U.S. patent application number 10/125814 was filed with the patent office on 2002-08-15 for variable loop gain in double loop power control systems.
Invention is credited to Schiff, Leonard N..
Application Number | 20020111144 10/125814 |
Document ID | / |
Family ID | 22672595 |
Filed Date | 2002-08-15 |
United States Patent
Application |
20020111144 |
Kind Code |
A1 |
Schiff, Leonard N. |
August 15, 2002 |
Variable loop gain in double loop power control systems
Abstract
An apparatus and method for using a variable loop gain in a
double-loop power control system to control the power of a forward
link signal sent by a gateway to a user terminal to compensate for
fading in a wireless communications system. In one embodiment the
invention includes the steps of detecting fast fading in the
forward link signal and informing the gateway of the fast fading;
and at the gateway, reducing the loop gain of the power control
loop when fast fading is indicated. In another embodiment, the
invention includes the steps of, at the gateway, detecting fast
fading in a reverse link signal received from the user terminal and
reducing the loop gain of the power control loop when fast fading
is indicated.
Inventors: |
Schiff, Leonard N.; (San
Diego, CA) |
Correspondence
Address: |
QUALCOMM Incorporated
5775 Morehouse Drive
San Diego
CA
92121-1714
US
|
Family ID: |
22672595 |
Appl. No.: |
10/125814 |
Filed: |
April 18, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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10125814 |
Apr 18, 2002 |
|
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09183388 |
Oct 29, 1998 |
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Current U.S.
Class: |
455/69 ;
455/522 |
Current CPC
Class: |
H04W 52/36 20130101;
H04W 52/20 20130101; H04W 52/228 20130101; H04W 52/24 20130101;
H04W 52/12 20130101; H04W 52/225 20130101 |
Class at
Publication: |
455/69 ;
455/522 |
International
Class: |
H04B 001/00 |
Claims
What is claimed is:
1. An apparatus comprising: means coupled to a first station for
measuring a signal-to-noise ratio of a signal transmitted by a
second station; means for adjusting a transmitted signal power of
said signal as a function of a loop gain, said signal-to-noise
ratio, and a signal-to-noise ratio threshold; means coupled to said
first station for measuring a signal quality of the received
signal; means for adjusting said signal-to-noise ratio threshold as
a function of said signal quality and a signal quality threshold;
means coupled to said first station for measuring a fading rate of
said signal; and means for adjusting said loop gain as a function
of said fading rate and a fading rate threshold.
2. The apparatus of claim 1, wherein said signal quality and said
signal quality threshold are based on detection of error
events.
3. The apparatus of claim 2, wherein said error events are selected
from the group comprising error rate, bit error rate, and frame
error rate.
4. The apparatus of claim 1, further comprising: means for
determining that said fading rate is above said fading rate
threshold when multiple excursions of said signal-to-noise ratio
below said signal-to-noise ratio threshold occur between successive
adjustments of said transmitted signal power.
5. The apparatus of claim 1, further comprising: means for
accumulating a plurality of said signal-to-noise ratio measurements
to produce a time series; and a high-pass filter that filters said
time series to produce said fading rate.
6. An apparatus comprising: means coupled to a first station for
measuring a signal-to-noise ratio of a signal transmitted by a
second station; means for adjusting a transmitted signal power of
said signal as a function of a loop gain, said signal-to-noise
ratio, and a signal-to-noise ratio threshold; means coupled to said
first station for measuring a signal quality of the received
signal; means for adjusting said signal-to-noise ratio threshold as
a function of said signal quality and a signal quality threshold;
means coupled to said second station for measuring a fading rate of
a further signal transmitted by said first station; and means for
adjusting said loop gain as a function of said fading rate and a
fading rate threshold.
7. The apparatus of claim 6, wherein said signal quality and said
signal quality threshold are based on detection of error
events.
8. The apparatus of claim 7, wherein said error events are selected
from the group comprising error rate, bit error rate, and frame
error rate.
