U.S. patent application number 09/930411 was filed with the patent office on 2002-02-14 for method for detection of pilot tones.
Invention is credited to Hartmann, Ralf, Yang, Bin.
Application Number | 20020018532 09/930411 |
Document ID | / |
Family ID | 7897569 |
Filed Date | 2002-02-14 |
United States Patent
Application |
20020018532 |
Kind Code |
A1 |
Yang, Bin ; et al. |
February 14, 2002 |
Method for detection of pilot tones
Abstract
A method identifies a pulse sequence having known values and a
known length in a signal. According to this method, the
mathematical sign of the phase difference between samples of the
signal is used to estimate whether the transmitted pulse is a 1 or
a 0. Undersampling, carried out to a selectable extent, produces a
relatively insensitive response to adjacent channel interference.
The sum of the pulses in a window which is proportional to the
length of the pulse sequence and to the extent of the undersampling
is determined, with the pulse sequence being regarded as being
identified at the point in time at which the sum of the pulses in
this search window exceeds a threshold value.
Inventors: |
Yang, Bin; (M?uuml;nchen,
DE) ; Hartmann, Ralf; (M?uuml;nchen, DE) |
Correspondence
Address: |
LERNER AND GREENBERG, P.A.
POST OFFICE BOX 2480
HOLLYWOOD
FL
33022-2480
US
|
Family ID: |
7897569 |
Appl. No.: |
09/930411 |
Filed: |
August 15, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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09930411 |
Aug 15, 2001 |
|
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PCT/DE00/00301 |
Feb 1, 2000 |
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Current U.S.
Class: |
375/340 |
Current CPC
Class: |
H04L 27/22 20130101;
H04B 1/76 20130101; H04L 25/061 20130101 |
Class at
Publication: |
375/340 |
International
Class: |
H04L 027/06 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 15, 1999 |
DE |
199 06 293.5 |
Claims
We claim:
1. A method for identification of a pulse in a signal, which
comprises: a) obtaining samples of a signal at successive times k,
the sample times having a time difference of .DELTA.k.gtoreq.2
between the sample times k, the signal including a pulse sequence
having known values 0 and 1 and a known length; b) corresponding an
estimated symbol "1" to a phase difference of the signal when the
phase difference is in a range mod (.DELTA.k*.PI./2.2)-.PI./2 to
mod (.DELTA.k*.PI./2.2.PI.)+.PI./2; and corresponding an estimated
symbol "0" to the phase difference when the phase difference is not
in the range; c) filtering the estimated symbols by placing a
search window with a search window length equal to the known length
of the pulse sequence to be identified minus (.DELTA.k+1) over the
successively estimated symbols and by in each case forming a symbol
sum of the estimated symbols within the search window; d) comparing
the symbol sum with a sum threshold value; and e) indicating a
sought pulse sequence and a timing of the sought pulse sequence
when the symbol sum is at least equal to the sum threshold
value.
2. The method according to claim 1, which further comprises, before
carrying out step a), subjecting the samples to DC voltage
compensation.
3. The method according to claim 1, wherein .DELTA.k equals 2.
4. The method according to claim 1, wherein .DELTA.k equals 5.
5. The method according to claim 1, wherein the pulse sequence to
be identified is a sequence of 148 zeros.
6. The method according to claim 5, wherein the pulse sequence
identifies an organization channel in a mobile radio system.
7. The method according to claim 1, wherein the timing of the
sought pulse sequence occurs midway between a first time and last
time at which the symbol sum exceeds the sum threshold value.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application is a continuation of copending
International Application No. PCT/DE00/00301, filed Feb. 1, 2000,
which designated the United States.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The invention relates to a method for detection of pilot
tones. Pilot tones are sinusoidal oscillations at a known
frequency, which are used, for example, in communications systems,
in particular in mobile radio systems. A frequent task that occurs
in such mobile radio systems is to search for pilot tones.
