Note: Descriptions are shown in the official language in which they were submitted.
CA 02275156 1999-07-13
REVERSE LINK, TRANSMIT PODPER CORRECTION AND
LIMITATION IN A RADIOTELEPHONE SYSTEM
BACKGROUND OF THE INVENTION
This application is a divisional of Canadian patent
application Serial No. 2,158,577 filed February 17, 1995.
I. Field of the Invention
The present invention relates to radio communications.
More particularly, the present invention relates to power
control in a radiotelephone system.
II. Description of the Related Art
The Federal Communications Commission (FCC) governs the
use of the radio frequency (RF) spectrum. The FCC allocates
certain bandwidths within the RF spectrum for specific uses. A
user of an allocated bandwidth of the RF spectrum must take
measures to ensure that the radiated emissions inside and
outside of that bandwidth are maintained within acceptable
levels to avoid interfering with other users operating in the
same and or other bandwidths. These levels are governed by
both the FCC and the particular user groups of said bandwidth.
The 800 MHz cellular telephone system operates its forward
link, the cell to radiotelephone transmission, in the bandwidth
of 869.01 MHz to 893.97 MHz and the reverse link, the
radiotelephone to cell transmission, in the bandwidth of 824.01
MHz to 848.97 MHz. The forward and reverse link bandwidths are
split up into channels each of which occupies a 30 kHz
bandwidth. A particular user of the cellular system may
operate on one or several of these channels at a time. All
users of the system must ensure that they are compliant with
the level of radiated emissions allowable inside and outside of
the channel or channels that they have been assigned.
There are several different techniques of modulation that
can be used in the cellular telephone system. Two examples of
modulation techniques are frequency division multiple access
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(FDMA) and code division multiple access (CDMA).
The FDMA modulation technique generates signals that
occupy one channel at a time while the CDMA modulation
technique generates signals that occupy several channels. Both
of these techniques must control their return link radiated
emissions to within acceptable limits inside and outside of the
assigned channel or channels. For maximum system performance,
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users of the CDMA technique must carefully control the level of radiated
power inside the channels in which they are operating.
FIG. 1 shows a typical prior cellular radiotelephone. In both an FDMA
and a CDMA based radiotelephone, there exists the possibility of driving the
power amplifier (101) in the transmitter beyond a point where acceptable out
of channel radiated emissions are maintained. This is primarily due to the
increased distortion output levels of the power ,amplifier (101) at high
output powers. Also, driving the power amplifier (101) beyond a certain
point can cause interference internal to the radio. For example, PA
puncturing in CDMA affects synthesizer phase noise due to large current
transitions. Bath of these issues cause unacceptable radio performance.
Maintaining the proper on-channel output power can be difficult due
to several undesirable effects in the radiotelephone hardware. For example,
the CDMA based radio must implement a power control system that
operates over a very wide dynamic range, 80dB to 90dB, such that the
transmitted output power is linearly related to the received input power.
Closed loop and open loop power control together determine the
return link transmit energy, as disclosed in U.S. Patent No. 5,056,109 to
Gilhousen et al. and assigned to Qualcomm, Incorporated. Therefore, the
linear and nonlinear errors produced in both the receiver (103) and
transmitter (102) RF sections can cause unacceptable power control
performance. Also, both the FDMA and CDMA based radios must operate
on different channels while maintaining acceptable output power levels.
Variation in output power level and input power detection versus
frequency can cause an unacceptable amount of error in the amount of
return link transmitted energy.
These issues present significant problems to the designer of both
FDMA and CDMA based radiotelephones. There is a resulting need for an
effective, cost efficient means of correcting these problems.
SUMMARY OF THE INVENTION
The process of the present invention enables a radiotelephone to
operate in a linear fashion over a wide dynamic range while maintaining
acceptable transmit output power levels inside and outside of the return
link bandwidth. The forward and return link power are measured by power
detectors and input to an analog to digital converter accessible by both
control hardware and/or software. The closed loop power control setting is _
also monitored. The radiotelephone uses the detected power levels and
CA 02275156 1999-07-13
closed loop power control setting to index a set of correction
tables that indicate the reverse link transmit power error and
desired power amplifier biasing for the particular operating
point. The radiotelephone also determines if the transmitter
is operating above a maximum set point. The transmit gain and
power amplifier biasing of the radiotelephone are adjusted to
correct the undesired error and maintain the desired output
power.
