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Questions and Answers
What is one primary purpose of diversity techniques in wireless communications?
What is one primary purpose of diversity techniques in wireless communications?
What is meant by the 'diversity order' in the context of wireless communications?
What is meant by the 'diversity order' in the context of wireless communications?
Which of the following statements about diversity techniques is correct?
Which of the following statements about diversity techniques is correct?
How much improvement in signal quality can diversity techniques potentially provide?
How much improvement in signal quality can diversity techniques potentially provide?
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What is the relationship between the probability of a deep fade on one channel and multiple channels?
What is the relationship between the probability of a deep fade on one channel and multiple channels?
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What does the term $\Gamma$ in the probability distribution of SNR represent?
What does the term $\Gamma$ in the probability distribution of SNR represent?
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In Maximal Ratio Combining Diversity, how are the signals weighted before summation?
In Maximal Ratio Combining Diversity, how are the signals weighted before summation?
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What is the purpose of the integral $Pr{rM \leq r}$ in the context of probability distributions?
What is the purpose of the integral $Pr{rM \leq r}$ in the context of probability distributions?
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What does $P( rM )$ represent in the context provided?
What does $P( rM )$ represent in the context provided?
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In the equation $\gamma_M = \sum_{i=1}^{M} G_i \gamma_i$, what does $G_i$ represent?
In the equation $\gamma_M = \sum_{i=1}^{M} G_i \gamma_i$, what does $G_i$ represent?
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What does the term $\gamma_M^2$ imply in relation to the overall diversity gain?
What does the term $\gamma_M^2$ imply in relation to the overall diversity gain?
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Which of the following expressions corresponds to the overall received SNR using multiple branches?
Which of the following expressions corresponds to the overall received SNR using multiple branches?
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How does varying the mean powers of branches affect the overall diversity performance?
How does varying the mean powers of branches affect the overall diversity performance?
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What describes the principle of switched combining in signal processing?
What describes the principle of switched combining in signal processing?
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In equal gain combining, how are branch weights managed?
In equal gain combining, how are branch weights managed?
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What is the primary disadvantage of switched combining compared to selection combining?
What is the primary disadvantage of switched combining compared to selection combining?
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What is the main function of the RAKE receiver in CDMA systems?
What is the main function of the RAKE receiver in CDMA systems?
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In the context of equal gain combining, which equation best represents the combiner output?
In the context of equal gain combining, which equation best represents the combiner output?
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What is a characteristic feature of the feedback diversity method?
What is a characteristic feature of the feedback diversity method?
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Which combining technique is likely to utilize energy from all branches effectively?
Which combining technique is likely to utilize energy from all branches effectively?
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What is the implication of having a non-zero correlation among branches in Maximum Ratio Combining (MRC)?
What is the implication of having a non-zero correlation among branches in Maximum Ratio Combining (MRC)?
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What is a primary requirement for diversity in communication systems?
What is a primary requirement for diversity in communication systems?
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Which type of diversity involves transmitting information over multiple carrier frequencies?
Which type of diversity involves transmitting information over multiple carrier frequencies?
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In the context of space diversity, what results from large antenna spacing?
In the context of space diversity, what results from large antenna spacing?
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What is a key advantage of time diversity in communication systems?
What is a key advantage of time diversity in communication systems?
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Which effect does polarization diversity leverage to improve signal diversity?
Which effect does polarization diversity leverage to improve signal diversity?
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What characteristic do frequencies need to have to achieve effective frequency diversity?
What characteristic do frequencies need to have to achieve effective frequency diversity?
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What type of combining technique selects the branch with the highest instantaneous SNR?
What type of combining technique selects the branch with the highest instantaneous SNR?
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What is a common disadvantage of using time diversity in communications?
What is a common disadvantage of using time diversity in communications?
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What disadvantage does polarization diversity have that can affect its efficacy?
What disadvantage does polarization diversity have that can affect its efficacy?
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Which type of space diversity technique accounts for restricted vertical angle spread?
Which type of space diversity technique accounts for restricted vertical angle spread?
