Make six-sector work using smart antennas, Part 1

"Data" is the buzzword du jour on the lips of just about everyone in the wireless industry, and certainly there is ample reason to believe that non-voice traffic will play an increasingly large role in defining the technology, the business opportunities, and the challenges of the wireless future. What is often overlooked in the enthusiasm to gaze over the data horizon, however, is that growth in voice subscriber numbers and minutes of use show little sign of slowing. Although the impact of the data explosion on carriers' networks remains largely hypothetical, the challenge of providing sufficient network capacity to handle current (mainly voice) traffic is already a daunting reality for many operators. Whether one focuses on the impending demands of data traffic or the needs of more than 60 million current CDMA subscribers in the here and now, the issue of how to increase network capacity in a cost-effective, scalable manner is pivotal.

Among other approaches to this problem, six-sector cells and smart antennas are two ideas that have generated a great deal of interest among network operators. There is a synergy between these two concepts, and smart antenna technology can facilitate the successful implementation of six-sector in CDMA networks.

Six-sector theory and practice
In theory, increasing the number of sectors in a CDMA cell is a good way to increase its capacity. Everything else being equal, a six-sector cell should offer double the capacity of a three-sector cell with a similar coverage footprint.

Unfortunately, everything else is not equal: As the number of sectors increases, the total area of the softer handoff zones between sectors increases, which, in turn, increases the "handoff overhead" of the cell. Because mobiles in softer handoff require downlink transmit power from more than one sector at a time, handoff overhead exacts a direct cost in terms of cell capacity.

Similarly, the more sectors, the greater the likelihood of pilot pollution. As the number of strong pilots in any locale increases, the noise floor rises, with a direct and negative impact on capacity. Thus, although increasing the sectorization of a cell increases capacity in some ways, it decreases it in others, and in practice, cells with more than three sectors have generally not offered anywhere near the expected capacity payoff.

In addition, implementing six-sector with conventional antennas has typically entailed serious challenges. The sheer number of separate antennas required for a six-sector deployment means that the physical installation and alignment processes are painstaking and expensive. Iterative tower climbs are the order of the day in optimizing a conventional six-sector site, and that's only after the ever-more-difficult zoning battles have been waged and won.

The smart antenna six-sector solution
Smart antenna systems make six-sector a practical proposition. They do so in three principal ways: By reducing handoff overhead, by easing the implementation burden, and by facilitating successful optimization.

The size of the softer handoff zones between sectors in a CDMA cell is a function of the roll-off characteristics of the antennas employed. The sharper the main-lobe roll-off, the smaller the areas of overlap between sectors where mobiles will be in softer handoff. The software-defined sector patterns produced by the phased-array panel antennas of a smart antenna system display much sharper roll-off than do conventional antennas—so much sharper that the handoff overhead of a six-sector cell equipped with smart antennas can be roughly comparable to that of a typical three-sector site with conventional antennas. This means that the theoretical capacity gains offered by the additional sectors can actually be realized in practice with smart antennas.

Smart antenna systems ease the implementation burden of six-sector deployments, chiefly by reducing the number of antennas required on the tower. With conventional antennas, six-sector can require as many as eighteen antennas, each of which must be precisely aligned for the site to operate properly. With a smart antenna system, as few as three antennas do the job, regardless of whether the site is configured for three, four, five, or six sectors.

Smart antenna systems can significantly reduce the amount of additional equipment necessary to implement six-sector. Of course, six-sector requires two base station radios, but with conventional antennas, it also requires twice the power amplifiers, duplexers, filters, and cabling compared to three-sector (see Figure 1). Because a smart antenna system manages the RF signal flow from the base stations all the way to the antennas, no additional amplifiers, duplexers, filters or cabling are required (see Figure 2).

Finally, smart antennas make six-sector optimization much easier. Through remote, software control, network engineers can manipulate the gain and phase of each individual narrow beam comprising the smart antenna pattern, thereby "sculpting" the coverage footprint to manage pilot pollution and control the location of handoff regions. Smart antennas allow six-sector to be successfully deployed in sites where optimization issues would prevent it with conventional antennas.

Part 2 of this two-part series involves a case study of a commercial deployment of a smart antenna system.

About the author
Ji-Hae Yea is a network engineer at Metawave Communications Corp. He can be reached at 10735 Willows Road NE, Redmond, WA 98502; phone: 425-702-6573; Fax: 425-702-5679; e-mail: ji-hae.yea@metawave.com. (Back to top)