U.S. patent application number 17/193939 was filed with the patent office on 2022-09-08 for linearizing narrowband carriers with low resolution predistorters.
The applicant listed for this patent is MOTOROLA SOLUTIONS, INC.. Invention is credited to Gregory J. Buchwald, Dennis M. Drees, Rodney W. Hagen, In S. Kim, Arthur Christopher Leyh.
Application Number | 20220286151 17/193939 |
Document ID | / |
Family ID | 1000005476260 |
Filed Date | 2022-09-08 |
United States Patent
Application |
20220286151 |
Kind Code |
A1 |
Hagen; Rodney W. ; et
al. |
September 8, 2022 |
LINEARIZING NARROWBAND CARRIERS WITH LOW RESOLUTION
PREDISTORTERS
Abstract
Apparatus and method for linearizing narrowband carriers with
low resolution predistorters are provided. The method includes
amplifying, using a power amplifier, one or more broadcast carriers
and linearizing, using a predistorter coupled to the power
amplifier, the one or more broadcast carriers. The method also
includes determining, using an electronic processor, a composite
bandwidth of the one or more broadcast carriers and determining,
using the electronic processor, whether the composite bandwidth is
below a modulation bandwidth of the predistorter. The method
further includes controlling, using the electronic processor, a
pacification carrier generator coupled to the electronic processor
to combine a pacification carrier with the one or more broadcast
carriers when the composite bandwidth is below the minimum
modulation bandwidth of the predistorter.
Inventors: |
Hagen; Rodney W.; (Lake in
the Hills, IL) ; Buchwald; Gregory J.; (Crystal Lake,
IL) ; Drees; Dennis M.; (Deer Park, IL) ; Kim;
In S.; (Buffalo Grove, IL) ; Leyh; Arthur
Christopher; (Spring Grove, IL) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
MOTOROLA SOLUTIONS, INC. |
Chicago |
IL |
US |
|
|
Family ID: |
1000005476260 |
Appl. No.: |
17/193939 |
Filed: |
March 5, 2021 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H04B 17/318 20150115;
H04B 2001/0433 20130101; H04B 1/04 20130101; H04B 1/1036 20130101;
H04B 2001/0491 20130101; H04B 2001/0425 20130101 |
International
Class: |
H04B 1/04 20060101
H04B001/04; H04B 17/318 20060101 H04B017/318; H04B 1/10 20060101
H04B001/10 |
Claims
1. A radio frequency (RF) system for linearizing narrowband
carriers with low resolution predistorters, the system comprising:
a power amplifier configured to amplify one or more broadcast
carriers; a predistorter coupled to the power amplifier and
configured to linearize the one or more broadcast carriers; and a
pacification carrier generator configured to generate a
pacification carrier, wherein the pacification carrier is combined
with the one or more broadcast carriers to satisfy a minimum
modulation bandwidth of the predistorter.
2. The RF system of claim 1, further comprising: an electronic
processor coupled to the pacification carrier and configured to:
determine a composite bandwidth of the one or more broadcast
carriers, determine whether the composite bandwidth is below a
minimum modulation bandwidth of the predistorter, and control the
pacification carrier generator to combine the pacification carrier
with the one or more broadcast carriers when the composite
bandwidth is below the minimum modulation bandwidth of the
predistorter.
3. The RF system of claim 2, wherein the electronic processor is
configured to control the pacification carrier generator to stop
generation of the pacification carrier when the composite bandwidth
is above the minimum modulation bandwidth of the predistorter.
4. The RF system of claim 2, wherein the minimum modulation
bandwidth of the predistorter is above 1 MHz and wherein the
electronic processor is further configured to combine the
pacification carrier with the one or more broadcast carriers
resulting in a combined signal having a bandwidth greater than 1
MHz.
5. The RF system of claim 1, further comprising: a transmission
post filter configured to filter an amplified signal received from
the power amplifier, wherein the transmission post filter
attenuates the pacification carrier from the amplified signal.
6. The RF system of claim 1, wherein the pacification carrier is
generated at a power level 10 Decibels or more below a composite
power of the one or more broadcast carriers.
7. The RF system of claim 2, wherein the electronic processor is
further configured to scan through a plurality of frequencies of a
transmission band of the RF system; measure a received signal
strength indicator (RSSI) of signals detected at each of the
plurality of frequencies; determine a number of the plurality of
frequencies for which the RSSI exceeds a threshold RSSI; and
determine whether the number of the plurality of frequencies
exceeds a threshold number.
8. The RF system of claim 7, wherein the electronic processor is
configured to: determine that the composite bandwidth is below the
minimum modulation bandwidth when the number of the plurality of
frequencies does not exceed the threshold number; and determine
that the composite bandwidth exceeds the minimum modulation
bandwidth when the number of the plurality of frequencies exceeds
the threshold number.
9. The RF system of claim 7, further comprising a detection antenna
coupled to the electronic processor and configured to detect the
one or more broadcast carriers transmitted by the RF system,
wherein the electronic processor is configured to measure the RSSI
based on signals detected at the detection antenna.
10. The RF system of claim 7, further comprising a detection
coupler coupled to the electronic processor and configured to
detect the one or more broadcast carriers transmitted by the RF
system, wherein the electronic processor is configured to measure
the RSSI based on signals detected at the detection coupler.
11. The RF system of claim 2, wherein the electronic processor is
further configured to measure a characteristic of the one or more
broadcast carriers; compare the characteristic to a predetermined
characteristic threshold; determine that the composite bandwidth is
below the minimum modulation bandwidth when the characteristic does
not satisfy the predetermined characteristic threshold; and
determine that the composite bandwidth is above the minimum
modulation bandwidth when the characteristic satisfies the
predetermined characteristic threshold.
12. The RF system of claim 1, wherein the narrowband carriers
include 200 kHz internet of things (IoT) carriers.
13. A method for linearizing narrowband carriers with low
resolution predistorters, the method comprising: amplifying, using
a power amplifier, one or more broadcast carriers; linearizing,
using a predistorter coupled to the power amplifier, the one or
more broadcast carriers; and generating, using a pacification
carrier generator, a pacification carrier, wherein the pacification
carrier is combined with the one or more broadcast carriers to
satisfy a minimum modulation bandwidth of the predistorter.