[0004] For example, in digital mobile radio systems that operate in
accordance with the GSM/DCS1800/PCS1900 Standard, the radio traffic
is organized into organization channels. For a mobile station to
set up a connection to the network via a fixed station, it first
needs to detect and search for this organization channel. The
organization channel is detected by searching for specific pulse
sequences, which identify this organization channel. In the system
cited above, pulse sequences are referred to as frequency
correction bursts (FCB) and have a sequence of 148 zeros.
[0005] In the system under consideration here, the GMSK modulation
method (Gaussian Minimum Shift Keying) is used for transmission. In
this case, a carrier frequency FT (for example 900 MHz) is
modulated with the signal to be transmitted, that is to say in this
case, in particular, also with the FCB signal which is of specific
interest. The resultant frequency is FT+67.7 kHz, that is to say
67.7 kHz above the carrier frequency. The FCB pulse sequence of 148
zeros is thus converted to a pure sinusoidal signal. In the
baseband, this means that the phase difference between adjacent
samples is ideally (without channel distortion or noise) ninety
degrees (90.degree.), if it is assumed that sampling takes place at
the bit clock rate (4*67.7=270.8 kHz).
[0006] Various methods for FCB searching are known from the prior
art. For example, the article "Anfangssynchronisation der
Mobilstation im D-Netz" [Initial synchronization of mobile stations
in the D network] by G. Frank and W. Koch, PKI Tech. Report 1
(1990), pages 43-49 describes one method for FCB searching. In this
method, the FCB search starts with a frequency shift by multiplying
all the (I,Q) samples of the received signal by exp(-jk.pi./2).
Each sample Z at the time k can be represented, as a complex
number, in the form Z(k)=I(k)+jQ(k). This means that the received
signal is shifted downward by 67.7 kHz, so that its mid-frequency
after frequency shifting is 0 Hz. The signal is then low-pass
filtered. If this is the FCB signal, then it passes through the
filter; other signals are largely suppressed due to their wide
bandwidth. The magnitude of the filtered signal is then formed,
ideally resulting in an approximately rectangular pulse of the same
duration as an FCB signal. In contrast to this, the modulation with
random data bits in the rest of the time results in a signal
similar to noise. An optimum search filter can be specified for the
approximately rectangular pulse. This corresponds to sliding
averaging over the time period of an FCB. An FCB is regarded as
having been found when the maximum value of the filtered signal
exceeds a previously defined threshold. The position of the maximum
value marks the end of the detected FCB signal.
[0007] The method described in this article has the disadvantage
that the maximum value of the filtered signal depends on the
instantaneous signal amplitudes, and is therefore subject to severe
fading fluctuations. Therefore, adaptive amplitude control is
required for a reliable FCB search. The low-pass filter also must
have a high Q factor; therefore, its construction is complex.
Furthermore, this method is highly sensitive to frequency mistuning
between the mobile station and base station. Thus, in practice, the
maximum value has to be averaged over a number of observation
intervals.
[0008] A further method is described in the article
"Synchronisation einer Mobilstation im GSM-System DMCS 900
(D-Netz)" [Synchronization of a mobile station in the GSM DMCS 900
system (D network)] by H. Neuner, H. Bilitza, S. Grtner in Frequenz
[Frequency] 47 (1993) 3-4, pages 66-72. In this method, the phase
difference between every fourth sample of the received signal is
evaluated. The method is based on the observation that, ideally,
such phase differences are zero for the duration of an FCB signal.
Since, as already stated above, the phase difference between two
adjacent samples is 90.degree., the phase difference between four
samples is 4.times.90=360.degree., or 0.degree.. Interference
(fading) is taken into account with a validity range, which is
recalculated for each phase difference. An FCB signal is regarded
as having been found when a sufficiently large number of negligibly
small phase differences occur. One problem with this method is
determining the position of the FCB signal because only every
fourth sample is evaluated. Because the method described here makes
it necessary to determine the phase difference between samples, the
arctan function must be used in order to calculate the phase of the
sample from the quadrature components of the sampled received
signal. However, virtually no signal processors provide any
hardware support for this, so that the calculation is approximated
by a complex series development, which requires a considerable
amount of computation time.
