Abstract
RAKE
receiver is used in CDMA-based(Code
Division Multiple Access) systems and can combine Multipath components, which
are time-delayed versions of the original signal transmission. Combining is
done in order to improve the signal to noise ration at the receiver. 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.
This can be done due to multipath components are practically uncorrelated from
another when their relative propagation delay exceeds a chip period. This paper
presents the basics of RAKE receiver technique, implementation, and design in
cellular systems. Also the usage of RAKE receiver is introduced in CDMA-based
systems such as IS-95 and WCDMA (Wideband Code Division Multiple Access).
INTRODUCTION
In
CDMA (Code Division Multiple Access) spread spectrum systems, the chip rate is
typically much greater than the flat fading bandwidth of the channel. Where as
conventional modulation techniques require an equalizer to undo the intersymbol
interference (ISI) between adjacent symbols, CDMA spreading codes are designed
to provide very low correlation between successive chips. Thus, propagation
delay spread in the radio channel merely provides multiple versions of the
transmitted signal at the receiver. If these multipath components are delayed
in time by more than one chip duration, they appear like uncorrelated noise at
a CDMA receiver, and equalization is not required.
RAKE
receiver, used specially in CDMA cellular systems, can combine multipath
components, which are time-delayed versions of the original signal
transmission. This combining is done in order to improve the signal to noise
ratio (SNR) at the receiver. RAKE receiver attempts to collect the timeshifted
versions of the original signal by providing a separate correlation receiver
for each of the multipath signals. This can be done due to multipath components
are practically uncorrelated from another when their relative propagation delay
exceeds a chip period.
RAKE
RECEIVER
Due
to reflections from obstacles a radio channel can consist of many copies of
originally transmitted signals having different amplitudes, phases, and delays.
If the signal components arrive more than duration of one chip apart from each
other, a RAKE receiver can be used to resolve and combine them. The RAKE receiver
uses a multipath diversity principle. It is like a rake that rakes the energy
from the multipath propagated signal components.
Multipath Channel
Model
Multipath
can occur in radio channel in various ways such as, reflection and diffraction
from buildings, and scattering from trees.
An
M-ray multipath model is shown in Figure 4, which is an extension to the
multipath channel model presented in . Each of the M paths has an independent
delay,, and an independent complex time-variant gain, G.
RAKE Receiver Block
Diagram
When
a signal is received in a matched filter over a multipath channel, the multiple
delays appear at the receiver, as depicted in Figure 4. The RAKE receiver uses
several baseband correlators to individually process several signal multipath
components. The correlator outputs are combined to achieve improved
communications reliability and performance.
Bit
decisions based only a single correlation may produce a large bit error rate as
the multipath component processed in that correlator can be corrupted by
fading. In a RAKE receiver, if the output from one correlator is corrupted by
fading, the others may not be, and the corrupted signal may be discounted
through the weighting process.
Impulse
response measurements of the multipath channel profile are executed through a
matched filter to make a successful de-spreading. It reveals multipath channel
peaks and gives timing and RAKE finger allocations to different receiver
blocks. Later it tracks and monitors these peaks with a measurement rate
depending on speeds of mobile station and on propagation environment. The
number of available RAKE fingers depends on the channel profile and the chip
rate. The higher the chip rate, the more resolvable paths there are, but higher
chip rate will cause wider bandwidth. To catch all the energy from the channel
more RAKE fingers are needed. A very large number of fingers lead to combining
losses and practical implementation problems.
Downlink
The
searcher receiver scans the time domain about the desired signal’s expected
time of arrival for multipath pilot signals from the same cell site and pilot
signals and their multipaths from other cell sites. Searching the time domain
on the downlink signals is simplified because the pilot channel permits the
coherent detection of signals. 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. In turn, 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.
If
a another cell site pilot signal becomes significantly stronger than the
current pilot signal, the control processor
initiates handover procedures during which the downlinks of both cell
sites transmit at the same call data on all their traffic channels. When both
sites handle the call, additional space diversity or macro diversity is
obtained.
The
data from all three digital receivers are combined for improved resistance to
fading. Different base stations or sectors are distinguished by different short
PN code offsets.
The
downlink performs coherent post-detection combining after ensuring that the
data streams are time-aligned; performance is not compromised by using
post-detection combining because the modulation technique is linear. Coherent
combining is possible because the pilot signal from each base station provides
a coherent phase reference that can be tracked by the digital data receivers.
Uplink
On
the uplink, 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, acquired before voice or data
transmission begins on the link, using s special preamble for that purpose.