9. The apparatus of claim 6, wherein said means for measuring a
fading rate comprises means for measuring a further signal-to-noise
ratio of said further signal.
10. The apparatus of claim 9, further comprising means for
determining that said fading rate is above said fading rate
threshold when multiple excursions of said further signal-to-noise
ratio below a further signal-to-noise ratio threshold occur between
successive adjustments of said transmitted signal power.
11. The apparatus of claim 6, wherein said means for measuring a
fading rate comprises: means for accumulating a plurality of said
further signal-to-noise ratio measurements to produce a time
series; and a high-pass filter that filters said time series to
produce said fading rate.
12. A method comprising the steps of: measuring, at a first
station, a signal-to-noise ratio of a signal transmitted by a
second station; adjusting a transmitted signal power of said signal
as a function of a loop gain, said signal-to-noise ratio, and a
signal-to-noise ratio threshold; measuring, at said first station,
a signal quality of the received signal; adjusting said
signal-to-noise ratio threshold as a function of said signal
quality and a signal quality threshold; measuring, at said first
station, a fading rate of said signal; and adjusting said loop gain
as a function of said fading rate and a fading rate threshold.
13. The method of claim 12, wherein said signal quality and said
signal quality threshold are based on detection of error
events.
14. The method of claim 13, wherein said error events are selected
from the group comprising error rate, bit error rate, and frame
error rate.
15. The method of claim 12, further comprising the step of
determining that said fading rate is above said fading rate
threshold when multiple excursions of said signal-to-noise ratio
below said signal-to-noise ratio threshold occur between successive
adjustments of said transmitted signal power.
16. The method of claim 12, further comprising the steps of:
accumulating a plurality of said signal-to-noise ratio measurements
to produce a time series; and high-pass filter in said time series
to produce said fading rate.
17. A method comprising the steps of: measuring, at a first
station, a signal-to-noise ratio of a signal transmitted by a
second station; adjusting a transmitted signal power of said signal
as a function of a loop gain, said signal-to-noise ratio, and a
signal-to-noise ratio threshold; measuring, at said first station,
a signal quality of the received signal; adjusting said
signal-to-noise ratio threshold as a function of said signal
quality and a signal quality threshold; measuring, at said second
station, a fading rate of a further signal transmitted by said
first station; and adjusting said loop gain as a function of said
fading rate and a fading rate threshold.
18. The method of claim 17, wherein said signal quality and said
signal quality threshold are based on detection of error
events.
19. The method of claim 18, wherein said error events are selected
from the group comprising error rate, bit error rate, and frame
error rate.
20. The method of claim 17, wherein said step of measuring a fading
rate comprises measuring a further signal-to-noise ratio of said
further signal.
21. The method of claim 20, further comprising the step of
determining that said fading rate is above said fading rate
threshold when multiple excursions of said further signal-to-noise
ratio below a further signal-to-noise ratio threshold occur between
successive adjustments of said transmitted signal power.
22. The method of claim 17, wherein said step of measuring a fading
rate comprises the steps of: accumulating a plurality of said
further signal-to-noise ratio measurements to produce a time
series; and high-pass filter in said time series to produce said
fading rate.
Description
[0001] This patent application is a continuation of U.S. patent
application Ser. No. 09/183,388, filed Oct. 29, 1998, pending,
which is incorporated herein by reference.
BACKGROUND OF THE INVENTION
[0002] I. Field of the Invention
[0003] The present invention relates to wireless communication
systems. More specifically, the present invention relates to a
novel and improved system and method of power control in a wireless
communications system.
[0004] II. Related Art
[0005] Wireless communication networks are enjoying notable
popularity in all aspects of business, industry and personal life.
As such, portable, hand-held communication devices have experienced
widespread growth in recent years. Portable devices such as
cellular phones are now commonplace with business and personal
users alike. Additionally, advanced systems, such as satellite
communications systems using portable, hand held and mobile phones,
are currently being deployed.
[0006] In wireless communication systems, signals are subject to
fading. Fading occurs when environmental factors diminish the power
of a signal during its transmission from transmitter to receiver.
