RAKE%20Receiver

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RAKE Receiver

• Marcel Bautista
• February 12, 2004

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Propagation of Tx Signal
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Multipath

• Multipath occurs when RF signals arrive at a
  location via different transmission paths due to
  the reflection of the transmitted signal from
  fixed and moving objects.
• The combination of the direct and reflected
  signals most often leads to significant signal
  loss due to mutual cancellation.

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RAKE Receiver Basic Idea

• The RAKE receiver was designed to equalize the
  effects of multipath.
• It uses a combination of correlators, code
  generators, and delays, or fingers, to spread
  out the individual echo signals of the multipath.
• Each signal is then delayed according to peaks
  found in the received signal.

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Impulse Response Measurement
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RAKE Receiver Continued

• The same symbols obtained via different paths are
  then combined together using the corresponding
  channel information using a combining scheme like
  maximum ratio combining (MRC).
• The combined outputs are then sent to a simple
  decision device to decide on the transmitted
  bits.

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RAKE Receiver Block Diagram
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Another Block Diagram
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Maximum Ratio Combining of Symbols

• MRC corrects channel phase rotation and weighs
  components with channel amplitude estimate.
• The correlator outputs are weighted so that the
  correlators responding to strong paths in the
  multipath environment have their contributions
  accented, while the correlators not synchronizing
  with any significant path are suppressed.

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End Result of RAKE Receiver

• By simulating a multipath environment through a
  parallel combination of correlators and delays,
  the output behaves as if there existed a single
  propogation path between the transmitter and
  receiver.

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Gaussian Minimum Shift Keying
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Gaussian Minimum Shift Keying

• GMSK is based on minimum shift keying which is a
  special form of frequency shift keying.
• Minimum shift keying (MSK) is generated as
  follows

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Gaussian Minimum Shift Keying

• GMSK is similar to MSK except it incorporates a
  premodulation Gaussian LPF
• Used extensively in 2nd generation digital
  cellular and cordless telephone apps. such as GSM

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GMSK Block Diagram

• h( ) Gaussian impulse response
• b( ) rectangular pulse train
• p( ) smoothed (Gaussian filtered) pulse train

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GMSK Impulse Response, Pulse Width

• B -3dB bandwidth of the Gaussian filter
• Pulse shape characterized by 3dB bandwidth times
  the bit period, BTb
• Pulse width increases as BTb decreases

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GMSK Example
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GMSK Improvement

• Achieves smooth phase transitions between signal
  states which can significantly reduce bandwidth
  requirements

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GMSK Tradeoffs

• There are no well-defined phase transitions to
  detect for bit synchronization at the receiving
  end.
• With smoother phase transitions, there is an
  increased chance in intersymbol interference
  which increases the complexity of the receiver.

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GMSK Tradeoffs Continued

• A compromise between spectral efficiency and
  time-domain performance must be made.