U.S. patent application number 14/231199 was filed with the patent office on 2014-08-14 for system and method for anticipatory receiver switching based on signal quality estimation.
This patent application is currently assigned to AVIAT U.S., INC.. The applicant listed for this patent is Aviat U.S., Inc.. Invention is credited to Richard Bourdeau, Tjo San Jao.
Application Number | 20140226706 14/231199 |
Document ID | / |
Family ID | 38876653 |
Filed Date | 2014-08-14 |
United States Patent
Application |
20140226706 |
Kind Code |
A1 |
Jao; Tjo San ; et
al. |
August 14, 2014 |
SYSTEM AND METHOD FOR ANTICIPATORY RECEIVER SWITCHING BASED ON
SIGNAL QUALITY ESTIMATION
Abstract
In various embodiments, a first and second complex multiplier
may be configured to receive an input signal and provide a baseband
I component signal and a baseband Q component signal, respectively.
A first and second filter may be configured to filter the baseband
I component signal and the baseband Q component signal,
respectively. An equalizer may be configured to equalize the
filtered baseband I component signal and the filtered baseband Q
component signal. A carrier recovery portion may be configured to
generate a reference signal based on the equalized filtered
baseband I component signal and the equalized filtered baseband Q
component signal. A first and second multilevel comparator may be
configured to receive the equalized filtered baseband I component
signal from the carrier recovery portion and provide an output I
and receive the equalized filtered baseband Q component signal and
provide an output Q signal for further modulation.
Inventors: |
Jao; Tjo San; (Beaconfield,
CA) ; Bourdeau; Richard; (Saint-Laurent, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Aviat U.S., Inc. |
Santa Clara |
CA |
US |
|
|
Assignee: |
AVIAT U.S., INC.
Santa Clara
CA
|
Family ID: |
38876653 |
Appl. No.: |
14/231199 |
Filed: |
March 31, 2014 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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13342850 |
Jan 3, 2012 |
8687737 |
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14231199 |
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12503805 |
Jul 15, 2009 |
8090056 |
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13342850 |
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11452216 |
Jun 14, 2006 |
7570713 |
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12503805 |
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Current U.S.
Class: |
375/232 ;
375/349 |
Current CPC
Class: |
H04B 7/0882 20130101;
H04L 1/0045 20130101; H04B 17/309 20150115; H04L 1/22 20130101;
H04B 7/0814 20130101; H04L 27/34 20130101; H04L 1/06 20130101; H04L
1/04 20130101; H04B 7/0888 20130101 |
Class at
Publication: |
375/232 ;
375/349 |
International
Class: |
H04B 7/08 20060101
H04B007/08 |
Claims
1-12. (canceled)
13. A system, comprising: a first estimating circuit for estimating
a first signal quality metric of a first signal, the first signal
quality metric being based on a distance signal that is a function
of a distance between a real symbol location of a real symbol of
the first signal and an associated predetermined ideal symbol
location of an ideal symbol; a comparing circuit for comparing the
first signal quality metric to a predetermined threshold to
determine whether the first signal quality metric meets a signal
quality, and for providing an alarm indicator when the first signal
quality metric does not meet the signal quality; a flag circuit for
determining whether the alarm indicator from the comparing circuit
satisfies an alarm condition; and a switching circuit for switching
from the first signal to a second signal in response to
satisfaction of the alarm condition, the second signal being
substantially independent of the first signal.
14. The system of claim 13, wherein the first and second signals
are baseband signals.
15. The system of claim 13, wherein the switching circuit is
configured to switch from the first signal to the second signal
based on one or more of: intermediate frequency (IF) spectrum
information, IF slope information, bit-error-rate (BER)
information, signal degradation information, and signal outage
information.
16. The system of claim 15, wherein the signal degradation
information is provided by a forward-error-coding (FEC)
process.
17. The system of claim 13, wherein the switching circuit is
configured to switch from the first signal to the second signal
according to a switching request priority associated with the
comparison of the first signal quality metric to the predetermined
threshold and other signal quality information.
18. The system of claim 17, wherein the switching request priority
comprises bit-error-rate (BER), frame loss, signal degradation,
intermediate frequency (IF) slope, or low receiver signal level
(RSL) sensed at a radio antenna port.
19. The system of claim 13, wherein the switching circuit comprises
a differential absolute delay equalizer (DADE) circuit configured
to compensate for delay between the first signal and the second
signal.
20. The system of claim 19, wherein the DADE circuit is configured
to dynamically correct the delay if a first receiver associated
with the first signal experiences multipath fading.
21. The system of claim 13, wherein the flag circuit is configured
to determine whether the alarm indicator satisfies the alarm
condition continuously, or at a predetermined interval.