One measurement that quantifies fading is the signal-to-noise ratio
(SNR) of the received signal as measured at the receiver. Systems
have been developed to adjust the transmitted power of the signal
to compensate for fading. One such system is known as "single-loop"
power control.
[0007] In a single-loop power control system, the receiver monitors
the SNR of the received signal and sends commands to the
transmitter to adjust the transmitted power so as to maintain a
specified "threshold" SNR at the receiver. Conventional single-loop
power control systems generally employ two or three types of such
commands. One type of command instructs the transmitter to increase
the transmitted power. Another type of command instructs the
transmitter to decrease the transmitted power. The amount by which
the transmitted power is increased or decreased in response to such
a command is referred to as the "gain" of the loop. In some
systems, a third type of command is used to instruct the
transmitter to maintain the transmitted power at the current
level.
[0008] Single-loop power control works well in an environment with
slow fading. In slow fading, there is no substantial fading during
the time required for a power control command to reach the
transmitter and the resulting signal-to-noise ratio to be measured
at the receiver, known as the "period" of the loop. One example of
a slow fading environment is one having only thermal noise as
signal interference.
[0009] However, in a signal environment with medium-speed fading,
single-loop power control is inadequate. In medium-speed fading,
there is substantial fading during a single loop period. One
example of a medium-speed fading environment is where the
transmitter or receiver is moving rapidly past stationary
obstructions, causing rapid changes in signal attenuation. In such
a medium-speed fading environment, the threshold SNR may be
insufficient to ensure signal quality. This is because the loop is
too slow to respond to the rapid variations in the SNR of the
received signal.
[0010] In digital communication systems, the adequacy of the
threshold SNR can be quantified by the ratio of information bits
received in error to the total number of bits received. This ratio
is generally computed repeatedly for each frame. The ratio thus
computed is known as the "frame error rate" (FER) of the signal.
One type of system developed to address this problem is known as a
"double-loop" power control system.
[0011] In a double-loop power control system, the single-loop power
control system described above is used as the "inner" loop. The SNR
threshold used by the inner loop is modified by an "outer" loop
based on the FER of the received signal. For example, when the FER
rises above a predetermined FER threshold, the threshold SNR is
increased by a fixed, predetermined amount. This process continues
until the FER falls below the FER threshold.
[0012] One consideration in double-loop power control systems is
the selection of the magnitude of the fixed gain employed by the
inner loop. The selection of this gain is a trade-off between two
conflicting considerations. In a mediumspeed fading environment,
rapid loop response is required. This consideration argues for a
large inner-loop gain. With a large inner-loop gain, fewer loop
periods are required to change the threshold SNR by a large amount.
However, in a slow fading signal environment, a large gain will
result in large SNR oscillations about the threshold SNR. These
oscillations waste transmitter power. Thus a fixed inner-loop gain
is not suitable for applications in which the signal will
experience both fast fading and slow fading.
[0013] Furthermore, fixed gain systems experience difficulty in
fast fading signal environments. In fast fading, the SNR
experiences several large oscillations within a single outer-loop
period (that is, the time required to adjust the SNR threshold
based on one or more FER measurements). Fast fading oscillations
are typically on the order of hundreds of hertz. In such an
environment, the response time of the inner loop is no longer
important because the inner loop cannot possibly keep up with the
fading. What is needed is a double-loop power control system where
the inner-loop gain can be varied to suit the speed of the
fading.
SUMMARY OF THE INVENTION
[0014] The present invention is an apparatus and method for
adjusting the power of a signal sent by a transmitter to a receiver
to compensate for fading in a wireless communications system. In
one embodiment, the method includes the steps of measuring, at a
first station, a signal-to-noise ratio of a signal transmitted by a
second station; adjusting a transmitted signal power of the signal
as a function of a loop gain, the signal-to-noise ratio, and a
signal-to-noise ratio threshold; measuring, at the first station, a
signal quality of the received signal; adjusting the
signal-to-noise ratio threshold as a function of the signal quality
and a signal quality threshold; measuring, at the first station, a
fading rate of the signal; and adjusting the loop gain as a
function of the fading rate and a fading rate threshold.
