U.S. patent application number 09/860035 was filed with the patent office on 2002-03-28 for method and apparatus for calibration of a wireless transmitter.
Invention is credited to Becker, Donald W., Moerder, Karl E..
Application Number | 20020037705 09/860035 |
Document ID | / |
Family ID | 26803137 |
Filed Date | 2002-03-28 |
United States Patent
Application |
20020037705 |
Kind Code |
A1 |
Moerder, Karl E. ; et
al. |
March 28, 2002 |
Method and apparatus for calibration of a wireless transmitter
Abstract
When an installed wireless remote unit is operating in the
field, routine operation is interrupted to enter a calibration
mode. In calibration mode, the compression point of a circuit
element is determined by applying a series of different drive
levels to the circuit element. Based upon the determination of the
compression point, a maximum transmission point is selected for the
remote unit. Upon resumption of normal operation, the remote unit
uses the maximum transmission point to limit the signal power level
transmitted over the wireless link. As time passes or operating
conditions change, the calibration process is re-executed.
Inventors: |
Moerder, Karl E.; (Poway,
CA) ; Becker, Donald W.; (Rancho Santa Fe,
CA) |
Correspondence
Address: |
KNOBBE MARTENS OLSON & BEAR LLP
620 NEWPORT CENTER DRIVE
SIXTEENTH FLOOR
NEWPORT BEACH
CA
92660
US
|
Family ID: |
26803137 |
Appl. No.: |
09/860035 |
Filed: |
May 17, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09860035 |
May 17, 2001 |
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09407641 |
Sep 28, 1999 |
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6256483 |
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60105953 |
Oct 28, 1998 |
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Current U.S.
Class: |
455/127.1 |
Current CPC
Class: |
H03G 3/3047 20130101;
H04W 52/52 20130101; H04W 52/36 20130101; H04B 17/13 20150115; H03G
3/3042 20130101 |
Class at
Publication: |
455/115 |
International
Class: |
H03C 001/62; H04B
017/00 |
Claims
What is claimed is:
1. A method of calibrating transmission circuitry of an installed
communication unit, comprising the steps of: interrupting routine
operation; generating a driving signal at a first level which
drives a circuit element into saturation; measuring a first signal
power level produced by said circuit element; setting said level of
said driving signal to a second level that is lower than said first
driving signal level; measuring a second signal power level
produced by said circuit element; comparing said first signal power
level to said second signal power level; and if said second signal
power level is less than said first signal power level by more than
a predetermined amount, using said second driving signal level as a
maximum drive level for said circuit element upon resuming routine
operation.
2. The method of calibrating transmission circuitry of claim 1,
wherein said step of interrupting routine operation is executed in
response to the passage of time.
3. The method of calibrating transmission circuitry of claim 1,
further comprising the step of transmitting an output of said
circuit element within a frequency bandwidth associated with
reception at a unit which houses said circuit element.
4. An apparatus for calibrating transmission circuitry of an
installed communication unit, comprising: means for interrupting
routine operation; means for generating a driving signal at a first
level which drives a circuit element into saturation; means for
measuring a first signal power level produced by said circuit
element; means for setting said level of said driving signal to a
second level that is lower than said first driving signal level;
means for measuring a second signal power level produced by said
circuit element; and means for comparing said first signal power
level to said second signal power level, and, if said second signal
power level is less than said first signal power level by more than
a predetermined amount, using said lower value as a maximum drive
level for said circuit element upon resuming routine operation.
5. The apparatus for calibrating transmission circuitry of claim 4,
further comprising means for measuring a passage of time and
interrupting routine operation in response to a passage of a
predetermined amount of time.
6. The apparatus for calibrating transmission circuitry of claim 4,
further comprising means for transmitting a signal produced by said
circuit element within a frequency bandwidth that enables signal
reception at a unit which houses said circuit element.
7. A method of limiting the output power of a wireless unit,
comprising the steps of: interrupting routine operation of an
installed communication unit; selecting a maximum transmission
point for said installed communication unit by applying a series of
different drive levels to a circuit element within the installed
communication unit to determine a compression point of the circuit
element; and resuming routine operation using said maximum
transmission point to limit a transmission power of said installed
communication unit
8. The method of calibrating transmission circuitry of claim 7,
wherein said step of selecting said maximum transmission point for
said installed communication unit further comprises the step of
measuring a series of relative signal power levels produced by said
circuit element.
Description
RELATED APPLICATIONS
[0001] The present application claims priority to copending
provisional application having the same title, application Ser. No.
60/105,953, filed Oct. 28, 1998, and copending utility application
having the same title, application Ser. No. 09/470,641, filed Sep.
28, 1999, which is hereby incorporated by reference.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] This invention relates generally to wireless transmitters.
More specifically, the invention relates to power control in a
wireless transmitter.
[0004] 2. Description of the Related Art
[0005] The use of wireless communication systems for the
transmission of digital data is becoming more and more pervasive.
In a wireless system, the most precious resource in terms of cost
and availability is typically the wireless link itself. Therefore,
one major design goal in designing a communication system
comprising a wireless link is to efficiently use the wireless
link.
[0006] In a system in which multiple units compete for finite
system resources, in order for multiple remote units to access
common system resources, the wireless link is divided into a series
of channels. Channelization can be achieved by one of a variety of
well-known techniques such as time division multiple access (TDMA),
code division multiple access (CDMA), frequency division multiple
access (FDMA) or a combination of these. Each of these
channelization techniques, to some extent, limits the frequency
bandwidth of the signal transmitted from the remote unit.
[0007] In addition, each of these channelization techniques
requires the use of power control to some extent to determine the
power level at which the remote unit transmits. If the remote unit
signal arrives at the hub station at a signal level that is too
low, the system performance level may be inadequate to support
communication due to excessive errors caused by thermal noise and
interference. If the remote unit signal arrives at the hub station
at a signal level that is too high, the remote unit generates
unnecessary interference to other system users.
