Rake Receiver CDMA

8/10/2019 Rake Receiver CDMA

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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

Rake Receiver CDMA

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