The Rake Receiver Principle

17 Pages·2012·0.21 MB·English

Recent Patents on Electrical Engineering 2012, 5, 55-71 55
The Rake Receiver Principle: Past, Present and Future
Konstantinos B. Baltzis*
RadioCommunications Laboratory, Section of Applied and Environmental Physics, Department of Physics, Aristotle
University of Thessaloniki, Thessaloniki, Greece
Received: September 12, 2011; Accepted: October 17, 2011; Revised: November 11, 2011
Abstract: In order to enhance the performance of wireless communication systems, we usually employ diversity combin-
ing techniques. Rake reception is a diversity method that gained popularity mainly due to its use in second and third gen-
eration mobile systems. This paper reviews important and recent advances in rake diversity and its applications in wireless
communications. First, we provide some theoretical background of rake reception focusing on the initial idea proposed in
the late 50's. Next, we present selected patent and scientific literature on rake receiver design. Recent patents, patent ap-
plications and scientific articles on the application of rake diversity in code division multiple access systems, ultra-
wideband communications and channel estimation are then discussed. Some more general applications of rake reception
in wireless communications are also examined. Future perspectives of rake diversity are finally explored.
Keywords: CDMA, channel estimation, diversity, rake, receiver, ultra-wideband, wireless communications.
A common way to mitigate the effects of multipath fad-
1. INTRODUCTION
ing is through the use of diversity combining techniques, that
Wireless communications have experienced an extreme is, by sending the same data over independent paths which,
growth since the early 90's. Wireless systems have been in- obviously, have a lower probability of experiencing deep
troduced in several areas of everyday life and provide serv- fades simultaneously [3]. In diversity reception, the received
ices that include a multimedia traffic mix of voice, data and signal samples fade in an uncorrelated manner. As a result,
video each having different transfer requirements. Moreover, the probability of all the samples being below a given
high mobility and global connectivity are usually required. threshold level at the same time is much lower than the prob-
With the rapid development of wireless networks, technolo- ability of any individual one to be below this level.
gies and applications that demand large quantities of traffic
Among different diversity techniques such as frequency,
such as video conferencing, remote medical imaging, video
temporal, spatial, polarization, etc. diversity [3-7], a popular
sharing, online documents storage and online gaming, also
method, especially in code division multiple access (CDMA)
emerged [1].
communications, is rake diversity [8, 9]. Rake reception was
In wireless communications, the propagation medium proposed about half a century ago [10, 11]. In general, the
influences all system components ranging from modulation method uses baseband correlators, also called fingers, to in-
and channel coding up to higher layer protocols. The radio dividually process multipath signal components that are
channel is susceptible to noise, interference and other im- properly combined to improve the signal-to-noise ratio
pediments that change stochastically in time. These impair- (SNR) and reduce the probability of deep fades. In spread
ments comprise additive and multiplicative components. The spectrum systems, rake fingers collect resolvable signal mul-
first arise from noise generated within the receiver1 (thermal tipaths. Then, the received signal components are suitably
and shot noise) and external sources (cosmic radiation, at- combined after despreading by a local copy of the delayed
mospheric effects, electrical appliances, etc.) while the sec- version of the transmitter’s spreading code [8, 9, 12]. The
ond from processes generated by propagating waves on their name rake arises from the notion that this receiver resembles
way from the transmitter to the receiver [2]. Multiplicative a garden rake and rakes up the multipath energy on its fin-
noise reduces system performance significantly and it is usu- gers.
ally described by three partially separable effects, path loss,
This paper provides an overview of recent and important
shadowing and multipath fading.
developments in rake receivers and their application in wire-
less communications. Within this context, selected patents,
patent applications and scientific articles are presented.
*Address correspondence to this author at the RadioCommunications Labo-
Critical issues related to the design, implementation and op-
ratory, Section of Applied and Environmental Physics, Department of Phys-
timization of rake receivers are thoroughly discussed. Spe-
ics, Aristotle University of Thessaloniki, Thessaloniki, Greece Tel: +30
2310 998285; Fax: [phone]; E-mail: [email protected] cial care has been paid to the presentation of recent literature
on rake reception in CDMA and wideband CDMA
(WCDMA) systems due to the worldwide deployment of
(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)
these technologies in the second and, primarily, in the third
1 Within this context, the term “receiver” refers to the reception unit that includes one
generation mobile systems (2G and 3G, respectively)
or more rake receivers, unless otherwise stated; the term “rake receiver” refers exclu-
sively to a rake receiver unit.
1874-4761/12 $100.00+.00 © 2012 Bentham Science Publishers
56(cid:1)(cid:1)(cid:1) Recent Patents on Electrical Engineering, 2012, Vol. 5, No. 1 Konstantinos B. Baltzis
[13,14]. In the last decade, ultra-wideband (UWB) commu- portion to the square roots of their SNRs. MRC gives the
nications have emerged as a promising technology for high- maximum SNR under the assumption of independent finger
speed short-range radio systems [15]. In these systems, the signals [19] but it is no longer an optimum criterion if we
extremely large signal bandwidth motivates the use of rake consider the correlation between them [20] (in general, the
schemes as long as signal multipaths can be finely resolved correlation between the desired signal components decreases
at the receiver [16]. Representative applications of rake di- the diversity gain and the correlation between the total inter-
versity in channel estimation are also examined. In this re- ference components decreases the total combined average
view paper, we finally present methods and apparatuses that SIR). On the other hand, in equal gain combing [21], the
employ rake reception in processes such as sequence detec- symbol decision statistics for each finger are summed with
tion, antenna tuning, signal-to-interference ratio (SIR) esti- equal gains. A different strategy is followed in selection
mation, baseband frequency error correction, etc. combining [22]. In this method, we demodulate the multi-
path component with the highest SNR and ignore the rest.
The rest of the paper is organized as follows: Section 2
The improvement of this method, the generalized selection
provides a short historical background and discusses basic
combining, considers more than one multipath component
principles of rake diversity. Next, in the main part of the
(again, the ones with the highest SNRs) [23]. The aforemen-
paper, we overview patents and scientific articles in rake
tioned criteria give adequate results and are commonly used
receiver design and the applications of rake reception in
in structures with uniform finger distribution. However, in
wireless communications. A brief discussion on current and
structures with unequal finger spacing the calculation of the
future developments of rake diversity in modern communica-
decision variable with a combining rule based on the maxi-
tions completes this study.
mum likelihood criterion [24] usually provides better results
[12, 25-28].
