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RAKE Receiver
12/7/2004 Tommi Heikkilä
S-72.333 Postgraduate Course in Radio Communications
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Agenda
• Introduction
• Multipath Channel
• M-finger RAKE Receiver
• RAKE Receiver Blocks
• RAKE Receiver Requirements
• RAKE Receiver in IS-95 System
• RAKE Receiver in WCDMA System
• Conclusions
• References
• Homework
12/7/2004 Tommi Heikkilä
S-72.333 Postgraduate Course in Radio Communications
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Introduction (1)
• In CDMA spread spectrum systems, the chip rate
is typically much greater than the flat fading
bandwidth of the channel
– Conventional modulation and receiver techniques
require an equalizer to undo the ISI between
adjacent symbols
– CDMA spreading codes are designed to provide
very low correlation between successive chips
• Propagation delay spread in the radio channel
provides multipath signal at the receiver
– If multipath components are delayed in time by
more than one chip duration (1/Rc), they appear
like uncorrelated noise at a CDMA receiver, and
equalization is not required
-> RAKE type correlator receiver can be used!!!
12/7/2004 Tommi Heikkilä
S-72.333 Postgraduate Course in Radio Communications
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Introduction (2)
• RAKE receiver, used specially in CDMA
cellular systems, can combine multipath
components
– To improve the signal to noise ratio (SNR)
at the receiver
– Provides a separate correlation receiver for
each of the multipath signals
– Multipath components are practically
uncorrelated when their relative propagation
delay exceeds one chip period
• The basic idea of A RAKE receiver was
first proposed by Price and Green and
patented in 1956
12/7/2004 Tommi Heikkilä
S-72.333 Postgraduate Course in Radio Communications
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Multipath Channel
• Due to reflections from obstacles a radio
channel can consist of many copies of
originally transmitted signals having
– different amplitudes, phases, and delays
• Multipath can occur in radio channel in
various ways
– Reflection, diffraction, scattering
• The RAKE receiver uses a multipath
diversity principle
– It rakes the energy from the multipath
propagated signal components
• M-ray multipath model can be used
– Each of the M paths has an independent Additive
delay, t, and an independent complex time- Gaussian
G1(t) Noise
variant gain, G Delay 1
G2 (t) r(t)
– t(t) is transmitted signal Delay 2
t(t) .
– r(t) is received signal .
Multiple
.
GM (t) Access
12/7/2004 Tommi Heikkilä Delay M Interference
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M-finger RAKE Receiver (1)
• RAKE receiver utilizes multiple correlators to separately detect M
strongest multipath components
• Each correlator detects a time-shifted version of the original
transmission, and each finger correlates to a portion of the signal, which
is delayed by at least one chip in time from the other fingers
• The outputs of each correlator are weighted to provide better estimate of
the transmitted signal than is provided by a single component
– Outputs of the M correlators are denoted as Z1, Z2,…, and ZM
– Outputs are weighted by 1, 2,…, and M, respectively
• Demodulation and bit decisions are then based on the weighted outputs
of the M correlators m (t) Z 1
1 1
Correlator 1
2
m2(t) Z2 Z’ T Z > m’(t)
Correlator 2 (.)dt
<
r(t) . o
.
. M
mM(t) ZM
Correlator M
12/7/2004 Tommi Heikkilä
S-72.333 Postgraduate Course in Radio Communications
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M-finger RAKE Receiver (2)
• The weighting coefficients are based on the power or the SNR (Signal-
to-Noise Ratio) from each correlator output
– If the power or SNR is small out of a particular correlator, it will be assigned
a small weighting factor, .
– If maximal-ratio combining is used, following can be written for Z’
M
Z' m 1
m Z m
• The weighting coefficients, m, are normalized to the output signal
power of the correlator 2
Z m
m M 2
m 1 Z m
• There are many ways to generate the weighting coefficients
– Due to Multiple Access Interference, RAKE fingers with strong multipath
amplitudes will not necessarily provide strong output after correlation
– Choosing weighting coefficients based on the actual outputs of the correlator
yields to better RAKE performance
12/7/2004 Tommi Heikkilä
S-72.333 Postgraduate Course in Radio Communications
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RAKE Receiver Blocks
• Matched filter Input
RF signal I
– Impulse Response Measurement Correlator
Q
Phase
rotator
Delay
equalizer
I
– Largest peaks to RAKE fingers
Output
Code Channel
generators estimators Q
– Timing to delay equalizer Finger 1
Combiner
– Tracks and monitors peaks with a
Finger 2
Finger 3
measurement rate depending on Timing and finger allocation
speeds of mobile station and on Matched
filter
propagation environment
• Code Generators
– PN codes for the user or channel
• Correlator • Delay Equalizer
– Despreading and integration of – Compensates delay for the
user data symbols
difference in the arrival times of the
• Channel Estimator symbols in each finger
– Channel state estimate
• Combiner
– Channel effect corrections
– Adding of the channel compensated
• Phase Rotator symbol
– Phase correction
12/7/2004 Tommi Heikkilä – Multipath diversity against fading
S-72.333 Postgraduate Course in Radio Communications
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RAKE Receiver Requirements (1)
• RAKE receiver has to know
– Multipath delays -> time delay synchronization
– Phases of the multipath components -> carrier phase synchronization
– Amplitudes of the multipath components -> amplitude tracking
– Number of multipath components -> RAKE allocation
• Time delay synchronization is based on correlation measurements
– Delay acquisition
– Delay tracking by feedback loops (delay-locked loops, DLL)
• Due to fading channels conventional phase-locked loop (PLL) cannot
be used in carrier and amplitude tracking
• Number of available fingers depends on the channel profile and the
chip rate
– The higher the chip rate, the more resolvable paths there are
– A very large number of fingers lead to combining losses and practical
implementation problems
• The main challenges for RAKE receivers operating in fading
channels are in receiver synchronization
12/7/2004 Tommi Heikkilä
S-72.333 Postgraduate Course in Radio Communications
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RAKE Receiver Requirements (2)
• High bandwidth (5 MHz in WCDMA) and dynamic interference
inherent to WCDMA requires that RF and IF parts have to operate
linearly with large dynamic range.