According to a first aspect, the invention provides a
method for limiting transmit power of a radio operating in a
radio communication system, the radio communication system
comprising at least one base station that transmits signals
including power control commands to the radio, the radio
comprising a variable gain amplifier and a maximum gain
setting, the method comprising the steps of: determining an
open loop power control value in response to a signal received
from the at least one base station; determining a gain adjust
signal in response to the transmitted power control commands;
combining the open loop power control value and the gain adjust
signal to produce a summation signal; comparing the summation
signal to the maximum gain setting; if the summation signal is
greater than or equal to the maximum gain setting, adjusting
the variable gain amplifier in response to the maximum gain
setting; and if the summation signal is less than the maximum
gain setting, adjusting the variable gain amplifier in response
to the summation signal.
According to a second aspect, the invention provides a
method for limiting transmit power of a radio operating in a
cellular environment, the cellular environment comprising a
plurality of cells that transmit power control commands to the
radio, the radio comprising a variable gain amplifier and a
maximum gain setting, the method comprising the steps of:
determining an open loop power control value in response to a
signal received from at least one cell of the plurality of
cells; determining a gain adjust signal in response to the
transmitted power control commands; combining the open loop
power control value and the gain adjust signal to produce a
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summation signal; adjusting the maximum gain setting in
response to a temperature of the variable gain amplifier;
comparing the adjusted maximum gain setting to the summation
signal; if the summation signal is greater than or equal to the
adjusted maximum gain setting, prohibiting the gain adjust
signal from increasing in response to the transmitted power
control commands; if the summation signal is greater than or
equal to the adjusted maximum gain setting, adjusting the
variable gain amplifier in response to the adjusted maximum
gain setting; and if the summation signal is less than the
adjusted maximum gain setting, adjusting the variable gain
amplifier in response to the summation signal.
According to a third aspect, the invention provides a
method for limiting transmit power of a radio operating in a
cellular environment, the cellular environment comprising a
plurality of cells that transmit power control commands to the
radio, the radio comprising a variable gain amplifier, a
maximum gain setting, and a power limiting accumulator, the
method comprising the steps of: the variable gain amplifier
transmitting a signal; determining a gain adjust signal in
response to the transmitted power control commands; detecting a
power value of the transmitted signal; digitizing the power
value; comparing the digitized power value to the maximum gain
setting; if the digitized power value is greater than the
maximum gain setting, decreasing the gain of the variable gain
amplifier; and if the digitized power value is greater than the
maximum gain setting, prohibiting the gain adjust signal from
increasing in response to the transmitted power control
commands.
According to a fourth aspect, the invention provides a
method for limiting transmit power of a radio operating in a
cellular environment, the cellular environment comprising a
plurality of cells that transmit power control commands to the
radio, the radio comprising a variable gain amplifier, a
maximum gain setting, and a power control command accumulator
that generates a gain adjust signal, the method comprising the
steps of: the variable gain amplifier transmitting a signal;
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determining the gain adjust signal in response to the
transmitted power control commands; detecting a power value of
the transmitted signal; digitizing the power value; comparing
the digitized power value to the maximum gain setting; if the
digitized power value is greater than the maximum gain setting,
decreasing the gain adjust signal by a predetermined amount for
every predetermined unit of time until the gain adjust signal
is less than the maximum gain setting; and if the digitized
power value is less than or equal to the maximum gain setting,
varying the gain of the variable gain amplifier in response to
the gain adjust signal.
According to a fifth aspect, the invention provides a
method for limiting transmit power of a radio operating in a
cellular environment, the cellular environment comprising a
plurality of cells that transmit power control commands to the
radio, the radio comprising a variable gain amplifier, a
maximum gain setting, and a power limiting accumulator, the
method comprising the steps of: the variable gain amplifier
transmitting a signal; determining a gain adjust signal in
response to the transmitted power control commands; detecting a
power value of the transmitted signal; digitizing the power
value; determining a difference between the digitized power
value and the maximum gain setting; integrating the difference
to generate a gain control signal, the gain control signal
being limited to a predetermined range; adjusting the variable
gain amplifier with the gain control signal; and if the gain
control signal is less than a predetermined value, prohibiting
the gain adjust signal from increasing the variable gain
amplifier in response to the transmitted power control
commands.