[0008] FIG. 1 is a schematic diagram illustrating a wireless
satellite communication system. A hub station 10 provides digital
data transfer capabilities to a plurality of remote units such as a
remote unit 14. The hub station sends signals over an uplink
forward channel 20 to a satellite 12. The satellite 12 repeats the
signal and transmits it over a downlink forward channel 22. The
remote unit 14 receives the signal and processes it. The remote
unit 14 sends a signal over an uplink reverse channel 24 to the
satellite 12. The satellite 12 repeats the signal and forwards it
over a downlink reverse channel 26.
[0009] A link budget is a design tool used to determine the level
at which signals are transmitted over the system. For example, a
link budget is used to determine a nominal level at which the
remote unit 14 transmits the reverse link signal over the uplink
reverse channel 24 based upon the expected path loss experienced
over the uplink reverse channel 24. The satellite 12 may amplify
the signal before forwarding it over the downlink reverse channel
26. The link budget estimates the expected path loss of the uplink
and downlink reverse channels 24 and 26. In addition, the link
budget estimates the expected interference and noise levels
introduced by the uplink and downlink reverse channels 24 and 26 as
well as noise introduced by the satellite 12 and the hub station 10
such as due to the noise figure of these units. In addition, the
link budget estimates the expected variations of these parameters.
Using the link budget, a system designer determines a nominal and
worst case power level at which the remote unit transmits.
[0010] In a communication system which comprises fixed location
remote units and which uses a geosynchronous satellite, the path
loss of the wireless link channel is fairly consistent over time.
However, weather conditions may vary the path loss to some extent.
Depending on the frequency at which the system operates, adverse
weather conditions, such as heavy fog, snow, rain or hail, may
increase the path loss by several decibels (dB) or more. Therefore,
in order to operate efficiently, most wireless systems include a
power control loop to control the level at which signals are
transmitted and received in the system. For example, the hub
station 10 monitors the signal-to-noise ratio of a signal received
from the remote unit 14 over the reverse link channels 24 and 26
and notifies the remote unit 14 if the signal-to-noise ratio of the
signal falls below a predetermined level. In response, the remote
unit 14 increases the power level at which it is transmitting. If
the hub station 10 determines that the path loss has decreased, the
hub station notifies the remote unit 14 over the forward link
channels 20 and 22 and the remote unit 14 decreases the level at
which it is transmitting.
[0011] In order to reduce the distortion of the reverse link
signal, the remote unit 14 is typically designed to comprise a
class A power amplifier. Class A power amplifiers provide a high
degree of linearity over a substantial range of output power. In
order to operate linearly, class A amplifiers require substantially
more supply power than the power level of the radio frequency (RF)
signals which they produce. A class A amplifier draws the same
supply power regardless of the output power which it is producing.
Therefore, the size and heat dissipation capabilities of a class A
power amplifier may be significant, as well as the cost of their
operation. Typically, the size of a class A amplifier doubles for
each 3 decibels (dB) extra of power which it is capable of
producing. In addition, the cost of the power amplifier increases
significantly for each 3 dB extra of power capability. Therefore,
it is advantageous to use a link budget to determine the maximum
power output level which the remote is required to transmit and to
limit the capability of the power amplifier based upon the
determination.
[0012] FIG. 2 is a graph showing the characteristics of a typical
class A amplifier. The horizontal axis represents the RF input
power level in units of decibels referred to 1 milliwatt (dBm). The
vertical axis represents the RF output power level of the amplifier
in the same units. The gain of the amplifier illustrated by curve
32 is approximately 54 dB. For example, when the input drive level
is -30 dBm, the output power is 24 dBm. As the input level is
increased in 1 dB steps, the output level also increases in 1 dB
steps as is characteristic of a linear amplifier. However, at some
point, the output power stops tracking the input power on a
one-to-one basis. For example, data point 36 on curve 32 represents
the point at room temperature at which the gain of the amplifier
has decreased by approximately 1 dB to 53 dB of gain. At this
point, the input to the amplifier is approximately -19.5 dBm and
the output is approximately 33.5 dBm. As the input drive level is
increased further beyond the 1 dB compression point, the output of
the amplifier does not increase significantly above 34 dBm.
[0013] The non-linearities introduced by use of a class A amplifier
close to and beyond the 1 dB compression point cause distortion in
the modulated signal which causes increased interference levels in
adjacent channels for the non-constant envelop signals generated by
many modern communication systems. FIG. 3 is a spectrum plot
showing the distortion caused when a modulated signal is amplified
by a power amplifier which has the characteristics shown by curve
32 of FIG. 2. The horizontal axis represents frequency and the
vertical axis represents power level relative to the power level of
the modulated signal. The horizontal axis is measured in terms of
channels. Channel 1 represents the channel in which the remote unit
is operating. Channels 2-8 correspond to other channels in the
system. The spectrum plot shown in FIG. 3 is a single side-band
plot. However, it may be assumed that the spectrum created is
relatively symmetric about the left-most axis as shown in FIG.
3.
[0014] Curve 40 represents the spectrum output by the power
amplifier when the modulated drive level is -30 dBm. Referring
again to FIG. 2, one can see that the power amplifier is quite
linear in this region. The modulation bandwidth is limited to less
than the bandwidth of the first channel. However, the modulated
signal introduces some interference to adjacent channels. For
example, in channel 2, the interference level generated is at least
35 dB lower than the intended signal level in channel 1. The
interference level continues to drop in channels 3-8.
[0015] Curve 42 represents the output of the power amplifier when
the input power level is -20 dBm. Referring again to FIG. 2, one
can see that operating at -20 dBm is approaching operation at the 1
dB compression point and the amplifier has begun to exhibit
increased non-linearity. Due to the non-linearities, the
interference level generated by the power amplifier in channel 2
has increased by approximately 10 dB. In addition, the interference
level generated in channels 3-8 has also increased. Curves 46, 48,
50, 52 and 54 represent the output of the power amplifier when the
input power level is -18, -14, -10, 0 and 10 dBm, respectively. As
the input power level increases, the non-linearities continue to
increase causing a corresponding increase in the interference level
in the adjacent channels. Referring again to FIG. 2, it can be seen
that increasing the drive level does not significantly increase the
output level once the 1 dB compression point has been reached. For
these reasons, it is advantageous to limit the input power level
such that the amplifier is operated in the linear region.