[0009] A third method from the prior art is a method that was
developed by Dr. Ralf Hartmann at Siemens AG, which is similar to
the Frank and Koch method. This method uses two frequency-selective
comb filters, one of which filters passes FCB signals at the
frequency 67.7 kHz without any attenuation, while the other filter
completely blocks FCB signals. Magnitudes, and then sliding
averages, are formed from both filtered signals. The quotient of
the two averages is then formed, and is compared with a previously
defined threshold value. If the quotient is below the threshold
value, then an FCB is regarded as having been found. The position
of the quotient minimum marks the end of the FCB signal.
[0010] This method already has been used successfully in chip sets
for GSM mobile telephones. Because the quotient formation process
results in insensitivity to amplitude fluctuations, the amplitude
control required in the Frank and Koch method is not necessary.
However, the division process required for quotient formation
likewise still requires a relatively large amount of computation
time. Furthermore, the method is sensitive to frequency mistuning.
In the event of frequency mistuning, one filter can no longer pass
the signal through completely, while the other filter no longer
completely blocks the signal. This means that the quotient minimum
value rises considerably and the threshold value, which is
configured for the best case of minimum frequency mistuning, is no
longer suitable, so that the entire FCB search becomes
uncertain.
[0011] A further method for searching for such pilot tones is known
from German Patent Application DE 197 43 191, corresponding to U.S.
patent application Ser. No. 09/539,239 filed on Mar. 30, 2000. The
inventors are named R. Hartmann and B. Yang and the invention is
entitled, "Verfahren zur Suche nach Pilottonen," [Method for
searching for pilot tones] (date of application Sep. 30, 1997).
This method uses what is referred to as differential symbol
estimation. In this case, the exact phase differences between
successive (I,Q) samples of the received signal are not determined,
as in the method by Neuner, Bilitza, and Grtner. Instead of this,
all that is investigated is to determine whether the phase
differences between successive samples are in the interval (0,
.pi.) or (-.pi., 0). Both cases correspond to a transmitted symbol
of 1 ("+1") or 0 ("-1") from the GMSK modulator. Because a FCB
signal has 148 zeros is changed to 147 ones after differential
coding at the transmitter end, and a virtually equal number of ones
and zeros occur outside the FCB signal, then it is possible to
search for an FCB signal by searching for a long, cohesive block of
ones.
[0012] The advantage of the differential symbol estimation is its
simple implementation. If I(k) represents the in-phase component
and Q(k) represents the quadrature component of the baseband sample
at the time k, then, in this method, the mathematical sign of the
expression Q(k)*I(k-1)-I(k)*Q(k-1) ideally reflects the transmitted
signal exactly. Because fading of the sampled signal occasionally
leads to false symbol estimates, the estimated symbols (1 or 0) are
filtered using what is referred to as a match filter. This means
that a search window of fixed length is placed over the estimated
symbols and the number of ones within the window is counted, in the
form of a sliding addition process. The maximum of the signal
filtered in this way is then compared with a threshold value, and
the presence of an FCB signal is deduced if the threshold value is
exceeded.
[0013] This additional filtering makes the method described there
for searching for pilot tones relatively insensitive to amplitude
fluctuations, to a poor signal-to-noise ratio and to frequency
mistuning. However, interference from an adjacent channel does
represent a problem with this algorithm. If nothing is currently
being transmitted in the frequency channel on which a search is
currently being carried out for a pilot tone, that is to say for an
FCB signal, but a powerful broadband signal is being transmitted on
the adjacent channel, then residues from this signal can frequently
also be found in the frequency channel to be investigated. This
residual signal can then be confused with a pilot tone in the form
of an FCB pulse sequence in the investigated frequency channel.