2. RAKE DIVERSITY: HISTORICAL BACKGROUND
AND FOUNDING PRINCIPLES In Price and Green’s proposal, fingers spacing equals to
the inverse of the signal bandwidth but in several rake re-
The conventional rake receiver as introduced by Price
ceiver implementations the fingers are unequally spaced. In
and Green [10, 11], comprises a tapped delay line (TDL)
fact, the relation between fingers positions and the correla-
with L taps spaced at distances equal to the inverse of the
tion coefficients of the rake combined signals [27] justify a
signal bandwidth W. In this structure, the TDL detects indi-
non-uniform finger distribution. Different implementations
vidual signal multipaths and collects their energy that falls
of the correlators have also been proposed. For example, in
within the span L/W. The detected signals are added after
several direct sequence spread spectrum (DS/SS) systems,
weighting by a factor maximizing the total SNR. A simpli-
each correlator is matched to the desired user’s spreading
fied block diagram of the receiver is illustrated in Fig. (1)
sequence and performs a convolution between this sequence
where r(t) is the signal at the receiver front-end and Z is the
and the finger signal [8]. A detailed discussion of these is-
decision variable determined from the combination of the
sues follows in the next Section.
fingers outputs x, i=1,2..L.
i
In general, a rake receiver combats fading effects to 3. SELECTED PATENTS AND SCIENTIFIC ARTI-
process the temporal diversity by using multipath signal re- CLES ON RAKE DIVERSITY
ception [17]. However, the separate detection, weighting and
The increased popularity of spread spectrum technology
recombination of signals that differ in their arrival time is
and the rapid development of 3G communication systems
equivalent to doing the same thing in frequency allowing us
have resulted in the publication of a vast amount of scientific
to consider rake reception as another form of frequency di-
papers and patents on rake diversity and its applications.
versity [3].
However, to the best of the author’s knowledge, there are no
Figure (2) depicts a typical postdetection rake diversity published articles that review patents on rake diversity. In
combining receiver for L-fold diversity. In this figure this paper, we focus on published patents and patent applica-
a,(cid:1), l=1,2..Lare the attenuation factor and the phase shift tions and study them as a measure of innovation.
l l
This Section represents the author’s best attempt to iden-
of the lth diversity channel and g , l=1,2..L are the weight-
l tify significant and recent inventions on rake reception and
ing coefficient in each branch.
its application in wireless communication systems. Repre-
In the previous structure, the most important unit of a sentative patents and patent applications granted by the
rake receiver is the combiner. Several different strategies United States Patent and Trademark Office (USPTO) [29]
(criteria) have been employed for its implementation and the and the European Patent Office (EPO) [30] are included
calculation of the decision variable. Among them, maximal herein. Selected articles in the scientific literature are also
ratio combining (MRC), equal gain combining (EGC), selec- referenced. In order to improve the clarity of the exposition,
tion combining (SC), generalized selection combining (GSC) we divide the Section into five parts. Subsection 3.1 dis-
and the maximum likelihood (ML) criterion are the most cusses rake receiver design and presents important and re-
popular. In brief2, in maximal ratio combining [18], the cent rake receiver implementations. In Subsection 3.2, we
phases of each diversity signal are added together and made highlight patents and patent applications related to rake di-
equally coherent while their envelopes are weighted in pro- versity in spread spectrum communication systems. Next, we
discuss rake reception in ultra-wideband wireless communi-
(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1) cations. Subsection 3.4 reviews novel inventions and meth-
2 The mathematical analysis of this issue is beyond the scope of the paper; an introduc- ods in radio channel estimation. A collection of miscellane-
tory but detailed analysis is provided in [8].
The Rake Receiver Principle Recent Patents on Electrical Engineering, 2012, Vol. 5, No. 1 57

Fig. (1). The TDL rake receiver model.

g
Diversity 1

channel 1
(a ,φ )
1 1
g
Diversity 2
channel 2
(a ,φ )
2 2

er
n
bi
Transmitted Diversity gl om Decision
signal C Variable(s)
channell

(a,φ)
l l

g
Diversity L

channel L
(a ,φ )
L L
Fig. (2). Postdetection diversity receiver model.
ous inventions completes our study. In each subsection, the larged with narrower finger spacing over the full range of the
innovations are listed in a chronological order, unless other- delay spread, the receiver may overcome this degradation by
wise stated. collecting more energy [12]. Based on the above, Kostic and
Pavlovic [31] proposed a sub-chip resolution rake receiver
3.1. Practical Implementations of Rake Receivers for DS/SS communication systems. This invention resolved
multipath components spaced closer than a chip interval al-
In the conventional DS/SS rake receiver, finger spacing
lowing the detection of multipaths spaced closer than the
is set equal to the chip period. In general, when finger spac-
inverse of their bandwidth. In that implementation, the esti-
ing is less than the chip duration, performance degrades due
mator utilized a constrained iterative deconvolution tech-
to the increase in the amount of correlation between total
nique based on the projection onto convex sets (POCS)
interference. However, if the number of rake fingers is en-
method [32]. In a matched filter embodiment, the delay be-
58(cid:1)(cid:1)(cid:1) Recent Patents on Electrical Engineering, 2012, Vol. 5, No. 1 Konstantinos B. Baltzis
tween any two sequential taps was set lower than the chip Lin [41] suggested a rake receiver that comprised a mul-
duration. tiplexer, a searcher, two switch fabrics, several fingers, a
combiner, a decoder, a channel estimator and a rake finger
In fact, the non-uniform finger assignment schemes are
controller. The number of rake fingers that were assigned for
very popular in the scientific and patent literature. For exam-
processing multipaths was determined dynamically. In order
ple, in [12], optimum finger positions were determined by
to reduce system complexity and increase system perform-
the joint maximization of the sum of squares of average re-
ance, the fingers generated a search request each time a link
ceived power in each finger and minimization of the sum of
was established. This search responded to a specific antenna
squares of correlation between each pair of fingers. The
for which the quality of the output signal was adequate; the
authors in [20] set the finger spacing less than the chip dura-
searcher might serve multiple base station antennas also.
tion and enhanced receiver performance by using a combin-
ing rule based on ML criterion. An embodiment in which The idea of a rake reception method that produces a sub-
fingers were locked on the strongest multipath components set of channel estimates from a set of received signal paths
on the basis of their instantaneous or the average power was and chooses, in accordance with at least one criterion, the
proposed in [33]. Recently, a rake receiver that receives sig- best signal paths only from these estimates is discussed in
nal from two or more antennas was proposed [34]. This re- [42]. In an embodiment, the author suggested the limitation
ceiver comprised a despreading unit that was adapted to allo- of the average remaining uncaptured signal energy as a best-
cate rake fingers to a number of delay positions. Finger allo- signal-path selection criterion. The method suits better for
cation was determined from a delay position unit which es- units with limited processing resources, e.g. mobile termi-
timated optimum delays by using direction-of-arrival (DOA) nals, which operate in rich fading environments.
