by Andrew Singer, Director, Technical Marketing, Celwave
Many cellular and PCS system operators have been experimenting with polarization diversity receive systems. These polarization diversity receive systems have been considered as substitutes for proven space diversity receive systems. Before such a change is made in the diversity scheme used by an operator, it is important 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. By horizontally spacing two antennas at least ten wavelengths apart an improvement can be expected. 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 is then used to either mix both signals together, or select the best one.
Benefits of using space diversity are the proven improvement in uplink performance for reception of both mobiles and portables. The negative of space diversity would be that a suitable structure is required to allow for the ten wavelength spacing of the two receive antennas.
During the last two years operators have been experimenting with the use of polarization diversity techniques. Polarization diversity is quite different from space diversity. Polarization diversity is based on the concept that in high multipath environments, the signal received at the base station, from a portable, 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 is typically different. By using two receive antennas with orthogonal polarizations and combining these signals, an improvement in uplink performance can be expected. The two receive antennas do not need to spaced apart horizontally to accomplish this, so that they may be mounted under the same radome.
Polarization diversity has been proven to work similarly to space diversity in high multipath environments, such as dense urban areas, when communicating with portables. The same performance is not experienced when communicating to mobiles. As early as 1990 experimenters such as Rodney Vaughan were testing the concept of polarization diversity systems. He concluded that "For suburban base stations, the dominance of the vertical polarization makes the diversity gain rather small - only a couple of dB at the 99.5% probability level. In urban environments, however, the diversity gain is nearly 7 dB at the 99.5% level, offering much promise for system design using polarization diversity." (1)
Benefits of polarization diversity include, not needing the large structures associated with space diversity techniques and the related ease of obtaining a suitable site. Potential negatives of polarization diversity include that it is only completely effective in high multipath environments, such as dense urban environments and the potential for long term intermodulation issues in PCS systems. These intermodulation issues will be discussed later in this paper.
Operators should be aware that certain manufacturers have been promoting polarization diversity as performing better than space diversity in all environments. The astute operator should note that were high multipath environments do not exist, the performance of the polarization diversity antennas may not be as good as the space diversity system. Additionally, 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. Any operator considering polarization diversity would be well advised to compare polarization diversity in their environment to see how it compares to space diversity, before accepting any biased statements in regard to the relative performance of each.
Electrical characteristics of polarization diversity antennas that operators should look for include good cross-polarization discrimination and orthogonality of polarization. The better the "cross-pol" characteristics, the better the diversity gain of the antenna. Operators should look for antennas that provide at least -20 dB cross-polarization discrimination. Similarly, orthogonality is important, but difficult to measure. Be wary of companies claiming superior orthoganlity performance without substantiating these claims. The port-to-port isolation should be at least -30 dB, to assure proper isolation of the receiver from transmitter side band noise.
Another cautionary note for PCS operators in particular concerns potentially serious long term intermodulation issues with the current polarization diversity designs being promoted for PCS systems. Some advertisements have actually claimed that polarization diversity antennas improve IM performance and reduce dropped calls. These are obviously ludicrous claims. We have measured several manufacturers polarization diversity antennas for intermodulation products. The basic test consists of two 20 watt carriers 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 -100 dBM. None of these antennas passed this standard IM test. If a system operator were to use any of these antennas in their system, they have the potential to suffer serious intermodulation problems once the system is fully loaded. This potential will be even higher at co-located sites. The operator must realize that when they duplex transmitters and receivers on one antenna, they lose any added isolation protection from intermodulation products. Polarization diversity antennas for PCS systems are typically designed for duplex operation. Even when placing one transmitter on each port of the antenna, the current density at a given point in time, can be high on the opposite dipole element.
The astute operator will also consider the robustness of a manufacturers mechanical design. Antenna manufacturers will typically use one of three design techniques; discrete elements connected with cable and solder joints, PC board technology or monolithic construction. Of particular concern are designs using PC board technology. While they often are the lowest cost to produce, and thus attractive to some manufacturers, there can be several design deficiencies with PC board antennas. Due to the losses in the PC board material, PC board antennas do not have as high an antenna efficiency as other designs. This will lead to lower gains for a given aperture relative to other designs. PC board antennas are typically not DC grounded and due to their thin conductors they can fail quickly in lightening prone areas relative to other designs. Finally, PC board antennas are often not as environmentally robust as other design techniques.
While polarization diversity is a useful technique in the proper environment, operators need to be aware that they may not have optimized systems if it is used in environments without the necessary multipath. Before assuming that polarization diversity works in a particular environment, an operator would be wise to perform field testing to compare space diversity and polarization diversity. Additionally, PCS operators need to 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.
It is now the end of 1998 and after being fortunate enough speak with numerous operators about the performance of polarization diversity antennas, one can form a clearer picture of their performance. In dense urban environments where significant multipath exists polarization diversity antennas appear to offer reasonable performance relative to space diversity antenna systems. However, in suburban and rural environments they do not offer the same system performance as space diversity antenna systems.
(1) Polarization Diversity in Mobile Communications, Rodney Vaughan, IEEE Transactions on Vehicular Technology, VOL . 39, NO. 3, August 1990