[0016] The response characteristics of a class A amplifier vary as
a function of frequency, temperature and aging. For example, curve
30 of FIG. 2 represents the characteristics of the class A
amplifier at a relatively high operating temperature. Notice that
the 1 dB compression point has fallen to approximately -22 dBm of
input power and 31 dBm of output power. Likewise, curve 34
represents the characteristics of the amplifier significantly below
room temperature. The 1 dB compression point has increased slightly
with comparison to curve 32. Similar curves could be generated to
represent the frequency and aging response of the amplifier.
[0017] In addition to the variations of an amplifier over time,
frequency and aging, each amplifier produced by a common process
exhibits different characteristics due to process variations as
well as other factors. FIG. 4 is a graph showing typical variations
in operating curves of amplifiers constructed from a common
process. Curve 32 represents the average performance at room
temperature as also shown in FIG. 2. Curve 60 represents a power
amplifier which exhibits the one-sigma low side process variation
characteristics. Notice that the 1 dB compression point is
approximately -23 dBm and that the maximum output power of the
amplifier is approximately 2 dB less than shown in curve 32. Also
note that the gain of the amplifier represented by curve 60 is
lower than shown in curve 32 even in the linear region. The curve
64 represents a power amplifier which exhibits the one-sigma high
side process variation. Note that the 1 dB compression point is
approximately -19 dBm and that the gain of the amplifier is higher
than the other two curves shown even in the linear region.
[0018] In order to design a link budget which is sufficient to
operate over process variations, temperature variations, frequency
variations and aging, the maximum output power must be limited in
view of the worse case 1 dB compression point over all of these
factors. Therefore, referring again to curve 60 of FIG. 4,
according to the prior art, the maximum output power of the system
should be limited to approximately 29 dBm in order to accommodate
variations over frequency, temperature and aging of the one sigma
low side power amplifier. Thus, even though the majority of power
amplifiers are capable of producing 5 dB more than this maximum at
room temperature and some of the amplifiers are capable of
producing approximately 7 dB more output power at room temperature,
under the prior art, each remote unit's output power is limited
according to the worse-case scenario.
[0019] Typical prior art remote units use a detector to sense the
output power level. When the output power level exceeds the
predetermined maximum, the detector alerts the remote unit and the
level at which the power amplifier is driven is limited to the
current drive level. Because the output of the detector itself can
vary according to process gains, frequency, temperature and aging,
some margin must also be built into the system to accommodate for
these variations. Alternatively, complicated detector calibration
mechanisms can be incorporated into factory testing in order to
account for some of these variations. In general, however, the
maximum output power must be further limited in order to
accommodate variations in the detector as well. In conjunction with
the limitations imposed by the variations in the power amplifiers,
these limitations significantly decrease the efficiency with which
the average power amplifier is used during routine operation.
[0020] Therefore, there has been a long-felt need in the industry
to develop an efficient power control mechanism in a wireless
transmitter.
SUMMARY OF THE INVENTION
[0021] When an installed wireless remote unit is operating in the
field, routine operation is interrupted to enter a calibration
mode. In calibration mode, the compression point of a circuit
element is determined by applying a series of different drive
levels to the circuit element. Based upon the determination of the
compression point, a maximum transmission point is selected for the
remote unit. Upon resumption of normal operation, the remote unit
uses the maximum transmission point to limit the signal power level
transmitted over the wireless link. As time passes or operating
conditions change, the calibration process is re-executed.
BRIEF DESCRIPTION OG THE DRAWINGS
[0022] The features, objectives, and advantages of the invention
will become more apparent from the detailed description set forth
below when taken in conjunction with the drawings wherein like
parts are identified with like reference numerals throughout and
wherein:
[0023] FIG. 1 is a schematic diagram illustrating a wireless
satellite communication system.
[0024] FIG. 2 is a graph showing the characteristics of a typical
class A amplifier.
[0025] FIG. 3 is a spectrum plot showing the distortion caused when
a modulated signal is amplified by a power amplifier which has the
characteristics shown by curve 32 of FIG. 2.
[0026] FIG. 4 is a graph showing typical variations of operating
curves produced by amplifiers constructed from a common
process.
[0027] FIG. 5 is a block diagram showing an outdoor unit and an
indoor unit of a remote unit in accordance with the invention.
[0028] FIG. 6 is a graph showing the characteristics of a typical
schottky diode detector with respect to temperature.
[0029] FIG. 7 is a flow chart showing the process for calibration
of transmission circuitry of an installed communication unit during
routine operation.
[0030] FIG. 8 is a representational drawing illustrating the
advantages that can be gained through transmission of a modulated
signal.
[0031] FIG. 9 is a spectral plot showing a random access reverse
link channel and a broadcast forward link channel.
[0032] FIG. 10 is a block diagram illustrating an exemplifying
system in which the invention may be embodied.
Detailed Description of the Invention
[0033] FIG. 5 is a block diagram showing an outdoor unit 68 and an
indoor unit 90 of a remote unit in accordance with the invention. A
satellite dish 70 is used to transmit and receive signals via a
satellite transponder and is directly coupled to the outdoor unit
68.
[0034] The outdoor unit 68 comprises circuit elements which are
located in close proximity to the satellite dish 70. In one
embodiment, the satellite dish 70 is located on the roof of a
building and the outdoor unit 68 is mounted directly to the
satellite dish 70 assembly on the exterior of the building. The
outdoor unit 64 comprises a low noise amplifier 74 which is located
in close proximity to the satellite dish 70 in order to reduce the
path loss between the satellite dish 70 and the low noise amplifier
74 and to preserve the system noise figure. Likewise, the outdoor
unit 68 comprises a power amplifier 82 which is located in close
proximity to the satellite dish 70 in order to reduce the path loss
between the power amplifier 82 and the satellite dish 70 and avoid
increasing the required output of the power amplifier 82.
[0035] The indoor unit 90 comprises circuitry which need not be
located in close proximity to the satellite dish 70. In one
embodiment, the indoor unit is located at a convenient position
which allows more ready access by the system administrator. The
indoor unit 90 is connected to the outdoor unit 68 by a length of
cable.
[0036] The division of the communication unit into the outdoor unit
68 and the indoor unit 90 allows them to be separately manufactured
and factory tested. For example, the outdoor unit 68 is preferably
manufactured by an RF design house. The indoor unit 90 is
preferably manufactured by a company which specializes in
communications. In one embodiment, the indoor unit 90 is designed
to be compatible with a variety of different outdoor units which
operate at a variety of different frequencies and which are
manufactured by a variety of different vendors. When in place in
the field, the outdoor unit and indoor unit can be separately
serviced, replaced or upgraded.
[0037] Within the outdoor unit 68, the satellite dish 70 is coupled
to an orthomode transducer (OMT) 72 or other device or method for
providing separation of signals. The OMT 72 couples the receive
signals from the satellite dish 70 to the low noise amplifier 74.
The OMT 72 also couples transmit signals from the power amplifier
82 to the satellite dish 70. The low noise amplifier 74 is coupled
to a mixer 76 which down-converts the received signal to an
intermediate frequency. The output of the mixer 76 is coupled to
the indoor unit 90 via a length of cable. Within the indoor unit
90, a receive processing unit 92 performs further analog and
digital signal processing on the received signal. An indoor unit
controller 98 controls the receive processing unit 92 as well as
other components of the indoor unit 90.
[0038] A signal generation unit 94 is controlled by the indoor unit
controller 98 and creates the transmit signal waveform. For
example, the signal generation unit 94 receives data from system
users and encodes the data for transmission. The signal generation
unit 94 also modulates the encoded data, converts the modulated
signal to an analog waveform and translates it to an intermediate
frequency.
[0039] The modulated IF output signal from the signal generation
unit 94 is coupled to a variable gain amplifier 96. The gain of the
variable gain amplifier 96 is controlled by the indoor unit
controller 98. The output IF signal from the variable gain
amplifier 96 is passed over a length of cable to the outdoor unit
68. Within the outdoor unit 68, a mixer 80 converts the IF signal
to a RF frequency suitable for transmission. For example, in one
embodiment, the RF center frequency is within the Ku band, Ka band
or any other suitable band. The output of the mixer 80 is input
into the power amplifier 82. In the preferred embodiment, the power
amplifier 82 is a class A amplifier. When enabled, the power
amplifier 82 outputs a high power RF signal to the satellite dish
70 via the OMT 72.
[0040] In addition to the transmit and receive path circuitry, the
outdoor unit 68 comprises a detector 84, a microprocessor 86 and a
temperature sensor 88. The microprocessor 86 is used to control the
functions of the outdoor unit 68 and to provide information to the
indoor unit controller 98. In the preferred embodiment, the
microprocessor 86 is a low functionality part which does not
greatly impact the price or size of the outdoor unit 68. In one
embodiment, the microprocessor 86 is an AT9054433 from Atmel
Corporation of San Jose, Calif. The microprocessor 86 is coupled to
the indoor unit controller 98 and responds to commands therefrom.
For example, the indoor unit controller 98 can request that the
microprocessor 86 identify itself. In response, the microprocessor
86 forwards a unique identification number which uniquely
identifies the outdoor unit 68. The identification can include, for
example, the manufacturer, model, revision number, and capabilities
of the outdoor unit 68.
[0041] In one embodiment, the microprocessor 86 comprises a 10-bit
analog-to-digital (A/D) converter. The microprocessor 86 is coupled
to the temperature sensor 88. In the preferred embodiment, the
temperature sensor 88 outputs an analog signal level indicative of
the operating temperature of the outdoor unit 86. The
microprocessor 86 converts the analog temperature indication to a
digital value using the analog-to-digital converter and provides
the value to the indoor unit controller 98 upon request
therefore.
[0042] The power detector 84 is loosely coupled to the output of
the power amplifier 82 such as through a coupler or power splitter.
In the preferred embodiment, the power detector 84 is a schottky
diode detector (or other detector-type) and outputs an analog
signal level indicative of the RF signal power level produced by
the power amplifier 82. FIG. 6 is a graph showing the
characteristics of a typical schottky diode detector over
temperature. The vertical axis of FIG. 6 is the detected output in
volts. The horizontal axis is the amplitude of the RF input signal
in volts. The curve 160 represents the response of the detector at
-20C. The curve 162 represents the response of the detector at 25C.
The curve 164 represents the response of the detector at +50C. As
can be seen by FIG. 6, the response of the detector is neither
linear nor constant over temperature. As shown in the graph, a
single output voltage may be mapped to a wide range of input
amplitudes depending upon the operating temperature of the
detector.
[0043] In one embodiment, the microprocessor 86 comprises an
onboard oscillator which is used to create a clock by which data is
transferred to the indoor unit 90. The onboard oscillator may cause
undesirable in-band spurs in the transmit signal when operating. In
one embodiment, the on-board oscillator is disabled during normal
transmissions and is enabled during the calibration process in
order to convert the analog values to digital values. In addition,
during periods of idleness, the oscillator is enabled for other
functions.
[0044] In a system which transfers digital data, remote units tend
to generate bursty data. The bursty data is characterized in that
it has a high peak-to-average traffic ratio, meaning that large
blocks of data are transferred during short periods of time
interposed between significantly longer periods of idleness.
[0045] Referring back again to FIG. 1, although only one remote
unit 14 is shown in FIG. 1, the reverse link resources are shared
between a great number of remote units in the preferred embodiment.
If the power amplifiers within the plurality of remote units remain
active while the remote units are idle, the combined interference
caused by the amplified noise from the idle remote units can create
a significant amount of interference to the remote units which are
currently transmitting. In order to decrease this interference, the
power amplifiers in the idle remote units are disabled so that they
do not amplify and transmit noise while idle. The microprocessor 86
is coupled to the power amplifier 82 such that it can disable the
power amplifier during periods of idleness.
[0046] In one embodiment, the outdoor unit 68 has a fixed gain. The
output power of the remote unit is varied by varying the input
power to the outdoor unit 68. For example, as shown in FIG. 5, the
variable gain amplifier 96 sets the output power of the indoor unit
90. In order to limit the interference to other remote units
operating in the same system, it is advantageous to limit the power
level at which the remote unit transmits its signal. Typically,
such systems are implemented using feedback from the hub station.
For example, the remote unit initially transmits at a relatively
low signal level. Based upon a signal quality at which the reverse
link signal reaches the hub station, the hub station sends commands
over the forward link channel requesting the remote unit to
increase the signal power level. When adverse weather conditions
increase the path loss on the reverse link channel, the hub station
sends a command over the forward link channel which orders an
increase in signal power level transmitted by the remote unit. In
response, the indoor unit 90 increases the gain setting of the
variable gain amplifier 96.
[0047] As noted above, however, it is advantageous to limit the
maximum signal power level at which the remote unit transmits so
that the power amplifier operates within the linear region in order
to avoid creating excess interference to the system. In order to
limit the signal power level transmitted from the fixed gain
outdoor unit 68, the signal power level produced by the indoor unit
90 must be limited. Therefore, in one embodiment, the indoor unit
comprises a memory unit 100 in which is stored a maximum gain
setting for the variable gain amplifier 96.
[0048] When the hub station sends a command for the remote unit to
increase the signal power level at which it is transmitting, the
message is received by the remote unit and processed by the receive
processing unit 92. The command is forwarded to the indoor unit
controller 98. In response, the indoor unit controller 98 increases
the gain setting of the variable gain amplifier 96 unless the
current gain setting is equal to or exceeds the maximum gain
setting stored in the memory unit 100.
[0049] As noted above, it is difficult to select a maximum gain
setting which allows efficient use of the outdoor unit resources
while providing margin to account for part-to-part variations and
variations over time, temperature, frequency and aging. The present
invention comprises a means for determining the maximum gain
setting such that the capabilities of the outdoor unit are fully
exercised while avoiding non-linear operation.
[0050] FIG. 7 is a flow chart showing calibration of transmission
circuitry of an installed communication unit during routine
operation. Flow begins in start block 110. In block 112, the
outdoor unit 68 measures the current temperature using the
temperature sensor 88. The microprocessor 86 converts the analog
level to a digital value and forwards it to the indoor unit
controller 96. In block 114, the indoor unit controller 98 compares
the temperature to a table of temperatures stored in the memory
unit 100. If there is a valid maximum gain setting for this
temperature stored in the memory unit 100, the indoor unit
controller 98 uses the stored maximum gain setting in block 116 and
flow continues back to block 112. If there is no valid maximum gain
setting for the current temperature, flow continues in block 118.
In block 118, the indoor unit controller 98 interrupts routine
operation and commands the signal generation unit 94 to generate a
test sequence, thus, initiating the calibration sequence. In block
120, the indoor unit controller 98 enables the power amplifier 82
and increases the gain setting of the variable gain amplifier 96 so
that it produces a signal power level large enough to drive the
power amplifier 82 deep into saturation. For example, referring
again to FIG. 4, the drive level to the power amplifier 82 exceeds
-16 dBm such that both low performance and high performance parts
are driven deep into saturation.
[0051] In block 122, the detector 84 measures the signal power
level output by the power amplifier 82. The microprocessor 86
converts the analog detector value into a digital value and, then,
passes it to the indoor unit controller 98. In block 124, the
indoor unit controller 98 decreases the gain setting of the
variable gain amplifier 96 so that its output signal power level is
decreased by some incremental amount. For example, the indoor unit
controller 98 decreases the gain setting of the variable gain
amplifier 96 to drop its output signal power level by approximately
1 dB. In block 126, the detector 84 once again measures the output
signal power level from the power amplifier 82. Once again, the
microprocessor 86 digitizes the analog detector value and passes
the value to the indoor unit controller 98. In block 128, the
indoor unit controller 98 compares the detected current signal
power level to the saturated signal power level in block 128. If
the current level is approximately equal to the saturated level,
flow continues back to block 124. For example, if the current level
plus two decibels is greater than the saturated level, flow
continues back to block 124 and the signal power level produced by
the variable gain amplifier 96 is further decreased. If the current
value is less than the saturated level by some incremental amount,
flow continues to block 130. In block 130 in the indoor unit
controller 98 stores the current temperature and the current gain
setting of the variable gain amplifier 96 in the memory unit 100.
The stored value becomes the maximum gain setting corresponding to
the current operating conditions.
[0052] In one embodiment, the steps shown in FIG. 7 and described
generally above are implemented in a series of processing units
which are embodied in memory or custom application specific
integrated circuits (ASIC) or executed by a microprocessor. In
another embodiment, the steps are implemented in software and
executed, for example, by a digital signal processor (DSP) or
embodied in hardware in a field programmable gate array (FPGA) or
in an ASIC.
[0053] In order to implement the steps shown in FIG. 7, the
variable gain amplifier 96 should be designed with enough gain
"headroom" so that the variable gain amplifier 96 continues to
linearly increase the driving signal level as the power amplifier
82 enters saturation. Likewise, the detector 84 must have enough
power handling capacity and linearity to provide accurate
measurements as the power amplifier 82 enters saturation.
[0054] Operation according to FIG. 7 illustrates a specific
implementation of the general principles of the invention. When an
installed unit is operating in the field, routine operation is
interrupted to enter a calibration mode. In a TDMA system,
calibration mode can be entered between transmission slots. In a
TDMA, FDMA or CDMA or other type of system, calibration mode can be
entered during a natural period of idleness which occurs between
the transmission bursts. If the length of the period of idleness
does not allow sufficient time for the calibration to be completed,
the calibration process can be executed over a series of disjoint
periods of calibration. Alternatively, transmission of data can be
interrupted or suspended in order to accommodate the calibration
process. In one embodiment, the remote unit continues to receive
data from the wireless link during calibration mode.
[0055] In calibration mode, the compression point of a circuit
element is determined by applying a series of different drive
levels to the circuit element. Based upon the determination of the
compression point, a maximum transmission point is selected for the
remote unit. Upon resumption of normal operation, the remote unit
uses the maximum transmission point to limit the signal power level
transmitted over the wireless link. As time passes or operating
conditions change, the calibration process is re-executed.