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Which method uses multiple received copies by directly combining them together?
Which method uses multiple received copies by directly combining them together?
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What computational technique evaluates the expectation of a phase difference in signals?
What computational technique evaluates the expectation of a phase difference in signals?
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What is a significant disadvantage of using frequency diversity?
What is a significant disadvantage of using frequency diversity?
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What does selection combining focus primarily on in a communication system?
What does selection combining focus primarily on in a communication system?
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Study Notes
Overcoming Channel Impairments
- Diversity techniques can be used to overcome deep fading and channel coding
- Diversity techniques can significantly improve the link with little added cost
- Diversity decisions are made by the receiver and are unknown to the transmitter
Diversity Concept
- If one radio path undergoes a deep fade, another independent path may have a strong signal
- By having more than one path to select from, both the instantaneous and average SNRs at the receiver can be improved, often by as much as 20 dB to 30 dB
- Diversity order is how many independent copies, or links, are used to improve the system.
Diversity Motivation
- Reduces the effects of fast fading
- Multiple branches with independent fading are required
- Low correlation between branches is required
- The mean powers of the branches should be similar
- An efficient combiner is required
Diversity Examples
- Spatial Diversity: MIMO, beamforming, smart antenna, space time coding, horizontal and vertical combining
- Frequency Diversity: Transmitting information on more than one carrier frequency, separated by more than the coherence bandwidth to avoid experiencing the same fades.
- Time Diversity: Repeats transmission at time spacings exceeding channel coherence time.
- Polarization diversity
- Multi-user diversity
Space Diversity
- Large antenna spacing or large scatterer spacing produce large path length differences
- Variations in path length cause different multipath combining at each antenna
Analysis of Space Diversity
- Phase difference is calculated as: exp( jkd sin )
- The correlation of signals from multiple scatterers is evaluated by calculating the expectation of the signals.
Horizontal Space Diversity
- Antenna spacing and scatterer spacing create a more direct path of signal transmission, leading to more signal quality.
Vertical Space Diversity
- Restricted vertical angle spread requires greater separation in vertical direction to achieve good diversity.
Polarization Diversity
- Scattering shifts and decorrelates polarization
- Advantage: very compact
- Disadvantage: Unequal branch power leads to less diversity gain
- Theoretical model of polarization diversity: the signal arrives at the base station x r1 cos(t 1 ) and y r2 cos(t 2 ).
Time Diversity
- Retransmit with time separation
- Advantage: Only one receiver is needed.
- Disadvantage: Wastes bandwidth and adds delay.
Frequency Diversity
- Uses a wideband channel and simultaneous transmissions
- Wastes power and bandwidth
- Equalizers help to manage this.
Combining Techniques
- Selection diversity: The receiver branch with the highest instantaneous SNR is connected to the demodulator. Using antennas, the best sampled signal is sent for demodulation.
- Maximal ratio combining: All branch signals are weighted according to their signal voltage to noise power ratio and summed.
- Feedback diversity: The best of M signals is used until a threshold criteria is not met. Then the scanning (selection diversity) is restarted.
- Equal gain combining: All branch weights are set to unity and signals are co-phased. This makes use of energy in all branches.
Combining Techniques Performance
- Selection diversity performance: M branch selection diversity is calculated as PrSNR r 1 Pr1 ,...., M r .
- Maximal ratio combining performance: M branch MRC is calculated as: r r/ M ( r / )k 1 Pr{rM r} p( rM )drM 1 e 0 k 1 ( k 1)!M 1 rM / rM e P( rM ) M ( M 1)!
RAKE Receiver
- The receiver implementation uses multiple correlators to detect time-shifted versions of the signal.
- Each finger of the RAKE correlates to a portion of the signal that is delayed by at least one chip in time.
- RAKE receiver is well equipped for multipath environments.
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Description
This quiz explores techniques used to overcome channel impairments through diversity methods. It covers concepts such as spatial diversity and the benefits of using multiple paths for signal improvement. Test your knowledge on how diversity techniques can enhance communication systems by improving the signal-to-noise ratio (SNR).
