The spatial diversity antenna system is constructed by physically separating two receiving base station antennas. Here I write on spatial diversity antenna system criteria, configuration.
SPATIAL DIVERSITY ANTENNA SYSTEMS
- Once they are separated far enough, both antennas receive independent fading signals. As a result, the signals captured by the antennas are most likely uncorrelated.
- The further apart are the antennas, the more likely that the signals are uncorrelated.
- The types of the configuration used in GSM networks are:
- horizontal separation
- vertical separation
- Composite separation.
TYPICAL SPATIAL ANTENNA DIVERSITY CONFIGURATIONS
CRITERIA FOR SELECTING TYPE OF SPATIAL SEPARATION
Branch correlation
- The physical limitation of the supporting structure should also be considered while selecting the spatial diversity antenna configuration. For example, if a wide framework is not permitted on top of a mounting tower, vertical separation is an alternative to be considered.
- To achieve the required correlation coefficient (r £0.7) different configurations require different separations.
- The separation indicated in Table below shows that low values of correlation are more easily obtained with horizontal rather than vertical separation.
- That is why most of the diversity antenna systems in GSM networks use horizontal separation.
Signal level difference
- A system using horizontally separated diversity antennas has a symmetrical configuration and is therefore able to provide balanced signal strengths.
- A system using vertically separated antennas needs large separation to meet the required correlation.
- The consequence is that the two antennas have different antenna height gains, which may result in imbalance between the two signal strengths.
Angular dependence
- Angular dependence reflects the dependence of the performance of a diversity antenna system on the angular position of a mobile relative to the boresight of the antenna.
- Horizontally separated antenna system has high dependence on the mobile’s angular position.
- The effective separation reduces as the mobile moves away from the antenna boresight.
- As the mobile is 90° off the antenna boresight, the effective separation becomes zero.
- In such a case, the signals from two antennas are very likely coherent which will then lead to a deterioration of the diversity performance.
- Most of the GSM cell sites are 3 sectored cell sites.
- The maximum angular offset is therefore approximately 60°.
- Simulation shows that the performance of a horizontally separated antenna system experiences noticeable deterioration only when the angular offset exceeds 70°.
PROS AND CONS OF HORIZONTAL CONFIGURATION
Advantages
- Easier to achieve low values of correlation and balance between the signals. Hence widely used.
Disadvantages
- High angular dependence. The impact is however marginal for sectorised applications.
- Require sizable head frame on the supporting structure.
PROS AND CONS OF VERTICAL CONFIGURATION
Advantages
- Slim supporting structure.
- Angular independence
Disadvantages
- Require large separation for low values of correlation.
- May cause imbalance between the two diversity branches.
- Generally not used.