[35] and channel dispersion information.
The invention in [43] uses signal impairment correlations
Usually, a rake receiver comprises a matched filter for for the generation of rake combining weights and SIR esti-
the processing of the received spread spectrum signal. How- mates. This idea was initially suggested by Bottomley et al.
ever, the receiver in [36] included not one but three matched in 2000 [25, 44]. In that receiver, the so-called generalized
filters. The first two filters were matched to the spreading rake (G-RAKE), fingers positions were determined from the
codes of the pilot and signal data while the third one esti- maximization of the decision statistic’s SNR using ML
mated the channel frequency response and combined the principles. The set of delays that optimized the performance
received data signals from different paths prior decision es- criterion was obtained from a search in a window of potential
timation. In the proposed embodiment, the filters were Fast delays that spanned from several chip periods before the first
Fourier Transformation (FFT) ones [37]. The receiver could arriving multipath component to several chip periods after
be used in both the uplink and downlink channels. In the first the last one. A simpler strategy suggested the positioning of
case, it employed a multi-stage parallel interference cancella- a subset only of the fingers on the multipaths to collect en-
tion process to remove multiple access interference. In the ergy while the remaining ones were placed based on the
downlink operation, it firstly estimated the interference of a strongest taps of the inverse channel filter so as to suppress
pilot signal and then subtracted it from the received signal interference (for example, these fingers could be placed as
prior data detection. mirrors to the rest). With this placement, the receiver ap-
proximated inverse channel filtering, thereby cancelling the
Patent application US20030103556A1 [38] proposes a
noise coloration caused by the channel and achieving partial
rake receiver that shares resources between demodulating
interference cancellation. It has to be noticed that the G-
branches without using extra hardware in its fingers. In a
RAKE receiver technology has been incorporated in U350
first embodiment, time deskew buffers [39] that storage the
and U360 HSDPA/EDGE3 Ericsson mobile platforms [45].
recovered symbols are shared between the demodulating
fingers; in a second one, not only the buffers but also a phase In a recent invention [46], an adaptive G-RAKE has been
compensator are shared. In the first case, the required mem- proposed. In that receiver, fingers positions were selected
ory area is decreased to approximately the inverse of the from a candidate set by using a maximal-weight criterion and
number of the demodulating fingers; in the second embodi- weight computation included estimation of an impairment
ment, receiver size is further decreased. Compared to con- covariance matrix using a parametric approach. The G-
ventional rake structures, the proposed receiver uses less RAKE in [47] uses a subset of the available fingers to de-
circuitry and has smaller size. modulate the incoming multipaths. The rest of them are lo-
cated in the vicinity of the multipath center at a resolution
An adaptive multimode receiver that employs dynamic
finer than a chip period and they are utilized for optimal fin-
pilot signal searching and multipath reception and combining
ger placement and weight estimation. Dynamic allocation of
capability is presented in [40]. The receiver includes a net-
fingers is employed in [48] also. This G-RAKE places a sub-
work interface, several adaptive multimode fingers and a
set of its fingers according to their signal strength and the
multimode processor. In a first stage, the fingers and the
rest of them on the basis of the estimated SIR at the com-
processor respond to a first mode signal and configure a path
biner output. Patent US 7688881B2 [49] suggests a sophisti-
reception functional mode. In a second stage, the fingers are
cated G-RAKE design in which the determination of im-
responsive to a second mode signal and configure a searcher
pairment correlations between the delays of interest is based
functional mode providing the receiver with acquisition, traf-
on the generation of kernel functions as samples of a net
fic and idle modes. This receiver supports both downlink and
channel response. This response is derived from the received
uplink signal reception.
(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)
3 The acronyms HSDPA and EDGE stand for High-Speed Downlink Packet Access
and Enhanced Data rates for GSM Evolution, respectively.
The Rake Receiver Principle Recent Patents on Electrical Engineering, 2012, Vol. 5, No. 1 59
signal that is taken at well defined chip sampling phases for above certain received signal spreading factor, colored inter-
delays within the set of the delays of interest. In this em- ference suppression does not improve system performance.
bodiment, an impairment correlation estimation circuit is In this case, simpler strategies can be followed. A low-
configured to obtain correlation estimates by convolving complexity receiver that selectively operates with or without
each kernel function with itself and with each of the rest. The colored interference suppression depending on current oper-
convolutions can be performed in the frequency domain via ating conditions was proposed in [54]. In the first case, the
FFTs or through manipulation of the kernel functions to receiver generates the output signals from a set of fingers
form interleaved, usually zero padded, functions. In another signals by combining some of the output estimates; in the
G-RAKE proposal [50], the receiver utilizes a square-root second case, it generates them from a set of fingers signals
covariance matrix that represents impairment and data co- based on channel estimates only.
variance for the communication channel. In this implementa-
Kang [55] invented a rake receiver which includes a
tion, symbol detection uses weights calculated from the
searcher for checking the intensity levels of the received
aforementioned covariance matrix and a net channel re-
signals, detecting the valid paths through which signals with
sponse. The calculation of a reconstructed version of the
intensity levels above a predetermined level are received and
desired signal from the detected symbols allows the genera-
outputting information about these paths. The fingers that
tion of an updated communication signal by subtracting this
correspond to the valid paths are selected from a controller.
reconstructed signal from the communication signal and the
The receiver is suitable for mobile terminals due to its en-
covariance matrix update. An interesting method for the re-
hanced ability for power consumption reduction.
