The key to capacity: Smart antennas unlock cell site sectorization so wireless operators can manage and distribute CDMA traffic loading more effectively

The number of code division multiple access subscribers surpassed 1.5 million this year, so it's no wonder cellular operators are racing to expand their digital networks.

Smart antenna-based spectrum management systems offer wireless operators a cost-effective way to expand CDMA service. Dual-mode CDMA and AMPS smart antenna systems let operators get more capacity from their CDMA and analog infrastructure.

CDMA capacity gains of up to 40% enable operators not only to expand service quickly and economically, but to delay the implementation of another CDMA carrier, which is a CDMA channel or carrier band of 1.23 MHz. When the extra carrier eventually becomes necessary, network operators can use the same smart antenna platform to clear spectrum without adding analog cell sites or degrading analog performance.

Operators deploying dual-mode smart antenna technology can use a single set of antennas for both their analog and CDMA networks, yet they can optimize each network's performance independently.

Smart antennas can play a key role in increasing CDMA capacity because they allow operators to manage and distribute traffic loading more effectively.

The geographic distribution of traffic across a CDMA network-even within a single cell-varies considerably. In a typical three-sector cell, the traffic density in the most heavily loaded sector is often more than twice that in the least-loaded sector.

As a result, some cells may have sectors that are fully loaded and where traffic is blocking up, while other sectors of the same cell are well below peak loading and have spare capacity. On a larger scale, high-traffic areas such as highway interchanges, urban centers and shopping centers create hot spots that strain capacity even while network resources go unused in low-traffic areas. This variability in traffic density creates network inefficiencies that hit a wireless operator's bottom line. To make matters worse, CDMA performance is extremely susceptible to variations in traffic density over time.

Flexibility adds value CDMA-compatible smart antennas are designed to help wireless operators cope with variable traffic levels and the network inefficiencies they cause. Connecting to the existing CDMA base station through standard RF ports, the systems feature a 12-beam antenna array integrated with beam-forming hardware and software.

A PC-based configuration tool lets users control the system's operation and manipulate the antenna's radiation pattern.

Smart antennas allow operators to control and optimize CDMA coverage with flexibility and precision. Working within a three-sector configuration, operators can adjust sector orientation by pointing angles in 30 degrees increments, select from sector beamwidths of 60 degrees, 90 degrees, 120 degrees, 180 degrees and 240 degrees, and change gain settings to expand or contract coverage in highly localized areas-all without climbing a tower or mounting another custom antenna.

Using the flexible configuration options of smart antennas, operators can tailor a CDMA cell's coverage to fit its unique traffic distribution. For example, operators can significantly benefit from matching usage levels with an appropriate sector beamwidth.

A relatively narrow beamwidth-60 degrees to 90 degrees horizontal, for instance-might cover a heavy handoff area or a highway corridor, while a moderate beamwidth-90 degrees horizontal-might serve a suburban or light urban area. In low-traffic areas such as mountains, water or rural environments, antennas with wider beamwidths (90 degrees to 180 degrees +) can provide the most effective use of network resources.

Operators also can use smart antennas to respond quickly to time-varying traffic patterns. Using the system's operation, administration and maintenance software, operators can adjust sector configurations on demand and within minutes.

As a result, operators can modify a cell's operation based on the time of day or day of the week, or to accommodate an anticipated surge in call volume from a sporting or community event. In this way, smart antenna technology helps operators avoid the coverage holes that arise from cell breathing.

Load leveling plays traffic cop The ability to adjust a cell's sector configuration based on local traffic patterns is the key to unlocking more CDMA capacity. By using smart antennas to change the size or orientation of sectors, operators can shift the traffic load from an overtaxed sector to one or more underused sectors-in effect, routing network capacity where and when it's required (Figure 1).

Network designs show that using smart antennas to balance traffic loads on a typical network can increase capacity up to 40%. Load leveling allows operators to get more capacity from existing network resources-both the infrastructure they've invested in and the spectrum they've allocated to CDMA.

For an example of the benefits of load leveling, compare the sector configurations in Figures 2 and 3. When conventional sector antennas are used (Figure 2), traffic loads across the three sectors vary greatly, with a single sector handling traffic for the office park and most of the residential area.

By contrast, after deploying a smart antenna (Figure 3), the cell's sectors have been resized and reoriented so the traffic load is more evenly distributed among the three sectors. Each sector's beamwidth has been altered to best accommodate the peak traffic load in that sector.

The narrow sector covering the office buildings maximizes capacity for a high-traffic area, while the relatively wide sector covering the water provides adequate capacity for a low-traffic area. Operators can accomplish these adjustments with minimal impact to neighbor lists and the pseudo-random number reuse plan.

A cell's optimal sector configuration will depend on its unique traffic patterns as well as on the local terrain and RF conditions. The flexibility of a smart antenna system lets operators pursue various strategies for alleviating network hot spots.