[0015] In one embodiment, the method includes the steps of
measuring, at a first station, a signal-to-noise ratio of a signal
transmitted by a second station; adjusting a transmitted signal
power of the signal as a function of a loop gain, the
signal-to-noise ratio, and a signal-to-noise ratio threshold;
measuring, at the first station, a signal quality of the received
signal; adjusting the signal-to-noise ratio threshold as a function
of the signal quality and a signal quality threshold; measuring, at
the second station, a fading rate of a further signal transmitted
by the first station; and adjusting the loop gain as a function of
the fading rate and a fading rate threshold.
[0016] One advantage of the present invention is that it mitigates
the effects of fast fading.
BRIEF DESCRIPTION OF THE FIGURES
[0017] The features, objects, and advantages of the present
invention will become more apparent from the detailed description
set forth below when taken in conjunction with the drawings in
which like reference characters identify corresponding elements
throughout and wherein:
[0018] FIG. 1 is a block diagram illustrating an exemplary
communication system;
[0019] FIGS. 2 and 3 are block diagrams illustrating the
transceivers of FIG. 1 in greater detail;
[0020] FIG. 4 is a flowchart depicting the operation of an inner
power control loop according to a preferred embodiment of the
present invention;
[0021] FIG. 5 is a flowchart depicting the operation of an outer
power control loop according to a preferred embodiment of the
present invention; and
[0022] FIG. 6 is a flowchart depicting the operation of a
variable-gain inner double-loop power control loop according to a
preferred embodiment of the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0023] The present invention is an apparatus and method for double
loop power control in a wireless communications system. In a
preferred embodiment, the present invention operates within a
code-division multiple-access (CDMA) communications system. Power
control loops operating within such systems are disclosed in U.S.
Pat. No. 6,185,432, entitled "System and Method for Selecting Power
Control Modes" issued Feb. 6, 2001 and U.S. Pat. No. 6,259,928,
entitled "System and Method for Optimized Power Control" issued
Jul. 10, 2001, filed herewith which are assigned to the assignee of
the present invention, and incorporated by reference herein. Other
examples of techniques for power control in such communication
systems are found in U.S. Pat. Nos. 5,383,219, entitled "Fast
Forward Link Power Control In A Code Division Multiple Access
System," issued Jan. 17, 1995; 5,396,516, entitled "Method And
System For The Dynamic Modification Of Control Parameters In A
Transmitter Power Control System," issued Mar. 7, 1995; and
5,267,262, entitled "Transmitter Power Control System," issued Nov.
30, 1993, which are incorporated herein by reference.
[0024] I. Example Environment
[0025] Before describing the invention in great detail, it is
useful to describe an example environment in which the invention
can be implemented. The present invention can be implemented in any
wireless communication system, especially one in which it is
desirable to control the amount of power provided by a transmitter.
Such environments include, without limitation, cellular
communication systems, personal communication systems, satellite
communication systems, and many others.
[0026] FIG. 1 is a diagram illustrating an exemplary communication
system 100. Referring to FIG. 1, system 100 has two transceivers
102 and 104. Transceiver 102 has a transmitter 106 and a receiver
108. Transceiver 104 has a transmitter 112 and a receiver 110.
[0027] Data or other information is transmitted between the
transceivers over a transmission channel 122. In satellite,
cellular and other wireless communication systems, channel 122 is a
wireless link. In satellite communication systems, channel 122
includes one or more relay satellites. Channel 122 is a two-way
channel that includes a "forward" signal 116 and a "reverse" signal
118.
[0028] In some environments, channel 122 is a packetized data path
in which the data is transmitted in data packets. This is often the
case where the information is in the form of digital data. In other
environments, analog data is modulated onto a carrier and
transmitted across channel 122.