duction of computational complexity of the G-RAKE re-
ceiver was also suggested in [51]. The proposed method The receiver in [56] comprises a demodulator which is
simplifies the calculations for pre-whitening by using two (or adapted to receive and process one or more input samples
more) signals with two (or more) antennas. The method es- that correspond to the same finger during a single clock pe-
timates and formulates in a matrix form the correlation effect riod. This structure is also adapted for each finger to accu-
of the antennas. The derived matrix is used to generate an mulate multiple processed input samples, form symbols and
uncorrelated received signal matrix; the inverse of this ma- provide them for all fingers. In fact, this method replaces the
trix is used to pre-whiten the noise. A G-RAKE receiver for physical finger instances with simplified time-multiplexed
multiple-input multiple-output (MIMO) systems is proposed fingers that process each path component during a different
in [52]. In this embodiment, the intermediate combining processing cycle. As a result, the fingers share the shame
weights are determined on the basis of a simplified impair- demodulating circuit that reduces noticeably receiver com-
ment correlation representation that excludes channel reuse plexity,
cross-stream interference. The same weights are also used in
In the femtocell environment [1], the limited number of
reporting channel quality information for one or more nodes
multipaths and the terminals’ low mobility and speed allow
of the MIMO operation providing a computationally efficient
the utilization of rake receivers with a small number of fin-
and robust basis for computing final combining weights that
gers without fine finger tracking. In this case, the coarse tim-
consider channel reuse cross-stream interference. According
ing search process usually introduces a sampling error. In
to the inventors, their receiver reduces the severity of the
order to overcome this, Muirhead [57] suggested a rake re-
problems that arise from the impairments correlations due to
ceiver with multiple rake fingers assigned to the same path at
the cross-stream interference produced from the channel
a granularity of less than one chip. These fingers provide
reuse.
additional energy for others that are sub-optimally sampled.
In a new type of generalized rake receivers, the paramet- Preferably, the embodiment comprises a coarse timing
ric G-RAKEs, the impairment correlations are decomposed searcher that identifies signal paths by detecting two signal
into constituent elements and then modeled. The structure of peaks in each received multipath component.
the modeled terms is known while the short-term impairment
A high-order rake receiver that mitigates the impact of
correlations can be measured from a set of despread pilot
Doppler frequency shift [58] is proposed in [59]. In this em-
values. As a result, the impairment correlation estimation
bodiment, rake fingers have two blocks. The first block sub-
task is reduced to the determination of the appropriate model
tracts the slowly varying phase and frequency signal compo-
fitting parameters, known as scaling parameters. However,
nents by removing a phase and a portion of the signal fre-
the strong dependence of the scaling parameters on the total
quency component. The remaining frequency variation is
energy per chip period and the thermal noise makes difficult
removed in the second block. This structure is a second-
the appropriate update of their values, especially in high data
order receiver; however, for higher-order receiver diversity
rate systems. An invention that tries to give a solution in this
this process can be repeatedly extended. It has to be noticed
problem was proposed in [53]. In this receiver, the modeled
that each finger can be configured to account for different
impairment correlations that are used to generate impairment
Doppler shift on the communications paths. According to the
correlation estimates for combining weight computation are
authors’ claims, the proposed receiver is mostly suitable for
estimated on a transmission interval basis. As such, the in-
systems that experience significant Doppler frequency shift
terval-to-interval changes in the signal conditions do not
such as systems where the transmitter travels with high
influence the estimation of the scaling parameters.
speeds, e.g. a jet fighter. In another invention [60], the im-
The G-RAKE receiver estimates the impairments cross- pact of Doppler shift is reduced by using a compensator in
correlations of the received multipaths and using them gen- each finger instead of using only one compensator (a con-
erates the combining weights to provide colored noise sup- trolled oscillator in a feedback loop, for example) in the re-
pression. However, below certain received signal qualities or ceiver. The compensators are implemented either in hard-
60(cid:1)(cid:1)(cid:1) Recent Patents on Electrical Engineering, 2012, Vol. 5, No. 1 Konstantinos B. Baltzis
ware or in software but hybrid solutions are also feasible. In signals at each finger output. Then, the optimum combining
general, a compensator comprises a filter, an amplitude nor- coefficients are calculated from the fading coefficient vector
malizer and two arithmetical modules that multiply input and the autocorrelation matrix of the received, or alterna-
signals with conjugated previous inputs and output symbols tively, the noise plus interference, signal for each pair of
with previous outputs. fingers. The rake receivers in [68] are coupled to each of the
antennas. In this invention, the interference that is correlated
Recently, McPherson [61] proposed a rake receiver that
between a finger associated with the first antenna and a fin-
determines symbol values by detecting signal peak locations
ger associated with the second antenna is minimized with
in the fingers. Each finger includes one or more despreaders
respect to the desired signal by optimally combining all fin-
for despreading time-staggered versions of the signal at the
gers signals using MRC. In [69], the first rake receiver tar-
receiver front-end and a frequency corrector downstream
gets in channel coefficients estimation while the second one
with a phase locked loop (PLL) [62]. The last module oper-
produces the desired data sequence. The receiver further
ates at a rate lower than the signal rate and it is usually im-
comprises a space time transmit diversity (STTD) derotator
plemented by a microprocessor. Symbols are detected by
[70] that processes the received signals from the two anten-
despreading time-staggered versions of the received signal at
nas in two consecutive symbol periods. The signal at the
its rate and performing frequency correction after despread-
derotator’s input is formed by the combination of the outputs
ing using the PLL. Optionally, each finger includes a control
of the two rake receivers and the conjugated channel estima-
and a data output path for outputting control and data and a
tions from the antennas. These components are linearly com-
buffer upstream from the despreader for buffering the time-
bined to achieve sufficient statistics for a decision to be
staggered versions of the received signal. The data output
made. The symbol sequence that is obtained from the derota-
path may further include a phase adjust module which is
tor is down-sampled by a factor of two; in this case, one
coupled to the control output path so as to correct symbol
sample is taken while the second is discarded.
rate phase differences between control and data symbols.