22. A method, comprising: estimating a first signal quality metric
of a first signal, the first signal quality metric being based on a
distance signal that is a function of a distance between a real
symbol location of a real symbol of the first signal and an
associated predetermined ideal symbol location of an ideal symbol;
comparing the first signal quality metric to a predetermined
threshold to determine whether the first signal quality metric
meets a signal quality; providing an alarm indicator when the first
signal quality metric does not meet the signal quality; determining
whether the alarm indicator satisfies an alarm condition; and
switching from the first signal to a second signal in response to
satisfaction of the alarm condition, the second signal being
substantially independent of the first signal.
23. The method of claim 22, wherein the first and second signals
are baseband signals.
24. The method of claim 22, wherein the switching from the first
signal to the second signal comprises switching from the first
signal to the second signal based on one or more of: intermediate
frequency (IF) spectrum information, IF slope information,
bit-error-rate (BER) information, signal degradation information,
and signal outage information.
25. The method of claim 24, wherein the signal degradation
information is provided by a forward-error-coding (FEC)
process.
26. The method of claim 22, wherein the switching from the first
signal to the second signal comprises switching from the first
signal to the second signal according to a switching request
priority associated with the comparison of the first signal quality
metric to the predetermined threshold and other signal quality
information.
27. The method of claim 26, wherein the switching request priority
comprises bit-error-rate (BER), frame loss, signal degradation,
intermediate frequency (IF) slope, or low receiver signal level
(RSL) sensed at a radio antenna port.
28. The method of claim 22, wherein the switching from the first
signal to the second signal comprises compensating for delay
between the first signal and the second signal.
29. The method of claim 28, wherein the compensating for delay
between the first signal and the second signal comprises
dynamically correcting the delay if a first receiver associated
with the first signal experiences multipath fading.
30. The method of claim 22, wherein the determining whether the
alarm indicator satisfies the alarm condition comprises determining
whether the alarm indicator satisfies the alarm condition
continuously, or at a predetermined interval.
31. A system, comprising: means for estimating a first signal
quality metric of a first signal, the first signal quality metric
being based on a distance signal that is a function of a distance
between a real symbol location of a real symbol of the first signal
and an associated predetermined ideal symbol location of an ideal
symbol; means for comparing the first signal quality metric to a
predetermined threshold to determine whether the first signal
quality metric meets a signal quality; means for providing an alarm
indicator when the first signal quality metric does not meet the
signal quality; means for determining whether the alarm indicator
satisfies an alarm condition; and means for switching from the
first signal to a second signal in response to satisfaction of the
alarm condition, the second signal being substantially independent
of the first signal.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] The present application is a continuation of, and claims
priority under 35 U.S.C. .sctn.120 to, U.S. patent application Ser.
No. 13/342,850, filed Jan. 3, 2012, entitled "System and Method for
Anticipatory Receiver Switching Based on Signal Quality
Estimation," now U.S. Pat. No. 8,687,737, which is a continuation
of, and claims priority under 35 U.S.C. .sctn.120 to, U.S. patent
application Ser. No. 12/503,805, filed Jul. 15, 2009, entitled
"System and Method for Anticipatory Receiver Switching Based on
Signal Quality Estimation," now U.S. Pat. No. 8,090,056, which is a
continuation of, and claims priority under 35 U.S.C. .sctn. 120 to,
U.S. patent application Ser. No. 11/452,216, filed Jun. 14, 2006,
entitled "System and Method for Anticipatory Receiver Switching
Based on Signal Quality Estimation," now U.S. Pat. No. 7,570,713,
each of which is incorporated herein by reference.
BACKGROUND
[0002] Radio communication systems are becoming more reliable, and
the Mean Time Between Failure (MTBF) associated therewith is high.
However, in microwave radio transmission, the associated
transmission link may be long and multipath may be frequently
encountered. Multipath refers to multiple transmission paths
between transmit and receive antennas of a communication system.
Multipath may result in both frequency-selective fading and
space-selective fading. Frequency-selective fading generally
indicates that a channel varies with frequency. Space-selective
fading generally indicates that a channel is dependent upon the
position of the respective transmit and receive antennas. When
multipath interferes with reception of a radio transmission signal,
the received signal is distorted causing errors in the
corresponding demodulated data stream.
[0003] FIG. 1A illustrates an arrangement of symbol points in
accordance with a 4 QAM (Quadrature Amplitude Modulation) method on
an I-Q coordinate plane. With reference to FIG. 1A, a symbol point
of a received signal corresponds to any of four signal points
positioned concentrically on the I-Q coordinate plane. Therefore,
it is possible to transmit at one time 2 bits of data representing
any of the four signal points.