14. The method of claim 13, further comprising: determining, using
an electronic processor, a composite bandwidth of the one or more
broadcast carriers; determining, using the electronic processor,
whether the composite bandwidth is below a minimum modulation
bandwidth of the predistorter; and controlling, using the
electronic processor, the pacification carrier generator coupled to
the electronic processor to combine the pacification carrier with
the one or more broadcast carriers when the composite bandwidth is
below the minimum modulation bandwidth of the predistorter.
15. The method of claim 14, further comprising controlling, using
the electronic processor, the pacification carrier generator to
stop generation of the pacification carrier when the composite
bandwidth is above the minimum modulation bandwidth of the
predistorter.
16. The method of claim 14, wherein the minimum modulation
bandwidth of the predistorter is above 1 MHz, the method further
comprising: combining, using the electronic processor, the
pacification carrier with the one or more broadcast carriers
resulting in a combined signal having a bandwidth greater than 1
MHz.
17. The method of claim 13, further comprising: attenuating, using
a transmission post filter, the pacification carrier from an
amplified signal.
18. The method of claim 13, wherein the pacification carrier is
generated at a power level 10 Decibels or more below a composite
power of the one or more broadcast carriers.
19. The method of claim 14, further comprising: scanning, using the
electronic processor, through a plurality of frequencies of a
transmission band of the RF system; measuring, using the electronic
processor, a received signal strength indicator (RSSI) of signals
detected at each of the plurality of frequencies; determining,
using the electronic processor, a number of the plurality of
frequencies for which the RSSI exceeds a threshold RSSI; and
determining, using the electronic processor, whether the number of
the plurality of frequencies exceeds a threshold number.
20. The method of claim 19, further comprising: determining, using
the electronic processor, that the composite bandwidth is below the
minimum modulation bandwidth when the number of the plurality of
frequencies does not exceed the threshold number; and determining,
using the electronic processor that the composite bandwidth exceeds
the minimum modulation bandwidth when the number of the plurality
of frequencies exceeds the threshold number.
21. The method of claim 19, further comprising: measuring, using a
detection antenna coupled to the electronic processor, the RSSI
based on signals detected at the detection antenna.
22. The method of claim 19, further comprising: measuring, using a
detection coupler, the RSSI based on signals detected at the
detection coupler.
23. The method of claim 14, further comprising: measuring, using
the electronic processor, a characteristic of the one or more
broadcast carriers; comparing, using the electronic processor, the
characteristic to a predetermined characteristic threshold;
determining, using the electronic processor, that the composite
bandwidth is below the minimum modulation bandwidth when the
characteristic does not satisfy the predetermined characteristic
threshold; and determining, using the electronic processor, that
the composite bandwidth is above the minimum modulation bandwidth
when the characteristic satisfies the predetermined characteristic
threshold.
24. The method of claim 13, wherein the narrowband carriers include
200 kHz internet of things (IoT) carriers.
Description
BACKGROUND OF THE INVENTION
[0001] Radio frequency (RF) transmission sites include one or more
power amplifiers. The power amplifiers amplify broadcast carriers
before the broadcast carriers are transmitted. Ideal power
amplifiers duplicate the input signal at the output with higher
power and few, if any, distortion products. However, power
amplifiers available in the market do not provide ideal
performance. Rather, power amplifiers introduce distortion products
(for example, 3.sup.rd order and 5.sup.th order harmonics) into the
output signals. Predistorters, for example, Radio frequency Power
Amplifier Linearizers are used to remove the distortion products
from the output. Adaptive predistorters use a feedback loop to
detect the distortion products in the output and introduce
cancellation products with the effective inverse function of an
AM-AM and AM-PM distortion in the input to cancel the distortion
products in the output.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS
[0002] The accompanying figures, where like reference numerals
refer to identical or functionally similar elements throughout the
separate views, together with the detailed description below, are
incorporated in and form part of the specification, and serve to
further illustrate embodiments of concepts that include the claimed
invention, and explain various principles and advantages of those
embodiments.
[0003] FIG. 1 is a block diagram of a radio frequency (RF) system
in accordance with some embodiments.
[0004] FIG. 2 is a flowchart of a method for linearizing carriers
with low resolution predistorters in the RF system of FIG. 1 in
accordance with some embodiments.
[0005] FIG. 3 is a flowchart of a method for linearizing carriers
with low resolution predistorters in the RF system of FIG. 1 in
accordance with some embodiments.
[0006] FIG. 4 is a graphical representation of received signal
strength indicators of one or more broadcast carriers of the RF
system of FIG. 1 in accordance with some embodiments.
[0007] FIG. 5 is a flowchart of a method for linearizing carriers
with low resolution predistorters in the RF system of FIG. 1 in
accordance with some embodiments.
[0008] FIG. 6 is a flowchart of a method for linearizing carriers
with low resolution predistorters in the RF system of FIG. 1 in
accordance with some embodiments.
[0009] FIG. 7 is a flowchart of a method for determining a
composite bandwidth of one or more broadcast carriers of the RF
system of FIG. 1 in accordance with some embodiments.
[0010] FIGS. 8A-8C are graphical representations illustrating
example carrier and distortion energy products of carrier signals
of the RF system of FIG. 1 in accordance with some embodiments.
[0011] Skilled artisans will appreciate that elements in the
figures are illustrated for simplicity and clarity and have not
necessarily been drawn to scale. For example, the dimensions of
some of the elements in the figures may be exaggerated relative to
other elements to help to improve understanding of embodiments of
the present invention.
[0012] The apparatus and method components have been represented
where appropriate by conventional symbols in the drawings, showing
only those specific details that are pertinent to understanding the
embodiments so as not to obscure the disclosure with details that
will be readily apparent to those of ordinary skill in the art
having the benefit of the description herein.
DETAILED DESCRIPTION OF THE INVENTION
[0013] Predistorters currently available in the market are tailored
for the particular application of the RF system. For this reason,
relative to narrowband land mobile radio (LMR), most predistorters
have a low resolution tailored to a particular minimum and maximum
bandwidth of the RF system. A low resolution predistorter is, for
example, a predistorter having a specific range of bandwidth over
which the predistorter is efficient in cancelling intermodulation
distortion (IMD) products. The specific range of bandwidth may be
limited to a lower end and a higher end of bandwidth of a
particular group of signals (e.g., LTE signals).