In the published patent and scientific literature, we find
3.2. Rake Diversity and (W)CDMA Communication Sys- several studies that discuss the reduction of power consump-
tems tion in wireless receivers. In fact, this is a very important
requirement towards enhancing system performance, espe-
The first popular wireless communication system that
cially in mobile terminals. Typical rake receiver implementa-
used the spread spectrum technology was the IS-95. This 2G
tions that successfully reduce power consumption have al-
standard was formally adopted by the U.S. Telecommunica-
ready been presented in the previous subsection. This issue
tion Industry Association (TIA) in 1993 and revised, with
has been also addressed in network level; see, for example,
minor improvements, in 1995 [63]. In IS-95, both base sta-
[71-73]. A mobile terminal that operates in the idle mode
tion and mobile receivers used rake reception. In the base
must be able to receive incoming and support outgoing calls
station, the receiver usually included four or five fingers and
while moving between adjacent cells. In this case, most of
combined their outputs non-coherently; on the other hand,
the power is consumed during the acquisition process. An
the mobile receiver comprised a three-finger rake and per-
invention that reduces power consumption during acquisition
formed coherent demodulation [8]. The application of
is suggested in [71]. In the proposed embodiment, the incom-
CDMA technology in second generation communication
ing baseband signals feed a matched filter that is driven by a
systems showed [64] that CDMA not only did not perform as
supply voltage control. The rake fingers despread the incom-
well as was originally promised, but also provided perform-
ing signal components only when an electric power calcula-
ance inferior to other digital technologies such as time divi-
tion circuit detects that the filter output exceeds a predeter-
sion multiple access. Despite this failure, a significant frac-
mined threshold. The reduction of consumed power is due to
tion of the industry appeared to be committed to pursuing
the fact that the matched filter works intermittently and the
variations of CDMA to future applications. In fact, 3G and
correlator tracks and receives signals only after the acquisi-
third generation and beyond (B3G) mobile systems use
tion is performed. According to the inventors, the use of a
WCDMA as multiple access technique and employ coherent
matched filter, a sample and hold circuit, a multiplier and an
rake reception in both forward and reverse link [13,65]. Im-
adder, prior to the rake fingers, reduces not only power con-
portant inventions related to rake reception in 2G, 3G and
sumption but also the acquisition time. The reduction of
B3G systems follow.
power consumption when a mobile terminal operates in the
In a practical CDMA system, we usually employ both idle mode is also addressed in [72]. In this embodiment, Jep-
antenna and rake diversity. In [66], for example, the mobile sen proposed that some of the rake fingers of a mobile re-
unit has two antennas. The received signal at the second an- ceiver that are not used for the reception of paging informa-
tenna is delayed by one or more chip periods and the signals tion can be used for the monitoring of the neighboring cells.
from the two antennas are combined in a summer and fed the This can be achieved by a variety of methods; for example,
rake receiver input. However, when we detect multipath en- monitoring can be performed during the regular reception of
ergy to a point where much of the received signal energy in paging information between the reception of relevant paging
the assigned rake fingers is not accounted, this signal is ig- parts or in one power-up and power-down cycle when mo-
nored. In other inventions [67-69], the reception unit com- bile operation employs a periodic paging reception cycle.
prises not only two antennas but two rake receivers also. In The monitoring of neighboring cells during the reception of
[67], the interference that is correlated between the fingers in the paging blocks reduces noticeably the total power-up time
the first rake with the fingers in the second rake is minimized and the power consumption. In [73], the mobile terminal
with respect to the desired signal by optimally combining the comprises an environmental variation estimator. This estima-
The Rake Receiver Principle Recent Patents on Electrical Engineering, 2012, Vol. 5, No. 1 61
tor is connected to the rake searcher and provides an estimate detector jointly detects the transmitted symbols by using the
of the rate at which the mobile environment changes. By rake combining weights which have been calculated using a
providing this estimate, the duty cycle of the searcher can be noise covariance matrix, the signals at the rake receiver out-
optimized reducing the overall power consumption. The put and the generated cross-correlations. At the final stage,
authors of this invention suggest the utilization of this esti- the receiver performs trellis decoding [79]. This decoder
mator in channel tracking also. calculates branch metrics associated with the state transitions
by using the obtained symbols at the rake outputs and the
In cellular CDMA networks, the base stations usually
cross-correlations between the spreading codes.
transmit a pilot signal with a known bit sequence on a pilot
channel so as the mobile terminal to determine the distortion The authors in [80] proposed a processing method of the
level in the traffic channel. A drawback of this method is the Dedicated Pilot (DP) and Transmit Power Control (TPC) bits
estimation error due to the simultaneous reception of signals [81] in WCDMA rake receivers. First, Dedicated Pilot
from a plurality of base stations. A method that partially Channel (DPCH) symbols are received, quantized and com-
solves this problem was proposed in [74]. In that method, an pensated. Next, we process the manipulated DPCH symbols
estimate for the traffic channel amplitude was generated are processed on the basis of the DPCH slot format and pro-
based on a comparison with received pilot signals for each duce DPCH pilot symbols in a common format. Finally, the
channel of the rake receiver. The generated estimates DP and TPC bits are processed either in parallel using sepa-
weighted the incoming signals prior to their combination rate processing branches or in series by applying timing con-
with weight signals in other channels. The receiver in [75] trol to common processing modules.
comprises two parallel connected rake receivers that receive
Despite its complexity, blind spreading factor detection
the same signal. The delays of the fingers of the first rake
was usually applied in 2G systems. For example, in an IS-95
receiver are matched to a propagation path delay. The fingers
application [82], a Viterbi decoder ran for each rate or
of the second rake produce an estimate of the multipath in-
spreading factor hypothesis and formulated the decision met-
terference generated by each finger in the first rake. Then,
rics. However, a major drawback of these techniques that
the estimated multipaths are scaled and subtracted from the
makes their use ineffective or even impractical in high data
first rake so as to reduce multipath interference. A proper
rate WCDMA systems is their significant latency. A novel
adjustment of the scaling coefficients reduces multipath in-
solution for WCDMA communications has been recently
terference.
provided. Wang [83] proposed two embodiments that use
The aforementioned inventions were developed for sec- rake receivers and perform blind spreading factor detection.
ond generation wireless communications systems. In contrast In the first, the received signal is despread by using an initial
to them, WCDMA systems support services with different spreading factor smaller than the smallest possible one that
traffic requirements. In a WCDMA receiver embodiment could be used to spread the signal. The despread values are
[76], two paths, a high-rate and a low-rate one are employed. then combined using combining weights and an actual
The first path despreads multi-rate physical channels with spreading factor is estimated based on a most likely data bit
variable spreading factor while the second one despreads repetition factor. Then, the detected data symbol repletion
physical channels with fixed spreading factor. The signal in factor is used to combine the partially combined signal val-
the high-rate path is resolved by high-rate rake fingers that ues prior to decoding. In the second proposal, the received
multiply symbols with channel estimation; then, a combiner signal is despread by using a spreading factor smaller than
combines the multipath components. The low-rate path com- the smallest possible one that is used to spread the signal.
prises low-rate fingers. In this case, a processor performs Next, a rake receiver combines the weights of the received
channel estimation and coherently combines the calculated signal and a combiner generates the partially despread and
estimates with the fingers outputs. combined signal using the calculated weights. The receiver
further includes a blind spreading factor detector that esti-
In the previous subsection, we have extensively dis-
mates the actual spreading factor of the signal based on a
cussed generalized rake reception. In [77], Fulghum and
most likely data bit repetition factor.
