they will face much tougher and more complex business and technical challenges than they ever have confronted in the past.

Tight control of spending — while simultaneously enabling new applications — is a prerequisite for an attractive business case. By using repeaters effectively, operators deploying cdma2000 and WCDMA networks can do both.

Davis is Repeater Technologies (www.repeaters.com) director of marketing.