by William C. Drach
This paper will address why operators need CAEDT antennas, the benefits that these antennas provide and their ability to solve wireless system problems
Today wireless carriers and system optimizers face a maze of factors that must be considered when deploying, maintaining, and expanding their wireless networks. Factors that can have a negative impact on wireless system performance include pilot pollution, co-channel interference, carrier-to-interference ratio, passive intermodulation distortion, co-location, scarce site real estate, difficult geographical terrain, and demanding system deployment rates.
All of these factors can contribute to an increase in subscriber churn and thus negatively impact the bottom line of wireless carriers. Subscriber churn is directly correlated to insufficient calling area coverage, poor call quality, and dropped call rates, which are all affected by the previously defined factors. Furthermore as current wireless networks evolve into the next generation (2G, 2.5G, 3G) systems, much more demand will be placed on system performance.
This paper will show how the deployment of Continuously Adjustable Electrical Down Tilt (CAEDT) antennas that offer both vertical and polarization diversity can single handedly address most of the factors that impact wireless network performance and subscriber churn.
In order to successfully traverse the maze of obstacles that affect network performance, system architects and optimizers should have control over as many variables as possible. Some of these variables include antenna-azimuth-beam-width selection, down link power, pilot tone power, neighbor lists and receiver sensitivity.
One very powerful variable that directly affects network capacity is cell size. In general, the larger the cell size, the lower the capacity. Having the ability to reduce the cell radius increases capacity, but only if soft hand-off and softer hand-off are optimized. If hand-off is not optimized, the capacity will be wasted by single mobiles communicating unnecessarily with several base stations simultaneously. In a perfect world a wireless system architect could place base stations at equal spacing creating a uniform matrix of coverage.
Because site location is limited by terrain, real estate, and zoning issues, there is always a compromise between cell size and cell overlap. The cell size can be reduced by mechanically down tilting (MDT) the antenna, however, this only reduces the radius of the horizontal radiation pattern directly in front of the antenna. The horizontal radiation pattern perpendicular to the front of the antenna does not change. This phenomenon is commonly known as "peanuting" because the azimuthal radiation pattern looks much like a peanut.
An antenna with CAEDT capability avoids this problem in both space and polarization diversity systems and is always preferred over a mechanical downtilt antenna (MDT).
Solving Co-channel Interference and Pilot Pollution with CAEDT Antennas
The peanuting effect caused by MDT antennas creates problems with pilot pollution in CDMA systems and co-channel interference in cellular (AMPS/TDMA) systems.
Extensive field-testing has demonstrated that CAEDT antennas are an excellent choice for CDMA systems. Drive tests of CDMA systems employing CAEDT show significant improvement in frame error rate, and elimination of dropped calls.
With CAEDT antennas, increased carrier to inference ratio can be achieved. Interference caused by other cell sites on the horizon can be reduced by down tilting the vertical pattern and lowering the amount of gain on horizon. Further reductions in inference can be achieved with the elimination of the peanuting pattern. MDT antennas provide little help little if the interference is on horizon and perpendicular to the antenna's bore sight. (See "Electrical Downtilt Through Beam Steering Versus Mechanical Downtilt," by G. Wilson, IEEE 07803-0673-2/92, Vehicular Technology Conference 1992.)
Public Safety Interference Solution
Interference between commercial mobile radio services (CMRS) and public safety wireless networks (PSWN) will only get worse with re-farming of spectrum by the FCC in the 800 and 700 MHZ bands. More digital modulation schemes are being deployed that include TDMA, FDMA, CDMA, WCDMA, OFMD, 2slotTDMA, 4slotTDMA, C4FM, C2PSK, and DCMA. These digital systems will not only cause sideband noise to fall in the receive bands of PSWN systems but will generate active and passive intermodulation interference which will fall into the receive bands of CMRS systems.
