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Diversity

Diversity. Diversity  primary goal is to reduce depth & duration of signal fades in flat fading channel Flat fading  B s < B c  no ISI from multipath ! Spatial diversity  multiple Rx antennas (usually) Use multiple Rx antennas in mobile or base station

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Diversity

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  1. Diversity • Diversity  primary goal is to reduce depth & duration of signal fades in flat fading channel • Flat fading Bs < Bc no ISI from multipath ! • Spatial diversity  multiple Rx antennas (usually) • Use multiple Rx antennas in mobile or base station • One antenna with signal null while another antenna may have signal peak • Mobile or base station Rx’s (microscopic) • Selection, equal gain, maximal ratio combining • Multiple Tx antennas (macroscopic) • Spread spectrum CDMA ECE 4730: Lecture #18

  2. Diversity Techniques • Polarization Diversity • Polarization : orientation of electric field with respect to Earth • Spatial diversity at base station less practical b/c of larger separation distances between antennas • Antenna separation > 20-30 l needed to have uncorrelated signals • Multipath signals originate on ground in vicinity of mobile • similar (correlated) signal paths for smaller l separations ECE 4730: Lecture #18

  3. Diversity Techniques • Polarization Diversity • Early radio days  mobile unit in car with vertical whip antenna  V polarization • Hand tilting of mobile unit alters polarization • Use orthogonal () polarizations in base station Tx & Rx antennas • Corresponds to M = 2 performance • Currently being deployed in some cellular applications • Does NOT require large antenna separation! • More aesthetically pleasing  prevents “antenna farm” look • Likely to have significant performance impact for in-building wireless systems  e.g. WLAN ECE 4730: Lecture #18

  4. Diversity Techniques • Polarization diversity • Multiple Tx/Rx antenna polarizations • Usually M = 2  orthogonal V & H • Replace base station spatial diversity (20-30 l separation) • Co-locate antennas • Two other types of diversity  Frequency & Time ECE 4730: Lecture #18

  5. Frequency Diversity • Frequency Diversity  transmit same information on more than one channel • Separation between channels > channel BW (Bc) to obtain uncorrelated signals • Lower probability that simultaneous fading occurs on both channels • Significantly degrades BW efficiency • 2  more BW than w/o frequency diversity • NOT practical for cellular systems ECE 4730: Lecture #18

  6. Frequency Diversity • 1:N frequency protection switching • Form of frequency diversity that requires less total BW • 1 standby channel for N regular channels • Microwave LOS systems (point-to-point) • Signal fades  switch to standby • Used for critical traffic • Cellular systems • Guard channels reserved for handoffs! ECE 4730: Lecture #18

  7. Frequency Diversity • Spread Spectrum Modulation • Another form of frequency diversity • Effectively combats signal fading • Signal power spread over large BW = Wss where Wss > Bs • Frequency selective fading affects only small % of total Tx BW (Wss) • Note that this is true if and only ifBs < Bc !! ECE 4730: Lecture #18

  8. Frequency Diversity • UNII Band Applications • Unlicensed National Information Infrastructure • Large amount of unlicensed spectrum in 56 GHz range • Some devices will likely exploit large BW and use true frequency diversity to combat fading • True = same information on multiple channels ECE 4730: Lecture #18

  9. Time Diversity • Time Diversity  transmit same information at different time spacings • Time spacing > Tc for independent signals • If < Tc then multiple signals may undergo same fade • Main disadvantage is that BW efficiency is significantly worsened • BW must  to obtain sameRd (data rate) • If data stream repeated twice then either 1) BW doubles for same Rdor 2)Rd halves for same BW ECE 4730: Lecture #18

  10. Time Diversity • Channel coding is a form of time diversity • Redundant data bits added to original data stream • Usually more BW efficient than pure time diversity since the # extra bits < # data bits • Fundamental tradeoff between signal power and BW • Example: Add coding bits to data stream but keep same data rate • For same RcTsmust and  BW  • But coding will correct errors allowing weaker signal power for same BER ECE 4730: Lecture #18

  11. Time Diversity • RAKE Rx • Powerful form of time diversity available in spread spectrum (DS) systems  CDMA • Propagation delays in MRC provide multiple copies of Tx signals delayed in time • If time delay between multiple signals > chip period of spreading sequence (Tc)  multipath signals are uncorrelated (independent) ** RAKE Rx is time diversity Rx that collects time-shifted versions of original Tx signal ** ECE 4730: Lecture #18

  12. Rake Rx ECE 4730: Lecture #18

  13. Rake Rx • Rake Rx Combining • Used exclusively in mobile Rx • M branches or “fingers” = # correlation Rx’s • Separately detect M strongest signals • Weighted sum computed from M branches • Faded signal  low weight • Strong signal  high weight • Overcome fading signal in single branch ECE 4730: Lecture #18

  14. CDMA Rx Issues • Practical implementation problems for CDMA (DS-SSM) • Forward link  macroscopic spatial diversity used in CDMA to combat large-scale fading • Multiple base stations serve mobile unit • Mobile Rx has small # branches (~ M = 3) • If # of strong signals (LOS + multipath) > M then noise floor of RAKE Rx  substantially • Extra correlated signals in each branch cannot be selected and properly despread and are  interference ECE 4730: Lecture #18

  15. CDMA Rx Issues • Reverse link  CDMA users share same channel BW • One base station Rx for each mobile • Codes from other users ~uncorrelated • To achieve large # users sharing same channel BW  each user must contribute same amount of power @ base station Rx • Near/Far problem • Much more critical in CDMA than 1G Analog or 2G TDMA • One user can have severe impact on all users in cell ECE 4730: Lecture #18

  16. CDMA Rx Issues • Near/Far Problem • Nearby mobile users with large Tx power raise noise floors in base station Rx’s serving mobile unitsfar away • Noise floor in correlation Rx is cumulative effect of all CDMA users • For maximum reverse-link CDMA capacity all users must contribute same amount of power @ base station Rx • If a nearby user dominates base station Rx it will severely degrade signals from other users in the cell • Requires perfect mobile unit Tx power control to achieve max. channel capacity and overall effective BW efficiency • CDMA vs. TDMA capacity arguments ECE 4730: Lecture #18

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