[0029] In the example of a cellular communication system,
transceiver 102 is a hand-held or mobile cellular telephone and
transceiver 104 is a base station at the local cell site that is
providing service in the phone's current area. In the example of a
satellite communication system, transceiver 102 is a hand-held,
mobile, or fixed transceiver (for example, a satellite telephone)
and transceiver 104 is located in a terrestrial gateway. In the
satellite communication system example, a satellite is used to
relay signals between the transceivers 102 and 104 over channel
122.
[0030] The present invention is described in terms of this example
environment. Description in these terms is provided for convenience
only. It is not intended that the invention be limited to
application in this example environment. In fact, after reading the
following description, it will become apparent to a person skilled
in the relevant art how to implement the invention in alternative
environments.
[0031] II. Power Control
[0032] The present invention is a system and method for adjusting
transmitted signal power to compensate for fast fading in a
wireless communications system. In fast fading, several signal
fades occur during a single outer-loop period. Thus the outer loop
is ineffective to mitigate fast fading. Such fast fading is often
superimposed upon a slower fading trend. The inventors have found
that one good solution to the fast fading case is to ignore the
fast (high-frequency) component of the fading and instead track any
slower (low-frequency) components of the fading. According to the
present invention, when fast fading is detected, the system
attempts to track the underlying slow fading rather than attempting
to track the fast fading. In a preferred embodiment of the present
invention, this is accomplished by using a small inner loop
gain.
[0033] FIG. 2 is a block diagram illustrating transceiver 102 in
greater detail. Transceiver 102 includes transmitter 106, receiver
108, a measurement element 202, a processor 204, a memory 206, a
data destination 210 and a data source 212. In operation, receiver
108 receives signal 116 and passes it to data destination 210. Data
destination 210 can be any element that makes use of the data, such
as a CODEC, MODEM, digital signal processor, and the like. Receiver
108 may perform certain tasks, such as demodulation, on signal 116,
as is well-known in the art.
[0034] Measurement element 202 makes certain measurements of the
characteristics of signal 116, as is described in detail below. For
example, these measurements include measurements of SNR, signal
quality based on the presence of one or more error events (such as
FER) and fading rate (FR). In a preferred embodiment, measurement
element 202 includes a SNR measurement circuit 214 and a frame
error measurement circuit 216. SNR measurement circuit 214 obtains
measurements of the SNR of received signal 116. Frame error
measurement circuit 216 obtains measurements of the error rate, or
one or more other error events, of received signal 116. Circuits
that accomplish these functions are well-known in the relevant
arts. These measurements are passed to a processor 204, which can
be any processor known in the art or developed hereafter. Processor
204 employs a memory 206 for storage of data, such as the SNR, FER,
and FR measurements, and other values, such as thresholds for
comparison with these measurements.
[0035] Data source 212 generates data for transmission. Data source
212 can include elements such as CODECs, MODEMs, digital signal
processors, and the like, as is well-known in the art. Transmitter
106 receives data from data source 212 and performs such tasks as
modulation. Processor 204 may be implemented using hardware,
software or a combination thereof and may be implemented as a
computer system or other processing system. In one embodiment,
processor 204 is implemented as one or more computer systems. In
another embodiment, processor 204 is implemented primarily in
hardware using, for example, hardware components such as
application specific integrated circuits (ASICs). Implementation of
a hardware state machine so as to perform the functions described
herein will be apparent to persons skilled in the relevant art(s).
In yet another embodiment, processor 204 is implemented using a
combination of both hardware and software.
[0036] FIG. 3 is a block diagram illustrating transceiver 104 in
greater detail. Transceiver 104 includes transmitter 112, receiver
110, a measurement element 302, a processor 304, a memory 306, a
data destination 310 and a data source 312. In operation, receiver
110 receives signal 118 and passes it to data destination 310. Data
destination 310 can be any element that makes use of the data, such
as a CODEC, MODEM, digital signal processor, and the like. Receiver
110 may perform certain tasks, such as demodulation, on signal 118,
as is well-known in the art.