In some cases-for example, hot spots greater than a sector width-the greatest capacity gains may be achieved by splitting the traffic load between two sectors. On the other hand, highly concentrated hot spots may be served most efficiently by focusing a narrow sector on the traffic.

With smart antennas, operators can test several antenna configurations quickly and easily from the switch without installing multiple antennas.

Smart antenna systems also can help operators better understand what is happening in their CDMA networks. Along with increased sectorization control, smart antenna systems provide RF engineers with the ability to measure and report CDMA performance statistics in 30 degrees increments around the cell site. Using this data, engineers can better assess network needs, determine optimum sector configurations and eventually reduce the time and money spent on drive testing.

The unconventional route Besides being a more immediate, flexible and cost-effective solution to boosting CDMA capacity than several conventional infrastructure solutions, smart antennas offer several advantages for improved network optimization. Other approaches may generate new engineering challenges.

Converting from a three-sector to a six-sector configuration is one solution. It requires a long-term, permanent investment in additional hardware and change-out of antennas, and it raises compatibility issues with analog sectorization.

Increased sectorization also can cause or intensify pilot pollution problems and reduce network efficiency because of higher handoff overhead.

Smart antennas, by contrast, not only increase network capacity but allow operators to reduce pilot pollution by sculpting cell coverage in 30 degrees increments.

Cell splitting is another method for alleviating network hot spots. But adding another cell site requires re-optimization of a substantial portion of the network, a lengthy and labor-intensive process. And few cellular operators overlaying CDMA on an AMPS network relish the thought of adding CDMA capacity cells.

By getting more capacity from an operator's existing network infrastructure, smart antennas enable wireless operators to postpone deployment of the ultimate capacity boost-adding another CDMA carrier.

The load-balancing benefits of smart antennas let operators delay the sizable investment in the infrastructure required to add another carrier. With added radios, amplifiers and antennas, that expense can exceed $200,000 per cell site. Moreover, smart antennas can alleviate highly localized hot spots, an application for which adding another carrier is not a viable solution.

Eventually, operators will expand CDMA capacity by adding another CDMA carrier. At this point, the major hurdle facing operators is clearing spectrum without disrupting analog service.

Cellular operators, unlike their PCS competitors, must use a portion of their existing spectrum to deploy CDMA service within their networks.

This requires taking analog channels out of service to clear spectrum for the CDMA carrier. Operators must displace analog channels each time they add another CDMA carrier. Indeed, today's digital migration is an ongoing, repetitive process that will continue well beyond 2000.

To make matters worse, most operators must implement a geographic guard zone, usually one cell wide, surrounding the core CDMA service area.

This guard zone requirement means operators must clear spectrum and cut into analog capacity in a much larger geographic area than just the core CDMA service area.

Operators that have successfully cleared spectrum for their initial CDMA carriers now are finding that doing it a second time around is more difficult. Any excess spectrum already has been brought into service, and in many cases, analog traffic is simply not going away. Many operators don't consider adding capacity by building analog cell sites, either because of budgetary priorities, community resistance or the inability to put sites in optimal locations.

So what does an operator do? Dual-mode smart antenna systems offer operators a single platform for managing capacity and spectrum in an integrated CDMA/AMPS network.

Smart antennas can double AMPS capacity, allowing operators to support more analog traffic on fewer channels. In analog systems, they provide superior interference isolation.

As a result, operators can increase network capacity by tightening frequency reuse and increasing trunking efficiency. Consequently, when the time comes to deploy a CDMA carrier-whether it's a first, second, third or subsequent carrier-operators can use smart antenna technology to clear spectrum for digital service without investing in new cell sites or jeopardizing analog quality.

In addition, these dual-mode spectrum management systems use the same physical structure for CDMA and analog signals but allow operators to configure different sector beamwidths and orientation for the two networks. This gives operators the flexibility to adjust optimization parameters for CDMA and analog networks independently, and it spares them the cost, permitting and tower-loading concerns of erecting an extra set of antennas.

Dual-mode systems also use the same linear power amplifiers for CDMA and analog signals, giving operators a flexible and cost-effective solution for meeting the increased power requirements of expanded CDMA service.

The migration to CDMA networks is not just a behind-the-scenes process endured only by operators. Although subscribers may not attribute poor service directly to digital migration, they are affected by lack of capacity, dropped calls and redirection to analog. And if subscribers are dissatisfied, there are probably four or five competitors waiting to snatch them up.

Given this scenario, maximizing CDMA and AMPS capacity while performing optimization as precisely and economically as possible has become even more critical to operators' profitability. Whether an operator is experiencing analog growth, digital growth or a combination, dual-mode smart antenna technology helps accommodate increased subscriber demand quickly, economically and with considerable flexibility.