• In practical RAKE receivers synchronization sets some requirements
– Automatic Gain Control (AGC) loop is needed to keep the receiver at
the dynamic range of the A/D converter (Analog-to-Digital).
– AGC must be fast and accurate enough to keep receiver at the linear
range
– Frame-by-frame data range change may set higher AGC and A/D
converter requirements
– The high sampling rates of few tens of MHz and high dynamics of the
input signal (80 dB) require fast A/D converters and high resolution
12/7/2004 Tommi Heikkilä
S-72.333 Postgraduate Course in Radio Communications
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RAKE Receiver in IS-95 System (1)
• In the implementation of the IS-95 system, the mobile receiver
employs
– A “searcher” receiver (matched filter)
– Three digital data receivers that act as fingers of a RAKE
– The PN chip rate of 1,2288 MHz allows for resolution of multipaths at
time intervals of 1,2288 x 10-6 s = 0,814 ms
• Downlink
– The searcher receiver scans the time domain about the desired signal’s
expected time of arrival for multipath cell’s pilot signals
– Pilot channel permits the coherent detection of signals via phase offsets
– The search receiver indicates to the mobile phone’s control processor
where, in time, the strongest replicas of the signal can be found, and their
respective signal strengths
– The control processor provides timing and PN code information to the
tree digital data receivers, enabling each of them to track and demodulate
a different signal
– The data from all three digital receivers are combined for improved
resistance to fading
12/7/2004 Tommi Heikkilä
S-72.333 Postgraduate Course in Radio Communications
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RAKE Receiver in IS-95 System (2)
• Uplink
– If the base station receiver uses two antennas for space diversity
reception, and there are four digital data receivers available for
tracking up to four multipath components of a particular
subscriber’s signal
– The searcher receiver at the base station can distinguish the
desired mobile signal by means of its unique scrambling long PN
code offset
– Code offset is acquired before voice or data transmission begins
– No coherent phase reference like the downlink’s pilot signal
– Must be demodulated and combined non-coherently
12/7/2004 Tommi Heikkilä
S-72.333 Postgraduate Course in Radio Communications
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RAKE Receiver in WCDMA System (1)
• RAKE receiver functions 1. 1
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1. Channel delay estimation or
4
0
0 20 40 60 80 1 00 1 20 14 0
Impulse Response (IR)
Measurement for multipath Impulse Response Finger
components
measurement Allocation
finger 1
2. RAKE receiver finger
f inger 2
finger n
Matched
Filters
allocation based on the S ymbol R ate
User
Datarate
Soft
channel delay estimation
ANT A/D Symbol
Symbol Rate
Chiprate Rate Combiner Decision
Correlator
3. RAKE receiver fingers to C hannel
Estimation
perform the descrambling and
despreading operations
4. Adaptive Channel Estimation
5. Maximal-Ratio Combining
12/7/2004 Tommi Heikkilä
S-72.333 Postgraduate Course in Radio Communications
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RAKE Receiver in WCDMA System (2)
• Channel Delay Estimation
– Yields both timing and phase information of the channel
– Impulse Response Measurement (IRM) is performed by using Matched
Filter (MF) type of correlators
– Length of the matched filter determines the total delay spread that can be
resolved (length of 64 chips time period for RACH and 32 chips time
period for DPCCH)
– Add averaging and thresholding, and the channel estimator is complete
– Requires a lot of processing -> power consumption
– The accuracy of the IR measurement is ¼ chip (65,1 ns)
12/7/2004 Tommi Heikkilä
S-72.333 Postgraduate Course in Radio Communications
15
RAKE Receiver in WCDMA System (3)
• Finger Allocation
– Defines the optimal finger delay positions for despreading that maximize
the receiver performance (also RX div and SHO)
– Also eliminates unnecessary changes in the finger time positions
between successive allocations
– Multipath component is kept on the same RAKE finger as long as
possible to maximize the performance of channel estimation and
maximal-ratio combining
– In the finger allocation procedure also the shape of the channel impulse
response is taken into account when defining the optimum finger delay
positions
– Chip rate of 3,84 MHz offers about 260 ns RAKE finger allocation