According to a sixth aspect, the invention provides a
method for limiting transmit power of a radio operating in a
cellular environment, the cellular environment comprising a
plurality of cells that transit power control commands to the
radio, the radio comprising a variable gain amplifier and a
power limiting accumulator, the method comprising the steps of:
receiving a signal from at least one of the plurality of cells;
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determining a power level of the received signal; determining a
closed loop power control value in response to the received
signal; generating a limiting gain control setting in response
to the closed loop power control value and the power level, the
limiting gain control setting being within a predetermined
range; combining the closed loop power control value, the power
level, and the limiting gain control setting to generate a gain
control signal; and adjusting the variable gain amplifier in
response to the gain control signal.
According to a seventh aspect, the invention provides a
method for limiting transmit power of a radio operating in a
radio communications system, the radio communications system
comprising a plurality of base stations that transmit power
control commands to radio, the radio comprising a variable gain
amplifier and a maximum gain setting, the method comprising the
steps of: receiving a signal from at least one of the plurality
of base stations; generating a received power level signal in
response to the received signal; generating a closed loop power
control signal in response to the received signal; combining
the received power level signal and the closed loop power
control signal to produce a summation signal; comparing the
summation signal to the maximum gain setting; adjusting the
variable gain amplifier in response to the maximum gain setting
if the summation signal is greater than or equal to the maximum
gain setting; and adjusting the variable gain amplifier in
response to the summation signal if the summation signal is
less than the maximum gain setting.
According to an eighth aspect, the invention provides a
method for limiting transmit power of a radio operating in a
cellular environment, the cellular environment comprising a
plurality of cells that transmit power control commands to the
radio, the radio comprising a variable gain amplifier, a
maximum gain setting, and a power limiting accumulator, the
method comprising the steps of: receiving a signal from at
least one of the plurality of cells; generating a received
power level signal in response to the received signal;
generating a closed loop power control signal in response to
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the received signal; digitizing the received power level
signal; comparing the digitized received power level signal to
the maximum gain setting; decreasing the gain of the variable
gain amplifier if the digitized received power level signal is
greater than the maximum gain setting; and prohibiting the
closed loop power control signal from changing in response to
the power control commands if the digitized received power
level signal is greater than the maximum gain setting.
According to a ninth aspect, the invention provides a
method for limiting transmit power of a radio operating in a
cellular environment, the cellular environment comprising a
plurality of cells that transmit power control commands to the
radio, the radio comprising a variable gain amplifier, a
maximum gain setting, and a power control command accumulator,
the method comprising the steps of: receiving a signal from at
least one of the plurality of cells; generating a received
power level signal in response to the received signal;
generating a closed loop power control signal in response to
the power control commands; digitizing the received power level
signal; comparing the digitized received power level signal to
the maximum gain setting; decreasing the closed loop power
control signal by a predetermined amount for every
predetermined unit of time until the closed loop power control
signal is less than the maximum gain setting if the digitized
received power level signal is greater than the maximum gain
setting; and varying the gain of the variable gain amplifier in
response to the closed loop power control signal if the
digitized received power level signal is less than or equal to
the maximum gain setting.
According to a tenth aspect, the invention provides a
method for limiting transmit power of a radio operating in a
cellular environment, the cellular environment comprising a
plurality of cells that transmit power control commands to the
radio, the radio comprising a variable gain amplifier, a
maximum gain setting, and a power limiting accumulator, the
method comprising the steps of: receiving a signal from at
least one of the plurality of cells; generating a received
- 3d -
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power level signal in response to the received signal;
generating a closed loop power control signal in response to
the power control commands; digitizing the received power level
signal; determining a difference between the digitized receive
power level signal and the maximum gain setting; integrating
the difference to generate a gain control signal, the gain
control signal being limited to a predetermined range;
adjusting the variable gain amplifier with the gain control
signal; and prohibiting the closed loop power control signal
from changing the variable gain amplifier in response to the
power control commands if the gain control signal is less than
a predetermined value.