[0056] Referring again to FIG. 4, several advantages of operation
according to FIG. 7 become more readily apparent. When the power
amplifier drive level is increased in block 120, if the power
amplifier 82 has the characteristics shown by curve 64, the
detected signal power level is approximately 36.25 dBm. If the
power amplifier 82 has the characteristics shown by curve 32 or
curve 60, the detector signal power level measures approximately
34.25 or 32.25 dBm, respectively. When the drive level is decreased
from -16 dBm to -17 dBm in block 124, the signal power level
detected by the detector remains fairly constant for all three
curve characteristics. Likewise, at -18 dBm, the signal power level
remains fairly constant for all three curve characteristics.
However, as the input power drops to -19 dBm, the signal power
level of an amplifier with the characteristics of curve 64 begins
to decrease. At -20 dBm, the signal power level of an amplifier
with the characteristics of curve 64 has dropped almost 1 decibel
below the saturated level. At -20 dBm, the signal power level of a
power amplifier with the characteristics of curve 32 has just begun
to decrease from the saturated level. However, at -20 dBm, the
signal power level produced by a power amplifier with the
characteristics of the curve 60 remains nearly equal to the
saturated level. At -21 dBm of input power, the signal power level
of an amplifier with the characteristics of curve 64 has fallen 2
dB below the saturated value. As such, the gain setting of the
variable gain amplifier 96 is recorded as the maximum gain setting.
In a like manner, as the input drive level is further decreased,
the signal power level output by amplifiers with the
characteristics of curve 32 and 60 also begins to fall. The maximum
gain setting corresponding to curve 32 is approximately equal to
-23 dBm and the maximum gain setting corresponding to curve 60 is
approximately -24 dBm. In this way, each power amplifier can be
used in accordance with its capabilities.
[0057] In certain embodiments, it is advantageous to increase or
decrease the step size by which the drive level is incremented such
that it is more or less than 1 dB. In addition, in certain
embodiments, it is advantageous to increase or decrease the amount
by which the signal power level of the power amplifier must drop in
order to determine the maximum gain setting such that it is more or
less than 2 dB. Several factors can be used to determine the
optimal value of these two variables. As shown in FIG. 6, the
detector 84 does not have a perfectly linear response. In addition,
the analog-to-digital converter within the microprocessor 86
introduces quantization errors when it converts the analog signal
level to digital value. Therefore, some error is introduced by the
accuracy with which the detector 84 measures the signal power
level. In addition, the response of the variable gain amplifier 96
to changes in the gain setting is not perfectly linear and also
introduces some error into the measurement. Therefore, it is
advantageous to include some additional margin to compensate for
any errors introduced by these factors.
[0058] One advantage of operation according to the process of FIG.
7 is that it is not necessary that the detector make an accurate
and absolute determination of the signal power level output by the
amplifier. It is only necessary that the detector measures a
relative change in the output power. Likewise, the signal power
level at which the variable gain amplifier 96 produces the IF drive
signal need not be absolute. It is only necessary that the variable
gain amplifier 96 output power change relative to the previous
output level in response to the changing gain settings.
[0059] In one embodiment, additional factors which influence the
outdoor unit characteristics are stored in the memory unit 100 in
order to more precisely identify the current operating conditions.
For example, if the outdoor unit is capable of operation in several
frequency bands, in one embodiment, the data stored in the memory
unit 100 is a function of both temperature and frequency or
frequency band. The identification number of the outdoor unit is
also stored in the memory unit 100. If a new outdoor unit is
connected to the indoor unit, the calibration points must be
retaken using the new outdoor unit. Other factors which can be
considered in determining the current operating conditions are
indoor unit temperature, the satellite or satellite position, the
presence of rain or other environmental conditions or the rate of
change of the temperature.
[0060] FIG. 8 is a representational drawing illustrating the
advantages that can be gained through transmission of a modulated
signal. If a constant wave (CW) signal is output from the power
amplifier 82, the detector 84 produces a single analog value
representative of the signal power level. The analog-to-digital
converter within the microprocessor 86 converts the analog detector
output to the closest digital value. For example, in FIG. 8A, a
range of analog detector output voltages surrounding the level x is
indicated. Any detector output within this range generates a
digital value of x at the output of the analog-to-digital
converter. Essentially, any analog value between x-1/2 and x+1/2
maps into a value of x at the output of the analog-to-digital
converter. For example, as shown in FIG. 8B, if the detector output
is x+1/4, the output of the analog-to-digital converter is x. Error
introduced in this manner is referred to as quantization error.
[0061] However, if a modulated signal is output from the power
amplifier 82, the signal power level produced by the amplifier
varies over time due to the modulation. For example, in a simple
scenario, the modulated signal power level varies evenly plus or
minus one step size due to the modulation. If a series of analog
detector level values are input to the analog-to-digital converter,
a variety of corresponding digital values are produced. FIG. 8C
illustrates a conversion of a modulated signal. Block 140
represents an even distribution of signal power levels produced by
the power amplifier 82 centered about a value of x+1/4 and fanning
a range of plus or minus one analog-to-digital converter step size.
Those detector level values which fall in the region below x -1/2
as shown by the shaded region 142 map into a value of x-1. Those
detector level values which are between x-1/2 and x+1/2 map into
the value x as shown by shaded region 144. Those detector level
values which are between x+1/2 and x+11/4 map into x+1 as shown by
shaded region 146. As can be seen by examination of the relative
size of areas 142, 144 and 146, 1/8 of the samples produce a value
of x-1, one half of the samples produce a value of x, and 3/8
samples produce a value of x+1. The average of these values is
x+1/4 which accurately reflects the average value of the signal
power level. The same effect is achieved by use of other
distributions, such as a Gaussian distribution, of the signal power
level. Thus, by sending a modulated signal and averaging a set of
converted detector level samples, a more accurate estimation of the
actual signal power level can be determined, thus, lessening the
error caused by quantization of the analog detector level
output.
[0062] In one embodiment, the remote units communicate with the hub
station over the reverse link using a set of random access
channels. For example, in one embodiment, the reverse link operates
in accordance with assignee's co-pending application entitled
METHOD AND APPARATUS FOR MULTIPLE ACCESS IN A COMMUNICATION SYSTEM,
application Ser. No.______ [Attorney Docket No. TACHYON.018CP2],
filed concurrently herewith, the entirety of which is hereby
incorporated by reference..
