Space vs. Polarization Diversity

Many cellular and PCS carriers have been experimenting with polarization-diversity receive systems as substitutes for proven space-diversity receive systems. But before you make this change in your diversity scheme, you need to gain a clear understanding of the trade-offs involved between space-diversity and polarization-diversity techniques.

For several years, both cellular and PCS operators have been using space-diversity reception techniques to improve uplink performance. They expect to see improvements by horizontally spacing two antennas at least 10 wavelengths apart. This is based on the concept that during a deep fade at one antenna location, the fade will not be as severe at the other receive-antenna location. A diversity combiner then is used to either mix both signals together or select the best one.

There are clear benefits to using space diversity. It offers proven improvement in uplink performance for reception of both mobiles and portables. But a suitable structure is required to allow for the 10-wavelength spacing of the two receive antennas.

During the last two years, carriers have been experimenting with the use of polarization-diversity techniques. Polarization diversity is different from space diversity. It is based on the concept that in high multipath environments, the signal from a portable received at the base station will have varying polarization. The mechanism of decorrelation for the different polarizations is the multipath reflections encountered by a signal traveling between the portable and base station. The reflection coefficient encountered by each polarization typically is different. You can improve uplink performance by using two receive antennas with orthogonal polarizations and combining these signals. Because the two receive antennas do not need to be spaced apart horizontally to accomplish this, you can mount them under the same radome.

When communicating with portables, polarization diversity works like space diversity in high multipath environments such as dense urban. However, it doesn't offer the same performance when communicating to mobiles.

As early as 1990, experimenters such as Rodney Vaughan were testing the concept of polarization-diversity systems.

"For suburban base stations, the dominance of the vertical polarization makes the diversity gain rather small -- only a couple of decibels at the 99.5% probability level," he said. "In urban environments, however, the diversity gain is nearly 7dB at the 99.5% level, offering much promise for system design using polarization diversity."

Polarization diversity does have its benefits. It's easy to obtain a suitable site because you will not need the large structures that are required for space-diversity techniques. But polarization diversity is completely effective only in high multipath environments.

Some manufacturers have promoted polarization diversity as performing better than space diversity in all environments. But note that where high multipath environments do not exist, the performance of the polarization- diversity antennas may not be as good as the space-diversity system.

Also, polarization diversity does not appear to be effective for communicating to mobiles using vertically polarized antennas. It is best used in systems with a high percentage of portables. If you are considering polarization diversity, evaluate polarization diversity in its environment to see how it compares with space diversity before making any assumptions about performance.

PCS providers also should consider the potential for long-term intermodulation (IM) issues with the current polarization-diversity designs. The basic test consists of two 20W cameras simultaneously applied to the antenna and then looking at third- and fifth-order IM products. To be considered acceptable, the antenna should have a third-order IM product below -100dBm. None of these antennas passed this standard IM test. If a system carrier used any of these antennas in its system, it potentially could suffer serious intermodulation problems once the system is fully loaded.

Realize that when you duplex transmitters and receivers on one antenna, you lose additional isolation protection from inter-modulation products. Polarization-diversity antennas for PCS systems all are designed for duplex operation. In addition, none of the current antennas on the market passes a basic IM test.

Polarization diversity is a useful technique in the proper environment. But be aware -- you may not have optimized systems if it is used in an environment without the necessary multipath. Before assuming that polarization diversity works in a particular environment, perform field testing to compare space diversity and polarization diversity. Polarization diversity antennas do not have as high of a front-to-back ratio as some optimized vertically polarized antennas. Many companies also are not aware that the front-to-back ratio of polarization diversity antennas typically is measured incorrectly.

Also, be aware of the potential long-term IM problems with current PCS polarization-diversity antenna offerings. Undoubtedly, manufacturers will develop new polarization-diversity antenna products in the future that do not suffer from these potential, long-term IM issues.