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Overcoming Channel Impairments in Diversity Techniques
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Overcoming Channel Impairments in Diversity Techniques

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Questions and Answers

What is one primary purpose of diversity techniques in wireless communications?

  • To increase training overhead for receivers
  • To simplify signal processing at the transmitter
  • To eliminate all channel impairments completely
  • To reduce the effects of fast fading (correct)

    • What is meant by the 'diversity order' in the context of wireless communications?

  • The number of independent signal paths used (correct)
  • The total transmission power available
  • The number of users in a network
  • The length of each transmission link

    • Which of the following statements about diversity techniques is correct?

  • They require significant training overhead.
  • They only work with highly correlated signal paths.
  • They are only applicable in urban environments.
  • They can improve both instantaneous and average SNRs. (correct)

    • How much improvement in signal quality can diversity techniques potentially provide?

    <p>20 dB to 30 dB</p> Signup and view all the answers

    What is the relationship between the probability of a deep fade on one channel and multiple channels?

    <p>The probability multiplies with the number of channels (pN).</p> Signup and view all the answers

    What does the term $\Gamma$ in the probability distribution of SNR represent?

    <p>The mean SNR on each branch</p> Signup and view all the answers

    In Maximal Ratio Combining Diversity, how are the signals weighted before summation?

    <p>According to their signal voltage to noise power ratios</p> Signup and view all the answers

    What is the purpose of the integral $Pr{rM \leq r}$ in the context of probability distributions?

    <p>To evaluate the probability that the maximum ratio is below a threshold</p> Signup and view all the answers

    What does $P( rM )$ represent in the context provided?

    <p>The probability density function of the maximum SNR</p> Signup and view all the answers

    In the equation $\gamma_M = \sum_{i=1}^{M} G_i \gamma_i$, what does $G_i$ represent?

    <p>The gain associated with the $i^{th}$ branch</p> Signup and view all the answers

    What does the term $\gamma_M^2$ imply in relation to the overall diversity gain?

    <p>It is the square of the maximum combined SNR</p> Signup and view all the answers

    Which of the following expressions corresponds to the overall received SNR using multiple branches?

    <p>$\gamma_M = \sum_{i=1}^{M} G_i \gamma_i$</p> Signup and view all the answers

    How does varying the mean powers of branches affect the overall diversity performance?

    <p>Affects the probability distribution of SNR thresholds</p> Signup and view all the answers

    What describes the principle of switched combining in signal processing?

    <p>Only one signal is received until it drops below a threshold.</p> Signup and view all the answers

    In equal gain combining, how are branch weights managed?

    <p>Weights are uniform, set to unity across all branches.</p> Signup and view all the answers

    What is the primary disadvantage of switched combining compared to selection combining?

    <p>Performance is always worse than selection combining.</p> Signup and view all the answers

    What is the main function of the RAKE receiver in CDMA systems?

    <p>To combine multiple delayed versions of a signal.</p> Signup and view all the answers

    In the context of equal gain combining, which equation best represents the combiner output?

    <p>$y = (s r_1 e^{j heta_1} + n_1)e^{-j heta_1} + (s r_2 e^{j heta_2} + n_2)e^{-j heta_2}$</p> Signup and view all the answers

    What is a characteristic feature of the feedback diversity method?

    <p>It receives the best signal until it falls below a threshold.</p> Signup and view all the answers

    Which combining technique is likely to utilize energy from all branches effectively?

    <p>Equal gain combining</p> Signup and view all the answers

    What is the implication of having a non-zero correlation among branches in Maximum Ratio Combining (MRC)?

    <p>It complicates the combining process.</p> Signup and view all the answers

    What is a primary requirement for diversity in communication systems?

    <p>Efficient combiner</p> Signup and view all the answers

    Which type of diversity involves transmitting information over multiple carrier frequencies?

    <p>Frequency diversity</p> Signup and view all the answers

    In the context of space diversity, what results from large antenna spacing?

    <p>Large path length differences</p> Signup and view all the answers

    What is a key advantage of time diversity in communication systems?

    <p>Requires only one receiver</p> Signup and view all the answers

    Which effect does polarization diversity leverage to improve signal diversity?

    <p>Scattering shifts</p> Signup and view all the answers

    What characteristic do frequencies need to have to achieve effective frequency diversity?

    <p>Wide separation</p> Signup and view all the answers

    What type of combining technique selects the branch with the highest instantaneous SNR?

    <p>Selection diversity</p> Signup and view all the answers

    What is a common disadvantage of using time diversity in communications?

    <p>Adds delay</p> Signup and view all the answers

    What disadvantage does polarization diversity have that can affect its efficacy?

    <p>Inequal branch powers</p> Signup and view all the answers

    Which type of space diversity technique accounts for restricted vertical angle spread?

    <p>Vertical space diversity</p> Signup and view all the answers

    Which method uses multiple received copies by directly combining them together?

    <p>Maximal ratio combining</p> Signup and view all the answers

    What computational technique evaluates the expectation of a phase difference in signals?

    <p>Angle-of-arrival PDF</p> Signup and view all the answers

    What is a significant disadvantage of using frequency diversity?

    <p>Wastes power and bandwidth</p> Signup and view all the answers

    What does selection combining focus primarily on in a communication system?

    <p>Best signal reception at any moment</p> Signup and view all the answers

    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 PrSNR  r  1  Pr1 ,...., 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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    Related Documents

    EE 552/452 Wireless Communications (and Networks) 2007 Class Notes PDF

    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).

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