SUMMARY OF THE INVENTION
[0014] It is accordingly an object of the invention to provide a
method for detection of pilot tones that overcomes the
hereinafore-mentioned disadvantages of the heretofore-known devices
of this general type and that improves the above method of
differential symbol estimation such that it is not sensitive to
interference from adjacent channels.
[0015] With the foregoing and other objects in view, there is
provided, in accordance with the invention, a method for
identification of a pulse in a signal. The first step of the method
is obtaining samples of a signal at successive times k. The sample
times has a time difference of .DELTA.k.gtoreq.2 between the sample
times k. The signal includes a pulse sequence having known values 0
and 1 and a known length. The next step is corresponding an
estimated symbol "1" to a phase difference of the signal when the
phase difference is in a range mod (.DELTA.k*.PI./2.2)-.PI./2 to
mod (.DELTA.k*.PI./2.2H)+.PI./2, and corresponding an estimated
symbol "0" to the phase difference when the phase difference is not
in the range. The next step is filtering the estimated symbols by
placing a search window with a search window length equal to the
known length of the pulse sequence to be identified minus
(.DELTA.k+1) over the successively estimated symbols, and by in
each case forming a symbol sum of the estimated symbols within the
search window. The next step is comparing the symbol sum with a sum
threshold value. The next step is indicating a sought pulse
sequence and a timing of the sought pulse sequence when the symbol
sum is at least equal to the sum threshold value.
[0016] In accordance with a further mode of the invention, the
method includes, before obtaining samples, subjecting the samples
to DC voltage compensation.
[0017] In accordance with a further mode of the invention, in the
method, .DELTA.k equals 2.
[0018] In accordance with a further mode of the invention, in the
method, .DELTA.k equals 5.
[0019] In accordance with a further mode of the invention, the
pulse sequence to be identified is a sequence of 148 zeros. Such a
pulse sequence can identify an organization channel in a mobile
radio system.
[0020] In accordance with a further object of the invention, the
timing of the sought pulse sequence occurs midway between a first
time and last time at which the symbol sum exceeds the sum
threshold value.
[0021] The method according to the invention uses the idea of
undersampling, in which, instead of using successive (I,Q) samples,
samples located further apart from one another are used to
calculate the phase differences. Such undersampling artificially
increases overlapping (aliasing) of the residual signal spectra
from an adjacent channel. This aliasing changes an originally
colored residual signal spectrum to an approximately white
spectrum. The residual signal thus behaves like noise and then has
scarcely any similarity with the sought FCB signal. The FCB signal
itself has a narrowband spectrum that is scarcely influenced by the
aliasing effect.
[0022] Other features which are considered as characteristic for
the invention are set forth in the appended claims.
[0023] Although the invention is illustrated and described herein
as embodied in a method for detection of pilot tones, it is
nevertheless not intended to be limited to the details shown, since
various modifications and structural changes may be made therein
without departing from the spirit of the invention and within the
scope and range of equivalents of the claims.
[0024] The construction and method of operation of the invention,
however, together with additional objects and advantages thereof
will be best understood from the following description of specific
embodiments when read in connection with the accompanying
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0025] FIG. 1a is a graph plotting the upper and lower adjacent
channel of thee investigated channel with their respective signal
spectra;
[0026] FIG. 1b is a graph plotting the amplitude response of the
baseband filter in the mobile station;
[0027] FIG. 1c is a graph plotting the residual signal spectra
after baseband filtering; and
[0028] FIG. 2 is a flow chart of the method for detection of pilot
tones.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0029] In all the figures of the drawing, sub-features and integral
parts that correspond to one another bear the same reference symbol
in each case.
[0030] Referring now to the figures of the drawings in detail and
first, particularly to FIGS. 1a to 1c thereof, there is shown the
problems of interference in the investigated channel from its
adjacent channels.
[0031] FIG. 1a shows the situation in which no signal is being
transmitted on the channel m to be investigated: that is, to say
the channel on which a search for a pilot tone is intended to be
carried out. Simultaneously, a powerful broadband signal is being
transmitted on its lower adjacent channel m-1 and on its upper
adjacent channel m+l. The frequency channel separation is 200 kHz
in mobile radio systems that comply with the GSM/DCS1800/PCS1900
Standard. The FCB signal in the lower channel m-1 and in the upper
channel m+1 is in each case represented as a vertical line, with an
arrow on it.
[0032] FIG. 1b shows the amplitude response of the baseband filter
in the mobile station. The mobile station is searching for the
pilot tone.
[0033] FIG. 1c shows the result that is obtained when the signals
shown in FIG. 1a are filtered using a baseband filter that has the
amplitude response shown in FIG. 1b. As can be seen, the baseband
filter is not able to completely suppress the broadband signals
from the two adjacent channels and the FCB signal from the lower
adjacent channel. Unfortunately, the broadband residual signal from
the upper adjacent channel occurs in a frequency band in which the
FCB signal for the channel m can also be found. The method
described in Patent Application DE 197 43 191, which was mentioned
above, for searching for pilot tones can thus not distinguish such
a residual signal from the FCB signal in its own channel, and the
residual signal would be incorrectly detected as an FCB signal.
[0034] The method according to the invention can prevent this
incorrect detection of the FCB signal. FIG. 2 uses a block diagram
to show the method according to the invention for detection of
pilot tones.
[0035] The signal in the channel m to be investigated is sampled at
individual times. Each sample Z of the signal received at the time
k can in this case be represented, in complex form, as
Z(k)=I(k)+jQ(k). In this case, I(k) is the in-phase component of
the baseband sample at the time k, and Q(k) is the quadrature
component of the baseband sample at the time k.
[0036] The two components I(k), Q(k) are subjected to offset
compensation in the block 1. This compensates for any possible DC
voltage components in the values I(k), Q(k). This may be done, for
example, by using a notch filter or block-oriented
compensation.
[0037] Differential symbol estimation is then carried out in the
blocks 2a and 2b. Until now, successive samples I(k), Q(k) have
been used for differential symbol estimation. In the undersampling
differential symbol estimation process according to the invention,
samples located further apart from one another are used to
calculate the phase differences. Undersampling is therefore carried
out. In general terms, I(k), Q(k) and I(k-.DELTA.k), Q(k-.DELTA.k)
are used to form the phase differences, where
.DELTA.k.gtoreq.2.
[0038] Undersampling with .DELTA.k=2 is chosen in the block 2a. To
this end, a check is completed to determine whether the phase
difference between I(k)+j*Q(k) and I(k-2)+j*Q(k-2) represents a
phase difference of .DELTA.k*.PI./2=.PI., that is to say whether it
is in the interval (.PI./2, 3.PI./2). This can be done by a simple
check of the value I(k)*I(k-1)+Q(k)*Q(k-2). If this value is less
than zero, then the estimated symbol a(k) represents a one. If this
value is greater than or equal to zero, then the estimated symbol
a(k) represents a zero.
[0039] In contrast, undersampling with .DELTA.k=5 is carried out in
the block 2b. To this end, a check is carried out to determine
whether the phase difference between I(k)+j*Q(k) and
I(k-15)+j*Q(k-5) represents a phase difference of
.DELTA.k*.PI./2=5*.PI./2, which is equivalent to .PI./2, that is to
say whether it is in the interval (0, .PI./2). This may be done by
a simple check of the value Q(k)*I(k-5)-I(k)*Q(k-5). If this value
is greater than zero, then the estimated symbol a(k) represents a
one. If this value is less than or equal to zero, then the
estimated symbol a(k) represents a zero.
[0040] As already mentioned, such undersampling results in
artificially increased overlapping (aliasing) of the residual
signal spectra from the adjacent channels. This aliasing effect
converts an originally colored residual signal spectrum, which is
present without this aliasing effect after filtering using the
baseband filter, to an approximately white spectrum. The residual
signal thus behaves like white noise and has scarcely any
similarity to the FCB signal, so that erroneous detection is
avoided.
[0041] The actual FCB signal has a narrowband spectrum, which is
scarcely influenced by the aliasing effect.
[0042] The aliasing effect becomes greater, the larger the chosen
value of .DELTA.k. However, on the other hand, there are reasons
against choosing an excessively large value for .DELTA.k:
[0043] a) An excessively large value of .DELTA.k also leads to a
broad spectrum in the actual FCB signal, which could lead to
failure to detect correct FCB signals.
[0044] b) The undersampling increases the effect of frequency
mistuning. For example, frequency mistuning of 20 kHz implies a
phase shift of (20/270.833)*360.degree. 26.6.degree. for adjacent
(I,Q) values (.DELTA.k=1). If .DELTA.k=2 is chosen, then frequency
mistuning of 20 kHz corresponds to a phase shift of
2*26.6.degree.=53.2.degree.. For this reason, .DELTA.k should be
chosen to be as small as possible so that even FCB signals with
major frequency mistuning can be detected correctly.
[0045] c) It is necessary to ensure that an FCB signal from the
lower adjacent channel (see FIG. 1c) is not shifted by the
undersampling process to approximately the same frequency band as
an FCB signal in the channel m. In this case, the mobile station
that is carrying out the process of detecting the pilot tones would
not be able to distinguish FCB signals from the channel m-1 and
from the channel m from one another. Such a situation occurs, for
example, if .DELTA.k=4.
[0046] As can be seen, the requirements mentioned above are
partially contradictory. Extensive tests have shown that .DELTA.k=2
and .DELTA.k=5 represent two sensible compromises for
GSM/DCS1800/PCS1900 systems.
[0047] The choice of .DELTA.k=2, as shown in block 2a in FIG. 2,
allows the detection of FCB signals that have major frequency
mistuning, and is thus suitable for initial synchronization of a
mobile station and base station. However, a small residual risk of
adjacent channel interference remains, since the undersampling is
not sufficient.
[0048] The choice of .DELTA.k=5, as shown in block 2b in FIG. 2,
prevents adjacent channel interference completely. Only FCB signals
with minor frequency mistuning can be detected here for this
reason. This operating mode is therefore particularly highly
suitable for monitoring adjacent cells in a mobile radio network
during radio operation.
[0049] Fading interference can occasionally lead to incorrect
symbol estimates in the above checks. For this reason, the
estimated symbols a (1 or 0) are filtered using a match filter in
the blocks 3a and 3b. This means that a search window of
predetermined length L is placed over the estimated symbols. In
this case, the number of ones within the search window is counted
in the form of a sliding addition process. The formula for this
purpose can be expressed as follows:
q(k)=q(k-1)+a(k)-a(k-L)
[0050] where q(k) is the symbol sum, a(k) is the symbol estimated
above, and L is the length of the search window. If the FCB signal
has 148 zeros, which become 147 ones at the transmission end after
differential coding, then L is calculated to be L=148-(.DELTA.k+1).
Thus, if .DELTA.k=2, this results in L=145, as is represented in
the block 3a, and if .DELTA.k=5, it results in L=142, as is
illustrated in the block 3b.
[0051] In both cases, the maximum of the symbol sum q(k) formed in
the blocks 3a and 3b is then compared with a threshold value S, and
the presence of an FCB signal is deduced if the threshold value is
exceeded. The position of the FCB signal can then also be deduced
from the position of the maximum. For example, the point in time
which is between the times at which the symbol sum (q) exceeds the
sum threshold value (S) for the first time and for the last time
can be quoted as the timing of the sought pulse sequence.
[0052] The method according to the invention detects pilot tones.
In particular, the method searches for pulse sequences that are
referred to as FCB signals, which identify an organization channel
in mobile radio systems. The method improves the prior art by being
relatively insensitive to interference from adjacent channels.
* * * * *