[0004] Advances in radio communication systems, however, require
data transmission of larger volume at higher speeds. Accordingly,
multiple value (M-ary) modulation methods having values larger than
the 4 QAM modulation method described above have been developed. As
an example of such an M-ary modulation method, 16 QAM is commonly
employed in data communications. FIG. 1B is an illustration of an
arrangement of symbol points in accordance with the 16 QAM
modulation method on the I-Q coordinate plane. With reference to
FIG. 1B, a symbol point of a received signal corresponds to any of
a total of 16 signal points on the coordinate plane, arranged four
by four in a lattice form in each quadrant of the I-Q coordinate
plane. Therefore, it is possible to transmit at one time 4 bits of
data representing any of the 16 signal points.
[0005] When a modulation method having a larger M-ary number is
employed and the communication environment of the propagation path
is defective (i.e., if the propagation path has severe
interference, noise or encounters multi-path), symbol points may be
recognized erroneously since the interval between each of the
symbol points is narrow and the symbol points are arranged tightly
in the respective modulation method, as may be seen from the
arrangement of symbol points of FIG. 1B. Therefore, though this
method has a communication speed faster than the 4 QAM modulation
method illustrated in FIG. 1A, it is more susceptible to reception
errors.
[0006] In a radio communication environment prone to multipath,
several known techniques may be implemented to mitigate the effects
of multipath. Prior art techniques commonly used to protect a
signal path include switching from an online channel, receiver or
antenna to a standby channel, receiver or antenna by 1+1 Frequency
Diversity (FD) or 1+1 Space Diversity (SD) or combination
thereof.
[0007] SD may commonly be provided by utilizing multiple receive
antennas separated by a sufficient distance to take advantage of
space-selective fading. With reference to FIG. 2, a prior art
method is illustrated that utilizes SD with two receive antennas
including a separate receiver connected to each receive antenna. A
pair of antennas 201, 202 are coupled to respective receivers 203,
205 that demodulate the signals received at each antenna. An
antenna selection circuit 209 accepts the demodulated output of the
receivers and provides control to an antenna switch 207 to select a
data set having the least amount of error.
[0008] The aforementioned diversity techniques and examples,
however, are not always appropriate for evaluating the
communication quality of the propagation path. For example,
different radio reception apparatuses employ different methods of
reception and performances. Qualities of components, such as
filters used in the reception apparatuses, vary and such
differences and variations have an influence on the quality of
communication. Conventional parameters such as reception level,
frame error rate, and carrier to interference ratios (CIR) do not
reflect such quality or performances of the reception apparatuses.
Further, as may be seen from a comparison of the modulation methods
shown in FIGS. 1A and 1B, even when there is no reception error
with a modulation method having smaller M-ary values, it is
unpredictable whether there arises reception error or not with
another modulation method (16, 64, 128, 256 QAM) having a larger
M-ary values (i.e., having dense symbol points on the I-Q plane).
As a result, special and complicated procedures and hardware are
necessary to measure the respective conventional parameters of the
propagation path during communication.
[0009] Thus, there is a need in the art for a system and method of
selecting antennas or receivers in a multipath environment without
incurring any errors in the received signal.
[0010] Accordingly, it is an object of the present subject matter
to obviate many of the deficiencies in the prior art and to provide
a novel method of switching from a first receiver receiving a
constant bit rate signal to a second receiver receiving the
constant bit rate signal, where the constant bit rate signal
received by the first and second receivers is converted to a first
baseband signal and a second baseband signal. The method further
comprises the steps of estimating a signal quality metric of the
first baseband signal, comparing the signal quality metric to a
predetermined threshold, and switching from the first baseband
signal to the second baseband signal if the signal quality metric
is greater than the threshold.
[0011] It is also an object of the present subject matter to
provide a novel method of switching from a first receiver receiving
a constant bit rate signal to a second receiver receiving the
constant bit rate signal, where the constant bit rate signal
received by the first and second receivers is converted to a first
baseband signal and a second baseband signal, respectively. The
method further comprises the steps of estimating a first signal
quality metric of the first baseband signal, estimating a second
signal quality metric of the second baseband signal, comparing the
first signal quality metric to the second signal quality metric,
and switching from the first baseband signal to the second baseband
signal if the first signal quality metric is greater than the
second signal quality metric.
[0012] It is another object of the present subject matter to
provide a novel system for switching from a first receiver
receiving a constant bit rate signal to a second receiver receiving
the constant bit rate signal. The system comprises a first
converting circuit for converting the signal from the first
receiver to a first baseband signal and a second converting circuit
for converting the signal from the second receiver to a second
baseband signal. The system further comprises an estimating circuit
for estimating a first signal quality metric of the first baseband
signal, a comparing circuit for comparing the first signal quality
metric to a predetermined threshold, and a switching circuit for
switching from the first baseband signal to the second baseband
signal if the first signal quality metric is greater than the
predetermined threshold.