According to an eleventh aspect, the invention provides a
radio performing transmit power calibration operating in a
cellular environment comprising a plurality of cells that
transmit power control commands to the radio, the radio
receiving signals through a variable gain receive amplifier the
radio comprising: a receive power detector, coupled to the
receive amplifier, for generating a received power level
signal; a saturating accumulator, coupled to the receive
amplifier, for generating a closed loop power control signal in
response to the power control commands; a power limiting
circuit, coupled to the receive power detector and the
saturating accumulator, for generating a limiting gain control
setting in response to the closed loop power control signal and
the received power level signal, the limiting gain control
setting being within a predetermined range; a signal combiner,
coupled to the receive power detector, the saturating
accumulator and the power limiting circuit, for combining the
received power level signal, the closed loop power control
signal, and the limiting gain control setting to generate a
transmit gain control signal; and a transmit amplifier having a
variable gain and a control input coupled to the signal
combiner, the variable gain adjusting in response to the
transmit gain control signal.
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BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 shows a block diagram of a typical prior art
radiotelephone frequency section for use in a radiotelephone
system.
Fig. 2 shows a block diagram of the preferred embodiment
power control correction implementation.
Fig. 3 shows a block diagram of the power limiting control
section as related to Fig. 2.
Fig. 4 shows a block diagram of the closed loop power
control section as related to Fig. 2.
Fig. 5 shows a block diagram of the PA limit threshold
control section as related to Fig. 2.
Fig. 6 shows an alternate embodiment of the present
invention that employs a power limiting control system based on
accumulator feedback control.
Fig. 7 shows an alternate embodiment of the present
invention that employs a power limiting control system based on
the closed loop power control accumulator.
Fig. 8 shows an alternate embodiment of the present
invention that employs a power limiting control system based on
integral feedback control.
Fig. 9 shows an alternate embodiment of the present
invention that employs a power limiting control system based
on a measure of receive power and the closed loop power control
setting to estimate output power.
DETAILED DESCRIPTION OF THE PREFERRED EM80DIMENTS
The process of the present invention provides power
control correction for a mobile radiotelephone as well as
maintaining acceptable in and out of band maximum emission
levels. This is accomplished by real-time
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compensation utilizing a set of correction tables that are generated during
the production testing of each radiotelephone.
FIG. 2 shows a block diagram of a CDMA radiotelephone with the
preferred embodiment power control correction implementation. FIGs. 3, 4,
and 5 detail specific blocks of FIG. 2. The radiotelephone is comprised of a
receive linearization section, transmit linearization section, power amplifier
bias control section, and power limiting control section.
The receive linearization section includes an automatic gain control
(AGC) section. The signal input to the AGC section is received on the
forward link and amplified by a low noise amplifier (LNA) (211). The output
of the LNA (211) is input to a variable gain amplifier (212). The variable
gain
ampliEler (212) produces a signal that is converted to a digital signal using
an
analog to digital converter (ADC) (213).
The power of the digitized received signal is next computed by a
digital power detector (214). The power detector (214) includes an integrator
that integrates the detected power with respect to a reference voltage. In the
preferred embodiment, this reference voltage is provided by the radio's
demodulator to indicate the nominal value at which the demodulator
requires the loop to lock in order to hold the power level constant. The
demodulator requires this value for optimum performance since a power
level too far out of the optimum range will degrade the performance of the
demodulator. The power detector (214) performs the integration, thus
generating an AGC setpoint. The setpoint and a receive frequency index are
. input to a receiver linearizing table (216).
The AGC setpoint and the frequency index are used to address the
linearizer (216), thus accessing the proper calibration value. This
calibration
value is then output to a digital to analog converter (215) that generates the
analog representation of the receive AGC setting.
The analog value adjusts the biasing of the variable gain amplifier
(212). The control of the variable gain amplifier (212) forces the receive AGC
loop to close such that the input to the receiver linearizing table (216)
follows a predetermined straight line with respect to RF input power. This
linearization removes the undesired linear and non-linear errors in
addition to variations versus frequency that would otherwise be apparent at
the input to the receiver linearizing table (216) in the receiver. These
errors
and variations would contribute to errors in the transmitter.
In order to reduce the error in the receive and transmit chains versus
frequency; the receive and transmit linearizers utilize the frequency index
that specifies the current center frequency on which the receive and
CA 02275156 1999-07-13
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transmit chains are operating. During factory calibration of the
radiotelephone, the linearizers are loaded with values, in addition to the
previously mentioned calibration values, that are indexed by frequency to
correct the errors related to operating center frequency. .
