space diversity

Spatial diversity is one of several diversity methods. With spatial diversity, the same radio signal is transmitted over different radio links to the receiving unit and received by several antennas. Techniques in which space diversity is used are multi- antenna systems.

The aim of space diversity is to combine the signals received by the different antennas in such a way that an optimum received signal is available to the receiver. Since the radio signals are applied to the antennas with different phase angles and received signal strengths, these characteristics must be taken into account when combining the radio signals.

The different space diversity methods

There are different space diversity methods. The relatively simple switched combining( SC) works with one antenna until the input field strength falls below a threshold. Then the system switches to another antenna with a higher field strength. Another method, Selection Combining (SC), operates with multiple antennas and uses the channel with the best signal-to- noise (S/N) ratio. When an SC system uses three antennas, the antenna gain improves by 2.6 dB. Space Time Coding( STC), which is used in Mobile-WiMAX, reduces interference.

A more difficult space diversity technique is equal gain combining( EGC). In this technique, the diverse received signals are shifted to a uniform phase and their levels are added.

Maximum Ratio Combining( MRC) is the technique with the best results, but it is also the most complex. In addition to phase alignment and symbol correction as in the EGC method, the MRC technique also has a weighting of the individual signals. Thus, signals with a high signal-to-noise ratio are weighted higher than those with a lower one.

In general, it can be stated that the spatial diversity depends primarily on the number of antennas and that a noticeable improvement can be achieved with the selection techniques.