rake
DESCRIPTION
Rake ReceiverTRANSCRIPT
AJAL.A.JASST PROFESSOR METS SCHOOL OF ENGG MALA
RAKE RECEIVER
"To trip twice on the same rake", which means "to repeat the same silly mistake". Russian saying
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Why the name rake receiver ?
The rake receiver is so named because it reminds the function of a garden rake, each finger collecting symbol energy similarly to how a rake collect leaves.
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Steel Rake
Leaf Rake
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SNOW RAKE
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GARDEN RAKE
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GARDEN RAKE
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Multi-path Energy Capture
In multi-path environments, the RMS delay spreads for a given channel can be large (14 ns for CM3, 25 ns ).
Un captured multi-path energy results in loss in performance of the communication device.
One method for energy collection is to use a RAKE receiver.Sampled
Matched-FilterOutput
w1 w2 w3 wN
RAKEoutput
t
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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 – Multipath fading
Rake receiver mitigates multipath fading effect Multipath fading is a major cause of unreliable wireless channel
characteristic
x(t)
y(t) = a0x(t)y(t) = a0x(t)+a1x(t-d1)y(t) = a0x(t)+a1x(t-d1)+a2x(t-d2)
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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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Overview of Rake Receiver
A rake receiver is a radio receiver designed to counter the effects of multipath fading. It does this by using several "sub-receivers" each delayed slightly in order to tune in to the individual multipath components.
Each component is decoded independently, but at a later stage combined in order to make the most use of the different transmission characteristics of each transmission path.
This could very well result in higher SNR (or Eb/No) in a multipath environment than in a "clean" environmen
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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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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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Fading in CDMA System ...Fading in CDMA System ...
Because CDMA has high time-resolution,different path delay of CDMA signals
can be discriminated.Therefore, energy from all paths can be summed
by adjusting their phases and path delays.This is a principle of RAKE receiver.
Path Delay
Po
we
r path-1
path-2
path-3
CDMAReceiver
CDMAReceiver
•••
Synchron
ization
Add
er
Path Delay
Po
we
r
CODE Awith timing of path-1
path-1
Po
we
r
path-1
path-2
path-3
Path Delay
Po
we
r
CODE Awith timing of path-2
path-2
interference from path-2 and path-3
•••
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Fading in CDMA System (continued)
Fading in CDMA System (continued)
In CDMA system, multi-path propagation improves the signal quality by use of RAKE receiver.
Time
Po
we
r Detected Power
RAKEreceiver
Less fluctuation of detected power, because of adding all
energy .
Po
we
r
path-1
path-2
path-3
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Rake finger selection
Delay ( )
1
( ) i
Lj
rake i ii
h t a e t
Channel estimation circuit of Rake receiver selects strongest samples (paths) to be processed in the Rake fingers:
In the Rake receiver example to follow, we assume L = 3.
1 2 3
Only one path chosen, since adjacent paths may be correlated
L = 3
Only these paths are constructively utilized
in Rake fingers
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Received multipath signal
Received signal consists of a sum of delayed (and weighted) replicas of transmitted signal.
All replicas are contained in received signal and cause interference
:
Signal replicas: same signal at different delays, with different amplitudes and phases
Summation in channel <=> “smeared” end result
Blue samples (paths) indicate signal replicas detected in Rake fingersGreen samples (paths) only cause interference
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Rake receiver
Finger 1Finger 1
Finger 2Finger 2
Channel estimationChannel estimation
Received baseband multipath signal (in ELP signal domain)
Finger 3Finger 3
Output signal
(to decision
circuit)
Rake receiver Combining (MRC)
(Generic structure, assuming 3 fingers)
WeightingWeighting
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Rake Receiver Blocks
Correlator
Finger 1
Finger 2
Finger 3
Combiner
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DelayDelay
Rake finger processing
Tdt
Received signal
To MRC
Tdt if
Stored code sequenceStored code sequence
(Case 1: same code in I and Q branches)
I branch
Q branch
I/QI/Q
Output of finger: a complex signal value for each detected bit
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Correlation vs. matched filtering
TdtReceived
code sequence
Received code sequence
Stored code sequenceStored code sequence
Basic idea of correlation:
Same result through matched filtering and sampling:
Received code sequence
Received code sequence
Matched filter
Matched filter
Sampling at t = T
Sampling at t = T
Sam
e e
nd
resu
lt !
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Architecture(1)
Conventional Rake Receiver
)}3]3[()2]2[()1]1[{(
)3]3[()2]2[()1]1[(
)]3[(3)]2[(2)]1[(1
WnXWnXWnXPN
WnXPNWnXPNWnXPN
PNnXWPNnXWPNnXW
T1
T2
T2
∫
∫
∫
W1
W2
W3
5-PathPre-Carrier Recovery
CarrierRecovery
Loop
Channel out
DifferentialDecoder
Rake Receiver
SymbolDLL
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Architecture(2)
Proposed Rake Receiver» correlator move behind
multipliers
» 3 fingers adopted W1
W2
W3
T1
T2
T2
∫
)}3]3[()2]2[()1]1[{(
)3]3[()2]2[()1]1[(
)]3[(3)]2[(2)]1[(1
WnXWnXWnXPN
WnXPNWnXPNWnXPN
PNnXWPNnXWPNnXW
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Rake finger processing
1
i n
Lj j
i i n nnn i
r t z t v t w t
a e s t a e s t w t
Correlation with stored code sequence has different impact on different parts of the received signal
= desired signal component detected in i:th Rake finger
= other signal components causing interference
= other codes causing interference (+ noise ... )
z t
v t
w t
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Rake finger processing
Illustration of correlation (in one quadrature branch) with desired signal component (i.e. correctly aligned code sequence)
Desired component
Stored sequence
After multiplication
Strong positive/negative “correlation result” after integration
“1” bit “0” bit “0” bit
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Rake finger processing
Illustration of correlation (in one quadrature branch) with some other signal component (i.e. non-aligned code sequence)
Other component
Stored sequence
After multiplication
Weak “correlation result” after integration
“1” bit “0” bit
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Rake finger processing
Mathematically:
0
2
0
1 0 0
i
n
T
i
Tj
i
T TLj
n n inn i
C z t v t w t s t dt
a e s t dt
a e s t s t dt w t s t dt
Correlation result for bit between
Interference from same signal
Interference from other signals
Desired signal
0, t
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Rake finger processing
Set of codes must have both: - good autocorrelation properties (same code sequence) - good cross-correlation properties (different sequences)
2
0
1 0 0
i
n
Tj
i i
T TLj
n n inn i
C a e s t dt
a e s t s t dt w t s t dt
Large
Small Small
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DelayDelay
Rake finger processing
Tdt
Received signal
Tdt
Stored I code sequenceStored I code sequence
(Case 2: different codes in I and Q branches)
I branch
Q branch
I/QI/Q
Stored Q code sequenceStored Q code sequence
i
To MRC for I signal
To MRC for Q signal
Required: phase synchronization
if
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Rake finger processing
Case 1: same code in I and Q branches
Case 2: different codes in I and Q branches
- for purpose of easy demonstration only
- the real case in IS-95 and WCDMA
- no phase synchronization in Rake fingers
- phase synchronization in Rake fingers
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Phase synchronization
I/QI/Q
i
When different codes are used in the quadrature branches (as in practical systems such as IS-95 or WCDMA), phase synchronization is necessary.
Phase synchronization is based on information within received signal (pilot signal or pilot channel).
Signal in I-branch
Pilot signalPilot signal
Signal in Q-branch
I
Q
Note: phase synchronization must
be done for each finger separately!
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Weighting
Maximum Ratio Combining (MRC) means weighting each Rake finger output with a complex number after which the weighted components are summed “on the real axis”:
3
1
i ij ji i
i
Z a e a e
Component is weighted
Phase is aligned
Rake finger output is complex-valued
real-valued
(Case 1: same code in I and Q branches)
Instead of phase alignment: take
absolute value of finger outputs ...
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Phase alignment
The complex-valued Rake finger outputs are phase-aligned using the following simple operation:
1i ij je e
Before phase alignment:
ije
ije
1
After phase alignment:
Phasors representing complex-valued Rake
finger outputs
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Maximum Ratio Combining
The idea of MRC: strong signal components are given more weight than weak signal components.