resolution -> better multipath combining than in IS-95
– Allocation frequency in normal operation mode is one allocation for a
code channel in every 25 ms (accuracy of ¼ chip)
– Code tracking with accuracy of 1/8 chip is further used in RAKE fingers
to track and compensate small delay deviations in multipath component
timing (UE movement, TX timing)
12/7/2004 Tommi Heikkilä
S-72.333 Postgraduate Course in Radio Communications
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RAKE Receiver in WCDMA System (4)
• Finger Descrambling and Despreading
– The despreading operation for DPDCH (Dedicated Physical Data
Channel) and DPCCH (Dedicated Physical Control Channel) is
performed in RAKE fingers to recover the receiver wideband signal
to symbol level information
– Multiplying of incoming signal by complex conjugate of scrambling
code and channelization code and accumulating the results over
symbol periods
– In the base station receiver 8 fingers are allocated for each code
channel (i.e. 8 multipath components can be despread for a single
user)
– Code tracking is used to track and compensate small deviations in
multipath component delays i.e. the Code tracking performs the fine
adjustment of the delay used in the despreading
– Tracking for every finger with accuracy of 1/8 chip (32,5 ns)
– Delay updating by code tracking is performed once in each or every
second 10 ms radio frame
12/7/2004 Tommi Heikkilä
S-72.333 Postgraduate Course in Radio Communications
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RAKE Receiver in WCDMA System (5)
• Adaptive Estimation
– Goal is to estimate the characteristics of the time-variant channel
– In WCDMA the solution is Pilot Symbol Aided + adaptive filtering
– Is used to remove distortion caused by radio channel and it is based on
the known pilot symbols on DPCCH
– Filter adapts to the Doppler power spectrum (both frequency and the
shape of the spectrum)
– The estimation is done for each finger separately
– The use of adaptive filter ensures good performance in all kind of
propagation conditions
– In the case of multiple receiver antennas the performance of channel
estimation is further improved by combining the power spectrum
information available from different receiver antennas
– The combining process is based on maximal-ratio combining, which
decreases the effect of additive noise, which can further be decreased by
channel decoding
12/7/2004 Tommi Heikkilä
S-72.333 Postgraduate Course in Radio Communications
18
RAKE Receiver in WCDMA System (6)
• Maximal-Ratio Combining (MRC)
– Is the optimal form of diversity combining
because it yields the maximal SNR
achievable
– It requires the exact knowledge 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 Finger Allocation
result of multiplication is summed together Delay estimations
1 2 n
into the “combined” symbol RAKE finger bank MRC combiner
– QPSK in WCDMA carries information in
r1
Finger 1
w1
phase RX RF ADC Finger 2
r2
+
Output
– MRC corrects channel phase rotation and
w2
rn
Finger n
weights components with amplitude wn
estimate
Channel
estimator Pilots
12/7/2004 Tommi Heikkilä
S-72.333 Postgraduate Course in Radio Communications
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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 and
performance
• RAKE receiver is used in CDMA based systems such
as IS-95 and WCDMA
12/7/2004 Tommi Heikkilä
S-72.333 Postgraduate Course in Radio Communications
20
References
• Rappaport, Wireless Communications Principles and
Practice, Prentice Hall, New Jersey, 1996, pp. 336-338.
• Tero Ojanperä, Ramjee Prasad, Wideband CDMA for
Third Generation Mobile Communications, Norwood MA,
USA, Artect House Inc., 1998, 439 pp.
• Simon Haykin, Michael Moher: Modern Wireless
Communications, Prentice Hall 2005, pp. 258-338.
• Samuel C. Yang, CDMA RF System Engineering,
Norwood MA, USA, Artech House Inc., 1998, 280 pp.
• Mikko Järvelä, RAKE training slides by Nokia,
03.04.2001, Oulu.
12/7/2004 Tommi Heikkilä
S-72.333 Postgraduate Course in Radio Communications
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Homework
1. Explain shortly the basic functions in a RAKE receiver.
2. How many fingers can a Mobile Station RAKE receiver’s
matched filter or “searcher” allocate from a following
multipath tapped delay line channel in WCDMA and IS-
95 systems? Don’t guess!
Tap 1 2 3 4 5 6
Avg. 0 -1,5 -6,0 -4,5 -9,0 -15,5
power (dB)
Relative 0 310 500 1090 2430 2510
delay (ns)
12/7/2004 Tommi Heikkilä
S-72.333 Postgraduate Course in Radio Communications