During
soft handover from one base station site to another, the voice data that are
selected could result from combining up to eight multipath components, four at
each site. The uplink transmission, not having a coherent phase reference like
the downlink’s pilot signal, must be demodulated and combined non-coherently;
maximal-ratio combining can be done by weighting each path’s symbol statistics in
proportion to the path’s relative power prior to demodulation and decoding
decision.
RAKE
RECEIVER IN WCDMA SYSTEM
A
basic implementation of RAKE receiver presented in Figure 5 despreads data from
different multipath components, combines the multipath components, and detects
combined data to soft bits. A WCDMA base station RAKE receiver contains the
following functions to enable the receiving of CDMA type of multipath signals.
1.
Channel delay estimation for multipath components. This can also called as
Impulse Response (IR) Measurement.
2.
RAKE receiver finger allocation based on the channel delay estimation
3.
RAKE receiver fingers to perform the descrambling and despreading operations
4.
Adaptive channel estimation
5.
Maximal-Ratio Combining (MRC)
Channel Delay
Estimation
The
channel Impulse Response Measurement (IRM) is performed by using Matched Filter
(MF) type of correlators that correlate the received signal with known
reference code sequence such as pilot channel code. The MF resources contain
shorter filters (length of 64 chips time period for RACH and 32 chips time
period for DPCCH), which can be concatenated in time domain to enable the
proper delay estimation also in large cells with large delay spreads (e.g.
hilly terrain environments).
To
improve the delay estimation performance and to increase signal to noise ratio
the results of MFs are further processed by coherent and non-coherent
averaging. The length of the coherent IR averaging is typically one time slot
while the noncoherent averaging is typically done over radio frames. The length
of the averaging operations can be selected by parameterization. The accuracy
of the IR measurement is ¼ chip (65.1 ns).
RAKE Receiver Finger
Allocation
The
purpose of the RAKE finger allocation procedure is to define the optimal finger
delay positions that maximize the receiver performance. The allocation
procedure defines the correct delay positions for despreading (in RAKE fingers)
the received wideband signal to symbol level information. In the case of
receiver antenna diversity the finger allocation procedure combines information
from separate receiver antennas. In softer handover the allocation procedure
defines the optimal finger delay positions by taking into account the
information from all the sectors involved in the handover situation.
The
finger allocation procedure contains algorithms, which eliminate the
unnecessary changes in the finger time positions between successive
allocations. Thus the despreading of a certain 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. It has
been confirmed that the allocation must be done differently for the channels
where the taps are very close to each others (so called "fat finger")
than for channels with clearly separate taps.
Typically
the allocation frequency in normal operation mode is one allocation for a code
channel in every 25 ms (accuracy of 1/4 chip), which is enough for all the
practical situations. 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. The change in the timing can be caused by the movement of the
UE or by the transmission timing adjustment of the UE
RAKE Receiver 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. The tracking is done for every finger and the accuracy is 1/8
chip. Like in the main finger allocation procedure the shape of the channel
impulse response is taken into account when defining the despreading timings.
Typically
the delay updating by code tracking is performed once in each or every second
10 ms radio frame.
Adaptive Channel
Estimation
The
goal adaptive channel estimation is to estimate the characteristics of the
time-variant channel. In WCDMA the solution is Pilot Symbol Aided plus adaptive
filtering.
The
channel estimation is used to remove distortion caused by radio channel and it
is based on the known pilot symbols on DPCCH. The channel estimator 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.
The advantage of adaptive filter coefficients compared to use of fixed
coefficient is evident since the solution with fixed coefficients would perform
well only in a constricted set 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.
Practical RAKE
Receiver Requirements
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. 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
(Analog-to-Digital) 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.
Automatic
Frequency Control (AFC) loop compensates for drift of the local oscillator and
possibly compensates the Doppler shifts. Synchronization is required for
channel impulse response measurements and scanning for RAKE finger allocation.
Also channel delay tracking needs synchronization for fine-adjustment and
tracking of multipath components.
CONCLUSION
This
paper has introduced the basic operation and requirements of RAKE receiver used
in CDMA based systems such as IS-95 and WCDMA. 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. The RAKE receiver uses
several baseband correlators to individually process several signal multipath
components. The correlator outputs are combined to achieve improved
communications reliability and performance.
The
basic functions of RAKE receiver are Channel delay estimation for multipath
components, RAKE receiver finger allocation, descrambling and despreading
operations, adaptive channel estimation, and Maximal-Ratio Combining.
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