[0037] Measurement element 302 makes certain measurements of the
characteristics of signal 118, as is described in detail below. For
example, these measurements include measurement of fading rate
(FR). In a preferred embodiment, measurement element 302 includes a
SNR measurement circuit 314. SNR measurement circuit 314 obtains
measurements of the SNR of received signal 118. Circuits that
accomplish this function are well-known in the relevant arts. These
measurements are passed to transmitter 112 and/or data source 312
for transmission as part of signal 116. These measurements are
passed to processor 304, which can be any processor known in the
art or developed hereafter. Processor 304 employs a memory 306 for
storage of data, such as the measurements, and other values, such
as thresholds for comparison with these measurements.
[0038] Data source 312 generates data for transmission. Data source
312 can include elements such as CODECs, MODEMs, digital signal
processors, and the like, as is well-known in the art. Transmitter
112 receives data from data source 312 and performs such tasks as
modulation before transmitting signal 116. Transmitter 112 also
includes a variable-gain amplifier 308 for amplifying the power of
the signal prior to transmission to produce signal 116. The gain of
amplifier 308 is controlled by processor 304.
[0039] FIGS. 4-6 are flowcharts depicting the operation of the
present invention according to a preferred embodiment. FIG. 4
depicts the operation of the inner power control loop of the
present invention. The function of the inner power control loop is
to adjust the signal power transmitted by transmitter 112. In a
preferred embodiment, the transmitted signal power is adjusted
according to the level of signal power received at receiver 108, as
described below.
[0040] Transmitter 112 transmits signal 116 over channel 122.
Signal 116 is received by receiver 108. The process begins with the
measurement by measurement element 202 of the power of signal 116,
as shown in step 402. In a preferred embodiment, measurement
element 202 measures the signal-to-noise ratio (SNR) of signal 116.
More specifically, the present invention measures the quantity
Eb/No, where Eb is energy per bit and No is noise density in units
of power/cycle. Of course, other measures of signal power can be
used without departing from the scope of the present invention. In
a preferred embodiment, SNR is measured for every frame of received
data.
[0041] In communication system 100, a predetermined SNR level,
referred to as the "SNR threshold," is associated with receiver
108. The SNR threshold represents the minimum SNR at which signals
should be received by receiver 108 in order to ensure data quality.
The SNR threshold can be selected according to methods that are
well-known in the relevant arts. One such method is to select a SNR
that will keep data errors under a certain percentage, such as one
percent. In step 404, receiver 108 compares the SNR measured in
step 402 to the SNR threshold.
[0042] If the measured SNR is lower than the SNR threshold, then
transmitter 106 of transceiver 102 transmits an "increase power"
command to transceiver 104, as shown in step 406. In a preferred
embodiment, the command is transmitted as part of signal 118 over
channel 122. In response, transmitter 112 increases the signal
power of the signal 116 by a predetermined amount, referred to as
the "gain" of the inner loop, or "inner loop gain." In a preferred
embodiment, the value of the inner loop gain, and the value of the
signal gain applied by amplifier 308, are stored in memory 306. The
value of the signal gain is manipulated by processor 304.
[0043] If the measured SNR exceeds the SNR threshold, then
transmitter 106 of transceiver 102 transmits a "decrease power"
command to transceiver 104, as shown in step 408. In response,
transmitter 112 decreases the signal power of signal 116 by the
inner loop gain. In either case, the process resumes at step
402.
[0044] FIG. 5 depicts the operation of the outer power control loop
of the present invention (also referred to as the "outer loop").
The function of the outer power control loop is to adjust the SNR
threshold of receiver 108. In a preferred embodiment, the SNR
threshold is adjusted according to the quality of the received
signal. In a preferred embodiment, the quality of the signal is
considered not only for the current frame, but also for a certain
number of previous frames. Also, in a preferred embodiment, the
measure of signal quality used is the presence of one or more error
events, for example using a frame error rate (FER). However, other
measures of signal quality, such as parity checks, can be used
without departing from the scope of the present invention. In
addition, other methods of evaluating the signal quality history,
such as averages and weighted averages, can be used.