[0014] In one example, the 900 MHz land mobile radio (LMR) band was
originally assigned for carrying only narrowband LMR carriers. The
Federal Communications Commission (FCC) has reassigned the 900 MHz
LMR band for simultaneous use with 3 MHz bandwidth LTE signals, 1.4
MHz bandwidth LTE signals, and 200 kHz Narrowband Internet of
Things (IoT) signals. The RF systems operating in the 900 MHz LMR
band are configured for transmitting signals using one or all of
the 3 MHz LTE signals, the 1.4 MHz LTE signals, and the 200 kHz
Narrowband IoT signals.
[0015] Predistorters specifically designed for LTE bandwidth are
configured to linearize signals having a bandwidth greater than 1
MHz. The minimum bandwidth at which the predistorter provides
desired performance is referred to as the minimum modulation
bandwidth of the predistorter. Accordingly, the minimum modulation
bandwidth for the predistorter designed for LTE signals is 1 MHz.
In this situation, currently available predistorters for the LTE
signals can handle one of the 3 MHz LTE signals and the 1.4 MHz LTE
signals when transmitted individually, or any of the 3 MHz LTE
signals, the 1.4 MHz LTE signals, and the 200 kHz Narrowband IoT
signals when transmitted together. However, when the 200 kHz
Narrowband IoT signals are transmitted individually, the output
signal performance may deteriorate because the bandwidth of the
transmitted signals is below the minimum modulation bandwidth of
the predistorter.
[0016] One solution for the above-noted problem is to use higher
resolution predistorters that can handle higher bandwidth range
including the 200 kHz narrowband and the 3 MHz LTE signals.
However, such predistorters are very expensive and are typically
not configured to operate with other components of the 900 MHz band
LMR RF systems and LTE systems. Additionally, development of new
high resolution predistorters configured for operation in the 900
MHz band LMR RF systems and configured to handle the LTE signals
may take time for development, market testing, and launch. This
results in a delay in implementing much needed infrastructure
systems for organizations using the 900 MHz band LMR RF
systems.
[0017] Accordingly, there is a need for a method and apparatus for
linearizing Narrowband IoT carriers with low-resolution
predistorters.
[0018] One embodiment provides a radio frequency system for
linearizing narrowband IoT carriers with low resolution
predistorters. The system includes a power amplifier configured to
amplify one or more broadcast carriers and a predistorter coupled
to the power amplifier and configured to linearize the one or more
broadcast carriers. The system also includes a pacification carrier
generator configured to generate a pacification carrier and an
electronic processor coupled to the pacification carrier. The
electronic processor is configured to determine a composite
bandwidth of the one or more broadcast carriers and determine
whether the composite bandwidth is below a modulation bandwidth of
the predistorter. The electronic processor is also configured to
control the pacification carrier generator to combine the
pacification carrier with the one or more broadcast carriers when
the composite bandwidth is below the minimum modulation bandwidth
of the predistorter.
[0019] Another embodiment provides a method for linearizing
narrowband IoT carriers with low resolution predistorters. The
method includes amplifying, using a power amplifier, one or more
broadcast carriers and linearizing, using a predistorter coupled to
the power amplifier, the one or more broadcast carriers. The method
also includes determining, using an electronic processor, a
composite bandwidth of the one or more broadcast carriers and
determining, using the electronic processor, whether the composite
bandwidth is below a modulation bandwidth of the predistorter. The
method further includes controlling, using the electronic
processor, a pacification carrier generator coupled to the
electronic processor to combine a pacification carrier with the one
or more broadcast carriers when the composite bandwidth is below
the minimum modulation bandwidth of the predistorter.
[0020] FIG. 1 is a simplified block diagram of an example RF system
100 implementing the method for linearizing Narrowband IoT carriers
with low-resolution predistorters. In the example illustrated, the
RF system 100 includes carrier generators 105, a predistorter 110,
a power amplifier 115, a transmission post filter 120, an
electronic processor 125, a memory 130, and a pacification carrier
generator 135. FIG. 1 illustrates only one example embodiment of
the RF system 100. The RF system 100 may include more or fewer
components and may perform additional functions other than those
described herein.
[0021] The carrier generators 105 generate one or more broadcast
carriers 140. The one or more broadcast carriers 140 include, for
example, a 3 MHz LTE carrier, a 1.4 MHz LTE carrier, and/or a 200
kHz Narrowband IoT. The example illustrated in FIG. 1 shows the
carrier generators 105 generating the 200 kHz Narrowband IoT
carrier for explaining the concepts in the present disclosure.
Although not illustrated, modulators are provided in the carrier
generators 105 or the RF system 100. The modulators modulate
symbols, information, or messages on to the one or more broadcast
carriers 140 before being transmitted.
[0022] The predistorter 110 receives the one or more broadcast
carriers 140 from the carrier generators 105 and provides
predistorted signals 145 to the power amplifier 115. The
predistorted signals 145 include cancellation products (that is,
the effective inverse functions of the AM-AM and AM-PM distortion)
introduced by the predistorter 110 to cancel any intermodulation
distortion due to non-linearity of the power amplifier 115. When
amplifying the one or more broadcast carriers 140, the cancellation
products ideally cancels out any intermodulation distortion having
a positive amplitude introduced by the power amplifier 115. The
predistorter 110 receives the output signal of the power amplifier
115 through a predistorter coupler 150 forming a feedback loop. The
predistorter 110 optimizes the cancellation products based on the
feedback received over the feedback loop. The predistorter 110 has
an associated minimum modulation bandwidth. The predistorter 110
provides desired predistortion performance when the composite
bandwidth of the one or more broadcast carriers 140 exceeds the
minimum modulation bandwidth. In some embodiments, the predistorter
110 is deactivated until a signal having a composite bandwidth
greater than the minimum modulation bandwidth is provided to the
predistorter 110.
[0023] The power amplifier 115 amplifies the predistorted signals
145 at an input of the power amplifier 115 to amplified signals 155
at the output of the power amplifier 115. The power amplifier 115
reproduces input signals with a higher amplitude at the output.
However, the power amplifier 115 may also introduce intermodulation
distortion (IMD) in the amplified signals 155. As discussed above,
the IMD is cancelled or reduced by the predistorter 110.
[0024] The transmission post filter 120 is, for example, a low-pass
filter, a high-pass filter, or a band-pass filter. The transmission
post filter 120 removes any noise (including, for example, a
pacification carrier as further described below) from the amplified
signals 155 outside of the functional bandwidth of the RF system
100. The transmission post filter 120 filters the amplified signals
155 received from the power amplifier 115 to output the
transmission signals 160.