[0013] It is still an object of the present subject matter to
provide a novel system of switching from a first receiver receiving
a constant bit rate signal to a second receiver receiving the
constant bit rate signal comprising a first converting circuit for
converting the signal from the first receiver to a first baseband
signal and a second converting circuit for converting the signal
from the second receiver to a second baseband signal. The system
also comprises a first estimating circuit for estimating a first
signal quality metric of the first baseband signal, a second
estimating circuit for estimating a second signal quality metric of
the second baseband signal, a comparing circuit for comparing the
first signal quality metric to the second signal quality metric,
and a switching circuit for switching from the first baseband
signal to the second baseband signal if the first signal quality
metric is greater than the second signal quality metric.
[0014] These and many other objects and advantages of the present
invention will be readily apparent to one skilled in the art to
which the invention pertains from a perusal of the claims, the
appended drawings, and the following detailed description of the
preferred embodiments.
SUMMARY OF THE INVENTION
[0015] In various embodiments, a system comprises a first and
second complex multiplier, a first and second filter, an equalizer,
a carrier recovery portion, and a first and second multilevel
comparator. The first complex multiplier may be configured to
receive an input signal and provide a baseband I component signal.
The second complex multiplier may be configured to receive the
input signal and provide a baseband Q component signal. The first
filter may be configured to filter the baseband I component signal.
The second filter may be configured to filter the baseband Q
component signal. The equalizer may be configured to equalize the
filtered baseband I component signal and to equalize the filtered
baseband Q component signal. The carrier recovery portion may be
configured to generate a reference signal based on the equalized
filtered baseband I component signal and the equalized filtered
baseband Q component signal. The first multilevel comparator may be
configured to receive the equalized filtered baseband I component
signal from the carrier recovery portion and provide an output I
signal for further demodulation. The second multilevel comparator
may be configured to receive the equalized filtered baseband Q
component signal from the carrier recovery portion and provide an
output Q signal for further demodulation.
[0016] The system may further comprise a lookup table (LUT) wherein
the LUT provides a sine signal to the first complex multiplier and
a cosine signal to the second complex multiplier. The sine signal
and the cosine signal may be based on the reference signal from the
carrier recovery portion. Further, the sine signal may be
substantially the same carrier frequency and is in phase with the
input signal and the cosine signal may be substantially the same
carrier frequency and is in quadrature phase with the input
signal.
[0017] The system may also further comprise a signal quality
estimation circuit configured to estimate a signal quality based on
the filtered baseband I component signal and the filtered baseband
Q component signal. The signal quality estimation circuit may be
configured to estimate a signal quality based on the equalized
filtered baseband I component signal and the equalized filtered
baseband Q component signal. Further, the signal quality estimation
circuit may be configured to estimate a signal quality based on the
equalized filtered baseband I component signal received form the
carrier recovery portion and the equalized filtered baseband Q
component signal received from the carrier recovery portion.
[0018] The signal quality estimation circuit may comprise a
distance calculator, an average calculator, and a comparator. The
distance calculator may be configured to determine a distance
between an ideal symbol and a received symbol of the equalized
filtered baseband I component signal and the equalized filtered
baseband Q component signal. The average calculator may be
configured to determine an average distance between based on the
distance from the distance calculator. The comparator may be
configured to compare the average distance to a predetermined
threshold. The signal quality estimation circuit may be further
configured to generate an alert signal when a quality is low based
on the comparison of the average distance to the predetermined
threshold.
[0019] In various embodiments, a method may comprise multiplying an
input signal with a sine signal to provide a baseband I component
signal, multiplying the input signal with a cosine signal to
provide a baseband Q component signal, filtering the baseband I
component signal, filtering the baseband Q component signal,
equalizing the filtered baseband I component signal, equalizing the
filtered baseband Q component signal, generating a reference signal
based on the equalized filtered baseband I component signal and the
equalized filtered baseband Q component signal, comparing the
equalized filtered baseband I component signal to provide an output
I signal for further demodulation, and comparing the equalized
filtered baseband Q component signal to provide an output Q signal
for further demodulation.
[0020] A system may comprise a means for multiplying an input
signal with a sine signal to provide a baseband I component signal,
a means for multiplying the input signal with a cosine signal to
provide a baseband Q component signal, a means for filtering the
baseband I component signal, a means for filtering the baseband Q
component signal, a means for equalizing the filtered baseband I
component signal, a means for equalizing the filtered baseband Q
component signal, a means for generating a reference signal based
on the equalized filtered baseband I component signal and the
equalized filtered baseband Q component signal, a means for
comparing the equalized filtered baseband I component signal to
provide an output I signal for further demodulation, and a means
for comparing the equalized filtered baseband Q component signal to
provide an output Q signal for further demodulation.
BRIEF DESCRIPTION OF THE FIGURES
[0021] FIGS. 1a and 1b illustrate a representation of arrangements
of symbol points of 4 QAM and 16 QAM, respectively, on an I-Q
coordinate plane.
[0022] FIG. 2 is a simplified block diagram of a prior art
receiving apparatus that includes two receivers and compares the
received signals after demodulation to select an antenna for
reception.
[0023] FIG. 3 is a simplified block diagram of a signal quality
estimation circuit according to an embodiment of the present
subject matter.
[0024] FIG. 4 is a functional block diagram of a signal quality
estimation method according to an embodiment of the present subject
matter.
[0025] FIG. 5 is an illustration of the ideal constellation points
of a 4 QAM modulation method having a received symbol in the (1,1)
quadrant.
[0026] FIG. 6 is a simplified block diagram of a 1+1 SD
configuration according to an embodiment of the present subject
matter.
[0027] FIG. 7 is a simplified block diagram of a 1+1 FD
configuration according to an embodiment of the present subject
matter.
DETAILED DESCRIPTION OF THE FIGURES
[0028] With reference to the figures where like elements have been
given like numerical designations to facilitate an understanding of
the present invention, the various embodiments of a system and
method for switching from a first receiver to a second receiver
without error in a received signal by utilizing signal quality
estimation are described. Switching according to embodiments of the
present subject matter may be conducted in conjunction with 1+1 or
1 for N protected radio links or other transmission media to
thereby protect against signal path disturbance, equipment
degradation or equipment failure and to achieve high data
availability carried through such media. Furthermore, embodiments
of the present subject matter estimate signal quality of a received
signal derived from the decision directed coherent demodulation of
M-ary/M-state QAM/PSK (i.e., 4 QAM (PSK) to 256 QAM (PSK) or
higher).
[0029] A system and method according to the present subject matter
may utilize an exemplary signal quality estimation circuit
illustrated by FIG. 3. With reference to FIG. 3, a functional block
diagram of a front end portion of a demodulator comprising a
demodulator circuit 300 is illustrated in accordance with an
embodiment of the present subject matter. While the demodulator
circuit 300 is described herein with application to M-ary
quadrature amplitude modulation (QAM), it is to be understood that
the demodulator circuit 400 may be utilized with various types of
demodulators such as those applicable to phase shift keying (PSK)
and multiple values thereof (i.e., M-state), quadrature phase shift
keying (QPSK), as well as QAM, and multiple values thereof (i.e.,
M-ary).
[0030] The demodulator circuit 300 receives a modulated IF signal
310. The modulated IF signal 310 is provided to complex multipliers
320, 322 which provide baseband I and Q component signals 324 and
326, respectively. A sine/cosine look-up table (LUT) 330 provides
sine and cosine signals, sin(x) and cos(x), 316 and 318
respectively, to the complex multipliers 320, 322. The phase
components of the sine and cosine signals are such that the
multiplication of the modulated IF signal 310 to form I and Q
components followed by an appropriate filtering 340, 341 (i.e., low
pass, loop or other well known filters) provide I and Q signals 342
and 344, respectively.
[0031] The signals 342, 344 are provided to an equalizer 350 which
processes the signals and passes equalized signals to a carrier
recovery portion 360 of the demodulator circuit 300. Broadly, the
equalizer 350 processes and/or filters the received signals 342,
344 to reduce or eliminate intersymbol interference (ISI). An
equalizer according to an embodiment of the present subject matter
may conduct maximum likelihood sequence estimation, filter
equalization (i.e., linear, non-linear filters), or other forms of
equalization known in the art. The carrier recovery portion 360, as
is known in the art, creates a reference signal for input into the
LUT 330 which, in turn, outputs a signal 316 having a frequency
that is substantially the same as the carrier frequency and is in
phase with the carrier signal, and outputs a signal 318 having a
frequency that is substantially the same as the carrier frequency
but is out of phase (quadrature) with the carrier signal. The
carrier recovery portion 360 provides equalized I and Q signals 362
and 364, respectively, from the equalizer 350 to multilevel
comparators 372, 374 and a signal quality estimation circuit 376.
Of course, the demodulator circuit 300 may include various other
components such as a slope equalizer, a DC offset compensation
circuit, converters, gain compensation circuits, clock recovery
circuits, demapper circuits, forward error correction circuits, and
combinations thereof, as well as other demodulator components known
in the art. Furthermore, the sensing connections for the signal
quality estimation circuit 376 may be moved to the output of the
filters 340, 341 or the output of the equalizer 350. The
demodulator circuit 300 may thus accurately estimate the signal
quality for different modulation levels and different FEC coding
schemes such as 2D TCM, 4D TCM, Reed Solomon code, and
concatenation of TCM and Reed Solomon code. Of course, the
demodulator circuit 300 may be implemented through hardware or
software or a combination of both hardware and software.
[0032] FIG. 4 is a functional block diagram of a signal quality
estimation circuit 376 according to an embodiment of the present
subject matter. With reference to FIG. 4, an estimation circuit
continuously monitors the quality of the received signals 362, 364
provided by the carrier recovery portion 360 and declares an alarm
434 when the quality is below a certain level. As represented by
block 410, the signal quality estimation circuit determines the
distance between a received symbol and an ideal symbol that would
have been received if there was no impairment of the signal. This
impairment may be the result of thermal noise, signal distortion
due to multipath fading (selective fading), interference, etc. As
represented by block 420, the signal quality estimation circuit
determines an average distance between the distances between
successive received symbols and the corresponding ideal symbols. As
represented by block 430, the signal quality estimation circuit
compares the average distance to a predetermined threshold.
[0033] For example, FIG. 5 illustrates ideal constellation points
of a 4 QAM constellation with a received symbol 502 in the (1,1)
quadrant. A signal quality estimation circuit according to an
embodiment of the present subject matter may determine a signal
quality metric as a function of the distance (d) between the
received symbol location 502 and a predetermined ideal symbol
location 504. For example, the higher the distance (d), the lower
the quality of the baseband signal. By monitoring successive
received symbols, determining the distance between the successive
received symbols and the predetermined ideal symbol location
associated therewith 410, the estimation circuit may determine an
average distance 420 for the measured distance determinations and
compare 430 the average distance against a predetermined threshold
432. If the predetermined threshold 432 is exceeded then the
estimation circuit produces an alarm 434. A further embodiment of
the signal quality determination according to the present subject
matter comprises determining, for each of n symbols of a signal, a
distance between a location of the symbol and an associated
predetermined ideal symbol location, determining a sum of the
distances for the n symbols, and dividing the sum by n. The sum
divided by n will be compared to the predetermined threshold 432
and an alarm produced 434 if the predetermined threshold is
exceeded.
[0034] FIG. 6 is a simplified block diagram of a communications
system employing a 1+1 SD configuration according to an embodiment
of the present subject matter. With reference to FIG. 6, a
communications system 600 employing a 1+1 SD configuration is
illustrated where a constant bit rate signal or RF carrier is being
transmitted while two antennas 604, 606 are utilized to receive the
signal. Each antenna provides its respective received signal to an
associated receiver 610, 612. The antennas 604, 606 may be
physically displaced, to thereby achieve a space diversity effect.
Any of the receivers may operate in a full duplex communication
system and may also comprise part of a communication node that
transmits and receives constant bit rate data in a full duplex mode
with other communication nodes (not shown).
[0035] FIG. 7 is a simplified block diagram of a communications
system employing a 1+1 FD configuration according to an embodiment
of the present subject matter. With reference to FIG. 7, a
communications system 700 employing a 1+1 FD configuration is
illustrated where data is transmitted redundantly through two
different carriers via a single antenna 704. The antenna 704
provides a received constant bit rate signal to a splitter
circulator 702 which splits and distributes the signal to
associated receivers 610, 612. Any of the receivers may operate in
a full duplex communication system and may also comprise part of a
communication node that transmits and receives constant bit rate
data in a full duplex mode with other communication nodes (not
shown).
[0036] With reference to FIGS. 6 and 7, modulated signals are
provided by the receivers 610, 612 to associated demodulators 620,
622 to recover and demodulate the data. The signals may be
converted to baseband signals in components of the demodulators
620, 622 or by a separate converting circuit (not shown). After
further signal processing, the two redundant data streams may be
provided by the demodulators 620, 622 to a Differential Absolute
Delay Equalizer (DADE) circuit 640. Alarm information provided by
the signal quality estimation circuit 376 from the respective
demodulator may be supplied to a radio controller or control
processing unit (CPU) 630. Embodiments of the CPU 630 may poll the
alarm signal 434 provided by the signal quality estimation circuit
376 at predetermined intervals, e.g., 1 msec, 5 msec, 10 msec. Of
course, the CPU 630 may receive a continuous signal from the alarm
signal 434 or may poll the alarm signal 434 at different intervals
than those identified and such examples are not intended to limit
the scope of the claims appended hereto. The CPU 630 utilizes the
alarm information alone or in combination with other information
such as IF spectrum or signal slope information, BER information,
signal degrade information based on data derived from FEC decoding
processes, and signal outage information to provide a switching
decision to the DADE circuit or another suitable switching circuit
based on a preset switching request priority. A switching request
priority for a receiver switching algorithm according to an
embodiment of the present subject matter, listed from highest
priority to lowest priority, may comprise: force switched, BER or
frame loss alarm, signal degrade alarm, IF slope alarm, and
receiver signal level (RSL) low alarm sensed at a radio antenna
port.
[0037] The DADE circuit 640 compensates for any delay between the
two data streams provided by the receivers and aligns the data
streams so that a data transition from an active channel to a
standby channel occurs without error. For example, when switching
occurs, the two data streams provided by the receivers must be
aligned so that the transition of data from one receiver to the
data from a second receiver occurs without error. The DADE circuit
may comprise two delay flip-flop buffers. One buffer delays data
signals from one receiver and the second buffer delays data signals
from the second receiver. Since the signal carried by the two
receivers possesses disparate paths (i.e., the signals pass through
different hardware), there exists some delay between the two
signals. The DADE circuit may constantly monitor and align the two
data streams from the two receivers. The active channel or receiver
buffer will be read and outputted at the center of the buffer while
the data from the standby channel or receiver is altered to possess
the same delay as that of the active channel or receiver. The DADE
circuit may also dynamically correct the delay alignment if one or
both receivers experience a multipath (selective) fading that sifts
the data around. When switching occurs, the selected buffer outputs
the data to the following circuit for further processing. Since the
two incoming signals to the DADE circuit possess different delay
and phase, the read clock for both buffers is synchronized by the
incoming clock of the selected channel. In the event of a sudden
phase jump of the read clock resulting in a phase hit and
synchronization loss for the following circuit, the phase
transition of the read clock should be slow enough to mitigate any
phase hits. This may be achieved by appropriately utilizing a phase
locked loop.
[0038] An exemplary communications system according to an
embodiment of the present subject matter receives a constant bit
rate signal at the antennas which is then provided to the
receivers. The constant bit rate signal received by the receivers
is converted to a first baseband signal and a second baseband
signal, respectively, and the demodulator estimates a signal
quality metric of the first baseband signal, compares the signal
quality metric to a predetermined threshold, and provides alarm
information to the CPU. The CPU evaluates the alarm information
provided by the demodulator and provides switching control
information to the DADE circuit or other suitable switching circuit
if the signal quality metric exceeds a predetermined threshold to
thereby switch from the first baseband signal to the second
baseband signal. The demodulator may estimate the first signal
quality metric by determining a first distance between a location
of a first symbol of the first baseband signal and a first
predetermined ideal symbol location, determining a second distance
between a location of a second symbol of the first baseband signal
and a second predetermined ideal symbol location, and determining
an average distance for the first and second distance
determinations whereby the average distance may be compared to the
predetermined threshold to generate the alarm information.
[0039] In a further embodiment, the demodulator may estimate the
first signal quality metric by determining, for each of n symbols
of the first baseband signal, a distance between a location of the
symbol and an associated predetermined ideal symbol location,
determining a sum of the distances for the n symbols, and dividing
the sum by n whereby the sum divided by n may be compared to a
predetermined threshold to generate the alarm information.
[0040] A communications system according to a further embodiment of
the present subject matter receives a constant bit rate signal at
the antennas which are then provided to the receivers. The constant
bit rate signal received by the receivers is converted to a first
baseband signal and a second baseband signal, respectively. A first
demodulator estimates a signal quality metric of the first baseband
signal, and a second demodulator estimates a signal quality metric
of the second baseband signal. The CPU compares the first signal
quality metric to the second signal quality metric and provides
switching control information to the DADE circuit or another
suitable switching circuit if the signal quality metric exceeds a
predetermined threshold to thereby switch from the first baseband
signal to the second baseband signal. The first demodulator may
estimate the first signal quality metric by determining a first
distance between a location of a first symbol of the first baseband
signal and a first predetermined ideal symbol location, determining
a second distance between a location of a second symbol of the
first baseband signal and a second predetermined ideal symbol
location, and determining a first average distance for the first
and second distance determinations. The second demodulator may
estimate the second signal quality metric by determining a third
distance between a location of a first symbol of the second
baseband signal and a third predetermined ideal symbol location,
determining a fourth distance between a location of a second symbol
of the second baseband signal and a fourth predetermined ideal
symbol location, and determining a second average distance for the
third and fourth distance determinations whereby the first average
distance may be compared to the second average distance to generate
the alarm information.
[0041] In a further embodiment, the first demodulator may estimate
the first signal quality metric by determining, for each of n
symbols of the first baseband signal, a distance between a location
of the symbol and an associated predetermined ideal symbol
location, determining a first sum of the distances for the n
symbols, and dividing the first sum by n. Similarly, the second
demodulator may estimate the second signal quality metric by
determining, for each of n symbols of the second baseband signal, a
distance between a location of the symbol and an associated
predetermined ideal symbol location, determining a second sum of
the distances for the n symbols, and dividing the second sum by n
whereby the first sum divided by n is compared to the second sum
divided by n to generate the alarm information. Of course, the
first and second signal quality metrics may be compared to
predetermined thresholds rather than to each other. For example,
the first signal quality metric may be compared to a predetermined
threshold and if the first signal quality metric is greater than a
predetermined threshold then the second signal quality metric may
be compared to a predetermined threshold. If the second signal
quality metric is less than the predetermined threshold, then the
CPU may provide switching control information to the DADE circuit
or another suitable switching circuit to thereby switch from the
first baseband signal to the second baseband signal. However, if
the second signal quality metric is greater than or equal to the
predetermined threshold, the first signal quality metric may be
compared to the second signal quality metric, and the CPU may
provide switching control information to the DADE circuit or
another suitable switching circuit to thereby switch from the first
baseband signal to the second baseband signal if the first signal
quality metric is greater than the second signal quality
metric.
[0042] For example, when multipath occurs to a wide band spectrum,
a slope with various steepness will occur to the affected spectrum
while selective fading may sweep through the spectrum. With proper
adaptive slope equalization, the slope in the spectrum may be
corrected to a certain extent. When slope equalization reaches its
correction limit, a slope alarm will be declared, which may be used
to switch to a standby receiver. However, when the spectrum is
narrow, relatively flat fading occurs during multipath occurrence.
Thus, slope alarm may not be as reliable against multipath
occurrences for narrow band signal.
[0043] By way of further example, FEC decoding generates syndromes
for convolutional decoders or corrected symbols in Reed Solomon
decoders when transmission errors occur, which may be used to
trigger receiver switching. However, comparing the relative signal
quality between the two may be a relatively slow process. Thus,
switching from this information may be too slow to ensure a
reliable errorless switch for lower capacity data during fast
fading periods. Thus, other events may be used to initiate receiver
switching, such as BER alarm, etc. Thus, in another aspect of the
present subject matter, a signal quality degrade threshold to
trigger the switching may be set such that an alarm will be
triggered before any error appears at an FEC decoder output.
[0044] In a multipath environment, selective fading may sweep
through a receiver antenna with progressive speed. However, with a
1+1 protected FD system 700 according to an embodiment of the
present subject matter, the two RF carriers are sufficiently
separated in frequency and thus, the signal distortion will occur
to one carrier and then to the next carrier, but not both
simultaneously. Furthermore, with a 1+1 protected SD system 600
according to an embodiment of the present subject matter, the two
receive antennas 604, 606 are properly displaced and thus,
selective fading will affect one of the two antennas, but not both
simultaneously. Thus, with FD or SD systems or combinations
thereof, error free data transmission can be ensured even during
severe path disturbance period.
[0045] In a further aspect of the present subject matter, the
system may incorporate Adaptive Time Domain Equalization (ATDE)
and/or forward error correction (FEC) such that the alarm
information and subsequent receiver switching will occur before any
error occurs to output data during signal path fading (selective or
flat fading). Thus, in conjunction with a DADE circuit, switching
from a degraded active channel to a good standby channel may be
completed without any error.
[0046] Although embodiments of the present subject matter have been
described with application to 4 QAM, it is to be understood that
the present subject matter is applicable for M-ary QAM systems and
M-state PSK systems, i.e., embodiments of the present subject
matter may be utilized in any M-ary QAM demodulator (M=4, 8, 16,
32, 64, 128, 256, etc.) and may be utilized in any M-state PSK
demodulator (M=2, 4, 8, 16, 32, 64, 128, 256, etc.). Furthermore,
it is to be understood that embodiments of the present subject
matter may also be utilized in any non conventional 2 dimensional
(I-Q) modulation.
[0047] As shown by the various configurations and embodiments
illustrated in FIGS. 1-7, the system and method for receiver
switching based on signal quality estimation according to the
present subject matter may be utilized for selecting antennas,
channels or receivers in a multipath environment without incurring
any errors in a received signal.
[0048] While preferred embodiments of the present subject matter
have been described, it is to be understood that the embodiments
described are illustrative only and that the scope of the invention
is to be defined solely by the appended claims when accorded a full
range of equivalence, many variations and modifications naturally
occurring to those of skill in the art from a perusal hereof.
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