[0063] In contrast, the hub station communicates with the remote
units on a broadcast channel. FIG. 9 is a spectral plot showing a
random access reverse link channel and a broadcast forward link
channel. The random access reverse link channels 150A-150N, each
originating from a different remote unit, randomly block one
another in the natural course of system operation. However, the
broadcast forward link channel 152 can be designed so that they do
not block one another and, therefore, are more tolerant of
additional interference. In addition, the forward link signals are
typically sent at a higher power rate than the reverse link
signals.
[0064] The random access reverse link channels are typically
transmitted about 10 dB lower power spectral density than the
broadcast forward link channel. The hub station has a significantly
larger antenna than the remote units and, therefore, it is able to
detect the reverse link signal at a smaller power spectral density.
Therefore, in one embodiment, when the calibration process is
executed by the remote unit, the transmit frequency of the outdoor
unit is changed so that the calibration output 154 falls within the
forward link channel bandwidth. In this way, interference with the
more susceptible random access reverse link channels 150A-150N is
avoided. In order to provide minimum interference to the broadcast
forward link channel 152, it is advantageous to modulate the
calibration output 154 transmission rather than to transmit a
constant waveform (CW). By modulating the calibration output 154
transmission, the energy transmitted into the forward link channel
152 is spread over a range of frequencies and, therefore, the
system is not burden with a strong, single tone signal.
[0065] In another embodiment, the output of the amplifier 82 during
the calibration process is dumped into a dummy load so that a
majority of the energy output by the power amplifier 82 is not
transmitted over the wireless link. In yet another embodiment, the
remote unit transmits within the random access reverse link
channels during the calibration process. In another embodiment, the
remote unit continues to transmit user data during the calibration
process rather than a dummy sequence.
[0066] In the embodiment shown in FIG. 7, the calibration process
is triggered by changes in the operating temperature of the outdoor
unit 68. The calibration values remain stored in the memory unit
100 and available for use should conditions return to a previously
calibrated state. In one embodiment, each calibration entry in the
memory unit 100 is time-stamped and expires after a predetermined
duration. After a set of stored values expires, the corresponding
operating state characteristics are re-calibrated the next time
such conditions exist.
[0067] In an alternative embodiment, the current characteristics of
the power amplifier are calibrated intermittently. In one
embodiment, for example, the characteristics of the power amplifier
is calibrated every eight minutes. The indoor unit uses the most
recently collected data until the next calibration data is
collected. This embodiment has the advantage of eliminating use of
the temperature sensor. However, this embodiment requires more
frequent calibration and is less accurate during periods of rapid
temperature change.
[0068] The invention has been described herein with reference to a
satellite system. The same techniques can be applied to other
operating environments. For example, the invention can be
implemented in a cellular or other terrestrial wireless
communication system--either in the base station or remote unit.
The invention can be implemented in systems which communicate using
a variety of modulation and access techniques. For example, the
invention may be implemented in a FDMA, CDMA or TDMA type system.
Likewise, the description above was presented with reference to a
power amplifier. However, the teachings can be directly applied to
calibrate of other types of circuit elements (such as mixers) or a
set of circuit elements whether they exhibit class A, class AB,
class B or other operating characteristics.
[0069] In one embodiment, when the maximum gain setting for the
current operating conditions is achieved, another mechanism is used
to increase the signal level with which the signal reaches the hub
station. For example, in one embodiment, when the maximum gain
setting is reached, the indoor unit controller 98 commands the
signal generation unit 94 to forward a message to the hub station
indicating that subsequent remote unit transmissions will have a
decreased data rate. By reducing the data rate in half, the energy
devoted to each information bit is doubled if the transmission
power remains constant. If the hub station once again requests an
increase in the transmission power, the data rate can be further
reduced. As link conditions improve and the hub station requests
that the remote unit decrease the power at which it is
transmitting, the indoor unit 98 commands the signal generation
unit 94 to generate a message for the hub station indicating that
subsequent transmissions from the remote unit will have an
increased data rate.
[0070] In the method of FIG. 7, the compression point of the power
amplifier 82 is determined by initially driving the power amplifier
into saturation and, then, incrementally dropping the input drive
level until the amplifier comes out of saturation. In another
embodiment, the 1 dB compression point is determined by
incrementally increasing the drive level and detecting a decrease
in the gain of the power amplifier 92 as it enters saturation. For
example, in such an embodiment, to determine the 1 dB compression
point of a power amplifier having the characteristics shown in FIG.
4, initially, the power amplifier is driven at -26 dBm. A
corresponding measurement is taken by the detector 84. The indoor
unit controller 98 commands the variable gain amplifier 86 to
increase its gain by approximately 1 dB. Again, the detector 84 is
used to make a measurement of the output power. The 1 dB
compression point of the power amplifier 82 is determined when the
measured signal power level indicates that the gain of the power
amplifier 82 has fallen by 1 dB in comparison to the first
measurement which is taken.
[0071] This process has different requirements than the process
shown in FIG. 7. For example, notice that the detector curve shown
in FIG. 6 is not linear. Therefore, as the detector 84 attempts to
measure the signal power level output by the amplifier 82, some
variation in the measurements is due to errors introduced by the
detector 84. In addition, the variable gain amplifier 96 also has a
non-linear response which further adds to the measurement errors.
This type of measurement requires a larger range of detector values
as well as a larger range of drive level values which causes a
corresponding increase in measurement errors due to non-linearities
or an increase in on the performance requirements of these
elements.
[0072] In contrast, the method of FIG. 7 requires the use of the
detector over only a relatively small range of power levels, thus,
decreasing the impact of the non-linearities on the measurements.
In addition, because the range of detected signal power levels is
decreased, the range over which the analog-to-digital converter
produces corresponding digital values is also decreased. For this
reason, each step of the analog-to-digital converter output
corresponds to a smaller change in signal power level, therefore,
increasing the accuracy with which the signal power level is
measured.
[0073] The invention introduces a number of benefits into the
system. For example, the modular nature of the indoor unit and the
outdoor unit allow them to be separately built, tested and
calibrated and also separately serviced and replaced in the field.
However, it is not necessary that the invention be embodied in two
separate units. The advantages of the invention will also benefit a
system which is housed in a single body.
[0074] The calibration of the installed system at the current
operating conditions in the midst of routine operation has several
advantages. For example, each unit takes advantage of its full
operating capabilities at the current operating conditions. The
system does not require the measurement of an absolute signal power
level which can be difficult and inaccurate. The invention allows
certain changes to be automatically compensated for in the field.
For example, if the length of cable which connects the indoor and
outdoor units is increased in length, the insertion loss of the
cable typically increases. Such changes are automatically accounted
for during the next calibration process.
[0075] The invention also has advantages in comparison with factory
calibration. Factory calibration only calibrates for conditions as
they exist in the calibration test set up. Such testing requires
the fixed mating of indoor and outdoor units as well as the cables
which connect them. Although factory calibration can be conducted
over a variety of temperatures and frequencies, factory testing
does not account for aging. In addition, factory testing does not
account for environmental factors to which the unit may be
subjected. For example, the impedance match between the outdoor
unit and the satellite dish to which it is connected can affect the
signal power level produced by the outdoor units. It is typically
not practical to factory test a unit in conjunction with a
satellite dish with which it is permanent coupled.
[0076] Although the invention can be used in many systems, FIG. 10
is a block diagram illustrating an exemplifying system in which the
invention may be embodied. The system in FIG. 10 provides
high-speed, reliable Internet communication service over a
satellite link.
[0077] In particular, in FIG. 10, content servers 200 are coupled
to the Internet 202 which is in turn coupled to a hub station 204
such that the hub station 204 can request and receive digital data
from the content servers 200. The hub station 204 also communicates
via satellite 206 with a plurality of remote units 208A-208N. For
example, the hub station 204 transmits signals over a forward
uplink 210 to the satellite 206. The satellite 206 receives the
signals from the forward uplink 210 and re-transmits them on a
forward downlink 212. Together, the forward uplink 210 and the
forward downlink 212 are referred to as the forward link. The
remote units 208A-208N monitor one or more channels which comprise
the forward link in order to receive remote-unit-specific and
broadcast messages from the hub station 204.
[0078] In a similar manner, the remote units 208A-208N transmit
signals over a reverse uplink 214 to the satellite 206. The
satellite 206 receives the signals from the reverse uplink 214 and
re-transmits them on a reverse downlink 216. Together, the reverse
uplink 214 and the reverse downlink 216 are referred to as the
reverse link. The hub station 204 monitors one or more channels
which comprise the reverse link in order to extract messages from
the remote units 208A-208N. For example, in one embodiment of the
exemplifying system, the reverse link carries multiple access
channels in accordance with assignee's above referenced co-pending
application Serial No.______ [Attorney Docket No.
TACHYON.018CP2].
[0079] In one embodiment of the exemplifying system, each remote
unit 208A-208N is coupled to a plurality of system users. For
example, in FIG. 10, the remote unit 208A is shown as coupled to a
local area network 216 which in turn is coupled to a group of user
terminals 218A-218N. The user terminals 218A-218N may be one of
many types of local area network nodes such as a personal or
network computer, a printer, digital meter reading equipment or the
like. When a message is received over the forward link intended for
one of the user terminals 218A - 218N, the remote unit 208A
forwards it to the appropriate user terminal 218 over the local
area network 216. Likewise, the user terminals 218A-218N can
transmit messages to the remote unit 208A over the local area
network 216.
[0080] In one embodiment of the exemplifying system, the remote
units 208A-208N provide Internet service to a plurality of users.
For example, assume that the user terminal 218A is a personal
computer which executes browser software in order to access the
World Wide Web. When the browser receives a request for a web page
or embedded object from the user, the user terminal 218A creates a
request message according to well-known techniques. The user
terminal 218A forwards the request message over the local area
network 216 to the remote unit 208A, also using wellknown
techniques. Based upon the request message, the remote unit 208A
creates and transmits a wireless link request over a channel within
the reverse uplink 214 and the reverse downlink 216. The hub
station 204 receives the wireless link request over the reverse
link. Based upon the wireless link request, the hub station 204
passes a request message to the appropriate content server 200 over
the Internet 202.
[0081] In response, the content server 200 forwards the requested
page or object to the hub station 204 over the Internet 202. The
hub station 204 receives the requested page or object and creates a
wireless link response. The hub station transmits the wireless link
response over a channel within the forward uplink 210 and forward
downlink 212. For example, in one embodiment of the exemplifying
system, the hub station 204 operates in accordance with assignee's
co-pending application entitled Alan Gordon's Application,
application Serial No. ______ [Attorney Docket No. TACHYON.046A],
and assignee's co-pending application entitled Mark Abumeri's
Application, application Serial No. ______ [Attorney Docket No.
TACHYON.039A], each of which is filed concurrently herewith and the
entirety of which is hereby incorporated by reference. In one
embodiment, the invention is used to determine the assigned data
rate as described in the just-mentioned patent application.
[0082] The remote unit 208A receives the wireless link response and
forwards a corresponding response message to the user terminal 218A
over the local area network 216. In one embodiment of the
exemplifying system, the process of retrieving a web page or object
is executed in accordance with assignee's co-pending application
entitled DISTRIBUTED SYSTEM AND METHOD FOR PREFETCHING OBJECTS,
application Ser. No. 09/129,142[Attorney Docket No. TACHYON.001A2],
filed Aug. 5, 1998, the entirety of which is hereby incorporated by
reference. In this way, a bi-directional link between the user
terminal 218A and the content servers 200 is established.
[0083] In view of the description above, a myriad of alternative
embodiments will be readily apparent to one skilled in the art. For
example, the analog detector may be replaced with a detector which
produces a digital output indication. In addition, the temperature
sensor may also be replaced with a sensor which produces a digital
output indication. In such a case, in one embodiment, these
elements are coupled directly to the indoor unit controller. In yet
another embodiment, gain variation is achieved through the use of a
variable attenuator which can be located in either the indoor unit
or the outdoor unit.
[0084] The invention may be embodied in other specific forms
without departing from its spirit or essential characteristics. The
described embodiment is to be considered in all respects only as
illustrative and not restrictive and the scope of the invention is,
therefore, indicated by the appended claims rather than by the
foregoing description. All changes which come within the meaning
and range of equivalency of the claims are to be embraced within
their scope.
* * * * *