5 The AGC setpoint is the open loop power control signal for the radio.
In the preferred embodiment, this is the power control performed by the
radio by itself without control input from the cells. As the power of the
signal received from the cell increases, the radio decreases its transmit
power. This output power control is accomplished by the AGC setpoint that
is filtered by a low pass filter (21~.
The transmit section includes a digital summer (210) that combines
the AGC setpoint and a closed loop power control setting (206). The output
of the summer (210) is fed into a power control limiting section (205). The
operation of the power control limiting section (205) and the closed loop
power control section (206), illustrated in FIGs. 3 and 4 respectively, will
be
discussed subsequently in greater detail.
The output of the power control limiting section (205), along with the
transmit frequency index, are used to address values stored in a transmitter
linearizing table (204). The transmitter linearizing table (204) contains
values determined from production testing of the radiotelephone. The
selected value is input to a digital to analog converter (203) whose output,
an analog representation of the digital value input, controls a variable gain
amplifier (202).
The biasing of the variable gain amplifier (202) is adjusted by the
analog calibration value to a point such that the input to the transmitter
linearizing table (204) follows a predetermined straight line with respect to
transmitted RF output power. This linearization removes the undesired
linear and non-linear errors along with variations versus frequency in the
transmitter. This, combined with the previously mentioned receive
linearization, greatly reduces the open and closed loop power control errors
due to RF performance imperfections.
The power amplifier (PA) bias control section (218) controls the bias
point of the transmit PA (201) based on the transmit gain setting such that
the transmit sidebands for the given gain setting are optimized versus PA
36 (201) current consumption. This allows a battery powered telephone to
maximize talk time by reducing PA (201) current consumption at lower
output powers while still maintaining acceptable sideband levels at higher
output power levels. _
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The power control limiting section (205) is illustrated in FIG. 3. The
power control limiting section (205) controls the closed loop power' control
and transmit gain settings when the output of the transmit gain summer
(210) corresponds to a transmit output power level which is egual to or
greater than the intended maximum output power. The maximum gain
setting is determined by the PA limit threshold control section (209).
The threshold control section (209) determines the maximum gain
setting based on a nominal value that is modified by a real-time
measurement of the transmitted output power. The measurement' is
accomplished by an analog power detector (20~ whose output transformed
into a digital signal by an analog to digital converter (208). The digitized
power value is then input to the threshold control section (209).
The threshold control section, detailed in FIG. 5, operates by the high
power detector (HDET) linearizer (501) scaling the input digitized power
value in order to match the numerology of the digital transmit gain control
section. The scaled output from the linearizer (501) is subtracted (502) from
the nominal maximum gain setting. This maximum gain setting can be
hard coded into the radio during assembly or input during manufacturing
and testing of the radio.
The difference of the maximum gain setting and the scaled output
power is then added, by the adder (503), to the maximum gain setting. The
sum of these signals is then used as the corrected maximum gain setting.
.This real-time modification of the detected power helps mitigate the errors
introduced by temperature variations and aging of the transmitter PAs. In
other words, if the difference between the maximum gain setting and the
real-time measured power value is 0, then no correction is necessary. If
there is a difference between the two, the difference is used to correct the
maximum gain setting.
Referring to FIG. 3, a digital comparator (301) detects when the output
of the transmit gain summer (210) equals or exceeds the maximum gain
setting. The comparator (301) controls a 2:1 multiplexes (302) that outputs
the maximum allowable setting when the output of the summer (210)
exceeds the maximum allowable setting. When the output of the summer
(210) is less than the maximum allowable setting, the multiplexes (302)
outputs the direct output of the summer (210). This prohibits the
transmitter from exceeding its maximum operating point.
The closed loop power control section (206), illustrated in FIG. 4,
accumulates the power control commands sent on the forward link by the
controlling radiotelephone cell site and outputs a gain adjust signal. The
CA 02275156 1999-07-13
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7
power control commands are collected in an accumulator (401). The op-
eration of the accumulator (401) is controlled by the power control limiting
section (205) when the transmit power amplifier (201) is outputting the
maximum allowable power. .
When the output of the summer (210) changes from being less than
to equal or greater than the maximum allowable setting, the output of the
closed loop power control accumulator (401) is latched into a flip-flop (402).
