Rake

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

2

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.

3

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:



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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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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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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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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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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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


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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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


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


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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



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

50

RAKE DEMODULATOR

RAKE demodulator for signal transmitted through a frequency selective channel.

51

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

Rake

Technology

rake receiver finger

use of rake receiver

rake receiver example

principle of rake receiver

steel rake leaf rake

snow rake

path delay power path

end result of rake receiver