[0025] The electronic processor 125 may be coupled to the carrier
generators 105, the predistorter 110, the power amplifier 115, and
the pacification carrier generator 135 over one or more control
and/or data buses. In some embodiments, the electronic processor
125 is implemented as a microprocessor with separate memory, for
example, memory 130. In other embodiments, the electronic processor
125 is implemented as a microcontroller or digital signal processor
(with memory 130 on the same chip). In other embodiments, the
electronic processor 125 is implemented using multiple electronic
processors. In addition, the electronic processor 125 may be
implemented partially or entirely as, for example, a field
programmable gate array (FPGA), an application specific integrated
circuit (ASIC), and the like and the memory 130 may not be needed
or be modified accordingly. In the example, the electronic
processor 125 is illustrated as a separate component. However, the
electronic processor 125 may be included in the same integrated
circuit as the predistorter 110, the same integrated circuit as the
power amplifier 115, and/or the like.
[0026] In the example illustrated, the memory 130 includes
non-transitory, computer readable memory that stores instructions
that are received and executed by the electronic processor 125 to
carry out the functionality of the RF system 100. The memory 130
may include, for example, a program storage area and a data storage
area. The program storage area and the data storage area may
include combinations of different types of memory, for example,
read-only memory and random-access memory.
[0027] The pacification carrier generator 135 is coupled to
electronic processor 125 and generates a pacification carrier based
on control signals received from electronic processor 125. The
pacification carrier generator 135 combines the pacification
carrier with the one or more broadcast carriers 140 using the
pacification coupler 165. The pacification carrier is used to
increase the composite bandwidth of the one or more broadcast
carriers 140 to above the minimum modulation bandwidth of the
predistorter 110.
[0028] Specifically, the pacification carrier is generated in a
frequency range and bandwidth to create distortion products that
fall within the resolution range of the predistorter 110. No
information is, however, modulated onto the pacification carrier.
In some embodiments, the pacification carrier is generated in a
radio frequency range that lies outside the pass band of the
transmission post filter 120. Accordingly, the transmission post
filter 120 attenuates the pacification carrier from the amplified
signal 155. The pacification carrier is also generated at a lower
power level than the one or more broadcast carriers 140. For
example, the pacification carrier is generated at a power level 10
Decibels or more below a composite power of the one or more
broadcast carriers 140. The use of the pacification carrier as
described herein is advantageous and can be differentiated over,
for example, pilot tone injection techniques. Pilot tone injection
techniques rely on a pilot tone being injected into the signals to
measure how well the various components are performing via the
level of the pilot tone observed in an output. However, a pilot
tone is not generated in response to determining a composite
bandwidth. In contrast, the pacification carrier as described
herein is generated to activate the predistorter 110 and create
distortion products that fall within the resolution range of the
low resolution predistorter 110, beneficially enabling the
pacification carrier to increase the composite bandwidth of the one
or more broadcast carriers.
[0029] In some embodiments, the RF system 100 optionally includes a
carrier coupler 170 and/or a transmission coupler 175. The carrier
coupler 170 provides information regarding the one or more
broadcast carriers 140 generated by the carrier generators 105 to
the electronic processor 125. The transmission coupler 175 provides
information regarding the transmission signals 160 to the
electronic processor 125. That is, the carrier coupler 170 and/or
the transmission coupler 175 are coupled to the electronic
processor 125 through a receiver 185 and detect the one or more
broadcast carriers 140 transmitted by the RF system 100. The
receiver 185 detects and measures one or more characteristics of
the one or more broadcast carriers 140 based on the signals
received from the carrier coupler 170 and/or the transmission
coupler 175.
[0030] In some embodiments, an integration time, that is, a time
taken to determine the composite bandwidth depends on the one or
more characteristics. The receiver 185 samples a predetermined
bandwidth of the coupled spectrum and determines the integration
time to determine the composite bandwidth. In one example, the
integration time is 15 milliseconds representing 32 swept samples
of the spectrum from the receiver 185. The 32 swept samples may be
considered sufficient to determine an overall operating bandwidth.
In some embodiments, individual spectrum raster size may alter the
number of processed samples and the integration time. In some
embodiments, a-priori knowledge of possible multiple carrier
configuration may also increase the integration time. For example,
the integration time may be increased to 40 milliseconds or longer
due to an expanded processing time of the spectrum. Therefore, the
sampling period, the processing time and delay to result (that is,
determination of composite bandwidth), number of integrated samples
considered per decision, external input of potential matrix table
of expected signals to be considers, and the like may be considered
in the selection of the one or more characteristics by which the
composite bandwidth is determined.
[0031] The electronic processor 125 communicates with the receiver
185 to determine a composite bandwidth of the one or more broadcast
carriers 140 based on the characteristic of the one or more
broadcast carriers 140. Specifically, the electronic processor 125
compares the measured characteristic to a predetermined
characteristic threshold. The electronic processor 125 determines
that the composite bandwidth of the one or more broadcast carriers
140 is below the minimum modulation bandwidth of the predistorter
110 when the characteristic does not satisfy the predetermined
characteristic threshold. In some embodiments, the electronic
processor 125 also determines that the one or more broadcast
carriers 140 meet a minimum qualifying amplitude threshold. The
electronic processor 125 determines that the composite bandwidth of
the one or more broadcast carriers 140 is above the minimum
modulation bandwidth of the predistorter 110 when the
characteristic satisfies the one or more predetermined
characteristic thresholds. The electronic processor 125 controls
the pacification carrier based on the qualifying composite
bandwidth of the one or more broadcast carriers 140 as further
discussed below. Specifically, example methods are provided below
that illustrate how the one or more characteristics of the one or
more broadcast carriers 140 is used to determined the composite
bandwidth of the one or more broadcast carriers 140.
[0032] In some embodiments, the RF system 100 optionally includes a
detection antenna 180. The electronic processor 125 is coupled to
the detection antenna 180 through the receiver 185 to detect the
one or more broadcast carriers 140 transmitted from the RF system
100. The electronic processor 125 determines the composite
bandwidth of the one or more broadcast carriers 140 based on the
signals detected at the receiver 185 through the detection antenna
180. In one example, the detection antenna 180 is a pre-existing
antenna of the RF system 100 that is repurposed to detect signals
transmitted by the RF system 100. Additional description for the RF
system 100 is provided below with respect to FIGS. 2-8C. The
description below may refer back to the components of the RF system
with corresponding designator numerals as shown in FIG. 1.