While the output of the summer (210) is equal to or greater than the
maximum allowable setting, as determined by the comparator (403) and
NAND gate (404) circuit, an AND gate (405) masks off any closed loop power
control up commands that would force the accumulator (401) above the flip-
flop's (402) latched value. This prevents the accumulator from saturating
during power limiting yet allows the closed loop power control setting to
change anywhere below the latched value.
An alternate embodiment of the process of the present invention is
illustrated in FIG. 6. In this embodiment, a power limiting control system is
employed based on accumulator feedback control. The system operates by
first measuring the output power of the power amplifier (609) using a power
detector (610). The detected power is then digitized by an ADC (611) and
compared to a maximum allowable setting by the comparator (601). If the
output power is greater than the maximum setting, the power limiting
accumulator (602) begins turning power down by reducing the gain of the
. variable gain amplifier (608). If the output power is less than the maximum
setting the power limiting accumulator (602) returns to a OdB correction
value.
In this embodiment, a closed loop power control limiting function
(604 and 605), similar to the preferred embodiment, is employed. However,
the trigger for the closed loop power control limiting function is a
comparator (603) that detects when the power limiting accumulator (602) is
limiting the output power by comparing the accumulator (602) output to
OdB with the comparator (603). The linearizing compensation tables, similar
to the tables in the preferred embodiment, are added into the transmit gain
control using a summer (606).
In another alternate embodiment, illustrated in FIG. 7, a power
limiting control system is employed that is based on the closed loop power
control accumulator (702). The system operates by first measuring the
output power of the power amplifier (705) using a power detector (706). The
detected power is digitized (70~ and compared to a maximum allowable
setting by the comparator (701). If the output power is greater than the
..-.. ~ ~ CA 02275156 1999-07-13 '~ ;_-..
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maximum setting, the closed loop power control accumulator (702) is
modified to turn the amplifier (704) power down by one step each~1.25 ms
until the output power is less than the maximum setting. If the output
power is less than the maximum setting, the closed loop power control
accumulator is not modified. The linearizing compensation tables, similar
to the preferred embodiment, are added into the transmit gain control using
a summer (703).
In yet another embodiment, illustrated in FIG. 8, a power limiting
control system is employed that is based on integral feedback control. The
system operates by first measuring the output power of the power amplifier
(808) using a power detector (809). The detected power is digitized (810) and
input to an integrator (801) that follows the equation:
1
K ~ J(Setpoint - Detected)dt.
The integrator (801), generating a gain control signal, saturates at OdB
and -63dB of correction. The gain control signal is thus limited within a
range. If the output power is greater than the setpoint, the integrator turns
down the output power of the amplifier (807) at a rate based on the
integration constant K until the setpoint is reached. The integrator is
allowed to turn power down by as much as 63dB. If the output power is less
than the setpoint, the output of the integrator (801) will be forced to zero,
thus not adjusting output power.
In this embodiment, a closed loop power control limiting function
(803 and 804), similar to the preferred embodiment, is employed. The trigger
for the closed loop power control limiting function, however, is a
comparator (802) that detects when the power limiting integrator (801) is
limiting the output power. The linearizing compensation tables, similar to
the preferred embodiment, are added into the transmit gain control using a
summer (805).
In still another embodiment, illustrated in FIG. 9, a power limiting
control system is employed that is based only on a measure of receive
power, as determined by the Rx power lookup table (902), and the closed
loop power control setting as opposed to actual output power. The transmit
power limiting and closed loop power control limiting function (901) can be
implemented with either the preferred embodiment using the saturating
accumulator (903) or one of the alternate embodiments. However, only the
CA 02275156 1999-07-13 ~' ,
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9
receive power and closed loop power control setting are used to estimate
transmit output power.
In summary, the process of the present invention ensures that the
transmitted sidebands and synthesizer phase noise of a radio transmitter
remains within a predetermined specification by limiting the maximum
output power. This power limitation is accomplished by a control loop
including a calibration look-up table. Therefore, a radiotelephone using the
process of the present invention would not exceed it's nominal maximum
power level due to the cell issuing too many power turn-up commands. The
radiotelephone limits the power output even when the cell erroneously
decides the radiotelephone power should be increased.
WE CLAIM:
