Wireless traffic and spectrum demand continues to grow at an exponential rate as user minutes increase, subscriber numbers grow, MP3 and multimedia phones are deployed and new data-rich applications like cell phone TV and personal digital assistant (PDA) gaming catch on. Operators, in turn, are scrambling to meet the network capacity and coverage demands for today’s revenue generating services while simultaneously investing in new broadband data network deployments

The mobile wireless customer is a fickle one. With many competitive carriers to choose from and the added convenience of number portability, users show little allegiance to their providers as indicated by industry subscriber churn numbers which remain at 25%-30% turnover per year. Recognizing this, operators continue to aggressively invest in their networks to improve capacity and quality and ensure a better-quality experience for the end user.

The latest network spectral-efficiency technologies such as enhanced power control, and Adaptive Rate voice codecs, have largely been deployed and exhausted. Carriers—seeking to avoid costly alternatives of building new cell sites or procuring spectrum—are looking for alternatives that will profitably see them through the dramatic growth in network traffic and optimize their available spectrum in transitioning to new broadband network technologies. Non-invasive, low-cost spatial-processing techniques such as site designs with Higher Order Sectorization are getting a fresh look from network operators for both existing and new broadband network deployments.

Understanding and Accommodating Traffic Growth

According to recent Cellular Telecommunications Industry Association (CTIA) reports, new subscriber growth and increased usage have boosted the average traffic per U.S. cell site 25 percent annually over the last five years (Figure 1). And that’s just the national average. In the metropolitan markets which generate the highest operator revenues, this growth in cell site traffic can double every two years. This trend stands to continue, with wireless usage becoming more predominant and the introduction of new, higher-speed multimedia services.

Figure 1: Cell Site Traffic Growth

If that comprised the whole story of the capacity challenge facing carriers and network planners, addressing network growth needs would be simpler. In fact, it’s complicated by the fact that network-capacity requirements are not uniform—not only across the continent, but also within a given local market.

The highest site traffic demand (and, therefore, capacity constraints) is found within only a small percentage of the overall network (Figure 2). For example, a recent study of select U.S. markets showed that only 25 percent of the site sectors in a mobile network typically serviced more than 50 percent of the overall network traffic. Furthermore, it is frequently true that in high-traffic sites, only one out of the three sectors of a typical tri-sector site may be heavily loaded requiring relief.

Figure 2: Network Traffic Distribution

Even with the latest digital air interfaces and other new network features, carriers still need to resort to adding new cell sites or purchasing additional spectrum to add significant spectral capacity. The problem here is that neither option is a particularly simple—or inexpensive—endeavor. In the urban markets where additional capacity is most needed, it is complex to gain the litany of necessary permissions and satisfy zoning concerns to lease new cell site space. For a new macro cell site, the process of site acquisition to service light-up can run up to two years, and initial capital costs alone can run $250,000. While purchasing an additional spectrum for a given market is a large upfront investment to solve the problem in a subset of the network.

The challenge is to find targeted alternatives that allow operators to cost effectively enhance network capacity and quality in identified congestion points while using their existing site infrastructure and available spectrum.

Higher Order Sectorization for Capacity Enhancement

With carriers seeking to address traffic growth today and ensure that degraded service quality doesn’t run off the fickle customers, a spatial-processing technique for enhanced capacity and coverage receiving renewed interest in the industry is Higher Order Sectorization sites beyond the typical three sectors. Today, Higher Order Sectorization, also commonly referred to as ‘six (6)sector’ sites, is an increasingly promising solution for current Global System for Mobile Communications (GSM), Enhanced Data for GSM Evolution (EDGE) and Code Division Multiple Access (CDMA2000) network evolution—and is planned for Universal Mobile Telecommunications Systems (UMTS) and other next-generation deployment models.

A recent CDMA Development Group (CDG) whitepaper (see Delivering Voice and Data: Comparing CDMA2000 and GSM/GPRS/EDGE/UMTS, http://www.cdg.org/resources/white_papers.asp), identified ‘6 sector’ sites as one of the key voice and data capacity advancements available for the 2006 and 2007 timeframes. Depending upon which of the above technologies is being compared, the CDG study points to a potential 70% to 100% improvement in voice capacity, and 50% to 70% improvement in data throughput above current network baselines.

Beyond the original concept of ‘six sector’ only sites, Higher Order Sectorization today also incorporates the broader requirements and feasibility of evolving an existing network by adding additional sectors to improve capacity and service quality within network hotspots as required, without altering and re-engineering the overall network (Figure 2).

Figure 3: Network Evolution to Higher Order Sectorization

Today, most base stations offered by the established vendors support up to six sectors, and with advancements in dynamic frequency planning, tools, and sophisticated network features for automated site optimization, a number of the key enablers are now present. In addition, moving to multi-sector sites is compatible with network-based techniques (Adaptive Rate voice codes, enhanced power control, fractional re-use, etc.) for enhancing capacity and quality.

The significant potential cost savings of enhancing capacity and quality using existing network infrastructure through High Order Sectorization has long been desired by network operators. However, beyond many of the above key Base Station enablers now in place, past industry experience in multi-sector site deployments using standard antennas with narrower beamwidths failed to deliver the anticipated performance gains, and were impeded by site implementation challenges.

Replacing a standard antenna in one sector with two narrower, symmetrical-beam antennas might have delivered only a 30- to 35-percent increase in site capacity because of excess handover areas and coverage gaps. Then there are the operational hurdles to consider. Installing, aligning and optimizing additional antennas are complex and expensive—and might even require additional leased space. The less-than-anticipated performance gains and operational challenges discouraged previous carrier attempts in adopting Higher Order Sectorization.

Along with the aforementioned advancements in Base Station features and network planning support, what’s been needed is matching improvements in RF pattern and antenna design to enable optimal performance and deployment ease.

Innovation in ‘Intelligent RF’

Today, new adaptive array technology can now been used to address the previous application performance and deployment shortfalls in moving to Higher Order Sectorization sites. A new type of Base Station array is now available in the marketplace, the Bi-Sector Array, which has been specifically designed for multi-sector site applications in today’s generation of networks.

The Bi-Sector Array’s unique asymmetric patterns and single dual-sector panel addresses key network design and implementation issues that to date have inhibited operators from applying Higher Order Sectorization for increased network capacity and quality. By intelligently shaping and optimizing the azimuth radio frequency (RF) patterns, the Bi-Sector Array reduces overlap between adjacent sectors and aids suppression of inter-sector interference (Figure 3). And the composite dual paired beams produce coverage that matches (and, in some instances, betters) that of today’s standard, 65-degree beam width sector.

Figure 4: Azimuth RF Pattern Comparison

The operational headaches are lessened, as well. Existing tower or rooftop-mounted sector antennas are replaced with a single antenna facet. Sector pattern alignment is pre-set, ensuring optimal handover overlap and alignment. Standard RF cables and connections link the Bi-Sector Array to existing base-station site equipment, and the solution is compatible with any existing Tower-Mount and Low-Noise Amplifiers.

Conclusion

The Bi-Sector Array’s “drop-and-insert” capability supports affordable, need-driven, non-disruptive network expansion in Higher Order Sectorization site designs. Network operators can realize the required capacity and coverage gains in a matter of mere weeks without compromising service quality, building new cell sites or dramatically altering existing site infrastructure. Upgrades are quick and cost-effective, and the network operator remains in position to support legacy and future handsets and remain compatible with future base-station technologies.

With wireless usage exploding—in terms of both number of users and variety of application-rich, high-speed services—carriers need cost-effective, incremental means for accommodating growth in targeted, high-traffic areas of their networks. Non-invasive, low-cost spatial-processing techniques such as Higher Order Sectorization sites enabled by intelligently designed Bi-Sector Arrays for maximum RF performance, delivers the relief they require.

Ross Ernst is Vice President of Marketing for TenXc Wireless.