Antenna technique boosts capacity and coverage, reduces interference Electronically controlled, directive antennas help cellular site operators to reduce interference, increase capacity, improve coverage (both in-building and rural) and optimize hand-offs
Cellular operators and system planners constantly strive to improve their networks. Once basic RF coverage has been achieved and the time comes to add capacity, interference control becomes the biggest concern. Traditional solutions to the capacity vs. interference dilemma are frequency planning (including sectorization), antenna pattern manipulation and the addition of cell sites. Base site vendors also strive to improve network performance by enhancing call-processing algorithms. Unfortunately, most of the opportunities for large improvements were exploited years ago, and newer improvements tend to be incremental because the cellular site infrastructures are tenacious.
Conventional optimization System designers are generally familiar with the theoretical flat-earth model for RF propagation and frequency planning. The N = 7 frequency reuse version of this “perfect” model targets 17dB of carrier-to-interference ratio (C/I) for acceptable call quality. Still, the model does not account for: * terrain height deviations * azimuthally dependent path losses * log-normal fading caused by large obstacles * variable power output levels for different mobile units * hand-off hysteresis
Inserting these factors in the ideal propagation model quickly shows that deterministic impairment conditions are common, and that they affect quality. These factors force system designers and optimizers to use all available techniques to recover from these serious quality impairments.
The primary tool for interference control has been sectorization. Unfortunately, with sectorization, quality improvements come at the expense of capacity. A three-sectored cell site suffers a capacity reduction of about 30%, compared to an omnidirectional site. To regain capacity lost from sectorization, additional sites are built. Considering an initial investment as high as $1,100,000 per cell site (plus ongoing maintenance, spare equipment, site leasing, T1 and telephone line leasing), the multiyear cost of new sites profoundly affects operating profits. In addition, zoning and litigation processes that always make new site availability tenuous cannot be ignored.
Trunk-hunting sequences are used to interleave the assignments of radios that might normally be interferers. Unfortunately, this method’s effectiveness is reduced during peak usage hours. Antenna downtilting works with sites that are a few hundred feet above average terrain, but this method’s benefits are limited for the vast majority of sites (which are about 100 feet or less in height). Other tools, such as dynamic channel lockout and inner-outer tiers, provide further flexibility, but still are technical bandages, used in an attempt to cure the symptoms rather than the disease.
New approach Instead of trying to “patch-up” existing cellular base sites with minor fixes, we decided to change the nature of the problem with a smart antenna system. A site is modified to accommodate many discrete antenna beams (typically three panel antennas with four 30ø beams for a total of 12 beams), each of which can be accessed by any site radio by means of electronic switches. (See Figure 1 on page 80.)
In the smart antenna system, RF signals from cellular phones are detected on the 30ø antenna beams by dedicated scanning receivers. The beam selections are made by dedicated microcontrollers via electronic switches at rates high enough to effectively track multipath signals in severely cluttered environments. The best two receive beams per signal are then connected by the antenna’s switch matrix to the radio’s diversity inputs. The transmit signal from each radio is routed by the switch matrix to the best beam for transmission.
The advantage of narrow beams is that the downlink power is concentrated in the mobile’s direction, and uplink signals are angularly discriminated from interference. (See Figure 2 on page 80.)
Our system uses 12 narrow antenna beams, either to retrofit a towertop or rooftop application or to create a truly distributed microcellular configuration. The narrow azimuthal beamwidths provide increased antenna gains (compared to typical panels). This increased gain allows absolute power to be reduced on the downlink while improving signal reception on the uplink. The increased gain can manifest itself by coverage extension for rural sites or by improved in-building coverage in urban settings. To solve different operators’ problems, seven multibeam antennas are manufactured with 13dBd to 19dBd gain for both vertical and dual polarization.
Omni implementation Cellular operators and system planners have a proven method for dealing with C/I problems that occur when many channels are added to omnidirectional sites: sectorization. Sectorization comes with a high price, though. For an average C/I improvement of 6.5dB, a cell site converted from omnidirectional to three-sector will exhibit a 30% to 40% capacity decrease.
The cost-effective, technically sound alternative to sectorization is the omnidirectional trunking smart antenna system. For about the same price as sectorization, a site can be converted to a smart system with significant improvements over a sectored site.
The omnidirectionally configured smart system has the following advantages over a three-sectored site: * no trunking losses. * 5dB to 13dB C/I improvement over three sectors (Users have measured C/I improvements of 20dB compared to omnidirectional sites.) * improved sensitivity and coverage due to additional antenna gain. * reduced downlink interference with other sites due to the narrow beams.
On a typical site, these advantages result in a decrease in the number of dropped calls and a corresponding improvement in marginal calls. In addition to these advantages, the smart antenna system does not have cavity combiners, thereby removing the constraints of 21-channel spacing imposed on frequency planners. Other benefits of removing cavity combiners include remote tuning of radios and, possibly, dynamic channel allocation.
Sectorized implementation Dramatic improvements in capacity_between 30% and 40%_can be achieved by converting sectorized sites into smart antenna omnidirectional sites. Yet operators and system engineers have worked hard to optimize sites using sectorization, frequency planning, antenna downtilting, channel interleaving and channel lockout to achieve the present performance. They are reluctant to risk all these iterative gains on a new technology.
