4.4 diversity combining techniques
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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.
4.4 diversity combining techniques
- 2. Lecture Videos are available for this slides at www.youtube.com/gurukula Support by Subscribing to my Channel
- 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