[0045] A type of error often encountered is of the "burst" type.
Burst errors are characterized by short duration. In general, the
duration of a burst error is less than the inner loop period.
Therefore, the inner loop cannot compensate for these errors. For
this reason, it is desirable to isolate the inner loop from the
effects of burst errors. Burst errors are also characterized by
errors in multiple consecutive frames. The outer loop uses this
characteristic to detect burst errors. When the outer loop detects
errors in multiple consecutive frames, it determines that a burst
error has occurred. When a burst error is detected, the outer loop
does not change the SNR threshold of the inner loop. The outer loop
changes the SNR threshold of the inner loop only in response to
non-burst type errors, thereby isolating the inner loop from burst
errors.
[0046] Referring to FIG. 5, the process begins by making
measurements of a quantity indicative of the presence of errors,
for example FER, as shown in step 502. The process determines
whether or not errors are present in the current frame using the
results of such measurements, as shown in step 504. If no errors
are present in the current frame, as indicated by the "N" branch in
step 504, then transceiver 102 decreases the SNR threshold by a
predetermined amount, as shown in step 506. However, if errors are
present in the current frame, as indicated by the "Y" branch in
step 504, then the quality history of the received signal is
reviewed, as shown in step 508. In a preferred embodiment, the
error history comprises a predetermined number of previous frames
N. Of course, the error history can be selected in other ways
without departing from the scope of the present invention. The
error history is preserved in memory 206. If any of the previous N
frames contain an error, then transceiver 102 decreases the SNR
threshold by the outer loop gain, as shown in step 506, subject to
a desired frame or timing delay as discussed below.
[0047] However, if the previous N frames contain no errors, then
transceiver 102 increases the SNR threshold, as shown in step 506.
In a preferred embodiment, two change values are employed: one for
decreasing the SNR threshold, and the other for increasing the SNR
threshold. The change value for decreasing the SNR threshold is
small, so that the SNR threshold, and through the action of the
inner loop, the transmitted signal power, is gradually reduced in
error-free environments. Conversely, the change value for
increasing the SNR threshold is large, so that the SNR threshold,
and through the action of the inner loop, the transmitted signal
power, is quickly increased in error-prone environments.
[0048] In addition, it has been found that it is generally not
desirable to quickly change the SNR threshold for a certain number
of frames after an increase has been made regardless of the
presence or not of errors, at least in some systems. Therefore, in
one embodiment, an initial increase in the SNR occurs when
encountering a frame error after a certain number of error free
frames as just described, but for a preselected number of frames Z
after this adjustment no additional increase is allowed to occur.
That is, the detection or not of frame errors does not provide a
mechanism for selecting further increases in the threshold value
until Z frames or frame periods have occurred after an increase.
This is shown by optional step 512 which is positioned between
quality history checking step 508 and threshold adjustment step
510. In step 512, a check is made to see whether or not Z data
frames have been processed since the last SNR threshold increase.
The frame count for this processing step is initially set equal to
Z so that the first time an adjustment is requested it is made, as
shown by set/reset step 514. Subsequent adjustments will then be
determined from the frame count.
[0049] Whenever Z frames have not yet been processed or passed
through, the SNR threshold is allowed to decrease by a fairly small
amount, or at a low rate, for each of the Z frames, as shown in
step 516. That is, during or at the end of each frame period the
SNR threshold level is decremented or decreased by a small
percentage or amount, say on the order of 0.004 dB. Those skilled
in the art will readily recognize that other amounts may be used
including 0 dB, as desired. Processing then returns to step 512
where measurements continue and so forth. Once the requisite number
of preselected frames Z have been reached, the SNR threshold is
again increased in step 510 (or decreased in step 506) instead of
decremented in step 516. Once an SNR threshold increase occurs, the
frame count used in step 512 is reset to zero and the counting
process beings anew until Z frames have again passed.