CAEDT antennas can reduce interference for PSWN and other CMRS operators by reshaping their coverage area and reducing antenna gain on horizon, so as to reduce the amount of inference into PSWN fringe areas. This process could also eliminate the need for a costly transmit filter in the CMRS system. For more information on this topic visit; http://www.apco911.org
Solution to the NIMBY Problem
Issues that make site acquisition and approval difficult include lengthy zoning processes, aesthetics that are less than acceptable to "NIMBY" (Not In My Back Yard) protestors, perceived health issues and environmental backlash. All of this has lead to a decrease in the speed of construction. Polarization diversity antennas can improve the aesthetics by replacing the traditional AMPS platform with three polarization diversity antennas around a monopole. Polarization diversity antennas also reduce tower loading by reducing the number of antennas.
Very careful consideration is required when choosing polarization diversity over space diversity in an antenna. Coverage to portables in environments with significant multipath is about as good as with space diversity, but in suburban environment where there is much less multipath, system performance will suffer. If an operator has a situation that can take full advantage of polarization diversity, the CAEDT antenna option should be selected.
If mechanical downtilting is used on slant 45 polarized antennas the polarization quality ratio will suffer along with the horizontal pattern tracking. The distortion in horizontal pattern tracking will cause large gain variations between polarizations, which in turn will cause deep signal fades and thus dropped calls. In order to achieve effective polarization diversity in a down tilt scenario one should always use electrical down tilt, and mechanically down tilting should be avoided.
Solving Geographical Terrain Problems with CAEDT Antennas
There are many geographical obstacles that must be overcome in a wireless network. Following are a few examples.
Water produces about 10dB/decade/mile of free space propagation loss compared to 20dB/decade/mile on flat earth. This fact can cause significant trouble for system planners who have base stations near bodies of water. Lakes, ponds, oceans, bays, rivers, and streams can all produce unpredictable propagation effects. For example, if a CDMA base station is located on the edge of a bay, and is intended to service a highly populated town center, it can suffer from dropped calls caused by pilot pollution from a base station on the other side of the bay. The interfering signal propagates across the water about twice as far as it would across land.
This problem can be eliminated with a CAEDT antenna, by down tilting the receiving antenna on the cell sector that faces the water. This reduces the excessive converge radius of the water facing sector, which reduces the pilot signal strength from the interfering base station, while maintaining the intended coverage, and thus eliminates the dropped calls.
Just as water can cause unpredictable propagation variables, so can manmade structures. Alleys, corridors, skyscrapers, highways and bridges can all produce undesirable propagation phenomena. Again, this type of obstacle can be reduced or eliminated with CAEDT antennas. By deploying a system using CAEDT antennas, cell sizes can be precisely controlled to eliminate many sources of interference and pilot pollution.
Large deviations in elevation are found in nature in the form of hills, mountains and valleys, and caused by man in the form of tall buildings and structures. All of these deviations in elevation create difficult cell planning. System planners often use mechanical down tilt on mountains and tall buildings only to have problems with pilot pollution and interference, case by a peanuting pattern. The solution is to use CAEDT antennas to precisely down tilt the coverage area into the valley or street below.
Solving Celestial Noise Problems with CAEDT Antennas
The flux of radio emission from the sun is centered at about 2.8 GHz, significant RF noise is generated in the wireless frequency band. In addition to the sun's radio emissions its ultraviolet emissions greatly influences the earth's upper atmosphere and ionosphere. These two solar emissions have a direct impact on the ambient noise level of all RF receivers. We are currently at the peak of the three-year cycle. I make this point to demonstrate that wireless system must be designed to be flexible in order to coexist with sun. (For more information visit NASA's solar disturbances predication page; http://science.msfc.nasa.gov/ssl/pad/solar/predict.htm,)
Solution to Passive Intermodulation Caused by Surroundings
It can be shown that there is a calculable probability of passive intermodulation distortion produced as a function of the cell radius. (See L. Sh. Alter, Probability of Intermodulation Interference of Land Mobile Cellular Radio System, EMC 2000, Pg 679, Radio Research and Development Institute, St. Petersburg, Russian Federation). By deploying a system using CAEDT antennas, cell radius can be precisely controlled to eliminate some sources of passive intermodulation caused by the sites surroundings.