Cairns suggested that the G-RAKE receivers for HSDPA
systems could be configured to determine a traffic-to-pilot The inventions in [84, 85] were proposed to mitigate the
gain scaling parameter as part of the impairment correlation effect of inter-cell and intra-cell interference in WCDMA
estimation and the combining weights generation processes. wireless systems. Patent application US20110151888A1 [84]
By this way, the receiver circuit accounts effectively for gain provides a programmable inter-cell and intra-cell interfer-
differences between the pilot and the traffic channels of the ence suppression module for WCDMA communication sys-
transmitted WCDMA signals. According to the two inven- tems. In the proposed method, the received signal is utilized
tors, the accounting of this difference enables proper de- for the estimation of channel state information (CSI) that
modulation of traffic signals such as quadrature amplitude includes estimates of both channel coefficients and rake fin-
modulated (QAM) ones. gers positions. The estimated CSI and a delayed version of
the received signal produce a reduced interference signal that
Patent US7787522B2 [78] proposes a diversity reception
is suitably formatted for post-processing. The receiver in
scheme that comprises a rake receiver, a code correlator and
[85] is designed and/or adapted through the repeated use of
a multi-code joint detector. In this implementation, the rake
symbol-estimate weighting, subtractive cancellation with a
receiver despreads the received signals using selected
stabilizing step-size and mixed-decision symbol estimates. It
spreading codes and separates them into two or more output
comprises a front-end processor that includes a combiner, a
signals and the code correlator generates cross-correlations
despreader and a symbol estimator. The front-wend proces-
between the aforementioned spreading codes. Then, the joint
62(cid:1)(cid:1)(cid:1) Recent Patents on Electrical Engineering, 2012, Vol. 5, No. 1 Konstantinos B. Baltzis
sor may also include a synthesizer that synthesizes estimated 3.3. The Rake Receiver in UWB Systems
rake finger signals and a subtraction module for performing
Ultra-wideband is a promising technology for wireless
per-antenna subtraction of a sum of the synthesized signals
communications that supports applications ranging from
and produce an error signal. The receiver may be imple-
video streaming to sensor networks and utilizes radio signals
mented either explicitly in software or programmed hard-
with a bandwidth which range from hundreds of megahertz
ware or implicitly in standard rake-based hardware at the
up to several gigahertz [97]. A major difference between
finger level or at the user or subchannel symbol level, that is,
UWB and narrowband communication systems resides in the
outside the rake. The proposed invention can be used not
characteristics of the propagation medium. In particular, the
only in wireless communications but also in signal process-
UWB channel is a highly frequency-selective fading one
ing.
because the bandwidth of an ultra-wideband signal is much
In [86], an interference cancelling receiver combines data wider than the channel coherence bandwidth. Aside from
from multiple paths after aligning them to transmitter timing this, the clustering effect [98] that is common in UWB sig-
and then uses a rake receiver to compute symbol estimates. nals results in high-resolution arrival times of the multipaths
These estimates are used for the generation of interference [99].
estimates which are combined after re-aligning the interfer-
In an UWB system, the special needs for effective cap-
ence estimates to receiver timing. For each segment of inter-
ture of multipath energy impose several technical and im-
ference cancelled data, two or more segments of symbol es-
plementation difficulties that make conventional rake receiv-
timates are computed. The proposed intervention can be real-
ers ineffective or even impractical [100, 101]. In fact, the
ized in both hardware and software. Hardware embodiments
most important limiting factors in conventional rake recep-
may include Application Specific Integrated Circuits
tion are the rapid fall of the incremental improvement of the
(ASICs), Field Programmable Gate Arrays (FPGAs), Digital
diversity gain with the number of fingers and the tradeoff
Signal Processors (DSPs) [87] and/or other circuitry while
between system performance and parameters such as hard-
software and firmware embodiments are implemented in
ware complexity, computational requirements and power
programming or assembly languages.
consumption [102, 103]. However, rake diversity remains an
The receiver in [88] comprises a searcher, a rake re- option for UWB communications [104], especially for im-
ceiver, circuits for the detection of the maximum signal en- pulse radio ultra-wideband (IR-UWB) systems [105]. In IR-
ergy among individual multipaths in a cluster, a module for UWB radio, the rake receiver’s ability to exploit multipath
the computation of the time-of-arrival (ToA) [89] of the in- diversity, its relatively low complexity compared to other
coming multipaths and a cluster tracker that computes timing UWB receivers, the multipath diversity inherent in the IR
adjustment signal based on the power level of the received signals and the high processing gain makes rake reception an
signal components. The cluster tracker comprises timing effective solution, especially in low-data-rate systems [106].
control circuitry for the computation of a timing reference In the following paragraphs, selected patents and scientific
signal based on a timing adjustment signal, a code generator articles show the application range and the merits of the use
for the generation of a code based on a start time associated of rake diversity in UWB communications.
with the received signal cluster and a code identification
In an interesting, mainly due to its simplicity, proposal
module. The proposed receiver receives a portion of the in-
[107], a rake receiver with single bit detection is optimized
coming multipaths at time instants within a specific duration.
for UWB signal reception. In this invention, channel estima-
The authors claim that their invention provides improved
tion is achieved by calculating the probability density func-
management, controlling and combination of the received
tion (PDF) of a single bit quantizer output instead of using a
multipaths and reduces the processing overhead in fingers
complex analog amplitude scheme that involves multi-bit
managing. Also, they support that the receiver mitigates the
detector quantization with increased computational complex-
impact of finger merging, that is, the case in which a com-
ity. In addition to this, the use of a single quantization bit
mon individual path signal is assigned to more than one fin-
simplifies signal processing and reduces cost implementation
gers due to paths that are closely spaced together4.
