By Group 14

*Rake & Pre-Rake

Technique

Propagation of TX Signal

Path Delay

Po

we

r path-1

path-2

path-3

With low time-resolution,different signal paths cannot be discriminated.

•••These signals sometimes strengthen,

and sometimes cancel out each other, depending on their phase relation.••• This is “fading”.

•••In this case, signal quality is damaged

when signals cancel out each other.In other words, signal quality is dominated

by the probability for detected power to be weaker than minimum required level.

This probability exists with less than two paths.

Time

Po

we

r

Detected Power

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

CDMAReceive

r

CDMAReceive

r

•••

Synchron

ization

Add

erPath 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

•••

Time

Po

we

r Detected Power

Less fluctuation of detected power, because of adding all

energy .

Three kinds of systems: BER performance

S/N

BER

Frequency-selective channel (no equalization or Rake)

Flat fading channel

AWGN channel

(no fading)

Frequency-selective channel (equalization or Rake)

“BER floor”

To start with: multipath channel

in which case the received (equivalent low-pass) signal is of the form

Suppose a signal s (t) is transmitted. A multipath channel

containing L paths can be presented (in equivalent low-pass signal domain) in form of its impulse response

1

0

( ) ( ) ( ) m

Mj

m mm

r t s t h t a e s t

1

0

( ) m

Mj

m mm

h t a e t

.

Impulse Response Measurement

RAKE Receiver Block Diagram

Another Block Diagram

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

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 from other paths causing interference

= other codes causing interference (+ noise ... )

z t

v t

w t

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

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

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

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!

Maximum Ratio Combining of Symbols

• Is the optimal form of diversity combining because it yields the maximal SNR achievable.

• It requires the exactknowledge of SNRs as well as the phases of the diversity signals

• The output symbols from different RAKE fingers are multiplied with complex conjugate of the channel estimate and the result of multiplication is summed together into the “combined” symbol

• QPSK in WCDMA carries information in phase

• MRC corrects channel phase rotation and weights components with amplitudeestimate.

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 ...

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

Rake Receiver Conclusions

• RAKE receiver attempts to collect the time-shifted versions of the original signal by providing a separate correlation receiver for each of the multipath signals

• RAKE receiver uses several baseband correlators to individually process several signal multipath components• The correlator outputs are (MRC) combined to

achieve improved communications reliability andperformance

• RAKE receiver is used in CDMA based systems such as IS-95 and WCDMA

Pre-Rake Technique• The pre-rake system is proposed for Time division duplex (TDD)

application.• The pre-rake is attracting the attention for wireless portable

communication due to achieving the same diversity gain of a rake receiver but using an ordinary matched filter at the receiver of the UE.

• This fact decrease the complexity of the mobile portable unit and matches the low power requirement at the receiver.

Problem of using Rake Receiver at

UE

Pre-Rake Concept• During the uplink, the base station estimates the channel impulse

response (delays and gains) of the multipath channel.• Since a TDD system uses the same carrier frequency in both

the uplink and the downlink, the channel impulse response is almost the same in both directions.

• During the downlink the previously estimated parameters are used in transmitting the signal via the pre-rake transmitter.

• This is done by transmitting L signals (L is number ofresolved paths) each having one of the previously estimated parameters in the uplink.

• When the signal is convolved with the channel impulse response it produces a strong peak at the o/p of the channel which is equivalent to the rake receiver output.

• Thus the portable unit does not need to estimate the channel impulse response and can only use one matched filter to get o/p

Pre-Rake Technique Benefits• UE now use only one Matched filter

so less complexity.• UE now doesn’t need to make hard channel

estimation so less power consumption.

• This technique doesn’t mean removing Rake receiver from BTS for example as it is not an issue if BTS has complexity and power consumption.

• Pre-rake is used effectively in conjunction with the CDMA/TDD technique.

Example of Simulated Results

• Pre-Rake vs. Rake for random & Orthogonal Codes having Processing gain = 64:and number of users=20and number of rake and pre-rake paths is equal to 5

• Rake is better for Orthogonal codes.

• Pre-rake is better for random codes .