Solution to Dynamic Changes in the Environment
Up to this point we have discussed obstacles that are static. What about obstacles that are dynamic? We live in a world of continual, fast paced change, which means one can not afford to build a wireless network that can not be adapted to deal with new variables. Dynamic environmental obstacles such as new buildings and structures, demolished buildings and structures, new cell sites (both in and outside ones network), new FCC regulations, natural disasters, and even celestial disturbances, will require a system to change.
Probably the best investment an operator can make in his wireless network is the deployment of CAEDT antennas either initially or as a replacement to existing antennas. The added cost of this feature is far less than the potential cost of future churn caused by dynamic environmental obstacles. To illustrate this, imagine an operator with a newly built network that works fine for about one year. Suddenly the operator begins to receive complaints about dropped calls. A drive test of the trouble spot determines the cause to be from co-channel or pilot pollution interference. This new inference was generated by one of the previously mentioned dynamic environmental obstacles.
The operator is now busy building a new network and does not need this problem. It is determined that reducing the radius of one of the sector's coverage will fix the problem.
If the operator has had the foresight to deploy CAEDT antennas in his network, he simply sends a technician to the site to add a few more degrees of down tilt to the affected antenna(s). Had he installed a fixed tilt or mechanical downtilt antenna on this cell site, he would need to try downtilt the antenna mechanically and hope the resulting peanuting pattern does not aggravate the problem.
Or, he could remove and replace the fixed electrical tilt antenna for another one with more fixed electrical tilt and hope that the tilt angle is correct, since the choice of fixed electrical tilt antennas is limited.
Another major problem with removing and replacing antennas of course is the site will need to be turned off, meaning that network performance, with all its consequences, will be impaired. CAEDT antennas permit the site to remain operational during adjustment.
CAEDT Antennas Permit System Cost Reduction
In addition to the increased revenue caused by reduced subscriber churn, further equipment cost savings can be achieved by the operator. Future site costs are reduced by the need to install only one antenna and by eliminating the need to swap one fixed electrical down tilt model for a different down tilt model at a later date. Other economies can be achieved by reduced inventory management costs since fewer antenna models are required. Volume discounts on single models and continued cellsite operation during adjustments are other cost saving avenues with CAEDT antennas.
Factors to Consider When Selecting A CAEDT Antenna Vendor (PIM, warranty, reliability, efficiency).
When selecting a continuously adjustable electrical down tilting antenna one should be very careful. Because there are several ways to produce phase shift and thus electrical down tilt, the phase shift method should be thoroughly evaluated. Most importantly there should be no moving metallic parts in the antenna. Because PIM is generated by currents flowing over metal discontinuities, and currents are induced by electric and magnetic (EM) fields, and since the EM fields are strong on the inside of the antenna, PIM could be generated by moving metallic parts in and around the antenna. It is safe to say that moving metallic parts will be a source of PIM, sooner or later. One of the best ways to produce phase shift is to use only dielectric materials. Dielectrics (insulators) by definition cannot generate PIM. These materials are the optimum choice for producing phase shift and thus CAEDT.
In addition to the phase shifting mechanism used in CAEDT antennas, there are other important features to look for in vendors -
- In vertically polarized CAEDT antennas one should look for high front to back ratio, high upper sidelobe suppression, good null fill, and VSWR<=1.5:1.
- In orthogonal polarization CAEDT antennas one needs to add more specifications like, high port-to-port isolation, high polarization quality ratio, and good horizontal pattern tracking. Not only are these specifications important at the minimum tilt value, but at all tilt values. The vendor should specify the worst specification over the full range of tilt. The last thing an operator needs is for an upper sidelobe to "grow" as they downtilt the antenna.
- Some manufactures even claim that the upper sidelobe always grows when an antenna is electrically downtilted. While it is true that some designs will exhibit this characteristic, quality CAEDT antennas do not do this.
- Finally, care should be taken to select a CAEDT antenna vendor, who production tests each and every unit for VSWR, PIM, and pattern, and overall tilt settings. This is the only way to guarantee that the antenna performs to the advertised specification.
CAEDT antennas represent an excellent investment for wireless system operators because they provide a solution to most system optimization problems, thereby improving network performance, reducing churn, and contributing to system profitability.