[0033] FIG. 2 is a flowchart of an example method 200 for
linearizing Narrowband IoT carriers with low-resolution
predistorters. In the example illustrated, the method 200 includes
amplifying, using the power amplifier 115, the one or more
broadcast carriers 140 (at block 210). As discussed above, the
carrier generators 105 generate one or more broadcast carriers 140.
The one or more broadcast carriers 140 are amplified by the power
amplifier 115 for transmission.
[0034] The method 200 includes linearizing, using the predistorter
110, the one or more broadcast carriers 140 (at block 220). The
predistorter 110 receives the one or more broadcast carriers 140
from the carrier generators 105 and the feedback signal from the
predistorter coupler 150. The predistorter 110 introduces
cancellation products into the one or more broadcast carriers 140
which cancel any IMD products due to non-linearity of the power
amplifier 115.
[0035] The method 200 also includes combining, using the
pacification carrier generator 135, the pacification carrier with
the one or more broadcast carriers 140 (at block 230). The
combination resulting in composite bandwidth greater than
modulation bandwidth of the predistorter 110 thereby activating the
predistorter 110. The pacification carrier is combined with the one
or more broadcast carriers 140 resulting in a composite bandwidth
that is above the minimum modulation bandwidth of the predistorter
110. For example, the minimum modulation bandwidth of the
predistorter 110 is 1 MHz. When the one or more broadcast carriers
140 includes a 200 kHz Narrowband carrier, the composite bandwidth
of the one or more broadcast carriers 140 is 200 kHz. The composite
bandwidth of 200 kHz is well below the minimum modulation bandwidth
of the predistorter 110. Due to the low resolution of the
predistorter 110, the predistorter 110 may not be capable or
efficient at removing the IMD products in the output signals for
broadcast carrier with bandwidth below 1 MHz.
[0036] The generated pacification carrier may have a frequency that
is sufficiently separated from the carrier signals to increase the
composite bandwidth of the one or more broadcast carriers 140 to
above the minimum modulation bandwidth of the predistorter 110. For
example, when a narrow band IoT signal is centered at 937.9 MHz and
has an upper end of the bandwidth located at 938 MHz, the
pacification carrier may be generated with a frequency of 937 MHz
and combined with the one or more broadcast carriers 140 resulting
in a combined signal having a bandwidth greater than 1 MHz. As
discussed above, the pacification carrier is generated in a
frequency range that is filtered out by the transmission post
filter 120 such that the pacification carrier is not transmitted by
the RF system 100.
[0037] The blocks 210, 220, 230 may not be performed in the order
illustrated in FIG. 2. As can be understood by one of ordinary
skill in the art, the blocks 210, 220, 230 may be performed in any
order or simultaneously. One advantage of method 200 is that
predistorters currently available in the market or currently being
implemented in live RF systems may be used to linearize Narrowband
IoT carriers in the 900 MHz LMR band.
[0038] FIG. 3 is a flowchart of an example method 300 for
linearizing Narrowband IoT carriers with low-resolution
predistorters. In the example illustrated, the method 300 includes
amplifying, using the power amplifier 115, the one or more
broadcast carriers 140 (at block 310). As discussed above, the
carrier generators 105 generate one or more broadcast carriers 140.
The one or more broadcast carriers 140 are amplified by the power
amplifier 115 for transmission.
[0039] The method 300 includes linearizing, using the predistorter
110, the one or more broadcast carriers 140 (at block 320). The
predistorter 110 receives the one or more broadcast carriers 140
from the carrier generators 105 and the feedback signal from the
predistorter coupler 150. The predistorter 110 introduces
cancellation products into the one or more broadcast carriers 140
which cancel any IMD products due to non-linearity of the power
amplifier 115.
[0040] The method 300 includes determining, using the electronic
processor 125, a composite bandwidth of the one or more broadcast
carriers 140 (at block 330). As discussed above, the receiver 185
detects the one or more characteristics of the one or more
broadcast carriers 140 using, for example, the carrier coupler 170,
the transmission coupler 175, and/or the detection antenna 180. In
some embodiments, other methods may be used to detect the one or
more characteristics of the one or more broadcast carriers 140. The
characteristic is indicative of the composite bandwidth of the one
or more broadcast carriers 140. The one or more characteristics
are, for example, the composite bandwidth of the one or more
broadcast carriers 140, a bandwidth of each individual carrier of
the one or more broadcast carriers 140, a received signal strength
indicator (RSSI) of the one or more broadcast carriers 140, a
composite power of the one or more broadcast carriers 140, a
difference between the highest detected frequency and the lowest
detected frequency of the one or more broadcast carriers 140,
and/or the like. The electronic processor 125 determines a
composite bandwidth of the one or more broadcast carriers 140 based
on the one or more characteristics. In some embodiments, the
electronic processor 125 may have a-priori knowledge of the
composite bandwidth of the current transmission of the RF system
100.
[0041] The method 300 includes determining, using the electronic
processor 125 whether the composite bandwidth of the one or more
broadcast carriers 140 is below the minimum modulation bandwidth of
the predistorter 110 (at block 340). The electronic processor 125
compares the composite bandwidth to a minimum modulation bandwidth
threshold to determine whether the composite bandwidth is below the
minimum modulation bandwidth of the predistorter 110.
[0042] When the composite bandwidth is below the minimum modulation
bandwidth, the method 300 includes combining, using the
pacification carrier generator 135, the pacification carrier with
the one or more broadcast carriers 140 (at block 350). The
combination resulting in composite bandwidth greater than the
minimum modulation bandwidth of the predistorter 110. The
combination described in block 350 is similar to the combination
described in block 230 of method 200.
[0043] The blocks 310-350 of method 300 may not be performed in the
order illustrated in FIG. 3. Some or all of the block 310-350 may
be performed in a different order or simultaneously. The method 300
provides an additional advantage of turning on the pacification
carrier only when needed to increase the composite bandwidth while
turning off the pacification carrier when the composite bandwidth
is already above the minimum modulation bandwidth. This provides
additional power savings for the RF system 100 and can allow for
more optimal linearization when the pacification carrier is not
added.