[0050] This gradual decrease process or period allows the system to
"settle" before further action is taken and assures a more
predictable and reproducible response to signal conditions. In
addition, due to the bursty nature of some errors and the minimum
amount of delay in implementing the power increase commands
encountered in some systems (satellite) or situations, making short
term requests for power won't help, or have a desired impact.
However, waiting a few frames does help decrease the amount of
power used.
[0051] After the preselected number of Z frames has passed,
adjustment of the SNR threshold occurs as before. Detection of
errors again causes an increase in the SNR threshold, provided the
previous N frames contain no errors. In a preferred embodiment, Z
is selected to be six frames after the frame generating or
triggering an increase in SNR threshold, during which no additional
increase occurs and a gradual decrease is implemented. However,
those skilled in the art will understand that other values can be
chosen according to known response characteristics of the
communication system in which the invention is employed.
[0052] FIG. 6 is a flowchart depicting the operation of a
variable-gain inner double-loop power control loop according to a
preferred embodiment of the present invention. In step 602,
receiver 108 measures a fading rate of a signal received from
transceiver 104A. According to a preferred embodiment, receiver 108
measures the SNR of the received signal several times over the
course of each frame to produce a series of measurements. This
series is applied to a high-pass filter to detect rapid changes in
SNR, which indicate the presence of fast fading. The output of the
high-pass filter, referred to as the "fading rate" is compared to a
predetermined fading rate threshold, as shown in step 604.
[0053] When the fading rate does not exceed the threshold, as
indicated by the "N" branch from step 604, the fading is not
sufficiently rapid to be characterized as fast. In that case, the
inner-loop gain is set to a first predetermined gain level G1, as
shown in step 606.
[0054] When the high-frequency content of the fading exceeds the
threshold, as indicated by the "Y" branch in step 604, the fading
is sufficiently rapid to be characterized as fast. In that case,
the inner-loop gain is set to a second predetermined gain level G2,
as shown in step 608. In either case, processing resumes at step
602.
[0055] In a preferred embodiment, first gain level G1 is much
larger than second gain level G2. In other words, the inner-loop
gain applied during fast fading is much less than the inner-loop
gain applied otherwise. The result is that, during fast fading, the
power control loop does not attempt to track the rapid signal power
level fluctuations caused by the fast fading, but rather tracks the
slower power level fluctuations caused by slower fading. In one
embodiment, G1 is approximately 0.5 dB, and G2 is approximately 0.1
dB.
[0056] In one embodiment, the user terminal detects fast fading in
the signal transmitted from the gateway to the user terminal. In
this embodiment, the user terminal reports the fast fading
condition to the gateway, which responds by adjusting the
inner-loop gain of the power control loop. Referring to FIG. 1,
fast fading in signal 116 is detected by transceiver 102 in step
604. Transceiver 102 detects fast fading in signal 116 by
evaluating fluctuations in its SNR, as described in detail above.
Fast fading is detected by evaluating the SNR. Transmitter 106 then
transmits a command to receiver 110 to adjust the innerloop gain.
In accordance with that command, transceiver 104 adjusts the
innerloop gain in steps 606 and 608.
[0057] In another embodiment, the gateway infers that fast fading
exists in the signal transmitted from the gateway to the user
terminal by detecting fast fading of a signal transmitted by the
user terminal to the gateway. Referring to FIG. 1, the fading rate
of signal 116 is inferred by transceiver 104 by evaluating the SNR
fluctuations in signal 118 in steps 602 and 604, as described in
detail above. Transceiver 104 then adjusts the inner-loop gain in
steps 606 and 608 based on that evaluation.
[0058] III. Conclusion
[0059] The previous description of the preferred embodiments is
provided to enable any person skilled in the art to make or use the
present invention. The various modifications to these embodiments
will be readily apparent to those skilled in the art, and the
generic principles defined herein may be applied to other
embodiments without the use of the inventive faculty. Thus, the
present invention is not intended to be limited to the embodiments
shown herein but is to be accorded the widest scope consistent with
the principles and novel features disclosed herein.
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