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

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.

Angular dependence

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.

ANTENNA SPATIAL CONFIGURATIONS