Antenna Systems for Polarization Diversity

Antenna Systems for Polarization Diversity

During the last couple of years, antennas for mobile networks have sprouted up like gladioli all over our downtown areas. For aesthetic reasons and due to the fact that it has become more expensive to acquire the rights to install antennas on buildings and towers, methods to reduce antenna size and numbers have attracted immense interest. Traditionally, fading is reduced in mobile phone systems by implementing space diversity on the uplink, that is, using two separate receive antennas for each sector. With dual-polarized antennas, this number can be reduced to one.

Claes Beckman and Ulrik Wahlberg
Allgon System AB
Toby, Sweden

When a mobile phone is moving in a radio environment, the received signal will experience fluctuations in amplitude.1 These fluctuations can be explained by three different effects: path loss, shadowing and multipath fading.2 Multipath fading is caused by multiple reflections from local scatterers such as houses and buildings so that the signals take several paths between the transmitting and the receiving antenna, as shown in Figure 1 . The received signal becomes a vector summation of all received rays. If two rays have a phase difference of 180¡, a cancellation occurs and the call may be dropped.

Diversity Systems

One way to reduce the effects from multipath fading on the uplink is to implement a diversity combining technique. Diversity combining in some systems is also used in the mobile (for example, the Japanese Digital Cellular system), but it is more efficient and cost effective to implement it at the base station.

For diversity combining, two or more transmission paths must be received, all carrying the same message but exhibiting independent fading characteristics. The mean signal levels of the different paths also must be somewhat comparable. There are several methods to achieve two or more transmission paths, including space, angle, frequency, time and polarization diversity.1,2

If the signals received at the base station have comparable mean signal strengths they can be combined. Generally, four different methods are used to combine the received signals,2,3 including selection, maximum ratio, equal gain and switched. Selection and maximum ratio are the two most commonly used methods. In selection diversity, the receiver selects only the path with the highest signal-to-noise ratio (SNR); in maximum-ratio combining, all received paths are combined in an optimum way using both amplitude and phase.

As a measure of improvement in signal quality when using a diversity combining technique, the term diversity gain is used, which is defined as the difference in signal level between one reference signal and the signal received at the output of the diversity combiner. Unfortunately, no real diversity gain standards exist for which reference signal or signal probability to use, meaning that the measure is not absolute.

In mobile phone networks, space diversity is used mainly at the base station. This scheme uses two separate antennas for each sector positioned approximately 20 wavelengths apart to receive independently fading signals. Thus, the antenna systems become large. For example, for the Global System for Mobile communications, a horizontal separation of approximately 6 m(20 l) is used normally, as shown in Figure 2 . If the two antennas for each sector are positioned with vertical separation, an even wider distance is required.

Polarization Diversity

Polarization diversity has been discussed for 20 years, but has become of real interest only recently. The main reason for this shift is that the method does not require any extra bandwidth or physical separations between the antennas. With polarization diversity, only one dual-polarized antenna is used, as shown in Figure 3 . However, the two polarizations must be orthogonal, for example, horizontal/vertical or ±45¡ slanted, as shown in Figure 4 . The method is based on the fact that two orthogonal polarizations provide almost uncorrelated signals in a scattering environment.

A study was conducted recently investigating the use of different dual-polarized base station antenna configurations to determine their uplink performances. The study revealed that, in general, the diversity gain received from polarization diversity is approximately 1 dB less than the gain received from spatial diversity. However, the results were strongly dependent on the environment and the inclination angle of the transmitting antenna. In the suburban area and with the terminal in talk position, the results were indistinguishable.

When comparing the horizontal/ vertical antenna with the ±45¡ slanted antenna, the ±45¡ antenna was found to perform better, independent of the combination scheme and especially in urban areas. The main reason for this result is the horizontal/vertical system has large differences in mean signal levels whereas the ±45¡ system has approximately equal signal strengths. In this study, the cross-correlation coefficient between polarization branches was low for both systems (< 0.3).4 This result indicates that cross correlation is not, by itself, a good enough measure to describe the performance of a polarization diversity system.

Mutual Coupling Effects

Unfortunately, all dual-polarized antennas suffer from cross coupling between their two ports. To specify how well such an antenna separates its two polarizations, the terms antenna isolation and cross-polarization discrimination (XPD) are used normally. Seeing the antenna as having two orthogonal elements and two ports, isolation is defined as the ratio between the power fed into one port to the power received at the other. On the other hand, the XPD is defined as the ratio between the power received in the orthogonal, or cross-polar, port to the power received at the co-polar port when the antenna is excited with a wave polarized as in the co-polar antenna element. This XPD measure is also a function of angle.

In order for the system to work well on transmit, the isolation in the antenna is often required to be at least 30 dB. However, for the uplink, the coupling can be described by XPD only. In the study, attempts were made to analyze the cross-coupling effects on the diversity gain through simulations. The results from a worst-case scenario, namely the one when the incoming signals add together 180¡ out of phase, are shown in Figure 5 . The diversity gain from selection combining is plotted as a function of XPD. The signals were measured in downtown Stockholm. The diversity gain was referred to the +45¡ branch and calculated at a 90 percent signal probability.

As can be seen, the diversity gain is dependent on the cross coupling in the antenna, indicating that the XPD is indeed of importance for a well-functioning polarization diversity scheme. However, the effect may not be as strong for maximum-ratio combining.

Conclusion

With the introduction of new mobile phone networks such as the personal communications network in Europe and personal communications service in North America, ways of reducing the size and numbers of the antennas needed for deployment have become important issues for manufacturers and operators in the industry. Polarization diversity has proved to be one fruitful way of achieving these goals. In order to obtain good diversity performance, high performance requirements must be placed on the antennas. Since the XPD may be crucial to polarization diversity, the antenna radiation patterns should always include co- and cross-polar field components.5 n

References

1. T. Carey, "Fading and Multipath Testing in Communications Systems," Microwave Journal, Vol. 39, No. 11, November 1996, pp. 90-98.

2. P.C.F. Eggers, J. Toftgerd and A.M. Oprea, "Antenna Systems for Base Station Diversity in Urban Small and Micro Cells," IEEE Journal on Selected Areas in Communications, Vol. 11, No. 7, September 1993, pp. 1046-1057.

3. R. Vaughan, "Polarization Diversity in Mobile Communications," IEEE Transactions on Vehicular Technology, Vol. 39, No. 3, August 1990, pp. 177-186.

4. U. Wahlberg, "Polarization Diversity for Cellular Base Stations at 1800 MHz," Allgon System AB Technical Report No. T1017, January 1997.

5. B. Lindmark, "A Beamwidth Definition for Slanted Dual-polarized Base Station Antennas," Microwave Journal, Vol. 40, No. 5, May 1997, pp. 324-328.