[0044] In some embodiments, the composite bandwidth of the one or
more broadcast carriers 140 can be determined based on the RSSI of
the transmitted signals. The electronic processor 125 measures the
RSSI via the receiver 185 of the transmitted signals using the
detection antenna 180, the carrier coupler 170, or the transmission
coupler 175. FIG. 4 illustrates an example RSSI signature 400 of
the signals transmitted by a 900 MHz band LMR Radio RF system 100.
Specifically, FIG. 4 illustrates the RSSI signature of the 3 MHz
LTE carrier 430, the RSSI signature of the 1.4 MHz LTE carrier 420,
and the RSSI signature of the 200 kHz Narrowband IoT carrier 410
with a center frequency of 833.6 MHz. The 200 kHz Narrowband IoT
carrier exhibits an RSSI spike above 40 decibels (dB) over a range
of four frequencies (that is, between 833.4-833.7 MHz). The 3 MHz
LTE carrier and the 1.4 MHz LTE carrier exhibit RSSI spikes above
40 dB over a larger range of frequencies, for example, over six
frequencies. Based on these characteristics of the broadcast
carriers, one can determine whether the composite bandwidth of the
one or more broadcast carriers 140 is greater than the minimum
modulation bandwidth if the detected RSSI is above 40 dB for six
consecutive frequencies. This is because the inclusion of the 1.4
MHz and/or the 3 MHz LTE carrier in the one or more broadcast
carriers 140 means that the composite bandwidth of the one or more
broadcast carriers 140 is above the minimum modulation bandwidth
of, for example, 1 MHz. FIGS. 5 and 6 illustrate example methods of
using the RSSI signature of the one or more broadcast carriers 140
to determine whether the composite bandwidth is below the minimum
modulation bandwidth of the predistorter 110.
[0045] FIG. 5 is a flowchart of one example method 500 for
linearizing Narrowband IoT carriers with low resolution
predistorters. In the example illustrated, the method 500 includes
detecting, using the electronic processor 125, input power (at
block 510). The input power is, for example, the power input to the
predistorter 110 or the power amplifier 115. In some embodiments,
the electronic processor 125 detects the input power using the
carrier coupler 170, the detection antenna 180, or the transmission
coupler 175 via the receiver 185. In other embodiments, the
electronic processor 125 has a-priori knowledge of the input power
based on the transmission schedule of the RF system 100.
[0046] The method 500 includes determining, using the electronic
processor 125, whether the input power is greater than a threshold
power (at block 520). The input power indicates whether the RF
system 100 is currently transmitting RF signals. The threshold
power may be preset into the memory 130. The threshold power is
preset to a value that indicates that the RF system 100 is
operational and is transmitting RF signals (for example, LTE and/or
Narrowband signals). When the input power is below the threshold
power, the method 500 returns to block 505 to continuously check
for input power.
[0047] When the input power is greater than threshold power, the
method 500 includes scanning, using the electronic processor 125,
through a plurality of frequencies of a transmission band of the RF
system 100 (at block 530). As shown in FIG. 4, the transmission
band is the 900 MHz LMR band. The scanning begins at 832 MHz and
proceeds through 835 MHz in predetermined steps, for example, in
steps of 100 kHz. The method 500 includes measuring, using the
electronic processor 125, a received signal strength indicator
(RSSI) of signals detected at each of the plurality of frequencies
(at block 540). In some embodiments, the electronic processor 125
uses the detection antenna 180 to measure the RSSI based on signals
detected at the detection antenna 180. In other embodiments, the
electronic processor 125 uses one or both of the carrier coupler
170 and the transmission coupler 175 (for example, a detection
coupler) to measure the RSSI based on signals detected at the one
or both couplers 170, 175.
[0048] The method 500 further includes determining, using the
electronic processor 125, a number of the plurality of frequencies
for which the RSSI exceeds a threshold RSSI (at block 550). The
threshold RSSI is, for example, 40 dB as illustrated in FIG. 4. The
threshold RSSI may be prestored in the memory 130 based on the
characteristics of the RF system 100. The electronic processor 125
compares the measured RSSI at the scanned frequencies to determine
whether the RSSI at the selected frequencies exceeds the threshold
RSSI.
[0049] The method 500 includes determining, using the electronic
processor 125, whether the number of plurality of frequencies
exceeds a threshold number (at block 560). As discussed above with
respect to FIG. 4, when the number of plurality of frequencies
reaches six or increases over six, the electronic processor 125
determines that the composite bandwidth of the one or more
broadcast carriers 140 is greater than the minimum modulation
bandwidth of the predistorter 110.
[0050] When the number of plurality of frequencies exceeds the
threshold number, the method 500 includes turning or keeping, using
the electronic processor 125, the pacification carrier off (at
block 570). The electronic processor 125 determines that the
composite bandwidth of the one or more broadcast carriers 140
exceeds the minimum modulation bandwidth of the predistorter 110
when the number of plurality of frequencies exceeds the threshold
number. The electronic processor 125 controls the pacification
carrier generator 135 to turn off or keep the pacification carrier
off for the current transmission of the RF system 100.
Specifically, the electronic processor 125 controls the
pacification carrier generator 135 to stop generation of the
pacification carrier when the composite bandwidth is above the
minimum modulation bandwidth of the predistorter 110.
[0051] When the number of plurality of frequencies does not exceed
the threshold number, the method 700 includes combining, using the
pacification carrier generator 135, the pacification carrier with
the one or more broadcast carriers 140 (at block 580). The
electronic processor 125 determines that the composite bandwidth of
the one or more broadcast carriers 140 is below the minimum
modulation bandwidth of the predistorter 110 when the number of
plurality of frequencies does not exceed the threshold number. The
electronic processor 125 controls the pacification carrier
generator 135 to generate the pacification carrier and combine the
pacification carrier with the one or more broadcast carriers 140
using the pacification coupler 165. The method 500 repeats for each
transmission of the RF system 100.
[0052] FIG. 6 is a flowchart of one example method 600 for
linearizing Narrowband IoT carriers with low resolution
predistorters. The method 600 is similar to the method 500 with
like numeral referring to like blocks. For conciseness, explanation
of blocks that were already described with respect to FIG. 5 are
omitted below. In the example illustrated, the method 600 includes
detecting the input power (at block 510) and determining whether
the input power is greater than the threshold power (at block 520).