Fortunately, the smart antenna system still has considerable advantages in a network when configured for sectorized implementation. Extensive field trials with sectorized smart systems carrying commercial traffic resulted in a reduction in dropped calls ranging from 50% to 70%. The combination of reduced interference and additional gain of the smart antenna can provide as much as 13dB improvement on all calls. Anyone who has experienced a call with C/I at 17dB vs. 12dB knows that the implications for the subscriber are dramatic. The voice-quality data gathered during our trials confirm these improvements. In some marginal areas, the voice was unintelligible on the standard sectorized system. Simultaneously, on the smart antenna system, the call had high audio quality.
Multiple field trials show that the improvement in C/I and improved sensitivity allow a frequency reuse of NK 5 4 with a sectorized smart antenna system. The tighter frequency reuse means that network capacity can be increased about 90%. Alternatively, large sections of the cellular bandwidth can be dedicated to digital systems, and smart antenna improvements can be used to maintain the quality and capacity of the advanced mobile phone system (AMPS) or narrowband AMPS (NAMPS) system.
Straightforward implementation Our smart antenna system is an appliqu‚ and does not require digital interfaces with the site controllers, the switching office or the voice channel controllers. It has a straightforward RF interface to the base site radios.
The system can be used in an uplink-only configuration for improving uplink coverage. This configuration allows perimeter sites to carry calls as much as 40% farther, which reduces call failures, provides better hand-offs and increases roamer revenues. This configuration also provides interference protection from adjacent carriers.
The retro-targeting version provides capacity increases, bidirectional interference improvement, coverage enhancement and reduced absolute output power (for a fixed effective radiated power [ERP]). The fixed, multibeam antenna system (uplink or retro-targeting) provides hand-off characteristic optimization, another benefit of significance to site operators. Because the system can have 15ø or 30ø antenna beamwidths and each of the antenna beams has its own low-noise amplifier (LNA), the LNA gain can be adjusted to optimize hand-offs in any direction. In existing systems with 120ø sectors, a single hand-off threshold applies to every sector. This causes serious problems, because the optimum hand-off level for a mobile on an elevated freeway in one direction might be 10dB or 15dB higher than for a mobile unit in a dense cluster of buildings in a different direction. One of three conditions results: either the mobile on the freeway drags a call into the adjacent cellular site, the mobile in the building cluster is constantly requesting hand-offs, or both of them hand off in a non-optimal manner. With this system, both mobile units’ signals can be compensated to optimize hand-offs and to greatly reduce hand-off dragging.
The multibeam antennas used with this system are passive. The system design avoids towertop active elements to enhance reliability and maintainability.
The system uses a new linear power amplifier (LPA) configuration (called a spatial transform amplifier) that “load-shares” all of the transmit linear power amplifiers. With this scheme, an individual amplifier failure has minimal effect on site performance and will not cause service to be lost on any base site radios. The amplifier can then be repaired when technicians are on routine duty rather than on an emergency call.
Integration with future technologies Operators intending to use time-division, multiple-access (TDMA) modulation, Global System for Mobile Communications (GSM) or code-division, multiple-access (CDMA) technologies will be able to continue to use that hardware in parallel with the analog smart products until smart systems for those technologies are commercially practical. We are working with multiple original equipment manufacturers (OEMs) on methods of sharing antenna systems and linear power amplifiers between our product and other systems. Our system does not impede the installation of these other technologies in the near term. In fact, it actually provides capacity increases that may free spectrum for these technologies.
Additional functions Remote node option _ The system is a true microcell controller. It does not differentiate between multiple beam inputs and multiple microcell inputs. This remote node option allows antennas to be placed inside convention centers, in parking garages and in “coverage holes.” Antennas can be connected to the base site via fiber-optic, microwave or millimeterwave links without radios dedicated to the remote locations. The common pool of radios at the base site is available to the remote node as if it were another antenna beam.
Remote diagnostics _ The system allows the operator to remotely monitor and record beam selections and signal strengths for each radio at the cell site in real time. This remote interface also can be used for monitoring internal and external power supply voltages, transmit amplifier status and site alarms, and for reprogramming the antenna system. The smart system’s supervisory controller is designed to integrate easily with network operations centers, providing alarm messaging and complete remote control.
Redundant operation _ The system includes no single component that, were it to fail, could force a cell site off-line. Distributed control, load-sharing power supplies, distributed frequency references, load-sharing LPAs and load-sharing LNAs make the system extremely reliable. Line-replaceable units (LRU) are used in addition to subsystem components, so in the event of a failure, they can be replaced in a matter of minutes. New LRUs are automatically recognized and programmed without additional steps required by the operators.
Visually unobtrusive antennas _ The system uses one multibeam antenna per face. On an existing tower or building top, these panels can be mounted on the towers themselves and do not have to be mounted on the traditional “top hat.” The antennas are slightly larger than standard panels, but because only one is used instead of three or four, zoning and landlord issues are minimized. When sites are converted to the smart system, the “top hat” can be removed from the towers and replaced with three panel antennas mounted directly on a monopole. For new sites, use of smart antennas without the unsightly top hat may be necessary to gain zoning approval.
Conclusion The smart antenna system is a breakthrough tool for network planners and operators. It provides the first real alternative to sectorizing and can be used to achieve network capacity increases as high as 90%. The system can be run in parallel with new technologies and eventually will be integrated with those technologies.
The system dramatically increases analog network capacity, interference control, in-building coverage, hand-off optimization, site reliability, rural coverage and microcellular architectures.
These improvements can reduce the number of new sites that have to be built, can improve performance and can have a direct influence on the profits of network operators.
