Basically Diversity Antenna required because of Signal Rich at mobile or BTS by reflection and this change value of its level and quality either good or bed. Let’s understand in detail.
NEED OF DIVERSITY
- In a typical cellular radio environment, the communication between the cell site and mobile is not by a direct radio path but via many paths.
- The direct path between the transmitter and the receiver is obstructed by buildings and other objects.
- Hence the signal that arrives at the receiver is either by reflection from the flat sides of buildings or by diffraction around man made or natural obstructions.
- When various incoming radio waves arrive at the receiver antenna, they combine constructively or destructively, which leads to a rapid variation in signal strength.
- The signal fluctuations are known as ‘multipath fading’.
- Multipath propagation causes large and rapid fluctuations in a signal
- These fluctuations are not the same as the propagation path loss.
Multipath causes three major things
- Rapid changes in signal strength over a short distance or time.
- Random frequency modulation due to Doppler Shifts on different multipath signals.
- Time dispersion caused by multipath delays
- These are called “fading effects
- Multipath propagation results in small-scale fading.
- Diversity techniques have been recognized as an effective means which enhances the immunity of the communication system to the multipath fading. GSM therefore extensively adopts diversity techniques that include.
CONCEPT OF DIVERSITY ANTENNA SYSTEMS
- Spatial and polarization diversity techniques are realized through antenna systems.
- A diversity antenna system provides a number of receiving branches or ports from which the diversified signals are derived and fed to a receiver. The receiver then combines the incoming signals from the branches to produce a combined signal with improved quality in terms of signal strength or signal-to-noise ratio (S/N).
- The performance of a diversity antenna system primarily relies on the branch correlation and signal level difference between branches.
CORRELATION BETWEEN BRANCHES
- The branch correlation coefficient (r) represents the degree of similarity between the signals from two different receiving branches.
- The correlation coefficient ranges from 0 to 1.
- r=1 means the signals from two different branches behave exactly the same. In this case, the signals are coherent.
- r=0 means the signals from two different branches behave completely different. In this case, the signals are uncorrelated.
- To achieve the best performance, a diversity antenna system is required to provide uncorrelated signals.
- For r=1, the diversity antenna becomes ineffective in combating the multipath fading.
- In reality, however, it is not always practical to have a diversity antenna system which guarantees r=0. Extensive research in this field has revealed that a diversity antenna system can perform satisfactorily provided that r £0.7.
SIGNAL LEVEL DIFFERENCE
- The second key parameter for a good diversity antenna system is the mean signal level difference.
- The difference is a statistical parameter which indicates the balance of the signal strengths from the two receiving branches.
- In a real system, the statistical balance can be verified by comparing the mean values of the two signals measured over a lengthy period.
- If the ratio betn the median values is 0dB, the two receiving branches are statistically balanced.
- The performance of the diversity system will deteriorate while the ratio increases or decreases from 0dB.