and required processing power. It has to be noticed that sin-
Last but not least, we present a spread spectrum MIMO gle bit quantization incurs a penalty of about 2dB compared
transceiver that employs rake diversity in signal reception to an ideal infinite resolution analog to digital (A/D) conver-
[95]. In this system, the transmission antennas are spaced by tor. However, this loss is easily recovered because the
at least one-quarter wavelength (a lower spacing degrades method allows a significant number of rake fingers while
significantly system performance) and preferably as far apart maintaining reasonable power consumption. In the proposed
as possible. In the receiver, the antennas are coupled to fil- embodiment, the rake receiver has two stages, a serial and a
ters which are matched to the code sequences of the trans- parallel one; the single bit quantizer is connected at the first
mitted signals and a rake receiver combines the detected stage.
spread spectrum signals and the detected multipaths in con-
The authors in [108] proposed a pilot-channel-assisted
junction with a space-diversity combiner. System perform-
log-likelihood ratio SC rake for long-range low-rate UWB
ance is further enhanced with erasure decoding [96].
applications. The receiver uses CSI and constructs an effec-
tive pilot channel that instantaneously updates the channel
information which is required from the rake receiver. The
(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)(cid:1)
pilot and data channels are generated from quadrature sinusoi-
4 In this case, rake fingers can not track the individual multipath components when they
move away from each other. As a result, the receiver loses any benefits obtained from dal bursts with Gaussian envelope and they are transmitted in
the assignment of multiple fingers to process the received signal (for a detailed discus- parallel. The joint maximization of the SNR at the receiver
sion on finger merging and prevention methods, see [90-94]).
The Rake Receiver Principle Recent Patents on Electrical Engineering, 2012, Vol. 5, No. 1 63
output and minimization of the mean square error (MSE) and the third one forwards the output of the second switch to
[24] of channel estimation optimizes system parameters. The the demodulators. The register stores the synchronization
limited number of fingers balances the suboptimal perform- position value and the summer adds the demodulators’ out-
ance of the receiver. puts and forwards the result to the equalizer when exceeds a
predefined threshold. Finally, the equalizer removes in-
The coherent detection scheme in [109] employed hy-
tersymbol interference, calculates symbol decision values
brid-selection/maximal-ratio combining (H-S/MRC) diver-
and feeds the Viterbi decoder.
sity [110, 111], also known as selective-rake reception, and
used pilot aided channel estimation information. In the pro- An IR-UWB receiver that minimizes energy consump-
posed embodiments, a detect-and-forward UWB relay was tion by finding the optimum packet length and operating
equipped with one or two antennas. In the first case, the relay with the optimum number of rake fingers is proposed in
operates either as a receiver or as a transmitter and the relay- [121]. The receiver is able to choose between coherent and
ing strategy is as follows: In a first time slot, the relay re- noncoherent demodulation after the signal passes through the
ceives the transmitted signal but the receiver is in idle mode; rake fingers and an MRC combiner. In the first case, the re-
then, in a second time slot, the relay transmits the detected ceived signal feeds a matched filter which is configured from
data to the receiver. In this case, the relay exploits multipath a template pulse. In noncoherent detection, the received sig-
diversity by using the selective-rake scheme and reduces nal is either correlated with the previously received signal or
path losses (if it is placed in a correct position). In the two- it is simply multiplied by itself (envelope detection). The
antenna relay system, the relay not only exploits multipath authors further suggest a differential modulation scheme that
diversity but also performs spatial diversity reception [7] and cooperates with both detection schemes. For example, a dif-
spatial transmit diversity [112]. ferential binary phase shift keying (DBPSK) modulated sig-
nal can be noncoherently detected by a correlation with the
The authors in [113] proposed a novel digital modulation
previously received signal so that only the difference be-
scheme for DS/SS UWB communications, the co-called
tween the two signals appear at the A/D converter output. In
quadrature binary orthogonal keying (QBOK), and a rake
coherent detection, each DBPSK modulated signal can be
receiver with analog emphasis based on a six-port circuit
detected individually through a matched filter; then, the dif-
[114-116]. QBOK differs from conventional M-ary bior-
ference between two adjacent bits is measured in the digital
thogonal keying (MBOK) [117] in that the second compo-
domain. Coherent detection requires that a pulse generator, a
nent of the signal space is derived from the modulation of the
synchronizer and a channel estimator are active during the
selected code using the Hilbert transform [118] of the modu-
template pulse generation. However, in noncoherent detec-
lating impulse signal. The receiver comprises a six-port net-
tion, these three units are not used due to the absence of tem-
work with two input ports (received signal and local oscilla-
plate pulses (in general, coherent detection yields an im-
tor) and four output ports which are linear combinations of
proved performance but at the cost of hardware complexity
the inputs. The outputs signals are additively mixed in zero-
and power consumption). In the two detection schemes, both
bias schottky diodes [119] and combined in pairs so as to
hard and soft decision combining can be used. The first con-
obtain the I-Q channels. In the proposed embodiment, corre-
sumes less energy at the cost of lower performance. On the
lation does not involve a local oscillator but it is imple-
other hand, soft decision combining methods demand a high-
mented by four integrators which are cascaded after the di-
resolution A/D converter and additional memory units.
odes. Significant advantages of this architecture are the pas-
sive mixing that it uses one less template generator because By using higher order metrics of the generalized Gaus-
the second component of the signal space is generated with sian distribution approximation [122], Beaulieu and Shao
hybrid couplers inside the six-port network. proposed two novel receiver architectures for time-hopping
(TH) ultra-wideband systems [123]. The key idea behind that
Patent US7649962B2 [120] proposes a DS/SS UWB re-
invention was the selection of a parameter p and the genera-
ceiver that comprises an A/D converter, a correlator, a rake
tion of a first set of partial statistics by using p so as to mod-
receiver, an equalizer and a Viterbi decoder. In this embodi-
ify the exponential order of the noise plus multiple access
ment, the rake receiver includes a channel estimator, a track-
interference PDF approximation. The first receiver uses a
ing module, two switches and multiple demodulators with
constant detection threshold while the second one introduces
parallel processing structure. The channel estimator assumes
a second degree of freedom in the p-order metric and sets the
negligible Doppler frequency spread and long channel co-
detection threshold adaptively. In the same patent, the inven-
herence time and estimates CSI during a preamble transfer
tors propose a rake receiver for TH UWB systems. This re-
period. The tracking module detects the received signal
ceiver adopts the aforementioned p-order metrics in each of
variations and then adjusts and stores the estimated synchro-
its fingers and calculates the decision variable using MRC.