The signal value after Maximum Ratio Combining is:
2 2 21 2 3Z a a a
(Case 1: same code in I and Q branches)
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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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Maximum Ratio Combining
Output signals from the Rake fingers are already phase aligned (this is a benefit of finger-wise phase synchronization).
Consequently, I and Q outputs are fed via separate MRC circuits to the quaternary decision circuit (e.g. QPSK demodulator).
(Case 2: different codes in I and Q branches)
Quaternarydecisioncircuit
Quaternarydecisioncircuit
Finger 1Finger 1
Finger 2Finger 2
MRC
MRC
MRC
MRC
:
I
Q
I
Q
I
Q
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Maximum Ratio Combining Diversity
Various techniques are known to combine the signals from multiple diversity branches.
In Maximum Ratio combining each signal branch is multiplied by a weight factor that is proportional to the signal amplitude. That is, branches with strong signal are further amplified, while weak signals are attenuated.
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Rake Receiver - Functions
Ideally the function of rake receiver is to aggregate the signal terms with proper delay compensation
y(t) = a0x(t)+a1x(t-d1)+a2x(t-d2)
r(t) = a0x(t-tdealy)+a1x(t-d1-dest1)+a2x(t-d2-dest2)
= (a0+a1+a2) * x(t-tdelay)
Rake receiver
delaytdelayt
We need to know delay spread of received signal that randomly varies
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Rake Receiver – Detect Delay Spread
Scan the received signal in frame buffer while computing correlation with scrambling code sequence.
Received signalCorrelation
window
Correlation Result
a0
a1
a2
0 d1 d2
0 1 1 2 2[ ] [ ] [ ] [ ]y n a x n a x n d a x n d
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Rake Receiver – Overall Architecture
Searcher
Descrambler/Despreader
Descrambler/Despreader
Descrambler/Despreader
combiner
Delay
Delay
Delay
r(t)
d1, d2, d3 a1, a2, a3
Detects delay spread
Compensates propagation delay recombine signal terms without delay
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Multipath Diversity: Rake Receiver
Instead of considering delay spread as an issue, use multipath signals to recover the original signal
Used in IS-95 CDMA, 3G CDMA, and 802.11
Invented by Price and Green in 1958» R. Price and P. E. Green, "A
communication technique for multipath channels," Proc. of the IRE, pp. 555--570, 1958
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Multipath Diversity: Rake Receiver
Use several "sub-receivers" each delayed slightly to tune in to the individual multipath components
Each component is decoded independently, but at a later stage combined
LOS pulsemultipathpulses
»This could very well result in
higher SNR in a multipath environment
than in a "clean" environment
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Rake Receiver: Matched Filter Impulse response measurement Tracks and monitors peaks with a measurement rate
depending on speeds of mobile station and on propagation environment
Allocate fingers: largest peaks to RAKE fingers
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Rake Receiver: Combiner
The weighting coefficients are based on the power or the SNR from each correlator output
If the power or SNR is small out of a particular finger, it will be assigned a smaller weight:
M
ii
mm
Z
Z
1
2
2
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RAKE DEMODULATOR
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RAKE DEMODULATOR
RAKE demodulator for signal transmitted through a frequency selective channel.
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The demodulator structure shown in above Figure is called a RAKE demodulator.
Because this demodulator has equally spaced taps with tap coefficients that essentially collect all the signal components in the received signal, its operation has been likened to that of an ordinary garden rake.
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Let us use the correlator structure that is illustrated in Figure
The received signal is passed through a tapped delay-line filter with tap spacing of 1/W, as in the channel model
The number of taps is selected to match the total number of resolvable signal components.
At each tap, the signal is multiplied with each of the two possible transmitted signals s1(t) and s2(t), and, then, each multiplier output is phase corrected and weighted by multiplication with c (t), n = 1, 2, …, L.
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DANG YOU
AJAL.A.J ASST PROFESSOR METS SCHOOL OF ENGG MALA
MOB 0-8907305642