When the input power is greater than threshold power, the method
600 includes reading, using the electronic processor 125, a
predistorter bandwidth (at block 610). The electronic processor 125
communicates with the predistorter 110 using a communication bus to
receive inputs from the predistorter 110. The predistorter 110
provides a current bandwidth to the electronic processor 125.
[0053] The method 600 includes determining, using the electronic
processor 125, whether the predistorter bandwidth is greater than
threshold bandwidth (at block 620). The threshold bandwidth may be
set to the minimum modulation bandwidth, just higher than
modulation bandwidth, or just lower than modulation bandwidth of
the predistorter 110. The threshold bandwidth is stored in the
memory 130. The electronic processor 125 compares the predistorter
bandwidth to the threshold bandwidth. When the predistorter
bandwidth is greater than the threshold bandwidth, the method 600
proceeds to block 570.
[0054] When the predistorter bandwidth is less than the threshold
bandwidth, the method 600 includes combining, using the
pacification carrier generator 135, the pacification carrier with
the one or more broadcast carriers 140 (at block 630). The
electronic processor 125 controls the pacification carrier
generator 135 to generate the pacification carrier and combine the
pacification carrier with the one or more broadcast carriers 140
using the pacification coupler 165. After combining the
pacification carrier with the one or more broadcast carrier, the
method 600 proceeds to block 530 to scan through the plurality of
frequencies of the transmission band. The method 600 repeats for
each transmission of the RF system 100.
[0055] It should be noted that the blocks in methods 500 and 600
may not be performed in the order illustrated unless otherwise
specified. Rather, the blocks may be performed in any order or
simultaneously. In addition, all blocks need not be performed. For
example, blocks 510 and 520 may be omitted in some instances.
[0056] FIG. 7 is a flowchart of an example method 700 for scanning
through a plurality of frequencies to determine a composite
bandwidth of the one or more broadcast carriers 140. Specifically,
the method 700 provides an example of how the process described in
blocks 530 to 560 is carried out. The method 700 includes scanning,
using the electronic processor 125, to a first frequency of the
transmission band (at block 710). In the example illustrated, the
transmission band is the 900 MHz LMR band. As shown in FIG. 4, the
scanning begins at 832 MHz and the detection antenna 180 is set to
detect signals at that frequency. The method 700 includes
measuring, using the electronic processor 125, RSSI at the selected
frequency (at block 720). The electronic processor 125 uses the
detection antenna 180 to detect a signal transmitted at the
selected frequency. The electronic processor 125 measures the
signal strength (that is, the RSSI) of the detected signal. As
discussed above, the electronic processor 125 may use the couplers
170, 175 to detect and measure RSSI at the selected
frequencies.
[0057] The method 700 includes determining, using the electronic
processor 125, whether the RSSI at the scanned frequency is greater
than the threshold RSSI (at block 730). The threshold RSSI is, for
example, 40 dB as illustrated in FIG. 4. The threshold RSSI may be
prestored in the memory 130 based on the characteristics of the RF
system 100. The electronic processor 125 compares the measured RSSI
at the scanned frequency to determine whether the RSSI at the
selected frequency exceeds the threshold RSSI.
[0058] When the measured RSSI at the selected frequency is below
the threshold RSSI, the method 700 includes determining, using the
electronic processor 125, whether an end of frequencies in the
transmission band is reached (at block 740). The electronic
processor 125 detects whether all the frequencies in the
transmission band, for example, from 832 MHz to 835 MHz have been
scanned. When the end of frequencies in the transmission band is
not reached, the method 700 includes scanning, using the electronic
processor 125, to the next frequency in the transmission band (at
block 750). The method 700 continues to scan the frequencies until
the end of the frequencies in the transmission band is reached. The
method then proceeds to block 720 to measure the RSSI at the
selected frequency.
[0059] When the measured RSSI at the selected frequency is above
the threshold RSSI, the method 700 includes incrementing, using the
electronic processor 125, an RSSI counter (at block 760). The RSSI
counter is maintained in the electronic processor 125 to track the
number of times the RSSI exceeds the threshold RSSI in the
transmission band.
[0060] After the RSSI counter is incremented, the method 700
includes determining, using the electronic processor 125, whether
the RSSI counter is greater than the counter threshold (at block
770). As discussed above with respect to FIG. 4, when the RSSI
counter reaches six or increases over six, the electronic processor
125 determines that the composite bandwidth of the one or more
broadcast carriers 140 is greater than the minimum modulation
bandwidth of the predistorter 110. When the RSSI counter is greater
than the counter threshold, the method 700 includes turning or
keeping, using the electronic processor 125, the pacification
carrier off (at block 570). The electronic processor 125 controls
the pacification carrier generator 135 to turn off or keep the
pacification carrier off for the current transmission of the RF
system 100. The electronic processor 125 resets the RSSI counter
for the next transmission of the RF system 100.
[0061] When the RSSI counter is not greater than the counter
threshold, the method 700 returns to block 740 to determine whether
an end of frequencies in the transmission range is reached. When
the end of frequencies in the transmission range is reached at
block 740 and the RSSI counter does not exceed the counter
threshold, the method 700 includes combining, using the
pacification carrier generator 135, the pacification carrier with
the one or more broadcast carriers 140 (at block 580). The
electronic processor 125 controls the pacification carrier
generator 135 to generate the pacification carrier and combine the
pacification carrier with the one or more broadcast carriers 140
using the pacification coupler 165. The electronic processor 125
resets the RSSI counter for the next transmission of the RF system
100. The method 700 repeats for each transmission of the RF system
100.
[0062] FIGS. 8A-8C illustrate examples of carrier and distortion
energy products of the one or more broadcast carriers 140. The term
"low resolution" when used with respect to a predistorter 110 is
used from the perspective of a narrowband broadcast carrier
application (for example, NB-IoT application). This contrasts with
the minimum modulation bandwidth for which the given predistorter
110 was designed for (for example, LTE signals having a bandwidth
between 1.4 MHz and 20 MHz). For the LTE spectrum, the predistorter
110 would not be considered low resolution.
[0063] FIG. 8A illustrates example energy products when the one or
more broadcast carriers 140 includes a 1.4 MHz LTE signal (that is,
broadcast carrier Y). Region 810 between C' and B' is the minimum
modulation bandwidth of a given predistorter 110 for a broadcast
carrier Y. The broadcast carrier Y is determined in the region 810.
Region 820 between B and A and between C and D represents the
bandwidth of where the distortion energy X will reside from the
broadcast carrier Y. Region 830 between B' and B and between C and
C' represents a guard band. Energy of the spectrum in the region
830 may be ignored by the predistorter 110. Specifically, the
predistorter 110 may not discern whether the energy is from the
broadcast carrier Y or the distortion energy X.
[0064] FIG. 8B illustrates example energy products when the one or
more broadcast carriers 140 includes only a narrow bandwidth 200
kHz carrier (that is, broadcast carrier Y). The composite bandwidth
of the broadcast carrier Y does not meet the minimum modulation
bandwidth of the predistorter 110. The distortion products X and
the broadcast carrier Y lie inside the minimum modulation bandwidth
of the predistorter 110. The predistorter 110 in this example may
not discern the distortion energy X from the broadcast carrier
energy Y. No distortion energy X is present in the region 820.
Therefore, the predistorter 110 may not determine whether the ratio
of broadcast carrier energy Y to distortion energy X is improving
with the linearization applied. That is, the predistorter 110 does
not have the resolution to resolve the individual distortion energy
X and the narrow bandwidth broadcast carrier energy Y.
[0065] FIG. 8C illustrates example energy products when the one or
more broadcast carriers 140 including only the narrow bandwidth 200
kHz carrier (that is, the broadcast carrier Y) is combined with a
pacification carrier Z. The pacification carrier Z is generated in
a frequency spectrum sufficiently separated from the narrowband
carrier Y such that the bandwidth of the resultant composite signal
satisfies the minimum modulation bandwidth of the predistorter 110.
The resultant distortion products X of the composite signals fall
in the region 820. The distortion energy X can now be discerned
from the broadcast carrier Y. Distortion energy is also present in
the region 820. Therefore, the predistorter can now determine
whether the ratio of the broadcast carrier energy Y to the
distortion energy X is improving with linearization applied. That
is, a linearization solution can be found, optimized, and applied
to the composite signals. The low-resolution predistorter 110 can
therefore be used in applications where higher resolution is
desired.
[0066] In the foregoing specification, specific embodiments have
been described. However, one of ordinary skill in the art
appreciates that various modifications and changes can be made
without departing from the scope of the invention as set forth in
the claims below. Accordingly, the specification and figures are to
be regarded in an illustrative rather than a restrictive sense, and
all such modifications are intended to be included within the scope
of present teachings.
[0067] The benefits, advantages, solutions to problems, and any
element(s) that may cause any benefit, advantage, or solution to
occur or become more pronounced are not to be construed as a
critical, required, or essential features or elements of any or all
the claims. The invention is defined solely by the appended claims
including any amendments made during the pendency of this
application and all equivalents of those claims as issued.
[0068] Moreover in this document, relational terms such as first
and second, top and bottom, and the like may be used solely to
distinguish one entity or action from another entity or action
without necessarily requiring or implying any actual such
relationship or order between such entities or actions. The terms
"comprises," "comprising," "has," "having," "includes,"
"including," "contains," "containing" or any other variation
thereof, are intended to cover a non-exclusive inclusion, such that
a process, method, article, or apparatus that comprises, has,
includes, contains a list of elements does not include only those
elements but may include other elements not expressly listed or
inherent to such process, method, article, or apparatus. An element
proceeded by "comprises . . . a," "has . . . a," "includes . . .
a," or "contains . . . a" does not, without more constraints,
preclude the existence of additional identical elements in the
process, method, article, or apparatus that comprises, has,
includes, contains the element. The terms "a" and "an" are defined
as one or more unless explicitly stated otherwise herein. The terms
"substantially," "essentially," "approximately," "about" or any
other version thereof, are defined as being close to as understood
by one of ordinary skill in the art, and in one non-limiting
embodiment the term is defined to be within 10%, in another
embodiment within 5%, in another embodiment within 1% and in
another embodiment within 0.5%. The term "coupled" as used herein
is defined as connected, although not necessarily directly and not
necessarily mechanically. A device or structure that is
"configured" in a certain way is configured in at least that way,
but may also be configured in ways that are not listed.
[0069] It will be appreciated that some embodiments may be
comprised of one or more generic or specialized processors (or
"processing devices") such as microprocessors, digital signal
processors, customized processors and field programmable gate
arrays (FPGAs) and unique stored program instructions (including
both software and firmware) that control the one or more processors
to implement, in conjunction with certain non-processor circuits,
some, most, or all of the functions of the method and/or apparatus
described herein. Alternatively, some or all functions could be
implemented by a state machine that has no stored program
instructions, or in one or more application specific integrated
circuits (ASICs), in which each function or some combinations of
certain of the functions are implemented as custom logic. Of
course, a combination of the two approaches could be used.
[0070] Moreover, an embodiment can be implemented as a
computer-readable storage medium having computer readable code
stored thereon for programming a computer (for example, comprising
a processor) to perform a method as described and claimed herein.
Examples of such computer-readable storage mediums include, but are
not limited to, a hard disk, a CD-ROM, an optical storage device, a
magnetic storage device, a ROM (Read Only Memory), a PROM
(Programmable Read Only Memory), an EPROM (Erasable Programmable
Read Only Memory), an EEPROM (Electrically Erasable Programmable
Read Only Memory) and a Flash memory. Further, it is expected that
one of ordinary skill, notwithstanding possibly significant effort
and many design choices motivated by, for example, available time,
current technology, and economic considerations, when guided by the
concepts and principles disclosed herein will be readily capable of
generating such software instructions and programs and ICs with
minimal experimentation.
[0071] The Abstract of the Disclosure is provided to allow the
reader to quickly ascertain the nature of the technical disclosure.
It is submitted with the understanding that it will not be used to
interpret or limit the scope or meaning of the claims. In addition,
in the foregoing Detailed Description, it can be seen that various
features are grouped together in various embodiments for the
purpose of streamlining the disclosure. This method of disclosure
is not to be interpreted as reflecting an intention that the
claimed embodiments require more features than are expressly
recited in each claim. Rather, as the following claims reflect,
inventive subject matter lies in less than all features of a single
disclosed embodiment. Thus, the following claims are hereby
incorporated into the Detailed Description, with each claim
standing on its own as a separately claimed subject matter.
* * * * *