This document discusses different techniques for achieving diversity in wireless communications and combining received signals: 1. Selection diversity techniques select the strongest signal from multiple antennas, either based on received signal strength (RSSI) or bit error rate (BER). Combining diversity techniques combine all received signals. 2. Combining diversity techniques include maximal ratio combining (MRC), which weights signals by amplitude, and equal gain combining (EGC), which weights all signals equally after phase correction. MRC achieves better performance than EGC when signals are highly faded. 3. The document compares the advantages and disadvantages of different selection criteria and combining techniques. It also describes switched and feedback selection diversity approaches.

1. DIVERSITY
COMBINING
TECHNIQUES

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3. Diversity Achieved. How to Resolve?
Selection
Diversity
Combining
Diversity

4. Signal Combining Techniques
• Selection diversity
 Best signal is selected, and the rest are discarded.
• Selection
• Switched
• Feed back
• Combining diversity
All signals are combined together and then it is decoded.
• Maximal ratio combining
• Equal gain diversity

5. Selection Diversity
• There are 2 selection criteria:
• RSSI – Received Signal Strength Indication
• BER – Bit Error Rate
• The receiver selects the signal with the
largest instantaneous power

6. RSSI – Driven Selection Diversity

7. BER – Driven Selection Diversity
• Working…
• We first transmit the Training Sequence. (ie) Know signal / sequence.
• The Rx then demodulates the signal from each antenna and compares /
correlates it with the transmit signal.
• The antenna which received the smallest BER is chosen as the best and
that signal is processed further.

8. BER – Driven Selection Diversity

9. Merits and Demerits of RSSI and BER
• RSSI
• Merits:
• Only one RF Chain is used
• Process is done on only one signal
• Easy to implement
• Demerits:
• Waste of signal energy by discarding (n-1) received signals
• Not an optimum method
• BER
• Demerits:
• More number of Rx are used.
• Implementation is complex
• Training sequence is to be repeated again and again.
• Tradeoff between the duration of training and BER should be
maintained.

10. Switched Selection Diversity
• The main drawback of the selection diversity is its criteria.
• RSSSI and BER has to be monitored continuously on all
branches.
• Leads to complex designs and heavy hardware requirements.
These drawbacks are eliminated by switched selection diversity.

11. • In this method the selection criteria is monitored
only in the active branches.
• If it falls below a certain threshold value, then the
receiver switches to a different antenna.
• Case 1: All branches have equal power then the
selection of active branch is RANDOM.
• Case 2: All branches are below threshold level,
then the receiver just switches back and forth
until an active line is detected.
The performance of the switched diversity is
worse than the selection diversity. Hence it is not
considered widely.

12. Feed Back Diversity
• Also called scanning diversity.
• This is a combination of selection and switched diversity.
• All the available channels are scanned first in a fixed sequence
until one is found above the threshold level.
• The signal is received from that antenna until it falls off the
threshold value and scanning process is initiated again.

13. 2. Combining Diversity
• It exploits all available copies of signals. Each signal
copy is multiplied by a (complex) weight and then
added up.
• Weight = phase correction + weight of amplitude
• Phase correction is done to make the signals
coherent.
• Amplitude weighting has 2 methods:
• Maximal ratio combining (MRC).
• Equal gain combining (EGC).

14. Maximum Ratio Combining
• This method weighs all signal copies by their amplitude.
• They also does the phase correction for different antennas.
Merits:
• Output are acceptable even when all the received signals are faded
highly.

15. Equal Gain Combining
• This method weighs all the signals with equal
amplitude and performs phase correction to give
equal gain diversity.

16. MRC Vs EGC