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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 locationvia different transmissionpaths due to the reflection ofthe transmitted signal fromfixed and moving objects.
The combination of thedirect and reflected signalsmost often leads to
significant signal loss due tomutual 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 individualecho signals of the multipath.
Each signal is then delayed according topeaks 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 acombining scheme like maximum ratiocombining (MRC).
The combined outputs are then sent to a
simple decision device to decide on thetransmitted 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 weighscomponents with channel
amplitude estimate. The correlator outputs are
weighted so that thecorrelators responding tostrong paths in themultipath environmenthave their contributions
accented, while thecorrelators notsynchronizing with anysignificant path aresuppressed.
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End Result of RAKE Receiver By simulating a multipath environment
through a parallel combination of
correlators and delays, the outputbehaves as if there existed a singlepropogation path between the
transmitter and receiver.
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Gaussian Minimum ShiftKeying
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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 asfollows:
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Gaussian Minimum Shift
Keying GMSK is similar to MSK except it
incorporates a premodulation Gaussian
LPF Used extensively in 2ndgeneration
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 by3dBbandwidth times the bit period, BTb
Pulse width increases as BTbdecreases
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GMSK Example
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GMSK Improvement
Achieves smooth phase transitionsbetween signal states which can
significantly reduce bandwidthrequirements
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GMSK Tradeoffs
There are no well-defined phasetransitions to detect for bit
synchronization at the receiving end. With smoother phase transitions, there
is an increased chance in intersymbol
interference which increases thecomplexity of the receiver.
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GMSK Tradeoffs Continued
A compromise between spectralefficiency and time-domain
performance must be made
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