nization position values. The first switch forwards to the
tracking module the value of either the A/D converter or the Finally, the “zonal” UWB receiver [124] has been pro-
correlator while the second one works in a similar manner posed for TH UWB signals and for signals that originally
but forwards the selected value to the demodulators. By us- had pulses with duration less than one nsec, signal band-
ing the output of the second switch, the estimated channel width greater than 500 MHz or signal bandwidth greater than
estimation and the synchronization position value, the de- 15% of the carrier frequency. In order to produce partial sta-
modulators demodulate the received signals. The receiver tistics for an information bit, the receiver performs correla-
further comprises three buffers, a register and a summer. The tions in the received signal. Next, each partial statistic is
first buffer supplies the switches with the A/D converter out- transformed into a new one on the basis of its initial value
put, the second buffer stores the channel estimation value and either keeps its value or sets it to zero. The new partial
64(cid:1)(cid:1)(cid:1) Recent Patents on Electrical Engineering, 2012, Vol. 5, No. 1 Konstantinos B. Baltzis
statistics are accumulated and produce the decision statistics search windows reduces memory size due to the smaller
upon which a decision is made. In an alternative, the new amount of information that is stored during correlation calcu-
partial statistics keep their initial values or they are weighted lations.
versions of them. In the same published patent, the inventors
Recently, Zhang and Lin [126] suggested a searching
have proposed an advanced rake receiver that comprised a
method and apparatus with reduced requirements on storage
zonal UWB receiver in each finger.
area. In their proposal, the path searcher comprised a multi-
plier, an adder and two buffers, the first for storing a copy of
3.4. Rake Reception and Channel Estimation
the local sequence and the second for storing data of the
The propagation medium has a profound impact on the sampled signal. The first buffer was longer than the local
performance of wireless communication systems. Knowl- sequence while the length of the second buffer was kn/Ts
edge of channel parameters is requisite for the operation and with k the number of the sampled bits, n the length of the
improved performance of wireless receivers. However, CSI local sequence (in bits) and Ts the sampling interval meas-
is not a priori known at the receiver, which poses significant ured in chips. In this method, the received signal was ini-
constraints and difficulties in wireless systems design, im- tially sampled and quantized. Next, the local sequence was
plementation and optimization. In practice, channel identifi- hold in the first buffer (the sequence was repeated until the
cation includes the estimation of CSI so as to determine cer- storage area was filled) and a part of the received data was
tain channel parameters. Among different techniques, rake hold in the second one. Then, the data in the second buffer
diversity is employed in channel estimation quite often. In were incrementally moved against the data in the first buffer
the following paragraphs, we discuss methods and appara- with increment equal to the sample interval. In each step, the
tuses for wireless channel estimation based on rake recep- bits in the second buffer were multiplied with the corre-
tion. sponding bits in the first buffer. In the end, the results of all
the multiplications were added and stored. In a second em-
In general, a path searcher calculates delay profile data
bodiment, the length of the first buffer was set to w–1+n bits,
from the baseband signal(s) and stores them so as to be
with w the number of the performed correlations. The
equalized at a specific time interval, e.g. one symbol period,
authors incorporated the proposed path searcher in a
of the signal(s). Therefore, it requires a great number of path
WCDMA receiver that further included a rake receiver, a
search sample points that correspond to oversampling times
radio frequency receiver and an A/D combiner.
in one symbol. In diversity systems, this number further in-
creases resulting in a significant enlargement of the circuit In mobile terminals, channel estimation usually involves
scale. Considering the above, Saito et al. [125] invented a the utilization of a known bit sequence that is either time
path search method for wireless CDMA communication sys- multiplexed with the information (pilot symbols) or transmit-
tems. The proposed invention offered improved tracking ted as a separate pilot channel [12]. The main drawback of
accuracy without significant cost in hardware complexity the first approach is the insufficiency of the pilot symbols to
and power consumption. In this method, the searcher detects provide accurate information in a single time slot. Thus, we
an effective path from the demodulated baseband signals and usually prefer the second method but neither this approach
obtains the exact timing at which fingers receive the incom- provides an exact measurement of the physical transmission
ing multipaths. In order to do so, it calculates the delay pro- channel. For example, a conventional channel estimation
file data and detects their peak value. In the proposed em- method on the basis of a common pilot channel ignores the
bodiment, the path searcher comprises a window controller, transmission power regulation [127-129] causing losses in
a sampling circuit, a spreading code generator, several corre- the processing of the combined signal. The invention in
lators, a peak detector, an assignment controller and a mem- [130] faces this problem successfully. This method calcu-
ory unit. The window controller defines a search range for lates one or more path weights using CSI obtained from the
calculating the delay profile data and splits in two parts. common pilot channel. Next, the same path weights are mul-
Next, it sets the center of the first window at the peak posi- tiplied with a correction factor that depends on the ratio of
tion of the delay profile obtained from the peek detector. The two gain estimations, a data-channel-specific to a pilot-
controller further sets two independent windows out of the channel-based one. As a result, the new path weights take
range of the first one and selects them alternately. The sam- into account the influence of the transmission power regula-
pling circuit samples the baseband signal while the spreading tion in the data channel. Power regulation is normally carried
code generator produces the desired code. Then, the correla- out on a time slot basis. An advantage of the aforementioned
tors correlate the sampled signals and the generated spread- method is that it allows the generation of correctly weighted
ing code within the range of the two windows. The distribu- data symbols over the length of a code word, even when this
tion of the calculated correlation values represents the delay word has been transmitted in time slots with different power
profile of the reception signal. The peak detector compares levels.
the calculated correlation values, which are stored in the Patent US7286593B1 [131] provides a channel estimator
memory unit, and detects the peaks with values that exceed a
for the weighting coefficient of a rake finger. The proposed
predetermined threshold. At the same time, it obtains infor-
method is a three-step process. First, we set the parameters
mation about the timing of the peak appearances and sup-
of a filter that minimizes the MSE of channel estimation.
plies it to the assignment controller which, in turn, forwards Under the assumption of Jake’s spectrum [132] for each
the timing information to the rake fingers. The proposed Doppler frequency, we average the channel estimation MSE
method consumes less power compared to conventional path spectrum over all the frequencies. In the second step, we
search techniques because it uses only a subset of the corre-
calculate a low-complexity filter by performing a discrete-
lators during path searching. Also, the alternate use of the